Effects of Al and Cu addition on pressureless sintering behavior and phase transformation of β-FeSi2

The preparation of beta-FeSi2 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 B-FeSi2 have been investigated. In die 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 or 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 Al68.7Si11.3. 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 beta phase to the alpha and epsilon 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 alpha and epsilon phase to the beta phase around 1130K during cooling after the sintering, which suggests that the subsequent annealing for die formation of the beta phase in the conventional method can be omitted. The beta phase transformation during cooling is considered to consist of the following two mechanisms. (1) Cu addition accelerates the decomposition reaction alpha --> beta + (Cu-Si)(liquid) instead of the ordinary alpha --> beta + Si. (2) The Cu-Si liquid phase spreads over the particles and facilitates the reaction epsilon + Si --> B by supplying Si atoms to the epsilon phase through the liquid.