Multinuclear solid state NMR investigations of crystallization processes in glass ceramics of the SiO2-Al2O3-K2O-Na2O-CaO-P2O5-F system

The crystallization of needle-like apatite from a glass ceramic of composit ion (mole.%) 61.5 SiO2-9.4 Al2O3-9.2 Na2O-7.7 K2O-6.0 CaO-1.9 P2O5-2.5 F wi th minor additional components of CeO2, ZrO2, TiO2, Li2O and B2O3, was moni tored by high-resolution solid state NMR utilizing the magnetic moments of H-1 F-19, Al-27, and P-31. Systematic studies of bulk and powder samples su bjected to well-defined annealing protocols indicate the utility in particu lar of F-19 magic-angle spinning (MAS) NMR to reveal the chemical changes o ccurring as a function of heating time and temperature. The parent glasses contain multiple fluorine sites, which act as joint reservoirs in the forma tion of the apatite phase crystallizing at temperatures exceeding 700 degre esC. The spectra suggest further that small amounts of this phase even form at lower annealing temperature.