Effect of Co substitution on thermoelectric properties of FeSi

We have studied Fe1-xCoxSi (0 ≤ x ≤ 1) crystals using the electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) measurements, to examine the influence of Co substitution on the electrical and thermal transport properties of FeSi. Notably, a metal-insulator transition occurs at x = 0.3 for the Fe1-xCoxSi series, as the closure of band-gap of FeSi occurs upon Co substitution. Besides, the substituent (Co) induced a significant variation in the Seebeck coefficient, including two sign crossovers. These observations are ascribed to the asymmetric electronic band structure of the Fe1-xCoxSi compounds. The analysis of thermal conductivity demonstrated that Co substitution has a strong effect on the low-temperature phonon thermal transport of FeSi via boundary scattering and point-defect scattering. Most intriguingly, the substituted end compound CoSi has the largest room temperature thermoelectric figure of merit of about 0.06 which is four orders of magnitude larger than that of undoped FeSi.