Motivated by the advantages of two-electrode flash sintering over normal sintering, we have investigated the effect of an external electric field on the viscosity of glass. The results show remarkable electric field-induced softening (EFIS), as application of DC field significantly lowers the softening temperature of glass. To establish the origin of EFIS, the effect is compared for single vs. mixed-alkali silicate glasses with fixed mole percentage of the alkali ions such that the mobility of alkali ions is greatly reduced while the basic network structure does not change much. The sodium silicate and lithium-sodium mixed alkali silicate glasses were tested mechanically in situ under compression in external electric field ranging from 0 to 250 V/cm in specially designed equipment. A comparison of data for different compositions indicates a complex mechanical response, which is observed as field-induced viscous flow due to a combination of Joule heating, electrolysis and dielectric breakdown.
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...
According to Joule’s well-known first law, application of electric field across a homogeneous solid should produce heat uniformly in proportion to the square of electrical current. Here we report strong departure from this expectation for common, homogeneous ionic solids such as alkali silicate glasses when subjected even to moderate fields (~100 V/cm). Unlike electronically conducting metals and semiconductors, with time the heating of ionically conducting glass becomes extremely inhomogeneous with the formation of a nanoscale alkali-depletion region, such that the glass melts near the anode, even evaporates, while remaining solid elsewhere. In situ infrared imaging shows and finite element analysis confirms localized temperatures more than thousand degrees above the remaining sample depending on whether the field is DC or AC. These observations unravel the origin of recently discovered electric field induced softening of glass. The observed highly inhomogeneous temperature profile point to the challenges for the application of Joule’s law to the electrical performance of glassy thin films, nanoscale devices, and similarly-scaled phenomena.
Electric field‐induced softening (EFIS) is a recently discovered phenomenon leading to significant reduction in the furnace temperature at which glass softens under the application of DC voltage. Unfortunately, it is accompanied by local compositional changes due to migration of ions that could limit its usefulness. To overcome this drawback, we have investigated the same phenomenon using AC voltage, that is, AC‐EFIS on a sodium disilicate glass and a 50/50 mixed lithium‐sodium disilicate glass of very different ionic resistivity yet similar network structure. The results show that the magnitude of EFIS temperature reduction is significantly greater for AC compared to DC for both glass compositions. The enhancement of EFIS under AC voltage appears to be due to a more uniform power dissipation and self‐healing of changes than under DC voltage. This uniformity allows for the overall sample temperature to increase throughout the bulk and provides a better technique for practical applications than the DC case which produces potentially undesirable changes, especially in the anode region.
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