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At least 1,765 records · Page 98

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NASA NTRS1981

Metal/silicon interface formation - The Ni/Si and Pd/Si systems

The valence level spectra of the Ni/Si and Pd/Si systems have been investigated using high resolution X-ray photoelectron spectroscopy. Temperature dependence studies for Ni deposited on thin thermal SiO2 demonstrate the importance of metal aggregation effects in the interpretation of binding energies as chemical shifts. Temperature studies for the Ni/Si system indicate that substantial chemical interaction occurs at the interface at temperatures as low as 100 K. These studies also show the presence of Ni in interstitial voids in the Si near the interface. A comparative study of the core and valence band features for the Ni and Pd silicides provides many valuable insights and a self-consistent picture of the attendent valence charge redistribution and its influence on the observed chemical shifts.

Grunthaner, P. J.↗

NASA NTRS1981

A general purpose computer-to-computer interface Xerox Sigma computer-to-DEC PDP 11 computer

The Sigma/PDP 11 Interface is a general-purpose system for interconnecting a Xerox Sigma series computer and a DEC PDP 11 series computer. Although the unit is designed to operate in a real-time environment, it can be used in nonreal-time modes as well. Several features, designed to facilitate using the system in varied applications are included in the unit; for example, the features can be used to isolate software or hardware faults. This paper contains descriptions of the hardware and software operations of the interface and examples of real-time applications in the research environment at Ames Research Center.

Fetter, J. L.↗

NASA NTRS1982

Behavior of the Si/SiO2 interface observed by Fowler-Nordheim tunneling

Thin-oxide (40-50 A) metal oxide semiconductor (MOS) structures are shown to exhibit, before large levels of electron tunnel injection, the near-ideal behavior predicted for a uniform trapezoidal barrier with thick-oxide properties. The oscillatory field dependence caused by electron-wave interference at the Si/SiO2 interface suggests an abrupt, one-monolayer barrier transition (approximately 2.5 A) consistent with earlier work. After tunnel injection of 10 to the 17th - 5 x 10 to the 18th electrons/sq cm, the barrier undergoes appreciable degradation, leading to enhanced tunneling conductance. Reproducible behavior is observed among different samples. This effect is found to be consistent with the generation of positive states in the region of the oxide near the Si/SiO2 interface (less than 20 A), where the tunneling electrons emerge into the oxide conduction band.

Maserjian, J.↗

NASA NTRS1982

Refraction at a curved dielectric interface - Geometrical optics solution

The transmission of a spherical or plane wave through an arbitrarily curved dielectric interface is solved by the geometrical optics theory. The transmitted field is proportional to the product of the conventional Fresnel's transmission coefficient and a divergence factor (DF), which describes the cross-sectional variation (convergence or divergence) of a ray pencil as the latter propagates in the transmitted region. The factor DF depends on the incident wavefront, the curvatures of the interface, and the relative indices of the two media. Explicit matrix formulas for calculating DF are given, and its physical significance is illustrated via examples.

Lee, S.-W.↗

NASA NTRS1984

Self-Checking Memory Interface

Memory-interface integrated circuit not only detects errors in data from other circuits but also detects errors within itself. Memory-interface chip encodes 16-bit words with Hamming code for single-error correction or double-error detection. Chip used in fault-tolerant computers under development by NASA.

Sievers, M. W.↗

NASA NTRS1982

RF Front End Interface and AGC Modification

The latest RF front end built by Burhans (Mini-L-82) was successfully interfaced to the Ohio University LORAN-C receiver. In order for the front end to operate optimally, modifications were made to existing automatic gain control (AGC) circuitry already developed for the Ohio University LORAN-C receiver. The hardware modifications to the AGC and other interface circuitry, as well as some preliminary results are discussed.

Yost, S. R.↗

NASA NTRS1981

Toward multidisciplinary use of LANDSAT: Interfacing computerized LANDSAT analysis systems with geographic information systems

The LANDSAT-geographic information system (GIS) interface must summarize the results of the LANDSAT classification over the same cells that serve as geographic referencing units for the GIS, and output these summaries on a cell-by-cell basis in a form that is readable by the input routines of the GIS. The ZONAL interface for cell-oriented systems consists of two primary programs. The PIXCEL program scans the grid of cells and outputs a channel of pixels. Each pixel contains not the reflectance values but the identifier of the cell in which the center of the pixel is located. This file of pixelized cells along with the results of a pixel-by-pixel classification of the scene produced by the LANDSAT analysis system are input to the CELSUM program which then outputs a cell-by-cell summary formatted according to the requirements of the host GIS. Cross-correlation of the LANDSAT layer with the other layers in the data base is accomplished with the analysis and display facilities of the GIS.

Myers, W. L.↗

NASA NTRS1983

Convection effects at solid-liquid interfaces: Influence of gravity

The stability of the flow between two vertical, infinite, rigid, coaxial cylinders held at different temperatures is analyzed by linear stability theory. For a Prandtl number of 22.8 and a radius ratio of 0.02, the flow is unstable to an axisymmetric perturbation at a critical Grashof number of 2150; the wave speed of the instability is comparable to the maximum velocity of the unperturbed flow. When the rigid outer cylinder-fluid interface is replaced by a crystal-melt interface which can change shape, two new modes of instability occur at lower Grashof numbers.

Glicksman, M. E.↗

NASA NTRS1983

Design of a microprocessor-based Control, Interface and Monitoring (CIM unit for turbine engine controls research

High speed minicomputers were used in the past to implement advanced digital control algorithms for turbine engines. These minicomputers are typically large and expensive. It is desirable for a number of reasons to use microprocessor-based systems for future controls research. They are relatively compact, inexpensive, and are representative of the hardware that would be used for actual engine-mounted controls. The Control, Interface, and Monitoring Unit (CIM) contains a microprocessor-based controls computer, necessary interface hardware and a system to monitor while it is running an engine. It is presently being used to evaluate an advanced turbofan engine control algorithm.

Delaat, J. C.↗

NASA NTRS1983

Interface states and internal photoemission in p-type GaAs metal-oxide-semiconductor surfaces

An interface photodischarge study of p-type GaAs metal-oxide-semiconductor (MOS) structures revealed the presence of deep interface states and shallow donors and acceptors which were previously observed in n-type GaAs MOS through sub-band-gap photoionization transitions. For higher photon energies, internal photoemission was observed, i.e., injection of electrons to the conduction band of the oxide from either the metal (Au) or from the GaAs valence band; the threshold energies were found to be 3.25 and 3.7 + or - 0.1 eV, respectively. The measured photoemission current exhibited a thermal activation energy of about 0.06 eV, which is consistent with a hopping mechanism of electron transport in the oxide.

Kashkarov, P. K.↗

NASA NTRS1983

Generalized modal shock spectra within indeterminate interface

It is noted that the generalized modal shock spectra method had limitations on the degree of structural static determinacy of the spacecraft-to-launch-vehicle interface. The 'interface modes' is used here to remove these practical limitations. The governing differential equations are derived and then shown to be valid either for integrating the modal models of two or more substructures that have been previously obtained separately or for removing a substructure from a previously available system modal model. Emphasis is placed on the integration problems in the context of the generalized modal shock spectra approach. A minimum of information the modal response of a previously obtained transient analysis of the unloaded launch vehicle is exploited in order to define an idealized modal impulse function and derive approximate explicit expressions for an estimate of the bounds of the spacecraft response. In addition, a numerical model is solved in order to compare the present results with the transient solution.

Salama, M.↗

NASA NTRS1982

Effect of heat transfer of melt/solid interface shape and solute segregation in Edge-Defined Film-Fed growth - Finite element analysis

The effects of the heat transfer environment in Edge-Defined Film-Fed Growth on melt-solid interface shape and lateral dopant segregation are studied by finite-element analysis of two-dimensional models for heat and mass transfer. Heat transfer configurations are studied that correspond to the uniform surroundings assumed in previous models and to lowand high-speed growth systems. The maximum growth rate for a silicon sheet is calculated and the range of validity of one-dimensional heat transfer models is established. The lateral segregation that results from curvature of the solidification interface is calculated for two solutes, boron and aluminum. In this way, heat transfer is linked directly to the uniformity of the product crystal.

Ettouney, H. M.↗

NASA NTRS1983

Techniques for improving the Si-SiO2 interface characterization

The two techniques which have provided most of the information on interface states in MIS-C (metal-insulator-semiconductor-capacitor) structures are the 'quasi-static method' and the 'conductance method'. Sher et al. (1979) and Su et al. (1980) have suggested a number of improvements concerning these methods. The present investigation has the objective to extend the earlier results and to offer a new tentative interpretation of the data. A critical review is conducted of the data collection and reduction techniques for the quasi-static method, taking into account the sample, the quasi-static capacitance, and the surface potential. In connection with a discussion of the conductance method, attention is given to parallel conductance and capacitance measurements, interface-state densities, time constants, and measurements on a (110) surface orientation.

Sher, A.↗

NASA NTRS1983

The control of float zone interfaces by the use of selected boundary conditions

The main goal of the float zone crystal growth project of NASA's Materials Processing in Space Program is to thoroughly understand the molten zone/freezing crystal system and all the mechanisms that govern this system. The surface boundary conditions required to give flat float zone solid melt interfaces were studied and computed. The results provide float zone furnace designers with better methods for controlling solid melt interface shapes and for computing thermal profiles and gradients. Documentation and a user's guide were provided for the computer software.

Foster, L. M.↗

NASA NTRS1984

Stability of finite difference models containing two boundaries or interfaces

The stability of finite difference models of hyperbolic initial boundary value problems is connected with the propagation and reflection of parasitic waves. Wave propagation ideas are applied to models containing two boundaires or interfaces, where repeated reflection of trapped wave packets is a potential new source of instability. Various known instability phenomena are accounted for in a unified way. Results show: (1) dissipativity does not ensure stability when three or more formulas are concatenated at a boundary or internal interface; (2) algebraic GKS instabilities can be converted by a second boundary to exponential instabilities only when an infinite numerical reflection coefficient is present; and (3) GKS-stability and P-stability can be established in certain problems by showing that all numerical reflection coefficients have modulus less than 1.

Trefethen, L. N.↗

NASA NTRS1983

Satellite services handbook. Interface guidelines

Satellite interfaces for on orbit servicing, both manned and unmanned are identified, and is intended to be used by designers of space vehicles, both foreign and domestic. A primary concern is for design of interfaces with the astronaut in the loop, especially extravehicular activity, but also intravehicular activity and operations that are remote but have man-in-the-loop. The main emphasis is on servicing in low earth orbits from the Space Shuttle and also from the Space Station or other platforms.

Source record↗

NASA NTRS1983

Radial segregation induced by natural convection and melt/solid interface shape in vertical Bridgman growth

The roles of natural convection in the melt and the shape of the melt/solid interface on radial dopant segregation are analyzed for a prototype of vertical Bridgman crystal growth system by finite element methods that solve simultaneously for the velocity field in the melt, the shape of the solidification isotherm, and the temperature distribution in both phases. Results are presented for crystal and melt with thermophysical properties similar to those of gallium-doped germanium in Bridgman configurations with melt below (thermally destabilizing) and above (stabilizing) the crystal. Steady axisymmetric flow are classified according to Rayleigh number as either being nearly the growth velocity, having a weak cellular structure or having large amplitude cellular convention. The flows in the two Bridgman configurations are driven by different temperature gradients and are in opposite directions. Finite element calculations for the transport of a dilute dopant by these flow fields reveal radial segregation levels as large as sixty percent of the mean concentration. Segregation is found most severe at an intermediate value of Rayleigh number above which the dopant distribution along the interface levels as the intensity of the flow increases.

Chang, C. J.↗

NASA NTRS1983

Is structural interface standardization beneficial?

Factors applicable to fixed angle, large field and fixed angle, large building flat plate photovoltaic (PV) generator arrays are discussed in the context of standardization. It is concluded that structural interface standardization may be highly desirable in any one major project, but not at this time in the overall PV industry. Attempts to mandate such standardization will act as a deterrent to long-range improvements. In specific projects, structural standardization should be defined at the largest practical interface, leaving the maximum possible freedom to the module and array manufacturer. There is a corollary area, however, where detailed standards would benefit the industry; the matter of Standard Practices. Work being done towards definition of acceptable/desirable practices in materials, finishes, fastening and locking methods, grounding techniques, lightning protection, etc., and in handling the environmental ranges, should be continued.

Dombert, W. E.↗