Heterogeneous crystallization of dicalcium phosphate dihydrate on titaniumsurfaces

The kinetics of nucleation and crystal growth of dicalcium phosphate dihydr ate (DCPD, CaHPO4. 2H(2)O) on titanium powders with different grain sizes h ave been investigated using the constant composition crystal growth method at 37 degrees C and pH = 5.50. Nucleation is independent of titanium powder size while crystal growth rate is strongly size-dependent. A minimum relat ive supersaturation ratio, sigma(min), required in the system for a detecta ble crystal growth rate to occur, taking into account the presence of the f oreign substrate in the system, was determined. Based on the kinetic data a nd on the various characterizations by X-ray photoelectron spectroscopy and by scanning electron microscopy, we propose a crystal growth kinetic equat ion as a function of two parameters: the size of the substrate and the exce ss of supersaturation (sigma-sigma(min)) which is squared, indicating a spi ral growth mechanism. (C) 1999 Kluwer Academic Publishers.