Electron Microscopy Research Based On High Performance Zintl Phase CaAg_xZn_(1-x)/2Sb System

With the development of science and technology,energy and environmental issues have become more and more topics of concern to people.Thermal energy and electric energy are the most important forms of energy in our social life.As a green new energy source,thermal power has received more and more attention from the country.Thermoelectric materials have become a promising new energy material with their unique characteristics of mutual conversion of heat and electric energy.It can use industrial,domestic waste heat and solar energy for thermoelectric power generation.At present,the relatively low thermoelectric conversion efficiency is the main reason that restricts the wide application of thermoelectric materials.Among thermoelectric materials,Zintl phase compound is a typical"electron-crystal phonon glass"material,and it is possible to improve the conductivity of Zintl phase thermoelectric materials and reduce the thermal conductivity at the same time by controlling the conductivity and thermal conductivity of different regions.Based on the structural advantages of Zintl phase compounds,in order to further improve the thermoelectric conversion efficiency,the crystal structure is usually changed and defects are introduced.Spherical aberration correction scanning transmission electron microscope is an indispensable method for the study of defect structure.This article mainly uses spherical aberration correction electron microscope JEM-ARM 200F and high resolution electron microscope JEM-2010 to characterize the microstructure of several CaAg_xZn_(1-x)/2Sb system thermoelectric materials,and in-depth study of thermoelectricity combined with physical property analysis The intrinsic relationship between performance and microstructure.The main research contents of this paper are as follows:1.Cs-corrected transmission electron microscope was used to study the atomic scale of CaAgSb samples.A large number of modulation structures were found in the CaAgSb sample.Through Cal Atom software analysis,it was found that the reason for the modulation structure was uneven segregation of Ca atoms in the sample.There are also a large number of banding structures in the CaAgSb sample,which run through the whole grain.By analysis,it was found that the structure of the band was caused by cell stretching,which included three atomic layers being uniformly stretched.The widely distributed defect structure provides a guarantee for the subsequent performance optimization of CaAg_xZn_(1-x)/2Sb system.2.CaAg_0.2Zn_0.4Sb was prepared by doping Zn at theAgsites of CaAgSb.An ultra-low lattice thermal conductivity of?0.4 W m^-1·K^-1 is obtained in the entire measurement temperature range,and the ZT value is improved compared with CaAgSb.Using a Cs-corrected electron microscope,the microstructure of the sample was studied on the atomic scale.A large number of defects of different dimensions were observed,including zero-dimensional point defects,one-dimensional dislocations,two-dimensional grain boundaries/twin boundaries,and three-dimensional nanoprecipitation phases.These defects inhibit the transmission of phonons in a large frequency range.By elucidating the scattering mechanism of phonons,our results promote the understanding of phonon transmission in Zintl phase compounds,thereby providing ideas for subsequent performance optimization.