Formation mechanism of nano-sized hydroxyapatite powders through spray pyrolysis of a calcium phosphate solution containing polyethylene glycol

Nano-sized hydroxyapatite powders were synthesized by spray pyrolysis of a calcium phosphate solution containing polyethylene glycol (PEG) and a subsequent dewaxing process. Calcium phosphate solutions (Ca/P ratio of 1.67) were prepared by dissolving calcium nitrate tetrahydrate and diammonium hydrogen phosphate in deionized water with and without PEG and subsequently adding nitric acid. When the calcium phosphate solution was sprayed at 1100 °C at a carrier gas flow rate of 40 L/min, micron-sized hollow hydroxyapatite particles with dense or collapsed shells were obtained. However, when the calcium phosphate solution containing PEG was sprayed under the same conditions, micron-sized porous hydroxyapatite particles consisting of loosely packed hydroxyapatite crystals containing randomly placed carbons were obtained. When these hydroxyapatite particles were dewaxed at 650 °C for 3 h, they were completely disintegrated into nano-sized hydroxyapatite particles, whereas the micron-sized hollow hydroxyapatite particles without carbons remained intact. After sintering at 1100 °C for 3 h, the relative densities of hydroxyapatite disks made from micron- and nano-sized powders were 78.5 ± 1.7% and 99.8 ± 0.3%, respectively. The practical implication of these results is that highly sinterable nano-sized hydroxyapatite powders can be synthesized through spray pyrolysis of a calcium phosphate solution containing PEG.