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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 217 records · Page 12
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NASA NTRS2017

Array Phase Shifters: Theory and Technology

Phase shifters are linear one- or two-port devices for adjusting the reflection or insertion carrier phase of a band-limited signal, nominally from 0 to 2π radians. A perfect phase shifter would have: no insertion loss, a voltage standing wave ratio of 1:1, arbitrarily high power handling capability, linear phase-versus-frequency response, an arbitrarily small footprint, radiation immunity, no DC power consumption, and of course be free. Remarkably, real phase shifters can approach some of these idealized attributes. New processing techniques hold promise to significantly reduce manufacturing cost (see "Trends" at the end of this chapter).

Romanofsky, Robert R.↗
NASA NTRS1981

The carbon monoxide fourth positive bands in the Venus dayglow. I - Synthetic spectra

Recent observations of the Venus dayglow spectrum from 1200 to 1800 A are discussed, and identifications of the CO4+ bands are given using synthetic spectra. The main excitation mechanism is found to be resonant and fluorescent scattering of solar radiation, so the theory for this process is covered in some detail. A synthetic fluorescence spectrum is generated with the aid of high-resolution solar flux data, and emission rate factors are calculated. The influence of solar emission line excitation on this spectrum is demonstrated. In particular, it is shown that the uncertainties in interpreting the Venus dayglow are overcome by the identification of CO4+ bands in the v prime = 14 progression resulting from the fluorescent scattering by CO of solar Lyman alpha radiation.

Durrance, S. T.↗
NASA NTRS1974

Time variation of the H2O maser and infrared continuum in late-type stars

Observations of the variability of the 1.35-cm H2O and 2.2-micron emission from eight late M stars are described. A comparison of the microwave and infrared variations is of interest because the bulk of the stellar energy is radiated in the near-infrared and because at least one theory (Litvak 1969) hypothesizes near-infrared maser pumping via the 2.7-micron H2O vibration-rotation band. Variations at the two wavelengths appear to be correlated and possible explanations for this correlation are discussed.

Schwartz, P. R.↗
NASA NTRS1975

Widths of atomic 4s and 4p vacancy states, 46 less than or equal to Z less than or equal to 50

Auger and X-ray photoelectron spectra involving N1, N2, and N3 vacancy states of Pd, Ag, Cd, In, and Sn were measured and compared with results of free atom calculations. As previously observed in Cu and Zn Auger spectra that involve 3d-band electrons, free-atom characteristics with regard to widths and structure were found in the Ag and Cd M4-N4,5N4,5 and M5-N4,5N4,5 Auger spectra that arise from transitions of 4d-band electrons. Theoretical N1 widths computed with calculated free-atom Auger energies agree well with measurements. Theory however predicts wider N2 than N3 vacancy states (as observed for Xe), while the measured N2 and N3 widths are nearly equal to each other and to the average of the calculated N2 and N3 widths. The calculations are made difficult by the exceedingly short lifetime of some 4p vacancies and by the extreme sensitivity of super-Coster-Kronig rates, which dominate the deexcitation, to the transition energy and to the fine details of the atomic potential.

Hsiungchen, M.↗
NASA NTRS1985

TDRSS S-shuttle unique receiver equipment

Beginning with STS-9, the Tracking and Date Relay Satellite system (TDRSS) will start providing S- and Ku-band communications and tracking support to the Space Shuttle and its payloads. The most significant element of this support takes place at the TDRSS White Sands Ground Terminal, which processes the Shuttle return link S- and Ku-band signals. While Ku-band hardware available to other TDRSS users is also applied to Ku-Shuttle, stringent S-Shuttle link margins have precluded the application of the standard TDRSS S-band processing equipment to S-Shuttle. It was therfore found necessary to develop a unique S-Shuttle Receiver that embodies state-of-the-art digital technology and processing techniques. This receiver, developed by Motorola, Inc., enhances link margins by 1.5 dB relative to the standard S-band equipment and its bit error rate performance is within a few tenths of a dB of theory. An overview description of the Space Shuttle Receiver Equipment (SSRE) is presented which includes the presentation of block diagrams and salient design features. Selected, measured performance results are also presented.

Weinberg, A.↗
NASA NTRS1979

Studies of magnetostriction and spin polarized band structures of rare earth intermetallics

Anisotropic magnetostriction measurements of R6Fe23, R = (Tb, Dy, Ho, and Er) were carried out from 77 K to room temperature. Magnetic fields up to 2.1 Tesla were applied. All the compounds exhibited large magnetostrictions at 77 K, the largest effect being obtained for Tb6Fe23. Saturation magnetostriction values for the compounds were also determined for 77 K and room temperature. Results of the temperature dependence of magnetostriction for Er6Fe23 are in good agreement with Callen and Callen's single ion theory. Therefore, the main sources of magnetostriction in this compound is the Er ion. The spin-up and spin-down electronic energy bands, the density of states and the magnetic moments of YCo5, SmCo5, and GdCo5 were calculated by the spin polarized augmented plane wave technique. The calculations obtained show the origin of the moment, provide good estimates of its magnitude and variation, and the reasons for those variations. They also show the important role of partial charge transfer and of d-d electronic coupling. Calculations for LaNi5 and GdNi5 systems are discussed.

Wallace, W. E.↗
NASA NTRS1972

Ultrasonic investigation of the superconducting properties of the Nb-Mo system

The superconducting properties of single crystals of Nb and two alloys of Nb with Mo were investigated by ultrasonic techniques. The results of measurements of the ultrasonic attenuation and velocities as a function of temperature, Mo composition, crystallographic direction, and ultrasonic frequency are reported. The attenuation and small velocity changes associated with the superconductivity of the samples are shown to be dependent on the sample resistivity ratio which varied from 4.3 for Nb-9% Mo to 6500 for pure Nb. The ultrasonic attenuation data are analyzed in terms of the superconducting energy gap term of the BCS theory. A new model is proposed for the analysis of ultrasonic attenuation in pure superconductors with two partially decoupled energy bands. To analyze the attenuation in pure superconducting Nb, the existence of two energy gaps was assumed to be associated with the two partially decoupled energy bands. One of the gaps was found to have the normal BCS value of 3.4 and the other gap was found to have the anomalously large value of 10. No experimental evidence was found to suggest that the second energy gap had a different transition temperature. The interpretation of the results for the Nb-Mo alloys is shown to be complicated by the possible existence of a second superconducting phase in Nb-Mo alloys with a transition temperature of 0.35 of the transition temperature of the first phase. The elastic constants of Nb and Nb-Mo alloys are shown to be approximately independent of Mo composition to nine atomic percent Mo. These results do not agree with the current microscopic theory of transition temperature for the transition elements.

Lacy, L. L.↗
NASA NTRS1984

IUE observations of weak G-band stars

High- and low-resolution ultraviolet spectra of several weak G-band giants have been obtained with the IUE satellite, to derive Be abundances and to search for degenerate companions. The abundances of Be in these stars are about log epsilon (Be) approximately -0.5. These values are in good agreement with the Be abundances of Hyades giants, and are consistent with standard theories of post-main-sequence Be depletion in stars. However, the Be abundances do not fit with the large Li abundances in weak G-band stars. Post-main-sequence production of Li or element segregation in main-sequence stars may explain the high Li abundance in weak G-band stars. No direct evidence is seen for the presence of hot subluminous companions. The peculiar abundances in these stars probably are not due to binary mass transfer.

Parthasarathy, M.↗
NASA NTRS1980

Calculated and measured performance of a 'near ideal' locally reacting duct liner in grazing incidence

The calculated acoustic response of a nearly ideal (locally reacting and linear) duct liner has been compared with its measured performance. The ceramic honeycomb liner structure was such that the local reaction assumption was well satisfied over the frequency range of interest (0.5-3.0 kHz). In addition, the high repeatability of the measured impedance values over a large SPL range verified the assumption of linearity. A finite element algorithm was used to compute the duct liner transmission properties in terms of the liner's normal incidence impedance and the duct termination impedance using a plane wave source. Very good agreement between theory and experiment was observed in general, but required accurate normal impedance values in the high attenuation frequency bands of the liner's response.

Parrott, T. L.↗
NASA NTRS1982

Microstructure and particulate properties of the surfaces of Io and Ganymede - Comparison with other solar system bodies

The phase curves of Io and Ganymede between 0 and 40 deg solar phase angle have been compiled from Voyager photopolarimeter and ground-based observations. Modeling the data with the Lumme-Bowell theory made it possible to determine improved estimates of the surface texture and particulate properties. Accurate V-band zero-phase geometric albedos, phase integrals, and Bond albedos are also obtained. The properties of the particles on Io's surface appear to be different from those of other solar system objects whose surfaces are totally controlled by impact cratering processes. Sulfur and SO2 frost, previously identified by spectral analysis, seem to exist in a physical form not commonly found on earth. The agglomeration of volcanic ash into aggregates on Io's surface is postulated.

Pang, K. D.↗
NASA NTRS1987

Nonlinear susceptibilities of finite conjugated organic polymers

Tight-binding calculations of the length dependence of the third-order molecular hyperpolarizability for polyenes and polyynes are reported. The pi-electron wave functions were determined by exploiting the limited translational symmetry of the molecules. Perturbation theory was used to calculate the longitudinal component of the electronic nonresonant hyperpolarizability. This is the first two-'band' calculation of third-order hyperpolarizabilities on finite pi-electron systems of varying length. In contrast to the results of the one-'band' models, the hyperpolarizability densities increase rapidly and then, after about 10-15 repeating units, approach an asymptotic value.

Beratan, David N.↗
NASA NTRS1987

A millimeter wave Josephson mixer employing a high-T(c) GdBaCuO point contact

A Josephson effect heterodyne mixer for the millimeter wave band was investigated employing high-T(c) GdBaCuO point contacts. Mixer performance was in qualitative agreement with theory. A mixing response was observed up to 55 K, the highest operating temperature achieved for such a device to date. The voltage separation of RF-induced steps gave a value of h/2e = 2.08 x 10 to the -15th V s, which is in excellent agreement with the value expected for Cooper pairs. In addition, the temperature dependence of the I(0)R product was found to agree with Bardeen-Cooper-Schrieffer theory in the weak coupling limit.

Olsson, H. K.↗
NASA NTRS1990

Infrared spectroscopy and Mie scattering of acetylene aerosols formed in a low temperature diffusion cell

A method is described for forming and spectroscopically characterizing cryogenic aerosols formed in a low temperature gas cell. By adjusting the cell pressure, gas composition and flow rate, the size distribution of aerosol particles can be varied over a wide range. The combination of pressure and flow rate determine the residence time of the aerosols in the cell and hence the time available for the particles to grow. FTIR spectroscopy, over the range from 600/cm to 6000/cm, is used to characterize the aerosols. The particle size distribution can be varied so that, at one extreme, the spectra show only absorption features associated with the infrared active vibrational bands and, at the other, they display both absorption and Mie scattering. In the latter case, Mie scattering theory is used to obtain semiquantitative aerosol size distributions, which can be understood in terms of the interplay between nucleation and condensation. In the case of acetylene aerosols, the infrared spectra suggest that the particles exist in the high temperature cubic phase of the solid.

Dunder, T.↗
NASA NTRS2004

Computational Confirmation of the Carrier for the "XCN" Interstellar Ice Bank: OCN(-) Charge Transfer Complexes

Recent experimental studies provide evidence that carrier for the so-called XCN feature at 2165 cm(exp -1) (4.62 micron) in young stellar objects is an OCN(-)/NH4(+) charge transfer (CT) complex that forms in energetically processed interstellar icy grain mantles. Although other RCN nitriles and RCN iosonitriles have been considered, Greenberg's conjecture that OCN(-) is associated with the XCN feature has persisted for over 15 years. In this work we report a computational investigation that thoroughly confirms the hypothesis that the XCN feature observed in laboratory studies can result from OCN(-)/NH4(+) CT complexes arising from HNCO and NH3, in a water ice environment. Density functional theory calculations with theory calculations with HNCO, NH3, and up to 12 waters reproduce seven spectroscopic measurements associated with XCN: the band origin of the asymmetric stretching mode of OCN(-), shifts due to isotopic substitutions of C, N, O, and H, and two weak features. However, very similar values are also found for the OCN(-)/NH4(+) CT complex arising from HOCN and NH3. In both cases, the complex forms by barrierless proton transfer from HNCO or HOCN to NH3 during the optimization of the solvated system. Scaled B3LYP/6-31+G** harmonic frequencies for HNCO and HOCN cases are 2181 and 2202 cm(exp -1), respectively.

Park, J.-Y.↗
NASA NTRS2000

Experiment Evaluation of Bifurcation in Sands

The basic principles of bifurcation analysis have been established by several investigators, however several issues remain unresolved, specifically how do stress level, grain size distribution, and boundary conditions affect general bifurcation phenomena in pressure sensitive and dilatant materials. General geometrical and kinematics conditions for moving surfaces of discontinuity was derived and applied to problems of instability of solids. In 1962, the theoretical framework of bifurcation by studying the acceleration waves in elasto-plastic (J2) solids were presented. Bifurcation analysis for more specific forms of constitutive behavior was examined by studying localization in pressure-sensitive, dilatant materials, however, analyses were restricted to plane deformation states only. Bifurcation analyses were presented and applied to predict shear band formations in sand under plane strain condition. The properties of discontinuous bifurcation solutions for elastic-plastic solids under axisymmetric and plane strain loading conditions were studied. The study focused on theory, but also references and comparisons to experiments were made. The current paper includes a presentation of a summary of bifurcation analyses for biaxial and triaxial (axisymmetric) loading conditions. The Coulomb model is implemented using incremental piecewise scheme to predict the constitutive relations and shear band inclination angles. Then, a comprehensive evaluation of bifurcation phenomena is presented based on data from triaxial experiments performed under microgravity conditions aboard the Space Shuttle under very low effective confining pressure (0.05 to 1.30 kPa), in which very high peak friction angles (47 to 75 degrees) and dilatancy angles (30 to 31 degrees) were measured. The evaluation will be extended to include biaxial experiments performed on the same material under low (10 kPa) and moderate (100 kPa) confining pressures. A comparison between the behavior under biaxial and triaxial loading conditions will be presented, and related issues concerning influence of confining pressure will be discussed.

Alshibi, Khalid A.↗
NASA NTRS1983

Mirror instability and origin of morningside auroral structure

Auroral optical imagery shows marked differences between auroral features of the evening and morning sectors: the separation between diffuse and discrete auroras in the evening sector is not distinct in the morning sector, which is dominated by auroral patches and multiple banded structures aligned along some direction. Plasma distribution function signatures also show marked differences: downward electron beams and inverted-V signatures prefer the evening sector, while the electron spectra on the morning sector are similar to the diffuse aurora. A theory of morningside auroras consistent with these features was constructed. The theory is based on modulation of the growth rates of electron cyclotron waves by the mirror instability, which is in turn driven by inward-convected ions that have become anisotropic. This modulation produces alternating bands of enhanced and reduced electron precipitation which approximate the observed multiple auroral bands and patches of the morning sector.

Chiu, Y. T.↗
NASA NTRS1972

A theory for energetic electron lifetimes within the plasmasphere.

The removal of radiation belt electrons through pitch-angle scattering by the observed obliquely propagating plasmaspheric whistler wave band is investigated. Electron lifetimes are calculated as a function of L and energy. The effects of high latitude interactions and of oblique wave propagation with the resulting diffusion at all cyclotron harmonics are quantitatively incorporated into the calculations. The inclusion of these effects appears to extend the role of whistler-driven electron pitch-angle diffusion to provide an explanation of the positioning of the quiet time radiation belt electron slot and the electron losses within the slot region.

Lyons, L. R.↗
NASA NTRS1974

Tunneling in thin MOS structures

Recent results on tunneling in thin MOS structures are described. Thermally grown SiO2 films in the thickness range of 22-40 A have been shown to be effectively uniform on an atomic scale and exhibit an extremely abrupt oxide-silicon interface. Resonant reflections are observed at this interface for Fowler-Nordheim tunneling and are shown to agree with the exact theory for a trapezoidal barrier. Tunneling at lower fields is consistent with elastic tunneling into the silicon direct conduction band and, at still lower fields, inelastic tunneling into the indirect conduction band. Approximate dispersion relations are obtained over portions of the silicon-dioxide energy gap and conduction band.

Maserjian, J.↗