Carbonate hydroxyapatite gel monolith formation and drying.

The effect of carbonate in reducing the crystal size of precipitated hydroxyapatite by approximately an order of magnitude has not been used previously in the preparation of gel monoliths for the fabrication of carbonate hydroxyapatite ceramics. The aim of this study was to devise a method whereby gel monoliths of carbonate hydroxyapatite could be repeatably produced without cracking. A precipitation reaction was used for the preparation of carbonate hydroxyapatities with carbonate contents of 5.8 and 7.8 wt%. Biaxial vacuum filtration was used to form disc shaped monoliths. The rate of filtration of a 7.8 wt% carbonate hydroxyapatite sol was measured throughout the gelation process. Gel monoliths were dried slowly in air and the mass and dimensions of the gel were recorded once approximately every 24 hours. Using this data, the permeability, water volume fraction with time, rate of water loss, gelation point and gel density were determined. The pore size distribution was measured using mercury porosimetry for a carbonate apatite gel and a pressed powder pellet of a commercial hydroxyapatite. In tact monoliths were formed with masses up to 9.9 g. It was found that gelation behaviour was independent of monolith size and carbonate content and the final green density of all monoliths was 37%. Gelation was found to occur at 50-55 vol% water. Gel monoliths were found to have a monomodal pore size distribution with a mean pore size of 9.1 nm, whereas a pressed pellet of hydroxyapatite had a bimodal pore size distribution.