A study was made of the high-temperature behavior of hydroxyapatite ceramics. It was found, from its ionic conduction properties, that the material underwent partial dehydration of its lattice hydroxide ions. Because the ceramics had been sintered at temperatures above 1200C without destroying the apatite structure, the dehydration was interpreted as being an unstable phenomenon of aging. The evolution of the instability of the dehydration was reflected by the time-dependent characteristics of the conductivity. The latter exhibited an up-and-down change at temperatures above 700C. The conduction was proved to be purely protonic by performing measurements in a H concentration cell, and it was noted that the protonic conductivity increased to a value of 0.001S/cm in the initial stages of aging. The aging was shown to be reversible with regard to the deuteration of fully aged material. The uptake of OD- into specimens was confirmed by means of infra-red spectroscopic analysis after exposure to deuterium oxide vapor. On the basis of the results, a conduction model was proposed that was consistent with the aging behavior. The present study also demonstrated the importance of a supply of H2O vapor to the ambient during sintering, because the lattice hydroxide ions of the ceramic material were considerably dehydrated during sintering in air at high temperatures.

K.Yamashita, K.Kitagaki, T.Umegaki: Journal of the American Ceramic Society, 1995, 78[5], 1191-7