The Study Of Electronic Structure And Transport Properties For Mg₂Si_xSn_1-x Thermoelectric Materials

With the environment getting worse in the world,energy and the environment have attracted more and more attention.With about two-thirds of all used energy being lost as waste heat,there is a compelling need for high-performance thermoelectric materials that can directly and reversibly convert heat to electricity.So far,most of important thermoelectric materials are composed of some toxicity elements,as Pb,Sb,Te,Bi et al,and which are rare,and/or expensive.If these materials are produced widely,the environment will be with a heavy burden.Mg₂Si_xSn_1-x thermoelectric compound is a typical green thermoelectric material,with the composite elements being abundant,non-toxicity,and potential applications as advanced thermoelectric materials in the mid-temperature range.However,it has been reported the maximum dimensionless figure of merit?ZT?is 1.55,which still falls short of the generally desired threshold value of 2.In this paper,in order to explore the physical mechanism of electric transports,and search some methods for enhancing thermoelectric properties,the electric and transport properties of Mg₂Si_xSn_1-x are calculated systematically by first principle methods based on the density functional theory.Our results will supply some useful information for designing high performance thermoelectric materials.Firstly,the electronic and thermoelectric properties of Mg2 Si and Mg2 Sn have been investigated using the full potential linearized augment plane wave method with the generalized gradient approximation?GGA?and generalized gradient approximation plus modified Becke and Johnson potential?GGA+mBJ?.Our results show that the GGA+mBJ calculations are consistent with experimental results.We estimated the relaxation time from both experimental data and electron-phonon coupling properties.And more detailed information of the electrical conductivity,power factor and ZT are given.Moreover,the difference of thermoelectric properties between Mg2 Si and Mg2 Sn are expressed through electronic structure differences.The electronic and thermoelectric properties of Mg2 Si and Mg2 Sn under hydrostatic pressure have been investigated.The hydrostatic pressure is simulated by applying equiaxed strain method.The band structure and thermoelectric parameters have been calculated and analyzed in detail.From the band structure one can see that the bottom of conduction band presents significant changes under strain effect.When the strain is up to 0.02 for Mg2 Si and down to 0.04 for Mg2 Sn,conduction bands occur degeneracy states at the center of Brillouin zone,and corresponding to thermoelectric properties are enhanced significantly.We can conclude the hydrostatic pressure have a slightly influence on the thermoelectric properties of electronic doping.The electronic and thermoelectric properties of Mg2Si0.3Sn0.7 are calculated.The random alloy configurations are simulated by special quasirandom structures method with considering local environment effects.It is satisfactory that our result agrees well with experiment data at room temperature,which verifying the reliability of our calculation.Moreover,the seekbeck coefficient,electric thermoeletcirc and ZT are studied by combining the semiclassical Boltzmann theory and single parabolic band model?SPB?.The results show the solid solution method and convergence of conduction bands are effective strategy for enhancing the thermoelectric properties of n-Mg2Si0.3Sn0.7 alloy.The solid solution method is effective strategy for decreasing thermal conductivity of Mg2Si0.3Sn0.7 alloy.Lastly,the intrinsic lattice thermal conductivity of Mg2Si0.4Sn0.6 is calculated.And the most significant is that the data of inherent lattice thermoelectric conductivities at other temperatures are supplied,contributing to better understanding the nature of materials.The relation between size and thermal conductivity is investigated as well for enhanced the dimensionless figure of merit ZT by nanostructuring.And the thermoeletcitic parameteris are calculated and analyzed through SPB model.The results show the bipolar effect plays a very important role in thermoelectric application,and the sharp enhanced ZT will be obtained when it is suppressed.