Abstract
Hydroxyapatite (HAp) precursors were prepared by chelation of calcium ions with organic phosphonic acid by using calcium nitrate hydrate (Ca(NO3)2·4H2O) and 2-phosphonobutane-1.2.4-tricarboxylic acid (C7H11PO9; PBTA) in distilled water in the temperature range of 60-130°C. HAp powder was then synthesized by heating the precursors at various temperatures. HAp precursors were pectized in a three-step reaction. First, calcium ions were chelated with carboxyl groups in PBTA in an early reaction. Second, chelation of calcium ions with phosphonic acid and/or the dehydrated condensation of carboxyl groups with phosphonic acid occurred at and above 80°C. Finally, dehydrated condensation of carboxyl groups proceeded in the range of 100-130°C. The homogeneity of the HAp precursors improved with an increase in their preparation temperature, which accelerated gelation. Powder obtained after heating the HAp precursor prepared at 130°C (preHAp-130) had a single crystalline HAp phase at 1000°C. In addition, the crystallization temperature of preHAp-130 was 200°C, which was lower than that of HAp precursors prepared using other polymerized complex methods. Powder obtained by heating HAp precursors at and below 800°C contained carbonate apatite, in which carbonate ions replaced hydroxyl and phosphate sites in the HAp crystal lattice. The Ca/P molar ratio of the HAp powder was higher than that in the stoichiometric composition of HAp (Ca/P = 1.67), because the organic components of the HAp precursors were not pyrolyzed; however, the powder obtained by heating preHAp-130 at 1000°C had stoichiometric HAp composition due to the thermal decomposition of the precursors. The particle size of the HAp powder prepared by heating preHAp-130 decreased with an increase in the heating temperature, and when the HAp particles were heated at and above 800°C, a three-dimensional network with micro and nanopores was formed.
