Enhancing the thermoelectric properties for hot-isostatic-pressed Bi2Te3 nano-powder using graphite nanoparticles

Abstract

Bismuth telluride (Bi2Te3) is a promising thermoelectric material produced commercially. However, its poor electrical conductivity and low figure of merit, caused by grain boundaries and high thermal conductivity, limit its effectiveness in powder metallurgy production. Herein, effects of adding Graphite nanoparticles (GTNPs) to Bi2Te3 on thermoelectric properties were studied. Three ratios of GTNPs (0.2, 0.35, 0.5 wt%) were added to ball-milled Bi2Te3 nano-powder. The hot isostatic pressing (HIP) sintering technique was employed to prepare the pristine Bi2Te3 and the BT-xGTNPs samples for testing. The crystallographic measurements showed a reduction in the crystallinity of the BT-xGTNPs samples compared to the pure Bi2Te3, whereas the electron microscopy measurements showed smaller grain sizes. This was also confirmed with an increase in the samples’ relative density implying the formation of nano-sized grains. Full electrical, thermal, and thermoelectric measurements were performed and comprehensively discussed in this report for all samples in the temperature range from room temperature (RT) to 570 K. The measurements demonstrated an enhancement for x = 0.35 wt% GTNPs at 540 K up to 43% in the power factor and 51% in the ZT compared to pristine Bi2Te3, which was attributed to the optimum grain size, the lower grain boundaries, and better electrical and thermal conductivity aroused from the precise addition of GTNPs. The best electrical conductivity of ~ 8.2 × 104 S/m and lowest thermal conductivity of ~ 1 W/m·K for the Bi2Te3-containing 0.35 wt% GTNPs at RT even though the sample with 0.5 wt% attained the highest Seebeck coefficient of 154 µV/T at 540 K.