Kinetic fragility of melts of conditional glass former oxides

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Conditional glass former oxides (CGFs) do play an essential role as components in the production of highly specialized and optical glasses. Their readiness for glass formation by melt-quenching is strongly limited due to immediate crystallization and in practice they form glasses only by binary or multicomponent mixtures with other oxides. However, the origin for limited glass formation of CGFs based on structural and/or kinetic criteria is still not satisfactorily settled. The objective of the research proposal is therefore to study the relation between enthalpy relaxation and viscous flow ranged by the strong–fragile pattern of kinetic fragility and its structural origin. Experiments are based on the capability of modern flash differential scanning calorimeter (FDSC) to quench a small droplet (few microns in size) of melt at high rate (up to 50,000 K s-1) and to simultaneously determine characteristic temperatures of the glass transition from which, in a second step, viscosity and kinetic fragility are obtained. Parallel shift factors are provided by performing a compositional loop linking viscosity with calorimetric data of modifier containing CGF glasses. With the use of FDSC and parallel shift factors, the work hypothesis is to verify expectations, that enthalpy relaxation and viscous flow at the glass transition of fragile CGFs are decoupled. Work will be focussed on three CGFs: the metalloid oxide Sb2O3, the transition metal oxide V2O5, and the heavy metal oxide Bi2O3 that are selected due to the attractive properties in glasses, thermal stability of their melt and the different electronic configuration of the metal/metalloid cation. Structural information on the near- and mid-range order will be gained by collection of Raman intensities from the small glassy droplet, for which special equipment has been built up. From it, the degree of dynamic heterogeneity will be deduced and correlated to the viscometric/calorimetric results. The issues dealt with by the work hypothesis are furthermore elucidated by comparative research in terms of highlighting the expected non-linear relation between glass transition temperature and modifier fraction of CGF melts when approaching inpurity level. In turn, temperature-dependent constraint theory will be used to provide a quantitative connection between the connectivity of the CGF glass network and the compositional dependence of the glass transition temperature.