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- ItemThermodynamic approach to viscosity in the glass transition(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1994) Conradt, ReinhardThe glass transition is re-investigated by means of the formahsm of linear nonequilibrium thermodynamics. The process is treated for melts cooled down below their liquidus temperatures in terms of three events. These events are, in the order of decreasing temperature: by-passing of crystallization, freezing-in of stress relaxation, and freezing-in o f near-range structural relaxation. Conditions for the viscosity levels are derived at which each of these events is accomplished. The model is tested against data of five one-component, three simple binary and ternary, and three multicomponent systems. Within the scope of this test, it is confirmed that the model correctly describes the viscosity levels typical of the glass transition, as well as their dependence on the cooling rate. An explanation for the 10¹³dPa*s rule at Tg is included.
- ItemExperimental study on the effect of cullet and batch water content on the melting behavior of flint and amber container glass batches(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2004) Dubois, Olivier; Conradt, ReinhardThe melting behavior of two commereial flint and amber glass batches with varied cullet content (0, 40, 65 % and 0, 40, 76 %, respectively) and batch water additions (0 to 5 %) was investigated. The cullet was a blend of up to five industrial types and grades, including ground cullet. Local temperature, electrical conductivity, and thermal diffusivity were determined in 200 g scale tests as a function of temperature, while the vertical distribution of the former two properties was determined by 10 kg tests in 5 cm high batch piles charged on top of a pre-heated glass melt (7 kg). The set of methods applied was established as a service tool for the quantitative comparison of the batch-to-melt conversion of different batches in general. For the flint and amber glass batches investigated, cullet additions primarily improved the thermal behavior, with moderate versus high additions required for flint and amber, respectively. The silica dissolution in cullet-free batches was enhanced by batch water additions, but retarded again with increasing cullet eontent. The presence of phonolite besides batch water and cullet yielded especially negative results.
- ItemLow Pressure evaporation from boron containing glass melts by the transpiration method(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2004) Dacko, Dawid; Wilsmann, Wolfgang; Conradt, ReinhardGlass melting is a well-established process, but it does not seem to be well understood under low-pressure conditions. Melting under low pressure has been suggested as a potential support of the refining process. This makes sense only if the evaporation form the glass melt is kept within acceptable limits. Α transpiration apparatus equipped for operation at low pressure, termed low pressure transpiration method, was used to investigate the evaporation from boron containing glass melts. We obtained information on the weight loss as a function of temperature, time, and pressure. We investigated an alkali borosilicate glass with acomposition in mol % of: 79SiO₂,7 Na₂O, 10 B₂O₃, 3 AI₂O₃, 1 minor constituents. At the beginning of glass melting, the evaporation mechanism was detected as surface controlled. Later, a diffusion controlled stationary behaviour was observed. The ratio of sodium oxide to boron oxide in the evaporated gas as determined by chemical analysis ranged from 1.25 to 1.4.
- ItemOn the relation between the crystalline and glassy State in multi-component oxide Systems(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2003) Conradt, Reinhard[no abstract available]
- ItemThermodynamics of glass melts and thermo-chemistry of glass forming batches(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2004) Conradt, ReinhardThe paper deals with the thermodynamics of glass melts and glass forming batches. In a first step, an improved approach to the heat capacities of multi-component glass melts by the combination of a thermochemical model and the Adam-Gibbs equation is presented. Then, the heat demand of dolomite decomposition is reinvestigated. Finally, a proposition is made for the modelling of the batch-to-melt reaction path, yielding Information on both calcoric and rheological properties.