Influence of Copper Addition on the Rapid Production of β-FeSi<SUB>2</SUB> by Spark-Plasma Sintering

Abstract

Thermoelectric materials of Fe 0.91 Mn 0.09 Si 2 + x mass%Cu (0 ≤ x ≤ 1.5) were produced by spark-plasma sintering (SPS) and their thermoelectric properties were measured. The densification was completed in one minute after the temperature reached 900°C. Liquid phase sintering probably occurred during the heating because the addition of copper enabled the specimen to be highly densified. The relative density of the specimen with 1.5%Cu was 94%, while that of the specimen without copper was 90%. Copper also promotes transformation to the β-phase. The e-phase ratio: f e = I e /(I e + I β ) × 100, (I e = intensity of e(210), I β = intensity of β(220, 202)) was calculated from the XRD results in order to easily evaluate the amount of the e-phase in the specimen. In a temperature range from 670 to 730°C, copper allows the annealing time to be shortened by about a half the annealing time without copper. However, an excessive amount of copper and an excessive annealing time had a bad influence on the Seebeck coefficient and the power factor, though the e-phase ratio of the specimen decreased. The optimum composition is Fe 0.91 Mn 0.09 Si 2 + 1%Cu, and the optimum annealing conditions are thirty minutes at 700°C. The power factor of the optimum specimen is about 1.6 times greater than that of the specimen annealed for sixty minutes at 670°C without copper. Thus, it is concluded that the SPS method is extremely effective for the rapid production of β-FeSi 2 .