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Study on the High Temperature Interfacial Stability of Ti/Mo/Yb0.3Co4Sb12 Thermoelectric Joints

Study on the High Temperature Interfacial Stability of Ti/Mo/Yb0.3Co4Sb12 Thermoelectric Joints applied sciences Article Study on the High Temperature Interfacial Stability of Ti/Mo/Yb Co Sb Thermoelectric Joints 0.3 4 12 Ming Gu, Shengqiang Bai *, Xugui Xia, Xiangyang Huang, Xiaoya Li, Xun Shi and Lidong Chen State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China; ftgm@mail.sic.ac.cn (M.G.); xgxia@mail.sic.ac.cn (X.X.); xyhuang@hotmail.com (X.H.); xyli@mail.sic.ac.cn (X.L.); xshi@mail.sic.ac.cn (X.S.); cld@mail.sic.ac.cn (L.C.) * Correspondence: bsq@mail.sic.ac.cn; Tel.: +86-021-6916-3686 Received: 31 August 2017; Accepted: 13 September 2017; Published: 15 September 2017 Abstract: To improve the interfacial stability at high temperatures, n-type skutterudite (SKD) thermoelectric joints with sandwich structures of Ti/Mo/Yb Co Sb were successfully designed 0.3 4 12 and fabricated. In this structure, Mo and Ti were introduced as the barrier layer with the goal of suppressing the interfacial diffusion and the buffer layer with the goal of enhancing the bonding strength, respectively. To evaluate the high temperature interfacial behavior of the Ti/Mo/Yb Co Sb joints, thermal shocking between 0 C and 600 C and isothermal aging 0.3 4 12 at a temperature range of 550 C to 650 C were carried out in vacuum. During the isothermal aging process, Ti penetrates across the Mo layer, and finally diffuses into the Yb Co Sb matrix. 0.3 4 12 By increasing the isothermal aging time, Ti continuously diffuses and reacts with the elements of Sb and Co in the matrix, consequently forming the multilayer-structured intermetallic compounds of Ti Sb/Ti Sb/TiCoSb. Diffusion kinetics was investigated and it was found that the interfacial 3 2 evolution of the Ti/Mo/Yb Co Sb joints was a diffusion-controlling process. During the diffusion 0.3 4 12 process, the formed Mo-Ti buffer layer acts as a damper, which greatly decelerates the diffusion of Ti towards the Yb Co Sb matrix at high temperatures. Meanwhile, it was found that the 0.3 4 12 increase in the contact resistivity of the joints mainly derives from the inter-diffusion between Ti and Yb Co4Sb . As a result, the Ti/Mo/Yb Co Sb joint demonstrates the excellent stability of the 0.3 12 0.3 4 12 interfacial contact resistivity. Service life prediction was made based on the stability of the contact resistivity, and it was found that the Ti/Mo/Yb Co Sb joint is qualified for practical applications 0.3 4 12 at 550 C. Keywords: thermoelectric joint; skutterudite; diffusion kinetics; contact resistivity; service life prediction 1. Introduction Unlike traditional power generation systems, a thermoelectric power generator (TEG) produces electrical power based on the Seebeck effect, via the movement of charge carriers within thermoelectric materials under a temperature difference. Using the heat from radioisotopes, TEGs can supply electrical power for tens of years, and are now the only choice of power source for deep space explorers which fly far from the sun and therefore can no longer obtain sufficient sunlight energy [1,2]. In the past decade, TEGs have been regarded as one of the most promising candidates to recover waste heat from industries or automobiles and turn them into electricity [3,4], which can benefit from the remission of the fossil energy shortage and global warming. Recently, TEGs have become one of the research focuses, and are expected to act as power suppliers of wearable electronic equipment [5]. Intrinsically, the performance of a TEG depends on the property of the thermoelectric (TE) material, which is referred to as the dimensionless figure of merit of ZT. Among the investigated Appl. Sci. 2017, 7, 952; doi:10.3390/app7090952 www.mdpi.com/journal/applsci Appl. Sci. 2017, 7, 952 2 of 10 various systems of TE materials, silicides [6–9], half-heuslers [10,11], and tellurides (e.g., of Bi [12–14], Pb [15–18] and Ge [19,20]) have been widely studied and some have been practically applied in TEGs. Besides, a number of materials featuring excellent TE properties (high ZT value) and a relatively lower cost, low toxicity, and good mechanical properties, such as filled skutterudite (SKD), also attract extensive attention and have been actively pursued for being converted into TEGs [21–26]. Thermoelectric joints, consisting of TE materials and electrodes, are the fundamental parts and also the key components of a TEG. In practical applications, the performance of a TEG not only depends on the property of TE materials, but also depends on the temperature difference between the hot- and cold-sides of the TE material bulks. Higher hot-side temperatures usually lead to a larger temperature difference, further enhancing the performance of the TEG. Meanwhile, a higher hot-side temperature also encourages interfacial diffusion and accelerates the degradation of the contact property, especially the contact (electrical) resistivity, which definitely deteriorates the performance and stability of the TEGs. Practically, the high temperature TEGs are expected to perform stably for a couple of years or even longer. For example, to make full use of the TE property, the hot side temperature of the CoSb -based SKD joints should be 500 C or higher. At such a high temperature, the major component in the SKD material, antimony, will react fiercely with most electrode materials (Cu, Ni, and Al, etc.) and then form single- or multi-layer interfacial intermetallics (IMC). The performance and stability of the TE joints are sensitively influenced by the microstructure and properties of the interface between the TE materials and electrode. To improve the interfacial stability of the SKD joints, a diffusion barrier layer is usually inserted between the electrode and the SKD material. Fan et al. firstly reported using Ti as the barrier layer in the CoSb [27]. Zhao et al. systematically studied the diffusion kinetics of the Ti/CoSb interface at high temperatures and predicted a service life of over 20 years for Ti/CoSb joints 3 3 at 500 C, using interfacial shearing strength as the criteria [28,29]. In Zhao’s work, when the working temperature increased to 550 C, the diffusion speed at the Ti/CoSb interface increased by one order and the predicted service life dropped to within one year [28]. In 2014, similar interfacial evolution in Ti/Yb Co Sb joints was found and the method was developed to improve the interfacial stability 0.6 4 12 of CoSb -based n-type SKD joints by introducing a Ti-Al barrier layer [30]. However, the uneven distributions of Ti and Al at the interfaces leave the accurate measurements of the thickness of the interfacial diffusion layer incomplete. Therefore, the diffusion kinetics and life prediction were also not studied. Afterwards, Fan et al. designed a Ti-Mo mixture as the barrier layer for the n-type Yb Co Sb matrix and found that the dispersion of the chemically stable Mo particles (15 at %) 0.3 4 12 disturbs the inter-diffusion between the Ti and Yb Co Sb matrix, which results in an improved 0.3 4 12 interfacial stability at 550 C. Still, the diffusion kinetics and life prediction were not studied [31]. Besides, Bae et al. studied the stability of the power generation characteristics of the Mo-Cu/Ti/SKD joints in terms of vibration and thermal cycling, and found that Ti helps to achieve good interfacial bonding [32]. In this paper, to develop an n-type SKD TE joint that is qualified for practical applications, we designed and fabricated n-type SKD joints with the structure of Ti/Mo/Yb Co Sb . In this 0.3 4 12 structure, Ti is reserved as the bonding layer for its active bonding behavior with common electrode materials including Cu and Ni. Furthermore, Ti is also expected to buffer the interfacial thermal stress. To suppress the activity of Ti at a high temperature and also barrier the inter-diffusion between TE materials and the electrode, Mo is chosen as the barrier layer. Both Mo and Ti were deposited onto the Yb Co Sb surface by magnetron sputtering to achieve flat interfaces. Because of the evident 0.3 4 12 difference between the CTEs of Mo and Yb Co Sb , the thickness of the Mo layer was controlled to 0.3 4 12 no more than 4.5 m to decrease the thermal stress at high temperatures. To reveal the interfacial diffusion kinetics of the Ti/Mo/Yb Co Sb joints and predict their 0.3 4 12 service life, the above n-type SKD TE joints were thermal shocked between 0 C and 600 C and isothermally aged between 550 C and 650 C. After each thermal shocking or isothermal aging step, the interfacial microstructures and the interfacial contact resistivity were both investigated. Based on Appl. Sci. 2017, 7, 952 3 of 10 the measurement results, the diffusion kinetics were investigated and the service life was predicted at different temperatures. 2. Materials and Methods The fabrication of the n-type Yb Co Sb powder followed the same process described in 0.3 4 12 reference [33]. To fabricate samples for the measurement of the coefficient of thermal expansion (CTE), Yb Co Sb powder was loaded into a graphite die (F30 mm) and consolidated in a spark 0.3 4 12 plasma sintering (SPS) system at 620 C under a pressure of 60 MPa and held for 10 min. Ti powder (4 N, 45 m) was loaded into a graphite die (F30 mm) and consolidated at 1100 C under a pressure of 60 MPa and held for 20 min in the same SPS system. The final dimension of the samples for CTE measurement was 4  4  20 mm . To fabricate the Ti/Mo/Yb Co Sb joints, Yb Co Sb 0.3 4 12 0.3 4 12 powder was consolidated following the same sintering process as mentioned above. The surfaces of the resulting Yb Co Sb bulks were polished and cleaned ultrasonically before they were dried 0.3 4 12 and loaded into the chamber of a magnetron sputtering system. The chamber was evacuated to 3  10 Pa and heated to 150 C before Ar was introduced. Mo was sputtered at 1.5 Pa under 1.5 kW for 50 min. Then, Ti was sputtered at 1.5 Pa under 3 kW for 120 min. The resulting samples were cut into joints with a final dimension of about 3  3  6 mm . The joints were then sealed in quartz ampoules under vacuum. In the thermal shocking test, the ampoules were moved from room temperature into a furnace at 600 C and kept inside for 5 min. Then, they were taken out into a water tank at 0 C for another 5 min. In the isothermal aging test, the ampoules were kept at 550 C, 600 C, and 650 C for different time periods of time. The CTE were measured with a Linseis-L75VS dilatometer (Princeton Junction, Princeton, NJ, USA). The interfacial microstructure of the joints was investigated by a scanning electronic microscope (ZEISS, SUPRA-55-SAPPHIRE, Oberkochen, Germany), and the chemical composition of the interfacial diffusion layers was analyzed using an Energy Dispersive Spectrometer (OXFORD, X-MaxN, Oxfordshire, UK). The contact resistivity of the joints was measured on the homemade four-probe platform (Haida Tech, Beijing, China) mentioned in our previous paper [34]. All the data for the diffusion layer thickness and the contact resistivity presented in this paper are the root mean square of three parallel samples, and the error bars are the root mean square errors. 3. Results and Discussion 3.1. Thermal Shocking Figure 1 shows the variation of the CTE with temperature for Yb Co Sb , Ti, Mo, and Cu & Ni, 0.3 4 12 which are common electrode materials. Clearly the CTE mismatch between Ti and Yb Co Sb (or Cu 0.3 4 12 & Ni) is much less than that between Mo and Yb Co Sb (or Cu & Ni). Based on Figure 2a,b, in the 0.3 4 12 as prepared Ti/Mo/Yb Co Sb joints, the bonding between Ti, Mo and Yb Co Sb is good with 0.3 4 12 0.3 4 12 the thickness of the Mo layer being about 4.5 m and the interfacial diffusion being neglectable at both the Ti/Mo and the Mo/Yb Co Sb interfaces. The Ti/Mo/Yb Co Sb joints then experienced 0.3 4 12 0.3 4 12 thermal shocking for 10 cycles between 0 C and 600 C in vacuum. Figure 2c demonstrates that the bonding between Ti, Mo, and Yb Co Sb remains good after 10 cycles of thermal shocking. 0.3 4 12 Besides, the contact resistivity of the joints before and after thermal shocking was measured to be 2 2 around 3.9 Wcm and 3.7 Wcm , respectively. Based on the above results, it can be concluded that in spite of evident CTE mismatch between Mo and Yb Co Sb , the interfacial bonding of 0.3 4 12 the Ti/Mo/Yb Co Sb joints is quite strong and can survive severe thermal shocking. For these 0.3 4 12 Ti/Mo/Yb Co Sb joints, because the Mo layer thickness is very small, the interfacial thermal stress 0.3 4 12 from CTE mismatch is controlled at a relatively low level. Moreover, the Ti layer also helps to balance the CTE mismatch between Mo and Yb Co Sb and release the interfacial thermal stress. 0.3 4 12 Appl. Sci. 2017, 7, 952 4 of 11 Appl. Appl. Sci. Sci. 2017 2017 , 7, , 7 952 , 952 4 of 11 4 of 10 Figure 1. Variation of the CTE with temperature for Yb0.3Co4Sb12, Ti, Mo, Cu, and Ni, Data for Mo, Figure 1. Variation of the CTE with temperature for Yb Co Sb , Ti, Mo, Cu, and Ni, Data for Mo, Cu, 0.3 4 12 Cu, and Ni are from references [35–37] respectively. Figure 1. Variation of the CTE with temperature for Yb0.3Co4Sb12, Ti, Mo, Cu, and Ni, Data for Mo, and Ni are from references [35–37] respectively. Cu, and Ni are from references [35–37] respectively. Figure Figure 2. 2. The interfacial The interfacial micr microstructure ostructure of of the the TTi/Mo/Yb i/Mo/Yb 0.3Co Co 4SbSb 12 joints ( joints a), as prepar (a), as prepar ed, a ed, Ba a Back ck 0.3 4 12 Figure 2. The interfacial microstructure of the Ti/Mo/Yb0.3Co4Sb12 joints (a), as prepared, a Back Scattering Scattering Electr Electron (BSE) gra on (BSE) graph, ph, ( (b),bas ), as prepared, a prepared, a line line scan,scan, and ( and (c), after c), a 10fter 10 cycles cy of thermal cles of therm shocking al Scattering Electron (BSE) graph, (b), as prepared, a line scan, and (c), after 10 cycles of thermal between shocking betw 0 C and een 0 °C and 6 600 C, a BSE 00 °C, a graph. BSE graph. shocking between 0 °C and 600 °C, a BSE graph. 3.2. Isothermal Aging 3.2. Isothermal Aging 3.2. Isothermal Aging 3.2.1. Pretreatment 3.2.1. 3.2.1. Pret Pretr re eatment atment The Ti/Mo/Yb0.3Co4Sb12 joints were then pretreated by isothermal aging at 550 °C. Figure 3a,b The The Ti/Mo/Yb Ti/Mo/Yb 0.3Co Co 4SbSb 12 joints were then pretrea joints were then pretreated ted by i bysotherm isothermal al agi aging ng at at 55550 0 °C. Fi C. Figur gure e 3a 3,b a,b 0.3 4 12 show the BSE graph with spot testing results and line scan results across the interfaces of the joints show show t the he BSE BSE gr graph aph w with ith s spot pot ttesting esting rre esults sults and andlin line e sc scan an rr eesults sults acros across s the the int interfaces erfaces of the of the joints joints after 16 days aging, respectively. Figure 3c shows the distribution of the Ti content in the joint and it after 16 days aging, respectively. Figure 3c shows the distribution of the Ti content in the joint and it after 16 days aging, respectively. Figure 3c shows the distribution of the Ti content in the joint and it clearly reveals that some Ti atoms have penetrated across the Mo layer and reached the Yb0.3Co4Sb12 clearly reveals that some Ti atoms have penetrated across the Mo layer and reached the Yb0.3Co4Sb12 clearly reveals that some Ti atoms have penetrated across the Mo layer and reached the Yb Co Sb 0.3 4 12 matrix. All the measurement results confirm that the inter-diffusion at the Mo/Yb0.3Co4Sb12 interface matrix. All the measurement results confirm that the inter-diffusion at the Mo/Yb0.3Co4Sb12 interface matrix. All the measurement results confirm that the inter-diffusion at the Mo/Yb Co Sb interface is is neglectable. However, Ti diffuses across the Ti/Mo interface and moves into the Mo layer, 0.3 4 12 is neglectable. However, Ti diffuses across the Ti/Mo interface and moves into the Mo layer, neglectable. consequently form However ing a new Mo-T , Ti diffuses acr i layer with oss the Ti/Mo a cont interface ent gradient. and moves In the Mo into -Ti the layMo er, the con layer, consequently tent of consequently forming a new Mo-Ti layer with a content gradient. In the Mo-Ti layer, the content of Ti decreases from about 10 at % Ti at the Ti/Mo-Ti interface to about 5 at % Ti at the forming a new Mo-Ti layer with a content gradient. In the Mo-Ti layer, the content of Ti decreases Ti decre Mo-Ti/Yb ase 0.3s from Co4Sb12 int abeout rfa 1 ce. The cont 0 at % aT cti resist at th ive Ti/ ity of about Mo-Ti int 3.8 μΩ erface t ·cm for t o ab he pretreated out 5 at joints is % Ti at the from about 10 at % Ti at the Ti/Mo-Ti interface to about 5 at % Ti at the Mo-Ti/Yb Co Sb interface. 0.3 4 12 almost the same value as that of the as prepa 2 red ones, which indicates that the penetration of the Ti Mo-Ti/Yb0.3Co4Sb12 interface. The contact resistivity of about 3.8 μΩ·cm for the pretreated joints is The contact resistivity of about 3.8 Wcm for the pretreated joints is almost the same value as that of atoms into the Mo layer almost does not affect the apparent contact resistivity of the joints. almost the same value as that of the as prepared ones, which indicates that the penetration of the Ti the as prepared ones, which indicates that the penetration of the Ti atoms into the Mo layer almost does Furthermore, if one prolongs the pretreatment time, the inter-diffusion between the Ti and atoms into the Mo layer almost does not affect the apparent contact resistivity of the joints. not affect the apparent contact resistivity of the joints. Furthermore, if one prolongs the pretreatment Yb0.3Co4Sb12 matrix will surely begin. Furthermore, if one prolongs the pretreatment time, the inter-diffusion between the Ti and time, the inter-diffusion between the Ti and Yb Co Sb matrix will surely begin. 0.3 4 12 Yb0.3Co4Sb12 matrix will surely begin. Based on the above results, to simplify the study on the interfacial stability, all the following Ti/Mo/Yb Co Sb joints were pretreated at 550 C for 16 days before they were further 0.3 4 12 isothermally aged. Correspondingly, the structure of the joints after the pretreatments is referred to as Ti/Mo-Ti/Yb Co Sb and the end of the pretreatment is set to be the beginning of the 0.3 4 12 isothermal aging. Appl. Sci. 2017, 7, 952 5 of 11 Appl. Sci. 2017, 7, 952 5 of 10 Appl. Sci. 2017, 7, 952 5 of 11 Figure 3. The interfacial microstructure of the Ti/Mo/Yb0.3Co4Sb12 joints pretreated at 550 °C for 16 days. (a) the BSE graph and spot testing, (b) line scan results, (c) the distribution of Ti. Based on the above results, to simplify the study on the interfacial stability, all the following Ti/Mo/Yb0.3Co4Sb12 joints were pretreated at 550 °C for 16 days before they were further isothermally aged. Correspondingly, the structure of the joints after the pretreatments is referred to as Figure 3. The interfacial microstructure of the Ti/Mo/Yb0.3Co4Sb12 joints pretreated at 550 °C for Figure 3. The interfacial microstructure of the Ti/Mo/Yb Co Sb joints pretreated at 550 C for 0.3 4 12 Ti/Mo-Ti/Yb0.3Co4Sb12 and the end of the pretreatment is set to be the beginning of the isothermal 16 16 days. ( days. (a)athe ) the BS BSEE g graph raph and spot and spot testing, testing(,b () bline ) line s scan can re results, sults, (c ( )cthe ) the distribution distribution of of T Ti. i. aging. Based on the above results, to simplify the study on the interfacial stability, all the following 3.2.2. Diffusion Kinetics of the Interface 3.2.2. Diffusion Kinetics of the Interface Ti/Mo/Yb0.3Co4Sb12 joints were pretreated at 550 °C for 16 days before they were further isothermally The pretreated joints were then isothermally aged at 550 C and the inter-diffusion between Ti The pretreated joints were then isothermally aged at 550 °C and the inter-diffusion between Ti aged. Correspondingly, the structure of the joints after the pretreatments is referred to as and Yb Co Sb occured at the Mo-Ti/Yb Co Sb interface. The BSE graphs of the interfacial cross and Yb0.3 0.3Co4 4Sb12 12 occured at the Mo-Ti/Yb0.3 0.3 Co4Sb 4 1212 interface. The BSE graphs of the interfacial cross Ti/Mo-Ti/Yb0.3Co4Sb12 and the end of the pretreatment is set to be the beginning of the isothermal section for the joints after 20-day and 50-day aging are shown in Figure 4a,b. After being aged for section for the joints after 20-day and 50-day aging are shown in Figure 4a,b. After being aged for aging. 50 days, the diffusion layer is about 0.3 m thick and consists of about 90 at % Ti and 10 at % Sb 50 days, the diffusion layer is about 0.3 μm thick and consists of about 90 at % Ti and 10 at % Sb without any sign of Mo. The extremely slow growth-speed of the diffusion layer implies that the new without any sign of Mo. The extremely slow growth-speed of the diffusion layer implies that the 3.2.2. Diffusion Kinetics of the Interface Mo-Ti layer formed in the pretreatment effectively dampens the diffusion of Ti atoms before they reach new Mo-Ti layer formed in the pretreatment effectively dampens the diffusion of Ti atoms before The pretreated joints were then isothermally aged at 550 °C and the inter-diffusion between Ti the Yb Co Sb matrix. Besides, Figure 4c,d show that there is no evident change in the distribution they reach the Yb0.3Co4Sb12 matrix. Besides, Figure 4c,d show that there is no evident change in the 0.3 4 12 and Yb0.3Co4Sb12 occured at the Mo-Ti/Yb0.3Co4Sb12 interface. The BSE graphs of the interfacial cross of Ti and Mo in the formed Mo-Ti layer, compared to the pretreated joints. distribution of Ti and Mo in the formed Mo-Ti layer, compared to the pretreated joints. section for the joints after 20-day and 50-day aging are shown in Figure 4a,b. After being aged for 50 days, the diffusion layer is about 0.3 μm thick and consists of about 90 at % Ti and 10 at % Sb without any sign of Mo. The extremely slow growth-speed of the diffusion layer implies that the new Mo-Ti layer formed in the pretreatment effectively dampens the diffusion of Ti atoms before they reach the Yb0.3Co4Sb12 matrix. Besides, Figure 4c,d show that there is no evident change in the distribution of Ti and Mo in the formed Mo-Ti layer, compared to the pretreated joints. Figure 4. The interfacial evolution of the pretreated joints isothermally aged at 550 °C. (a) BSE graph, Figure 4. The interfacial evolution of the pretreated joints isothermally aged at 550 C. (a) BSE graph, 20 days; (b) BSE graph, 50 days; (c) distribution of Mo, 50 days; (d) distribution of Ti, 50 days. 20 days; (b) BSE graph, 50 days; (c) distribution of Mo, 50 days; (d) distribution of Ti, 50 days. The pretreated joints were also isothermally aged at 600 C and 650 C. It was found that the growth rate of the diffusion layer between Ti and Yb Co Sb increases with an increasing aging 0.3 4 12 temperature. According to Figure 5, after being aged at 600 C for 41 days, the diffusion layer at the Figure 4. The interfacial evolution of the pretreated joints isothermally aged at 550 °C. (a) BSE graph, 20 days; (b) BSE graph, 50 days; (c) distribution of Mo, 50 days; (d) distribution of Ti, 50 days. Appl. Sci. 2017, 7, 952 6 of 11 Appl. Sci. 2017, 7, 952 6 of 11 The pretreated joints were also isothermally aged at 600 °C and 650 °C. It was found that the The pretreated joints were also isothermally aged at 600 °C and 650 °C. It was found that the Appl. Sci. 2017, 7, 952 6 of 10 growth rate of the diffusion layer between Ti and Yb0.3Co4Sb12 increases with an increasing aging growth rate of the diffusion layer between Ti and Yb0.3Co4Sb12 increases with an increasing aging temperature. According to Figure 5, after being aged at 600 °C for 41 days, the diffusion layer at the temperature. According to Figure 5, after being aged at 600 °C for 41 days, the diffusion layer at the Mo-Ti/ Mo-Ti/Yb Yb0.3Co Co 4SbSb 12 interf interface ace reaches reaches ababout out 2 μ 2m a mn and d consists of consists of two sub-l two sub-layers ayers of Ti of T 3iSb an Sb and d Ti T2iSb Sb. . 0.3 4 12 3 2 Mo-Ti/Yb0.3Co4Sb12 interface reaches about 2 μm and consists of two sub-layers of Ti3Sb and Ti2Sb. Figure 6 shows that after being aged for nine days at 650 °C, the total thickness of the diffusion layer Figure 6 shows that after being aged for nine days at 650 C, the total thickness of the diffusion layer Figure 6 shows that after being aged for nine days at 650 °C, the total thickness of the diffusion layer at at tthe he Mo-Ti/ Mo-Ti/Yb Yb0.3Co Co 4Sb Sb 12 interf interface ace grows to a grows tob about out 4.5 4.5 μm. Be m. Beside side the sub-layer the sub-layers s of Ti of T3iSb an Sb and d Ti T2 iSb, Sb, 0.3 4 12 3 2 at the Mo-Ti/Yb0.3Co4Sb12 interface grows to about 4.5 μm. Beside the sub-layers of Ti3Sb and Ti2Sb, there also appears a sign of TiCoSb in the front of the diffusion layer. there also appears a sign of TiCoSb in the front of the diffusion layer. there also appears a sign of TiCoSb in the front of the diffusion layer. Figure 5. The interfacial microstructure of the pretreated joints isothermally aged at 60  0 °C for 41 Figure Figure 5. 5. The The interfacial interfacial microstructure of t microstructure of theh pr e pretreat etreated ed j joints oints isothermally aged at isothermally aged at 600 60 C 0 °C for 41 for 41 days. days. (a) BSE graph and spot testing, (b) distribution of Ti, (c) distribution of Mo. ( day a) BSE s. (agraph ) BSE graph and and spot testing, spot te( sb ti)ndistribution g, (b) distribution of T of Ti, (c) distribution i, (c) distribution o of Mo. f Mo. Figure 6. The interfacial microstructure of the pretreated joints isothermally aged at 650 °C for nine Figure 6. The interfacial microstructure of the pretreated joints isothermally aged at 650 °C for nine Figure 6. The interfacial microstructure of the pretreated joints isothermally aged at 650 C for nine days. (a) BSE graph and spot testing, (b) distribution of Ti, (c) distribution of Mo. days. (a) BSE graph and spot testing, (b) distribution of Ti, (c) distribution of Mo. days. (a) BSE graph and spot testing, (b) distribution of Ti, (c) distribution of Mo. To reveal the diffusion kinetics, we analyzed the relationship between the total diffusion layer To reveal the diffusion kinetics, we analyzed the relationship between the total diffusion layer To reveal the diffusion kinetics, we analyzed the relationship between the total diffusion layer thickness, aging time, and aging temperature. Figure 7a shows that at all the three aging thickness, aging time, and aging temperature. Figure 7a shows that at all the three aging tempera thickness, tures, the tota aging time, and l thic aging kness of the temperatur diff e. usi Figur on la e yer is 7a shows linea that r wi atth the squa all the three re root of aging temperatur the agines, g temperatures, the total thickness of the diffusion layer is linear with the square root of the aging the total thickness of the diffusion layer is linear with the square root of the aging time, which indicates time, which indicates that the evolution of the Mo-Ti/Yb0.3Co4Sb12 interface is a diffusion-controlling time, which indicates that the evolution of the Mo-Ti/Yb0.3Co4Sb12 interface is a diffusion-controlling process that thebetw evolution een 550 °C an of the Mo-T d 650 °C. i/YbThe Co refore Sb , the interface diffusion is alayer diffusion-contr thickness colling an be expressed process between by the 0.3 4 12 process between 550 °C and 650 °C. Therefore, the diffusion layer thickness can be expressed by the 550 C and 650 C. Therefore, the diffusion layer thickness can be expressed by the following parabolic following parabolic equation [38,39]: following parabolic equation [38,39]: equation [38,39]: 0.5 0.5 YY-( = Dt) (1) 0.5 0 (1) YY-( = Dt) Y Y0 = (Dt) (1) where t is the aging time, and Y0 and Y are the total diffusion layer thickness at the origin of aging where t is the aging time, and Y0 and Y are the total diffusion layer thickness at the origin of aging where t is the aging time, and Y and Y are the total diffusion layer thickness at the origin of aging and and after being aged for a time period of t, respectively. D is the growth rate of the diffusion layer, and after being aged for a time period of t, respectively. D is the growth rate of the diffusion layer, after being aged for a time period of t, respectively. D is the growth rate of the diffusion layer, which is which is a function of the aging temperature. By fitting the data of the diffusion layer thickness as a which is a function of the aging temperature. By fitting the data of the diffusion layer thickness as a a function of the aging temperature. By fitting the data of the diffusion layer thickness as a function of 0.5 0.5 function of t , D , which is the slope of the fitting result, can be obtained. 0.5 0.5 0.5 0.5 function of t , D , which is the slope of the fitting result, can be obtained. t , D , which is the slope of the fitting result, can be obtained. To determine the activation energy for the interfacial diffusion, the Arrhenius relationship was used, D = D exp( ) (2) RT where D is the growth constant, Q is the apparent activation energy for the growth of the total diffusion layer, T is the temperature in Kelvin, and R is the gas constant, 8.314 J/(molK). Equation (2) can be transformed into: Q 1000 ln D =  + D (3) 1000R T By plotting and fitting lnD against 1000/T, Q can be obtained. Table 1 shows detailed D values and relations between the diffusion layer thickness and aging time. As is shown in Figure 7b, based on Table 1 and Equation (3), the activation energy Q for the growth of the diffusion layer at the Appl. Sci. 2017, 7, 952 7 of 10 Mo-Ti/Yb Co Sb interface is calculated to be about 447.5 kJ/mol at the temperature range of 0.3 4 12 550 C to 650 C. Combined with the above results, the variation of the diffusion layer thickness at the Appl. Sci. 2017, 7, 952 7 of 11 Mo-Ti/Yb Co Sb interface can be predicted over the above temperature range. 0.3 4 12 Figure 7. (a) Variation of the diffusion layer thickness of the pretreated joints with the square root of Figure 7. (a) Variation of the diffusion layer thickness of the pretreated joints with the square root of aging time between 550 °C and 650 °C. The inset is the enlargement of the result at 550 °C. aging time between 550 C and 650 C. The inset is the enlargement of the result at 550 C. (b) Variation (b) Variation of lnD with 1000/T between 550 °C and 650 °C. of lnD with 1000/T between 550 C and 650 C. To determine the activation energy for the interfacial diffusion, the Arrhenius relationship Table 1. Fitting results of the growth rate and the relationship between diffusion layer thickness and was used, aging time (s). 19 2 DD=− exp( ) (2) Temperature/ C D/10 m /s Y /m RT 4 0.5 550 0.21 1.45  10 t 4 0.5 where D0 is the growth constant, Q is the apparent activation energy for the growth of the total 600 9.53 9.76  10 t 4 0.5 650 251 50.1  10 t diffusion layer, T is the temperature in Kelvin, and R is the gas constant, 8.314 J/(mol·K). Equation (2) can be transformed into: 3.2.3. Stability of the Contact Resistivity and Service Life Prediction Q 1000 ln D =− ⋅ + D (3) To study the high temperature stability of1000 theRT electrical properties of these joints, the contact resistivity of the joints was measured after each different thermal aging time. Figure 8 exhibits the By plotting and fitting lnD against 1000/T, Q can be obtained. Table 1 shows detailed D values variation of the interfacial contact resistivity of the joints with the square root of the aging time between and relations between the diffusion layer thickness and aging time. As is shown in Figure 7b, based 550 C and 650 C. At the aging temperature of 550 C, the contact resistivity of the joint increases very on Table 1 and Equation (3), the activation energy Q for the growth of the diffusion layer at the slowly and reaches about 4.5 Wcm after being aged for 50 days. Increasing the aging temperature to Mo-Ti/Yb0.3Co4Sb12 interface is calculated to be about 447.5 kJ/mol at the temperature range of 550 °C 2 2 600 C and 650 C, the contact resistivity reaches 6.8 Wcm and 10 Wcm after 41 days and nine to 650 °C. Combined with the above results, the variation of the diffusion layer thickness at the days, respectively. In contrast to the previous reports [26,29–31], the stability of the contact resistivity Mo-Ti/Yb0.3Co4Sb12 interface can be predicted over the above temperature range. of the Ti/Mo/Yb Co Sb joints is significantly improved. 0.3 4 12 The variation of the contact resistivity was then fitted with the square root of the aging time and a Table 1. Fitting results of the growth rate and the relationship between diffusion layer thickness and linear relationship between them was found (See Table 2). Based on Tables 1 and 2, it is clear that the aging time (s). increase in the contact resistivity of the Ti/Mo-Ti/Yb Co Sb joints corresponds well to the increase 0.3 4 12 −19 2 Temperature/°C D/10 m /s Y/μm in the thickness of the interfacial diffusion layer at the Mo-Ti/Yb Co Sb interface. In another word, 0.3 4 12 −4 0.5 550 0.21 1.45 × 10 ·t it is the growth of the interfacial diffusion layer at the Mo-Ti/Yb Co Sb interface that dominates 0.3 4 12 −4 0.5 600 9.53 9.76 × 10 ·t the stability of the interfacial contact resistivity of the Ti/Mo-Ti/Yb Co Sb joints. 0.3 4 12 −4 0.5 650 251 50.1 × 10 ·t The performance of the TE joints, as well as that of the TEGs, is sensitive to the contact resistivity, and therefore, the contact resistivity is a valuable criteria for the service life prediction of the TE 3.2.3. Stability of the Contact Resistivity and Service Life Prediction joints. We assume that a Ti/Mo/Yb Co Sb joint fails when the interfacial contact resistivity reaches 0.3 4 12 2 2 10 Wcm or 20 Wcm , which are low values and satisfy most practical applications, and the service To study the high temperature stability of the electrical properties of these joints, the contact life of the joint is then predicted in Table 2. At the service temperature of 550 C, the pretreated resistivity of the joints was measured after each different thermal aging time. Figure 8 exhibits the Ti/Mo/Yb Co Sb joints can endure about 21 years before the contact resistivity reaches 10 Wcm . variation of 0.3 th 4e i12 nterfacial contact resistivity of the joints with the square root of the aging time Even when increasing the service temperature to 600 C, the joints can survive for 3.9 years before the between 550 °C and 650 °C. At the aging temperature of 550 °C, the contact resistivity of the joint contact resistivity reaches 20 Wcm . 2 increases very slowly and reaches about 4.5 μΩ·cm after being aged for 50 days. Increasing the 2 2 aging temperature to 600 °C and 650 °C, the contact resistivity reaches 6.8 μΩ·cm and 10 μΩ·cm after 41 days and nine days, respectively. In contrast to the previous reports [26,29–31], the stability of the contact resistivity of the Ti/Mo/Yb0.3Co4Sb12 joints is significantly improved. Appl. Sci. 2017, 7, 952 8 of 11 Appl. Sci. 2017, 7, 952 8 of 10 Figure 8. Variation of the interfacial contact resistivity of the joints with the square root of the aging Figure 8. Variation of the interfacial contact resistivity of the joints with the square root of the aging time between 550 C and 650 C. time between 550 °C and 650 °C. Table 2. Fitting results of the relationship between the contact resistivity and the aging time, and The variation of the contact resistivity was then fitted with the square root of the aging time and predicted service life (The prediction is based on the assumption that a joint fails when the contact a linear relationship between them was found (See Table 2). Based on Tables 1 and 2, it is clear that 2 2 resistivity reaches 10 Wcm (a) and 20 Wcm (b)). the increase in the contact resistivity of the Ti/Mo-Ti/Yb0.3Co4Sb12 joints corresponds well to the increase in the thickness of the interfacial diffusion layer at the Mo-Ti/Yb0.3Co4Sb12 interface. In (a) (b) Temperature/ C Contact Resistivity/Wcm Predicted Service Life /day Predicted Service Life /day another word, it is the growth of the interfacial diffusion layer at the Mo-Ti/Yb0.3Co4Sb12 interface 4 0.5 550 2.37  10 t + 3.91 7642 53,346 that dominates the stability of the interfacial contact resistivity of the Ti/Mo-Ti/Yb0.3Co4Sb12 joints. 4 0.5 600 14.1  10 t + 3.99 201 1431 4 0.5 650 11 75 63.5  10 t + 3.76 Table 2. Fitting results of the relationship between the contact resistivity and the aging time, and predicted service life (The prediction is based on the assumption that a joint fails when the contact 4. Conclusions 2 2 resistivity reaches 10 μΩ·cm (a) and 20 μΩ·cm (b)). In this paper, we successfully designed and fabricated n-type SKD thermoelectric joints with 2 (a) (b) Temperature/°C Contact Resistivity/μΩ∙cm Predicted Service life /day Predicted Service life /day the sandwich structures of Ti/Mo/Yb Co Sb . With a controlled Mo layer thickness and the −4 0.5 0.3 4 12 550 2.37 × 10 ·t + 3.91 7642 53,346 buffering of the Ti layer, the interfaces −4 0.5 of the joints bond well and can survive severe thermal shocking. 600 14.1 × 10 ·t + 3.99 201 1431 −4 0.5 650 63.5 × 10 ·t + 3.76 11 75 During isothermal aging at the temperature range of 550 C to 650 C, Ti penetrates across the Mo layer and diffuses into the Yb Co Sb matrix. The interfacial evolution at the formed Mo-Ti/Yb Co Sb 0.3 4 12 0.3 4 12 The performance of the TE joints, as well as that of the TEGs, is sensitive to the contact interface is found to be a diffusion-controlling process. Diffusion kinetics between Ti and Yb Co Sb 0.3 4 12 resistivity, and therefore, the contact resistivity is a valuable criteria for the service life prediction of at the Mo-Ti/Yb Co Sb interface were investigated and the apparent activation energy for the 0.3 4 12 the TE joints. We assume that a Ti/Mo/Yb0.3Co4Sb12 joint fails when the interfacial contact resistivity growth of the diffusion layer was obtained. Meanwhile, it was also found that the inter-diffusion 2 2 reaches 10 μΩ·cm or 20 μΩ·cm , which are low values and satisfy most practical applications, between Ti and Yb Co Sb is the decisive factor in the rise of the contact resistivity. Because of the 0.3 4 12 and the service life of the joint is then predicted in Table 2. At the service temperature of 550 °C, damping of the formed Mo-Ti layer, the diffusion of Ti towards the Yb Co Sb is greatly decelerated 0.3 4 12 the pretreated Ti/Mo/Yb0.3Co4Sb12 joints can endure about 21 years before the contact resistivity and the inter-diffusion between the Ti and the Yb Co Sb matrix is extremely slow. As a result, 0.3 4 12 reaches 10 μΩ·cm . Even when increasing the service temperature to 600 °C, the joints can survive the Ti/Mo/Yb Co Sb joints demonstrate the excellent stability of the contact resistivity. Based on 0.3 4 12 for 3.9 years before the contact resistivity reaches 20 μΩ·cm . the stability of the contact resistivity, service life prediction of the joints was made, and the result shows that the Ti/Mo/Yb Co Sb joint is a qualified choice and a promising candidate for practical 0.3 4 12 4. Conclusions applications at 550 C and 600 C in vacuum, respectively. In this paper, we successfully designed and fabricated n-type SKD thermoelectric joints with Acknowledgments: The authors acknowledge the financial supports from the National Natural Science the sandwich structures of Ti/Mo/Yb0.3Co4Sb12. With a controlled Mo layer thickness and the Foundation of China (Contract No. 51404236, 51572282 and 51372261). buffering of the Ti layer, the interfaces of the joints bond well and can survive severe thermal Author Contributions: Ming Gu conceived, designed, and performed the experiments, analyzed the data and shocking. During isothermal aging at the temperature range of 550 °C to 650 °C, Ti penetrates across wrote the paper. Shengqiang Bai instructed the designing of the experiments, the analyses of the data, and the writing of the paper. Xugui Xia helped to perform the experiments and analyses of the data. Xiangyang Huang the Mo layer and diffuses into the Yb0.3Co4Sb12 matrix. The interfacial evolution at the formed and Xiaoya Li supported the analyses of the data through valuable discussions. Xun Shi and Lidong Chen Mo-Ti/Yb0.3Co4Sb12 interface is found to be a diffusion-controlling process. Diffusion kinetics supported and provided guideline of the work. between Ti and Yb0.3Co4Sb12 at the Mo-Ti/Yb0.3Co4Sb12 interface were investigated and the apparent Conflicts of Interest: The authors declare no conflict of interest. activation energy for the growth of the diffusion layer was obtained. Meanwhile, it was also found Appl. Sci. 2017, 7, 952 9 of 10 References 1. Mohamed, S.E.G.; Saber, H.H. 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[CrossRef] 39. Sun, P.; Andersson, C.; Wei, X.; Cheng, Z.; Shangguan, D.; Liu, J. Study of interfacial reactions in Sn–3.5Ag–3.0Bi and Sn–8.0Zn–3.0Bi sandwich structure solder joint with Ni(P)/Cu metallization on Cu substrate. J. Alloys Compd. 2007, 437, 169–179. [CrossRef] © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Applied Sciences Multidisciplinary Digital Publishing Institute

Study on the High Temperature Interfacial Stability of Ti/Mo/Yb0.3Co4Sb12 Thermoelectric Joints

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applied sciences Article Study on the High Temperature Interfacial Stability of Ti/Mo/Yb Co Sb Thermoelectric Joints 0.3 4 12 Ming Gu, Shengqiang Bai *, Xugui Xia, Xiangyang Huang, Xiaoya Li, Xun Shi and Lidong Chen State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China; ftgm@mail.sic.ac.cn (M.G.); xgxia@mail.sic.ac.cn (X.X.); xyhuang@hotmail.com (X.H.); xyli@mail.sic.ac.cn (X.L.); xshi@mail.sic.ac.cn (X.S.); cld@mail.sic.ac.cn (L.C.) * Correspondence: bsq@mail.sic.ac.cn; Tel.: +86-021-6916-3686 Received: 31 August 2017; Accepted: 13 September 2017; Published: 15 September 2017 Abstract: To improve the interfacial stability at high temperatures, n-type skutterudite (SKD) thermoelectric joints with sandwich structures of Ti/Mo/Yb Co Sb were successfully designed 0.3 4 12 and fabricated. In this structure, Mo and Ti were introduced as the barrier layer with the goal of suppressing the interfacial diffusion and the buffer layer with the goal of enhancing the bonding strength, respectively. To evaluate the high temperature interfacial behavior of the Ti/Mo/Yb Co Sb joints, thermal shocking between 0 C and 600 C and isothermal aging 0.3 4 12 at a temperature range of 550 C to 650 C were carried out in vacuum. During the isothermal aging process, Ti penetrates across the Mo layer, and finally diffuses into the Yb Co Sb matrix. 0.3 4 12 By increasing the isothermal aging time, Ti continuously diffuses and reacts with the elements of Sb and Co in the matrix, consequently forming the multilayer-structured intermetallic compounds of Ti Sb/Ti Sb/TiCoSb. Diffusion kinetics was investigated and it was found that the interfacial 3 2 evolution of the Ti/Mo/Yb Co Sb joints was a diffusion-controlling process. During the diffusion 0.3 4 12 process, the formed Mo-Ti buffer layer acts as a damper, which greatly decelerates the diffusion of Ti towards the Yb Co Sb matrix at high temperatures. Meanwhile, it was found that the 0.3 4 12 increase in the contact resistivity of the joints mainly derives from the inter-diffusion between Ti and Yb Co4Sb . As a result, the Ti/Mo/Yb Co Sb joint demonstrates the excellent stability of the 0.3 12 0.3 4 12 interfacial contact resistivity. Service life prediction was made based on the stability of the contact resistivity, and it was found that the Ti/Mo/Yb Co Sb joint is qualified for practical applications 0.3 4 12 at 550 C. Keywords: thermoelectric joint; skutterudite; diffusion kinetics; contact resistivity; service life prediction 1. Introduction Unlike traditional power generation systems, a thermoelectric power generator (TEG) produces electrical power based on the Seebeck effect, via the movement of charge carriers within thermoelectric materials under a temperature difference. Using the heat from radioisotopes, TEGs can supply electrical power for tens of years, and are now the only choice of power source for deep space explorers which fly far from the sun and therefore can no longer obtain sufficient sunlight energy [1,2]. In the past decade, TEGs have been regarded as one of the most promising candidates to recover waste heat from industries or automobiles and turn them into electricity [3,4], which can benefit from the remission of the fossil energy shortage and global warming. Recently, TEGs have become one of the research focuses, and are expected to act as power suppliers of wearable electronic equipment [5]. Intrinsically, the performance of a TEG depends on the property of the thermoelectric (TE) material, which is referred to as the dimensionless figure of merit of ZT. Among the investigated Appl. Sci. 2017, 7, 952; doi:10.3390/app7090952 www.mdpi.com/journal/applsci Appl. Sci. 2017, 7, 952 2 of 10 various systems of TE materials, silicides [6–9], half-heuslers [10,11], and tellurides (e.g., of Bi [12–14], Pb [15–18] and Ge [19,20]) have been widely studied and some have been practically applied in TEGs. Besides, a number of materials featuring excellent TE properties (high ZT value) and a relatively lower cost, low toxicity, and good mechanical properties, such as filled skutterudite (SKD), also attract extensive attention and have been actively pursued for being converted into TEGs [21–26]. Thermoelectric joints, consisting of TE materials and electrodes, are the fundamental parts and also the key components of a TEG. In practical applications, the performance of a TEG not only depends on the property of TE materials, but also depends on the temperature difference between the hot- and cold-sides of the TE material bulks. Higher hot-side temperatures usually lead to a larger temperature difference, further enhancing the performance of the TEG. Meanwhile, a higher hot-side temperature also encourages interfacial diffusion and accelerates the degradation of the contact property, especially the contact (electrical) resistivity, which definitely deteriorates the performance and stability of the TEGs. Practically, the high temperature TEGs are expected to perform stably for a couple of years or even longer. For example, to make full use of the TE property, the hot side temperature of the CoSb -based SKD joints should be 500 C or higher. At such a high temperature, the major component in the SKD material, antimony, will react fiercely with most electrode materials (Cu, Ni, and Al, etc.) and then form single- or multi-layer interfacial intermetallics (IMC). The performance and stability of the TE joints are sensitively influenced by the microstructure and properties of the interface between the TE materials and electrode. To improve the interfacial stability of the SKD joints, a diffusion barrier layer is usually inserted between the electrode and the SKD material. Fan et al. firstly reported using Ti as the barrier layer in the CoSb [27]. Zhao et al. systematically studied the diffusion kinetics of the Ti/CoSb interface at high temperatures and predicted a service life of over 20 years for Ti/CoSb joints 3 3 at 500 C, using interfacial shearing strength as the criteria [28,29]. In Zhao’s work, when the working temperature increased to 550 C, the diffusion speed at the Ti/CoSb interface increased by one order and the predicted service life dropped to within one year [28]. In 2014, similar interfacial evolution in Ti/Yb Co Sb joints was found and the method was developed to improve the interfacial stability 0.6 4 12 of CoSb -based n-type SKD joints by introducing a Ti-Al barrier layer [30]. However, the uneven distributions of Ti and Al at the interfaces leave the accurate measurements of the thickness of the interfacial diffusion layer incomplete. Therefore, the diffusion kinetics and life prediction were also not studied. Afterwards, Fan et al. designed a Ti-Mo mixture as the barrier layer for the n-type Yb Co Sb matrix and found that the dispersion of the chemically stable Mo particles (15 at %) 0.3 4 12 disturbs the inter-diffusion between the Ti and Yb Co Sb matrix, which results in an improved 0.3 4 12 interfacial stability at 550 C. Still, the diffusion kinetics and life prediction were not studied [31]. Besides, Bae et al. studied the stability of the power generation characteristics of the Mo-Cu/Ti/SKD joints in terms of vibration and thermal cycling, and found that Ti helps to achieve good interfacial bonding [32]. In this paper, to develop an n-type SKD TE joint that is qualified for practical applications, we designed and fabricated n-type SKD joints with the structure of Ti/Mo/Yb Co Sb . In this 0.3 4 12 structure, Ti is reserved as the bonding layer for its active bonding behavior with common electrode materials including Cu and Ni. Furthermore, Ti is also expected to buffer the interfacial thermal stress. To suppress the activity of Ti at a high temperature and also barrier the inter-diffusion between TE materials and the electrode, Mo is chosen as the barrier layer. Both Mo and Ti were deposited onto the Yb Co Sb surface by magnetron sputtering to achieve flat interfaces. Because of the evident 0.3 4 12 difference between the CTEs of Mo and Yb Co Sb , the thickness of the Mo layer was controlled to 0.3 4 12 no more than 4.5 m to decrease the thermal stress at high temperatures. To reveal the interfacial diffusion kinetics of the Ti/Mo/Yb Co Sb joints and predict their 0.3 4 12 service life, the above n-type SKD TE joints were thermal shocked between 0 C and 600 C and isothermally aged between 550 C and 650 C. After each thermal shocking or isothermal aging step, the interfacial microstructures and the interfacial contact resistivity were both investigated. Based on Appl. Sci. 2017, 7, 952 3 of 10 the measurement results, the diffusion kinetics were investigated and the service life was predicted at different temperatures. 2. Materials and Methods The fabrication of the n-type Yb Co Sb powder followed the same process described in 0.3 4 12 reference [33]. To fabricate samples for the measurement of the coefficient of thermal expansion (CTE), Yb Co Sb powder was loaded into a graphite die (F30 mm) and consolidated in a spark 0.3 4 12 plasma sintering (SPS) system at 620 C under a pressure of 60 MPa and held for 10 min. Ti powder (4 N, 45 m) was loaded into a graphite die (F30 mm) and consolidated at 1100 C under a pressure of 60 MPa and held for 20 min in the same SPS system. The final dimension of the samples for CTE measurement was 4  4  20 mm . To fabricate the Ti/Mo/Yb Co Sb joints, Yb Co Sb 0.3 4 12 0.3 4 12 powder was consolidated following the same sintering process as mentioned above. The surfaces of the resulting Yb Co Sb bulks were polished and cleaned ultrasonically before they were dried 0.3 4 12 and loaded into the chamber of a magnetron sputtering system. The chamber was evacuated to 3  10 Pa and heated to 150 C before Ar was introduced. Mo was sputtered at 1.5 Pa under 1.5 kW for 50 min. Then, Ti was sputtered at 1.5 Pa under 3 kW for 120 min. The resulting samples were cut into joints with a final dimension of about 3  3  6 mm . The joints were then sealed in quartz ampoules under vacuum. In the thermal shocking test, the ampoules were moved from room temperature into a furnace at 600 C and kept inside for 5 min. Then, they were taken out into a water tank at 0 C for another 5 min. In the isothermal aging test, the ampoules were kept at 550 C, 600 C, and 650 C for different time periods of time. The CTE were measured with a Linseis-L75VS dilatometer (Princeton Junction, Princeton, NJ, USA). The interfacial microstructure of the joints was investigated by a scanning electronic microscope (ZEISS, SUPRA-55-SAPPHIRE, Oberkochen, Germany), and the chemical composition of the interfacial diffusion layers was analyzed using an Energy Dispersive Spectrometer (OXFORD, X-MaxN, Oxfordshire, UK). The contact resistivity of the joints was measured on the homemade four-probe platform (Haida Tech, Beijing, China) mentioned in our previous paper [34]. All the data for the diffusion layer thickness and the contact resistivity presented in this paper are the root mean square of three parallel samples, and the error bars are the root mean square errors. 3. Results and Discussion 3.1. Thermal Shocking Figure 1 shows the variation of the CTE with temperature for Yb Co Sb , Ti, Mo, and Cu & Ni, 0.3 4 12 which are common electrode materials. Clearly the CTE mismatch between Ti and Yb Co Sb (or Cu 0.3 4 12 & Ni) is much less than that between Mo and Yb Co Sb (or Cu & Ni). Based on Figure 2a,b, in the 0.3 4 12 as prepared Ti/Mo/Yb Co Sb joints, the bonding between Ti, Mo and Yb Co Sb is good with 0.3 4 12 0.3 4 12 the thickness of the Mo layer being about 4.5 m and the interfacial diffusion being neglectable at both the Ti/Mo and the Mo/Yb Co Sb interfaces. The Ti/Mo/Yb Co Sb joints then experienced 0.3 4 12 0.3 4 12 thermal shocking for 10 cycles between 0 C and 600 C in vacuum. Figure 2c demonstrates that the bonding between Ti, Mo, and Yb Co Sb remains good after 10 cycles of thermal shocking. 0.3 4 12 Besides, the contact resistivity of the joints before and after thermal shocking was measured to be 2 2 around 3.9 Wcm and 3.7 Wcm , respectively. Based on the above results, it can be concluded that in spite of evident CTE mismatch between Mo and Yb Co Sb , the interfacial bonding of 0.3 4 12 the Ti/Mo/Yb Co Sb joints is quite strong and can survive severe thermal shocking. For these 0.3 4 12 Ti/Mo/Yb Co Sb joints, because the Mo layer thickness is very small, the interfacial thermal stress 0.3 4 12 from CTE mismatch is controlled at a relatively low level. Moreover, the Ti layer also helps to balance the CTE mismatch between Mo and Yb Co Sb and release the interfacial thermal stress. 0.3 4 12 Appl. Sci. 2017, 7, 952 4 of 11 Appl. Appl. Sci. Sci. 2017 2017 , 7, , 7 952 , 952 4 of 11 4 of 10 Figure 1. Variation of the CTE with temperature for Yb0.3Co4Sb12, Ti, Mo, Cu, and Ni, Data for Mo, Figure 1. Variation of the CTE with temperature for Yb Co Sb , Ti, Mo, Cu, and Ni, Data for Mo, Cu, 0.3 4 12 Cu, and Ni are from references [35–37] respectively. Figure 1. Variation of the CTE with temperature for Yb0.3Co4Sb12, Ti, Mo, Cu, and Ni, Data for Mo, and Ni are from references [35–37] respectively. Cu, and Ni are from references [35–37] respectively. Figure Figure 2. 2. The interfacial The interfacial micr microstructure ostructure of of the the TTi/Mo/Yb i/Mo/Yb 0.3Co Co 4SbSb 12 joints ( joints a), as prepar (a), as prepar ed, a ed, Ba a Back ck 0.3 4 12 Figure 2. The interfacial microstructure of the Ti/Mo/Yb0.3Co4Sb12 joints (a), as prepared, a Back Scattering Scattering Electr Electron (BSE) gra on (BSE) graph, ph, ( (b),bas ), as prepared, a prepared, a line line scan,scan, and ( and (c), after c), a 10fter 10 cycles cy of thermal cles of therm shocking al Scattering Electron (BSE) graph, (b), as prepared, a line scan, and (c), after 10 cycles of thermal between shocking betw 0 C and een 0 °C and 6 600 C, a BSE 00 °C, a graph. BSE graph. shocking between 0 °C and 600 °C, a BSE graph. 3.2. Isothermal Aging 3.2. Isothermal Aging 3.2. Isothermal Aging 3.2.1. Pretreatment 3.2.1. 3.2.1. Pret Pretr re eatment atment The Ti/Mo/Yb0.3Co4Sb12 joints were then pretreated by isothermal aging at 550 °C. Figure 3a,b The The Ti/Mo/Yb Ti/Mo/Yb 0.3Co Co 4SbSb 12 joints were then pretrea joints were then pretreated ted by i bysotherm isothermal al agi aging ng at at 55550 0 °C. Fi C. Figur gure e 3a 3,b a,b 0.3 4 12 show the BSE graph with spot testing results and line scan results across the interfaces of the joints show show t the he BSE BSE gr graph aph w with ith s spot pot ttesting esting rre esults sults and andlin line e sc scan an rr eesults sults acros across s the the int interfaces erfaces of the of the joints joints after 16 days aging, respectively. Figure 3c shows the distribution of the Ti content in the joint and it after 16 days aging, respectively. Figure 3c shows the distribution of the Ti content in the joint and it after 16 days aging, respectively. Figure 3c shows the distribution of the Ti content in the joint and it clearly reveals that some Ti atoms have penetrated across the Mo layer and reached the Yb0.3Co4Sb12 clearly reveals that some Ti atoms have penetrated across the Mo layer and reached the Yb0.3Co4Sb12 clearly reveals that some Ti atoms have penetrated across the Mo layer and reached the Yb Co Sb 0.3 4 12 matrix. All the measurement results confirm that the inter-diffusion at the Mo/Yb0.3Co4Sb12 interface matrix. All the measurement results confirm that the inter-diffusion at the Mo/Yb0.3Co4Sb12 interface matrix. All the measurement results confirm that the inter-diffusion at the Mo/Yb Co Sb interface is is neglectable. However, Ti diffuses across the Ti/Mo interface and moves into the Mo layer, 0.3 4 12 is neglectable. However, Ti diffuses across the Ti/Mo interface and moves into the Mo layer, neglectable. consequently form However ing a new Mo-T , Ti diffuses acr i layer with oss the Ti/Mo a cont interface ent gradient. and moves In the Mo into -Ti the layMo er, the con layer, consequently tent of consequently forming a new Mo-Ti layer with a content gradient. In the Mo-Ti layer, the content of Ti decreases from about 10 at % Ti at the Ti/Mo-Ti interface to about 5 at % Ti at the forming a new Mo-Ti layer with a content gradient. In the Mo-Ti layer, the content of Ti decreases Ti decre Mo-Ti/Yb ase 0.3s from Co4Sb12 int abeout rfa 1 ce. The cont 0 at % aT cti resist at th ive Ti/ ity of about Mo-Ti int 3.8 μΩ erface t ·cm for t o ab he pretreated out 5 at joints is % Ti at the from about 10 at % Ti at the Ti/Mo-Ti interface to about 5 at % Ti at the Mo-Ti/Yb Co Sb interface. 0.3 4 12 almost the same value as that of the as prepa 2 red ones, which indicates that the penetration of the Ti Mo-Ti/Yb0.3Co4Sb12 interface. The contact resistivity of about 3.8 μΩ·cm for the pretreated joints is The contact resistivity of about 3.8 Wcm for the pretreated joints is almost the same value as that of atoms into the Mo layer almost does not affect the apparent contact resistivity of the joints. almost the same value as that of the as prepared ones, which indicates that the penetration of the Ti the as prepared ones, which indicates that the penetration of the Ti atoms into the Mo layer almost does Furthermore, if one prolongs the pretreatment time, the inter-diffusion between the Ti and atoms into the Mo layer almost does not affect the apparent contact resistivity of the joints. not affect the apparent contact resistivity of the joints. Furthermore, if one prolongs the pretreatment Yb0.3Co4Sb12 matrix will surely begin. Furthermore, if one prolongs the pretreatment time, the inter-diffusion between the Ti and time, the inter-diffusion between the Ti and Yb Co Sb matrix will surely begin. 0.3 4 12 Yb0.3Co4Sb12 matrix will surely begin. Based on the above results, to simplify the study on the interfacial stability, all the following Ti/Mo/Yb Co Sb joints were pretreated at 550 C for 16 days before they were further 0.3 4 12 isothermally aged. Correspondingly, the structure of the joints after the pretreatments is referred to as Ti/Mo-Ti/Yb Co Sb and the end of the pretreatment is set to be the beginning of the 0.3 4 12 isothermal aging. Appl. Sci. 2017, 7, 952 5 of 11 Appl. Sci. 2017, 7, 952 5 of 10 Appl. Sci. 2017, 7, 952 5 of 11 Figure 3. The interfacial microstructure of the Ti/Mo/Yb0.3Co4Sb12 joints pretreated at 550 °C for 16 days. (a) the BSE graph and spot testing, (b) line scan results, (c) the distribution of Ti. Based on the above results, to simplify the study on the interfacial stability, all the following Ti/Mo/Yb0.3Co4Sb12 joints were pretreated at 550 °C for 16 days before they were further isothermally aged. Correspondingly, the structure of the joints after the pretreatments is referred to as Figure 3. The interfacial microstructure of the Ti/Mo/Yb0.3Co4Sb12 joints pretreated at 550 °C for Figure 3. The interfacial microstructure of the Ti/Mo/Yb Co Sb joints pretreated at 550 C for 0.3 4 12 Ti/Mo-Ti/Yb0.3Co4Sb12 and the end of the pretreatment is set to be the beginning of the isothermal 16 16 days. ( days. (a)athe ) the BS BSEE g graph raph and spot and spot testing, testing(,b () bline ) line s scan can re results, sults, (c ( )cthe ) the distribution distribution of of T Ti. i. aging. Based on the above results, to simplify the study on the interfacial stability, all the following 3.2.2. Diffusion Kinetics of the Interface 3.2.2. Diffusion Kinetics of the Interface Ti/Mo/Yb0.3Co4Sb12 joints were pretreated at 550 °C for 16 days before they were further isothermally The pretreated joints were then isothermally aged at 550 C and the inter-diffusion between Ti The pretreated joints were then isothermally aged at 550 °C and the inter-diffusion between Ti aged. Correspondingly, the structure of the joints after the pretreatments is referred to as and Yb Co Sb occured at the Mo-Ti/Yb Co Sb interface. The BSE graphs of the interfacial cross and Yb0.3 0.3Co4 4Sb12 12 occured at the Mo-Ti/Yb0.3 0.3 Co4Sb 4 1212 interface. The BSE graphs of the interfacial cross Ti/Mo-Ti/Yb0.3Co4Sb12 and the end of the pretreatment is set to be the beginning of the isothermal section for the joints after 20-day and 50-day aging are shown in Figure 4a,b. After being aged for section for the joints after 20-day and 50-day aging are shown in Figure 4a,b. After being aged for aging. 50 days, the diffusion layer is about 0.3 m thick and consists of about 90 at % Ti and 10 at % Sb 50 days, the diffusion layer is about 0.3 μm thick and consists of about 90 at % Ti and 10 at % Sb without any sign of Mo. The extremely slow growth-speed of the diffusion layer implies that the new without any sign of Mo. The extremely slow growth-speed of the diffusion layer implies that the 3.2.2. Diffusion Kinetics of the Interface Mo-Ti layer formed in the pretreatment effectively dampens the diffusion of Ti atoms before they reach new Mo-Ti layer formed in the pretreatment effectively dampens the diffusion of Ti atoms before The pretreated joints were then isothermally aged at 550 °C and the inter-diffusion between Ti the Yb Co Sb matrix. Besides, Figure 4c,d show that there is no evident change in the distribution they reach the Yb0.3Co4Sb12 matrix. Besides, Figure 4c,d show that there is no evident change in the 0.3 4 12 and Yb0.3Co4Sb12 occured at the Mo-Ti/Yb0.3Co4Sb12 interface. The BSE graphs of the interfacial cross of Ti and Mo in the formed Mo-Ti layer, compared to the pretreated joints. distribution of Ti and Mo in the formed Mo-Ti layer, compared to the pretreated joints. section for the joints after 20-day and 50-day aging are shown in Figure 4a,b. After being aged for 50 days, the diffusion layer is about 0.3 μm thick and consists of about 90 at % Ti and 10 at % Sb without any sign of Mo. The extremely slow growth-speed of the diffusion layer implies that the new Mo-Ti layer formed in the pretreatment effectively dampens the diffusion of Ti atoms before they reach the Yb0.3Co4Sb12 matrix. Besides, Figure 4c,d show that there is no evident change in the distribution of Ti and Mo in the formed Mo-Ti layer, compared to the pretreated joints. Figure 4. The interfacial evolution of the pretreated joints isothermally aged at 550 °C. (a) BSE graph, Figure 4. The interfacial evolution of the pretreated joints isothermally aged at 550 C. (a) BSE graph, 20 days; (b) BSE graph, 50 days; (c) distribution of Mo, 50 days; (d) distribution of Ti, 50 days. 20 days; (b) BSE graph, 50 days; (c) distribution of Mo, 50 days; (d) distribution of Ti, 50 days. The pretreated joints were also isothermally aged at 600 C and 650 C. It was found that the growth rate of the diffusion layer between Ti and Yb Co Sb increases with an increasing aging 0.3 4 12 temperature. According to Figure 5, after being aged at 600 C for 41 days, the diffusion layer at the Figure 4. The interfacial evolution of the pretreated joints isothermally aged at 550 °C. (a) BSE graph, 20 days; (b) BSE graph, 50 days; (c) distribution of Mo, 50 days; (d) distribution of Ti, 50 days. Appl. Sci. 2017, 7, 952 6 of 11 Appl. Sci. 2017, 7, 952 6 of 11 The pretreated joints were also isothermally aged at 600 °C and 650 °C. It was found that the The pretreated joints were also isothermally aged at 600 °C and 650 °C. It was found that the Appl. Sci. 2017, 7, 952 6 of 10 growth rate of the diffusion layer between Ti and Yb0.3Co4Sb12 increases with an increasing aging growth rate of the diffusion layer between Ti and Yb0.3Co4Sb12 increases with an increasing aging temperature. According to Figure 5, after being aged at 600 °C for 41 days, the diffusion layer at the temperature. According to Figure 5, after being aged at 600 °C for 41 days, the diffusion layer at the Mo-Ti/ Mo-Ti/Yb Yb0.3Co Co 4SbSb 12 interf interface ace reaches reaches ababout out 2 μ 2m a mn and d consists of consists of two sub-l two sub-layers ayers of Ti of T 3iSb an Sb and d Ti T2iSb Sb. . 0.3 4 12 3 2 Mo-Ti/Yb0.3Co4Sb12 interface reaches about 2 μm and consists of two sub-layers of Ti3Sb and Ti2Sb. Figure 6 shows that after being aged for nine days at 650 °C, the total thickness of the diffusion layer Figure 6 shows that after being aged for nine days at 650 C, the total thickness of the diffusion layer Figure 6 shows that after being aged for nine days at 650 °C, the total thickness of the diffusion layer at at tthe he Mo-Ti/ Mo-Ti/Yb Yb0.3Co Co 4Sb Sb 12 interf interface ace grows to a grows tob about out 4.5 4.5 μm. Be m. Beside side the sub-layer the sub-layers s of Ti of T3iSb an Sb and d Ti T2 iSb, Sb, 0.3 4 12 3 2 at the Mo-Ti/Yb0.3Co4Sb12 interface grows to about 4.5 μm. Beside the sub-layers of Ti3Sb and Ti2Sb, there also appears a sign of TiCoSb in the front of the diffusion layer. there also appears a sign of TiCoSb in the front of the diffusion layer. there also appears a sign of TiCoSb in the front of the diffusion layer. Figure 5. The interfacial microstructure of the pretreated joints isothermally aged at 60  0 °C for 41 Figure Figure 5. 5. The The interfacial interfacial microstructure of t microstructure of theh pr e pretreat etreated ed j joints oints isothermally aged at isothermally aged at 600 60 C 0 °C for 41 for 41 days. days. (a) BSE graph and spot testing, (b) distribution of Ti, (c) distribution of Mo. ( day a) BSE s. (agraph ) BSE graph and and spot testing, spot te( sb ti)ndistribution g, (b) distribution of T of Ti, (c) distribution i, (c) distribution o of Mo. f Mo. Figure 6. The interfacial microstructure of the pretreated joints isothermally aged at 650 °C for nine Figure 6. The interfacial microstructure of the pretreated joints isothermally aged at 650 °C for nine Figure 6. The interfacial microstructure of the pretreated joints isothermally aged at 650 C for nine days. (a) BSE graph and spot testing, (b) distribution of Ti, (c) distribution of Mo. days. (a) BSE graph and spot testing, (b) distribution of Ti, (c) distribution of Mo. days. (a) BSE graph and spot testing, (b) distribution of Ti, (c) distribution of Mo. To reveal the diffusion kinetics, we analyzed the relationship between the total diffusion layer To reveal the diffusion kinetics, we analyzed the relationship between the total diffusion layer To reveal the diffusion kinetics, we analyzed the relationship between the total diffusion layer thickness, aging time, and aging temperature. Figure 7a shows that at all the three aging thickness, aging time, and aging temperature. Figure 7a shows that at all the three aging tempera thickness, tures, the tota aging time, and l thic aging kness of the temperatur diff e. usi Figur on la e yer is 7a shows linea that r wi atth the squa all the three re root of aging temperatur the agines, g temperatures, the total thickness of the diffusion layer is linear with the square root of the aging the total thickness of the diffusion layer is linear with the square root of the aging time, which indicates time, which indicates that the evolution of the Mo-Ti/Yb0.3Co4Sb12 interface is a diffusion-controlling time, which indicates that the evolution of the Mo-Ti/Yb0.3Co4Sb12 interface is a diffusion-controlling process that thebetw evolution een 550 °C an of the Mo-T d 650 °C. i/YbThe Co refore Sb , the interface diffusion is alayer diffusion-contr thickness colling an be expressed process between by the 0.3 4 12 process between 550 °C and 650 °C. Therefore, the diffusion layer thickness can be expressed by the 550 C and 650 C. Therefore, the diffusion layer thickness can be expressed by the following parabolic following parabolic equation [38,39]: following parabolic equation [38,39]: equation [38,39]: 0.5 0.5 YY-( = Dt) (1) 0.5 0 (1) YY-( = Dt) Y Y0 = (Dt) (1) where t is the aging time, and Y0 and Y are the total diffusion layer thickness at the origin of aging where t is the aging time, and Y0 and Y are the total diffusion layer thickness at the origin of aging where t is the aging time, and Y and Y are the total diffusion layer thickness at the origin of aging and and after being aged for a time period of t, respectively. D is the growth rate of the diffusion layer, and after being aged for a time period of t, respectively. D is the growth rate of the diffusion layer, after being aged for a time period of t, respectively. D is the growth rate of the diffusion layer, which is which is a function of the aging temperature. By fitting the data of the diffusion layer thickness as a which is a function of the aging temperature. By fitting the data of the diffusion layer thickness as a a function of the aging temperature. By fitting the data of the diffusion layer thickness as a function of 0.5 0.5 function of t , D , which is the slope of the fitting result, can be obtained. 0.5 0.5 0.5 0.5 function of t , D , which is the slope of the fitting result, can be obtained. t , D , which is the slope of the fitting result, can be obtained. To determine the activation energy for the interfacial diffusion, the Arrhenius relationship was used, D = D exp( ) (2) RT where D is the growth constant, Q is the apparent activation energy for the growth of the total diffusion layer, T is the temperature in Kelvin, and R is the gas constant, 8.314 J/(molK). Equation (2) can be transformed into: Q 1000 ln D =  + D (3) 1000R T By plotting and fitting lnD against 1000/T, Q can be obtained. Table 1 shows detailed D values and relations between the diffusion layer thickness and aging time. As is shown in Figure 7b, based on Table 1 and Equation (3), the activation energy Q for the growth of the diffusion layer at the Appl. Sci. 2017, 7, 952 7 of 10 Mo-Ti/Yb Co Sb interface is calculated to be about 447.5 kJ/mol at the temperature range of 0.3 4 12 550 C to 650 C. Combined with the above results, the variation of the diffusion layer thickness at the Appl. Sci. 2017, 7, 952 7 of 11 Mo-Ti/Yb Co Sb interface can be predicted over the above temperature range. 0.3 4 12 Figure 7. (a) Variation of the diffusion layer thickness of the pretreated joints with the square root of Figure 7. (a) Variation of the diffusion layer thickness of the pretreated joints with the square root of aging time between 550 °C and 650 °C. The inset is the enlargement of the result at 550 °C. aging time between 550 C and 650 C. The inset is the enlargement of the result at 550 C. (b) Variation (b) Variation of lnD with 1000/T between 550 °C and 650 °C. of lnD with 1000/T between 550 C and 650 C. To determine the activation energy for the interfacial diffusion, the Arrhenius relationship Table 1. Fitting results of the growth rate and the relationship between diffusion layer thickness and was used, aging time (s). 19 2 DD=− exp( ) (2) Temperature/ C D/10 m /s Y /m RT 4 0.5 550 0.21 1.45  10 t 4 0.5 where D0 is the growth constant, Q is the apparent activation energy for the growth of the total 600 9.53 9.76  10 t 4 0.5 650 251 50.1  10 t diffusion layer, T is the temperature in Kelvin, and R is the gas constant, 8.314 J/(mol·K). Equation (2) can be transformed into: 3.2.3. Stability of the Contact Resistivity and Service Life Prediction Q 1000 ln D =− ⋅ + D (3) To study the high temperature stability of1000 theRT electrical properties of these joints, the contact resistivity of the joints was measured after each different thermal aging time. Figure 8 exhibits the By plotting and fitting lnD against 1000/T, Q can be obtained. Table 1 shows detailed D values variation of the interfacial contact resistivity of the joints with the square root of the aging time between and relations between the diffusion layer thickness and aging time. As is shown in Figure 7b, based 550 C and 650 C. At the aging temperature of 550 C, the contact resistivity of the joint increases very on Table 1 and Equation (3), the activation energy Q for the growth of the diffusion layer at the slowly and reaches about 4.5 Wcm after being aged for 50 days. Increasing the aging temperature to Mo-Ti/Yb0.3Co4Sb12 interface is calculated to be about 447.5 kJ/mol at the temperature range of 550 °C 2 2 600 C and 650 C, the contact resistivity reaches 6.8 Wcm and 10 Wcm after 41 days and nine to 650 °C. Combined with the above results, the variation of the diffusion layer thickness at the days, respectively. In contrast to the previous reports [26,29–31], the stability of the contact resistivity Mo-Ti/Yb0.3Co4Sb12 interface can be predicted over the above temperature range. of the Ti/Mo/Yb Co Sb joints is significantly improved. 0.3 4 12 The variation of the contact resistivity was then fitted with the square root of the aging time and a Table 1. Fitting results of the growth rate and the relationship between diffusion layer thickness and linear relationship between them was found (See Table 2). Based on Tables 1 and 2, it is clear that the aging time (s). increase in the contact resistivity of the Ti/Mo-Ti/Yb Co Sb joints corresponds well to the increase 0.3 4 12 −19 2 Temperature/°C D/10 m /s Y/μm in the thickness of the interfacial diffusion layer at the Mo-Ti/Yb Co Sb interface. In another word, 0.3 4 12 −4 0.5 550 0.21 1.45 × 10 ·t it is the growth of the interfacial diffusion layer at the Mo-Ti/Yb Co Sb interface that dominates 0.3 4 12 −4 0.5 600 9.53 9.76 × 10 ·t the stability of the interfacial contact resistivity of the Ti/Mo-Ti/Yb Co Sb joints. 0.3 4 12 −4 0.5 650 251 50.1 × 10 ·t The performance of the TE joints, as well as that of the TEGs, is sensitive to the contact resistivity, and therefore, the contact resistivity is a valuable criteria for the service life prediction of the TE 3.2.3. Stability of the Contact Resistivity and Service Life Prediction joints. We assume that a Ti/Mo/Yb Co Sb joint fails when the interfacial contact resistivity reaches 0.3 4 12 2 2 10 Wcm or 20 Wcm , which are low values and satisfy most practical applications, and the service To study the high temperature stability of the electrical properties of these joints, the contact life of the joint is then predicted in Table 2. At the service temperature of 550 C, the pretreated resistivity of the joints was measured after each different thermal aging time. Figure 8 exhibits the Ti/Mo/Yb Co Sb joints can endure about 21 years before the contact resistivity reaches 10 Wcm . variation of 0.3 th 4e i12 nterfacial contact resistivity of the joints with the square root of the aging time Even when increasing the service temperature to 600 C, the joints can survive for 3.9 years before the between 550 °C and 650 °C. At the aging temperature of 550 °C, the contact resistivity of the joint contact resistivity reaches 20 Wcm . 2 increases very slowly and reaches about 4.5 μΩ·cm after being aged for 50 days. Increasing the 2 2 aging temperature to 600 °C and 650 °C, the contact resistivity reaches 6.8 μΩ·cm and 10 μΩ·cm after 41 days and nine days, respectively. In contrast to the previous reports [26,29–31], the stability of the contact resistivity of the Ti/Mo/Yb0.3Co4Sb12 joints is significantly improved. Appl. Sci. 2017, 7, 952 8 of 11 Appl. Sci. 2017, 7, 952 8 of 10 Figure 8. Variation of the interfacial contact resistivity of the joints with the square root of the aging Figure 8. Variation of the interfacial contact resistivity of the joints with the square root of the aging time between 550 C and 650 C. time between 550 °C and 650 °C. Table 2. Fitting results of the relationship between the contact resistivity and the aging time, and The variation of the contact resistivity was then fitted with the square root of the aging time and predicted service life (The prediction is based on the assumption that a joint fails when the contact a linear relationship between them was found (See Table 2). Based on Tables 1 and 2, it is clear that 2 2 resistivity reaches 10 Wcm (a) and 20 Wcm (b)). the increase in the contact resistivity of the Ti/Mo-Ti/Yb0.3Co4Sb12 joints corresponds well to the increase in the thickness of the interfacial diffusion layer at the Mo-Ti/Yb0.3Co4Sb12 interface. In (a) (b) Temperature/ C Contact Resistivity/Wcm Predicted Service Life /day Predicted Service Life /day another word, it is the growth of the interfacial diffusion layer at the Mo-Ti/Yb0.3Co4Sb12 interface 4 0.5 550 2.37  10 t + 3.91 7642 53,346 that dominates the stability of the interfacial contact resistivity of the Ti/Mo-Ti/Yb0.3Co4Sb12 joints. 4 0.5 600 14.1  10 t + 3.99 201 1431 4 0.5 650 11 75 63.5  10 t + 3.76 Table 2. Fitting results of the relationship between the contact resistivity and the aging time, and predicted service life (The prediction is based on the assumption that a joint fails when the contact 4. Conclusions 2 2 resistivity reaches 10 μΩ·cm (a) and 20 μΩ·cm (b)). In this paper, we successfully designed and fabricated n-type SKD thermoelectric joints with 2 (a) (b) Temperature/°C Contact Resistivity/μΩ∙cm Predicted Service life /day Predicted Service life /day the sandwich structures of Ti/Mo/Yb Co Sb . With a controlled Mo layer thickness and the −4 0.5 0.3 4 12 550 2.37 × 10 ·t + 3.91 7642 53,346 buffering of the Ti layer, the interfaces −4 0.5 of the joints bond well and can survive severe thermal shocking. 600 14.1 × 10 ·t + 3.99 201 1431 −4 0.5 650 63.5 × 10 ·t + 3.76 11 75 During isothermal aging at the temperature range of 550 C to 650 C, Ti penetrates across the Mo layer and diffuses into the Yb Co Sb matrix. The interfacial evolution at the formed Mo-Ti/Yb Co Sb 0.3 4 12 0.3 4 12 The performance of the TE joints, as well as that of the TEGs, is sensitive to the contact interface is found to be a diffusion-controlling process. Diffusion kinetics between Ti and Yb Co Sb 0.3 4 12 resistivity, and therefore, the contact resistivity is a valuable criteria for the service life prediction of at the Mo-Ti/Yb Co Sb interface were investigated and the apparent activation energy for the 0.3 4 12 the TE joints. We assume that a Ti/Mo/Yb0.3Co4Sb12 joint fails when the interfacial contact resistivity growth of the diffusion layer was obtained. Meanwhile, it was also found that the inter-diffusion 2 2 reaches 10 μΩ·cm or 20 μΩ·cm , which are low values and satisfy most practical applications, between Ti and Yb Co Sb is the decisive factor in the rise of the contact resistivity. Because of the 0.3 4 12 and the service life of the joint is then predicted in Table 2. At the service temperature of 550 °C, damping of the formed Mo-Ti layer, the diffusion of Ti towards the Yb Co Sb is greatly decelerated 0.3 4 12 the pretreated Ti/Mo/Yb0.3Co4Sb12 joints can endure about 21 years before the contact resistivity and the inter-diffusion between the Ti and the Yb Co Sb matrix is extremely slow. As a result, 0.3 4 12 reaches 10 μΩ·cm . Even when increasing the service temperature to 600 °C, the joints can survive the Ti/Mo/Yb Co Sb joints demonstrate the excellent stability of the contact resistivity. Based on 0.3 4 12 for 3.9 years before the contact resistivity reaches 20 μΩ·cm . the stability of the contact resistivity, service life prediction of the joints was made, and the result shows that the Ti/Mo/Yb Co Sb joint is a qualified choice and a promising candidate for practical 0.3 4 12 4. Conclusions applications at 550 C and 600 C in vacuum, respectively. In this paper, we successfully designed and fabricated n-type SKD thermoelectric joints with Acknowledgments: The authors acknowledge the financial supports from the National Natural Science the sandwich structures of Ti/Mo/Yb0.3Co4Sb12. With a controlled Mo layer thickness and the Foundation of China (Contract No. 51404236, 51572282 and 51372261). buffering of the Ti layer, the interfaces of the joints bond well and can survive severe thermal Author Contributions: Ming Gu conceived, designed, and performed the experiments, analyzed the data and shocking. During isothermal aging at the temperature range of 550 °C to 650 °C, Ti penetrates across wrote the paper. Shengqiang Bai instructed the designing of the experiments, the analyses of the data, and the writing of the paper. Xugui Xia helped to perform the experiments and analyses of the data. Xiangyang Huang the Mo layer and diffuses into the Yb0.3Co4Sb12 matrix. The interfacial evolution at the formed and Xiaoya Li supported the analyses of the data through valuable discussions. Xun Shi and Lidong Chen Mo-Ti/Yb0.3Co4Sb12 interface is found to be a diffusion-controlling process. Diffusion kinetics supported and provided guideline of the work. between Ti and Yb0.3Co4Sb12 at the Mo-Ti/Yb0.3Co4Sb12 interface were investigated and the apparent Conflicts of Interest: The authors declare no conflict of interest. activation energy for the growth of the diffusion layer was obtained. Meanwhile, it was also found Appl. Sci. 2017, 7, 952 9 of 10 References 1. Mohamed, S.E.G.; Saber, H.H. 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Journal

Applied Sciences – Multidisciplinary Digital Publishing Institute

Published: Sep 15, 2017

References

  • High efficiency segmented thermoelectric unicouple for operation between 973 and 300 K
    Mohamed
  • Thierry Caillat, Efficient segmented thermoelectric unicouples for space power applications
    Mohamed