Thermal conductivity of cold compacted bismuth nanowires

In contrast to their macroscale counterparts, nanostructures may exhibit a higher thermoelectric figure of merit (performance) due to their enhanced Seebeck coefficient and/or reduced thermal conductivity. Nonetheless, practical use of individual nanostructures in larger scale thermoelectric applications may not be feasible. Instead, the use of compressed materials formed from these nanomaterials may offer unique opportunities. This work explores the effects of anisotropy and particle size and shape on the thermal transport behavior in compressed materials formed from bismuth nanowires. In its bulk and nanoscale forms, bismuth and its alloys are attractive candidates as thermoelectric materials. Free standing bismuth nanowires of two different diameters, 250 nm and 20 nm, were synthesized through a vapor deposition process and compressed into pellets. Bismuth microparticles (mean diameter 100 µm) were also compressed to the same pressures for comparison. The thermal conductivity of individual nanowires with 250 nm diameter was measured to be 6.1 ± 1.2 W/m K. The compressed nanowire pellets were found to be nearly thermally isotropic with thermal conductivity of approximately 1 W/m K. The microparticle pellets showed a higher degree of anisotropy as well as higher thermal conductivity. Results indicate that thermal characteristics are a function of pellet porosity, compression pressure, and the presence of an oxide layer on the particle surface. The increase in pressure initially reduces the porosity (from 100 to 500 MPa but further compression to 3000 MPa does not influence porosity) and enhances the thermal conductivity for the microparticle samples. The results also indicate the surface oxide is breached due to compression. Some anisotropy in thermal properties due to particle shape and the direction of compression was also observed.