Thermoelectric performance of n-type and p-type β-FeSi2 prepared by pressureless sintering with Cu addition

The n-type Co-doped and p-type Mn-doped beta -FeSi2 were synthesized by pressureless sintering with Cu addition, without a subsequent heat treatment for beta phase formation. The effects of Cu addition on the thermoelectric performance of n-type and p-type beta -FeSi2, were investigated and their performance was compared to the hot-pressed samples. In the case of the n-type pressurelessly sintered samples with 2 and 4 mass% Cu, the samples were mostly composed of the beta phase after sintering. On the other hand, in the case of the p-type pressurelessly sintered samples, a large amount of epsilon phase remained present in the samples even with 4 mass% Cu. The liquid phase sintering caused by Cu-Si liquid phase formation around 1075 K during heating significantly accelerated the compact densification of the n-type and p-type samples. SEM observation revealed that many small pores were dispersed in the pressurelessly sintered samples. In the case of the n-type Go-doped samples, these pores effectively decreased the thermal conductivity as compared to the hot-pressed samples. Besides that, Gu addition increased the thermoelectric power and decreased the electrical resistivity of the pressurelessly sintered and hot-pressed samples. The figure of merit obtained in the n-type pressurelessly sintered sample with 4 mass% Cu showed almost the same values as that obtained in the n-type hot-pressed sample without Cu. In the case of the p-type Mn-doped pressurelessly sintered samples, the large amount of metallic epsilon phase markedly deteriorated the thermoelectric power, particularly in the low temperature range, and decreased the electrical resistivity. These p-type samples showed lower values of the figure of merit than the p-type hot-pressed samples. (C) 2001 Elsevier Science B.V. All rights reserved.