Abstract
Polycrystalline bulk β-FeSi2 samples were prepared by heating a compact body of Fe powder in a Na-Si melt at various conditions, such as density of the Fe compact and the ratio of the source materials. Single-phase bulk β-FeSi2 samples were prepared by heating Fe compact with density below 3.9 g cm-3, Si powder with a Si/Fe molar ratio above 2.25, and Na with a Na/Si molar ratio above 0.33 at 800°C for 12 h. Fe powder reacts with Si dissolved in the Na-Si melt and forms ε-FeSi, followed by changing to β-FeSi2 due to the diffusion of Si while the Na-Si melt soaks into the compact via voids in the compact. Cobalt (Co)-doped β-FeSi2 bulk samples were prepared by heating compact bodies of powder mixtures of Fe powder with grain size of < 45 μm and Co powder with grain sizes of 1-2 μm or < 150 μm in a Na-Si melt. The Co-doped bulk sample prepared with Co powder of 1-2 μm grain size was homogeneous and showed thermoelectric properties better than those of the sample prepared with Co powder of < 150 μm in grain size. It is considered that Co-doped β-FeSi2 is formed by mutual diffusion between β-FeSi2 and Co silicides.
