Relationships Between Atomic Diffusion Mechanisms and Ensemble Transport Coefficients in Crystalline Polymorphs

Ionic transport in conventional ionic solids is generally considered to proceed via independent diffusion events or "hops''. This assumption leads to well-known Arrhenius expressions for transport coefficients, and is equivalent to assuming diffusion is a Poisson process. Using molecular dynamics simulations of the low-temperature B1, B3, and B4 AgI polymorphs, we have compared rates of ion-hopping with corresponding Poisson distributions to test the assumption of independent hopping in these common structure-types. In all cases diffusion is a non-Poisson process, and hopping is strongly correlated in time. In B1 the diffusion coefficient can be approximated by an Arrhenius expression, though the physical significance of the parameters differs from that commonly assumed. In low temperature B3 and B4 diffusion is characterised by concerted motion of multiple ions in short closed loops. Diffusion coefficients can not be expressed in a simple Arrhenius form dependent on single-ion free-energies, and intrinsic diffusion must be considered a many-body process

Molecular Dynamics Simulation of Coherent Interfaces in Fluorite Heterostructures

The standard model of enhanced ionic conductivities in solid electrolyte heterostructures follows from a continuum mean-field description of defect distributions that makes no reference to crystalline structure. To examine ionic transport and defect distributions while explicitly accounting for ion-ion correlations and lattice effects, we have performed molecular dynamics simulations of a model coherent fluorite heterostructure without any extrinsic defects, with a difference in standard chemical potentials of mobile fluoride ions between phases induced by an external potential. Increasing the offset in fluoride ion standard chemical potentials across the internal interfaces decreases the activation energies for ionic conductivity and diffusion and strongly enhances fluoride ion mobilities and defect concentrations near the heterostructure interfaces. Non-charge-neutral "space-charge" regions, however, extend only a few atomic spacings from the interface, suggesting a continuum model may be inappropriate. Defect distributions are qualitatively inconsistent with the predictions of the continuum mean-field model, and indicate strong lattice-mediated defect-defect interactions. We identify an atomic-scale "Frenkel polarisation" mechanism for the interfacial enhancement in ionic mobility, where preferentially oriented associated Frenkel pairs form at the interface and promote local ion mobility via concerted diffusion processes

An Evaluation of Vegetated Roofing Technology: Application at Air Force Plant Four, Building 15

The United States Air Force maintains thousands of facilities around the world. Many of these facilities have asphalt built up roofs or some other less than sustainable roofing system. In an effort to find roofing systems suitable for Air Force facilities that are both economically and environmentally friendly, this thesis investigated vegetated roofing as a possible alternative to conventional roofing systems. While vegetated roofs are a relatively new roofing system, they exhibit performance qualities that seem to be in line with Air Force needs. An investigation into the feasibility of vegetated roofing technology revealed that this roofing system has many positive economic and environmental characteristics that could benefit the United States Air Force and the Department of Defense. The potential use of this technology was researched specifically for application to building 15 at Air Force Plant 4 (AFP4) in Ft. Worth Texas. A combination of case studies, site visits, and a life cycle economic evaluation was used to compare vegetated roofing with conventional asphalt built up roofing that is typically used at AFP4. The research revealed multiple environmental benefits and few disadvantages. The life cycle costs combined with the environmental benefits of vegetated roofing show that this roofing system is indeed a feasible alternative for building 15

Density Functional Theory screening of gas-treatment strategies for stabilization of high energy-density lithium metal anodes

To explore the potential of molecular gas treatment of freshly cut lithium foils in non-electrolyte based passivation of high energy-density Li anodes, density functional theory (DFT) has been used to study the decomposition of molecular gases on metallic lithium surfaces. By combining DFT geometry optimization and Molecular Dynamics, the effects of atmospheric (N2, O2, CO2) and hazardous (F2, SO2) gas decomposition on Li(bcc) (100), (110), and (111) surfaces on relative surface energies, work functions, and emerging electronic and elastic properties are investigated. The simulations suggest that exposure to different molecular gases can be used to induce and control reconstructions of the metal Li surface and substantial changes (up to over 1 eV) in the work function of the passivated system. Contrary to the other considered gases, which form metallic adlayers, SO2 treatment emerges as the most effective in creating an insulating passivation layer for dosages <= 1 mono-layer. The substantial Li->adsorbate charge transfer and adlayer relaxation produce marked elastic stiffening of the interface, with the smallest change shown by nitrogen-treated adlayers

RevelsMD: Reduced Variance Estimators of the Local Structure in Molecular Dynamics

RevelsMD is a new open source Python library, which uses reduced variance force sampling based estimators to calculate 3D particle densities and radial distribution functions from molecular dynamics simulations. This short note describes the scientific background of the code, its utility and how it fits within the current zeitgeist in computational chemistry and materials science.Comment: 3 page note describing an open source cod

From Linear to Bottlebrush: Material Property Correlations of Acrylate Elastomers

This dissertation aims to advance the current understanding of graft polymer architectures and the new and unique properties that can be accessed by their application to elastomeric materials. Current accessible polymer materials are synthesized from linear polymer architectures, which have been implemented to great effect and restructured the economy and technology of the world. However, in the case of linear elastomers the only architectural control parameter that can be tuned is the crosslink density,??,which imposes a fundamental limitation on the accessible material properties limiting their modulus(?>100???),maximum extensibility(????<5), and strain-stiffening characteristics(?<0.20).Here we have studied graft polymer elastomers made from poly(butyl acrylate) (PBA) to draw correlations between their architectural parameters (sidechain length-???,grafting density-??, backbone length-???, and crosslink density-??) and their material properties. A series of loosely grafted comb polymers in the melt were examined by oscillatory shear rheology to study how the entanglement modulus of such architectures can be tuned by varying the ???, and ??. A model system created from PBA graft polymer elastomers was introduced and used to correlate the mechanical and swelling properties of graft polymer elastomers with their associated architectural parameters. Of particular note it was found that graft polymer architectures could provide a unique opportunity to develop tissue like gels, whose mechanical properties can be tuned independently of their solvent fraction. Lastly, the electroactive properties of bottlebrush elastomers were shown to be large strains at low operating voltages.Doctor of Philosoph

Facing Facts: Facial Injuries from Stand-up Electric Scooters

Background Stand-up electric scooters (SES) are a popular public transportation method. Numerous safety concerns have arisen since their recent introduction. Methods A retrospective chart review was performed to identify patients presenting to the emergency departments in Indianapolis, who sustained SES-related injuries. Results A total of 89 patients were included in our study. The average patient age was 29 ± 12.9 years in a predominantly male cohort (65.2%). No patient was documented as wearing a helmet during the event of injury. Alcohol intoxication was noted in 14.6% of accidents. Falling constituted the leading trauma mechanism (46.1%). Injuries were most common on Saturday (24.7%) from 14h00 to 21h59 (55.1%). Injury types included: abrasions/contusions (33.7%), fractures (31.5%), lacerations (27.0%), or joint injuries (18.0%). The head and neck region (H&N) was the most frequently affected site (42.7%). Operative management under general anesthesia was necessary for 13.5% of injuries. Nonoperative management primarily included conservative orthopedic care (34.8%), pain management with nonsteroidal anti-inflammatory drugs (NSAIDs) (34.8%) and/or opioids (4.5%), bedside laceration repairs (27.0%), and wound dressing (10.1%). Individuals sustaining head and neck injuries were more likely to be older (33.8 vs. 25.7 years, p=0.003), intoxicated by alcohol (29.0% vs. 3.9%, p=0.002), and requiring CT imaging (60.5% vs. 9.8%, p <0.001). Conclusion Although SESs provide a convenient transportation modality, unregulated use raises significant safety concerns. More data need to be collected to guide future safety regulations