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At least 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
NASA NTRS2001

Stability of Detached Grown Germanium Single Crystals

Detachment of the melt meniscus from the crucible during semiconductor Bridgman growth experiments has been observed in recent years, especially under microgravity experiments. Under earth conditions, the hydrostatic pressure counteracts the mechanism, whereby it is more difficult to achieve detached Bridgman growth. Attempts to get stable detached growth under terrestrial conditions have been discussed in the literature and have been the subject of recent experiments in our own group. The advantage of crystals grown without wall contact is obvious: In general, they possess a higher crystal quality than conventional Bridgman grown crystals with wall contact. However, due to the interaction of different parameters such as the wetting behavior of the melt with the crucible, and the dependence of the growth angle with the shape of the melt meniscus, the mechanism leading to detachment is very complicated and not completely understood. We have grown several doped and undoped Germanium crystals with the detached Bridgman and the normal Bridgman growth technique. Pyrolytic boron nitride containers were used for all growth experiments. In the detached grown crystals the typical gap thickness between the pBN crucible and the crystal is in the range of 10 to 100 micrometers, which was determined by performing profilometer measurements. Etch pit density measurements were also performed and a comparison between detached and attached grown crystals will be given. An interesting feature was detected on the surface of a detached grown crystal. Strong surface striations with an average axial distance of 0.5 mm were observed around the whole circumference. The maximum fluctuation of the gap thickness is in the range of 5-10 micrometers. These variations of the detached gap along the crystal axis can be explained by a kind of stiction of the melt/crucible interface and thus by a variation of the meniscus shape. This phenomenon leading to the fluctuation of the gap thickness will be discussed in detail.

Schweizer, M.↗
NASA NTRS2002

Stability of Detached Grown Germanium Single Crystals

Detachment of the melt meniscus from the crucible during semiconductor Bridgman growth experiments has been observed in recent years especially, under microgravity experiments. Under earth conditions, the hydrostatic pressure counteracts the mechanism, whereby it is more difficult to achieve detached Bridgman growth. Attempts to get stable detached growth under terrestrial conditions have been discussed in the literature and have been the subject of recent experiments in our own group. The advantage of crystals grown without wall contact is obvious: In general, they possess a higher crystal quality than conventional Bridgman grown crystals with wall contact. However, due to the interaction of different parameters such as the wetting behavior of the melt with the crucible, and the dependence of the growth angle with the shape of the melt meniscus, the mechanism leading to detachment is very complicated and not completely understood. We have grown several doped and undoped Germanium crystals with the detached Bridgman and the normal Bridgman growth technique. Pyrolytic boron nitride containers were used for all growth experiments. In the detached grown crystals the typical gap thickness between the pBN crucible and the crystal is in the range of 10 to 100 microns, which was determined by performing profilometer measurements. Etch pit density measurements were also performed and a comparison between detached and attached grown crystals will be given. An interesting feature was detected on the surface of a detached grown crystal. Strong surface striations with an average axial distance of 0.5mm were observed around the whole circumference. The maximum fluctuation of the gap thickness is in the range of 5-10 microns. These variations of the detached gap along the crystal axis can be explained by a kind of stiction of the melt/crucible interface and thus by a variation of the meniscus shape. This phenomenon leading to the fluctuation of the gap thickness will be discussed in detail.

Schweizer, M.↗
NASA NTRS1972

Injun 5 observations of magnetospheric electric fields and plasma convection.

Recent measurements of magnetospheric electric fields with the satellite Injun 5 have provided a comprehensive global survey of plasma convection at low altitudes in the magnetosphere. A persistent feature of these electric-field observations is the occurrence of an abrupt reversal in the convection electric field at auroral-zone latitudes. The plasma convection velocities associated with these reversals are generally directed E-W, away from the sun on the poleward side of the reversal, and toward the sun on the equatorward side of the reversal. Convection velocities over the polar cap region are normally less than those observed near the reversal region. The electric-field reversal is observed to be coincident with the 'trapping boundary' for electrons with energies E greater than 45 keV. Near local noon the region of anti-sunward convection poleward of the electric-field reversal/trapping boundary corresponds to the low-altitude extension of the polar-cusp plasma.

Gurnett, D. A.↗
NASA NTRS1979

An introduction to a unified approach to flexible rotor balancing

Two types of technique for flexible-rotor balancing are examined: the influence coefficient method, and the modal balancing method. Briefly, the influence coefficient method seeks those correction masses in a predetermined set of planes which will minimize measured vibration (readings) at a series of sensors and speeds, as predicted by influence coefficients relating vibration readings to mass additions; the influence coefficients are normally determined by a series of trial mass tests. The modal balancing method seeks to balance the rotor, one mode at a time, with a set of masses specifically selected not to disturb previously balanced lower modes, the sensitivity to this combination of masses is determined empirically by a series of trial mass tests. The two approaches are compared in supercritical-shaft balancing tests, and some common features are stressed for ultimate incorporation into a unified approach toward flexible-rotor balancing.

Parkinson, A. G.↗
NASA NTRS1986

Rarefied aerodynamics and upper atmosphere density from multiple orbiter flight measurements

Flight data from six flights of the experimental High Resolution Accelerometer Package (HIRAP), a micro-g accelerometer system, have been analyzed to produce rarefied flow aerodynamic coefficients of the Orbiter and freestream density in the altitude range from 60 km to 160 km. The direct measurements of the lift to drag ratio (L/D) are used to obtain individual normalized body-axis force coefficients in a least-squares regression scheme. An aerodynamics model based on an exponential function of Knudsen number is used for the individual coefficients. The calculated L/D from the flight determined coefficients agree with the multiple flight data measurements to within about 5 percent. Simultaneously, an upper altitude density variation is also obtained from the data. Density altitude profiles exhibit a wave feature on all flights. The wave has an amplitude of about + or - 25 percent relative to the 1962 Standard atmosphere model.

Buck, G. M.↗
NASA NTRS1986

Verification of a SEU model for advanced 1-micron CMOS structures using heavy ions

Modeling and test results are reported for 1 micron CMOS circuits. Analytical predictions are correlated with experimental data, and sensitivities to process and design variations are discussed. Unique features involved in predicting the SEU performance of these devices are described. The results show that the critical charge for upset exhibits a strong dependence on pulse width for very fast devices, and upset predictions must factor in the pulse shape. Acceptable SEU error rates can be achieved for a 1 micron bulk CMOS process. A thin retrograde well provides complete SEU immunity for N channel hits at normal incidence angle. Source interconnect resistance can be important parameter in determining upset rates, and Cf-252 testing can be a valuable tool for cost-effective SEU testing.

Cable, J. S.↗
NASA NTRS1996

Wind tunnel test results of a 1/8-scale fan-in-wing model

A 1/8-scale model of a fan-in-wing concept considered for development by Grumman Aerospace Corporation for the U.S. Army was tested in the Langley 14- by 22-Foot Subsonic Tunnel. Hover testing, which included height above a pressure-instrumented ground plane, angle of pitch, and angle of roll for a range of fan thrust, was conducted in a model preparation area near the tunnel. The air loads and surface pressures on the model were measured for several configurations in the model preparation area and in the tunnel. The major hover configuration change was varying the angles of the vanes attached to the exit of the fans for producing propulsive force. As the model height above the ground was decreased, there was a significant variation of thrust-removed normal force with constant fan speed. The greatest variation was generally for the height-to-fan exit diameter ratio of less than 2.5; the variation was reduced by deflecting fan exit flow outboard with the vanes. In the tunnel angles of pitch and sideslip, height above the tunnel floor, and wind speed were varied for a range of fan thrust and different vane angle configurations. Other configuration features such as flap deflections and tail incidence were evaluated as well. Though the V-tail empennage provided an increase in static longitudinal stability, the total model configuration remained unstable.

Wilson, John C.↗
NASA NTRS1959

NASA Reactor Facility Hazards Summary

The Lewis Research Center of the National Aeronautics and Space Administration proposes to build a nuclear research reactor which will be located in the Plum Brook Ordnance Works near Sandusky, Ohio. The purpose of this report is to inform the Advisory Committee on Reactor Safeguards of the U. S. Atomic Energy Commission in regard to the design Lq of the reactor facility, the characteristics of the site, and the hazards of operation at this location. The purpose of this research reactor is to make pumped loop studies of aircraft reactor fuel elements and other reactor components, radiation effects studies on aircraft reactor materials and equipment, shielding studies, and nuclear and solid state physics experiments. The reactor is light water cooled and moderated of the MTR-type with a primary beryllium reflector and a secondary water reflector. The core initially will be a 3 by 9 array of MTR-type fuel elements and is designed for operation up to a power of 60 megawatts. The reactor facility is described in general terms. This is followed by a discussion of the nuclear characteristics and performance of the reactor. Then details of the reactor control system are discussed. A summary of the site characteristics is then presented followed by a discussion of the larger type of experiments which may eventually be operated in this facility. The considerations for normal operation are concluded with a proposed method of handling fuel elements and radioactive wastes. The potential hazards involved with failures or malfunctions of this facility are considered in some detail. These are examined first from the standpoint of preventing them or minimizing their effects and second from the standpoint of what effect they might have on the reactor facility staff and the surrounding population. The most essential feature of the design for location at the proposed site is containment of the maximum credible accident.

Source record↗
NASA NTRS2019

Electron Diffusion Regions in Magnetotail Reconnection Under Varying Guide Fields

Kinetic structures of electron diffusion regions (EDRs) under finite guide fields in magnetotail reconnection are reported. The EDRs with guide fields 0.14–0.5 (in unit of the reconnecting component) are detected by the Magnetospheric Multiscale spacecraft. The key new features include the following: (1) cold inflowing electrons accelerated along the guide field and demagnetized at the magnetic field minimum while remaining a coherent population with a low perpendicular temperature, (2) wave fluctuations generating strong perpendicular electron flows followed by alternating parallel flows inside the reconnecting current sheet under an intermediate guide field, and (3) gyrophase bunched electrons with high parallel speeds leaving the X‐line region. The normalized reconnection rates for the three EDRs range from 0.05 to 0.3. The measurements reveal that finite guide fields introduce new mechanisms to break the electron frozen‐in condition.

Chen, L.‐J.↗
NASA NTRS1979

A unified kinetic model of the tangential magnetopause structure

In the self-consistent model of the tangential magnetopause, formulated in the present paper on the basis of the Vlasov-Maxwell equations, the plasmas on both sides are magnetized and the magnetic field is everywhere parallel to the magnetopause (i.e., the normal field component is zero) and rotates through an arbitrary angle across the magnetopause. It is shown that the thickness of the magnetopause is greater than the gyroradius of the plasma ions. The presence of a trapped particle population within the magnetopause is shown to be required to allow the magnetic field to rotate more than a certain critical angle (-90 degrees). The model proposed can reproduce the observed features of the tangential magnetopause structure by specifying boundary conditions on both sides of the magnetopause.

Lee, L. C.↗
NASA NTRS1989

Estimations of ABL fluxes and other turbulence parameters from Doppler lidar data

Techniques for extraction boundary layer parameters from measurements of a short-pulse CO2 Doppler lidar are described. The measurements are those collected during the First International Satellites Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE). By continuously operating the lidar for about an hour, stable statistics of the radial velocities can be extracted. Assuming that the turbulence is horizontally homogeneous, the mean wind, its standard deviations, and the momentum fluxes were estimated. Spectral analysis of the radial velocities is also performed from which, by examining the amplitude of the power spectrum at the inertial range, the kinetic energy dissipation was deduced. Finally, using the statistical form of the Navier-Stokes equations, the surface heat flux is derived as the residual balance between the vertical gradient of the third moment of the vertical velocity and the kinetic energy dissipation. Combining many measurements would normally reduce the error provided that, it is unbiased and uncorrelated. The nature of some of the algorithms however, is such that, biased and correlated errors may be generated even though the raw measurements are not. Data processing procedures were developed that eliminate bias and minimize error correlation. Once bias and error correlations are accounted for, the large sample size is shown to reduce the errors substantially. The principal features of the derived turbulence statistics for two case studied are presented.

Gal-Chen, Tzvi↗
NASA NTRS1994

A structural fabric defined by topographic lineaments: Correlation with Tertiary deformation of Ellesmere and Axel Heiberg Islands, Canadian Arctic

Digital topographic contours from four 1:250000 scale maps have been gridded to produce a digital elevation model for part of Ellesmere and Axel Heiberg islands in the Canadian Arctic Islands. Gradient calculations were used to define both east and west dipping slopes defining a pattern of lineaments that have been compared with mapped geological structures. In ice-covered areas, where geological mapping was not possible, well-defined topographic lineaments have been identified and are correlated to extensions of major structural features. The northeast-southwest patterns of both topographic lineaments and mapped structures are strongly unimodal and support a single compressive event oriented at 67 deg west of north. This orientation is coincidental with the convergence direction calculated from the kinematic poles of rotation for Greenland relative to North America between 56 and 35 Ma. A minor secondary peak at 70 east of north is observed for thrust and normal fault solutions and is not directly related to the predicted convergence direction. Whether this represents a unique phase of deformation or is a subcomponent of a single event is not known. The agreement of structural components, lineament orientations, and convergence direction suggests an overwhelming over print of Eurekan deformation on any preexisting structural fabric. This study confirms, for the first time, an excellent compatibility between geological and geophysical constraints for the timing and geometry of the Eurekan orogeny.

Oakey, Gordon↗
NASA NTRS1995

Interplanetary planar magnetic structures associated with expanding active regions

Planar magnetic structures are interplanetary objects whose magnetic field cannot be explained by Parker's solar wind model. They are characterized by two-dimensional structure of magnetic field that are highly variable and parallel to a plane which is inclined to the ecliptic plane. They appeared independently of interplanetary compression, solar flares, active prominences nor filament disappearances, but the sources often coincided with active regions. On the other hand, it has been discovered by the Yohkoh Soft X-ray telescope that active-region corona expand outwards at speeds of a few to a few tens of km/s near the Sun. The expansions occurred repeatedly, almost continually, even in the absence of any sizable flares. In the Yohkoh Soft X-ray images, the active-region corona seems to expand out into interplanetary space. Solar sources of interplanetary planar magnetic structures observed by Sakigake were examined by Yohkoh soft X-ray telescope. During a quiet period of the Sun from January 6 to November 11, 1993, there found 5 planar magnetic structures according to the criteria (absolute value of Bn)/(absolute value of B) less than 0.1 for planarity and (dB)/(absolute value of B) greater than 0.7 for variability of magnetic field, where Bn, dB, and the absolute value of B are field component normal to a plane, standard deviation, and average of the magnitude of the magnetic field, respectively. Sources of 4 events were on low-latitude (less than 5 degrees) active regions from which loop-like structures were expanding. The coincidence, 80%, is extremely high with respect to accidental coincidence, 7%, of Sakigake windows of solar wind observation with active regions. The last source was on loop-like features which seemed to be related with a mid-latitude (20 degrees) active region.

Nakagawa, Tomoko↗
NASA NTRS1984

Compressional features in the Tharsis Region of Mars: Orientations and Timing of Deformation

The Tharsis ridge system appears to form a great circle around the major volcanoes. The roughly circumferential orientation of these ridges could be interpreted as the formation of a ridge system in response to a single stress field with a center near Pavonis Mons. Wise, et al. (1979) plotted the perpendiculars to strikes of ridges on the eastern flanks of Tharsis as great circles on the upper hemisphere of an equal area net. In order to further test for circular symmetry and possible other influences on ridge orientations, normals to vector means of ridge orientations were calculated using over 1850 digitized ridge segments sampled in 10 degree boxes. Orientations of individual ridge segments were weighted by their lengths, and the magnitudes of normals to vectors means were weighted by cummulative length of the ridge segments. Normals to vector means with magnitudes less than 100 km are not shown. Assuming a single fold origin for ridges, the resulting plot shows compressive stress trajectories for the ridges in the Tharsis region. The averaged compressive stress orientation around Tharsis confirm the suggestion by Wise, et al. that the ridge system is not concentric to any single point.

Watters, T. R.↗
NASA NTRS1986

Process for preparing essentially colorless polyimide film containing phenoxy-linked diamines

A polyimide film that is approximately 90% transparent at 500 nm, useful for thermal protective coatings and solar cells, and the processes for preparing the same by thermal and chemical conversion are disclosed. An essential feature for achieving maximum optical transparency films requires utilizing recrystallized and/or sublimated specific aromatic diamines and dianhydride monomers and introducing phenoxy or thiophenyl separator groups and isomeric m,m' or o,p'-oriented diamines into the polymer molecular structure. The incorporation of these groups in the polymer structure serves to separate the chromaphoric centers and reduce the formation of inter-chain and intra-chain charge transfer complexes which normally cause absorptions in the UV-visible range. The films may be obtained by hand, brushing, casting, or spraying a layer of polyamic acid solutions onto a surface and thermally converting the applied layer to the polyimide, or the polyamic acid solution can be chemically converted to the polyimide, subsequentially dissolved in an organic solvent, and applied as a polyimide film layer with the solvent therein thermally removed.

Stclair, A. K.↗
NASA NTRS1991

Development of a Rayleigh scattering system for temperature measurements in combustor flows

A dual line laser induced Rayleigh scattering system has been developed to be used in high temperature gas flows such as in combustors, and preliminary tests have been performed in a hot air jet. The qualification measurements included the collection and analysis of Rayleigh signal for time-resolved gas temperature over the two lines of a copper-vapor laser. The dual line signal collection technique is a feature to be used in detecting and discriminating the surface scattered laser light in the Rayleigh signal. Optical fibers are used to transmit the laser beam to the optical probe bench and to carry the collected signal from the bench to the housing of the signal sensors. Incident laser intensity at the probe volume is measured for both lines at each pulse of the laser. This information is used to normalize the signal. Good agreement has been obtained between the two line Rayleigh scattering and thermocouple results in the particle free co-flowing air jet.

Otugen, M. V.↗
NASA NTRS1993

A two-stage (turbulent-drainage) mechanism for the emplacement of impact crater outflows on Venus

Several emplacement mechanisms were proposed for the unique flow features associated with 402 of the 912 impact craters found on Venus to date. Studies based on rheological models and laboratory results have suggested two separate depositional flow mechanisms: a turbulent emplacement and a lava like emplacement. Schultz proposed a progressive transition from the turbulent to the lava like mechanism during deceleration of some turbulent flows. He suggested that these composite flows were emplaced prior to the deposition of the normal ejecta. After an examination of the geomorphologic, stratigraphic, and topographic relations of the flows associated with the 402 craters, it is suggested that most of the outflows result from a two-stage emplacement process. In the first stage, a turbulent, proximal part is usually emplaced downrange of the impact site after the other ejecta materials. In the second stage, a distal part, composed of low-viscosity melt, slowly drains via dendritic channels from the proximal deposit and flows like lava. A few flows were identified that appear to have been emplaced prior to the ejecta; these flows likely result from a different mechanism.

Chadwick, D. J.↗
NASA NTRS1994

Cooling of the white dwarf in U Geminorum between outbursts

We have obtained far-ultraviolet (1150-1610 A) spectra of U Gem with the Faint Object Spectrograph on the Hubble Space Telescope approximately 13 and 70 days after the end of a normal dwarf nova outburst of the system. Both spectra appear dominated by the White Dwarf (WD) in U Gem. At 1400 A, the flux from U Gem declined approximately 28% between the two observations. Detailed comparison of the spectra with solar abundance WD models suggest that average surface temperature of the WD surface cooled from approximately 39,400 K to approximately 32,100 K between the two observations. The main features which are not fitted well by WD models are absorption due to N V lambda lambda-1239, 1243 and emission at Lyman-alpha. These observations provide unambiguous confirmation that the WD in U Gem cools during quiescence, as was suggested by analyses of far-ultraviolet spectra obtained with International Ultraviolet Explorer (IUE) and Hopkins Ultraviolet Telescope (HUT).

Long, Knox S.↗