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

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Alumosilicate glass-ceramics are functional materials native to different areas of present key technologies. Their efficiency is based on the interplay of unique thermal, optical and mechanical properties, which can be tailored by catalyzing the crystallization of alumosilicate solid solutions using nucleation agents (NA) of srilankite, pyrochlor and tohdite structures. Here the bottleneck is the significant lack of fundamental understanding of the rate determining interface and diffusion processes of heterogeneous nucleation with the follow-up of an empirically driven materials development. On one side indications of an interface controlled epitaxial mechanism were reported while on the other side an Al-gradient in the direct proximity of NA crystals was evidenced.Based on isothermal experiments (rare stable isotopes/SIMS and GIXRD and HT-GIXR, respectively) using NA coated parent glasses of labelled major elements (O-, Si-, Al-, Li-, K-, Sr-tracer), which for the first time allow a simultaneous determination of chemical and structural gradients as well as mechanical stresses with nano-metric resolution (SIMS) at the NA/alumosilicate interface, a complete set of kinetic data, including self-diffusion of major elements in the glass, diffusion through the liquid/solid interface during crystal growth as well as nucleation rates at the NA interface and growth rates perpendicular to the NA interface will be generated. In particular, via selection of different NA-coatings, i.e. Al-poor: srilankite-ss and Al-rich: pyrochlor- and tohdite-ss, special emphasis will be on the role of aluminium and the formation of Al profiles at the interface. It will be possible to gain insights into the heterogeneous nucleation mechanism of alumosilicate crystals in glass-ceramics, which will be utilized for numerical modelling of the processes. Results of the calculation will help to improve predictions of correlation between microstructure and macroscopic properties of alumosilicate glass-ceramics.