Progress in the rheology of glass melts - Α survey

The rheology of glass melts is not restricted to Newtonian viscous flow, but it involves also non-Newtonian flow and viscoelasticity The former one predominates in the steady-state flow at low deformation rates, the latter ones prevail during the initial State of stress generation and during the final State of flow and fracture under extreme deformation conditions. The whole ränge of rheological phenomena is demonstrated in this review article. Typical available experimental results and mathematical formulations are briefly given for single-phase glass melts. In this context the development of new equations gives rise to the introduction of a series of new definitions and conceptions which physically and mathematically characterize the different rheological behaviour of glass melts under various stress-strain conditions. The connection between flow behaviour and structural aspects is pointed out with a short reference to flow birefringence. Finally, an extension from the behaviour of a single-phase to a double-phase system is developed for the case of suspended crystals with defined sizes, concentrations and shapes within an inert glass melt (model for glassceramic melts)

Mg-bearing quartz solid solutions as structural intermediates between low and high quartz

Glass powder samples of cordierite composition (doped with 8 mol% TiO2) were heat-treated to produce a series of increasingly SiO2-enriched Mg-bearing quartz solid solutions (Qss). The obtained materials were then analyzed by X-ray diffraction: Rietveld structural refinements revealed that Mg-bearing Qss phases possess trigonal symmetry and a compositionally dependent intermediate structural arrangement between those of low and high quartz. High-temperature diffraction measurements were performed up to 700°C to characterize the thermal expansion behavior of the crystals. At SiO 2-rich compositions, a reversible high-to-intermediate inversion of the quartz structure is observed, which shifts with increasing stuffing to lower temperatures than the conventional 573°C for pure quartz. Similarities and differences to the better-established Li-bearing Qss are discussed in the text

Statistical approach to crystal nucleation in glass-forming liquids

In this work, methods of description of crystal nucleation by using the statistical approach are analyzed. Findings from classical nucleation theory (CNT) for the average time of formation of the first supercritical nucleus are linked with experimental data on nucleation in glass-forming liquids stemming from repetitive cooling protocols both under isothermal and isochronal conditions. It is shown that statistical methods of lifetime analysis, frequently used in medicine, public health, and social and behavioral sciences, are applicable to crystal nucleation problems in glass-forming liquids and are very useful tools for their exploration. Identifying lifetime with the time to nucleate as a random variable in homogeneous and non-homogeneous Poisson processes, solutions for the nucleation rate under steady-state conditions are presented using the hazard rate and related parameters. This approach supplies us with a more detailed description of nucleation going beyond CNT. In particular, we show that cumulative hazard estimation enables one to derive the plotting positions for visually examining distributional model assumptions. As the crystallization of glass-forming melts can involve more than one type of nucleation processes, linear dependencies of the cumulative hazard function are used to facilitate assignment of lifetimes to each nucleation mechanism

Viscosity and glass transition temperature of hydrous float glass

Viscosity of water-bearing float glass (0.03–4.87 wt% H2O) was measured in the temperature range of 573–1523 K and pressure range of 50–500 MPa using a parallel plate viscometer in the high viscosity range and the falling sphere method in the low viscosity range. Melt viscosity depends strongly on temperature and water content, but pressure up to 500 MPa has only minor influence. Consistent with previous studies on aluminosilicate compositions we found that the effect of dissolved water is most pronounced at low water content, but it is still noticeable at high water content. A new model for the calculation of the viscosities as a function of temperature and water content is proposed which describes the experimental data with a standard deviation of 0.22 log units. The depression of the glass tran- sition temperature Tg by dissolved water agrees reasonably well with the prediction by the model of Deubener [J. Deubener, R. Mu¨ ller, H. Behrens, G. Heide, J. Non-Cryst. Solids 330 (2003) 268]. Using water speciation measured by near-infrared spectroscopy we infer that although the effect of OH groups in reducing Tg is larger than that of H2O molecules, the difference in the contribution of both species is smaller than predicted by Deubener et al. (2003). Compared to alkalis and alkaline earth elements the effect of protons on glass fragility is small, mainly because of the relatively low concentration of OH groups (max. 1.5 wt% water dissolved as OH) in the glasses