Mechanism And Thermoelectric Properties Of Mg₂Si-Based Thermoelectric Materials By Microwave Irradiation

Thermoelectric material is a kind of functional materials which can convert heatinto electricity directly. The thermoelectric materials could be used as generators andrefrigeration. The thermoelectric generators can convert terrestrial heat and the wasteheat of industry to electricity directly. The thermoelectric devices own some advantagesas no pollution, no voice, and lightweight, small, portable and safety, which are ofpromising application value for thermoelectric generation and thermoelectric cooling,mainly used in the fields of low-grade heat power generation, aerospace andmicroelectronics. Magnesium silicide is a kind of middle temperature thermoelectricmaterials, owing to some advantages as the abundance of raw materials, low cost andenvironment friendly. However, the phase purity and microstructure of the productMg₂Si are difficult to control by conventional technique because of the easyvolatilization and oxidation of Mg. In this paper, the method of microwave-assistedsynthesis was used to realize the rapid reaction and densification of Mg₂Si basedthermoelectric materials. Microwave synthesizing is a method through using the heatingproduced by the couple of microwace with the microstucture of materials and whichhave these features on bulk heating, selective heating and non-therm effect. This methodis a very promising preparation method for fabricating many materials because it is fast,clean energy efficient and does not suffer from the disadvantages of the classicalpreparation technique. In this paper, the behaviour and mechanism of microwaveheating metal powders was investigated, and the kinetics and mechanism of microwavesynthesizing Mg₂Si was researched. The process, thermoelectric properties andmicrosture of microwave synthesizing Mg₂Si and Mg₂Si_1-xSn_xwere studies. On the baseof study, the magjor results of this paper are shown as following.（1） In the microwave heating of Mg₂Si-based thermoelectric materials, dielectriclosses was the main mode at lower temperature, while conductive losses would get therun upon at higher temperature.（2） The heating behavior of Mg, Si and Sn fixed powder was investigated undermicrowave irradiation. The microwave heating therm profiles of Mg₂Si–based compactsare dependent on composition, porosity and power. The faster of heating rate and higherof final temperature are. The larger of porosity is, the faster of heating rate is, And thehigher of output power is, the faster of heating rate is. The heating rate increases as the green density decreases （porosity increases）. Higher is the porosity, higher is the heatingrate for a particular choice of experimental variables, the temperature rise is restricted toa certain level, but the final temperature doesn’t depend on the porosity. The heatingrate Mg₂Si_1-xSn_xcompacts decreases at a particular power setting with the increase of x.（3） Mg₂Si_1-xSn_x（x=0、0.2、0.4、0.6、0.8） solid solutions were prepared bymicrowave irradiation and the morphologies, microstures and phase compositions of thesample were mainly studied using scanning electron microscope（SEM） with energydisperse spectroscopy analyzer（EDS） and X-ray diffraction（XRD）. The oxidation of Mgcan be rest rained by changing microwave heating programs, the volatility of Mg can bemade up by controlling excessive content of Mg. When the excessive content of Mg isabout8at%, the samples is kept under853K for30min, high purity Mg₂Si powders canbe obtained under this experimental conditions. XRD patterns show that Mg₂Si_1-xSn_xsolid solutions have been well formed under microwave irradiation and microstructurecharacterization revealed a continuous network of Sn particulates and fabricate self-assembled Mg₂（Si, Sn） composites. This method is a very promising preparationmethod for many materials because it is fast, clean energy efficient and does not sufferfrom the disadvantages of the classical preparation technique.（4）Test of thermoelectric properties demonstrate that the electrical conductivity ofMg₂Si-based thermoelectric materials increases monotonically with temperature,indicating semiconducting behavior and a negative Seebeck coefficient suggests that theobtained materials exhibit n-type conductivity. What is more, increase of Sn ispositively related to the absolute value of Seebeck coefficient and electricalconductivity for Mg₂Si_1-xSn_xalloy.The thermal conductivity of Mg₂Si sample dopedwith Sn element had been reduced by60%. A maximum figure of merit, ZT=0.26, wasobtained for Mg₂Si0.4Sn0.6at about500K, the figure of merit ZT achieved2times ofpure Mg₂Si at600K, and its ZTmaxwas0.13. Sn element is better than the heavy ones inthermoelectric properties of of Mg₂Si owing to some advantages as the abundance of itsraw materials and low cost. It is really a renewed approach to enhance thethermoelectric performances of Mg₂Si by doping Sn elements and using microwaveIrradiation technology.（5）Experiments of the kinetic analysis on Mg₂Si synthesizing were carried outby microwave irradiation and the curve of rate of conversion as the function of reactiontime is obtained based on former experiment. The kinetic analysis showed that theactivation energy Eain Mg-Si system decreased under microwave irradiation. The activation energy was102KJ/mol by microwave heating and was120¹90kJ/mol byconventional heating. The synthesizing time of Mg₂Si sample by microwave heatinghad been reduced by85⁹0%. The microwave could increase the activation energy andpre-exponential factor. The kinetic model of the reaction process has been establishedand the reaction mechanism of Mg-Si system belonged to the Kingsburg diffusionreaction shell-core model. The results show that the reaction process of Mg-Sithermoelectric materials could be expressed as:（i） Mg is melted and the surface of Siparticle is covered by Mg liquid;（ii） formation of Mg₂Si;（iii） Mg penetrate Mg₂Siphase and move to interface of Si particle;（iv） The following reaction occurs at Mg₂Si/Si side:2Mg+Si=Mg₂Si.（6）In comparison with the conventional heating processing, the results ofmicrowave direct synthesis of Mg₂Si-based thermoelectric materials show that thekinetic induced by microwave irradiation owing to microwave thermal effect and non-thermal effect. The results that it is possible by microwave heating to induce localizedsuperheating （eventually thermal runaway） which lead to localized reaction rateenhancements, PMF effect and microwave E-field has orientation and, which canaccelerate the diffusion rate of particles. So the reaction rate of Mg-Si by microwavequickly than that conventional heating.