Effects of Al and Cu Addition on Pressureless Sintering Behavior and Phase Transformation of β-FeSi<SUB>2</SUB>

Abstract

The preparation of β-FeSi 2 thermoelectric material by liquid phase sintering with addition of a low melting point metal has been tried. Aluminum and copper were selected as an additive. The effects of Al or Cu on the sintering behavior and the phase transformation of β-FeSi 2 have been investigated. In the case of sample with Al addition before mechanical grinding (MG), the liquid phase was not formed because pure Al disappeared by diffusion of Al atoms into the matrix before the sintering temperature reached the melting point of Al. On the other hand, in the case of the sample with Al addition after MG, Al-Si liquid phase was formed around 850 K, the eutectic point of Al 88.7 Si 11.3 . The liquid aluminum preferentially diffused into the solid phase and the pores were formed at previously occupied Al particle sites. This pore formation resulted in swelling of the compacts. Additionally, Al addition accelerated decomposition of the β phase to the α and e phases. In the case of the sample with Cu addition, Cu-Si liquid phase was formed around 1075 K. This liquid phase was highly effective for densification of the compacts. Cu addition caused the transformation from the mixture of α and e phase to the β phase around 1130 K during cooling after the sintering, which suggests that the subsequent annealing for the formation of the β phase in the conventional method can be omitted. The β phase transformation during cooling is considered to consist of the following two mechanisms. (1) Cu addition accelerates the decomposition reaction α → β + (Cu-Si) liquid instead of the ordinary α → β + Si. (2) The Cu-Si liquid phase spreads over the particles and facilitates the reaction e + Si → β by supplying Si atoms to the e phase through the liquid.