Microstructure And Thermoelectric Properties Of MgAgSb-based Alloys

MgAgSb alloy exhibits a low intrinsic lattice thermal conductivity from 300 K to 548 K.However,its carrier concentration is still relatively low that is far out of the optimum range,so the power factor and ZT value could be probably enhanced somewhat.Besides,the deep carrier and phonon transport mechanism is still unclear that limits the further enhancement of thermoelectric performance.Here proper doping could increase the carrier concentration and thus enhances power factor and ZT value.The influence of hot-press temperature,increasing Sb content as well as doping on the Mg site,including Li,Ca and Yb,on the microstructure,transport behavior and thermoelectric properties are investigated by means of XRD,TEM,ZEM-3,NANO FLASH and PPMS.The carrier and phonon transport mechanism are investigated by assuming single parabolic model with an acoustic phononscattering mechanism,Debye-Callaway model as well as structural analysis.Finally,the mechanism of doping could be revealed.It is demonstrated that hot press temperature has a significant influence on the grain size,phase composition and thermoelectric performance.When hot-press temperature is lower than 573 K,MgAg_0.97Sb_0.99 alloys are pure phase and have a small grain size ¹50 nm.When hot pressed temperature is 593 K,the grain size show the significant growth（1-5 μm）.When hot-press temperature is 613 K,low content of impurity phases can be observed.With increasing hot-pressed temperature,carrier concentration first decreases and then increases,leading to power factor first increasing and then decreasing.Simutaneously,total thermal conductivity is gradually increased and finally ZT value is decreased.Choosing proper alloy composition,namely tuning Sb content,could lead to the increased carrier concentration and power factor for MgAg_0.97Sb_0.99+x alloys.With x less than 0.005,MgAg_0.97Sb_0.99+x alloys are pure phase.However,Sb nanoinclusions could be found for MgAg_0.97Sb1.00 alloy.With increasing Sb content,carrier concentration first increases markedly and then decreases somewhat,but the power factor is gradually increased.Typically,the room-temperature power factor is increased from 18.4 μW cm-1 K-2 to 22.9 μW cm-1 K-2.Simultaneously,the lattice thermal conductivity is greatly increased that counteracts the enhancement of power factor.Finally,ZT value is almost no change with increasing Sb content,but high power factor of MgAg_0.97Sb_0.99+x alloy is beneficial for achieving high maximum output power density of TE device.It can be revealed that the optimal carrier concentration of MgAg_0.97Sb_0.99 alloy is around 9.0×1019 cm-3 using the theoretical model.The carrier concentration of undoped alloy is around 2.7×1019 cm-3 that is much lower than that of optimal value.Li doping obviously decreases the electrical resistivity and thus significantly enhances power factor.With increasing Li doping concentration,power factor of Mg1-x Lix Ag_0.97Sb_0.99 alloy first increases and then decreases.The peak value is achieved for x=0.01 with room-temperature value 24.0 μW cm-1 K-2.Simutaneously,lattice thermal conductivity is decreased first and then increased.Finally,average ZT from 300 K to 548 K is increased somewhat,from 1.0（x=0）to 1.1（x=0.01）.Ca or Yb doping could increases the carrier concentration,decreases the lattice thermal conductivity and enhances ZT for Mg1-x Mx Ag_0.97Sb_0.99 alloys（M=Ca or Yb）.With the increased Ca doping content,carrier concentration is gradually increased,leading to enhancing power factor.Simultaneously,lattice distortion reduces the lattice thermal conductivity.With the increased Yb doping content,carrier concentration is first increased and kept saturate.Thus,power factor first increases and then decreases somewhat.Simultaneously,lattice thermal conductivity first obviously decreases and then increases.Finally,a high peak ZT ¹.4 could be finally achieved at 548 K for x=0.005.Compared with corresponding Ca doping,Yb doping produces the strong lattice distortion leading to obvious reduction of lattice thermal conductivity and significant enhancement of ZT value.High energy ball milling in combination with quick hot press method could fabricate the nanostructured MgAg_0.97Sb_0.99 based alloys,generating high density of boundaries,stacking faults and dislocations that could obviously scatter the wide-frequency phonons.Since the carrier concentration of undoped MgAgSb is far lower than the theoretical optimum value,choosing proper alloy composition and doping element could increase the carrier concentration and thus enhance the power factor.Simultaneously,lattice distortion reduces the lattice thermal conductivity and finally ZT value could be enhanced.