A Study On Processing And Property Optimization Of SnTe-Based Lead-free Thermoelectric Materials

In today's world,the energy crisis and the deterioration of the environment are the two major problems.Thermoelectric materials can convert the waste heat of environment into electrical energy,and is beneficial to alleviate the warming of the Earth.It can play a positive role to solve the two problems.Among the thermoelectric materials,LAST attractes much attention due to the high thermoelectric properties.But,LAST materials containing large amounts of lead,which is not friendly to the environment.This study used tin instead of lead to prepare AgSnmSbTem+2 compounds,and improve the thermoelectric properties of AgSn_mSbTe_m+2 by optimizing process,micro structure regulation and nano-composite technology.Firstly,the preparation process was optimized.In this study,mechanical alloying and spark plasma sintering were used to the preparation of AgSn_mSbTe_m+2 materials.The study shows that the grain size of the materials can be reduced and the thermoelectric properties can be improved by properly prolonging the milling time and decreasing the sintering temperature.The ZT value can be enhanced by 60% through optimizing the processing parameters,resulting in a relatively high ZT up to 0.62 at 723 K when milled for 12 h and sintered at 743 K.Secondly,the chemical composition was optimized.The electrical and thermal transport properties of samples with different m values in a wide range were investigated for compositional optimization with the most enhanced thermoelectric performance.It was found that both electrical and thermal conductivity increase with increasing m value,but the Seebeck coefficient shows a mamimum at m=4,resulting in an optimal composition with a relatively high thermoelectric figure of merit up to ZT?0.79 at 723 K.Thirdly,the microstructure was regulated.AgSn4SbTe6 thermoelectric materials were fabricated by a process combining mechanical alloys?MA?and spark plasma sintering?SPS?,whose property improvement was realized by simply repeating the MA and SPS processes.Through the regulation of microstructure,the grain size of material can be refined,and the enhanced grain boundary energy filtering effect and Seebeck coefficient has also been improved.The effect of grain boundary on phonon scattering is enhanced,and the thermal conductivity of the material is reduced.The mamimum thermoelectric figure of merit?ZT?value was raised to 0.92 at 723 K,almost equivalent to the record one reported for this materials fabricated by a melting process.Finally,nano-composite technology was adopted.It is found that miming SiC nanoparticles into the AgSn4SbTe6 matrim effectively enhances its thermoelectric properties.SiC nanoinclusions can increase the Seebeck coefficient while increasing the electrical conductivity,in addition to its effect of reducing lattice thermal conductivity by the enhanced phonon scattering.A high dimensionless figure of merit?ZT?value of up to 1 at a relatively low temperature?723 K?is obtained in AgSn4SbTe6 incorporated with 1 vol% SiC nanoparticles,equivalent to the record one reported for this materials fabricated by a melting process.