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At least 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

O-18 tracer studies of Al2O3 scale formation on NiCrAl alloys

Diffusion processes in Al2O3 scales formed on NiCrAl + Zr alloys were studied by the proton activation technique employing the O-18 isotope as a tracer. The O-18 profiles identified a zone of oxide penetration beneath the external scale. Both this subscale formation and the outer Al2O3 scale thickness were shown by this technique to increase with Zr content in the alloy. Estimated kp's from scale thicknesses were in agreement with gravimetric measurements for various Zr levels. Alternate exposures in O-16 and O-18 revealed that oxygen inward transport was the primary growth mechanism. A qualitative analysis of these O-18 profiles indicated that the oxygen transport was primarily via short-circuit paths, such as grain boundaries.

Reddy, K. P. R.↗
NASA NTRS1982

Performance degradation and cleaning of photovoltaic arrays

NASA tests results from an 18 mo program of cleaning silicone-encapsulated and glass fronted solar cell panels in urban and desert environments to examine the effects of cleaning on module performance are reported. The panels were cleaned on weekly, monthly, quarterly, or semi-annual basis, while other panels of the same construction were not cleaned and served as controls. Commercially-available detergents and city water were employed for the tests, and the measurements were maintained of the modules' continuing short-circuit current output. The decay of the output was determined by least square regression analyses. Performance degradation was noticeably less in glass covered, rather than silicone-encapsulated modules which decayed faster in urban than in desert environments. Lower frequency cleanings are recommended where labor costs are high.

Sheskin, T. J.↗
NASA NTRS1983

Analysis of the interaction of an electron beam with back surface field solar cells

In this paper the short circuit current Isc induced by the electron beam of a scanning electron microscope in a back surface field solar cell will be determined theoretically. It will be shown that, in a configuration used previously for solar cells with an ohmic back surface, the Isc gives a convenient means for estimating the back surface recombination velocities and thus the quality of back surface field cells. Numerical data will be presented applicable to a point source model for the electron-hole pair generation.

Von Roos, O.↗
NASA NTRS1983

An EBIC equation for solar cells

When an electron beam of a scanning electron microscope (SEM) impinges on an N-P junction, the generation of electron-hole pairs by impact ionization causes a characteristic short circuit current I(sc) to flow. The I(sc), i.e., EBIC (electron beam induced current) depends strongly on the configuration used to investigate the cell's response. In this paper the case where the plane of the junction is perpendicular to the surface is considered. An EBIC equation amenable to numerical computations is derived as a function of cell thickness, source depth, surface recombination velocity, diffusion length, and distance of the junction to the beam-cell interaction point for a cell with an ohmic contact at its back surface. It is shown that the EBIC equation presented here is more general and easier to use than those previously reported. The effects of source depth, ohmic contact, and diffusion length on the normalized EBIC characteristic are discussed.

Luke, K. L.↗
NASA NTRS1983

Multiwavelength analyzer for the determination of diffusion lengths

The minority carrier diffusion length Ln in the base or substrate region is an important parameter which governs a solar cell's performance. The present investigation is concerned with the development of a multiwavelength analyzer (MWA) technique for the nondestructive spatial testing of polycrystalline solar cells. The MWA method is based on the utilization of the short-circuit current generated by two or more light-emitting diodes (LEDs) operating at different wavelengths and modulated 180 deg out-of-phase. For a determination of Ln by the MWA technique, it is necessary to know the value of the absorption coefficient.

Stafsudd, O. M.↗
NASA NTRS1985

Direct-Current Unbalance Detector

Short circuits to ground indicated by voltage changes. Current transducer connected in bridge circuit with floating 30-Vdc power supply measures voltage unbalance between positive and negative powerlines and ground. Detection circuit output swings higher or lower than normal balanced condition voltage to signify unbalance toward positive or negative side respectively. Circuit configuration adaptable to monitor 4- to 20-mA current loops often found in industrial controls.

Wagner, A. P.↗
NASA NTRS1983

Sputtering phenomena in ion thrusters

Sputtering effects in discharge chambers of ion thrusters are lifetime limiting in basically two ways: (1) ion bombardment of critical thruster components at energies sufficient to cause sputtering removes significant quantities of material; enough to degrade operation through adverse dimensional changes or possibly lead to complete component failure, and (2) metals sputtered from these intensely bombarded components are deposited in other locations as thin films and subsequently flake or peel off; the flakes then lodge elsewhere in the discharge chamber with the possibility of providing conductive paths for short circuiting of thruster components such as the ion optics. This experimental work has concentrated in two areas. The first has been to operate thrusters for multi-hour periods and to observe and measure the films found inside the thruster. The second was to simulate the environment inside the discharge chamber of the thruster by means of a dual ion beam system. Here, films were sputter deposited in the presence of a second low energy bombarding beam to simulate film deposition on thruster interior surfaces that undergo simultaneous sputtering and deposition. Mo presents serious problems for use in a thruster as far as film deposition is concerned. Mo films were found to be in high stress, making them more likely to peel and flake.

Robinson, R. S.↗
NASA NTRS1984

Advanced photovoltaic experiment (S0014)

The advanced photovoltaics-related experiments for investigating a portion of the solar spectrum and the effect of the space environment on photovoltaics. The information will be used to provide correlation between space and ground testing and also to provide for more accurate performance measurement in the laboratory. Specific objectives of these experiments are to provide information on the performance and endurance of advanced and conventional solar cells, to improve reference standards for photovoltaic measurements, and to measure the energy distribution in the extraterrestrial solar spectrum. Data to be obtained will include temperatures and short-circuit current of the samples. Six-point current-voltage (I-V) characteristics will be obtained for selected samples. These data will be recorded once a day during the flight. Orbit data will be correlated with preflight and postflight measurement of the samples.

Bradhorst, H. W., Jr.↗
NASA NTRS1984

Investigation of critical surface degradation effects on coatings and solar cells developed in Germany (S1002)

Various coatings developed in the FRG (i.e., second-surface mirrors with interference filters with and without conductive layers, conductive layers on solar-cell covers, and selective absorber coatings) have been qualified by accelerated tests under simulated space environment conditions. Experiments with coatings and solar cells have shown, however, that the thermo-optical behavior can differ considerably when performed on the ground and in space because of the great difficulty in simulating the space environment realistically. The objective of this experiment is to qualify these coatings under realistic space environment conditions. In addition, the experiment will provide design criteria, techniques, and test methods to insure control of the combined space and spacecraft environment effects, such as contamination, electrical conductance, and optical degradation, on the coatings. Data to be measured include the temperature of the samples, the electrical resistance of the conductive layers of the samples, the short circuit current of the solar-cell modules, and the deposition of contaminants on the samples (using quartz crystal microbalances (QCM's)).

Preuss, L.↗
NASA NTRS1983

Process Research On Polycrystalline Silicon Material (PROPSM)

The performance-limiting mechanisms in large-grain (greater than 1 to 2 mm in diameter) polycrystalline silicon solar cells were investigated by fabricating a matrix of 4 sq cm solar cells of various thickness from 10 cm x 10 cm polycrystalline silicon wafers of several bulk resistivities. Analysis of the illuminated I-V characteristics of these cells suggests that bulk recombination is the dominant factor limiting the short-circuit current. The average open-circuit voltage of the polycrystalline solar cells is 30 to 70 mV lower than that of co-processed single-crystal cells; the fill-factor is comparable. Both open-circuit voltage and fill-factor of the polycrystalline cells have substantial scatter that is not related to either thickness or resistivity. This implies that these characteristics are sensitive to an additional mechanism that is probably spatial in nature. A damage-gettering heat-treatment improved the minority-carrier diffusion length in low lifetime polycrystalline silicon, however, extended high temperature heat-treatment degraded the lifetime.

Culik, J. S.↗
NASA NTRS1984

Model of solar cell proton damage

A short circuit current (I sub sc) degradation model for gallium arsenide (GaAs) solar cells irradiated by protons from low energy to 100 MeV was developed. This model was found to be satisfactory in predicting the I sub sc degradation of GaAs cells, but not that of silicon (Si) cells. A modification to the aforementioned model that preserves the optical wavelength dependency in the photogeneration of minority carrier pairs was described herein. Spectral Si and GaAs response and I sub sc for the Si and GaAs solar cells were discussed, and also were presented in graph form. The overall predictability of the modified model was deemed to be satisfactory.

Yaung, J. Y.↗
NASA NTRS1984

Equivalent electron fluence for solar proton damage in GaAs shallow junction cells

The short-circuit current reduction in GaAs shallow junction heteroface solar cells was calculated according to a simplified solar cell damage model in which the nonuniformity of the damage as a function of penetration depth is treated explicitly. Although the equivalent electron fluence was not uniquely defined for low-energy monoenergetic proton exposure, an equivalent electron fluence is found for proton spectra characteristic of the space environment. The equivalent electron fluence ratio was calculated for a typical large solar flare event for which the proton spectrum is PHI(sub p)(E) = A/E(p/sq. cm) where E is in MeV. The equivalent fluence ratio is a function of the cover glass shield thickness or the corresponding cutoff energy E(sub c). In terms of the cutoff energy, the equivalent 1 MeV electron fluence ratio is r(sub p)(E sub c) = 10(9)/E(sub c)(1.8) where E(sub c) is in units of KeV.

Wilson, J. W.↗
NASA NTRS1983

Surface and allied studies in silicon solar cells

Two main results are presented. The first deals with a simple method that determines the minority-carrier lifetime and the effective surface recombination velocity of the quasi-neutral base of silicon solar cells. The method requires the observation of only a single transient, and is amenable to automation for in-process monitoring in manufacturing. This method, which is called short-circuit current decay, avoids distortion in the observed transient and consequent inacccuracies that arise from the presence of mobile holes and electrons stored in the p/n junction spacecharge region at the initial instant of the transient. The second main result consists in a formulation of the relevant boundary-value problems that resembles that used in linear two-port network theory. This formulation enables comparisons to be made among various contending methods for measuring material parameters of p/n junction devices, and enables the option of putting the description in the time domain of the transient studies in the form of an infinite series, although closed-form solutions are also possible.

Lindholm, F. A.↗
NASA NTRS1984

The effects of lithium counterdoping on radiation damage and annealing in n(+)p silicon solar cells

Boron-doped silicon n(+)p solar cells were counterdoped with lithium by ion implantation and the resultant n(+)p cells irradiated by 1 MeV electrons. Performance parameters were determined as a function of fluence and a deep level transient spectroscopy (DLTS) study was conducted. The lithium counterdoped cells exhibited significantly increased radiation resistance when compared to boron doped control cells. Isochronal annealing studies of cell performance indicate that significant annealing occurs at 100 C. Isochronal annealing of the deep level defects showed a correlation between a single defect at E sub v + 0.43 eV and the annealing behavior of short circuit current in the counterdoped cells. The annealing behavior was controlled by dissociation and recombination of this defect. The DLTS studies showed that counterdoping with lithium eliminated three deep level defects and resulted in three new defects. The increased radiation resistance of the counterdoped cells is due to the interaction of lithium with oxygen, single vacancies and divacancies. The lithium-oxygen interaction is the most effective in contributing to the increased radiation resistance.

Weinberg, I.↗
NASA NTRS1984

Definition, analysis and development of an optical data distribution network for integrated avionics and control systems. Part 2: Component development and system integration

Fiber optic transmission is emerging as an attractive concept in data distribution onboard civil aircraft. Development of an Optical Data Distribution Network for Integrated Avionics and Control Systems for commercial aircraft will provide a data distribution network that gives freedom from EMI-RFI and ground loop problems, eliminates crosstalk and short circuits, provides protection and immunity from lightning induced transients and give a large bandwidth data transmission capability. In addition there is a potential for significantly reducing the weight and increasing the reliability over conventional data distribution networks. Wavelength Division Multiplexing (WDM) is a candidate method for data communication between the various avionic subsystems. With WDM all systems could conceptually communicate with each other without time sharing and requiring complicated coding schemes for each computer and subsystem to recognize a message. However, the state of the art of optical technology limits the application of fiber optics in advanced integrated avionics and control systems. Therefore, it is necessary to address the architecture for a fiber optics data distribution system for integrated avionics and control systems as well as develop prototype components and systems.

Yen, H. W.↗
NASA NTRS1984

Radiation effects in silicon and gallium arsenide solar cells using isotropic and normally incident radiation

Several types of silicon and gallium arsenide solar cells were irradiated with protons with energies between 50 keV and 10 MeV at both normal and isotropic incidence. Damage coefficients for maximum power relative to 10 MeV were derived for these cells for both cases of omni-directional and normal incidence. The damage coefficients for the silicon cells were found to be somewhat lower than those quoted in the Solar Cell Radiation Handbook. These values were used to compute omni-directional damage coefficients suitable for solar cells protected by coverglasses of practical thickness, which in turn were used to compute solar cell degradation in two proton-dominated orbits. In spite of the difference in the low energy proton damage coefficients, the difference between the handbook prediction and the prediction using the newly derived values was negligible. Damage coefficients for GaAs solar cells for short circuit current, open circuit voltage, and maximum power were also computed relative to 10 MeV protons. They were used to predict cell degradation in the same two orbits and in a 5600 nmi orbit. Results show the performance of the GaAs solar cells in these orbits to be superior to that of the Si cells.

Anspaugh, B. E.↗
NASA NTRS1983

Inherent overload protection for the series resonant converter

The overload characteristics of the full bridge series resonant power converter are considered. This includes analyses of the two most common control methods presently in use. The first of these uses a current zero crossing detector to synchronize the control signals and is referred to as the alpha controller. The second is driven by a voltage controlled oscillator and is referred to as the gamma controller. It is shown that the gamma controller has certain reliability advantages in that it can be designed with inherent short circuit protection. Experimental results are included for an 86 kHz converter using power metal-oxide-semiconductor field-effect transistors (MOSFETs).

King, R. J.↗
NASA NTRS1985

Measuring Recombination Velocities in Solar Cells

Electron-beam scanning method suggested for determining back-surface recombination velocity, SB, of back-surface-field (BSF) solar photovoltaic cell. Recombination velocity important because smaller value correlated with smaller saturation current and therefore higher cell efficiency. Method involves analyzing short-circuit current measured while electron beam scanned along edge of cell.

Vonroos, O.↗