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Piezoresistive Characteristics of Nanocarbon Composite Strain Sensor by Its Longitudinal Pattern Design

Piezoresistive Characteristics of Nanocarbon Composite Strain Sensor by Its Longitudinal Pattern... applied sciences Article Piezoresistive Characteristics of Nanocarbon Composite Strain Sensor by Its Longitudinal Pattern Design 1 , † 1 , † 1 1 1 Sung-Yong Kim , Baek-Gyu Choi , Gwang-Won Oh , Chan-Jung Kim , Young-Seok Jung , 2 3 4 1 , Jin-Seok Jang , Kwan-Young Joung , Jun-Ho Suh and Inpil Kang * Department of Mechanical and Design Engineering, Pukyong National University, Busan 48513, Korea; ksy1357@pukyong.ac.kr (S.-Y.K.); cbg8901@pukyong.ac.kr (B.-G.C.); 201955212@pukyong.ac.kr (G.-W.O.); cjkim@pknu.ac.kr (C.-J.K.); yousjung@pknu.ac.kr (Y.-S.J.) Mechanical Components and Materials R&D Group, Korea Institute of Industrial Technology (KITECH), Daegu 42994, Korea; jsjang@kitech.re.kr Innovative Smart Manufacturing R&D Department, Korea Institute of Industrial Technology (KITECH), Cheonan 31056, Korea; j6044@kitech.re.kr Department of Mechanical Engineering, Pusan National University, Busan 46241, Korea; junhosuh@pusan.ac.kr * Correspondence: ipkang@pknu.ac.kr; Tel.: +82-51-629-6971 † These authors contributed equally as a first author to this work. Abstract: For an engineering feasibility study, we studied a simple design to improve NCSS (nanocar- bon composite strain sensor) sensitivity by using its geometric pattern at a macro scale. We fabricated bulk- and grid-type sensors with different filler content weights (wt.%) and different sensor lengths and investigated their sensitivity characteristics. We also proposed a unit gauge factor model of Citation: Kim, S.-Y.; Choi, B.-G.; Oh, NCSS to find a correlation between sensor length and its sensitivity. NCSS sensitivity was improved G.-W.; Kim, C.-J.; Jung, Y.-S.; Jang, proportional to its length incremental ratio and we were able to achieve better linear and consistent J.-S.; Joung, K.-Y.; Suh, J.-H.; Kang, I. data from the grid type than the bulk type one. We conclude that the longer sensor length results Piezoresistive Characteristics of Nanocarbon Composite Strain Sensor in a larger change of resistance due to its piezoresistive unit summation and that sensor geometric by Its Longitudinal Pattern Design. pattern design is one of the important issues for axial load and deformation measurement. Appl. Sci. 2021, 11, 5760. https://doi.org/10.3390/ Keywords: carbon nanotube; strain sensor; piezoresistive mechanism; sensor pattern design app11135760 Academic Editor: David Charles Barton 1. Introduction Due to their versatility and exceptional mechanical and electrical properties, nanocar- Received: 11 May 2021 bon materials have been studied for use in reinforced composites as well as in trans- Accepted: 18 June 2021 ducers [1–3]. The incorporation of nanocarbon materials, with their pertinent electrical Published: 22 June 2021 properties, allows for applications as sensors in a wide range of fields [4–11]. A nanocarbon composite can be used as a sensory material due to the electrically Publisher’s Note: MDPI stays neutral conductive fillers in the matrix. Having electrical conductivity, the nanocarbon composite with regard to jurisdictional claims in has a piezoresistive feature that can be used like a commercial foil strain gauge. We reported published maps and institutional affil- the comparable strain sensing performance of a nanocarbon composite strain sensor (NCSS) iations. to a commercial foil strain gauge [12]. Although the strain sensing performances of the two sensors are similar, the piezoresistive mechanism of the NCSS is more complicated than that of the metallic type gauge [13]. The piezoresistive mechanism of the NCSS is related to various factors, such as the properties of the matrix, the content and arrangement of the Copyright: © 2021 by the authors. fillers, and the geometric shape of the composite. Licensee MDPI, Basel, Switzerland. In particular, the piezoresistivity is directly related to the NCSS sensitivity, and it may This article is an open access article be controllable via the fabrication process variables. During the sensor fabrication process, distributed under the terms and we believe that sensor irregularities generally occur due to filler randomness, including conditions of the Creative Commons Attribution (CC BY) license (https:// quality and dispersion, which dominate a sensor ’s electrical properties. Such irregularities creativecommons.org/licenses/by/ may hamper the ability to achieve uniform performance of an individual sensor. To 4.0/). Appl. Sci. 2021, 11, 5760. https://doi.org/10.3390/app11135760 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 5760 2 of 10 predict nanoscale internal filler behavior, researchers have studied physical models [14–17], computer-based molecular dynamics, and other types of simulations [18–21]. Some researchers have studied variations in NCSS sensitivity based on fabrication methodology. Lee et al. fabricated a single-walled carbon nanotube (SWCNT) film strain sensor using the spray method in a grid form and studied the sensor resistance and sensitiv- ity characteristics according to the number of grids and the thickness of the sensor [22]. In addition, Huang et al. fabricated a SWCNT nano strain sensor by alcohol catalytic chemical vapor deposition (ACCVD) and studied the sensor sensitivity characteristics according to the SWCNT growth time and the number of sensor grids [23]. Wang et al. used the spray deposition modeling (SDM) method to fabricate carbon nanotube (CNT) sensors with varying thicknesses and investigated their piezoresistive properties [24]. Kong et al. fabricated a composite sensor using carbon black and PDMS (Polydimethylsiloxane), and proposed an application case with different patterns and shapes for the sensor [25]. There have been other similar macroscopic approaches based on sensor pattern stud- ies. Li et al. fabricated multi-walled carbon nanotube (MWCNT) film sensors using a solution/filtration method, and they tested piezoresistivity with an optimized aspect ratio to obtain a proper signal from the sensor [26]. Xu and Allen fabricated a MWCNT/PDMS strain sensor. They showed the correlation between the initial resistance according to sensor thickness and sensor sensitivity [27]. Other studies have investigated improvements to NCSS sensitivity by using hybrid nanocarbon composites or functionalized nanocarbon fillers [28–30]. Since the quality control of most nanocomposites remains a challenge at the mass production level, the electrical characteristics of nanocarbon composites are also hard to consistently control. Consequently, most fabricated NCSSs may not have identical piezoresistive properties in terms of the nanocomposite process at the nano or micro scales. However, a commercial strain sensor yields uniform performance according to specifications, and a customer can expect reliable output from the sensor to meet the desired purpose. Unlike conventional strain sensors, NCSSs tend to involve many complexities at micro and macro scales and, furthermore, they are difficult to fabricate. Therefore, for an engineering feasibility study, we used a simple design to improve NCSS sensitivity by using its macroscale geometric pattern. We fabricated bulk- and grid-type sensors with different filler contents (wt.%) and different sensor lengths, and we investigated their sensitivity characteristics. We also proposed a unit gauge factor model of NCSS to determine the correlation between sensor length and its sensitivity. 2. Experimental Program 2.1. NCSS Fabrication Process To examine the percolation threshold and sensitivity of nanocarbon composite sensors, we fabricated the samples with five weight ratios (0.125, 0.25, 0.35, 0.5, and 1 wt.%) via the process shown in Figure 1. We used MWCNTs (Hanhwa Chemical Co., Korea, CM-280) as internal electrical fillers, epoxy (Kukdo Chemical Co., Korea, YD-128) as base material, and methylene chloride (Samchun Pure Chemical Co., Korea, purity 99.8%) as a solvent. The MWCNTs have a length of 180~200 m, a diameter of 10~15 nm, and an aspect ratio of 12,000~20,000. Appl. Sci. 2021, 11, 5760 3 of 10 Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 9 Figure 1. Figure 1. Sche Schematic matic ddiagram iagram of the nanocar of the nanocarbon bon composite composite fabrication pro fabrication pr cess ocess. . MWCNT: MWCNT mu : lti- multi- walled carbon nanotube. walled carbon nanotube. To obtain the effective electrical performance of the manufactured composite material, To obtain the effective electrical performance of the manufactured composite mate- the solution was dispersed at about 40 W for 30 min using a sonicator (Branson Co., rial, the solution was dispersed at about 40 W for 30 min using a sonicator (Branson Co., Brookfield, CT, USA, 450). To control the mixing viscosity with the hardener (Kukdo Brookfield, CT, USA, 450). To control the mixing viscosity with the hardener (Kukdo Chemical Co., Korea, Jeffamine), the dispersed solution was evaporated in a programmable Chemical Co., Korea, Jeffamine), the dispersed solution was evaporated in a programma- oven (Lab Companion, Daejeon, Korea, OF-01E) at 80 C. After that, the mixture was ble oven (Lab Companion, Daejeon, Korea, OF-01E) at 80 °C. After that, the mixture was degassed in a vacuum oven at 0 atm and 50 C for about 10 min to remove residual bubbles. degassed in a vacuum oven at 0 atm and 50 °C for about 10 min to remove residual bub- The mixture was injected into silicone molds using a syringe. Silicone molds were prepared bles. The mixture was injected into silicone molds using a syringe. Silicone molds were in various casting widths (8, 10, 12 mm), thicknesses (0.6, 1.2, 2.4, 3.6 mm), and lengths (30, prepared in various casting widths (8, 10, 12 mm), thicknesses (0.6, 1.2, 2.4, 3.6 mm), and 40, 50 mm) to fabricate different sizes of sensors. The degassed mixture in the mold was lengths (30, 40, 50 mm) to fabricate different sizes of sensors. The degassed mixture in the cured at 80 C using the programable oven. To complete the sensor samples, the casted mold was cured at 80 °C using the programable oven. To complete the sensor samples, samples were separated from the molds, and electrical wires were connected to the samples the casted samples were separated from the molds, and electrical wires were connected to with conductive epoxy (CANS, Japan, Elcoat-P-100). the samples with conductive epoxy (CANS, Japan, Elcoat-P-100). 2.2. Test Setup 2.2. Test Setup To investigate the electrical characteristics and sensitivity of the NCSS, the experimen- To investigate the electrical characteristics and sensitivity of the NCSS, the experi- tal apparatus shown in Figure 2 was constructed. The NCSS was tightly bonded with an mental apparatus shown in Figure 2 was constructed. The NCSS was tightly bonded with adhesive epoxy on top of a thick steel cantilever (300 mm 25 mm 2 mm). The center of an adhesive epoxy on top of a thick steel cantilever (300 mm × 25 mm × 2 mm). The center all sensor samples was located 50 mm from the fixed end of the cantilever. We installed a of all sensor samples was located 50 mm from the fixed end of the cantilever. We installed pair of NCSSs on the cantilever to double-check the outputs. The sensor-installed cantilever a pair of NCSSs on the cantilever to double-check the outputs. The sensor-installed canti- was mounted on an optical table by fixtures. We manually bent the free end of cantilever lever was mounted on an optical table by fixtures. We manually bent the free end of can- at steps of 20 mm from 140 mm to 140 mm, and the deflection was measured with a tilever at steps of 20 mm from −140 mm to 140 mm, and the deflection was measured with laser sensor (KEYENCE Co., Seongnam-si, Korea, IL-300). When a displacement is applied a laser sensor (KEYENCE Co., Seongnam-si, Korea, IL-300). When a displacement is ap- to the free end of the cantilever, strain is generated on the sensor attached to the upper plied to the free end of the cantilever, strain is generated on the sensor attached to the part of the cantilever. The induced strain changes into sensor resistance change due to upper part of the cantilever. The induced strain changes into sensor resistance change due its piezoresistive characteristics. The sensor resistance was measured by a multi-meter to its piezoresistive characteristics. The sensor resistance was measured by a multi-meter (KEYSIGHT technologies Co., Santa Rosa, CA, USA, 34465A) with the two-probes method. (KEYSIGHT technologies Co., Santa Rosa, CA, USA, 34465A) with the two-probes The induced strain was later calculated in terms of the deflection by strain and bending method. The induced strain was later calculated in terms of the deflection by strain and relation equation. bending relation equation. Appl. Sci. 2021, 11, 5760 4 of 10 Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 9 Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 9 Figure 2. Experimental setup for the electrical and sensitivity characteristics of the nanocarbon Figure 2. Experimental setup for the electrical and sensitivity characteristics of the nanocarbon Figure 2. Experimental setup for the electrical and sensitivity characteristics of the nanocarbon composite strain sensor (NCSS). com composite posite strai strain n sensor ( sensor (NCSS). NCSS). 3. Results 3. Results 3. Results 3.1. Percolation Threshold and Sensor Sensitivity Characteristics 3.1. Percolation Threshold and Sensor Sensitivity Characteristics 3.1. Percolation Threshold and Sensor Sensitivity Characteristics We varied the percolation threshold to find an appropriate piezoresistivity boundary We varied the percolation threshold to find an appropriate piezoresistivity boundary We varied the percolation threshold to find an appropriate piezoresistivity boundary and to check the uniformity of the samples fabricated in each batch. We fabricated the sam- and to check the uniformity of the samples fabricated in each batch. We fabricated the and to check the uniformity of the samples fabricated in each batch. We fabricated the ples at five different MWCNT weight fractions and measured their resistances, as shown samples at five different MWCNT weight fractions and measured their resistances, as samples at five different MWCNT weight fractions and measured their resistances, as in Figure 3a. The fabrication procedure was repeated two times to obtain reliable data. shown in Figure 3a. The fabrication procedure was repeated two times to obtain reliable shown in Figure 3a. The fabrication procedure was repeated two times to obtain reliable The resistances changed rapidly below 0.35 wt.%, and we decided on which piezoresistive data. The resistances changed rapidly below 0.35 wt.%, and we decided on which piezo- data. The resistances changed rapidly below 0.35 wt.%, and we decided on which piezo- design to use based on these results. resistive design to use based on these results. resistive design to use based on these results. Figure 3. (a) Percolation threshold of the samples and (b) sensitivity characteristics (according to Figure 3. (a) Percolation threshold of the samples and (b) sensitivity characteristics (according to Figure 3. (a) Percolation threshold of the samples and (b) sensitivity characteristics (according to wt.%) of the NCSS electrodes. wt.%) of the NCSS electrodes. wt.%) of the NCSS electrodes. When the samples were fabricated using a different process with the same recipe, When the samples were fabricated using a different process with the same recipe, When the samples were fabricated using a different process with the same recipe, their their electrical properties were not identical under 0.35 wt.%, as shown in Figure 3a. This their electrical properties were not identical under 0.35 wt.%, as shown in Figure 3a. This electrical properties were not identical under 0.35 wt.%, as shown in Figure 3a. This may may be due to the relationship between the electrical conducting path and the filler-load- may be due to the relationship between the electrical conducting path and the filler-load- be due to the relationship between the electrical conducting path and the filler-loading ing fraction in composites. In the case of higher filler content, we might be able to obtain ing fraction in composites. In the case of higher filler content, we might be able to obtain fraction in composites. In the case of higher filler content, we might be able to obtain similar electrical conductivity from the samples due to their high electrical filler density, similar electrical conductivity from the samples due to their high electrical filler density, similar electrical conductivity from the samples due to their high electrical filler density, as as shown for 0.5 wt.% in Table 1. However, in the case of lower filler content, the electrical as shown for 0.5 wt.% in Table 1. However, in the case of lower filler content, the electrical shown for 0.5 wt.% in Table 1. However, in the case of lower filler content, the electrical conducting path may vary widely due to their lower filler-loading fraction, as shown for conducting path may vary widely due to their lower filler-loading fraction, as shown for conducting path may vary widely due to their lower filler-loading fraction, as shown for 0.35 wt.% in Table 1. We also fabricated sensor samples with different lengths for cases 0.35 wt.% in Table 1. We also fabricated sensor samples with different lengths for cases 0.35 wt.% in Table 1. We also fabricated sensor samples with different lengths for cases with with both 0.35 and 0.5 wt.% filler to test the piezoresistivity in terms of the geometrical with both 0.35both and 0.35 0.5 wt.% fi and 0.5llwt.% er to test the pi filler to test ezthe oresi pisezor tivity esistivity in terms of in terms the geometri of the geometrical cal factors factors of the sensors. We prepared samples with lengths of 30, 40, and 50 mm and fixed factors of the sensors. We prepared samples with lengths of 30, 40, and 50 mm and fixed of the sensors. We prepared samples with lengths of 30, 40, and 50 mm and fixed their their width (10 mm) and thickness (1.2 mm), as shown in Table 1. The resistance change their width (10 mm) and thickness (1.2 mm), as shown in Table 1. The resistance change Appl. Sci. 2021, 11, 5760 5 of 10 width (10 mm) and thickness (1.2 mm), as shown in Table 1. The resistance change ratio (normalized resistance, Rn) of the sensors were also increased according to their length, as shown in Figure 3b. This indicates that the sensor length may correlate with sensitivity, and the length should therefore be one of the factors considered in sensor design. Table 1. The resistance of the sensor according to the length and wt.% at no load. Sensor 0.35 wt.% 0.5 wt.% T = 1.2, W = 10 (mm) L = 30 L = 40 L = 50 L= 30 L = 40 L = 50 Sample 1 (kW) 9.47 11.37 14.33 1.53 1.87 2.33 Sample 2 (kW) 8.63 10.89 14.10 1.44 1.87 2.50 3.2. Length and Sensitivity Correlation Based on Piezoresistive Effect To determine the correlation between sensor length and sensitivity, we proposed a unit gauge factor model of NCSS. The gauge factor (G. F.) is defined in Equation (1), and the strain (") of the cantilever is given in Equation (2). DR G.F. = = (1) # # 3c(L a) # = y(L) (2) Here, c is the height from the center of the cantilever, L is the length of the cantilever, a is the distance from the fixed end of the cantilever to the center of the sensor, and y (L) is the displacement applied to the end of the cantilever. Substituting the above equation into the general gauge factor equation, we yield the following. DR L DR G.F. = = (3) 3c(La) 3c L a y L R ( ) ( ) y L ( ) Because the fillers dispersed in the matrix construct the electrical conductive paths with contact resistance, its conductivity is less than that of a metal. Therefore, we assumed that the NCSS is an electrical conductive series based on an electrical conductivity model of nanocomposites and tried to determine its piezoresistive sensitivity mechanism based on this assumption. The NCSS is considered as a chain series of piezoresistive units having the same geometric size (Figure 4a). The electrical resistance of the sensor can be expressed as a linear summation of each unit, as shown in Figure 4b. Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 9 ∆ +∆ +∆ + + (6) G. F. = = = Thus, the electrical resistance change of the composite is dominated by ∆ [12], and the resistance due to the tunneling effect can be expressed as follows in Equation (7) [31]: = √2 (7) where h is Planck’s constant, d is the spacing between fillers, A is the cross-sectional area of the tunnel, e is the quantum of electricity, m is the mass of electrons, and is the height of the matrix barrier. The amount of change in the tunneling resistance is generated expo- nentially by the applied strain of each single unit part. Accordingly, the amount of change in the tunneling resistance with respect to the strain of the entire sensor line, that is, the sensitivity to the length of the nanocarbon composite, may be expressed as follows: G. F. ∆ ∆ ∆ + + = + + +⋯ (8) = ; = =⋯= From the above hypothesis, in this study, we supposed that the NCSS is a chain series Appl. Sci. 2021, 11, 5760 6 of 10 of individual piezoresistive units, and the whole piezoresistive change is a linear summa- tion of the units. Figure 4. Unit gauge factor model of NCSS: (a) Single piezoresistive unit (top) and chain series of Figure 4. Unit gauge factor model of NCSS: (a) Single piezoresistive unit (top) and chain series of the units with linear summation (bottom) under uniaxial load; and (b) an electrical conductivity the units with linear summation (bottom) under uniaxial load; and (b) an electrical conductivity schematic concept model of nanocomposite. schematic concept model of nanocomposite. As strains happen across all of the units of the NCSS, the gauge factor with respect to the strain can be expressed as follows. DR DR DR S S S 0 1 2 R R R S S S 0 1 2 G.F. = + + (4) # # # S S S 0 2 In addition, the electrical resistance model of the nanocarbon composite, shown in Figure 4b, can be expressed as follows: R = R + R + R (5) contact CNTcomposites f iller tunnel where R is the electrical resistance of the fillers in the composite, R is the resistance f iller tunnel between the fillers inside the composite, and R is the contact resistance between fillers. contact Substituting the above equation into the gauge factor equation, the gauge factor of the nanocarbon composite sensor can be expressed as follows. DR +DR +DR contact f iller tunnel DR R +R +R R contact N f iller tunnel G.F. = = = (6) CNTcomposites # # # Thus, the electrical resistance change of the composite is dominated by DR [12], tunnel and the resistance due to the tunneling effect can be expressed as follows in Equation (7) [31]: h d 4pd R = p exp 2ml (7) tunnel Ae 2ml where h is Planck’s constant, d is the spacing between fillers, A is the cross-sectional area of the tunnel, e is the quantum of electricity, m is the mass of electrons, and l is the height of the matrix barrier. The amount of change in the tunneling resistance is generated exponentially by the applied strain of each single unit part. Accordingly, the amount of Appl. Sci. 2021, 11, 5760 7 of 10 change in the tunneling resistance with respect to the strain of the entire sensor line, that is, the sensitivity to the length of the nanocarbon composite, may be expressed as follows: Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 9 DR DR DR tunnelS tunnelS tunnelS 1 2 R +R +R R R f illerS tunnelS contactS S S 0 0 0 1 2 G.F. = + + + CNTcomposites # # # S S S 0 1 2 (8) DR S tunneln 3.3. Improvement of Piezoresistive C = haracteristics by Usin ; R g Sensor = R Pa = tter  n = R S S S # 0 1 S n=S Based on the first experiment described in Section 3.2, we hypothesized that the sen- sitivity of the NCSS is closely related to its length because structural deformation brings From the above hypothesis, in this study, we supposed that the NCSS is a chain whole piezoresistive change from entire units of the sensor. We designed three grid sen- series of individual piezoresistive units, and the whole piezoresistive change is a linear sors via a thin polyimide film mask. The mask pattern was designed by CAD (Computer summation of the units. Aided Design) program and was fabricated by laser cutting process. The CNT ink was 3.3. Improvement of Piezoresistive Characteristics by Using Sensor Pattern manually printed through the mask on the cantilever, as shown in the lower right-hand corner of Figure 5. To secure sensor electrical stability with better conductivity, we con- Based on the first experiment described in Section 3.2, we hypothesized that the servatively used the 0.5 wt.% to fabricate the grid sensor samples. sensitivity of the NCSS is closely related to its length because structural deformation brings We measured each resistance change with respect to beam bending five times and whole piezoresistive change from entire units of the sensor. We designed three grid sensors aver via a aged thin the polyimide measured film values after mask. The elim mask in pattern ating the m was a designed ximum aby nd CAD minimum (Computer data points Aided . Fig Design) ure 5 sho program ws the res and was ults. When comp fabricated byarin laser g the am cuttingount of ch process. The ange CNT in the norm ink was alized re manually - printed through the mask on the cantilever, as shown in the lower right-hand corner of sistance with respect to the strain of the sensor samples shown in Figures 3 and 5, it can b Figur e seen t e 5.hT at o t secur he 40 e-m sensor m paelectrical ttern typestability (G.F.: 0.with 87) ha better s a la conductivity rger change in t , we h conservatively e normalized resistance used the 0.5 for wt.% the same to fabricate strain than the grid the 40- sensor mm bulk samples. type (G.F.: 0.22). Figure 5. Grid-type NCSS experiment: (a) grid-type NCSS samples and (b) sensitivity characteristic Figure 5. Grid-type NCSS experiment: (a) grid-type NCSS samples and (b) sensitivity characteristic of the grid-type NCSS with respect to length pattern change. of the grid-type NCSS with respect to length pattern change. We measured each resistance change with respect to beam bending five times and As we expected from the above, the NCSS sensitivity improvement is exactly pro- averaged the measured values after eliminating the maximum and minimum data points. portional to its incremental length ratio. We obtained higher sensitivity in cases where the Figure 5 shows the results. When comparing the amount of change in the normalized sensor had a longer length or denser pattern per unit area. We also achieved more linear, resistance with respect to the strain of the sensor samples shown in Figures 3 and 5, it can higher sensitivity and consistent data from the grid-type NCSS than from the bulk-type be seen that the 40-mm pattern type (G.F.: 0.87) has a larger change in the normalized sensor. Additionally, we deduced that NCSS strain output may be expressed by the sum- resistance for the same strain than the 40-mm bulk type (G.F.: 0.22). mation of the entire sensor covered surface. According to our literature survey, the strain As we expected from the above, the NCSS sensitivity improvement is exactly pro- sensitivity of a conventional foil strain gauge is only related to its material properties and portional to its incremental length ratio. We obtained higher sensitivity in cases where is not much affected by its length [32], which is a notable piezoresistive feature of the the sensor had a longer length or denser pattern per unit area. We also achieved more NCSS. We conclude that since NCSSs are affected by the overall strain change in a given linear, higher sensitivity and consistent data from the grid-type NCSS than from the bulk- direction, the geometric pattern design is one of the most important variables in axial load type sensor. Additionally, we deduced that NCSS strain output may be expressed by the and deformation measurements. summation of the entire sensor covered surface. According to our literature survey, the 4. Conclusions In this study, the piezoresistive characteristics of NCSSs were experimentally studied in terms of the sensor’s geometric length to determine its sensitivity to the design. We fabricated bulk- and grid-type sensors with different filler contents (wt.%) and different sensor lengths. Appl. Sci. 2021, 11, 5760 8 of 10 strain sensitivity of a conventional foil strain gauge is only related to its material properties and is not much affected by its length [32], which is a notable piezoresistive feature of the NCSS. We conclude that since NCSSs are affected by the overall strain change in a given direction, the geometric pattern design is one of the most important variables in axial load and deformation measurements. 4. Conclusions In this study, the piezoresistive characteristics of NCSSs were experimentally studied in terms of the sensor ’s geometric length to determine its sensitivity to the design. We fabricated bulk- and grid-type sensors with different filler contents (wt.%) and different sensor lengths. In the first experiment, we used silicone molds to prepare samples with three different lengths (30, 40, 50 mm) and two different filler compositions (0.35 and 0.5 wt.%). We measured the sensor resistance change with respect to the bending strain variation of a cantilever. For both compositions, we obtained higher sensitivity at longer sensor lengths, which suggested the possibility of using the geometric design to control the sensitivity. We deduced that sensor length may correlate with sensitivity, and we proposed a unit gauge factor model of NCSSs to explain the proportional relationship between their length and sensitivity. We supposed that the NCSS is a chain series of individual piezoresistive units and that the whole piezoresistive change can be a linear summation of these units. To verify our hypothesis, we performed a second experiment with more sophisticated samples. We designed grid-type sensors by using thin polyimide film masks. We printed the grid- type NCSS (0.5 wt.% filler) on a cantilever and repeated the same test. Results indicated that the improvement in NCSS sensitivity was exactly proportional to its incremental length ratio (because structural deformation brings piezoresistive change from whole units of the sensor). We were able to obtain higher sensitivity in the cases where the sensor had a longer length and denser pattern per unit area. We also achieved more linear and consistent data from the grid-type NCSS than from the bulk-type sensor. From the analysis, we concluded that the greater sensor length brings a greater change in resistance due to its piezoresistive unit summation. Eventually, the sensitivity of NCSSs directly relates to their length, and the patternized length can be used to easily control the sensor sensitivity. We also found that the fine sensor pattern can improve its performance, allowing better results to be achieved. Sensor geometric pattern design is one of the most important aspects of axial load and deformation measurements. For further study, we are studying advanced sensitivity design considering the three-dimensional pattern variables of the NCSS as well. Author Contributions: Conceptualization: S.-Y.K., B.-G.C., and I.K.; methodology: S.-Y.K., B.-G.C., C.-J.K., Y.-S.J., J.-S.J., J.-H.S., and I.K.; formal analysis: S.-Y.K. and B.-G.C.; investigation: S.-Y.K. and B.-G.C.; data curation: S.-Y.K., B.-G.C., and G.-W.O.; writing—original draft preparation: S.-Y.K. and B.-G.C.; writing—review and editing: C.-J.K., Y.-S.J., J.-S.J., K.-Y.J., and J.-H.S.; visualization: S.-Y.K., B.-G.C., and G.-W.O.; project administration: I.K.; funding acquisition: I.K. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by a Research Grant of Pukyong National University (2019). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data available in a publicly accessible repository. Conflicts of Interest: The authors declare no conflict of interest. Appl. Sci. 2021, 11, 5760 9 of 10 References 1. Salvado, R.; Lopes, C.; Araujo, P.; Gorski, M.; Velez, F.J.; Gomez, J.C.; Krzywon, R. Carbon fiber epoxy composites for both strengthening and health monitoring of structures. Sensors 2015, 15, 10753–10770. [CrossRef] 2. Arash, B.; Park, H.S.; Rabczuk, T. Mechanical properties of carbon nanotube reinforced polymer nanocomposites: A coarse- grained model. Compos. B. Eng. 2015, 80, 92–100. [CrossRef] 3. Cho, B.G.; Hwang, S.H.; Park, Y.B. Fabrication and characterization of carbon nanotube/carbon fiber/polycarbonate multiscale hybrid composites. Compos. Res. 2016, 29, 269–275. [CrossRef] 4. 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Piezoresistive Characteristics of Nanocarbon Composite Strain Sensor by Its Longitudinal Pattern Design

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applied sciences Article Piezoresistive Characteristics of Nanocarbon Composite Strain Sensor by Its Longitudinal Pattern Design 1 , † 1 , † 1 1 1 Sung-Yong Kim , Baek-Gyu Choi , Gwang-Won Oh , Chan-Jung Kim , Young-Seok Jung , 2 3 4 1 , Jin-Seok Jang , Kwan-Young Joung , Jun-Ho Suh and Inpil Kang * Department of Mechanical and Design Engineering, Pukyong National University, Busan 48513, Korea; ksy1357@pukyong.ac.kr (S.-Y.K.); cbg8901@pukyong.ac.kr (B.-G.C.); 201955212@pukyong.ac.kr (G.-W.O.); cjkim@pknu.ac.kr (C.-J.K.); yousjung@pknu.ac.kr (Y.-S.J.) Mechanical Components and Materials R&D Group, Korea Institute of Industrial Technology (KITECH), Daegu 42994, Korea; jsjang@kitech.re.kr Innovative Smart Manufacturing R&D Department, Korea Institute of Industrial Technology (KITECH), Cheonan 31056, Korea; j6044@kitech.re.kr Department of Mechanical Engineering, Pusan National University, Busan 46241, Korea; junhosuh@pusan.ac.kr * Correspondence: ipkang@pknu.ac.kr; Tel.: +82-51-629-6971 † These authors contributed equally as a first author to this work. Abstract: For an engineering feasibility study, we studied a simple design to improve NCSS (nanocar- bon composite strain sensor) sensitivity by using its geometric pattern at a macro scale. We fabricated bulk- and grid-type sensors with different filler content weights (wt.%) and different sensor lengths and investigated their sensitivity characteristics. We also proposed a unit gauge factor model of Citation: Kim, S.-Y.; Choi, B.-G.; Oh, NCSS to find a correlation between sensor length and its sensitivity. NCSS sensitivity was improved G.-W.; Kim, C.-J.; Jung, Y.-S.; Jang, proportional to its length incremental ratio and we were able to achieve better linear and consistent J.-S.; Joung, K.-Y.; Suh, J.-H.; Kang, I. data from the grid type than the bulk type one. We conclude that the longer sensor length results Piezoresistive Characteristics of Nanocarbon Composite Strain Sensor in a larger change of resistance due to its piezoresistive unit summation and that sensor geometric by Its Longitudinal Pattern Design. pattern design is one of the important issues for axial load and deformation measurement. Appl. Sci. 2021, 11, 5760. https://doi.org/10.3390/ Keywords: carbon nanotube; strain sensor; piezoresistive mechanism; sensor pattern design app11135760 Academic Editor: David Charles Barton 1. Introduction Due to their versatility and exceptional mechanical and electrical properties, nanocar- Received: 11 May 2021 bon materials have been studied for use in reinforced composites as well as in trans- Accepted: 18 June 2021 ducers [1–3]. The incorporation of nanocarbon materials, with their pertinent electrical Published: 22 June 2021 properties, allows for applications as sensors in a wide range of fields [4–11]. A nanocarbon composite can be used as a sensory material due to the electrically Publisher’s Note: MDPI stays neutral conductive fillers in the matrix. Having electrical conductivity, the nanocarbon composite with regard to jurisdictional claims in has a piezoresistive feature that can be used like a commercial foil strain gauge. We reported published maps and institutional affil- the comparable strain sensing performance of a nanocarbon composite strain sensor (NCSS) iations. to a commercial foil strain gauge [12]. Although the strain sensing performances of the two sensors are similar, the piezoresistive mechanism of the NCSS is more complicated than that of the metallic type gauge [13]. The piezoresistive mechanism of the NCSS is related to various factors, such as the properties of the matrix, the content and arrangement of the Copyright: © 2021 by the authors. fillers, and the geometric shape of the composite. Licensee MDPI, Basel, Switzerland. In particular, the piezoresistivity is directly related to the NCSS sensitivity, and it may This article is an open access article be controllable via the fabrication process variables. During the sensor fabrication process, distributed under the terms and we believe that sensor irregularities generally occur due to filler randomness, including conditions of the Creative Commons Attribution (CC BY) license (https:// quality and dispersion, which dominate a sensor ’s electrical properties. Such irregularities creativecommons.org/licenses/by/ may hamper the ability to achieve uniform performance of an individual sensor. To 4.0/). Appl. Sci. 2021, 11, 5760. https://doi.org/10.3390/app11135760 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 5760 2 of 10 predict nanoscale internal filler behavior, researchers have studied physical models [14–17], computer-based molecular dynamics, and other types of simulations [18–21]. Some researchers have studied variations in NCSS sensitivity based on fabrication methodology. Lee et al. fabricated a single-walled carbon nanotube (SWCNT) film strain sensor using the spray method in a grid form and studied the sensor resistance and sensitiv- ity characteristics according to the number of grids and the thickness of the sensor [22]. In addition, Huang et al. fabricated a SWCNT nano strain sensor by alcohol catalytic chemical vapor deposition (ACCVD) and studied the sensor sensitivity characteristics according to the SWCNT growth time and the number of sensor grids [23]. Wang et al. used the spray deposition modeling (SDM) method to fabricate carbon nanotube (CNT) sensors with varying thicknesses and investigated their piezoresistive properties [24]. Kong et al. fabricated a composite sensor using carbon black and PDMS (Polydimethylsiloxane), and proposed an application case with different patterns and shapes for the sensor [25]. There have been other similar macroscopic approaches based on sensor pattern stud- ies. Li et al. fabricated multi-walled carbon nanotube (MWCNT) film sensors using a solution/filtration method, and they tested piezoresistivity with an optimized aspect ratio to obtain a proper signal from the sensor [26]. Xu and Allen fabricated a MWCNT/PDMS strain sensor. They showed the correlation between the initial resistance according to sensor thickness and sensor sensitivity [27]. Other studies have investigated improvements to NCSS sensitivity by using hybrid nanocarbon composites or functionalized nanocarbon fillers [28–30]. Since the quality control of most nanocomposites remains a challenge at the mass production level, the electrical characteristics of nanocarbon composites are also hard to consistently control. Consequently, most fabricated NCSSs may not have identical piezoresistive properties in terms of the nanocomposite process at the nano or micro scales. However, a commercial strain sensor yields uniform performance according to specifications, and a customer can expect reliable output from the sensor to meet the desired purpose. Unlike conventional strain sensors, NCSSs tend to involve many complexities at micro and macro scales and, furthermore, they are difficult to fabricate. Therefore, for an engineering feasibility study, we used a simple design to improve NCSS sensitivity by using its macroscale geometric pattern. We fabricated bulk- and grid-type sensors with different filler contents (wt.%) and different sensor lengths, and we investigated their sensitivity characteristics. We also proposed a unit gauge factor model of NCSS to determine the correlation between sensor length and its sensitivity. 2. Experimental Program 2.1. NCSS Fabrication Process To examine the percolation threshold and sensitivity of nanocarbon composite sensors, we fabricated the samples with five weight ratios (0.125, 0.25, 0.35, 0.5, and 1 wt.%) via the process shown in Figure 1. We used MWCNTs (Hanhwa Chemical Co., Korea, CM-280) as internal electrical fillers, epoxy (Kukdo Chemical Co., Korea, YD-128) as base material, and methylene chloride (Samchun Pure Chemical Co., Korea, purity 99.8%) as a solvent. The MWCNTs have a length of 180~200 m, a diameter of 10~15 nm, and an aspect ratio of 12,000~20,000. Appl. Sci. 2021, 11, 5760 3 of 10 Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 9 Figure 1. Figure 1. Sche Schematic matic ddiagram iagram of the nanocar of the nanocarbon bon composite composite fabrication pro fabrication pr cess ocess. . MWCNT: MWCNT mu : lti- multi- walled carbon nanotube. walled carbon nanotube. To obtain the effective electrical performance of the manufactured composite material, To obtain the effective electrical performance of the manufactured composite mate- the solution was dispersed at about 40 W for 30 min using a sonicator (Branson Co., rial, the solution was dispersed at about 40 W for 30 min using a sonicator (Branson Co., Brookfield, CT, USA, 450). To control the mixing viscosity with the hardener (Kukdo Brookfield, CT, USA, 450). To control the mixing viscosity with the hardener (Kukdo Chemical Co., Korea, Jeffamine), the dispersed solution was evaporated in a programmable Chemical Co., Korea, Jeffamine), the dispersed solution was evaporated in a programma- oven (Lab Companion, Daejeon, Korea, OF-01E) at 80 C. After that, the mixture was ble oven (Lab Companion, Daejeon, Korea, OF-01E) at 80 °C. After that, the mixture was degassed in a vacuum oven at 0 atm and 50 C for about 10 min to remove residual bubbles. degassed in a vacuum oven at 0 atm and 50 °C for about 10 min to remove residual bub- The mixture was injected into silicone molds using a syringe. Silicone molds were prepared bles. The mixture was injected into silicone molds using a syringe. Silicone molds were in various casting widths (8, 10, 12 mm), thicknesses (0.6, 1.2, 2.4, 3.6 mm), and lengths (30, prepared in various casting widths (8, 10, 12 mm), thicknesses (0.6, 1.2, 2.4, 3.6 mm), and 40, 50 mm) to fabricate different sizes of sensors. The degassed mixture in the mold was lengths (30, 40, 50 mm) to fabricate different sizes of sensors. The degassed mixture in the cured at 80 C using the programable oven. To complete the sensor samples, the casted mold was cured at 80 °C using the programable oven. To complete the sensor samples, samples were separated from the molds, and electrical wires were connected to the samples the casted samples were separated from the molds, and electrical wires were connected to with conductive epoxy (CANS, Japan, Elcoat-P-100). the samples with conductive epoxy (CANS, Japan, Elcoat-P-100). 2.2. Test Setup 2.2. Test Setup To investigate the electrical characteristics and sensitivity of the NCSS, the experimen- To investigate the electrical characteristics and sensitivity of the NCSS, the experi- tal apparatus shown in Figure 2 was constructed. The NCSS was tightly bonded with an mental apparatus shown in Figure 2 was constructed. The NCSS was tightly bonded with adhesive epoxy on top of a thick steel cantilever (300 mm 25 mm 2 mm). The center of an adhesive epoxy on top of a thick steel cantilever (300 mm × 25 mm × 2 mm). The center all sensor samples was located 50 mm from the fixed end of the cantilever. We installed a of all sensor samples was located 50 mm from the fixed end of the cantilever. We installed pair of NCSSs on the cantilever to double-check the outputs. The sensor-installed cantilever a pair of NCSSs on the cantilever to double-check the outputs. The sensor-installed canti- was mounted on an optical table by fixtures. We manually bent the free end of cantilever lever was mounted on an optical table by fixtures. We manually bent the free end of can- at steps of 20 mm from 140 mm to 140 mm, and the deflection was measured with a tilever at steps of 20 mm from −140 mm to 140 mm, and the deflection was measured with laser sensor (KEYENCE Co., Seongnam-si, Korea, IL-300). When a displacement is applied a laser sensor (KEYENCE Co., Seongnam-si, Korea, IL-300). When a displacement is ap- to the free end of the cantilever, strain is generated on the sensor attached to the upper plied to the free end of the cantilever, strain is generated on the sensor attached to the part of the cantilever. The induced strain changes into sensor resistance change due to upper part of the cantilever. The induced strain changes into sensor resistance change due its piezoresistive characteristics. The sensor resistance was measured by a multi-meter to its piezoresistive characteristics. The sensor resistance was measured by a multi-meter (KEYSIGHT technologies Co., Santa Rosa, CA, USA, 34465A) with the two-probes method. (KEYSIGHT technologies Co., Santa Rosa, CA, USA, 34465A) with the two-probes The induced strain was later calculated in terms of the deflection by strain and bending method. The induced strain was later calculated in terms of the deflection by strain and relation equation. bending relation equation. Appl. Sci. 2021, 11, 5760 4 of 10 Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 9 Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 9 Figure 2. Experimental setup for the electrical and sensitivity characteristics of the nanocarbon Figure 2. Experimental setup for the electrical and sensitivity characteristics of the nanocarbon Figure 2. Experimental setup for the electrical and sensitivity characteristics of the nanocarbon composite strain sensor (NCSS). com composite posite strai strain n sensor ( sensor (NCSS). NCSS). 3. Results 3. Results 3. Results 3.1. Percolation Threshold and Sensor Sensitivity Characteristics 3.1. Percolation Threshold and Sensor Sensitivity Characteristics 3.1. Percolation Threshold and Sensor Sensitivity Characteristics We varied the percolation threshold to find an appropriate piezoresistivity boundary We varied the percolation threshold to find an appropriate piezoresistivity boundary We varied the percolation threshold to find an appropriate piezoresistivity boundary and to check the uniformity of the samples fabricated in each batch. We fabricated the sam- and to check the uniformity of the samples fabricated in each batch. We fabricated the and to check the uniformity of the samples fabricated in each batch. We fabricated the ples at five different MWCNT weight fractions and measured their resistances, as shown samples at five different MWCNT weight fractions and measured their resistances, as samples at five different MWCNT weight fractions and measured their resistances, as in Figure 3a. The fabrication procedure was repeated two times to obtain reliable data. shown in Figure 3a. The fabrication procedure was repeated two times to obtain reliable shown in Figure 3a. The fabrication procedure was repeated two times to obtain reliable The resistances changed rapidly below 0.35 wt.%, and we decided on which piezoresistive data. The resistances changed rapidly below 0.35 wt.%, and we decided on which piezo- data. The resistances changed rapidly below 0.35 wt.%, and we decided on which piezo- design to use based on these results. resistive design to use based on these results. resistive design to use based on these results. Figure 3. (a) Percolation threshold of the samples and (b) sensitivity characteristics (according to Figure 3. (a) Percolation threshold of the samples and (b) sensitivity characteristics (according to Figure 3. (a) Percolation threshold of the samples and (b) sensitivity characteristics (according to wt.%) of the NCSS electrodes. wt.%) of the NCSS electrodes. wt.%) of the NCSS electrodes. When the samples were fabricated using a different process with the same recipe, When the samples were fabricated using a different process with the same recipe, When the samples were fabricated using a different process with the same recipe, their their electrical properties were not identical under 0.35 wt.%, as shown in Figure 3a. This their electrical properties were not identical under 0.35 wt.%, as shown in Figure 3a. This electrical properties were not identical under 0.35 wt.%, as shown in Figure 3a. This may may be due to the relationship between the electrical conducting path and the filler-load- may be due to the relationship between the electrical conducting path and the filler-load- be due to the relationship between the electrical conducting path and the filler-loading ing fraction in composites. In the case of higher filler content, we might be able to obtain ing fraction in composites. In the case of higher filler content, we might be able to obtain fraction in composites. In the case of higher filler content, we might be able to obtain similar electrical conductivity from the samples due to their high electrical filler density, similar electrical conductivity from the samples due to their high electrical filler density, similar electrical conductivity from the samples due to their high electrical filler density, as as shown for 0.5 wt.% in Table 1. However, in the case of lower filler content, the electrical as shown for 0.5 wt.% in Table 1. However, in the case of lower filler content, the electrical shown for 0.5 wt.% in Table 1. However, in the case of lower filler content, the electrical conducting path may vary widely due to their lower filler-loading fraction, as shown for conducting path may vary widely due to their lower filler-loading fraction, as shown for conducting path may vary widely due to their lower filler-loading fraction, as shown for 0.35 wt.% in Table 1. We also fabricated sensor samples with different lengths for cases 0.35 wt.% in Table 1. We also fabricated sensor samples with different lengths for cases 0.35 wt.% in Table 1. We also fabricated sensor samples with different lengths for cases with with both 0.35 and 0.5 wt.% filler to test the piezoresistivity in terms of the geometrical with both 0.35both and 0.35 0.5 wt.% fi and 0.5llwt.% er to test the pi filler to test ezthe oresi pisezor tivity esistivity in terms of in terms the geometri of the geometrical cal factors factors of the sensors. We prepared samples with lengths of 30, 40, and 50 mm and fixed factors of the sensors. We prepared samples with lengths of 30, 40, and 50 mm and fixed of the sensors. We prepared samples with lengths of 30, 40, and 50 mm and fixed their their width (10 mm) and thickness (1.2 mm), as shown in Table 1. The resistance change their width (10 mm) and thickness (1.2 mm), as shown in Table 1. The resistance change Appl. Sci. 2021, 11, 5760 5 of 10 width (10 mm) and thickness (1.2 mm), as shown in Table 1. The resistance change ratio (normalized resistance, Rn) of the sensors were also increased according to their length, as shown in Figure 3b. This indicates that the sensor length may correlate with sensitivity, and the length should therefore be one of the factors considered in sensor design. Table 1. The resistance of the sensor according to the length and wt.% at no load. Sensor 0.35 wt.% 0.5 wt.% T = 1.2, W = 10 (mm) L = 30 L = 40 L = 50 L= 30 L = 40 L = 50 Sample 1 (kW) 9.47 11.37 14.33 1.53 1.87 2.33 Sample 2 (kW) 8.63 10.89 14.10 1.44 1.87 2.50 3.2. Length and Sensitivity Correlation Based on Piezoresistive Effect To determine the correlation between sensor length and sensitivity, we proposed a unit gauge factor model of NCSS. The gauge factor (G. F.) is defined in Equation (1), and the strain (") of the cantilever is given in Equation (2). DR G.F. = = (1) # # 3c(L a) # = y(L) (2) Here, c is the height from the center of the cantilever, L is the length of the cantilever, a is the distance from the fixed end of the cantilever to the center of the sensor, and y (L) is the displacement applied to the end of the cantilever. Substituting the above equation into the general gauge factor equation, we yield the following. DR L DR G.F. = = (3) 3c(La) 3c L a y L R ( ) ( ) y L ( ) Because the fillers dispersed in the matrix construct the electrical conductive paths with contact resistance, its conductivity is less than that of a metal. Therefore, we assumed that the NCSS is an electrical conductive series based on an electrical conductivity model of nanocomposites and tried to determine its piezoresistive sensitivity mechanism based on this assumption. The NCSS is considered as a chain series of piezoresistive units having the same geometric size (Figure 4a). The electrical resistance of the sensor can be expressed as a linear summation of each unit, as shown in Figure 4b. Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 9 ∆ +∆ +∆ + + (6) G. F. = = = Thus, the electrical resistance change of the composite is dominated by ∆ [12], and the resistance due to the tunneling effect can be expressed as follows in Equation (7) [31]: = √2 (7) where h is Planck’s constant, d is the spacing between fillers, A is the cross-sectional area of the tunnel, e is the quantum of electricity, m is the mass of electrons, and is the height of the matrix barrier. The amount of change in the tunneling resistance is generated expo- nentially by the applied strain of each single unit part. Accordingly, the amount of change in the tunneling resistance with respect to the strain of the entire sensor line, that is, the sensitivity to the length of the nanocarbon composite, may be expressed as follows: G. F. ∆ ∆ ∆ + + = + + +⋯ (8) = ; = =⋯= From the above hypothesis, in this study, we supposed that the NCSS is a chain series Appl. Sci. 2021, 11, 5760 6 of 10 of individual piezoresistive units, and the whole piezoresistive change is a linear summa- tion of the units. Figure 4. Unit gauge factor model of NCSS: (a) Single piezoresistive unit (top) and chain series of Figure 4. Unit gauge factor model of NCSS: (a) Single piezoresistive unit (top) and chain series of the units with linear summation (bottom) under uniaxial load; and (b) an electrical conductivity the units with linear summation (bottom) under uniaxial load; and (b) an electrical conductivity schematic concept model of nanocomposite. schematic concept model of nanocomposite. As strains happen across all of the units of the NCSS, the gauge factor with respect to the strain can be expressed as follows. DR DR DR S S S 0 1 2 R R R S S S 0 1 2 G.F. = + + (4) # # # S S S 0 2 In addition, the electrical resistance model of the nanocarbon composite, shown in Figure 4b, can be expressed as follows: R = R + R + R (5) contact CNTcomposites f iller tunnel where R is the electrical resistance of the fillers in the composite, R is the resistance f iller tunnel between the fillers inside the composite, and R is the contact resistance between fillers. contact Substituting the above equation into the gauge factor equation, the gauge factor of the nanocarbon composite sensor can be expressed as follows. DR +DR +DR contact f iller tunnel DR R +R +R R contact N f iller tunnel G.F. = = = (6) CNTcomposites # # # Thus, the electrical resistance change of the composite is dominated by DR [12], tunnel and the resistance due to the tunneling effect can be expressed as follows in Equation (7) [31]: h d 4pd R = p exp 2ml (7) tunnel Ae 2ml where h is Planck’s constant, d is the spacing between fillers, A is the cross-sectional area of the tunnel, e is the quantum of electricity, m is the mass of electrons, and l is the height of the matrix barrier. The amount of change in the tunneling resistance is generated exponentially by the applied strain of each single unit part. Accordingly, the amount of Appl. Sci. 2021, 11, 5760 7 of 10 change in the tunneling resistance with respect to the strain of the entire sensor line, that is, the sensitivity to the length of the nanocarbon composite, may be expressed as follows: Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 9 DR DR DR tunnelS tunnelS tunnelS 1 2 R +R +R R R f illerS tunnelS contactS S S 0 0 0 1 2 G.F. = + + + CNTcomposites # # # S S S 0 1 2 (8) DR S tunneln 3.3. Improvement of Piezoresistive C = haracteristics by Usin ; R g Sensor = R Pa = tter  n = R S S S # 0 1 S n=S Based on the first experiment described in Section 3.2, we hypothesized that the sen- sitivity of the NCSS is closely related to its length because structural deformation brings From the above hypothesis, in this study, we supposed that the NCSS is a chain whole piezoresistive change from entire units of the sensor. We designed three grid sen- series of individual piezoresistive units, and the whole piezoresistive change is a linear sors via a thin polyimide film mask. The mask pattern was designed by CAD (Computer summation of the units. Aided Design) program and was fabricated by laser cutting process. The CNT ink was 3.3. Improvement of Piezoresistive Characteristics by Using Sensor Pattern manually printed through the mask on the cantilever, as shown in the lower right-hand corner of Figure 5. To secure sensor electrical stability with better conductivity, we con- Based on the first experiment described in Section 3.2, we hypothesized that the servatively used the 0.5 wt.% to fabricate the grid sensor samples. sensitivity of the NCSS is closely related to its length because structural deformation brings We measured each resistance change with respect to beam bending five times and whole piezoresistive change from entire units of the sensor. We designed three grid sensors aver via a aged thin the polyimide measured film values after mask. The elim mask in pattern ating the m was a designed ximum aby nd CAD minimum (Computer data points Aided . Fig Design) ure 5 sho program ws the res and was ults. When comp fabricated byarin laser g the am cuttingount of ch process. The ange CNT in the norm ink was alized re manually - printed through the mask on the cantilever, as shown in the lower right-hand corner of sistance with respect to the strain of the sensor samples shown in Figures 3 and 5, it can b Figur e seen t e 5.hT at o t secur he 40 e-m sensor m paelectrical ttern typestability (G.F.: 0.with 87) ha better s a la conductivity rger change in t , we h conservatively e normalized resistance used the 0.5 for wt.% the same to fabricate strain than the grid the 40- sensor mm bulk samples. type (G.F.: 0.22). Figure 5. Grid-type NCSS experiment: (a) grid-type NCSS samples and (b) sensitivity characteristic Figure 5. Grid-type NCSS experiment: (a) grid-type NCSS samples and (b) sensitivity characteristic of the grid-type NCSS with respect to length pattern change. of the grid-type NCSS with respect to length pattern change. We measured each resistance change with respect to beam bending five times and As we expected from the above, the NCSS sensitivity improvement is exactly pro- averaged the measured values after eliminating the maximum and minimum data points. portional to its incremental length ratio. We obtained higher sensitivity in cases where the Figure 5 shows the results. When comparing the amount of change in the normalized sensor had a longer length or denser pattern per unit area. We also achieved more linear, resistance with respect to the strain of the sensor samples shown in Figures 3 and 5, it can higher sensitivity and consistent data from the grid-type NCSS than from the bulk-type be seen that the 40-mm pattern type (G.F.: 0.87) has a larger change in the normalized sensor. Additionally, we deduced that NCSS strain output may be expressed by the sum- resistance for the same strain than the 40-mm bulk type (G.F.: 0.22). mation of the entire sensor covered surface. According to our literature survey, the strain As we expected from the above, the NCSS sensitivity improvement is exactly pro- sensitivity of a conventional foil strain gauge is only related to its material properties and portional to its incremental length ratio. We obtained higher sensitivity in cases where is not much affected by its length [32], which is a notable piezoresistive feature of the the sensor had a longer length or denser pattern per unit area. We also achieved more NCSS. We conclude that since NCSSs are affected by the overall strain change in a given linear, higher sensitivity and consistent data from the grid-type NCSS than from the bulk- direction, the geometric pattern design is one of the most important variables in axial load type sensor. Additionally, we deduced that NCSS strain output may be expressed by the and deformation measurements. summation of the entire sensor covered surface. According to our literature survey, the 4. Conclusions In this study, the piezoresistive characteristics of NCSSs were experimentally studied in terms of the sensor’s geometric length to determine its sensitivity to the design. We fabricated bulk- and grid-type sensors with different filler contents (wt.%) and different sensor lengths. Appl. Sci. 2021, 11, 5760 8 of 10 strain sensitivity of a conventional foil strain gauge is only related to its material properties and is not much affected by its length [32], which is a notable piezoresistive feature of the NCSS. We conclude that since NCSSs are affected by the overall strain change in a given direction, the geometric pattern design is one of the most important variables in axial load and deformation measurements. 4. Conclusions In this study, the piezoresistive characteristics of NCSSs were experimentally studied in terms of the sensor ’s geometric length to determine its sensitivity to the design. We fabricated bulk- and grid-type sensors with different filler contents (wt.%) and different sensor lengths. In the first experiment, we used silicone molds to prepare samples with three different lengths (30, 40, 50 mm) and two different filler compositions (0.35 and 0.5 wt.%). We measured the sensor resistance change with respect to the bending strain variation of a cantilever. For both compositions, we obtained higher sensitivity at longer sensor lengths, which suggested the possibility of using the geometric design to control the sensitivity. We deduced that sensor length may correlate with sensitivity, and we proposed a unit gauge factor model of NCSSs to explain the proportional relationship between their length and sensitivity. We supposed that the NCSS is a chain series of individual piezoresistive units and that the whole piezoresistive change can be a linear summation of these units. To verify our hypothesis, we performed a second experiment with more sophisticated samples. We designed grid-type sensors by using thin polyimide film masks. We printed the grid- type NCSS (0.5 wt.% filler) on a cantilever and repeated the same test. Results indicated that the improvement in NCSS sensitivity was exactly proportional to its incremental length ratio (because structural deformation brings piezoresistive change from whole units of the sensor). We were able to obtain higher sensitivity in the cases where the sensor had a longer length and denser pattern per unit area. We also achieved more linear and consistent data from the grid-type NCSS than from the bulk-type sensor. From the analysis, we concluded that the greater sensor length brings a greater change in resistance due to its piezoresistive unit summation. Eventually, the sensitivity of NCSSs directly relates to their length, and the patternized length can be used to easily control the sensor sensitivity. We also found that the fine sensor pattern can improve its performance, allowing better results to be achieved. Sensor geometric pattern design is one of the most important aspects of axial load and deformation measurements. For further study, we are studying advanced sensitivity design considering the three-dimensional pattern variables of the NCSS as well. Author Contributions: Conceptualization: S.-Y.K., B.-G.C., and I.K.; methodology: S.-Y.K., B.-G.C., C.-J.K., Y.-S.J., J.-S.J., J.-H.S., and I.K.; formal analysis: S.-Y.K. and B.-G.C.; investigation: S.-Y.K. and B.-G.C.; data curation: S.-Y.K., B.-G.C., and G.-W.O.; writing—original draft preparation: S.-Y.K. and B.-G.C.; writing—review and editing: C.-J.K., Y.-S.J., J.-S.J., K.-Y.J., and J.-H.S.; visualization: S.-Y.K., B.-G.C., and G.-W.O.; project administration: I.K.; funding acquisition: I.K. All authors have read and agreed to the published version of the manuscript. 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Journal

Applied SciencesMultidisciplinary Digital Publishing Institute

Published: Jun 22, 2021

Keywords: carbon nanotube; strain sensor; piezoresistive mechanism; sensor pattern design

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