Research On The Microstructure And Properties Of Directionally Solidified Mg₂?Si,Sn? Based Thermoelectric Materials

Thermoelectric materials can provide the energy conversion between heat and electricity.This new type of functional material has a broad application prospects in the fields of aerospace,power generation,and refrigeration,because of its pollution-free,noiseless,reliable,and responsive.In recent years,medium temperature thermoelectric material Mg₂?Si,Sn?has received great attention due to its low cost and non-toxic.However,the chemical activity and volatile of Mg,as well as the great melting point difference of the elements,results in the difficulty in synthesis and preparation.Concerning this issue,high temperature gradient directional solidification?HGDS?method is used in this paper to prepare high quality Mg₂?Si,Sn?crystal by controlling the planar interface growth.First principles calculation is employed to research the carrier transport properties and predict the optimal thermoelectric performance.The results provide theoretical basis for the preparation of high performance Mg₂?Si,Sn?thermoelectric materials.Mg₂Sn binary alloy is firstly researched including directional solidification process parameters and mechanism,properties of crystal growth,and microstructure morphology.Combining with the first principles calculation,the electronic structure,mechanical properties,thermodynamic and kinetic parameters of Mg₂Sn single crystal are studied.Based on the results of Mg₂Sn,the influence of band convergence in Mg₂Si_1-xSn_x single crystal is discussed.Additionally,thermoelectric performance is optimized on the basis of the anisotropic Seebeck coefficient.We obtained the following results:At high temperature gradient(200 Kcm^-1),and the single-phase Mg₂Sn crystal is obtained at 1?ms^-1.The thermoelectric performance is dramatically improved comparing with the crystals prepared by conventional Bridgman and Radio-frequency?RF?induction melting method.The maximum of Seebeck coefficient is-261?VK^-1,and the increased values result in the power factor enhanced about 3 times.The thermal conductivity also significantly reduced without the influence of the second phase Sn.First principles calculation is employed to make a basic research for Mg₂Sn crystal.The dominated conduction type is electron in the intrinsic condition for Mg₂Sn.The atoms arrangement in[111]orientation is Sn-Mg-Mg-Sn,including the strong Mg-Sn covalent bonds and weaker Mg-Mg bonds.This complex arrangement of chemical bond is helpful to obtained higher thermoelectric performance.Under the ground state,the brittle Mg₂Sn crystal shows the non-metallic features.The anisotropic factor of elastic constants indicates that,the crystal along[111]orientation is expected to get better mechanical properties.The Seebeck coefficient of Mg₂Sn single crystal prepared by HGDS shows a strong anisotropy in the direction of growth direction[111]and its perpendicular direction11 0 and 112,and appears the phenomenon of orientation-dependent conduction sign reverse?OCSR?.The electrical and thermal conductivities are isotropic.In addition,first principles calculation results indicates that,ternary Mg₂Si_1-xSn_x solid solutions are direct bandgap semiconductors,and the extreme points of valence band and conduction band fall on the G point of the first Brillouin zone.The distance between the two lowest conduction bands?E is changed with the variation of Sn contents.The minimum of?E is 0.008 eV at x=0.625,which indicates convergence of the two conduction bands.The convergent band is beneficial to the improvement of Seebeck coefficient.Prediction results of theoretical calculation indicate that,the maximum of Seebeck coefficient for n and p type conduction is-520?VK^-1 and 345?VK^-1,respectively.The corresponding value of power factor is 6.7 mWm^-1K^-2 and 4.5mWm^-1K^-2,and the n-type doping is more suitable for the Mg₂Si_1-xSn_x solid solutions.The single-phase Mg₂Si_1-xSn_x single crystals are obtained at x=0.65 and 0.75 by HGDS method.The maximum of Seebeck coefficient and power factor for undoped Mg₂Si_1-xSn_x single crystal is-518?VK^-1 and 2.56 mWm^-1K^-1,respectively.It increase more than 2.5 times compared with the nanocrystalline prepared by solid state reaction method.The Seebeck coefficients of Sb-doped Mg₂Si_0.35Sn_0.65 single crystals in growth direction[111]are higher than its perpendicular direction[11 0]in the measured temperature range,and the maximum is-265?VK^-1.The top value of power factor of1.25 at%Sb doped crystal is 5.56 mWm^-1K^-2 at T=600K,and this result improved about 20%comparing with the maximum of previous reports.The corresponding value of ZT is 1.3 at730 K.The results of research show that,the method of growing single crystal using the anisotropic Seebeck coefficient is a effective way to improve the performance of Mg₂Si_1-xSn_x thermoelectric materials.