Preparation of submicrometer-sized porous spherical hydroxyapatite agglomerates by ultrasonic spray pyrolysis technique

Abstract

Microstructural changes during the heating of submicrometer-sized spherical hydroxyapatite (Ca10(PO4)6(OH)2: HAp) agglomerates were examined. The starting powder was prepared by the spray pyrolysis of calcium phosphate (Ca/P ratio = 1.67) solution containing 0.0167–1.5 mol·dm−3 Ca(NO3)2, 0.0100–0.9 mol·dm−3 (NH4)2HPO4 and concentrated HNO3 at 600°C using an ultrasonic vibrator. The resulting powders were composed of spherical agglomerates; the agglomerate diameter was reduced with decreasing concentration of spraying solution. The mean diameter of agglomerates obtained by the spray pyrolysis of solution with the lowest concentrations, i.e., 0.0167 mol·dm−3 Ca(NO3)2 and 0.0100 mol·dm−3 (NH4)2HPO4 (abbreviated as 0.0167/0.0100), was as small as 0.34 µm. The spherical agglomerates became hollow following a heat treatment at 800°C for 10 min. The incorporation of 0.015 mol·dm−3 citric acid into 0.0167/0.0100 solution contributed to the formation of hollow spherical agglomerates following spray pyrolysis. Furthermore, the heat treatment of these agglomerates at 800°C for 10 min increased the amount of pores with mean diameter of 0.2 µm, due to the burn-out of residual carbon.