To provide bioelectrets with controlled electrical energy, the polarization characteristics of stoichiometric and nonstoichiometric hydroxyapatite (HA) ceramics were investigated in terms of crystal structures using Rietvelt analysis technique. HA ceramics with and without hydroxide ion defects were prepared at 1250°C for 2 h under streaming steam (for stoichiometric HA; w-HA) and air (for hydroxide-deficient HA; a-HA). Polarization of ceramics was done at 400°C for 1 h under a 5 kVcm-1 DC electrical field. Based on a result of thermally stimulated depolarization current (TSDC) measurement, the charge storage (Q) of polarized ceramics was significantly influenced by sintering atmosphere; Q value of a-HA was about 7 times larger than that of w-HA. Rietvelt analysis of XRD spectra suggests that a dipole formation of a-HA was probably due to the proton displacement form hydroxide ions to phosphate ions, while w-HA was likely polarized by proton migration between hydroxide ions. The dominant usage of phosphate ion as a proton localization site should contribute for the improvement of polarization property of a-HA.
