Study On Filling And Preparation Mechanism Of Supersaturated Ce_xCo₄Sb₁₂ Thermoelectric Material

Thermoelectric material is a kind of functional material which can realize the mutual conversion of heat energy and electric energy,so it has a wide application prospect in waste heat recovery and semiconductor refrigeration.Facing the problems of global warming and energy exhaustion,in response to the call of Made in China 2025,the recovery of waste heat from thermoelectric generators has the greatest potential in the energy recovery of automotive systems.Thermoelectric devices have the outstanding advantages of pollution-free,noise-free,small size,high accuracy,long life and easy control.However,due to the insufficiency of thermoelectric properties of thermoelectric materials and the large contact resistance of thermoelectric joints,large-scale commercial applications can not be realized at present.Up to now,the filling mechanism of Ce in Ce_xCo₄Sb₁₂ thermoelectric materials and the influence law of its filling limit have not been determined.Ce,the most abundant rare earth element,the lowest price and the most potential commercial application,was selected as filling element in this project.The influence of temperature and cooling rate on the filling limit of Ce in Ce_xCo₄Sb₁₂ thermoelectric material during heat treatment process was studied.It was found that increasing the heat preservation temperature and cooling rate of heat treatment process could improve the filling limit of Ce in Ce_xCo₄Sb₁₂ thermoelectric materials.The average filling amount of Ce_xCo₄Sb₁₂ thermoelectric material obtained by holding at 820?for 100h and quenching can reach 0.177,the lattice constant can reach 9.055,and the corresponding lattice thermal conductivity can be as low as 0.9W/?m*K?.At the same time,Ce_xCo₄Sb₁₂ thermoelectric materials with different nominal components?x=0.08,0.2,0.3,0.5?,different cooling rates?furnace cooling/quenching?,different quenching temperatures?700/740/780/820??were characterized by Ce filling amount,lattice constant,microstructure and thermoelectric properties.It was found that the ZT value increases with the increase of theoretical Ce filling and the decrease of impurity phase number.Finally,the Ce_0.3Co₄Sb₁₂ thermoelectric material samples with the maximum ZT=0.99 at 550?are prepared.Thereafter,on the basis of the data of Ce filling amount,lattice constant,microstructure and thermoelectric properties,with the help of Co-Sb and Ce-Sb binary phase diagram,the phases'mechanism of formation and transformation in Ce_xCo₄Sb₁₂thermoelectric material during smelting-solidification-heat preservation-cooling process were analyzed,and the influence mechanism of Ce filling limit in Ce_xCo₄Sb₁₂thermoelectric material was also discussed.In the process of smelting and solidification of Ce_xCo₄Sb₁₂ thermoelectric materials,the system is homogeneous liquid phase at1150?.Beta phase CoSb first precipitates from the liquid phase when the temperature is lower than 1065?.When the temperature is lower than 931?,the liquid phase reacts with CoSb phase at the interface of CoSb to form a gamma phase CoSb₂.When the temperature is lower than 876?,the liquid phase reacts with CoSb₂ phase at the interface of CoSb₂ to form a delta phase CoSb₃.CeSb₂ begins to precipitate from the liquid phase when the temperature is lower than about 756?,and disappears completely when the temperature is lower than 624?,forming eutectic structure of CeSb₂ and Sb?s,s?with a very small amount of Co dissolved.Finally,the ingots with the highest Sb content,the second CoSb content,the second CoSb₂,CoSb₃ and CeSb₂ content decreasing in turn were obtained.During the heat treatment process,the Sb?s,s?dissolved with a small amount of Co was re-transformed into liquid phase,and the peritectic reaction continued with the diffusion of CoSb and CoSb₂ phase,eventually uniform COSb₃ phase was formed in the long heat preservation process.In this process,CeSb₂ also dissolves into the liquid phase,and further generates the diffusion of Ce atoms into CoSb₃ phase,eventually forming the element-filled Ce_xCo₄Sb₁₂ phase.The influence of heat treatment process on the FFL of Ce in Ce_xCo₄Sb₁₂ thermoelectric materials is that the increase of heat preservation temperature will promote the dissolution of CeSb₂ and the diffusion of Ce into Ce_xCo₄Sb₁₂ phase,while the increase of cooling rate will inhibit the separation of Ce from Ce_xCo₄Sb₁₂ phase,thus the Ce_xCo₄Sb₁₂ phase with higher filling amount and the Ce_xCo₄Sb₁₂ thermoelectric materials with higher ZT value can be obtained.