Electrical conductivity of polycrystalline hydroxyapatite and its application to electret formation

Hydroxyapatite (HAp), a type of calcium phosphate found in bones and teeth, exhibits proton conductivity. In this study, its conductivity is investigated by measuring the complex permittivity of polycrystalline hydroxyapatite samples obtained via a sintering process. The dielectric dispersions measured from samples with different grain sizes are successfully explained using the Maxwell-Wagner interfacial polarization model. The results show that polycrystalline hydroxyapatite consists of both a high impedance grain boundary and a proton-conductive grain that has a conductivity of about 1.0 × 10− 7 S cm− 1 at 573 K. The interfacial polarization is confirmed using thermally stimulated depolarization current measurements. Additionally, electrets are prepared with the polycrystalline HAp via a poling process, and it is demonstrated that the electret formation is caused by the interfacial polarization.