9,102 research outputs found
High intensity 5 eV O-atom exposure facility for material degradation studies
An atomic oxygen exposure facility was developed for studies of material degradation. The goal of these studies is to provide design criteria and information for the manufacture of long life (20 to 30 years) construction materials for use in low Earth orbit. The studies that are being undertaken will provide: (1) absolute reaction cross sections for the engineering design problems, (2) formulations of reaction mechanisms for use in the selection of suitable existing materials and the design of new more resistant ones, and (3) the calibration of flight hardware (mass spectrometers, etc.) in order to directly relate experiments performed in low Earth orbit to ground based investigations. The facility consists of a CW laser sustained discharge source of O-atoms, an atomic beam formation and diagnostics system, a spinning rotor viscometer, and provision for using the system for calibration of actual flight instruments
A temperature-controlled device for volumetric measurements of Helium adsorption in porous media
We describe a set-up for studying adsorption of helium in silica aerogels,
where the adsorbed amount is easily and precisely controlled by varying the
temperature of a gas reservoir between 80 K and 180 K. We present validation
experiments and a first application to aerogels. This device is well adapted to
study hysteresis, relaxation, and metastable states in the adsorption and
desorption of fluids in porous media
Mass spectrometers and atomic oxygen
The likely role of atmospheric atomic oxygen in the recession of spacecraft surfaces and in the shuttle glow has revived interest in the accurate measurement of atomic oxygen densities in the upper atmosphere. The Air Force Geophysics Laboratory is supplying a quadrupole mass spectrometer for a materials interactions flight experiment being planned by the Johnson Space Center. The mass spectrometer will measure the flux of oxygen on test materials and will also identify the products of surface reactions. The instrument will be calibrated at a new facility for producing high energy beams of atomic oxygen at the Los Alamos National Laboratory. The plans for these calibration experiments are summarized
Phase Bubbles and Spatiotemporal Chaos in Granular Patterns
We use inelastic hard sphere molecular dynamics simulations and laboratory
experiments to study patterns in vertically oscillated granular layers. The
simulations and experiments reveal that {\em phase bubbles} spontaneously
nucleate in the patterns when the container acceleration amplitude exceeds a
critical value, about , where the pattern is approximately hexagonal,
oscillating at one-fourth the driving frequency (). A phase bubble is a
localized region that oscillates with a phase opposite (differing by ) to
that of the surrounding pattern; a localized phase shift is often called an
{\em arching} in studies of two-dimensional systems. The simulations show
that the formation of phase bubbles is triggered by undulation at the bottom of
the layer on a large length scale compared to the wavelength of the pattern.
Once formed, a phase bubble shrinks as if it had a surface tension, and
disappears in tens to hundreds of cycles. We find that there is an oscillatory
momentum transfer across a kink, and this shrinking is caused by a net
collisional momentum inward across the boundary enclosing the bubble. At
increasing acceleration amplitudes, the patterns evolve into randomly moving
labyrinthian kinks (spatiotemporal chaos). We observe in the simulations that
and subharmonic patterns emerge as primary instabilities, but that
they are unstable to the undulation of the layer. Our experiments confirm the
existence of transient and patterns.Comment: 6 pages, 12 figures, submitted to Phys. Rev. E on July 1st, 2001. for
better quality figures, visit http://chaos.ph.utexas.edu/research/moo
Continuum-type stability balloon in oscillated granular layers
The stability of convection rolls in a fluid heated from below is limited by
secondary instabilities, including the skew-varicose and crossroll
instabilities. We observe a stability boundary defined by the same
instabilities in stripe patterns in a vertically oscillated granular layer.
Molecular dynamics simulations show that the mechanism of the skew-varicose
instability in granular patterns is similar to that in convection. These
results suggest that pattern formation in granular media can be described by
continuum models analogous to those used in fluid systems.Comment: 4 pages, 6 ps figs, submitted to PR
Nonlinear Competition Between Small and Large Hexagonal Patterns
Recent experiments by Kudrolli, Pier and Gollub on surface waves,
parametrically excited by two-frequency forcing, show a transition from a small
hexagonal standing wave pattern to a triangular ``superlattice'' pattern. We
show that generically the hexagons and the superlattice wave patterns bifurcate
simultaneously from the flat surface state as the forcing amplitude is
increased, and that the experimentally-observed transition can be described by
considering a low-dimensional bifurcation problem. A number of predictions come
out of this general analysis.Comment: 4 pages, RevTex, revised, to appear in Phys. Rev. Let
EXAFS study of lead-free relaxor ferroelectric BaTi(1-x)Zr(x)O3 at the Zr K-edge
Extended X-ray absorption fine structure (EXAFS) experiments at the Zr K-edge
were carried out on perovskite relaxor ferroelectrics BaTi(1-x)Zr(x)O3 (BTZ) (x
= 0.25, 0.30, 0.35), and on BaZrO3 for comparison. Structural information up to
4.5 A around the Zr atoms is obtained, revealing that the local structure
differs notably from the average Pm-3m cubic structure deduced from X-ray
diffraction. In particular, our results show that the distance between Zr atoms
and their first oxygen neighbors is independent of the Zr substitution rate x
and equal to that measured in BaZrO3, while the X-ray cubic cell parameter
increases linearly with x. Furthermore, we show that the Zr atoms tend to
segregate in Zr-rich regions. We propose that the relaxor behavior in BTZ is
linked to random elastic fields generated by this particular chemical
arrangement, rather than to random electric fields as is the case in most
relaxors.Comment: 13 pages, 12 figures, 4 tables. Submitted to Phys. Rev.
Instabilities and disorder of the domain patterns in the systems with competing interactions
The dynamics of the domains is studied in a two-dimensional model of the
microphase separation of diblock copolymers in the vicinity of the transition.
A criterion for the validity of the mean field theory is derived. It is shown
that at certain temperatures the ordered hexagonal pattern becomes unstable
with respect to the two types of instabilities: the radially-nonsymmetric
distortions of the domains and the repumping of the order parameter between the
neighbors. Both these instabilities may lead to the transformation of the
regular hexagonal pattern into a disordered pattern.Comment: ReVTeX, 4 pages, 3 figures (postscript); submitted to Phys. Rev. Let
The rheological behavior of CO2 ice: application to glacial flow on Mars
Vast amounts of solid CO2 reside in topographic basins of the south polar layered deposits (SPLD) on Mars and exhibit morphological features indicative of glacial flow. Previous experimental studies showed that coarse-grained CO2 ice is 1–2 orders of magnitude weaker than water ice under Martian polar conditions. Here we present data from a series of deformation experiments on high-purity, fine-grained CO2 ice over a broader range of temperatures than previously explored (158–213 K). The experiments confirm previous observations of highly non-linear power-law creep at larger stresses, but also show a transition to a previously-unseen linear-viscous creep regime at lower stresses. We examine the viscosity of CO2 within the SPLD and predict that the CO2-rich layers may be stronger than previously thought. We also predict that CO2 ice flows much more readily than H2O ice on steep flanks of SPLD topographic basins, allowing the CO2 to pond as observed.National Aeronautics and Space Administration (NASA) NNH16ZDA001N-SS
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