Rapid production of β-FeSi2 by spark-plasma sintering

Thermoelectric materials of Fe0.91Mn0.09Si2 were produced with two methods (i.e., hot pressing (HP) and spark-plasma sintering (SPS)) and their thermoelectric properties were compared. The relative density of the samples sintered at the same temperature for thirty minutes was similar. The relative density of the SPS specimen reached 90% in five minutes at 1173 K, while the HP specimen reached 90% in thirty minutes at 1173 K. The optimum condition of the heat treatment was found to be for 3.6 ks at 943 K for the transformation to the β-phase. The peritectoid reaction (α + ɛ → β) mainly occurred at and above 953 K, and the eutectoid reaction mainly occurred at 943 K. The β-phase ratio of the HP specimen was larger than that of the SPS specimen because the peritectoid reaction occurred during longer cooling period from 1173 K to 943 K. Consequently, the Seebeck coefficient of the SPS specimen was slightly smaller than that of the HP specimen. However, the figure of merit was very similar because the thermal conductivity of the SPS specimen was smaller than that of the HP specimen. Thus, the SPS method is superior to the HP method with respect to mass-production because the production time is significantly shorter.