The effect of crystal grain size on thermoelectric properties of sintered β-FeSi2

The effects of crystal grain sizes on the thermoelectric properties were investigated for sintered β-FeSi₂ doped with Mn or Co. Sintered materials of β-FeSi₂ doped with Mn or Co were prepared by the cold-pressing and sintering technique. The mean powder size d, sintered grain size C_s and annealed grain size C_a decreased exponentially with increasing ball-milling duration. The mean crystal grain sizes (C_s and C_a) of FeSi₂ doped with Mn were larger than that of Co-doped specimen after sintering and annealing. The Seebeck coefficient α and electrical resistivity ρ of Mn-doped p-type FeSi₂ increases with C_a. On the other hand, α of Co-doped n-type FeSi₂ is independent of C_a, while ρ decreases with increasing C_a. The Hall mobilities of p- and n-type FeSi₂ specimens were approximately independent of C_a. For p-type FeSi₂, the carrier concentration n_H decreased with increasing C_a, whereas n_H of n-type FeSi₂ increased with C_a. This result shows that the C_a dependence of the α and ρ is similar to the nH dependence. For p-type FeSi₂, the increase in nH with decreasing C_a is due to the holes formed in grain boundaries. For n-type FeSi₂, the decrease in n₁₁ with decreasing C_a is due to the carrier compensation.