Influence of CaO/SiO2 ratio on the structure of mold flux glasses: preliminary results

teel production is made through continuous casting, with the need to reduce the frictional force between molten steel (which is solidifying on the outer shell) and the crystallisator. Mold fluxes are especially made for this purpose: they are synthetic oxy- fluoride glass-ceramic systems used also to control the amount of heat transfer rate [1, 2]. Mold fluxes form various layers, in contact with molten steel, the most interesting one being the interface between the amorphous and the liquid one. In this work, the structure of the amorphous layer is studied by means of total scattering, with spallation neutrons and high energy X-rays. Future work will include the study of the liquid phase, to be compared with the amorphous one, and the direct study of the interface, for which there is a proposal already submitted to ISIS. The performance of these materials is ideal when the ratio CaO/SiO2 is larger than one, but the relationship between this empirical value and actual structure of the amorphous layer is yet to be fully understood. The samples were prepared with a CaO/SiO2 varying from 0.65 to 1.3, and with different amount of fluorine (which also has important effects on viscosity), in order to check how the structural features of the materials vary with the chemical composition. The data collection was performed at ISIS (beamline NIMROD) and at ESRF (beamline ID11), both ideally suited for amorphous materials. Studying the structure of amorphous materials is not easy and requires specific knowledge and some good statical indicators for the quality of the fit between the experimental data and the model for the structure. With no periodicity and with short-length order, amorphous materials are challenging. The structural model obtained for an amorphous material can therefore be one of the possible models, and understanding its reliability can be particularly intriguing and difficult. The structural information were obtained using the EPSR (Empirical Potential Structure Refinement) approach [3], using a large cell of 50000 atoms, for better statistics. The results showed that the structure varies, as expected, with composition, getting proportionally more different from a standard alumino-silicate glass with increasing CaO/SiO2 ratio. Those with a ratio lower than one are, in fact, very similar to each other in terms of coordination numbers, bond lengths and bond angles. It’s only when we reach a ratio larger than one and a significant amount of fluorine that things start to change. The most interesting result is shown in figure 1, where the size of Ca-Ca clusters (with a clustering distance from 1 to 4 Å) increase dramatically as soon as CaO/SiO2 becomes larger than 1. This is in good agreement with DCS results and with viscosity measurements. The relationship between this structural feature and the performance of the mold flux seems to be clear, but it needs to be confirmed by the results obtained from the same samples in the melted state [1] K.C. Mills (2016). ISIJ International, 56, 14-23. [2] J. Cho, H. Shibata, T. Emi, M. Suzuki (1998). ISIJ international, 38, 440-446. [3] Soper AK (2010) EPSRshell: a users guide. ISIS DisorderedMaterial Group, Didcot (UK)