Coatings for directional eutectics

Coating compositions were evaluated for oxidation protection of directionally solidified composite alloy NiTaC-13. These coatings included three NiCrAlY compositions (30-5-1, 25-10-1 and 20-15-1), two FeCrAlY compositions (30-5-1 and 25-10-1), a CoCrAlY composition (25-10-1), and one duplex coating, Ni-35Cr + Al. Duplicate pin samples of each composition were evaluated using two cyclic furnace oxidation tests of 100 hours at 871 C and 500 hours at 1093 C. The two best coatings were Ni-20Cr-15Al-lY and Ni-35Cr + Al. The two preferred coatings were deposited on pins and were evaluated in detail in .05 Mach cyclic burner rig oxidation to 1093 C. The NiCrAlY coating was protective after 830 hours of cycling, while the duplex coating withstood 630 hours. Test bars were coated and cycled for up to 500 hours. Tensile tests indicated no effect of coatings on strength. In 871 C air stress rupture, a degradation was observed for coated relative to bare material. The cycled NiCrAlY coating offered excellent protection with properties superior to the bare cycled NiTaC-13 in 1093 C air stress rupture

Deducing radiation pressure on a submerged mirror from the Doppler shift

Radiation pressure on a flat mirror submerged in a transparent liquid, depends not only on the refractive index n of the liquid, but also on the phase angle psi_0 of the Fresnel reflection coefficient of the mirror, which could be anywhere between 0^{\circ} and 180^{\circ}. Depending on the value of psi_0, the momentum per incident photon picked up by the mirror covers the range between the Abraham and Minkowski values, i.e., the interval (2\hbarw_0/nc,2n\hbarw_0/c). Here \hbar is the reduced Planck constant, w_0 is the frequency of the incident photon, and c is the speed of light in vacuum. We argue that a simple experimental setup involving a dielectric slab of refractive index n, a vibrating mirror placed a short distance behind the slab, a collimated, monochromatic light beam illuminating the mirror through the slab, and an interferometer to measure the phase of the reflected beam, is all that is needed to deduce the precise magnitude of the radiation pressure on a submerged mirror. In the proposed experiment, the transparent slab plays the role of the submerging liquid (even though it remains detached from the mirror at all times), and the adjustable gap between the mirror and the slab simulates the variable phase-angle psi_0. The phase of the reflected beam, measured as a function of time during one oscillation period of the mirror, then provides the information needed to determine the gap-dependence of the reflected beam's Doppler shift and, consequently, the radiation pressure experienced by the mirror.Comment: 9 pages, 2 figures, 13 equation

Amplitude expansion of the binary phase field crystal model

Amplitude representations of a binary phase field crystal model are developed for a two dimensional triangular lattice and three dimensional BCC and FCC crystal structures. The relationship between these amplitude equations and the standard phase field models for binary alloy solidification with elasticity are derived, providing an explicit connection between phase field crystal and phase field models. Sample simulations of solute migration at grain boundaries, eutectic solidification and quantum dot formation on nano-membranes are also presented.Comment: 11 pages, 8 figure

Atomistic spin-model based on a new spin-cluster expansion technique: Application to the IrMn3/Co interface

In order to derive tensorial exchange interactions and local magnetic anisotropies in itinerant magnetic systems, an approach combining the Spin-Cluster Expansion with the Relativistic Disordered Local Moment scheme is introduced. The theoretical background and computational aspects of the method are described in detail. The exchange interactions and site resolved anisotropy contributions for the IrMn3/Co(111) interface, a prototype for an exchange bias system, are calculated including a large number of magnetic sites from both the antiferromagnet and ferromagnet. Our calculations reveal that the coupling between the two subsystems is fairly limited to the vicinity of the interface. The magnetic anisotropy of the interface system is discussed, including effects of the Dzyaloshinskii-Moriya interactions that appear due to symmetry breaking at the interface.Comment: 10 pages, 6 figure

Boundary conditions at spatial infinity for fields in Casimir calculations

The importance of imposing proper boundary conditions for fields at spatial infinity in the Casimir calculations is elucidated.Comment: 8 pages, 1 figure, submitted to the Proceedings of The Seventh Workshop QFEXT'05 (Barcelona, September 5-9, 2005

Radio Sources in the 2dF Galaxy Redshift Survey. I. Radio Source Populations

We present the first results from a study of the radio continuum properties of galaxies in the 2dF Galaxy Redshift Survey, based on thirty 2dF fields covering a total area of about 100 square degrees. About 1.5% of galaxies with b(J) < 19.4 mag are detected as radio continuum sources in the NRAO VLA Sky Survey (NVSS). Of these, roughly 40% are star-forming galaxies and 60% are active galaxies (mostly low-power radio galaxies and a few Seyferts). The combination of 2dFGRS and NVSS will eventually yield a homogeneous set of around 4000 radio-galaxy spectra, which will be a powerful tool for studying the distriibution and evolution of both AGN and starburst galaxies out to redshift z=0.3.Comment: 14 pages, 7 figures, accepted for publication in PAS

Electromagnetic Oscillations in a Driven Nonlinear Resonator: A New Description of Complex Nonlinear Dynamics

Many intriguing properties of driven nonlinear resonators, including the appearance of chaos, are very important for understanding the universal features of nonlinear dynamical systems and can have great practical significance. We consider a cylindrical cavity resonator driven by an alternating voltage and filled with a nonlinear nondispersive medium. It is assumed that the medium lacks a center of inversion and the dependence of the electric displacement on the electric field can be approximated by an exponential function. We show that the Maxwell equations are integrated exactly in this case and the field components in the cavity are represented in terms of implicit functions of special form. The driven electromagnetic oscillations in the cavity are found to display very interesting temporal behavior and their Fourier spectra contain singular continuous components. To the best of our knowledge, this is the first demonstration of the existence of a singular continuous (fractal) spectrum in an exactly integrable system.Comment: 5 pages, 3 figure

Distribution of the local density of states, reflection coefficient and Wigner delay time in absorbing ergodic systems at the point of chiral symmetry

Employing the chiral Unitary Ensemble of random matrices we calculate the probability distribution of the local density of states for zero-dimensional ("quantum chaotic") two-sublattice systems at the point of chiral symmetry E=0 and in the presence of uniform absorption. The obtained result can be used to find the distributions of the reflection coefficent and of the Wigner time delay for such systems.Comment: 4 pages, 3 figure

Fiscal year 1976 progress report on a feasibility study evaluating the use of surface penetrators for planetary exploration

The feasibility of employing penetrators for exploring Mars was examined. Eight areas of interest for key scientific experiments were identified. These include: seismic activity, imaging, geochemistry, water measurement, heatflow, meteorology, magnetometry, and biochemistry. In seven of the eight potential experiment categories this year's progress included: conceptual design, instrument fabrication, instrument performance evaluation, and shock loading of important components. Most of the components survived deceleration testing with negligible performance changes. Components intended to be placed inside the penetrator forebody were tested up to 3,500 g and components intended to be placed on the afterbody were tested up to 21,000 g. A field test program was conducted using tentative Mars penetrator mission constraints. Drop tests were performed at two selected terrestrial analog sites to determine the range of penetration depths for anticipated common Martian materials. Minimum penetration occurred in basalt at Amboy, California. Three full-scale penetrators penetrated 0.4 to 0.9 m into the basalt after passing through 0.3 to 0.5 m of alluvial overburden. Maximum penetration occurred in unconsolidated sediments at McCook, Nebraska. Two full-scale penetrators penetrated 2.5 to 8.5 m of sediment. Impact occurred in two kinds of sediment: loess and layered clay. Deceleration g loads of nominally 2,000 for the forebody and 20,000 for the afterbody did not present serious design problems for potential experiments. Penetrators have successfully impacted into terrestrial analogs of the probable extremes of potential Martian sites

Ethane steam reforming over a platinum/alumina catalyst: effect of sulphur poisoning

In this study we have examined the adsorption of hydrogen sulfide and methanethiol over platinum catalysts and examined the effect of these poisons on the steam reforming of ethane. Adsorption of hydrogen sulfide was measured at 293 and 873 K. At 873 K the adsorbed state of hydrogen sulfide in the presence of hydrogen was SH rather than S, even though the Pt:S ratio was unity. The effect of 11.2 ppm hydrogen sulfide or methanethiol on the steam reforming of ethane was studied at 873 K and 20 barg. Both poisons deactivated the catalyst over a number of hours, but methanethiol was found to be more deleterious, reducing the conversion by almost an order of magnitude, possibly due to the co-deposition of sulfur and carbon. Changes in the selectivity revealed that the effect of sulfur was not uniform on the reactions occurring, with the production of methane reduced proportionally more than the other products, due to the surface sensitivity of the hydrogenolysis and methanation reactions. The water-gas shift reaction was affected to a lesser extent. No regeneration was observed when hydrogen sulfide was removed from the feedstream in agreement with adsorption studies. A slight regeneration was observed when methanethiol was removed from the feed, but this was believed to be due to the removal of carbon rather than sulfur. The overall effect of sulfur poisoning was to reduce activity and enhance hydrogen selectivity