13,368 research outputs found
Assessing carbon-based anodes for lithium-ion batteries: A universal description of charge-transfer binding
Many key performance characteristics of carbon-based lithium-ion battery
anodes are largely determined by the strength of binding between lithium (Li)
and sp2 carbon (C), which can vary significantly with subtle changes in
substrate structure, chemistry, and morphology. Here, we use density functional
theory calculations to investigate the interactions of Li with a wide variety
of sp2 C substrates, including pristine, defective, and strained graphene;
planar C clusters; nanotubes; C edges; and multilayer stacks. In almost all
cases, we find a universal linear relation between the Li-C binding energy and
the work required to fill previously unoccupied electronic states within the
substrate. This suggests that Li capacity is predominantly determined by two
key factors -- namely, intrinsic quantum capacitance limitations and the
absolute placement of the Fermi level. This simple descriptor allows for
straightforward prediction of the Li-C binding energy and related battery
characteristics in candidate C materials based solely on the substrate
electronic structure. It further suggests specific guidelines for designing
more effective C-based anodes. The method should be broadly applicable to
charge-transfer adsorption on planar substrates, and provides a
phenomenological connection to established principles in supercapacitor and
catalyst design.Comment: accepted by Physical Review Letter
Fermionic and Bosonic Stabilizing Effects for Type I and Type II Dimension Bubbles
We consider two types of "dimension bubbles", which are viewed as 4d
nontopological solitons that emerge from a 5d theory with a compact extra
dimension. The size of the extra dimension varies rapidly within the domain
wall of the soliton. We consider the cases of type I (II) bubbles where the
size of the extra dimension inside the bubble is much larger (smaller) than
outside. Type I bubbles with thin domain walls can be stabilized by the
entrapment of various particle modes whose masses become much smaller inside
than outside the bubble. This is demonstrated here for the cases of scalar
bosons, fermions, and massive vector bosons, including both Kaluza-Klein zero
modes and Kaluza-Klein excitation modes. Type II bubbles expel massive particle
modes but both types can be stabilized by photons. Plasma filled bubbles
containing a variety of massless or nearly massless radiation modes may exist
as long-lived metastable states. Furthermore, in contrast to the case with a
"gravitational bag", the metric for a fluid-filled dimension bubble does not
exhibit a naked singularity at the bubble's center.Comment: 17 pages, no figs; to appear in Phys.Rev.
Ruthenium atomically dispersed in carbon outperforms platinum toward hydrogen evolution in alkaline media.
Hydrogen evolution reaction is an important process in electrochemical energy technologies. Herein, ruthenium and nitrogen codoped carbon nanowires are prepared as effective hydrogen evolution catalysts. The catalytic performance is markedly better than that of commercial platinum catalyst, with an overpotential of only -12 mV to reach the current density of 10 mV cm-2 in 1 M KOH and -47 mV in 0.1 M KOH. Comparisons with control experiments suggest that the remarkable activity is mainly ascribed to individual ruthenium atoms embedded within the carbon matrix, with minimal contributions from ruthenium nanoparticles. Consistent results are obtained in first-principles calculations, where RuCxNy moieties are found to show a much lower hydrogen binding energy than ruthenium nanoparticles, and a lower kinetic barrier for water dissociation than platinum. Among these, RuC2N2 stands out as the most active catalytic center, where both ruthenium and adjacent carbon atoms are the possible active sites
Hydrogen effects on nanomechanical behavior of additively manufactured 316L stainless steels
Additive manufacturing (AM) has received considerable attention in recent years due to its ability to produce complex engineering components with reduced cost and waste, which simply cannot be made with conventional manufacturing processes. It has been reported that AM 316L austenitic stainless steel (SS) has excellent mechanical properties and possibly even breaks the strength-ductility trade-off. For practical industrial application, it is necessary to investigate the AM steel\u27s resistance to hydrogen embrittlement which is unavoidable in most strucral applications. In this work, we explore the hydrogen effects on nanomechanical responses of AM 316L SS (such as hardness, strain rate sensitivity, activation volume). The obtained results will be compared with those of conventional 316L SS and discussed in terms of hydrogen effect on plastic deformation and microstructure.
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Formation of Millisecond Pulsars from Accretion Induced Collapse and Constraints on Pulsar Gamma Ray Burst Models
We study accretion induced collapse of magnetized white dwarfs as an origin
of millisecond pulsars. We apply magnetized accretion disk models to the
pre-collapse accreting magnetic white dwarfs and calculate the white dwarf spin
evolution. If the pulsar magnetic field results solely from the flux-frozen
fossil white dwarf field, a typical millisecond pulsar is born with a field
strength . The uncertainty in the field strength is
mainly due to the uncertain physical parameters of the magnetized accretion
disk models. A simple correlation between the pulsar spin and the
magnetic field , , is
derived for a typical accretion rate \sim 5\times 10^{-8}M_{\sun}/yr. This
correlation remains valid for a wide pre-collapse physical conditions unless
the white dwarf spin and the binary orbit are synchronized prior to accretion
induced collapse. We critically examine the possibility of spin-orbit
synchronization in close binary systems. Using idealized homogeneous ellipsoid
models, we compute the electromagnetic and gravitational wave emission from the
millisecond pulsars and find that electromagnetic dipole emission remains
nearly constant while millisecond pulsars may spin up rather than spin down as
a result of gravitational wave emission. We also derive the physical conditions
under which electromagnetic emission from millisecond pulsars formed by
accretion induced collapse can be a source of cosmological gamma-ray bursts. We
find that relativistic beaming of gamma-ray emission and precession of
gamma-ray emitting jets are required unless the dipole magnetic field strengths
are G; such strong dipole fields are in excess of those allowed from
the accretion induced collapse formation process except in spin-orbit
synchronization.Comment: 36 pages, AASLATEX, 4 ps figures, Ap
Statistical-Mechanical Measure of Stochastic Spiking Coherence in A Population of Inhibitory Subthreshold Neurons
By varying the noise intensity, we study stochastic spiking coherence (i.e.,
collective coherence between noise-induced neural spikings) in an inhibitory
population of subthreshold neurons (which cannot fire spontaneously without
noise). This stochastic spiking coherence may be well visualized in the raster
plot of neural spikes. For a coherent case, partially-occupied "stripes"
(composed of spikes and indicating collective coherence) are formed in the
raster plot. This partial occupation occurs due to "stochastic spike skipping"
which is well shown in the multi-peaked interspike interval histogram. The main
purpose of our work is to quantitatively measure the degree of stochastic
spiking coherence seen in the raster plot. We introduce a new spike-based
coherence measure by considering the occupation pattern and the pacing
pattern of spikes in the stripes. In particular, the pacing degree between
spikes is determined in a statistical-mechanical way by quantifying the average
contribution of (microscopic) individual spikes to the (macroscopic)
ensemble-averaged global potential. This "statistical-mechanical" measure
is in contrast to the conventional measures such as the "thermodynamic" order
parameter (which concerns the time-averaged fluctuations of the macroscopic
global potential), the "microscopic" correlation-based measure (based on the
cross-correlation between the microscopic individual potentials), and the
measures of precise spike timing (based on the peri-stimulus time histogram).
In terms of , we quantitatively characterize the stochastic spiking
coherence, and find that reflects the degree of collective spiking
coherence seen in the raster plot very well. Hence, the
"statistical-mechanical" spike-based measure may be used usefully to
quantify the degree of stochastic spiking coherence in a statistical-mechanical
way.Comment: 16 pages, 5 figures, to appear in the J. Comput. Neurosc
Pulse Shape Discrimination Techniques in Scintillating CsI(Tl) Crystals
There are recent interests with CsI(Tl) scintillating crystals for Dark
Matter experiments. The key merit is the capability to differentiate nuclear
recoil (nr) signatures from the background -events due to
ambient radioactivity on the basis of their different pulse shapes. One of the
major experimental challenges is to perform such pulse shape analysis in the
statistics-limited domain where the light output is close to the detection
threshold. Using data derived from measurements with low energy 's and
nuclear recoils due to neutron elastic scatterings, it was verified that the
pulse shapes between -events are different. Several methods of
pulse shape discrimination are studied, and their relative merits are compared.
Full digitization of the pulse shapes is crucial to achieve good
discrimination. Advanced software techniques with mean time, neural network and
likelihood ratios give rise to satisfactory performance, and are superior to
the conventional Double Charge method commonly applied at higher energies.
Pulse shape discrimination becomes effective starting at a light yield of about
20 photo-electrons. This corresponds to a detection threshold of about 5 keV
electron-equivalence energy, or 4050 keV recoil kinetic energy, in realistic
experiments.Comment: 20 pages, 7 figure
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