378 research outputs found
Atomistic Insights into the Effects of Doping and Vacancy Clustering on Li-Ion Conduction in the Li<sub>3</sub>OCl Antiperovskite Solid Electrolyte
Solid-state batteries are currently attracting increased attention because of their potential for significant improvements in energy density and safety as compared to liquid electrolyte-based batteries. Lithium-rich antiperovskites, such as Li3OCl, are of particular interest, but the effects of doping on lithium mobility are not fully understood at the atomic level. Here, we investigate the impact of divalent cation (Mg2+, Ca2+, Sr2+, and Ba2+) and Fâ doping on the ion conduction properties of Li3OCl, using both defect simulation and molecular dynamics techniques. Our results show that the F-doped system has a low conductivity and high activation barriers. This is attributable to high binding energies, which leads to the formation of stable dopantâvacancy pairs, preventing long-range lithium-ion mobility. In contrast to the F-doped system, Mg dopants (shown to be the most favorable dopant on the Li+ site) have lower binding energies to lithium vacancies, yielding higher lithium-ion conductivities and lower migration energies. Our results indicate a viable doping strategy to improve the electrochemical performance of antiperovskite solid electrolytes
The non-dipolar magnetic fields of accreting T Tauri stars
Models of magnetospheric accretion on to classical T Tauri stars often assume
that stellar magnetic fields are simple dipoles. Recently published surface
magnetograms of BP Tau and V2129 Oph have shown, however, that their fields are
more complex. The magnetic field of V2129 Oph was found to be predominantly
octupolar. For BP Tau the magnetic energy was shared mainly between the dipole
and octupole field components, with the dipole component being almost four
times as strong as that of V2129 Oph. From the published surface maps of the
photospheric magnetic fields we extrapolate the coronal fields of both stars,
and compare the resulting field structures with that of a dipole. We consider
different models where the disc is truncated at, or well-within, the Keplerian
corotation radius. We find that although the structure of the surface magnetic
field is particularly complex for both stars, the geometry of the larger scale
field, along which accretion is occurring, is somewhat simpler. However, the
larger scale field is distorted close to the star by the stronger field
regions, with the net effect being that the fractional open flux through the
stellar surface is less than would be expected with a dipole magnetic field
model. Finally, we estimate the disc truncation radius, assuming that this
occurs where the magnetic torque from the stellar magnetosphere is comparable
to the viscous torque in the disc.Comment: 14 pages, 8 figures. Figures are reduced resolutio
Radio Astronomy in LSST Era
A community meeting on the topic of "Radio Astronomy in the LSST Era" was
hosted by the National Radio Astronomy Observatory in Charlottesville, VA (2013
May 6--8). The focus of the workshop was on time domain radio astronomy and sky
surveys. For the time domain, the extent to which radio and visible wavelength
observations are required to understand several classes of transients was
stressed, but there are also classes of radio transients for which no visible
wavelength counterpart is yet known, providing an opportunity for discovery.
From the LSST perspective, the LSST is expected to generate as many as 1
million alerts nightly, which will require even more selective specification
and identification of the classes and characteristics of transients that can
warrant follow up, at radio or any wavelength. The LSST will also conduct a
deep survey of the sky, producing a catalog expected to contain over 38 billion
objects in it. Deep radio wavelength sky surveys will also be conducted on a
comparable time scale, and radio and visible wavelength observations are part
of the multi-wavelength approach needed to classify and understand these
objects. Radio wavelengths are valuable because they are unaffected by dust
obscuration and, for galaxies, contain contributions both from star formation
and from active galactic nuclei. The workshop touched on several other topics,
on which there was consensus including the placement of other LSST "Deep
Drilling Fields," inter-operability of software tools, and the challenge of
filtering and exploiting the LSST data stream. There were also topics for which
there was insufficient time for full discussion or for which no consensus was
reached, which included the procedures for following up on LSST observations
and the nature for future support of researchers desiring to use LSST data
products.Comment: Conference summary, 29 pages, 1 figure; to be published in the Publ.
Astron. Soc. Pacific; full science program and presentations available at
http://science.nrao.edu/science/event/RALSST201
Nonradial and nonpolytropic astrophysical outflows IX. Modeling T Tauri jets with a low mass-accretion rate
Context: A large sample of T Tauri stars exhibits optical jets, approximately
half of which rotate slowly, only at ten per cent of their breakup velocity.
The disk-locking mechanism has been shown to be inefficient to explain this
observational fact.
Aims: We show that low mass accreting T Tauri stars may have a strong stellar
jet component that can effectively brake the star to the observed rotation
speed.
Methods: By means of a nonlinear separation of the variables in the full set
of the MHD equations we construct semi- analytical solutions describing the
dynamics and topology of the stellar component of the jet that emerges from the
corona of the star.
Results: We analyze two typical solutions with the same mass loss rate but
different magnetic lever arms and jet radii. The first solution with a long
lever arm and a wide jet radius effectively brakes the star and can be applied
to the visible jets of T Tauri stars, such as RY Tau. The second solution with
a shorter lever arm and a very narrow jet radius may explain why similar stars,
either Weak line T Tauri Stars (WTTS) or Classical T Tauri Stars (CTTS) do not
all have visible jets. For instance, RY Tau itself seems to have different
phases that probably depend on the activity of the star.
Conclusions: First, stellar jets seem to be able to brake pre-main sequence
stars with a low mass accreting rate. Second, jets may be visible only part
time owing to changes in their boundary conditions. We also suggest a possible
scenario for explaining the dichotomy between CTTS and WTTS, which rotate
faster and do not have visible jets
Boot-insole effects on comfort and plantar loading at the heel and fifth metatarsal during running and turning in soccer
Plantar loading may influence comfort, performance and injury risk in soccer boots. This study investigated the effect of cleat configuration and insole cushioning levels on perception of comfort and in-shoe plantar pressures at the heel and fifth metatarsal head region. Nine soccer academy players (age 15.7 ± 1.6 years; height 1.80 ± 0.40 m; body mass 71.9 ± 6.1 kg) took part in the study. Two boot models (8 and 6 cleats) and two insoles (Poron and Poron/gel) provided four footwear combinations assessed using pressure insoles during running and 180° turning. Mechanical and comfort perception tests differentiated boot and insole conditions. During biomechanical testing, the Poron insole generally provided lower peak pressures than the Poron/gel insole, particularly during the braking step of the turn. The boot model did not independently influence peak pressures at the fifth metatarsal, and had minimal influence on heel loads. Specific boot-insole combinations performed differently (P < 0.05). The 8-cleat boot and the Poron insole performed best biomechanically and perceptually, but the combined condition did not. Inclusion of kinematic data and improved control of the turning technique are recommended to strengthen future research. The mechanical, perception and biomechanical results highlight the need for a multi-faceted approach in the assessment of footwear
The impact of contact tracing in clustered populations
The tracing of potentially infectious contacts has become an important part of the control strategy for many infectious diseases, from early cases of novel infections to endemic sexually transmitted infections. Here, we make use of mathematical models to consider the case of partner notification for sexually transmitted infection, however these models are sufficiently simple to allow more general conclusions to be drawn. We show that, when contact network structure is considered in addition to contact tracing, standard âmass actionâ models are generally inadequate. To consider the impact of mutual contacts (specifically clustering) we develop an improvement to existing pairwise network models, which we use to demonstrate that ceteris paribus, clustering improves the efficacy of contact tracing for a large region of parameter space. This result is sometimes reversed, however, for the case of highly effective contact tracing. We also develop stochastic simulations for comparison, using simple re-wiring methods that allow the generation of appropriate comparator networks. In this way we contribute to the general theory of network-based interventions against infectious disease
Redox Chemistry and the Role of Trapped Molecular O<sub>2</sub>in Li-Rich Disordered Rocksalt Oxyfluoride Cathodes
In the search for high energy density cathodes for next-generation lithium-ion batteries, the disordered rocksalt oxyfluorides are receiving significant attention due to their high capacity and lower voltage hysteresis compared with ordered Li-rich layered compounds. However, a deep understanding of these phenomena and their redox chemistry remains incomplete. Using the archetypal oxyfluoride, Li2MnO2F, we show that the oxygen redox process in such materials involves the formation of molecular O2 trapped in the bulk structure of the charged cathode, which is reduced on discharge. The molecular O2 is trapped rigidly within vacancy clusters and exhibits minimal mobility unlike free gaseous O2, making it more characteristic of a solid-like environment. The Mn redox process occurs between octahedral Mn3+ and Mn4+ with no evidence of tetrahedral Mn5+ or Mn7+. We furthermore derive the relationship between local coordination environment and redox potential; this gives rise to the observed overlap in Mn and O redox couples and reveals that the onset potential of oxide ion oxidation is determined by the degree of ionicity around oxygen, which extends models based on linear Li-O-Li configurations. This study advances our fundamental understanding of redox mechanisms in disordered rocksalt oxyfluorides, highlighting their promise as high capacity cathodes
Redox Chemistry and the Role of Trapped Molecular O<sub>2</sub>in Li-Rich Disordered Rocksalt Oxyfluoride Cathodes
In the search for high energy density cathodes for next-generation lithium-ion batteries, the disordered rocksalt oxyfluorides are receiving significant attention due to their high capacity and lower voltage hysteresis compared with ordered Li-rich layered compounds. However, a deep understanding of these phenomena and their redox chemistry remains incomplete. Using the archetypal oxyfluoride, Li2MnO2F, we show that the oxygen redox process in such materials involves the formation of molecular O2 trapped in the bulk structure of the charged cathode, which is reduced on discharge. The molecular O2 is trapped rigidly within vacancy clusters and exhibits minimal mobility unlike free gaseous O2, making it more characteristic of a solid-like environment. The Mn redox process occurs between octahedral Mn3+ and Mn4+ with no evidence of tetrahedral Mn5+ or Mn7+. We furthermore derive the relationship between local coordination environment and redox potential; this gives rise to the observed overlap in Mn and O redox couples and reveals that the onset potential of oxide ion oxidation is determined by the degree of ionicity around oxygen, which extends models based on linear Li-O-Li configurations. This study advances our fundamental understanding of redox mechanisms in disordered rocksalt oxyfluorides, highlighting their promise as high capacity cathodes.</p
Accretion discs, low-mass protostars and planets: probing the impact of magnetic fields on stellar formation
Whereas the understanding of most phases of stellar evolution made
considerable progress throughout the whole of the twentieth century, stellar
formation remained rather enigmatic and poorly constrained by observations
until about three decades ago, when major discoveries (e.g., that protostars
are often associated with highly collimated jets) revolutionized the field. At
this time, it became increasingly clearer that magnetic fields were playing a
major role at all stages of stellar formation.
We describe herein a quick overview of the main breakthroughs that
observations and theoretical modelling yielded for our understanding of how
stars (and their planetary systems) are formed and on how much these new worlds
are shaped by the presence of magnetic fields, either those pervading the
interstellar medium and threading molecular clouds or those produced through
dynamo processes in the convective envelopes of protostars or in the accretion
discs from which they feed.Comment: Proceedings of CNRS/PNPS astrophysical school on "stellar magnetic
fields", EAS Publications Serie
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