33,293 research outputs found
Fabrication process development of SiC/superalloy composite sheet for exhaust system components
A chemical compatibility study was conducted between SiC filament and the following P/M matrix alloys: Waspaloy, Hastelloy-X, NiCrAlY, Ha-188, S-57, FeCrAlY, and Incoloy 800. None of the couples demonstrated sufficient chemical compatibility to withstand the minimum HIP consolidation temperatures (996 C) or intended application temperature of the composite (982 C). However, Waspaloy, Haynes 188, and Hastelloy-X were the least reactive with SiC of the candidate alloys. Chemical vapor deposited tungsten was shown to be an effective diffusion barrier between the superalloy matrix and SiC filament providing a defect-free coating of sufficient thickness. However, the coating breaks down when the tungsten is converted into intermetallic compounds by interdiffusion with matrix constituents. Waspaloy was demonstrated to be the most effective matrix alloy candidate in contact with the CVD tungsten barrier because of its relatively low growth rate constant of the intermediate compound and the lack of formation of Kirkendall voids at the matrix-barrier interface. Fabrication methods were developed for producing panels of uniaxial and angle ply composites utilizing CVD tungsten coated filament
Biased random satisfiability problems: From easy to hard instances
In this paper we study biased random K-SAT problems in which each logical
variable is negated with probability . This generalization provides us a
crossover from easy to hard problems and would help us in a better
understanding of the typical complexity of random K-SAT problems. The exact
solution of 1-SAT case is given. The critical point of K-SAT problems and
results of replica method are derived in the replica symmetry framework. It is
found that in this approximation for .
Solving numerically the survey propagation equations for K=3 we find that for
there is no replica symmetry breaking and still the SAT-UNSAT
transition is discontinuous.Comment: 17 pages, 8 figure
Spin-orbit coupled j=1/2 iridium moments on the geometrically frustrated fcc lattice
Motivated by experiments on the double perovskites La2ZnIrO6 and La2MgIrO6,
we study the magnetism of spin-orbit coupled j=1/2 iridium moments on the
three-dimensional, geometrically frustrated, face-centered cubic lattice. The
symmetry-allowed nearest-neighbor interaction includes Heisenberg, Kitaev, and
symmetric off-diagonal exchange. A Luttinger-Tisza analysis shows a rich
variety of orders, including collinear A-type antiferromagnetism, stripe order
with moments along the [111]-direction, and incommensurate non-coplanar
spirals, and we use Monte Carlo simulations to determine their magnetic
ordering temperatures. We argue that existing thermodynamic data on these
iridates underscores the presence of a dominant Kitaev exchange, and also
suggest a resolution to the puzzle of why La2ZnIrO6 exhibits `weak'
ferromagnetism, but La2MgIrO6 does not.Comment: 5 pages, 5 figs, significantly revised to address referee comments,
to appear in PRB Rapid Com
The Thermal Evolution of Ices in the Environments of Newly Formed Stars: The CO_2 Diagnostic
Archival data from the Infrared Spectrometer of the Spitzer Space Telescope are used to study the 15 ÎŒm absorption feature of solid CO_2 toward 28 young stellar objects (YSOs) of approximately solar mass. Fits to the absorption profile using laboratory spectra enable categorization according to the degree of thermal processing of the ice matrix that contains the CO_2. The majority of YSOs in our sample (20 out of 28) are found to be consistent with a combination of polar (H_2O-rich) and nonpolar (CO-rich) ices at low temperature; the remainder exhibit profile structure consistent with partial crystallization as the result of significant heating. Ice-phase column densities of CO_2 are determined and compared with those of other species. Lines of sight with crystallization signatures in their spectra are found to be systematically deficient in solid-phase CO, as expected if CO is being sublimated in regions where the ices are heated to crystallization temperatures. Significant variation is found in the CO2 abundance with respect to both H_2O (the dominant ice constituent) and total dust column (quantified by the extinction, AV ). YSOs in our sample display typically higher CO_2 concentrations (independent of evidence for thermal processing) in comparison to quiescent regions of the prototypical cold molecular cloud. This suggests that enhanced CO_2 production is driven by photochemical reactions in proximity to some YSOs, and that photoprocessing and thermal processing may occur independently
Immune compromise in HIV-1/HTLV-1 coinfection with paradoxical resolution of CD4 lymphocytosis during antiretroviral therapy: a case report
Human immunodeficiency virus type-1 (HIV-1) and human T lymphotropic virus type-1 (HTLV-1) infections have complex effects on adaptive immunity, with specific tropism for, but contrasting effects on, CD4 T lymphocytes: depletion with HIV-1, proliferation with HTLV-1. Impaired T lymphocyte function occurs early in HIV-1 infection but opportunistic infections (OIs) rarely occur in the absence of CD4 lymphopenia. In the unusual case where a HIV-1 infected individual with a high CD4 count presents with recurrent OIs, a clinician is faced with the possibility of a second underlying comorbidity. We present a case of pseudo-adult T cell leukemia/lymphoma (ATLL) in HIV-1/HTLV-1 coinfection where the individual fulfilled Shimoyama criteria for chronic ATLL and had pulmonary Mycobacterium kansasii, despite a high CD4 lymphocyte count. However, there was no evidence of clonal T-cell proliferation by T-cell receptor gene rearrangement studies nor of monoclonal HTLV-1 integration by high-throughput sequencing. Mutually beneficial interplay between HIV-1 and HTLV-1, maintaining high level HIV-1 and HTLV-1 viremia and proliferation of poorly functional CD4 cells despite chronicity of infection is a postulated mechanism. Despite good microbiological response to antimycobacterial therapy, the patient remained systemically unwell with refractory anemia. Subsequent initiation of combined antiretroviral therapy led to paradoxical resolution of CD4 T lymphocytosis as well as HIV-1 viral suppression and decreased HTLV-1 proviral load. This is proposed to be the result of attenuation of immune activation post-HIV virological control. This case illustrates the importance of screening for HTLV-1 in HIV-1 patients with appropriate clinical presentation and epidemiological risk factors and explores mechanisms for the complex interactions on HIV-1/HTLV-1 adaptive immunity
Fragmentation of Nuclei at Intermediate and High Energies in Modified Cascade Model
The process of nuclear multifragmentation has been implemented, together with
evaporation and fission channels of the disintegration of excited remnants in
nucleus-nucleus collisions using percolation theory and the intranuclear
cascade model. Colliding nuclei are treated as face--centered--cubic lattices
with nucleons occupying the nodes of the lattice. The site--bond percolation
model is used. The code can be applied for calculation of the fragmentation of
nuclei in spallation and multifragmentation reactions.Comment: 19 pages, 10 figure
Simplifying Random Satisfiability Problem by Removing Frustrating Interactions
How can we remove some interactions in a constraint satisfaction problem
(CSP) such that it still remains satisfiable? In this paper we study a modified
survey propagation algorithm that enables us to address this question for a
prototypical CSP, i.e. random K-satisfiability problem. The average number of
removed interactions is controlled by a tuning parameter in the algorithm. If
the original problem is satisfiable then we are able to construct satisfiable
subproblems ranging from the original one to a minimal one with minimum
possible number of interactions. The minimal satisfiable subproblems will
provide directly the solutions of the original problem.Comment: 21 pages, 16 figure
Detection Techniques of Microsecond Gamma-Ray Bursts using Ground-Based Telescopes
Gamma-ray observations above 200 MeV are conventionally made by
satellite-based detectors. The EGRET detector on the Compton Gamma Ray
Observatory (CGRO) has provided good sensitivity for the detection of bursts
lasting for more than 200 ms. Theoretical predictions of high-energy gamma-ray
bursts produced by quantum-mechanical decay of primordial black holes (Hawking
1971) suggest the emission of bursts on shorter time scales. The final stage of
a primordial black hole results in a burst of gamma-rays, peaking around 250
MeV and lasting for a tenth of a microsecond or longer depending on particle
physics. In this work we show that there is an observational window using
ground-based imaging Cherenkov detectors to measure gamma-ray burst emission at
energies E greater than 200 MeV. This technique, with a sensitivity for bursts
lasting nanoseconds to several microseconds, is based on the detection of
multi-photon-initiated air showers.Comment: accepted for publication in the Astrophysical Journa
A method to quantitatively evaluate Hamaker constant using the jump-into-contact effect in Atomic Force microscopy
We find that the jump-into-contact of the cantilever in the atomic force
microscope (AFM) is caused by an inherent instability in the motion of the AFM
cantilever. The analysis is based on a simple model of the cantilever moving in
a nonlinear force field. We show that the jump-into-contact distance can be
used to find the interaction of the cantilever tip with the surface. In the
specific context of the attractive van der Waals interaction, this method can
be realized as a new method of measuring the Hamaker constant for materials.
The Hamaker constant is determined from the deflection of the cantilever at the
jump-into-contact using the force constant of the cantilever and the tip radius
of curvature, all of which can be obtained by measurements. The results have
been verified experimentally on a sample of cleaved mica, a sample of Si wafer
with natural oxide and a silver film, using a number of cantilevers with
different spring constants. We emphasize that the method described here is
applicable only to surfaces that have van der Waals interaction as the
tip-sample interaction. We also find that the tip to sample separation at the
jump-into-contact is simply related to the cantilever deflection at this point,
and this provides a method to exactly locate the surface.Comment: 11 pages, 4 figures, 1 tabl
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