Preparation of various calcium-phosphate powders by ultrasonic spray freeze-drying technique

Various calcium-phosphate powders with Ca/P ratios ranging from 1.00 to 1.67 were prepared by an ultrasonic spray freeze-drying (USFD) technique: (i) 100 cm(3) of aqueous solution containing 0.500 mol.dm(-3) of calcium acetate (Ca(CH3COO)(2)), 0.300-0.500 mol.dm(-3) of trimethyl phosphate (PO(OCH3)(3)) and 1.77 mol.dm(-3) of acetic acid (CH3COOH) were refluxed at 80 degrees C for 100 h; (ii) the droplets with diameters as small as 2-3 mu m, which were formed by the ultrasonic spray of aqueous solution, were introduced into a chamber chilled by liquid nitrogen to freeze the droplets; (iii) the water ice in the frozen droplets was sublimed under reduced pressures; and (iv) the freeze-dried materials were heated at 900 degrees C for 1 h to form calcium phosphates. The beta-calcium orthophosphate (beta-Ca-3(PO4)(2)) and carbonate-containing hydroxyapatite powders with stoichiometric Ca/P ratios were prepared from the solutions with Ca/P ratio of 1.44 and 1.67, respectively. (C) 2000 Elsevier Science Ltd. All rights reserved.