Doping Modification And Mechanism For CaMg₂Bi₂ Based Thermoelectric Materials

As a new type thermoelectric material,p-type CaMg₂Bi₂ compound has attracted more and more attention because it meets the characteristics of“electron-crystal phonon-glass”of ideal thermoelectric materials.However,the current research obstacle is that the influence mechanism of inevitably formative Bi second phase on the thermoelectric performance is unclear.And the intrinsic low carrier concentration and high lattice thermal conductivity result in a low ZT value,which limits its practical applications.In this work,through the first-principles calculation combined with experimental verification,the physical essence of Bi second phase’s influence on thermoelectric performance was firstly identified.The improvement of ZT are achieved by constructing point defect and modulating band structure.Meanwhile,this work systematically studied the effect of the change of Bi content and Zn content,alkali metal acceptor doping,alkaline earth metal equivalent doping and Ba doping on the microstructure,band structure and thermoelectric transport properties of CaMg₂Bi₂-based thermoelectric materials.The first-principles calculation results show that in CaMg₂Bi₂thermoelectric materials,the cation vacancy formation energy decreases when the chemical environment changes from poor Bi to rich Bi,which will increase the carrier concentration.Guided by the calculation results,this work designed the CaMg₂Bi_x compounds with different Bi contents.The experimental finding indicates that the carrier concentration increases with increasing Bi content,making the electrical conductivity increase and the Seebeck coefficient decrease and the power factor increase.At the same time,the CaMg₂Bi_x compounds with the Bi second phase are naturally cooled and quenched after high-temperature annealing,respectively.It is found that the content of the Bi second phase in the quenched samples significantly decreased,revealing the high-temperature redissolving of the Bi second phase.Attributing to the dynamic doping effect caused by the redissolving of Bi second phase,the ZT value of CaMg₂Bi_1.98sample reaches 0.85 at 873 K,which is 5.5 times higher than that of the single phase CaMg₂Bi_1.94.On the basis the Bi second phase induced dynamic doping,the thermoelectrical properties could be further improved by Zn doping.When Zn atom replaces Ca site,the weakening of Chemical bond could reduce sound velocity,and the introduction of point defect scattering could hinder phonon transport.The combination of the two leads to the reduction of lattice thermal conductivity.The lattice thermal conductivity of Ca_0.85Zn_0.15Mg₂Bi_1.98compound is as low as 0.6 W m^-1 K^-1 at 873 K;When Zn atom replaces Mg sites,the cation vacancy formation energy decreases,resulting in the increased carrier concentration,the electrical conductivity and power factor.Meanwhile,the covalence in the polyanion layer is weakened,causing the decrease of sound velocity.Combined with enhanced phonon scattering,the lattice thermal conductivity of Zn doped compounds(0.7 W m^-1 K^-1@873K)is lower than CaMg₂Bi_1.98.Finally,the ZT values of Ca_0.85Zn_0.15Mg₂Bi_1.98 and CaMg_1.9Zn_0.1Bi_1.98 compounds reach 0.96 at 873 K.It is pointed out that introducing holes through alkali acceptor doping could effectively increase the carrier concentration of CaMg_1.9Zn_0.1Bi_1.98 compound.The increased carrier concentration can shield the polarized optical phonon scattering and improve the mobility.Thus,the power factor and the ZT value are significantly promoted in the measure temperature range.When Na content is0.005,the average power factor and average ZT value are 14.3μW cm^-1 K^-2 and0.6（300 K-873 K）,respectively.Using Yb element with high electronegativity to replace Ca site could reduce the formation energy of cation vacancy and increase the carrier concentration.Finally,the electrical conductivity and power factor is improved.In addition,the introduced point defects hinder phonon transport and reduce the lattice thermal conductivity.The lattice thermal conductivity of Ca_0.65Yb_0.35Mg_1.9Zn_0.1Bi_1.98 compound at 773 K is as low as 0.5 W m^-1 K^-1,and the ZT value is as high as 1.0.The results show that Ba doping could reduce the crystal field splitting energy of Ca_0.995Na_0.005Mg_1.9Zn_0.1Bi_1.98 compound and improve the band degeneracy and the effective mass,leading to a balance between the conductivity and Seebeck coefficient.The higher power factor is maintained.The point defects introduced by Ba doping further hinder the phonon transmission and reduce the lattice thermal conductivity.Simultaneously,with the further reduction of Zn content to adjust the band structure,the conductivity remains unchanged.But the Seebeck coefficient is slightly increase,resulting in the increased power factor.Finally,the ZTave of（Ca0.75Ba0.25）0.995Na0.005Mg1.95Zn0.05Bi1.98 compound reaches0.85 in the range of 300 K-873 K,which is the highest value of CaMg₂Bi₂system at present.