Molten alkali chloride salts are a critical component in concentrated solar power and nuclear applications. Despite their ubiquity, the extreme chemical reactivity of molten alkali chlorides at high temperatures has presented a significant challenge in characterizing atomic structures and dynamic properties experimentally. Here, we investigate molten NaCl by performing high-temperature molecular dynamics simulations using a Gaussian approximation potential (GAP) trained on density functional theory (DFT) data sets. Our GAP model, trained with 1000 atomic configurations, arrives near DFT accuracy with a mean absolute error of 1.5 meV/atom, thus enabling fast analysis of high-temperature salt properties at large length (5000 ion pairs) and time (>1 ns) scales, currently inaccessible to ab initio simulations. Calculated structure factors and diffusion constants from our GAP model simulations show excellent agreement with experiments. Our results indicate that GAP models are able to capture the many-body interactions required to accurately model ionic systems.

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We report a study investigating the effects of thermal annealing on the optical properties of Si-Ge alloy-core silica-cladded fibers. Low temperature fiber draw was performed with a laboratory-made draw tower at 1760 °C that minimizes impurity diffusion from cladding to the core. As a post-drawing process, Si-Ge core fibers were annealed in a box furnace to alter the core structure. Microstructural and optical properties of fibers were investigated, and transmission losses were measured as 28 dB/cm at 6.1 µm. Numerical studies were performed to analyze the experimental results and to find the optimum structure for low loss semiconductor-core glass-cladded fibers.