3,996 research outputs found
Catalytic graphitization of three-dimensional wood-derived porous scaffolds
A catalytic technique to enhance graphite formation in nongraphitizing carbons was adapted to work with three-dimensional wood-derived scaffolds. Unlike many synthetic graphite precursors, wood and other cellulosic carbons remain largely disordered after high temperature pyrolysis. Using a nickel nitrate liquid catalyst and controlled pyrolysis conditions, wood-derived scaffolds were produced showing similar graphitic content to traditional pitch-based graphite while retaining the high-aspect ratio pores of the precursor wood microstructure. Graphite formation was studied as a function of processing time and pyrolysis temperature, and the resulting carbons were analyzed using x-ray diffraction, Raman spectroscopy, x-ray photoelectron spectroscopy, and electron microscopy techniques
On the Use of Approximation Methods for Microcrack Shielding Problems
There is experimental evidence that stress-induced microcracking near a macrocrack tip enhances the fracture toughness of brittle materials. In considering the interaction of the macrocrack with multiple microcracks using a discrete model, it is essential to use approximation methods in order to keep the amount of the computation to a tractable level. However, when crack distances are small, the results of the approximation methods can be significantly different from the numerical solution based upon the exact formulation. The results obtained by these approximation methods will be compared with the numerical solution to show the applicability ranges in which the errors are acceptably small. The use of results obtained by the approximation methods outside applicability ranges in literature is shown to lead to incorrect conclusions concerning microcrack shielding
Long Distance Coupling of a Quantum Mechanical Oscillator to the Internal States of an Atomic Ensemble
We propose and investigate a hybrid optomechanical system consisting of a
micro-mechanical oscillator coupled to the internal states of a distant
ensemble of atoms. The interaction between the systems is mediated by a light
field which allows to couple the two systems in a modular way over long
distances. Coupling to internal degrees of freedom of atoms opens up the
possibility to employ high-frequency mechanical resonators in the MHz to GHz
regime, such as optomechanical crystal structures, and to benefit from the rich
toolbox of quantum control over internal atomic states. Previous schemes
involving atomic motional states are rather limited in both of these aspects.
We derive a full quantum model for the effective coupling including the main
sources of decoherence. As an application we show that sympathetic ground-state
cooling and strong coupling between the two systems is possible.Comment: 14 pages, 5 figure
Synthetic crossed-lamellar microstructures in oxide ceramics
A process has been developed to produce a crossed-lamellar-type microstructure in mullite combining tape casting, oriented lamination, and templated grain growth. Ceramic laminates were produced with aligned rod-like grains with the alignment direction varying from layer-to-layer with abrupt interfaces between layers. Other designed microstructures are also possible using this process
Status of center dominance in various center gauges
We review arguments for center dominance in center gauges where vortex
locations are correctly identified. We introduce an appealing interpretation of
the maximal center gauge, discuss problems with Gribov copies, and a cure to
the problems through the direct Laplacian center gauge. We study correlations
between direct and indirect Laplacian center gauges.Comment: Presented by S. Olejnik at the NATO Advanced Research Workshop
"Confinement, Topology, and other Non-Perturbative Aspects of QCD", Jan.
21-27, 2002, Stara Lesna, Slovakia. 10 pages, 3 figures (8 EPS files), uses
crckapb.st
Low Thermal Expansion Coatings for Carbon/Carbon Composites
Two classes of materials are considered for low expansion protective
coatings for carbon/carbon composites. They include composites containing
particles which undergo allotropic phase transformations accompanied by
negative volume changes with increasing temperature and anisotropic oxides
which demonstrate low expansion due to microcracking. Conditions for
failure of coatings by either cracking or spallation due to thermal
mismatch are evaluated
Remarks on the Gribov Problem in Direct Maximal Center Gauge
We review the equivalence of maximal center gauge fixing to the problem of
finding the best fit, to a given lattice gauge field, by a thin vortex
configuration. This fit is necessarily worst at the location of P-plaquettes.
We then compare the fits achieved in Gribov copies generated by (i)
over-relaxation; (ii) over-relaxation after Landau gauge preconditioning; and
(iii) simulated annealing. Simulated annealing yields the best fit if all links
on the lattice are included, but the situation changes if we consider only the
lattice volume exterior to P-plaquettes. In this exterior region, the fit is
best for Gribov copies generated by over-relaxation, and worst for Gribov
copies generated after Landau gauge preconditioning. The two fitting criteria
(including or not including the P-plaquettes) yield string tensions differing
by -34% to +20% respectively, relative to the full string tension. Our usual
procedure (``quenched minimization'') seems to be a compromise between these
criteria, and yields string tensions at an intermediate value close to the full
string tension.Comment: 14 pages, 6 figure
Crack stability in the fracture of cementitious materials
The aim of the present study is to investigate the stability of crack propagation in cementitious materials. Tests were conducted on bend specimens in three-point and four-point loading conditions. Three-point bend specimens showed stable crack growth for mortar, normal strength and high strength concrete specimens. Alternatively, four-point bend specimens showed catastrophic failure for mortar and quasi-catastrophic failure for normal strength and high strength concrete specimens. Results will be discussed in relation to brittleness number model and specific microstructural features including the interfacial transition zone between the cement paste and the aggregate and the attendant toughening mechanisms
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