AC electric field‐induced softening of alkali silicate glasses

Abstract

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.

Development of highly inhomogeneous temperature profile within electrically heated alkali silicate glasses

“…Manifestations of localized Joule heating are most clearly seen as physical softening and large deformation of glass samples, which should be otherwise solid at the ambient temperature of the furnace wherein the EFIS experiment is performed. Therefore, we summarize the key observations of EFIS first, which are similar to those reported previously: When heating the sample at a constant rate and applied voltage, thermally stimulated polarization current exhibited a large step-change at some temperature wherein 30 seconds or less, the glass softens and deforms under the applied load 19–21 . As typical examples of the electrical current and power through glass samples, experimental data are shown in Supplementary Fig.…”

“…The recent discovery of EFIS of glass 19–21 , the present observations of localized heating and thermal runaway (Figs 2–5), and the FEA modeling (Figs 5–8) clearly demonstrate that classic macro-scale Joule’s law for homogeneous samples does not apply to the electrical heating of common glasses, indeed any ionically conducting solid, when usual metal or graphite electrodes are employed. The macro-scale asymmetry of temperature evolution has been reported also during flash sintering of oxygen anion conducting yttria-stabilized zirconia ceramics 31,32 .…”

“…By comparison, the use of DC voltage leads to extreme localized heating and glass softening, while AC contributes to uniform heating and gradual softening. As discussed in previous AC-EFIS work 21 , the application of an AC voltage creates two processes that the mobile cations undergo. In the first half-cycle of the AC voltage, the electrode has a temporary positive bias that drives mobile cations away from the electrode/glass interface into the bulk.…”

“…The differences in EFIS as observed between DC and AC applied voltages are believed to be due to the difference in power dissipation during EFIS for the two cases, which leads to self-heating and thermal runaway 21 . A comparison is made for NS with 150 V DC in Fig.…”

“…For example, glass has been processed in a variety of innovative ways to create waveguides, enable through-glass vias for use as dielectric components, or even create surface structures for use with chip-on-glass applications, integrated circuits, etc 14–16 . Glass softening via EFIS at much lower furnace temperatures (T furnace ) than that obtainable through conventional heating has been demonstrated using both externally applied AC and DC 19–21 . Flash sintering of glass powder is believed to be intimately related to EFIS 22,23 .…”