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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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

On the cause of the flat-spot phenomenon observed in silicon solar cells at low temperatures and low intensities

A model is presented that explains the "flat-spot" (FS) power loss phenomenon observed in silicon solar cells operating deep space (low temperature, low intensity) conditions. Evidence is presented suggesting that the effect is due to localized metallurgical interactions between the silicon substrate and the contact metallization. These reactions are shown to result in localized regions in which the PN junction is destroyed and replaced with a metal-semiconductor-like interface. The effects of thermal treatment, crystallographic orientation, junction depth, and metallurization are presented along with a method of preventing the effect through the suppression of vacancy formation at the free surface of the contact metallization. Preliminary data indicating the effectiveness of a TiN diffusion barrier in preventing the effect are also given.

Weizer, V. G.↗
NASA NTRS1982

On the cause of the flat-spot phenomenon observed in silicon solar cells at low temperatures and low intensities

A model is presented that explains the 'flat-spot' power-loss phenomenon observed in silicon solar cells operating under deep space (low temperature, low intensity) conditions. Evidence is presented suggesting that the effect is due to localized metallurgical interactions between the silicon substrate and the contact metallization. These reactions are shown to result in localized regions in which the PN junction is destroyed and replaced with a metal-semiconductor-like interface. The effects of thermal treatment, crystallographic orientation, junction depth, and metallization are presented along with a method of preventing the effect through the suppression of vacancy formation at the free surface of the contact metallization. Preliminary data indicating the effectiveness of a TiN diffusion barrier in preventing the effect are also given.

Weizer, V. G.↗
NASA NTRS1982

On the cause of the flat-spot phenomenon observed in silicon solar cells at low temperatures and low intensities

A model that explains the flat-spot power loss phenomenon is presented. Evidence suggests that the effect is due to localized metallurgical interactions between the silicon substrate and the contact metallization. These reactions are shown to result in localized regions in which the P-N junction is destroyed and replaced with a metal semiconductor-like interface. The effects of thermal treatment, crystallographic orientation, junction depth, and metallization are shown along with a method of preventing the effect through the suppression of vacancy formation at the free surface of the contact metallization. Data indicating the effectiveness of a TiN diffusion barrier in preventing the effect are also given.

Weizer, V. G.↗
NASA NTRS1983

Low frequency dielectric relaxation in boracites

In order to elucidate the nature of the imperfections which adversely affect pyroelectric processes in boracites, the thermal and dispersive characteristics of the low frequency dielectric response in iron-iodide boracite (Fe3B7O13I) and copper-chloride boracite (Cu3B7O13Cl) have been investigated. These characteristics were measured as a function of crystallographic orientation and applied field in both the ferroelectric and paraelectric states. The low frequency dielectric relaxation of 100 line oriented multi-domain copper-chloride boracite clearly indicates the dipole nature of the lattice imperfections. The activation energies calculated from a noninteracting Debye model, are 0.53 eV in the ferroelectric phase and 0.10 eV in the paraelectric phase.

Kim, Q.↗
NASA NTRS1986

Anisotropy of high temperature strength in precipitation-hardened nickel-base superalloy single crystals

The anisotropy of high temperature strength of nickel-base superalloy, Alloy 454, in service for advanced jet engine turbine blades and vanes, was investigated. Crystallographic orientation dependence of tensile yield strength, creep and creep rupture strength was found to be marked at about 760C. In comparison with other single crystal data, a larger allowance in high strength off-axial orientation from the 001 axis, and relatively poor strength at near the -111 axis were noted. From transmission electron microscopy the anisotropic characteristics of this alloy were explained in terms of available slip systems and stacking geometries of gamma-prime precipitate cuboids which are well hardened by a large tantalum content. 100 cube slip was considered to be primarily responsible for the poor strength of the -111 axis orientation replacing the conventional 111 plane slip systems.

Nakagawa, Y. G.↗
NASA NTRS1987

Solidification Effects in MAR-M246(Hf) Alloy

Fatigue properties degraded with crystallographic orientations greater than 10 degrees from {001} axis. Influence of solidification and heat-treatment parameters on structure and fatigue properties of nickel-based superalloy MAR-M246(Hf) described in 24-page report. Superalloys have high strength and corrosion resistance at temperatures up to 1,400 degrees C; their uses range from petrochemical equipment to marine, industrial, aircraft, and vehicular gas turbines.

Johnston, M. H.↗
NASA NTRS1987

Enhanced beam coupling modulation using the polarization properties of photorefractive GaAs

Observation is reported of a rotation in the polarization of the two photorefractive recording beams in GaAs for a configuration with the internally generated space-charge field along the line 110 crystallographic orientation. This rotation is a result of simultaneous constructive and destructive beam coupling in each beam for the optical electric field components along the two electrooptically induced principal dielectric axes of the crystal. By turning one of the beams on and off, the intensity of the other beam after the crystal and a polarization analyzer can be modulated by as much as 500 percent. This result is of particular importance for optical information processing applications.

Partovi, Afshin↗
NASA NTRS1988

Solidification-Rate Effects In MAR-M-246+Hf Alloy

Under slower solidification, primary-dendrite-arm spacing increases. Report discusses experiments on influence of solidification rates on crystallographic orientation and mechanical properties of superalloy MAR-M-246+Hf. Specimens grown in directional-solidification furnace, visually examined for microstructure, and stretched to failure in tensile-testing machine. Back-reflection Laue x-ray photographs taken to determine growth orientations.

Hamilton, David↗
NASA NTRS1988

Streaking images that appear only in the plane of diffraction in undoped GaAs single crystals: Diffraction imaging (topography) by monochromatic synchrotron radiation

Streaking images restricted to the direction of the diffraction (scattering) vector have been observed on transmission through undoped GaAs. These disruption images (caused by the reduction of diffraction in the direction of observation) appear both in the forward and in Bragg diffracted directions in monochromatic synchrontron radiation diffraction imaging. This previously unobserved phenomenon can be explained in terms of planar defects (interfaces) or platelets which affects the absorption coefficient in anomalous transmission. Such regions of the crystal look perfect despite the presence of imperfections when the scattering vector is not perpendicular to the normal of the platelets. The observed crystallographic orientation of these interfaces strongly indicates that they are antiphase boundaries.

Kuriyama, Masao↗
NASA NTRS1987

Liquid phase epitaxy growth of GaAs/GaAlAs multi-quantum well structures

Experiments in liquid phase epitaxial fabrication of thin GaAs/GaAlAs layers over a planar substrates have been carried out. Layer thicknesses smaller than 300 A were routinely obtained, with the best result being 120 A. Interface sharpness between the layers is approximately 10 A, which is comparable to OMCVD results, but the layers' thicknesses are usually not uniform. Of the experimental parameters, the growth time and the cooling rate seem to have the largest effect on the obtained layer thickness, while the growth temperature and the substrate crystallographic orientation produce less noticeable effects. Quantum effects in the grown layers were observed by photoluminescence measurements.

Cser, J.↗
NASA NTRS1989

Auger electron intensity variations in oxygen-exposed large grain polycrystalline silver

Auger electron spectroscopic studies of the grains in oxygen-charged polycrystal-line silver show significant intensity variations as a function of crystallographic orientation. These intensity variations were observed by studies of the Auger images and line scans of the different grains (randomly selected) for each silver transition energy. The results can be attributed to the diffraction of the ejected Auger electrons and interpreted by corresponding changes in the electron mean-free path for inelastic scattering and by oxygen atom accumulation in the subsurface. The subsurface (second layer) octahedral sites increased in size because of surface relaxation and serve as a stable reservoir for the dissolved oxygen.

Lee, W. S.↗
NASA NTRS1989

The low cycle fatigue deformation response of a single-crystal superalloy at 650 C

The cyclic stress-strain response and the associated deformation structure of the single crystal nickel-base superalloy PWA 1480 were investigated. Specimens of various crystallographic orientations were tested in low-cycle fatigue (LCF) at 650 C, resulting in a significant tension-compression anisotropy in initial yield strength associated with the shearing of gamma-prime precipitates by dislocation pairs, and a LCF cyclic hardening of the crystals associated with dislocation interactions occurring in the gamma phase. In specimens deforming by slip on a single slip system, dislocations of the primary slip system accumulated in the gamma matrix and formed seesile entanglements. In specimens deforming by slip on several slip systems, the dislocations of the different operative slip systems intersected in the gamma matrix and formed sessile arrangements.

Gabb, T. P.↗
NASA NTRS1991

Growth and characterization of high-performance photorefractive BaTiO3 crystals

Barium titanate has been used for many nonlinear optical applications primarily because it has high grain and high self-pumped phase conjugate reflectivities. However, barium titanate has had a relatively slow response time, and thus low sensitivity. Therefore, it has not been suited to real-time operations. In this report we will describe the modifications in crystal growth, doping, reduction, and poling that have produced barium titanate crystals with the fastest photorefractive response time reported to date, approximately 21 microseconds with a beam-coupling gain coefficient of 38.7 cm(exp -1) and the highest sensitivity reported to date of 3.44 cm(exp 3)/kJ. The sensitivity of these barium titanate crystals is comparable to or greater than other photorefractive oxides. We will show, for the first time, beam-coupling in barium titanate at video frame rates. We infer from response time measurements that barium titanate has a phonon limited mobility. Also, photorefractive response time measurements as a function of the crystallographic orientation and grating wave vector for our cobalt-doped oxygen reduced crystals indicate that their faster response time arise because of an increase in the free carrier lifetime.

Warde, C.↗
NASA NTRS1992

Phase transformations in 40-60-GPa shocked gneisses from the Haughton Crater (Canada): An Analytical Transmission Electron Microscopy (ATEM) study

In order to better understand phase transformations, chemical migration, and isotopic disequilibrium in highly shocked rocks, we have performed a microprobe and an ATEM study on gneisses shocked up to 60 GPa from the Haughton Crater. This study reveals the following chemical and structural characteristics: (1) SiO2 dominant areas are formed by a mixture of pure SiO2 polycrystalline quartz identified by electron diffraction pattern and chemical analysis and a silica-rich amorphous phase containing minor amounts of aluminium, potassium, and iron; (2) Areas with biotitelike composition are formed by less than 200-nm grains of iron-rich spinels embedded in a silica-rich amorphous phase that is very similar to the one described above; (3) Layers with feldsparlike composition are constituted by 100-200-nm-sized alumina-rich grains (the indexation of the crystalline structure is under progress) and the silica-rich amorphous phase; (4) Zones characterized by the unusual Al/Si ratio close to 1 are formed by spinel grains (200-nm-sized) embedded in the same silica-rich amorphous phase; and (5) The fracturated sillimanites contain domains with a lamellar structure, defined by the intercalation of 100-nm-wide lamellae of mullite crystals and of a silica-rich amorphous phase. These mullite crystals preserved the crystallographical orientation of the preshock sillimanite. All compositional domains, identified at the microprobe scale, can thus be explained by a mixture in different proportion between the following phases: (1) a silica-rich amorphous phase, with minor Al and K; (2) quartz crystals; (3) spinel crystals and alumina-rich crystals; (4) sillimanite; and (5) mullite. Such mixtures of amorphous phases and crystals in different proportions explain disturbed isotope systems in these rocks and chemical heterogeneities observed on the microprobe.

Martinez, I.↗
NASA NTRS1994

Shocked materials from the Dutch Peak diamictite, Utah

Evidence of shock metamorphism in the Dutch Peak diamictite in the Sheeprock Mountains, Utah, is reported. The Dutch Peak diamictite is of Proterozoic age and is a minor part of the Dutch Peak formation. A shocked sample, specimen A250, was collected during a brief visit of the Harker Canyon area of the Sheeprock Mountains. This sample consists of equant, anhedral grains of quartz, K-feldspar, and plagioclase. The crystallographic orientation of 244 lamellae systems in 106 grains was measured. It is presently difficult to evaluate the significance of this single specimen. Without additional and substantial field work, and petrographic characterization of this formation, a number of scenarios for the presence of a shocked clast and the emplacement of the entire formation remain viable.

Hoerz, F.↗
NASA NTRS1994

Convection and morphological stability during directional solidification

For growth of a vicinal face at constant velocity, the effect of anisotropic interface kinetics on morphological stability is calculated for a binary alloy. The dependence of the interface kinetic coefficient on crystallographic orientation is based on the motion and density of steps. Anisotropic kinetics give rise to traveling waves along the crystal-melt interface, and can lead to a significant enhancement of morphological stability. The stability enhancement increases as the orientation approaches a singular orientation and as the solidification velocity increases. Shear flows interact with the traveling waves and, depending on the direction of the flow, may either stabilize or destabilize the interface. Specific calculations are carried out for germanium-silicon alloys.

Coriell, Sam R.↗
NASA NTRS1994

Midfit dislocation generation mechanisms in InGaAs/GaAs heterostructures

An experimental investigation of misfit dislocation generation mechanisms at an InGaAs/GaAs heterointerface is reported. InGaAs epitaxial layers were grown by low-pressure oragnometallic vapor-phase epitaxy on patterned and unpatterned GaAs substrate having etch-pit densities (EPD) of 200, 1400, and 10,000 cm(exp -2). After epitaxial growth, the samples were annealed at temperatures between 650 and 750 C, and analyzed by optical and transmission electron microscopy. For the range of substrate EPD studied, it was found that the substrate EPD controls the onset of misfit dislocation generation for low-temperature epitaxy (less than 600 C) on unpatterned substrates. When epilayers were annealed at 750 C, the density of misfit dislocations was independent of the substrate EPD. These studies also show that the dominant misfit dislocation generation mechanism for films grown on patterned substrates is nucleation at the growth-mesa edge. The density of preexisting threading dislocations has little influence on misfit dislocation generation for films selectively deposited within 100 x 100 sq micrometer growth windows. For selective heteroepitaxy, misfit dislocation generation strongly depends on the crystallographic orientation of the growth-mesa edge.

Kui, J.↗
NASA NTRS1998

Crystal Growth of Mixed Optical Materials With the Automatic Czochralski Puller

Most of optical and semiconductor single crystalline materials produced in the electronic industry is grown with the Czochralski technique. This is the only technique capable of producing large high quality single crystals in a reasonable amount of time. The technique originated, and took its name, from pioneering work by Polish engineer J. Czochralski in 1917, who pulled single crystal wires (fibers) of low melting point metals from the melt in order to determine the maximum speeds at which they could be crystallized. The technique has been modified and its application has been extended first to the growth of silicon and later to a very wide range of compound semiconductors, metals, oxides and halides. It is now the dominant technique for the wide range of materials in the electronic industries. Czochralski growth is initiated by dipping a seed crystal of a chosen crystallographic orientation onto the surface of a melt contained in the heated crucible. The seed is then slowly withdrawn, solidifying the adherent column of the melt in steady state at an average rate given by the sum of the pulling rate and the rate of lowering the solid-liquid interface due to the depletion of the melt in the crucible. The control of the desired crystal diameter is achieved by closely controlling the heat balance at the solid-liquid interface.

Loutts, George B.↗