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Microstructural changes taking place during the heating of spherical calcium-phosphate (Ca10(PO4)6(OH)2 (HAp) and β-Ca3(PO4)2) agglomerates prepared by spray-pyrolysis and freeze-drying techniques have been reviewed. The powders are prepared (1) by the spray-pyrolysis of calcium phosphate (Ca/P ratio=1.50 and 1.67) solution containing Ca(NO3)2, (NH4)2HPO4 and concentrated HNO3 at 600°C, and (2) by the flash freezing of droplets (Ca(CH3COO)2, PO(OCH3)3 and concentrated HNO3) in a cryogen, subliming of water ice under reduced pressures, and finally heat-treating of freeze-dried powder at 900°C for 1 h. In both techniques the resulting powders are composed of spherical agglomerates, reflecting the outward form of the starting droplets. The spray-pyrolyzed and heat-treated powder is generally composed of hollow spherical agglomerates, except for the case of ultrasonic spraying of droplets from 0.5 mol · dm-3 Ca(NO3)2/0.3 mol · dm-3 (NH4)2HPO4 or higher concentration where dense spherical agglomerates are produced. The freeze-dried β-Ca3(PO4)2 and HAp powders are prepared from solutions with Ca/P ratios of 1.44 and 1.67, respectively, and comprise of agglomerates with weakly-bonded particles.

MICROSTRUCTURES OF SPHERICAL CALCIUM-PHOSPHATE AGGLOMERATES PREPARED BY SPRAY-PYROLYSIS AND FREEZE-DRYING TECHNIQUES