Sodium-carbonate co-substituted hydroxyapatite ceramics

Powders of sodium-carbonate co-substituted hydroxyapatite, having sodium content in the range of 0.25–1.5 wt.% with a 0.25 wt.% step, were prepared by a precipitation-solid state reaction route. Compacts of the powders were sintered in a CO2 flow (4 mL/min) at 1100 °C for 2 h. The sintered ceramics contained sodium and carbonate ions in the ranges of 0–1.5 wt.% and 1.3–6 wt.%, respectively, which are typical impurity concentrations in biological apatite. A relationship between sodium and carbonate contents and the type of carbonate substitution was found. The total carbonate content progressively increased with the sodium content. The obtained ceramics showed an AB-type carbonate substitution. However, the substitution became more B-type as the sodium content increased. As a result, the carbonation was almost B-type (94 %) for the highest sodium content (1.5 wt.%)

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)

Phase evolution during heat treatment of amorphous calcium phosphate derived from fast nitrate synthesis

The phase evolution in amorphous calcium phosphate (ACP, with a Ca/P ratio of 1 : 1), derived from the fast nitrate synthesis using different conditions, was studied in temperature range 20-980°C. ACP crystallized within 600-700°C and the phase composition depended on the synthesis duration. It was firstly revealed that for an extremely short synthesis (1min) two metastable phases α′-CPP and α′-TCP of the high-temperature calcium pyrophosphate α-CPP and tricalcium phosphate α-TCP were crystallized. For a longer synthesis (5min), α′- CPP and minor β-CPP crystallized. The metastable phases gradually transformed to stable polymorphs β-CPP and β-TCP above 800°C, and a biphasic mixture β-CPP/β-TCP or β-CPP formed at 980°C. The crystallization of the metastable phases was attributed to the Ostwald step rule. A mechanism for the formation of TCP (Ca/P = 1.5) from ACP (Ca/P = 1) was proposed. The prepared powders of β-CPP/β-TCP, β-CPP or initial ACP were fine-grained and would have enhanced sinterability. Contribution to the densification was demonstrated due to the thermal transformation of the metastable polymorphs into stable phases having higher densities

Determination of the Ca/P ratio in calcium phosphates during the precipitation of hydroxyapatite using X-ray diffractometry

The applicability of the X-ray powder diffraction method to the determination of phase composition and Ca/P ratio in precipitates during the nitrous wet synthesis of hydroxyapatite (HA) has been shown. The plotted dependences of the phase composition and the Ca/P ratio on the synthesis time can be used as initial data for the development of new and simple processing routes of calcium phosphate ceramics based on HA of any desired composition