76 research outputs found
Reactive Molecular Dynamics Simulations of Shock Through a Single Crystal of Pentaerythritol Tetranitrate
Large-scale molecular dynamics simulations and the reactive force field ReaxFF were used to study shock-induced initiation in crystalline pentaerythritol tetranitrate (PETN). In the calculations, a PETN single crystal was impacted against a wall, driving a shockwave back through the crystal in the [100] direction. Two impact speeds (4 and 3 km/s) were used to compare strong and moderate shock behavior. The primary difference between the two shock strengths is the time required to exhibit the same qualitative behaviors with the lower impact speed lagging behind the faster impact speed. For both systems, the shock velocity exhibits an initial deceleration due to onset of endothermic reactions followed by acceleration due to the onset of exothermic reactions. At long times, the shock velocity reaches a steady value. After the initial deceleration period, peaks are observed in the profiles of the density and axial stress with the strongly shocked system having sharp peaks while the weakly shocked system developed broad peaks due to the slower shock velocity acceleration. The dominant initiation reactions in both systems lead to the formation of NO_2 with lesser quantities of NO_3 and formaldehyde also produced
Secular variation of activity in comets 2P/Encke and 9P/Tempel 1
We compare production rates of H20 derived from International Ultraviolet Explorer (IUE) spectra from multiple apparitions of 2 comets, 2P/Encke and 9P/Tempel 1, whose orbits are in near-resonance with that of the Earth. Since model-induced errors are primarily a function of observing geometry, the close geometrical matches afforded by the resonance condition results in the cancellation of such errors when taking ratios of production rates. Giving careful attention to the variation of model parameters with solar activity, we find marginal evidence of change in 2P/Encke: a 1-sigma pre-perihelion decrease averaging 4%/revolution over 4 apparitions from 1980-1994, and a 1-sigma post-perihelion increase of 16%/revolution for 2 successive apparitions in 1984 and 1987. We find for 9P/Tempel 1, however, a 7-sigma decrease of 29%/revolution over 3 apparitions from 1983-1994, even after correcting for a tracking problem which made the fluxes systematically low. We speculate on a possible association of the character of long-term brightness variations with physical properties of the nucleus, and discuss implications for future research
Ab initio Molecular Dynamics Simulations of the Initial Stages of Solid-electrolyte Interphase Formation on Lithium Ion Battery Graphitic Anodes
The decomposition of ethylene carbonate (EC) during the initial growth of
solid-electrolyte interphase (SEI) films at the solvent-graphitic anode
interface is critical to lithium ion battery operations. Ab initio molecular
dynamics simulations of explicit liquid EC/graphite interfaces are conducted to
study these electrochemical reactions. We show that carbon edge terminations
are crucial at this stage, and that achievable experimental conditions can lead
to surprisingly fast EC breakdown mechanisms, yielding decomposition products
seen in experiments but not previously predicted.Comment: 5 pages, 4 figure
Exploring the volatile composition of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON) with ALMA
Comets formed in the outer and cold parts of the disk which eventually
evolved into our Solar System. Assuming that the comets have undergone no major
processing, studying their composition provides insight in the pristine
composition of the Solar Nebula. We derive production rates for a number of
volatile coma species and explore how molecular line ratios can help constrain
the uncertainties of these rates. We analyse observations obtained with the
Atacama Large Millimetre/Submillimetre Array of the volatile composition of the
comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON) at heliocentric
distances of ~1.45 AU and ~0.56 AU, respectively. Assuming a Haser profile with
constant outflow velocity, we model the line intensity of each transition using
a 3D radiative transfer code and derive molecular production rates and parent
scale lengths. We report the first detection of CS in comet ISON obtained with
the ALMA array and derive a parent scale length for CS of ~200 km. Due to the
high spatial resolution of ALMA, resulting in a synthesised beam with a size
slightly smaller than the derived parent scale length, we are able to
tentatively identify CS as a daughter species, i.e., a species produced in the
coma and/or sublimated from icy grains, rather than a parent species. In
addition we report the detection of several CH3OH transitions and confirm the
previously reported detections of HCN, HNC and H2CO as well as dust in the coma
of each comet, and report 3sigma upper limits for HCO+. We derive molecular
production rates relative to water of 0.2% for CS, 0.06-0.1% for HCN,
0.003-0.05% for HNC, 0.1-0.2% for H2CO and 0.5-1.0% for CH3OH, and show that
the modelling uncertainties due to unknown collision rates and kinematic
temperatures are modest and can be mitigated by available observations of
different transitions of HCN.Comment: 10 pages, 4 figures, 2 tables. Accepted for publication in A&
Automatic Coarse Graining of Polymers
Several recently proposed semi--automatic and fully--automatic
coarse--graining schemes for polymer simulations are discussed. All these
techniques derive effective potentials for multi--atom units or super--atoms
from atomistic simulations. These include techniques relying on single chain
simulations in vacuum and self--consistent optimizations from the melt like the
simplex method and the inverted Boltzmann method. The focus is on matching the
polymer structure on different scales. Several ways to obtain a time-scale for
dynamic mapping are discussed additionally. Finally, similarities to other
simulation areas where automatic optimization are applied as well are pointed
out.Comment: 17 pages, 5 figure
Segmental dynamics in a blend of alkanes: Nuclear magnetic resonance experiments and molecular dynamics simulation
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Horizontal Ionospheric Electron Density Gradients Observed by FORMOSAT-3/COSMIC TIP: Spatial Distributions and Effects on VLF Wave Propagation at Mid-Latitudes
We investigate the spatial variability of electron densities in the nightside ionosphere and its effects on very-low frequency (VLF) wave propagation using a suite of instruments from the FORMOSAT-3/Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) spacecraft.We use observations from the Tiny Ionospheric Photometer (TIP) instruments to infer the horizontal electron density gradients along each satellite track. We demonstrate that the OI 1356 _ radiance measured by the TIP instruments tracks the horizontal electron density structure well with high spatial resolution and unprecedented sensitivity. Accurate measurements of the horizontal electron density gradients are important for improving retrieved electron density profiles from GPS occultation and other tomographic remote sensing techniques. The processes underlying the variability in the large-scale, nightside electron density gradients are the main drivers of ionospheric weather. TIP observations reveal significant variability in both the small and large scale structure of the nightside ionosphere. The relative intensities, relative widths, and latitudinal separation of the northern and southern ionization crests of the Appleton anomalies show a high degree of longitudinal variation.We demonstrate how the TIP observations can be used to measure the horizontal gradient of the refractive index of whistler-mode VLF waves propagating in a cold, collisionless plasma. These measurements are critical for understanding how gradients in electron density associated with ionospheric structure such as depletions and the Appleton anomalies affect VLF wave propagation through the equatorial and mid-latitude ionosphere
Coordinated Ionospheric Reconstruction CubeSat Experiment (CIRCE) mission overview
The Coordinated Ionospheric Reconstruction Cubesat Experiment (CIRCE) is a joint US/UK mission consisting of two 6U CubeSats actively maintaining a lead-follow configuration in the same low Earth orbit with a launch planned for the 2020 timeframe. These nanosatellites will each feature multiple space weather payloads. From the US, the Naval Research Laboratory will provide two 1U Triple Tiny Ionospheric Photometers (Tri-TIPs) on each satellite, observing the ultraviolet 135.6 nm emission of atomic oxygen at nighttime. The primary objective is to characterize the twodimensional distribution of electrons in the Equatorial Ionization Anomaly (EIA). The methodology used to reconstruct the nighttime ionosphere employs continuous UV photometry from four distinct viewing angles in combination with an additional data source, such as in situ plasma density measurements, with advanced image space reconstruction algorithm tomography techniques. From the UK, the Defence Science and Technology Laboratory (Dstl) is providing the In-situ and Remote Ionospheric Sensing suite consisting of an Ion/Neutral Mass Spectrometer, a triple-frequency GPS receiver for ionospheric sensing, and a radiation environment monitor. We present our mission concept, simulations illustrating the imaging capability of the Tri-TIP sensor suite, and a range of science questions addressable via these measurements
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