Quantitative Studies on the Microstructure Evolution and Its Impact on the Viscosity of a Molten Al2O3-Na3AlF6 System by Raman Spectroscopy and Theoretical Simulations

The microstructure of the molten alumina-cryolite system is an important aspect in the dissolution behavior of alumina (alpha-Al2O3) in cryolite (Na3AlF6) molten salt and impacts the physical properties of the molten system. In this work, the symmetric stretching vibrational wavenumbers of aluminum nonbridging fluorine (AlFx3-x) and aluminum -oxygen -fluorine (Al2OFy4-y, and Al2O2Fz2-z) bonds in the high wavenumber range, as well as their corresponding Raman scattering cross sections (RSCS), were first analyzed and determined by quantum chemistry (QC) ab initio calculations. The species in binary Al2O3-Na3AlF6 melts with varying Al2O3 contents from 0 to 6.0 wt% were then qualitatively and quantitatively analyzed using in situ high -temperature Raman spectroscopy in conjunction with first principles calculation from room temperature to melt. The species of AlF4-, AlF52- and AlF63- were found to be present and predominant in the melts with Al2O3 contents ranging from 0 to 6.0 wt%. The content of Al2OF62- species tended to increase with Al2O3 content from 0 to 3.0 wt%, while Al2O2F42- species only existed in the melts with an Al2O3 concentration of more than 3.0 wt%. Finally, the abundance of various species in the molten Al2O3-Na3AlF6 system was further correlated to the melt viscosity. The contribution to viscosity was primarily determined by the distribution of AlFx3-x (x = 4, 5, and 6) and Al2OF62- species, and the contribution ability of corresponding species to viscosity was Al2OF62-, AlF63-, AlF52- and AlF4- from largest to smallest.