Si-substituted hydroxyapatite nanopowders by precipitation: Synthesis, thermal stability and sintering behavior

Synthetic hydroxyapatites incorporating small amounts of Si have shown improved biological performances in terms of enhanced bone apposition, bone in-growth and cell-mediated degradation. This paper reports a systematic investigation on Si-substituted hydroxyapatite (Si 1.40 wt%) nanopowders produced following two different conventional wet methodologies: (a) precipitation of Ca(NO3)2.4H2O and (b) titration of Ca(OH)2. The influence of the synthesis process on composition, thermal behaviour and sinterability of the resulting nanopowders is studied. Samples were characterised by electron microscopy, induced coupled plasma atomic emission spectroscopy, thermal analysis, infrared spectroscopy, N2 adsorption measurements, X-ray diffraction and dilatometry. Semicrystalline Si-substituted hydroxyapatite powders made up of needle-like nanoparticles were obtained, the specific surface area ranged between 84 and 110 m2/g. Pure and Sisubstituted hydroxyapatite nanopowders derived from Ca(NO3)2.4H2O decomposed around 1000 °C. Si-substituted hydroxyapatite nanopowders obtained from Ca(OH)2 were thermally stable up to 1200 °C and showed a distinct decreased thermal stability with respect to the homologous pure sample. Sisubstituted hydroxyapatites exhibited higher sintering temperature and increased total shrinkage with respect to pure powders. Nanostructured dense ceramics were obtained by sintering at 1100 °C Sisubstituted hydroxyapatites derived from Ca(OH)2.