High-pressure-assisted Preparation And Thermoelectric Performance Optimization Of Ca₉Co₁₂O₂₈ Oxide Thermoelectric Materials

Since the 21st century,the acceleration of global industrialization leads to the accelerated consumption of non-renewable fossil energy,which makes the problem of environmental pollution and energy shortage more serious.However,most of the fossil energy is dissipated in the form of waste heat in the process of conversion and utilization.Through thermoelectric conversion(TEC)technology,waste heat can be directly converted into electric energy,so as to improve the utilization efficiency of energy and effectively solve the problems of enviromental pollution.High-performance thermoelectric(TE)materials need to have a large Seebeck coefficient(S),high electrical conductivity,and low thermal conductivity,but these three parameters often change non-cooperatively,and only the lattice thermal conductivity can be adjusted independently.The lattice thermal conductivity accounts for more than90%of the total thermal conductivity.Therefore,reducing thermal conductivity has gradually become the research trend of TE performance optimization.The existing methods to reduce the thermal conductivity are increasing porosity,heavy ion doping,nanocrystallization and high entropy,etc.These methods all reduce thermal conductivity by increasing the scattering of phonons,but these modification methods have their inherent defects.For example,the improvement of porosity will deteriorate the electrical properties,and the nano-preparation is difficult and so on.In this study,high-pressure technology was used to densify Ca₉Co₁₂O₂₈with several GPa high pressures through a domestic cubic high-pressure apparatus(CHPA)to improve electrical conductivity;continue to pressurize,crush the grains,and introduce a large number of interfaces to scatter thermally conductive phonons.Compared with traditional modulation methods,we use pressure as a new modulation variable,which is expected to obtain new material properties.The main research contents and results are as follows:First,the single-phase Ca₉Co₁₂O₂₈TE powder was synthesized by solid state method.High temperature resintering(HTR)experiments were carried out at different high temperature close to its decomposition temperature and investigated their properties.We found that the specimen which resintered at 920?achieved the maximum power factor(PF)of 1.514?W/(cm·K~2)and the highest TE figure of merit(ZT)value of 0.132 at 750?,and its PF is 66.01%higher than that without HTR.We also found that the specimen which resintered at 905?achieved the maximum S of 179.963?V/K at 705?,the highest of all the specimens.And its S was 14.58%higher than that without HTR on average,which indicated that 905?may be the optimal temperature for Ca₉Co₁₂O₂₈to obtain performance optimization.Second,The pure Ca₉Co₁₂O₂₈TE material powder is pressed into compact tablets under pressure of 200 MPa,2.0 GPa,2.5 GPa,3.0 GPa,3.5 GPa and 4.0 GPa,respectively.The electrical conductivity,Seebeck coefficient,thermal conductivity and hardness are tested.The properties of specimens under different high pressures are compared and analyzed.It is found that the relative density of the specimens under different high pressures can reach 95%approximately.The highest PF of the specimens is 2.171?W/(cm·K~2)at 3.5 GPa,which is 2.4 times that at 200 MPa.At the same time,the maximum Vickers hardness of the specimens under 3.5 GPa is 127.7 MPa,while that under 200 MPa is only 16.8 MPa,indicating that the hardness under 3.5 GPa is increased by about 6.6 times.More importantly,it is found that the thermal conductivity of the specimens decreases with pressure in the range of 2.5 GPa-4.0 GPa,which proves that high pressure can refine the heat transfer performance effectively.This study lays the foundation for the synthesis of new-type Ca₉Co₁₂O₂₈TE materials with high machinability,high power factor,low thermal conductivity and high TE figure of merit with high-pressure technology.In summary,we first prepared pure phase Ca₉Co₁₂O₂₈TE powder by solid-state method,and carried out high temperature resintering experiments in extreme high temperature environment,and found out the optimal high temperature sintering temperature.Secondly,we introduced high-pressure conditions into the modification and preparation process of Ca₉Co₁₂O₂₈TE materials by using a domestic CHPA,and obtained a new type of Ca₉Co₁₂O₂₈TE materials with high density,high strength and high power factor.Finally,we demonstrate the effectiveness of high-pressure modulated thermal conductivity of Ca₉Co₁₂O₂₈TE materials,potentially discovering a new technical approach that can coordinately control the three parameters(S,?,?)of thermoelectric properties,which lays a foundation for obtaining the new Ca₉Co₁₂O₂₈TE material with high ZT value...