Effects of Cobalt Substitution on Crystal Structure and Thermoelectric Properties of Melt-Grown Higher Manganese Silicides.

To improve the thermoelectric (TE) properties of melt-grown higher manganese silicides MnSi_γ, dissipation of MnSi precipitates that deteriorate the electrical conductivity is required. We have investigated the effects of light cobalt (Co) substitution on TE properties and MnSi precipitates of MnSi_γ. A 4% substitution of Mn with Co is an effective approach to eliminate MnSi precipitates from melt-grown MnSi_γ, which is confirmed by powder x-ray diffraction and energy-dispersive spectroscopy measurements. Furthermore, this light Co substitution leads to increase of the hole carrier concentration, resulting in a great increase in the electrical conductivity from 24 × 10³ S/m to 54 × 10³ S/m at 700 K. The resulting power factor exhibits 1.9 × 10⁻³ W/mK² around 700 K. Moreover, the lattice thermal conductivity is greatly decreased by partial Co substitution compared with that of Co-free MnSi_γ. Consequently, the dimensionless figure-of-merit zT of (Mn_1−xCo_x)Si_γ samples increases from 0.27 for x = 0 to 0.50 for x = 0.04 in the vicinity of 800 K. [ABSTRACT FROM AUTHOR]