Thermal behaviour of sulphate-bearing multicomponent silicate-phosphate glasses

The present study provides a deep insight into the implications of sulphate addition on glass forming properties of materials from the SiO2-P2O5-K2O-MgO/CaO/MgOCaO systems, by combining: preliminary investigation of as-synthesized materials (XRD, XRF, SEM-EDS), detailed characteristics of their thermal behaviour (DSC), estimation of various glass-stability parameters, in-depth analysis of the course of crystallization and an identification of the crystallized products (XRD, Raman imaging technique). Performed investigations revealed that obtaining amorphous materials is only possible in the system containing MgO as a alkaline earth glass modifier, whereas melts of other tested compositions display high tendency to crystallize during cooling. Thermal investigation showed that the influence of SO3 addition on thermal properties of studied materials strongly depends on their composition, but generally such additive increased values of glass transformation temperature and decreased vitreous state transformation range and onset of the crystallization process, compared to the base samples, which is a signal of decrease in thermal stability. Examination of devitrificates revealed the character and distribution of domains present in the structure of studied materials. It was showed that irrespective of composition of the base glass, it is K2SO4 that precipitates once the sulphate capacity of glass is exceeded and exists in the studied materials as groupings either distributed in the voids of silicate phase, or surrounding the remnants of the gas bubbles embodied within the glass body. The results demonstrated in the present study provide guiding suggestions for development of SO3-containing silicate-phosphate glasses intended especially for applications where release of bioactive ions is sought.