Preparation Of Mg₂Si Based Thermoelectric Materials Doped With Multi Nano Components And Diffusion Bonding With Ni/Cu Composite Electrode

Thermoelectric material is a kind of functional material which can realize the direct mutual conversion between heat and electricity.The thermoelectric module can be used for power generation and refrigeration with advantages of less pollution,no moving parts,and high safety and reliability.Hence,there is a promising prospect for further research and application of thermoelectric materials.As one type of important medium-temperature（400⁹00K）thermoelectric materials,Mg₂ Si based thermoelectric materials possess superior material factors and are of low cost since there are abundant raw materials in the earth crust.However,it’s difficult to get pure product due to the oxidation and volatilization of pure Mg,so exploring an original and feasible preparing method for Mg₂ Si based thermoelectric materials is a crucial research direction in this field.Besides,research on the properties and application of Mg₂ Si based thermoelectric materials are at the primary stage,and seeking for efficient waysto improve and optimize the thermoelectric performance and application link are key orientations in current research.Considering the above-mentioned difficulties,this paper prepared Mg₂ Si based materials with the field activated and pressure assisted synthesis（FAPAS）method and MgH2 had been used instead of pure Mg as raw material to react with nano silicon,tin or both.This paper explored the feasibility of the synthesis process,and properly solved problems in traditional methods such as high reaction temperature,severe oxidation of Mg,and uncontrollable components and so on.Based on this,silicon nanowires and nano Mg₂ Si powder were combined with Mg₂ Si to prepare composite materials,and the decoupling and optimization of the thermoelectric properties of Bi doped samples were realized.Based on the relationship among phase component,microstructure and transport properties,the thermoelectric transport mechanism was analyzed by the SPB model.Finally,aiming at the application problem of the output electrode of the thermoelectric generator,the diffusion bonding of Mg₂Si-Cu/Ni composite electrode was studied.The morphology and formation of the diffusion layer of the interface in different processes were analyzed,and its thermal contact properties,mechanical properties and thermal stability were studied.The major results are shown as follows:（1）Using MgH2 as raw material,a one-step synthesis method（FAPAS）was applied to prepare pure Mg₂ Si based thermoelectric materials,in which the solid state reaction and the densification of the powder were simultaneouslycompleted.The incorporation of a small amount of nano Si particles and 1at.%Bi can effectively improve the electrical conductivity and reduce the thermal conductivity of Mg₂ Si based materials.The maximum ZT value of 2at.% Bi doped Mg₂Si0.6Sn0.4 samples reached 1.29 at 770 K by optimizing the solid solution and Bi doping.The annealing treatment of the doped samples exhibited good thermodynamic stability.（2）Mg₂Si-SiNW was successfully prepared by chemical etching followed by external mixing method.The possibility to decouple the relationship between electrical parameters of the intrinsic Mg₂ Si had been explored by introducing SiNW,and thus exploring new ways to improve its thermoelectric properties.The results showed that the thermoelectric properties of the samples havd been greatly improved with the introduction of SiNW.However,the Bi doping weakened the effect of energy filtering and the phonon scattering from the SiNW in the intrinsic Mg₂ Si materials.The carrier transportation is selective in the micro/nano crystalline Mg₂ Si samples.The carrier mobility increased with the increase of micro crystalline,while the thermal conductivity of the samples decreased with the increase of nano crystalline.（3）SPB model had guiding significance in determining transportation mechanism and optimizing ZT value of thermoelectric materials.In this study the transportation mechanism of Bi-doped Mg₂ Si based samples was analyzed by SPB model,and the results indicated that there was a relationship between peak ZT and reduced Fermi energy,η,which is near the band edge and relatedwith effective mass and mobility.The best carrier concentration of Bi-doped Mg₂SiSiNW0.005 samples increased with temperature.When it reached 775 K,the experimental value was far off the predicted one by SPB model due to the effects of other energy bands.The peak ZT value of the Mg₂Si0.6Sn0.4Bix sample increased at first and then decreased with temperature,in which the best carrier concentration varied in a small range.The peak ZT had reached 775 K with the carrier concentration of 1×1020cm-3.（4）One-step synthesis method was adopted to realize the Mg₂ Si synthesis and the bonding with the Cu/Ni composite electrode.The influences of technological parameters such as temperature and time on the interface structure and new phase formation were investigated.The shear strength and contact resistance were characterized.The result manifested that a reactive sintering temperatures higher than 1023 K with a no less than 15 min hold time provided the best properties and long-term thermal stability.