Study On The Synergistic Optimization And Mechanism Of Thermoelectric Performance Of Mgagsb Based Alloys

MgAgSb-based alloy,as promising candidate of Bi₂Te₃ thermoelectric materials,by virtue of its rich elements,environment friendly and intrinsically low thermal conductivity characteristics has drawn a great deal of attention for low temperature thermoelectric applications.However,the sintering pressures used in synthesis process of MgAgSb-based alloys by different researchers are different and the significant effects of sintering pressures on their microstructure and thermoelectric performance still remain unclear.Additionally,the thermoelectric performance of MgAgSb alloys still room for further optimization due to its lower intrinsic carrier concentration and lack of introduction of defects.Finally,MgAgSb-based alloy has been extensively studied as near-room temperature thermoelectric material in the temperature range from 300 K to 573 K,but its thermoelectric performance and carrier transport mechanism at cryogenic temperatures from 173 K to 323 K have not been reported in detail so far.Hence,the significant effects of sintering pressure on microstructure and thermoelectric performance of MgAgSb alloy have been studied systematically in our work,and realizes the synergistic optimization of thermoelectric performance of MgAgSb alloy via Zr doping.Furthermore,the effect on thermoelectric performance and carrier transport mechanism of MgAgSb alloy could be revealed by different heat treatment time at cryogenic temperatures.The achieved results are summarized as following:（1）MgAgSb alloys with optimized the element ratio were successfully prepared by one-step ordinary planetary ball milling and spark plasma sintering,followed by a heat treatment process.The results show that the phase composition,micro-pores and crystallinity can be rationally engineered via simply manipulating the spark plasma sintering pressure.The crystallinity and grain size of MgAgSb alloy gradually increase,the impurity phases of Ag₃Sb and Sb and pore size gradually decrease with increasing sintering pressure from 20 MPa to 80 MPa.The Hall carrier mobility dependence relation with temperature transform fromμvs T^-1for MgAg_0.95Sb_0.99with a sintering pressure of 20 MPa toμvs T^-3/2for ones with a sintering pressure of 80 MPa,resulting in a gradually increasing electrical conductivity.Simultaneously,a lowest lattice thermal conductivity～0.452 W m^-1K^-1at 423 K is achieved in the MgAg_0.95Sb_0.99with a sintering pressure of 80 MPa,which can be ascribed to the reduction of the second phases with ultrahigh thermal conductivity and the introduction of a large number of micro-pores and nano pores that can strengthen the phonon scattering to reduce the lattice thermal conductivity.As a consequence,the thermoelectric performance of MgAg_0.95Sb_0.99sample can be synergistically optimized by simply manipulating the sintering pressure and a peak ZT～1.22 at 473 K was achieved in MgAg_0.95Sb_0.99sample synthesized with a sintering pressure of 80 MPa.（2）The synergistic optimization of thermoelectric performance of MgAgSb alloys is achieved via Zr doping on Mg site.The carrier concentrations of Mg_0.98Zr_0.025Ag_0.95Sb_0.98sample at 323K is optimized to～4.7×10¹⁹cm^-3by Zr doping.Hence,the maximum power factor at 523 K significantly increases from～1925.44μW m^-1K^-2of MgAg_0.95Sb_0.98sample to～2243.79μW m^-1K^-2of sample Mg_0.98Zr_0.02Ag_0.95Sb_0.98sample.In addition,the nanoscale and mesoscale Zr precipitation are formed by Zr doping and introduces nano pores,micro pores and Zr point defects,which can act as the phonon scattering center that can strengthen scattering phonons of different frequencies significantly to reduce the lattice thermal conductivity.So,an ultralow lattice thermal conductivity～0.390 W m^-1K^-1at 423 K is achieved in the Mg_0.98Zr_0.02Ag_0.95Sb_0.98.The results demonstrate well that electrical and thermal transport performance of MgAgSb alloy can be synergistically optimized via Zr doping.As a result,a peak ZT～1.30 at 473 K and a high average ZT～1.17 from 323 K to 573 K are obtained in the Mg_0.98Zr_0.02Ag_0.95Sb_0.98sample,simultaneously,the maximum theoretical output power density and conversion efficiency are～3.936 W cm^-2and 10.7%,respectively.（3）The effect of different heat treatment time on the thermoelectric performance and carrier transport mechanism of MgAgSb alloy are first reported at cryogenic temperatures.The results show that the purity and crystallinity of MgAgSb alloy can be significantly improved by adjusting the heat treatment time.The carrier mobility at323K can be significantly enhanced to～83.6 cm²V^-1s^-1for MgAg_0.95Sb_0.99sample with heat-treated 10 days.Hence,the maximum power factor at 173 K is enhanced dramatically from～538.05μW m^-1K^-2for MgAg_0.95Sb_0.99sample with heat-treated 0 days to～1747.11μW m^-1K^-2for ones with heat-treating 10 days,2.3 times higher,simultaneously,the maximum theoretical output power density and conversion efficiency are～0.783 W cm^-2and 6.83%,respectively.In addition,the simulation calculation shows that the maximum output power and the maximum energy conversion efficiency of the thermoelectric device composed of Ag₂Se and MgAg_0.95Sb_0.99sample are～37.40 m W and 3.10%,respectively.The results reveal that MgAgSb-based alloy has great prospects for thermoelectric applications at cryogenic temperatures.