Densification additives for hydroxyapatite ceramics

The present work is aimed at the elucidation of the role played by CaCl2 and NH4NO3 (the latter is a by-product of the solution synthesis of hydroxyapatite, hereinafter referred to as HAp) in the densification of nano-sized HAp powder in the course of the pressureless sintering. Nanocrystalline HAp powder was fabricated via the wet-precipitation technique by the dropwise addition of an (NH4)2HPO4 solution to a Ca(NO3)2 mother solution at a pre-adjusted pH at 60 °C. The pH of the aqueous mixture was maintained at a constant value (either 7 or 9) by the addition of an appropriate amount of NH4OH. The Ca/P ratio was set to 1.67, 1.61, and 1.48; 10 wt% of CaCl2 was added to dry HAp powder. NH4NO3 remaining in unwashed HAp powder can act as a fluxing agent that promotes partial melting at a relatively low temperature (150–250 °C) thus allowing the particles to rearrange into a denser packing. Several mechanisms of the CaCl2 action as a densification additive might be envisaged: (i) a decrease in the melting temperature; (ii) the surface wetting of grains; (iii) a change in the growth morphology owing to the high-temperature surfactant properties; (iv) a possible reaction with HAp on the surface of grains giving rise to the decomposition of HAp and yielding chlorapatite (ClAp), which can convert back to HAp over a wide temperature range and at any level of H2O.