To exploit the excellent properties of rare earth oxide materials and develop suitable materials for the surface layer of thermal/ environmental barrier coatings, the work aims to design and synthesize (Nd0.2Sm0.2Gd0.2Yb0.2Y0.2)3NbO7 high-entropy oxides. The high-entropy oxide was prepared by Sol-Gel method and high-temperature sintering technology with Nd2O3, Sm2O3, Gd2O3, Yb2O3, Y2O3 and Nb2Cl5 as origin chemicals. The lattice-structure, microstructure, element composition, thermophysical and room-temperature mechanical properties of achieved high-entropy oxide were investigated. The XRD and Raman spectra of naono-powder confirmed the single crystal structure for this high-entropy ceramics, and the corresponding bulk sample was also confirmed for the single pyrochlore-type lattice through XRD and Raman analysis. The bulk sample featured the densified micro-structure and well-distributed elements. The relationship curve between the specific heat capacity and temperature of the high-entropy ceramics showed that they were basically proportional to each other. The value of specific heat capacity ranged from 0.372 5 J/(kg·℃) at room temperature to 0.477 96 J/(kg·℃) at 1 000 ℃. Thermal diffusivity of the high-entropy ceramics decreased with the increasing temperature, which was consistent with the phonon thermal conduction mechanism exhibited by most oxide materials. The thermal diffusivity ranged from 0.31 mm2/s at 1 000 ℃ to 0.42 mm2/s at room temperature. The high-entropy design effectively reduced both room-temperature and high-temperature thermal conductivity of the ceramic material, with measured values of 1.07 W/(m·K) at room temperature and 1.03 W/(m·K) at 1 000 ℃. High-entropy ceramics showed no phase transformation or lattice change when the temperature increased from room temperature to 1 200 ℃, demonstrating excellent phase stability, which was conducive to extending the service life of thermal barrier coatings. Thermal expansion coefficient of the high-entropy was 10.14×10-5 K-1, which met the thermal expansion requirements for thermal barrier coating applications. Mechanical property characterization revealed a Young's modulus of 208 GPa. The lower elastic modulus increased the strain limit, thereby extending the thermal cycling service life of the applied components. Meanwhile, the lower elastic modulus was conducive to slowing down the phonon propagation speed and enhancing the thermal insulation performance. The micro-hardness of high-entropy ceramics was approximately 9.27 GPa, and the fracture toughness was about 1.67 MPa·m1/2 at ambient conditions. The micro-hardness and fracture toughness will be improved by process optimization or element adjustment in the future. The results indicate that the obtained high-entropy oxides are of single pyrochlore-type lattice. Its relative-density is higher than 90%, exhibiting even-element distribution. Owing to the aggravated phonon scattering caused by lattice-distortion, mass variation and introduced oxygen-vocation from multi-type rare-earth cation doping, its thermal conductivity at room and high-temperature decreased. Because of decreased electro-negativity-difference value between metal cations and oxygen ions, its thermal expansion coefficient is higher than that of YSZ, which is disadvantageous to thermal/environmental barrier coatings. Its Young's modulus is in the same order with those of YSZ, while the micro-hardness and the fracture-toughness are low.
Key words
thermal/environmental barrier coatings /
high-entropy ceramics /
rare earth niobates /
thermophysical properties /
mechanical properties
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Funding
The National Natural Science Foundation of China (52101392); Shandong Provincial Natural Science Foundation (ZR2024QE253); Universities of Shandong Province of china (2020KJA014); Shandong NarralScience Foundation (ZR20200D081); Science and Technology Support Plan for Youth Innovation (ZR2020ME130)
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