Regulating Electron-phonon Transport In PbTe-and GeTe-based Medium-temperature Thermoelectric Materials

With the development of our society,the demand for energy is increasing,which promotes the development of clean,efficient and sustainable energy.Thermoelectric materials have attracted intensive attention because of their ability to directly convert waste heat into electricity without pollution.Due to the wide application prospect of the mid-temperature thermoelectric materials,it attracts many researchers.Many studies are focused on optimization of PbTe and GeTe considering their competitive performance in the medium temperature region.Most studies focus on the enhancement of peak z Ts,but the efficiency of thermoelectric devices depends on the average z T in a wide temperature range.Therefore,we choose PbTe and GeTe as the research objects in this thesis.The electrical and thermal transport properties were optimized respectively.Various effective optimization methods were adopted to improve the average z T in a wide temperature range.The key problems restricting the improvement of average z T are solved and the mechanism of performance optimization is explored in detail.The main results of this thesis are as follows:The effect of Eu alloying in PbTe on carrier scattering mechanism in the near room temperature region was revealed.Combined with band convergence and defect phonon scattering,the average z T of PbTe was improved over a wide temperature range.Eu alloying can suppress the Pb vacancies and Mg/Pb point defects.By adjusting Eu content,the scattering mechanism can be regulated to improve the room-temperature mobility and optimize the room-temperature electrical transport properties.Meanwhile,Na doping is used to optimize the carrier concentration,Mg alloying is used to achieve band convergence,and Na,Mg and Eu are induced to realize defect engineering,simultaneously.These optimization methods work together to improve the average z T in the wide temperature range.Finally,a high room-temperature z T of 0.3 can be obtained for the Eu_0.02Mg_0.06Na_0.03Pb_0.89Te sample,and the peak z T can reach 2.0 at 773 K.The average z T is 1.34 from room temperature to 873 K.The thermoelectric performance can be enhanced by Al&Sb and Ga&Bi co-doping in cubic and rhombohedral GeTe-based materials,respectively.Due to the low formation energy of Ge vacancy in intrinsic GeTe,there is a high concentration of Ge vacancies.Therefore,it is important to find effective dopants that can reduce the concentration of Ge vacancies.Aluminum,as a p-type dopant in GeTe,will increase the hole concentration and lattice thermal conductivity;thus,it has long been considered as an unfavorable dopant for the opti mization of GeTe-based materials.However,when Al and Sb were co-doped into GeTe,compared with Sb doping,the hole concentration of the material can be reduced more effectively.Meanwhile,the lattice thermal conductivity can be reduced by more dislocations and nanoprecipitates,realizing the optimization of cubic GeTe.Eventually,a maximum z T of～2.0 at 773 K was achieved in Al&Sb-codoped Al_0.01Sb_0.1Ge_0.89Te,and the average z T can reach 1.2 in the temperature range of 323K-823 K.Ga is an n-type doped element that can effectively reduce the hole concentration in GeTe.Ga&Bi co-doping can effectively optimize the electrical and thermal transport properties in GeTe.As the doping efficiency of Bi is higher than that of Sb,a small amount of Bi doping makes the change of phase transition temperature less obvious than that of Sb doping.Finally,the temperature for achieving peak z T is lower than the phase transition temperature,and the thermoelectric properties of rhombohedral GeTe were impoved.The peak z T of Ga_0.02Bi_0.06Ge_0.92Te sample reaches 2.0 at 623 K.And the average z T reaches 1.02for rhombohedral GeTe in the temperature range from 300 K to 623 K.The mechanism of improved thermoelectric properties of multi-component GeTe-based materials by alloying PbTe and Ag Sb Te₂ was investigated.The phase structure was transformed from rhombohedral into cubic phase at room temperature by alloying PbTe and Ag Sb Te₂,which improved the symmetry of structure at room temperature.Meanwhile,band convergence and point defect scattering was used to improve the thermoelectric properties of GeTe-based materials over a wide temperature range.Firstly,GeTe was alloyed with Ag Sb Te₂ to achieve band convergence and lower phase transition temperature,and the lattice thermal conductivity can be reduced by nanoprecipitates.On this basis,PbTe alloying can further transform the phase structure from rhombohedral to cubic at room temperature.Contributed by the band structure modification,the Seebeck coefficient can be increased.Moreover,intense phonon scattering occurs due to the introduced point defects at the Ge site.The lattice thermal conductivity was effectively reduced to as low as～0.47 W m^-1 K^-1.As a result,a high z T of～0.7 at323 K and～2.4 at 723 K can be achieved in(Pb_0.15Ge_0.85Te)_0.8（Ag Sb Te₂）_0.2,leading to a record-high average z T of～1.8.High-entropy alloys Pb_0.25Ge_0.25Sn_0.25Mn_0.25Te have been successfully designed and prepared to enhance the thermoelectric performance by band degeneracy and high entropy.It can be realized by alloying GeTe with PbTe,Sn Te and Mn Te.The equivalent elements alloying can achieve band degeneracy.With an increasing number of components,the effective mass and Seebeck coefficient increased.The Seebeck coefficient can be enhanced to 103μV K^-1.Meantime,the lattice thermal conductivity decreased with the increasing configurational entropy.The peak z T enhanced to 1.4 at 823 K.However,the carrier concentration of the sample is as high as 1.5×10²¹ cm^-3,which is higher than the optimal carrier concentration(～10²⁰ cm^-3).Ga is selected to reduce the carrier concentration and further enhance phonon scattering.A high z T of 0.3 at 300 K and 1.52 at 823 K,and an average z T of 1.0 from 323 to 823 K were achieved in Ga_0.025(Pb_0.25Ge_0.25Sn_0.25Mn_0.25)_0.975Te.