Influence of Pre-annealing on the Microstructure and Thermoelectric Properties of Mn + Al Doped Iron Disilicide Prepared by Rapid Solidification and Hotpressing

Abstract

Iron disilicide based thermoelectric materials with the designed composition Fe1-xMnxAlySi2-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 800°C for only 10 hours, the high temperature phases (α-Fe2Si5 and ε-FeSi) in the rapidly solidified powders Fe1-xMnxAlySi2-y have been completely transformed to β-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 (Fe0.94Mn0.06Al0.02Si1.98) that is hot pressed from pre-annealed powders.