Fabrication And Performance Of Mg₂Si-based Thermoelectric Materials By Microwave-assisted Solid State Reaction

Thermoelectric material is a kind of functional materials which can convertheat into electricity directly. The thermoelectric module can be used as generatorand refrigeration with advantages as clean, noiseless, lightweighted, small andsafe. It has promising to further research and application in the filed ofthermoelectric generation and thermoelectric cooling. As one type of importantmedium-temperature（400~900K）thermoelectric materials, Mg2Si1-xSnxbasedsolid solutions have attracted considerable interest for waste heat recovery dueof their abundant and lowcost chemical constituents, environmentally friendly,low density and so on. However, oxidation and evaporation are the mainproblems that limit the its enhancement of ZT values.Based on the difficulties mentioned above, in this research, the synthesis ofn-type Mg2Si1-xSnxbased solid solution thermoelectric materials was achievedby microwave-assisted solid state reaction followed by the field activated and pressure assisted synthesis (FAPAS), in which MgH2was used as reactantinstead of Mg. In this paper, the behaviour and mechanism of microwaveheating metal powders was investigated, the effect of microwave synthesisprocess and percent of Bi dopant on the thermoelectric properties and microstureof the product of Mg2Si1-xSnxwere discussed. The major results are shown asfollowing：（1）MgH2, Sn and Si powder can be rapidly heated to get reaction undermicrowave radiation, and the product is pure single phase without MgO peakswere detected by XRD; Higher the microwave power accelerates the reactionbetween MgH2and silicon, but no obvious change in purity and particle size ofproduct Mg2Si were observed.(2) Compact, pure and single Mg2Si1-xSnxsolid solution was successfullyobtained by microwave radiation followed by FAPAS sintering, with increasingcontent of Sn, the electrical conductivity of the Mg2Si1-xSnxsolid solutionincreased while its absolute value of the Seebeck coefficient decreased;Meanwhile, it’s the thermal conductivity is reduced from4~6Wm-1K-1(pureMg2Si and Mg2Sn) to a minimum of1~2Wm-1K-1. The highest ZT value of0.2was obtained at480K for Mg2Si0.4Sn0.6sample.(3) Bi doping significantly increased electron concentration and optimizedthe Seebeck coefficient, which result effectively improvement the power factor.Meanwhile, Bi doping significantly increases the degree of disorder and densityof point defects in the crystal lattice, which severely reduce the lattice thermal conductivity. The ZT value reached to0.68at700K for the solid solutionMg2Si0.4Sn0.6-yBiywith y=0.03.