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

Microstructural changes during the heating of submicrometer-sized spherical hydroxyapatite (Ca-10(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)(2)HPO4 and concentrated HNO3 at 600 degrees 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)(2)HPO4 (abbreviated as 0.0167/0.0100), was as small as 0.34 mu m. The spherical agglomerates became hollow following a heat treatment at 800 degrees 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 degrees C for 10 min increased the amount of pores with mean diameter of 0.21 mu m, due to the burn-out of residual carbon. (C)2010 The Ceramic Society of Japan. All rights reserved.