Design And Thermal Conductivity Mechanisms Of Rare-Earth Zirconate High-Entropy Ceramics

With the continuous development and progress of aviation engine and gas turbine technology,the higher requirements have been put forward for the working temperature,insulation performance and service life of thermal barrier coatings（TBCs）.The current thermal barrier coating material is Y₂O₃ partially stabilized Zr O₂（YSZ）,which cannot meet the application requirements above 1200℃due to high-temperature phase transformation,accelerated sintering and decreased insulation performance.Rare earth zirconate high-entropy ceramics are considered as the novel thermal barrier coating materials with potential applications,and have also been the research hotspot in recent years.However,the increase in thermal conductivity of rare earth zirconate high-entropy ceramics at high temperatures seriously affects the insulation performance,which is not conducive to practical applications in the high-temperature field.Therefore,it is of great significance to suppress the increase in high-temperature thermal conductivity of rare earth zirconate high-entropy ceramics.In addition,there is currently a lack of comprehensive and in-depth research on the high-temperature thermal conductivity mechanism,the reasons for the increase in thermal conductivity and the methods to reduce high-temperature thermal conductivity of rare earth zirconate high-entropy ceramics.The(La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xTi_x)₂O₇（x=0～0.5）and(La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xCe_x)₂O₇（x=0～0.5）high-entropy ceramics with two crystal structures of pyrochlore and defective fluorite were prepared by the solid-state reaction method in this study.The structural evolution behavior and thermal conductivity of these high-entropy ceramics were systematically studied.According to the theory of phonon,photon and electronic heat conduction,the high-temperature thermal conduction mechanism was mainly studied and the reason for the increase in high-temperature thermal conductivity was revealed.Furthermore,based on the clarified high-temperature thermal conduction mechanism,the oxides（CoCrFeO₄,MnCr₂O₄,MnO₂,MnFe₂O₄and MnAl₂O₄）with high emissivity were further incorporated into the(La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_0.7Ti_0.3)₂O₇ high-entropy matrix for in-situ reaction to prepare single phase or multiphase high-entropy ceramics,and focused on studying their structure,thermal conductivity and mechanism,ultimately achieving the goal of suppressing the increase in high-temperature thermal conductivity.In addition,according to the performance requirements of thermal barrier coating materials,the high-temperature phase stability,thermal expansion coefficient and mechanical properties of the above rare earth zirconate high-entropy ceramics were evaluated.The main research findings are as follows:（1）The grain size,the order degree of pyrochlore structure and the disorder degree of defective fluorite structure increase with the rise of Ti⁴⁺and Ce⁴⁺content at the B site for the(La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xTi_x)₂O₇ and(La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xCe_x)₂O₇ high-entropy ceramics.In addition,the high-temperature（1450℃）phase stability of(La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xTi_x)₂O₇（x=0～0.5）high-entropy ceramics is superior to(La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xCe_x)₂O₇（x=0.1～0.5）high-entropy ceramics.（2）At room temperature～600℃,the thermal conductivities of(La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xTi_x)₂O₇ high-entropy ceramics decrease with the increase of Ti⁴⁺content,with a minimum of 1.20～1.35 W m^-1 K^-1（x=0.5）,while the thermal conductivities of(La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xCe_x)₂O₇ high-entropy ceramics increase with increasing Ce⁴⁺content,with the highest thermal conductivity of the x=0.5 component(1.50～1.87 W m^-1 K^-1).Moreover,the thermal conductivities of(La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xTi_x)₂O₇ and(La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xCe_x)₂O₇ high-entropy ceramics decrease with increasing temperature,indicating the phonon thermal conduction behavior.However,above 600℃,the thermal conductivities of high-entropy ceramics doped with Ti⁴⁺and Ce⁴⁺at the B site exhibit an abnormal increase with the increase of temperature and doping content(Ti⁴⁺and Ce⁴⁺content).Especially,the thermal conductivities of high-entropy ceramics doped with Ce⁴⁺increase drastically at 1000℃,with the highest thermal conductivity being 2.62 W m^-1 K^-1（x=0.5）,much higher than its room temperature thermal conductivity.（3）As the doping amount of Ti⁴⁺and Ce⁴⁺at the B site increases,the room temperature transmittance increases and the infrared emissivity at 1000℃decreases for(La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xTi_x)₂O₇ and(La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xCe_x)₂O₇ high-entropy ceramics,which leads to a decrease in thermal radiation shielding performance and an increase in photon thermal conductivity,ultimately promoting an abnormal increase in their actual thermal conductivity at high temperatures.In addition,the doping of Ce⁴⁺at the B site significantly increases the electrical conductivity of(La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂Zr₂O₇ high-entropy ceramics,and the increase in electrical conductivity（electronic thermal conductivity）with temperature and Ce⁴⁺content also promotes a drastic increase in the actual thermal conductivity of(La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xCe_x)₂O₇（x=0.1～0.5）high-entropy ceramics.While the doping of Ti⁴⁺at the B site is beneficial for reducing the contribution of electronic thermal conductivity to the actual thermal conductivity for(La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xTi_x)₂O₇（x=0.1～0.5）high-entropy ceramics.（4）The average thermal expansion coefficient（room temperature～1300℃）of(La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xTi_x)₂O₇ high-entropy ceramics changes little with the increase of Ti⁴⁺content,ranging from 10.65×10^-6 to 10.83×10^-6 K^-1.While the average thermal expansion coefficient of(La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xCe_x)₂O₇ high-entropy ceramics increases with the rise of Ce⁴⁺content,reaching a maximum of 11.07×10^-6 K^-1,which is close to the thermal expansion coefficient of YSZ ceramic.In addition,the hardness and elastic modulus of(La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xTi_x)₂O₇and(La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xCe_x)₂O₇high-entropy ceramics show a decreasing trend with the increase of x.The hardness and elastic modulus of(La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_0.5Ce_0.5)₂O₇high-entropy ceramics are the lowest,with 9.16±0.17GPa and 197.9±4.0 Gpa,respectively,which are close to La₂Zr₂O₇ ceramics.However,the fracture toughness does not change significantly with the increase of x,ranging from 2.5 to 3.0MPa·m^1/2,which is higher than La₂Zr₂O₇ ceramic.（5）Both CoCrFeO₄ and MnCr₂O₄ dissolve into the(La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_0.7Ti_0.3)₂O₇high-entropy matrix,forming the high-entropy ceramics with the pyrochlore structure,while the MnO₂,MnFe₂O₄ and MnAl₂O₄undergo chemical reaction with the high-entropy matrix to generate high-entropy ceramics with a mixed structure of pyrochlore and perovskite.Compared to single phase high-entropy ceramics,the generated multiphase high-entropy ceramics have a significant increase in thermal expansion coefficient,but their mechanical properties are relatively low.Among them,the multiphase high-entropy ceramics doped with MnO₂ have the highest thermal expansion coefficient(11.48×10^-6 K^-1),but the hardness（8.87±0.16 Gpa）,elastic modulus（112.57±2.25 Gpa）and fracture toughness(1.93±0.05 MPa·m^1/2)are the lowest.In addition,the high-entropy ceramics doped with CoCrFeO₄,MnCr₂O₄ and MnAl₂O₄ exhibit excellent high-temperature（1450℃）phase stability.（6）The incorporation of the high emissivity oxides（MnO₂,CoCrFeO₄,MnCr₂O₄,MnFe₂O₄and MnAl₂O₄）into the(La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_0.7Ti_0.3)₂O₇high-entropy matrix effectively reduces the room temperature transmittance,improves the high-temperature infrared emissivity,and also reduces the high-temperature electrical conductivity,thereby reducing its the photon and electronic thermal conductivity at high temperatures,ultimately suppressing the increase in high-temperature thermal conductivity of rare earth zirconate high-entropy ceramics.Among them,the high-entropy ceramics doped with MnO₂ and MnFe₂O₄ exhibit the most significant reduction in thermal conductivity,with thermal conductivity of 1.21 W m^-1 K^-1 and 1.36 W m^-1 K^-1 at 1000℃,respectively.