Fabrication and Characterization of Flexible Thermoelectric Generators Using Micromachining and Electroplating Techniques

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Article Fabrication and Characterization of Flexible Thermoelectric Generators Using Micromachining and Electroplating Techniques

Wnag-Lin Lee 1, Po-Jen Shih 2, Cheng-Chih Hsu 3 and Ching-Liang Dai 1,* 1 Department of Mechanical Engineering, National Chung Hsing University, Taichung 402, Taiwan; [email protected] 2 Department of Biomedical Engineering, National Taiwan University, Taipei 106, Taiwan; [email protected] 3 Department of Electro-Optical Engineering, National United University, Miaoli 360, Taiwan; [email protected] * Correspondence: [email protected]; Tel.: +886-4-2284-0433

Received: 20 August 2019; Accepted: 29 September 2019; Published: 30 September 2019

Abstract: This study involves the fabrication and measurement of a flexible thermoelectric generator (FTG) using micromachining and electroplating processes. The area of the FTG is 46 17 mm2, and × it is composed of 39 thermocouples in series. The thermoelectric materials that are used for the FTG are copper and nickel. The fabrication process involves patterning a silver seed layer on the polymethyl methacrylate (PMMA) substrate using a computer numerical control (CNC) micro-milling machine. Thermoelectric materials, copper and nickel, are deposited on the PMMA substrate using an electroplating process. An epoxy polymer is then coated onto the PMMA substrate. Acetone solution is then used to etch the PMMA substrate and to transfer the thermocouples to the flexible epoxy film. The FTG generates an output voltage (OV) as the thermocouples have a temperature difference (∆T) between the cold and hot parts. The experiments show that the OV of the FTG is 4.2 mV at ∆T of 5.3 K and the output power is 429 nW at ∆T of 5.3 K. The FTG has a voltage factor of 2 2 2 1 1 µV/mm K and a power factor of 19.5 pW/mm K . The FTG reaches a curvature of 20 m− .

Keywords: thermoelectric generator; ﬂexibility; micromachining; electroplating; thermocouple

1. Introduction Many recent studies have focused on the development of thermoelectric generators [1–8]. Thermoelectric generators are used to convert waste heat into electrical power, which is a green energy. It is difficult to use a non-flexible thermoelectric generator for a heat source with a curved surface because it does not fit the curved surface. However, a flexible thermoelectric generator can be used for a heat source with a curved surface and has more applications than a non-flexible thermoelectric generator. Flexible thermoelectric generators can be used as a wearable device to fit the curves of human skin. Various fabrication methods were used for thermoelectric generators. For instance, Lee [9] used a screen printing method to produce a thermoelectric generator using the thermoelectric materials, ZnSb and CoSb3, on an alumina substrate. The output voltage (OV) for the thermoelectric generator was 10 mV at ∆T of 50 K, and its output power per unit area was 1.7 µW/mm2 at temperature difference (∆T) of 50 K. The power factor of the thermoelectric generator was 0.68 pW/mm2K2. Phaga [10] developed a thermoelectric generator that had 31 thermocouples in series. The thermocouple materials were p-Ca3Co4O9 and n-CaMnO3 that were deposited by the solid state reaction method. The area of the thermoelectric generator was 6.45 cm2. The OV for the thermoelectric generator was 121.7 mV at ∆T of 140 K and the output power was 1.47 µW at ∆T of 140 K. The voltage factor

Micromachines 2019, 10, 660; doi:10.3390/mi10100660 www.mdpi.com/journal/micromachines Micromachines 2019, 10, 660 2 of 10 for the thermoelectric generator was 1.35 µV/mm2K and the power factor was 0.16 pW/mm2K2. Big-Alabo [11] produced a thermoelectric generator using low pressure chemical vapor deposition and a micro-fabrication process [12]. Ge and SiGe were the thermoelectric materials. The thermoelectric generator had an estimated power density of 111 nW/cm2 and a power factor of 0.0035 µW/cm2K2. Itoigawa [13] proposed a flexible thermoelectric generator that was fabricated on a polyimide substrate using a micro-fabrication process. The thermoelectric materials for the generator were copper and nickel. The generator contained 380 thermocouples in series, and its area was about 50 50 mm2. × The generator had a bending radius of curvature of 9 mm. The OV and output power were 16 µV/K per thermocouple and 4.1 pW/K2 per thermocouple, respectively. Therefore, the thermoelectric generator had a voltage factor of 2.43 µV/mm2K and a power factor of 0.64 pW/mm2K2. A flexible thermoelectric generator, developed by Lu [14], was fabricated using physical mixing and drop casting processes. The thermoelectric materials for the generator were Te/poly(3,4-ethylenedioxythiophene) and poly(styrenesulfonate) /Cu7Te4 ternary composite films. The flexible thermoelectric generator (FTG) consisted of eight thermopiles with Ag paste and the area was about 25 40 mm2. The OV × for the generator was 31.2 mV at ∆T of 39 K and the maximum output power was 94.7 nW at ∆T of 39 K. The FTG had a power factor of 112.3 µW/mK2. Ding [15] manufactured a flexible thermoelectric generator on a nylon membrane. The thermoelectric material of the generator was n-type Ag2Se. The FTG contained four legs of the Ag2Se film that were connected with Ag paste on a nylon substrate and the area of the FTG was about 20 20 mm2. The OV and the maximum output power for × the FTG were 18 mV and 460 nW, respectively, at ∆T of 30 K. The voltage factor, the FTG was 1.5 µV/mm2K and the power factor was 1.2 pW/mm2K2. Selvan [16] manufactured a thermoelectric generator on a polyimide substrate using a microfabrication process. Cobalt and copper were used as negative and positive thermoelectric materials for the thermoelectric generator. The generator was a flexible sandwiched planar structure. The power factor for the thermoelectric generator was 6.6 10 3 µW/cm2K2 at ∆T of 44.2 K. A flexible thermoelectric generator, presented by Jo [17], was × − made on a polydimethylsiloxane substrate to harvest heat energy from a human body. The materials for the thermocouples in the generator were n-type and p-type Bi2Te3, which were manufactured using dispenser printing. The area of the thermoelectric generator was 50 50 mm2. The OV for the × generator was 7 mV at ∆T of 19 K and the output power was 2.1 µW at ∆T of 19 K. The thermoelectric generator had a voltage factor of 0.15 µV/mm2K and an output power of 2.33 pW/mm2K2. Oh [18] proposed a flexible thermoelectric generator for self-powered wearable electronics. The materials for the FTG were p-type NbSe2 and n-type WS2 nanosheet films. The flexible thermoelectric generator produced an output power of 38 nW at the ∆T of 60 K and the performance was stable after 100 bending cycles and 100 stretching cycles at a 50% strain. Kim [19] used an optimized composite film with tungsten disulfide nanosheets and single wall carbon nanotubes to fabricate a flexible thermoelectric generator on a rubber substrate with pre-strain. The FTG kept its performance after 10,000 stretching cycles at a 30% strain. These thermoelectric generators [13–17] are flexible and have more applications than non-flexible thermoelectric generators [9–11]. This study uses a low cost electroplating process that allows easy fabrication to manufacture a flexible thermoelectric generator on an epoxy substrate. The power factor for the FTG exceeds that for the FTG’s that were developed by Itoigawa [13], Lu [14], Ding [15], and Jo [17].

2. Design for the Thermoelectric Generator Figure1 shows the schematic structure of the flexible thermoelectric generator. The FTG is composed of 39 thermocouples in series. Each of the thermocouple is made of copper and nickel. The thermocouples have a hot part and a cold part. Each thermocouple is 7 mm long, 0.5 mm wide, and 0.1 mm thick. The area of the FTG is 46 17 mm2. The FTG uses the Seebeck effect to generate an × output voltage if the hot and cold parts of the thermocouples have different temperatures. The output voltage for the FTG is given by [20], U = m(αc αn)∆T (1) 0 − Micromachines 2019, 10, x 3 of 10

MicromachinesMicromachines2019 2019,,10 10,, 660x 33 ofof 1010 where U0 represents the OV for the FTG, m is the number of thermocouples in the FTG, αc is the Seebeck coefficient for copper, αn is the Seebeck coefficient for nickel and ΔT is temperature difference where U0 represents the OV for the FTG, m is the number of thermocouples in the FTG, αc is the wherebetweenU therepresents hot part the and OV cold for theparts FTG, of mtheis thermocouples. the number of thermocouples Equation (1) shows in the FTG, that αthatis thethe SeebeckOV for Seebeck0 coefficient for copper, αn is the Seebeck coefficient for nickel and ΔT is temperaturec difference the FTG is proportional to the number of thermocouples (m), the difference in the Seebeck coefficients coebetweenfficient the for hot copper, part αandn is cold the Seebeck parts of coe theffi cientthermocouples. for nickel and Equation∆T is temperature (1) shows that diff erencethat the between OV for (αc − αn) for the thermocouple materials, and ΔT for the thermocouples [21]. If there is an increase in thethe hotFTG part is proportional and cold parts to the of thenumber thermocouples. of thermocouples Equation (m), (1) the shows difference that that in the the Seebeck OV for coefficients the FTG is the three parameters, m, αc − αn and ΔT, the OV for the FTG increases. To increase the temperature proportional to the number of thermocouples (m), the difference in the Seebeck coefficients (αc αn) (αc − αn) for the thermocouple materials, and ΔT for the thermocouples [21]. If there is an increase− in differencefor the thermocouple between the materials, hot and and cold∆ Tpartsfor the of thermocouplesthermocouples, [21 the]. Iflength there isof anthe increase thermocouples in the three is the three parameters, m, αc − αn and ΔT, the OV for the FTG increases. To increase the temperature extended to 7 mm. In the design, thermocouple materials are copper and nickel. The Seebeck parameters,difference betweenm, αc αthen and hot∆ Tand, the cold OV parts for the of FTG thermocouples, increases. To the increase length the of temperature the thermocouples difference is coefficient of copper− is 1.83 μV/K, and the Seebeck coefficient of nickel is −19.5 μV/K [22]. The betweenextended the to hot 7 mm. and cold In the parts design, of thermocouples, thermocouple the lengthmaterials of the are thermocouples copper and nickel. is extended The toSeebeck 7 mm. thermocoupleIn the design, number thermocouple of the materialsFTG is 39. areThe copper values and m = nickel.39 and Theαc − Seebeckαn = 21.33 coe μV/Kfficient are ofsubstituted copper is coefficient of copper is 1.83 μV/K, and the SeebeckΔ coefficient of nickel is −19.5 μV/K [22]. The 1.83into µEquationV/K, and (1), the the Seebeck relation coe betweenfficient of the nickel OV isand19.5 T µofV /theK[ 22FTG]. The is obtained. thermocouple Figure number 2 shows of the thermocouple number of the FTG is 39. The values m = 39 and αc − αn = 21.33 μV/K are substituted simulated OV of the FTG. In this computation, the− temperature difference of thermocouples changes FTGinto isEquation 39. The (1), values the mrelation= 39 and betweenαc α nthe= 21.33OV andµV/ KΔT are of substitutedthe FTG is intoobtained. Equation Figure (1), 2 the shows relation the from zero to 5.5 K. As shown in Figure− 2, the change between the OV and ΔT was linear relation. The betweensimulated the OV OV ofand the ∆FTG.T of In the this FTG computation, is obtained. th Figuree temperature2 shows thedifference simulated of thermocouples OV of the FTG. changes In this simulated OV of the FTG was 4.6 mV at ΔT of 5.5 K. computation,from zero to 5.5 the K. temperature As shown in di Figurefference 2, ofthe thermocouples change between changes the OV from and zeroΔT was to 5.5linear K. Asrelation. shown The in Figuresimulated2, the OV change of the between FTG was the 4.6 OV mV and at ΔT∆ Tofwas 5.5 K. linear relation. The simulated OV of the FTG was 4.6 mV at ∆T of 5.5 K.

Figure 1. Schematic structure of the flexible thermoelectric generator. Figure 1. Schematic structure of the ﬂexible thermoelectric generator. Figure 1. Schematic structure of the flexible thermoelectric generator.

Figure 2. Simulated output voltage for the ﬂexible thermoelectric generator. Figure 2. Simulated output voltage for the flexible thermoelectric generator. The output power of the FTG depends on the internal resistance and the external load. When the Figure 2. Simulated output voltage for the flexible thermoelectric generator. internalThe resistanceoutput power of the of the FTG FTG equals depends its external on the inte load,rnal the resistance FTG has and a maximum the external output load. power.When theThe internal maximum resistance output powerof the ofFTG the equals FTG is its given external by [23 load,], the FTG has a maximum output power. The output power of the FTG depends on the internal resistance and the external load. When The maximum output power of the FTG is given by [23], the internal resistance of the FTG equals its external load,2 the FTG has a maximum output power. U0 The maximum output power of the FTG is givenPmax =by [23], (2) 4Rg

Micromachines 2019, 10, x 4 of 10

Micromachines 2019, 10, x 4 of 10 U 2 P = 0 (2) max 4R2 Micromachines 2019, 10, 660 U0g 4 of 10 Pmax = (2) where Pmax is the maximum output power for the FTG,4R Ug 0 is the OV for the FTG, and Rg is the internal resistance of the FTG. Equation (2) shows that the maximum output power of the FTG is proportional where PPmaxmax isis the the maximum maximum output output power for the FTG,FTG, U00 is the OV for the FTG, and Rgg isis the internal toresistance the square ofof the of the FTG. OV Equation and is inversely (2) shows proportional that thethe maximum to the internal output power resistance of the [24]. FTG If is the proportional OV for the FTGto the is square squareincreased of of the theand OV OVthe and andinternal is is inversely inversely resistance proportional proportional is decreased, to to the the the internal maximum internal resistance resistance output [24].power [24 If]. theincreases. If theOV OVfor The the for maximumtheFTG FTG is increased is output increased and power andthe internalfor the the internal FTG resistance resistanceis calculated. is decreased, is decreased, The internal the maximum the resistance maximum output of output the power FTG power increases.is assumed increases. The to beThemaximum 10.3 maximum Ω. Theoutput value output power R powerg = 10.3for thefor Ω and theFTG FTGthe is calculated.simulated is calculated. OVThe The ininternal Figure internal resistance 2 are resistance substituted of the of the FTG into FTG is Equation assumed is assumed (2). to The maximum output power for the FTG is calculated and the simulated results are shown in tobe be10.3 10.3 Ω.Ω The. The value value RgR =g 10.3= 10.3 Ω Ωandand the the simulated simulated OV OV in in Figure Figure 22 are are substituted substituted into into Equation (2). FigureThe maximummaximum 3. The temperature output output power power difference for for the the FTG forFTG is the calculated is thermocouples calculated and theand simulatedchanges the simulated from results zero results are to shown 5.5 are K. inshown Figure Figure in33. showsTheFigure temperature that3. The the temperaturerelationship diﬀerence forbetweendifference the thermocouples the for output the thermocouples po changeswer and from temperature changes zero to 5.5from difference K. zero Figure to is3 5.5showsnonlinear. K. Figure that The the 3 Δ simulatedrelationshipshows that output the between relationship power the outputfor betweenthe powerFTG isthe and508 output nW temperature at po werT of and5.5 diﬀ K. erencetemperature is nonlinear. difference The simulatedis nonlinear. output The powersimulated for theoutput FTG power is 508 for nW the at ∆FTGT of is 5.5 508 K. nW at ΔT of 5.5 K.

Figure 3. Simulated output power for a flexible thermoelectric generator. Figure 3. Simulated output power for a flexible thermoelectric generator. Figure 3. Simulated output power for a flexible thermoelectric generator. 3. Fabrication Fabrication of of the Thermoelectric Generator 3. FabricationThe flexibleflexible of thermoelectric thethermoelectric Thermoelectric generators generators Generator were were fabricated fabricated on an epoxy on an substrate epoxy using substrate an electroplating using an electroplatingtechnique.The flexible The technique. structure thermoelectric The of structure the FTGgenerators consistedof the FTG were of consisted thermocouplesfabricated of thermocouples on inan series. epoxy in The series.substrate materials The usingmaterials for thean forthermocoupleselectroplating the thermocouples technique. were copper were The copper and structure nickel. and of nickel. Figure the FTG4 Figure illustrates consisted 4 illustrates the of thermocouples process the process flow for inflow the series. FTG. for theThe FTG. materials for the thermocouples were copper and nickel. Figure 4 illustrates the process flow for the FTG.

Figure 4. Process flow for a flexible thermoelectric generator: (a) Defining a silver seed layer; (b) Figure 4. Process flow for a flexible thermoelectric generator: (a) Defining a silver seed layer; (b) electroplating copper; (c) electroplating nickel; (d) coating epoxy polymer; (e) etching polymethyl electroplating copper; (c) electroplating nickel; (d) coating epoxy polymer; (e) etching polymethyl methacrylateFigure 4. Process substrate; flow (forf) paintinga flexible silver; thermoelectric (g) packaging. generator: (a) Defining a silver seed layer; (b) methacrylate substrate; (f) painting silver; (g) packaging. electroplating copper; (c) electroplating nickel; (d) coating epoxy polymer; (e) etching polymethyl Figuremethacrylate4a shows substrate; a seed (f layer) painting of silver silver; on (g) the packaging. PMMA substrate. The silver seed layer was coated onto the PMMA substrate, and the silver layer was patterned using a computer numerical control (CNC) micro-milling machine. Figure5 shows an image of the silver layer pattern that is defined by Micromachines 2019, 10, x 5 of 10 Micromachines 2019, 10, x 5 of 10 Micromachines 2019, 10, x 5 of 10 Figure 4a shows a seed layer of silver on the PMMA substrate. The silver seed layer was coated onto Figurethe PMMA 4a shows substrate, a seed andlayer the of silver layeron the was PMMA patterned substrate. using The a computersilver seed numerical layer was controlcoated Micromachinesonto the PMMA2019, 10 substrate,, 660 and the silver layer was patterned using a computer numerical control5 of 10 onto the PMMA substrate, and the silver layer was patterned using a computer numerical control (CNC) micro-milling machine. Figure 5 shows an image of the silver layer pattern that is defined by (CNC) micro-milling machine. Figure 5 shows an image of the silver layer pattern that is defined by the CNC micro-milling machine. Figure 4b shows that a copper layer is deposited onto the silver thelayer. CNC An micro-millingelectroplating process machine. with Figure Figure CuSO 44bb4 solution showsshows thatthat and aa a copper coppercurrent layer layerdensity isis deposited ofdeposited 4 A/dm2 onto ontofor 45 thethe min silversilver was layer. An electroplating process with CuSO4 solution and a current density of 4 A/dm2 for 45 min was layer. An electroplating process with CuSO4 solution and a current density of 4 A/dm2 for2 45 min was layer.utilized An to electroplating deposit the copper process layer with onto CuSO the4 solution silver layer. and aThe current thickness density of ofthe 4 Acopper/dm forlayer 45 minwas utilized to deposit the copper layer onto the silver layer. The thickness of the copper layer was was100 μ utilizedm. Figure to deposit4c shows the that copper a nickel layer layer onto is the electr silveroplated layer. onto The the thickness silver layer. of the An copper electroplating layer was 100process µμm. withFigure NiSO 44cc showsshows4 solution thatthat and aa nickelnickel a current layerlayer density isis electroplatedelectr ofoplated 4 A/dm ontoonto2 for thethe 45silver silvermin was layer.layer. employed AnAn electroplatingelectroplating to deposit process with NiSO4 solution and a current density of 4 A/dm2 for 45 min was employed to deposit process with NiSO4 solution and a current density of 4 A/dm2 for 45 min was employed to deposit processthe nickel with layer NiSO onto4 solution the silver and layer. a current The thickness density of the 4 A /nickeldm for layer 45 minwas was100 μ employedm. Figure to4d deposit shows the nickel layer onto the silver layer. The thickness of the nickel layer was 100 µμm. Figure 4d shows thethat nickel an epoxy layer is ontocoated the onto silver the layer. PMMA The substrate. thickness Epoxy of the is nickel used layerbecause was the 100 PMMAm. Figure substrate4d shows is not that an epoxy is coated onto the PMMA substrate. Epoxy is used because the PMMA substrate is not thatflexible an epoxyso epoxy is coated is used onto to the replace PMMA the substrate. PMMA Epoxysubstrate. is used Figure because 4e shows the PMMA etching substrate the PMMA is not flexible so epoxy is used to replace the PMMA substrate. Figure 4e shows etching the PMMA flexiblesubstrate. so The epoxy FTG isused was immersed to replace thein an PMMA acetone substrate. solution Figure for 304 emin shows to etch etching the PMMA the PMMA substrate substrate. and substrate. The FTG was immersed in an acetone solution for 30 min to etch the PMMA substrate and Thestructure FTG wasof the immersed thermocouples in an acetone was transferred solution for to 30 th mine epoxy to etch substrate. the PMMA Figure substrate 4f shows and structure that silver of structure of the thermocouples was transferred to the epoxy substrate. Figure 4f shows that silver thepaint thermocouples connects the nickel was transferred and the copper to the to epoxy form substrate. the thermocouples Figure4f showsin series. that Figure silver 4g paint shows connects that a paint connects the nickel and the copper to form the thermocouples in series. Figure 4g shows that a thethin nickel epoxy and layer the is coppercoated toonto form the the FTG thermocouples to protect the inthermocouples. series. Figure 4Thisg shows layer thatprevents a thin damage epoxy thin epoxy layer is coated onto the FTG to protect the thermocouples. This layer prevents damage layerfrom dust is coated and steam. onto the Figure FTG to6 shows protect an the image thermocouples. of the flexible This thermoelectric layer prevents generator. damage Figure from dust 7 shows and from dust and steam. Figure 6 shows an image of the flexible thermoelectric generator. Figure 7 shows steam.that the Figure FTG is6 showsflexible. an image of the flexible thermoelectric generator. Figure7 shows that the FTG thatis flexible. the FTG is flexible.

Figure 5. The pattern of the silver seed layer. Figure 5. The pattern of the silver seed layer. Figure 5. TheThe pattern pattern of the silver seed layer.

Figure 6. An image of the flexible thermoelectric generator. Figure 6. An image of the flexible ﬂexible thermoelectric generator. Figure 6. An image of the flexible thermoelectric generator.

Figure 7. Demonstration of the flexibility ﬂexibility of the ﬂexibleflexible thermoelectric generator. Figure 7. Demonstration of the flexibility of the flexible thermoelectric generator. Figure 7. Demonstration of the flexibility of the flexible thermoelectric generator.

Micromachines 2019, 10, x 6 of 10 Micromachines 2019, 10, 660 6 of 10 4.Micromachines Results and 2019 Discussion, 10, x 6 of 10

4.4. ResultsResultsA heater, andand a DiscussionDiscussion cooler, a power supply, an infrared thermometer and a digital multimeter were used to test the OV for the FTG. The heater was placed at the hot part of the thermocouples and the cooler AA heater,heater, a a cooler, cooler, a a power power supply, supply, an an infrared infrared thermometer thermometer and and a digitala digital multimeter multimeter were were used used to was placed at the cold part of the thermocouples. The power supply provided power to the heater testto test the the OV OV for for the the FTG. FTG. The The heater heater was was placed placed at the at hotthe parthot part of the of thermocouplesthe thermocouples and theand cooler the cooler was and cooler. The heater acted as a heat source for the hot part of the thermocouples and the cooler placedwas placed at the at cold the partcold ofpart the of thermocouples. the thermocouples. The powerThe power supply supply provided provided power power to the to heater the heater and acted as a heat sink for the cold part of the thermocouples. An infrared thermometer was used to cooler.and cooler. The heater The heater acted asacted a heat as sourcea heat forsource the hotfor partthe hot of the part thermocouples of the thermocouples and the cooler and the acted cooler as a measure the temperature difference between the hot and cold parts of the thermocouples. A digital heatacted sink as a for heat the sink cold for part the of cold the thermocouples. part of the thermocouples. An infrared thermometerAn infrared thermometer was used to measurewas used the to multimeter recorded the OV for the FTG. temperaturemeasure the ditemperaturefference between difference the hotbetween and cold the hot parts and of cold the thermocouples. parts of the thermocouples. A digital multimeter A digital Figure 8 shows the measured OV for the flexible thermoelectric generator. The temperature recordedmultimeter the recorded OV for the the FTG. OV for the FTG. difference of the thermocouples in the FTG was varied between zero and 5.3 K. The results show that FigureFigure8 8shows shows the the measured measured OV OV for for the the flexible flexible thermoelectric thermoelectric generator. generator. The The temperature temperature the OV for the FTG is 2.6 mV at ΔT of 5.3 K. The internal resistance was measured using a digital didifferencefference ofof thethe thermocouplesthermocouples inin thethe FTGFTG waswas variedvaried betweenbetween zerozero andand 5.35.3 K.K. TheThe resultsresults showshow thatthat multimeter. The value was 10.3 Ω. Equation (2) is used to calculate the maximum output power for thethe OVOV forfor thethe FTGFTG isis 2.62.6 mVmV atat ∆ΔTT ofof 5.35.3 K.K. TheThe internalinternal resistanceresistance waswas measuredmeasured usingusing aa digitaldigital the FTG. The internal resistance Rg = 10.3 Ω and the measured results for the FTG OV in Figure 8 are multimeter.multimeter. TheThe valuevalue waswas 10.310.3 ΩΩ.. EquationEquation (2)(2) isis usedused toto calculatecalculate thethe maximummaximum outputoutput powerpower forfor substituted into Equation (2) to calculate the maximum output power. Figure 9 shows the measured thethe FTG.FTG. TheThe internalinternal resistanceresistanceR Rg == 10.3 ΩΩ and the measured results forfor thethe FTGFTG OVOV inin FigureFigure8 8 are are maximum output power for the flexibleg thermoelectric generator. The measured maximum output substitutedsubstituted intointo EquationEquation (2)(2) toto calculatecalculate thethe maximummaximum outputoutput power.power. FigureFigure9 9 shows shows the the measured measured power for the FTG was 165 nW at ΔT of 5.3 K. maximummaximum outputoutput powerpower forfor thethe flexibleflexible thermoelectricthermoelectric generator.generator. TheThe measuredmeasured maximummaximum outputoutput powerpower forfor thethe FTGFTG waswas 165165 nWnW atat∆ ΔTT ofof 5.35.3 K.K.

Figure 8. Measurements for the output voltage of the flexible thermoelectric generator. Figure 8. Measurements for the output voltage of the ﬂexible thermoelectric generator. Figure 8. Measurements for the output voltage of the flexible thermoelectric generator.

Figure 9. Measurements for the output power of the ﬂexible thermoelectric generator. Figure 9. Measurements for the output power of the flexible thermoelectric generator. As shown in Figures2 and8, the simulated OV for the FTG is 4.3 mV at ∆T of 5.3 K and the measuredAs shown OVFigure is in 2.6 9.Figures mV Measurements at ∆ 2T andof 5.3 8, for K. the the A comparisonsimulated output power OV of of thefor th e simulatedthe flexible FTG thermoelectric is and 4.3 measuredmV at generator.ΔT results of 5.3 showsK and that the themeasured OV for OV the FTGis 2.6 has mV an at error ΔT of percentage 5.3 K. A comparison of 40%. As shownof the si inmulated Figures 3and and measured9, this error results results shows in a As shown in Figures 2 and 8, the simulated OV for the FTG is 4.3 mV at ΔT of 5.3 K and the largethat the diﬀ OVerence for the between FTG has the an simulated error percentage and the measured of 40%. As maximum shown in output Figures power. 3 and The9, this thermoelectric error results measured OV is 2.6 mV at ΔT of 5.3 K. A comparison of the simulated and measured results shows that the OV for the FTG has an error percentage of 40%. As shown in Figures 3 and 9, this error results

Micromachines 2019, 10, x 7 of 10 Micromachines 2019, 10, 660 7 of 10 in a large difference between the simulated and the measured maximum output power. The thermoelectric properties of the copper and nickel in the thermocouples depend on the current properties of the copper and nickel in the thermocouples depend on the current density during density during the electroplating process. To reduce the error and improve the thermoelectric the electroplating process. To reduce the error and improve the thermoelectric properties of the properties of the thermocouples, this study uses different values of current density to electroplate the thermocouples, this study uses different values of current density to electroplate the copper and nickel copper and nickel for the FTG and measures the OV. Figure 10 shows the measured OV for the for the FTG and measures the OV. Figure 10 shows the measured OV for the flexible thermoelectric flexible thermoelectric generator for different electroplating current densities. Four current densities generator for different electroplating current densities. Four current densities are used to electroplate are used to electroplate the copper and nickel onto the FTG: 0.5 A/dm2, 1 A/dm2, 2 A/dm2 and 4 A/dm2. the copper and nickel onto the FTG: 0.5 A/dm2, 1 A/dm2, 2 A/dm2 and 4 A/dm2. The results show The results show that the OV for the FTG increases as the current density that is used for that the OV for the FTG increases as the current density that is used for electroplating is decreased. electroplating is decreased. As shown in Figure 10, the FTG produces the greatest OV at a current As shown in Figure 10, the FTG produces the greatest OV at a current density of 0.5 A/dm2. The results density of 0.5 A/dm2. The results (0.5 A/dm2) are in agreement with the simulation results. At a (0.5 A/dm2) are in agreement with the simulation results. At a current density of 0.5 A/dm2, the FTG current density of 0.5 A/dm2, the FTG produces an OV of 4.2 mV for a value of ΔT of 5.3 K. The voltage produces an OV of 4.2 mV for a value of ∆T of 5.3 K. The voltage factor for the FTG was 1 µV/mm2K. factor for the FTG was 1 μV/mm2K. The values U0 = 4.2 mV, ΔT = 5.3 K and m = 39 are substituted into The values U0 = 4.2 mV, ∆T = 5.3 K and m = 39 are substituted into Equation (1) to evaluate the Equation (1) to evaluate the difference in the Seebeck coefficient for the thermocouple materials. The difference in the Seebeck coefficient for the thermocouple materials. The result shows that the difference result shows that the difference in the Seebeck coefficients (αc − αn) for the thermocouple materials is in the Seebeck coefficients (αc αn) for the thermocouple materials is 20.3 µV/K. 20.3 μV/K. −

Figure 10. Output voltage of the flexible thermoelectric generator at different current density. Figure 10. Output voltage of the flexible thermoelectric generator at different current density. The results in Figure 10 are substituted into Equation (2) to calculate the maximum output power for theThe FTG. results Figure in Figure 11 shows 10 are the substituted maximum into output Equa powertion (2) for to the calculate flexible the thermoelectric maximum output generator power for forvarious the FTG. values Figure of current 11 shows density the for maximum electroplating. output The power results for show the flexible that the thermoelectric maximum output generator power for thevarious FTG values increases of current as the current density density for electrop of electroplatinglating. The isresults decreased. show Asthat shown the maximum in Figure output11, the powerFTG produces for the theFTG greatest increases output as the power current at a currentdensity densityof electroplating of 0.5 A/dm is 2decreased.. The results As (0.5 shown A/dm in2) 2 Figureare in agreement 11, the FTG with produces the simulation the greatest results. output At apower current at densitya current of density 0.5 A/dm of2 0.5, the A/dm FTG. produces The results an (0.5 A/dm2) are in agreement with the simulation results. At a current density of 0.5 A/dm2, the FTG2 2 outputMicromachines power 2019 of, 42910, x nW for a value of ∆T of 5.3 K. The power factor for the FTG was 19.5 pW/mm8 ofK 10. produces an output power of 429 nW for a value of ΔT of 5.3 K. The power factor for the FTG was 19.5 pW/mm2K2. The relationship between the curvature and resistance of the FTG is used to characterize the flexibility of the FTG. A digital multimeter was used to record the change in the resistance of the FTG when a force is applied to change its curvature. Figure 12 shows the relationship between the curvature and the resistance of the FTG. For a curvature of less than 10 m−1, the resistance of the FTG remains constant at 10.3 Ω. The resistance of the FTG increases from 10.3 to 330 Ω as the curvature of the FTG increase from 10 to 20 m−1. The limit of curvature for the FTG is 20 m−1, which corresponds to a radius of curvature that is 50 mm.

Figure 11. Output power of the ﬂexible thermoelectric generator at diﬀerent current density. Figure 11. Output power of the flexible thermoelectric generator at different current density.

Figure 12. Relationship between resistance and curvature for the FTG.

Table 1 lists the performance for various thermoelectric generators. These thermoelectric generators, which were fabricated by Itoigawa [13], Lu [14], Ding [15], Selvan [16], and Jo [17], were flexible. As shown in Table 1, the power factor for the FTG that was produced by Selvan [16] is 66 pW/mm2K2 which is the greatest power factor. The power factor for the FTG that was produced by Lu [14] is 0.064 pW/mm2K2, which is the lowest power factor. The power factor for the FTG that is produced by this study is 19.5 pW/mm2K2, which is a higher figure than that for the FTG’s that were produced by Itoigawa [13], Lu [14], Ding [15], and Jo [17].

Table 1. Performances of various thermoelectric generators.

Flexibility Voltage Factor Power Factor Authors (μV/mm2K) (pW/mm2K2) Lee [9] No - 0.68 Phaga [10] No 1.35 0.16 Big-Alabo [11] No - 35 Itoigawa [13] Yes 2.43 0.64 Lu [14] Yes 0.8 0.064 Ding [15] Yes 1.5 1.2 Selvan [16] Yes - 66 Jo [17] Yes 0.15 2.33 This work Yes 1 19.5

Micromachines 2019, 10, x 8 of 10

Micromachines 2019, 10, 660 8 of 10

The relationship between the curvature and resistance of the FTG is used to characterize the ﬂexibility of the FTG. A digital multimeter was used to record the change in the resistance of the FTG when a force is applied to change its curvature. Figure 12 shows the relationship between the curvature 1 and the resistance of the FTG. For a curvature of less than 10 m− , the resistance of the FTG remains constant at 10.3 Ω. The resistance of the FTG increases from 10.3 to 330 Ω as the curvature of the FTG increase from 10 to 20 m 1. The limit of curvature for the FTG is 20 m 1, which corresponds to a radius Figure 11. Output− power of the flexible thermoelectric generator at− different current density. of curvature that is 50 mm.

Figure 12. Relationship between resistance and curvature for the FTG. Figure 12. Relationship between resistance and curvature for the FTG. Table1 lists the performance for various thermoelectric generators. These thermoelectric generators, Table 1 lists the performance for various thermoelectric generators. These thermoelectric which were fabricated by Itoigawa [13], Lu [14], Ding [15], Selvan [16], and Jo [17], were flexible. generators, which were fabricated by Itoigawa [13], Lu [14], Ding [15], Selvan [16], and Jo [17], were As shown in Table1, the power factor for the FTG that was produced by Selvan [ 16] is 66 pW/mm2K2 flexible. As shown in Table 1, the power factor for the FTG that was produced by Selvan [16] is which is the greatest power factor. The power factor for the FTG that was produced by Lu [14] is 66 pW/mm2K2 which is the greatest power factor. The power factor for the FTG that was produced 0.064 pW/mm2K2, which is the lowest power factor. The power factor for the FTG that is produced by by Lu [14] is 0.064 pW/mm2K2, which is the lowest power factor. The power factor for the FTG that is this study is 19.5 pW/mm2K2, which is a higher figure than that for the FTG’s that were produced by produced by this study is 19.5 pW/mm2K2, which is a higher figure than that for the FTG’s that were Itoigawa [13], Lu [14], Ding [15], and Jo [17]. produced by Itoigawa [13], Lu [14], Ding [15], and Jo [17]. Table 1. Performances of various thermoelectric generators. Table 1. Performances of various thermoelectric generators. Authors Flexibility Voltage Factor (µV/mm2K) Power Factor (pW/mm2K2) Flexibility Voltage Factor Power Factor Authors Lee [9] No -(μV/mm2K) (pW/mm2K2) 0.68 Phaga [10] No 1.35 0.16 Big-Alabo [11]Lee [9] No No - - 0.68 35 Itoigawa [13]Phaga Yes[10] No 2.43 1.35 0.16 0.64 Lu [14] Yes 0.8 0.064 Ding [15]Big-Alabo Yes [11] No 1.5 - 35 1.2 Selvan [16]Itoigawa Yes [13] Yes - 2.43 0.64 66 Jo [17] Yes 0.15 2.33 This workLu [14] Yes Yes 1 0.8 0.064 19.5 Ding [15] Yes 1.5 1.2 The internal resistanceSelvan [16] of the thermoelectric Yes generator - that was fabricated 66 by Peng [21] was 8 kΩ, and the internal resistanceJo [17] of the thermoelectric Yes generator 0.15 that was produced 2.33 by Yang [23] was 8 kΩ. The thermoelectric generatorThis work that was produced Yes by Kao 1 [24] had an internal 19.5 resistance of 2.45 kΩ. The internal resistance of the FTG that is produced by this study is 10.3 kΩ, which is a lower figure than those for the thermoelectric generators that were produced by Peng [21], Yang [23], and Kao [24].

5. Conclusions A ﬂexible thermoelectric generator was fabricated on an epoxy substrate using an electroplating process. The FTG contained 39 thermocouples in series. These thermocouples were composed of the thermoelectric materials, copper, and nickel. The electroplating process, which is low cost and Micromachines 2019, 10, 660 9 of 10 allows easy processing, was used to deposit the copper and the nickel. The FTG is quite ﬂexible, with a 1 maximum curvature of 20 m− . The experiments showed that the OV and maximum output power for the FTG were 4.2 mV and 429 nW, respectively, for a value of ∆T of 5.3 K. The FTG had a voltage factor of 1 µV/mm2K and a power factor of 19.5 pW/mm2K2.

Author Contributions: Conceptualization, W.-L.L. and C.-L.D.; Methodology, W.-L.L. and P.-J.S.; Validation, P.-J.S., C.-C.H. and C.L.; Formal Analysis, P.-J.S. and C.-C.H.; Investigation, W.-L.L.; Data Curation, W.-L.L.; P.-J.S. and C.-C.H.; Writing-Original Draft Preparation, W.-L.L. and C.-L.D.; Writing-Review & Editing, C.-L.D.; Supervision, C.-L.D.; Project Administration, C.-L.D.; Funding Acquisition, C.-L.D. Acknowledgments: The authors would like to thank National Center for High-performance Computing (NCHC) for simulation; Taiwan Semiconductor Research Institute (TSRI) for fabrication and the Ministry of Science and Technology (MOST) of the Republic of China for financially supporting this research under Contract Nos. MOST 108-2221-E-005 -065 -MY2. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Mirhosseini, M.; Rezania, A.; Rosendahl, L. Effect of heat loss on performance of thin film thermoelectric; a mathematical model. Mat. Res. Exp. 2019, 6, 096450. [CrossRef] 2. Piggott, A. Detailed transient multiphysics model for fast and accurate design, simulation and optimization of a thermoelectric generator (TEG) or thermal energy harvesting device. J. Electron. Mat. 2019, 48, 5442–5452. [CrossRef] 3. Ahn, D.; Choi, K. Performance evaluation of thermoelectric energy harvesting system on operating rolling stock. Micromachines 2018, 9, 359. [CrossRef] 4. Burton, M.R.; Mehraban, S.; Beynon, D.; McGettrick, J.; Watson, T.; Lavery, N.P.; Carnie, M.J. 3D printed SnSe thermoelectric generators with high figure of merit. Adv. Eng. Mat. 2019, 9, 1900201. [CrossRef] 5. Jang, W.; Cho, H.A.; Choi, K.; Park, Y.T. Manipulation of p-/n-Type thermoelectric thin films through a layer-by-layer assembled carbonaceous multilayer structure. Micromachines 2018, 9, 628. [CrossRef] [PubMed] 6. Zeng, W.; Tao, X.M.; Lin, S.P.; Lee, C.; Shi, D.L.; Lam, K.H.; Huang, B.L.; Wang, Q.M.; Zhao, Y. Defect-engineered reduced graphene oxide sheets with high electric conductivity and controlled thermal conductivity for soft and flexible wearable thermoelectric generators. Nano Eng. 2018, 54, 163–174. [CrossRef] 7. Liu, H.C.; Zhang, J.K.; Shi, Q.F.; He, T.Y.Y.; Chen, T.; Sun, L.N.; Dziuban, J.A.; Lee, C. Development of a thermoelectric and electromagnetic hybrid energy harvester from water flow in an irrigation system. Micromachines 2018, 9, 395. [CrossRef] 8. Fukuie, K.; Iwata, Y.; Iwase, E. Design of substrate stretchability using origami-like folding deformation for flexible thermoelectric generator. Micromachines 2018, 9, 315. [CrossRef][PubMed] 9. Lee, H.B.; Yang, H.J.; We, J.H.; Kim, K.; Choi, K.C.; Cho, B.J. Thin-film thermoelectric module for power generator applications using a screen-printing method. J. Electron. Mat. 2011, 40, 615–619. [CrossRef] 10. Phaga, P.; Vora-Ud, A.; Seetawan, T. Invention of low cost thermoelectric generators. Proced. Eng. 2012, 32, 1050–1053. [CrossRef] 11. Big-Alabo, A. Performance evaluation of Ge/SiGe-based thermoelectric generator. Phys. E Low-Dimens. Syst. Nanostruct. 2019, 108, 202–205. [CrossRef] 12. Dai, C.L.; Chiou, J.H.; Lu, M.S.C. A maskless post-CMOS bulk micromachining process and its application. J. Micromech. Microeng. 2005, 15, 2366–2371. [CrossRef] 13. Itoigawa, K.; Ueno, H.; Shiozaki, M.; Toriyama, T.; Sugiyama, S. Fabrication of fexible thermopile generator. J. Micromech. Microeng. 2005, 15, 233–238. [CrossRef] 14. Lu, Y.; Qiu, Y.; Jiang, Q.L.; Cai, K.F.; Du, Y.; Song, H.J.; Gao, M.Y.; Huang, C.J.; He, J.Q.; Hu, D.H.

Preparation and characterization of Te/Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate)/Cu7Te4 ternary composite ﬁlms for ﬂexible thermoelectric power generator. ACS Appl. Mater. Interfac. 2018, 10, 42310–42319. [CrossRef][PubMed]

15. Ding, Y.F.; Qiu, Y.; Cai, K.F.; Yao, Q.; Chen, S.; Chen, L.D.; He, J.Q. High performance n-type Ag2Se ﬁlm on nylon membrane for ﬂexible thermoelectric power generator. Nat. Commun. 2019, 10, 841. [CrossRef] [PubMed] Micromachines 2019, 10, 660 10 of 10

16. Selvan, K.V.; Rehman, T.; Saleh, T.; Ali, M.S.M. Copper-cobalt thermoelectric generators: Power improvement through optimized thickness and sandwiched planar structure. IEEE Trans. Electron Dev. 2019, 66, 3459–3465. [CrossRef] 17. Jo, S.E.; Kim, M.K.; Kim, M.S.; Kim, Y.J. Flexible thermoelectric generator for uuman body heat energy harvesting. Electron. Lett. 2012, 48, 1015–1016. [CrossRef] 18. Oh, J.Y.; Lee, J.H.; Han, S.W.; Chae, S.S.; Bae, E.J.; Kang, Y.H.; Choi, W.J.; Cho, S.Y.; Lee, J.O.; Baik, H.K.; et al. Chemically exfoliated transition metal dichalcogenide nanosheet-based wearable thermoelectric generators. Energ. Environ. Sci. 2016, 9, 1696–1705. [CrossRef] 19. Kim, J.Y.; Oh, J.Y.; Lee, T.I. Multi-dimensional nanocomposites for stretchable thermoelectric applications. Appl. Phys. Lett. 2019, 114, 043902. [CrossRef] 20. Chen, Y.W.; Wu, C.C.; Hsu, C.C.; Dai, C.L. Fabrication and testing of thermoelectric CMOS-MEMS microgenerators with CNCs ﬁlm. Appl. Sci. 2018, 8, 1047. [CrossRef] 21. Peng, S.W.; Shih, P.J.; Dai, C.L. Manufacturing and characterization of a thermoelectric energy harvester using the CMOS-MEMS technology. Micromachines 2015, 6, 1560–1568. [CrossRef] 22. Haynes, W.M. CRC Handbook of Chemistry and Physics, 95th ed.; CRC Press: Boca Raton, FL, USA, 2014. 23. Yang, M.Z.; Wu, C.C.; Dai, C.L.; Tsai, W.J. Energy harvesting thermoelectric generators manufactured using the complementary metal oxide semiconductor process. Sensors 2013, 13, 2359–2367. [CrossRef][PubMed] 24. Kao, P.H.; Shih, P.J.; Dai, C.L.; Liu, M.C. Fabrication and characterization of CMOS-MEMS thermoelectric micro generators. Sensors 2010, 10, 1315–1325. [CrossRef][PubMed]

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