Effect of Silica Addition on Compositional Gradient within Porous Spherical Calcium Phosphate Agglomerates Prepared by Spray Pyrolysis

摘要

Phase changes during the heat treatment of spray-pyrolyzed/calcined calcium phosphate agglomerates with silica addition have been investigated. The calcium phosphate solution (1 dm~3), whose chemical composition corresponded to that of calcium orthophosphate (Ca_3(PO_4)_2; TCP, Ca/P ratio = 1.50), was prepared by using 0.150 mol-dm~(-3) Ca(NO_3)_2, 0.100 mol-dm~(-3) (NH_4)_2HPO_4, 0.1 mass% colloidal silica and 30 cm3 concentrated nitric acid. The mixed solution was spray-pyrolyzed at 600°C using an air-liquid nozzle and further calcined at 600°C for 10 min. The spray-pyrolyzed/ calcined powder with silica addition was composed of hollow spherical agglomerates with diameters of 1 to 8 mu m, in contrast to the formation of dense agglomerates with no silica addition. The heat treatment of spray-pyrolyzed/calcined powder with silica addition at 800°C for 10 min resulted in a notable compositional gradient from the outside to inside of the shell, i.e., (i) CaO-P_2O_5-SiO_2 layer with Ca/P ratio being 1.44 (outermost region of the shell), (ii) hydroxyapatite (Ca_(10)(PO_4)_6(OH)_2; HAp) layer and (iii) beta-TCP (innermost region of the shell). On further heating up to 900°C for 10 min, HAp and beta-TCP were found to be randomly situated within the shells of the spherical agglomerates.