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

Abstract

The sintering of silicon doped calcium phosphate ceramics prepared from a basic colloidal hydroxyapatite (Ca 5(PO 4) 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 (Ca 3(P 0.9Si 0.1O 3.95) 2 or Si–TCP sat) and α-TCP (Ca 3(PO 4) 2). Si–TCP sat has the same crystalline space group (P2 1/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 SiO 2:mol HA (0.2:1) and 1 mol SiO 2: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–TCP sat or β-TCP. A thermodynamic model for the formation of Si–TCP sat predicts a nucleation temperature of 795 °C, in close agreement with experiment. If sufficient silicon is available, the α-TCP transforms to Si–TCP sat during extended sintering. In the absence of sufficient silicon, the α-TCP transforms to β-TCP.