Search NASA⌕ Search

SEARCH · Search NASA

Results for “recombination layers”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Thermal coatings for titanium-aluminum alloys

Titanium aluminides and titanium alloys are candidate materials for use in hot structure and heat-shield components of hypersonic vehicles because of their good strength-to-weight characteristics at elevated temperature. However, in order to utilize their maximum temperature capability, they must be coated to resist oxidation and to have a high total remittance. Also, surface catalysis for recombination of dissociated species in the aerodynamic boundary layer must be minimized. Very thin chemical vapor deposition (CVD) coatings are attractive candidates for this application because of durability and very light weight. To demonstrate this concept, coatings of boron-silicon and aluminum-boron-silicon compositions were applied to the titanium-aluminides alpha2 (Ti-14Al-21Nb), super-alpha2 (Ti-14Al-23-Nb-2V), and gamma (Ti-33Al-6Nb-1Ta) and to the titanium alloy beta-21S (Ti-15Mo-3Al-3Nb-0.2Si). Coated specimens of each alloy were subjected to a set of simulated hypersonic vehicle environmental tests to determine their properties of oxidation resistance, surface catalysis, radiative emittance, and thermal shock resistance. Surface catalysis results should be viewed as relative performance only of the several coating-alloy combinations tested under the specific environmental conditions of the LaRC Hypersonic Materials Environmental Test System (HYMETS) arc-plasma-heated hypersonic wind tunnel. Tests were also conducted to evaluate the hydrogen transport properties of the coatings and any effects of the coating processing itself on fatigue life of the base alloys. Results are presented for three types of coatings, which are as follows: (1) a single layer boron silicon coating, (2) a single layer aluminum-boron-silicon coating, and (3) a multilayer coating consisting of an aluminum-boron-silicon sublayer with a boron-silicon outer layer.

Cunnington, George R.↗
NASA NTRS1994

InP solar cell with window layer

The invention features a thin light transmissive layer of the ternary semiconductor indium aluminum arsenide (InAlAs) as a front surface passivation or 'window' layer for p-on-n InP solar cells. The window layers of the invention effectively reduce front surface recombination of the object semiconductors thereby increasing the efficiency of the cells.

Jain, Raj K.↗
NASA NTRS1972

Metallic ions in the equatorial ionosphere

Four positive ion composition measurements of the equatorial E region made at Thumba, India, are presented. During the day, the major ions between 90 and 125 km are NO(+) and O2(+). A metallic ion layer centered at 92 km is observed, and found to contain Mg(+), Fe(+), Ca(+), K(+), Al(+), and Na(+) ions. The layer is explained in terms of a similarly shaped latitude distribution of neutral atoms which are photoionized and charge-exchanged with NO(+) and O2(+). Three body reactions form molecular metallic ions which are rapidly lost by dissociative ion-electron recombination. Nighttime observations show downward drifting of the metallic ion layer caused by equatorial dynamo effects. These ions react and form neutral metals which exchange charges with NO(+) and O2(+) to produce an observed depletion of those ions within the metallic ion region.

Aikin, A. C.↗
NASA NTRS1987

Intrusions in the active layer of channeled-substrate-planar laser diodes

Channeled-substrate-planar laser diodes from various wafers were observed to develop a node or dark spot in their near-field patterns after failure during lifetesting. 'Node' devices from each wafer were angle-lapped and stained in an attempt to uncover a physical mechanism for this common failure symptom. Optical microscopic examination of the beveled cross sections revealed that an intrusion, in the form of a spike at the tip of the zinc diffusion front, had penetrated into the active layer of these diodes. The node observed in the near-field pattern appears to be in a position correspondig to the location of the spike in the active region. In addition, cathodoluminescence measurements on other 'node' diodes revealed a dark line region approximately 2-micron-wide running parallel to and in the middle of the 5-micron contact stripe. It is believed that this dark line region is representative of an area of nonradiative recombination which occurs in the portion of the active layer containing the intrusion.

Slavin, S. E.↗
NASA NTRS1990

Effect of dislocations on the open-circuit voltage, short-circuit current and efficiency of heteroepitaxial indium phosphide solar cells

Excellent radiation resistance of indium phosphide solar cells makes them a promising candidate for space power applications, but the present high cost of starting substrates may inhibit their large scale use. Thin film indium phosphide cells grown on Si or GaAs substrates have exhibited low efficiencies, because of the generation and propagation of large number of dislocations. Dislocation densities were calculated and its influence on the open circuit voltage, short circuit current, and efficiency of heteroepitaxial indium phosphide cells was studied using the PC-1D. Dislocations act as predominant recombination centers and are required to be controlled by proper transition layers and improved growth techniques. It is shown that heteroepitaxial grown cells could achieve efficiencies in excess of 18 percent AMO by controlling the number of dislocations. The effect of emitter thickness and surface recombination velocity on the cell performance parameters vs. dislocation density is also studied.

Jain, Raj K.↗
NASA NTRS1992

Modeling of the near-electrode regions of arcjets. I - Coupling of the flowfield to the non-equilibrium boundary layers

A model for arc attachment at an anode in contact with a flowing thermal plasma is described. Current limitations are identified which arise when the convective transport of ions into the electrical, thermal and hydrodynamic boundary layers is incapable of supplying the loss of ions through three-body recombination in the volume, and catalytic recombination on the electrode surface. It is shown that the anode current where this condition is achieved can be exceeded provided there is a significant Joule heating in the boundary layer. The saturation current (current density above which the anode must emit ions) is found to substantially increase with increasing free stream velocity and anode temperature.

Cappelli, Mark A.↗
NASA NTRS2000

Descending Layer Variability Over Arecibo

Descending layers of ionization over Arecibo exhibit very diverse behavior from night to night that does not appear to be strongly correlated to geomagnetic activity, solar forcing, or average semidiurnal tidal winds. On some nights, three or more distinct layers are observed to form near 170 km over timescales of approx. 2 hours. Rather than descending smoothly over periods of several hours, these layers stall, abruptly disappear, or even reverse direction in the midst of their descent. The time scales for their disappearance are examined and compared to loss rates arising from diffusion and recombination. Diffusion alone is found to be too slow to account for the observations, but recombination is fast enough provided that the convergent wind shear that forms the layer is relatively weak coincident with their disappearance. The continuity equation is solved in conjunction with a time sequence of radar profiles to estimate the vertical drift and horizontal neutral wind consistent with the observed behavior. The resultant wind field is northward, has an average speed of approx. 80 m/ s, and varies significantly near the altitude where the layers are observed. These inferred winds are consistent with the presence of the observed layers, and their magnitudes as obtained from the classical continuity and momentum equations are reasonable for this altitude range.

Earle, G. D.↗
NASA NTRS1981

Study of radiation induced deep-level defects and annealing effects in the proton irradiated AlGaAs-GaAs solar cells

The radiation induced deep-level defects and the recombination parameters in the proton irradiated AlGaAs-GaAs p-n junction solar cells were investigated over a wide range of proton energies (from 50 KeV to 10 MeV) and proton fluences (from 10 to the 10th to 10 to the 13th P/sq cm), using DLTS, I-V, C-V, and SEM-EMIC measurement techniques. The measurements were used to determine the defect and recombination parameters such as defect density and energy level, carrier lifetimes, and the hole diffusion lengths in the GaAs LPE layers. Results show that a good correlation was obtained between the measured defect parameters and the dark recombination current as well as the performance parameters of the solar cells. The most damages to the cell were produced by the 200 KeV protons. In addition, the effects of low temperatures (200 to 400 C) thermal annealing on the deep-level defects and the dark current of the 200 KeV proton irradiated samples were examined.

Li, S. S.↗
NASA NTRS1989

Flash-photoionized nebulae

Under conditions of high radiation intensity and low gas density, recombination may be neglected in determining the ionization state of a photoionized gas. Calculations of the ionization structure of nebulae in this 'flash-photoionized' regime are reported. Very hard spectra of ionizing ultraviolet radiation may be produced by filtration of the ionizing flux through a neutral hydrogen layer which preferentially absorbs photons just above the hydrogen photoionization threshold. Fluxes with these hard spectra produce gas layer in which helium is largely doubly ionized while hydrogen is largely neutral. Such a layer leads to anomalously high ratios of He II to H I recombination line strengths. These results are applied to the problem of the spectrum of the arc in the cluster of galaxies A370. It is found that the spectrum may possibly be reconciled with the light echo model.

Katz, J. I.↗
NASA NTRS1992

A three-dimensional axisymmetric photochemical flow model of the cometary 'inner' shock layer

Assuming the Newtonian thin layer approximation to describe the structure of the shock layer between the cometary 'ionopause' and the inner shock, a 3D axisymmetric photochemical flow model of this layer is constructed. While sources of ions in this layer are the flow across the inner shock and photoionization of neutrals within it, the sinks are the flow into the flanks and dissociative recombination, the latter being the dominant one. For Halley's comet at the time of the Giotto encounter, the calculated thickness of the layer is very small, typically about 100 km for expected values of the dissociative-recombination coefficient. This is not inconsistent with the observations. The lateral flow speed near the point of encounter (inbound) is about 0.9 km/s, while the sonic line is at an angle of about 50 deg to the sun-comet line. Testing the validity of this model will have to await a cometary rendezvous mission such as the proposed CRAF/Cassini mission.

Damas, M. C.↗
NASA NTRS1968

Survey of the Relationship Between Theory and Experiment for Convective Heat Transfer From Rocket Combustion Gases

The problem of predicting the distribution of heat flux in a rocket engine is complicated by a flow field and thermodynamics so perturbed by as yet indescribable combustion effects that it continues to be necessary to utilize a simplified theoretical model against which to compare experimental results in order to gain insight. The most successful model applied to this problem is that of a compressible, turbulent, accelerating boundary layer flow with an arbitrary point of origin, and chemical reaction restricted to equilibrium recombination or diffusion-limited recombination on a catalytic wall. Analysis of this model was not possible without making several assumptions about the nature of the skin friction coefficient relationship and Reynold's analogy in an accelerating boundary layer flow. The paper summarizes the analysis of this model (together with detailed treatment of the special problems of variable properties, chemical reaction, and sources of transport properties), and describes several closed-form equations that have also been developed and applied to the problem. Described are results of several experiments with hot air flow made for the purpose of checking the validity of the assumptions in the boundary layer analysis. Most significant, however, are presentations of sample distributions of local heat flux measured in rocket engines operating with a wide range of conditions, propellants, injectors and sizes. When compared with predictions, these data show the significant range of deviation in heat flux that results from the combustion effects and which precludes a generally accurate prediction theory or correlation equation until the combustion process itself is correlated.

D R Bartz↗
NASA NTRS1984

Results of a comprehensive study of the photochemistry of N2(+) in the ionosphere

The improved match that can be obtained between the Atmospheric Explorer (AE) data on the ionospheric F layer N2(+) abundance and theoretical predictions by changing the dissociative recombination rate coefficient is demonstrated. Historically, models have overestimated the N2(+) concentration. It is shown that the calculated enhancement is due to charge exchange between 0(+)(2D) with N2. Increasing alpha by a factor of 2-3, at least in the orbits examined, can augment the destruction of vibrationally excited N2(+) ions. However, the validity of the correction is dependent on the availability of further laboratory data.

Abdou, W. A.↗
NASA NTRS1986

Improved High/Low Junction Silicon Solar Cell

Air-mass-zero open-circuit voltage increased to 650 mV. Oxide-chargeinduced high/low emitter solar cell, electron accumulation layer induced by positive charge. High/low emitter suppresses dark emitter recombination current, resulting in power-conversion efficiencies significantly higher than previously achieved.

Neugroschel, A.↗
NASA NTRS2000

Thermal Control and Enhancement of Heat Transport Capacity of Two-Phase Loops With Electrohydrodynamic Conduction Pumping

There are three kinds of electrohydrodynamics (EHD) pumping based on Coulomb force: induction pumping, ion-drag pumping, and pure conduction pumping. EHD induction pumping relies on the generation of induced charges. This charge induction in the presence of an electric field takes place due to a non-uniformity in the electrical conductivity of the fluid which can be caused by a non-uniform temperature distribution and/or an inhomogeneity of the fluid (e.g. a two-phase fluid). Therefore, induction pumping cannot be utilized in an isothermal homogeneous liquid. In order to generate Coulomb force, a space charge must be generated. There are two main mechanisms for generating a space charge in an isothermal liquid. The first one is associated with the ion injection at a metal/liquid interface and the related pumping is referred to as ion-drag pumping. Ion-drag pumping is not desirable because it can deteriorate the electrical properties of the working fluid. The second space charge generation mechanism is associated with the heterocharge layers of finite thickness in the vicinity of the electrodes. Heterocharge layers result from dissociation of the neutral electrolytic species and recombination of the generated ions. This type of pumping is referred to as pure conduction pumping. This project investigates the EHD pumping through pure conduction phenomenon. Very limited work has been conducted in this field and the majority of the published papers in this area have mistakenly assumed that the electrostriction force was responsible for the net flow generated in an isothermal liquid. The main motivation behind this study is to investigate an EHD conduction pump for a two-phase loop to be operated in the microgravity environment. The pump is installed in the liquid return passage (isothermal liquid) from the condenser section to the evaporator section. Unique high voltage and ground electrodes have been designed that generate sufficient pressure heads with very low electric power requirements making the EHD conduction pumping attractive to applications such as two-phase systems (e.g. capillary pumped loops and heat pipes). Currently, the EHD conduction pump performance is being tested on a two-phase loop under various operating conditions in the laboratory environment. The simple non-mechanical and lightweight design of the EHD pump combined with the rapid control of performance by varying the applied electric field, low power consumption, and reliability offer significant advantages over other pumping mechanisms; particularly in reduced gravity applications.

Seyed-Yagoobi, J.↗
NASA NTRS1984

Molecular processes in a high temperature shock layer

Models of the shock layer encountered by an Aeroassisted Orbital Transfer Vehicle require as input accurate cross sections and rate constants for the atomic and molecular processes that characterize the shock radiation. From the estimated atomic and molecular densities in the shock layer and the expected residence time of 1 m/s, it can be expected that electron-ion collision processes will be important in the shock model. Electron capture by molecular ions followed by dissociation, e.g., O2(+) + e(-) yields 0 + 0, can be expected to be of major importance since these processes are known to have high rates (e.g., 10 to the -7th power cu/cm/sec) at room temperature. However, there have been no experimental measurements of dissociative recombination (DR) at temperatures ( 12000K) that are expected to characterize the shock layer. Indeed, even at room temperature, it is often difficult to perform experiments that determine the dependence of the translational energy and quantum yields of the product atoms on the electronic and vibrational state of the reactant molecular ions. Presented are ab initio quantum chemical studies of DR for molecular ions that are likely to be important in the atmospheric shock layer.

Guberman, S. L.↗
NASA NTRS1987

Laser induced OMCVD growth of AlGaAs on GaAs

A major factor limiting the efficiency of the GaAs-GaAlAs solar cell is the rate of recombination at the GaAs-AlGaAs interface. Evidence has been previously reported which indicates that recombination at this interface can be greatly reduced if the AlGaAs layer is grown at lower than normal temperatures. The authors examine the epitaxial growth of AlGaAs on GaAs using a horizontal OMCVD reactor and an excimer laser operating in the UV (lambda = 193 nm) region. The growth temperatures were 450 and 500 C. The laser beam was utilized in two orientations: 75 deg angle of incidence and parallel to the substrate. Film composition and structure were determined by Auger electron spectroscopy (AES) and transmission electron microscopy (TEM). Auger analysis of epilayers grown at 500 C with the laser impinging show no carbon or oxygen contamination of the epitaxial layers or interfaces. TEM diffraction patterns of these same epilayers exhibit single crystal (100) zone axis patterns.

Wilt, David M.↗
NASA NTRS1991

The effect of process conditions on the performance of epitaxial InP solar cells

Indium phosphide solar cells have a higher resistance to electron irradiation than Si or GaAs cells of comparable junction depth. As a result, there is much interest in the use of this material for space applications. Cells of this material were made in bulk InP by a number of techniques, including ion implantation, direct diffusion in sealed ampoules, and by open tube diffusion. However, it is generally considered that the epitaxial approach will be superior to all of these techniques. The epitaxy of InP is considerably more difficult than that of gallium arsenide, for a number of reasons. Perhaps the most important is the fact that the native oxides of Indium are extremely difficult to remove, as compared to that of Gallium. In addition, thermal treatments for the desorption of these oxides often result in the formation of phosphorus vacancies and free indium on the surface. Thus, inadequate sample preparation before epitaxy, poor reactor cleaning procedures, or poor transition procedures between the growth of successive layers, all give rise to trap phenomena and to high interface recombination velocities. Moreover, the lifetime of the grown material is dominated by the occurrence of native defects, so that it is a strong function of growth parameters. These problems are of special interest to the fabrication of solar cells, where long life-time, combined with the absence of traps, is highly desirable. A study of this problem is described using a non-invasive diagnostic technique which was developed.

Borrego, J. M.↗