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 th ermoelectric power in various FeSi2 and Fe2Si5 based alloys was examin ed. The peritectoid reaction (a+epsilon-->beta) in FeSi2 alloys was in itially enhanced by the addition of Cu but it became slower for longer annealing times. The retained metallic epsilon was harmful for the th ermoelectric 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 w ere attained after the elimination of epsilon. In the case of Fe2Si5 b ased alloys, almost a single phase of cu structure was formed by conve ntional solidification. The beta phase was only formed by the eutectoi d reaction (alpha-->beta+Si). Differential thermal analysis, X-ray dif fraction and microscopic observation clearly confirmed that the eutect oid reaction rate was drastically enhanced by the addition of a small amount of Cu and its rate decreased with decreasing Cu content. The ra te 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 sti ll very effective even in Mn of Co doped alloys. The thermoelectric po wer of these alloys increased very quickly with annealing time. Their final values decreased with Cu content and saturated at 0.2 at.% Cu. T he value of the 0.1 at.% Cu added alloy was higher than that of both t he conventional p- and n-type FeSi2 based alloys. These results sugges t that the Fe2Si5 alloys with a small amount of Cu may be attractive a s new thermoelectric materials. (C) 1997 Elsevier Science S.A.