Engineering a bimodal grain microstructure in Bi2Te2.85Se0.15 bulk materials via Ti3AlC2-assisted microstructural modification

A bimodal grain microstructure was successfully engineered in Bi 2 Te 2.85 Se 0.15 bulk materials through the introduction of Ti 3 AlC 2 using a combined mechanical alloying (MA) and spark plasma sintering (SPS) approach. During sintering, the highly conductive Ti 3 AlC 2 phase triggered localized Joule heating, which accelerated grain growth in its vicinity and formed coarse-grained regions, whereas areas farther away retained a fine-grained microstructure. Furthermore, partial diffusion of Al atoms from Ti 3 AlC 2 into the Bi 2 Te 2.85 Se 0.15 matrix further promoted grain coarsening, ultimately resulting in a bimodal grain microstructure. Density functional theory (DFT) calculations revealed that Al atoms preferentially substitute Bi sites, but do not significantly affect the bandgap and carrier concentration near the Fermi level. As a result, the (Ti 3 AlC 2 ) 0.05 Bi 2 Te 2.85 Se 0.15 sample achieved a maximum ZT value of 0.71 and an average ZT value of 0.62 over 300-475 K, corresponding to improvements of 70% and 82%, respectively, compared with the Ti 3 AlC 2 -free sample. The present findings demonstrate that microstructure-focused design, rather than chemical doping, is also an effective strategy for optimizing the transport properties of Bi 2 Te 3 -based thermoelectric materials.