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Electro-thermal poling of alkali-containing glasses has been known to enhance various physical and chemical properties due to the formation of an alkali ion depletion layer. We have investigated the role of alkali ion migration in depletion layer formation by in situ impedance spectroscopy, poling current measurements and ToF-SIMS on two binary alkali (lithium and sodium) disilicate glasses and two mixed alkali lithium-sodium disilicate glasses. Typically, the depletion layer is formed within a few minutes, reaching a thickness of the order of 100 nm while its impedance continues to increase for the duration of poling. Its electrical conductivity is six or more orders of magnitude lower than that of the bulk glass; by comparison the dielectric constant is lower, approaching the value for silica containing a few percent alkali oxide. Two processes contribute to the formation of a depletion layer: a relatively fast initial process arising from alkali ion migration, followed by a slower process of either electrolysis or gaseous oxygen evolution near the anode. Implications of electro-thermal poling for the mechanism of recently discovered electric field-induced softening of glass are discussed. Electro-thermal poling was primarily developed to induce secondorder nonlinear (SONL) optical susceptibility in glasses by application of DC electric fields. [1][2][3][4] In recent years, interest in this technique has expanded beyond SONL to enhance a variety of biological, physical and chemical properties of glass.5-18 For example, electro-thermal poling has been reported to modify a glass' affinity to atmospheric water at the anode region.18 Electro-thermal poling generally comprises of four main processing steps. First, a glass is heated to a predetermined poling temperature (T p ) below the glass transition temperature (T g ), which allows for increased ionic conductivity while retaining the preformed dimensions. Electrodes on opposite sides of the glass sample are then used to apply a DC voltage (V p ) at T p . After sufficient charge flow has occurred, the glass is then cooled to ambient while still applying the DC voltage to 'freeze' ionic displacements. Finally, the applied voltage is removed at ambient temperature where ionic conductivity is significantly lower to prevent ionic migration back toward original positions. Modification of properties is largely effected by the formation of an alkali ion depletion layer at the anode due to charge transport of ions during these steps.Recently, electric field-induced softening (EFIS) of glass was reported to reduce furnace temperature required for glass softening. 19EFIS is a processing technique where a compressive load is applied to a rectangular glass block inside a furnace. The furnace is then heated at a constant rate while electrodes are used to apply an external electric field across the two parallel faces of the block. The observed reduction in furnace temperature, compared to zero-field conditions, occurs as a result of Joule heating, electrolysis and dielectric b...

The structure and lithium ion transport properties of grain boundaries in the Ohara lithium-ion conductive glass ceramic (LI-CGC) were studied by means of nonlinear impedance spectroscopy and high-resolution transmission electron microscopy (HR-TEM). Ac voltages up to 0.8 V (rms) per single boundary could be applied without any irreversible changes of the lithium ion transport properties. Although the activation energy of the grain boundary resistance is only 30 meV higher than the activation energy of the grain resistance, large ac voltages >500 mV per single grain boundary were needed for reducing the grain boundary resistance to a level similar to the grain resistance. Consequently, we argue that the higher activation energy of the grain boundary resistance is not caused by a single barrier (e.g., a single space charge barrier), but by a number of serial barriers. By comparing the nonlinear grain boundary conductivity of the LI-CGC with that of other thin ion conducting layers, we estimate an average electrical grain boundary thickness of 7.5 nm. The HR-TEM images indicate that layers with a thickness in this range exist between grains with similar crystal lattice orientations.

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