A Reactive Force Field Approach to Modeling Corrosion of NiCr Alloys in Molten FLiNaK Salts

The interface between NiCr alloys and FLiNaK molten salt exhibits complex corrosion behavior, mainly driven by intricate chemical interactions involving Cr and F$\mathrm{^-}$ ions. Understanding these dynamic reactions is crucial for developing effective corrosion mitigation strategies to ensure the long-term durability of Ni-based alloy components in molten salt technologies. However, obtaining molecular-level understanding through experiments is challenging. To address this, we utilize reactive molecular dynamics simulations enabled by a reactive force field, ReaxFF, to investigate detailed reaction dynamics at the atomic level. Since there is currently no available force field involving fluoride salt and Ni-based alloys, we first present the development of the ReaxFF parameter set for Ni/Cr/F/Li/Na/K based on extensive first-principles calculations. With this force field, we achieve a strong agreement for the structure of FLiNaK molten salt by comparing the pair distribution functions with experimental and simulation results. Furthermore, it successfully reproduces the experimental phenomenon of Cr dissolution in fluoride salt, with the corrosion rate depending on the alloy and salt compositions. Particularly, it reveals that increasing the concentration of Li can enhance the formation of a compact double layer, mitigating Cr dissolution. This work enables a fundamental understanding of the interfacial behavior between fluoride salt and NiCr alloys

Role of bilayer graphene microstructure on the nucleation of WSe2 overlayers

PubMed ID37368885Over the past few years, graphenegrown by chemical vapordeposition(CVD) has gained prominence as a template to grow transition metaldichalcogenide (TMD) overlayers. The resulting two-dimensional (2D)TMD/graphene vertical heterostructures are attractive for optoelectronicand energy applications. However, the effects of the microstructuralheterogeneities of graphene grown by CVD on the growth of the TMDoverlayers are relatively unknown. Here, we present a detailed investigationof how the stacking order and twist angle of CVD graphene influencethe nucleation of WSe2 triangular crystals. Through thecombination of experiments and theory, we correlate the presence ofinterlayer dislocations in bilayer graphene with how WSe2 nucleates, in agreement with the observation of a higher nucleationdensity of WSe2 on top of Bernal-stacked bilayer grapheneversus twisted bilayer graphene. Scanning/transmission electron microscopy(S/TEM) data show that interlayer dislocations are present only inBernal-stacked bilayer graphene but not in twisted bilayer graphene.Atomistic ReaxFF reactive force field molecular dynamics simulationsreveal that strain relaxation promotes the formation of these interlayerdislocations with localized buckling in Bernal-stacked bilayer graphene,whereas the strain becomes distributed in twisted bilayer graphene.Furthermore, these localized buckles in graphene are predicted toserve as thermodynamically favorable sites for binding WSe x molecules, leading to the higher nucleation densityof WSe2 on Bernal-stacked graphene. Overall, this studyexplores synthesis-structure correlations in the WSe2/graphene vertical heterostructure system toward the site-selectivesynthesis of TMDs by controlling the structural attributes of thegraphene substrate

Oxidation of tungsten at room temperature irradiated by oxygen plasma

The performance of tungsten as a fusion material depends on its surface properties, which are strongly affected by the interaction with impurities, either already contained in the bulk or coming from the plasma. Plasma-facing components, such as the divertor and the first wall of the current fusion reactors, as well as many of the laboratory experiments, are operating at or are cooled down to room temperatures. We performed a detailed molecular dynamics study of the processes of oxidation of a tungsten surface at 300 K when it was cumulatively irradiated by a low-energy oxygen plasma. We developed and used a ReaxFF W-O-H classical potential, capable of treating reactive interactions of all atoms in the system, including polar interactions present when O interacts with tungsten. We also studied the properties of the oxidized layers and their response in the form of reflection, retention, and chemical sputtering to the impact of oxygen