Review on texturization effects in thermoelectric oxides

Rishi Prasad; Shekhar D. Bhame

doi:10.1007/s40243-019-0163-y

Review on texturization effects in thermoelectric oxides

Prasad, Rishi; Bhame, Shekhar D. 2020-01-09 00:00:00 Sustainable energy sources and energy-harvesting technologies have been researched for decades. Thermoelectric conversion is currently one of the primary foci in this area. Thermoelectric research has been concentrated into two parts—(i) strate- gies to enhance the efficiency of existing thermoelectric materials and (ii) development of new materials with promising thermoelectric parameters. Although such strategies have led to the improvement of thermoelectric non-oxide-based materi- als, the limitations possessed by them does not allow to be used at high temperatures. Due to the same reason, oxide-based materials have gained much attention. Here, we discuss about the oxide thermoelectric materials in detail and the effect of texturization on their morphology and transport properties. There is a lot of scope available for such class of materials for high-temperature applications. Keywords Thermoelectric review · Oxide thermoelectrics · Effect of texturization · Thermoelectric figure of merit Introduction cooling devices is 200–250 million$ per year that attracts a lot of research in this area [2]. Renewable energy has been one of the very widely studied Ioffe in the 1930s proposed the idea that semiconductors topics since the past few decades due to the rising problem act as better thermoelectric materials than metals that were of fulfilling the energy demands. The increasing demand previously used [3]. Though Altenkirch (in 1909) was the for oil and electricity is a much-known fact that cannot be first to establish a mathematical expression which relates ignored. On a global average, more than 70% of the energy physical properties of thermoelectric materials and the effi- generated is wasted one way or the other. TE energy harvest- ciency of simplified thermopile or thermoelectric generators, ing is promising in providing opportunities to harvest the his equation contained various parameters like electromo- waste heat that is rejected out of the automobile exhausts, tive force, thermal, and electrical resistances etc., Ioffe was chimneys of factories, etc. In this review, we will concentrate the first to quantify Z (Thermoelectric figure of merit) by only on thermal energy harvesting. Thermoelectric materi- integrating all the variables introduced by Altenkirch into als can be used as a source for both power generation and one term as Z [4, 5]. cooling devices [1]. Metal-based thermoelectric conversion The maximum energy conversion efficiency is given by materials were studied earlier followed by semiconducting ƞ and is defined as: max materials. Since metallic TE materials are not ideal for high- T −T , M−1 hot cold max= temperature applications as they are prone to corrosion and cold hot (1) M+ hot hamper the conductivity, oxide-based thermoelectric materi- 1/2 als have gained much attention. The world market of ther- where M = (1 + ZT ) , T being the average temperature, avg avg Z is the thermoelectric figure of merit, and T and T are moelectric generators is 25–50 million$ per year and that for hot cold temperatures of the hot and cold end of the module, respec- tively [6, 7]. The potential of a thermoelectric material for the application is measured with the help of a term called as * Shekhar D. Bhame the dimensionless thermoelectric figure of merit (ZT) which Shekhar.bhame@sitpune.edu.in measures the maximum heat conversion into electricity. ZT is given by: Symbiosis International (Deemed University), Symbiosis Institute of Technology, Lavale, Pune, Maharashtra 412115, India Vol.:(0123456789) 1 3 3 Page 2 of 22 Materials for Renewable and Sustainable Energy (2020) 9:3 S K = + + = + LT + , tot lat ele bip lat bip (5) ZT = T, (2) + + e l bip wher e K is a Boltzmann constant, m* is den - where S (V/K) is the Seebeck coefficient, σ (S/m) is electri- sity of states effective mass, h is the Planks constant –34 2 cal conductivity, and , , and (W/m K) are the elec- (6.6260 × 10 m kg/s), n being carrier concentration, e is e l bip –19 tronic, lattice, and bipolar contribution to thermal conduc- charge of an electron (1.6021 × 10 coulombs), µ is carrier tivity due to bipolar effects [8 , 9]. The power factor S σ is mobility, τ is relaxation time, K , , , being total, tot lat ele bip optimized in narrow gap semiconductors as a function of lattice, electronic and bipolar contribution to thermal con- 19 20 3 –8 2 carrier concentration (typically ~ 10 –10 carriers/cm ). ductivity, and L is Lorentz number (2.44 × 10 W Ω/K ) [11, Figure 1 shows the optimization of ZT by tuning the carrier 12]. is usually observed for relatively narrow band-gap bip concentration for Bi Te [10]. It was observed that maximum materials, which leads to the decrease of TE efficiency. This 2 3 ZT could be obtained on tuning the carrier concentration to be effect is the consequence of the diffusion of both electrons 19 20 3 between ~ 10 to ~ 10 carriers/cm . and holes in the same direction. The minority conductors get Since Wiedemann–Franz law relates electrical and thermal thermally exited through the bandgap leading to the cancel- conductivities as directly proportional quantities ( = LT σ), lation of net charge in the conduction band. They reduce the the quantities S, σ, and for a conventional 3D crystalline Seebeck coefficient as the minority charge carriers contrib- system cannot be varied independently to give a high ZT value. ute significantly. This is generally observed for very nar - Because what is desired is a high Seebeck coefficient, high row bandgap semiconductors (E < 0.5 eV) [13, 14]. What electrical conductivity, and low thermal conductivity. If elec- is commonly observed is for an ideal metal, they possess trical conductivity is low for a material, carrier concentration high electrical conductivity and low thermal conductivity. has to be increased. As we increase the carrier concentration, Glasses exhibit high thermal conductivity and low electrical electrical conductivity would increase. This, in turn, would conductivity. Since a mixture of these two properties would reduce the Seebeck coefficient and increase the electrical con- be optimum for a thermoelectric material, such materials tribution to thermal conductivity. The complex relationships are termed as “phonon glass electron crystal” since it would of these parameters can be summarized in the Fig. 3 and also possess high thermal conductivity like glass and still possess in the form of the equation as: high electrical conductivity like an ordered crystal. It has been reported by Dresselhaus that as the dimension of mate- 8K 2∕3 B rial decreases and approaches a nanometer scale, it possibly (3) S = m T , 3eh 3n causes differences in the density of states and gives rise to quantum confinement effects allowing new methods to tailor S, σ, and K values independently to achieve high ZT [15]. ne = ne = , (4) Although there are significant reports of various class of thermoelectric materials exhibiting low thermal conductiv- ity and high electronic conductivity leading to high power factor and ZT, as shown in Fig. 2. Operating them at a high temperature for waste heat conversion is still a concern. A number of concerns such as toxicity, low melting point, high oxidation tendency, and stability at high temperature are yet to be addressed in detail. Figure 2 shows the development of high ZT materials over the year for non-oxide-based systems adapted from Zhang et al. [11]. The extreme left indicates the state-of-the-art materials, mid-portion indicates the improvement till close to 2010, and the last part shows very high ZT obtained in the recent decade. Few of the newly included materials other than the reference quoted above are PbTe–SrTe, Cu Se/CNT, PbSe + 2%HgSe, and Na-doped SnS. Oxides are considered to be more stable with a high melt- ing point, relatively less toxicity, and cheap. During the ini- tial days of development, thermoelectric community had ignored this class of materials due to the low conductivity Fig. 1 Optimization of ZT by tuning the carrier concentration for of carriers and ionic nature. However, due to the features that Bi Te . Image adapted with permission from Snyder et al. [10], 2 3 Springer Nature possess, they have been investigated since past 2 decades. 1 3 Materials for Renewable and Sustainable Energy (2020) 9:3 Page 3 of 22 3 Fig. 2 The development of high ZT materials over the year for non-oxide-based systems [adapted with permission from Zhang et al. [11], Elsevier] SrTiO Strontium titanium oxide (STO) is an n-type perovskite oxide. They have a cubical crystal structure and belong to the space group of pm3 m [17] with a very high melting point of 2353 K. Since they have a very high melting point, it is a potential high-temperature thermoelectric candidate [18]. In Fig. 3, the periwinkle blue sphere indicates Sr atoms, brown indicates Ti atoms, and black sphere indicates oxy- gen atoms. STO has a wide bandgap of 3.2 eV. STO shows promising Seebeck coefficient, electrical conductivity, car - rier mobility, and large effective mass (m* ~ 10m ). The high Seebeck coefficient is attributed to the sixfold degeneracy of Fig. 3 Crystal structure of STO simulated using VESTA 3d-t2g conduction band of Ti [19]. The carrier concentration of STO can be controlled by adequate doping of elements to transform it from insulating to semiconducting with a con- 21 −3 Their electronic properties can be tuned by manipulating centration of ~ 10 cm [20]. It has been seen that Nb and their crystal structures, chemical compositions, and dop- La doping makes way for STO single crystals as an effec- ing concentrations. As thermoelectric oxides exhibit high tive thermoelectric material. Okuda et al. reported very high 2 covalent and ionic character, they tend to show high thermal Seebeck coefficient (2800–3600 µW/K m) and power factor 2 conductivity. Hence, doping them with suitable elements (28–36 µV/(K cm) for La-substituted STO (Sr La TiO ) 1−x x 3 is necessary. It has been seen that, in solids that show poor which was comparable to Bi Te [21]. However, since they 2 3 thermal conductivity, the phonon mean free path is in the show high thermal conductivity (6–12 W/m/K for single- range of interatomic distance. Hence, doping the right ele- crystalline STO) obtaining a high ZT was limited [22], since ment not only alters electronic properties but also alters the then, a lot of studies has been done on reducing the thermal thermal properties [16]. A number of oxide-based thermo- conductivity to enhance ZT. In 2003, Muta et al. tried to electric materials have been reported. A few promising ones understand the effect of rare-earth element doping in the to mention are SrTiO, Ca Co O, Na CoO , ZnO, In O , STO crystal. Not much difference in electrical conductiv - 3 3 4 9 x 2 2 3 and BiCuSeO. ity was seen on varying the substituent rare-earth element. 1 3 3 Page 4 of 22 Materials for Renewable and Sustainable Energy (2020) 9:3 However, in the case of thermal conductivity, the dopant had was decreased, an increase in power factor up to 75% was a strong influence. La and Sm showed a decrease in thermal reported. It has to be noted that the reduction of thermal conductivity as temperature increased. Gd, Dy, and Y doping conductivity was most effective in the case of Fe inclusion led to the least thermal conductivity without much depend- than the Cu inclusion as Fe acted as a better phonon scatter- ence on the temperature. This trend was attributed to tem- ing centre [22]. Electrons confined in quantum wells with perature-independent phonon impurity scattering. The order dimensionality narrower than de-Broglie wavelength results of thermal conductivity on doping is La > Sm > Gd > Y > Dy in 2D electron gas that exhibit exotic electronic properties. with Dy being the element of substitution with least thermal 2D electron well enhances Seebeck coefficient for STO as conductivity (~ 2.2 W/m/K) which naturally led to better ZT carrier electrons are more localized in STO than in heavy than the rest as Seebeck coefficient and electrical conductiv - metals as it is an insulator [18]_ENREF_93_ENREF_93. ity was almost the same [23]. Since Nb and La have one of High Seebeck coefficient has been seen for one atom thick the most structural tolerance for doping in STO and since it STO layers containing 2D electron gas (2DEG). Seebeck possesses highest electrical conductivity, B site substitution coefficient of up to 850 µV/K leading to a ZT of 2.4 2DEG with Nb gained much attention [20]. Few other rare-earth and ZT of ~ 0.24 (including the barrier layer) was reported eff substitutions in the STO matrix along with reduced sinter- by Ohta et al. [29]. Very recently, Nb-doped SrTiO /rGO ing time were reported by Kovalivisky et al. Pr, Nd, and composite was explored. The cationic vacancies, oxygen Sm incorporation showed promising electronic conductivity vacancies due to rGO, and heterointerfaces lead to increased as compared to La, Ce, Gd, Dy, and Y doping. However, phonon scattering. Mobility of charge carriers dramatically small cation elements like Dy and Y gave rise to better See- increased due to the cationic vacancies and heterointerfaces. beck coefficient. Oxygen-deficient perovskite layers present These features lead to a high power factor of ~ 1.98 mW/ in Nd-, Sm-, and Dy-doped STO led to phonon scattering K m promising ZT of 0.29 at 1180 K [30]. Tkach et al. leading to ZT as high as 0.42 at 1190–1225 K [24]. Tungsten in 2018 made use of a unique two-step firing process to substitution in the STO was performed by Kovalivisky et al. obtain bimodal distribution of 20% Nb-doped STO fine Tungsten substitution at the B site greatly increased the elec- particles (particles smaller size ~ 270 nm) with a grain size trical conductivity without affecting the Seebeck coefficient of ~ 2.3 µm and 59 µm while for the coarse particles (par- to a great extent. Tungsten is interesting as it can donate two ticles with larger size ~ 800 nm) exhibited an average grain electrons per W cation and also scatters the phonons due to size of ~ 8 µm. The lower number of grain boundaries within mass fluctuation in STO. While a ZT of 0.24 was reached at the large connected grains in fine size STN20 leads to better 1300 K by substituting 0.06 portion of W, on codoping W electrical conductivity (328 S/cm at 970 K). Energy l fi tering and Nb in STO (Sr Ti Nb W O ) resulted in a ZT effects in the fine size STN20 further reportedly lead to an 0.97 0.8 0.17 0.03 3±δ of 0.28 at 1270 K. Better performance due to Nb codoping increase in Seebeck coefficient with temperature. The power 2 was assigned to the phase separation and phonon scattering factor of this material obtained was 13.4 µW/K cm which [25]. Kovalivisky et al. reported the effect of Pr substitu- is comparable with that of state-of-the-art thermoelectric tion. Results show that Pr leads to an increase in electrical material Bi Te and finally leading to a ZT of 0.36 at 970 K 2 3 conductivity reducing the Seebeck coefficient. At the same [31]. Meanwhile, a much simpler approach was followed by time, thermal conductivity reduction was also observed and Ekren et al. where a fixed amount (0.5 wt%) of B O and 2 3 maximum reduction in thermal conductivity was found for 0.3 wt% ZrO were composited with Sr Nb TiO to obtain 2 0.9 0.1 3 x = 0.30 in Sr Pr TiO . However, a ZT of ~ 0.34 at 1173 K 1400 S/cm at room temperature and ~ 200 S/cm at 1273 K. 1−x x 3 was obtained for x = 0.10 [26]. Improvising the electrical A drastic improvement in electrical conductivity was seen conductivity in oxide-based system is crucial and this was on just 0.3 wt% addition of ZrO . The dislocation density addressed by Akin et al. by creating the oxygen vacancies. enhancement on addition of ZrO that leads to increased They could successfully improvise the electrical conduc- oxygen vacancies thereby increasing the carrier concentra- tivity by creating A site vacancies in La-doped STO lead- tion (also helps in reduction in thermal conductivity) hold ing to oxygen loss that leads to improvement in electrical responsible for this improvement. The addition of ZrO also conductivity [27]. However, clarity on the influence of such lead to the formation of uniform-sized smaller grains lead- vacancies on thermoelectric properties was further estab- ing to barely irregular grain boundaries that lead to carrier lished by Lu et al. They obtained a ZT of 0.4 at 973 K (for mobility improvement. Lowest Seebeck coefficient and high- x = 0.15 in Sr La TiO ) by tuning the A site and oxy- est power factor(~ 1000 µV/m. K ) were obtained for the 1−3x/2 x 3−δ 3+ 4+ gen vacancies along with mixed valent Ti and Ti that same composition leading to a ZT of 0.37 at 1015 K [32]. To held responsible for the improvement [28]. Most recently tune the thermal conductivity, substituting Ca and Ba at the in 2018, Srivastava et al. reported promising ZT of 0.36 A site has resulted in lowering the same [33, 34]. However, it (900 K) and 0.38 (1000 K) for Cu and Fe inclusion, respec- was reported that these dopants also supresses the power fac- tively, in Sr La Ti Nb O system. As the resistivity tor, thereby hampering the thermoelectric performance [35]. 0.8 0.067 0.8 0.2 3−δ 1 3 Materials for Renewable and Sustainable Energy (2020) 9:3 Page 5 of 22 3 Though there were other reports of doping STO, addition of structure. This represents a misfit structure with a rock salt secondary phases, and tuning the A and B site vacancies, no disordered layer of Ca CoO sandwiched between CdI type 2 3 2 further improvement in ZT as reported by Kovalivisky et al. CoO layers stalked along c-axis with [40, 41]. These two [24] has been seen till date. layers share the same lattice parameters. a = 4.8270(5) Å, c = 10.8300(2) Å, and β = 98.136(1) Å. Misfit of unit cell Eec ff t of texturization along the b-axis changes the lattice parameter b values as b = 4.5615(2) Å, b = 2.8173(1) Å for Ca CoO and CoO 1 2 2 3 2 Quin et al. followed a Reactive Template Grain Growth layer, respectively [42]. The cobaltates are said to reduce Method (RTGG) assisted tape casting method whereby a thermal conductivity due to phonon scattering from inco- Ruddlesden–Popper-type phase SrO(SrTiO ) material was herent boundaries. Thermoelectric properties of CCO-349 3 n synthesised that has a multi-layered structure. They con- (whisker-based single crystals) showed a high ZT of 1.2–2.7 sist of alternating layer of perovskite STO and superlattice for at 873 K. These results show that there is a lot of poten- layer of SrO stalked along the c-axis. Where they could tial for such materials in thermoelectric applications [43]. achieve a Seebeck Coefficient of − 399 µV/k at 1073 K ZT of 0.87 at 973 K was reported by Shikano and Funahashi without any dopant as compared to the highest Seebeck for single-crystalline (Ca CoO ) CoO misfit oxide [44]. 2 3 0.7 2 coefficient reported by Gd/Sm doped SrO(SrTiO ) syn- Due to the complexity of the production of single crystals, 3 n thesised by conventional solid-state-assisted hot-pressing development of polycrystalline materials is of importance. method (− 350 µV/k at 1073 K) [36]. Such an improve- Polycrystalline material synthesis has also been reported for ment was achieved by controlling the microstructure and these materials with various substitutions like Eu [45], Lu annealing temperature. Promising Seebeck coefficient has [46], Ho, and Ga [47] to give promising ZT values. Bis- been reported for SrT iO textured using the RTGG tech- muth has been considered as one among the most promising nique by Gao et al. where the texturization induced a low dopant. It has shown to increase electrical conductivity and thermal conductivity of 2.1/Wm/K at 1027 K and Seebeck Seebeck coefficient. Bismuth also leads to a reduced ther - coefficient of ~ 313 to ~ 328 µV/K within the temperature mal conductivity due to the larger size of Bi atoms and also range of 230 K to 1027 K [37]. The reported thermoelectric their corresponding mass [48]. Silver is another promising parameters in all the cases were found to be higher for the dopant that leads to an increase in electrical conductivity textured samples. and reduction in thermal conductivity because of the heavy ion. Whereas for Ag that is added as a composite mixture Ca Co O (CCO‑349) with CCO-349, Ag acts as a connector between the cobaltate 3 4 9 grains leading to a decrease in the scattering of charge car- The challenges of creating novel TE oxides have led to riers leading to increasing conductivity at grain boundaries. investigating a lot of materials. Layered cobaltates such as While doping of Ag ions increases the thermopower and Ca Co O (CCO-349) and Na CoO (NCO- × 12) have been charge mobility along with charge concentration, composite 3 4 9 x 2 found to be promising candidates for thermoelectricity. One decreases thermopower due to metal Ag at grain bounda- of the major steps in thermoelectric oxide materials was the ries. However, a balance between doping and compositing discovery of Na CoO by Terasaki et al. in 1997. This mate- in the same system (Ca Ag Co O /Ag-10 wt%) can lead x 2 2.7 0.3 4 9 rial possesses a unique conduction mechanism. The charge to a promising ZT of 0.5 at 1000 K [49]. Very recently, the carriers are transported via the CoO layer for p-type con- highest ZT of 0.74 has been achieved by Saini et al. at 800 K duction and phonon path is mediated through the N a layer. for Tb doped CCO-349 system. This is the highest ZT ever They possess a highly anisotropic structure with a Seebeck reported for a polycrystalline oxide-based system [50]. coefficient of 200 μΩ cm in the plane and 100 μV/K resistiv - 2 ity giving rise to a power factor of 50 μW/(K cm) which was comparable to that of the traditional material Bi Te with Eec ff t of texturization 2 3 2 40 μW/(K cm) [38]. This anisotropy refers to the in-plane anisotropy caused because of hexagonal lattice distortion by On following the thermoforging process by Pravel et al. on- the square RS-type layer, CCO-349 exhibits a larger thermo- calcium cobalt oxide (CCO-349), a decrement in electrical power meanwhile shows very less resistivity. They have ther- resistivity was seen on increasing the applied pressure dur- mal stability to up to 973 K. High carrier concentration and ing the process which increases grain alignment. This study low thermal conductivity make it a good example of elec- made way for further exploration of thermoforging and the tron crystal phonon glass-type material [39]. In the layered influence of applied pressure on CCO-349 system [51]. On cobaltate system, only calcium cobaltate has one cation with a comparative study of the effect of sintering technique on different oxidation state. A general representation of this the thermoelectric performance, Liu et al. found that SPS system is [CaCoO] [CoO ]. Where RS represents rock salt leads to higher degree of grain orientation. RS 2 1 3 3 Page 6 of 22 Materials for Renewable and Sustainable Energy (2020) 9:3 A sheet-like grains with 1–2 µm can be seen in both Densification and texturing are found maximum in HP sam- the cases due to the layered structure. However, a mas- ple, and thus, resistivity is minimum (6.1 mΩ cm at 300 K), sive grain alignment and reduction in pore size can be seen and Seebeck coefficient of 550 µW/m/K at 850 K in the case in the case of SPS (Fig. 4b, c). This grain alignment not of HP which was claimed to be one of the lowest resistivity only leads to greater density of material (90%-SPS as com- and highest S reported along the ab plane [55]. Since the pared to 70%-CS), but also holds responsible for the higher synthesized product was only 0.5 mm thick, module making electrical conductivity. The measured electrical conduc- would be a challenge. Hence, they stalked the multi-layers tivity for the CS sample was 34 S/cm at 973 K, whereas of CCO. Each layer of CCO was hot-pressed and polished. SPS sample exhibited an electrical conductivity of 118 S/ They were stalked together, hot-pressed again, and was cm. Although Seebeck coefficient variation was less, the cut along ab and c direction. Thermoelectric parameters higher difference in electrical conductivity leads to a high- measured parallel and perpendicular to c-axis show highly –4 2 power factor of 3.51 × 10 W/m/K (SPS) as compared to anisotropic nature of the sample. Finally, a ZT of 0.16 was –4 2 1.1551 × 10 W/m/K (CS). These factors lead to 34.5% realized along the ab plane [56]. To obtain a better power increment in ZT of SPS sintered sample as compared to the factor, optimizing stress while pressing multi-layers of CCO CS sample [52]. Lin et al. performed a unique combination was later identified. Better power factor (595 µW/m/K ) at of SPS and dynamic forging technique leading improved higher stress level (30 MPa) was realized by the team at texturization over the just SPS processed sample. It has been 900 K along the ab plane [57, 58]. The anisotropic resistiv- seen that this technique allows in lowering the resistivity of ity behaviour was also explored later using a modified SPS the sample without affecting much of Seebeck coefficient. technique called as SPT. In this technique, a pre-sintered To reduce thermal conductivity, lanthanum was doped in the pellet is kept in a die of bigger diameter in the second step CCO matrix. La leads to scattering of phonon as it is a heavy for free deformation of pellet resulting in better texturiza- mass element leading to mass fluctuation in the matrix. tion and platelet formation along the axis perpendicular to Finally, ZT of 0.26 at 975 K was reported [53]. Neodymium the pressing direction [59]. An improvement of 40–60% substitution in the CCO matrix along with texturization (170–180 µW/m/K ) in power factor was reported in 2016 efforts was done by Parvel et al. in 2007. Their results show as a result of microwave sintering. Microwave sintering that Nd increased the decomposition temperature of CCO. resulted in an increase in density as well as the growth of Also, thermoforging led to an increase in electrical con- elongated grains. The main reason for the enhancement is ductivity of the sample by 2.5-fold. The decrease in carrier assigned to better electrical conductivity due to low porosity concentration with Nb substitution holds responsible for the and texturing [60]. Strontium-doped CCO349 synthesised improvement in Seebeck coefficient and thermopower [54]. by hot-pressing exhibited higher electrical conductivity and A comparative study of conventional sintering, hot-pressing lower Seebeck Coefficient. Resistivity close to those sintered and spark plasma sintering, their texturization effects, and by SPS method (6 mΩ cm) was obtained for the hot-pressed influence on thermoelectric parameters of CCO-349 were Sr-doped (6.5 mΩ cm) sample at 1073 K. Presence of Sr done by Kenfaui et al. The micrographs obtained for these and Ca carbonate impurities on the surface of the sample methods revealed that there were a platelet formation and along with grain alignment and good conductivity between high degree of orientation in the case of the hot-pressed sam- the grains was attributed to be responsible for the improved ple. The order of degree of orientation was: HP > SPS > CS. electrical conductivity. Almost 20% higher PF (1.16 mW/ Fig. 4 Scanning electron microscopy of the fracture surface of CS (a) and SPS (b, c) materials 1 3 Materials for Renewable and Sustainable Energy (2020) 9:3 Page 7 of 22 3 2 K m at 800 °C) were exhibited by the Sr-doped samples as (not in the case of Ti) which also led to the Seebeck coef- a result of high chemical pressure effects. Though there was ficient improvement and has been reported in [69, 70]. Also, significant improvement in electrical conductivity and power compositing NCO with Ag has led to enhancement of ther- factor, ZT was estimated to be 0.29 at 1073 K due to the high moelectric properties as seen in the work by Seetawan et al. thermal conductivity (4.4 W/K m) [61]. ZT as good as 0.36 A 2% doping at the cobalt site exhibited marginally lower at 1073 K were reported for CCO synthesised by calcin- resistivity (66 µΩ cm) over the complete temperature range ing the nitrate hydrate precursors followed by cold pressing as compared to the other compositions. A reduction in ther- at 200 MPa and sintering at 1173 K. The porous structure mal conductivity was also observed mainly due to the single- clearly has lead to the reduction in thermal conductivity, phase NCO and due to the Ag presence, while the other which, for a highly dense sample, is a bane [62]. compositions exhibited mixed phase. These factors com- bined to gather with a high power factor of 0.31 mW/m/K Na CoO (NCO) at 973 K leading to a ZT of 0.124 at 973 K [71]. It has been x 2 seen that out of the transition-metal element addition (Ni, Great interest in misfit oxide-based thermoelectric mate- Fe, Mn, and Cu) into the cobalt site, only Ni and Cu were rials was seen after the reporting very high thermopower a promising dopant which rules out the possibility of other of ~ 100 μV/K by Terasaki et al. in 1997 for Na Co O [38]. elements mentioned as an effective dopant for enhanced x 2 4 Since the thermal conductivity exhibited by this lamellar thermoelectric performance [72–74]. Addition of Zn has based material was very low, it is considered to be fitting also led to enhancement in thermopower [75]. Krasutskaya into the PGEC concept [63]. This oxide has a misfit double- et al. recently reported a ZT of 1.57 at 1100 K for x = 0.89 layer structure. One layer consists of the CdI type-CoO with a low thermal conductivity of 0.829 mW/(mK ) which 2 2 ions with edge-shared octahedra and Na ions stalked along is one of the highest ZT reported for polycrystalline NCO- c-axis on the other. While CoO layer acts as an electron based system [76]. By doping Co site with a minute fraction reservoir and is responsible for electronic conductivity and of Fe (Na Co Fe O ) increased thermopower along 0.71 0.95 0.05 2 thermopower, the N a ion layer acts as an electron donor to the ab plane to a greater extent and was reported recently by the CoO layer and also holds responsible for reducing the Richter et al.[77]. Behera et al. reported a low-temperature thermal conductivity. The Na ions are intercalated between synthesis by nitrate–citrate combustion method which fol- the CoO layers. The concentration of Na ions can be varied lowed a low heat treatment strategy to obtain a ZT of ~ 0.01 to manipulate the number of conduction electrons on the Co at 780 K for the La Na CoO -based compound. The 0.85 0.15 3 layers [64]. When x = 0, the electrovalency of cobalt is + 3 thermopower of this compound was found to be promising with 5 electrons occupying the ground state of the t g band. (~ 374 µV/K at 318 K) due to strong electron correlation and And when x = 1, the electrovalency of cobalt is + 4 with a degeneracy of 3d orbitals of Co. However, thermal activation fully filled ground state t g band [65]. They crystallize in of holes due to Na (induces p-type conductivity) content at the hexagonal structure with a space group of P63/mmc. In higher temperature reduces the thermopower [78]. these family of oxides, since carrier density does not con- tribute to the power factor, tuning the same would leave no Eec ff t of texturization difference in the power factor. This class of materials was then found to be a promising candidate for high-temperature The hydrothermal method was reported by Zhang et al. for thermoelectrics as they exhibited good chemical stability the synthesis of NCO so as to develop materials with ori- and non-toxicity as compared to the other state-of-the-art ented growth which would add on to the texturing effects materials like Bi Te and PbTe [66]. Reports on polycrys- leading to improvements in thermoelectric performance. A 3 3 talline NaCo O with Cu doping at the Co site prepared by strong anisotropic behaviour was exhibited by the HT sam- 2 4 solid-state mixing method following hot pressing exhib- ple as the grain boundary density was different along in- ited ZT of 0.88 at 573 K for Na(Co Cu ) O (x = 0.05). plane and out-of-plane direction. However, the CSS sample 1−x x 2 4 Though the thermal conductivity was 30% higher for the failed to exhibit such a behaviour as a result of higher grain hot-pressed sample, a reduction in room temperature electri- size and lower degree of grain alignment. Hence, in-plane cal resistivity from 2 to 1.1 mΩ cm (for NCO) as a result of electrical conductivity and Seebeck coefficient was much hot pressing holds responsible for this improvement [67]. At higher (Fig. 5a) along the complete temperature range as low temperature, for sodium-deficient system, intercalated compared to the out-of-plane direction resulting in a drastic water results in superconducting transition. High Seebeck improvement in power factor of ~ 0.43 from ~ 0.08 mW/m K coefficient of ~ 120 µV/K was reported by tailoring the Na at 993 K (Fig. 5b). Due to the high orientation degree of (x = 0.15) stoichiometry in Na Co O [68]. Later on, adding NCO-based materials, the grain boundary density is more x 2 4 metallic phase of (Ag, Ti, Rh, Pd) into this system was tried along the out-of-plane direction. As a result of grain to increase the electronic conductivity which was successful boundary scattering due to the high grain density, thermal 1 3 3 Page 8 of 22 Materials for Renewable and Sustainable Energy (2020) 9:3 Fig. 5 Electrical conductivity (a), Seebeck coefficient (a), power factor (b), thermal conductivity (c), and thermoelectric figure of merit (d) vari- ations with respect to temperature for the NCO-based material under two pressing directions conductivity decreases (also leads to reduction in electrical various applications such as UV-light emitters, solar cells, conductivity) (Fig. 5c). However, NCO prepared by HT and flat panel display, light-emitting devices, piezoelectric trans- cut along the in-plane direction exhibits maximum ZT of ducers, brake linings, cosmetics, dental cements, lubricants, 0.064 at 923 K (Fig. 5d) due to the high power factor and paints, phosphors, and other products due to its low cost, low thermal conductivity (~ 5.2 W/mK at 923 K) [79]. The abundance, non-toxicity, and thermal/chemical stability transport properties of layered cobaltates are highly aniso- [80–82]. Wurtzite structure is the most common structure tropic due to the layered structure. Hence, texturization finds exhibited by ZnO with a space group of P6 mc. However, potential in the improvement of their properties as it would zinc blend structure with a space group of Fm3m is also seen lead to a higher degree of grain alignment, thereby increas- under pressure. A typical wurtzite crystal structure is shown ing the electrical conductivity. in Fig. 6. Here, the brown sphere indicates zinc atom and blue indicates an oxygen atom. ZnO Elements such as Aluminium, Indium, Gallium, etc. are doped in the ZnO matrix to optimise the wide bandgap of ZnO is an n-type wide band-gap semiconductor having ZnO for applications in the field of solar cells, thermoelec- bandgap of 3.37 eV, and has been used extensively for tric, and optoelectronic applications [83–85]. A high carrier 1 3 Materials for Renewable and Sustainable Energy (2020) 9:3 Page 9 of 22 3 were explored and was seen to exhibit one of the highest ZT of 0.52 at 1000 K for any n-type oxide-based material with 1% doping. The phonon scattering from multiple scale scat- tering sources such as point defects, grain boundary, ZnO nanoprecipitates, and micropores leads to very low thermal conductivity and can be attributed to as the main reason for the high ZT reported [92]. Further optimisation of dopant and size of nanoparticle could also lead way for better ZT [93]. Lately, Al and Zr codoping have been found to be a promising combination in the ZnO matrix which would sup- press the thermal conductivity and boost electrical conduc- tivity. Zn Al Zr O showed a ZT of 0.12 at 1173 K. 0.997 0.002 0.005 This mixed combination was seen to exhibit the lowest ther- mal conductivity and better ZT than the individual-doping compositions [94]. Dy doping has been tried in the ZnO matrix by Anju et al. Since Dy possess higher ionic radii and since they produce localised magnetic moment due to Fig. 6 Crystal structure of wurtzite-type ZnO simulated with VESTA unpaired electron, it can effectively act as the phonon scat- terer and lead to reduction in thermal conductivity. On the 20 −3 concentration (> 10 cm ) is exhibited by the group III addition of 1% Dy atoms, thermal conductivity was reduced elements which makes them ideal dopants. Out of the group to up to 2 W/mK at 900 K. Also, since Dy leads to remark- III elements, Al has been identified to be the most abun- able increase in electrical conductivity, i.e., up to 1000 times dant and also is a shallow donor making it one of the most as compared to pristine ZnO at 373 K. These factors lead to suitable dopants to increase the n-type character [80]. ZnO an improved ZT of 0.11 at 923 K [95]. exhibits a harmonically vibrating lattice as a result of the ionic and covalent nature leading to high thermal conduc- tivity (~ 40 W/m/K ). M. Ohtaki in 1996 identified ZnO to Eec ff t of texturization be a potential thermoelectric material exhibiting a PF of –4 2 8–15 × 10 W/m K for aluminium-doped zinc oxide and The thermoelectric properties exhibited by pure and doped leading to ZT of 0.3 at 1273 K. Though they possess high zinc oxide are discussed above. However, it is known that thermal conductivity due to the ionic character of ZnO, due texturization could lead to an improvement in the thermo- to higher carrier mobility and carrier concentration, the elec- electric parameters. Han et al. synthesised aluminium-doped trical conductivity shot up without much decrement in ther- ZnO by various methods to obtain rods, platelets, and nano- mopower [86]. Observation of enhancement in thermopower particles. SPS technique was used for sintering and induce of Al-doped ZnO-based systems was reported by Ohtaki as texturing in the sample (Fig. 7). The relation between vari- a result of phonon scattering by the nanovoids—leading to ous morphologies on the thermoelectric parameters after suppressed thermal conductivity [87] to give a ZT of 0.55 texturing was then explored [96]. to 0.57 at 1273 K, thermionic effects due to the nanovo - It was seen that the rods exhibited maximum electrical ids and suspected carrier energy filtering by the nanovoids conductivity (Fig. 8a) and carrier mobility (Fig. 8b). One to deliver a ZT of up to 0.6 at 1250 K [88]. Later, dually of the reasons for this improvement is the aligning of rods doped ZnO (Ga, Al) set a benchmark by exhibiting a ZT of along the perpendicular direction leading to a reduction in 0.65 at 1247 K where Ga acted as the scattering centres to number of grain boundary, thereby reducing the scattering reduce thermal conductivity, whereas Al acted as an agent of charge carriers, hence proving that microstructure and to increase the carrier mobility which increases the electri- morphology plays an important role. The parallelly cut rods cal conductivity [89]. More recently, Jood et.al reported a and platelets exhibited least thermal conductivity which ZT of ~ 0.44 for 1000 k that reached thermal conductivity again is a proof of anisotropic behaviour (Fig. 8c). Paral- of ~ 2 W/mK. However, further improvements are necessary lelly cut samples exhibit maximum grain boundaries lead- to obtain better ZT values [90]. Very recently, Zhakharchuk ing to maximum phonon scattering. However, nanoparticles et al. reported a slightly improved ZT of close to 0.12 for exhibit the least thermal conductivity due to impurity scat- Zn Al Zr O at 1200 K. While new scattering inter- tering and grain boundary scattering. Though nanoparticles 0.994 0.003 0.003 faces and suppression of thermal conductivity were due to exhibit poor electrical conductivity, carrier mobility, and zirconia, enhancement of electrical transport properties carrier concentration as compared to the other morpholo- mainly led to this enhancement [91]. Recently, Cd Zn O gies, since they exhibit the least thermal conductivity, and 1−x x 1 3 3 Page 10 of 22 Materials for Renewable and Sustainable Energy (2020) 9:3 Fig. 7 Illustration of the con- solidation process for a rods, b platelets, and c particles which were cut parallel and perpen- dicular to the pressing direction for measurements the ZT exhibited by nanoparticles is maximum (ZT = 0.30 at a consequence of Zn vacancies created as they are the pref- 3+ 1223 K) (Fig. 8d) [96]. erential site for incorporation of In ions. Their ordering further leads to (ZnO) .In O (IZO)-type phases [101]. It k 2 3 In O based ceramics was identified by Ohta et al. in 1996 that such series could 2 3 be potential thermoelectric material. They reported that ZT Homologous In O –ZnO-based system has been widely would increase with increase in the concentration of ZnO 2 3 studied for their thermoelectric performance as they pos- (i.e., k = 3, 5, 7 and 9 with maximum ZT at k = 9). ZT values sess excellent structural and chemical stability even at of 0.11 at 900 K have been reported by Bernik et al. for k = 5 higher temperature [97]. They are indexed by rhombohedral as a consequence of sintering, pre-reacted mixture, and raw symmetry. Homologous (ZnO) .In O belong to the space mixture in 1:1 ratio leading to lower thermal conductivity k 2 3 group of R3m for odd values of k and P63/mmc for even [102]. values of k [98]. Particularly, the wide bandgap exhibited The representation of the crystal structure of In O , 2 3 by these compounds leads to better thermoelectric param- ZnO, and (ZnO) In O is given in Fig. 9. The brown sphere 5 2 3 eters as they would resist the thermal excitation of electrons indicates Zn atoms, light blue indicates oxygen atoms, and at higher temperature [99]. They exhibit a layered struc- the periwinkle blue indicates In atom. The introduction ture with alternating layers of edge-sharing In–O octahe- of point defects in In O results in a glass-like thermal 2 3 dral layers and In–Zn–O layers arrange in a zigzag manner conductivity. The lattice thermal conductivity could be [100]. The n-type conductivity exhibited by this compound reduced by 60% and extraordinary low lattice thermal is dominant through hexagonal lattice than across the ab conductivity of 1.2 W/mK at 973 K was also reported. plane as the main in-plane conducting path is In–O layer However, point defect engineering by co-doping Zn and and In–Zn–O being the out-of-plane conducting path [97, Ce in the In O system (In Zn Ce O ) leads to a much 2 3 2−2x x x 3 99]. Local charge deficiencies in the crystal are created as better thermoelectric parameter (ZT = 0.44 for x = 0.12). 1 3 Materials for Renewable and Sustainable Energy (2020) 9:3 Page 11 of 22 3 Fig. 8 Electrical conductivity (a), carrier mobility (b), thermal conductivity (c), and thermoelectric figure of merit (d) of Zn Al O (x = 0.02) 1−x x measured for samples cut parallel and perpendicular to pressing direction Various other elements were doped in ZnO for thermo- in (ZnO) (In Al )O and k = 5 which was because of the k 1−x x electric applications such as Ca, In, Ni, Nb, Dy, Yb, Y, and increment in carrier mobility due to Al and reduction in Gd which have led to a decrease in thermal conductivity. thermal conductivity [104]. It was identified that Dy, Yb, and Gd are the most suit- Berardan et al. later explored the potential of Germa- able dopants, since they possess higher mean atomic mass nium doped In O (In Ge O ) as a thermoelectric mate- 2 3 2−x x 3 leading to a low heat capacity which is directly related to rial. It was observed that a decrease of fivefold in resistivity thermal conductivity. Kosir et al. studied this system for of the sample was seen on the addition of just x = 0.002 of varying k values of 5, 11, and 18. It was identified that Ge in the In O matrix. The resistivity of the sample fur- 2 3 total thermal conductivity was minimum for least k value. ther decreased on addition of more Ge which resulted in Similarly, promising electrical conductivity was seen in a decrease in resistivity from ~ 25 mΩ cm to ~ 1.5 mΩ cm the case of k = 5, while 11 and 18 exhibited less electrical at x = 0.015 measured at 1000 K. It was also pointed out conductivity. The reason for this trend was attributed to that Ge addition selectively reduces thermal conductivity the In planar defect density. Higher the defect density, (3–0.6 W/mK) without affecting charge carrier mobility Zn lower was the thermal conductivity and better electrical altogether giving rise to a ZT of 0.45 at 1273 K [105]. Since conductivity. It was also seen that sintering the composite In is a rare element, reducing the In content is of importance. at 1773 K (ZT ~ 0.125) resulted in a better ZT than for the Bhame et al. were successful in reducing the indium con- material sintered at 1573 K (ZT ~ 0.11) at 973 K [103]. tent and still obtain thermoelectric parameters comparable Later, in the same year, they reported that the addition to the doped In O system. The fluorite-type structure of 2 3 of Al into homologous IZO with k = 5 results in a slight Ga In Sn O resulted in interesting thermoelectric per- 3−x 5+x 2 16 increment in ZT. However, the improvement was very little formance [106]. Zhou et al. observed that in the In–Sn–O (ZT ~ 0.13) which was for the composition with x = 0.01 system, a lower density of the final sample reduces electrical 1 3 3 Page 12 of 22 Materials for Renewable and Sustainable Energy (2020) 9:3 at 1027 K. The textured samples were obtained by Tani et al. with the help of RTGG method (Fig. 10). A high degree of anisotropy was seen in the electrical conductivity measure- ments. The samples cut perpendicular to the casting direc- tion (c plane) exhibited less conductivity as compared to the sample cut parallel showing the effect of misalignment distribution of grains. Greater number of grain boundaries is seen in the case of perpendicularly cut sample, whereas long grains and fewer grain boundaries in parallelly cut samples lead to higher electrical conductivity. However, a huge dif- ference between the textured (~ 2 W/mK) and non-textured (~ 5 W/mK) sample is shown in the case of thermal conduc- tivity due to the high density exhibited by them regardless of the direction in which the sample is cut [98]. Isobe et al. synthesised Y-doped IZO5 (((ZnO) In Y ) O ) by RTGG method to obtain textured sam- 0.97 0.03 2 3 ples. SEM images revealed longer grains and grain orienta- tion for the perpendicularly cut sample as compared to the parallel one. Although the sintered specimens exhibited less density, the electrical conductivity was seen to be greater for the textured samples. Textured samples exhibited half the thermal conductivity exhibited by the non-textured speci- mens. The thermal conductivity exhibited by Y-doped non- textured (impurity scattering) specimen was also exhibited by the textured undoped samples (scattering due to pores). However, combining both the effects textured Y-doped IZO Fig. 9 Schematic representation of crystal structure for parent oxides samples exhibited the maximum ZT of 0.33 at 1073 K. Such (In O , ZnO) and (ZnO) In O 2 3 5 2 3 a ZT was possible due to the samples retaining high electri- cal conductivity in spite of low density [109]. conductivity. However, reactive sintering with gallium addi- Kaga et al. reported a ZT of 0.31 for textured Ca-doped tion seemed to overcome this by densifying the sample fur- IZO as compared to the non-textured sample that showed ther. It resulted in an increase in density to up to 90% of a ZT of 0.23 at 1053 K. RTGG technique was followed bulk density. Ga In Sn O and SnO secondary phases were for texturization. RTGG method introduces bigger grains 2 6 2 16 2 seen as a result of reactive sintering of the precursors. How- and higher degree of alignment than the reference sample. ever, a large number of localized grain boundaries among Electrical conductivity measurements show that there was a secondary phases result in grain growth and increase the certain degree of anisotropy present in the sample. Samples density. Though thermal conductivity increases with Ga cut parallel to ab-axis exhibited better electronic conduc- addition, finally with a minute addition of Ga, ZT of ~ 0.23 tivity. Though at temperatures close to room temperature, was reported at 1000 K [107]. Combe et al. reported a ZT of the thermal conductivity is less for c-plane measurements, 0.3 at 1000 K for the Ge doped In O system. They pointed at high-temperature both exhibited almost similar thermal 2 3 out a decrease in thermal conductivity at high temperature conductivity leading to ZT of 0.31 [110]. after the solubility limit of Ge in the In O system (less than 2 3 0.5 at%). The decrease in thermal conductivity was attrib- BiCuSeO uted to the homogeneously distributed insulating second- ary phases of In Ge O observed after the solubility limit. BiCuSeO is an interesting class of mixed anionic com- 2 2 7 Though electrical resistivity increased after the solubility pounds made of oxide-based chalcogenide atoms. The oxide limit, the total effect contributed to an enhanced ZT [108]. and chalcogenide atoms are indirectly bonded through the cationic atoms. This leads to an alternating insulating and 2+ Eec ff t of texturization conducting fluorite-like layers of oxide (Bi O ) and chalco- 2 2 2− gen (Cu Se ) , respectively, along the c-axis. This naturally 2 2 Among the homologous IZO series, k = 5 resulted in maxi- leads to better carrier mobility along with the a, b planes mum ZT and has been mentioned already. Texturization than along the c plane resulting in anisotropy. These features efforts were made on this composition to obtain ZT of 0.18 make them a suitable thermoelectric material and were first 1 3 Materials for Renewable and Sustainable Energy (2020) 9:3 Page 13 of 22 3 Fig. 10 The methodology followed for the synthesis of textured IZO (a). b SEM of the parallelly cut, c perpendicularly cut sample processed by RTGG method, and d reference sample Se atom, periwinkle blue sphere represents the Bi atom, and black sphere represents the oxygen atoms. At room tempera- 18 −3 ture, their carrier concentration is reported to be 10 cm and carrier mobility is 22/cm /V/s [114]. A huge leap in the electrical conductivity for this class of material was achieved in 2010 when Sr was doped with BiCuSeO. The electrical conductivity increased from 470 S/m to 4.8 × 10 S/m at 293 K on Sr substitution at the Bi site by a proportion of just 0.15 at%. This drastic increase was assigned to be respon- 2+ sible due to the carriers that are induced in the (Bi O ) 2 2 layer that acts as the charge reservoir by partially substi- 3+ 2+ tuting Bi with Sr and transferring it to the conducting 2− (Cu Se ) layer. As the thermal conductivity of this mate- 2 2 Fig. 11 Crystal structure of BiCuSeO generated using VASP rial is by default very low (~ 0.65 W/mK at 873 K) along with increased electrical conductivity and Seebeck coeffi- cient (~ 225 µV/K) leads to a ZT of 0.76 at 873 K [115]. reported in 2010 with Sr doping [11, 111, 112]. They usually Very soon on realizing the potential of oxychalcogenides, especially BiCuSeO, Ca doping at the Bi site resulted in crystallize in the tetragonal system with a = b = 3.9273 and c = 8.9293 as unit cell dimensions and belongs to the space a reduction of thermal conductivity and improved power factor as a result of improvised electrical conductivity as group of P4/nmm. The figure shows the crystal structure of BiCuSeO exhibiting a tetrahedral symmetry [113]. compared to the undoped sample. The replacement of Bi with Ca lead to an increase in the hole concentration hence Figure 11 represents the BiCuSeO crystal structure. The 2+ orange sphere represents a Cu atom, blue sphere represents transferring charge carriers from charge reservoir (Bi O ) 2 2 1 3 3 Page 14 of 22 Materials for Renewable and Sustainable Energy (2020) 9:3 2− layer to the conducting (Cu Se ) layer. The overall ZT for electrical conductivity, whereas Ca played the role to reduce 2 2 Bi Ca CuSeO is reported to be 0.9 at 923 K [116]. thermal conductivity. Since Pb has a delocalised 6 s orbital 0.0925 0.075 However, in 2014, Pei et al. reported the highest ZT of all that has a lone pair of electrons and due to an increased times till the date for undoped BCSO exhibiting a ZT of 1.4 effective mass, Seebeck coefficient and electrical conductiv - at 923 K. Such a huge leap from the previously reported ZT ity improve better than any other elemental doping. On the for pristine sample (0.70 at 773 K as in [117]) was mainly other hand, Ca induces both mass fluctuation (difference in because of the improvement of power factor (Fig. 12c) due mass), size, and strain field fluctuation (difference in inter - to modulation doping strategy (MD). MD strategy has been atomic coupling force) leading to minimum thermal con- used to increase the carrier mobility. They could improve ductivity. Though the solubility limit of Pb is 6–7%, higher mobility by twofold without deteriorating the carrier concen- amount of Pb exhibited formation of nanoprecipitates which tration. This leads to a high electrical conductivity without lead to reduction in lattice thermal conductivity. Though and resulted in a very high PF of 10 µW/cm/K at 923 K. the pristine BCSO are known to exhibit high Seebeck coef- However, MD had no much influence on the Seebeck coef- ficient, upon dual doping, it decreases to 175 µV/K at 873 K ficient. These factors along with a decrease in K (~ 0.25 W/ which three times is lower. Thus, the improvement in elec- lat mK) lead to ZT of 1.4 at 923 K (Fig. 12f) [118]. trical conductivity coupled with a moderate Seebeck coef- –3 Previous reports show that single-element doping has ficient gives rise to a promising power factor of 1.0 × 10 / 2 –4 2 led to the improvement in either electrical conductivity or Wm/K for x = 0.06 as compared to 2.2 × 10 /Wm/K at thermal conductivity. An improvement in both has been 873 K. The formation of Bi-rich nanoinclusions and CaO observed, but very moderate. To combine both the effects, contributes to phonon scattering leading to a reduction in lat- i.e., reduction in thermal conductivity and increase in elec- tice thermal conductivity from ~ 3.8 to ~ 2.9 W/mK at 873 K. trical conductivity, Liu et al. carried out dual-doping tech- All the factors combined together lead to a high ZT of ~ 1.5 nique, whereby Pb acted as agent for the improvement in at 873 K [119]. ZT as good as 1.09 was reported in 2016 by Das et al. for the pristine sample of BaCuSeO. However, the high ZT was attributed to a reduction in thermal conductivity 0.19 W/mK at 773 K due to all length scale phonon scat- tering by micrometre-sized grains, point defects, naturally layered structure, and oxygen vacancies [120]. Replacing Sb at the Bi site and Te in Se site by a proportion of 8 mol% (Bi Sb CuSe Te ) remarkably increased the ZT to 0.92 0.08 0.92 0.08 1.19 at 873 K. This increase in ZT was due to the increase in band covalency and increase in carrier concentration due to Sb/Te co-doping. Another reason was attributed to the smaller grain size due to long hours (16 h) of ball milling and Cu vacancies created which lead to an increase in hole concentration eventually leading to better electrical conduc- tivity. Sb/Te co-doping was also responsible for the reduc- tion in thermal conductivity as it leads to mass fluctuation and strain field fluctuation by the point defects and nano- structurization [121]. Eec ff t of texturization Since BCSO is a highly anisotropic material, texturizing these materials would lead to an improvement in the trans- port properties with respect to the direction of measurement. Hot forging is a method adopted for inducing the texturiza- tion in samples. Hot forging leads to higher order of grain alignment which was seen in BCSO with Ba doping. Prefer- ential orientation of grains was seen to get shifted from (102) Fig. 12 The electrical conductivity (a), seebeck coefficient (b), power to (003) direction. It was seen that as the number of hot forg- factor (c), thermal conductivity (d, e), and thermoelectric figure of merit (f) of modulation doped BCSO ing steps increases, the orientation along (003) became more prominent. While the non-textured (0 T) samples exhibited 1 3 Materials for Renewable and Sustainable Energy (2020) 9:3 Page 15 of 22 3 least anisotropy during the transport property measure- orientation resulting in close to 187 µV/K at 923 K for all ments, the textured samples exhibited maximum anisot- the samples, power factor for the 3 T⊥ sample was the high- ropy (increased with number of hot forging steps). Since est (8 µW/cm/K AT 923 K). Though thermal conductivity the grains arrange themselves in the form of platelets along would increase with hot forging, the ratio of electrical to the a- and b-axis, i.e., perpendicular to pressing direction thermal conductivity was the highest for 3 T⊥ resulting in a evident from the SEM, z-Euler images and inverse pole fig- high ZT of ~ 1.4 at 923 K [112]. ures, as shown in Fig. 13. The inverse pole figures especially highlight the high grain orientation along the c-axis for 3 T⊥ sample as compared to 0 T⊥. The grain boundary density Other oxides decreases in this direction leading to increase in both electri- cal conductivity and thermal conductivity. In addition to the oxide-based systems discussed above, However, along the parallel cut direction, since the grain there are also a few other materials that are researched boundary density is greater, it leads to a decrease in trans- upon. One class of them is the double perovskites. Double port properties. Electrical conductivity increases from 450 perovskites are an interesting class of materials that exhibit to 700 S/cm at room temperature for three times hot forged half-metallic ground state in which conduction electrons are sample that was cut perpendicular direction. While Hall fully spin-polarized. The potential of such double perovs- effect showed not much variation in carrier concentration, kite-based materials for thermoelectric application is high- so the improvement in electrical conductivity was attributed lighted by Roy et al. and high Seebeck coefficient (290 µV/K to an increase in carrier mobility (2/cm/V/s for 0 T⊥ and at 1124 K) for material like S r TiFeO has been reported 2 6 4/cm/V/s for 3 T⊥). The transport property improvement [122]. ZT of 0.29 at 1223 K was later reported by the team along the ⊥ cut samples was attributed to the higher grain for p-type Ba Sr TiCoO (x = 0.2). This material exhibited x 2−x 6 alignment as confirmed from the XRD. While the Seebeck glass-like behaviour. Thermal conductivity was reported to coefficient was seen to be independent of any such grain be within 0.01 to 0.55 W/mK which is very low as compared Fig. 13 The microstructure (a, e, h, k), grain size distribution (b, f, i, l), z-Euler images (c, g, j, m) of 0, 1, 2, and 3 T⊥ samples, respectively, and the inverse pole figures (d, n) for 0 and 3 T⊥ samples 1 3 3 Page 16 of 22 Materials for Renewable and Sustainable Energy (2020) 9:3 Fig. 14 Representation of the development of thermoelectric oxides over the years to the state-of-the-art materials existing in the market. The centres, metallic electrical conductivity is shown due to the low thermal conductivity, however, was attributed to the combination of Sr TiFeO and Sr TiMoO . On adding Ba 2 6 2 6 phonon scattering in the crystal due to multiple cations, alongside increases the Seebeck coefficient at the cost of the transition from relaxor to the glassy state after Burns electrical conductivity leading to such high-power factor val- temperature and shorter grain size [123]. Increasing Ba ues [125]. Pseudobrookites (Fe TiO ) have been known for 2 5 composition to x = 0.25 showed a very high Seebeck coef- their high Seebeck coefficient of -100 to – 370 µV/K within ficient of ~ 800 µV/K at 1123 K at the cost of compromising the range of 286–1000 K and low thermal conductivity electricidal conductivity. However, the author suggests that (1–1.5 W/mK within the temperature range of 673–1000 K) aliovalent substitution could tune the electrical conductiv- [126]. Efforts have been done to improvise their electrical ity of such materials [124]. The promising power factor conductivity to obtain a promising ZT. Improvement in the of ~ 35 µW/mK at 1100 K was reported for a composition electrical conductivity, thermal conductivity deuteration, of n-type Ba Sr Fe Mo O . While thermal conductivity and power factor was reported by et al. recently as a result of 0.1 1.9 0.5 0.5 6 is taken care by the grain size and cationic phonon scattering optimizing the percentage of Ti content in F e Ti O . An 2−x 1+x 5 improvised ZT of 0.014 at 1000 K was obtained at x = 0.25 1 3 Materials for Renewable and Sustainable Energy (2020) 9:3 Page 17 of 22 3 leading to a few microcracks which reduce the thermal con- ductivity without affecting the electrical conductivity to a great extent as in the case of x = 0.5 [127]. Since oxides, in general, show high resistivity, oxygen deficiencies were created using vacuum sintering in a helium atmosphere to improve the electrical conductivity of Ni-doped SnO . Power factor as good as 20.195 µV/K at 950 K was obtained for 3at% doped SnO revealing the high-temperature perfor- mance potential of this material [128]. On a summary, the following graph depicts the different promising thermoelectric oxides reported over the years. The development of thermoelectric n and p-type oxides is given in Fig. 14. The various oxides labelled in Fig. 14 are–In–Sn–O [129], (Ca, Bi)MnO [130], (Ca,In)MnO 3 3 [131], (Zn, Al)O [86], (Zn, Al)O nanovoid [88, 132], (Zn, Al, Ga)O [89], (Ca, Eu) Co O [45], (Ca, Ho) Co O [133], 3 4 9 3 4 9 (Ca, Lu) Co O [46], CaMn Nb O [134], Ca (Co, Ga) O 3 4 9 1−x x 3 4 9 [47], (Ca, Ag) Co O /Ag [49], (Ca, Ag, Lu) Co O [58], 3 4 9 3 4 9 Na(Co,Cu) O [67], Na(Co,Zn) O [135], NaCo O [136], 2 4 2 4 2 4 Bi Sr Co O [137], (Sr,Dy)TiO [23], Ca(Mn,Nb)O [134], 2 2 2 y 3 (In,Zn,Ge)O [138], (Zn,Al)O [90], PPP/(Zn.Ni)O [139], (Zn. In)O [140], TiC O @TiO -TiO [141], In(Zn,Sn)O [142], 0.1 0.9 y 2 BiCu0.90SeO [143], (Bi,Sr)CuSeO [117], (Bi,Ba)CuSeO [112], (Bi,Na)CuSeO [144], (Bi,Pb)CuSeO [144], (Bi,Ba) CuSeO [112], (Bi,Pb)CuSeO [145], Modulation BiCuSeO [118], (Bi,Ba)CuSeO [146], and (Bi,Ca,Pb)CuSeO [119]. The following table highlights the overall effect of tex- turization on thermoelectric parameters of various materials as discussed above: It can be seen from the table that texturization has played a vital role in improving the thermoelectric parameters. However, further studies combining methodologies such as nanostructurisation, modulation doping, and texturization could lead to further improvements. It is clear that ZnO- based materials lack attention and improvements could be made by doping and texturing, since they can be used for high-temperature applications (Table 1). Summary The effect of nanostructurisation and texturization of ther - moelectric ceramics and their credibility for the improve- ment of figure of merit is discussed. The most typical thermoelectric materials and their heat-recovery range are summarized. Thermoelectric parameters can be tuned by texturization using various methods such as reactive tem- plate grain growth method, hot press, spark plasma sinter- ing, hotforging, etc. The representation in Fig. 15 makes it clear that more studies on ZnO, In O Ca Co O and other 2 3 3 4 9 oxide-based systems have to be done in the future as they 1 3 Table 1 A comparison of effect of texturization on thermoelectric parameters for the promising oxide-based materials (*%change in PF) Material Measuring tem- Before texturization After texturization % Change References perature (K) 2 2 σ, S/cm |S|, µV/K PF, µW/cm/K Κ, /Wm/K ZT σ, S/cm |S|, µV/K PF, µW/cm/K Κ, /Wm/K ZT SrTiO 1027 140 170 4 4.7 0.08 50 280 3.9 2 0.14 57.14 [36, 148] Ca Co O 973 30 186 1 – – 118 175 3.5 – – 28.57* [52] 3 4 9 Na CoO 923 64 200 2.2 4.4 0.038 90 220 4.3 4.5 0.064 59.37 [79] x 2 ZnO 1000 – 300 – 2 0.44 – 200 – 3.2 0.30 − 68.1 [90, 96] In O 1053–1273 162/9.1 170 – 2.2 0.23 251/11 160 2.15 2.4 0.31 74.19 [110] 2 3 BiCuSeO 923 200 180 6.3 0.48 1.1 230 187 8.1 0.58 1.38 79.71 [112] * resembles % change in PF which is mentioned at the top of table 3 Page 18 of 22 Materials for Renewable and Sustainable Energy (2020) 9:3 Fig. 15 Thermoelectric materials classified based on environmental friendliness and heat-recovery range [adapted and modified from Ohtaki et al. [147]] included in the article’s Creative Commons licence, unless indicated are promising for mid to high-temperature heat-recovery otherwise in a credit line to the material. If material is not included in applications. Figure 15 is a representation of various ther- the article’s Creative Commons licence and your intended use is not moelectric materials classified based on the heat-recovery permitted by statutory regulation or exceeds the permitted use, you will range (adapted from [147] with modification). The times need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativ ecommons .or g/licenses/b y/4.0/. ahead are expected to see a very robust alternative to the existing alloy-based thermoelectric modules based on oxide materials. A lot of work is already going on in developing the thermoelectric modules using non-metallic materials. References Certainly, the progress in nanostructuring and techniques to sinter the powders without significantly affecting the nano- 1. Zhang, D.-B., Zhang, B.-P., Ye, D.-S., Zhu, Y.-B.: Thermoelec- tric properties of textured Zn Al O ceramics prepared by spark structure will impact the performance of newly developed 1–x x plasma sintering of hydrothermally synthesized rod-shaped pow- thermoelectric materials. ders. Ceram. Int. 41(10), 14627–14634 (2015) 2. Vining, C.B.: An inconvenient truth about thermoelectrics. Nat. Acknowledgements The authors would like to acknowledge the Mater. 8(2), 83 (2009) Department of Science & Technology-Science & Engineering Research 3. Vedernikov, M., Iordanishvili, E.: AF Ioffe and origin of modern Board (DST-SERB) for the funds received under the Grant number: semiconductor thermoelectric energy conversion. In: Thermo- SERB/F/3112/2014-15. R. Prasad would like to acknowledge Symbio- electrics, 1998. Proceedings ICT 98. XVII International Confer- sis International (Deemed University) for the award of Senior Research ence on 1998, pp. 37–42. IEEE Fellowship. 4. Takabatake, T., Suekuni, K., Nakayama, T., Kaneshita, E.: Pho- non-glass electron-crystal thermoelectric clathrates: experiments Funding This study was funded by Department of Science & Tech- and theory. Rev. Mod. Phys. 86(2), 669 (2014) nology–Science & Engineering Research Board (DST-SERB), India 5. Polozine, A., Sirotinskaya, S., Schaeffer, L.: History of develop- (Grant Number: SERB/F/3112/2014-15). ment of thermoelectric materials for electric power generation and criteria of their quality. Mater. Res. 17(5), 1260–1267 (2014) 6. Aswal, D.K., Basu, R., Singh, A.: Key issues in development of Compliance with ethical standards thermoelectric power generators: high figure-of-merit materials and their highly conducting interfaces with metallic intercon- Conflict of interest The authors declare that we have no conflict of nects. Energy Convers. Manag. 114, 50–67 (2016) interest. 7. Salvador, J.R., Cho, J.Y., Ye, Z., Moczygemba, J.E., Thomp- son, A.J., Sharp, J.W., Koenig, J.D., Maloney, R., Thompson, Open Access This article is licensed under a Creative Commons Attri- T., Sakamoto, J.: Conversion efficiency of skutterudite-based bution 4.0 International License, which permits use, sharing, adapta- thermoelectric modules. PCCP 16(24), 12510–12520 (2014) tion, distribution and reproduction in any medium or format, as long 8. Tripathi, M., Bhandari, C.: High-temperature thermoelectric as you give appropriate credit to the original author(s) and the source, performance of Si–Ge alloys. J. Phys. Condens. Matter 15(31), provide a link to the Creative Commons licence, and indicate if changes 5359 (2003) were made. The images or other third party material in this article are 1 3 Materials for Renewable and Sustainable Energy (2020) 9:3 Page 19 of 22 3 9. Rowe, D., Bhandari, C.: Modern Thermoelectrics. Reston Publ. 28. Lu, Z., Zhang, H., Lei, W., Sinclair, D.C., Reaney, I.M.: High- Co., Reston (1983). (ISBN 0-8359-4593-6) figure-of-merit thermoelectric La-doped A-site-deficient 10. Snyder, G.J., Toberer, E.S.: Complex thermoelectric materials. SrTiO ceramics. Chem. Mater. 28(3), 925–935 (2016) Nat. Mater. 7(2), 105 (2008) 29. Ohta, H., Kim, S., Mune, Y., Mizoguchi, T., Nomura, K., Ohta, 11. Zhang, X., Zhao, L.-D.: Thermoelectric materials: energy con- S., Nomura, T., Nakanishi, Y., Ikuhara, Y., Hirano, M.: Giant version between heat and electricity. J. Materiomics 1(2), 92–105 thermoelectric Seebeck coefficient of a two-dimensional elec- (2015) tron gas in SrTiO . Nat. Mater. 6(2), 129 (2007) 12. Weidenkaff, A.: Thermoelectricity for future sustainable 30. Okhay, O., Zlotnik, S., Xie, W., Orlinski, K., Gallo, M.J.H., energy technologies. In: European Physical Journal Web of Otero-Irurueta, G., Fernandes, A.J., Pawlak, D.A., Weidenkaff, Conferences 2017 A., Tkach, A.: Thermoelectric performance of Nb-doped SrTiO 13. Zhao, L., Wu, H., Hao, S., Wu, C.-I., Zhou, X., Biswas, K., enhanced by reduced graphene oxide and Sr deficiency coopera- He, J., Hogan, T.P., Uher, C., Wolverton, C.: All-scale hierar- tion. Carbon 143, 215–222 (2018) chical thermoelectrics: MgTe in PbTe facilitates valence band 31. Tkach, A., Resende, J., Saravanan, K.V., Costa, M.E.V., Díaz- convergence and suppresses bipolar thermal transport for high Chao, P., Guilmeau, E., Okhay, O., Vilarinho, P.M.: Abnormal performance. Energy Environ. Sci. 6(11), 3346–3355 (2013) grain growth as a method to enhance the thermoelectric perfor- 14. Gong, J., Hong, A., Shuai, J., Li, L., Yan, Z., Ren, Z., Liu, mance of Nb-doped strontium titanate ceramics. ACS Sustain. J.-M.: Investigation of the bipolar effect in the thermoelectric Chem. Eng. 6, 15988–15994 (2018) material CaMg Bi using a first-principles study. PCCP 18(24), 32. Ekren, D., Azough, F., Gholinia, A., Day, S.J., Hernandez- 2 2 16566–16574 (2016) Maldonado, D., Kepaptsoglou, D.M., Ramasse, Q.M., Freer, R.: 15. Dresselhaus, M.S., Chen, G., Tang, M.Y., Yang, R., Lee, H., Enhancing the thermoelectric power factor of Sr 0.9 Nd 0.1 T iO Wang, D., Ren, Z., Fleurial, J.P., Gogna, P.: New directions for through control of the nanostructure and microstructure. J. Mater. low-dimensional thermoelectric materials. Adv. Mater. 19(8), Chem. A 6(48), 24928–24939 (2018) 1043–1053 (2007) 33. Muta, H., Ieda, A., Kurosaki, K., Yamanaka, S.: Substitution 16. Ren, G.-K., Lan, J.-L., Ventura, K.J., Tan, X., Lin, Y.-H., effect on the thermoelectric properties of alkaline earth titanate. Nan, C.-W.: Contribution of point defects and nano-grains to Mater Lett 58(30), 3868–3871 (2004) thermal transport behaviours of oxide-based thermoelectrics. 34. Muta, H., Kurosaki, K., Yamanaka, S.: Thermoelectric proper- Comput. Mater. 2, 16023 (2016) ties of doped BaT iO –SrTiO solid solution. J. Alloy Compd. 3 3 17. Kinaci, A., Sevik, C., Çağın, T.: Electronic transport proper- 368(1–2), 22–24 (2004) ties of SrTiO and its alloys: Sr La TiO and SrTi M O 35. Yamamoto, M., Ohta, H., Koumoto, K.: Thermoelectric phase 3 1–x x 3 1–x x 3 (M=Nb, Ta). Phys. Rev. B 82(15), 155114 (2010) diagram in a CaT iO –SrTiO –BaTiO system. Appl. Phys. Lett. 3 3 3 18. Ohta, H.: Thermoelectrics based on strontium titanate. Mater. 90(7), 072101 (2007) Today 10(10), 44–49 (2007) 36. Wang, Y., Lee, K.H., Ohta, H., Koumoto, K.: Thermoelectric 19. Shang, P.-P., Zhang, B.-P., Liu, Y., Li, J.-F., Zhu, H.-M.: Prep- properties of electron doped SrO ( SrTiO ) n (n = 1, 2) ceramics. aration and thermoelectric properties of La-doped SrTiO J. Appl. Phys. 105(10), 103701 (2009) ceramics. J. Electron. Mater. 40(5), 926–931 (2011) 37. Gao, F., Yang, S., Li, J., Qin, M., Zhang, Y., Sun, H.: Fabrica- 20. Ohta, S., Nomura, T., Ohta, H., Koumoto, K.: High-tempera- tion, dielectric, and thermoelectric properties of textured SrTiO ture carrier transport and thermoelectric properties of heavily ceramics prepared by RTGG method. Ceram. Int. 41(1), 127–135 La-or Nb-doped Sr Ti O single crystals. J. Appl. Phys. 97(3), (2015) 034106 (2005) 38. Terasaki, I., Sasago, Y., Uchinokura, K.: Large thermoelectric 21. Okuda, T., Nakanishi, K., Miyasaka, S., Tokura, Y.: Large ther- power in N aCo O single crystals. Phys. Rev. B 56(20), R12685 2 4 moelectric response of metallic perovskites: Sr La TiO (0 (1997) 1−x x 3 %3c ~ x %3c ~ 0.1). Phys. Rev. B 63(11), 113104 (2001) 39. Hébert, S., Flahaut, D., Pelloquin, D., Martin, C., Maignan, A., 22. Srivastava, D., Norman, C., Azough, F., Schäfer, M.C., Hejtmanek, J.: Cobalt oxides as potential thermoelectric ele- Guilmeau, E., Freer, R.: Improving the thermoelectric prop- ments: the influence of the dimensionality. In: Thermoelectrics, erties of SrTiO -based ceramics with metallic inclusions. J. 2003 Twenty-Second International Conference on-ICT 2003, pp. Alloys Compd. 731, 723–730 (2018) 161–166. IEEE 23. Muta, H., Kurosaki, K., Yamanaka, S.: Thermoelectric proper- 40. Miyazaki, Y., Onoda, M., Oku, T., Kikuchi, M., Ishii, Y., Ono, Y., ties of rare earth doped SrT iO . J. Alloys Compd. 350(1–2), Morii, Y., Kajitani, T.: Modulated structure of the thermoelec- 292–295 (2003) tric compound [Ca CoO ] 0.62CoO . J. Phys. Soc. Jpn. 71(2), 2 3 2 24. Kovalevsky, A., Yaremchenko, A., Populoh, S., Thiel, P., Fagg, 491–497 (2002) D., Weidenkaff, A., Frade, J.: Towards a high thermoelectric 41. Pravarthana, D., Lebedev, O., Hebert, S., Chateigner, D., Salva- performance in rare-earth substituted SrTiO : effects provided dor, P., Prellier, W.: High-throughput synthesis of thermoelectric by strongly-reducing sintering conditions. Phys. Chem. Chem. Ca Co O films. Appl. Phys. Lett. 103(14), 143123 (2013) 3 4 9 Phys. 16(48), 26946–26954 (2014) 42. Wu, T., Tyson, T.A., Chen, H., Bai, J., Wang, H., Jaye, C.: A 25. Kovalevsky, A., Populoh, S., Patrício, S., Thiel, P., Ferro, M., structural change in Ca Co O associated with enhanced ther- 3 4 9 Fagg, D., Frade, J., Weidenkaff, A.: Design of SrTiO -based moelectric properties. J. Phys. Condens. Matter 24(45), 455602 thermoelectrics by tungsten substitution. J. Phys. Chem. C (2012) 119(9), 4466–4478 (2015) 43. Funahashi, R., Matsubara, I., Ikuta, H., Takeuchi, T., Mizutani, 26. Kovalevsky, A., Yaremchenko, A., Populoh, S., Weidenkaff, U., Sodeoka, S.: An oxide single crystal with high thermoelectric A., Frade, J.: Enhancement of thermoelectric performance performance in air. Jpn. J. Appl. Phys. 39(11B), L1127 (2000) in strontium titanate by praseodymium substitution. J. Appl. 44. Shikano, M., Funahashi, R.: Electrical and thermal properties of Phys. 113(5), 053704 (2013)single-crystalline (Ca CoO ) CoO with a Ca Co O structure. 2 3 0.7 2 3 4 9 27. Akin, I., Li, M., Lu, Z., Sinclair, D.C.: Oxygen-loss in A-site Appl. Phys. Lett. 82(12), 1851–1853 (2003) deficient Sr 0.85 La 0.10 TiO perovskite. RSC Adv. 4(61), 45. Wang, D., Chen, L., Yao, Q., Li, J.: High-temperature thermo- 32549–32554 (2014) electric properties of Ca3Co4O9+ δ with Eu substitution. Solid State Commun. 129(9), 615–618 (2004) 1 3 3 Page 20 of 22 Materials for Renewable and Sustainable Energy (2020) 9:3 46. Nong, N., Liu, C.-J., Ohtaki, M.: High-temperature thermoelec- thermoelectric properties. J. Eur. Ceram. Soc. 39(4), 1237– tric properties of late rare earth-doped Ca3Co4O9+ δ. J. Alloys 1244 (2019) Compd. 509(3), 977–981 (2011) 63. Takahata, K., Iguchi, Y., Tanaka, D., Itoh, T., Terasaki, I.: Low 47. Nong, N., Liu, C.-J., Ohtaki, M.: Improvement on the high tem- thermal conductivity of the layered oxide (Na, Ca) Co O : 2 4 perature thermoelectric performance of Ga-doped misfit-layered another example of a phonon glass and an electron crystal. Phys. Ca 3 Co 4−xGa x O 9+ δ (x= 0, 0.05, 0.1, and 0.2). J. Alloys Rev. B 61(19), 12551 (2000) Compd. 491(1–2), 53–56 (2010) 64. Roger, M., Morris, D., Tennant, D., Gutmann, M., Goff, J., Hoff- 48. Li, S., Funahashi, R., Matsubara, I., Ueno, K., Sodeoka, S., mann, J.-U., Feyerherm, R., Dudzik, E., Prabhakaran, D., Boo- Yamada, H.: Synthesis and thermoelectric properties of the new throyd, A.: Patterning of sodium ions and the control of electrons oxide materials Ca Bi Co O + δ (0.0%3c x%3c 0.75). Chem. in sodium cobaltate. Nature 445(7128), 631 (2007) 3-x x 4 9 Mater. 12(8), 2424–2427 (2000) 65. Li, Q.: On the thermoelectric properties of layered cobaltates. 49. Wang, Y., Sui, Y., Cheng, J., Wang, X., Su, W.: Comparison MRS Online Proc. Libr. Arch. 886 (2005) of the high temperature thermoelectric properties for Ag-doped 66. Thermoelectrics, O.: Research Signpost/ed. by K. Koumoto, I. and Ag-added Ca Co O . J. Alloys Compd. 477(1–2), 817–821 Terasaki, N. Murayama. Trivandrum, India (2002). 3 4 9 (2009) 67. Yakabe, H., Kikuchi, K., Terasaki, I., Sasago, Y., Uchinokura, 50. Saini, S., Yaddanapudi, H.S., Tian, K., Yin, Y., Magginetti, D., K.: Thermoelectric properties of transition-metal oxide NaCo/ Tiwari, A.: Terbium ion doping in Ca Co O : a step towards sub 2/O/sub 4/system. In: Thermoelectrics, 1997. Proceedings 3 4 9 high-performance thermoelectric materials. Sci. Rep. 7, 44621 ICT’97. XVI International Conference on 1997, pp. 523–527. (2017) IEEE 51. Prevel, M., Lemonnier, S., Klein, Y., Hébert, S., Chateigner, 68. Motohashi, T., Naujalis, E., Ueda, R., Isawa, K., Karppinen, D., Ouladdiaf, B., Noudem, J.: Textured Ca Co O thermo- M., Yamauchi, H.: Simultaneously enhanced thermoelectric 3 4 9 electric oxides by thermoforging process. J. Appl. Phys. 98(9), power and reduced resistivity of N a Co O by controlling Na x 2 4 093706 (2005) nonstoichiometry. Appl. Phys. Lett. 79(10), 1480–1482 (2001) 52. Liu, Y., Lin, Y., Shi, Z., Nan, C.W., Shen, Z.: Preparation of 69. Seetawan, T., Amornkitbamrung, V., Burinprakhon, T., Ca Co O and improvement of its thermoelectric properties by Maensiri, S., Kurosaki, K., Muta, H., Uno, M., Yamanaka, S.: 3 4 9 spark plasma sintering. J. Am. Ceram. Soc. 88(5), 1337–1340 Thermoelectric power and electrical resistivity of Ag-doped (2005) Na Co O . J. Alloys Compd. 407(1–2), 314–317 (2006) 1.5 2 4 53. Lin, Y.-H., Lan, J., Shen, Z., Liu, Y., Nan, C.-W., Li, J.-F.: 70. Kurosaki, K., Muta, H., Uno, M., Yamanaka, S.: Thermo- High-temperature electrical transport behaviors in textured electric properties of N aCo O . J. Alloys Compd. 315(1–2), 2 4 Ca Co O -based polycrystalline ceramics. Appl. Phys. Lett. 234–236 (2001) 3 4 9 94(7), 072107 (2009) 71. Seetawan, T., Amornkitbamrung, V., Burinprakhon, T., Maen- 54. Prevel, M., Reddy, E.S., Perez, O., Kobayashi, W., Terasaki, siri, S., Kurosaki, K., Muta, H., Uno, M., Yamanaka, S.: Ther- I., Goupil, C., Noudem, J.G.: Thermoelectric properties of sin- moelectric properties of N a Co O /Ag composites. J. Alloys x 2 4 tered and textured Nd-substituted Ca Co O ceramics. Jpn. J. Compd. 414(1–2), 293–297 (2006) 3 4 9 Appl. Phys. 46(10R), 6533 (2007) 72. Park, K., Jang, K.: Improvement in high-temperature thermo- 55. Kenfaui, D., Chateigner, D., Gomina, M., Noudem, J.G.: Tex- electric properties of NaCo O through partial substitution of 2 4 ture, mechanical and thermoelectric properties of Ca Co O Ni for Co. Mater. Lett. 60(8), 1106–1110 (2006) 3 4 9 ceramics. J. Alloys Compd. 490(1–2), 472–479 (2010) 73. Yao, Q., Wang, D., Chen, L., Shi, X., Zhou, M.: Effects of 56. Kenfaui, D., Lenoir, B., Chateigner, D., Ouladdiaf, B., Gomina, partial substitution of transition metals for cobalt on the high- M., Noudem, J.G.: Development of multilayer textured temperature thermoelectric properties of Ca Co O . J. Appl. 3 4 9+δ Ca Co O materials for thermoelectric generators: influence Phys. 97(10), 103905 (2005) 3 4 9 of the anisotropy on the transport properties. J. Eur. Ceram. 74. Terasaki, I., Tsukada, I., Iguchi, Y.: Impurity-induced transi- Soc. 32(10), 2405–2414 (2012) tion and impurity-enhanced thermopower in the thermoelectric 57. Kenfaui, D., Chateigner, D., Gomina, M., Noudem, J.G.: Ani-oxide NaCo Cu O . Phys. Rev. B 65(19), 195106 (2002) 2–x x 4 sotropy of the mechanical and thermoelectric properties of hot- 75. Tsai, P.-H., Zhang, T., Donelson, R., Tan, T.T., Li, S.: Power pressed single-layer and multilayer thick Ca Co O ceramics. factor enhancement in Zn-doped N a CoO . J. Alloys Compd. 3 4 9 0.8 2 Int. J. Appl. Ceram. Technol. 8(1), 214–226 (2011) 509(16), 5183–5186 (2011) 58. Van, N.N., Pryds, N.: Nanostructured oxide materials and mod- 76. Krasutskaya, N., Klyndyuk, A., Evseeva, L., Tanaeva, S.: Syn- ules for high-temperature power generation from waste heat. thesis and properties of N a CoO (x = 0.55, 0.89) oxide ther- x 2 Adv. Nat. Sci. Nanosci. Nanotechnol. 4(2), 023002 (2013) moelectrics. Inorg. Mater. 52(4), 393–399 (2016) 59. Noudem, J.G., Kenfaui, D., Chateigner, D., Gomina, M.: 77. Richter, R., Shopova, D., Xie, W., Weidenkaff, A., Lecher - Toward the enhancement of thermoelectric properties of lamel- mann, F.: Thermopower enhancement from engineering the lar Ca Co O by edge-free spark plasma texturing. Scr. Mater. Na CoO interacting fermiology via Fe doping. Adv. Con- 3 4 9 0.7 2 66(5), 258–260 (2012) dens. Matter Phys. 2018, 1–7 (2018) 60. Miyazawa, K., Amaral, F., Kovalevsky, A., Graça, M.: Hybrid 78. Behera, S., Kamble, V.B., Vitta, S., Umarji, A.M., Shivaku- microwave processing of Ca Co O thermoelectrics. Ceram. mara, C.: Synthesis, structure and thermoelectric properties 3 4 9 Int. 42(8), 9482–9487 (2016)of La Na CoO perovskite oxides. Bull. Mater. Sci. 40(7), 1-x x 3 61. Torres, M., Costa, F., Flahaut, D., Touati, K., Rasekh, S., Fer- 1291–1299 (2017) reira, N., Allouche, J., Depriester, M., Madre, M., Kovalevsky, 79. Zhang, W., Liu, P., Wang, Y., Zhu, K., Tai, G., Liu, J., Wang, A.: Significant enhancement of the thermoelectric performance J., Yan, K., Zhang, J.: Textured N a CoO ceramics sintered x 2 in Ca Co O thermoelectric materials through combined stron- from hydrothermal platelet nanocrystals: growth mechanism 3 4 9 tium substitution and hot-pressing process. J. Eur. Ceram. Soc. and transport properties. J. Electron. Mater. 47(7), 4070–4077 39(4), 1186–1192 (2019) (2018) 62. Bittner, M., Kanas, N., Hinterding, R., Steinbach, F., Groen- 80. Look, D.C.: Progress in ZnO materials and devices. J. Electron. eveld, D., Wemhoff, P., Wiik, K., Einarsrud, M.-A., Feldhoff, Mater. 35(6), 1299–1305 (2006) A.: Triple-phase ceramic 2D nanocomposite with enhanced 1 3 Materials for Renewable and Sustainable Energy (2020) 9:3 Page 21 of 22 3 81. Huang, M.H., Mao, S., Feick, H., Yan, H., Wu, Y., Kind, H., 99. Peng, H., Song, J.-H., Hopper, E.M., Zhu, Q., Mason, T.O., Free- Weber, E., Russo, R., Yang, P.: Room-temperature ultraviolet man, A.J.: Possible n-type carrier sources in In2O3 (ZnO) k. nanowire nanolasers. Science 292(5523), 1897–1899 (2001) Chem. Mater. 24(1), 106–114 (2011) 82. Pearton, S., Norton, D., Ip, K., Heo, Y., Steiner, T.: Recent 100. Wen, J., Wu, L., Zhang, X.: A unique arrangement of atoms for progress in processing and properties of ZnO. Prog. Mater Sci. the homologous compounds InM O (ZnO) m (M= Al, Fe, Ga, 50(3), 293–340 (2005) and In). J. Appl. Phys. 111(11), 113716 (2012) 83. Serier, H., Gaudon, M., Ménétrier, M.: Al-doped ZnO pow- 101. Košir, M., Podlogar, M., Daneu, N., Rečnik, A., Guilmeau, E., dered materials: Al solubility limit and IR absorption proper- Bernik, S.: Phase formation, microstructure development and ties. Solid State Sci. 11(7), 1192–1197 (2009) thermoelectric properties of (ZnO) k In O ceramics. J. Eur. 2 3 84. Hasnidawani, J., Azlina, H., Norita, H., Bonnia, N., Ratim, S., Ceram. Soc. 37(8), 2833–2842 (2017) Ali, E.: Synthesis of ZnO nanostructures using sol–gel method. 102. Bernik, S., Košir, M., Guilmeau, E.: Microstructure and thermo- Proc. Chem. 19, 211–216 (2016) electric characteristics of (ZnO) k× In O : based ceramics (k= 5 2 3 85. Ogi, T., Hidayat, D., Iskandar, F., Purwanto, A., Okuyama, and 11). Zaštita materijala 57(2), 318–325 (2016) K.: Direct synthesis of highly crystalline transparent conduct- 103. Košir, M., Podlogar, M., Daneu, N., Rečnik, A., Guilmeau, E., ing oxide nanoparticles by low pressure spray pyrolysis. Adv. Bernik, S.: Phase formation, microstructure development and Powder Technol. 20(2), 203–209 (2009) thermoelectric properties of (ZnO) kIn O ceramics. J. Eur. 2 3 86. Ohtaki, M., Tsubota, T., Eguchi, K., Arai, H.: High-tempera- Ceram. Soc. 37(8), 2833–2842 (2017) ture thermoelectric properties of (Zn Al ) O. J. Appl. Phys. 104. Košir, M., Čeh, M., Ow-Yang, C.W., Guilmeau, E., Bernik, S.: 1−x x 79(3), 1816–1818 (1996) Structural features and thermoelectric properties of Al-doped 87. Ohtaki, M., Araki, K.: Thermoelectric properties and ther-(ZnO) 5In O homologous phases. J. Am. Ceram. Soc. 100(8), 2 3 mopower enhancement of Al-doped ZnO with nanosized pore 3712–3721 (2017) structure. J. Ceram. Soc. Jpn. 119(1395), 813–816 (2011) 105. Bérardan, D., Guilmeau, E., Maignan, A., Raveau, B.: In O : Ge, 2 3 88. Ohtaki, M., Hayashi, R.: Enhanced thermoelectric performance a promising n-type thermoelectric oxide composite. Solid State of nanostructured ZnO: a possibility of selective phonon scat- Commun. 146(1–2), 97–101 (2008) tering and carrier energy filtering by nanovoid structure. In: 106. Bhame, S., Zhou, T., Guilmeau, E., Kinemuchi, Y., Delorme, F., Thermoelectrics, 2006. ICT’06. 25th International Conference Raveau, B.: Synthesis and thermoelectric properties of oxygen on 2006, pp. 276–279. IEEE deficient fluorite derivative Ga In Sn O . J. Appl. Phys. 3–x 5+x 2 16 89. Ohtaki, M., Araki, K., Yamamoto, K.: High thermoelectric 108(9), 093708 (2010) performance of dually doped ZnO ceramics. J. Electron. Mater. 107. Zhou, T., Bhame, S., Guilmeau, E., Marinel, S., Raveau, B.: 38(7), 1234–1238 (2009) Enhanced densification and thermoelectric performance of 90. Jood, P., Mehta, R.J., Zhang, Y., Peleckis, G., Wang, X., Siegel, In Sn O by reactive sintering in the In-Sn-Ga-O System. J. 4 3 12 R.W., Borca-Tasciuc, T., Dou, S.X., Ramanath, G.: Al-doped Am. Ceram. Soc. 94(11), 3733–3737 (2011) zinc oxide nanocomposites with enhanced thermoelectric prop- 108. Combe, E., Bhame, S.D., Guilmeau, E., Boschini, F., Cloots, R.: erties. Nano Lett. 11(10), 4337–4342 (2011) Synthesis of In Ge O nanopowders for thermoelectric applica- 2–x x 3 91. Zakharchuk, K., Widenmeyer, M., Alikin, D.O., Xie, W., Popu- tions. J. Mater. Res. 27(2), 500–505 (2012) loh, S., Mikhalev, S.M., Tselev, A., Frade, J., Weidenkaff, A., 109. Isobe, S., Tani, T., Masuda, Y., Seo, W.-S., Koumoto, K.: Ther- Kovalevsky, A.: Self-forming nanocomposite concept for ZnO- moelectric performance of yttrium-substituted (ZnO) 5In O 2 3 based thermoelectrics. J. Mater. Chem. A 6, 13386–13396 improved through ceramic texturing. Jpn. J. Appl. Phys. 41(2R), (2018) 731 (2002) 92. Zha, X., Gao, L., Bai, H., Fu, N., Wang, J., Fu, G., Wang, S.: 110. Kaga, H., Asahi, R., Tani, T.: Thermoelectric properties of highly Towards a high thermoelectric performance in CdO ceramics textured Ca-doped (ZnO) mIn O ceramics. Jpn. J. Appl. Phys. 2 3 by introducing multiscale scattering centers via ZnCO3 doping. 43(10R), 7133 (2004) Mater. Res. Express 5(3), 035908 (2018) 111. Luu, S.D., Vaqueiro, P.: Layered oxychalcogenides: Structural 93. Prasad, R., Bhame, S.D., Walke, P.: Structural and optical studies chemistry and thermoelectric properties. J. Materiomics 2(2), on nanosized thermoelectric oxide-Zn Al O. Int. J. Nanosci. 131–140 (2016) 1−x x 17(5), 1850038 (2018) 112. Sui, J., Li, J., He, J., Pei, Y.-L., Berardan, D., Wu, H., Dragoe, 94. Zakharchuk, K.V., Tobaldi, D.M., Xiao, X., Xie, W., Mikhalev, N., Cai, W., Zhao, L.-D.: Texturation boosts the thermoelectric S.M., Martins, J.F., Frade, J.R., Weidenkaff, A., Kovalevsky, performance of BiCuSeO oxyselenides. Energy Environ. Sci. A.V.: Synergistic effects of zirconium-and aluminum co-doping 6(10), 2916–2920 (2013) on the thermoelectric performance of zinc oxide. J. Eur. Ceram. 113. Lee, C., An, T.-H., Gordon, E.E., Ji, H.S., Park, C., Shim, J.-H., Soc. 39, 1222–1229 (2018) Lim, Y.S., Whangbo, M.-H.: Seebeck coefficients of layered 95. Paulson, A., Sabeer, N.M., Pradyumnan, P.: A synergetic BiCuSeO phases: analysis of their hole-density dependence and approach of band gap engineering and reduced lattice thermal quantum confinement effect. Chem. Mater. 29(5), 2348–2354 conductivity for the enhanced thermoelectric property in Dy ion (2017) doped ZnO. J. Alloy Compd. 786, 581–587 (2019) 114. Zhao, L.-D., He, J., Berardan, D., Lin, Y., Li, J.-F., Nan, C.-W., 96. Han, L., Van Nong, N., Zhang, W., Hung, L.T., Holgate, T., Dragoe, N.: BiCuSeO oxyselenides: new promising thermoelec- Tashiro, K., Ohtaki, M., Pryds, N., Linderoth, S.: Effects of mor - tric materials. Energy Environ. Sci. 7(9), 2900–2924 (2014) phology on the thermoelectric properties of Al-doped ZnO. RSC 115. Zhao, L., Berardan, D., Pei, Y., Byl, C., Pinsard-Gaudart, L., Adv. 4(24), 12353–12361 (2014) Dragoe, N.: Bi Sr CuSeO oxyselenides as promising thermo- 1–x x 97. Jia, J., Ow-Yang, C., Inan Akmehmet, G., Nakamura, S.-I., Kato, electric materials. Appl. Phys. Lett. 97(9), 092118 (2010) K., Shigesato, Y.: Formation of homologous In2O3 (ZnO) m thin 116. Pei, Y.-L., He, J., Li, J.-F., Li, F., Liu, Q., Pan, W., Barreteau, C., films and its thermoelectric properties. J. Vac. Sci. Technol. A Berardan, D., Dragoe, N., Zhao, L.-D.: High thermoelectric per- Vac. Surf. Films 34(4), 041507 (2016) formance of oxyselenides: intrinsically low thermal conductivity 98. Tani, T., Isobe, S., Seo, W.-S., Koumoto, K.: Thermoelectric of Ca-doped BiCuSeO. NPG Asia Mater. 5(5), e47 (2013) properties of highly textured (ZnO) 5In O ceramics. J. Mater. 117. Li, F., Li, J.-F., Zhao, L.-D., Xiang, K., Liu, Y., Zhang, B.-P., Lin, 2 3 Chem. 11(9), 2324–2328 (2001) Y.-H., Nan, C.-W., Zhu, H.-M.: Polycrystalline BiCuSeO oxide 1 3 3 Page 22 of 22 Materials for Renewable and Sustainable Energy (2020) 9:3 as a potential thermoelectric material. Energy Environ. Sci. 5(5), Thermoelectrics, 2006. ICT’06. 25th International Conference 7188–7195 (2012) on 2006, pp. 62–65. IEEE 118. Pei, Y.-L., Wu, H., Wu, D., Zheng, F., He, J.: High thermoelectric 134. Bocher, L., et al.: CaMn1− x Nb x O3 (x≤ 0.08) perovskite-type performance realized in a BiCuSeO system by improving car- phases as promising new high-temperature n-type thermoelectric rier mobility through 3D modulation doping. J. Am. Chem. Soc. materials. Inorg. Chem. 47(18), 8077–8085 (2008) 136(39), 13902–13908 (2014) 135. Ito, M., Nagira, T., Oda, Y., Katsuyama, S., Majima, K., Nagai, 119. Liu, Y., Zhao, L.D., Zhu, Y., Liu, Y., Li, F., Yu, M., Liu, D.B., H.: Effect of partial substitution of 3d transition metals for Co on Xu, W., Lin, Y.H., Nan, C.W.: Synergistically optimizing elec- the thermoelectric properties of N a Co O . Mater. Trans. 43(3), x 2 4 trical and thermal transport properties of BiCuSeO via a dual- 601–604 (2002) doping approach. Adv. Energy Mater. 6(9), 1502423 (2016) 136. Ohtaki, M., Maeda, E.: Microstructures and thermoelectric prop- 120. Das, S., Chetty, R., Wojciechowski, K., Suwas, S., Mallik, R.C.: erties of N aCo O prepared by double-step sintering. J. Jpn. Soc. 2 4 Thermoelectric properties of Sn doped BiCuSeO. Appl. Surf. Powder Powder Metall. 47(11), 1159–1164 (2000) Sci. 418, 238–245 (2017) 137. Funahashi, R., Shikano, M.: B i Sr Co O whiskers with high 2 2 2 y 121. Feng, B., Li, G., Pan, Z., Hu, X., Liu, P., Li, Y., He, Z., Fan, thermoelectric figure of merit. Appl. Phys. Lett. 81 (8), 1459– X.A.: Enhancing thermoelectric and mechanical performances 1461 (2002) in BiCuSeO by increasing bond covalency and nanostructuring. 138. Cheng, B., Fang, H., Lan, J., Liu, Y., Lin, Y.H., Nan, C.W.: J. Solid State Chem. 265, 306–313 (2018) Thermoelectric performance of Zn and Ge Co-doped In O fine- 2 3 122. Roy, P., Bose, I., Maiti, T.: Synthesis and characterization of grained ceramics by the spark plasma sintering. J. Am. Ceram. Sr TiMO (M = Fe, Co) double perovskites for high temperature Soc. 94(8), 2279–2281 (2011) 2 6 thermoelectric applications. Integr. Ferroelectr. 174(1), 34–42 139. Wu, Z.-H., Xie, H.-Q., Zhai, Y.-B.: Enhanced thermoelectric (2016) figure of merit in nanostructured ZnO by nanojunction effect. 123. Saxena, M., Roy, P., Acharya, M., Bose, I., Tanwar, K., Maiti, Appl. Phys. Lett. 103(24), 243901 (2013) T.: Enhanced thermoelectric figure-of-merit in environmentally 140. Jood, P., Mehta, R.J., Zhang, Y., Borca-Tasciuc, T., Dou, S.X., benign Ba Sr TiCoO double perovskites. Appl. Phys. Lett. Singh, D.J., Ramanath, G.: Heavy element doping for enhanc- x 2-x 6 109(26), 263903 (2016) ing thermoelectric properties of nanostructured zinc oxide. RSC 124. Roy, P., Waghmare, V., Maiti, T.: Environmentally friendly Ba Adv. 4(13), 6363–6368 (2014) Sr TiFeO double perovskite with enhanced thermopower for 141. Ou, C., Hou, J., Wei, T.-R., Jiang, B., Jiao, S., Li, J.-F., Zhu, H.: 2–x 6 high temperature thermoelectric power generation. RSC Adv. High thermoelectric performance of all-oxide heterostructures 6(60), 54636–54643 (2016) with carrier double-barrier filtering effect. NPG Asia Mater. 7 (5), 125. Saxena, M., Maiti, T.: Evaluation of Ba doped Sr TiFe Mo e182 (2015) 2 0.5 0.5 O double perovskites for high temperature thermoelectric power 142. Ahmad, A., Umer, M., Tan, X., Liu, R., Mohmad, F., Hussain, generation. Scr. Mater. 155, 85–88 (2018) M., Ren, G.-K., Lin, Y.-H.: High-temperature electrical and 126. Chen, C., Giovannelli, F., Duclère, J.-R., Delorme, F.: Thermo- thermal transport behaviors of In O -based ceramics by Zn-Sn 2 3 electric properties of Fe (Ti Nb) O pseudobrookite ceram- co-substitution. J. Appl. Phys. 123(24), 245108 (2018) 2 1−x x 5 ics with low thermal conductivity. J. Eur. Ceram. Soc. 37(15), 143. Liu, Y., Zhao, L.-D., Liu, Y., Lan, J., Xu, W., Li, F., Zhang, B.-P., 4681–4685 (2017) Berardan, D., Dragoe, N., Lin, Y.-H.: Remarkable enhancement 127. Chen, C., Giovannelli, F., Delorme, F.: Thermoelectric properties in thermoelectric performance of BiCuSeO by Cu deficiencies. of Fe Ti O solid solutions: Influence of microcracking and J. Am. Chem. Soc. 133(50), 20112–20115 (2011) 2-x 1+x 5 Nb substitution. Ceram. Int. 44(17), 21794–21799 (2018) 144. Li, J., Sui, J., Pei, Y., Meng, X., Berardan, D., Dragoe, N., Cai, 128. Paulson, A., Sabeer, N.M., Pradyumnan, P.: Enhanced ther- W., Zhao, L.-D.: The roles of Na doping in BiCuSeO oxysele- moelectric property of oxygen deficient nickel doped SnO for nides as a thermoelectric material. J. Mater. Chem. A 2(14), high temperature application. Mater. Res. Express 5(4), 045511 4903–4906 (2014) (2018) 145. Lan, J.L., Liu, Y.C., Zhan, B., Lin, Y.H., Zhang, B., Yuan, X., 129. Ohtaki, M.: Thermoelectric properties and applicability of Zhang, W., Xu, W., Nan, C.W.: Enhanced thermoelectric proper- In_2O_3-based mixed metal oxides for thermoelectric conver- ties of Pb-doped BiCuSeO ceramics. Adv. Mater. 25(36), 5086– sion at high temperature. In: Proceedings of The 12th Interna- 5090 (2013) tional Conference on Thermoelecrics, K. Matsuura ed., Yoko- 146. Li, J., Sui, J., Pei, Y., Barreteau, C., Berardan, D., Dragoe, N., hama, XII, ICT 1993, pp. 212–217 Cai, W., He, J., Zhao, L.-D.: A high thermoelectric figure of 130. Ohtaki, M., Koga, H., Eguchi, K., Arai, H.: Thermoelectric merit ZT %3e 1 in Ba heavily doped BiCuSeO oxyselenides. properties of (Ca Bi) MnO perovskite-type oxides as a new Energy Environ. Sci. 5(9), 8543–8547 (2012) 1−x x 3 material for high temperature thermoelectric conversion. In: AIP 147. Ohtaki, M.: Oxide thermoelectric materials for heat-to-electricity Conference Proceedings 1994, vol. CONF-940830 direct energy conversion. Novel Carbon Resour. Sci. Newsl. 3, 131. Ohtaki, M., Koga, H., Tokunaga, T., Eguchi, K., Arai, H.: Elec- 1–6 (2010) trical transport properties and high-temperature thermoelectric 148. Hanbo, L., Ganhong, Z., Zhenxiang, D., Zhenheng, Y., Haiqiu, performance of (Ca M) MnO (M=Y, La, Ce, Sm, In, Sn, Sb, W., Yongqing, M.: Thermoelectric properties of Sr La TiO 0.9 0.1 3 0.9 0.1 3 Pb, Bi). J. Solid State Chem. 120(1), 105–111 (1995).and Sr La Ti O with 15% Ag addition. J. Rare Earths 32(4), 2.7 0.3 2 7 132. Hayashi, R., Ohtaki, M.: Selective phonon scattering and ther- 314–319 (2014) moelectric properties of ZnO-based oxide with nanovoid struc- ture. In: Preprints of Annual Meeting of The Ceramic Society Publisher’s Note Springer Nature remains neutral with regard to of Japan Preprints of Fall Meeting of The Ceramic Society of jurisdictional claims in published maps and institutional affiliations. Japan Annual Meeting of The Ceramic Society of Japan, 2005, pp. 256–256. The Ceramic Society of Japan (2005) 133. Nong, N., Ohtaki, M.: Thermoelectric properties and local electronic structure of rare earth-doped Ca Co O . In: 3 2 6 1 3 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Materials for Renewable and Sustainable Energy Springer Journals http://www.deepdyve.com/lp/springer-journals/review-on-texturization-effects-in-thermoelectric-oxides-DGPlXGImMj

Loading next page...

References (149)

Ichiro Terasaki, Ichiro Tsukada, Y. Iguchi (2002)
Impurity-induced transition and impurity-enhanced thermopower in the thermoelectric oxide NaCo 2 − x Cu x O 4
Physical Review B, 65
E. Combe, S. Bhame, E. Guilmeau, F. Boschini, R. Cloots (2012)
Synthesis of In_2-xGe_xO_3 nanopowders for thermoelectric applications
Journal of Materials Research, 27
Jing Li, J. Sui, Y. Pei, C. Barreteau, D. Bérardan, N. Dragoe, W. Cai, Jiaqing He, Li-dong Zhao (2012)
A high thermoelectric figure of merit ZT > 1 in Ba heavily doped BiCuSeO oxyselenides
Energy and Environmental Science, 5
Cong Chen, F. Giovannelli, F. Delorme (2018)
Thermoelectric properties of Fe2-xTi1+xO5 solid solutions: Influence of microcracking and Nb substitution
Ceramics International
(2002)
Research Signpost/ed
J. Noudem, D. Kenfaui, D. Chateigner, M. Gomina (2012)
Toward the enhancement of thermoelectric properties of lamellar Ca3Co4O9 by edge-free spark plasma texturing
Scripta Materialia, 66
Xiao Zhang, Li-dong Zhao (2015)
Thermoelectric materials: Energy conversion between heat and electricity
Journal of Materiomics, 1
A. Kinaci, C. Sevik, T. Çagin (2010)
Electronic transport properties ofSrTiO3and its alloys:Sr1−xLaxTiO3andSrTi1−xMxO3(M=Nb,Ta)
Physical Review B, 82
T. Seetawan, V. Amornkitbamrung, T. Burinprakhon, S. Maensiri, K. Kurosaki, H. Muta, M. Uno, S. Yamanaka (2006)
Thermoelectric properties of NaxCo2O4/Ag composites
Journal of Alloys and Compounds, 414
A. Kovalevsky, A. Yaremchenko, S. Populoh, A. Weidenkaff, J. Frade (2013)
Enhancement of thermoelectric performance in strontium titanate by praseodymium substitution
Journal of Applied Physics, 113
T. Motohashi, E. Naujalis, R. Ueda, K. Isawa, M. Karppinen, H. Yamauchi (2001)
Simultaneously enhanced thermoelectric power and reduced resistivity of NaxCo2O4 by controlling Na nonstoichiometry
Applied Physics Letters, 79
K. Zakharchuk, D. Tobaldi, Xingxing Xiao, W. Xie, S. Mikhalev, J. Martins, J. Frade, A. Weidenkaff, A. Kovalevsky (2019)
Synergistic effects of zirconium- and aluminum co-doping on the thermoelectric performance of zinc oxide
Journal of the European Ceramic Society
C. Vining (2009)
An inconvenient truth about thermoelectrics.
Nature materials, 8 2
H. Ohta (2007)
Thermoelectrics based on strontium titanate
Materials Today, 10
T. Okuda, K. Nakanishi, S. Miyasaka, Y. Tokura (2001)
Large thermoelectric response of metallic perovskites: Sr 1 − x La x TiO 3 ( 0 x 0 . 1 )
Physical Review B, 63
M. Košir, M. Čeh, C. Ow‐Yang, E. Guilmeau, S. Bernik (2017)
Structural features and thermoelectric properties of Al‐doped (ZnO)5In2O3 homologous phases
Journal of the American Ceramic Society, 100
H. Yakabe, K. Kikuchi, I. Terasaki, Y. Sasago, K. Uchinokura (1997)
Thermoelectric properties of transition-metal oxide NaCo/sub 2/O/sub 4/ system
XVI ICT '97. Proceedings ICT'97. 16th International Conference on Thermoelectrics (Cat. No.97TH8291)
K. Kurosaki, H. Muta, M. Uno, S. Yamanaka (2001)
Thermoelectric properties of NaCo2O4
Journal of Alloys and Compounds, 315
H. Muta, A. Ieda, K. Kurosaki, S. Yamanaka (2004)
Substitution effect on the thermoelectric properties of alkaline earth titanate
Materials Letters, 58
H. Muta, K. Kurosaki, S. Yamanaka (2004)
Thermoelectric properties of doped BaTiO3-SrTiO3 solid solution
Journal of Alloys and Compounds, 368
M. Vedernikov, E.K. Iordanishvili (1998)
A.F. Ioffe and origin of modern semiconductor thermoelectric energy conversion
Seventeenth International Conference on Thermoelectrics. Proceedings ICT98 (Cat. No.98TH8365)
K. Miyazawa, F. Amaral, A. Kovalevsky, M. Graça (2016)
Hybrid microwave processing of Ca3Co4O9 thermoelectrics
Ceramics International, 42
Chunlin Ou, Jungang Hou, Tian‐Ran Wei, B. Jiang, S. Jiao, Jingfeng Li, Hongmin Zhu (2015)
High thermoelectric performance of all-oxide heterostructures with carrier double-barrier filtering effect
Npg Asia Materials, 7
M. Shikano, R. Funahashi (2003)
Electrical and thermal properties of single-crystalline (Ca2CoO3)0.7CoO2 with a Ca3Co4O9 structure
Applied Physics Letters, 82
L. Bocher, M. Aguirre, D. Logvinovich, A. Shkabko, R. Robert, M. Trottmann, A. Weidenkaff (2008)
CaMn(1-x)Nb(x)O3 (x < or = 0.08) perovskite-type phases as promising new high-temperature n-type thermoelectric materials.
Inorganic chemistry, 47 18
Ipek Akin, M. Li, Zhilun Lu, D. Sinclair (2014)
Oxygen-loss in A-site deficient Sr0.85La0.10TiO3 perovskite
RSC Advances, 4
M. Ohtaki, Eishi Maeda (2000)
Microstructures and Thermoelectric Properties of NaCo2O4 Prepared by Double-step Sintering
Journal of The Japan Society of Powder and Powder Metallurgy, 47
Masahiro Yamamoto, H. Ohta, K. Koumoto (2007)
Thermoelectric phase diagram in a CaTiO3–SrTiO3–BaTiO3 system
Applied Physics Letters, 90
M. Ohtaki, H. Koga, T. Tokunaga, K. Eguchi, H. Arai (1995)
Electrical transport properties and high-temperature thermoelectric performance of (Ca0.9M0.1)MnO3 (M = Y, La, Ce, Sm, In, Sn, Sb, Pb, Bi)
IEEE Journal of Solid-state Circuits, 120
Nong Van, N. Pryds (2013)
Nanostructured oxide materials and modules for high-temperature power generation from waste heat
Advances in Natural Sciences: Nanoscience and Nanotechnology, 4
M. Torres, F. Costa, D. Flahaut, K. Touati, S. Rasekh, N. Ferreira, J. Allouche, M. Depriester, M. Madre, A. Kovalevsky, J. Diez, A. Sotelo (2019)
Significant enhancement of the thermoelectric performance in Ca3Co4O9 thermoelectric materials through combined strontium substitution and hot-pressing process
Journal of the European Ceramic Society
Zhilun Lu, Huairuo Zhang, W. Lei, D. Sinclair, I. Reaney (2016)
High-Figure-of-Merit Thermoelectric La-Doped A-Site-Deficient SrTiO3 Ceramics
Chemistry of Materials, 28
M. Dresselhaus, Gang Chen, M. Tang, Ronggui Yang, Hohyun Lee, Dezhi Wang, Zhifeng Ren, J. Fleurial, P. Gogna (2007)
New Directions for Low‐Dimensional Thermoelectric Materials
Advanced Materials, 19
T. Seetawan, V. Amornkitbamrung, T. Burinprakhon, S. Maensiri, K. Kurosaki, H. Muta, M. Uno, S. Yamanaka (2006)
Thermoelectric power and electrical resistivity of Ag-doped Na1.5Co2O4
Journal of Alloys and Compounds, 407
Li-dong Zhao, Jiaqing He, D. Bérardan, Yuanhua Lin, Jingfeng Li, C. Nan, N. Dragoe (2014)
BiCuSeO oxyselenides: new promising thermoelectric materials
Energy and Environmental Science, 7
Li-dong Zhao, D. Bérardan, Y. Pei, C. Byl, L. Pinsard-Gaudart, N. Dragoe (2010)
Bi1−xSrxCuSeO oxyselenides as promising thermoelectric materials
Applied Physics Letters, 97
P. Shang, Boping Zhang, Yong Liu, Jingfeng Li, Hongmin Zhu (2011)
Preparation and Thermoelectric Properties of La-Doped SrTiO3 Ceramics
Journal of Electronic Materials, 40
F. Gao, Shengjie Yang, Jinjin Li, Mengjie Qin, Yong Zhang, Huajun Sun (2015)
Fabrication, dielectric, and thermoelectric properties of textured SrTiO3 ceramics prepared by RTGG method
Ceramics International, 41
Y. Liu, Yuanhua Lin, S. Zhan, C. Nan, Z. Shen (2005)
Preparation of Ca3Co4O9 and Improvement of its Thermoelectric Properties by Spark Plasma Sintering
Journal of the American Ceramic Society, 88
Li-dong Zhao, Haijun Wu, Shiqiang Hao, Chunrui Wu, Xiaoyuan Zhou, K. Biswas, Jiaqing He, T. Hogan, C. Uher, C. Wolverton, V. Dravid, M. Kanatzidis (2013)
All-scale hierarchical thermoelectrics: MgTe in PbTe facilitates valence band convergence and suppresses bipolar thermal transport for high performance
Energy and Environmental Science, 6
N. Nong, M. Ohtaki (2006)
Thermoelectric properties and local electronic structure of rare earth-doped Ca3Co2O6
2006 25th International Conference on Thermoelectrics
(2010)
Oxide thermoelectric materials for heat-to-electricity direct energy conversion
T. Tani, Shinya Isobe, W. Seo, K. Koumoto (2001)
Thermoelectric properties of highly textured (ZnO)5In2O3 ceramics
Journal of Materials Chemistry, 11
T. Ogi, Darmawan Hidayat, Ferry Iskandar, A. Purwanto, K. Okuyama (2009)
Direct synthesis of highly crystalline transparent conducting oxide nanoparticles by low pressure spray pyrolysis
Advanced Powder Technology, 20
Wei Zhang, Pengcheng Liu, Yifeng Wang, K. Zhu, G. Tai, Jinsong Liu, Jing Wang, K. Yan, Jianhui Zhang (2018)
Textured NaxCoO2 Ceramics Sintered from Hydrothermal Platelet Nanocrystals: Growth Mechanism and Transport Properties
Journal of Electronic Materials, 47
G. Ren, J. Lan, Kyle Ventura, X. Tan, Yuanhua Lin, C. Nan (2016)
Contribution of point defects and nano-grains to thermal transport behaviours of oxide-based thermoelectrics
, 2
R. Funahashi, I. Matsubara, H. Ikuta, T. Takeuchi, U. Mizutani, S. Sodeoka (2000)
An Oxide Single Crystal with High Thermoelectric Performance in Air
Japanese Journal of Applied Physics, 39
D. Pravarthana, O. Lebedev, D. Chateigner, S. Hébert, P. Salvador, W. Prellier (2013)
High-throughput synthesis of thermoelectric Ca3Co4O9 films
Applied Physics Letters, 103
K. Takahata, Y. Iguchi, D. Tanaka, T. Itoh, I. Terasaki (2000)
Low thermal conductivity of the layered oxide ( N a , C a ) Co 2 O 4 : Another example of a phonon glass and an electron crystal
Physical Review B, 61
N. Nong, Chia‐Jyi Liu, M. Ohtaki (2010)
Improvement on the high temperature thermoelectric performance of Ga-doped misfit-layered Ca3Co4−xGaxO9+δ (x = 0, 0.05, 0.1, and 0.2)
Journal of Alloys and Compounds, 491
P. Roy, Imon Bose, T. Maiti (2016)
Synthesis and characterization of Sr2TiMO6 (M = Fe,Co) double perovskites for high temperature thermoelectric applications
Integrated Ferroelectrics, 174
H. Muta, K. Kurosaki, S. Yamanaka (2003)
Thermoelectric properties of rare earth doped SrTiO3
Journal of Alloys and Compounds, 350
Michael Huang, S. Mao, H. Feick, Haoquan Yan, Yiying Wu, H. Kind, E. Weber, R. Russo, P. Yang (2001)
Room-Temperature Ultraviolet Nanowire Nanolasers
Science, 292
T. Takabatake, K. Suekuni, T. Nakayama, E. Kaneshita (2014)
Phonon-glass electron-crystal thermoelectric clathrates : Experiments and theory
Reviews of Modern Physics, 86
Cong Chen, F. Giovannelli, J. Duclère, F. Delorme (2017)
Thermoelectric properties of Fe 2 (Ti 1−x Nb x )O 5 pseudobrookite ceramics with low thermal conductivity
Journal of The European Ceramic Society, 37
Y. Pei, Haijun Wu, Di Wu, Fengshan Zheng, Jiaqing He (2014)
High thermoelectric performance realized in a BiCuSeO system by improving carrier mobility through 3D modulation doping.
Journal of the American Chemical Society, 136 39
Mandvi Saxena, P. Roy, M. Acharya, Imon Bose, Khagesh Tanwar, T. Maiti (2016)
Enhanced thermoelectric figure-of-merit in environmentally benign BaxSr2-xTiCoO6 double perovskites
Applied Physics Letters, 109
L. Han, N. Nong, Wei Zhang, L. Hung, T. Holgate, K. Tashiro, M. Ohtaki, N. Pryds, S. Linderoth (2014)
Effects of morphology on the thermoelectric properties of Al-doped ZnO
RSC Advances, 4
A. Kinaci, C. Sevik, T. Çagin (2010)
Electronic transport properties of SrTi03 and its alloys: Sr1-xLaxTiO3 and SrTi1-xMxO3 (M=Nb,Ta)
Physical Review B, 82
A. Paulson, N. Sabeer, P. Pradyumnan (2019)
A synergetic approach of band gap engineering and reduced lattice thermal conductivity for the enhanced thermoelectric property in Dy ion doped ZnO
Journal of Alloys and Compounds
P. Jood, Rutvik Mehta, Yanliang Zhang, G. Peleckis, Xiaolin Wang, R. Siegel, T. Borca-Tasciuc, S. Dou, G. Ramanath (2011)
Al-doped zinc oxide nanocomposites with enhanced thermoelectric properties.
Nano letters, 11 10
Bo Feng, Guangqiang Li, Zhao Pan, Xiaoming Hu, Peihi Liu, Yawei Li, Zhu He, Xi’an Fan (2018)
Enhancing thermoelectric and mechanical performances in BiCuSeO by increasing bond covalency and nanostructuring
Journal of Solid State Chemistry
M. Ohtaki, K. Araki, Kiyoshi Yamamoto (2009)
High Thermoelectric Performance of Dually Doped ZnO Ceramics
Journal of Electronic Materials, 38
Yuanhua Lin, J. Lan, Zhijian Shen, Yuheng Liu, C. Nan, Jingfeng Li (2009)
High-temperature electrical transport behaviors in textured Ca3Co4O9-based polycrystalline ceramics
Applied Physics Letters, 94
A. Kovalevsky, A. Yaremchenko, S. Populoh, P. Thiel, D. Fagg, A. Weidenkaff, Anke Weidenkaff, J. Frade (2014)
Towards a high thermoelectric performance in rare-earth substituted SrTiO3: effects provided by strongly-reducing sintering conditions.
Physical chemistry chemical physics : PCCP, 16 48
Dongli Wang, Lidong Chen, Q. Yao, Jianguo Li (2004)
High-temperature thermoelectric properties of Ca3Co4O9+δ with Eu substitution
Solid State Communications, 129
Yifeng Wang, Kyu Lee, H. Ohta, K. Koumoto (2009)
Thermoelectric properties of electron doped SrO(SrTiO3)n (n=1,2) ceramics
Journal of Applied Physics, 105
Kyeongsoon Park, K. Jang (2006)
Improvement in high-temperature thermoelectric properties of NaCo2O4 through partial substitution of Ni for Co
Materials Letters, 60
Raphael Richter, D. Shopova, W. Xie, A. Weidenkaff, F. Lechermann (2016)
Thermopower enhancement from engineering the Na$_{0.7}$CoO$_2$ interacting fermiology via Fe doping
arXiv: Materials Science
Y. Miyazaki, M. Onoda, T. Oku, M. Kikuchi, Y. Ishii, Y. Ono, Y. Morii, T. Kajitani (2002)
Modulated Structure of the Thermoelectric Compound [Ca2CoO3]0.62CoO2
Journal of the Physical Society of Japan, 71
O. Okhay, Sebastian Złotnik, W. Xie, K. Orliński, Maria Gallo, G. Otero-Irurueta, A. Fernandes, D. Pawlak, A. Weidenkaff, A. Tkach (2019)
Thermoelectric performance of Nb-doped SrTiO3 enhanced by reduced graphene oxide and Sr deficiency cooperation
Carbon
A. Kovalevsky, S. Populoh, S. Patrício, P. Thiel, M. Ferro, D. Fagg, J. Frade, A. Weidenkaff (2015)
Design of SrTiO3-Based Thermoelectrics by Tungsten Substitution
Journal of Physical Chemistry C, 119
Mikiko Ito, T. Nagira, Y. Oda, S. Katsuyama, K. Majima, H. Nagai (2002)
Effect of Partial Substitution of 3d Transition Metals for Co on the Thermoelectric Properties of NaxCo2O4
Materials Transactions, 43
R. Prasad, S. Bhame, P. Walke (2018)
Structural and Optical Studies on Nanosized Thermoelectric Oxide-Zn1−xAlxO
International Journal of Nanoscience
M. Roger, D. Morris, D. Tennant, M. Gutmann, J. Goff, J. Hoffmann, R. Feyerherm, E. Dudzik, D. Prabhakaran, A. Boothroyd, N. Shannon, Bella Lake, P. Deen (2005)
Patterning of sodium ions and the control of electrons in sodium cobaltate
Nature, 445
Dai-Bing Zhang, Boping Zhang, Ye Daosheng, Yongbin Zhu (2015)
Thermoelectric properties of textured Zn1−xAlxO ceramics prepared by spark plasma sintering of hydrothermally synthesized rod-shaped powders
Ceramics International, 41
I. Terasaki, Y. Sasago, K. Uchinokura (1997)
Large thermoelectric power in NaCo 2 O 4 single crystals
Physical Review B, 56
A. Polozine, S. Sirotinskaya, L. Schaeffer (2014)
History of Development of Thermoelectric Materials for Electric Power Generation and Criteria of their Quality
Materials Research-ibero-american Journal of Materials, 17
Fu Li, Jingfeng Li, Li-dong Zhao, Kai-Lun Xiang, Yong Liu, Boping Zhang, Yuanhua Lin, C. Nan, Hongmin Zhu (2012)
Polycrystalline BiCuSeO oxide as a potential thermoelectric material
Energy and Environmental Science, 5
M. Košir, M. Podlogar, N. Daneu, A. Rečnik, E. Guilmeau, S. Bernik (2017)
Phase formation, microstructure development and thermoelectric properties of (ZnO)kIn2O3 ceramics
Journal of The European Ceramic Society, 37
Li Hanbo, Zheng Ganhong, Dai Zhenxiang, Yuan Zhenheng, Wang Hai-qiu, Mao Yongqing (2014)
Thermoelectric properties of Sr0.9La0.1TiO3 and Sr2.7La0.3Ti2O7 with 15% Ag addition
Journal of Rare Earths, 32
Qiang Li (2006)
On the Thermoelectric Properties of Layered Cobaltates
MRS Proceedings, 886
Y. Pei, Jiaqing He, Jingfeng Li, Fu Li, Qijun Liu, W. Pan, C. Barreteau, D. Bérardan, N. Dragoe, Li-dong Zhao (2013)
High thermoelectric performance of oxyselenides: intrinsically low thermal conductivity of Ca-doped BiCuSeO
Npg Asia Materials, 5
M. Ohtaki, H. Koga, K. Eguchi, H. Arai (1994)
Thermoelectric properties of (Ca1−xBix)MnO3 perovskite‐type oxides as a new material for high temperature thermoelectric conversion
, 316
K. Zakharchuk, M. Widenmeyer, D. Alikin, W. Xie, S. Populoh, S. Mikhalev, A. Tselev, J. Frade, A. Weidenkaff, A. Kovalevsky (2018)
A self-forming nanocomposite concept for ZnO-based thermoelectrics
Journal of Materials Chemistry, 6
A. Weidenkaff (2017)
Thermoelectricity for future sustainable energy technologies
, 148
Zihua Wu, Huaqing Xie, Yongbiao Zhai (2013)
Enhanced thermoelectric figure of merit in nanostructured ZnO by nanojunction effect
Applied Physics Letters, 103
S. Ohta, T. Nomura, H. Ohta, K. Koumoto (2005)
High-temperature carrier transport and thermoelectric properties of heavily La- or Nb-doped SrTiO3 single crystals
Journal of Applied Physics, 97
M. Ohtaki, K. Araki (2011)
Thermoelectric properties and thermopower enhancement of Al-doped ZnO with nanosized pore structure
Journal of the Ceramic Society of Japan, 119
D. Kenfaui, D. Chateigner, M. Gomina, J. Noudem (2011)
Anisotropy of the Mechanical and Thermoelectric Properties of Hot‐Pressed Single‐Layer and Multilayer Thick Ca3Co4O9 Ceramics
International Journal of Applied Ceramic Technology, 8
Chia-Jyi Liua, M. Ohtakib (2010)
Jul 10, 2020 High-temperature thermoelectric properties of late
H. Kaga, R. Asahi, T. Tani (2004)
Thermoelectric Properties of Highly Textured Ca-Doped (ZnO)mIn2O3 Ceramics
Japanese Journal of Applied Physics, 43
Sukanti Behera, V. Kamble, S. Vitta, A. Umarji, C. Shivakumara (2017)
Synthesis, structure and thermoelectric properties of La1-xNaxCoO3\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mat
Bulletin of Materials Science, 40
P. Roy, V. Waghmare, T. Maiti (2016)
Environmentally friendly BaxSr2−xTiFeO6 double perovskite with enhanced thermopower for high temperature thermoelectric power generation
RSC Advances, 6
T. Zhou, S. Bhame, E. Guilmeau, S. Marinel, B. Raveau (2011)
Enhanced Densification and Thermoelectric Performance of In4Sn3O12 by Reactive Sintering in the In‐Sn‐Ga‐O System
Journal of the American Ceramic Society, 94
M. Prével, E. Reddy, O. Pérez, W. Kobayashi, I. Terasaki, C. Goupil, J. Noudem (2007)
Thermoelectric Properties of Sintered and Textured Nd-Substituted Ca3Co4O9 Ceramics
Japanese Journal of Applied Physics, 46
Shinya Isobe, T. Tani, Y. Masuda, W. Seo, K. Koumoto (2002)
Thermoelectric Performance of Yttrium-substituted (ZnO)5In2O3 Improved through Ceramic Texturing
Japanese Journal of Applied Physics, 41
R. Hayashi, M. Ohtaki (2005)
Selective phonon scattering and thermoelectric properties of ZnO-based oxide with nanovoid structure
B. Cheng, H. Fang, J. Lan, Yong Liu, Yong Liu, Yuanhua Lin, C. Nan (2011)
Thermoelectric Performance of Zn and GeCo-Doped In2O3 Fine-Grained Ceramics by the Spark Plasma Sintering
Journal of the American Ceramic Society, 94
Yong Liu, Li-dong Zhao, Yaochun Liu, J. Lan, Wei Xu, Fu Li, Boping Zhang, D. Bérardan, N. Dragoe, Yuanhua Lin, C. Nan, Jingfeng Li, Hongmin Zhu (2011)
Remarkable enhancement in thermoelectric performance of BiCuSeO by Cu deficiencies.
Journal of the American Chemical Society, 133 50
H. Ohta, Sungwng Kim, Yoriko Mune, T. Mizoguchi, K. Nomura, S. Ohta, T. Nomura, Yuki Nakanishi, Y. Ikuhara, M. Hirano, H. Hosono, K. Koumoto (2007)
Giant thermoelectric Seebeck coefficient of a two-dimensional electron gas in SrTiO3.
Nature materials, 6 2
N. Krasutskaya, A. Klyndyuk, L. Evseeva, S. Tanaeva (2016)
Synthesis and properties of NaxCoO2 (x = 0.55, 0.89) oxide thermoelectrics
Inorganic Materials, 52
S. Bhame, T. Zhou, E. Guilmeau, Y. Kinemuchi, F. Delorme, B. Raveau (2010)
Synthesis and thermoelectric properties of oxygen deficient fluorite derivative Ga3−xIn5+xSn2O16
Journal of Applied Physics, 108
L. Bocher, M. Aguirre, D. Logvinovich, A. Shkabko, R. Robert, M. Trottmann, A. Weidenkaff (2008)
CaMn1-xNbxO3 (x ≤ 0.08) Perovskite-Type Phases as Promising New High-Temperature n-Type Thermoelectric Materials.
ChemInform, 39
J. Salvador, J. Cho, Z. Ye, Joshua Moczygemba, A. Thompson, J. Sharp, J. Koenig, Ryan Maloney, T. Thompson, J. Sakamoto, Hsin Wang, A. Wereszczak (2014)
Conversion efficiency of skutterudite-based thermoelectric modules.
Physical chemistry chemical physics : PCCP, 16 24
J. Hasnidawani, H. Azlina, H. Norita, N. Bonnia, S. Ratim, E. Ali (2016)
Synthesis of ZnO Nanostructures Using Sol-Gel Method
Procedia Chemistry, 19
S. Hébert, D. Flahaut, D. Pelloquin, C. Martin, A. Maignan, J. Hejtmánek (2003)
Cobalt oxides as potential thermoelectric elements: The influence of the dimensionality
Proceedings ICT'03. 22nd International Conference on Thermoelectrics (IEEE Cat. No.03TH8726)
Sayan Das, R. Chetty, K. Wojciechowski, S. Suwas, R. Mallik (2017)
Thermoelectric properties of Sn doped BiCuSeO
Applied Surface Science, 418
M. Ohtaki, R. Hayashi (2006)
Enhanced Thermoelectric Performance of Nanostructured ZnO: A possibility of selective phonon scattering and carrier energy filtering by nanovoid structure
2006 25th International Conference on Thermoelectrics
S. Pearton, D. Norton, K. Ip, Y. Heo, T. Steiner (2003)
Recent progress in processing and properties of ZnO
Superlattices and Microstructures, 34
Y. Liu, Li-dong Zhao, Yingcai Zhu, Yaochun Liu, Fu Li, Meijuan Yu, Da-bo Liu, W. Xu, Yuanhua Lin, C. Nan (2016)
Synergistically Optimizing Electrical and Thermal Transport Properties of BiCuSeO via a Dual‐Doping Approach
Advanced Energy Materials, 6
D. Bérardan, E. Guilmeau, A. Maignan, B. Raveau (2008)
In2O3:Ge, a promising n-type thermoelectric oxide composite
Solid State Communications, 146
Jing Li, J. Sui, Y. Pei, Xianfu Meng, D. Bérardan, N. Dragoe, W. Cai, Li-dong Zhao (2014)
The roles of Na doping in BiCuSeO oxyselenides as a thermoelectric material
Journal of Materials Chemistry, 2
Q. Yao, Dadi Wang, L. Chen, Xuefeng Shi, Ming-xing Zhou (2005)
Effects of partial substitution of transition metals for cobalt on the high-temperature thermoelectric properties of Ca3Co4O9+δ
Journal of Applied Physics, 97
Dursun Ekren, F. Azough, A. Gholinia, S. Day, D. Hernández-Maldonado, D. Kepaptsoglou, Q. Ramasse, R. Freer (2018)
Enhancing the thermoelectric power factor of Sr0.9Nd0.1TiO3 through control of the nanostructure and microstructure
Journal of Materials Chemistry, 6
M Tripathi, C Bhandari (2003)
High-temperature thermoelectric performance of Si-Ge alloys
Journal of Physics: Condensed Matter, 15
Xinyu Zha, Linjie Gao, Hong-Chang Bai, Nian Fu, Jianglong Wang, G. Fu, Shufang Wang (2018)
Towards a high thermoelectric performance in CdO ceramics by introducing multiscale scattering centers via ZnCO3 doping
Materials Research Express, 5
S. Bernik, M. Košir, E. Guilmeau (2016)
Microstructure and thermoelectric characteristics of (ZnO)k×In2O3: Based ceramics (k = 5 and 11)
, 57
D. Look (2006)
Progress in ZnO materials and devices
Journal of Electronic Materials, 35
(1993)
Thermoelectric properties and applicability of In_2O_3-based mixed metal oxides for thermoelectric conversion at high temperature
J. Lan, Yaochun Liu, Bin Zhan, Yuanhua Lin, Boping Zhang, Xun Yuan, Wenqing Zhang, Wei Xu, C. Nan (2013)
Enhanced Thermoelectric Properties of Pb‐doped BiCuSeO Ceramics
Advanced Materials, 25
Abid Ahmad, M. Umer, X. Tan, Rui Liu, Fidah Mohmad, M. Hussain, G. Ren, Yuanhua Lin (2018)
High-temperature electrical and thermal transport behaviors of In2O3-based ceramics by Zn-Sn co-substitution
Journal of Applied Physics
J. Jia, C. Ow‐Yang, Güliz Akmehmet, Shin‐ichi Nakamura, Kunihisa Kato, Y. Shigesato (2016)
Formation of homologous In2O3(ZnO)(m) thin films and its thermoelectric properties
Journal of Vacuum Science and Technology, 34
M. Prével, S. Lemonnier, Y. Klein, S. Hébert, D. Chateigner, B. Ouladdiaf, J. Noudem (2005)
Textured Ca3Co4O9 thermoelectric oxides by thermoforging process
Journal of Applied Physics, 98
Mandvi Saxena, T. Maiti (2018)
Evaluation of Ba doped Sr2TiFe0.5Mo0.5O6 double perovskites for high temperature thermoelectric power generation
Scripta Materialia
P. Jood, P. Jood, Rutvik Mehta, Yanliang Zhang, Yanliang Zhang, T. Borca-Tasciuc, S. Dou, David Singh, G. Ramanath (2014)
Heavy element doping for enhancing thermoelectric properties of nanostructured zinc oxide
RSC Advances, 4
Jing Wen, Lili Wu, Xitian Zhang (2012)
A unique arrangement of atoms for the homologous compounds InMO3(ZnO)m (M = Al, Fe, Ga, and In)
Journal of Applied Physics, 111
A. Tkach, João Resende, K. Saravanan, M. Costa, P. Díaz-Chao, E. Guilmeau, O. Okhay, P. Vilarinho (2018)
Abnormal Grain Growth as a Method To Enhance the Thermoelectric Performance of Nb-Doped Strontium Titanate Ceramics
ACS Sustainable Chemistry & Engineering
Tao Wu, T. Tyson, Haiyan Chen, J. Bai, Hsin Wang, C. Jaye (2012)
A structural change in Ca3Co4O9 associated with enhanced thermoelectric properties
Journal of Physics: Condensed Matter, 24
S. Saini, Haritha Yaddanapudi, K. Tian, Yinong Yin, D. Magginetti, A. Tiwari (2017)
Terbium Ion Doping in Ca3Co4O9: A Step towards High-Performance Thermoelectric Materials
Scientific Reports, 7
M. Bittner, N. Kanas, R. Hinterding, F. Steinbach, D. Groeneveld, P. Wemhoff, K. Wiik, M. Einarsrud, A. Feldhoff (2019)
Triple-phase ceramic 2D nanocomposite with enhanced thermoelectric properties
Journal of the European Ceramic Society
(2012)
Polycrystalline BiCuSeO oxide Materials for Renewable and Sustainable Energy (2020) 9:3 1 3 3 Page 22 of 22 as a potential thermoelectric material
P. Tsai, Tianshu Zhang, R. Donelson, T. Tan, Sean Li (2011)
Power factor enhancement in Zn-doped Na0.8CoO2
Journal of Alloys and Compounds, 509
A. Paulson, N. Sabeer, P. Pradyumnan (2018)
Enhanced thermoelectric property of oxygen deficient nickel doped SnO2 for high temperature application
Materials Research Express, 5
Son Luu, P. Vaqueiro (2016)
Layered oxychalcogenides: Structural chemistry and thermoelectric properties
Journal of Materiomics, 2
H. Serier, M. Gaudon, M. Ménétrier (2009)
Al-doped ZnO powdered materials: Al solubility limit and IR absorption properties
Solid State Sciences, 11
D. Srivastava, C. Norman, F. Azough, M. Schäfer, E. Guilmeau, R. Freer (2018)
Improving the thermoelectric properties of SrTiO3-based ceramics with metallic inclusions
Journal of Alloys and Compounds, 731
J. Gong, A. Hong, J. Shuai, L. Li, Z. Yan, Z. Ren, J. Liu (2016)
Investigation of the bipolar effect in the thermoelectric material CaMg2Bi2 using a first-principles study.
Physical chemistry chemical physics : PCCP, 18 24
G. Snyder, E. Toberer (2008)
Complex thermoelectric materials.
Nature materials, 7 2
M. Ohtaki, Toshiki Tsubota, K. Eguchi, H. Arai (1996)
High‐temperature thermoelectric properties of (Zn1−xAlx)O
Journal of Applied Physics, 79
Siwen Li, R. Funahashi, I. Matsubara, K. Ueno, S. Sodeoka, Hiroyuki Yamada (2000)
Synthesis and Thermoelectric Properties of the New Oxide Materials Ca3-xBixCo4O9+δ (0.0 < x < 0.75).
ChemInform, 31
R. Funahashi, M. Shikano (2002)
Bi2Sr2Co2Oy whiskers with high thermoelectric figure of merit
Applied Physics Letters, 81
D. Aswal, R. Basu, Ajay Singh (2016)
Key issues in development of thermoelectric power generators: High figure-of-merit materials and their highly conducting interfaces with metallic interconnects
Energy Conversion and Management, 114
D. Kenfaui, B. Lenoir, D. Chateigner, Bachir Ouladdiaf, M. Gomina, J. Noudem (2012)
Development of multilayer textured Ca3Co4O9 materials for thermoelectric generators: Influence of the anisotropy on the transport properties
Journal of The European Ceramic Society, 32
D. Kenfaui, D. Chateigner, M. Gomina, J. Noudem (2010)
Texture, mechanical and thermoelectric properties of Ca3Co4O9 ceramics
Journal of Alloys and Compounds, 490
Yang Wang, Y. Sui, Jinguang Cheng, Xianjie Wang, W. Su (2009)
Comparison of the high temperature thermoelectric properties for Ag-doped and Ag-added Ca3Co4O9
Journal of Alloys and Compounds, 477
Changhoon Lee, Tae-Ho An, E. Gordon, H. Ji, Chan Park, J. Shim, Y. Lim, M. Whangbo (2017)
Seebeck Coefficients of Layered BiCuSeO Phases: Analysis of Their Hole-Density Dependence and Quantum Confinement Effect
Chemistry of Materials, 29
Haowei Peng, J. Song, E. Hopper, Qimin Zhu, T. Mason, A. Freeman (2012)
Possible n–type carrier sources in In2O3(ZnO)k
Chemistry of Materials, 24
riss Kenfaui, D. Chateigner, M. Gomina, J. Noudem (2009)
exture , mechanical and thermoelectric properties of Ca 3 Co 4 O 9 ceramics

Publisher: Springer Journals
Copyright: Copyright © The Author(s) 2020
ISSN: 2194-1459
eISSN: 2194-1467
DOI: 10.1007/s40243-019-0163-y
Publisher site: See Article on Publisher Site

Abstract

Sustainable energy sources and energy-harvesting technologies have been researched for decades. Thermoelectric conversion is currently one of the primary foci in this area. Thermoelectric research has been concentrated into two parts—(i) strate- gies to enhance the efficiency of existing thermoelectric materials and (ii) development of new materials with promising thermoelectric parameters. Although such strategies have led to the improvement of thermoelectric non-oxide-based materi- als, the limitations possessed by them does not allow to be used at high temperatures. Due to the same reason, oxide-based materials have gained much attention. Here, we discuss about the oxide thermoelectric materials in detail and the effect of texturization on their morphology and transport properties. There is a lot of scope available for such class of materials for high-temperature applications. Keywords Thermoelectric review · Oxide thermoelectrics · Effect of texturization · Thermoelectric figure of merit Introduction cooling devices is 200–250 million$ per year that attracts a lot of research in this area [2]. Renewable energy has been one of the very widely studied Ioffe in the 1930s proposed the idea that semiconductors topics since the past few decades due to the rising problem act as better thermoelectric materials than metals that were of fulfilling the energy demands. The increasing demand previously used [3]. Though Altenkirch (in 1909) was the for oil and electricity is a much-known fact that cannot be first to establish a mathematical expression which relates ignored. On a global average, more than 70% of the energy physical properties of thermoelectric materials and the effi- generated is wasted one way or the other. TE energy harvest- ciency of simplified thermopile or thermoelectric generators, ing is promising in providing opportunities to harvest the his equation contained various parameters like electromo- waste heat that is rejected out of the automobile exhausts, tive force, thermal, and electrical resistances etc., Ioffe was chimneys of factories, etc. In this review, we will concentrate the first to quantify Z (Thermoelectric figure of merit) by only on thermal energy harvesting. Thermoelectric materi- integrating all the variables introduced by Altenkirch into als can be used as a source for both power generation and one term as Z [4, 5]. cooling devices [1]. Metal-based thermoelectric conversion The maximum energy conversion efficiency is given by materials were studied earlier followed by semiconducting ƞ and is defined as: max materials. Since metallic TE materials are not ideal for high- T −T , M−1 hot cold max= temperature applications as they are prone to corrosion and cold hot (1) M+ hot hamper the conductivity, oxide-based thermoelectric materi- 1/2 als have gained much attention. The world market of ther- where M = (1 + ZT ) , T being the average temperature, avg avg Z is the thermoelectric figure of merit, and T and T are moelectric generators is 25–50 million$ per year and that for hot cold temperatures of the hot and cold end of the module, respec- tively [6, 7]. The potential of a thermoelectric material for the application is measured with the help of a term called as * Shekhar D. Bhame the dimensionless thermoelectric figure of merit (ZT) which Shekhar.bhame@sitpune.edu.in measures the maximum heat conversion into electricity. ZT is given by: Symbiosis International (Deemed University), Symbiosis Institute of Technology, Lavale, Pune, Maharashtra 412115, India Vol.:(0123456789) 1 3 3 Page 2 of 22 Materials for Renewable and Sustainable Energy (2020) 9:3 S K = + + = + LT + , tot lat ele bip lat bip (5) ZT = T, (2) + + e l bip wher e K is a Boltzmann constant, m* is den - where S (V/K) is the Seebeck coefficient, σ (S/m) is electri- sity of states effective mass, h is the Planks constant –34 2 cal conductivity, and , , and (W/m K) are the elec- (6.6260 × 10 m kg/s), n being carrier concentration, e is e l bip –19 tronic, lattice, and bipolar contribution to thermal conduc- charge of an electron (1.6021 × 10 coulombs), µ is carrier tivity due to bipolar effects [8 , 9]. The power factor S σ is mobility, τ is relaxation time, K , , , being total, tot lat ele bip optimized in narrow gap semiconductors as a function of lattice, electronic and bipolar contribution to thermal con- 19 20 3 –8 2 carrier concentration (typically ~ 10 –10 carriers/cm ). ductivity, and L is Lorentz number (2.44 × 10 W Ω/K ) [11, Figure 1 shows the optimization of ZT by tuning the carrier 12]. is usually observed for relatively narrow band-gap bip concentration for Bi Te [10]. It was observed that maximum materials, which leads to the decrease of TE efficiency. This 2 3 ZT could be obtained on tuning the carrier concentration to be effect is the consequence of the diffusion of both electrons 19 20 3 between ~ 10 to ~ 10 carriers/cm . and holes in the same direction. The minority conductors get Since Wiedemann–Franz law relates electrical and thermal thermally exited through the bandgap leading to the cancel- conductivities as directly proportional quantities ( = LT σ), lation of net charge in the conduction band. They reduce the the quantities S, σ, and for a conventional 3D crystalline Seebeck coefficient as the minority charge carriers contrib- system cannot be varied independently to give a high ZT value. ute significantly. This is generally observed for very nar - Because what is desired is a high Seebeck coefficient, high row bandgap semiconductors (E < 0.5 eV) [13, 14]. What electrical conductivity, and low thermal conductivity. If elec- is commonly observed is for an ideal metal, they possess trical conductivity is low for a material, carrier concentration high electrical conductivity and low thermal conductivity. has to be increased. As we increase the carrier concentration, Glasses exhibit high thermal conductivity and low electrical electrical conductivity would increase. This, in turn, would conductivity. Since a mixture of these two properties would reduce the Seebeck coefficient and increase the electrical con- be optimum for a thermoelectric material, such materials tribution to thermal conductivity. The complex relationships are termed as “phonon glass electron crystal” since it would of these parameters can be summarized in the Fig. 3 and also possess high thermal conductivity like glass and still possess in the form of the equation as: high electrical conductivity like an ordered crystal. It has been reported by Dresselhaus that as the dimension of mate- 8K 2∕3 B rial decreases and approaches a nanometer scale, it possibly (3) S = m T , 3eh 3n causes differences in the density of states and gives rise to quantum confinement effects allowing new methods to tailor S, σ, and K values independently to achieve high ZT [15]. ne = ne = , (4) Although there are significant reports of various class of thermoelectric materials exhibiting low thermal conductiv- ity and high electronic conductivity leading to high power factor and ZT, as shown in Fig. 2. Operating them at a high temperature for waste heat conversion is still a concern. A number of concerns such as toxicity, low melting point, high oxidation tendency, and stability at high temperature are yet to be addressed in detail. Figure 2 shows the development of high ZT materials over the year for non-oxide-based systems adapted from Zhang et al. [11]. The extreme left indicates the state-of-the-art materials, mid-portion indicates the improvement till close to 2010, and the last part shows very high ZT obtained in the recent decade. Few of the newly included materials other than the reference quoted above are PbTe–SrTe, Cu Se/CNT, PbSe + 2%HgSe, and Na-doped SnS. Oxides are considered to be more stable with a high melt- ing point, relatively less toxicity, and cheap. During the ini- tial days of development, thermoelectric community had ignored this class of materials due to the low conductivity Fig. 1 Optimization of ZT by tuning the carrier concentration for of carriers and ionic nature. However, due to the features that Bi Te . Image adapted with permission from Snyder et al. [10], 2 3 Springer Nature possess, they have been investigated since past 2 decades. 1 3 Materials for Renewable and Sustainable Energy (2020) 9:3 Page 3 of 22 3 Fig. 2 The development of high ZT materials over the year for non-oxide-based systems [adapted with permission from Zhang et al. [11], Elsevier] SrTiO Strontium titanium oxide (STO) is an n-type perovskite oxide. They have a cubical crystal structure and belong to the space group of pm3 m [17] with a very high melting point of 2353 K. Since they have a very high melting point, it is a potential high-temperature thermoelectric candidate [18]. In Fig. 3, the periwinkle blue sphere indicates Sr atoms, brown indicates Ti atoms, and black sphere indicates oxy- gen atoms. STO has a wide bandgap of 3.2 eV. STO shows promising Seebeck coefficient, electrical conductivity, car - rier mobility, and large effective mass (m* ~ 10m ). The high Seebeck coefficient is attributed to the sixfold degeneracy of Fig. 3 Crystal structure of STO simulated using VESTA 3d-t2g conduction band of Ti [19]. The carrier concentration of STO can be controlled by adequate doping of elements to transform it from insulating to semiconducting with a con- 21 −3 Their electronic properties can be tuned by manipulating centration of ~ 10 cm [20]. It has been seen that Nb and their crystal structures, chemical compositions, and dop- La doping makes way for STO single crystals as an effec- ing concentrations. As thermoelectric oxides exhibit high tive thermoelectric material. Okuda et al. reported very high 2 covalent and ionic character, they tend to show high thermal Seebeck coefficient (2800–3600 µW/K m) and power factor 2 conductivity. Hence, doping them with suitable elements (28–36 µV/(K cm) for La-substituted STO (Sr La TiO ) 1−x x 3 is necessary. It has been seen that, in solids that show poor which was comparable to Bi Te [21]. However, since they 2 3 thermal conductivity, the phonon mean free path is in the show high thermal conductivity (6–12 W/m/K for single- range of interatomic distance. Hence, doping the right ele- crystalline STO) obtaining a high ZT was limited [22], since ment not only alters electronic properties but also alters the then, a lot of studies has been done on reducing the thermal thermal properties [16]. A number of oxide-based thermo- conductivity to enhance ZT. In 2003, Muta et al. tried to electric materials have been reported. A few promising ones understand the effect of rare-earth element doping in the to mention are SrTiO, Ca Co O, Na CoO , ZnO, In O , STO crystal. Not much difference in electrical conductiv - 3 3 4 9 x 2 2 3 and BiCuSeO. ity was seen on varying the substituent rare-earth element. 1 3 3 Page 4 of 22 Materials for Renewable and Sustainable Energy (2020) 9:3 However, in the case of thermal conductivity, the dopant had was decreased, an increase in power factor up to 75% was a strong influence. La and Sm showed a decrease in thermal reported. It has to be noted that the reduction of thermal conductivity as temperature increased. Gd, Dy, and Y doping conductivity was most effective in the case of Fe inclusion led to the least thermal conductivity without much depend- than the Cu inclusion as Fe acted as a better phonon scatter- ence on the temperature. This trend was attributed to tem- ing centre [22]. Electrons confined in quantum wells with perature-independent phonon impurity scattering. The order dimensionality narrower than de-Broglie wavelength results of thermal conductivity on doping is La > Sm > Gd > Y > Dy in 2D electron gas that exhibit exotic electronic properties. with Dy being the element of substitution with least thermal 2D electron well enhances Seebeck coefficient for STO as conductivity (~ 2.2 W/m/K) which naturally led to better ZT carrier electrons are more localized in STO than in heavy than the rest as Seebeck coefficient and electrical conductiv - metals as it is an insulator [18]_ENREF_93_ENREF_93. ity was almost the same [23]. Since Nb and La have one of High Seebeck coefficient has been seen for one atom thick the most structural tolerance for doping in STO and since it STO layers containing 2D electron gas (2DEG). Seebeck possesses highest electrical conductivity, B site substitution coefficient of up to 850 µV/K leading to a ZT of 2.4 2DEG with Nb gained much attention [20]. Few other rare-earth and ZT of ~ 0.24 (including the barrier layer) was reported eff substitutions in the STO matrix along with reduced sinter- by Ohta et al. [29]. Very recently, Nb-doped SrTiO /rGO ing time were reported by Kovalivisky et al. Pr, Nd, and composite was explored. The cationic vacancies, oxygen Sm incorporation showed promising electronic conductivity vacancies due to rGO, and heterointerfaces lead to increased as compared to La, Ce, Gd, Dy, and Y doping. However, phonon scattering. Mobility of charge carriers dramatically small cation elements like Dy and Y gave rise to better See- increased due to the cationic vacancies and heterointerfaces. beck coefficient. Oxygen-deficient perovskite layers present These features lead to a high power factor of ~ 1.98 mW/ in Nd-, Sm-, and Dy-doped STO led to phonon scattering K m promising ZT of 0.29 at 1180 K [30]. Tkach et al. leading to ZT as high as 0.42 at 1190–1225 K [24]. Tungsten in 2018 made use of a unique two-step firing process to substitution in the STO was performed by Kovalivisky et al. obtain bimodal distribution of 20% Nb-doped STO fine Tungsten substitution at the B site greatly increased the elec- particles (particles smaller size ~ 270 nm) with a grain size trical conductivity without affecting the Seebeck coefficient of ~ 2.3 µm and 59 µm while for the coarse particles (par- to a great extent. Tungsten is interesting as it can donate two ticles with larger size ~ 800 nm) exhibited an average grain electrons per W cation and also scatters the phonons due to size of ~ 8 µm. The lower number of grain boundaries within mass fluctuation in STO. While a ZT of 0.24 was reached at the large connected grains in fine size STN20 leads to better 1300 K by substituting 0.06 portion of W, on codoping W electrical conductivity (328 S/cm at 970 K). Energy l fi tering and Nb in STO (Sr Ti Nb W O ) resulted in a ZT effects in the fine size STN20 further reportedly lead to an 0.97 0.8 0.17 0.03 3±δ of 0.28 at 1270 K. Better performance due to Nb codoping increase in Seebeck coefficient with temperature. The power 2 was assigned to the phase separation and phonon scattering factor of this material obtained was 13.4 µW/K cm which [25]. Kovalivisky et al. reported the effect of Pr substitu- is comparable with that of state-of-the-art thermoelectric tion. Results show that Pr leads to an increase in electrical material Bi Te and finally leading to a ZT of 0.36 at 970 K 2 3 conductivity reducing the Seebeck coefficient. At the same [31]. Meanwhile, a much simpler approach was followed by time, thermal conductivity reduction was also observed and Ekren et al. where a fixed amount (0.5 wt%) of B O and 2 3 maximum reduction in thermal conductivity was found for 0.3 wt% ZrO were composited with Sr Nb TiO to obtain 2 0.9 0.1 3 x = 0.30 in Sr Pr TiO . However, a ZT of ~ 0.34 at 1173 K 1400 S/cm at room temperature and ~ 200 S/cm at 1273 K. 1−x x 3 was obtained for x = 0.10 [26]. Improvising the electrical A drastic improvement in electrical conductivity was seen conductivity in oxide-based system is crucial and this was on just 0.3 wt% addition of ZrO . The dislocation density addressed by Akin et al. by creating the oxygen vacancies. enhancement on addition of ZrO that leads to increased They could successfully improvise the electrical conduc- oxygen vacancies thereby increasing the carrier concentra- tivity by creating A site vacancies in La-doped STO lead- tion (also helps in reduction in thermal conductivity) hold ing to oxygen loss that leads to improvement in electrical responsible for this improvement. The addition of ZrO also conductivity [27]. However, clarity on the influence of such lead to the formation of uniform-sized smaller grains lead- vacancies on thermoelectric properties was further estab- ing to barely irregular grain boundaries that lead to carrier lished by Lu et al. They obtained a ZT of 0.4 at 973 K (for mobility improvement. Lowest Seebeck coefficient and high- x = 0.15 in Sr La TiO ) by tuning the A site and oxy- est power factor(~ 1000 µV/m. K ) were obtained for the 1−3x/2 x 3−δ 3+ 4+ gen vacancies along with mixed valent Ti and Ti that same composition leading to a ZT of 0.37 at 1015 K [32]. To held responsible for the improvement [28]. Most recently tune the thermal conductivity, substituting Ca and Ba at the in 2018, Srivastava et al. reported promising ZT of 0.36 A site has resulted in lowering the same [33, 34]. However, it (900 K) and 0.38 (1000 K) for Cu and Fe inclusion, respec- was reported that these dopants also supresses the power fac- tively, in Sr La Ti Nb O system. As the resistivity tor, thereby hampering the thermoelectric performance [35]. 0.8 0.067 0.8 0.2 3−δ 1 3 Materials for Renewable and Sustainable Energy (2020) 9:3 Page 5 of 22 3 Though there were other reports of doping STO, addition of structure. This represents a misfit structure with a rock salt secondary phases, and tuning the A and B site vacancies, no disordered layer of Ca CoO sandwiched between CdI type 2 3 2 further improvement in ZT as reported by Kovalivisky et al. CoO layers stalked along c-axis with [40, 41]. These two [24] has been seen till date. layers share the same lattice parameters. a = 4.8270(5) Å, c = 10.8300(2) Å, and β = 98.136(1) Å. Misfit of unit cell Eec ff t of texturization along the b-axis changes the lattice parameter b values as b = 4.5615(2) Å, b = 2.8173(1) Å for Ca CoO and CoO 1 2 2 3 2 Quin et al. followed a Reactive Template Grain Growth layer, respectively [42]. The cobaltates are said to reduce Method (RTGG) assisted tape casting method whereby a thermal conductivity due to phonon scattering from inco- Ruddlesden–Popper-type phase SrO(SrTiO ) material was herent boundaries. Thermoelectric properties of CCO-349 3 n synthesised that has a multi-layered structure. They con- (whisker-based single crystals) showed a high ZT of 1.2–2.7 sist of alternating layer of perovskite STO and superlattice for at 873 K. These results show that there is a lot of poten- layer of SrO stalked along the c-axis. Where they could tial for such materials in thermoelectric applications [43]. achieve a Seebeck Coefficient of − 399 µV/k at 1073 K ZT of 0.87 at 973 K was reported by Shikano and Funahashi without any dopant as compared to the highest Seebeck for single-crystalline (Ca CoO ) CoO misfit oxide [44]. 2 3 0.7 2 coefficient reported by Gd/Sm doped SrO(SrTiO ) syn- Due to the complexity of the production of single crystals, 3 n thesised by conventional solid-state-assisted hot-pressing development of polycrystalline materials is of importance. method (− 350 µV/k at 1073 K) [36]. Such an improve- Polycrystalline material synthesis has also been reported for ment was achieved by controlling the microstructure and these materials with various substitutions like Eu [45], Lu annealing temperature. Promising Seebeck coefficient has [46], Ho, and Ga [47] to give promising ZT values. Bis- been reported for SrT iO textured using the RTGG tech- muth has been considered as one among the most promising nique by Gao et al. where the texturization induced a low dopant. It has shown to increase electrical conductivity and thermal conductivity of 2.1/Wm/K at 1027 K and Seebeck Seebeck coefficient. Bismuth also leads to a reduced ther - coefficient of ~ 313 to ~ 328 µV/K within the temperature mal conductivity due to the larger size of Bi atoms and also range of 230 K to 1027 K [37]. The reported thermoelectric their corresponding mass [48]. Silver is another promising parameters in all the cases were found to be higher for the dopant that leads to an increase in electrical conductivity textured samples. and reduction in thermal conductivity because of the heavy ion. Whereas for Ag that is added as a composite mixture Ca Co O (CCO‑349) with CCO-349, Ag acts as a connector between the cobaltate 3 4 9 grains leading to a decrease in the scattering of charge car- The challenges of creating novel TE oxides have led to riers leading to increasing conductivity at grain boundaries. investigating a lot of materials. Layered cobaltates such as While doping of Ag ions increases the thermopower and Ca Co O (CCO-349) and Na CoO (NCO- × 12) have been charge mobility along with charge concentration, composite 3 4 9 x 2 found to be promising candidates for thermoelectricity. One decreases thermopower due to metal Ag at grain bounda- of the major steps in thermoelectric oxide materials was the ries. However, a balance between doping and compositing discovery of Na CoO by Terasaki et al. in 1997. This mate- in the same system (Ca Ag Co O /Ag-10 wt%) can lead x 2 2.7 0.3 4 9 rial possesses a unique conduction mechanism. The charge to a promising ZT of 0.5 at 1000 K [49]. Very recently, the carriers are transported via the CoO layer for p-type con- highest ZT of 0.74 has been achieved by Saini et al. at 800 K duction and phonon path is mediated through the N a layer. for Tb doped CCO-349 system. This is the highest ZT ever They possess a highly anisotropic structure with a Seebeck reported for a polycrystalline oxide-based system [50]. coefficient of 200 μΩ cm in the plane and 100 μV/K resistiv - 2 ity giving rise to a power factor of 50 μW/(K cm) which was comparable to that of the traditional material Bi Te with Eec ff t of texturization 2 3 2 40 μW/(K cm) [38]. This anisotropy refers to the in-plane anisotropy caused because of hexagonal lattice distortion by On following the thermoforging process by Pravel et al. on- the square RS-type layer, CCO-349 exhibits a larger thermo- calcium cobalt oxide (CCO-349), a decrement in electrical power meanwhile shows very less resistivity. They have ther- resistivity was seen on increasing the applied pressure dur- mal stability to up to 973 K. High carrier concentration and ing the process which increases grain alignment. This study low thermal conductivity make it a good example of elec- made way for further exploration of thermoforging and the tron crystal phonon glass-type material [39]. In the layered influence of applied pressure on CCO-349 system [51]. On cobaltate system, only calcium cobaltate has one cation with a comparative study of the effect of sintering technique on different oxidation state. A general representation of this the thermoelectric performance, Liu et al. found that SPS system is [CaCoO] [CoO ]. Where RS represents rock salt leads to higher degree of grain orientation. RS 2 1 3 3 Page 6 of 22 Materials for Renewable and Sustainable Energy (2020) 9:3 A sheet-like grains with 1–2 µm can be seen in both Densification and texturing are found maximum in HP sam- the cases due to the layered structure. However, a mas- ple, and thus, resistivity is minimum (6.1 mΩ cm at 300 K), sive grain alignment and reduction in pore size can be seen and Seebeck coefficient of 550 µW/m/K at 850 K in the case in the case of SPS (Fig. 4b, c). This grain alignment not of HP which was claimed to be one of the lowest resistivity only leads to greater density of material (90%-SPS as com- and highest S reported along the ab plane [55]. Since the pared to 70%-CS), but also holds responsible for the higher synthesized product was only 0.5 mm thick, module making electrical conductivity. The measured electrical conduc- would be a challenge. Hence, they stalked the multi-layers tivity for the CS sample was 34 S/cm at 973 K, whereas of CCO. Each layer of CCO was hot-pressed and polished. SPS sample exhibited an electrical conductivity of 118 S/ They were stalked together, hot-pressed again, and was cm. Although Seebeck coefficient variation was less, the cut along ab and c direction. Thermoelectric parameters higher difference in electrical conductivity leads to a high- measured parallel and perpendicular to c-axis show highly –4 2 power factor of 3.51 × 10 W/m/K (SPS) as compared to anisotropic nature of the sample. Finally, a ZT of 0.16 was –4 2 1.1551 × 10 W/m/K (CS). These factors lead to 34.5% realized along the ab plane [56]. To obtain a better power increment in ZT of SPS sintered sample as compared to the factor, optimizing stress while pressing multi-layers of CCO CS sample [52]. Lin et al. performed a unique combination was later identified. Better power factor (595 µW/m/K ) at of SPS and dynamic forging technique leading improved higher stress level (30 MPa) was realized by the team at texturization over the just SPS processed sample. It has been 900 K along the ab plane [57, 58]. The anisotropic resistiv- seen that this technique allows in lowering the resistivity of ity behaviour was also explored later using a modified SPS the sample without affecting much of Seebeck coefficient. technique called as SPT. In this technique, a pre-sintered To reduce thermal conductivity, lanthanum was doped in the pellet is kept in a die of bigger diameter in the second step CCO matrix. La leads to scattering of phonon as it is a heavy for free deformation of pellet resulting in better texturiza- mass element leading to mass fluctuation in the matrix. tion and platelet formation along the axis perpendicular to Finally, ZT of 0.26 at 975 K was reported [53]. Neodymium the pressing direction [59]. An improvement of 40–60% substitution in the CCO matrix along with texturization (170–180 µW/m/K ) in power factor was reported in 2016 efforts was done by Parvel et al. in 2007. Their results show as a result of microwave sintering. Microwave sintering that Nd increased the decomposition temperature of CCO. resulted in an increase in density as well as the growth of Also, thermoforging led to an increase in electrical con- elongated grains. The main reason for the enhancement is ductivity of the sample by 2.5-fold. The decrease in carrier assigned to better electrical conductivity due to low porosity concentration with Nb substitution holds responsible for the and texturing [60]. Strontium-doped CCO349 synthesised improvement in Seebeck coefficient and thermopower [54]. by hot-pressing exhibited higher electrical conductivity and A comparative study of conventional sintering, hot-pressing lower Seebeck Coefficient. Resistivity close to those sintered and spark plasma sintering, their texturization effects, and by SPS method (6 mΩ cm) was obtained for the hot-pressed influence on thermoelectric parameters of CCO-349 were Sr-doped (6.5 mΩ cm) sample at 1073 K. Presence of Sr done by Kenfaui et al. The micrographs obtained for these and Ca carbonate impurities on the surface of the sample methods revealed that there were a platelet formation and along with grain alignment and good conductivity between high degree of orientation in the case of the hot-pressed sam- the grains was attributed to be responsible for the improved ple. The order of degree of orientation was: HP > SPS > CS. electrical conductivity. Almost 20% higher PF (1.16 mW/ Fig. 4 Scanning electron microscopy of the fracture surface of CS (a) and SPS (b, c) materials 1 3 Materials for Renewable and Sustainable Energy (2020) 9:3 Page 7 of 22 3 2 K m at 800 °C) were exhibited by the Sr-doped samples as (not in the case of Ti) which also led to the Seebeck coef- a result of high chemical pressure effects. Though there was ficient improvement and has been reported in [69, 70]. Also, significant improvement in electrical conductivity and power compositing NCO with Ag has led to enhancement of ther- factor, ZT was estimated to be 0.29 at 1073 K due to the high moelectric properties as seen in the work by Seetawan et al. thermal conductivity (4.4 W/K m) [61]. ZT as good as 0.36 A 2% doping at the cobalt site exhibited marginally lower at 1073 K were reported for CCO synthesised by calcin- resistivity (66 µΩ cm) over the complete temperature range ing the nitrate hydrate precursors followed by cold pressing as compared to the other compositions. A reduction in ther- at 200 MPa and sintering at 1173 K. The porous structure mal conductivity was also observed mainly due to the single- clearly has lead to the reduction in thermal conductivity, phase NCO and due to the Ag presence, while the other which, for a highly dense sample, is a bane [62]. compositions exhibited mixed phase. These factors com- bined to gather with a high power factor of 0.31 mW/m/K Na CoO (NCO) at 973 K leading to a ZT of 0.124 at 973 K [71]. It has been x 2 seen that out of the transition-metal element addition (Ni, Great interest in misfit oxide-based thermoelectric mate- Fe, Mn, and Cu) into the cobalt site, only Ni and Cu were rials was seen after the reporting very high thermopower a promising dopant which rules out the possibility of other of ~ 100 μV/K by Terasaki et al. in 1997 for Na Co O [38]. elements mentioned as an effective dopant for enhanced x 2 4 Since the thermal conductivity exhibited by this lamellar thermoelectric performance [72–74]. Addition of Zn has based material was very low, it is considered to be fitting also led to enhancement in thermopower [75]. Krasutskaya into the PGEC concept [63]. This oxide has a misfit double- et al. recently reported a ZT of 1.57 at 1100 K for x = 0.89 layer structure. One layer consists of the CdI type-CoO with a low thermal conductivity of 0.829 mW/(mK ) which 2 2 ions with edge-shared octahedra and Na ions stalked along is one of the highest ZT reported for polycrystalline NCO- c-axis on the other. While CoO layer acts as an electron based system [76]. By doping Co site with a minute fraction reservoir and is responsible for electronic conductivity and of Fe (Na Co Fe O ) increased thermopower along 0.71 0.95 0.05 2 thermopower, the N a ion layer acts as an electron donor to the ab plane to a greater extent and was reported recently by the CoO layer and also holds responsible for reducing the Richter et al.[77]. Behera et al. reported a low-temperature thermal conductivity. The Na ions are intercalated between synthesis by nitrate–citrate combustion method which fol- the CoO layers. The concentration of Na ions can be varied lowed a low heat treatment strategy to obtain a ZT of ~ 0.01 to manipulate the number of conduction electrons on the Co at 780 K for the La Na CoO -based compound. The 0.85 0.15 3 layers [64]. When x = 0, the electrovalency of cobalt is + 3 thermopower of this compound was found to be promising with 5 electrons occupying the ground state of the t g band. (~ 374 µV/K at 318 K) due to strong electron correlation and And when x = 1, the electrovalency of cobalt is + 4 with a degeneracy of 3d orbitals of Co. However, thermal activation fully filled ground state t g band [65]. They crystallize in of holes due to Na (induces p-type conductivity) content at the hexagonal structure with a space group of P63/mmc. In higher temperature reduces the thermopower [78]. these family of oxides, since carrier density does not con- tribute to the power factor, tuning the same would leave no Eec ff t of texturization difference in the power factor. This class of materials was then found to be a promising candidate for high-temperature The hydrothermal method was reported by Zhang et al. for thermoelectrics as they exhibited good chemical stability the synthesis of NCO so as to develop materials with ori- and non-toxicity as compared to the other state-of-the-art ented growth which would add on to the texturing effects materials like Bi Te and PbTe [66]. Reports on polycrys- leading to improvements in thermoelectric performance. A 3 3 talline NaCo O with Cu doping at the Co site prepared by strong anisotropic behaviour was exhibited by the HT sam- 2 4 solid-state mixing method following hot pressing exhib- ple as the grain boundary density was different along in- ited ZT of 0.88 at 573 K for Na(Co Cu ) O (x = 0.05). plane and out-of-plane direction. However, the CSS sample 1−x x 2 4 Though the thermal conductivity was 30% higher for the failed to exhibit such a behaviour as a result of higher grain hot-pressed sample, a reduction in room temperature electri- size and lower degree of grain alignment. Hence, in-plane cal resistivity from 2 to 1.1 mΩ cm (for NCO) as a result of electrical conductivity and Seebeck coefficient was much hot pressing holds responsible for this improvement [67]. At higher (Fig. 5a) along the complete temperature range as low temperature, for sodium-deficient system, intercalated compared to the out-of-plane direction resulting in a drastic water results in superconducting transition. High Seebeck improvement in power factor of ~ 0.43 from ~ 0.08 mW/m K coefficient of ~ 120 µV/K was reported by tailoring the Na at 993 K (Fig. 5b). Due to the high orientation degree of (x = 0.15) stoichiometry in Na Co O [68]. Later on, adding NCO-based materials, the grain boundary density is more x 2 4 metallic phase of (Ag, Ti, Rh, Pd) into this system was tried along the out-of-plane direction. As a result of grain to increase the electronic conductivity which was successful boundary scattering due to the high grain density, thermal 1 3 3 Page 8 of 22 Materials for Renewable and Sustainable Energy (2020) 9:3 Fig. 5 Electrical conductivity (a), Seebeck coefficient (a), power factor (b), thermal conductivity (c), and thermoelectric figure of merit (d) vari- ations with respect to temperature for the NCO-based material under two pressing directions conductivity decreases (also leads to reduction in electrical various applications such as UV-light emitters, solar cells, conductivity) (Fig. 5c). However, NCO prepared by HT and flat panel display, light-emitting devices, piezoelectric trans- cut along the in-plane direction exhibits maximum ZT of ducers, brake linings, cosmetics, dental cements, lubricants, 0.064 at 923 K (Fig. 5d) due to the high power factor and paints, phosphors, and other products due to its low cost, low thermal conductivity (~ 5.2 W/mK at 923 K) [79]. The abundance, non-toxicity, and thermal/chemical stability transport properties of layered cobaltates are highly aniso- [80–82]. Wurtzite structure is the most common structure tropic due to the layered structure. Hence, texturization finds exhibited by ZnO with a space group of P6 mc. However, potential in the improvement of their properties as it would zinc blend structure with a space group of Fm3m is also seen lead to a higher degree of grain alignment, thereby increas- under pressure. A typical wurtzite crystal structure is shown ing the electrical conductivity. in Fig. 6. Here, the brown sphere indicates zinc atom and blue indicates an oxygen atom. ZnO Elements such as Aluminium, Indium, Gallium, etc. are doped in the ZnO matrix to optimise the wide bandgap of ZnO is an n-type wide band-gap semiconductor having ZnO for applications in the field of solar cells, thermoelec- bandgap of 3.37 eV, and has been used extensively for tric, and optoelectronic applications [83–85]. A high carrier 1 3 Materials for Renewable and Sustainable Energy (2020) 9:3 Page 9 of 22 3 were explored and was seen to exhibit one of the highest ZT of 0.52 at 1000 K for any n-type oxide-based material with 1% doping. The phonon scattering from multiple scale scat- tering sources such as point defects, grain boundary, ZnO nanoprecipitates, and micropores leads to very low thermal conductivity and can be attributed to as the main reason for the high ZT reported [92]. Further optimisation of dopant and size of nanoparticle could also lead way for better ZT [93]. Lately, Al and Zr codoping have been found to be a promising combination in the ZnO matrix which would sup- press the thermal conductivity and boost electrical conduc- tivity. Zn Al Zr O showed a ZT of 0.12 at 1173 K. 0.997 0.002 0.005 This mixed combination was seen to exhibit the lowest ther- mal conductivity and better ZT than the individual-doping compositions [94]. Dy doping has been tried in the ZnO matrix by Anju et al. Since Dy possess higher ionic radii and since they produce localised magnetic moment due to Fig. 6 Crystal structure of wurtzite-type ZnO simulated with VESTA unpaired electron, it can effectively act as the phonon scat- terer and lead to reduction in thermal conductivity. On the 20 −3 concentration (> 10 cm ) is exhibited by the group III addition of 1% Dy atoms, thermal conductivity was reduced elements which makes them ideal dopants. Out of the group to up to 2 W/mK at 900 K. Also, since Dy leads to remark- III elements, Al has been identified to be the most abun- able increase in electrical conductivity, i.e., up to 1000 times dant and also is a shallow donor making it one of the most as compared to pristine ZnO at 373 K. These factors lead to suitable dopants to increase the n-type character [80]. ZnO an improved ZT of 0.11 at 923 K [95]. exhibits a harmonically vibrating lattice as a result of the ionic and covalent nature leading to high thermal conduc- tivity (~ 40 W/m/K ). M. Ohtaki in 1996 identified ZnO to Eec ff t of texturization be a potential thermoelectric material exhibiting a PF of –4 2 8–15 × 10 W/m K for aluminium-doped zinc oxide and The thermoelectric properties exhibited by pure and doped leading to ZT of 0.3 at 1273 K. Though they possess high zinc oxide are discussed above. However, it is known that thermal conductivity due to the ionic character of ZnO, due texturization could lead to an improvement in the thermo- to higher carrier mobility and carrier concentration, the elec- electric parameters. Han et al. synthesised aluminium-doped trical conductivity shot up without much decrement in ther- ZnO by various methods to obtain rods, platelets, and nano- mopower [86]. Observation of enhancement in thermopower particles. SPS technique was used for sintering and induce of Al-doped ZnO-based systems was reported by Ohtaki as texturing in the sample (Fig. 7). The relation between vari- a result of phonon scattering by the nanovoids—leading to ous morphologies on the thermoelectric parameters after suppressed thermal conductivity [87] to give a ZT of 0.55 texturing was then explored [96]. to 0.57 at 1273 K, thermionic effects due to the nanovo - It was seen that the rods exhibited maximum electrical ids and suspected carrier energy filtering by the nanovoids conductivity (Fig. 8a) and carrier mobility (Fig. 8b). One to deliver a ZT of up to 0.6 at 1250 K [88]. Later, dually of the reasons for this improvement is the aligning of rods doped ZnO (Ga, Al) set a benchmark by exhibiting a ZT of along the perpendicular direction leading to a reduction in 0.65 at 1247 K where Ga acted as the scattering centres to number of grain boundary, thereby reducing the scattering reduce thermal conductivity, whereas Al acted as an agent of charge carriers, hence proving that microstructure and to increase the carrier mobility which increases the electri- morphology plays an important role. The parallelly cut rods cal conductivity [89]. More recently, Jood et.al reported a and platelets exhibited least thermal conductivity which ZT of ~ 0.44 for 1000 k that reached thermal conductivity again is a proof of anisotropic behaviour (Fig. 8c). Paral- of ~ 2 W/mK. However, further improvements are necessary lelly cut samples exhibit maximum grain boundaries lead- to obtain better ZT values [90]. Very recently, Zhakharchuk ing to maximum phonon scattering. However, nanoparticles et al. reported a slightly improved ZT of close to 0.12 for exhibit the least thermal conductivity due to impurity scat- Zn Al Zr O at 1200 K. While new scattering inter- tering and grain boundary scattering. Though nanoparticles 0.994 0.003 0.003 faces and suppression of thermal conductivity were due to exhibit poor electrical conductivity, carrier mobility, and zirconia, enhancement of electrical transport properties carrier concentration as compared to the other morpholo- mainly led to this enhancement [91]. Recently, Cd Zn O gies, since they exhibit the least thermal conductivity, and 1−x x 1 3 3 Page 10 of 22 Materials for Renewable and Sustainable Energy (2020) 9:3 Fig. 7 Illustration of the con- solidation process for a rods, b platelets, and c particles which were cut parallel and perpen- dicular to the pressing direction for measurements the ZT exhibited by nanoparticles is maximum (ZT = 0.30 at a consequence of Zn vacancies created as they are the pref- 3+ 1223 K) (Fig. 8d) [96]. erential site for incorporation of In ions. Their ordering further leads to (ZnO) .In O (IZO)-type phases [101]. It k 2 3 In O based ceramics was identified by Ohta et al. in 1996 that such series could 2 3 be potential thermoelectric material. They reported that ZT Homologous In O –ZnO-based system has been widely would increase with increase in the concentration of ZnO 2 3 studied for their thermoelectric performance as they pos- (i.e., k = 3, 5, 7 and 9 with maximum ZT at k = 9). ZT values sess excellent structural and chemical stability even at of 0.11 at 900 K have been reported by Bernik et al. for k = 5 higher temperature [97]. They are indexed by rhombohedral as a consequence of sintering, pre-reacted mixture, and raw symmetry. Homologous (ZnO) .In O belong to the space mixture in 1:1 ratio leading to lower thermal conductivity k 2 3 group of R3m for odd values of k and P63/mmc for even [102]. values of k [98]. Particularly, the wide bandgap exhibited The representation of the crystal structure of In O , 2 3 by these compounds leads to better thermoelectric param- ZnO, and (ZnO) In O is given in Fig. 9. The brown sphere 5 2 3 eters as they would resist the thermal excitation of electrons indicates Zn atoms, light blue indicates oxygen atoms, and at higher temperature [99]. They exhibit a layered struc- the periwinkle blue indicates In atom. The introduction ture with alternating layers of edge-sharing In–O octahe- of point defects in In O results in a glass-like thermal 2 3 dral layers and In–Zn–O layers arrange in a zigzag manner conductivity. The lattice thermal conductivity could be [100]. The n-type conductivity exhibited by this compound reduced by 60% and extraordinary low lattice thermal is dominant through hexagonal lattice than across the ab conductivity of 1.2 W/mK at 973 K was also reported. plane as the main in-plane conducting path is In–O layer However, point defect engineering by co-doping Zn and and In–Zn–O being the out-of-plane conducting path [97, Ce in the In O system (In Zn Ce O ) leads to a much 2 3 2−2x x x 3 99]. Local charge deficiencies in the crystal are created as better thermoelectric parameter (ZT = 0.44 for x = 0.12). 1 3 Materials for Renewable and Sustainable Energy (2020) 9:3 Page 11 of 22 3 Fig. 8 Electrical conductivity (a), carrier mobility (b), thermal conductivity (c), and thermoelectric figure of merit (d) of Zn Al O (x = 0.02) 1−x x measured for samples cut parallel and perpendicular to pressing direction Various other elements were doped in ZnO for thermo- in (ZnO) (In Al )O and k = 5 which was because of the k 1−x x electric applications such as Ca, In, Ni, Nb, Dy, Yb, Y, and increment in carrier mobility due to Al and reduction in Gd which have led to a decrease in thermal conductivity. thermal conductivity [104]. It was identified that Dy, Yb, and Gd are the most suit- Berardan et al. later explored the potential of Germa- able dopants, since they possess higher mean atomic mass nium doped In O (In Ge O ) as a thermoelectric mate- 2 3 2−x x 3 leading to a low heat capacity which is directly related to rial. It was observed that a decrease of fivefold in resistivity thermal conductivity. Kosir et al. studied this system for of the sample was seen on the addition of just x = 0.002 of varying k values of 5, 11, and 18. It was identified that Ge in the In O matrix. The resistivity of the sample fur- 2 3 total thermal conductivity was minimum for least k value. ther decreased on addition of more Ge which resulted in Similarly, promising electrical conductivity was seen in a decrease in resistivity from ~ 25 mΩ cm to ~ 1.5 mΩ cm the case of k = 5, while 11 and 18 exhibited less electrical at x = 0.015 measured at 1000 K. It was also pointed out conductivity. The reason for this trend was attributed to that Ge addition selectively reduces thermal conductivity the In planar defect density. Higher the defect density, (3–0.6 W/mK) without affecting charge carrier mobility Zn lower was the thermal conductivity and better electrical altogether giving rise to a ZT of 0.45 at 1273 K [105]. Since conductivity. It was also seen that sintering the composite In is a rare element, reducing the In content is of importance. at 1773 K (ZT ~ 0.125) resulted in a better ZT than for the Bhame et al. were successful in reducing the indium con- material sintered at 1573 K (ZT ~ 0.11) at 973 K [103]. tent and still obtain thermoelectric parameters comparable Later, in the same year, they reported that the addition to the doped In O system. The fluorite-type structure of 2 3 of Al into homologous IZO with k = 5 results in a slight Ga In Sn O resulted in interesting thermoelectric per- 3−x 5+x 2 16 increment in ZT. However, the improvement was very little formance [106]. Zhou et al. observed that in the In–Sn–O (ZT ~ 0.13) which was for the composition with x = 0.01 system, a lower density of the final sample reduces electrical 1 3 3 Page 12 of 22 Materials for Renewable and Sustainable Energy (2020) 9:3 at 1027 K. The textured samples were obtained by Tani et al. with the help of RTGG method (Fig. 10). A high degree of anisotropy was seen in the electrical conductivity measure- ments. The samples cut perpendicular to the casting direc- tion (c plane) exhibited less conductivity as compared to the sample cut parallel showing the effect of misalignment distribution of grains. Greater number of grain boundaries is seen in the case of perpendicularly cut sample, whereas long grains and fewer grain boundaries in parallelly cut samples lead to higher electrical conductivity. However, a huge dif- ference between the textured (~ 2 W/mK) and non-textured (~ 5 W/mK) sample is shown in the case of thermal conduc- tivity due to the high density exhibited by them regardless of the direction in which the sample is cut [98]. Isobe et al. synthesised Y-doped IZO5 (((ZnO) In Y ) O ) by RTGG method to obtain textured sam- 0.97 0.03 2 3 ples. SEM images revealed longer grains and grain orienta- tion for the perpendicularly cut sample as compared to the parallel one. Although the sintered specimens exhibited less density, the electrical conductivity was seen to be greater for the textured samples. Textured samples exhibited half the thermal conductivity exhibited by the non-textured speci- mens. The thermal conductivity exhibited by Y-doped non- textured (impurity scattering) specimen was also exhibited by the textured undoped samples (scattering due to pores). However, combining both the effects textured Y-doped IZO Fig. 9 Schematic representation of crystal structure for parent oxides samples exhibited the maximum ZT of 0.33 at 1073 K. Such (In O , ZnO) and (ZnO) In O 2 3 5 2 3 a ZT was possible due to the samples retaining high electri- cal conductivity in spite of low density [109]. conductivity. However, reactive sintering with gallium addi- Kaga et al. reported a ZT of 0.31 for textured Ca-doped tion seemed to overcome this by densifying the sample fur- IZO as compared to the non-textured sample that showed ther. It resulted in an increase in density to up to 90% of a ZT of 0.23 at 1053 K. RTGG technique was followed bulk density. Ga In Sn O and SnO secondary phases were for texturization. RTGG method introduces bigger grains 2 6 2 16 2 seen as a result of reactive sintering of the precursors. How- and higher degree of alignment than the reference sample. ever, a large number of localized grain boundaries among Electrical conductivity measurements show that there was a secondary phases result in grain growth and increase the certain degree of anisotropy present in the sample. Samples density. Though thermal conductivity increases with Ga cut parallel to ab-axis exhibited better electronic conduc- addition, finally with a minute addition of Ga, ZT of ~ 0.23 tivity. Though at temperatures close to room temperature, was reported at 1000 K [107]. Combe et al. reported a ZT of the thermal conductivity is less for c-plane measurements, 0.3 at 1000 K for the Ge doped In O system. They pointed at high-temperature both exhibited almost similar thermal 2 3 out a decrease in thermal conductivity at high temperature conductivity leading to ZT of 0.31 [110]. after the solubility limit of Ge in the In O system (less than 2 3 0.5 at%). The decrease in thermal conductivity was attrib- BiCuSeO uted to the homogeneously distributed insulating second- ary phases of In Ge O observed after the solubility limit. BiCuSeO is an interesting class of mixed anionic com- 2 2 7 Though electrical resistivity increased after the solubility pounds made of oxide-based chalcogenide atoms. The oxide limit, the total effect contributed to an enhanced ZT [108]. and chalcogenide atoms are indirectly bonded through the cationic atoms. This leads to an alternating insulating and 2+ Eec ff t of texturization conducting fluorite-like layers of oxide (Bi O ) and chalco- 2 2 2− gen (Cu Se ) , respectively, along the c-axis. This naturally 2 2 Among the homologous IZO series, k = 5 resulted in maxi- leads to better carrier mobility along with the a, b planes mum ZT and has been mentioned already. Texturization than along the c plane resulting in anisotropy. These features efforts were made on this composition to obtain ZT of 0.18 make them a suitable thermoelectric material and were first 1 3 Materials for Renewable and Sustainable Energy (2020) 9:3 Page 13 of 22 3 Fig. 10 The methodology followed for the synthesis of textured IZO (a). b SEM of the parallelly cut, c perpendicularly cut sample processed by RTGG method, and d reference sample Se atom, periwinkle blue sphere represents the Bi atom, and black sphere represents the oxygen atoms. At room tempera- 18 −3 ture, their carrier concentration is reported to be 10 cm and carrier mobility is 22/cm /V/s [114]. A huge leap in the electrical conductivity for this class of material was achieved in 2010 when Sr was doped with BiCuSeO. The electrical conductivity increased from 470 S/m to 4.8 × 10 S/m at 293 K on Sr substitution at the Bi site by a proportion of just 0.15 at%. This drastic increase was assigned to be respon- 2+ sible due to the carriers that are induced in the (Bi O ) 2 2 layer that acts as the charge reservoir by partially substi- 3+ 2+ tuting Bi with Sr and transferring it to the conducting 2− (Cu Se ) layer. As the thermal conductivity of this mate- 2 2 Fig. 11 Crystal structure of BiCuSeO generated using VASP rial is by default very low (~ 0.65 W/mK at 873 K) along with increased electrical conductivity and Seebeck coeffi- cient (~ 225 µV/K) leads to a ZT of 0.76 at 873 K [115]. reported in 2010 with Sr doping [11, 111, 112]. They usually Very soon on realizing the potential of oxychalcogenides, especially BiCuSeO, Ca doping at the Bi site resulted in crystallize in the tetragonal system with a = b = 3.9273 and c = 8.9293 as unit cell dimensions and belongs to the space a reduction of thermal conductivity and improved power factor as a result of improvised electrical conductivity as group of P4/nmm. The figure shows the crystal structure of BiCuSeO exhibiting a tetrahedral symmetry [113]. compared to the undoped sample. The replacement of Bi with Ca lead to an increase in the hole concentration hence Figure 11 represents the BiCuSeO crystal structure. The 2+ orange sphere represents a Cu atom, blue sphere represents transferring charge carriers from charge reservoir (Bi O ) 2 2 1 3 3 Page 14 of 22 Materials for Renewable and Sustainable Energy (2020) 9:3 2− layer to the conducting (Cu Se ) layer. The overall ZT for electrical conductivity, whereas Ca played the role to reduce 2 2 Bi Ca CuSeO is reported to be 0.9 at 923 K [116]. thermal conductivity. Since Pb has a delocalised 6 s orbital 0.0925 0.075 However, in 2014, Pei et al. reported the highest ZT of all that has a lone pair of electrons and due to an increased times till the date for undoped BCSO exhibiting a ZT of 1.4 effective mass, Seebeck coefficient and electrical conductiv - at 923 K. Such a huge leap from the previously reported ZT ity improve better than any other elemental doping. On the for pristine sample (0.70 at 773 K as in [117]) was mainly other hand, Ca induces both mass fluctuation (difference in because of the improvement of power factor (Fig. 12c) due mass), size, and strain field fluctuation (difference in inter - to modulation doping strategy (MD). MD strategy has been atomic coupling force) leading to minimum thermal con- used to increase the carrier mobility. They could improve ductivity. Though the solubility limit of Pb is 6–7%, higher mobility by twofold without deteriorating the carrier concen- amount of Pb exhibited formation of nanoprecipitates which tration. This leads to a high electrical conductivity without lead to reduction in lattice thermal conductivity. Though and resulted in a very high PF of 10 µW/cm/K at 923 K. the pristine BCSO are known to exhibit high Seebeck coef- However, MD had no much influence on the Seebeck coef- ficient, upon dual doping, it decreases to 175 µV/K at 873 K ficient. These factors along with a decrease in K (~ 0.25 W/ which three times is lower. Thus, the improvement in elec- lat mK) lead to ZT of 1.4 at 923 K (Fig. 12f) [118]. trical conductivity coupled with a moderate Seebeck coef- –3 Previous reports show that single-element doping has ficient gives rise to a promising power factor of 1.0 × 10 / 2 –4 2 led to the improvement in either electrical conductivity or Wm/K for x = 0.06 as compared to 2.2 × 10 /Wm/K at thermal conductivity. An improvement in both has been 873 K. The formation of Bi-rich nanoinclusions and CaO observed, but very moderate. To combine both the effects, contributes to phonon scattering leading to a reduction in lat- i.e., reduction in thermal conductivity and increase in elec- tice thermal conductivity from ~ 3.8 to ~ 2.9 W/mK at 873 K. trical conductivity, Liu et al. carried out dual-doping tech- All the factors combined together lead to a high ZT of ~ 1.5 nique, whereby Pb acted as agent for the improvement in at 873 K [119]. ZT as good as 1.09 was reported in 2016 by Das et al. for the pristine sample of BaCuSeO. However, the high ZT was attributed to a reduction in thermal conductivity 0.19 W/mK at 773 K due to all length scale phonon scat- tering by micrometre-sized grains, point defects, naturally layered structure, and oxygen vacancies [120]. Replacing Sb at the Bi site and Te in Se site by a proportion of 8 mol% (Bi Sb CuSe Te ) remarkably increased the ZT to 0.92 0.08 0.92 0.08 1.19 at 873 K. This increase in ZT was due to the increase in band covalency and increase in carrier concentration due to Sb/Te co-doping. Another reason was attributed to the smaller grain size due to long hours (16 h) of ball milling and Cu vacancies created which lead to an increase in hole concentration eventually leading to better electrical conduc- tivity. Sb/Te co-doping was also responsible for the reduc- tion in thermal conductivity as it leads to mass fluctuation and strain field fluctuation by the point defects and nano- structurization [121]. Eec ff t of texturization Since BCSO is a highly anisotropic material, texturizing these materials would lead to an improvement in the trans- port properties with respect to the direction of measurement. Hot forging is a method adopted for inducing the texturiza- tion in samples. Hot forging leads to higher order of grain alignment which was seen in BCSO with Ba doping. Prefer- ential orientation of grains was seen to get shifted from (102) Fig. 12 The electrical conductivity (a), seebeck coefficient (b), power to (003) direction. It was seen that as the number of hot forg- factor (c), thermal conductivity (d, e), and thermoelectric figure of merit (f) of modulation doped BCSO ing steps increases, the orientation along (003) became more prominent. While the non-textured (0 T) samples exhibited 1 3 Materials for Renewable and Sustainable Energy (2020) 9:3 Page 15 of 22 3 least anisotropy during the transport property measure- orientation resulting in close to 187 µV/K at 923 K for all ments, the textured samples exhibited maximum anisot- the samples, power factor for the 3 T⊥ sample was the high- ropy (increased with number of hot forging steps). Since est (8 µW/cm/K AT 923 K). Though thermal conductivity the grains arrange themselves in the form of platelets along would increase with hot forging, the ratio of electrical to the a- and b-axis, i.e., perpendicular to pressing direction thermal conductivity was the highest for 3 T⊥ resulting in a evident from the SEM, z-Euler images and inverse pole fig- high ZT of ~ 1.4 at 923 K [112]. ures, as shown in Fig. 13. The inverse pole figures especially highlight the high grain orientation along the c-axis for 3 T⊥ sample as compared to 0 T⊥. The grain boundary density Other oxides decreases in this direction leading to increase in both electri- cal conductivity and thermal conductivity. In addition to the oxide-based systems discussed above, However, along the parallel cut direction, since the grain there are also a few other materials that are researched boundary density is greater, it leads to a decrease in trans- upon. One class of them is the double perovskites. Double port properties. Electrical conductivity increases from 450 perovskites are an interesting class of materials that exhibit to 700 S/cm at room temperature for three times hot forged half-metallic ground state in which conduction electrons are sample that was cut perpendicular direction. While Hall fully spin-polarized. The potential of such double perovs- effect showed not much variation in carrier concentration, kite-based materials for thermoelectric application is high- so the improvement in electrical conductivity was attributed lighted by Roy et al. and high Seebeck coefficient (290 µV/K to an increase in carrier mobility (2/cm/V/s for 0 T⊥ and at 1124 K) for material like S r TiFeO has been reported 2 6 4/cm/V/s for 3 T⊥). The transport property improvement [122]. ZT of 0.29 at 1223 K was later reported by the team along the ⊥ cut samples was attributed to the higher grain for p-type Ba Sr TiCoO (x = 0.2). This material exhibited x 2−x 6 alignment as confirmed from the XRD. While the Seebeck glass-like behaviour. Thermal conductivity was reported to coefficient was seen to be independent of any such grain be within 0.01 to 0.55 W/mK which is very low as compared Fig. 13 The microstructure (a, e, h, k), grain size distribution (b, f, i, l), z-Euler images (c, g, j, m) of 0, 1, 2, and 3 T⊥ samples, respectively, and the inverse pole figures (d, n) for 0 and 3 T⊥ samples 1 3 3 Page 16 of 22 Materials for Renewable and Sustainable Energy (2020) 9:3 Fig. 14 Representation of the development of thermoelectric oxides over the years to the state-of-the-art materials existing in the market. The centres, metallic electrical conductivity is shown due to the low thermal conductivity, however, was attributed to the combination of Sr TiFeO and Sr TiMoO . On adding Ba 2 6 2 6 phonon scattering in the crystal due to multiple cations, alongside increases the Seebeck coefficient at the cost of the transition from relaxor to the glassy state after Burns electrical conductivity leading to such high-power factor val- temperature and shorter grain size [123]. Increasing Ba ues [125]. Pseudobrookites (Fe TiO ) have been known for 2 5 composition to x = 0.25 showed a very high Seebeck coef- their high Seebeck coefficient of -100 to – 370 µV/K within ficient of ~ 800 µV/K at 1123 K at the cost of compromising the range of 286–1000 K and low thermal conductivity electricidal conductivity. However, the author suggests that (1–1.5 W/mK within the temperature range of 673–1000 K) aliovalent substitution could tune the electrical conductiv- [126]. Efforts have been done to improvise their electrical ity of such materials [124]. The promising power factor conductivity to obtain a promising ZT. Improvement in the of ~ 35 µW/mK at 1100 K was reported for a composition electrical conductivity, thermal conductivity deuteration, of n-type Ba Sr Fe Mo O . While thermal conductivity and power factor was reported by et al. recently as a result of 0.1 1.9 0.5 0.5 6 is taken care by the grain size and cationic phonon scattering optimizing the percentage of Ti content in F e Ti O . An 2−x 1+x 5 improvised ZT of 0.014 at 1000 K was obtained at x = 0.25 1 3 Materials for Renewable and Sustainable Energy (2020) 9:3 Page 17 of 22 3 leading to a few microcracks which reduce the thermal con- ductivity without affecting the electrical conductivity to a great extent as in the case of x = 0.5 [127]. Since oxides, in general, show high resistivity, oxygen deficiencies were created using vacuum sintering in a helium atmosphere to improve the electrical conductivity of Ni-doped SnO . Power factor as good as 20.195 µV/K at 950 K was obtained for 3at% doped SnO revealing the high-temperature perfor- mance potential of this material [128]. On a summary, the following graph depicts the different promising thermoelectric oxides reported over the years. The development of thermoelectric n and p-type oxides is given in Fig. 14. The various oxides labelled in Fig. 14 are–In–Sn–O [129], (Ca, Bi)MnO [130], (Ca,In)MnO 3 3 [131], (Zn, Al)O [86], (Zn, Al)O nanovoid [88, 132], (Zn, Al, Ga)O [89], (Ca, Eu) Co O [45], (Ca, Ho) Co O [133], 3 4 9 3 4 9 (Ca, Lu) Co O [46], CaMn Nb O [134], Ca (Co, Ga) O 3 4 9 1−x x 3 4 9 [47], (Ca, Ag) Co O /Ag [49], (Ca, Ag, Lu) Co O [58], 3 4 9 3 4 9 Na(Co,Cu) O [67], Na(Co,Zn) O [135], NaCo O [136], 2 4 2 4 2 4 Bi Sr Co O [137], (Sr,Dy)TiO [23], Ca(Mn,Nb)O [134], 2 2 2 y 3 (In,Zn,Ge)O [138], (Zn,Al)O [90], PPP/(Zn.Ni)O [139], (Zn. In)O [140], TiC O @TiO -TiO [141], In(Zn,Sn)O [142], 0.1 0.9 y 2 BiCu0.90SeO [143], (Bi,Sr)CuSeO [117], (Bi,Ba)CuSeO [112], (Bi,Na)CuSeO [144], (Bi,Pb)CuSeO [144], (Bi,Ba) CuSeO [112], (Bi,Pb)CuSeO [145], Modulation BiCuSeO [118], (Bi,Ba)CuSeO [146], and (Bi,Ca,Pb)CuSeO [119]. The following table highlights the overall effect of tex- turization on thermoelectric parameters of various materials as discussed above: It can be seen from the table that texturization has played a vital role in improving the thermoelectric parameters. However, further studies combining methodologies such as nanostructurisation, modulation doping, and texturization could lead to further improvements. It is clear that ZnO- based materials lack attention and improvements could be made by doping and texturing, since they can be used for high-temperature applications (Table 1). Summary The effect of nanostructurisation and texturization of ther - moelectric ceramics and their credibility for the improve- ment of figure of merit is discussed. The most typical thermoelectric materials and their heat-recovery range are summarized. Thermoelectric parameters can be tuned by texturization using various methods such as reactive tem- plate grain growth method, hot press, spark plasma sinter- ing, hotforging, etc. The representation in Fig. 15 makes it clear that more studies on ZnO, In O Ca Co O and other 2 3 3 4 9 oxide-based systems have to be done in the future as they 1 3 Table 1 A comparison of effect of texturization on thermoelectric parameters for the promising oxide-based materials (*%change in PF) Material Measuring tem- Before texturization After texturization % Change References perature (K) 2 2 σ, S/cm |S|, µV/K PF, µW/cm/K Κ, /Wm/K ZT σ, S/cm |S|, µV/K PF, µW/cm/K Κ, /Wm/K ZT SrTiO 1027 140 170 4 4.7 0.08 50 280 3.9 2 0.14 57.14 [36, 148] Ca Co O 973 30 186 1 – – 118 175 3.5 – – 28.57* [52] 3 4 9 Na CoO 923 64 200 2.2 4.4 0.038 90 220 4.3 4.5 0.064 59.37 [79] x 2 ZnO 1000 – 300 – 2 0.44 – 200 – 3.2 0.30 − 68.1 [90, 96] In O 1053–1273 162/9.1 170 – 2.2 0.23 251/11 160 2.15 2.4 0.31 74.19 [110] 2 3 BiCuSeO 923 200 180 6.3 0.48 1.1 230 187 8.1 0.58 1.38 79.71 [112] * resembles % change in PF which is mentioned at the top of table 3 Page 18 of 22 Materials for Renewable and Sustainable Energy (2020) 9:3 Fig. 15 Thermoelectric materials classified based on environmental friendliness and heat-recovery range [adapted and modified from Ohtaki et al. [147]] included in the article’s Creative Commons licence, unless indicated are promising for mid to high-temperature heat-recovery otherwise in a credit line to the material. If material is not included in applications. Figure 15 is a representation of various ther- the article’s Creative Commons licence and your intended use is not moelectric materials classified based on the heat-recovery permitted by statutory regulation or exceeds the permitted use, you will range (adapted from [147] with modification). The times need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativ ecommons .or g/licenses/b y/4.0/. ahead are expected to see a very robust alternative to the existing alloy-based thermoelectric modules based on oxide materials. A lot of work is already going on in developing the thermoelectric modules using non-metallic materials. References Certainly, the progress in nanostructuring and techniques to sinter the powders without significantly affecting the nano- 1. Zhang, D.-B., Zhang, B.-P., Ye, D.-S., Zhu, Y.-B.: Thermoelec- tric properties of textured Zn Al O ceramics prepared by spark structure will impact the performance of newly developed 1–x x plasma sintering of hydrothermally synthesized rod-shaped pow- thermoelectric materials. ders. Ceram. Int. 41(10), 14627–14634 (2015) 2. Vining, C.B.: An inconvenient truth about thermoelectrics. Nat. Acknowledgements The authors would like to acknowledge the Mater. 8(2), 83 (2009) Department of Science & Technology-Science & Engineering Research 3. Vedernikov, M., Iordanishvili, E.: AF Ioffe and origin of modern Board (DST-SERB) for the funds received under the Grant number: semiconductor thermoelectric energy conversion. In: Thermo- SERB/F/3112/2014-15. R. Prasad would like to acknowledge Symbio- electrics, 1998. Proceedings ICT 98. XVII International Confer- sis International (Deemed University) for the award of Senior Research ence on 1998, pp. 37–42. IEEE Fellowship. 4. Takabatake, T., Suekuni, K., Nakayama, T., Kaneshita, E.: Pho- non-glass electron-crystal thermoelectric clathrates: experiments Funding This study was funded by Department of Science & Tech- and theory. Rev. Mod. Phys. 86(2), 669 (2014) nology–Science & Engineering Research Board (DST-SERB), India 5. Polozine, A., Sirotinskaya, S., Schaeffer, L.: History of develop- (Grant Number: SERB/F/3112/2014-15). ment of thermoelectric materials for electric power generation and criteria of their quality. Mater. Res. 17(5), 1260–1267 (2014) 6. Aswal, D.K., Basu, R., Singh, A.: Key issues in development of Compliance with ethical standards thermoelectric power generators: high figure-of-merit materials and their highly conducting interfaces with metallic intercon- Conflict of interest The authors declare that we have no conflict of nects. Energy Convers. Manag. 114, 50–67 (2016) interest. 7. Salvador, J.R., Cho, J.Y., Ye, Z., Moczygemba, J.E., Thomp- son, A.J., Sharp, J.W., Koenig, J.D., Maloney, R., Thompson, Open Access This article is licensed under a Creative Commons Attri- T., Sakamoto, J.: Conversion efficiency of skutterudite-based bution 4.0 International License, which permits use, sharing, adapta- thermoelectric modules. PCCP 16(24), 12510–12520 (2014) tion, distribution and reproduction in any medium or format, as long 8. Tripathi, M., Bhandari, C.: High-temperature thermoelectric as you give appropriate credit to the original author(s) and the source, performance of Si–Ge alloys. J. Phys. Condens. Matter 15(31), provide a link to the Creative Commons licence, and indicate if changes 5359 (2003) were made. The images or other third party material in this article are 1 3 Materials for Renewable and Sustainable Energy (2020) 9:3 Page 19 of 22 3 9. Rowe, D., Bhandari, C.: Modern Thermoelectrics. Reston Publ. 28. Lu, Z., Zhang, H., Lei, W., Sinclair, D.C., Reaney, I.M.: High- Co., Reston (1983). (ISBN 0-8359-4593-6) figure-of-merit thermoelectric La-doped A-site-deficient 10. Snyder, G.J., Toberer, E.S.: Complex thermoelectric materials. SrTiO ceramics. Chem. Mater. 28(3), 925–935 (2016) Nat. Mater. 7(2), 105 (2008) 29. Ohta, H., Kim, S., Mune, Y., Mizoguchi, T., Nomura, K., Ohta, 11. Zhang, X., Zhao, L.-D.: Thermoelectric materials: energy con- S., Nomura, T., Nakanishi, Y., Ikuhara, Y., Hirano, M.: Giant version between heat and electricity. J. Materiomics 1(2), 92–105 thermoelectric Seebeck coefficient of a two-dimensional elec- (2015) tron gas in SrTiO . Nat. Mater. 6(2), 129 (2007) 12. Weidenkaff, A.: Thermoelectricity for future sustainable 30. Okhay, O., Zlotnik, S., Xie, W., Orlinski, K., Gallo, M.J.H., energy technologies. In: European Physical Journal Web of Otero-Irurueta, G., Fernandes, A.J., Pawlak, D.A., Weidenkaff, Conferences 2017 A., Tkach, A.: Thermoelectric performance of Nb-doped SrTiO 13. Zhao, L., Wu, H., Hao, S., Wu, C.-I., Zhou, X., Biswas, K., enhanced by reduced graphene oxide and Sr deficiency coopera- He, J., Hogan, T.P., Uher, C., Wolverton, C.: All-scale hierar- tion. Carbon 143, 215–222 (2018) chical thermoelectrics: MgTe in PbTe facilitates valence band 31. Tkach, A., Resende, J., Saravanan, K.V., Costa, M.E.V., Díaz- convergence and suppresses bipolar thermal transport for high Chao, P., Guilmeau, E., Okhay, O., Vilarinho, P.M.: Abnormal performance. Energy Environ. Sci. 6(11), 3346–3355 (2013) grain growth as a method to enhance the thermoelectric perfor- 14. Gong, J., Hong, A., Shuai, J., Li, L., Yan, Z., Ren, Z., Liu, mance of Nb-doped strontium titanate ceramics. ACS Sustain. J.-M.: Investigation of the bipolar effect in the thermoelectric Chem. Eng. 6, 15988–15994 (2018) material CaMg Bi using a first-principles study. PCCP 18(24), 32. Ekren, D., Azough, F., Gholinia, A., Day, S.J., Hernandez- 2 2 16566–16574 (2016) Maldonado, D., Kepaptsoglou, D.M., Ramasse, Q.M., Freer, R.: 15. Dresselhaus, M.S., Chen, G., Tang, M.Y., Yang, R., Lee, H., Enhancing the thermoelectric power factor of Sr 0.9 Nd 0.1 T iO Wang, D., Ren, Z., Fleurial, J.P., Gogna, P.: New directions for through control of the nanostructure and microstructure. J. Mater. low-dimensional thermoelectric materials. Adv. Mater. 19(8), Chem. A 6(48), 24928–24939 (2018) 1043–1053 (2007) 33. Muta, H., Ieda, A., Kurosaki, K., Yamanaka, S.: Substitution 16. Ren, G.-K., Lan, J.-L., Ventura, K.J., Tan, X., Lin, Y.-H., effect on the thermoelectric properties of alkaline earth titanate. Nan, C.-W.: Contribution of point defects and nano-grains to Mater Lett 58(30), 3868–3871 (2004) thermal transport behaviours of oxide-based thermoelectrics. 34. Muta, H., Kurosaki, K., Yamanaka, S.: Thermoelectric proper- Comput. Mater. 2, 16023 (2016) ties of doped BaT iO –SrTiO solid solution. J. Alloy Compd. 3 3 17. Kinaci, A., Sevik, C., Çağın, T.: Electronic transport proper- 368(1–2), 22–24 (2004) ties of SrTiO and its alloys: Sr La TiO and SrTi M O 35. Yamamoto, M., Ohta, H., Koumoto, K.: Thermoelectric phase 3 1–x x 3 1–x x 3 (M=Nb, Ta). Phys. Rev. B 82(15), 155114 (2010) diagram in a CaT iO –SrTiO –BaTiO system. Appl. Phys. Lett. 3 3 3 18. Ohta, H.: Thermoelectrics based on strontium titanate. Mater. 90(7), 072101 (2007) Today 10(10), 44–49 (2007) 36. Wang, Y., Lee, K.H., Ohta, H., Koumoto, K.: Thermoelectric 19. Shang, P.-P., Zhang, B.-P., Liu, Y., Li, J.-F., Zhu, H.-M.: Prep- properties of electron doped SrO ( SrTiO ) n (n = 1, 2) ceramics. aration and thermoelectric properties of La-doped SrTiO J. Appl. Phys. 105(10), 103701 (2009) ceramics. J. Electron. Mater. 40(5), 926–931 (2011) 37. Gao, F., Yang, S., Li, J., Qin, M., Zhang, Y., Sun, H.: Fabrica- 20. Ohta, S., Nomura, T., Ohta, H., Koumoto, K.: High-tempera- tion, dielectric, and thermoelectric properties of textured SrTiO ture carrier transport and thermoelectric properties of heavily ceramics prepared by RTGG method. Ceram. Int. 41(1), 127–135 La-or Nb-doped Sr Ti O single crystals. J. Appl. Phys. 97(3), (2015) 034106 (2005) 38. Terasaki, I., Sasago, Y., Uchinokura, K.: Large thermoelectric 21. Okuda, T., Nakanishi, K., Miyasaka, S., Tokura, Y.: Large ther- power in N aCo O single crystals. Phys. Rev. B 56(20), R12685 2 4 moelectric response of metallic perovskites: Sr La TiO (0 (1997) 1−x x 3 %3c ~ x %3c ~ 0.1). Phys. Rev. B 63(11), 113104 (2001) 39. Hébert, S., Flahaut, D., Pelloquin, D., Martin, C., Maignan, A., 22. Srivastava, D., Norman, C., Azough, F., Schäfer, M.C., Hejtmanek, J.: Cobalt oxides as potential thermoelectric ele- Guilmeau, E., Freer, R.: Improving the thermoelectric prop- ments: the influence of the dimensionality. In: Thermoelectrics, erties of SrTiO -based ceramics with metallic inclusions. J. 2003 Twenty-Second International Conference on-ICT 2003, pp. Alloys Compd. 731, 723–730 (2018) 161–166. IEEE 23. Muta, H., Kurosaki, K., Yamanaka, S.: Thermoelectric proper- 40. Miyazaki, Y., Onoda, M., Oku, T., Kikuchi, M., Ishii, Y., Ono, Y., ties of rare earth doped SrT iO . J. Alloys Compd. 350(1–2), Morii, Y., Kajitani, T.: Modulated structure of the thermoelec- 292–295 (2003) tric compound [Ca CoO ] 0.62CoO . J. Phys. Soc. Jpn. 71(2), 2 3 2 24. Kovalevsky, A., Yaremchenko, A., Populoh, S., Thiel, P., Fagg, 491–497 (2002) D., Weidenkaff, A., Frade, J.: Towards a high thermoelectric 41. Pravarthana, D., Lebedev, O., Hebert, S., Chateigner, D., Salva- performance in rare-earth substituted SrTiO : effects provided dor, P., Prellier, W.: High-throughput synthesis of thermoelectric by strongly-reducing sintering conditions. Phys. Chem. Chem. Ca Co O films. Appl. Phys. Lett. 103(14), 143123 (2013) 3 4 9 Phys. 16(48), 26946–26954 (2014) 42. Wu, T., Tyson, T.A., Chen, H., Bai, J., Wang, H., Jaye, C.: A 25. Kovalevsky, A., Populoh, S., Patrício, S., Thiel, P., Ferro, M., structural change in Ca Co O associated with enhanced ther- 3 4 9 Fagg, D., Frade, J., Weidenkaff, A.: Design of SrTiO -based moelectric properties. J. Phys. Condens. Matter 24(45), 455602 thermoelectrics by tungsten substitution. J. Phys. Chem. C (2012) 119(9), 4466–4478 (2015) 43. Funahashi, R., Matsubara, I., Ikuta, H., Takeuchi, T., Mizutani, 26. Kovalevsky, A., Yaremchenko, A., Populoh, S., Weidenkaff, U., Sodeoka, S.: An oxide single crystal with high thermoelectric A., Frade, J.: Enhancement of thermoelectric performance performance in air. Jpn. J. Appl. Phys. 39(11B), L1127 (2000) in strontium titanate by praseodymium substitution. J. Appl. 44. Shikano, M., Funahashi, R.: Electrical and thermal properties of Phys. 113(5), 053704 (2013)single-crystalline (Ca CoO ) CoO with a Ca Co O structure. 2 3 0.7 2 3 4 9 27. Akin, I., Li, M., Lu, Z., Sinclair, D.C.: Oxygen-loss in A-site Appl. Phys. Lett. 82(12), 1851–1853 (2003) deficient Sr 0.85 La 0.10 TiO perovskite. RSC Adv. 4(61), 45. Wang, D., Chen, L., Yao, Q., Li, J.: High-temperature thermo- 32549–32554 (2014) electric properties of Ca3Co4O9+ δ with Eu substitution. Solid State Commun. 129(9), 615–618 (2004) 1 3 3 Page 20 of 22 Materials for Renewable and Sustainable Energy (2020) 9:3 46. Nong, N., Liu, C.-J., Ohtaki, M.: High-temperature thermoelec- thermoelectric properties. J. Eur. Ceram. Soc. 39(4), 1237– tric properties of late rare earth-doped Ca3Co4O9+ δ. J. Alloys 1244 (2019) Compd. 509(3), 977–981 (2011) 63. Takahata, K., Iguchi, Y., Tanaka, D., Itoh, T., Terasaki, I.: Low 47. Nong, N., Liu, C.-J., Ohtaki, M.: Improvement on the high tem- thermal conductivity of the layered oxide (Na, Ca) Co O : 2 4 perature thermoelectric performance of Ga-doped misfit-layered another example of a phonon glass and an electron crystal. Phys. Ca 3 Co 4−xGa x O 9+ δ (x= 0, 0.05, 0.1, and 0.2). J. Alloys Rev. B 61(19), 12551 (2000) Compd. 491(1–2), 53–56 (2010) 64. Roger, M., Morris, D., Tennant, D., Gutmann, M., Goff, J., Hoff- 48. Li, S., Funahashi, R., Matsubara, I., Ueno, K., Sodeoka, S., mann, J.-U., Feyerherm, R., Dudzik, E., Prabhakaran, D., Boo- Yamada, H.: Synthesis and thermoelectric properties of the new throyd, A.: Patterning of sodium ions and the control of electrons oxide materials Ca Bi Co O + δ (0.0%3c x%3c 0.75). Chem. in sodium cobaltate. Nature 445(7128), 631 (2007) 3-x x 4 9 Mater. 12(8), 2424–2427 (2000) 65. Li, Q.: On the thermoelectric properties of layered cobaltates. 49. Wang, Y., Sui, Y., Cheng, J., Wang, X., Su, W.: Comparison MRS Online Proc. Libr. Arch. 886 (2005) of the high temperature thermoelectric properties for Ag-doped 66. Thermoelectrics, O.: Research Signpost/ed. by K. Koumoto, I. and Ag-added Ca Co O . J. Alloys Compd. 477(1–2), 817–821 Terasaki, N. Murayama. Trivandrum, India (2002). 3 4 9 (2009) 67. Yakabe, H., Kikuchi, K., Terasaki, I., Sasago, Y., Uchinokura, 50. Saini, S., Yaddanapudi, H.S., Tian, K., Yin, Y., Magginetti, D., K.: Thermoelectric properties of transition-metal oxide NaCo/ Tiwari, A.: Terbium ion doping in Ca Co O : a step towards sub 2/O/sub 4/system. In: Thermoelectrics, 1997. Proceedings 3 4 9 high-performance thermoelectric materials. Sci. Rep. 7, 44621 ICT’97. XVI International Conference on 1997, pp. 523–527. (2017) IEEE 51. Prevel, M., Lemonnier, S., Klein, Y., Hébert, S., Chateigner, 68. Motohashi, T., Naujalis, E., Ueda, R., Isawa, K., Karppinen, D., Ouladdiaf, B., Noudem, J.: Textured Ca Co O thermo- M., Yamauchi, H.: Simultaneously enhanced thermoelectric 3 4 9 electric oxides by thermoforging process. J. Appl. Phys. 98(9), power and reduced resistivity of N a Co O by controlling Na x 2 4 093706 (2005) nonstoichiometry. Appl. Phys. Lett. 79(10), 1480–1482 (2001) 52. Liu, Y., Lin, Y., Shi, Z., Nan, C.W., Shen, Z.: Preparation of 69. Seetawan, T., Amornkitbamrung, V., Burinprakhon, T., Ca Co O and improvement of its thermoelectric properties by Maensiri, S., Kurosaki, K., Muta, H., Uno, M., Yamanaka, S.: 3 4 9 spark plasma sintering. J. Am. Ceram. Soc. 88(5), 1337–1340 Thermoelectric power and electrical resistivity of Ag-doped (2005) Na Co O . J. Alloys Compd. 407(1–2), 314–317 (2006) 1.5 2 4 53. Lin, Y.-H., Lan, J., Shen, Z., Liu, Y., Nan, C.-W., Li, J.-F.: 70. Kurosaki, K., Muta, H., Uno, M., Yamanaka, S.: Thermo- High-temperature electrical transport behaviors in textured electric properties of N aCo O . J. Alloys Compd. 315(1–2), 2 4 Ca Co O -based polycrystalline ceramics. Appl. Phys. Lett. 234–236 (2001) 3 4 9 94(7), 072107 (2009) 71. Seetawan, T., Amornkitbamrung, V., Burinprakhon, T., Maen- 54. Prevel, M., Reddy, E.S., Perez, O., Kobayashi, W., Terasaki, siri, S., Kurosaki, K., Muta, H., Uno, M., Yamanaka, S.: Ther- I., Goupil, C., Noudem, J.G.: Thermoelectric properties of sin- moelectric properties of N a Co O /Ag composites. J. Alloys x 2 4 tered and textured Nd-substituted Ca Co O ceramics. Jpn. J. Compd. 414(1–2), 293–297 (2006) 3 4 9 Appl. Phys. 46(10R), 6533 (2007) 72. Park, K., Jang, K.: Improvement in high-temperature thermo- 55. Kenfaui, D., Chateigner, D., Gomina, M., Noudem, J.G.: Tex- electric properties of NaCo O through partial substitution of 2 4 ture, mechanical and thermoelectric properties of Ca Co O Ni for Co. Mater. Lett. 60(8), 1106–1110 (2006) 3 4 9 ceramics. J. Alloys Compd. 490(1–2), 472–479 (2010) 73. Yao, Q., Wang, D., Chen, L., Shi, X., Zhou, M.: Effects of 56. Kenfaui, D., Lenoir, B., Chateigner, D., Ouladdiaf, B., Gomina, partial substitution of transition metals for cobalt on the high- M., Noudem, J.G.: Development of multilayer textured temperature thermoelectric properties of Ca Co O . J. Appl. 3 4 9+δ Ca Co O materials for thermoelectric generators: influence Phys. 97(10), 103905 (2005) 3 4 9 of the anisotropy on the transport properties. J. Eur. Ceram. 74. Terasaki, I., Tsukada, I., Iguchi, Y.: Impurity-induced transi- Soc. 32(10), 2405–2414 (2012) tion and impurity-enhanced thermopower in the thermoelectric 57. Kenfaui, D., Chateigner, D., Gomina, M., Noudem, J.G.: Ani-oxide NaCo Cu O . Phys. Rev. B 65(19), 195106 (2002) 2–x x 4 sotropy of the mechanical and thermoelectric properties of hot- 75. Tsai, P.-H., Zhang, T., Donelson, R., Tan, T.T., Li, S.: Power pressed single-layer and multilayer thick Ca Co O ceramics. factor enhancement in Zn-doped N a CoO . J. Alloys Compd. 3 4 9 0.8 2 Int. J. Appl. Ceram. Technol. 8(1), 214–226 (2011) 509(16), 5183–5186 (2011) 58. Van, N.N., Pryds, N.: Nanostructured oxide materials and mod- 76. Krasutskaya, N., Klyndyuk, A., Evseeva, L., Tanaeva, S.: Syn- ules for high-temperature power generation from waste heat. thesis and properties of N a CoO (x = 0.55, 0.89) oxide ther- x 2 Adv. Nat. Sci. Nanosci. Nanotechnol. 4(2), 023002 (2013) moelectrics. Inorg. Mater. 52(4), 393–399 (2016) 59. Noudem, J.G., Kenfaui, D., Chateigner, D., Gomina, M.: 77. Richter, R., Shopova, D., Xie, W., Weidenkaff, A., Lecher - Toward the enhancement of thermoelectric properties of lamel- mann, F.: Thermopower enhancement from engineering the lar Ca Co O by edge-free spark plasma texturing. Scr. Mater. Na CoO interacting fermiology via Fe doping. Adv. Con- 3 4 9 0.7 2 66(5), 258–260 (2012) dens. Matter Phys. 2018, 1–7 (2018) 60. Miyazawa, K., Amaral, F., Kovalevsky, A., Graça, M.: Hybrid 78. Behera, S., Kamble, V.B., Vitta, S., Umarji, A.M., Shivaku- microwave processing of Ca Co O thermoelectrics. Ceram. mara, C.: Synthesis, structure and thermoelectric properties 3 4 9 Int. 42(8), 9482–9487 (2016)of La Na CoO perovskite oxides. Bull. Mater. Sci. 40(7), 1-x x 3 61. Torres, M., Costa, F., Flahaut, D., Touati, K., Rasekh, S., Fer- 1291–1299 (2017) reira, N., Allouche, J., Depriester, M., Madre, M., Kovalevsky, 79. Zhang, W., Liu, P., Wang, Y., Zhu, K., Tai, G., Liu, J., Wang, A.: Significant enhancement of the thermoelectric performance J., Yan, K., Zhang, J.: Textured N a CoO ceramics sintered x 2 in Ca Co O thermoelectric materials through combined stron- from hydrothermal platelet nanocrystals: growth mechanism 3 4 9 tium substitution and hot-pressing process. J. Eur. Ceram. Soc. and transport properties. J. Electron. Mater. 47(7), 4070–4077 39(4), 1186–1192 (2019) (2018) 62. Bittner, M., Kanas, N., Hinterding, R., Steinbach, F., Groen- 80. Look, D.C.: Progress in ZnO materials and devices. J. Electron. eveld, D., Wemhoff, P., Wiik, K., Einarsrud, M.-A., Feldhoff, Mater. 35(6), 1299–1305 (2006) A.: Triple-phase ceramic 2D nanocomposite with enhanced 1 3 Materials for Renewable and Sustainable Energy (2020) 9:3 Page 21 of 22 3 81. Huang, M.H., Mao, S., Feick, H., Yan, H., Wu, Y., Kind, H., 99. Peng, H., Song, J.-H., Hopper, E.M., Zhu, Q., Mason, T.O., Free- Weber, E., Russo, R., Yang, P.: Room-temperature ultraviolet man, A.J.: Possible n-type carrier sources in In2O3 (ZnO) k. nanowire nanolasers. Science 292(5523), 1897–1899 (2001) Chem. Mater. 24(1), 106–114 (2011) 82. Pearton, S., Norton, D., Ip, K., Heo, Y., Steiner, T.: Recent 100. Wen, J., Wu, L., Zhang, X.: A unique arrangement of atoms for progress in processing and properties of ZnO. Prog. Mater Sci. the homologous compounds InM O (ZnO) m (M= Al, Fe, Ga, 50(3), 293–340 (2005) and In). J. Appl. Phys. 111(11), 113716 (2012) 83. Serier, H., Gaudon, M., Ménétrier, M.: Al-doped ZnO pow- 101. Košir, M., Podlogar, M., Daneu, N., Rečnik, A., Guilmeau, E., dered materials: Al solubility limit and IR absorption proper- Bernik, S.: Phase formation, microstructure development and ties. Solid State Sci. 11(7), 1192–1197 (2009) thermoelectric properties of (ZnO) k In O ceramics. J. Eur. 2 3 84. Hasnidawani, J., Azlina, H., Norita, H., Bonnia, N., Ratim, S., Ceram. Soc. 37(8), 2833–2842 (2017) Ali, E.: Synthesis of ZnO nanostructures using sol–gel method. 102. Bernik, S., Košir, M., Guilmeau, E.: Microstructure and thermo- Proc. Chem. 19, 211–216 (2016) electric characteristics of (ZnO) k× In O : based ceramics (k= 5 2 3 85. Ogi, T., Hidayat, D., Iskandar, F., Purwanto, A., Okuyama, and 11). Zaštita materijala 57(2), 318–325 (2016) K.: Direct synthesis of highly crystalline transparent conduct- 103. Košir, M., Podlogar, M., Daneu, N., Rečnik, A., Guilmeau, E., ing oxide nanoparticles by low pressure spray pyrolysis. Adv. Bernik, S.: Phase formation, microstructure development and Powder Technol. 20(2), 203–209 (2009) thermoelectric properties of (ZnO) kIn O ceramics. J. Eur. 2 3 86. Ohtaki, M., Tsubota, T., Eguchi, K., Arai, H.: High-tempera- Ceram. Soc. 37(8), 2833–2842 (2017) ture thermoelectric properties of (Zn Al ) O. J. Appl. Phys. 104. Košir, M., Čeh, M., Ow-Yang, C.W., Guilmeau, E., Bernik, S.: 1−x x 79(3), 1816–1818 (1996) Structural features and thermoelectric properties of Al-doped 87. Ohtaki, M., Araki, K.: Thermoelectric properties and ther-(ZnO) 5In O homologous phases. J. Am. Ceram. Soc. 100(8), 2 3 mopower enhancement of Al-doped ZnO with nanosized pore 3712–3721 (2017) structure. J. Ceram. Soc. Jpn. 119(1395), 813–816 (2011) 105. Bérardan, D., Guilmeau, E., Maignan, A., Raveau, B.: In O : Ge, 2 3 88. Ohtaki, M., Hayashi, R.: Enhanced thermoelectric performance a promising n-type thermoelectric oxide composite. Solid State of nanostructured ZnO: a possibility of selective phonon scat- Commun. 146(1–2), 97–101 (2008) tering and carrier energy filtering by nanovoid structure. In: 106. Bhame, S., Zhou, T., Guilmeau, E., Kinemuchi, Y., Delorme, F., Thermoelectrics, 2006. ICT’06. 25th International Conference Raveau, B.: Synthesis and thermoelectric properties of oxygen on 2006, pp. 276–279. IEEE deficient fluorite derivative Ga In Sn O . J. Appl. Phys. 3–x 5+x 2 16 89. Ohtaki, M., Araki, K., Yamamoto, K.: High thermoelectric 108(9), 093708 (2010) performance of dually doped ZnO ceramics. J. Electron. Mater. 107. Zhou, T., Bhame, S., Guilmeau, E., Marinel, S., Raveau, B.: 38(7), 1234–1238 (2009) Enhanced densification and thermoelectric performance of 90. Jood, P., Mehta, R.J., Zhang, Y., Peleckis, G., Wang, X., Siegel, In Sn O by reactive sintering in the In-Sn-Ga-O System. J. 4 3 12 R.W., Borca-Tasciuc, T., Dou, S.X., Ramanath, G.: Al-doped Am. Ceram. Soc. 94(11), 3733–3737 (2011) zinc oxide nanocomposites with enhanced thermoelectric prop- 108. Combe, E., Bhame, S.D., Guilmeau, E., Boschini, F., Cloots, R.: erties. Nano Lett. 11(10), 4337–4342 (2011) Synthesis of In Ge O nanopowders for thermoelectric applica- 2–x x 3 91. Zakharchuk, K., Widenmeyer, M., Alikin, D.O., Xie, W., Popu- tions. J. Mater. Res. 27(2), 500–505 (2012) loh, S., Mikhalev, S.M., Tselev, A., Frade, J., Weidenkaff, A., 109. Isobe, S., Tani, T., Masuda, Y., Seo, W.-S., Koumoto, K.: Ther- Kovalevsky, A.: Self-forming nanocomposite concept for ZnO- moelectric performance of yttrium-substituted (ZnO) 5In O 2 3 based thermoelectrics. J. Mater. Chem. A 6, 13386–13396 improved through ceramic texturing. Jpn. J. Appl. Phys. 41(2R), (2018) 731 (2002) 92. Zha, X., Gao, L., Bai, H., Fu, N., Wang, J., Fu, G., Wang, S.: 110. Kaga, H., Asahi, R., Tani, T.: Thermoelectric properties of highly Towards a high thermoelectric performance in CdO ceramics textured Ca-doped (ZnO) mIn O ceramics. Jpn. J. Appl. Phys. 2 3 by introducing multiscale scattering centers via ZnCO3 doping. 43(10R), 7133 (2004) Mater. Res. Express 5(3), 035908 (2018) 111. Luu, S.D., Vaqueiro, P.: Layered oxychalcogenides: Structural 93. Prasad, R., Bhame, S.D., Walke, P.: Structural and optical studies chemistry and thermoelectric properties. J. Materiomics 2(2), on nanosized thermoelectric oxide-Zn Al O. Int. J. Nanosci. 131–140 (2016) 1−x x 17(5), 1850038 (2018) 112. Sui, J., Li, J., He, J., Pei, Y.-L., Berardan, D., Wu, H., Dragoe, 94. Zakharchuk, K.V., Tobaldi, D.M., Xiao, X., Xie, W., Mikhalev, N., Cai, W., Zhao, L.-D.: Texturation boosts the thermoelectric S.M., Martins, J.F., Frade, J.R., Weidenkaff, A., Kovalevsky, performance of BiCuSeO oxyselenides. Energy Environ. Sci. A.V.: Synergistic effects of zirconium-and aluminum co-doping 6(10), 2916–2920 (2013) on the thermoelectric performance of zinc oxide. J. Eur. Ceram. 113. Lee, C., An, T.-H., Gordon, E.E., Ji, H.S., Park, C., Shim, J.-H., Soc. 39, 1222–1229 (2018) Lim, Y.S., Whangbo, M.-H.: Seebeck coefficients of layered 95. Paulson, A., Sabeer, N.M., Pradyumnan, P.: A synergetic BiCuSeO phases: analysis of their hole-density dependence and approach of band gap engineering and reduced lattice thermal quantum confinement effect. Chem. Mater. 29(5), 2348–2354 conductivity for the enhanced thermoelectric property in Dy ion (2017) doped ZnO. J. Alloy Compd. 786, 581–587 (2019) 114. Zhao, L.-D., He, J., Berardan, D., Lin, Y., Li, J.-F., Nan, C.-W., 96. Han, L., Van Nong, N., Zhang, W., Hung, L.T., Holgate, T., Dragoe, N.: BiCuSeO oxyselenides: new promising thermoelec- Tashiro, K., Ohtaki, M., Pryds, N., Linderoth, S.: Effects of mor - tric materials. Energy Environ. Sci. 7(9), 2900–2924 (2014) phology on the thermoelectric properties of Al-doped ZnO. RSC 115. Zhao, L., Berardan, D., Pei, Y., Byl, C., Pinsard-Gaudart, L., Adv. 4(24), 12353–12361 (2014) Dragoe, N.: Bi Sr CuSeO oxyselenides as promising thermo- 1–x x 97. Jia, J., Ow-Yang, C., Inan Akmehmet, G., Nakamura, S.-I., Kato, electric materials. Appl. Phys. Lett. 97(9), 092118 (2010) K., Shigesato, Y.: Formation of homologous In2O3 (ZnO) m thin 116. Pei, Y.-L., He, J., Li, J.-F., Li, F., Liu, Q., Pan, W., Barreteau, C., films and its thermoelectric properties. J. Vac. Sci. Technol. A Berardan, D., Dragoe, N., Zhao, L.-D.: High thermoelectric per- Vac. Surf. Films 34(4), 041507 (2016) formance of oxyselenides: intrinsically low thermal conductivity 98. Tani, T., Isobe, S., Seo, W.-S., Koumoto, K.: Thermoelectric of Ca-doped BiCuSeO. NPG Asia Mater. 5(5), e47 (2013) properties of highly textured (ZnO) 5In O ceramics. J. Mater. 117. Li, F., Li, J.-F., Zhao, L.-D., Xiang, K., Liu, Y., Zhang, B.-P., Lin, 2 3 Chem. 11(9), 2324–2328 (2001) Y.-H., Nan, C.-W., Zhu, H.-M.: Polycrystalline BiCuSeO oxide 1 3 3 Page 22 of 22 Materials for Renewable and Sustainable Energy (2020) 9:3 as a potential thermoelectric material. Energy Environ. Sci. 5(5), Thermoelectrics, 2006. ICT’06. 25th International Conference 7188–7195 (2012) on 2006, pp. 62–65. IEEE 118. Pei, Y.-L., Wu, H., Wu, D., Zheng, F., He, J.: High thermoelectric 134. Bocher, L., et al.: CaMn1− x Nb x O3 (x≤ 0.08) perovskite-type performance realized in a BiCuSeO system by improving car- phases as promising new high-temperature n-type thermoelectric rier mobility through 3D modulation doping. J. Am. Chem. Soc. materials. Inorg. Chem. 47(18), 8077–8085 (2008) 136(39), 13902–13908 (2014) 135. Ito, M., Nagira, T., Oda, Y., Katsuyama, S., Majima, K., Nagai, 119. Liu, Y., Zhao, L.D., Zhu, Y., Liu, Y., Li, F., Yu, M., Liu, D.B., H.: Effect of partial substitution of 3d transition metals for Co on Xu, W., Lin, Y.H., Nan, C.W.: Synergistically optimizing elec- the thermoelectric properties of N a Co O . Mater. Trans. 43(3), x 2 4 trical and thermal transport properties of BiCuSeO via a dual- 601–604 (2002) doping approach. Adv. Energy Mater. 6(9), 1502423 (2016) 136. Ohtaki, M., Maeda, E.: Microstructures and thermoelectric prop- 120. Das, S., Chetty, R., Wojciechowski, K., Suwas, S., Mallik, R.C.: erties of N aCo O prepared by double-step sintering. J. Jpn. Soc. 2 4 Thermoelectric properties of Sn doped BiCuSeO. Appl. Surf. Powder Powder Metall. 47(11), 1159–1164 (2000) Sci. 418, 238–245 (2017) 137. Funahashi, R., Shikano, M.: B i Sr Co O whiskers with high 2 2 2 y 121. Feng, B., Li, G., Pan, Z., Hu, X., Liu, P., Li, Y., He, Z., Fan, thermoelectric figure of merit. Appl. Phys. Lett. 81 (8), 1459– X.A.: Enhancing thermoelectric and mechanical performances 1461 (2002) in BiCuSeO by increasing bond covalency and nanostructuring. 138. Cheng, B., Fang, H., Lan, J., Liu, Y., Lin, Y.H., Nan, C.W.: J. Solid State Chem. 265, 306–313 (2018) Thermoelectric performance of Zn and Ge Co-doped In O fine- 2 3 122. Roy, P., Bose, I., Maiti, T.: Synthesis and characterization of grained ceramics by the spark plasma sintering. J. Am. Ceram. Sr TiMO (M = Fe, Co) double perovskites for high temperature Soc. 94(8), 2279–2281 (2011) 2 6 thermoelectric applications. Integr. Ferroelectr. 174(1), 34–42 139. Wu, Z.-H., Xie, H.-Q., Zhai, Y.-B.: Enhanced thermoelectric (2016) figure of merit in nanostructured ZnO by nanojunction effect. 123. Saxena, M., Roy, P., Acharya, M., Bose, I., Tanwar, K., Maiti, Appl. Phys. Lett. 103(24), 243901 (2013) T.: Enhanced thermoelectric figure-of-merit in environmentally 140. Jood, P., Mehta, R.J., Zhang, Y., Borca-Tasciuc, T., Dou, S.X., benign Ba Sr TiCoO double perovskites. Appl. Phys. Lett. Singh, D.J., Ramanath, G.: Heavy element doping for enhanc- x 2-x 6 109(26), 263903 (2016) ing thermoelectric properties of nanostructured zinc oxide. RSC 124. Roy, P., Waghmare, V., Maiti, T.: Environmentally friendly Ba Adv. 4(13), 6363–6368 (2014) Sr TiFeO double perovskite with enhanced thermopower for 141. Ou, C., Hou, J., Wei, T.-R., Jiang, B., Jiao, S., Li, J.-F., Zhu, H.: 2–x 6 high temperature thermoelectric power generation. RSC Adv. High thermoelectric performance of all-oxide heterostructures 6(60), 54636–54643 (2016) with carrier double-barrier filtering effect. NPG Asia Mater. 7 (5), 125. Saxena, M., Maiti, T.: Evaluation of Ba doped Sr TiFe Mo e182 (2015) 2 0.5 0.5 O double perovskites for high temperature thermoelectric power 142. Ahmad, A., Umer, M., Tan, X., Liu, R., Mohmad, F., Hussain, generation. Scr. Mater. 155, 85–88 (2018) M., Ren, G.-K., Lin, Y.-H.: High-temperature electrical and 126. Chen, C., Giovannelli, F., Duclère, J.-R., Delorme, F.: Thermo- thermal transport behaviors of In O -based ceramics by Zn-Sn 2 3 electric properties of Fe (Ti Nb) O pseudobrookite ceram- co-substitution. J. Appl. Phys. 123(24), 245108 (2018) 2 1−x x 5 ics with low thermal conductivity. J. Eur. Ceram. Soc. 37(15), 143. Liu, Y., Zhao, L.-D., Liu, Y., Lan, J., Xu, W., Li, F., Zhang, B.-P., 4681–4685 (2017) Berardan, D., Dragoe, N., Lin, Y.-H.: Remarkable enhancement 127. Chen, C., Giovannelli, F., Delorme, F.: Thermoelectric properties in thermoelectric performance of BiCuSeO by Cu deficiencies. of Fe Ti O solid solutions: Influence of microcracking and J. Am. Chem. Soc. 133(50), 20112–20115 (2011) 2-x 1+x 5 Nb substitution. Ceram. Int. 44(17), 21794–21799 (2018) 144. Li, J., Sui, J., Pei, Y., Meng, X., Berardan, D., Dragoe, N., Cai, 128. Paulson, A., Sabeer, N.M., Pradyumnan, P.: Enhanced ther- W., Zhao, L.-D.: The roles of Na doping in BiCuSeO oxysele- moelectric property of oxygen deficient nickel doped SnO for nides as a thermoelectric material. J. Mater. Chem. A 2(14), high temperature application. Mater. Res. Express 5(4), 045511 4903–4906 (2014) (2018) 145. Lan, J.L., Liu, Y.C., Zhan, B., Lin, Y.H., Zhang, B., Yuan, X., 129. Ohtaki, M.: Thermoelectric properties and applicability of Zhang, W., Xu, W., Nan, C.W.: Enhanced thermoelectric proper- In_2O_3-based mixed metal oxides for thermoelectric conver- ties of Pb-doped BiCuSeO ceramics. Adv. Mater. 25(36), 5086– sion at high temperature. In: Proceedings of The 12th Interna- 5090 (2013) tional Conference on Thermoelecrics, K. Matsuura ed., Yoko- 146. Li, J., Sui, J., Pei, Y., Barreteau, C., Berardan, D., Dragoe, N., hama, XII, ICT 1993, pp. 212–217 Cai, W., He, J., Zhao, L.-D.: A high thermoelectric figure of 130. Ohtaki, M., Koga, H., Eguchi, K., Arai, H.: Thermoelectric merit ZT %3e 1 in Ba heavily doped BiCuSeO oxyselenides. properties of (Ca Bi) MnO perovskite-type oxides as a new Energy Environ. Sci. 5(9), 8543–8547 (2012) 1−x x 3 material for high temperature thermoelectric conversion. In: AIP 147. Ohtaki, M.: Oxide thermoelectric materials for heat-to-electricity Conference Proceedings 1994, vol. CONF-940830 direct energy conversion. Novel Carbon Resour. Sci. Newsl. 3, 131. Ohtaki, M., Koga, H., Tokunaga, T., Eguchi, K., Arai, H.: Elec- 1–6 (2010) trical transport properties and high-temperature thermoelectric 148. Hanbo, L., Ganhong, Z., Zhenxiang, D., Zhenheng, Y., Haiqiu, performance of (Ca M) MnO (M=Y, La, Ce, Sm, In, Sn, Sb, W., Yongqing, M.: Thermoelectric properties of Sr La TiO 0.9 0.1 3 0.9 0.1 3 Pb, Bi). J. Solid State Chem. 120(1), 105–111 (1995).and Sr La Ti O with 15% Ag addition. J. Rare Earths 32(4), 2.7 0.3 2 7 132. Hayashi, R., Ohtaki, M.: Selective phonon scattering and ther- 314–319 (2014) moelectric properties of ZnO-based oxide with nanovoid struc- ture. In: Preprints of Annual Meeting of The Ceramic Society Publisher’s Note Springer Nature remains neutral with regard to of Japan Preprints of Fall Meeting of The Ceramic Society of jurisdictional claims in published maps and institutional affiliations. Japan Annual Meeting of The Ceramic Society of Japan, 2005, pp. 256–256. The Ceramic Society of Japan (2005) 133. Nong, N., Ohtaki, M.: Thermoelectric properties and local electronic structure of rare earth-doped Ca Co O . In: 3 2 6 1 3

Journal

Materials for Renewable and Sustainable Energy – Springer Journals

Published: Jan 9, 2020

Get 20M+ Full-Text Papers For Less Than $1.50/day. Start a 14-Day Trial for You or Your Team.

Learn More →

Review on texturization effects in thermoelectric oxides

Review on texturization effects in thermoelectric oxides

Get 20M+ Full-Text Papers For Less Than $1.50/day. Start a 14-Day Trial for You or Your Team.

Learn More →

Review on texturization effects in thermoelectric oxides

Review on texturization effects in thermoelectric oxides

References (149)

Abstract

Journal

Recommended Articles

There are no references for this article.

Our policy towards the use of cookies