Preparation and characterisation of monoclinic hydroxyapatite and its precipitated carbonate apatite intermediate

Five 100 g batches of a carbonate apatite (the intermediate) were produced by heating an aqueous slurry of CaCO3 and CaHPO4 with an overall Ca/P mole ratio of 5/3 with vigorous stirring. Each intermediate produced by boiling off water was heated in vacuum at 1100°C to remove carbonate, then steamed at 900°C to ensure complete hydroxylation. Comparison of calculated and observed X-ray diffraction patterns showed final products containing 50–100 wt% monoclinic hydroxyapatite (remainder hexagonal). Rietveld refinements in P63/m gave structures similar to several hydroxyapatite standards, including NIST SRM 2910, although there was no evidence from X-ray diffraction that the latter was in the monoclinic form. Refinements from standards and final products were slightly different from published single crystal data for Holly Springs hydroxyapatite. This is attributed to known impurities in mineral hydroxyapatite and indicates that parameters from the Rietveld refinements are closer to the true values for pure hydroxyapatite. Rietveld refinements for intermediates showed small, but significant differences from the final product, the largest being in O1x, O2x and O(H)z. All P–O bond lengths were shorter than in the final product, resulting in a 3.2% lower PO4 tetrahedron volume. The occupancies of P and Ca(2) were reduced. These differences are attributed to partial replacement of PO43− by CO32− ions

The implementation of the boundary element method on a network of transputers

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Dry mechanosynthesis of nanocrystalline calcium deficient hydroxyapatite: Structural characterisation

Dry mechanosynthesis is an efficient technique to synthesise nanocrystalline calcium deficient hydroxyapatites (CDHA). The mechanisms underlying a mechanochemical reaction are different from those triggering a dissolution mediated process, and this can have an effect on the structural features of the product. In this work, a nanocrystalline CDHA with Ca/P molar ratio of 1.5 obtained by means of dry mechanosynthesis of calcium oxide and dicalcium phosphate dihydrate was analysed. Spectroscopic techniques confirmed the presence of hydrogen phosphate (HPO4 2−) groups and a non-apatitic environment of the phosphate ions and disordered hydroxyl groups due to the nanometric size of the crystals. Lattice parameters of mechanosynthesised CDHA showed a small increase in the a lattice parameter (9.4418(20)A˚ ) and a small decrease in the c lattice parameter (6.8745(17)A˚ ), in agreement with the values reported in the literature for precipitated CDHAs. A prolonged milling resulted in an increase of the crystallinity of the CDHA and its partial decomposition into -TCP by the loss of OH− and HPO4 2− groups