Thermal behaviour of functionally graded sodium-containing calcium phosphates

Hydroxyapatite (HA), ß-tricalcium phosphate (ß-TCP) and their mixtures have been successfully applied in orthopedic, oral and maxillofacial surgery [1]. Calcium phosphate (CP) bioactive ceramics based on HA is usually fabricated of appropriate powders. Unfortunately, mechanical properties of such material are in general worse than those of human bone. Therefore, there is a necessity for development of new ceramic CP composites with improved characteristics. An effective biomaterial must simultaneously possess various, often contradictory, properties [2]. A functionally graded material (FGM) with gradients in composition, phase and microstructure from the surface to the interior can better satisfy such combined requirements. Recently, FGM's of sodium-containing calcium phosphates were prepared expecting them to have improved circumstances for bone formation and bonding [3-4]. The aim of this work was to study the thermal behavior of FGM's of sodium - containing calcium phosphates prepared by an original route. The initial powder was prepared by a simple one-step precipitation using Ca(NO3)2 and Na3PO4 reactants. The synthesis reaction resulted, besides HA, in the by product NaNO3. However, the latter was not washed out of the precipitate but rather used as a source of sodium for ceramics preparation. An original molding step was used resulted in gradient distribution of NaNO3 in compacts. They were sintered under usual conditions. It has been found that the linear (or volume) shrinkage and the density slightly changed in the fired compacts below 1100°C. However, the consolidation abruptly increased at 11000C. The poor sintering is associated with NaNO3 decomposition during heating and firing at temperatures below 11000C. At higher temperatures, reactions between HA particles and products of NaNO3 decomposition occurred and dif¬fusion processes were intensified. Due to the gradient distribution of NaNO3 in the compacts, the indicated thermal processes resulted in formation of functionally graded ceramics (having in particular a phase gradient from the surface to the interior as Na3Ca6(PO4)5\to ß- rhenanite\to Na+ solid solution in HA).