The effect of silicon doping on the transformation of amorphous calcium phosphate to silicon-substituted α-tricalcium phosphate by heat treatment

Abstract

In the present study, the effects of silicon dopant contents on the phase composition of silicon-substituted hydroxyapatite (Si-HA) powders were investigated. Si-HA as-prepared samples, with varying silicon (Si) dopant contents, were prepared by an aqueous precipitation method. The phase transformation of the as-prepared samples during sintering was also discussed. The analyses of the as-prepared powders confirmed the formation of well-crystallized Si-HA at low Si dopant concentrations (≤1.5wt% Si). Contrarily, silicon-substituted amorphous calcium phosphate (Si-ACP) and amorphous silica formed at high Si dopant concentrations (>1.5wt% Si). Upon sintering at 1000°C, Si-HA (≤1.5wt% Si) underwent no phase transformations, whereas Si-ACP (>1.5wt% Si) transformed to biphasic silicon-substituted α-tricalcium phosphate (Si-α-TCP) and HA (no additional phases such as β-TCP, silicocarnotite or CaO were detected). Rietveld refinement on X-ray diffraction patterns was conducted to determine the percentage contents of ACP and differences of lattice parameters between pure α-TCP and Si-α-TCP. X-ray photoelectron spectroscopy measurements suggested the formation of amorphous silica and decomposition of the Si-ACP to Si-α-TCP were correlated. The results confirmed that Si doping led to formation of Si-ACP, which primarily transformed to Si-α-TCP upon sintering. The current study highlights a possible account of recrystallization of Si-ACP directly to Si-α-TCP at low sintering temperatures.