Abstract
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, sintered grain size Cs and annealed grain size Ca decreased exponentially with increasing ball-milling duration. The mean crystal grain sizes (Cs and Ca) of FeSi2 doped with Mn were larger than that of Co-doped specimen after sintered and annealing. The Seebeck coefficient α and electrical resistivity ρ of Mn-doped p-type FeSi2 increases with Ca. On the other hand, α of Co-doped n-type FeSi2 is independent of Ca, while ρ decreases with increasing Ca. The Hall mobilities of p- and n-type FeSi2 specimens were approximately independent of Ca. For p-type FeSi2, the carrier concentration nH decreased with increasing Ca, whereas nH of n-type FeSi2 increased with Ca. This result shows that the Ca dependence of the α and ρ is similar to the nH dependence. For p-type FeSi2, the increase in nH with decreasing Ca is due to the holes formed in grain boundaries. For n-type FeSi2, the decrease in nH with decreasing Ca is due to the carrier compensation. Thermal conductivity of p- and n-type FeSi2 specimens decrease with decreasing Ca, because the phonons are scattered in grain boundaries. The effective maximum power (Pef) at the temperature difference 800 K for p-type Fe0.93Mn0.07Si2 decreases with Ca. On the other hand, and Pef for n-type Fe0.985Co0.015Si2 increases with Ca. It was found that p- and n-type FeSi2 showed high thermoelectric performance at ball-milled powder sizes of 1.10 μm and 2.12 μm, respectively.
