Thermal Instability and Proton Conductivity of Ceramic Hydroxyapatite at High Temperatures

Abstract

In the study of the high‐temperature behavior of ceramic hydroxyapatite (HAp), it was found in relation to its ionic conduction properties that HAp underwent partial dehydration of its lattice hydroxide ions. Considering that HAp ceramics are sintered above 1200°C without destruction of the apatitic structure, the dehydration was interpreted as an unstable phenomenon of aging. The evolution of instability of dehydration was reflected in the time‐dependent characteristics of conductivity, which exhibited up‐and‐down change of 103 S·cm−1 above 700°C. The conduction was proved purely protonic by measurements of a hydrogen concentration cell, and it was noted that the protonic conductivity was increased to a high value of 10−3 S·cm−1 at the initial stage of the aging. The aging phenomenon was demonstrated to be reversible in the deuteration of fully aged HAp; the uptake of OD− inside the specimen was confirmed by infrared spectroscopic analysis after exposure to deuterium oxide vapor. Based on those results, a conduction model was proposed consistent with the aging phenomenon. The present study also showed the importance of the supply of H2O vapor to the ambient during sintering, for the lattice hydroxide ions of ceramic HAp were considerably dehydrated in sintering in air at high temperatures.

Conduction properties of non-stoichiometric hydroxyapatite whiskers for biomedical use

“…As briefly described in the introduction section, conductivity in sintered HA has been reported to be the result of the migration of protons supplied by OH -ions in the HA structure. The activation energy of the proton migration through OH -ions in HA is a relatively higher value of around 1 eV, 1) which shows good agreement not only with the activation energy of sintered HA in this study (1.03 eV) but also the values of the whisker compact (1.03 eV) and powder compact (1.09 eV) for temperatures above 600°C. This suggests that the conductivity of the whiskers and powders is also attributed to the protons in the OH -ions in this temperature region.…”

“…We previously reported that some types of bioceramics with relatively high ionic conductivity could be turned into electrets under a direct electric field at an elevated temperature. 1),2) The polarized biomaterials were experimentally proved as new functionalized biomaterials that can actively manipulate biological phenomena. It is also probable that a variety of biomaterials can be subjected to the poling technique because poling is a convenient post-treatment process.…”

Numerical osteobonding evaluation of electrically polarized hydroxyapatite ceramics

“…8,9 The obtained HA specimens with a size of 10.0 ϫ 5.0 ϫ 0.7 3 were identified by conventional X-ray diffractometry (XRD) and FT-IR spectrometry and electrically polarized in a dc field of 1.0 kVcm Ϫ1 with a pair of Pt electrodes in air at 300°C for 1 h. [5][6][7] The polarization charge confirmation of one of the three samples chosen at random was examined by the thermally stimulated current (TSC) method with a HewlettPackard 4140B pA meter 4 [ Fig.1(a)]. Polarized and nonpolarized HA specimens sterilized by an ethylene oxide gas method were implanted in the femoral and tibial diaphyses of 10 male New Zealand white rabbits.…”

Electric poling of cement composites of hydroxyapatite whiskers with chitosan and their chemical properties in simulated body fluid

“…20) Impedance measurements were carried out in air using an impedance analyzer (16048A Hewlett Packard) over a frequency range of 100 Hz1 MHz. A four-probe alternate-current (ac) method with platinum ion blocking electrodes and attached platinum wires was used to measure the complex conductivity.…”

“…6 (e)], rather lower than that for HAp ceramics. 20) The result may imply that protons move among the lattice sites of OH, HPO 4 and H 2 O molecules due to poling.…”