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Crystallisation of alumosilicates in glass-ceramics: interface processes and diffusion of the main constituents

Subject Area Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
Term from 2019 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 424949604
 
Final Report Year 2023

Final Report Abstract

Aluminosilicate glass-ceramics are functional materials that have long outgrown traditional applications such as cooktops, mirror telescope substrates and fireplace screens and now appear to be indispensable in various areas of key technologies. They are produced by controlled crystallisation from previously melted glass and thus combine the technological advantages of hot forming of glasses with the advantages of thermal microstructure tuning of ceramics (socalled "ceramisation"). Their performance is based on the interplay of thermal, optical and mechanical properties, which can be tailored by triggering the crystallisation of aluminosilicate solid solutions with nucleating agents. A bottleneck in this respect is the lack of a fundamental understanding of the rate-determining interfacial and diffusion processes of heterogeneous nucleation and the resulting empirically guided material development. The research hypothesis is based on the imitation of these processes, which in practice take place on crystals a few nanometres in size, by vapour deposition of thin layers on a nucleation agent-free glass with the advantage of being able to mark relevant elements of the glass (Al, Si and O) with isotopes. In principle, a 2D layer structure enables the simultaneous investigation of diffusion processes and crystal growth (vapour deposition of nucleating TiO2, ZrTiO4). The results show that this "stacking technique" is quite suitable for shedding light on kinetic processes, as it was found in a crystallisation-resistant NAS6 glass (Na2O⋅Al2O3⋅6SiO2) that Al, when present together with Ti, exhibits a diffusivity that is two orders of magnitude higher than that of Ti and O. This finding supports structural analyses suggesting the formation of mobile edge-linked [[V]Al[IV- VI TiO4.5, 5, 5.5]]^2-, 3-,4- complexes, which are also expected to be present in the LAS4 glass (Li2O⋅Al2O3⋅4SiO2). Here, however, the crystallisation rate is significantly higher (or the evaporated nucleating agent is more effective), so that Al, Si and O diffusivities could not be considered independently of the crystallisation of the glass. Furthermore, the addition of the refining agent and a slight Al deficiency resulted in a low-viscosity melt at the crystallisation front leading to an increasing growth rate in isothermal experiments, while oxygen diffusion in the "channels" of this melt between the crystalline domains found a fast path. In addition, for the NAS6 glass, the activation enthalpies of Al (Ti, O) diffusion were found to be only half that of viscosity in the same temperature range, which re-evaluates the beneficial role of [V]Al in switching the oxygen coordination from IV (network-forming) to VI (network-modifying) of titanium ions in TiO2 and ZrTiO4 nucleated aluminosilicate glasses.

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