Characterization of hydroxyapatite powders prepared by ultrasonic spray-pyrolysis technique

The hydroxyapatite (HAp) powder was prepared by the ultrasonic spray-pyrolysis technique; the characterization of the resulting powders was performed. Five kinds of the starting solutions with the Ca/P ratio of 1.67 were prepared by mixing Ca(NO3)(2), (NH4)(2)HPO4 and HNO3; the concentrations of Ca2+ and PO43- were in the ranges of 0.10 to 0.90 mol.dm(-3) and 0.06 to 0.54 mol.dm(-3), respectively. These solutions were sprayed into the heating zone to prepare the HAp powders. The heating zone was composed of two electric furnaces; the lower furnace was used for the evaporation of the solvent from the droplets (300-500 degrees C) and the upper furnace for the pyrolysis of the precipitated metal salts (750-900 degrees C). The easily sinterable HAp powder was prepared by spray-pyrolysing the solution with Ca2+ (0.50 mol.dm(-3)) and PO43- (0.30 mol.dm(-3)) at the temperatures of 800 degrees C (the upper furnaces) and 400 degrees C (the lower furnaces). The resulting powder was composed of the spherical particles with diameters of similar to 1 mu m or below. Even without the calcination and grinding operations, the relative densities of the compacts fired at 1150 and 1200 degrees C for 5 h attained maxima similar to 95%. The microstructure of the sintered compacts appeared to be uniform; the average grain size was similar to 3 mu m. The activation energies for the grain growth of the sintered HAp compacts were 120 to 147 kJ.mol(-1).K-1. (C) 1999 Kluwer Academic Publishers.