Polycrystalline BiSb Alloys with Enhanced Thermoelectric Performance: The Role of Titanium Doping and Band Engineering

Abstract

Bismuth antimonide (BiSb) alloys are promising thermoelectric materials for cooling applications. However, their performance in polycrystalline form and the impact of doping remain underexplored. Here, we report a systematic study of the thermoelectric properties of titanium-doped polycrystalline Bi_1-xTi_xSb (x = 0 – 0.0025), complemented by density functional theory (DFT) calculations. Contrary to conventional wisdom, the polycrystalline samples exhibit higher electrical conductivity than a single-crystal reference. Ti doping enhances the Seebeck coefficient by increasing the density-of-states effective mass, leading to a 50% improvement in the power factor for x = 0.0015 at 300 K. Simultaneously, the thermal conductivity is markedly reduced due to the combined effects of grain boundary and point defect scattering, reaching a value of 2.14 W m^-1 K^-1 for x = 0.0015. Consequently, a peak zT of ~ 0.21 is obtained at 300 K, a fivefold increase over single-crystal BiSb. DFT calculations reveal that Ti doping induces the convergence of heavy and light conduction bands, resulting in increased valley degeneracy and enhanced density-of-states near the Fermi level, which is identified as the primary mechanism for the significant enhancement of the Seebeck coefficient. These findings underscore the untapped potential of polycrystalline BiSb alloys and the critical role of targeted doping in optimizing their thermoelectric performance for near-room-temperature applications.