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At least 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Fault structures in rapidly quenched Ni-Mo binary alloys

Fault structures in two Ni-Mo alloy ribbons (Ni-28 at. pct Mo and Ni-35 at. pct Mo) cast by a free jet chill block melt spinning process were studied. Thin foils for TEM studies were made by electrochemical thinning using an alcohol/butyl cellosolve/perchloric acid mixture in a twin jet electropolishing device. The samples displayed typical grains containing linear faulted regions on the wheelside of the two alloy ribbons. However, an anomalous diffraction behavior was observed upon continuous tilting of the sample: the network of diffraction spots from a single grain appeared to expand or contract and rotate. This anomalous diffraction behavior was explained by assuming extended spike formation at reciprocal lattice points, resulting in a network of continuous rel rods. The validity of the model was confirmed by observations of a cross section of the reciprocal lattice parallel to the rel rods.

Jayaraman, N.↗
NASA NTRS1987

An improved main reflector design for compact range applications

Compact ranges offer many advantages over other types of ranges, and as a result much effort is being directed toward the improvement of their performance. The use of concave edge contours and blended rolled edge terminations to reduce the unwanted energy diffracted into the target zone from the termination of the main reflector is discussed. The proposed shaping of the edge contour minimizes the diffracted fields by virtue of reducing the spread factor; whereas, the blended rolled edge terminations reduce the diffracted fields by creating a smooth transition in the reflected field. Two design examples are treated in order to illustrate these concepts.

Pistorius, Carl W. I.↗
NASA NTRS1987

RCS of a coated circular waveguide terminated by a perfect conductor

The radar cross section (RCS) of a circular waveguide terminated by a perfect electric conductor is calculated by the geometrical theory of diffraction for the rim diffraction and by a physical optics approximation for the interior irradiation. The interior irradiation is generally more than 10 dB higher than the rim diffraction for a/lambda equal to or greater than 1 (a is the waveguide radius, lambda is the free-space wavelength). At low frequencies (a/lambda about 1), the interior irradiation can be significantly reduced over a broad range of incident angle if the interior waveguide wall is coated with a thin layer (1 percent of the radius) of lossy magnetic material. Our theoretical prediction is confirmed by measurements. At higher frequencies (a/lambda about 3), a thin layer of coating is effective for the case of near axial incidence, provided that a good transition of the TE(11) mode near the waveguide opening to the HE(11) mode inside the waveguide is made. A thicker layer of coating is required for the RCS reduction over wider incident angle.

Lee, Choon S.↗
NASA NTRS1987

Phase purity of NiCo2O4, a catalyst candidate for electrolysis of water

NiCo2O4 is shown to be difficult to obtain as a pure phase, and may never have been so obtained. High resolution x-ray diffractometry is required for its precise characterization. Film XRD is not likely to show the asymmetry in the spinel diffraction lines, caused by poorly crystallized NiO, as seen in diffractometer traces. The Co3O4 which is expected to accompany NiO as an impurity in NiCo2O4 syntheses has the same diffraction pattern as the binary oxide. Firings of the co-precipitated hydroxides at 300, 350, and 400 C, including one in pure O2, failed to produce single phase cobaltate. Scanning electron microscopy showed all the sintered products to range over several orders of magnitude in agglomerate/particle size. Surface areas by BET were all in the range 40 to 110 m sq/g, equivalent to particles of 200 to 100 Angstrom diameter. The spinel diffraction line breadths were compatible with those approximate dimensions.

Singer, J.↗
NASA NTRS1989

The crystalline phases present in carbon cathodes of discharged Li/SOCl2-LiAlCl4 cells

The X-ray diffraction patterns of 100 percent discharged Schawinigan black cathodes from Li/SOCl2-LiAlCl4 cells were obtained using a high resolution Guinier camera. The previous assignments of the diffraction lines to Li2O2 and rhombohedral sulfur are all found to be incorrect; all sharp Bragg diffraction lines not assignable to anhydrous LiCl can be assigned to LiCl.H2O.

Williams, R. M.↗
NASA NTRS1988

Real-time sensing of optical alignment

The Large Deployable Reflector and other future segmented optical systems may require autonomous, real-time alignment of their optical surfaces. Researchers have developed gratings located directly on a mirror surface to provide interferometric sensing of the location and figure of the mirror. The grating diffracts a small portion of the incident beam to a diffractive focus where the designed diagnostics can be performed. Mirrors with diffraction gratings were fabricated in two separate ways. The formation of a holographic grating over the entire surface of a mirror, thereby forming a Zone Plate Mirror (ZPM) is described. Researchers have also used computer-generated hologram (CGH) patches for alignment and figure sensing of mirrors. When appropriately illuminated, a grid of patches spread over a mirror segment will yield a grid of point images at a wavefront sensor, with the relative location of the points providing information on the figure and location of the mirror. A particular advantage of using the CGH approach is that the holographic patches can be computed, fabricated, and replicated on a mirror segment in a mass production 1-g clean room environment.

Stier, Mark T.↗
NASA NTRS1990

On diamond, graphitic and amorphous carbons in primitive extraterrestrial solar system materials

Carbon is among the most abundant elements in the universe and carbon chemistry in meteorites and comets is an important key to understanding many Solar System and interstellar processes. Yet, the mineralogical properties and interrelations between various structural forms of elemental carbon remain ambiguous. Crystalline elemental carbons include rhombohedral graphite, hexagonal graphite, cubic diamond, hexagonal diamond (i.e., lonsdaleite or carbon-2H) and chaoite. Elemental carbon also occurs as amorphous carbon and poorly graphitized (or turbostratic) carbon but of all the forms of elemental carbon only graphite is stable under physical conditions that prevail in small Solar System bodies and in the interstellar medium. The recent discovery of cubic diamond in carbonaceous chondrites and hexagonal diamond in chondritic interplanetary dust particles (IDPs) have created a renewed interest in the crystalline elemental carbons that were not formed by shock processes on a parent body. Another technique, Raman spectroscopy, confirms a widespread occurrence of disordered graphite in the Allende carbonaceous chondrite and in chondritic IDPs. Elemental carbons have also been identified by their characteristic K-edge features in electron energy loss spectra (EELS). However, the spectroscopic data do not necessarily coincide with those obtained by selected area electron diffraction (SAED). In order to interpret these data in terms of rational crystalline structures, it may be useful to consider the principles underlying electron diffraction and spectroscopic analyses. Electron diffraction depends on electron scattering, on the type of atom and the distance between atoms in a crystal lattice. Spectroscopic data are a function of the type of atom and the energy of bonds between atoms. Also, SAED is a bulk sampling technique when compared to techniques such as Raman spectroscopy or EELS. Thus, it appears that combined analyses provide contradictory results and that amorphous, or short-range ordered, carbon identified by conventional TEM imaging and SAED may show evidence for sp(3) bonds in EELS spectra. It is suggested that complex, nanometer-scale, mineralogical interrelations are common to all elemental carbons irrespective of their origin. The subsequent thermal history, or energy balance, will determine the ultimate microstructure.

Rietmeijer, Frans J. M.↗
NASA NTRS1990

Ternary and quaternary oxides of Bi, Sr, and Cu

Before the discovery of superconductivity in an oxide of Bi, Sr, and Cu, the system Bi-Sr-Cu-O had not been studied, although several solid phases had been identified in the two-component regions of the ternary system Bi2O3-SrO-CuO. The oxides Sr2CuO3, SrCu2O2, SrCuO2, and Bi2CuO4 were then well known and characterized, and the phase diagram of the binary system Bi2O3 -SrO had been established in the temperature range 620 to 1000 C. Besides nine solutions of compositions Bi(2-2x) Sr(x) O(3-2x) and different symmetries, this diagram includes three definite compounds of stoichiometries Bi(2)SrO4, Bi2Sr2O5, and Bi2Sr3O6 (x = 0.50, 0.67 and 0.75 respectively), only the second of which with known unit-cell of orthorhombic symmetry, dimensions (A) a = 14.293(2), b = 7.651(2), c = 6.172(1), and z = 4. The first superconducting oxide in the system Bi-Sr-Cu-O was initially formulated as Bi2Sr2Cu2O(7+x), with an orthorhombic unit-cell of parameters (A) a = 5.32, b = 26.6, c = 48.8. In a preliminary study the same oxide was formulated with half the copper content, Bi(2)Sr(2)CuO(6+x), and indexed its reflections assuming an orthorhombic unit-cell of dimensions (A) a = 5.390(2), b = 26.973(8), c = 24.69(4). Subsequent studies by diffraction techniques have confirmed the composition 2:2:1. A new family of oxygen-deficient perovskites, was characterized, after identifying by x ray diffraction the phases present in the products of thermal treatments of about 150 mixtures of analytical grade Bi2O3, Sr(OH)2-8H2O and CuO at different molar ratios. X ray diffraction data are presented for some other oxides of Bi and Sr, as well as for various quaternary oxides, among them an oxide of Bi, Sr, and Cu.

Casais, M. T.↗
NASA NTRS1990

Electromagnetic scattering from two-dimensional thick material junctions

The problem of the plane wave diffraction is examined by an arbitrary symmetric two dimensional junction, where Generalized Impedance Boundary Conditions (GIBCs) and Generalized Sheet Transition Conditions (GSTCs) are employed to simulate the slabs. GIBCs and GSTCs are constructed for multilayer planar slabs of arbitrary thickness and the resulting GIBC/GSTC reflection coefficients are compared with exact counterparts to evaluate the GIBCs/GSTCs. The plane wave diffraction by a multilayer material slab recessed in a perfectly conducting ground plane is formulated and solved via the Generalized Scattering Matrix Formulation (GDMF) in conjunction with the dual integral equation approach. Various scattering patterns are computed and validated with exact results where possible. The diffraction by a material discontinuity in a thick dielectric/ferrite slab is considered by modelling the constituent slabs with GSTCs. A non-unique solution in terms of unknown constants is obtained, and these constants are evaluated for the recessed slab geometry by comparison with the solution obtained therein. Several other simplified cases are also presented and discussed. An eigenfunction expansion method is introduced to determine the unknown solution constants in the general case. This procedure is applied to the non-unique solution in terms of unknown constants; and scattering patterns are presented for various slab junctions and compared with alternative results where possible.

Ricoy, M. A.↗
NASA NTRS1989

Application of super smooth optics to extra-solar planet detection

The observational constraints on direct detection of extrasolar planets around nearby stars are defined. The design of an instrument capable of meeting these goals requires the efficient reduction of diffracted and scattered light by about three orders of magnitude below the airy wings of the unapodized aperture. The diffraction reduction is achieved through the high-efficiency coronagraph, which uses Gaussian occulting masks in the first focus to concentrate better diffracted light in the pupil plane. Laboratory experiments confirm this high efficiency to the limits set by the experimental setup. Scattering due to midspatial scale figure error in the mirrors must also be reduced to a comparable level.

Terrile, Richard J.↗
NASA NTRS1989

Fabrication experiments on supersmooth optics for extrasolar planet detection

The direct detection of extrasolar planets by imaging will require reductions in scattered and diffracted light by factors in excess of 1000 within one arcsecond of a bright source. While diffraction can be reduced by a number of approaches, small angle scatter can only be reduced by controlling midspatial frequency figure errors. The surface requirements are reviewed and their meaning when compared to the data base of existing mirrors is considered. Experiments are discribed that were successful in reducing midspatial frequency figure so that the scatter level was 500 times less than diffraction for a 25-cm spherical mirror.

Ftaclas, C.↗
NASA NTRS1991

High frequency scattering by a smooth coated cylinder simulated with generalized impedance boundary conditions

Rigorous uniform geometrical theory of diffraction (UGTD) diffraction coefficients are presented for a coated convex cylinder simulated with generalized impedance boundary conditions. In particular, ray solutions are obtained which remain valid in the transition region and reduce uniformly to those in the deep lit and shadow regions. These involve new transition functions in place of the usual Fock-type integrals, characteristic to the impedance cylinder. A uniform asymptotic solution is also presented for observations in the close vicinity of the cylinder. As usual, the diffraction coefficients for the convex cylinder are obtained via a generalization of the corresponding ones for the circular cylinder.

Syed, Hasnain H.↗
NASA NTRS1991

Analytical solutions with Generalized Impedance Boundary Conditions (GIBC)

Rigorous uniform geometrical theory of diffraction (UTD) diffraction coefficients are presented for a coated convex cylinder simulated with generalized impedance boundary conditions. In particular, ray solutions are obtained which remain valid in the transition region and reduce uniformly to those in the deep lit and shadow regions. These involve new transition functions in place of the usual Fock-type integrals, characteristics to the impedance cylinder. A uniform asymptotic solution is also presented for observations in the close vicinity of the cylinder. The diffraction coefficients for the convex cylinder are obtained via a generalization of the corresponding ones for the circular cylinder.

Syed, H. H.↗
NASA NTRS1991

High-frequency techniques for RCS prediction of plate geometries

Radar cross section (RCS) prediction of several rectangular plate geometries is discussed using high-frequency techniques such as the Uniform Theory of Diffraction (UTD) for perfectly conducting and impedance wedges and the Method of Equivalent Currents (MEC). Previous reports have presented detailed solutions to the principal-plane scattering by a perfectly conducting and a coated rectangular plate and nonprincipal-plane scattering by a perfectly conducting plate. These solutions are briefly reviewed and a modified model is presented for the coated plate. Theoretical and experimental data are presented for the perfectly conducting geometries. Agreement between theory and experiment is very good near and at normal incidence. In regions near and at grazing incidence, the disagreement between the data vary according to diffraction distances and angles involved. It is these areas of disagreement which are of extreme interest as an explanation for the disagreement will yield invaluable insight into scattering mechanisms which are not yet identified as major contributors near and at grazing incidence. Areas of disagreement between theory and experiment are identified and examined in an attempt to better understand and predict near-grazing incidence, grazing incidence, and nonprincipal-plane diffractions.

Balanis, Constantine A.↗
NASA NTRS1990

Time domain version of the uniform GTD

The uniform geometrical theory of diffraction (UTD) solutions can be inversely transformed analytically to obtain a time-domain version of the UTD. The time-domain solutions are valid in the early time period where an observation time t is close to the time after the arrival of the first diffracted wavefront. Comparisons with GTD (geometrical theory of diffraction) and also with available rigorous results reveal that the UTD solutions are accurate for substantial early time periods while the GTD results are valid for very early time periods.

Veruttipong, Thavath Watt↗
NASA NTRS1991

Ternary and quaternary oxides of Bi, Sr and Cu

Before the discovery of superconductivity in an oxide of Bi, Sr, and Cu, the system Bi-Sr-Cu-O had not been studied, although several solid phases had been identified in the two-component regions of the ternary system Bi2O3-Si-O-CuO. The oxides Sr2CuO3, SrCu2O2, SrCuO2, and Bi2CuO4 were then well known and characterized, and the phase diagram of the binary system Bi2O3-SrO had been established in the temperature range 620 to 1000 C. Besides nine solutions of compositions Bi(2-2x) Sr(x) O(3-2x) and different symmetries, this diagram includes three definite compounds of stoichiometries Bi(2)BrO4. Bi2Sr2O5, and Bi2Sr3O6 (x - 0.50, 0.67 and 0.75 respectively), only the second of which with known unit-cell of orthorhombic symmetry, dimensions (A) a = 14.293(2), b = 7.651(2), c = 6.172(1), and z = 4. The first superconducting oxide in the system Bi-Sr-Cu-O was initially formulated as Bi2Sr2Cu2O(7+x), with an orthorhombic unit-cell of parameters (A) a = 5.32, b = 26.6, c = 48.8. In a preliminary study the same oxide was formulated with half the copper content, Bi(2)Sr(2)CuO(6+x), and index its reflections assuming an orthorhombic unit-cell of dimensions (A) a = 5.390(2), b = 26.973(8), c = 24.69(4). Subsequent studies by diffraction techniques have confirmed the composition 2:2:1. A new family of oxygen-deficient perovskites, was characterized, after identifying by x ray diffraction the phases present in the products of thermal treatments of about 150 mixtures of analytical grade Bi2O3, Sr(OH)2-8H2O and CuO at different molar ratios. X ray diffraction data are presented for some other oxides of Bi and Sr, as well as for various quaternary oxides, among them an oxide of Bi, Sr, and Cu.

Casais, M. T.↗
NASA NTRS1992

High-frequency techniques for RCS prediction of plate geometries

Given here are hard polarization diffraction coefficients for the special case of a wedge that is coated with a lossy dielectric on one face and is perfectly conducting on the other face. Expressions for plane-wave incidence, far-field observation; for cylindrical-wave incidence, far-field observation; and for plane-wave incidence, near-field observation are presented for regular diffractions. In addition, terms for surface-wave and surface-wave transition fields are presented. These expressions are derived from the Uniform Theory of Diffraction (UTD) coefficients for a generalized impedance wedge and are simplified for rapid calculation for the case of one perfectly conducting face. Results from a first-order model for a coated, rectangular plate are examined; and potential problems in extending this modeling approach to more complicated coated structures are identified.

Balanis, Constantine A.↗
NASA NTRS1991

Stray signal requirements for compact range reflectors based on RCS measurement errors

The authors present a performance criterion for compact range reflectors such that their edge diffracted stray signal levels meet a reasonable radar cross section (RCS) measurement error requirement. It is shown by example that one of the significant error sources is the diffracted fields emanating from the edges or junctions of the reflector. This measurement error is demonstrated by placing a diagonal square flat plate in the target zone and rotating it to appropriate angles. These angles are determined by bisecting the plane wave and stray signal directions. This results in a peak bistatic measurement of the edge diffracted stray signal. It is proposed that the diagonal flat plate be used to evaluate new reflector designs as well as existing systems. A reasonable stray signal performance level has been developed so that new reflector systems can be characterized in terms of an RCS measurement error requirement.

Lee, Teh-Hong↗