1,565 research outputs found
Effect of alloy chemistry and exposure conditions on the oxidation of titanium
Multiwall is a new thermal protection system concept for advanced space transportation vehicles. The system consists of discrete panels made up of multiple layers of foil gage metal. Titanium is the proposed candidate metal for multiwall panels in the reentry temperature range up to 675 C. Oxidation and embrittlement are the principal concerns related to the use of Ti in heat shield applications. The results of a broad study on the oxidation kinetics of several titanium alloys subjected to different exposure conditions are described. The alloys include commercially pure titanium, Ti-6Al-4V, and Ti-6Al-2Sn-4Zr-2Mo. Oxidation studies were performed on these alloys exposed at 704 C in 5-760 torr air pressure and 0 to 50% relative humidity. The resulting weight gains were correlated with oxide thickness and substrate contamination. The contamination depth and weight gains due to solid solutioning were obtained from microhardness depth profiles and hardness versus weight percent oxygen calibration data
Re-equilibration after quenches in athermal martensites:Conversion-delays for vapour to liquid domain-wall phases
Entropy barriers and ageing states appear in martensitic
structural-transition models, slowly re-equilibrating after temperature
quenches, under Monte Carlo dynamics. Concepts from protein folding and ageing
harmonic oscillators turn out to be useful in understanding these
nonequilibrium evolutions. We show how the athermal, non-activated delay time
for seeded parent-phase austenite to convert to product-phase martensite,
arises from an identified entropy barrier in Fourier space. In an ageing state
of low Monte Carlo acceptances, the strain structure factor makes
constant-energy searches for rare pathways, to enter a Brillouin zone `golf
hole' enclosing negative energy states, and to suddenly release entropically
trapped stresses. In this context, a stress-dependent effective temperature can
be defined, that re-equilibrates to the quenched bath temperature.Comment: 11 pages, 12 figures. Under process with Phys. Rev. B (2015
Microstructural Characterization of Aluminum-Lithium Alloys 1460 and 2195
Transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) techniques were employed to characterize the precipitate distributions in lithium-containing aluminum alloys 1460 and 2195 in the T8 condition. TEM examinations revealed delta prime and T1 as the primary strengthening precipitates in alloys 1460 and 2195 respectively. TEM results showed a close similarity of the Russian alloy 1460 to the U.S. alloy 2090, which has a similar composition and heat treatment schedule. DSC analyses also indicate a comparable delta prime volume fraction. TEM study of a fractured tensile sample of alloy 1460 showed that delta prime precipitates are sheared by dislocations during plastic deformation and that intense stress fields arise at grain boundaries due to planar slip. Differences in fracture toughness of alloys 1460 and 2195 are rationalized on the basis of a literature review and observations from the present study
A Catalog of MIPSGAL Disk and Ring Sources
We present a catalog of 416 extended, resolved, disk- and ring-like objects
as detected in the MIPSGAL 24 micron survey of the Galactic plane. This catalog
is the result of a search in the MIPSGAL image data for generally circularly
symmetric, extended "bubbles" without prior knowledge or expectation of their
physical nature. Most of the objects have no extended counterpart at 8 or 70
micron, with less than 20% detections at each wavelength. For the 54 objects
with central point sources, the sources are nearly always seen in all IRAC
bands. About 70 objects (16%) have been previously identified, with another 35
listed as IRAS sources. Among the identified objects, those with central
sources are mostly listed as emission-line stars, but with other source types
including supernova remnants, luminous blue variables, and planetary nebulae.
The 57 identified objects (of 362) without central sources are nearly all PNe
(~90%).which suggests that a large fraction of the 300+ unidentified objects in
this category are also PNe. These identifications suggest that this is
primarily a catalog of evolved stars. Also included in the catalog are two
filamentary objects that are almost certainly SNRs, and ten unusual compact
extended objects discovered in the search. Two of these show remarkable spiral
structure at both 8 and 24 micron. These are likely background galaxies
previously hidden by the intervening Galactic plane
Dynamic of a non homogeneously coarse grained system
To study materials phenomena simultaneously at various length scales,
descriptions in which matter can be coarse grained to arbitrary levels, are
necessary. Attempts to do this in the static regime (i.e. zero temperature)
have already been developed. In this letter, we present an approach that leads
to a dynamics for such coarse-grained models. This allows us to obtain
temperature-dependent and transport properties. Renormalization group theory is
used to create new local potentials model between nodes, within the
approximation of local thermodynamical equilibrium. Assuming that these
potentials give an averaged description of node dynamics, we calculate thermal
and mechanical properties. If this method can be sufficiently generalized it
may form the basis of a Molecular Dynamics method with time and spatial
coarse-graining.Comment: 4 pages, 4 figure
Coulomb Interactions and Nanoscale Electronic Inhomogeneities in Manganites
We address the issue of endemic electronic inhomogeneities in manganites
using extensive simulations on a new model with Coulomb interactions amongst
two electronic fluids, one localized (polaronic), the other extended
(band-like), and dopant ions. The long range Coulomb interactions frustrate
phase separation induced by the strong on site repulsion between the fluids. A
single quantum phase ensues which is intrinsically and strongly inhomogeneous
at a nano-scale, but homogeneous on meso-scales, with many characteristics
(including colossal responses)that agree with experiments. This, we argue, is
the origin of nanoscale inhomogeneities in manganites, rather than phase
competition and disorder related effects as often proposed.Comment: 4 pages, 3 figure
Tracing the energetics and evolution of dust with Spitzer : a chapter in the history of the Eagle Nebula
Context. The Spitzer GLIMPSE and MIPSGAL surveys have revealed a wealth of details about the Galactic plane in the infrared (IR)with orders of magnitude higher sensitivity, higher resolution, and wider coverage than previous IR observations. The structure of the interstellar medium (ISM) is tightly connected to the countless star-forming regions. We use these surveys to study the energetics and dust properties of the Eagle Nebula (M16), one of the best known star-forming regions.
Aims. We present MIPSGAL observations of M16 at 24 and 70 μm and combine them with previous IR data. The mid-IR image
shows a shell inside the well-known molecular borders of the nebula, as in the ISO and MSX observations from 15 to 21 μm. The morphologies at 24 and 70 μm are quite different, and its color ratio is unusually warm. The far-IR image resembles the one at 8 μm that enhances the structure of the molecular cloud and the "pillars of creation". We use this set of IR data to analyze the dust energetics and properties within this template for Galactic star-forming regions.
Methods. We measure IR spectral energy distributions (SEDs) across the entire nebula, both within the inner shell and the photodissociation regions (PDRs).We use the DUSTEM model to fit these SEDs and constrain the dust temperature, the dust-size distribution, and the radiation field intensity relative to that provided by the star cluster NGC 6611 (χ/χ0). Results. Within the PDRs, the inferred dust temperature (~35 K), the dust-size distribution, and the radiation field intensity (χ/χ0 < 1) are consistent with expectations. Within the inner shell, the dust is hotter (~70 K). Moreover, the radiation field required to fit the
SED is larger than that provided by NGC 6611 (χ/χ0 > 1). We quantify two solutions to this problem: (1) The size distribution of the dust in the shell is not that of interstellar dust. There is a significant enhancement of the carbon dust-mass in stochastically heated
very small grains. (2) The dust emission arises from a hot (~10^6 K) plasma where both UV and collisions with electrons contribute to the heating. Within this hypothesis, the shell SED may be fit for a plasma pressure p/k ~ 5 × 10^7 K cm^(−3).
Conclusions. We suggest two interpretations for the M16 inner shell: (1) The shell matter is supplied by photo-evaporative flows arising from dense gas exposed to ionized radiation. The flows renew the shell matter as it is pushed out by the pressure from stellar winds. Within this scenario, we conclude that massive-star forming regions such as M16 have a major impact on the carbon dustsize
distribution. The grinding of the carbon dust could result from shattering in grain-grain collisions within shocks driven by the dynamical interaction between the stellar winds and the shell. (2) We also consider a more speculative scenario where the shell is a supernova remnant. In this case, we would be witnessing a specific time in the evolution of the remnant where the plasma pressure and temperature would enable the remnant to cool through dust emission
Diluted Josephson-junction arrays in a magnetic field: phase coherence and vortex glass thresholds
The effects of random dilution of junctions on a two-dimensional
Josephson-junction array in a magnetic field are considered. For rational
values of the average flux quantum per plaquette , the superconducting
transition temperature vanishes, for increasing dilution, at a critical value
, while the vortex ordering remains stable up to , much
below the value corresponding to the geometric percolation threshold. For
, the array behaves as a zero-temperature vortex-glass.
Numerical results for from defect energy calculations are presented
which are consistent with this scenario.Comment: 4 pages, 4 figures, to appear in Phys. Rev.
A Survey of Nucleotide Cyclases in Actinobacteria: Unique Domain Organization and Expansion of the Class III Cyclase Family in Mycobacterium tuberculosis
Cyclic nucleotides are well-known second messengers involved in the regulation of
important metabolic pathways or virulence factors. There are six different classes
of nucleotide cyclases that can accomplish the task of generating cAMP, and four
of these are restricted to the prokaryotes. The role of cAMP has been implicated in
the virulence and regulation of secondary metabolites in the phylum Actinobacteria, which contains
important pathogens, such as Mycobacterium tuberculosis, M. leprae, M. bovis
and Corynebacterium, and industrial organisms from the genus Streptomyces.
We have analysed the actinobacterial genome sequences found in current databases
for the presence of different classes of nucleotide cyclases, and find that only class
III cyclases are present in these organisms. Importantly, prominent members such as
M. tuberculosis and M. leprae have 17 and 4 class III cyclases, respectively, encoded
in their genomes, some of which display interesting domain fusions seen for the
first time. In addition, a pseudogene corresponding to a cyclase from M. avium has
been identified as the only cyclase pseudogene in M. tuberculosis and M. bovis. The
Corynebacterium and Streptomyces genomes encode only a single adenylyl cyclase
each, both of which have corresponding orthologues in M. tuberculosis. A clustering
of the cyclase domains in Actinobacteria reveals the presence of typical eukaryote-like,
fungi-like and other bacteria-like class III cyclase sequences within this phylum,
suggesting that these proteins may have significant roles to play in this important
group of organisms
Performance Evaluation of wide Bandwidth RF Signal Generator Chip
The work in this paper is to give an overview of the compact wide band RF signal generator board design, emphasizing on the analyses and evaluation of the performance characteristics corresponding to the output signal purity and stability. The paper describes the design aspects involved in developing a reliable RF generating source which includes details regarding the factors that have taken care for optimum output power, spectral purity and noise performance. The simulation results obtained from the tool given by Maxim integrated are used as reference to evaluate the actual board when it is realised. These results are shown here for reference. Design aspects such as the power supply, noise filtering, loop filter component selection board layout consideration along with easy and compact form factor is considered. The board contains not only the signal generator device but also an FPGA from Xilinx to control the device, to make the board more useful for future applications; the board also has an SDRAM and an USB controller. This paper mainly concentrates on to MAX2870 signal generator and simulation results obtained by EE-Sim tool. Since the actual board is still in the process of being developed, the comparison of the actual performance to the simulation performance may not be possible at this point of time but definitely is in pipeline
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