Electronic conduction mechanism in thermoelectric semiconductor of boron-doped iron disilicides

Sintered materials of (1 - x)FeSi2 + xBSi(2) in the composition range of 0 less than or equal to x less than or equal to 0.08 were prepared by the hot-pressing technique. The solid solutions of Fe1 - xBxSi2, which were a beta-FeSi2 single-phase, were obtained in the range of x less than or equal to 0.03. The electrical resistivity (rho) and Seebeck coefficient (alpha) were measured as a function of temperature over the range from 77 to 1241 K. It was found that the solid solutions possess semiconducting properties below the semiconductor-to-metal transition temperature (T-c). Logarithmic rho versus temperature 1/T curves showed an S-shaped variation in the temperature range of 200 to 600 K for the beta-FeSi2 containing borun. The curves could be explained in terms of a small polaron model and a impurity conduction in which boron atoms formed a localized state as a donor in the beta-FeSi2. The energy gap at 0 K(E(g)o) decreased with increasing the boron concentration. The energy gap (E-g) estimated around T-c was smaller than the E-g0 and decreased with increasing the boron concentration. T-c was of 1241 K and independent of the boron concentration. From these facts it was found that the transition from semiconductor to metal phases was mainly caused by expanding the width of a narrow valence band with increasing hole concentrations. The alpha of beta-FeSi2 with borons showed a remarked peak larger than that of un-doped. and the peak temperature shifted to lower temperatures with increasing boron concentrations.