| Currently, the6–8wt.%Y2O3partially stabilized ZrO2(6–8YSZ) thermal barrier coatings (TBCs) topcoat ceramic is the most widely used material in the aerospace industry, which is stable below1200℃. However, the long-term exposure of the6–8YSZ coatings above1200℃will lead to the conversion of the phase constitution. The phase transformation results in a volume increase and thereby promotes the formation of cracks in the ceramics, finally leading to the spallation of TBCs, which is hard to meet the requirement of turbine inlet temperature rising. Therefore, it is urgently desired to develop new topcoat ceramics suitable for application higher than1200oC.In the present work, the (La1-xErx)2Zr2O7(x=0,0.02,0.04,0.06,0.08,0.10),(La1-2xErxYbx)2Zr2O7(x=0,0.01,0.02,0.03,0.04,0.05) and (La1-xYx)2Zr2O7(x=0,0.1,0.2) ceramics were synthesized by a solid reaction method, and the high-purity(≥99.99%) La2O3, ZrO2, Er2O3, Yb2O3and Y2O3were used as the starting materials in this study. The phase structures, phase stabilities, microstructures, bulk densities, thermal expansion coefficients, specific heat capacities and thermal diffusivities were identified by X-ray diffraction (XRD), differential scanning calorimetry(DSC), scanning electron microscope (SEM), Archimedes principle, high-temperature dilatometer, Neumann–Kopp rule and laser-flash apparatus (LFA), respectively.The XRD results indicated that (La1-xErx)2Zr2O7,(La1-2xErxYbx)2Zr2O7and (La1-xYx)2Zr2O7ceramics sintered at1600℃for5h were composed of the pure pyrochlore structure. In addition, the partial substitution of Ln3+(Er3+, Yb3+, Y3+) for La3+resulted in a decreased lattice parameter.The DSC results indicated that (La1-xErx)2Zr2O7,(La1-2xErxYbx)2Zr2O7and (La1-xYx)2Zr2O7ceramics sintered at1600℃for5h exhibited excellent phase stability from room temperature to1450℃.The SEM results indicated that the microstructure of these ceramics sintered at 1600℃for5h were similar, the grain boundaries were clean, the size of grains was several micrometers and the sintered ceramics were dense.The average thermal expansion coefficients of (La1-xErx)2Zr2O7,(La1-2xErxYbx)2Zr2O7and (La1-xYx)2Zr2O7ceramics were similar from room temperature to1300℃, ranged from9.57×10-6K-1to9.66×10-6K-1,9.39×10-6K-1to9.80×10-6K-1and9.52×10-6K-1to9.71×10-6K-1, respectively. With the increasing Ln3+(Er3+, Yb3+, Y3+) content, the specific heat capacities of (La1-xErx)2Zr2O7and (La1-2xErxYbx)2Zr2O7ceramics decreased from room temperature to1026.85℃, while that of (La1-xYx)2Zr2O7ceramics increased. In addition, Ln3+(Er3+, Yb3+, Y3+) doping substantially reduced the thermal diffusivity and thermal conductivity of La2Zr2O7ceramic between room temperature and1000℃, the thermal diffusivity and thermal conductivity decreased remarkably with increasing Ln3+(Er3+, Yb3+, Y3+) fraction. Especially, for the (La0.9Er0.1)2Zr2O7,(La0.9Er0.05Yb0.05)2Zr2O7and (La0.8Y0.2)2Zr2O7ceramics, the thermal conductivities showed a nearly temperature-independent behavior, their values ranged from1.27Wm-1K-1to1.35Wm-1K-1,1.22Wm-1K-1to1.32Wm-1K-1and1.28Wm-1K-1to1.44Wm-1K-1, respectively, which suggested a glass-like thermal conductivity. The extraordinarily low thermal conductivities could be ascribed to the extra strong source of phonon scattering, namely, rattlers scattering, and the rattlers were more efficient in scattering low frequency phonons.These results indicated promising potential applications of the (La1-xErx)2Zr2O7,(La1-2xYbxErx)2Zr2O7and (La1-xYx)2Zr2O7ceramics for high temperature thermal barrier coating topcoats. |
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