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, 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. A 4% substitution of Mn with Co is an effective approach to eliminate MnSi precipitates from melt-grown MnSi, 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 24x10(3)S/m to 54x10(3)S/m at 700K. The resulting power factor exhibits 1.9x10(-3)W/mK(2) around 700K. Moreover, the lattice thermal conductivity is greatly decreased by partial Co substitution compared with that of Co-free MnSi. Consequently, the dimensionless figure-of-merit zT of (Mn1-xCox)Si samples increases from 0.27 for x=0 to 0.50 for x=0.04 in the vicinity of 800K.