No abstract

Structural transformation and sintering processes of tricalcium phosphate (TCP) ceramics prepared from defective hydroxyapatite (Ca9HPO4(PO4)5OH) were studied by X-ray diffraction (XRD) and atomic force microscopy (AFM). Starting powders with Ca/P ratio approximately 1.5 were obtained by adding 0.5 l of 0.3 M H3PO4 solution to an equal volume of 0.45 M Ca(OH)2. In the prepared ceramics, the onset temperature for transformation of defective hydroxyapatite into TCP (witlokite) agrees with the onset temperature for sintering (800 degrees C). Sintering occurs through the formation of a fibrous structure, which resembles biological hard tissue. In the 1000-1200 degrees C range, these fibres coalesce into grains of up to 0.6 microm in size with a fibrous-laminar morphology. At the end of this sintering stage witlokite transforms into alphaTCP. At about 1450 degrees C, partial decomposition of alphaTCP into Ca2P2O7+Ca4P2O9 is observed. AFM observations suggest that Ca2P2O7 is segregated in the liquid state and increases the velocity of grain growth (up to 12 microm).

The effect of ultrasonic vibration on the particle size distribution of hydroxyapatite chemically precipitated from eggshells and H3PO4, or alternatively from calcium acetate and Na3PO4•12H20, was evaluated. For the case of hydroxyapatite aqueous suspensions with pfl>10 , with no particle-dispersing agent added, the average particle size decreased from 40 to 31.ƒÊm in five minutes by employing an ultrasonic power of 61.75 watts and a frequency of 20 KHz .