Thermochemical stability and microstructural evolution of Yb₂Si₂O₇ in high-velocity high-temperature water vapor

Thermochemical stability and microstructural evolution of Yb₂Si₂O₇ was studied in high-temperature high-velocity water vapor at temperatures between 1200–1400 °C. Two reactions were shown to occur in the steam environment: Yb₂Si₂O₇ reaction to form Yb₂SiO₅, and further Yb₂SiO₅ reaction to form Yb₂O₃. Parabolic rates of both reactions were observed, and similar reaction enthalpies were determined for each reaction; 207 kJ/mol and 205 kJ/mol, respectively. Densification of the product phase Yb₂SiO₅ shut off pore connectivity for gas transport to the reaction interface at gas velocities exceeding 115−125 m/s and for temperatures of 1300 °C and 1400 °C, resulting in reduced reaction rates at higher velocities. Outward gas diffusion by a silicon hydroxide species is predicted to govern ytterbium silicate reactions with high temperature water vapor. Microstructure changes at high temperatures and velocities were shown to greatly impact the long-term stability of Yb₂Si₂O₇.