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[18p-P15-2] Thermoelectric Properties of Semiconducting Single-walled Carbon Nanotube Sheets
Keywords:carbon nanotube, thermoelectric conversion, Seebeck coefficient
Single-walled carbon nanotubes (SWNTs) have attracted strong attention as thermoelectric (TE) material due to their extremely high electrical conductivity (σ), light weight, mechanical toughness and flexibility. However, SWNTs are produced as a mixture of metallic (m-) and semiconducting SWNTs (s-SWNTs). s-SWNT sheet is proved to have large Seebeck coefficient (S) theoretically and experimentally [1, 2], however, its improvement in ZT value is still uncertain compared with the unsorted SWNT sheet. In this study, we systematically characterized the TE properties of s-SWNT sheets with different ratio of m-SWNTs as well as the unsorted SWNT sheet to evaluate the necessity of s-SWNT extraction from its SWNT mixture. Density gradient ultracentrifugation (DGU) is used as the extraction method of commercial s- and m-SWNT [3]. However, DGU method may induce defects in s- and m-SWNTs. To cancel this factor, we mixed the extracted s- and m-SWNT in the ratio of 4:1, 2:1 and compared their ZT value with pure s- and m-SWNT. Raman and UV spectra were used to characterize the defect of the extracted SWNTs and the ratio to s- and m-SWNTs, respectively.
As for the experiment, s-SWNTs 2.0 mg (98% Nanointegris) and m-SWNT 1.0 mg (98% Nanointegris) were added to 0.5% SDBS solution and dispersed by bath sonicator (BRANSON, 1 hr) and probe sonicator (TOMY UD-200). The dispersion were filtrated and the free-standing s-:m-SWNT=2:1 sheet was obtained (2:1 mix). s:m-SWNT=4:1 sheet (4:1 mix), s- and m-SWNT sheets were made in the same fashion, using 2.4 mg s-SWNT and 0.6 mg s-SWNT, 3.0 mg s-SWNT and 3.0 mg m-SWNT respectively. In-plane electrical conductivity and Seebeck coefficient of were measured by ZEM-3 (ADVANCE RIKO) from 30 to 100 ºC. The specific heat capacity (Cp) was measured by differential scanning calorimetry (DSC) method using EXSTAR DSC 6200 (SII Nanotechnology) at the heating rate of 10 K min-1. In-plane thermal diffusivities (α) were evaluated using a Thermowave Analyzer TA (Bethel Co., Ltd.,). We found that unsorted SWNT sheet exhibits nearly 1.9 times higher electrical conductivity than the m-SWNT sheet, due to the defects induced during the DGU extraction of the m-SWNT, which was confirmed by the larger D band of m-SWNT than that of unsorted SWNT in Raman spectra. [1] Ferguson, A. J. et al. Nature Energy, 1, 16033 (2016) [2] Maniwa, Y. et al, Appl. Phys. Express, 7, 025103 (2014) [3] Liu, Y. et al, Chem. Soc. Rev., 40, 1324 (2011)
As for the experiment, s-SWNTs 2.0 mg (98% Nanointegris) and m-SWNT 1.0 mg (98% Nanointegris) were added to 0.5% SDBS solution and dispersed by bath sonicator (BRANSON, 1 hr) and probe sonicator (TOMY UD-200). The dispersion were filtrated and the free-standing s-:m-SWNT=2:1 sheet was obtained (2:1 mix). s:m-SWNT=4:1 sheet (4:1 mix), s- and m-SWNT sheets were made in the same fashion, using 2.4 mg s-SWNT and 0.6 mg s-SWNT, 3.0 mg s-SWNT and 3.0 mg m-SWNT respectively. In-plane electrical conductivity and Seebeck coefficient of were measured by ZEM-3 (ADVANCE RIKO) from 30 to 100 ºC. The specific heat capacity (Cp) was measured by differential scanning calorimetry (DSC) method using EXSTAR DSC 6200 (SII Nanotechnology) at the heating rate of 10 K min-1. In-plane thermal diffusivities (α) were evaluated using a Thermowave Analyzer TA (Bethel Co., Ltd.,). We found that unsorted SWNT sheet exhibits nearly 1.9 times higher electrical conductivity than the m-SWNT sheet, due to the defects induced during the DGU extraction of the m-SWNT, which was confirmed by the larger D band of m-SWNT than that of unsorted SWNT in Raman spectra. [1] Ferguson, A. J. et al. Nature Energy, 1, 16033 (2016) [2] Maniwa, Y. et al, Appl. Phys. Express, 7, 025103 (2014) [3] Liu, Y. et al, Chem. Soc. Rev., 40, 1324 (2011)