Slow coarsening of tetragonal zirconia nanocrystals in a phase-separated sodium borosilicate glass

A Zr ⁴⁺-containing sodium borosilicate glass (ZNBS) and a zirconium-free reference glass (NBS) were prepared by melt quenching. Thermal analysis revealed two glass transitions, indicating that both glasses exhibit phase separation into boron- and silica-rich domain structures already after quenching to room temperature. Electron micrographs showed that annealing of NBS and ZNBS glasses initiated the coarsening of these domains and the precipitation of metastable t-ZrO ₂ in the boron-rich matrix of the ZNBS glass. The coarsening kinetics of the silica-rich domains followed the theoretical predictions (power law exponent = 1/3), while the coarsening of the t-ZrO ₂ crystallites was much slower. The time evolution of the average size of t-ZrO ₂ nanocrystallites from small angle X-ray scattering (SAXS) and high-temperature X-ray diffraction (HTXRD) experiments revealed a coarsening exponent of ∼1/6. This slow coarsening of t-ZrO ₂ is assumed to be confined by the domain structure of the primary phase separation.