Dielectric properties and structure of hydroxyapatite ceramics sintered by different conditions

Several conditions for hydroxyapatite ceramic preparation were used: sintering at 1150 °C in air or under H₂O vapor flow, quenched in water at room temperature after sintering, or slow cooling inside the furnace. Depending on specific combinations of these preparation conditions, in the resulting ceramics significant differences are observed in the phase composition and dielectric properties, ranging from capacitive to a semiconducting response. Comparison between the experimental x-ray diffraction patterns and those calculated by the Rietveld method show that during sintering in air, approximately 40% of hydroxyapatite is transformed to tricalcium phosphate. The XRD analysis shows that this transformation could be possible using the following process: Ca₁₀(PO₄)₆(OH)₂= 2Ca₃(PO₄)₂+ Ca₂P₂O₇+ 2CaO + H₂O.