Phase formation and evolution in the silicon substituted tricalcium phosphate/apatite system

The sintering of silicon doped calcium phosphate ceramics prepared from a basic colloidal hydroxyapatite (Caw(5)(PO4)(3)OH or HA) precipitate mixed with silica over 800degreesC yields a phase mixture of tricalcium phosphate phases (TCP) designated Si-TCP, beta-TCP and a silicon substituted dehydrated apatite (Si-Ap). The Si-TCP phase is defined as a combination of a silicon stabilized TCP in which the silicon content attains a saturated value (Ca-3(P0.9Si0.1O3.95)(2) or Si-TCPsat) and alpha-TCP (Ca-3(PO4)(2)). Si-TCPsat has the same crystalline space group (P2(1)/a) as alpha-TCP, but with characteristically different lattice parameters due to the substitution of silicon in tetrahedral phosphorus sites. The nucleation and growth kinetics of Si-TCP in samples of composition 0.2 mol SiO2:mol HA (0.2:1) and 1 mol SiO2:mol HA (1:1) can be understood in terms of the initial growth of alpha-TCP at a silica-HA interface followed by a transformation to Si-TCPsat or beta-TCP. A thermodynamic model for the formation of Si-TCP,at predicts a nucleation temperature of 795'C, in close agreement with experiment. If sufficient silicon is available, the alpha-TCP transforms to Si-TCPsat during extended sintering. In the absence of sufficient silicon, the alpha-TCP transforms to beta-TCP. (C) 2004 Elsevier Ltd. All rights reserved.