Thermoelectric properties of Mn doped FeSi_x alloys hot pressed from nitrified rapidly solidified powders

Abstract

Manganese doped FeSi_x alloys (x=1.9, 2.0, 2.1, 2.3 and 2.5) with fine grain sizes of about a few micrometers and densifications over 99% have been prepared by vacuum hot-pressing at 975 °C/30 min and annealing at 800 °C/20 h from rapidly solidified powders. In addition to β-FeSi₂, a silicon phase was found in the samples in the form of rods of 200 nm in diameter as x>2 and also polyhedrons of several microns in size as x=2.5. Transport property measurements revealed that excess silicon decreases the Seebeck coefficient and the electric conductivity, but increases the thermal conductivity of the β-FeSi₂ based alloys, due to the high thermal but low electric conductivity of the silicon particles. A high figure of merit of 2.0×10^-4 K^-1 was obtained for the sample with x=2 at 600 °C. Low thermal conductivity of about 4 W(mK)^-1 between 400 °C and 800 °C for the samples without excess silicon suggests that some dispersed fine nitrides form during the preparation of the samples, which enhance the phonon scattering and reduce the thermal conductivity of the materials.