Effect of mixed grain sizes on thermoelectric performance of $Bi2Te3$ compound

Coarse (about $1μm$⁠) and fine (about 100 nm) $Bi2Te3$ particles were synthesized by mechanical alloying for different times, respectively. Their mixtures at different ratios were consolidated by spark plasma sintering to produce nano-/microstructured composites with the same chemical compositions. The Seebeck coefficient was significantly enhanced by addition of fine particles through the potential barrier scattering. The electrical resistivity increased rapidly when the weight fraction of fine particles exceeded about 60%, probably due to an effect similar to percolation phenomenon. However, the thermal conductivity was reduced almost linearly with increasing fraction of fine particles. Because of the combined effects of the potential barrier scattering, nonlinear change in electrical property and linear change in thermal transport property, a peak value of dimensionless figure of merit $(ZT)$ was achieved for the sample containing 60% fine particles. This study demonstrated the possibility to enhance $ZT$ value by introducing nonuniform microstructures even without changing chemical compositions.