The Study On Preparation And Thermoelectric Transport Properties Of Fine Silicide Materials

Thermoelectric material is a kind of energy materials which can convert heat to electricity directly or use electricity to cooling through the motion of internal carrier. Nowadays with the urgency of energy problems and the big development of thermoelectric materials, this field has attracted much attention of scientists. An important application of middle-temperature thermoelectric materials (400-800K) is to generate electricity from waste heat in industrial field and daily life. Silicide is one of the promising thermoelectric based on its high quality and abundance in shell. In this paper, transport properties of silicide have been investigated including preparation, choice of raw materials, doping modification and microstructure control.In this paper, the effect of current on transport properties during Field-Activated and Pressure-Assisted Method (FAPAS)was investigated by comparing three heating models, which is No Current Sintering (NCS), Low Current Sintering (LCS), and High Current Sintering (HCS). The results indicate that the fast heating rate, mainly determined by the current heating mode, combined with particular distribution of temperature field is very helpful for the emission of gas during sintering, which thus results in high density and high electric conductivity of samples; the presence of electric current and field promotes the mass migration among powders and reduces the grain size by lowering the reaction temperature. Fine nano-crystallinte leads to smaller thermal conductivity. Larger current and higher electric field can also make the grain grow in an orientation.MgH2powder was used as raw materials instead of Mg powders to prepare Mg2Si, and the parameters of the reaction between MgH2and Si powders including powders ratio, temperature, holding time, protective gas and by-products collection were investigated. The results indicate that due to the excellent stability of MgH2, pure Mg2Si products with nano grain is obtained due to low reaction temperature between MgH2and Si powders, which owns to self-decomposition of MgH2in reaction. And no MgO phase has been detected in powder and bulk materials. Both purity and nanocrystallization of products enhance the thermoelectric properties obviously, combined with the advantages of uniform phase and good repeatability.In this paper, the effect of doping on the thermoelectric properties of silicide was investigated. The synthesis time of β-FeSi2is largely shortened through the Cu doping. Both n-type and p-type β-FeSi2based thermoelectric materials were prepared respectively through Co and Mn doping and their power factor were greatly enhanced. To further the mechanism of temperature shift of optimum thermoelectric properties based on doping, the temperature characteristic of heavy metal Sb-doped, transition metal Y-doped and Sc-doped Mg2Si were investigated. The results indicates that the optimum temperature range of transition mental Y and Sc doped silicides is lower than that of Sb doped ones, so the optimum temperature of zT value could be pulled back to within600K. without the lost of thermoelectric transmission performance.