Effect of oxidation state of iron ions on the viscosity of alkali silicate melts

Understanding the changes in viscosity due to the different oxidation states of iron ions is important for simulating phenomena related to molten slags and also for understanding the structure of iron-oxidecontaining silicate melts. However, these viscosity changes are not well understood. Here, we show the viscosity changes of R₂O-SiO₂-FexO (R = Li, Na, or K melts due to changes in the oxidation states of the iron ions by systematically varying the oxygen partial pressure using several Ar-based gases at 1 773 K. The initial compositions of the samples were 30R₂O-60SiO ₂-10Fe₂O₃ (mol%), and the ratio of Fe ³⁺ to Fe²⁺ in the R₂O-SiO₂-FexO melts increased with increasing oxygen partial pressure in all samples. Meanwhile, the viscosity of all the R₂O-SiO₂-FexO melts decreased with increasing Fe²⁺ to total-Fe ratio. The data indicate that the increase in the amount of Fe²⁺ ions, which behave as network modifiers, would result in depolymerization of the silicate melts. It should also be noted that the viscosity of the melts increased in the order of alkali cationic radius (K > Na > Li when the ratios of Fe²⁺ to Fe ³⁺ in the melts were comparable. This was due to the change in the coordination structure of Fe³⁺ in the melts. Not only the oxidation state of the iron ions but also the coordination structure of Fe³⁺ may be important for understanding the viscosity.