The Waldemar T. Ziegler Award Winners 2016 Importance of Updraft Velocity to Cloud Ice Crystal Formation Computational Characterization of Disorder and Heterogeneity in Bulk MOF Structures

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Sullivan, Sylvia
Han, Rebecca
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Importance of Updraft Velocity to Cloud Ice Crystal Formation, Sylvia Sullivan - The number and size of ice crystals and liquid droplets within a cloud determine its radiative impact on the Earth. To reduce the uncertainty associated with this cloud radiative forcing, simulations were performed with two global climate models to understand which inputs to the cloud system are most influential on crystallization therein: the updraft velocity, as a source of supersaturation and equivalent to a temperature ramp, or the aerosol number, as a sink of supersaturation and equivalent to a seed loading. In particular, attribution metrics are calculated using an efficient and robust automatic differentiation technique. These metrics show that updraft velocity is the controlling factor for realistic formulations and that predictions of cloud ice crystal number will be improved by more frequent updraft measurements at low latitudes and altitudes.
Computational Characterization of Disorder and Heterogeneity in Bulk MOF Structures, Rebecca Han - Metal organic frameworks (MOFs) are an important class of microporous materials used in sieving, separations, and catalysis. Despite offering tunable pore size and functionalization, MOFs are limited in industrial applications by their tendency to degrade in humid and acid gas environments. We aim to characterize disorder and heterogeneity in bulk MOF structure based on the hypothesis that degradation begins at defect sites, as occurs in zeolites and metals. Two representative systems are considered, zeolitic imidazolate frameworks (ZIFs) and Zn2(1,4-BDC)2 (Dabco) (DMOF), because their ideal structures are wellstudied without much knowledge of the realistic materials. We establish evidence for thermodynamically favorable stacking faults in hypothetical ZIF polymorphs and correlate defect density to XRD spectra. We can also use simulations to elucidate the short range order in a threecomponent ZIF system and predict impact of linker distribution on pore window size or molecular diffusion. In comparison, DMOF is a mixed-linker framework inherently constructed of two ligand species. Our goal is to identify energetically accessible defects in the bulk structure and estimate their prevalence. This understanding will allow us to interpret literature observations of DMOF stability where the precise mechanisms remain unknown.
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