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At least 487 records · Page 27

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NASA NTRS1993

Lumped elements characterize Q in dielectric resonators

It has been earlier observed (Podcameni et al., 1981) that, as the coupling factor between a microstrip-coupled dielectric resonator and the line becomes much larger than unity, the unloaded quality factor (Q) of the resonator decreases. In this paper it is shown that this effect can be explained using lumped-element models of the coupling line, when the dielectric resonator is either overcoupled or undercoupled to the line.

Hearn, Chase P.↗

NASA NTRS1994

Insoluble, Low-Dielectric-Constant Polyimides

In process, insoluble, low-dielectric-constant polyimides produced by combining dianhydride of bis{4-(3,4-dicarboxyphenoxy) phenyl}perfluoroisopropylidene (BFDA) with fluorinated aromatic diamine; each molecule contains at least one or more CF3 groups. These fully fluorinated polyimides have dielectric constants in range of 2.4 to 2.5 at 10 GHz. Suitable for use as composite matrix resins in advanced aerospace applications. Potential use in microelectronics industry, particularly as insulating resins in production of printed-circuit boards.

St. Clair, Anne K.↗

NASA NTRS1994

Low-Dielectric-Constant Polyimide Fibers

In experiments performed at NASA Langley Research Center, low-dielectric-constant polyimide fibers produced by use of resin extrusion. These fibers also have high thermal stability and good tensile properties. Useful in industrial and aerospace applications in which fibers required to have dielectric constants less than 3, high thermal stability, and tensile properties in range of those of standard textile fibers. Potential applications include use in printed circuit-boards and in aircraft composites.

Dorogy, William E., Jr.↗

NASA NTRS1993

Cyanoresin, cyanoresin/cellulose triacetate blends for thin film, dielectric capacitors

Non-brittle dielectric films are formed by blending a cyanoresin such as cyanoethyl, hydroxyethyl cellulose (CRE) with a compatible, more crystalline resin such as cellulose triacetate. The electrical breakdown strength of the blend is increased by orienting the films by uniaxial or biaxial stretching. Blends of high molecular weight CRE with high molecular weight cyanoethyl cellulose (CRC) provide films with high dielectric constants.

Yen, Shiao-Ping↗

NASA NTRS1993

FD-TD modeling of 2-D dielectric waveguides for propagation and scattering of femtosecond optical solitons

Experimentalists have produced all-optical switches capable of 100-fs responses. To adequately model such switches, nonlinear effects in optical materials (both instantaneous and dispersive) must be included. In principle, the behavior of electromagnetic fields in nonlinear dielectrics can be determined by solving Maxwell's equations subject to the assumption that the electric polarization has a nonlinear relation to the electric field. However, until our previous work, the resulting nonlinear Maxwell's equations have not been solved directly. Rather, approximations have been made that result in a class of generalized nonlinear Schrodinger equations (GNLSE) that solve only for the envelope of the optical pulses. In this paper, we present first-time calculations from the vector nonlinear Maxwell's equations of femtosecond soliton propagation and scattering, including carrier waves, in two-dimensional systems of dielectric waveguides exhibiting the Kerr and Raman quantum effects. We use the finite-difference time-domain (FD-TD) method in an extension of our 1-D work. There, in a fundamental innovation, we treated the linear and nonlinear convolutions for the electric polarization as new dependent variables. By differentiating these convolutions in the time domain, we derived an equivalent system of coupled, nonlinear second-order ODE's. These equations together with Maxwell's equations form the system that is solved to determine the electromagnetic fields in inhomogeneous nonlinear dispersive media. Backstorage in time is limited to only that needed by the time-integration algorithm for the ODE's, rather than that needed to store the time-history of the kernel functions of the convolutions (1000-10,000 time steps). Thus, a 2-D nonlinear optics model from Maxwell's equations is now feasible.

Joseph, Rose↗

NASA NTRS1994

Dielectric measurements of selected ceramics at microwave frequencies

Dielectric measurements of strontium titanate and lead titanate zirconate ceramics are conducted at microwave frequencies using a cylindrical resonant cavity in the TE(sub 011) mode. The perturbations of the electric field are recorded in terms of the frequency shift and Q-changes of the cavity signal. Slater's perturbation equations are used to calculate e' and e" of the dielectric constant as a function of temperature and frequency.

Dahiya, J. N.↗

NASA NTRS1994

Role of dielectric constant in electrohydrodynamics of conducting fluids

Electrohydrodynamic sample distortion during continuous flow electrophoresis is an experiment to be conducted during the second International Microgravity Laboratory (IML-2) in July 1994. The specific objective of this experiment is the distortion caused by the difference in dielectric constant between the sample and surrounding buffer. Although the role of sample conductivity in electrohydrodynamic has been the subject of both flight and ground experiments, the separate role of dielectric constant, independent of sample conductivity, has not been measured. This paper describes some of the laboratory research and model development that will support the flight experiment on IML-2.

Rhodes, Percy H.↗

NASA NTRS1994

Low dielectric polyimide fibers

A high temperature resistant polyimide fiber that has a dielectric constant of less than 3 is presented. The fiber was prepared by first reacting 2,2-bis (4-(4aminophenoxy)phenyl) hexafluoropropane with 2,2-bis (3,4-dicarboxyphenyl) hexafluoropropane dianhydride in an aprotic solvent to form a polyamic acid resin solution. The polyamic acid resin solution is then extruded into a coagulation medium to form polyamic acid fibers. The fibers are thermally cured to their polyimide form. Alternatively, 2,2-bis(4-(4-aminophenoxy)phenyl) hexafluoropropane is reacted with 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride to form a polyamic acid, and the polyamic acid is chemically converted to its polyimide form. The polyimide is then dissolved in a solvent to form a polyimide resin solution, and the polyimide resin is extruded into a coagulation medium to form a polyimide wet gel filament. In order to obtain polyimide fibers of increased tensile properties, the polyimide wet gel filaments are stretched at elevated temperatures. The tensile properties of the fibers were measured and found to be in the range of standard textile fibers. Polyimide fibers obtained by either method will have a dielectric constant similar to that of the corresponding polymer, viz., less than 3 at 10 GHz.

Dorogy, William E., Jr.↗

NASA NTRS1994

Merging Magellan emissivity and SAR data for analysis of Venus surface dielectric properties

Integration of Magellan datasets for Venus is an important part of characterizing the geology of the surface. We analyze the global average beahvior of radar backscatter and emissivity and find that for incidence angles greater than 30 deg the two parameters can be modeled by the behavior of a single-dielectric surface (epsilon approximately 4.15) with variable roughness. Based on this result, we propse a model which relates the HH backscatter and H-polarized emissivity to values for a relative roughness fraction and dielectric constant.

Campbell, Bruce A.↗

NASA NTRS1996

Thin-Ribbon Tapered Couplers For Dielectric Waveguides

Thin-ribbon tapered couplers proposed for launching electro-magnetic waves into dielectric waveguides, which include optical fibers. Intended for use with ribbon dielectric waveguides designed for operation at millimeter or submillimeter wavelengths, made of high-relative-permittivity, low-loss materials and thicknesses comparable to or less than free-space design wavelengths. Coupling efficiencies exceeds those of older tapered couplers.

Otoshi, Tom Y.↗

NASA NTRS1996

Automated Monitoring Of Dielectric Properties Of Tree Trunks

Semiautomated instrumentation system called "dielectric monitoring system" (DMS) developed for measuring microwave permittivities of selected components of plants, in particular, of active xylems in tree trunks. System set up with coaxial probes inserted in tree trunks to measure dielectric properties. Can be left to operate unattended to gather data on permittivities as function of time.

Mcdonald, Kyle C.↗

NASA NTRS1997

W-Band Transmission MeasurementS and X-Band Dielectric Properties Measurements for a Radome Material Sample

This paper describes measurements which were performed on a sample of radome material in the Electromagnetic Properties Measurements Laboratory (EPML). The purpose of the measurements described in this paper was to determine the one-way transmission loss through the flat panel of radome material for a frequency range of 84 to 94 GHz, for varying incidence angles. The panel, which was manufactured by Norton Performance Plastics Corporation, was provided to the EPML by TRW. The size of the panel is 40 in x 36 in x 0.422 in and consists of a foam material with one side coated with a smooth white coating (this side will be referred to as the front side). The dielectric properties of the foam material from the inside of the panel were also determined at X-band (8.2-12.4 GHz). The W-band free space measurements are presented first, followed by the X-band dielectric properties measurements.

Cravey, Robin L.↗

NASA NTRS1997

Dielectric Spectroscopy Study of ZnSe Grown by Physical Vapor Transport

The dielectric properties of ZnSe samples grown by physical vapor transport were measured as a function of frequency. Differences can be seen in the dielectric properties of samples grown under different conditions. The spectra of heat treated samples were also acquired and were found to exhibit significant deviations from those of the as grown crystals.

Kokan, J.↗

NASA NTRS1997

Dielectric Spectroscopy Study of ZnSe Grown by Physical Vapor Transport

Kokan, Julie L.↗

NASA NTRS1999

The Conductor-Dielectric Junctions in a Low Density Plasma

A conductor-dielectric junction exposed to the space environment is a frequent spacecraft design feature. Due to spacecraft charging and/or solar array operation, the conductor can acquire a high potential with respect to the surrounding plasma. If this potential is positive the insulators adjacent to exposed conductors can collect current as if they were conductors themselves. This phenomenon, called snapover, results in a substantial increase in current collection, and may even result in a glow discharge if the potential is high enough. If a conductor has a negative potential, arcing can occur at the site of a junction. Both of these phenomena negatively affect spacecraft operation. To prevent negative consequences, the physical mechanisms of snapover and arc inception require investigation. In this paper, results are presented of an experimental and theoretical study of snapover, glow discharge, and arc phenomena for different materials immersed in argon or xenon plasmas. The effect of snapover is investigated for several metal-dielectric junctions: copper-teflon, copper-Kapton, copper-glass, aluminum-teflon, aluminum-Kapton, steel-teflon, anodized aluminum with pinholes, and copper-ceramics. I-V curves are measured and snapover inception voltages, essential parameters (increase in current and collection area due to secondary electrons), and glow discharge inception thresholds are determined. Optical spectra are obtained for glow discharges in both argon and xenon plasmas. These spectra provide information regarding atomic species entrapped in the glow region. Some spectral lines can be used to estimate plasma parameters in the discharge area. A video-camera and linear array were used to confirm that snapover inception is accompanied by very low intensity visible light emission. This result seems to be important for the estimate of the light pollution around spacecraft. Optical spectra (wavelengths 380-650 nm) of arcs are also obtained on a negatively biased chromic acid anodized aluminum plate immersed in low density argon and xenon plasmas. Analysis of these spectra confirms our earlier findings that aluminum atoms are ejected from the arc site. Moreover, it is found that chromium atoms are also quite abundant in the arc plasma. It is believed that the latter results contribute considerably to the understanding of processes of plasma contamination caused by arcing.

Vayner, Boris↗

NASA NTRS2000

BOREAS RSS-17 Dielectric Constant Profile Measurements

The BOREAS RSS-17 team acquired and analyzed imaging radar data from the ESA's ERS-1 over a complete annual cycle at the BOREAS sites in Canada in 1994 to detect shifts in radar backscatter related to varying environmental conditions. This data set consists of dielectric constant profile measurements from selected trees at various BOREAS flux tower sites. The relative dielectric constant was measured at C-band (frequency = 5 GHz) as a function of depth into the trunk of three trees at each site, Measurements were made during April 1994 with an Applied Microwave Corporation field PDP fitted with a 0.358-cm (0.141-inch) diameter coaxial probe tip. The data are available in tabular ASCII files. The data files are available on a CD-ROM (see document number 20010000884), or from the Oak Ridge National Laboratory (ORNL) Distributed Active Archive Center (DAAC).

Hall, Forrest G.↗

NASA NTRS2002

Preliminary Evaluation of Polyarylate Dielectric Films for Cryogenic Applications

Polymeric materials are used extensively on spacecraft and satellites in electrical power and distribution systems, as thermal blankets and optical surface coatings, as well as mechanical support structures. The reliability of these systems when exposed to the harsh environment of space is very critical to the success of the mission and the safety of the crew in manned-flight ventures. In this work, polyarylate films were evaluated for potential use as capacitor dielectrics and wiring insulation for cryogenic applications. Two grades of the film were characterized in terms of their electrical and mechanical properties before and after exposure to liquid nitrogen (-196 C). The electrical characterization consisted of capacitance and dielectric loss measure Cents in the frequency range of 50 Hz to 100 kHz, and volume and surface resistivities. The mechanical measurements performed included changes in tensile (Young's modulus, elongation-at-break, and tensile strength) and structural properties (dimensional change, weight, and surface morphology). The preliminary results, which indicate good stability of the polymer after exposure to liquid nitrogen, are presented and discussed.

Patterson, Richard L.↗

NASA NTRS2004

On the Dielectric Properties of the Martian-like Surface Sediments

We have undertaken laboratory electromagnetic characterization of the total set of minerals identified by TES on the Martian surface in order to investigate experimentally the dielectric properties of the sediments covering it in the frequency range from 1 to 30 MHz. Volcanic Rocks with a well defined mineralogy and petrology from potential terrestrial analogues sites have also been included in the study. Our primary objective is to evaluate the range of electrical and magnetic losses that may be encountered by the various Radar sounding and imaging experiments dedicated to map the Martian subsurface searching for underground water. The electromagnetic properties of these Mars-like materials will be presented as a function of various geophysical parameters, such as porosity, bulk density and temperature. The secondary objective, is to locate regions were surface dielectric conditions are suitable for subsurface sounding.

Heggy, E.↗