15,251 research outputs found
Management of Science
NASA concept, philosophy, and approach to science managemen
Stress State Required for Pyramidal Dislocation Movement in Zinc
A tension or compression stress in such a direction that basal slip is minimized can produce second-order pyramidal slip bands in zinc single crystals. The stress required to initiate pyramidal dislocation motion is not sensitive to temperature. However, dislocation velocity at a given stress is sensitive to temperature and the very small dislocation velocity at low temperatures has lead to an erroneous estimate of a ``starting stress'' for pyramidal dislocations. Dislocation velocity at a constant temperature was found to be a function of the magnitude, but not the sense of the resolved shear stress
Twinning and Slip in Zinc by Indentation
Observations of twinning and slip deformation caused by indentation of zinc reveal that extensive slip on the basal and second-order pyramidal systems takes place at loads up to 5 kg. Prismatic punching through 1-cm crystals is observed at indentation loads in excess of about 2.5 kg. It is concluded that the stress at the tip of the twins cannot be obtained by use of an elastic stress analysis
Theoretical and material studies on thin-film electroluminescent devices
The effect of surface nucleation processes on the quality of ZnS layers grown on (001) GaAs substrates by molecular beam epitaxy is reported. Reflection high energy electron diffraction indicated that nucleation at high temperatures produced more planar surfaces than nucleation at low temperatures, but the crystalline quality as accessed by x ray double crystal diffractometry is relatively independent of nucleation temperature. A critical factor in layer quality was the initial roughness of the GaAs surfaces
Learning a Hierarchical Latent-Variable Model of 3D Shapes
We propose the Variational Shape Learner (VSL), a generative model that
learns the underlying structure of voxelized 3D shapes in an unsupervised
fashion. Through the use of skip-connections, our model can successfully learn
and infer a latent, hierarchical representation of objects. Furthermore,
realistic 3D objects can be easily generated by sampling the VSL's latent
probabilistic manifold. We show that our generative model can be trained
end-to-end from 2D images to perform single image 3D model retrieval.
Experiments show, both quantitatively and qualitatively, the improved
generalization of our proposed model over a range of tasks, performing better
or comparable to various state-of-the-art alternatives.Comment: Accepted as oral presentation at International Conference on 3D
Vision (3DV), 201
Non-radial Oscillation Modes of Compact Stars with a Crust
Oscillation modes of isolated compact stars can, in principle, be a
fingerprint of the equation of state (EoS) of dense matter. We study the
non-radial high-frequency l=2 spheroidal modes of neutron stars and strange
quark stars, adopting a two-component model (core and crust) for these two
types of stars. Using perturbed fluid equations in the relativistic Cowling
approximation, we explore the effect of a strangelet or hadronic crust on the
oscillation modes of strange stars. The results differ from the case of neutron
stars with a crust. In comparison to fluid-only configurations, we find that a
solid crust on top of a neutron star increases the p-mode frequency slightly
with little effect on the f-mode frequency, whereas for strange stars, a
strangelet crust on top of a quark core significantly increases the f-mode
frequency with little effect on the p-mode frequency.Comment: 10 pages, 6 figure
Viscometric and misting properties of polymer-modified fuel
Solutions of polyisobutylenes L160, L200, B200, and B230 in Jet-A were prepared at concentrations up to 3000 ppm. These polymers have molecular weights in the range 5 to 9 x 1,00,000 and have previously been shown to induce anti-misting properties in Jet-A. In connection with the pumpability of such solutions, especially at low temperatures, the shear viscosity, eta, of these solutions was measured at temperatures 25 C, 0 C, and -25 C. Concentration-dependence of eta was very similar for all four polymer solutes, the increase of eta(c) at 3000 ppm being roughly four-fold (relative to Jet-A) for the L-series and five-fold for the B-series. This behavior prevailed at all temperatures, and there was no evidence of phase separation or other chemical instability at -25 C at any concentration. In the more practical c-range for anti-misting applications, say within 1000 ppm, the increase of eta(c) was only twofold
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