Influence of pre-annealing and Mn + Al double doping on the microstructure and thermoelectric properties of iron disilicide

Iron disilicide based thermoelectric materials with the designed composition Fe_1-xMn_xAl_ySi_2-y (x = 0.06, 0.08; y = 0, 0.02, 0.04) were prepared by rapid solidification and hot pressing (1193K, 100 MPa, 30 minutes). The effects of Mn+Al double doping were investigated. The influence of pre-annealing on the microstructure and the transport properties were also studied. X-ray diffraction and scanning electron microscopy show that after pre-annealing at 800degC for only 10 hours, the high temperature phases (alpha-Fe₂Si₅ and epsiv-FeSi) in the rapidly solidified powders Fe_1-xMn_x Al_ySi_2-y have been completely transformed to (beta-FeSi₂ phase for y = 0 and y = 0.02. It is found that the grain growth during hot pressing was suppressed for the pre-annealed samples. For the pressed compacts, the Seebeck coefficient was markedly enhanced by the pre-annealing treatment, while both the electrical conductivity and the thermal conductivity were decreased. Mn+Al double doping improves the electrical conductivity, but deteriorates the Seebeck coefficient and the thermal conductivity. The maximum of the figure of merit (ZT = 0.20) is obtained at 890 K for the sample double doped with y = 0.04 (Fe_0.94Mn_0.06Al_0.02Si_1.98) that is hot pressed from pre-annealed powders