Mechanical And Thermodynamic Calculations And Thermoelectric Properties Of Copper-rhodium-based Ternary Oxides

With the continuous development of industrialization,the energy crisis and industrial pollution are becoming more and more serious.This provides a very good opportunity for the development of thermoelectric materials.In the process of using traditional thermoelectric materials,the conversion of thermal energy to electrical energy is complicated and the energy conversion efficiency is low,resulting in waste of energy,which is closely related to the internal phase structure of the thermoelectric material.In recent years,the physicochemical properties of oxide thermoelectric materials at high temperatures have been extensively studied.Among them,Cu-Rh-O ternary oxides have potential applications in the fields of catalysis and electrochemistry,but their application in the actual reaction process of thermoelectric materials There is little interest in thermodynamic changes.With the increasing development of microelectronics and chip technology,the mechanical stability of thermoelectric materials is also an important factor to be considered in the manufacture of thermoelectric devices.Therefore,the reaction of Cu-Rh-O ternary oxide thermoelectric materials at different temperatures during the preparation process,and the mechanical stability at different pressures are of great significance for studying their thermoelectric properties.In this paper,the first-principles calculation method is used to calculate the elastic constants of Cu Rh₂O₄and Cu Rh O₂for the mechanical properties of Cu Rh₂O₄and Cu Rh O₂based on copper-rhodium-based oxides.Combined with the mechanical properties of Cu Rh O₂and the thermodynamic trends of Cu Rh₂O₄and Cu Rh O₂reaction,and combined with the experiment,the samples at different temperatures were obtained by the solid phase reaction sintering method.Doping Mg element to study its effect on the thermoelectric properties of Cu Rh₂O₄.Calculation and experimental results show that:1.Calculate the thermodynamic data of Cu Rh₂O₄and Cu Rh O₂by MS and Gibbs software respectively.The CASTEP package of MS was used to calculate the phonon spectrum value,and the obtained thermodynamic data was compared with the experimental manual data to verify the accuracy of the data.The heat capacity was calculated by Gibbs software.The heat capacity obtained by the phonon spectrum method and the heat capacity calculated by Gibbs software were compared with the manual thermodynamic data.It was found that the thermodynamic data obtained by MS CASTEP package to calculate the phonon spectrum was closer to the manual.2.By calculating the thermodynamic properties of each substance corresponding to the reaction of generating Cu Rh₂O₄and Cu Rh O₂,comparing the T*S with temperature change curve of each phase,the stability of Cu Rh₂O₄is better than that of Cu O and Rh₂O₃,Cu Rh O₂is more stable,but Rh₂O₃is less Stable,the stability of Cu₂O is between Cu Rh O₂and Rh₂O₃.Reaction Cu O+Rh₂O₃?Cu Rh₂O₄,the reaction free energy change value is less than 0 at 25??1174?,and the reaction free energy change value is greater than 0.At600°C,the reaction free energy reaches the minimum value,indicating 600?provides the most thermodynamic driving force for the reaction.For the reaction Cu₂O+Rh₂O₃?2Cu Rh O₂,the Gibbs free energy value is greater than 0 in the temperature range of 25??1540?,and as the temperature increases,the Gibbs free energy value increases.3.Calculate and analyze the mechanical properties of Cu Rh₂O₄and Cu Rh O2 by calculating the changes in the energy value corresponding to the unit cell volume changes,and at the same time use the Modulusball software based on MATLAB to characterize the anisotropy under different pressures through three-dimensional images.Calculations of the mechanical properties of Cu Rh₂O₄and Cu Rh O₂show that Cu Rh₂O₄has low anisotropy at low pressure and exhibits mechanical instability at 9.1GPa,and the plasticity does not change significantly with stress.The anisotropy of Cu Rh O₂varies greatly with pressure,and the anisotropy degree of 40GPa changes most obviously.According to the Born stability criterion of the rhombic crystal system,when the stress is greater than 62GPa and C44 is less than 0,the mechanical properties of Cu Rh O₂are unstable.4.X-ray,differential heat and XPS analysis show that the actual reaction temperature range of Cu O+Rh₂O₃?Cu Rh₂O₄is within the theoretically calculated temperature range,and the difference between the experiment and the calculation error is 174?.After increasing the reaction temperature,Cu Rh₂O₄undergoes a phase transformation at high temperature to form Cu Rh O2.By analyzing the Rh valence state of the mixed sample of Cu Rh O2 and Cu Rh₂O₄,it can be known that the singlet state of Rh exists.Combined with the calculation,the possibility of Cu₂O+Rh₂O₃?2Cu Rh O₂is excluded,and the high-temperature decomposition reaction of Cu Rh₂O₄is determined as:Cu Rh₂O₄?Cu Rh O₂+Rh+O₂.5.Cu Rh_2-2xMg_2xO₄(x=0,0.05,0.1,0.15,0.2,0.25,0.3)block was synthesized by solid phase sintering and SPS method.XRD pattern analysis showed that when Mg doping amount x<0.3,The overall X-ray diffraction peak of Cu Rh_2-2xMg_2xO₄shifted,indicating that the Mg element has been doped into the Cu Rh₂O₄lattice.The Seebeckk-resistance test system equipment was used to test the thermal conductivity,electrical conductivity and Seebeck coefficient of the sample.The results showed that Mg doping reduced the sample thermal conductivity,and the thermal conductivity decreased with the increase of the doping amount;Mg doping Impurities improve the electrical properties of Cu Rh₂O₄,and Mg doping increases the sample conductivity and ZT value.