Combustion synthesis of heterogeneous calcium phosphate bioceramics from calcium oxide and phosphate precursors

The solid-state reaction of combustion synthesis has been demonstrated to be capable of rapidly producing materials for biomedical applications. Reactant powders of CaO and P₂O₅ were pressed into cylinders and reacted by heating a tungsten filament in either an argon, CO₂ or N₂ atmosphere. Reaction systems examined were: (1) 3CaO + P₂O₅ → Ca₃(PO₄)₂ (TCP) and (2) 4CaO + P₂O₅ → Ca4P₂O₇ + O₂ (TTCP). The product pellets were bright white with a thin glassy appearing outer layer observed with a very porous interior (range of 5–1000 μm pore diameter) regardless of reaction atmosphere. No evidence of sintering is apparent such as intact precursor particles or necking between particles. In the TCP system characteristic XRD peaks for crystalline β-, α-TCP, and HA are noted while in the TTCP system tetracalcium phosphate, α-TCP, and HA are present. DSC analysis indicates the ignition temperature 450°C when P₂O₅ decomposes to PO₃ to react with CaO. SHS is a viable method to manufacture heterogeneous calcium phosphates from CaO and P₂O₅ precursors. The formation of HA and TCP, regardless of stoichiometry or atmosphere is due to thermodynamic energies of the products while other phases are preserved due to limited kinetics in the system’s rapid heating and cooling.