Synthesis And Thermoelectric Performance Of Mg₂^?(^?=si,Ge,Sn)and Mg₃X₂^V?X^V=sb,Bi?based Materials

Magnesium-based thermoelectric materials have lots of advantages,such as rich and environmentally friendly chemical constituents,low price,and low density.Among them,the Mg₂X^?(X^?=Si,Ge,Sn)-based and Mg₃X₂^V?X^V=Sb,Bi?-based materials have both achieved a breakthrough of zT>1.0,which are regarded as the most excellent and promising materials in the medium temperature region.In this thesis,two typical Mg-based thermoelectric materials have been studied systematically,mainly on how to optimize the synthesis processes and adjust the composition,in order to realize high TE performance.For the traditional Mg₂X^?-based materials,the research was focused on how to introduce point defect engineering to greatly suppress the intrinsic thermal conductivity,and at the same time effectively regulate the carrier concentration to maintain high power factor,so as to achieve the synchronous optimization of electroacoustic transport.Based on this,exploration was carried out for the controllable preparation and performance study of the 100-gram large-size sample required for commercialization.For the Mg₃X₂^V-based materials,we studied how to effectively change the transport mode,and used various methods such as doping and solid solution to adjust the carrier type and concentration,achieving a significant improvement in TE properties.In addition,for the poor P-type Mg-based materials,we made some attempts on increasing the hole concentration and introducing point defect scattering effect,hoping to enhance the thermoelectric properties.The main results of this thesis are as follows:1)Zn doping in high Sb-soluted Mg₂?Si,Sn?thermoelectric materials could improve the Seebeck coefficient effectively,therefore maintaining the high electrical properties of the materials.Meanwhile,the Zn-Sb double doping could largely reduce the thermal conductivity,which achieves significant increase of the overall thermoelectric properties of the materials.At 823 K,the maximum zT value of the materials reached 1.42 with the composition of Mg₂Si_0.4Sn_0.5Sb_0.1Zn_0.025,and this excellent performance has good experimental repeatality.2)N-type Mg₃?Sb,Bi?₂-based materials were prepared by mechanical alloying method combined with spark plasma sintering process.The introduction of Mg excess could decrease the content of Mg vacancies,and Te doping at the anion site could convert the intrinsic P type carrier transpot process to N type conduction.On this basis,the solid solution of Sb/Bi could further reduce the thermal conductivity,and the carrier concentrations could be optimized by the incorporation of Mn and In at Mg site,thereby achieving improvement of TE performance.At 780 K,an excellent performance of zT¹.1 was obtained in N type Mg₃?Sb,Bi?₂-based materials.3)P-type Mg₃?Sb,Bi?₂ materials were successfully prepared by the substitution of Ca at the Mg site and the increase of the Bi contents.The Analysis of transport mechanism shows that the introduction of Ca could reduce the formation energy of hole carriers by optimizing the energy band structure of the materials,and at the same time incorporate more point defect scattering to enhance the Phonon scattering,effectively reducing the lattice thermal conductivity.In addition,Sb solid solution and Ag doping in Mg₂Ge,and Zn doping in Mg₃Sb₂ could achieve P-type conduction as well.4)The thermal stability of Mg₂?Si,Sn?materials was systematically studied.Annealing process was used to achieve overall improvement of the zT values over the entire test temperature range.The high-performance Mg₂?Si,Sn?material samples were successfully obtained by the low-temperature solid-phase reaction method.And this method was successfully applied to the preparation of large-scale 100-gram Mg₂Si_0.35Sn_0.635Sb_0.015 cylindrical sample?diameter about 60 mm,thickness about 12mm?,and its phase,morphology,thermoelectric properties and uniformity of different regions were studied.