  Electro-thermal poling experiments were carried out on a 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 the 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. It is
shown that 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.
