β-phase transformation and thermoelectric power in FeSi 2 and Fe 2Si 5 based alloys containing small amounts of Cu

Abstract

The effects of Cu addition on the β phase formation rate and the thermoelectric power in various FeSi 2 and Fe 2Si 5 based alloys was examined. The peritectoid reaction ( a+ ε→ β) in FeSi 2 alloys was initially enhanced by the addition of Cu but it became slower for longer annealing times. The retained metallic ϵ was harmful for the thermoelectric power. The inherent thermoelectric properties of (FeSi 2) 99− X Mn 1Cu X ( X=0–1. O at.%), (FeSi 2) 99− X Co 1Cu X ( X=0–1.0 at.%) alloys were attained after the elimination of ϵ. In the case of eutectoid reaction ( α→ β+Si). Differential thermal analysis, X-ray diffraction and microscopic observation clearly confirmed that the eutectoid reaction rate was drastically enhanced by the addition of a small amount of Cu and its rate decreased with decreasing Cu content. The rate also depends on the annealing temperature and reached a maximum at about 1073 K for most alloys. The addition of only 0.1 at.% Cu was still very effective even in Mn or Co doped alloys. The thermoelectric power of these alloys increased very quickly with annealing time. Their final values decreased with Cu content and saturated at 0.2 at.% Cu. The value of the 0.1 at.% Cu added alloy was higher than that of both the conventional p- and a-type FeSi 2 based alloys. These results suggest that the Fe 2Si 5 alloys with a small amount of Cu may be attractive as new thermoelectric materials.