Optimization Of Thermoelectric Properties Of MgAgSb Based Alloys By Electroacoustic Transport Regulation

Due to the excellent thermoelectric and mechanical properties of MgAgSb-based thermoelectric materials have attracted the attention of the thermoelectrical community.However,it is difficult to synthesize pure phase of MgAgSb-based alloy by traditional methods.In addition,the carrier transport mechanism,the cause of the low intrinsic thermal conductivity and optimization methods of thermoelelctric properties also have not been explicated yet.Thus,there is still room to improve thermoelectric performance and confirm whether the theoretical calculation of MgAgSb is semiconductor or semi-metal.According to the above,this paper mainly focuses on the preparation of MgAgSb alloy,the optimization of carrier concentration and the influence of Zn doping and heat treating on its thermoelectric properties,electronic structure and mechanical properties.The following results were achieved:（1）For the first time,a high-purity MgAgSb alloy was synthesized successfully by ordinary planetary ball mill and spark plasma sintering.We improved the purity of the MgAgSb alloy and optimized the carrier concentration by Zn doping.All-scale hierarchical architectures consisted of high-density grain boundaries,nano-grains,dislocations,and nanopores significantly increased phonon scattering and achieved lower thermal conductivity.Through optimization of electrical and thermal properties,a maximum ZT value of 0.7 was obtained at 473 K for the sample doped with 3%Zn.（2）After Zn doping and heat treating,all-scale hierarchical architecture is formed,and it realizes full-range phonon scattering,which effectively reduces the lattice thermal conductivity of the material.According to the theoretical simulation calculations,it is demonstrated that Zn-doping can obtain lower sound velocity and higher the Grüneisen parameter,which leads to a reduction of the lattice thermal conductivity.Besides,Zn doping and heat treating can remarkably improve the electrical properties of the material by optimizing the Hall carrier concentration and Hall mobility.The electrical properties of the material are remarkably improved.A highest ZT value of 1.4 at 423 K is obtained in the sample of 3%Zn doping with heat treating for 10 days.In the meantime,a higher average ZT¹.3 is gained at 323 K of cold side with 548 K of hot side,which is one of the highest values of the currently reported MgAgSb based thermoelectric materials.（3）A higher purity of MgAgSb alloy is obtained through gradient annealing and prolonging heat treatment time.We use the first principle to calculate the band structure of?-MgAgSb and Zn-doped?-MgAgSb.The calculation results show that?-MgAgSb is a semi-metal.The effective density of MgAgSb is improved by Zn doping,which is beneficial to realize higher thermoelectric performance.According to the density functional theory,?-MgAgSb and Zn-doped?-MgAgSb have good mechanical properties,which are of great significance for the application of thermoelectric materials,and have the potential to replace Bi₂Te₃ as room-temperature power generation and solid-state refrigeration.