Proton conduction related electrical dipole and space charge polarization in hydroxyapatite

Abstract

Hydroxyapatite (HAp), well known as a biomaterial, is also known as a proton conductor. Its electrical properties are related strongly to its stability and surface properties. In particular, persistent electrical polarization related to proton conductivity of hydroxyapatite has a substantial influence on hydroxyapatite surface properties. The origins of polarizations were investigated in proton-defect-induced HAp ceramics using thermally stimulated depolarization current (TSDC) measurements. Two peaks were observed, indicating that the persistent polarization comprises polarization elements of two kinds. The TSDC response as a function of the applied electric field in polarization treatments indicated that the persistent polarization is dipole polarization and space charge polarization. The former, dipole polarization, increased continuously with increased defect concentration. The activation energies were 0.67–0.86 eV, which are comparable to proton conduction activation energy. Results show that dipole polarization consists of electrical dipoles of the defect pairs, which are formed through proton conduction. The activation energies of 1.01 ± 0.01 eV for space charge polarization are independent of the defect concentration, suggesting that these originated from protons trapped at the grain boundaries as a result of long-range proton conduction. The polarization of two different types is expected to exert different effects on the HAp surface properties.