Low thermal conductivity and high thermoelectric ...

Low thermal conductivity and high thermoelectric figure of merit in p-type Sb2Te3/poly(3,4-ethylenedioxythiophene) thermoelectric composites Wenwen Zheng, Peng Bi, Haochen Kang, Wei Wei, Fengming Liu, Jing Shi, Luxi Peng, Ziyu Wang, and Rui Xiong Citation: Applied Physics Letters 105, 023901 (2014); doi: 10.1063/1.4887504 View online: http://dx.doi.org/10.1063/1.4887504 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/105/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Thermal conductivity measurement of a Sb2Te3 phase change nanowire Appl. Phys. Lett. 104, 263103 (2014); 10.1063/1.4884604 Thermoelectric and mechanical properties of multi-walled carbon nanotube doped Bi0.4Sb1.6Te3 thermoelectric material Appl. Phys. Lett. 103, 221907 (2013); 10.1063/1.4834700 Enhanced thermoelectric performance in spark plasma textured bulk n-type BiTe2.7Se0.3 and p-type Bi0.5Sb1.5Te3 Appl. Phys. Lett. 102, 211901 (2013); 10.1063/1.4807771 Thermoelectric properties of p-type Bi0.5Sb1.5Te2.7Se0.3 fabricated by high pressure sintering method J. Appl. Phys. 112, 073708 (2012); 10.1063/1.4754840 High thermoelectric figure of merit in the Cu3SbSe4-Cu3SbS4 solid solution Appl. Phys. Lett. 98, 261911 (2011); 10.1063/1.3605246

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APPLIED PHYSICS LETTERS 105, 023901 (2014)

Low thermal conductivity and high thermoelectric figure of merit in p-type Sb2Te3/poly(3,4-ethylenedioxythiophene) thermoelectric composites Wenwen Zheng,1 Peng Bi,1 Haochen Kang,2 Wei Wei,3 Fengming Liu,4 Jing Shi,1 Luxi Peng,1 Ziyu Wang,1,5,a) and Rui Xiong1,a)

1 Key Laboratory of Artificial Micro-and Nano-structures of Ministry of Education and School of Physics and Technology, Wuhan University, Wuhan 430072, China 2 School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China 3 Hubei Cancer Hospital, Wuhan 430079, China 4 School of Science, Hubei University of Technology, Wuhan 430068, China 5 Dongfeng Commercial Vehicle Technology Center, Wuhan 430056, China

(Received 25 April 2014; accepted 25 June 2014; published online 14 July 2014) p-type Sb2Te3/poly(3,4-ethylenedioxythiophene) (PEDOT) thermoelectric composites are fabricated by embedding PEDOT into Sb2Te3 matrix. The grains of Sb2Te3 in the composites are found to be in micron degree and keep plate-like shapes. The measurements of thermoelectric properties show that the thermal conductivity j of the composites is about 0.14 W m1 K1 in the temperature range of 300–523 K, much lower than that of Sb2Te3 compounds. The maximum of dimensionless figure of merit of the composites reaches to 1.18 at 523 K, which is the highest value for the reported Sb2Te3/organic composites. It is suggested that the plate-like Sb2Te3 grains and the embedded PEDOTs may play a significant role in decreasing the thermal conductivity. Furthermore, results of the thermal cycling between the room temperature and 523 K for 50 cycles C 2014 AIP Publishing LLC. show that the composites are stable with j remaining a low value. V [http://dx.doi.org/10.1063/1.4887504]

Thermoelectric (TE) materials, which can effectively harvest electricity from heat for power generation, are expected to be a green option for global energy resources.1 The energy conversion efficiency of a TE material is explained in terms of a dimensionless figure of merit (ZT), ZT ¼ S2rT/j, where S is the Seebeck coefficient, r is the electrical conductivity, j is the thermal conductivity, and T is the absolute temperature. In order to compete with traditional energy device, ZT of TE materials should be improved to be higher than 3. However, in typical bulk semiconductors, the thermoelectric properties are strongly correlated with each other, making ZT enhancement very difficult. Following the concept of phonon glass-electron crystal (PGEC) proposed by Slack,2 low-dimensional and nanostructured TE materials have drawn much attention.3,4 In these nanostructured TE materials, the strong phonon scattering at the grain boundaries can reduce the thermal conductivity dramatically.5,6 Up to now, ZT of 2.4 have been obtained in Bi2Te3/Sb2Te3 nanolayer superlattice films in 2001.7 Different chemical methods have been used to prepare low-dimension and nanostructured TE materials, due to their relatively simple and low cost manufacturing processes.8,9 For example, Zhao et al. exploited a hydrothermal method to synthesize Bi2Te3 nanotubes, with the nano-composite yielding the maximum ZT about 1 at 450 K.10 Bi2Te3 flower-like nanostructures was fabricated by chemical self-assembly rout, obtaining ZT of 0.7 at 180 C.11 It is found that ZT of the reported nanostructured TE materials show much

a)

Authors to whom correspondence should be addressed. Electronic addresses: [email protected] and [email protected]

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dependence on the preparation method; new methods for preparing these nanostructured TE materials are still in demand. As a new strategy, composites of semiconductor and organic conducting polymer have attracted much attention. Thermoelectric property of such unique composites could be optimized by utilizing the bilateral benefits of the semiconductor and organic materials. Wang et al. prepared Bi2S3 nanotubes and poly(3,4-ethylenedioxythiophene) (PEDOT) composite powders, yielding the maximum of power factor 2.3 lW m1 K2, higher than pure PEDOT (0.445 lW m1 K2) and Bi2S3 (1.94 lW m1 K2).12 To further improve the ZT of such composite, the selective organic materials are at issue. One of the great potential organic materials is conducting polymer PEDOT, which has been extensively studied,13–15 possessing a low j of 0.05–0 .6 W m1 K1-one order of magnitude lower than that of the inorganic materials, high conductivity(2.2–4 104 S/m),16,17 excellent stability, and flexible mechanical properties. Sb2Te3 is a p-type narrow gap semiconductor with a band gap of 0.28 eV.18 The high electrical conductivity and low thermal conductivity of Sb2Te3 make it very suitable for being used as the p type legs in the TE devices. In the present study, nanostructured Sb2Te3 was selected as the semiconductor and PEDOT was used as the organic material. By dispersing PEDOT into Sb2Te3 bulk, a p type-p type mingling composites was synthesized. High power factor and low thermal conductivity was obtained. The ZT value could reach as high as 1.18 at 523 K, which was improved by 50% compared with pure Sb2Te3. Sb2Te3/PEDOT composites were synthesized by a lowcost and highly effective process. First, Sb2Te3 nanoplatelets

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were fabricated by using the solvothermal method with a stoichiometric of raw materials SbCl3, K2TeO3, NaOH, and polyvinylphrrolidone (PVP) dissolving in the solution of diethylene glycol (DEG). After stirring and heating at 240 C for 4h, dark suspensions of Sb2Te3 were formed. Then, the obtained suspensions were washed with isopropyl alcohol and acetone several times to remove the residues and dried in the oven at 100 C for 2 h. Subsequently, the dried Sb2Te3 powders were sintered by Spark Plasma Sintering (SPS) under 200 C and a pressure of 70 MPa. The sintered specimens were columns with dimension /15 mm. Then, a rectangular bar with size of 2.3 2.7 12 mm3 was made for Seebeck coefficient and electrical conductivity measurements, and a cubic chunk with size of 8 8 2 mm3 was made for thermal diffusivity measurement. In order to form the Sb2Te3/PEDOT composites, commercialized PEDOT:poly(styrenesulfonate) PSS solution (CLEVIOS PH1000, Heraeus) was chosen due to the fact that PEDOT can be processed in aqueous solution with PSS by appropriate ratio to obtain soluble polymer. The final samples of Sb2Te3/PEDOT composites were obtained by soaking Sb2Te3 into PEDOT:PSS solution and storing the solution at 4 C in refrigerator for 30 days. The phase structure of the Sb2Te3 powders was investigated by X-ray powder diffraction (XRD). The morphology and size of Sb2Te3 nanoplatelets were characterized by fieldemission scanning electron microscopy (FESEM), The Raman spectra was recorded on Confocal Raman Microspectroscopy (RM-1000, Renishaw) with 514.5 nm excitation laser wavelength. The electrical conductivity r and Seebeck coefficient S were measured on a commercial equipment (ZEM-3, ULVAC-RIKO) in a He atmosphere in a temperature range from 300 K to 523 K. The thermal conductivity j was calculated from specific heat (Cp), thermal diffusivity D, and density q using the relationship of j ¼ DCp q. The thermal diffusivity D was measured by Netzsch LFA 457 using laser flash diffusivity method. The specific heat Cp was obtained by differential scanning calorimetry apparatus (DSC-Q50), and the density q was measured by Archimede method. Fig. 1 shows the XRD patterns of Sb2Te3 powders and Sb2Te3/PEDOT composites. It can be seen that all the diffraction peaks of the powders sample can be indexed to wellcrystallized Sb2Te3 with space group R-3m (JCPDS No. 15–0874). No extra peak of elemental Te is detected, and the relative intensity of (015) plane reflection peak is much

stronger than other peaks, revealing that (015) plane is the preferred orientation. For the Sb2Te3/PEDOT composites, the peak positions remain almost the same but higher intensity of background can be observed, implying that PEDOT exists in Sb2Te3 crystalline matrix without changing the crystal structure of Sb2Te3. The morphology and size of the as-prepared Sb2Te3 powders are studied by FESEM, as shown in Fig. 2(a). It can be seen that the powders are composed of homogenous and uniform hexagonal nanosheets with sharp edges and flat surfaces, and most of the nanosheets have sizes of 600 nm or so. Fig. 2(b) shows the FESEM image of the cross section of Sb2Te3 sintered via SPS. It is clearly found that the grains of Sb2Te3 still remain thin and film-like shape, lots of grain boundaries are formed among these plate-like grains, which provide the possibility of PEDOT:PSS dispersing into Sb2Te3 main body. The cross-sectional view of the Sb2Te3/ PEDOT composites (Figs. 2(c) and 2(d)) shows quite different morphology compared with that of Sb2Te3—plate-like grains are no longer observed due to the filling of grain boundaries in Sb2Te3 matrix by PEDOT, after dipping the Sb2Te3 pellet in PEDOT:PSS solution for 30 days. The dark and gray contrast shown in Fig. 2(d) could be corresponding to PEDOT-rich and Sb2Te3-rich areas, respectively. The Raman spectra of Sb2Te3, Sb2Te3/PEDOT composites, and PEDOT in the range of 100–2500 cm1 is shown in Figure 3. All observed peaks in PEDOT are in good agreement with the previous study.19 The Sb2Te3/PEDOT composites, which were tested for 50 thermal cycling, shows the combination Raman results of pure Sb2Te3 and PEDOT. No extra peak is observed, indicating the stable coexistence of the two components. The peaks of the composites at 1429 cm1, 1532 cm1, and 1566 cm1 reveal the presence of the high conductivity form of PEDOT-quinoid structures,20 which have significant influence on the thermoelectric property of the composites. The temperature dependence of r of the Sb2Te3 sample and Sb2Te3/PEDOT composites are shown in Fig. 4(a). As can be seen, the electrical conductivity r of the Sb2Te3 sample decreases as the temperature increases from 300 K to 523 K, showing a metallic behavior. But for the Sb2Te3/PEDOT composites, the electrical conductivity performs differently: it

FIG. 1. XRD patterns of Sb2Te3 powders and Sb2Te3/PEDOT composites pellet.

FIG. 2. FESEM images of Sb2Te3 powders (a), the cross section of Sb2Te3 after SPS (b) and Sb2Te3/PEDOT composites ((c) and (d)).


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FIG. 3. Raman spectra of Sb2Te3, Sb2Te3/PEDOT composites and PEDOT.

first increases with the temperature increasing up to 375 K, showing a semiconductor behavior, and then decreases with temperature when temperature is higher than 375 K, showing a metallic property. In nanostructured Sb2Te3, varied values of electrical conductivities have been reported, for example, 0.323 103 S/m for Sb2Te3 thin films deposited by coevaporation,21 0.11 103 S/m for Sb2Te3 thin films by hydrothermal treatment,22 and (2.33–2.49) 104 S/m for Sb2Te3 nanosheets synthesized by microwave-assisted method.23 The difference in electrical conductivity is about 2–3 order of magnitudes, confirming that the structures and sizes of the grains and the grain boundaries can have much influence on the scattering of carriers. The electrical conductivity of the Sb2Te3 in our experiments is (2.0–1.8) 104 S/m in the temperature range of 300 K to 523 K, which is in the same order as the largest value reported. After dipping, the electrical conductivity of Sb2Te3/PEDOT composites reduces to (1.3–1.1) 104 S/m over the temperature range. The reduction of r is believed to result from the scattering of carriers by the embedded PEDOT.

FIG. 4. Temperature dependence of the thermoelectric properties of Sb2Te3 pure compound and Sb2Te3/PEDOT composites: (a) electrical conductivity r, (b) Seebeck coefficient S, (c) thermal conductivity j, (d) the dimensionless composites.

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The Seebeck coefficients (S) for Sb2Te3 and Sb2Te3/ PEDOT composites shown in Fig. 4(b) are positive over the entire temperature range, showing the p-type behavior. The linear temperature dependence of the Seebeck coefficient indicates the diffusive transport mechanism under the gradient temperature. It is noteworthy that the Seebeck coefficient of the PEDOT/Sb2Te3 composites is higher than that of the pure compound, and the value of 175 lV/K at 523 K is higher than that of p-type Ag(Pb1ySny)mSbTe2þm alloy (135 lV/K).24 The enhancement in Seebeck coefficient for Sb2Te3/PEDOT composites may be attributed to the filtering of low energy charge carriers by the addition of PEDOT embedded inside the Sb2Te3 grain boundaries. This energy filtering can decrease the concentration of carriers and thus increases the Seebeck coefficient and reduces the electrical conductivity of the composites. The temperature dependence of the thermal conductivity j is shown in Fig. 4(c). It is noticed that the thermal conductivity of Sb2Te3/PEDOT composites is very low, compared to that of the pure compound. It remains almost a constant, about 0.15 W m1 K1 over the temperature range from 300 K to 523 K. The reduced j of the Sb2Te3/PEDOT composites is lower than ever reported values of Sb2Te3 compounds or composites.25,26 Such low j is nearly equal to the typical thermal conductivity of PEDOT pellet (0.17 W m1 K1),27 demonstrating that the addition of PEDOT in Sb2Te3 makes a significant contribution to the reduction of j. According to the relationship of Wiedemann-Frantz-Lorenz, the thermal conductivity of a TE material includes two parts of contributions: thermal conductivity of carrier (Kel) and thermal conductivity of phonon (Kph), K ¼ Kel þ Kph. The thermal conductivity of carrier is Kel ¼ LrT, where L is the Lorenz factor. From Fig. 3(a), it is noticed that the electrical conductivity is reduced when adding PEDOT, resulting from the reduction of carrier concentration by the energy filtering effect. The phonon thermal conductivity relates with the transport and scattering of phonons. In this case, the nanostructure of Sb2Te3 can scatter heat carrying phonons. Moreover, the PEDOT distributed in the matrix can be regarded as scattering centers to scatter heat carrying phonons, and the large number of mesoscale boundaries between the two phases can also increase the scattering of phonons, which results in a decrease of thermal conductivity of phonon. The ZT values for Sb2Te3 and Sb2Te3/PEDOT composites are depicted in Fig. 4(d). The ZT of Sb2Te3 compound increases with temperature in range of 300 K to 523 K, reaching the maximum 0.6. The ZT of as-prepared Sb2Te3/ PEDOT composites exhibits the same trend as the pure phase, but outperforms it over the whole temperature range. The maximum ZT obtained for the composites is found to be 1.18 at 523 K, double of the value of the pure Sb2Te3 phase. Such high thermoelectric properties have seldom been reported on Sb2Te3/organic composites. The high ZT for our synthesized Sb2Te3/PEDOT composites is due to the high r and low j, resulting from strong phonon scattering by embedded PEDOT. Thermal stability was investigated on Sb2Te3/PEDOT composites after 50 periodic thermal cycling from room temperature to 523 K. Fig. 5 shows the dependence of j on times of thermal cycles. The value of j at 523 K increases slightly


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Science (No. 51172166), and National Science Fund for Talent Training in Basic Science (No. J1210061).

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FIG. 5. Thermal cycle of Sb2Te3/PEDOT composites (thermal conductivity measured at 523 K).

with number of thermal cycles. After cycling for 50 times, the thermal conductivity j of the composites increases from 0.148 W m1 K1 to 0.205 W m1 K1. This value is 38.7% higher than the as-prepared composites, but still far below that of the primary Sb2Te3 alloys. The ZT remains a high value 0.857 at 523 K with thermal cycles up to 50 times, higher than that of the pure Sb2Te3 samples. It seems that the thermal conductivity j increases to a saturation value with the increase of times of thermal cycles, implying good thermal stability of the Sb2Te3/PEDOT composites. The low j and high thermal stability makes Sb2Te3/PEDOT composites a promising TE material, where the operating temperature is periodically fluctuant. In summary, p-type PEDOT is integrated into hexagonal shaped p-type Sb2Te3 matrix. r of the composites does not subject to significant change while S increases. It is interesting to mention that the j of the composites is significantly reduced from 0.398 W m1 K1 to 0.148 W m1 K1 at 523 K, owing to dramatic scattering of heat carrying phonons by Sb2Te3 nanoparticles and the embedded PEDOT. The figure of merit for this organic and inorganic composites is 1.18, increasing 60% compared with our pure Sb2Te3 sample. The composites are stable and remain low j after 50 periodic thermal cycles in the temperature range between room temperature and 523 K. The authors would like to acknowledge the financial support from 973 Program (Nos. 2013CB632502 and 2012CB821404), Chinese National Foundation of Natural
