Hydroxyapatite Compacts Research Articles

The effects of sintering temperature on the properties of hydroxyapatite

The sintering behaviour of hydroxyapatite (HA), the resulting microstructure and properties are influenced not only by the characteristics and impurities of the raw materials but also were found to be dependent on the thermal history during the fabrication process. This work is concerned with the effects of grain size on the relative density and hardness. A commercially available HA powder was cold isostatically pressed at 200 MPa and sintered at temperatures ranging from 1000 to 1450°C with a dwell time of 2 hours. It has been found that, at the optimum sintering temperature of 1250°C where the material is composed of pure hydroxyapatite phase, the samples exhibited densities >99% of theoretical value and possessed a hardness value of 6.08 GPa. Decomposition of HA starts to occur at ∼1400°C with the formation of TCP phase. The change in hardness was found to be dependent on the relative density up to a certain grain size limit. However, above this grain size limit, no correlation exists between the two properties. Porosity and grain size were found to play an important role in determining the properties of sintered hydroxyapatite compacts.

Characterization of hydroxyapatite powders prepared by ultrasonic spray-pyrolysis technique

The hydroxyapatite (HAp) powder was prepared by the ultrasonic spray-pyrolysis technique; the characterization of the resulting powders was performed. Five kinds of the starting solutions with the Ca/P ratio of 1.67 were prepared by mixing Ca(NO3)2, (NH4)2HPO4 and HNO3; the concentrations of Ca2+ and PO4 3− were in the ranges of 0.10 to 0.90 mol · dm−3 and 0.06 to 0.54 mol · dm−3, respectively. These solutions were sprayed into the heating zone to prepare the HAp powders. The heating zone was composed of two electric furnaces; the lower furnace was used for the evaporation of the solvent from the droplets (300–500°C) and the upper furnace for the pyrolysis of the precipitated metal salts (750–900°C). The easily sinterable HAp powder was prepared by spray-pyrolysing the solution with Ca2+ (0.50 mol · dm−3) and PO4 3− (0.30 mol · dm−3) at the temperatures of 800°C (the upper furnaces) and 400°C (the lower furnaces). The resulting powder was composed of the spherical particles with diameters of ∼1 μm or below. Even without the calcination and grinding operations, the relative densities of the compacts fired at 1150 and 1200°C for 5 h attained maxima ∼95%. The microstructure of the sintered compacts appeared to be uniform; the average grain size was ∼3 μm. The activation energies for the grain growth of the sintered HAp compacts were 120 to 147 kJ · mol−1 · K−1.

Dynamic compaction of calcium phosphate biomaterials

The purpose of this study was to apply the dynamic compaction process to calcium phosphate biomaterials. this new technique is currently used to compact metallic powders at room temperature but has not been previously applied to biomaterials. A detailed study of hydroxyapatite compacts was carried out to determine shock compaction parameters. Low static precompaction (3.1 MPa) resulted in slight peripheral cracks. A compaction degree of about 70% and macrohardness of 51 Hv were achieved for a striker velocity of about 50 m/s. FTIR spectroscopy and X-ray diffraction showed no differences in structure and composition after dynamic compaction. Two other infrequently used biomaterials were also tested: an unstable octacalcium phosphate and β-calcium metaphosphate fibres. Scanning electron microscopy showed that dynamic compaction preserved the initial fibre structure of the material. No major structural or chemical changes were noted after shock consolidation. Our results show that dynamic compaction could extend the range of bioceramics.

水酸アパタイトの焼結に対する２，３のフッ化物の添加効果

Addition effects of some fluorides, NaF, MgF2, CaF2 and AIF3, on the densification and the mechanical properties (Bending strength, fracture toughness) of hydroxyapatite were investigated. Sintering treatments were performed by hot-pressing under 10 min-20 MPa. An addition amount of each fluoride was fixed at 2.5 wt%. Of above fluorides added, calcium fluoride brought on extremely good results for the sintering of hydroxyapatite. The sintered compacts having relative densities more than 98% were obtained from 900°C in the case of adding calcium fluoride. On the other hand, the hydroxyapatite single compacts were fully densified above 1200°C. Moreover, the hydroxyapatite compacts containing calcium fluoride had a little higher bending strengths and fracture toughnesses than the hydroxyapatite single compacts because of the difference of their fine structures or grain sizes. Addition of other fluorides, on the contrary, obstructed the densification of hydroxyapatite.