The effect of crystal grain size on thermoelectric properties of sintered /spl beta/-FeSi/sub 2/

The effects of crystal grain sizes on the thermoelectric properties were investigated for sintered /spl beta/-FeSi/sub 2/ doped with Mn or Co. Sintered materials of /spl beta/-FeSi/sub 2/ doped with Mn or Co were prepared by the cold-pressing and sintering technique. The mean powder size d, sintered grain size C/sub s/ and annealed grain size C/sub a/ decreased exponentially with increasing ball-milling duration. The mean crystal grain sizes (C/sub s/ and C/sub a/) of FeSi/sub 2/ doped with Mn were larger than that of Co-doped specimen after sintering and annealing. The Seebeck coefficient /spl alpha/ and electrical resistivity /spl rho/ of Mn-doped p-type FeSi/sub 2/ increases with C/sub a/. On the other hand, /spl alpha/ of Co-doped n-type FeSi/sub 2/ is independent of C/sub a/, while /spl rho/ decreases with increasing C/sub a/. The Hall mobilities of p- and n-type FeSi/sub 2/ specimens were approximately independent of C/sub a/. For p-type FeSi/sub 2/, the carrier concentration n/sub H/ decreased with increasing C/sub a/, whereas n/sub H/ of n-type FeSi/sub 2/ increased with C/sub a/. This result shows that the C/sub a/ dependence of the /spl alpha/ and /spl rho/ is similar to the nH dependence. For p-type FeSi/sub 2/, the increase in nH with decreasing C/sub a/ is due to the holes formed in grain boundaries. For n-type FeSi/sub 2/, the decrease in n/sub 11/ with decreasing C/sub a/ is due to the carrier compensation.