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Jul 18, 2017 - Thermostability of Hybrid Thermoelectric Materials. Consisting of Poly(Ni-ethenetetrathiolate), Polyimide and Carbon Nanotubes. Keisuke ...
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Thermostability of Hybrid Thermoelectric Materials Consisting of Poly(Ni-ethenetetrathiolate), Polyimide and Carbon Nanotubes Keisuke Oshima 1 , Shifumi Sadakata 2 , Hitoshi Asano 2 Naoki Toshima 3 1 2

3

*

ID

, Yukihide Shiraishi 2, * and

Graduate School of Engineering, Tokyo University of Science Yamaguchi, Daigakudori, SanyoOnoda, Yamaguchi 756-0884, Japan; [email protected] Department of Applied Chemistry, Faculty of Engineering, Tokyo University of Science Yamaguchi, Daigakudori, SanyoOnoda, Yamaguchi 756-0884, Japan; [email protected] (S.S.); [email protected] (H.A.) Department of Applied Chemistry, Tokyo University of Science, Yamaguchi 756-0884, Japan; [email protected] Correspondence: [email protected]; Tel.: +81-836-88-4580

Received: 31 May 2017; Accepted: 14 July 2017; Published: 18 July 2017

Abstract: Three-component organic/inorganic hybrid films were fabricated by drop-casting the mixed dispersion of nanodispersed-poly(nickel 1,1,2,2-ethenetetrathiolate) (nano-PETT), polyimide (PI) and super growth carbon nanotubes (SG-CNTs) in N-methylpyrrolidone (NMP) at the designed ratio on a substrate. The dried nano-PETT/PI/SG-CNT hybrid films were prepared by the stepwise cleaning of NMP and methanol, and were dried once more. The thermoelectric properties of Seebeck coefficient S and electrical conductivity σ were measured by a thin-film thermoelectric measurement system ADVANCE RIKO ZEM-3M8 at 330–380 K. The electrical conductivity of nano-PETT/PI/SG-CNT hybrid films increased by 1.9 times for solvent treatment by clearing insulated of polymer. In addition, the density of nano-PETT/PI/SG-CNT hybrid films decreased 1.31 to 0.85 g·cm−3 with a decrease in thermal conductivity from 0.18 to 0.12 W·m−1 ·K−1 . To evaluate the thermostability of nano-PETT/PI/SG-CNT hybrid films, the samples were kept at high temperature and the temporal change of thermoelectric properties was measured. The nano-PETT/PI/SG-CNT hybrid films were rather stable at 353 K and kept their power factor even after 4 weeks. Keywords: organic/inorganic hybrid thermoelectric materials; polyimide; poly(nickel 1,1,2,2ethenetetrathiolate); carbon nanotube

1. Introduction Energy is a key issue for human beings. The increasing world population, food shortages, and economic differences are also big issues, but these problems could be solved if we had enough sustainable energy. Presently, developed countries as well as developing countries obtain most of their energy from fossil fuels like coal, petroleum and natural gas. When we use these energy sources, more than half of the energy is lost without utilizing them in the planned form, which results in waste heat. For example, we use most of our energy in the form of electricity. In 2014, the percentage of electricity in total energy was 25.3% in Japan. However, the conversion efficiency to get electricity from fossil fuels was 42.2% in Japan in 2014, meaning that 57.8% of the energy from fossil fuels was lost as heat energy without its use. If we could acquire some electricity from these lost heat energies, we could reduce the consumption of fossil fuels and reduce the increasing worldwide temperature. For this reason, thermoelectric technology has become interesting in recent years.

Materials 2017, 10, 824; doi:10.3390/ma10070824

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The utilization of unused heat energy like natural heat and waste heat below 423 K has received much attention recently. About two thirds of the waste heat is at a low grade, i.e., below 423 K [1]. The efficiency of a thermoelectric material depends on a dimensionless figure of merit, ZT [2], calculated according to Equation (1): ZT = S2 σT/κ (1) where S is the Seebeck coefficient (V·K−1 ), σ is the electrical conductivity (S·m−1 ), T is the absolute temperature (K), and κ is the thermal conductivity (W·m−1 ·K−1 ). If materials have similar thermal conductivities, the parameter power factor, PF [3] (=S2 σ) is used to characterize thermoelectric performances. Thus, good thermoelectric materials possess large power factors and low thermal conductivities. Inorganic thermoelectric materials, such as Mg2 Si, and Bi2 Te3 , etc., have been the focus of research [4,5], while research related to organic thermoelectric materials [6–8] has received less attention, mainly because of their low figures of merit [9,10]. However, inorganic materials have several disadvantages, such as rarity, high cost in production, poor processability, and environmental problems due to their toxicity. On the other hand, organic thermoelectric materials, compared with inorganic thermoelectric materials, have many advantages, such as plenty of raw resources, easy processability into a versatile form, easy application of printing technology to fabricate devices with large areas, low cost in raw materials and device manufacturing, environmental friendliness, and so on. The most exciting results on organic thermoelectric materials involve thin films of polyphenylenevinylene (PPV) derivatives after stretching [11], poly(3,4-ethylene dioxythiophene) p-toluenesulfonate (PEDOT-Tos) at controlled oxidation level [12], and poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT-PSS) [13–15] after ethylene glycol treatment. Although devices made from PEDOT-based films show high performance as thermoelectric materials, their instability in an ambient environment is a serious disadvantage that limits their practical use. It is difficult to achieve higher thermoelectric performances using only organic materials such as conducting polymers [16–18]. Recently, hybrid materials using organic and inorganic materials have been studied for next-generation thermoelectric materials. The properties of novel hybrid types based on organic thermoelectric materials are different from those of inorganic thermoelectric materials, and it is therefore possible to enhance electrical conductivity and to reduce thermal conductivity. For example, hybrid organic materials show good thermoelectric performance by incorporating organic materials with carbon nanotubes (CNTs) [19–22], nanocrystals [23–26], or nanoparticles [27–29] as inorganic materials. CNTs were thought not to be very good thermoelectric materials because of their high thermal conductivity [30]. However, CNTs with high Seebeck coefficients were reported to have a good thermoelectric performance [31]. In addition, CNTs with organic ligands, such as tetraphenylphosphine, were recently demonstrated to have a relatively high negative Seebeck coefficient and low thermal conductivity, resulting in ZT = 0.07 [32]. Preliminarily, we have successfully developed new thermoelectric materials with high thermoelectric property by using nanodispersed-poly(Ni 1,1,2,2-ethenetetrathiolate) (nano-PETT), poly(vinylchloride) (PVC), and CNTs [33]. The nanoparticles of polymer complexes, nano-PETT, were able to be well dispersed in PVC hybrids, and played an important role for the smooth contact and good charge transport between CNT bundles in the films. In these cases, however, PVC was also not stable at high temperature. Thus, development of thermostable hybrid thermoelectric materials is strongly required for fabrication of thermoelectric devices. In this study, organic/inorganic hybrid thermoelectric materials were prepared using a heat resistant polymer; polyimide (PI), nano-PETT, and SG-CNTs. 2. Materials and Experimental Processes 2.1. Materials To synthesis nano-PETTs, we used 1,3,4,6-tetrathiapentalene-2,5-dione (TPD, Tokyo Chemical Industry Co., Ltd., Tokyo, Japan). Nickel (II) were selected as center metals (Wako Pure Chemical

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Ind., Ltd., Hiroshima, Japan). Polyimide (PI, TS-8, Solpit Industries Co., Ltd., Tokyo, Japan) for binder was used. SG-CNTs (Zeon Corp., Tokyo, Japan) were used to improve electrical conductivity and mechanical properties of polymer. 2.2. Preparation of Nano-PETT When the powders of the solid PETT, prepared according to the literature [34], were sonicated in NMP containing the surfactant for an hour, the solid Ni-PETT could not be dispersed in it at all. Thus, we used the surfactant during the synthesis of nano-PETT as follows: dodecyltrimethyl-ammonium bromide (6.9 g, 22.2 mmoL) and sodium methoxide (1.2 g, 22.2 mmoL) were dissolved in methanol (190 mL) by stirring the mixtures with a magnetic stirrer at room temperature under air overnight to produce a pale yellow solution. To this solution, 1,3,4,6-tetrathiapentalene-2.5-dione (1.0 g, 4.8 mmoL) was added. The mixture was refluxed for 12 h at 363 K in an oil bath and kept in the oil bath overnight after the refluxing. The color of the solution gradually changed from yellow to black. Then, nickel (II) chloride (4.8 mmoL) and methanol (10 mL) were added to this mixture. The mixture was refluxed for another 12 h, and then gradually cooled down by keeping the flask in an oil bath overnight. The produced black precipitates were separated by suction filtration membrane filter (polytetrafluoro-ethylene with pore size 0.1 µm, Toyo Roshi Kaisha Ltd., Tokyo, Japan), washed with 2 L water, methanol and a small amount of diethyl ether, and then dried under vacuum at 313 K through a night. The obtained black powders of surfactant-containing nano-PETT were able to be dispersed in NMP to produce a brown solution. The majority of the nano-PETT was in the range of 10–50 nm, suggesting that the size of nano-PETT is nearly homogeneous [35]. The results reveal that nano-PETT examined here has an average diameter of 37.6 nm. 2.3. Fabrication of Three-Component Hybrid Films: Nano-PETT/PI/SG-CNT The nano-PETT and SG-CNTs were added to an NMP solution containing PI, and dispersed in an ultrasonic homogenizer for 10 min. The dispersions of nano-PETT, PI, and SG-CNT were cast on a petri dish and/or polyimide substrate, and dried in air on a hot plate at 333 K for 12 h to obtain a three-component hybrid film, nano-PETT/PI/SG-CNT. 2.4. Characterization The film thickness was measured with a linear gage (model LGK-010, resolution: 0.1 µm, Mitsutoyo Corp., Kawasaki, Japan). Seebeck coefficient and electrical conductivity of nano-PETT/PI/SG-CNT were measured with a thermoelectric evaluation system (ZEM-3M8, ADVANCE RIKO, Inc., Yokohama, Japan) at 330–380 K under vacuum with helium gas. The values of the power factor for each of the films were calculated by the equation PF = S2 σ. The surface morphology of the film was observed with a field emission scanning electron microscope (FE-SEM, S-4800 Type2, Hitachi, Tokyo, Japan). Thermal conductivity, κ, was calculated by Equation (2) κ = αρCp

(2)

where α is the thermal diffusivity, ρ is the density calculated by measuring the weight and volume of the films, and Cp is the specific heat capacity at constant pressure measured using a Netzsch DSC 204 F1 Phoenix (Yokohama, Japan). The thermal diffusivity α was measured with a Netzsch LFA 447 Nanoflash (Yokohama, Japan) in a through-plane direction of self-standing films at 290 K. 3. Results and Discussion 3.1. Preparation and Characterization of Nano-PETT/PI/SG-CNT Hybrid Films Maniwa et al. reported that purified semiconducting single-walled CNTs had a very high Seebeck coefficient of 170 µV·K−1 at 350 K, which would make them suitable for use in flexible thermoelectric

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materials [31]. On the other hand, the SG-CNTs have many defects, and a poor thermoelectric performance Materials 2017, 10,by 824themselves. In order to improve the performance of SG-CNTs, three-component 4 of 9 nano-PETT/PI/SG-CNT hybrid films were prepared by a conventional drop-casting method from amethod mixed from dispersion of dispersion nano-PETT, and SG-CNTs in SG-CNTs NMP at the designed ratios on a ratios petri dish. a mixed ofPI nano-PETT, PI and in NMP at the designed on a The SEM images of nano-PETT/PI/SG-CNT hybrid films before and after solvent treatment are shown petri dish. The SEM images of nano-PETT/PI/SG-CNT hybrid films before and after solvent in Figure 1.areThe SEM in images hybrid films before cleaning (Figure 1a) treatment shown Figureof1.nano-PETT/PI/SG-CNT The SEM images of nano-PETT/PI/SG-CNT hybrid films before and after methanol cleaning (Figure 1b) reveal that they are composed complicated large masses. cleaning (Figure 1a) and after methanol cleaning (Figure 1b) reveal of that they are composed of We previouslylarge found that treatment of the organic/inorganic hybrid films with methanol could enhance complicated masses. We previously found that treatment of the organic/inorganic hybrid films the electrical conductivity [35].the In these nano-PETT/PI/SG-CNT hybrid films, however, the solubility with methanol could enhance electrical conductivity [35]. In these nano-PETT/PI/SG-CNT hybrid of PI inhowever, methanolthe was poor. When was used of methanol, theused nano-PETT/PI/SG-CNT films, solubility of PI NMP in methanol wasinstead poor. When NMP was instead of methanol, hybrid films exhibited good film. Furthermore, the SEMgood imagefilm. of nano-PETT/PI/SG-CNT hybrid films the nano-PETT/PI/SG-CNT hybrid films exhibited Furthermore, the SEM image of after the stepwise cleaning by NMP methanol (Figure 1d) showed fewand CNT bundles.(Figure The film’s nano-PETT/PI/SG-CNT hybrid filmsand after the stepwise cleaning by NMP methanol 1d) thickness decreased from 52.7 2.6 µm to 31.0 ±decreased 1.6 µm. from 52.7 ± 2.6 µm to 31.0 ± 1.6 µm. showed few CNT bundles. The±film’s thickness

Figure 1. SEM images of the hybrid films of nano-PETT/PI/SG-CNT before cleaning (a), after Figure 1. SEM images of the hybrid films of nano-PETT/PI/SG-CNT before cleaning (a), after methanol methanol(b); cleaning (b); after N-methylpyrrolidone (NMP)(c) cleaning (c) the andstepwise after the cleaning stepwise by cleaning cleaning after N-methylpyrrolidone (NMP) cleaning and after NMP by NMP and methanol (d). and methanol (d).

We next measured the thermal conductivity of the nano-PETT/PI/SG-CNT hybrid films in the We next measured the thermal conductivity of the nano-PETT/PI/SG-CNT hybrid films in the through-plane direction. Recently, anisotropy in thermal conductivity has become a hot topic in through-plane direction. Recently, anisotropy in thermal conductivity has become a hot topic in organic organic and hybrid thermoelectric materials [36]. Since the Seebeck coefficient and electrical and hybrid thermoelectric materials [36]. Since the Seebeck coefficient and electrical conductivity conductivity were measured in the in-plane direction, the thermal conductivity needs to be were measured in the in-plane direction, the thermal conductivity needs to be measured in the same measured in the same direction in order to obtain the correct thermoelectric figure-of-merit. direction in order to obtain the correct thermoelectric figure-of-merit. However, it is difficult to obtain However, it is difficult to obtain the correct thermal conductivity of the thin films in the in-plane the correct thermal conductivity of the thin films in the in-plane direction. The data used to calculate direction. The data used to calculate the thermal conductivity of the nano-PETT/PI/SG-CNT hybrid the thermal conductivity of the nano-PETT/PI/SG-CNT hybrid films before and after solvent cleaning films before and after solvent cleaning are summarized in Table 1. The density of are summarized in Table 1. The density of nano-PETT/PI/SG-CNT−3hybrid films decreased 1.31 to nano-PETT/PI/SG-CNT hybrid films decreased 1.31 to 0.85 g·cm , with −a1 decrease in thermal 0.85 g·cm−3 , with a decrease in thermal conductivity from 0.18 to 0.12 W·m ·K−1 before and after −1 before and conductivity from 0.18 to 0.12 W·m−1·K after the stepwise cleaning by NMP and the stepwise cleaning by NMP and methanol, respectively. It should be emphasized that the solvent methanol, respectively. It should be emphasized that the solvent treatment provides a low thermal treatment provides a low thermal conductivity, which ρ. could due to the low density, ρ. SEM It is conductivity, κ, which could be due to the low κ, density, It is be clearly demonstrated by the clearly demonstrated by the SEM photographs shown in Figure 1d that the films became porous after photographs shown in Figure 1d that the films became porous after the stepwise cleaning by NMP the cleaning NMP and in methanol, which could in many voids and low density. andstepwise methanol, which by could result many voids and lowresult density. Another reason for the low Another reason for the low thermal conductivity of the nano-PETT/PI/SG-CNT hybrid films is the thermal conductivity of the nano-PETT/PI/SG-CNT hybrid films is the utilization of PI, the thermal utilization of PI, the thermal conductivity conductivity of which is intrinsically low. of which is intrinsically low.

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Table 1. Thermal conductivity κ and related data of nano-PETT/PI/SG-CNT hybrid films at 290 K. Table 1. Thermal conductivity κ and related data of nano-PETT/PI/SG-CNT hybrid films at 290 K.

Pristine ρ/g·cm−3 2 −1 α/mm ρ/g ·cm−3·s 2 ·s−1−·K 1 −1 Cp/J·g α/mm − 1 − 1 −1 −1 Cp /J ·g ·K ·K κ/W·m

κ/W·m−1 ·K−1

Pristine

1.31 ± 0.06 0.13 0.03 1.31 ±±0.06 0.86 ± 0.06 0.13 ± 0.03 0.86 0.18±±0.06 0.01 0.18 ± 0.01

MeOH 1.12 ± 0.15 MeOH 0.14 ± 1.12 ±0.00 0.15 0.87 0.14±±0.01 0.00 0.87±±0.02 0.01 0.14 0.14 ± 0.02

Treated Treated NMP NMP + MeOH 1.12 ±NMP 0.08 0.85 ± 0.04 NMP + MeOH 0.131.12 ± 0.01 0.15 ± 0.00 ± 0.08 0.85 ± 0.04 0.800.13 ± 0.02 0.930.15 ± 0.04 ± 0.01 ± 0.00 ± 0.02 ± 0.04 0.120.80 ± 0.01 0.120.93 ± 0.02 0.12 ± 0.01

0.12 ± 0.02

3.2. Thermoelectric Properties of Nano-PETT/PI/SG-CNT Hybrid Films 3.2. Thermoelectric Properties of Nano-PETT/PI/SG-CNT Hybrid Films The thermoelectric properties of the sheet of SG-CNT, PI/SG-CNT and nano-PETT/PI/SG-CNT usedThe in this research are shown inofFigure 2. The SeebeckPI/SG-CNT coefficients and of SG-CNT, PI/SG-CNT and thermoelectric properties the sheet of SG-CNT, nano-PETT/PI/SG-CNT −1, 53 µV·K −1, and PI/SG-CNT nano-PETT/PI/SG-CNT (pristine) were 48 µV·K 50 µV·K−1, used in this research arehybrid shownfilms in Figure 2. The Seebeck coefficients of SG-CNT, − 1 − 1 −1 , respectively. The Seebeck coefficient not(pristine) depend on the 48 type themselves and nano-PETT/PI/SG-CNT hybriddid films were µVof·Ksheet. , 53The µV·CNTs K , and 50 µV·Khad −1. not an electrical conductivity ca. 98 S·cm When they were in PI, however, electrical respectively. The Seebeckof coefficient did depend on thedispersed type of sheet. The CNTsthe themselves −1 . as −1. In contrast, conductivity of the hybrids ofofPI/SG-CNT lowthey as 22 S·cm the electrical had an electrical conductivity ca. 98 S·cmwas When were dispersed in PI, however, the − 1 conductivity of nano-PETT/PI/SG-CNT hybrid films was higher than that of PI/SG-CNT films. In electrical conductivity of the hybrids of PI/SG-CNT was as low as 22 S·cm . In contrast, the electrical addition, we found that treatment of the nano-PETT/PI/SG-CNT hybrid byPI/SG-CNT stepwise cleaning conductivity of nano-PETT/PI/SG-CNT hybrid films was higher than films that of films. byaddition, NMP andwe methanol could enhance conductivity. A hybrid similarfilms enhancement in electrical In found that treatment of the the electrical nano-PETT/PI/SG-CNT by stepwise cleaning conductivity by treatment with a solvent such as DMSO and EG has been reported in in the case of by NMP and methanol could enhance the electrical conductivity. A similar enhancement electrical PEDOT films, the with solvent treatment considered to has enhance the alignment of the conductivity bywhere treatment a solvent suchwas as DMSO and EG been reported in the case of conducting polymer chains [37]treatment and/or towas remove the insulating materials from the surface of the PEDOT films, where the solvent considered to enhance the alignment of the conducting films [13].chains The electrical conductivity of nano-PETT/PI/SG-CNT films increased 1.9films times[13]. for polymer [37] and/or to remove the insulating materialshybrid from the surface of the solvent treatment by clearing insulated of polymer. The electrical conductivity is known to be The electrical conductivity of nano-PETT/PI/SG-CNT hybrid films increased 1.9 times for solvent proportional to the carrier concentration andelectrical carrier mobility. Theis addition nano-PETT treatment by clearing insulated of polymer. The conductivity known toof be the proportional to increased electrical conductivity, while the Seebeck coefficient remainedincreased constant.the This means the carrier the concentration and carrier mobility. The addition of the nano-PETT electrical that the nano-PETT does not work as an electron conductor, covers thethat defects of the SG-CNTs, conductivity, while the Seebeck coefficient remained constant.but This means the nano-PETT does which results in an increased mobility [38],ofalthough the which real mobility not be not work as an electron conductor,electron but covers the defects the SG-CNTs, results incould an increased measured for technical reasons.the Hereby, the power factor of measured nano-PETT/PI/SG-CNT hybrid Hereby, films is electron mobility [38], although real mobility could not be for technical reasons. ca. 2.0 times higher that of SG-CNT. We hybrid previously found the treatment of organic/inorganic the power factor of than nano-PETT/PI/SG-CNT films is ca.that 2.0 times higher than that of SG-CNT. hybrid films could enhance power factor [39]. When poly(methyl methacrylate) was the employed We previously found that thethe treatment of organic/inorganic hybrid films could enhance power as a binder, poly(methyl methacrylate) hybridwas films were very after methanol treatment. It is factor [39]. When poly(methyl methacrylate) employed asfragile a binder, poly(methyl methacrylate) important to select an optimal polymer, because the polymer added as a binder affects film hybrid films were very fragile after methanol treatment. It is important to select an optimal polymer, properties Thus, it is asstriking PI film is very effective forThus, obtaining high thermoelectric because the[40]. polymer added a binderthat affects properties [40]. it is striking that PI is very properties the hybrid films. effective forfor obtaining high thermoelectric properties for the hybrid films.

(S),(S), electrical conductivity (σ) and power factorfactor (PF) of SG-CNT sheet, Figure 2. Seebeck Seebeckcoefficient coefficient electrical conductivity (σ) and power (PF) of SG-CNT sheet, PI/SG-CNT and nano-PETT/PI/SG-CNT. ratios= 12/8, are and PI/SG-CNT = 12/8, and= PI/SG-CNT and nano-PETT/PI/SG-CNT. Mass ratios are Mass PI/SG-CNT nano-PETT/PI/SG-CNT nano-PETT/PI/SG-CNT = 9/3/8. films have a thickness of 10 µm. 9/3/8. All films have a thickness of All 10 µm.

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In order2017, to 10, evaluate the thermal long-term stability of nano-PETT/PI/SG-CNT hybrid Materials 824 6 of 9films (film thickness of 10 µm), samples were kept at 353 K and 423 K, and the temporal change of the In order to evaluate the thermal long-term stability of nano-PETT/PI/SG-CNT hybrid films (film In order to evaluate the thermal long-term stability of nano-PETT/PI/SG-CNT hybrid films (film thermoelectric properties in air was measured for 4Kweeks. between duration thickness of 10 µm), samples were kept at 353 and 423The K, relationship and the temporal change of the and thickness of 10 µm), samples were kept at 353 K and 423 K, and the temporal change of the thermoelectric in air was measured for 4 films weeks.isThe relationship between and of the power factor ofproperties nano-PETT/PI/SG-CNT hybrid shown in Figure 3. Theduration power factor thermoelectric properties in air was measured for 4 weeks. The relationship between and 1 ·K− 2duration the power factor of nano-PETT/PI/SG-CNT films from is shown 3.−The power of nano-PETT/PI/SG-CNT hybrid films at 353 Khybrid decreased 46 toin39Figure µW·m . Thefactor percentage the power factor of nano-PETT/PI/SG-CNT hybrid films is shown in Figure−13. −2The power factor of nano-PETT/PI/SG-CNT hybrid films at 353 K decreased from 46 to 39 µW·m ·K . The percentage of loss of weight loss from 0 day to 4 weeks K decreased was −15%. Moreover, the −1percentage of weight nano-PETT/PI/SG-CNT hybrid filmsat at 353 353 K from 46 to 39 µW·m ·K−2. The percentage of weight loss from 0 days to 4 weeks at 353 K was −15%. Moreover, the percentage of weight loss from from 0 day to 4 weeks was to −16% even at the higher temperature ofpercentage 423 K. This probably because weight loss from 0 days 4 weeks at 353 K was −15%. Moreover, the of is weight loss from 0 days to 4 weeks was −16% even at the higher temperature of 423 K. This is probably because PI PI may protect from even the decomposition at the higher temperature. We nextbecause prepared 0 days to 4nano-PETT weeks was −16% at the higher temperature of 423 K. This is probably PI the may protect nano-PETT from the decomposition at the higher temperature. We next prepared the nano-PETT/PI/SG-CNT hybrid with a film thickness of 3 µm by coatingWe thenext mixed dispersion may protect nano-PETT fromfilms the decomposition at the higher temperature. prepared the of nano-PETT/PI/SG-CNT hybrid films with a film thickness of 3 µm by coating the mixed dispersion nano-PETT/PI/SG-CNT hybrid films with a film thickness of 3asµm by coating the mixed dispersion nano-PETT, PI and SG-CNTs in NMP onNMP a polyimide substrate, shown Figure of nano-PETT, PI and SG-CNTs in on a polyimide substrate, asinshown in4 (inset Figurephotograph). 4 (inset of nano-PETT, PI and SG-CNTs in NMP on a polyimide substrate, as shown in Figure 4 These (inset thin Figurephotograph). 4 depicts the relationship duration and the power and factor thesefactor thin films. Figure 4 depictsbetween the relationship between duration theof power of these thin photograph). Figure 4 depicts the relationship between duration and the power factor of these thin nano-PETT/PI/SG-CNT hybrid films werehybrid ratherfilms stable at 353 K, and kept their factor films. These thin nano-PETT/PI/SG-CNT were rather stable at 353 K, power and kept theireven films. These thin nano-PETT/PI/SG-CNT hybrid films were rather stable at 353 K, and kept their factor even after 4 weeks. after 4power weeks. power factor even after 4 weeks.

Figure 3. Durability of power factor (PF) at 353 K and 423 K of nano-PETT/PI/SG-CNT hybrid films

FigureFigure 3. Durability of power factor (PF) atat 353 hybrid 3. Durability of power factor (PF) 353KKand and423 423K K of of nano-PETT/PI/SG-CNT nano-PETT/PI/SG-CNT hybrid filmsfilms (film thickness of 10 µm). (film thickness of 10 µm). (film thickness of 10 µm).

-1 -1 PF / µW K-2 PF / µW mm K-2 345345

100 100 80 80

353 K hold 353 K hold

60 60 40 40 20 20 0 0 0 0

1 2 3 1 Duration 2 / week 3

Duration / week

4 4

Figure 4. Durability of power factor (PF) at 353 K of nano-PETT/PI/SG-CNT hybrid films (film Figure 4. Durability of power factor (PF) at 353 K of nano-PETT/PI/SG-CNT hybrid films (film Figure 4. Durability thickness of 3 µm). of power factor (PF) at 353 K of nano-PETT/PI/SG-CNT hybrid films thickness of 3 µm).

(film thickness of 3 µm). In order to examine the thermostability of nano-PETT/PI/SG-CNT hybrid films, thermogravimetric In order to examine the thermostability of nano-PETT/PI/SG-CNT hybrid films, thermogravimetric analyses were performed under air. A thermal decomposition temperature of nano-PETT/PI/SG-CNT In orderwere toperformed examineunder theair.thermostability of nano-PETT/PI/SG-CNT hybrid films, analyses A thermal decomposition temperature of nano-PETT/PI/SG-CNT hybrid films (695 K) was higher than that of nano-PETT/PVC/SG-CNT hybrid films (476 K), as hybrid films (695 K) was were higherperformed than that of nano-PETT/PVC/SG-CNT hybrid films temperature (476 K), as of thermogravimetric analyses under air. A thermal decomposition shown in Figure 5. The thermal decomposition temperatures of nano-PETT, PI, and SG-CNT are 413 K, shown in Figure 5. The hybrid thermal decomposition temperatures nano-PETT, and SG-CNT are 413 K, nano-PETT/PI/SG-CNT films was higher of than that of PI, nano-PETT/PVC/SG-CNT 825 K, and 854 K, respectively. In spite(695 of theK)low decomposition temperature of nano-PETT, thermal 825 K, and 854 K, respectively. In spite of the low decomposition temperature of nano-PETT, thermal hybrid films (476 K),was as shown in Figure The thermal decomposition temperatures nano-PETT, decomposition suppressed in the 5. nano-PETT/PI/SG-CNT hybrid films. Previously,ofwe reported PI, decomposition was suppressed in the nano-PETT/PI/SG-CNT hybrid films. Previously, we reported and SG-CNT are 413 K, 825 K, and 854 K, respectively. In spite of the decomposition temperature that poly(sodium acrylate)(PAA)-protected Ag nanoclusters arelow much more stable than that poly(sodium acrylate)(PAA)-protected Ag nanoclusters are much more stable than

of nano-PETT, thermal decomposition was suppressed in the nano-PETT/PI/SG-CNT hybrid films. Previously, we reported that poly(sodium acrylate)(PAA)-protected Ag nanoclusters are much

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Ag nanoclusters, especially at high temperature morepoly(N-vinyl-2-pyrrolidone)(PVP)-protected stable than poly(N-vinyl-2-pyrrolidone)(PVP)-protected Ag nanoclusters, especially[41]. at high The increment of weight lossof between and PAA-Ag at nanoclusters 443 K (1.9%) was much temperature [41]. The increment weightPAA loss between PAAnanoclusters and PAA-Ag at 443 K (1.9%) smaller than that between PVP andPVP PVP-Ag (3.8%). The (3.8%). protection playspolymer an was much smaller than that between and nanoclusters PVP-Ag nanoclusters Thepolymer protection important role not only in protecting nanomaterials, but also in controlling functions [42,43]. plays an important role not only in protecting nanomaterials, but also in controlling functions [42,43]. In nano-PETT/PI/SG-CNT hybrid films, PI is the significant material, not only for binder but also for In nano-PETT/PI/SG-CNT hybrid films, PI is the significant material, not only for binder but also for long-term thermal stability. One of the biggest of the various advantages of hybrid thermoelectric long-term thermal stability. One of the biggest of the various advantages of hybrid thermoelectric materials is the low cost of fabrication for hybrid thermoelectric devices. It might be possible that materials is the low cost of fabrication forfilms hybrid thermoelectric devices. It might possible that these these nano-PETT/PI/SG-CNT hybrid allow the fabrication of devices bybe simple methods nano-PETT/PI/SG-CNT hybrid films allow the fabrication of devices by simple methods like printing. like printing.

Figure 5. Comparison of thermogravimetric analyses of nano-PETT/PI/SG-CNT (red line) and

Figure 5. Comparison of thermogravimetric analyses of nano-PETT/PI/SG-CNT (red line) and nano-PETT/PVC/SG-CNT (black line) hybrid films. Mass ratio: nano-PETT/polymer/SG-CNT = 9/3/8. nano-PETT/PVC/SG-CNT (black line) hybrid films. Mass ratio: nano-PETT/polymer/SG-CNT = 9/3/8. 4. Conclusions

In summary, we have developed a three-component organic/inorganic hybrid material of nano-PETT/PI/SG-CNT with high thermoelectric properties. The Seebeck coefficient and the −1 at 345 K, electrical conductivity nano-PETT/PI/SG-CNT were 45 organic/inorganic µV·K−1 and 226 S·cm In summary, we haveofdeveloped a three-component hybrid material of −1·K−2 at 345 K. The nano-PETT/PI/ respectively. The materials exhibit a high PF value of 46 µW·m nano-PETT/PI/SG-CNT with high thermoelectric properties. The Seebeck coefficient and the electrical −1 and −1 at factor SG-CNT of hybrid films are rather stable at 353 45 K, µV and·Kretain a negative power after conductivity nano-PETT/PI/SG-CNT were 226 S·cm 345 K,even respectively. 4 weeks. The stabilization of the nano-PETT/PI/SG-CNT hybrid films can be understood by The materials exhibit a high PF value of 46 µW·m−1 ·K−2 at 345 K. The nano-PETT/PI/ SG-CNTthe hybrid polymer effect of polyimide surrounding SG-CNTs. The developed nano-PETT/PI/SG-CNT hybrid films are rather stable at 353 K, and retain a negative power factor even after 4 weeks. The stabilization films are expected to be a candidate as hybrid materials for use in future thermoelectric devices, of thewhich nano-PETT/PI/SG-CNT hybrid films can be understood by the polymer effect of polyimide could have advantages in terms of flexibility and long lifetime. For energy harvesting surrounding The developed nano-PETT/PI/SG-CNT hybrid films are expected to be a purposes,SG-CNTs. the thermoelectric devices could be used to provide electricity in dark places, as the power candidate as of hybrid materials forbatteries. use in future thermoelectric which couldpolymers have advantages sources sensors instead of This novel concept of devices, the role of functional may in terms long lifetime.hybrid For energy harvesting purposes, the thermoelectric devices openofa flexibility new field inand organic/inorganic electronics.

4. Conclusions

could be used to provide electricity in dark places, as the power sources of sensors instead of batteries. Acknowledgments: This work was supported by a Grant-in-Aid for Scientific Research(C) (No. 15K04613) This novel concept of the role of functional polymers may open a new field in organic/inorganic from MEXT, NEDO and ZEON Corporation, Japan project for the “Development of high-density energy hybrid electronics. devices using nano carbon materials”. The authors express their sincere thanks to H. Anno and K. Okamoto for their kind assistance through the experiments.

Acknowledgments: This work was supported by a Grant-in-Aid for Scientific Research(C) (No. 15K04613) from Yukihide Shiraishi and Naoki Toshima conceivedofthe idea and wrote paper;using MEXT,Author NEDOContributions: and ZEON Corporation, Japan project for the “Development high-density energythe devices Keisuke materials”. Oshima, andThe Shifumi Sadakata performed the experiments; Hitoshiand Asano, Yukihide for Shiraishi, nano carbon authors express their sincere thanks to and H. Anno K. Okamoto their kind assistance through the experiments. and Naoki Toshima discussed the results and commented on the manuscript equally. Naoki Toshima conceived the idea and wrote the paper; Author Contributions: Conflicts of Interest: Yukihide The authorsShiraishi declare noand conflict of interest. Keisuke Oshima, and Shifumi Sadakata performed the experiments; and Hitoshi Asano, Yukihide Shiraishi, and Naoki Toshima discussed the results and commented on the manuscript equally. References Conflicts of Interest: The authors declare no conflict of interest. 1. Toshima, N. Recent progress of organic and hybrid thermoelectric materials. Synth. Met. 2017, 225, 3–21. 2.

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