Influence of environmentally friendly refining agents on residual glass properties and crystallisation kinetics of glass-ceramics

• Applicants: Professor Dr.-Ing. Joachim Deubener; Dr. Peter Fielitz
• Subject Area: Glass, Ceramics and Derived Composites
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 552468353

The "side effects" of the switch from toxic (Sb2O3/As2O3) to environmentally friendly refining agents (SnO2/CeO2) lead to quality losses in glass-ceramic production, such as a decline in transparency due to changes in microstructure development. This causes a loss of competitiveness for German specialty glass manufacturers compared to non-European suppliers with lower environmental standards. The aim of the project is therefore to clarify the influence of the environmentally friendly refining agents SnO2 and CeO2 on the crystallization of LAS glass-ceramics and on the associated formation of a residual glass melt. The approach taken is to simulate the processes occurring in practice in the volume and at the nano-level (crystallite sizes < 50 nm) by means of crystallization experiments on the surface of nucleation agent-free base glasses at the micro-level (crystallites >> 1 μm). Due to the low number density of crystals on the surface or the large distance between them, chemical gradients in the residual glass phase can be observed for longer times, i.e. up to larger concentration differences, in isothermal test series or developed to higher crystal phase proportions (> 95 vol.%). This creates favorable conditions that make it possible to detect non-stationary crystal growth kinetics (due to controlling long-range diffusion) in the first place. In addition, the approach allows the chemical composition of the residual glass melt to be calculated on the basis of validable assumptions about the crystal stoichiometry. This residual glass melt can then be produced and its viscosity can be compared with the crystal growth rate or the diffusivity of the network-building elements (Al, Si, O). The joint consideration of the mobilities of the refining elements (Sn, Ce) and the main components (Si, Al, O) provides the essential key to understanding the microstructure development. In our approach, the diffusion of the mentioned refining elements and main components is therefore initiated from "outside", i.e. sputtered tracer atom layers are applied in a 2D geometry as sources of these elements, and their mobilities are measured perpendicular to the surface using diffusion profiles (diffusion lengths >> 1 μm).

• DFG Programme: Research Grants