Engineering topics
Publications and source records attributed to Brady, Alexander B..
Using neutron scattering dynamics measurements to validate molecular dynamic simulations, it is found that the rates of solvent exchange are not likely to limit the rates of calcite crystal growth and dissolution.
In this work, the nucleation and growth of barium sulfate in nanoporous silica was investigated using in situ small-angle X-ray scattering and X-ray pair distribution function analysis, together with ex situ transmission and scanning transmission electron microscopy (TEM and STEM) imaging. We found that crystalline barite formation in micropores is likely preceded by a nonbulk barite phase in the nanopores, indicating a possible nonclassical nucleation pathway for barium sulfate under confinement. The nucleation of barium sulfate inside the nanopores stopped at ~12% of the pores filled and was seemingly limited by the formation of crystals near the exterior of the silica particles, which likely blocked subsequent solute transport into the interior of the nanopores. The growth rate of barium sulfate was fit using the Johnson–Mehl–Avrami–Kolmogorov equation and constrained using a growth rate of barite of ~1.0 × 10 –7 mol/m 2 /s, obtained from previous studies, but is consistent with TEM and STEM observations made here. The inferred nucleation rate of barium sulfate inside nanopores is estimated to be on the order of 1.0 × 10 9 nuclei/m 2 /s, which is 2 orders of magnitude higher than previous measurements on a planar silica substrate (~1.0 × 10 7 nuclei/m 2 /s). This implies that the ability of silica nanopores to promote barium sulfate nucleation is sufficiently high as to create a potentially self-limiting condition, where the nucleation reaction is shut down prematurely because rapid growth blocks reactant transport.
The speciation and reactivity of aqueous salt solutions are important for a wide variety of applications. However, application of this information is inhibited by broad disagreement about composition, mechanisms, and structure in concentrated solutions especially. Here, neutron diffraction with isotopic substitution measurements on aqueous zinc chloride solutions are used to calibrate molecular dynamics simulations that include effective electronic polarization. This allows us to probe the origin of speciation and reactivity of zinc chloride-water ion complexes, ZnCl x (H 2 O) y 2-x (x ≤ 4 and y ≤ 6), by comparing the reactivity of species in concentrated (4.5 m) and dilute (0.01 m) conditions. Within the concentrated solution, it is found that the extended solvation environment is dominated by solvent-separated ion complexes whose presence increases the free energy of activation for interconversion of species while simultaneously enhancing their thermodynamic stability. This concentration-dependent reactivity and stability suggests that other reactions, such as the nucleation of solid phases, will also be affected.
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Here, a comprehensive study on the prototype solid solution phase carbonitride MXene Ti 3 CN is conducted using nuclear magnetic resonance, electron spin resonance, total and quasi-elastic neutron scattering, combined with density functional theory-based electronic structure and molecular dynamic calculations. The combination of experiment and theory lead toward rational atomic structural models of Ti 3 CN. The remnant Al ions from the etching process significantly tune the interlayer spacing, distinct from the more typical MXene, Ti 3 C 2 , prepared similarly. Neutron scattering indicates the surface terminations of Ti 3 CN display high oxygen and fluorine concentrations and rather low hydroxyl and hydrogen concentrations. Calculations show that the structure including both the residual Al ions and mixed surface terminations give the best agreement with the measurements. The water molecules in Ti 3 CN are highly immobile, in strong contrast to those in Ti 3 C 2 . The analysis of the electronic structure suggests that the nitride MXene displays higher conductivity than the carbides. Finally, the absence of hydroxyl groups in terminations, the solid-solution in the anion sites, the remnants within layers, and immobile water altogether make the carbonitrides a unique series in the MXene family, implying a further exploration of their exotic properties and applications in energy storage.
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Hollandite α-MnO 2 , consisting of manganese–oxygen octahedra, has recently attracted attention due to its high theoretical capacity, yet it suffers capacity degradation during repeated (de)lithiation. Here we use a new conceptual approach to substitute one of the Mn in the tunnel wall via the form of Mn 0.875 M 0.125 O 2 (M = Ti, V, Cr, Nb, Ru), aiming to increase the lithiation potential and attain the theoretical capacity via the enhanced structural stability, with the ultimate goal of improved capacity retention upon repeated (de)lithiation. A bottom-up screening using density functional theory (DFT) was performed to identify the effect of the transition metal substitution on lithiation of α-MnO 2 . The calculations reveal that substitution with electron-accepting Cr ions results in a more significant increase in the lithiation potential of MnO 2 than the other substituents. In terms of structural stability, both Cr and Nb are capable of effectively stabilizing the tunnel structure of α-MnO 2 under increased levels of lithiation, thus providing the opportunity for significant increases in the cyclability and delivered capacity. In conclusion, our study not only discovers the new lithiation pathway and intermediates at the atomic level but also develops the key concepts to optimize the lithiation potential and structural durability for future α-MnO 2 -based materials. This approach opens a new avenue for materials design of 1D tunnel structured materials for use as stable host frameworks for electrochemical ion (de)insertion.
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