95 research outputs found
Stochastic Generation of Particle Structures with Controlled Degree of Heterogeneity
The recently developed void expansion method (VEM) allows for an efficient
generation of porous packings of spherical particles over a wide range of
volume fractions. The method is based on a random placement of the structural
particles under addition of much smaller "void-particles" whose radii are
repeatedly increased during the void expansion. Thereby, they rearrange the
structural particles until formation of a dense particle packing and introduce
local heterogeneities in the structure. In this paper, microstructures with
volume fractions between 0.4 and 0.6 produced by VEM are analyzed with respect
to their degree of heterogeneity (DOH). In particular, the influence of the
void- to structural particle number ratio, which constitutes a principal
VEM-parameter, on the DOH is studied. The DOH is quantified using the pore size
distribution, the Voronoi volume distribution and the density-fluctuation
method in conjunction with fit functions or integral measures. This analysis
has revealed that for volume fractions between 0.4 and 0.55 the void-particle
number allows for a quasi-continuous adjustment of the DOH. Additionally, the
DOH-range of VEM-generated microstructures with a volume fraction of 0.4 is
compared to the range covered by microstructures generated using previous
Brownian dynamics simulations, which represent the structure of coagulated
colloidal suspensions. Both sets of microstructures cover similarly broad and
overlapping DOH-ranges, which allows concluding that VEM is an efficient method
to stochastically reproduce colloidal microstructures with varying DOH.Comment: 10 pages, 7 figure
Microstructures and Mechanical Properties of Dense Particle Gels: Microstructural Characterization
The macroscopic mechanical properties of densely packed coagulated colloidal
particle gels strongly depend on the local arrangement of the powder particles
on length scales of a few particle diameters. Heterogeneous microstructures
exhibit up to one order of magnitude higher elastic properties and yield
strengths than their homogeneous counterparts. The microstructures of these
gels are analyzed by the straight path method quantifying quasi-linear particle
arrangements of particles. They show similar characteristics than force chains
bearing the mechanical load in granular material. Applying this concept to gels
revealed that heterogeneous colloidal microstructures show a significantly
higher straight paths density and exhibit longer straight paths than their
homogeneous counterparts.Comment: 7 pages, 9 figure
Control of Diastereoselectivity in C=O/C=N Reductive Cyclizations Using an Intramolecularly Tethered Hydrazone
Cyclic hydrazones are efficient ketyl radical acceptors in reductive coupling cyclizations mediated by samarium diiodide, affording cyclic amino alcohols with controlled stereochemistry at the new aminated stereocenter. This approach has been successfully applied to the stereoselective synthesis of a fully functionalized trehazolin cyclitol starting from D-glucose, where the required cyclic hydrazone was directly obtained by partial hydrazynolysis of a 1,2-cyclic carbonate.Financial support by the Ministry of Science and Technology of Spain (project BQU2000-1501-C02-01) and Fundación Ramón Areces (predoctoral fellowship to A. G.) are gratefully acknowledged.Peer reviewe
Fe-resonant valence band photoemission and oxygen NEXAFS study on La1-xSrxFe0.75Ni0.25O3-{\delta}
Iron resonant valance band photoemission spectra of Sr substituted
LaFe0.75Ni0.25 O3-{\delta} have been recorded across the Fe 2p - 3d absorption
threshold to obtain Fe specific spectral information on the 3d projected
partial density of states. Comparison with La1-xSrxFeO3 resonant VB PES
literature data suggests that substitution of Fe by Ni forms electron holes
which are mainly O 2p character. Substitution of La by Sr increases the hole
concentration to an extent that the eg structure vanishes. The variation of the
eg and t2g structures is paralleled by the changes in the electrical
conductivity
Strong and ductile platelet-reinforced polymer films inspired by nature: Microstructure and mechanical properties
The unique structure and mechanical properties of platelet-reinforced biological materials such as bone and seashells have motivated the development of artificial composites exhibiting new, unusual mechanical behavior. On the basis of designing principles found in these biological structures, we combined high-performance artificial building blocks to fabricate platelet-reinforced polymer matrix composites that exhibit simultaneously high tensile strength and ductility. The mechanical properties are correlated with the underlying microstructure of the composites before and after mechanical loading using transmission electron microscopy. The critical role of the strength of the platelet-polymer interface and its dependence on the platelet surface chemistry and the type of matrix polymer are studied. Thin multilayered films with highly oriented platelets were produced through the bottom-up layer-by-layer assembly of submicrometer-thin alumina platelets and either polyimide or chitosan as polymer matrix. The tensile strength and strain at rupture of the prepared composites exceeded that of nacre, whereas the elastic modulus reached values similar to that of lamellar bones. In contrast to the brittle failure of clay-reinforced composites of similar or higher strength and stiffness, our composites exhibit plastic deformation in the range of 2-90% before failure. In addition to the high reinforcing efficiency and ductility achieved, several toughening mechanisms were identified in fractured composites, namely friction, debonding, and formation of microcracks at the platelet-polymer interface, as well as plastic deformation and void formation within the continuous polymeric phase. The combination of high strength, ductility, and toughness was achieved by selecting platelets that exhibit an aspect ratio high enough to carry significant load but small enough to allow for fracture under the platelet pull-out mode. At high concentrations of platelets, the ductility gets lost because of out-of-plane misalignment of the platelets and incorporation of voids in the microstructure during processing. The designing principles applied in this study can potentially be extended to other types of platelets and polymers to obtain new, hybrid materials with tunable mechanical propertie
An electron hole doping and soft x-ray spectroscopy study on La1-xSrxFe0.75Ni0.25O3-{\delta}
The conductivity of the electron hole and polaron conductor
La1-xSrxFe0.75Ni0.25O3-{\delta}, a potential cathode material for intermediate
temperature solid oxide fuel cells, was studied for 0 <x < 1 and for
temperatures 300 K <T < 1250 K. In LaSrFe-oxide, an ABO3 type perovskite,
A-site substitu-tion of the trivalent La3+ by the divalent Sr2+ causes
oxidation of Fe3+ towards Fe4+, which forms conducting electron holes. Here we
have in addition a B-site substitution by Ni. The compound for x = 0.5 is
identified as the one with the highest conductivity ({\sigma} ~ 678 S/cm) and
lowest activation energy for polaron conductivity (Ep = 39 meV). The evolution
of the electronic structure was monitored by soft x-ray Fe and oxygen K-edge
spectroscopy. Homogeneous trend for the oxida-tion state of the Fe was
observed. The variation of the ambient temperature conductivity and activation
energy with relative Sr content (x) shows a correlation with the ratio of
(eg/eg+t2g) in Fe L3 edge up to x=0.5. The hole doping process is reflected by
an almost linear trend by the variation of the pre-peaks of the oxygen K-edge
soft x-ray absorption spectra
Аудит пожарной безопасности организации в системе МВД в г. Северске
Планируется проведение проверки соответствия установленных требованиям пожарной безопасности,подготовки рекомендаций по устранению выявленных нарушений,а так же разработка и реализация плана-программы пожарного аудита для здания УМВД РФ по ТО в г. Северске.It is planned to carry out checking the conformity to the established requirements of fire safety, the preparation of recommendations on removing the revealed violations, as well as the development and implementation of programs to audit buildings fire UMVD RF THEN in Seversk
Yielding of weakly attractive nanoparticle networks
We propose a scaling law that explicitly correlates the macroscopic yield stress of weakly attractive nanoparticle networks with features of the microscopic aggregates that form the percolating gel. The proposed law correctly predicts experimental data obtained from a model colloidal system covering a wide range of particle sizes, volume fractions and interparticle attractive forces. The agreement between theory and experiments supports the idea that yielding is ultimately caused by the rupture of a few interparticle bonds within the aggregates of the mechanically loaded network. © 2011 The Royal Society of Chemistry
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