Comparison of the Formation of Calcium and Barium Phosphates by the Conversion of Borate Glass in Dilute Phosphate Solution at Near Room Temperature

The formation of phosphates of calcium or barium at near room temperature by the direct conversion of borate glass in dilute phosphate solution was investigated. Borate glass particles (150-300 μm), with the composition 20Na2O·20AO·60B2O3 (mol%), where a is the alkali-earth metal Ca or Ba, were prepared by conventional processing, and immersed in 0.25M K2HPO4 solution at 37°C and with a starting pH value of 9.0 or 12.0. The effects of the borate glass composition and the solution pH on the rate of formation, the chemical composition, and the structure of the phosphate products formed in the conversion reaction were investigated using weight loss and pH measurements, X-ray diffraction, X-ray fluorescence, scanning electron microscopy, and Brunauer-Emmett-Teller surface area. At both pH values, the calcium borate glass particles were completely converted to hydroxyapatite, Ca10(PO4)6(OH)2, with a high surface area (160-170 m2/g). At pH=9.0, the barium borate glass particles converted completely to a barium phosphate product that was isostructural with BaHPO4, whereas at pH=12.0, the barium phosphate product was isostructural with Ba3(PO4)2, with both products having much lower surface areas (4-8 m2/g). The formation of the phosphate products was pseudomorphic, retaining the external shape of the original glass particles