Flash Sintering Preparation Of Mg_0.2(Ca_0.2)Co_0.2Ni_0.2Cu_0.2Zn_0.2O High Entropy Ceramics And Its Properties

High entropy oxide ceramics have attracted a lot of attention in the field of ceramics due to their simple crystal structure and excellent properties.Now it has been expanded from high-entropy oxide ceramics to high-entropy ultra-high temperature ceramics and high-entropy thermoelectric materials.At present,high-entropy oxide ceramic blocks are mainly prepared by traditional sintering methods,but the traditional preparation methods have problems such as long sintering time,high sintering temperature and high energy consumption.Flash sintering is an electric field-assisted sintering technique that has received a lot of attention in recent years and can achieve rapid sintering of ceramics at low temperatures,which has important applications.Liu et al.synthesized high-entropy oxide ceramics Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2O by flash sintering,but the properties of the high-entropy ceramics prepared by the flash sintering process and the reasons for the rapid transformation of the material phase are not clear.In this paper,the formation process and properties of high-entropy ceramics Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2O（HEO-1）and Ca_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2O（HEO-2）with the assistance of electric field are studied and analyzed by combining Joule heating theory and electric field excitation non-equilibrium defect theory.The main research contents and results are as follows:（1）The single-phase high-entropy oxide ceramic HEO-1 was successfully prepared at room temperature when the electric field strength was 50 V/cm and the current density was 300 m A/mm²,and the material phase transition took only 10 s.It was shown that flash sintering accelerated the movement and mixing of metal cations.（2）The microstructures and properties of flash-sintered and conventional sintered specimens were compared at the same blackbody radiation temperature.The relative density and hardness of the flash-sintered specimens were higher than those of the conventionally sintered specimens under all conditions,and the relative density and hardness of the flash-sintered HEO-1 specimens with a current density of 300 m A/mm²and a holding time of 60 s were 94%and 5.05 GPa,respectively,which were 22.8%higher and 3.95 GPa harder than those of the conventionally sintered HEO-1 specimens with a holding time of 2 h.The analysis showed that the increase in hardness was due to the reduction in porosity.（3）The dielectric constants of flash-sintered and conventionally sintered specimens were tested at the same blackbody radiation temperature,and the dielectric constants of flash-sintered HEO-1 specimens were as high as 2×10⁴,which was one order of magnitude higher than that of conventionally sintered specimens.It indicates that in addition to Joule heat,the non-equilibrium defects formed during flash sintering have a significant effect on the electrical properties of the flash sintered samples.（4）An attempt was made to synthesise the high enthalpy,high entropy oxide ceramic HEO-2 by flash sintering.high enthalpy high entropy oxide ceramics HEO-2were synthesized at room temperature when the current density exceeded 300 m A/mm²and the time for phase transition was only 30 s.The analysis showed that flash sintering could lead to the formation of single phase oxides in a non-equilibrium thermodynamic state for multiphase oxides.（5）The critical temperature for flash sintering of HEO-1 raw billets at different pretreatment temperatures in the flash sintering incubation stage was investigated.Within a certain range,the higher the pretreatment temperature,the higher the critical conditions for flash sintering.The analysis shows that both Joule heat and non-equilibrium defects have an important influence in the incubation stage of flash sintering.（6）The phase transition process of flash sintering synthesis of high entropy oxides was investigated.The phase transition of the high-entropy oxide occurs in the third stage of flash sintering.The phase transition occurs first at the positive end and then triggers a phase transition in the adjacent region until the phase transition is complete near the negative end.The electric field and temperature promote each other during this process,accelerating the phase transition.（7）The phase transition temperatures of two sintering methods,flash sintering and conventional sintering,were investigated.The phase transition temperature of HEO-1synthesized by flash sintering was shown to be lower than that of HEO-1 synthesized by conventional sintering by a counterfactual method.The analysis suggests that flash sintering may have increased the mixing entropy of the mixture oxide and therefore lowered the temperature of the phase transition.