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

We investigated the anisotropic thermoelectric properties of Bi_0.4Sb_1.6Te₃ (BST) composites with heavy metallic high-entropy alloy TaNb2HfZrTi (HEA_x) (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+HEA_x 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 λe is attributed to metallic HEA nanoparticles that improve electrical grain connectivity, particularly in the Pa-direction. The lattice thermal conductivities (κ_L) in the Pa-direction of the BST+HEA_x samples decreased at x = 0.1 vol% by significant phonon scattering, followed by an increase at higher HEA concentrations (0.5 to 1.0 vol%), which is consistent with the values of the phonon mean free path (λ_ph). The phonon mean free path (λ_ph) 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 (ZT) 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.