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 (Ca5(PO4)3OH or HA) precipitate mixed with silica over 800 °C yields a phase mixture of tricalcium phosphate phases (TCP) designated Si–TCP, β-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 (Ca3(P0.9Si0.1O3.95)2 or Si–TCPsat) and α-TCP (Ca3(PO4)2). Si–TCPsat has the same crystalline space group (P21/a) as α-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 α-TCP at a silica-HA interface followed by a transformation to Si–TCPsat or β-TCP. A thermodynamic model for the formation of Si–TCPsat predicts a nucleation temperature of 795 °C, in close agreement with experiment. If sufficient silicon is available, the α-TCP transforms to Si–TCPsat during extended sintering. In the absence of sufficient silicon, the α-TCP transforms to β-TCP.