Thermoelectric properties of sorted semiconducting single-walled carbon nanotube sheets

Abstract

Single-walled carbon nanotubes (SWNTs), especially their semiconducting type, are promising thermoelectric (TE) materials due to their high Seebeck coefficient. In this study, the in-plane Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (κ) of sorted semiconducting SWNT (s-SWNT) free-standing sheets with different s-SWNT purities are measured to determine the figure of merit ZT. We find that the ZT value of the sheets increases with increasing s-SWNT purity, mainly due to an increase in Seebeck coefficient while the thermal conductivity remaining constant, which experimentally proved the superiority of the high purity s-SWNT as TE materials for the first time. In addition, from the comparison between sorted and unsorted SWNT sheets, it is recognized that the difference of ZT between unsorted SWNT and high-purity s-SWNT sheet is not remarkable, which suggests the control of carrier density is necessary to further clarify the superiority of SWNT sorting for TE applications.

Keywords: 104 Carbon and related materials; 105 Low-Dimension (1D/2D) materials; 206 Energy conversion / transport / storage / recovery; 210 Thermoelectronics / Thermal transport / insulators; 50 Energy Materials; Seebeck coefficient; Semiconducting carbon nanotubes; figure of merit; thermal conductivity; thermoelectric conversion.

Graphical abstract

Figure 1.

In-plane (a) Seebeck coefficient, (b) electrical conductivity, and (c) PF of the as-purchased raw SWNT sheet (black), s-SWNT sheet (red), and m-SWNT sheet (blue) from 30 to 100 °C in a helium atmosphere at 0.01 MPa.

Figure 2.

(a) UV-vis-NIR absorption spectra of s-SWNT dispersions with various s-SWNT purities: 98% (red), 80% (black), 67% (green), 33% (blue), and 2% (purple). (b) The S₂₂ absorption peak area as a proportion of total peak area (S₂₂+M₁₁) as a function of the s-SWNT purity. The dotted lines was added as eye guide.

Figure 3.

In-plane (a) Seebeck coefficient and (b) electrical conductivity as a function of the s-SWNT purity (red circles). (c) Seebeck coefficient as a function of electrical conductivity. PF of s-SWNT sheets as a function of (d) s-SWNT purity and (e) electrical conductivity. The TE values of the unsorted SWNT sheet are also plotted (black squares). Inset in (a) shows the photograph of free-standing 98% s-SWNT sheet. The blue dotted lines were added as eye guide.

Figure 4.

In-plane (a) thermal conductivity and (b) ZT of s-SWNT sheets as a function of the s-SWNT purity (red circles). The TE values of the unsorted SWNT sheet are also plotted (black squares). The blue dotted lines were added as eye guide.