Investigation of bioglass–electrode interfaces after thermal poling

Electrical and electrochemical processes in a bioactive soda-lime phosphosilicate glass and in a bioabsorbable soda-lime phosphate glass during thermal poling were studied by means of thermally stimulated polarization and depolarization current measurements (TSPC and TSDC), ac impedance spectroscopy, and SEM/EDX analyses. The thermal poling was done by sputtering thin Pt electrode films onto the faces of the glass samples and by applying voltages up to 750 V to the electrodes at temperatures up to 513 K. The poling leads to the formation of interfacial layers under the electrodes which are responsible for two depolarization current peaks. The Na+ depletion layer under the anode causes an additional semicircle in a Nyquist plot of the ac impedance. The SEM/EDX profiles suggest that redox and transport processes of Na+ ions are responsible for the formation of the interfacial layers, while the Ca2+ ions are immobile under the poling conditions. From the EDX profile of the Na+ depletion under the anode, we calculate a hypothetical voltage drop in the depletion layer under the assumption that there is no charge compensation. Since the calculated voltage drop is much larger than the poling voltage, we conclude that the negative charge density due to Na+ depletion must be compensated, at least to a large extent, by the presence of other charge carriers, most likely protons.