Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Electrical resistivity”

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 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Thermoelectric properties of cerium monopnictides

Several cerium pnictides have been synthesized from the pure elements and hot pressed into test samples. Measurements of Seebeck coefficients and electrical resistivities were performed on these samples from room temperature to 1000 C. Cerium arsenide and cerium antimonide are n-type; cerium nitride changes from p-type to n-type conduction at 800 C. The materials are semimetals with resistivities below 1 mohm/cm. Cerium arsenide is the most favorable of the pnictides studied for high-temperature thermoelectric energy conversion, with an average power factor of 15 microW/cm K sq from 500 to 1000 C.

Danielson, L. R.↗
NASA NTRS1987

Upper critical field of high temperature Y(1.2)Ba(0.8)CuO(4-delta) superconductor

A 20-T high-field magnet is used to measure electrical resistance as a function of temperature in the Y(1.2)Ba(0.8)CuO(4-delta) superconductor. The temperature dependence of the critical field, Hc2(T), is obtained from the superconduction transition. A Hc2(O) value of 166T is determined which is the highest critical field yet reported. Results show Y(1.2)Ba(0.8)CuO(4-delta) to be a 90K Type-II superconductor, with a lower critical field Hc1(O) of about 0.2T and a penetration depth of about 290 A.

Hor, P. H.↗
NASA NTRS1987

Glass properties in the yttria-alumina-silica system

The glass formation region in the yttria-alumina-silica system was investigated. Properties of glasses containing 25 to 55 wt pct yttria were measured and the effect of the composition was determined. The density, refractive index, thermal-expansion coefficient, and microhardness increased with increasing yttria content. The dissolution rate in 1N HCl increased with increasing yttria content and temperature. These glasses were also found to have high electrical resistivity.

Hyatt, M. J.↗
NASA NTRS1987

A comparison of the bromination dynamics of various carbon and graphite fibers

The electrical resistance of four grades of pitch-based graphite fibers and three experimental organic vapor-derived fibers was determined in situ during bromination and subsequent exposure to ambient laboratory air. The results show that the least graphitic pitch-based fiber does not brominate significantly, and that bromination and debrominaton reactions proceed much slower for vapor-derived fibers than for pitch-based ones. It is suggested that this decreased reacton rate is primarily due to the differences in graphene plane orientation between the fiber types. The results also imply that the vapor-derived and pitch-based fibers produce true intercalation compounds.

Gaier, James R.↗
NASA NTRS1987

In situ deposits of copper and copper oxide containing condensation polyimide films

Novel copper-polyimide composites have been synthesized via simultaneous thermal decomposition of solid solutions of bis (trifluoroacetylacetonato) copper (II) and thermal cyclodehydration of polyimide acid. In contrast to conventional filled polymer composites which are prepared by dispersion of particles or fibers in a polymer matrix this study has yielded in general uniform Cu or CuO dispersions of very small particle size that reside near the film surface that was exposed to the atmosphere during curing. The nature of the copper deposit, the thickness of the copper deposit, and the polyimide overlayer which bonds the copper to the polymer substrate depend on the curing atmosphere used. A variety of analytical surface methods along with thermogravimetric analysis and variable temperature (surface and volume) electrical resistivity measurements have been used to characterize these thin, flexible copper doped polyimide films.

Porta, G. M.↗
NASA NTRS1987

Effect of high temperature annealing on the thermoelectric properties of GaP doped SiGe

Silicon-germanium alloys doped with GaP are used for thermoelectric energy conversion in the temperature range 300-1000 C. The conversion efficiency depends on Z = S-squared/rho lambda, a material's parameter (the figure of merit), where S is the Seebeck coefficient, rho is the electrical resistivity and lambda is the thermal conductivity. The annealing of several samples in the temperature range of 1100-1300 C resulted in the power factor P (= S-squared/rho) increasing with increased annealing temperature. This increase in P was due to a decrease in rho which was not completely offset by a drop in S-squared suggesting that other changes besides that in the carrier concentration took place. SEM and EDX analysis of the samples indicated the formation of a Ga-P-Ge rich phase as a result of the annealing. It is speculated that this phase is associated with the improved properties. Several reasons which could account for the improvement in the power factor of annealed GaP doped SiGe are given.

Vandersande, Jan W.↗
NASA NTRS1988

Characterization of microcomposite polyimide films

Thermally stable microcomposite polyimide films were prepared by the homogeneous incorporation of either a soluble metal salt or a metal complex into a poly(amide-acid)/N,N-dimethylacetamide solution. The thermal processing of films solvent cast onto glass substrates and cured in an appropriate atmosphere yields films containing not only a fairly uniform dispersion of submicrometer particles, but also a new surface layer of metal or metal oxide. These polymeric systems have been characterized using X-ray photoelectron spectroscopy, Auger electron spectroscopy with depth profiling via argon ion etching, transmission electron microscopy of ultramicrotomed cross-sections, scanning electron microscopy, elemental analysis, thermal analysis, and variable temperature electrical resistivity determinations. The data from these techniques were used cooperatively to develop a model for these microcomposite polyimide films.

Rancourt, J. D.↗
NASA NTRS1989

Surface Halogenation Of High-Temperature Superconductors

Surface halogenation experimental technique of postgrowth nonaqueous chemical processing of high-temperature superconductors. Research continues in use of technique to obtain superconducting and/or passivated surfaces, to etch bulk superconductors, and to make low-resistance electrical contacts. Most promising etchant is bromine dissolved in such polar nonaqueous solvents as alchols, acetone, or ether. Etchant appears to act only on surface without adversely affecting superconducting bulk of film.

Vasquez, Richard P.↗
NASA NTRS1989

Improving Silicon Carbide/Silicon Nitride Fibers

Experiments described in report indicate properties of silicon carbide/silicon nitride fibers change significantly with modifications of monomeric precursors. Monomers polymerized by heating to temperatures above 500 degree C. Fibers drawn from resin melt under stream of dry nitrogen, cured in environmental chamber at 100 degree C and 95 percent relative humidity until unmeltable, then placed under tension and pyrolyzed at 1,200 to 1,500 degree C to produce inorganic silicon carbide/silicon nitride fibers. Allows possibility of creating improved silicon carbide/silicon nitride fibers, replacing graphite fibers in composite materials requiring high electrical resistance and thermal stability.

Crouse, David J.↗
NASA NTRS1986

Development of a high temperature static strain sensor

The goal of this program is to develop an electrical resistance strain gage system which will accurately measure the static strains of superalloy blades and vanes in gas turbine engines running on a test stand. Accurate knowledge of these strains is essential to reaching the goals of the HOST program in the selection and experimental verification of the various theoretical models developed to understand and improve the performance of these engines. The specific objective is to develop a complete system capable of making strain measurements of up to + or - 10 percent of full scale during a 50 hour period at temperatures as high as 1250 K. In addition to survival and stability, attaining a low temperature coefficient of resistance, of the order of 20 ppm/K or less, was a major goal. This requirement arises from the presently unavoidable uncertainties in measurement of the exact temperatures inside gas turbines for use in making corrections for apparent strain due to temperature.

Hulse, Charles O.↗
NASA NTRS1986

The NASA Lewis Strain Gauge Laboratory: An update

Efforts continue in the development and evaluation of electrical resistance strain gauges of the thin film and small diameter wire type. Results obtained early in 1986 on some Chinese gauges and Kanthal A-1 gauges mounted on a Hastelloy-X substrate are presented. More recent efforts include: (1) the determination of the uncertainty in the ability to establish gauge factor, (2) the evaluation of sputtered gauges that were fabricated at Lewis, (3) an investigation of the efficacy of dual element temperature compensated gauges when using strain gauge alloys having large thermal coefficients of resistance, and (4) an evaluation of the practical methods of stabilizing gauges whose apparent strain is dependent on cooling rate (e.g., FeCrAl gauges).

Hobart, H. F.↗
NASA NTRS1988

Proterozoic Bushveld-Vredefort catastrophe: Possible causes and consequences

Bushveld Complex and Vredefort Dome are unique features, formed in close proximity during the same time interval, approximately 2 Ga. Both show evidence of catastrophic events in the shallow marine environment of the otherwise stable Kaapvaal Craton. Explanation by multiple impacts of an asteroid, brecciated by an inter-asteroidal collision and disintegrating in Earth's gravity field is supported by pseudotachylite, shatter cones, coesite, and stishovite at Vredefort but these shock phenomena were not found in the Bushveld Complex. The Bushveld Complex was formerly interpreted as a lopolith, a view incompatible with gravity, electrical resistivity, magnetic, and seismic-reflection data. It is outlined by five inward-dipping lobes of layered ultramafic-mafic plutonic rocks that partly coalesce to form a basin-like feature 400 km in diameter and 65,000 sq. km. in area, equivalent to a small lunar mare. The Bushveld Complex is orders of magnitudes larger than other proposed terrestrial impact structures and differs from them in important ways. Its principal members, in order of age, are Rooiberg Felsite, RLS, and Lebowa Granite. The Bushveld-Vredefort events occurred during the interval from neutral or reducing atmosphere to oxidizing atmosphere. This transition is usually related to the evolution of photosynthesizing organisms. If the impact hypothesis for Bushveld-Vredefort can be confirmed, it may represent a global catastrophe sufficient to contribute to environmental changes favoring aerobic photosynthesizing eukaryotes over anaerobic prokaryotes.

Elston, W. E.↗
NASA NTRS1988

Effect of processing parameters on the characteristics of high-Tc superconductor YBa2Cu3Oy

SEM, thermogravimetric analysis, powder X-ray diffraction,and measurements of electrical resistivity and magnetic susceptibility, are presently used to characterize the influence of sintering temperature, sintering and annealing atmospheres, and quench-rate on the properties of the YBa2Cu3Oy superconducting oxide. It is established that annealing in oxygen, together with slow cooling rates, are required for preparation of high-Tc superconductors with sharp transitions; rapid quenching from high temperature does not yield good superconductors, due to low oxygen content.

Bansal, Narottam P.↗
NASA NTRS1989

Thermal design of the Wide Field/Planetary Camera

The Wide Field/Planetary Camera is an imaging system developed by the Jet Propulsion Laboratory for the NASA Hubble Space Telescope currently scheduled to be launched in December 1989 aboard the space shuttle. The temperature control design of the instrument utilizes multilayered insulation, electric resistance heaters, aluminum/ammonia heat pipes, thermoelectric coolers, temperature control coatings, and space radiators. A feedback control system maintains stable sensor temperatures. Thermal capacitance maintains stable optics and electronics temperatures during transient conditions. Schedule slips and launch delays have allowed extensive thermal testing of the instrument. Six instrument thermal vacuum tests and a spacecraft thermal vacuum test were performed. Several modifications have been made to the instrument to correct icing and contamination problems that have been discovered during thermal vacuum testing. This paper describes the thermal design, last instrument thermal vacuum test, results, and thermal model correlation.

Garcia, R. D.↗
NASA NTRS1989

Do steady fast magnetic dynamos exist?

This paper considers the question of the existense of a steady fast kinematic magnetic dynamo for a conducting fluid with a steady velocity field and vanishingly small electrical resistivity. The analysis of examples of steady dynamos, found by considering the zero-resistivity dynamics, indicated that, for sufficiently small resistivity, dynamo action can indeed occur in steady smooth three-dimensional chaotic fluid flows and that fast dynamos should consequently be a typical occurrence for such flows.

Finn, John M.↗
NASA NTRS1989

Soliton and strong Langmuir turbulence in solar flare processes

The occurrence of modulational instability in the current sheet of a solar flare is investigated. Special attention is given to the plasma microinstability in this sheet and its relation to the flare process. It is found that solitons or strong Langmuir turbulence are likely to occur in the diffusion region under solar flare conditions in which the electric resistivity could be enhanced by several orders of magnitude in the region, resulting in significant heating and stochastic acceleration of particles. A numerical example is used to demonstrate the transition of the magnetic field velocity and plasma density from the outer MHD region into the diffusive region and then back out again with the completion of the energy conversion process. This is all made possible by an increase in resistivity of four to five orders of magnitude over the classical value.

Song, M. T.↗
NASA NTRS1990

Programmable Analog Memory Resistors For Electronic Neural Networks

Electrical resistance of new solid-state device altered repeatedly by suitable control signals, yet remains at steady value when control signal removed. Resistance set at low value ("on" state), high value ("off" state), or at any convenient intermediate value and left there until new value desired. Circuits of this type particularly useful in nonvolatile, associative electronic memories based on models of neural networks. Such programmable analog memory resistors ideally suited as synaptic interconnects in "self-learning" neural nets. Operation of device depends on electrochromic property of WO3, which when pure is insulator. Potential uses include nonvolatile, erasable, electronically programmable read-only memories.

Ramesham, Rajeshuni↗
NASA NTRS1990

Improved Thermal-Switch Disks Protect Batteries

Improved thermal-switch disks help protect electrical batteries against high currents like those due to short circuits or high demands for power in circuits supplied by batteries. Protects batteries against excessive temperatures. Centered by insulating fiberglass washer. Contains conductive polymer that undergoes abrupt increase in electrical resistance when excessive current raises its temperature above specific point. After cooling, polymer reverts to low resistance. Disks reusable.

Darcy, Eric↗