Novel Fabrication Mechanism And Performance Optimization Of Fast Ionic Conductor Thermoelectric Materials

Thermoelectric materials and corresponding thermoelectric conversion technology will play an important role in solving the increasingly prominent problems of environmental pollution and energy shortage because it can realize the direct conversion between thermal energy and electric energy.Focusing on the fast ionic conductor thermoelectric material under the new concept of"electron-crystal phonon-liquid",combined with the theoretical analysis of electrons,phonons and their transport process,it can be seen that there are three main contradictions need to be balanced in three aspects for fast ionic conductor thermoelectric materials:?i?The contradiction between material preparation technology and thermoelectric properties,it is necessary to ensure the excellent thermoelectric properties of materials while developing simple,efficient,economical,and environmentally friendly material preparation technology;?ii?The contradiction between electron and phonon coupling in thermoelectric transport,it is necessary to increase the power factor while reducing the lattice thermal conductivity;?iii?The contradiction between ZT and T or?35?T or I,it is necessary to improve the thermodynamic and kinetic stability of the material in high temperature and the chemical stability under large temperature difference or current while ensuring its high ZT.Obviously,for a material system where"liquid"ion migration behavior leads to excellent thermoelectric properties,efforts should be made to optimize or even make use of the ion migration behavior;for a material system where"liquid"ion migration behavior deteriorates thermoelectric properties,efforts should be made to avoid the ion migration behavior.This paper follows the above research idea.In order to balance the above three main contradictions,we designed Cu₂Se/BiCuSeO composite thermoelectric materials in the high temperature region,developed electric field assisted ultrafast preparation technology of Zn₄Sb₃ based thermoelectric materials in the intermediate temperature region,and ultrafast preparation technology of Ag₂Se thermoelectric materials in low temperature region via the room temperature dissociative adsorption reaction.Meanwhile,their internal new mechanisms and new effects have been explored in detail.In addition,the characteristics of electrical and acoustic transport process have been revealed.The main research contents and conclusions are as follows:?1?Bi_1-x-x Pb_xCuSeO compounds can be quickly prepared by self-propagating high-temperature synthesis?SHS?combined with spark plasma sintering?SPS?technology.Bulk materials with density of more than 99%can be obtained.The SHS process of BiCuSeO consisted two fast binary SHS process?2 Bi+3 Se=Bi₂Se₃ and2 Cu+Se=Cu₂Se?intimately coupled with two relatively slow solid-state diffusion reactions?Bi₂Se₃+2 Bi₂O₃?3 Bi₂SeO₂ and then Bi₂SeO₂+Cu₂Se=2 BiCuSeO?.The formation rate of Bi₂SeO₂ was the bottleneck in the SHS process of BiCuSeO.Importantly,we found that a small amount of PbO addition in the starting materials has greatly facilitated the formation of Bi₂SeO₂-based compounds,which shows that Pb doping in the Bi site significantly reduces the reaction activation energy,increases the combustion temperature and combustion wave propagation rate,and then promotes the combustion reaction.Pb is not only a combustion promoter,but also an effective dopant.On one hand,carriers transport in the bivalent band.As the doping ratio increases,the Fermi level moves deeper into the valence band,the two closely lying heavier bands start to participate in the electrical transport,and the effective mass of the density of states?m^*?increases gradually.On the other hand,factors,which including lattice anharmonicity?lone pair electrons?,strong interaction between optical phonons and acoustic phonons caused by low frequency vibration,point defect scattering and decrease of sound velocity due to lattice softening,lead to the very low lattice thermal conductivity of Bi1-xPbxCuSeO samples.Compared with other preparation methods,SHS technology can effectively control the components and minimize the deletion of Cu.As a result,the optimal carrier concentration of7?10²⁰cm^-3 with 6mol%of Pb doping yielded a maximum ZT of 1.2 at 923 K.In addition,the BiCuSeO based compounds prepared by SHS+SPS process not only have excellent thermoelectric properties,but also have good thermal stability and reproducibility.?2?In the above study,Bi₂SeO₂ compounds were involved in the SHS reaction to form Cu₂Se compounds,and finally BiCuSeO compounds were formed.On this basis,Cu₂Se_1+y+xBiCuSeO?y=0,0.005,0.01,0.015,0.02;x=0,0.05%,0.1%,0.3%,0.5%?composite materials were prepared in situ by the same SHS+SPS process as the BiCuSeO based compounds mentioned above.It is found that the phase composition,microstructure,thermoelectric transport process and chemical stability of Cu₂Se matrix can be significantly changed by the combination of very small amounts of BiCuSeO compounds.Specifically,when the proportion of?phase in Cu₂Se matrix is low,BiCuSeO itself will be decomposed into special Bi-Se-O compounds,and?phase will be injected into the system at the same time,accompanied by the increase of carrier concentration;when the proportion of?phase in the Cu₂Se matrix is high,BiCuSeO will absorb too much?phase in the system due to its great inclusiveness to the composition,accompanied by the decrease of carrier concentration.There are high concentration of micro-nano scale pores in the matrix of bulk composites,and the compounds containing Bi are enriched on the inner wall of the pores,showing coherent characteristics with the matrix.The change of composition and structure greatly affects the transport behavior of electrons and phonons,and the composites with lots of components show excellent thermoelectric properties.In particular,for Cu2Se1.005+0.1%BiCuSeO composites,its peak ZT is 2.7 at 973 K,and its average ZT is 1.42 in the range of 300⁹73 K.The chemical stability of the composites is significantly improved,which may be closely related to the special pinning structure of BiCuSeO in the Cu₂Se matrix and its high density of multi-scale pores.In particular,when Cu₂Se_1.005+0.1%BiCuSeO and Cu₂Se_1.02+0.3%BiCuSeO composites were charged at 673 K for 24 h,the shape of the bulk materials remained intact,and no migration of Cu to the surface and pulverization were found.?3?In the above studies,we have optimized the thermoelectric properties and chemical stability of Cu₂Se-based fast ionic conductors in the high temperature region from the point of view of composition regulation.On this basis,we chose Zn4Sb3-based fast ionic conductor materials with more complex crystal structure and more thermodynamic and kinetic instability factors to study,and developed the electric field-assisted rapid preparation technology for the first time.The thermoelectric properties and chemical stability of Zn₄Sb₃-based fast ionic conductor materials were also optimized from the technological point of view.In particular,(Zn_1-xCd_x)₄Sb₃ and(Zn_1-xGe_x)₄Sb₃ compounds can be obtained from stoichiometric ratio and uniform mixing raw powders under the pulse current for 60 s.The bulk materials have a relative density of more than 98%and a special Zn4Sb3-ZnSb core-shell structure.The reaction process is extremely special.Firstly,under the action of numerous electric fields formed between Zn powder particles and Sb powder particles,Zn₄Sb₃ thin films are formed due to element interdiffusion.Then,under the subsequent action of current,Zn²⁺can be migrated rapidly in the Zn₄Sb₃thin film network to achieve fast mass transmission.Finally,the reaction was completed in a short period of time.In the core-shell structure of Zn₄Sb₃-ZnSb,the decomposition of Zn4Sb3 occurs in the upstream part of the current and the re-synthesis of Zn4Sb3 occurs in the downstream part of the current.So that to a certain extent,Zn²⁺could not be able to migrate out of the ZnSb shell.This special mechanism combined with foreign element doping will hinder the migration of dynamic Zn²⁺at the micro-nano and atomic level at the same time,which greatly improves the thermodynamic?phase transition?and kinetic?ion migration?stability of Zn₄Sb₃-based materials.Finally,(Zn_1-xCd_x)₄Sb₃ and(Zn_1-xGe_x)₄Sb₃ samples have excellent thermoelectric properties.For Zn_3.96Cd_0.04Sb₃ and Zn_3.97Ge_0.03Sb₃ samples,ZT_max is 1.19 and 1.12 at 723 K,respectively.In addition,the Zn₄Sb₃-based compounds synthesized by this technique have good thermoelectric cycle transport properties and repeatability.At the same time,this technique can be successfully extended to the rapid synthesis of ZnSb,Cu2Se and Cu2S compounds.For ZnSb compounds,because Zn4Sb3 compounds appear in the intermediate stage of the reaction,Zn²⁺can migrate directionally in the Zn₄Sb₃ intermediate product layer with the assistance of electric field,and the mass transfer can be realized quickly.This is the key reason why ZnSb compounds can be prepared quickly in such warm conditions.For Cu₂Se and Cu₂S compounds,this is because Cu⁺can still migrate along the current direction in the product layer to achieve mass transmission when the temperature is higher than their phase transition temperature,396 K and 370 K,respectively.?4?In the above studies,we have optimized the thermoelectric properties and chemical stability of Cu₂Se and Zn₄Sb₃-based fast ionic conductors from the point of view of composition and process control,respectively.However,the optimal thermoelectric property region of Ag₂Se compound corresponds to its low temperature phase,not to the corresponding fast ionic conductor phase.Therefore,from the synthesis and densification of Ag₂Se materials to the test of its thermoelectric properties and application,all the processes should avoid the fast ion migration behaviour.That is,the heat treatment temperature does not exceed 407 K.Based on this consideration,we have developed room temperature synthesis and densification technology of Ag₂Se compounds.In particular,Ag powder and Se powder can undergo dissociative adsorption reaction at room temperature.When Se touches the surface of Ag crystal,the strong dissociative adsorption makes Se vapor not escape from the system,which will always drive the formation of Ag₂Se compounds.After the formation of Ag₂Se crystal nucleus,it will fall off from the surface of Ag in time,so that the saturated steam of Se with high mass transfer rate will further contact with the newly exposed Ag surface.Because the reaction temperature is room temperature,Ag₂Se grains can not grow further,so the final product grains are at the nanometer level.Cold-pressing the phase pure stoichiometric Ag₂Se powder at 450 MPa for 2 min led to a pellet with a relative density of 99%.The microstructure of this material is featured by multiscale nanostructures.The grain size is very close to that of the powder before cold pressing,which is about20²00nm.The composition of the cold-pressed bulk samples is uniformly distributed,and the carrier concentration is as low as 2.3 x 10¹⁸ cm^-3,which is close to the theoretical optimal value.Finally,the TE figure of merit of the material of ZT=1.2was obtained at 390 K.In addition,Ag₂Se bulk materials with high quality,good reproducibility and chemical stability can be obtained by room temperature dissociative adsorption reaction combined with cold pressing densification process.