Improved thermoelectric performance by grain connectivity in bismuth antimony telluride composite with metallic high-entropy alloy nanoparticles

We investigated the anisotropic thermoelectric properties of Bi0.4Sb1.6Te3 (BST) composites with heavy metallic high-entropy alloy TaNb2HfZrTi (HEAx) (x = 0, 0.1, 0.5, and 1.0 vol%), synthesized by ball-milling, mixing, and hot-press sintering method. The HEA additions below 1.0 vol% in the BST+HEAx samples do not contribute to the change of Seebeck coefficients, carrier concentrations, and Fermi energies. Besides, electrical conductivity in the parallel (Pa) direction to the press direction was significantly enhanced due to an increased carrier mean free path (λ e) from 10.4 nm (x = 0) to 13.6 nm (x = 0.5). This enhanced λ e is attributed to metallic HEA nanoparticles that improve electrical grain connectivity, particularly in the Pa-direction. The lattice thermal conductivities in the Pa-direction of the BST+HEAx samples decreased at x = 0.1 vol% by significant phonon scattering, followed by an increase at higher HEA concentrations (0.5 vol% to 1.0 vol%), which is consistent with the values of the phonon mean free path. The phonon mean free path and lattice thermal conductivity of the composites indicate the synergistic competition between the phonon scattering and the grain connectivity in the Pa-direction. As a result, the thermoelectric figure of merit in the Pa-direction improved from 1.09 (x = 0) to 1.33 (x = 0.1 vol%) at 350 K. These findings suggest that incorporating heavy metallic HEA nanoparticles is a promising strategy to improve the performance of the anisotropic thermoelectric materials and other systems sensitive to grain connectivity.