Date

2017

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Materials Science and Engineering

First Adviser

Himanshu Jain

Abstract

The advancement of materials engineering relies on our ability to develop innovative processing techniques or incrementally improve efficiency of existing ones. Glass manufacturing is an energy intensive process. Many efforts have been focused on increasing the energy efficiency of glass melting, however, very few have focused on additional energy savings through innovative glass forming techniques. Therefore, reduction in glass processing time and temperature could be readily accepted into already existing or new forming techniques. A similar scenario has recently occurred in the ceramics community. A novel ceramic sintering technique, named flash sintering, has greatly improved sintering of ceramics by application of an electric field at elevated temperatures. The technique has been demonstrated to successfully reduce sintering time and temperature on a wide variety of ceramic materials. The underlying mechanism of flash sintering is controversial. It remains unclear if Joule heating solely accounts for flash sintering or if an electric field enhances defect concentration and mobility. In this dissertation, an innovative glass forming technique named electric field-induced softening (EFIS) has been demonstrated to reduce furnace temperature and processing times for bulk glass softening using an applied voltage, similar to flash sintering. A family of binary and mixed alkali disilicate glasses were used to delineate the role of alkali ion migration. This study also identifies the underlying mechanism of EFIS and provides insight on how to tailor the technique for application. The reduction of furnace temperature and processing time for glass softening was quantitatively measured as compressive displacement upon viscous flow of the glass samples. The difference in temperature between conventional glass softening and EFIS was used to compare the effect of electric fields or more appropriately, applied voltage. It also allowed for quantitative comparison between glass compositions of varying glass transition temperatures. EFIS showed dependence on glass resistivity, applied voltage, frequency, current limit and heating rate. The formation of an alkali ion depletion layer was investigated due to EFIS being dependent on applied voltage rather than nominal external electric field. Electro-thermal poling and impedance spectroscopy was used to measure the kinetics of depletion layer formation along with its associated electrical and dielectric characteristics. The results from time-of-flight secondary ion mass spectroscopy (ToF-SIMS) showed a depletion layer thickness ranging from 50 nm to 200 nm depending on glass composition. The formation of the depletion layer also revealed two relaxation time constants (~20 s and ~1,000 s) which have been attributed to alkali ion migration followed by electrolysis of non-bridging oxygens. The electric and dielectric properties of the depletion layer are similar to those of fused silica. Polarization mechanisms during EFIS were identified by investigating thermally stimulated poling current (TSPC). Four distinct regions of the TSPC were observed using DC voltages while only three were observed using AC. In the case of DC, the four regions corresponded to dipolar polarization, alkali ion migration, anion migration/proton injection and dielectric breakdown, in order of increasing temperature. Photoemissions were observed as part of the dielectric breakdown process using UV-Vis spectrometer. The peak temperatures of thermally stimulated current and their associated activation energies of each peak were used to identify the dominate charge carrier process within each region. Activation energy for alkali ion diffusion was measured by both impedance spectroscopy and initial rise method. Secondary electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were used to measure depletion layer depth profiles up to 50 µm thick following EFIS. In the case of AC, the three regions of the thermally stimulated current are attributed to short-range oscillations of cations around the non-bridging oxygens, long-range migration oscillations of cations followed by thermal runaway, in order of increasing temperature. Finally, finite element analysis (FEA) simulated two major parts of EFIS. First, the formation of an alkali ion depletion layer was modeled. The electrical properties of the depletion layer were then calculated as a function of composition from parent to modified glass. The current density decay of depletion layer formation agrees with experimental results from electro-thermal poling. Second, thermal runaway was calculated using a fixed depletion layer thickness at various starting temperatures. FEA also calculated thermal runaway with depletion layer thicknesses ranging from 5 nm to 50 µm starting from the same temperature. A maximum calculated temperature was about 1,650°C within the model on the anode side of the glass. Experimental measurements of thermal runaway were recorded using infrared imaging cameras up to a maximum temperature of 2,000°C. Temperatures within the alkali disilicate glasses during EFIS readily exceeded 1,300°C nearest the anode. A maximum temperature was measured to be 1,868°C during softening. Results from simulated FEA and experimental measurements are in good qualitative agreement with the proposed mechanism of EFIS. Based on the above results, the overall mechanism that determines EFIS of alkali silicate glasses occurs as follows: i) dipolar polarization of alkali ions in the direction of the applied voltage, ii) long-range alkali ion migration toward the oppositely charge electrode, iii) formation of a highly resistive alkali ion depletion layer near the positively charge electrode creating a large internal electric field, iv) migration of anions toward the positive electrode along with proton injection into the depletion layer, v) mixed ionic-electronic conduction sustains large internal electric field strength, iv) at elevated temperatures the dielectric strength of the depletion layer decreases to the point where it is exceeded by the local internal electric field within the alkali ion depletion layer. Here, dielectric breakdown occurs leading to thermal runaway and subsequent heat transfer into the bulk ultimately leading to bulk glass softening.

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