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polyaniline composites with improved thermoelectric properties
Materials Letters 164 (2016) 132–135
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Semiconductor to metallic behavior transition in multi-wall carbon nanotubes/polyaniline composites with improved thermoelectric properties Yao Wang n, Shuangmei Zhang, Yuan Deng School of Materials Science and Engineering, Ministry of Education Key Laboratory of Aerospace Advanced Materials and Performance, Beihang University, Beijing 100191, China
art ic l e i nf o
a b s t r a c t
Article history: Received 21 September 2015 Received in revised form 22 October 2015 Accepted 28 October 2015 Available online 30 October 2015
Multi-wall carbon nanotubes/polyaniline (MWCNT/PANI) composites were prepared by the in situ polymerization at room temperature and in ice-water bath, respectively. Raman spectra and X-ray diffraction patterns showed that the molecular conformation and the degree of order of PANI were greatly affected by the synthesis temperature. In low temperature condition, quinoid units converted into benzenoid in PANI and meanwhile, the chain segments of MWCNT/PANI arranged in a more orderly arrangement, which resulted in a change of the conducting behavior from semiconductor to metallic and almost twice increase in conductivity. Power factor (PF) of both composites increased with the increase of MWCNTs content, and 66.7% higher PF value was obtained in metallic-like composite with 80 wt% MWCNTs loading. & 2015 Elsevier B.V. All rights reserved.
Keywords: Polymeric composites Carbon nanotubes Polyaniline Electrical properties Thermoelectric properties
1. Introduction Thermoelectric (TE) materials and devices are attracting increasing attention for their eco-friendly applications in power generation and refrigeration. The performance of TE materials is quantified by the TE figure of merit: ZT ¼ S2sT/κ, where S, s, T, and κ are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. S2s is the power factor (PF). Organic semiconductor materials offer advantages over inorganic semiconductors in thermoelectric applications, such as low density, low cost due to rich resources, high toughness and elasticity [1,2]. Furthermore, the intrinsically low thermal conductivities of conductive polymers make them stand out as potential candidates for high-performance thermoelectric materials [3,4]. Polyaniline (PANI) has gained special attention among conductive polymers due to its good processability, environmental stability, and tunable electrical properties. However, the relatively low conductivity ( 10 7 320 S/cm) and Seebeck coefficient of PANI compared to inorganic semiconductor TE materials have prevented it being applied as high-performance TE materials. The properties of PANI can be improved by selecting the method of preparation, the dopants, or by mixing with other materials to form composites [5–8]. Carbon nanotubes (CNTs) n
Corresponding author. E-mail address: [email protected] (Y. Wang).
http://dx.doi.org/10.1016/j.matlet.2015.10.138 0167-577X/& 2015 Elsevier B.V. All rights reserved.
showing excellent intrinsic electrical properties and structural characteristics have been employed as ideal fillers to improve the TE performances of PANI-based TE composites [9–11]. A maximum conductivity of PANI/CNT was reported to reach 769 S/cm due to the formation of an ordered PANI chain structure on the surface of CNTs induced by strong π–π conjugated interactions between carbon nanotubes and polyaniline [9]. Doping is the key to maximizing the TE power factor of organic TE materials because dopants can not only modify the conformation of conducting host molecules and thereby alter their carrier transport properties, but also typically increase the tunneling distance between these molecules and hence greatly reduce the rate of thermally activated hopping [12]. Meanwhile, doping PANI is strongly dependent on preparation conditions and temperature [1,13]. Thus, in this study, MWCNTs/PANI composites were synthesized via in situ polymerization at room temperature and in icewater bath, respectively; to tune the conformation of PANI molecules and morphology of polymer chains. Their effects on the conducting behavior and TE properties of the composites were investigated.
2. Experimental The MWCNTs (purity 497 wt% with diameter between 10– 20 nm, length more than 5 μm) were purchased from the Shenzhen Nanotech Port Co. Ltd. Aniline was purified through
Y. Wang et al. / Materials Letters 164 (2016) 132–135
distillation before use. All chemicals were of analytical grade and used as received. The PANI/MWCNTs composites were fabricated via in situ polymerization method with the following process. 2.328 g aniline was added in 100 ml of 0.5 M HCl and stirred until the aniline was completely dissolved in the solution. The CNTs were dispersed in the mixed solution by sonication for 30 min. Then the mixture was kept in ice-water bath and stirred. After 30 min, 5.705 g ammonium persulfate was dissolved in HCl solution and then added to the reaction mixture dropwise. Subsequently, the mixture was stirred for 4 h in ice-water bath. The product was filtered, washed repeatedly with distilled water and acetone, and dried at 60 °C. The obtained samples were marked as i-PANI/MWCNTs. The same procedure was used to make samples with polymerization reaction occurring at room temperature, marked as r-PANI/MWCNTs. X-ray diffraction (XRD) analysis was carried out on Smartlab (Rigaku). Raman spectroscopy measurement was carried out on a LabRAM HR Evolution (HORIBA) using an Ar–Kr laser in wavelength 632.8 nm. Scanning electron microscopy (SEM, JEOL JSM6010) was used to observe the morphologies of the composites. Electrical conductivity and Seebeck coefficient were measured on bulk samples made from cold-pressing by a ZEM-3 system (ULVAC-RIKO). Hall measurements were carried out on Physical Properties Measurement System.
3. Results and discussions Raman spectra of i-PANI/MWCNTs and r-PANI/MWCNTs composites are shown in Fig.1(a). The characteristic Raman peaks of
133
PANI changed greatly with different reaction temperatures. The strong peak at 1462 cm 1 of r-PANI/MWCNTs assigned to the C ¼ N stretching of the quinoid diamine units remarkably weakened in iPANI/MWCNTs. The peak assigned to C–H bending of benzoid rings located at 1168 cm 1 in i-PANI/MWCNTs slightly shifts to 1160 cm 1 in r-PANI/MWCNTs, which is the same C–H bending mode in the quinoid segments. A small shift of C–C stretching vibration mode of benzenoid rings at 1596 cm 1 to C ¼C stretching vibration in the quinoid segments at 1586 cm 1 was also observed. These changes of Raman modes unambiguously confirm the transformation of quinoid units to benzenoid rings in PANI as preparation temperature decreases to 0 °C. The mode at 1334 cm 1 is regarded as the characteristic vibration from radical cation C–N þ [9], whose intensity is stronger in i-PANI/MWCNTs than in r-PANI/MWCNTS, indicating higher doping level of i-PANI/ MWCNTs. Furthermore, the doping level could also be promoted by increasing MWCNTs contents in both i-PANI/MWCNTs and tPANI/MWCNTs. Meanwhile, the modes at 1391 and 1643 cm 1 are related to delocalized polarons in the extended polymeric conformation, which appeared only in i-PANI/MWCNTs demonstrating that molecular conformation of PANI became more expanded and the number and effective degree of delocalization of the polarons are enhanced in low-temperature fabricated i-PANI/ MWCNTs [9]. Therefore, the PANI molecular conformations are concluded and schematically illustrated in Fig.1(c) as preparation temperature changes. XRD patterns of PANI/MWCNTs composites are shown in Fig.1 (b). For PANI, three characteristic diffraction peaks locate at 2θ ¼15°, 20° and 25°, assigned to the periodicity of the repeat unit of the PANI chains, the periodicities perpendicular and parallel to
Fig. 1. (a) Raman spectra and (b) XRD patterns of PANI/MWCNTs composites with different weight fraction of MWCNTs. (* stands for the peak of CNTs) (c) Schematic illustration of PANI molecular conformations at different preparation temperature.
134
Y. Wang et al. / Materials Letters 164 (2016) 132–135
Fig. 2. Cross-sectional SEM images of (a) r-PANI/MWCNTs and (b) i-PANI/MWCNTs at 80 wt% CNTs content.
the polymer backbone chains, respectively. Much broadened and weakened diffraction peaks of r-PANI/MWCNTs compared with those of i-PANI/MWCNTs evidently suggest more ordered molecular arrangement in PANI/MWCNTs composites fabricated at low temperature. Cross-sectional SEM images of the bulk samples with 80 wt% CNTs content are shown in Fig.2. As seen from Fig. 2(a), partial CNTs were exposed from the PANI matrix and agglomerated in rPANI/MWCNTs. In comparison, i-PANI/MWCNTs bulk was much denser with little exposed and agglomerated CNTs [see Fig. 2(b)], which suggests good adhesion between CNTs and PANI polymer and well dispersion of CNTs in PANI matrix. The changes in conformations and degree of order of PANI molecule as well as morphology of the composites undoubtedly influence the carrier transport properties of the composites. As shown in Fig. 3(a), the conductivities of both PANI/MWCNTs composites increase significantly with increasing CNTs fraction. Due to the higher doping level, more delocalization of the polarons and more orderly arrangement of the polymer chains in i-PANI/ MWCNTs, increased mobility (μ) and carrier concentration (n) and hence much higher conductivity (s) are obtained as shown in Table 1. In contrast, Seebeck coefficient (S) of r-PANI/MWCNTs is
Table 1 Comparison of transport properties between i-PANI/MWCNTs and r-PANI/MWCNTs at 80 wt% CNTs content.
i-PANI/80% MWCNT r-PANI/80% MWCNT
s (S/m)
n (cm-3)
μ (cm2/V s)
2160 1200
1.47E þ 20 1.23E þ 20
0.917 0.608
larger than that of i-PANI/MWCNTs at low CNTs fraction. But S gradually decreases with the increase of CNTs fraction, while S of iPANI/MWCNTs shows the opposite trend. When CNTs content was up to 80 wt%, both PANI/MWCNTs composites show nearly equal value. The PF values of composites increase rapidly as CNTs content exceeds 40 wt%, and the PF value 0.355 μW/m K2 is obtained in 80 wt% i-PANI/MWCNTs, which is 66.7% higher than that of rPANI/MWCNTs, about 60 times larger than pristine PANI. Fig. 3(b) and (c) shows the temperature-dependent TE properties of i-PANI/MWCNTs and r-PANI/MWCNTs, respectively. The conductivity of i-PANI/MWCNTs decreases with increase of temperature, showing metallic-like conducting behavior, caused by the formation of ordered regions in the polyaniline molecular
Fig. 3. (a) Comparison of TE properties between i-PANI/MWCNTs and r-PANI/MWCNT with various MWCNTs fraction. Temperature-dependent TE properties of (b) i-PANI/ MWCNTs, and (c) r-PANI/MWCNTs with CNTs fraction 0, 40, 80 wt%, respectively.
Y. Wang et al. / Materials Letters 164 (2016) 132–135
structure. While the conductivity of r-PANI/MWCNTs increases with the increase of temperature, indicating semiconductor-like conducting behavior caused by the disordered regions in the PANI molecular structure [14]. As CNTs content increases, the variation of conductivities of the composites with temperature becomes flat. S and PF values of all the composites increase with increasing temperature. The maximum PF value 0.501 μW/m K2 is obtained in i-PANI/MWCNTs at 364 K.
135
Program for Basic Research of China (No. 2012CB933200 ), National Natural Science Foundation of China (Nos. 51172008 and 51002006), National Natural Science Fund Innovation Group (No. 51221163), and the Fundamental Research Funds for the Central Universities.
References 4. Conclusions
[1] [2] [3] [4]
Different preparation temperature was used to tune the molecular conformation and thereby TE properties of PANI/MWCNTs composites. At low preparation temperature, quinoid units converted into benzenoid inducing higher doping level and more delocalization of the polarons; meanwhile, more orderly arrangement of the polymer chains was achieved. Therefore, a change of the conducting behavior from semiconductor to metallic was observed. The greatly increased conductivity from 1200 to 2160 S/m results in a 66.7% enhancement in PF value of PANI/MWCNT composite with 80 wt% CNTs content at room temperature. The study provides a route to fabricate polymer based TE material with a potential application as flexible energy conversion devices.
[11]
Acknowledgments
[12] [13]
[5] [6] [7] [8] [9] [10]
[14]
The work was supported by the State Key Development
Y. Du, S.Z. Shen, K.F. Cai, P.S. Casey, Prog. Polym. Sci. 37 (2012) 820–841. J.H. Yang, H.L. Tip, A. Jen, Adv. Energy Mater. 3 (2013) 549–565. S. Pal, G. Balasubramanian, I.K. Puri, Appl. Phys. Lett. 102 (2013) 023109. H. Pang, Y.Y. Piao, Y.Q. Tan, G.Y. Jiang, J.H. Wang, Z.M. Li, Mater. Lett. 107 (2013) 150–153. K. Chatterjee, S. Ganguly, K. Kargupta, D. Banerjee, Synth. Met. 161 (2011) 275–279. W.J. Wang, S.P. Sun, S.J. Gu, H.W. Shen, Q.H. Zhang, J.J. Zhu, et al., RSC Adv. 4 (2014) 26810–26816. L.M. Wang, Q. Yao, H. Bi, F.Q. Huang, Q. Wang, L.D. Chen, J. Mater. Chem. A 2 (2014) 11107–11113. P. Limelette, B. Schmaltz, D. Brault, M. Gouineau, C. Autret-Lambert, S. Roger, et al., J. Appl. Phys. 115 (2014) 033712. Q. Yao, Q. Wang, L.M. Wang, L.D. Chen, Energy Environ. Sci. 7 (2014) 3801–3807. Q.L. Zhang, W.J. Wang, J.L. Li, J.J. Zhu, L.J. Wang, M.F. Zhu, et al., J. Mater. Chem. A 1 (2013) 12109–12114. K. Zhang, M. Davis, J.J. Qiu, L. Hope-Weeks, S. Wang, Nanotechnology 23 (2012) 385701. G.H. Kim, L. Shao, K. Zhang, K.P. Pipe, Nat. Mater. 12 (2013) 719–723. J. Liu, L.M. Zhang, L. He, X.F. Tang, J. Wuhan Univ. Technol. Mater. Sci. Ed. 18 (2003) 53–55. Q. Yao, Q. Wang, L.M. Wang, Y. Wang, J. Sun, H.R. Zeng, et al., J. Mater. Chem. A 2 (2014) 2634–2640.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Semiconductor to metallic behavior transition in multi-wall carbon nanotubes/polyaniline composites with improved thermoelectric properties Yao Wang n, Shuangmei Zhang, Yuan Deng School of Materials Science and Engineering, Ministry of Education Key Laboratory of Aerospace Advanced Materials and Performance, Beihang University, Beijing 100191, China
art ic l e i nf o
a b s t r a c t
Article history: Received 21 September 2015 Received in revised form 22 October 2015 Accepted 28 October 2015 Available online 30 October 2015
Multi-wall carbon nanotubes/polyaniline (MWCNT/PANI) composites were prepared by the in situ polymerization at room temperature and in ice-water bath, respectively. Raman spectra and X-ray diffraction patterns showed that the molecular conformation and the degree of order of PANI were greatly affected by the synthesis temperature. In low temperature condition, quinoid units converted into benzenoid in PANI and meanwhile, the chain segments of MWCNT/PANI arranged in a more orderly arrangement, which resulted in a change of the conducting behavior from semiconductor to metallic and almost twice increase in conductivity. Power factor (PF) of both composites increased with the increase of MWCNTs content, and 66.7% higher PF value was obtained in metallic-like composite with 80 wt% MWCNTs loading. & 2015 Elsevier B.V. All rights reserved.
Keywords: Polymeric composites Carbon nanotubes Polyaniline Electrical properties Thermoelectric properties
1. Introduction Thermoelectric (TE) materials and devices are attracting increasing attention for their eco-friendly applications in power generation and refrigeration. The performance of TE materials is quantified by the TE figure of merit: ZT ¼ S2sT/κ, where S, s, T, and κ are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. S2s is the power factor (PF). Organic semiconductor materials offer advantages over inorganic semiconductors in thermoelectric applications, such as low density, low cost due to rich resources, high toughness and elasticity [1,2]. Furthermore, the intrinsically low thermal conductivities of conductive polymers make them stand out as potential candidates for high-performance thermoelectric materials [3,4]. Polyaniline (PANI) has gained special attention among conductive polymers due to its good processability, environmental stability, and tunable electrical properties. However, the relatively low conductivity ( 10 7 320 S/cm) and Seebeck coefficient of PANI compared to inorganic semiconductor TE materials have prevented it being applied as high-performance TE materials. The properties of PANI can be improved by selecting the method of preparation, the dopants, or by mixing with other materials to form composites [5–8]. Carbon nanotubes (CNTs) n
Corresponding author. E-mail address: [email protected] (Y. Wang).
http://dx.doi.org/10.1016/j.matlet.2015.10.138 0167-577X/& 2015 Elsevier B.V. All rights reserved.
showing excellent intrinsic electrical properties and structural characteristics have been employed as ideal fillers to improve the TE performances of PANI-based TE composites [9–11]. A maximum conductivity of PANI/CNT was reported to reach 769 S/cm due to the formation of an ordered PANI chain structure on the surface of CNTs induced by strong π–π conjugated interactions between carbon nanotubes and polyaniline [9]. Doping is the key to maximizing the TE power factor of organic TE materials because dopants can not only modify the conformation of conducting host molecules and thereby alter their carrier transport properties, but also typically increase the tunneling distance between these molecules and hence greatly reduce the rate of thermally activated hopping [12]. Meanwhile, doping PANI is strongly dependent on preparation conditions and temperature [1,13]. Thus, in this study, MWCNTs/PANI composites were synthesized via in situ polymerization at room temperature and in icewater bath, respectively; to tune the conformation of PANI molecules and morphology of polymer chains. Their effects on the conducting behavior and TE properties of the composites were investigated.
2. Experimental The MWCNTs (purity 497 wt% with diameter between 10– 20 nm, length more than 5 μm) were purchased from the Shenzhen Nanotech Port Co. Ltd. Aniline was purified through
Y. Wang et al. / Materials Letters 164 (2016) 132–135
distillation before use. All chemicals were of analytical grade and used as received. The PANI/MWCNTs composites were fabricated via in situ polymerization method with the following process. 2.328 g aniline was added in 100 ml of 0.5 M HCl and stirred until the aniline was completely dissolved in the solution. The CNTs were dispersed in the mixed solution by sonication for 30 min. Then the mixture was kept in ice-water bath and stirred. After 30 min, 5.705 g ammonium persulfate was dissolved in HCl solution and then added to the reaction mixture dropwise. Subsequently, the mixture was stirred for 4 h in ice-water bath. The product was filtered, washed repeatedly with distilled water and acetone, and dried at 60 °C. The obtained samples were marked as i-PANI/MWCNTs. The same procedure was used to make samples with polymerization reaction occurring at room temperature, marked as r-PANI/MWCNTs. X-ray diffraction (XRD) analysis was carried out on Smartlab (Rigaku). Raman spectroscopy measurement was carried out on a LabRAM HR Evolution (HORIBA) using an Ar–Kr laser in wavelength 632.8 nm. Scanning electron microscopy (SEM, JEOL JSM6010) was used to observe the morphologies of the composites. Electrical conductivity and Seebeck coefficient were measured on bulk samples made from cold-pressing by a ZEM-3 system (ULVAC-RIKO). Hall measurements were carried out on Physical Properties Measurement System.
3. Results and discussions Raman spectra of i-PANI/MWCNTs and r-PANI/MWCNTs composites are shown in Fig.1(a). The characteristic Raman peaks of
133
PANI changed greatly with different reaction temperatures. The strong peak at 1462 cm 1 of r-PANI/MWCNTs assigned to the C ¼ N stretching of the quinoid diamine units remarkably weakened in iPANI/MWCNTs. The peak assigned to C–H bending of benzoid rings located at 1168 cm 1 in i-PANI/MWCNTs slightly shifts to 1160 cm 1 in r-PANI/MWCNTs, which is the same C–H bending mode in the quinoid segments. A small shift of C–C stretching vibration mode of benzenoid rings at 1596 cm 1 to C ¼C stretching vibration in the quinoid segments at 1586 cm 1 was also observed. These changes of Raman modes unambiguously confirm the transformation of quinoid units to benzenoid rings in PANI as preparation temperature decreases to 0 °C. The mode at 1334 cm 1 is regarded as the characteristic vibration from radical cation C–N þ [9], whose intensity is stronger in i-PANI/MWCNTs than in r-PANI/MWCNTS, indicating higher doping level of i-PANI/ MWCNTs. Furthermore, the doping level could also be promoted by increasing MWCNTs contents in both i-PANI/MWCNTs and tPANI/MWCNTs. Meanwhile, the modes at 1391 and 1643 cm 1 are related to delocalized polarons in the extended polymeric conformation, which appeared only in i-PANI/MWCNTs demonstrating that molecular conformation of PANI became more expanded and the number and effective degree of delocalization of the polarons are enhanced in low-temperature fabricated i-PANI/ MWCNTs [9]. Therefore, the PANI molecular conformations are concluded and schematically illustrated in Fig.1(c) as preparation temperature changes. XRD patterns of PANI/MWCNTs composites are shown in Fig.1 (b). For PANI, three characteristic diffraction peaks locate at 2θ ¼15°, 20° and 25°, assigned to the periodicity of the repeat unit of the PANI chains, the periodicities perpendicular and parallel to
Fig. 1. (a) Raman spectra and (b) XRD patterns of PANI/MWCNTs composites with different weight fraction of MWCNTs. (* stands for the peak of CNTs) (c) Schematic illustration of PANI molecular conformations at different preparation temperature.
134
Y. Wang et al. / Materials Letters 164 (2016) 132–135
Fig. 2. Cross-sectional SEM images of (a) r-PANI/MWCNTs and (b) i-PANI/MWCNTs at 80 wt% CNTs content.
the polymer backbone chains, respectively. Much broadened and weakened diffraction peaks of r-PANI/MWCNTs compared with those of i-PANI/MWCNTs evidently suggest more ordered molecular arrangement in PANI/MWCNTs composites fabricated at low temperature. Cross-sectional SEM images of the bulk samples with 80 wt% CNTs content are shown in Fig.2. As seen from Fig. 2(a), partial CNTs were exposed from the PANI matrix and agglomerated in rPANI/MWCNTs. In comparison, i-PANI/MWCNTs bulk was much denser with little exposed and agglomerated CNTs [see Fig. 2(b)], which suggests good adhesion between CNTs and PANI polymer and well dispersion of CNTs in PANI matrix. The changes in conformations and degree of order of PANI molecule as well as morphology of the composites undoubtedly influence the carrier transport properties of the composites. As shown in Fig. 3(a), the conductivities of both PANI/MWCNTs composites increase significantly with increasing CNTs fraction. Due to the higher doping level, more delocalization of the polarons and more orderly arrangement of the polymer chains in i-PANI/ MWCNTs, increased mobility (μ) and carrier concentration (n) and hence much higher conductivity (s) are obtained as shown in Table 1. In contrast, Seebeck coefficient (S) of r-PANI/MWCNTs is
Table 1 Comparison of transport properties between i-PANI/MWCNTs and r-PANI/MWCNTs at 80 wt% CNTs content.
i-PANI/80% MWCNT r-PANI/80% MWCNT
s (S/m)
n (cm-3)
μ (cm2/V s)
2160 1200
1.47E þ 20 1.23E þ 20
0.917 0.608
larger than that of i-PANI/MWCNTs at low CNTs fraction. But S gradually decreases with the increase of CNTs fraction, while S of iPANI/MWCNTs shows the opposite trend. When CNTs content was up to 80 wt%, both PANI/MWCNTs composites show nearly equal value. The PF values of composites increase rapidly as CNTs content exceeds 40 wt%, and the PF value 0.355 μW/m K2 is obtained in 80 wt% i-PANI/MWCNTs, which is 66.7% higher than that of rPANI/MWCNTs, about 60 times larger than pristine PANI. Fig. 3(b) and (c) shows the temperature-dependent TE properties of i-PANI/MWCNTs and r-PANI/MWCNTs, respectively. The conductivity of i-PANI/MWCNTs decreases with increase of temperature, showing metallic-like conducting behavior, caused by the formation of ordered regions in the polyaniline molecular
Fig. 3. (a) Comparison of TE properties between i-PANI/MWCNTs and r-PANI/MWCNT with various MWCNTs fraction. Temperature-dependent TE properties of (b) i-PANI/ MWCNTs, and (c) r-PANI/MWCNTs with CNTs fraction 0, 40, 80 wt%, respectively.
Y. Wang et al. / Materials Letters 164 (2016) 132–135
structure. While the conductivity of r-PANI/MWCNTs increases with the increase of temperature, indicating semiconductor-like conducting behavior caused by the disordered regions in the PANI molecular structure [14]. As CNTs content increases, the variation of conductivities of the composites with temperature becomes flat. S and PF values of all the composites increase with increasing temperature. The maximum PF value 0.501 μW/m K2 is obtained in i-PANI/MWCNTs at 364 K.
135
Program for Basic Research of China (No. 2012CB933200 ), National Natural Science Foundation of China (Nos. 51172008 and 51002006), National Natural Science Fund Innovation Group (No. 51221163), and the Fundamental Research Funds for the Central Universities.
References 4. Conclusions
[1] [2] [3] [4]
Different preparation temperature was used to tune the molecular conformation and thereby TE properties of PANI/MWCNTs composites. At low preparation temperature, quinoid units converted into benzenoid inducing higher doping level and more delocalization of the polarons; meanwhile, more orderly arrangement of the polymer chains was achieved. Therefore, a change of the conducting behavior from semiconductor to metallic was observed. The greatly increased conductivity from 1200 to 2160 S/m results in a 66.7% enhancement in PF value of PANI/MWCNT composite with 80 wt% CNTs content at room temperature. The study provides a route to fabricate polymer based TE material with a potential application as flexible energy conversion devices.
[11]
Acknowledgments
[12] [13]
[5] [6] [7] [8] [9] [10]
[14]
The work was supported by the State Key Development
Y. Du, S.Z. Shen, K.F. Cai, P.S. Casey, Prog. Polym. Sci. 37 (2012) 820–841. J.H. Yang, H.L. Tip, A. Jen, Adv. Energy Mater. 3 (2013) 549–565. S. Pal, G. Balasubramanian, I.K. Puri, Appl. Phys. Lett. 102 (2013) 023109. H. Pang, Y.Y. Piao, Y.Q. Tan, G.Y. Jiang, J.H. Wang, Z.M. Li, Mater. Lett. 107 (2013) 150–153. K. Chatterjee, S. Ganguly, K. Kargupta, D. Banerjee, Synth. Met. 161 (2011) 275–279. W.J. Wang, S.P. Sun, S.J. Gu, H.W. Shen, Q.H. Zhang, J.J. Zhu, et al., RSC Adv. 4 (2014) 26810–26816. L.M. Wang, Q. Yao, H. Bi, F.Q. Huang, Q. Wang, L.D. Chen, J. Mater. Chem. A 2 (2014) 11107–11113. P. Limelette, B. Schmaltz, D. Brault, M. Gouineau, C. Autret-Lambert, S. Roger, et al., J. Appl. Phys. 115 (2014) 033712. Q. Yao, Q. Wang, L.M. Wang, L.D. Chen, Energy Environ. Sci. 7 (2014) 3801–3807. Q.L. Zhang, W.J. Wang, J.L. Li, J.J. Zhu, L.J. Wang, M.F. Zhu, et al., J. Mater. Chem. A 1 (2013) 12109–12114. K. Zhang, M. Davis, J.J. Qiu, L. Hope-Weeks, S. Wang, Nanotechnology 23 (2012) 385701. G.H. Kim, L. Shao, K. Zhang, K.P. Pipe, Nat. Mater. 12 (2013) 719–723. J. Liu, L.M. Zhang, L. He, X.F. Tang, J. Wuhan Univ. Technol. Mater. Sci. Ed. 18 (2003) 53–55. Q. Yao, Q. Wang, L.M. Wang, Y. Wang, J. Sun, H.R. Zeng, et al., J. Mater. Chem. A 2 (2014) 2634–2640.