1,095 research outputs found
Thermal stress response of General Purpose Heat Source (GPHS) aeroshell material
A thermal stress test was conducted to determine the ability of the GPHS aeroshell 3 D FWPF material to maintain physical integrity when exposed to a severe heat flux such as would occur from prompt reentry of GPHS modules. The test was performed in the Giant Planetary Facility at NASA's Ames Research Center. Good agreement was obtained between the theoretical and experimental results for both temperature and strain time histories. No physical damage was observed in the test specimen. These results provide initial corroboration both of the analysis techniques and that the GPHS reentry member will survive the reentry thermal stress levels expected
Photovoltage in curved 1D systems
Curvature of quantum wire results in intrasubband absorption of
IR radiation that induces stationary photovoltage in presence of circular
polarization. This effect is studied in ballistic (collisionless) and kinetic
regimes. The consideration is concentrated on quantum wires with curved central
part. It is shown, that if mean free path is shorter than length of the curved
part the photovoltage does not depend on the wire shape, but on the total angle
of rotation of wire tangent. It is not the case when mean free path is finite
or large. This situation was studied for three specific shapes of wires: "hard
angle", "open book" and "-like".Comment: 12 pages, 1 figur
Degradation and reuse of radiative thermal protection system materials for the space shuttle
Three silicide coated columbium alloys and two cobalt alloys were subjected to identical simulated reentry profiling exposures in both static (controlled vacuum leak) and dynamic (hypersonic plasma shear) environments. Primary emphasis in the columbium alloy evaluation was on the Cb752 and C129Y alloys with a lesser amount on FS85. Commercial silicide coatings of the R512E and VH109 formulations were used. The coated specimens were intentionally defected to provide the types of coating flaws that are expected in service. Temperatures were profiled up to peak temperatures of either 2350 F or 2500 F for 15 minutes in each cycle
Variability in high-mass X-ray binaries
Strongly magnetized, accreting neutron stars show periodic and aperiodic
variability over a wide range of time scales. By obtaining spectral and timing
information on these different time scales, we can have a closer look into the
physics of accretion close to the neutron star and the properties of the
accreted material. One of the most prominent time scales is the strong
pulsation, i.e., the rotation period of the neutron star itself. Over one
rotation, our view of the accretion column and the X-ray producing region
changes significantly. This allows us to sample different physical conditions
within the column but at the same time requires that we have
viewing-angle-resolved models to properly describe them. In wind-fed high-mass
X-ray binaries, the main source of aperiodic variability is the clumpy stellar
wind, which leads to changes in the accretion rate (i.e., luminosity) as well
as absorption column. This variability allows us to study the behavior of the
accretion column as a function of luminosity, as well as to investigate the
structure and physical properties of the wind, which we can compare to winds in
isolated stars.Comment: 6 pages, 4 figures, accepted for publication in Astronomische
Nachrichten (proceedings of the XMM-Newton Workshop 2019
Nucleation and growth mechanism of ferroelectric domain-wall motion
The motion of domain walls is critical to many applications involving ferroelectric materials, such as fast high-density non-volatile random access memory. In memories of this sort, storing a data bit means increasing the size of one polar region at the expense of another, and hence the movement of a domain wall separating these regions. Experimental measurements of domain growth rates in the well-established ferroelectrics PbTiO3 and BaTiO3 have been performed, but the development of new materials has been hampered by a lack of microscopic understanding of how domain walls move. Despite some success in interpreting domain-wall motion in terms of classical nucleation and growth models, these models were formulated without insight from first-principles-based calculations, and they portray a picture of a large, triangular nucleus that leads to unrealistically large depolarization and nucleation energies. Here we use atomistic molecular dynamics and coarse-grained Monte Carlo simulations to analyse these processes, and demonstrate that the prevailing models are incorrect. Our multi-scale simulations reproduce experimental domain growth rates in PbTiO3 and reveal small, square critical nuclei with a diffuse interface. A simple analytic model is also proposed, relating bulk polarization and gradient energies to wall nucleation and growth, and thus rationalizing all experimental rate measurements in PbTiO3 and BaTiO3
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