Non-isothermal melt crystallization of cuspidine in CaO–SiO2–CaF2 based glasses

• Non-isothermal melt crystallization kinetics of cuspidine were investigated.
• Ozawa analysis is inappropriate to evaluate melt crystallization kinetics.
• Cuspidine crystallization is nucleation-controlled with negative activation energy.
• Morphology of cuspidine is faceted or dendritic depending on the undercooling.

Non-isothermal crystallization kinetics of cuspidine (Ca4Si2O7F2) in CaO–SiO2–CaF2 based glass system has been investigated using a DSC to understand and improve the performances of mold fluxes applied to commercial continuous casting of steels. It was found that the Ozawa analysis is not suitable to depict non-isothermal melt crystallization of glasses. Instead, the effective activation energy for non-isothermal crystallization was estimated using differential iso-conversional method of Friedman analysis. The effective activation energy of cuspidine formation for the glasses examined showed negative sign, from − 241 to − 652 kJ/mol. It remains constant over the range from 0.1 to 0.4 of the degree of crystallinity, whereas it increases as the degree of crystallinity exceeds 0.4. The negative effective activation energy indicates an anti-Arrhenius behavior for crystallization of the glasses investigated, which means that the melt crystallization for the commercial mold fluxes will be determined by thermodynamics of nucleation which is relevant to degree of undercooling. The crystalline morphology of cuspidine observed by SEM and a SHTT also supported the anti-Arrhenius kinetics during non-isothermal melt crystallization.