Preparation And Thermoelectric Properties Of CaMnO₃ Based Thermoelectric Materials

Thermoelectric materials are energy materials that can realize the conversion of heat energy to electricity,and vice versa.They are widely used in power generation and refrigeration.There are many advantages such as high reliability,no transmission parts and long service life for using thermoelectric devices cored with thermoelectric materials.Therefore,they have become a hotspot of research in the field of new energy materials in recent years.In this paper CaMnO₃ thermoelectric material system was selected as the research object,by means of microstructural optimization and doping modification to improve the thermoelectric properties of the materials.The following research contents included:1.The process conditions for preparing CaMnO₃ thermoelectric materials by coprecipitation were studied and the optimum process was determined.CaMnO₃ precursor powders were prepared by controlling the co-precipitation with pH value at 6.5,7.5 and 8.5respectively.By comparing the electrical transport performance of the samples,it was found that pH=8.5 was the effective to reduce the resistivity of the samples.Therefore,the optimal pH value for the preparation of CaMnO₃ by co-precipitation was determined to be 8.5.After sintering the CaMnO₃ thermoelectric materials at temperature within the range of 1150?to1300?,the material microstructure,density and electrical transport properties were compared to determine the optimum sintering temperature,which was determined as1250?.Then the Ca site of CaMnO₃ was modified by single element doping to study the influence of single doping modification on thermoelectric properties.The high-valent rare earth ion Dy doped Ca_1-xDy_xMnO₃?x=0.03,0.05,0.10,0.15?thermoelectric material was used as the research object to conduct experiments with different doping concentrations.Through the characterization and comparison of the phase structure,density and electrical transport performance of the sample,it was found that the Ca_0.9Dy_0.1MnO₃ sample obtained a high power factor of 3.01×10^-44 Wm^-1K^-2,which is 1.5 times that of the undoped CaMnO₃.2.On the basis of single-doping modification,CaMnO₃ thermoelectric materials were modified by Ca site Pr and Yb double-doping to improve their electrical transport properties and reduce their thermal conductivity.Through XRD analysis and Rietveld refinement,SEM,XPS,first-principles calculation and thermoelectric properties characterization,the effects of different Pr and Yb double doping concentration at Ca site on the microstructures,electronic structures and thermoelectric properties of Ca_1-2xPr_xYb_xMnO₃?x=0,0.01,0.02,0.03,0.04,0.05?were investigated,with the results obtained as follows:1)The cell parameters,Mn-O bond length and Mn-O bond angle of the samples doped with different Pr and Yb were obtained by XRD analysis and Rietveld refinement.The octahedron distortion of MnO₆ in doped CaMnO₃ cell structure caused by double doping of Pr and Yb was analyzed from the aspect of structure-determining properties of materials,which reduced the resistivity of double doped system.2)The first-principles calculation results of the electronic structure?density of states?explain theoretically the reason why the resistivity of CaMnO₃ decreases due to Pr and Yb doping.3)The small grain size of the double-doped sample prepared by coprecipitation increases the scattering of grain boundaries,resulting in the decrease of thermal conductivity of the double-doped sample.In addition,the structural distortion of MnO₆ octahedron and the huge difference between the mass of double-doped ions and calcium ions are another main cause of the decrease of thermal conductivity.The highest power factor of Ca_0.92Pr_0.04Yb_0.04MnO₃ as per calculation is 3.49×10^-44 Wm^-1K^-2,which implies that the electrical transport performance of Ca-doped thermoelectric materials is better than that of the single-doped.The maximum ZT value is 0.24,which is three times much as that of undoped CaMnO₃,and is more optimal than the ZT value of CaMnO₃materials reported in most documents.3.La and Sm,Dy and Yb were selected under the condition of optimum concentration x=0.04,to modify Ca sites of CaMnO₃ system by double doping with the same amount of different rare earth elements,for the study of the effects of Ca sites double doping with different rare earth elements on the fine phase structure,the micro-morphology,the electronic structure and the thermoelectric properties of CaMnO₃ thermoelectric materials.The main results concluded are as follows:1)The accurate crystal structure information of double doping of different rare earth elements were obtained through XRD analysis and Rietveld refinement,which revealed that the change of electrical transport properties caused by double doping is strongly structure-dependent.2)The results of first-principles calculation regarding the electronic structure?density of states?explain theoretically the cause of the change brought by the double doping with different elements to the electrical transport properties of CaMnO₃ thermoelectric materials.3)Double doping of rare earth elements at Ca site can effectively inhibit the growth of grains,and increase the scattering of grain boundaries as well as the density of samples.The results show that the optimal ZT value of Ca_0.92La_0.04Yb_0.04MnO₃ is 0.24,which indicates that elements with high molar mass play a more significant role in double doping of Ca sites.4.CaMnO₃ precursor powders were prepared by solid-state reaction method,then added Bi₂O₃ powders to the precursor powders as mineralizers to obtain Dense Bi₂O₃ ceramics at lower sintering temperatures.The impact of Bi₂O₃ addition on the fine structure,micro-morphology and thermoelectric properties of CaMnO₃ has also been discussed in this paper.The conclusions are as follows:X-ray diffraction shows that formation of solid solution takes place during sintering of Bi₂O₃ and CaMnO₃ at high temperature;Rietveld refinement results show that the cell parameters and cell volume of the CaMnO₃ system increase with the augment of Bi₂O₃ content,resulting in the increased Jahn-Teller distortion of MnO₆ octahedron following the adding of Bi₂O₃ content.At the same time,the grain size of CaMnO₃ ceramics declines when the addition of Bi₂O₃ is increased,while its density becomes greater and greater.It drops with a certain amount after it increases to certain extent.Therefore,with the increase of Bi₂O₃ content,the resistivity of the sample decreases,the absolute value of Seebeck coefficient decreases,the value of power factor increases,and the value of thermal conductivity decreases.In the whole temperature range,the compound CaMnO₃-5Bi₂O₃ possess the lowest thermal conductivity,0.63@973 K,and the maximum ZT value is 0.21@973 K,which is about three times as much as that of CaMnO₃ without Bi₂O₃.In this paper,the thermoelectric properties of CaMnO₃ thermoelectric materials were systematically studied by optimizing the preparation process,element doping,double element doping and solid solution formation.It is concluded that co-precipitation method with double doping of Ca sites can effectively improve the thermoelectric properties of CaMnO₃,while the adding of Bi₂O₃ into the precursor powder of CaMnO₃ will result in CaMnO₃thermoelectric materials with low thermal conductivity but high thermoelectric properties.The research methods in this paper will serve as a constructive reference for other similar thermoelectric materials systems.