JIAO Chunrong, JIAO Jian

(National Key Laboratory of Advanced Composites, AECC Beijing Institute of Aeronautical Materials, Beijing 100095, China)

Abstract: Ytterbium silicate powders were prepared by using hydrothermal method, with Na₂SiO₃ 9H₂O and YbCl₃·6H₂O as raw materials, water as solvent and NaF as mineralizer. The effects of hydrothermal conditions, including temperature and time, on phase structure and microstructure of the ytterbium silicate powders were studied, by using XRD and SEM. The hydrothermal reaction mechanism of the ytterbium silicate was elucidated. Finally, the powders were pressed into blocks to characterize their coefficients of thermal expansion. Ytterbium silicate powders with uniform morphology and high dispersion behavior could be obtained after hydrothermal reaction at 230-250 ℃ for 24 h. When the molar ratios of Yb/Si were 0.95 and 1.90, high purity ytterbium pyrosilicate (Yb₂Si₂O₇) and ytterbium monosilicate (Yb₂SiO₅) were formed, with coefficients of thermal expansion to be 3.0×10^-6 /K and 4.3×10^-6 /K, respectively, at 1200 ℃, while their thermal conductivity were both below 4.8 W/(m·K) at 1300 ℃. The powders exhibited properties comparable with those of SiC_f/SiC composite in thermal matching and heat-insulating characteristics, which meet the application requirements as environment barrier coatings.

Key words: hydrothermal reaction; ytterbium silicate powder; environmental barrier coating