6,488 research outputs found
An analyses of CD-ROM technology for spacelink
This study first examined the possibility of placing NASA CD's in the Spacelink Public Electronic Library (SPEL). The goal of this task was to place these materials on the Internet. The second part of the study addressed the possibility of using CD's as storage for portions of SPEL. These files would also be on the Internet and available to the users of Spacelink
The growth of small corrosion fatigue cracks in alloy 2024
The corrosion fatigue crack growth characteristics of small surface and corner cracks in aluminum alloy 2024 is established. The damaging effect of salt water on the early stages of small crack growth is characterized by crack initiation at constituent particle pits, intergranular microcracking for a less than 100 micrometers, and transgranular small crack growth for a micrometer. In aqueous 1 percent NaCl and at a constant anodic potential of -700 mV(sub SCE), small cracks exhibit a factor of three increase in fatigue crack growth rates compared to laboratory air. Small cracks exhibit accelerated corrosion fatigue crack growth rates at low levels of delta-K (less than 1 MPa square root of m) below long crack delta-K (sub th). When exposed to Paris regime levels of crack tip stress intensity, small corrosion fatigue cracks exhibit growth rates similar to that observed for long cracks. Results suggest that crack closure effects influence the corrosion fatigue crack growth rates of small cracks (a less than or equal to 100 micrometers). This is evidenced by similar small and long crack growth behavior at various levels of R. Contrary to the corrosion fatigue characteristics of small cracks in high strength steels, no pronounced chemical crack length effect is observed for Al by 2024 exposed to salt water
Optimization of aircraft interior panels
Eight different graphite composite panels were fabricated using four different resin matrices. The resin matrices included Hercules 71775, a blend of vinylpolystyrpyridine and bismaleimide, H795, a bismaleimide, Cycom 6162, a phenolic, and PSP 6022M, a polystyrylpyridine. Graphite panels were fabricated using fabric or unidirectional tape. This report describes the processes for preparing these panels and some of their mechanical, thermal and flammability properties. Panel properties are compared with state-of-the-art epoxy fiberglass composite panels
Contractions of Degenerate Quadratic Algebras, Abstract and Geometric
Quadratic algebras are generalizations of Lie algebras which include the
symmetry algebras of 2nd order superintegrable systems in 2 dimensions as
special cases. The superintegrable systems are exactly solvable physical
systems in classical and quantum mechanics. Distinct superintegrable systems
and their quadratic algebras can be related by geometric contractions, induced
by B\^ocher contractions of the conformal Lie algebra to itself. In 2 dimensions there are two kinds of quadratic algebras,
nondegenerate and degenerate. In the geometric case these correspond to 3
parameter and 1 parameter potentials, respectively. In a previous paper we
classified all abstract parameter-free nondegenerate quadratic algebras in
terms of canonical forms and determined which of these can be realized as
quadratic algebras of 2D nondegenerate superintegrable systems on constant
curvature spaces and Darboux spaces, and studied the relationship between
B\^ocher contractions of these systems and abstract contractions of the free
quadratic algebras. Here we carry out an analogous study of abstract
parameter-free degenerate quadratic algebras and their possible geometric
realizations. We show that the only free degenerate quadratic algebras that can
be constructed in phase space are those that arise from superintegrability. We
classify all B\^ocher contractions relating degenerate superintegrable systems
and, separately, all abstract contractions relating free degenerate quadratic
algebras. We point out the few exceptions where abstract contractions cannot be
realized by the geometric B\^ocher contractions
Exciton Trapping Is Responsible for the Long Apparent Lifetime in Acid-Treated MoS2
Here, we show that deep trapped "dark" exciton states are responsible for the
surprisingly long lifetime of band-edge photoluminescence in acid-treated
single-layer MoS2. Temperature-dependent transient photoluminescence
spectroscopy reveals an exponential tail of long-lived states extending
hundreds of meV into the band gap. These sub-band states, which are
characterized by a 4 microsecond radiative lifetime, quickly capture and store
photogenerated excitons before subsequent thermalization up to the band edge
where fast radiative recombination occurs. By intentionally saturating these
trap states, we are able to measure the "true" 150 ps radiative lifetime of the
band-edge exciton at 77 K, which extrapolates to ~600 ps at room temperature.
These experiments reveal the dominant role of dark exciton states in
acid-treated MoS2, and suggest that excitons spend > 95% of their lifetime at
room temperature in trap states below the band edge. We hypothesize that these
states are associated with native structural defects, which are not introduced
by the superacid treatment; rather, the superacid treatment dramatically
reduces non-radiative recombination through these states, extending the exciton
lifetime and increasing the likelihood of eventual radiative recombination
Microscopic dynamics of charge separation at the aqueous electrochemical interface
We have used molecular simulation and methods of importance sampling to study
the thermodynamics and kinetics of ionic charge separation at a liquid
water-metal interface. We have considered this process using canonical examples
of two different classes of ions: a simple alkali-halide pair, NaI, or
classical ions, and the products of water autoionization, HOOH, or
water ions. We find that for both ion classes, the microscopic mechanism of
charge separation, including water's collective role in the process, is
conserved between the bulk liquid and the electrode interface. Despite this,
the thermodynamic and kinetic details of the process differ between these two
environments in a way that depends on ion type. In the case of the classical
ion pairs, a higher free energy barrier to charge separation and a smaller flux
over that barrier at the interface, results in a rate of dissociation that is
40x slower relative to the bulk. For water ions, a slightly higher free energy
barrier is offset by a higher flux over the barrier from longer lived hydrogen
bonding patters at the interface, resulting in a rate of association that is
similar both at and away from the interface. We find that these differences in
rates and stabilities of charge separation are due to the altered ability of
water to solvate and reorganize in the vicinity of the metal interface.Comment: 6 pages, 3 figures + S
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