Grain refinement to improve thermoelectric and mechanical performance in n-type Bi2Te2.7Se0.3 alloys

High-performance thermoelectric (TE) materials are in demand to improve energy conversion efficiency and mitigate the energy crisis for the next generation. In this study, we produced fine-grained n-type Bi2Te2.7Se0.3 materials using high energy ball milling followed by spark plasma sintering techniques. The results demonstrated that milling time strongly influences the thermoelectric properties. The electrical conductivity gradually decreased with increased milling time due to a decrease in carrier mobility by the enhanced carrier scattering at high density grain boundaries. In contrast, the Seebeck coefficient significantly increased and then decreased with prolonged milling time, due to the variation in carrier concentration. A substantial decrease in lattice thermal conductivity (similar to 20% for 60 min) and suppression of bipolar thermal conductivity (similar to 50% for 120 min) resulted from intensified phonon and minority carrier scattering at refined grain boundaries, respectively. A maximum figure of merit (ZT) of 0.88 was achieved at 400 K for the sample ball milled for 60 min, which was about 27% higher than the GA sample. The grain refinement significantly increased the hardness from 53 to 86 Hv and compressive strength from 59.3 to 102.5 MPa, due to grain boundary strengthening. These results provide useful evidence for TE power generation and refrigeration applications.