Molecular dynamics investigation on structural and transport properties of Na3AlF6-Al2O3 molten salt

摘要

To study the effect of Al2O3 concentration on the local structure and transport properties of Na3AlF6-Al2O3 molten salt, molecular dynamics (MD) simulations with the Coulomb-Buckingham potential model were carried out. MD simulation results show [AlF4](-), [AlF6](2-) and [AlF6](3-) groups coexist in Na3AlF6-Al2O3 molten salt and [AlF6](3-) ions are the dominant species at low Al2O3 concentration of 1-2 wt%. The percentages of [AlF4](-), [AlF6](2-) and [AlF6](3-) groups are approximately equal at 3 wt% Al2O3 and [AlF4](-) ions become the major species at 4 wt% Al2O3. O atoms mainly exist in the form of bridge Ob up to 90%, worsening the fluidity and transport properties of Na3AlF6-Al2O3 molten salt. According to quantum chemical calculations, Al-F and Al-O bonds in the [Al2OF6](2-) groups have ionic characters as well as partial covalent characters due to the hybridization of F, O-2p and Al-3s (3p) orbitals, while Na-F and F F bonds are ionic. The orders of particles diffusion ability follow as Na+ > F- > Al3+ > O2-. In addition, Al2O3 acts as bridges connecting ionic structure networks and more Al2O3 can increase the polymerization degree of the local structure in Na3AlF6-Al2O3 molten salt, and the viscosity increase and ionic conductivity reduce accordingly. (C) 2016 Elsevier B.V. All rights reserved.