Thermal analyses to assess diffusion kinetics in the nano-sized interspaces between the growing crystals of a glass ceramics

According to a hypothesis by Rüssel and coworkers, the absence of Ostwald ripening during isothermal crystallization of lithium aluminosilicate (LAS) and other glass ceramics indicates the existence of a kinetic hindrance of atomic reorganization in the interstitial spaces between the crystals.Methods of Thermal Analysis (Differential Scanning Calorimetry (DSC), Dynamic Mechanical Analysis (DMA)) which are sensitive to the local atomic rearrangements in the interstitial spaces (including viscous flow) are applied to find support for the idea of kinetic hindrance and the formation of a core shell structure acting as diffusion barrier.Both the DSC-measured calorimetric glass transition and the DMA-measured viscoelastic properties indicate an increase in the time constants of atomic rearrangements and diffusion by at least two orders of magnitude during ceramization. This fits to the above idea.Based on these findings, thermo analytic studies have been performed in order to find out how Ostwald ripening may be provoked.

► Macroscopic, routine laboratory methods of the “Thermal Analysis” type (DSC, DMA) allow a rough description of the kinetics in the nano-sized interstitial spaces of glass ceramics. ► These macroscopic measurements support the idea of a rigid zone around the crystals which builds up during ceramization and is part of a negative feedback loop which finally stops crystal growth and Ostwald ripening within the time window of observation. ► Ostwald ripening may be provoked by thermally softening said rigid zone. Under certain conditions, this gives rise to a characteristic peak in the DSC.