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At least 1,369 records · Page 76
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NASA NTRS1981

Intraband optical conductivity sigma/omega,T/ of Cu, Ag, and Au - Contribution from electron-electron scattering

The frequency and temperature dependence of the intraband optical conductivity of the noble metals Cu, Ag and Au is measured and contributions of electron-electron scattering are assessed. Optical measurements were performed at temperatures of 77, 295 and 425 K to obtain values of the Drude electron scattering rate with a linear dependence on temperature which may be attributed to electron-phonon scattering, and a quadratic dependence on photon energy, which is suggestive of electron-electron scattering but is a factor of two to three times greater than would be expected. Comparison of the optical data with dc electrical and thermal resistivity data which also show behavior attributed to electron-electron scattering reveals discrepancies of up to an order of magnitude. Other possible mechanisms for the frequency dependence, including absorptance, electron-surface plasmon interactions, a two-carrier model, and a structure dependence are considered, and it is concluded that the frequency dependence in the Drude scattering rates of the noble metals is not yet quantitatively understood

Parkins, G. R.↗
NASA NTRS1983

Dimensional stability considerations for cryogenic metals

Work performed as part of an effort to identify, and where possible separate out, some of the factors that contribute to dimensional stability in cryogenic wind tunnel models is reported. Initial problems were encountered with two dimensional models made of 15-5 PH stainless steel, which warped significantly after being subjected to cryogenic testing in the 0.3 Meter Transonic Cryogenic Tunnel. Subsequently, an effort was undertaken to investigate the mechanisms that could cause model warpage during cryogenic testing. The two basic mechanisms that can lead to warpage are (1) metallurgical structural instability in which one phase transforms partially or fully into a second phase which has a different crystal structure and volume, and (2) deformation due to the creation, or relief, of unbalanced induced or residual stresses. In the case of the 15-5 PH airfoils, it is highly probable that metallurgical instability was responsible for most of the observed warpage. A particular specimen configuration was established for use in the systematic evaluation of the factors influencing warpage. Preliminary studies of a specimen made of VASCOMAX 200 suggest the possibility of manipulating the stresses in the surface layers by appropriate combinations of milling and grinding steps. This opens up the possibility of correcting or establishing the required surface profile of an airfoil.

Wigley, D. A.↗
NASA NTRS1992

The introduction of spurious models in a hole-coupled Fabry-Perot open resonator

A hemispherical open resonator has previously been used to make relative comparisons of the surface resistivity of metallic thin-film samples in the submillimeter wavelength region. This resonator is fed from a far-infrared laser via a small coupling hole in the center of the concave spherical mirror. The experimental arrangement, while desirable as a coupling geometry for monitoring weak emissions from the cavity, can lead to the introduction of spurious modes into the cavity. Sources of these modes are identified, and a simple alteration of the experimental apparatus to eliminate such modes is suggested.

Cook, Jerry D.↗
NASA NTRS1984

Secondary flow spanwise deviation model for the stators of NASA middle compressor stages

A model of the spanwise variation of deviation for stator blades is presented. Deviation is defined as the difference between the passage mean flow angle and the metal angle at the outlet of a blade element of an axial compressor stage. The variation of deviation is taken as the difference above or below that predicted by blade element, (i.e., two-dimensional) theory at any spanwise location. The variation of deviation is dependent upon the blade camber, solidity and inlet boundary layer thickness at the hub or tip end-wall, and the blade channel aspect ratio. If these parameters are known or can be calculated, the model provides a reasonable approximation of the spanwise variation of deviation for most compressor middle stage stators operating at subsonic inlet Mach numbers.

Roberts, W. B.↗
NASA NTRS1980

The core and the magnetic field of Uranus

The existence of a magnetic field on Uranus is suggested by the tentative observation of 0.5 MHz pulses from the planet which are analogous to the Jovian decametric radiation. Analysis of a recent three-layer model leads to the conclusion that the electrical conductivity, physical state, and thermal gradients are such that a thermally driven hydromagnetic dynamo is probably operating in the core. An investigation is conducted to see whether the considered structure is suitable for the generation of a field. It is found that the existence of a magnetic field on Uranus can be understood as being a natural result of gravitational differentiation in a liquid, metallic core. The presence of a heat source in the core implies that the central temperature is approximately 8400 K rather than 7000 K in the original model

Torbett, M.↗
NASA NTRS1971

Characteristic temperatures of the Mossbauer fraction and thermal-shift measurements in iron and iron salts.

Measurement of Mossbauer spectra in metallic iron, sodium nitroprusside, sodium ferrocyanide, and potassium ferrocyanide absorbers between 78 and 293 K. The temperature dependences of the Mossbauer fraction and the resonant velocity were fitted to Einstein and Debye lattice-vibration models. The characteristic temperatures of the models fitted to the Mossbauer fraction are consistently lower than those fitted to the resonant velocity showing the sensitivity of the Mossbauer fraction to low-frequency modes of vibration. The characteristic temperatures obtained from the resonant velocity are higher for the salts than for the metal, indicating the presence of higher-frequency modes in the salts. This interpretation is verified semiquantitatively by comparing the thermal-shift Debye temperatures of the salts to their infrared absorption frequencies. The Mossbauer fraction of potassium ferrocyanide shows a weaker temperature dependence than that expected for a harmonic solid, which suggests that potassium ferrocyanide is anharmonic in the temperature range studied.

Lafleur, L. D.↗
NASA NTRS2004

Chandra Observations and Models of the Mixed Morphology Supernova Remnant W44: Global Trends

We report on the Chandra observations of the archetypical mixed morphology (or thermal composite) supernova remnant, W44. As with other mixed morphology remnants, W44's projected center is bright in thermal X-rays. It has an obvious radio shell, but no discernable X-ray shell. In addition, X-ray bright knots dot W44's image. The spectral analysis of the Chandra data show that the remnant s hot, bright projected center is metal-rich and that the bright knots are regions of comparatively elevated elemental abundances. Neon is among the affected elements, suggesting that ejecta contributes to the abundance trends. Furthermore, some of the emitting iron atoms appear to be underionized with respect to the other ions, providing the first potential X-ray evidence for dust destruction in a supernova remnant. We use the Chandra data to test the following explanations for W44's X-ray bright center: 1.) entropy mixing due to bulk mixing or thermal conduction, 2.) evaporation of swept up clouds, and 3.) a metallicity gradient, possibly due to dust destruction and ejecta enrichment. In these tests, we assume that the remnant has evolved beyond the adiabatic evolutionary stage, which explains the X-ray dimness of the shell. The entropy mixed model spectrum was tested against the Chandra spectrum for the remnant's projected center and found to be a good match. The evaporating clouds model was constrained by the finding that the ionization parameters of the bright knots are similar to those of the surrounding regions. While both the entropy mixed and the evaporating clouds models are known to predict centrally bright X-ray morphologies, their predictions fall short of the observed brightness gradient. The resulting brightness gap can be largely filled in by emission from the extra metals in and near the remnant's projected center. The preponderance of evidence (including that drawn from other studies) suggests that W44's remarkable morphology can be attributed to dust destruction and ejecta enrichment within an entropy mixed, adiabatic phase supernova remnant. The Chandra data prompts a new question - by what astrophysical mechanisms are the metals distributed so inhomogeneously in the supernova remnant.

Shelton, R. L.↗
NASA NTRS2005

Physics-Based Spectra of Accretion Disks around Black Holes

The purpose of this grant was to begin the process of deriving the light output of accretion disks around black holes directly from the actual processes that inject heat into the accreting matter, rather than from guessed dependences of heating rate on physical parameters. At JHU, the effort has focussed so far on models of accretion onto "intermediate mass black holes", a possible class of black holes, examples of which may have recently been discovered in nearby galaxies. There, Krolik and his student (Yawei Hui) have computed stellar atmospheres for uniformly-heated disks around this class of black holes. Their models serve two purposes: they are the very first serious attempts to compute the spectrum from accreting black holes in this mass range; and a library of such models can be used later in this program as contrasts for those computed on the basis of real disk dynamics. The output from these local disk calculations has also been successfully coupled to a program that applies the appropriate relativistic transformations and computes photon trajectories in order to predict the spectrum received by observers located at different polar angles. The principal new result of these calculations is the discovery of potentially observable ionization edges of H-like C and O at frequencies near the peak in flux from these objects. Most of the grant money at UCSB was spent on supporting graduate student Shane Davis. In addition. some money was spent on supporting two other students: Ari Socrates (now a Hubble Fellow at Princeton), and Laura Melling. Davis spent the year constructing stellar atmosphere models of accretion disks appropriate for the high/soft (thermal) state of black hole X-ray binaries. As with AGN models published previously by our collaboration with NASA support. our models include a complete general relativistic treatment of both the disk structure and the propagation of photons from the disk to a distant observer. They also include all important continuum opacity sources, including Compton scattering and bound-free opacity from abundant metal species. The principal new result is that bound-free opacity is very significant in altering the continuum spectral shape, resulting for example in quite different "color correction factors" compared to those predicted previously. In addition, the models predict a relationship between luminosity and inner disk temperature that is, for the first time, in accord with that observed. The primary purpose of the grant was to incorporate more realistic accretion disk physics, learned largely from simulations, into such spectral models. The Davis et al. paper includes consideration of a vertical dissipation profile computed from radiation magneto-hydrodynamic simulations of MRI turbulence by N. J. Turner (2004). So long as the disk is effectively thick, such dissipation profiles do not affect the predicted spectrum significantly. (More work needs to be done on these simulations, however.) A potentially more serious issue is that MRI turbulence produces substantial inhomogeneities, as do photon bubble instabilities. These inhomogeneities can affect the spectra by enhancing the effects of absorption opacity over scattering opacity. We have done some preliminary Monte Carlo calculations to explore these effects.

Krolik, Julian H.↗
NASA NTRS2004

Interstellar Dust Models

A viable interstellar dust model - characterized by the composition, morphology, and size distribution of the dust grains and by the abundance of the different elements locked up in the dust - should fit all observational constraints arising primarily from the interactions of the dust with incident radiation or the ambient gas. As a minimum, these should include the average interstellar extinction, the infrared emission from the diffuse interstellar medium (ISM), and the observed interstellar abundances of the various refractory elements. The last constraint has been largely ignored, resulting in dust models that require more elements to be in the dust phase than available in the ISM. In this talk I will describe the most recent advances towards the construction of a comprehensive dust model made by Zubko, Dwek, and Arendt, who, for the first time, included the interstellar abundances as explicit constraints in the construction of interstellar dust models. The results showed the existence of many distinct models that satisfy the basic set of observational constraints, including bare spherical silicate and graphite particles, PAHs, as well as spherical composite particles containing silicate, organic refractories, water ice, and voids. Recently, a new interstellar dust constituent has emerged, consisting of metallic needles. These needles constitute a very small fraction of the interstellar dust abundance, and their existence is primarily manifested in the 4 to 8 micron wavelength region, where they dominate the interstellar extinction. Preliminary studies show that these models may be distinguished by their X-ray halos, which are produced primarily by small angle scattering off large dust particles along the line of sight to bright X-ray sources, and probe dust properties largely inaccessible at other wavelengths.

Dwek, Eli↗
NASA NTRS1974

A study of the diffusional behavior of a two-phase metal matrix composite exposed to a high temperature environment

The progress of diffusion-controlled filament-matrix interaction in a metal matrix composite where the filaments and matrix comprise a two-phase binary alloy system was studied by mathematically modeling compositional changes resulting from prolonged elevated temperature exposure. The analysis treats a finite, diffusion-controlled, two-phase moving-interface problem by means of a variable-grid finite-difference technique. The Ni-W system was selected as an example system. Modeling was carried out for the 1000 to 1200 C temperature range for unidirectional composites containing from 6 to 40 volume percent tungsten filaments in a Ni matrix. The results are displayed to show both the change in filament diameter and matrix composition as a function of exposure time. Compositional profiles produced between first and second nearest neighbor filaments were calculated by superposition of finite-difference solutions of the diffusion equations.

Tenney, D. R.↗
NASA NTRS1979

Galileo probe thermal protection: Entry heating environments and spallation experiments design

A valid procedure was developed for predicting wall heating and ablation rates about the probe forebody. Entropy layer effects on convective heating rate were analyzed and the computed results are given. A feasibility study to perform an experiment, the selection of a candidate test facility, and the definition of a test matrix are described. The material selection, fabrication, and evaluation of the metal containing carbon-carbon composites for use on the Galileo probe are summarized. The effect of various Jovian atmospheric models on entry heating environment is considered as well as the effect of the nonspherical shape of the planet on entry trajectory.

Balakrishnan, A.↗
NASA NTRS1978

A study of trends and techniques for space base electronics

A sputtering system was developed to deposit aluminum and aluminum alloys by the dc sputtering technique. This system is designed for a high level of cleanliness and for monitoring the deposition parameters during film preparation. This system is now ready for studying the deposition and annealing parameters upon double-level metal preparation. A technique recently applied for semiconductor analysis, the finite element method, was studied for use in the computer modeling of two dimensional MOS transistor structures. It was concluded that the method has not been sufficiently well developed for confident use at this time. An algorithm was developed for confident use at this time. An algorithm was developed for implementing a computer study which is based upon the finite difference method. The program which was developed was modified and used to calculate redistribution data for boron and phosphorous which had been predeposited by ion implantation with range and straggle conditions. Data were generated for 111 oriented SOS films with redistribution in N2, dry O2 and steam ambients.

Trotter, J. D.↗
NASA NTRS1982

The deep lunar electrical conductivity profile - Structural and thermal inferences

Simultaneous lunar surface and orbital magnetometer records are reexamined, to ascertain intervals which may be suitable for measuring lunar inductive response in the solar wind and terrestrial magnetosheath. Power spectral estimates of the response tangent to the lunar surface, defined in terms of transfer and gain functions, are obtained for the 0.0001-0.01 Hz frequency range. The maximum consistency of estimates from different time intervals is found when the initial analysis is limited to the tangential direction of maximum incident power, or that direction in which the ratio of signal to background noise is greatest. Spherically symmetric plasma confinement theory is used in the interpretation of transfer function data, by way of forward model calculations, under the assumption of continuous electrical conductivity increase with depth. Results are presented for internal electrical conductivity profile, metallic core radius, and selenotherm limits.

Hood, L. L.↗
NASA NTRS1987

Thermomechanical force application

The present work conducted in Summer 1987 continues investigations on Thermal Components for 1.8 K Space Cryogenics (Grant NAG 1-412 of 1986). The topics addressed are plug characterization efforts in a small pore size regime of sintered metal plugs, characterization in the nonlinear regime, temperature profiles in a heat supply unit for a fountain effect pump and modeling efforts.

Frederking, T. H. K.↗
NASA NTRS1990

Petrogenesis of mesosiderites. I - Origin of mafic lithologies and comparison with basaltic achondrites

New petrologic and trace element data on basaltic and gabbroic clasts in mesosiderites and basaltic achondrites, combined with existing petrologic and trace element data, have served as a basis for interpretation of the petrogenesis of mesosiderite clasts. Compared with the basaltic achondrites, the mesosiderite basaltic and gabbroic clasts contain more abundant modal tridymite and merrilite, and more commonly contain augite as a late magmatic phase. Their pyroxenes tend to be more MgO-rich, and have lower Fe/Mn ratios which are positively correlated with the Fe/Mg ratio. Some of the mesosiderite basaltic and gabbroic clasts contain xenocrystic plagioclase. The basaltic clasts commonly show superchondritic Eu/Sm ratios and slight LREE depletions. The cumulate gabbro clasts are extremely depleted in LIL, have very high Eu/Sm ratios and high Lu/Sm ratios. All of these features can be explained by a model in which the mafic lithologies of mesosiderites were formed by remelting of a mixed basalt-cumulate gabbro-metal source region near the parent body surface.

Mittlefehldt, David W.↗
NASA NTRS1990

Compositions of large metal nodules in mesosiderites - Links to iron meteorite group IIIAB and the origin of mesosiderite subgroups

The compositions of large metal nodules in 12 mesosiderites, ranging from 3 mm to 4 cm in minimum dimensions and from 10 to 200 mg/g in silicate content, were analyzed by neutron activation; also analyzed were two additional samples (Pennyweight Point and Murchison Downs) which were originally designated iron meteorites but were later classified as Mes nodules. It was found that mesosiderites subgroups classified on the basis of metal composition are virtually the same as those based on silicate composition. The general covariation of the silicate and the metal compositional characteristics suggests that these features were established in the same process or in linked processes. A mesosiderite formation model is proposed.

Hassanzadeh, Jamshid↗
NASA NTRS1991

Micromechanics for ceramic matrix composites

The fiber substructuring concepts and the micromechanics equations that are embedded in the Ceramic Matrix Composite Analyzer (CEMCAN) computer code are described as well as the code itself, its current features and capabilities, and some examples to demonstrate the code's versatility. The methodology is equally applicable to metal matrix and polymer matrix composites. The prediction of ply mechanical and thermal properties agree very well with the existing models in the Integrated Composite Analyzer and the Ceramic Matrix Composite Analyzer, lending credence to the fiber substructuring approach. Fiber substructuring can capture greater local detail than conventional unit-cell-based micromechanical theories. It offers promise in simulating complex aspects of micromechanics in ceramic matrix composites.

Murthy, P. L. N.↗