Partic. vol. 2 no. 5 pp. 200-206 (October 2004)
doi: 10.1016/S1672-2515(07)60059-1

Morphological and microstructural changes during the heating of spherical calcium orthophosphate agglomerates prepared by spray pyrolysis

Kiyoshi Itatani^a,*, Mari Abe^a,Tomohiro Umeda^b, Ian J. Davies^c, Seiichiro Koda^a

itatani@sophia.ac.jp

Abstract

The microstructural changes taking place during heating of calcium orthophosphate (Ca₃(PO₄)₂) agglomerates were examined in this study. The starting powder was prepared by the spray-pyrolysis of calcium phosphate (Ca/P ratio=1.50) solution containing 1.8 mol·L⁻¹ Ca(NO₃)₂, 1.2 mol·L^-1(NH₄)₂HPO₄ and concentrated HNO₃ at 600 °C, using an air-liquid nozzle. The spray-pyrolyzed powder was found to be composed of dense spherical agglomerates with a mean diameter of 1.3 μm. This powder was further heat-treated at a temperature between 800 and 1400 °C for 10 min. When the spray-pyrolyzed powder was heated up to 900 °C, only β-Ca₃(PO₄)₂ was detected, and the mean pore size of the spherical agglomerates increased via the (i) elimination of residual water and nitrates, (ii) rearrangement of primary particles within the agglomerates, (iii) coalescence of small pores (below 0.1 μm), and (iv) coalescence of agglomerates with diameters below 1 μm into the larger agglomerates. Among the heat-treated powders, pore sizes within the spherical agglomerates were observed to be the largest (mean diameter: 1.8 μm) for the powder heat-treated at 900 °C for 10 min. With an increase in heat-treatment temperature up to 1000 °C, the spherical agglomerates were composed of dense shells. Upon further heating up to 1400 °C, the hollow spherical agglomerates collapsed as a result of sintering via the phase transformation from β- to α-Ca₃(PO₄)₂ (1150 °C), thus leading to the formation of a three-dimensional porous network.

Keywords

spray-pyrolysis; calcium orthophosphate; hollow spherical agglomerates; heat-treatment; morphology; microstructure