Coarsening of needle-shaped apatite crystals in SiO2•Al2O3•Na2O•K2O•CaO•P2O5•F glass

Coarsening of needle-shaped apatite crystals was studied in a SiO2 . Al2O3 Na2O . K2O . CaO . P2O5 . F glass. Crystal numbers and size distributions were measured by image analysis of electron micrographs obtained from slightly etched fractures of powder compacts sintered from differently annealed glass powders. Results indicate that the growth of apatite needles is controlled by diffusion-limited Ostwald ripening. Thus, the mean needle diameter and length increase with t(1/3) while the crystal number decreases with t(-1) due to a constant volume fraction of apatite. An invariant reduced size distribution could be found for the diameters. The measurement of needle lengths is affected by several difficulties (e.g. random needle orientation). (C) 1999 Kluwer Academic Publishers.