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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 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Tribological characteristics of sputtered Au/Cr films on alumina substrates at elevated temperatures

Research to evaluate the tribological properties of alumina pins sliding against thin sputtered gold films deposited on alumina disk substrates is described. A 250 A thick chromium interlayer was first deposited onto the alumina test disks to enhance adhesion and high temperature wetting of the gold films. The Au/Cr films were tribotested in pure sliding in a pin-on-disk tribometer under a 4.9 N load at 1m/s. The test atmosphere was room air at temperatures of 25, 500, and 800 C and the test duration varied from 60 to 540 min. The use of the Au/Cr films reduced friction by about a factor of two compared to the unlubricated alumina sliding couple. The coatings prevented wear of the alumina substrate disks and reduced pin wear by one to two orders of magnitude. In addition, wear lives in excess of 200,000 sliding passes (9 hr) were observed during sliding at 800 C. The results suggest that these films show promise for the practical lubrication of many high temperature sliding components.

Benoy, P. A.↗
NASA NTRS1993

Organometallic chemical vapor deposition and characterization of ZnGeP2/GaP multiple heterostructures on GaP substrates

The growth of ZnGeP2/GaP double and multiple heterostructures on GaP substrates by organometallic chemical vapor deposition is reported. These epitaxial films were deposited at a temperature of 580 C using dimethylzinc, trimethylgallium, germane, and phosphine as source gases. With appropriate deposition conditions, mirror smooth epitaxial GaP/ZnGeP2 multiple heterostructures were obtained on (001) GaP substrates. Transmission electron microscopy (TEM) and secondary ion mass spectroscopy (SIMS) studies of the films showed that the interfaces are sharp and smooth. Etching study of the films showed dislocation density on the order of 5x10(exp 4)cm(sup -2). The growth rates of the GaP layers depend linearly on the flow rates of trimethylgallium. While the GaP layers crystallize in zinc-blende structure, the ZnGeP2 layers crystallize in the chalcopyrite structure as determined by (010) electron diffraction pattern. This is the first time that multiple heterostructures combining these two crystal structures were made.

Xing, G. C.↗
NASA NTRS1993

Ten Ghz YBa2Cu3O(7-Delta) Superconducting Ring Resonators on NdGaO3 Substrates

YBa2Cu3O(7-delta) thin films were formed on NdGaO3 substrates by laser ablation. Critical temperatures greater than 89 K and critical current densities exceeding 2 x 10(exp 8) Acm(sub -2) at 77 K were obtained. The microwave performance of films patterned into microstrip ring resonators with gold ground planes was measured. An unloaded quality factor six times larger than that of a gold resonator of identical geometry was achieved. The unloaded quality factor decreased below 70 K for both the superconducting and gold resonators due to increasing dielectric losses in the substrate. The temperature dependence of the loss tangent of NdGaO3 was extracted from the measurements.

To, H. Y.↗
NASA NTRS1994

Temperature Dependent Performance of Coplanar Waveguide (CPW) on Substrates of Various Materials

The attenuation (a) and effective dielectric constant (E(sub eff)) of Coplanar Waveguide (CPW) transmission lines on high-resistivity silicon and diamond substrates as a function of both temperature and frequency are presented. The technique used to obtain the values for a and E(sub eff) involves the use of a unique cryogenic probe station designed and built by NASA. Attenuation of gold CPW lines on diamond substrates is compared with that of superconducting CPW lines.

Taub, Susan R.↗
NASA NTRS1994

High-efficiency, deep-junction, epitaxial InP solar cells on (100) and (111)B InP substrates

We report on the development and performance of deep-junction (approximately 0.25 micron), graded-emitter-doped, n(sup +)-p InP solar cells grown by metallorganic chemical vapor deposition (MOCVD). A novel, diffusion-transport process for obtaining lightly-doped p-type base regions of the solar cell is described. The I-V data and external quantum-efficiency response of these cells are presented. The best active-area AMO efficiency for these deep-junction cells on (100)-oriented InP substrates is 16.8 percent, with a J(sub SC) of 31.8 mA/sq cm, a V(sub OC) of 0.843 V, and a fill-factor of 0.85. By comparison, the best cell efficiency on the (111)B-oriented InP substrates was 15.0 percent. These efficiency values for deep-junction cells are encouraging and compare favorably with performance of thin-emitter (0.03 micron) epitaxial cells as well as that of deep-emitter diffused cells. The cell performance and breakdown voltage characteristics of a batch of 20 cells on each of the orientations are presented, indicating the superior breakdown voltage properties and other characteristics of InP cells on the (111)B orientation. Spectral response, dark I-V data, and photoluminescence (PL) measurements on the InP cells are presented with an analysis on the variation in J(sub SC) and V(sub OC) of the cells. It is observed, under open-circuit conditions, that lower-V(sub OC) cells exhibit higher band-edge PL intensity for both the (100) and (111)B orientations. This anomalous behavior suggests that radiative recombination in the heavily-doped n(sup +)-InP emitter may be detrimental to achieving higher V(sub OC) in n(sup +)-p InP solar cells.

Venkatasubramanian, R.↗
NASA NTRS1995

Effects of long term space environment exposure on optical substrates and coatings

At the time of our Second LDEF Post-Retrieval Symposium report, we had completed cleaning and measuring the post flight spectral performance of all of the samples except for the fused silica substrate with an antireflection (SiO2/TiO2) coating. As was documented in that report, after cleaning, all of the samples returned to their original pre-flight performance, with the exception of the fused silica anti-reflectance coated sample, which had significant degraded performance. All normal methods of removing the contaminant from the surface of this sample failed except for oxygen plasma etch. At the time of the previous report, we had only exposed the sample to three hours of plasma etch. We have since continued the plasma etch for a total of 32 hours, after which time, the transmission of the sample almost reached its pre-flight values. The peak areas of the transmission curves remained shifted approximately 150 nm. Of interest is the fact that the control samples, which hadn't flown, did not exhibit any spectral shifting. In summary, after the 6 yr exposure to the space environment, all of the coatings and substrates returned to their pre-flight performance after cleaning, with the exception of the antireflection coated fused silica sample. On this sample, the spectral transmission curves shifted approximately 150 nm.

Vallimont, John↗
NASA NTRS1995

On the levels of enzymatic substrate specificity: Implications for the early evolution of metabolic pathways

The most frequently invoked explanation for the origin of metabolic pathways is the retrograde evolution hypothesis. In contrast, according to the so-called 'patchwork' theory, metabolism evolved by the recruitment of relatively inefficient small enzymes of broad specificity that could react with a wide range of chemically related substrates. In this paper it is argued that both sequence comparisons and experimental results on enzyme substrate specificity support the patchwork assembly theory. The available evidence supports previous suggestions that gene duplication events followed by a gradual neoDarwinian accumulation of mutations and other minute genetic changes lead to the narrowing and modification of enzyme function in at least some primordial metabolic pathways.

Lazcano, A.↗
NASA NTRS1994

Electrical properties of epitaxial 3C- and 6H-SiC p-n junction diodes produced side-by-side on 6H-SiC substrates

3C-SiC (beta-SiC) and 6H-SiC p-n junction diodes have been fabricated in regions of both 3C-SiC and 6H-SiC epitaxial layers which were grown side-by-side on low-tilt-angle 6H-SiC substrates via a chemical vapor deposition (CVD) process. Several runs of diodes exhibiting state-of-the-art electrical characteristics were produced, and performance characteristics were measured and compared as a function of doping, temperature, and polytype. The first 3C-SiC diodes which rectify to reverse voltages in excess of 300 V were characterized, representing a six-fold blocking voltage improvement over experimental 3C-SiC diodes produced by previous techniques. When placed under sufficient forward bias, the 3C-SiC diodes emit significantly bright green-yellow light while the 6H-SiC diodes emit in the blue-violet. The 6H-SiC p-n junction diodes represent the first reported high-quality 6H-SiC devices to be grown by CVD on very low-tilt-angle (less than 0.5 deg off the (0001) silicon face) 6H substrates. The reverse leakage current of a 200 micron diameter circular device at 1100 V reverse bias was less than 20 nA at room temperature, and excellent rectification characteristics were demonstrated at the peak characterization temperature of 400 C.

EPITAXIAL LAYER↗
NASA NTRS1995

Characterization of cobalt-dipped nickel electrodes with fibrex substrates

Nickel electrodes using fibrous substrates have poorer initial utilization of the active material than those using conventional nickel sinter substrates. Previous investigators had shown that utilization can be dramatically improved by dipping these electrodes in a cobalt solution immediately after the electrochemical impregnation, before formation and cycling is carried out. The present study looked at the gas evolution behavior of dipped and undipped electrodes, impedance curves, and the charge-discharge curves to try to understand the reasons for the improvement in utilization. Impedance measurements under open circuit conditions indicate that some of the improvement is due to a reduction in the ohmic resistance of the surface layer of the particles, in agreement with earlier work. The charge-discharge curves suggest that there may also be an additional increase in the ohmic resistance of the surface layer of the undipped electrode during charging.

Youngman, Carolyn A.↗
NASA NTRS1994

Method and apparatus for producing a substrate with low secondary electron emissions

The present invention is directed to a method and apparatus for producing a highly-textured surface on a copper substrate with only extremely small amounts of texture-inducing seeding or masking material. The texture-inducing seeding material is delivered to the copper substrate electrically switching the seeding material in and out of a circuit loop.

Jensen, Kenneth A.↗
NASA NTRS1996

IR Interference Filters Made Of Al Patterns On Si Substrates

Filters relatively thin and better accommodate convergent infrared beams. Infrared (IR) interference filters with pass bands centered at wavelength of about 70 micrometer made in form of aluminum patterns 1,000 Angstrom thick on both sides of silicon substrates 12 to 15 micrometer thick. Substrates, in turn, mounted on silicon frames 0.2 millimeter thick. Prototypes of class of miniature band-pass infrared filters small enough to be bonded to, and cooled with, infrared detectors.

Ksendzov, Alexander↗
NASA NTRS1996

The growth of low band-gap InAs on (111)B GaAs substrates

Growth on the (111)B orientation exhibits a number of advantageous properties as compared to the (100) during the early stages of strained-layer epitaxy. In accordance with a developing model of nucleation and growth, we have deposited thin (60 A - 2500 A), fully relaxed InAs films on (111)B GaAs substrates. Although thicker InAs films are subject to the formation of twin defects common to epitaxy on the (111)B orientation, appropriate control of the growth parameters can greatly minimize their density. Using this knowledge base, InAs films up to 2 microns in thickness with improved morphology and structural quality have been grown on (111)B GaAs substrates.

Welser, Roger E.↗
NASA NTRS1994

Reflection Properties of Metallic Gratings on ZnO Films over GaAs Substrates

A potential application for piezoelectric film deposited on GaAs substrates is the monolithic integration of surface acoustic wave (SAW) devices with GaAs electronics. Metallic gratings are basic elements required for the construction of such devices, and analyzing the reflectivity and the velocity change due to metallic gratings is often a critical design parameter. In this article, Datta and Hunsinger technique is extended to the case of a multilayered structure, and the developed technique is applied to analyze shorted and open gratings on ZnO films sputtered over (001)-cut (110)-propagating GaAs substrates. The analysis shows that zero reflectivity of shorted gratings can be obtained by a combination of the ZnO film and the metal thickness and the metalization ratio of the grating. Experiments are performed on shorted and an open gratings (with the center frequency of about 180 MHz) for three different metal thicknesses over ZnO films which are 0.8 and 2.6 micrometers thick. From the experiments, zero reflectivity at the resonant frequency of the grating is observed for a reasonable thickness (h/Alpha = 0.5%) of aluminum metalization. The velocity shift between the shorted and the open grating is also measured to be 0.18 MHz and 0.25 MHz for 0.8 and 1.6 micrometers respectively. The measured data show relatively good agreement with theoretical predictions.

Hickernell, Fred S.↗
NASA NTRS1996

Effect of Tilt Angle on the Morphology of SiC Epitaxial Films Grown on Vicinal (0001) SiC Substrates

In this study of 4H-SiC and 6H-SiC epitaxial films we found that film morphology was strongly dependent on the tilt angle of the substrate. Large surface steps (up to 25 nm high) due to step bunching were more prevalent at smaller tilt angles. Also, 4H films were more susceptible than 6H to 3C-SiC inclusions during growth. The lateral growth of steps from screw dislocations in low-tilt-angle substrates demonstrated that step bunching on the atomic scale was anisotropic with respect to growth direction for both 4H-SiC and 6H-SiC. A model explaining this behavior is presented. We observed and directly measured the Burgers vector of a 'super' screw dislocation in a 6H-SiC epilayer.

Powell, J. A.↗
NASA NTRS1998

Method and Apparatus for Producing a Substrate with Low Secondary Electron Emissions

The present invention is directed to a method and apparatus for producing a highly-textured surface on a copper substrate with only extremely small amounts of texture-inducing seeding of masking material. The texture-inducing seeding material is delivered to the copper substrate electrically switching the seeding material in and out of a circuit loop.

Jensen, Kenneth A.↗
NASA NTRS1998

Prebiotic Amino Acid Thioester Synthesis: Thiol-Dependent Amino Acid Synthesis from Formose substrates (Formaldehyde and Glycolaldehyde) and Ammonia

Formaldehyde and glycolaldehyde (substrates of the formose autocatalytic cycle) were shown to react with ammonia yielding alanine and homoserine under mild aqueous conditions in the presence of thiol catalysts. Since similar reactions carried out without ammonia yielded alpha-hydroxy acid thioesters, the thiol-dependent synthesis of alanine and homoserine is presumed to occur via amino acid thioesters-intermediates capable of forming peptides. A pH 5.2 solution of 20 mM formaldehyde, 20 mM glycolaldehyde, 20 mM ammonium chloride, 23 mM 3-mercaptopropionic acid, and 23 mM acetic acid that reacted for 35 days at 40 C yielded (based on initial formaldehyde) 1.8% alanine and 0.08% homoserine. In the absence of thiol catalyst, the synthesis of alanine and homoserine was negligible. Alanine synthesis required both formaldehyde and glycolaldehyde, but homoserine synthesis required only glycolaldehyde. At 25 days the efficiency of alanine synthesis calculated from the ratio of alanine synthesized to formaldehyde reacted was 2.1%, and the yield (based on initial formaldehyde) of triose and tetrose intermediates involved in alanine and homoserine synthesis was 0.3 and 2.1%, respectively. Alanine synthesis was also seen in similar reactions containing only 10 mM each of aldehyde substrates, ammonia, and thiol. The prebiotic significance of these reactions that use the formose reaction to generate sugar intermediates that are converted to reactive amino acid thioesters is discussed.

Weber, Arthur L.↗
NASA NTRS1999

RF Transmission Lines on Silicon Substrates

A review of RF transmission lines on silicon substrates is presented. Through measurements and calculated results, it is shown that attenuation is dominated by conductor loss if silicon substrates with a resistivity greater than 2500 Ohm-cm are used. Si passivation layers affect the transmission line attenuation; however, measured results demonstrate that passivation layers do not necessarily increase attenuation. If standard, low resistivity Si wafers must be used, alternative transmission lines such as thin film microstrip and Co-Planar Waveguide (CPW) on thick polyimide layers must be used. Measured results presented here show that low loss per unit length is achievable with these transmission lines.

Ponchak, George E.↗