8,767 research outputs found
Structurally optimized shells.
Shells, i.e., objects made of a thin layer of material following a surface,
are among the most common structures in use. They are highly efficient, in
terms of material required to maintain strength, but also prone to deformation
and failure. We introduce an efficient method for reinforcing shells, that is,
adding material to the shell to increase its resilience to external loads. Our
goal is to produce a reinforcement structure of minimal weight. It has been
demonstrated that optimal reinforcement structures may be qualitatively
different, depending on external loads and surface shape. In some cases, it
naturally consists of discrete protruding ribs; in other cases, a smooth shell
thickness variation allows to save more material.
Most previously proposed solutions, starting from classical Michell trusses,
are not able to handle a full range of shells (e.g., are restricted to
self-supporting structures) or are unable to reproduce this range of behaviors,
resulting in suboptimal structures.
We propose a new method that works for any input surface with any load
configurations, taking into account both in-plane (tensile/compression) and
out-of-plane (bending) forces. By using a more precise volume model, we are
capable of producing optimized structures with the full range of qualitative
behaviors. Our method includes new algorithms for determining the layout of
reinforcement structure elements, and an efficient algorithm to optimize their
shape, minimizing a non-linear non-convex functional at a fraction of the cost
and with better optimality compared to standard solvers.
We demonstrate the optimization results for a variety of shapes, and the
improvements it yields in the strength of 3D-printed objects
First-principles thermodynamic modeling of lanthanum chromate perovskites
Tendencies toward local atomic ordering in (A,A′)(B,B′)O_(3−δ) mixed composition perovskites are modeled to explore their influence on thermodynamic, transport, and electronic properties. In particular, dopants and defects within lanthanum chromate perovskites are studied under various simulated redox environments. (La_(1−x),Sr_x)(Cr_(1−y),Fe_y)O_(3−δ) (LSCF) and (La_(1−x),Sr_x)(Cr_(1−y),Ru_y)O_(3−δ) (LSCR) are modeled using a cluster expansion statistical thermodynamics method built upon a density functional theory database of structural energies. The cluster expansions are utilized in lattice Monte Carlo simulations to compute the ordering of Sr and Fe(Ru) dopant and oxygen vacancies (Vac). Reduction processes are modeled via the introduction of oxygen vacancies, effectively forcing excess electronic charge onto remaining atoms. LSCR shows increasingly extended Ru-Vac associates and short-range Ru-Ru and Ru-Vac interactions upon reduction; LSCF shows long-range Fe-Fe and Fe-Vac interaction ordering, inhibiting mobility. First principles density functional calculations suggest that Ru-Vac associates significantly decrease the activation energy of Ru-Cr swaps in reduced LSCR. These results are discussed in view of experimentally observed extrusion of metallic Ru from LSCR nanoparticles under reducing conditions at elevated temperature
Designing Volumetric Truss Structures
We present the first algorithm for designing volumetric Michell Trusses. Our
method uses a parametrization approach to generate trusses made of structural
elements aligned with the primary direction of an object's stress field. Such
trusses exhibit high strength-to-weight ratios. We demonstrate the structural
robustness of our designs via a posteriori physical simulation. We believe our
algorithm serves as an important complement to existing structural optimization
tools and as a novel standalone design tool itself
Recent advances in lightweight, filament-wound composite pressure vessel technology
A review of recent advances is presented for lightweight, high performance composite pressure vessel technology that covers the areas of design concepts, fabrication procedures, applications, and performance of vessels subjected to single cycle burst and cyclic fatigue loading. Filament wound fiber/epoxy composite vessels were made from S glass, graphite, and Kevlar 49 fibers and were equipped with both structural and nonstructural liners. Pressure vessels structural efficiencies were attained which represented weight savings, using different liners, of 40 to 60 percent over all titanium pressure vessels. Significant findings in each area are summarized
Ferromagnetic Ligand Holes in Cobalt Perovskite Electrocatalysts as Essential Factor for High Activity Towards Oxygen Evolution
The definition of the interplay between chemical composition, electro-magnetic configuration and
catalytic activity requires a rational study of the orbital physics behind active materials. Apart from
Coulomb forces, quantum spin exchange interactions (QSEI) are part of the potentials that differentiate
the activity of magnetic oxides, strongly correlated electrocatalysts, in electron transfer reactions.
Ferromagnetic (FM) cobalt oxides can show low overpotentials for the oxygen evolution reaction (OER)
and the La1XSrXCoO3d (0 r X r 1) family of perovskites is good ground to gain understanding of
the electronic interactions in strongly correlated catalysts. In this case, Sr-doping raises the OER activity
and the conductivity and increases FM spin moments. The efficiency of electrocatalysts based on
Earth-abundant 3d-transition metals correlates with the interrelated factors: mild-bonding energies,
the reduction of the electronic repulsions because of the QSEI in the open-shells, and enhanced
spin delocalization in FM ordering. The reason for the outstanding OER activity of SrCoO3d is the
accumulation of FM holes in the 3d–2p bonds, including the ligand orbitals, thus facilitating spinselected charge transport and production of triplet O2 moieties from the oxidation of diamagnetic
precursors. Spin-polarized oxygen atoms in the lattice can participate in O–O coupling and release of
O2 in a Mars–Van Krevelen mechanistic fashion. We show that the stabilizing FM QSEI decrease the
adsorption and activation energies during oxygen evolution and spin-dependent potentials are one of
the factors that govern the catalytic activity of magnetic compositions: spintro-catalysis
Tuning ion coordination preferences to enable selective permeation
Potassium (K-) channels catalyze K+ ion permeation across cellular membranes
while simultaneously discriminating their permeation over Na+ ions by more than
a factor of a thousand. Structural studies show bare K+ ions occupying the
narrowest channel regions in a state of high coordination by all 8 surrounding
oxygen ligands from the channel walls. As in most channels, the driving force
for selectivity occurs when one ion is preferentially stabilized or
destabilized by the channel compared to water. In the common view of mechanism,
made vivid by textbook graphics, the driving force for selectivity in K-
channels arises by a fit, whereby the channel induces K+ ions to leave water by
offering an environment like water for K+, in terms of both energy and local
structure. The implication that knowledge of local ion coordination in a liquid
environment translates to design parameters in a protein ion channel, producing
similar energetic stabilities, has gone unchallenged, presumably due in part to
lack of consensus regarding ion coordination structures in liquid water.
Growing evidence that smaller numbers and different arrangements of ligands
coordinate K+ ions in liquid water, however, raises new questions regarding
mechanism: how and why should ion coordination preferences change, and how does
that alter the current notions of ion selectivity? Our studies lead to a new
channelcentric paradigm for the mechanism of K+ ion channel selectivity.
Because the channel environment is not liquid-like, the channel necessarily
induces local structural changes in ion coordination preferences that enable
structural and energetic differentiation between ions.Comment: Main manuscript: 12 pages, 6 figures. Supplementary information: 10
pages, 7 figure
Kitaev interactions between j=1/2 moments in honeycomb Na2IrO3 are large and ferromagnetic: insights from ab initio quantum chemistry calculations
NaIrO, a honeycomb 5 oxide, has been recently identified as a
potential realization of the Kitaev spin lattice. The basic feature of this
spin model is that for each of the three metal-metal links emerging out of a
metal site, the Kitaev interaction connects only spin components perpendicular
to the plaquette defined by the magnetic ions and two bridging ligands. The
fact that reciprocally orthogonal spin components are coupled along the three
different links leads to strong frustration effects and nontrivial physics.
While the experiments indicate zigzag antiferromagnetic order in NaIrO,
the signs and relative strengths of the Kitaev and Heisenberg interactions are
still under debate. Herein we report results of ab initio many-body electronic
structure calculations and establish that the nearest-neighbor exchange is
strongly anisotropic with a dominant ferromagnetic Kitaev part, whereas the
Heisenberg contribution is significantly weaker and antiferromagnetic. The
calculations further reveal a strong sensitivity to tiny structural details
such as the bond angles. In addition to the large spin-orbit interactions, this
strong dependence on distortions of the IrO plaquettes singles out the
honeycomb 5 oxides as a new playground for the realization of
unconventional magnetic ground states and excitations in extended systems.Comment: 13 pages, 2 tables, 3 figures, accepted in NJ
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