39,539 research outputs found
Influence of inversion on Mg mobility and electrochemistry in spinels
Magnesium oxide and sulfide spinels have recently attracted interest as
cathode and electrolyte materials for energy-dense Mg batteries, but their
observed electrochemical performance depends strongly on synthesis conditions.
Using first principles calculations and percolation theory, we explore the
extent to which spinel inversion influences Mg ionic mobility in
MgMnO as a prototypical cathode, and MgInS as a potential solid
electrolyte. We find that spinel inversion and the resulting changes of the
local cation ordering give rise to both increased and decreased Mg
migration barriers, along specific migration pathways, in the oxide as well as
the sulfide.
To quantify the impact of spinel inversion on macroscopic Mg
transport, we determine the percolation thresholds in both MgMnO and
MgInS. Furthermore, we analyze the impact of inversion on the
electrochemical properties of the MgMnO cathode via changes in the
phase behavior, average Mg insertion voltages and extractable capacities, at
varying degrees of inversion. Our results confirm that inversion is a major
performance limiting factor of Mg spinels and that synthesis techniques or
compositions that stabilize the well-ordered spinel structure are crucial for
the success of Mg spinels in multivalent batteries
Deformation-induced accelerated dynamics in polymer glasses
Molecular dynamics simulations are used to investigate the effects of
deformation on the segmental dynamics in an aging polymer glass. Individual
particle trajectories are decomposed into a series of discontinuous hops, from
which we obtain the full distribution of relaxation times and displacements
under three deformation protocols: step stress (creep), step strain, and
constant strain rate deformation. As in experiments, the dynamics can be
accelerated by several orders of magnitude during deformation, and the history
dependence is entirely erased during yield (mechanical rejuvenation). Aging can
be explained as a result of the long tails in the relaxation time distribution
of the glass, and similarly, mechanical rejuvenation is understood through the
observed narrowing of this distribution during yield. Although the relaxation
time distributions under deformation are highly protocol specific, in each case
they may be described by a universal acceleration factor that depends only on
the strain.Comment: 15 pages, 15 figure
Towards a System Theoretic Approach to Wireless Network Capacity in Finite Time and Space
In asymptotic regimes, both in time and space (network size), the derivation
of network capacity results is grossly simplified by brushing aside queueing
behavior in non-Jackson networks. This simplifying double-limit model, however,
lends itself to conservative numerical results in finite regimes. To properly
account for queueing behavior beyond a simple calculus based on average rates,
we advocate a system theoretic methodology for the capacity problem in finite
time and space regimes. This methodology also accounts for spatial correlations
arising in networks with CSMA/CA scheduling and it delivers rigorous
closed-form capacity results in terms of probability distributions. Unlike
numerous existing asymptotic results, subject to anecdotal practical concerns,
our transient one can be used in practical settings: for example, to compute
the time scales at which multi-hop routing is more advantageous than single-hop
routing
Influence Maximization Meets Efficiency and Effectiveness: A Hop-Based Approach
Influence Maximization is an extensively-studied problem that targets at
selecting a set of initial seed nodes in the Online Social Networks (OSNs) to
spread the influence as widely as possible. However, it remains an open
challenge to design fast and accurate algorithms to find solutions in
large-scale OSNs. Prior Monte-Carlo-simulation-based methods are slow and not
scalable, while other heuristic algorithms do not have any theoretical
guarantee and they have been shown to produce poor solutions for quite some
cases. In this paper, we propose hop-based algorithms that can easily scale to
millions of nodes and billions of edges. Unlike previous heuristics, our
proposed hop-based approaches can provide certain theoretical guarantees.
Experimental evaluations with real OSN datasets demonstrate the efficiency and
effectiveness of our algorithms.Comment: Extended version of the conference paper at ASONAM 2017, 11 page
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