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 cm3 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°C for 100 h; (ii) the droplets with diameters as small as 2–3 μm, 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°C for 1 h to form calcium phosphates. The β-calcium orthophosphate (β-Ca3(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.