Glass sintering and crystallization assisted by electrical field

Abstract

Flash sintering is a promising ceramic-making technique using an electric field to enhance sintering that belongs to a set of electric or magnetic Field Assisted Sintering Techniques known as FAST. The boost with the applied voltage has resulted remarkably with ceramic as Y2O3-stabilized ZrO2, lowering the sintering time from hours to a few seconds and the temperature by hundred degrees compared to conventional sintering. The sintering of several ionic conductive polycrystalline ceramics have been successfully enhanced with this technique, but the understanding of the phenomenon is still a matter of debate, whether it is a result of Joule heating, a defect avalanche effect, photoluminescence, or change of crystal structure. Despite the success with crystalline ceramics, the attempts to flash sinter glass powder compacts have failed and only recently was achieved . We also successfully sintered powder compacts of a commercial soda-lime-silica window glass treated by heating up to 750 °C at 10 °C/min under different DC voltages (0 to 500 V). Electric field was observed to strongly influence glass sintering kinetics and relative densities higher than that obtained by conventional sintering above 300 V. Softening and foaming have to be avoided when the flash occurs by an electric field application. Currently, the effect of crystallization and the possibility to obtain glass-ceramics are open fields for research. Thus, the researcher will investigate the sintering of particle compacts of model stoichiometric glasses in the soda-lime-silica and/or lithium-alumina-silica systems, heat treated at constant rates at different applied electric fields. For these systems, the researcher will have to collect information in the literature or characterize the properties such as the dependence on temperature of the sinter-crystallization kinetics, viscosity, and electrical conductivity, which are important to evaluate the phenomenological basis of the electric-field sintering. The experimental results will be compared with analytical calculations using non-isothermal Clusters model of glass sintering by viscous flow with concurrent crystallization. The research activities still include the further development of the flash-sintering instrumentation to allow for AC treatments, the melting of designed composition and forming them into glasses, milling and granulometry control, making compacts, perform heat treatments with and without application of DC and AC electric field, and characterization of properties before and after the treatments by, e.g., powder granulometry, apparent and true densities, microstructure characterization by optical and electron microscopy, and X-ray diffraction.