Study On The Synthesis Method And Thermoelectric Properties Of N-type Mg₃Sb₂ Based Thermoelectric Materials

Mg₃Sb₂-based thermoelectric materials have been attracted more attention in the thermoelectric fields because of their low-cost,earth abundant and environment-friendly constituent elements.Mg₃Sb₂is an intrinsic p type semiconductor has poor electrical performance and thermoelectric properties because of its low intrinsic carrier concentration.Some results have shown that n type Mg₃Sb₂-based thermoelectric materials possess good thermoelectric properties,but how to efficiently and cheaply prepare n type Mg₃Sb₂-based thermoelectric materials are less systematically studied.In addition,the carrier transport mechanism is less explicated.Therefore,our work developed a synthesis method,which has the advantage of short synthesis period and low cost.Increasing the carrier concentration of n type Mg₃Sb₂-based thermoelectric materials by doping Bi/Se on Sb site and Mn/Y on Mg site,which enhances the power factor and thermoelectric properties.The following results are achieved:(1)High-performance n type Mg₃Sb₂-based thermoelectric materials were prepared bythe synthesis method of one-step ball milling and spark plasma sintering.The phase composition was analyzed by X-ray diffraction measurements,showing the highly pure phase of Mg₃Sb₂.Scanning electron microscopy was used to characterize the micromorphology of the samples,which shows that the surface micromorphology of the prepared materials have disordered nano layered crystal structures with a size of 50-100nm.Transmission electron microscopy was employed to investigate more details of microstructures which shows the grains with a size of 5-15 nm.As a result,our synthesis method is contributing to the highly pure phase and the nano crystal structure of Mg₃Sb₂-based thermoelectric materials,which contributes to the high electrical transport properties and the low thermal conductivity.(2)Optimizing the elemental composition of Mg₃Sb₂-based thermoelectric materials forthe n type conductivity.Increasing the proportion of Mg to compensate the vacancy produced by the volatilization of Mg during sintering process.Doping 0.5 ratio of Bi on Sb site,for causing lattice distortion and reducing lattice thermal conductivity.A small quantity of Se doping for optimizing carrier concentrations,which contribute to the increasing power factor and thermoelectric properties.Finally,the p type Mg₃Sb₂are converted into n type.By the optimization of the electrical and thermal properties,a peak ZT of 1.2 at 723 K was achieved in Mg_3.2Sb_1.5Bi_0.49Se_0.01sample.(3)Doping Mn or Y on Mg site for high performance.Mn doping increases the carrier concentration and the power factor,achieving a highest power factor of 2180?W m^-1K^-2in the sample of Mg_3.18Mn_0.02Sb_1.5Bi_0.49Se_0.01.At the same time,the impurity defects formed by Mn atoms in the Mg₃Sb₂lattice significantly enhance the phonon scattering,contributing to a lower lattice thermal conductivity.The lowest thermal conductivity of0.89 W m^-1K^-1was obtained in the sample of Mg_3.18Mn_0.02Sb_1.5Bi_0.49Se_0.01.Y doping can greatly enhance the carrier concentration,obtaining a highest value of 9.43×10¹⁹cm^-3in the sample of Mg_3.18Y_0.02Sb_1.5Bi_0.49Se_0.01,which is 8.5 times of the undoped sample.Because of the enhanced carrier concentration,the highest power factor 2450?W m^-1K^-2was obtained by Y doping,which is the highest power factor of the reported n type Mg₃Sb₂-based thermoelectric materials.Additionally,Mg₃Sb₂lattice produced greatly lattice distortion because of the ion size difference between Y and Mg atom,which enhances phonon scattering and is favorable for obtaining lower lattice thermal conductivity.The lowest lattice thermal conductivity 0.41 W m^-1K^-1was obtained in the sample of Mg_3.18Y_0.02Sb_1.5Bi_0.49Se_0.01.As a result,the highest ZT values of Mn and Y doped samples at 723K are 1.60 and 1.80,respectively,which is one of the highest values of n type Mg₃Sb₂-based thermoelectric materials.