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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 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Improvement of program to calculate electronic properties of narrow band gap materials

The program was improved by reprogramming it so it will run on both a SUN and a VAX. Also it is easily transportable as it is on a disk for use on a SUN. A computer literature search resulted in some improved parameters for Hg(1-x)Cd(x)Te and a table of parameters for Hg(1-x)Zn(x)Te. The effects of neutral defects were added to the program, and it was found, as expected, that they contribute very little to the mobility at temperatures of interest. The effect were added of varying the following parameters: dielectric constants, screening parameters, disorder energies, donor and acceptor concentrations, momentum matrix element, different expressions for energy gap, and transverse effective charge.

Patterson, James D.↗
NASA NTRS1991

Solar cycle variation in the subauroral electron temperature enhancement - Comparison of AE-C and DE 2 satellite observations

The elevation of the subauroral electron temperature is one of the phenomena showing the energy transfer from the magnetosphere and the response of the ionosphere. This study addresses solar cycle variations in the subauroral Te peak by comparing observations of the subauroral peak by the Atmosphere Explorer C (AE-C) satellite near solar minimum (1974 and 1977) with similar observations by the Dynamics Explorer 2 (DE 2) satellite near solar maximum (1981-1982). Te peaks with magnitudes sufficient to produce observable stable auroral red arc emissions occurred more frequently during solar maximum than in solar minimum, but the variation in the magnitudes and positions of these peaks with magnetic activity did not change significantly with solar cycle. These results are discussed in terms of the solar cycle changes in the ionosphere and the magnetospheric energy source.

Fok, M.-C.↗
NASA NTRS1991

Transport of gyration-dominated space plasmas of thermal origin. I - Generalized transport equations

Grad's 20-moment set of transport equations has been examined in the limit of strong external magnetic fields. This simplified set of equations describes the transport of mass and parallel momentum as well as the transport of parallel and perpendicular energy and heat flow in the magnetic field direction. The effect of collisions was calculated assuming a modified relaxation model. Wave speeds and normal modes of the simplified set of equations were examined for an ion and electron gas. It was found that four of the ten normal modes are electron thermal-heat waves which approximately decouple from the six ion waves in the system. When low-frequency waves are considered (slow-wave approximation), this allows the electron energy and heat flow equations to be solved separately from the ion equations and in a time-independent fashion. When this was done, it was found that, under certain conditions, these equations predict an infinite electron perpendicular temperature, Te-perpendicular, in the collisionless regime. This occurs whenever Te-perpendicular is greater than the parallel temperature, at any point along collisionless and diverging magnetic field lines. When applied appropriately, the significantly simplified set of equations derived here are well suited for application to a large variety of problems in planetary ionospheres and magnetospheres.

Gombosi, Tamas I.↗
NASA NTRS1991

Directional solidification of HgCdTe and HgZnTe in a transverse magnetic field

Hg(0.80)Cd(0.20)Te crystals were grown vertically in a transverse magnetic field by directional solidification. The effect of a magnetic field on the nature of fluid flow in the melts was investigated by measuring compositional variations along the axial and radial directions of the grown ingots. Magnetic field effects were shown to be significant over the entire field range employed (i.e., 2 to 5 kG). The axial compositional profiles (determined by precision density measurements) showed an abrupt decrease in the mole fraction of CdTe when the field was applied. Radial compositional mapping by IR transmission and X-ray energy dispersion spectrometry indicated that the solid-liquid interface evolved through three stages when the field was applied (i.e., from a radially symmetric concave interface to an off-center concave shape when the field was initially applied, then to a tilted plane, and, finally, to an off-center concave interface). The axial compositional profile of an Hg(0.84)Zn(0.16)Te ingot showed similar field effects.

Su, Ching-Hua↗
NASA NTRS1991

SP-100 space subsystems development progress

The space technology effort related to SP-100 subsystems is described in terms of the areas of primary focus and most significant progress. The SP-100 is briefly compared to the Voyager, and detailed descriptions of the converter subsystem, the heat-transport system, and the heat-rejection subsystem. Progress on the converter subsystem includes a high-voltage insulator composed of a single sapphire crystal, a compliant pad of coated niobium, an SiGe thermoelectric (TE) module, and a TE cell assembly. The test of the Nb1Zr piping related to the heat-transport subsystem is described, and the development is reported for the TEM pump and the gas separator. It is concluded that the critical technical issues related to the technologies have been addressed although further efforts are required. Future testing is described for the three major components of the space subsystems including the converter, pump, and the radiator.

Mondt, Jack F.↗
NASA NTRS1992

Correction of high-frequency noise-temperature inaccuracies

Deep-space mission data rates to Earth are limited by the system operating noise-temperature (T(sub op)) performance of the DSN. This article addresses some of the techniques and definitions used for measuring and reporting the effective noise temperature of receivers (Te) and T(sub op) of the DSN's ground receiving systems. Calibration loads are used to measure T(sub op) of the DSN antennas. At 32 GHz, a calibration load cooled to 2-K physical temperature requires a correction of 0.67 K to determine the noise temperature. Using corrected noise temperature for the calibration loads results in the correct values for T(sub op) such that the total system noise power can be defined by Pn = kT(sub op) B, as required for DSN telecommunications design control tables. T(sub op) and Te should not be converted to equivalent physical temperatures.

Stelzreid, C. T.↗
NASA NTRS1993

System performance and on-orbit operations of the Crystal Growth Furnace (CGF) on the first United States microgravity laboratory mission

The paper discusses the design, the performance, and on-orbit operations of the Crystal Growth Furnace (CGF) system flown for the first time aboard the Orbiter Columbia on STS-50. Four experiments were carried out: (1) crystal growth of (Hg,Zn)Te by chemical vapor transport, (2) seeded Bridgman growth of Zn-doped CdTe, (3) Bridgman growth of Se-doped GaAs, and (4) crystal growth of (Hg,Zn)Te by directional solidification. The results of a system performance data analysis demonstrate that all the elements of the CGF system functioned flawlessly. Seven out of eight samples carried on board yielded the best possible science return to date, surpassing previous microgravity material processing experiments in space.

Srinivas, R.↗
NASA NTRS1993

Study of tellurium precipitates in CdTe crystals

The effect of tellurium precipitates was studied in medium resistivity (10 exp 3-10 exp 6 ohm cm) undoped and Cl-doped CdTe using differential scanning calorimetry (DSC) and mid-infrared spectroscopy and the results were correlated with near-infrared microscopy photographs. When present in a significant quantity (about 0.25 wt pct), we show that Te precipitates are detectable using DSC measurements. In the mid-infrared, the contribution of the absorption by free-carriers is negligible, and therefore, the effect of the Te precipitates in these crystals can be considered uncoupled from the effects of Cd vacancies.

Jayatirtha, H. N.↗
NASA NTRS1993

C III spectra in WC Wolf-Rayet stars - Does collisional excitation dominate?

A direct comparison of the spectra emitted by an improved collisionally excited C III atomic model, with observations of C III spectra in Wolf-Rayet WC stars, shows agreement for UV, visible, and near-infrared lines including lines usually considered to be recombination lines. The agreement implies high-density and temperature source conditions corresponding to log (Ne Te) is greater than 16 as a lower limit, whereas most current modeling assumes log (Ne Te) is less than 15.5. This raises questions concerning the photoionization/recombination assumptions on which most WR modeling is based. Recent models are discussed from this point of view.

Kastner, S. O.↗
NASA NTRS1993

Initial development of a high-pressure crystal growth facility: Center director's discretionary fund

A low-cost, flexible, high-pressure (600 psi) system for crystal growth and related thermophysical properties measurements was designed, assembled, and tested. The furnace system includes a magnetically coupled translation mechanism that eliminates the need for a high-pressure mechanical feedthru. The system is currently being used for continuing crystal growth experiments and thermophysical properties measurements on several material systems including Hg(1-x)Cd(x)Te, Hg(1-x)Zn(x)Te, and Hg(1-x)Zn(x)Se.

Szofran, F. R.↗
NASA NTRS1995

Designing A Mode Converter For Use With A Gyrotron

Report describes process of designing corrugated, circular-cross-section length of waveguide converting input electromagnetic radiation at frequency of 34.5 GHz in TM(11) mode to output radiation in HE(11) mode. TM(11)-mode input radiation supplied by gyrotron generating continuous-wave power of 200 kW at 34.5 GHz in TE(01) mode followed by TE(01)-to-TM(11) mode converter. Together, gyrotron and mode converters constitute prototype high-power transmitter for long-distance free-space communication.

Hoppe, Daniel J.↗
NASA NTRS1995

Ulysses observations of electron and proton components in a magnetic cloud and related wave activity

In addition to a smooth rotation of the magnetic field vector, magnetic clouds have a low proton temperature T(sub p). Their expansion in the solar wind leads to depletion and therefore the ion component cools down. It has been shown recently that the electron component in magnetic clouds behaves differently: when the cloud expands, electron temperature Te anti correlates with density and therefore Te increases in the cloud, creating favorable conditions for the rise of ion-acoustic waves. For the magnetic cloud observed by Ulysses on June 10 - 12, 1993 at 4.64 AU at S 32.5 deg, we present observations for both electron and proton components and related plasma wave activity. Our results confirm the anti correlation between T(sub e) and electron density and also exhibit a high ratio of T(sub e)/T(sub P) in the cloud. Since Landau damping is not effective for T(sub e)/T(sub p) much greater than 1, Doppler shifted ion acoustic waves are expected in the cloud. Calculation of ion acoustic wave frequencies in the cloud and comparison with observed wave activity confirm this expectation. As in our previous work, we show that the electron component in the cloud obeys a polytropic law with gamma is less than 1 (gamma approximately equals 0.3-0.4). The dynamics of the magnetic cloud are determined to a large degree by the dominating electron pressure.

Osherovich, V. A.↗
NASA NTRS1996

Convective Influence on Radial Segregation During Unidirectional Solidification of the Binary Alloy HgCdTe

In order to simulate the space environment for basic research into the crystal growth mechanism, Hg(0.8)Cd(0.2)Te crystals were grown by the vertical Bridgman-Stockbarger method in the presence of an applied axial magnetic field. The influence of convection, by magneto hydrodynamic damping, on mass transfer in the melt and segregation at the solid-liquid interface was investigated by measuring the axial and radial compositional variations in the grown samples. The reduction of convective mixing in the melt through the application of the magnetic field is found to have a large effect on radial segregation and interface morphology in the grown crystals. Direct comparisons are made with a Hg(0.8)Cd(0.2)Te crystal grown without field and also in the microgravity environment of space during the second United States Microgravity Payload Mission (USMP-2).

Watring, D. A.↗
NASA NTRS1997

Design of Launch Vehicle Flight Control Augmentors and Resulting Flight Stability and Control (Center Director's Discretionary Fund Project 93-05, Part III)

This publication presents the control requirements, the details of the designed Flight Control Augmentor's (FCA's), the static stability and dynamic stability wind tunnel test programs, the static stability and control analyses, the dynamic stability characteristics of the experimental Launch Vehicle (LV) with the designed FCA's, and a consideration of the elastic vehicle. Dramatic improvements in flight stability have been realized with all the FCA designs; these ranged from 41 percent to 72 percent achieved by the blunt TE design. The control analysis showed that control increased 110 percent with only 3 degrees of FCA deflection. The dynamic stability results showed improvements with all FCA designs tested at all Mach numbers tested. The blunt TE FCA's had the best overall dynamic stability results. Since the lowest elastic vehicle frequency must be well separated from that of the control system, the significant frequencies and modes of vibration have been identified, and the response spectra compared for the experimental LV in both the conventional and the aft cg configuration. Although the dynamic response was 150 percent greater in the aft cg configuration, the lowest bending mode frequency decreased by only 2.8 percent.

Barret, C.↗
NASA NTRS1997

Characterization of Cadmium-Zinc Telluride Crystals Grown by 'Contactless' PVT Using Synchrotron White Beam Topography

Crystals of Cd(1-x)Zn(x)Te grown by Physical Vapor Transport (PVT) using self-seeding 'contactless' techniques were characterized using synchrotron radiation (reflection, transmission, and Laue back-reflection X-ray topography). Crystals of low (x = 0.04) and high (up to x approx. = 0.4) ZnTe content were investigated. Twins and defects such as dislocations, precipitates, and slip bands were identified. Extensive inhomogeneous strains present in some samples were found to be generated by interaction (sticking) with the pedestal and by composition gradients in the crystals. Large (up to about 5 mm) oval strain fields were observed around some Te precipitates. Low angle grain boundaries were found only in higher ZnTe content (x greater than or equal to 0.2) samples.

Palosz, W.↗
NASA NTRS1998

Modeling Climate-Biosphere Interactions in the Boreal Forest

The overall goal of this BOREAS Program was to develop, test, and apply a model of the carbon balance of boreal forest sites with a significant groundcover component (moss or lichen). The basic question addressed with this model was: What is the sensitivity of the boreal forest carbon balance to weather variability? More specifically: What are the differences in the sensitivities of carbon gains (photosynthesis) and carbon losses (respiration) of the various components of the ecosystem? Are there different seasonalities to their sensitivities (e.g., warmer springs will have one effect, warmer summers a different effect)? What are the effects of different patterns of successive weather years (wet/dry, warm/cool)? What, for example, would be the difference in effects of two "warmer than normal" months-one with each day warmer than normal, and the other with three normal weeks and one very hot week? Due to weather variability, how "noisy" will any carbon flux or carbon pool signal be that we might use to try to detect change? The project resulted in the development of a new boreal forest ecosystem model. This model was the first model in the BOREAS project to look closely at the role of mosses in the ecosystem carbon balance, and also was the first model in the BOREAS project to look closely at interannual variability in carbon fluxes. Along with the work of many other groups, TE-19 modeling analysis pointed to the need for a second, longer field season in 1996, with particular focus on the spring and fall transitions and on ground vegetation. BOREAS groups TE-19 (Frolking), TGB-1 (Crill) & TGB-3 (Moore & Roulet) analyzed BOREAS data and other published and unpublished data to develop a relationship between peatland ecosystem productivity and incoming radiation, which is quite distinct from the upland ecosystem relationships observed in other studies.

Frolking, Steve↗
NASA NTRS1995

Recent Developments in Quantum-Well Infrared Photodetectors

Intrinsic infrared (IR) detectors in the long wavelength range (8-20 Am) are based on an optically excited interband transition, which promotes an electron across the band gap (E(sub g)) from the valence band to the conduction band as shown. These photoelectrons can be collected efficiently, thereby producing a photocurrent in the external circuit. Since the incoming photon has to promote an electron from the valence band to the conduction band, the energy of the photon (h(sub upsilon)) must be higher than the E(sub g) of the photosensitive material. Therefore, the spectral response of the detectors can be controlled by controlling the E(sub g) of the photosensitive material. Examples for such materials are Hg(1-x), Cd(x), Te, and Pb(1-x), Sn(x), Te, in which the energy gap can be controlled by varying x. This means detection of very-long-wavelength IR radiation up to 20 microns requires small band gaps down to 62 meV. It is well known that these low band gap materials, characterized by weak bonding and low melting points, are more difficult to grow and process than large-band gap semiconductors such as GaAs. These difficulties motivate the exploration of utilizing the intersub-band transitions in multiquantum well (MQW) structures made of more refractory large-band gap semiconductors. The idea of using MQW structures to detect IR radiation can be explained by using the basic principles of quantum mechanics. The quantum well is equivalent to the well-known particle in a box problem in quantum mechanics, which can be solved by the time independent Schroudiner equation.

Gunapala, S. D.↗
NASA NTRS1998

Experiment 2: Vapor Transport Crystal Growth of Mercury Cadmium Telluride in Microgravity- USML-2

The new epitaxial growth experiments of Hg(l-x)Cd(x)Te on (100) CdTe substrates by chemical vapor transport (CVT), using HgI2 as a transport agent, were performed in the transient growth regime of this ternary, heteroepitaxial system at normal and reduced gravity during the USML-2 flight. The surface and interface morphology, the compositional and structural uniformity, and carrier mobility of the epitaxial layer and islands grown in microgravity are measurably improved relative to ground specimens. These observations demonstrate the effects of convective flow on the transport, deposition, and growth processes of this solid-vapor system even in the transient growth regime. The properties of the Hg(l-x)Cd(x)Te layer grown in a microgravity environment compare quite favorably to those of layers obtained by other techniques.

Wiedemeier, H.↗