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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 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Flux lattice melting in the high Tc superconductors

One of the important issues for technological application of the high temperature superconductors is their behavior in a magnetic field. A variety of experiments including electrical transport, mechanical oscillators, and magnetic decoration have suggested that these magnetic properties will make applications more difficult than originally anticipated. These experiments and their results are briefly discussed.

Bishop, D. J.↗
NASA NTRS1991

Self-consistent analysis of lattice-matched and pseudomorphic quantum-well emission transistors

A self-consistent analysis of the quantum-well emission transistor (QWET) is presented allowing an exact calculation of the device quantum properties. Poisson's and Schroedinger's equation are solved numerically using a finite-difference method on a self-consistent basis. Pseudomorphic AlGaAs/InGaAs designs with 15-20 percent excess In are suggested for improving the device performance. Design with doping in various parts of the QWET are also studied. This analysis reveals that the device performance is less optimistic than previously predicted by analytic approaches. By introducing the pseudomorphic channel principle, while maintaining a reasonably low Al content for the gate and collector layers, it is, however, possible to obtain satisfactory performance. Optimum pseudomorphic designs showed high current driving capability (200,000 A/sq cm), high transconductance (3S/mm), and small intrinsic delay time (2 ps).

Hong, Kyushik↗
NASA NTRS1991

Frequency-dependent characteristics and trap studies of lattice-matched (x = 0.53) and strained (x greater than 0.53) In(0.52)Al(0.48)As/In(x)Ga(1-x)As HEMT's

The low-frequency characteristics of InAlAs/In(x)Ga(1-x)As high-electron-mobility transistors (HEMTs) are studied, and trap densities are evaluated. The HEMTs' transconductance g(m) and output resistance R(ds) dispersion are smallest for 60 percent indium (In) content and largest for 53 percent In. The maximum dispersion for the 53 percent In sample is about 6 percent for g(m) and about 13.3 percent for R(ds), corresponding to lower values than observed in MESFETs. The R(ds) dispersion characteristics are weaker than in AlGaAs/GaAs HEMTs and manifest themselves primarily up to 100 kHz. An analysis of the dispersion results indicates that, unlike in the case of MESFETs, the channel region under the gate rather than the access regions is responsible for the dispersion. Interface state densities were extracted by the AC conductance method and were found to follow the same trend as the g(m) and R(ds) dispersion.

Ng, Geok-Ing↗
NASA NTRS1942

Diagrams for calculation of airfoil lattices

The field for curved blades is represented by a vortex series with a vortex removed at the blade point. Further, an example of calculation of a curved blade from this series is given, whereby the necessary accuracy required of the different methods in practice is shown according to the case considered.

Betz, Albert↗
NASA NTRS1994

Martian environmental simulation for a deployable lattice mast

The Mars Pathfinder mission (formerly Mars Environmental Survey or MESUR) is scheduled for launch in December 1996 and is designed to place a small lander on the surface of Mars. After impact, the lander unfolds to expose its solar panels and release a miniature rover. Also on board is the Imager for Mars Pathfinder (IMP) binocular camera which is elevated by a deployable mast to obtain a panoramic view of the landing area. The design of this deployable mast is based on similar designs which have a long and successful flight history. In the past when this type of self-deployable mast has been used, a rate limiter has been incorporated to control the speed of deployment. In this application, to reduce weight and complexity, it was proposed to eliminate the rate limiter so that the mast would deploy without restraint. Preliminary tests showed that this type of deployment was possible especially if the deployed length was relatively short, as in this application. Compounding the problem, however, was the requirement to deploy the mast at an angle of up to 30 degrees from vertical. The deployment process was difficult to completely analyze due to the effects of gravitational and inertial loads on the mast and camera during rapid extension. Testing in a realistic manner was imperative to verify the system performance. A deployment test was therefore performed to determine the maximum tilt angle at which the mast could reliably extend and support the camera on Mars. The testing of the deployable mast requires partial gravity compensation to simulate the smaller force of Martian gravity. During the test, mass properties were maintained while weight properties were reduced. This paper describes the testing of a deployable mast in a simulated Martian environment as well as the results of the tests.

Warden, Robert M.↗
NASA NTRS2001

System Identification of a Vortex Lattice Aerodynamic Model

The state-space presentation of an aerodynamic vortex model is considered from a classical and system identification perspective. Using an aerodynamic vortex model as a numerical simulator of a wing tunnel experiment, both full state and limited state data or measurements are considered. Two possible approaches for system identification are presented and modal controllability and observability are also considered. The theory then is applied to the system identification of a flow over an aerodynamic delta wing and typical results are presented.

Juang, Jer-Nan↗
NASA NTRS2002

Carbon Nanostructure Examined by Lattice Fringe Analysis of High Resolution Transmission Electron Microscopy Images

The dimensions of graphitic layer planes directly affect the reactivity of soot towards oxidation and growth. Quantification of graphitic structure could be used to develop and test correlations between the soot nanostructure and its reactivity. Based upon transmission electron microscopy images, this paper provides a demonstration of the robustness of a fringe image analysis code for determining the level of graphitic structure within nanoscale carbon, i.e. soot. Results, in the form of histograms of graphitic layer plane lengths, are compared to their determination through Raman analysis.

VanderWal, Randy L.↗
NASA NTRS2003

The Traps of using Conventional Methodology of Evaluation of Powder Diffractograms for Determination of the Lattice Parameters of Nanocrystals

Inherent errors introduced by the algorithm of standard procedures of evaluation of powder diffractograms when applied for nanopowders are shown. The implications of the errors are discussed in connection with some results reported in the literature. Alternate methodology of evaluation of powder diffractograms of nanocrystals is presented.

Curreri, Peter A.↗
NASA NTRS2003

Carbon Nanostructure Examined by Lattice Fringe Analysis of High Resolution Transmission Electron Microscopy Images

The dimensions of graphitic layer planes directly affect the reactivity of soot towards oxidation and growth. Quantification of graphitic structure could be used to develop and test correlations between the soot nanostructure and its reactivity. Based upon transmission electron microscopy images, this paper provides a demonstration of the robustness of a fringe image analysis code for determining the level of graphitic structure within nanoscale carbon, i.e., soot. Results, in the form of histograms of graphitic layer plane lengths, are compared to their determination through Raman analysis.

VanderWal, Randy L.↗
NASA NTRS2006

Growth and Properties of Lattice Matched GaAsSbN Epilayer on GaAs for Solar Cell Applications

The growth and properties of GaAsSbN single quantum wells (SQWs) are investigated in this work. The heterostructures were grown on GaAs substrates in an elemental solid source molecular beam epitaxy (MBE) system assisted with a RF plasma nitrogen source. A systematic study has been carried out to determine the influence of various growth conditions, such as the growth temperature and the source shutter-opening sequence, on the quality of the grown layers and the incorporation of N and Sb. The effects of ex situ and in situ annealing under As overpressure on the optical properties of the layers have also been investigated. Substrate temperature in the range of 450-470 C was found to be optimum. Simultaneous opening of the source shutters was found to yield sharper QW interfaces. N and Sb incorporations were found to depend strongly upon substrate temperatures and source shutter opening sequences. A significant increase in PL intensity with a narrowing of PL line shape and blue shift in emission energy were observed on annealing the GaAsSbN/GaAs SQW, with in situ annealing under As overpressure providing better results, compared to ex situ annealing.

Bharatan, Sudhakar↗