Thermoelectric properties of Mn doped FeSix alloys hot-pressed from nitrided rapidly solidified powders

Manganese doped FeSix 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 degrees C/30 min and annealing at 800 degrees C/20 h from rapidly solidified powders. In addition to beta-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 beta-FeSi2 based alloys, due to the high thermal but low electric conductivity of the silicon particles. A high figure of merit of 2.0 x 10(-4) K-1 was obtained for the sample with x = 2 at 600 degrees C. Low thermal conductivity of about 4W(m K)(-1) between 400 degrees C and 800 degrees 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.