Mechanism and kinetics of Na+ ion depletion under the anode during electro-thermal poling of a bioactive glass

Electro-thermal poling experiments were carried on a 46.4 SiO2 – 25.2 Na2O – 25.2 CaO – 3.2 P2O5 (46S4) bioactive glass, and the kinetics of the Na+ ion depletion layer formation under the anode was studied in situ by means of ac impedance spectroscopy. One important finding is a linear relation between the depletion layer thickness and the applied voltage, which is in contrast to the predictions of standard space charge theory. The average electric field in the layer is independent of the voltage and is close to the dielectric breakdown field of alkali ion conducting glasses. Furthermore, we observe that the thickness of the depletion layer is established on a much shorter time scale than its resistance. We explain these results by assuming that the huge electric fields created under the anode during Na+ ion depletion lead to a strong increase of the electronic mobility in the layer and to charge compensation via extraction of electrons. In the initial stages of the depletion process, a relative Na+ ion depletion of only 400 ppm is sufficient to generate electric fields of the order of the dielectric breakdown field.