Effect of dispersed Si-phase on thermoelectric properties of FeSi2 prepared by mechanical alloying and sintering

While melt-cast FeSi2 requires careful heat-treatment of homogenization and phase transformation to achieve thermoelectric beta-phase, the mechanical alloying(M/A) and sintering method is expected to simplify the process. By using the M/A method with elementary Fe and Si coarse powders, very fine powders consisting of FeSi- and Si-phases were obtained. Especially when excess Si was deliberately introduced in the starting materials, the desired microstructure containing finely distributed Si-phase in the matrix of beta-FeSi2 was produced by subsequent sintering. The volume fraction of dispersed Si-phase could be controlled within a range of 0.26 similar to 0.38 by varying the excess Si content. The shape of the dispersed the Si phase was something like the coral colony of fine finger-shaped protrusions (diameter less than or equal to 0.5 mu m and length less than or equal to 3 mu m). The size and volume fraction of the Si-phase in the beta-FeSi2 matrix was analysed being effective in controlling the thermal conductivity of the sintered mass due to the phonon scattering.