Engineering topics

Singh, J. J.

Publications and source records attributed to Singh, J. J..

At least 19 records

NASA NTRS1996

Electrorheological Fluids: Aerospace Applications

Electrorheological fluids (ERF) are an intriguing class of non-Newtonian industrial fluids. They consist of fine dielectric particles suspended in liquids of low dielectric constants. The objectives of this research were to select a particulate system such that: (1) its density can be varied to match that of the selected liquid, and (2) the dielectric constant of the particles and the liquids should be such that the critical fields needed for asymptotic increase in viscosity are less than or equal to 10 KV/cm. Synthetic Zeolite particles were selected as the solute/suspensions. Octoil oil was selected as the solvent. The results are summarized here.

Parmar, D. S.↗

NASA NTRS1996

Free Volume in Glassy Poly(arylene Ether Ketone)s

Amorphous polyarylene ether ketones were examined in the glassy state by positron annihilation lifetime spectroscopy ( PALS ) and in the melt by standard rheological techniques. Specimens were well-characterized fractions of two isomeric structures. PALS clearly shows that the polymer with meta linkages in its backbone contains larger voids (greater than 0.25 nm radius). Thus despite their similar bulk densities, the two materials must pack very differently on a local scale. On the other hand, the free volumes inferred from the WLF treatment of melt viscosity data are practically identical in both materials ca. 4% at T(sub g). The comparison between techniques sheds some light on the distribution of free volume.

Hinkley, J. A.↗

NASA NTRS1996

Microstructural Characterization of Thin Polyimide Films by Positron Lifetime Spectroscopy

Positron lifetimes have been measured in a series of thin aromatic polyimide films. No evidence of positronium formation was observed in any of the films investigated. All test films exhibited only two positron lifetime components, the longer component corresponding to the positrons annihilating at shallow traps. Based on these trapped positron lifetimes, free volume fractions have been calculated for all the films tested. A free volume model has been developed to calculate the dielectric constants of thin polyimide films. The experimental and the calculated values for the dielectric constants of the films tested are in reasonably good agreement. It has been further noted that the presence of bulky CF(sub 3) groups and meta linkages in the polyimide structure results in higher free volume fraction and, consequently, lower dielectric constant values for the films studied.

Eftekhari, A.↗

NASA NTRS1994

Morphology of Thermoset Polyimides by Positron Annihilation Spectroscopy

Thermoset polyimides have great potential for successfully meeting tough stress and temperature challenges in the advanced aircraft development program. However, studies of structure/property relationships in these materials have not been very successful so far. Positron annihilation spectroscopy has been used to investigate free volumes and associated parameters. It has been noted that the free volume correlates well with the molecular weight, cross-link density and thermal coefficient of expansion of these materials. Currently no other techniques are available for direct measurement of these parameters. Experimental results and their interpretations will be discussed.

Ranganathaiah, C.↗

NASA NTRS1993

Shear-induced surface alignment of polymer dispersed liquid crystal microdroplets on the boundary layer

Polymer dispersed liquid crystal thin films have been deposited on a glass substrate, utilizing the processes of polymerization and solvent evaporation induced phase separation. Liquid crystal microdroplets trapped on the upper surface of the thin film respond to the shear stress due to air or gas flow on the surface layer. Response to an applied step shear stress input on the surface layer has been measured by measuring the time response of the transmitted light intensity. Initial results on the measurements of the light transmission as a function of the air flow differential pressure indicate that these systems offer features suitable for boundary layer and gas flow sensors.

Parmar, D. S.↗

NASA NTRS1993

Gas flow-field induced director alignment in polymer dispersed liquid crystal microdroplets deposited on a glass substrate

Polymer dispersed liquid crystal thin films have been deposited on glass substrates by the processes of polymerization and solvent evaporation induced phase separation. The electron and the optical polarization microscopies of the films reveal that PDLC microdroplets formed during the process of phase separation near the top surface of the film remain exposed and respond to shear stress due to air or gas flow on the surface. Optical response of the film to an air flow-induced shear stress input on the free surface has been measured. Director orientation in the droplets changes with the applied shear stress leading to time varying transmitted light intensity. Director dynamics of the droplet for an applied step shear stress has been discussed from free energy considerations. Results on the measurement of light transmission as a function of the gas flow parameter unambiguously demonstrate the potential of these systems for use as boundary layer and gas flow sensors.

Parmar, D. S.↗

NASA NTRS1992

Free volume model for molecular weights of polymers

A free volume model has been developed for determining molecular weights of linear polymers. It is based on the size of free volume cells in two geometries of poly(arylene ether ketone)s. Free volume cell sizes in test samples were measured using positron lifetime spectroscopy. The molecular weights computed from free volume cell sizes are in good agreement with the values measured by gel permeation chromatography, with a low angle laser light scattering photometer as the detector. The model has been further tested on two atactic polystyrene samples, where it predicted the ratio of their molecular weights with reasonable accuracy.

Singh, J. J.↗

NASA NTRS1986

Investigation of the effects of cobalt ions on epoxy properties

The effects of Co(acac)sub x complexes on MY-720 epoxy properties have been investigated. It appears that Co2(+) ions form antibonding or nonbonding orbitals which increase the free volume and also reduce the cohesiveness of the host epoxy. The effects of Co2(+) ions, on the other hand, seem to result in increased Cohesiveness of the epoxy. The experimental values of magnetic moments of both types of ions in MY-720 suggest that the orbital momentum contributions of the (3d) electrons are partially conserved, though the effect is more pronounced for Co2(+) ions. The coordination environment of the cobalt ions in the host epoxy does not appear to be uniquely defined. These results indicate that the effects of metal ions on resin properties cannot be easily predicted on the basis of ligand field theory argument alone. Complex interactions between metal ions and host epoxy molecular structure suggest the desirability of parallel experimental investigations of electronic, magnetic, and mechanical properties of metal ion-containing epoxy samples for comparison with theory.

Singh, J. J.↗

NASA NTRS1986

Investigation of the effects of short chain processing additives on polymers

The effects of low level concentrations of several short chain processing additives on the properties of the 4,4'-bis(3,4-dicarboxyphenoxy) diphenylsulfide dianhydride (BDSDA)/4,4'-diaminodiphenyl ether (ODA)/1,3'-diaminobenzene (m-phenylene diamine) (MPA) (422) copolyimide were investigated. It was noted that 5 percent MPD/phthalic anhydride (PA) is more effective than 5 percent ODA/PA and BDSDA/aniline (AN) in strengthening the host material. However, the introduction of 10 percent BDSDA/AN produces disproportionately high effects on free volume and free electron density in the host copolyimide.

Singh, J. J.↗

NASA NTRS1986

Effect of annealing history on free volume in thermoplastics

Two different types of thermoplastic glassy polymers have been investigated for the effects of thermal annealing on their free volumes. It has been observed that free volumes in glassy polymers decrease asymptotically to a steady level after about four thermal anneals lasting for 24 hours at a temperature about 50 C below their glass transition temperatures. These results indicate that composites incorporating properly annealed thermoplastic matrices may not experience any additional internal stresses due to subsequent thermal excursions experienced while in service.

Singh, J. J.↗

NASA NTRS1986

Measurement of viscosity of gaseous mixtures at atmospheric pressure

Coefficients of viscosity of various types of gas mixtures, including simulated natural-gas samples, have been measured at atmospheric pressure and room temperature using a modified capillary tube method. Pressure drops across the straight capillary tube section of a thermal mass flowmeter were measured for small, well-defined, volume flow rates for the test gases and for standard air. In this configuration, the flowmeter provides the volumetric flow rates as well as a well-characterized capillary section for differential pressure measurements across it. The coefficients of viscosity of the test gases were calculated using the reported value of 185.6 micro P for the viscosity of air. The coefficients of viscosity for the test mixtures were also calculated using Wilke's approximation of the Chapman-Enskog (C-E) theory. The experimental and calculated values for binary mixtures are in agreement within the reported accuracy of Wilke's approximation of the C-E theory. However, the agreement for multicomponent mixtures is less satisfactory, possible because of the limitations of Wilkes's approximation of the classical dilute-gas state model.

Singh, J. J.↗

NASA NTRS1986

Technique for Measuring Gas Conversion Factors

Technique for measuring calibration conversion factors for hydrocarbon mass flowmeters applied to widely used type of commercial thermal mass flowmeter for hydrocarbon gases. Values of conversion factors for two common hydrocarbons measured using this technique in good agreement with empirical values cited by manufacturer. Similar agreement expected for all other hydrocarbons. Technique based on Nernst theorem for matching partial pressure of oxygen in combustion product gases with that in normal air. Simple, quick, and relatively safe, particularly for toxic/poisonous hydrocarbons.

Singh, J. J.↗

NASA NTRS1986

Liquid-Level Sensor for Containers in Motion

Nonintrusive technique monitors fluid contents of sealed vessels, regardless of fluid distribution inside vessels. Technique based on differences in cesium-137 gamma-ray attenuation coefficients in air and test liquids. Used to measure fluid content in closed containers on high-maneuvering aircraft where accelerations cause fluid to become distributed throughout container.

Singh, J. J.↗

NASA NTRS1986

Chromium Ions Improve Moisure Resistance of Epoxy Resins

Broad spectrum of thermosetting epoxy resins used on commercial and military aircraft, primarily as composite matrices and adhesives. In new technique, chromium-ion containing epoxy with improved resistance to moisture produced where chromium ions believed to prevent absorption of water molecules by coordinating themselves to hydroxyl groups on epoxy chain. Anticipated that improved epoxy formulation useful as composite matrix resin, adhesive, or casting resin for applications on commercial and advanced aircraft. Improvement made without sacrifice in mechanical properties of polymer.

St. Clair, A. K.↗

NASA NTRS1986

Fast-Response Oxygen-Monitoring and Control System

Oxygen sensor is Y2O3-stabilized ZrO2 ceramic disk maintained at 843 degrees C. Overall system response time reduced to about 0.2 second, equal to or less than 1 percent of tunnel run time. When test gas oxygen concentration differs from normal air concentration by 25 percent or more, alarm sounds, and emergency tunnel shutdown signal operates. New ZrO2 sensors intended for hypersonic-vehicle testing.

Singh, J. J.↗

NASA NTRS1986

Liquid-Level Monitor for Pressurized Vessels

Technique for monitoring water levels in pressurized stainless-steel cylinders, based on differences in gamma-ray attenuation coefficients in water and air, developed. Full-scale laboratory prototype system constructed to test technique. Technique usable with liquids other than water, since linear attenuation coefficients for intermediate-energy gamma rays in air considerably lower than in liquids. Also adaptable for continuous monitoring of liquid levels in resevoir systems and in underground storage tanks.

Singh, J. J.↗

NASA NTRS1985

New method for determining heats of combustion of gaseous hydrocarbons

As a spin off of a system developed for monitoring and controlling the oxygen concentration in the Langley 8-foot High Temperature Tunnel, a highly accurate on-line technique was developed for determining heats of combustion of natural gas samples. It is based on measuring the ratio m/n, where m is the (volumetric) flowrate of oxygen required to enrich the carrier air in which the test gas flowing at the rate n is burned, such that the mole fraction of oxygen in the combustion product gases equals that in the carrier air. The m/n ratio is directly related to the heats of combustion of the saturated hydrocarbons present in the natural gas. A measurement of the m/n ratio for the test gas can provide a direct means of determination of its heat of combustion by using the calibration graph relating the m/n values for pure saturated hydrocarbons with their heats of combustion. The accuracy of the technique is determine solely by the accuracy with which the flowrates m and n can be measured and is of the order of 2 percent in the present study. The theoretical principles and experimental results are discussed.

Singh, J. J.↗

NASA NTRS1985

Technique for measuring gas conversion factors

A method for determining hydrocarbon conversion factors for a flowmeter. A mixture of air, O2 and C sub x H sub y is burned and the partial paressure of O2 in the resulting gas is forced to equal the partial pressure of O2 in air. The flowrate of O2 flowing into the mixture is measured by flowmeter and the flowrate of C sub x H sub y flowing into the mixture is measured by the flowmeter conversion factor is to be determined. These measured values are used to calculate the conversion factor.

Singh, J. J.↗