Structure–property relationship amphoteric oxide systems via phase stability and ionic structural analysis

The effects of basicity and amphoteric oxides (Al₂O₃ and Fe_tO) on the structure–property relationships of CaO–SiO₂–(Al₂O₃ and Fe_tO) and CaO–SiO₂–Al₂O₃–Fe_tO slags were investigated to determine the constitutional effects on the structure of high-temperature ionic melts. The proportion of Qⁿ species, which is determined by Raman spectroscopy, and the viscosity measured by the rotating cylinder method are both correlated and shown together with the slag structure index (NBO/T) concept. The NBO/T of CaO-SiO₂ binary slags showed a linear relationship with basicity (CaO/SiO₂), including an inflection point at CaO/SiO₂ = 1.0 resulting from the stability and Qⁿ-dominant unit of the melt, which changes close to the wollastonite (CaSiO₃) congruent point. This inflection point changes with the increasing amphoteric oxide content (Al₂O₃ and Fe_tO) because of the change in the dominant polymeric unit (Si⁴⁺–O–Si⁴⁺→M⁴⁺–O–Si⁴⁺; M: Al and Fe), in accordance with the equilibrated primary phases. As the Al₂O₃ content increased, the viscosity and activation energy of slags both drastically increased owing to the change in the flow unit (Si–O–Si, Al–O–Si, and Al–O–Al). In contrast, as Fe_tO increased, the viscosity and activation energy (E_η) of slags decreased because of the change in the flow unit (Si–O–Si, Fe–O–Si, and Fe–O–Fe). Ultimately, the flow unit (T–O–T; T = Si, Al, and Fe) and activation energy of the slags were found to be closely related to the solid primary phase on the phase diagram, and the physical-property–structure relationship was determined from the phase stability.