Boron-Doped Single-Walled Carbon Nanotubes with Enhanced Thermoelectric Power Factor for Flexible Thermoelectric Devices

Abstract

We report a detailed experimental and theoretical study on thermoelectric properties of boron-doped single-walled carbon nanotubes (B-SWCNTs), which are versatile building blocks of flexible thermoelectric devices. Implantations of substitutional boron dopants (0.1−0.5 atom %) in SWCNTs are realized using thermal diffusion. The after-synthesis boron doping simultaneously improves the Seebeck coefficient (S) and electrical conductivity (σ) of SWCNT networks, leading to an S 2 σ value of 226 μW/mK 2 . First-principle calculations indicate that a few tenths atom % of substitutional boron atoms improve the S value of semi-conducting SWCNTs but reduce the electron conductance in individual SWCNTs. The high σ of B-SWCNT networks is attributed to the improved electrical transport between laterally contacted metallic and semi-conducting nanotubes. The produced B-SWCNTs are stable over high-temperature annealing or processing in liquid media, which inspired us to fabricate thermoelectric modules by a low-cost printing method. The modules demonstrate an increased thermoelectric efficiency by 76% compared to those with undoped SWCNTs. This work provides a feasible fabrication strategy and physical insights for B-SWCNT-based flexible thermoelectrics.

Boron Doping of SWCNTs as a Way to Enhance the Thermoelectric Properties of Melt-Mixed Polypropylene/SWCNT Composites

“…The Seebeck coefficients of solution prepared thin films of pristine SWCNT and B-SWCNT materials were 24 and 32 µV/K (at room temperature), respectively. The corresponding electrical conductivity and therefrom calculated PF were 145,000 S/m and 85 µW/(m•K 2 ) for pristine SWCNT films and 214,000 S/m and 226 µW/(m•K 2 ) for B-SWCNT films [23]. For pristine SWCNT and B-SWCNT powders, electrical conductivities of 1790 [18] and 1810 S/m and Seebeck coefficients of 39.6 ± 0.2 [18] and 42.2 ± 0.1 µV/K were measured.…”

“…The SWCNTs and diboron trioxide powders (B 2 O 3 , >98%, Sigma-Aldrich, St. Louis, MO, USA) were mechanically mixed at a weight ratio of 1:5 in a graphite crucible and then processed at 1300 • C in argon atmosphere as described elsewhere [23,28]. The boron-doped SWCNTs were boiled in excess deionized water and filtered to remove the unreacted B 2 O 3 .…”

“…No extra processes, such as ultrasonication, were performed in an attempt to further break the SWCNT bundles, as such processes could cause structural damage of B-SWCNTs and deterioration of their electrical properties. After boron doping, 8% of boron, in total, was introduced to the SWCNTs in the form of B 2 O 3 , BCO 2 , BC 2 O, BC 3 , and B 4 C, as characterized by X-ray photoelectron spectroscopy (XPS) [23,28]. Boron doping leads to an increase in electrical conductivity by 1.6 times (up to 2.2 × 10 5 S/m) compared to undoped SWCNTs (1.4 × 10 5 S/m), as measured in SWCNT films [23,28].…”

“…After boron doping, 8% of boron, in total, was introduced to the SWCNTs in the form of B 2 O 3 , BCO 2 , BC 2 O, BC 3 , and B 4 C, as characterized by X-ray photoelectron spectroscopy (XPS) [23,28]. Boron doping leads to an increase in electrical conductivity by 1.6 times (up to 2.2 × 10 5 S/m) compared to undoped SWCNTs (1.4 × 10 5 S/m), as measured in SWCNT films [23,28]. Freeze-dried B-SWCNTs in dry powder form were directly used for melt-mixing with PP polymer.…”

“…On the other hand, such kind of doping may become unstable if SWCNTs are processed at elevated temperatures or under vacuum conditions. A substitutional doping of SWCNTs with boron (B) atoms, where B replaces some of the carbon (C) atoms in the hexagonal lattice of SWCNT walls, is more stable and leads to an increase in both the Seebeck coefficient and electrical conductivity of SWCNT networks [23]. Furthermore, large amounts of B-doped SWCNTs can be produced with a post-synthesis treatment of pristine SWCNTs, which is important in regard to the application of such material in thermoelectric applications.…”