The effect of powder pre-treatment on the mechanical and thermoelectric properties of spark plasma sintered N-type bismuth telluride

The effect of N-type (Bi0.95 Sb0.05)2(Se0.05 Te0.95)3 powder pre-treatments, including annealing and powder oxide reduction, was determined in terms of both the mechanical and thermoelectric properties for bodies fabricated using spark plasma sintering (SPS). The uniaxial tensile strength was measured and the figure of merit “ZT” was calculated both parallel and perpendicular to the sintering pressure direction. The powder oxide reduction pre-treatment was carried out using 5% H2 ˗ 95% Ar (vol%) in an in-house built mechanically agitated fluidized bed reduction facility at 380 °C for 24 h, while annealing was carried out at the same temperature and holding time using high purity Ar. Both pre-treatments were compared to the as-manufactured baseline powder sample that was dried under vacuum for 24 h. It was determined that the oxide content decreased from 0.362 to 0.143 wt% after H2 reduction, while it decreased to 0.341 wt% after annealing. The increase in the mechanical properties was found to be mainly due to the annealing pre-treatment, which resulted in an increased relative density of the sintered samples from 97.0% to 98.0%. The annealing pre-treatment increased the characteristic uniaxial tensile strength from 30.4 to 34.1 MPa in the parallel direction and from 30.8 to 38.0 MPa in the perpendicular direction. The increase in the ZT is attributed to the increase in the Seebeck coefficient, which is in turn due to a decrease in the carrier concentration, as was found by examining the literature. The ZTmax (150 °C) increased from 0.54 to 0.63 versus the dried powder in both the parallel and perpendicular directions due to powder oxide reduction and 0.58 and 0.62 in the parallel and the perpendicular directions, respectively, due to annealing versus the dried powder.