The wetting characteristics of aluminum droplets on rough surfaces with molecular dynamics simulations.

In the present study, the impact of surface roughness on the wettability behavior of Al droplets has been investigated via molecular dynamics (MD) simulations. In this work, amorphous carbon (AC) and graphite substrates with different depths and widths were considered. The results show that the increased width of grooves causes the transition of the wetting state from Cassie to Wenzel. Thermodynamic property analysis results indicate that the solid-liquid adhesion and the work done for the removal of the Al droplet from the solid surface decrease as the roughness increases. However, the adhesion in the Wenzel wetting state is better than that in the Cassie wetting state. Therefore, the contact angle increases with the increased roughness in the Cassie wetting systems, while in the Wenzel wetting systems, the contact angle is less than that in other rough systems. In addition, due to the heterogeneity of the surfaces, the density of Al droplets in the solid-liquid interface is decreased with the increased roughness. The anisotropic spreading of Al liquid can be explained by the MSD curves along the X and Y directions.

Coalescence and wetting mechanism of Al droplets on different types of carbon for developing wettable cathodes: a molecular dynamics simulation.

So far, there have been few studies on the microscopic wetting behavior of aluminum liquid on cathode surfaces, which is critical for developing wettable cathode materials. In the present study, an investigation on the coalescence and wetting mechanism of Al droplets on different carbonaceous substrates has been performed via molecular dynamics (MD) simulation for developing wettable cathodes. The growth rate of liquid bridge, the mean squared displacement, the balanced contact angle, and the time of full coalescence were calculated to describe the coalescence and wetting of the Al droplets. The results illustrate the sequence of full coalescence time for the Al droplets: DG < HCNT < VCNT ≈ AC and the corresponding balanced contact angles were 47.98°, 53.32°, 55.02°, and 63.12°, respectively. Furthermore, the presence of defects on DG will increase the time of coalescence and the contact angle but the directions of defects have little influence. The free energy analysis indicates that the defects reduce the solid-liquid interaction and the work done for removing the Al droplet from the substrates so that the wettability is weaker than that for perfect graphene, which also explains the balanced wettability of Al droplets on the other substrates. In addition, the surface roughness increases the contact angle of Al liquid on AC (from 62° to 113°-120°) and hence, the wettability is changed from good to poor. In general, our results can improve the understanding of the wetting of AC and graphene by Al liquid at the atomic level, which can provide direction and theoretical guidance for further research on wettable cathodes.

Ionic micro-structure and transport properties of low-temperature aluminium electrolytes containing potassium cryolite and sodium cryolite.

Nowadays, low-temperature aluminium electrolytes are reported to have good prospects for application in the industrial process of aluminium production. In this paper, low-temperature electrolytes containing potassium cryolite and sodium cryolite with a cryolite ratio of 1.3 were investigated by using first-principles molecular dynamics simulation. This calculation reproduces the ionic structure of low-temperature 1.3(KF + NaF)-AlF3 electrolytes, indicating that [AlF4]-, [AlF5]2- and [AlF6]3- are the fundamental aluminum-fluoro clusters and [AlF5]2- is the predominant species. The calculated results for the ionic structure indicate that molten 1.3(KF + NaF)-AlF3 electrolytes have a high ionic polymerization degree, which is unfavorable for the transport properties of low-temperature 1.3(KF + NaF)-AlF3 electrolytes. Fortunately, increasing the mass fraction of NaF can effectively reduce the polymerization degree of ionic structure and thus improve the ionic conductivity of low-temperature 1.3(KF + NaF)-AlF3 electrolytes, which is an important guiding factor for the component selection of low-temperature electrolytes in future. Also, DFT calculations were adopted to further analyse the small aluminum-fluoro complexes. The calculated Raman spectrum of the aluminum-fluoro complexes is excellently consistent with literature results. Our calculated ionic conductivity falls in between the estimated value of the empirical equation of different literature studies, demonstrating that our results may be more precise than the literature results. This further proves the practicability of our modified N-E equation.

Ionic structure and transport properties of KF-NaF-AlF3 fused salt: a molecular dynamics study.

We used the first-principles molecular dynamics simulations combined with the interatomic potential molecular dynamics to study the ionic structure and transport properties of KF-NaF-AlF3 fused salt. Simulation results show that the ionic structure of KF-NaF-AlF3 fused salt is principally dominated by the distorted five-coordinated [AlF5]2- and six-coordinated [AlF6]3- groups. When melting to a liquid, a part of the six-coordinated [AlF6]3- group dissociated into the four-coordinated [AlF4]- and five-coordinated [AlF5]2- groups. Four, five and six-coordinated aluminum-fluoro complexes coexist in KF-NaF-AlF3 fused salt, while the tetrahedral [AlF4]- groups are relatively rare. The content of the bridging fluorine atom is relatively small, about 5-11%, which indicates that the polymerization degree of the ionic structure of the KF-NaF-AlF3 fused salt system is lower. The KF-NaF-AlF3 fused salt has better liquidity and ionic conductivity due to the high self-diffusion coefficients of all particles in the fused salt system. KF can effectively break the F atom bridges, which reduces the polymerization degree of the ionic structure of the fused salt system and increases its ionic conductivity.