Effect Of High Magnetic Fields On The Microstructures And Thermoelectric Properties Of Hypoeutectic Bi-Te Alloy

Thermoelectric material is a functional material that can make the heat into electrical energy directly,it is mainly used for thermoelectric cooling and power generators.And as one of the best thermoelectric material around room temperature,Bi2Te3 based thermoelectric materials are extensively applied in the field of biomedical,military weapon,electronic chips and other higher-quality segments.According to the results of previous reports,optimizing the phase composition and microstructure of Bi2Te3based alloys effective strategies to improve their thermoelectric performance.High magnetic field(HMF)is an extreme physical field with enhanced Lorenz force and magnetization effect.It has been found that the HMF can control evolution of microstructure during solidification processing.Consequently,in this work,HMF was introduced into the solidification process of hypoeutectic Bi-Te semiconductor alloy,and the effects of uniform magnetic field and gradient magnetic field on microstructure evolution and thermoelectric properties are investigated.In this paper,the crystal orientation,microstructure and phase composition of hypoeutectic Bi-Te alloy were analyzed by metallographic microscope,X ray diffractometer and scanning electron microscope(SEM);thermoelectric properties(Seebeck coefficient,resistivity,thermal conductivity,carrier concentration and mobility)were measured by Seebeck Coefficient/Electric Resistance Measuring System,laser thermal conductivity mater,and holzer effect tester.Conclusions can be reached as follow:(1)Effect of HMF on crystal orientation of hypoeutectic Bi-Te alloy.For the hypoeutectic Bi-Te alloy prepared under uniform HMF,due to the magnetic anisotropy effect,Bi2Te3 crystal showed the tendency of c axis to be oriented perpendicular to the direction of magnetic field,and degree of orientation increased with the increase of magnetic field intensity;for hypoeutectic Bi-Te alloy prepared under gradient magnetic field,the result of orientation is the same as that of under uniform magnetic field,but its orientation is not good comparing with the uniform magnetic field,and the degree of orientation decreased with the increase of magnetic field gradient.(2)Effect of HMF on microcosmic structure of hypoeutectic Bi-Te alloy.When it is free of magnetic field,Bi-Te alloy is composed of primary phase Bi2Te3 and eutectic structure(Bi2Te3+Te).For Bi-Te alloy prepared under uniform magnetic field,the width and spacing of primary phase narrowed down.When the magnetic field is zero,the orientation of primary phase is out of order.With application of uniform magnetic field,the primary phases were arranged along the direction of magnetic field.In the gradient magnetic field(B=8.8T),primary phase arranged along the direction of magnetic field,when BdB/dz is OT2/m and-177 T2/m,the width of primary phase change hardly.When magnetic field gradient is-385T2/m,the width and spacing of the primary phase was narrowed obviously,the stripe morphology is not clear.(3)Effect of HMF on thermoelectric transport properties of hypoeutectic Bi-Te alloy.Compared with magnetic field-free,hypoeutectic Bi-Te alloys prepared under uniform magnetic fields have lower resistivity,higher Seebeck coefficient,lower thermal conductivity.In general,the thermoelectric parameters increased with increasing the uniform magnetic field intensity.When the magnetic field intensity is 11.5T,hypoeutectic Bi-Te alloy reached the maximum ZT value at 370K.Compared with magnetic field-free condition,the effect of gradient magnetic field on thermoelectric properties was the same as the uniform magnetic field.But compared with uniform magnetic field under the same magnetic induction intensity,hypoeutectic Bi-Te alloys prepared under gradient magnetic field has higher resistivity,lower Seebeck coefficient,and thermal conductivity increased first and then decreased.