beta-phase transformation and thermoelectric power in FeSi2 and Fe2Si5 based alloys containing small amounts of Cu

The effects of Cu addition on the beta phase formation rate and the thermoelectric power in various FeSi2 and Fe2Si5 based alloys was examined. The peritectoid reaction (a+epsilon-->beta) in FeSi2 alloys was initially enhanced by the addition of Cu but it became slower for longer annealing times. The retained metallic epsilon was harmful for the thermoelectric power. The inherent thermoelectric properties of (FeSi2)(99-X)Mn1CuX(X=0-1.O at.%), (FeSi2)(99-X)Co1CuX (X=0-1.0 at.%) alloys were attained after the elimination of epsilon. In the case of Fe2Si5 based alloys, almost a single phase of cu structure was formed by conventional solidification. The beta phase was only formed by the eutectoid reaction (alpha-->beta+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 of 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 n-type FeSi2 based alloys. These results suggest that the Fe2Si5 alloys with a small amount of Cu may be attractive as new thermoelectric materials. (C) 1997 Elsevier Science S.A.