Depletion Layer Formation in Alkali Silicate Glasses by Electro-Thermal Poling

Abstract

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...

AC electric field‐induced softening of alkali silicate glasses

“…The initial rise using DC voltage gave activation energy values of 0.64 and 1.24 eV for NS and 5L5NS, respectively. These values, as summarized in Figure as a function of frequency, are close to previously reported thermally stimulated poling current activation values . The minor discrepancy is likely from samples being from different melt batches.…”

“…The spectra of photoemission collected during AC‐EFIS have the same main features as those observed in DC but of lower intensities as seen in Figure B. The description of the photoemissions was discussed in earlier work . To summarize these results, the photoemission for both AC and DC test conditions includes characteristic peaks of energies corresponding very well to alkali ion electron energy level transitions.…”

“…These compositions, as listed in Table , were chosen due to their similarity in network structure yet very large difference in electrical conductivity . From previous work and literature values, the 5L5NS glass composition has a higher DC resistivity by about five orders of magnitude than NS at room temperature …”

“…Differential scanning calorimetry (DSC) (NETZSCH 404/F3 microcalorimeter) was used to measure the glass transition temperature of both glass compositions as reported previously …”

“…A limitation of EFIS using DC voltages is that it introduces large compositional changes from migration, at both the anode and cathode, where alkali content is greatly reduced and increased, respectively. If such compositional changes are undesirable for final properties then DC voltage, which causes long‐range migration of ionic charge carriers toward oppositely charged electrodes, should be avoided . We attempt to overcome this drawback by employing AC voltage, anticipating that such compositional changes will be reduced as charges move in opposite directions every time the field changes direction with the frequency of AC signal.…”