The Effect Of Y₂O₃and Gd₂O₃co-doping On Structures And Properties Of La₂(Zr_0.7Ce0.3)₂O₇thermal Barrier Coatings

The most commonly used ceramic topcoat in thermal barrier coatings (TBCs) systemsis6-8wt%yttria-stabilized zirconia (YSZ) due to its excellent thermophysical properties,including low intrinsic thermal conductivity and high fracture toughness. However,sintering phenomena and phase transformation of YSZ above1000~1200℃lead to TBCsfailure, which is hard to meet the requirement of rapid development of the aerospaceindustry. Therefore, it is vital to search novel ceramic materials which provide the bestperformance in high-temperature applications such as excellent phase stability, low thermalconductivity and low sintering rate to substitute for6-8wt%YSZ.In the present work, the powders of the (La1-2xYxGdx)2(Zr0.7Ce0.3)2O7(x=0,0.05,0.10,0.15,0.20,0.25,0.30,0.35,0.40,0.45,0.50) samples were synthesized by solid statereaction with La2O3, ZrO2, CeO2, Y2O3, Gd2O3as the raw materials. The phasecompositions, microstructures, thermal expansion coefficients, thermal diffusioncoefficients, densities and specific heat capacities of the sintered specimens weredetermined by powder X-ray diffraction (XRD, Bruker, D8Advance), Scanning electronmicroscope (SEM), thermal dilatometer (DIL402C), laser-flash apparatus (LFA-427),Archimedes principle and Neumann-Kopp rule, respectively.The results of XRD patterns indicated that the (La1-2xYxGdx)2(Zr0.7Ce0.3)2O7(x=0,0.05,0.10,0.15,0.20,0.25) samples sintered at1600℃for5h exhibits pure pyrochlorestructure and the (La1-2xYxGdx)2(Zr0.7Ce0.3)2O7(x=0.30,0.35,0.40,0.45,0.50)compositions sintered at1600℃for5h ceramics are composed of the pure defectivefluorite structure. The lattice parameters of the (La1-2xYxGdx)2(Zr0.7Ce0.3)2O7graduallydecrease with the increase of x value, which also indicates that Y3+and Gd3+ions havedissolved into the lattice of La2(Zr0.7Ce0.3)2O7to form the (La1-2xYxGdx)2(Zr0.7Ce0.3)2O7solid solutions.The SEM results of the ceramics sintered at1600℃for5h indicated that the grainsgrew up uniformly, the grain size of the ceramics is smaller than5μm and the sintered ceramics are incomplete dense.The thermal expansion coefficients (TEC) of (La1-2xYxGdx)2(Zr0.7Ce0.3)2O7ceramicsranged from10.17×10-6K-1to11.11×10-6K-1, from room temperature to1300℃. Theresult indicated that Y2O3and Gd2O3play a significant role in increasing TEC of the(La1-2xYxGdx)2(Zr0.7Ce0.3)2O7. With the Ln2O3(Y2O3, Gd2O3) substitution, the specific heatcapacities of the (La1-2xYxGdx)2(Zr0.7Ce0.3)2O7ceramics increased from room temperatureto1100℃. Y3+/Gd3+co-doping significantly reduces the thermal conductivity ofLa2(Zr0.7Ce0.3)2O7at the whole temperature range and the thermal conductivity remarkablydecreased with increasing Y3+and Gd3+fraction, but then with the content of x>0.15risesslightly. At low temperature, the thermal conductivity decreased remarkably withincreasing Ln2O3(Y2O3, Gd2O3) content fraction and the values of(La1-2xYxGdx)2(Zr0.7Ce0.3)2O7(x=0,0.05,0.10,0.15,0.20) ranged from2.08Wm-1K-1to1.55Wm-1K-1. Furthermore, as temperature rises up, the thermal conductivities of the(La1-2xYxGdx)2(Zr0.7Ce0.3)2O7become gradually less sensitive to composition and thevalues of (La1-2xYxGdx)2(Zr0.7Ce0.3)2O7(x=0,0.05,0.10,0.15,0.20) ranged from1.41Wm-1K-1to1.33Wm-1K-1.