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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 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Photophysics of poly(2,3,4,5,6-pentafluoro styrene) film

The temperature-dependent steady-state emission, emission polarization anisotropy, and fluorescence-kinetics of poly(2,3,4,5,6-pentafluoro styrene) film are reported. Two interconverting excited-state conformations of the chromophore have been identified. The fluorescence of the higher energy conformation results from excitation on the red edge of the polymer absorption band at temperatures below 180 K. The energy barrier for conversion of the high energy conformer to the lower energy conformer is estimated to be E/hc = 27 + or - 7/cm. Electronic energy migration is not evident in this polymer.

O'Connor, Donald B.↗
NASA NTRS1990

Recent trends in stratospheric ozone - Implications of ozone and temperature correlation

The relation between ozone and temperature in the upper stratosphere is used to estimate the drift in the SBUV spectrometer on Nimbus-7 and the linear trend in ozone from 1979 to 1986. The ozone and temperature variabilities are coupled through chemical and dynamical processes. In the upper stratosphere they are anticorrelated through temperature-dependent loss rates, while in the lower stratosphere their variabilities are positively correlated through dynamical coupling. These relations are used to determine the ozone sensitivity to temperature and ozone trends in terms of temperature trends. It is shown that in the upper stratosphere (approximately 2 mbar), the eight-year trend in ozone is a manifestation of dynamical perturbations. It varies from about +4 percent at the equator to about -1 to -2 percent at high latitudes in both hemispheres.

Chandra, S.↗
NASA NTRS1991

Thermal protection for hypervelocity flight in earth's atmosphere by use of radiation backscattering ablating materials

A heat-shield-material response code predicting the transient performance of a material subject to the combined convective and radiative heating associated with the hypervelocity flight is developed. The code is dynamically interactive to the heating from a transient flow field, including the effects of material ablation on flow field behavior. It accomodates finite time variable material thickness, internal material phase change, wavelength-dependent radiative properties, and temperature-dependent thermal, physical, and radiative properties. The equations of radiative transfer are solved with the material and are coupled to the transfer energy equation containing the radiative flux divergence in addition to the usual energy terms.

Howe, John T.↗
NASA NTRS1991

Temperature dependence of optical properties of GaAs

The effect of temperature on the optical properties of GaAs was investigated using spectroscopic ellipsometry measurements, between room temperature and about 610 C in increments of 50 C, of pseudodielectric functions and related optical constants of GaAs. A quantitative analysis of the pseudodielectric function spectrum was carried out using a harmonic-oscillator approximation (HOA) to fit the measured dielectric functions. Good fits were obtained with this model, which provides a convenient means of reproducing the GaAs dielectric function at any temperature, by using the temperature-dependent oscillator parameters. The HOA analysis also provides information about band-gap variation with temperature. Using the measured optical constants at a number of fixed temperatures, an algorithm was developed for computing the dielectric function spectrum at an arbitrary temperature in the range 22-610 C.

Yao, Huade↗
NASA NTRS1992

Martian atmospheric chemistry during the time of low water abundance

The importance of odd hydrogen (or HO(x)) radicals in the catalytic recombination of carbon monoxide and oxygen in the Martian atmosphere is a well known fact. The inclusion of recent chemical kinetics data, specifically temperature-dependent CO2 absorption cross sections, into our one dimensional photochemical model shows that HO(x) is too efficient in this regard. The absorption cross sections of CO2 are smaller than previously assumed; this leads to a reduction in the photolysis rate of CO2 while the photolysis rate of H2O has increased. As a consequence the predicted mixing ratio of CO in our models is substantially less than the observed value of 6.5(10)(exp -4). Simultaneous measurements of water, ozone, and carbon monoxide were obtained in the Martian atmosphere in early Dec. 1990 (L(sub s) for Mars was 344 deg.).

Nair, Hari↗
NASA NTRS1992

Effects of fiber and interfacial layer architectures on the thermoplastic response of metal matrix composites

Examined here is the effect of fiber and interfacial layer morphologies on thermal fields in metal matrix composites (MMCs). A micromechanics model based on an arbitrarily layered concentric cylinder configuration is used to calculate thermal stress fields in MMCs subjected to spatially uniform temperature changes. The fiber is modelled as a layered material with isotropic or orthotropic elastic layers, whereas the surrounding matrix, including interfacial layers, is treated as a strain-hardening, elastoplastic, von Mises solid with temperature-dependent parameters. The solution to the boundary-value problem of an arbitrarily layered concentric cylinder under the prescribed thermal loading is obtained using the local/global stiffness matrix formulation originally developed for stress analysis of multilayered elastic media. Examples are provided that illustrate how the morphology of the SCS6 silicon carbide fiber and the use of multiple compliant layers at the fiber/matrix interface affect the evolution of residual stresses in SiC/Ti composites during fabrication cool-down.

Pindera, Marek-Jerzy↗
NASA NTRS1991

Comparison of two-way satellite time transfer and GPS common-view time transfer between OCA and TUG

For about one year the time scales UTC(OCA) and UTC(TUG) were compared by means of GPS and two-way satellite time transfer. At the end of the experiment both links were independently 'calibrated' by measuring the differential delays of the GPS receivers and of the satellite earth stations by transportation of a GPS receiver and of one of the satellite terminals. The results obtained by both methods differ by about 3 ns, but reveal a seasonal variation of about 8 ns peak-to-peak which is likely the result of a temperature-dependence of the delays of the GPS receivers used. For the comparison of both methods the stabilities of the timescales are of great importance. Unfortunately, during the last three months of the experiment a less stable clock had to be used for the generation of UTC(TUG).

D. Kirchner↗
NASA NTRS1991

Boundary-element shape sensitivity analysis for thermal problems with nonlinear boundary conditions

Implicit differentiation of the discretized boundary integral equations governing the conduction of heat in solid objects subjected to nonlinear boundary conditions is shown to generate an accurate and economical approach for the computation of shape sensitivities for this class of problems. This approach involves the employment of analytical derivatives of boundary-element kernel functions with respect to shape design variables. A formulation is presented that can consistently account for both temperature-dependent convection and radiation boundary conditions. Several iterative strategies are presented for the solution of the resulting sets of nonlinear equations and the computational performances examined in detail. Multizone analysis and zone condensation strategies are demonstrated to provide substantive computational economies in this process for models with either localized nonlinear boundary conditions or regions of geometric insensitivity to design variables. A series of nonlinear example problems are presented that have closed-form solutions.

Kane, James H.↗
NASA NTRS1991

Emission spectra and kinetics of copolymer films of styrene and 2,3,4,5,6-pentafluorostyrene

The temperature-dependent steady-state emission spectra and fluorescence kinetics of copolymer films of styrene and 2,3,4,5,6-pentafluorostyrene are reported. The polystyrene excimer emission is efficiently quenched in the presence of relatively small amounts of the pentafluorophenyl quencher in spite of the fact that direct energy transfer from the excimer to the quencher chromophores is not possible. The quenching of the polystyrene excimer emission at room temperature and of the polystyrene monomer emission at 13 K is attributed to the interception of the migrating monomer excitation by the quencher. The data suggest that this quenching mechanism is independent of temperature.

O'Connor, Donald B.↗
NASA NTRS1990

Dissociation of O(2-)2 defects into paramagnetic O(-) in wide band-gap insulators - A magnetic susceptibility study of magnesium oxide

The magnetic susceptibility of single-crystal MgO has been measured in the temperature range 300-1000 K, using a Faraday balance. The high-purity crystal (less than 100 ppm transition metals), grown from the melt in a H2O-containing atmosphere, was found to be paramagnetic due to the presence of defects on the O(2-) sublattice. The defects derive from OH(-) introduced into the MgO matrix by the dissolution of traces of H2O during crystal growth. The OH(-) converts into O(2-)2 and H2. Each O(2-)2 represents two coupled, spin-paired O(-) states. The observed strongly temperature-dependent paramagnetism can be described by three contributions that overlay the intrinsic diamagnetism of MgO and arise from the low level of transition-metal impurities, O(-) generated by 0(2-)2 dissociation, and O(-) states trapped by quenching from high temperatures from previous experiments.

Batllo, F.↗
NASA NTRS1989

Nonlinear triggered lightning models for use in finite difference calculations

Two nonlinear triggered lightning models have been developed for use in finite difference calculations. Both are based on three species of air chemistry physics and couple nonlinearly calculated air conductivity to Maxwell's equations. The first model is suitable for use in three-dimensional modeling and has been applied to the analysis of triggered lightning on the NASA F106B Thunderstorm Research Aircraft. The model calculates number densities of positive ions, negative ions, and electrons as a function of time and space through continuity equations, including convective derivative terms. The set of equations is closed by using experimentally determined mobilities, and the mobilities are also used to determine the air conductivity. Results from the model's application to the F106B are shown. The second model is two-dimensional and incorporates an enhanced air chemistry formulation. Momentum conservation equations replace the mobility assumption of the first model. Energy conservation equations for neutrals, heavy ions, and electrons are also used. Energy transfer into molecular vibrational modes is accounted for. The purpose for the enhanced model is to include the effects of temperature into the air breakdown, a necessary step if the model is to simulate more than the very earliest stages of breakdown. Therefore, the model also incorporates a temperature-dependent electron avalanche rate. Results from the model's application to breakdown around a conducting ellipsoid placed in an electric field are shown.

Rudolph, Terence↗
NASA NTRS1992

A filter for deep near-infrared imaging

The K passband (central wavelength 2.2 microns, FWHM 0.4 micron) is the longest wavelength standard near-IR passband through which deep ground-based imaging is possible. Thermal emission from telescope, instrument, and sky limits the depth to which such imaging can reach by producing strongly temperature-dependent backgrounds in the range 11-13.5 mag/sq arcsec. This paper describes how a passband, which is denoted as K-prime, located slightly shortward of the standard K passband (central wavelength 2.1 microns), yet still within the same atmospheric window, leads to a significantly lower thermal component of the background, reducing the background surface brightness by up to 0.9 mag sq arcsec, and thereby allowing deeper imaging to be obtained in the same integration time. The photometric differences between the K-prime filter and the standard K filter are discussed.

Wainscoat, Richard J.↗
NASA NTRS1992

Stress versus temperature dependence of activation energies for creep

The activation energy for creep at low stresses and elevated temperatures is associated with lattice diffusion, where the rate controlling mechanism for deformation is dislocation climb. At higher stresses and intermediate temperatures, the rate controlling mechanism changes from dislocation climb to obstacle-controlled dislocation glide. Along with this change in deformation mechanism occurs a change in the activation energy. When the rate controlling mechanism for deformation is obstacle-controlled dislocation glide, it is shown that a temperature-dependent Gibbs free energy does better than a stress-dependent Gibbs free energy in correlating steady-state creep data for both copper and LiF-22mol percent CaF2 hypereutectic salt.

Freed, A. D.↗
NASA NTRS1990

Laser-induced fluorescence of phosphors for remote cryogenic thermometry

Remote cryogenic temperature measurements can be made by inducing fluorescence in phosphors with temperature-dependent emissions and measuring the emission lifetimes. The thermographic phosphor technique can be used for making precision, noncontact, cryogenic-temperature measurements in electrically hostile environments, such as high dc electric or magnetic fields. The National Aeronautics and Space Administration is interested in using these thermographic phosphors for mapping hot spots on cryogenic tank walls. Europium-doped lanthanum oxysulfide (La2O2S:Eu) and magnesium fluorogermanate doped with manganese (Mg4FGeO6:Mn) are suitable for low-temperature surface thermometry. Several emission lines, excited by a 337-nm ultraviolet laser, provide fluorescence lifetimes having logarithmic dependence with temperature from 4 to above 125 K. A calibration curve for both La2O2S:Eu and Mg4FGeO6:Mn is presented, as well as emission spectra taken at room temperature and 11 K.

Beshears, D. L.↗
NASA NTRS1992

Numerical models of mantle convection

An overview of numerical methods describing the structure and dynamics of the mantle is presented with attention given to novel 3D modeling techniques. The paper reviews 3D spherical and Cartesian models for constant viscosity emphasizing the assumptions regarding style of convection, time dependence, and implications for the mantle. Similarly treated are 3D Cartesian models with temperature-dependent viscosities, and briefly examined are models that are based on compressibility, nonlinear viscosity, or plates. Extensive illustrations are presented detailing: (1) temperature variations from models of 3D thermal convection in spherical shells; (2) thermal anomalies in equatorial cross sections; and (3) temperature variations in a spherical shell heated from within. The discussion relates the numerical results of the models with real mantle-convection events, and the simulations are shown to yield increasingly realistic representations of material behavior.

Schubert, G.↗
NASA NTRS1992

Thermocapillary flow about an evaporating meniscus

The steady motion and thermal behavior of an evaporating superheated liquid in a small cavity bounded by isothermal sidewalls is examined. Scaling analyses and a two-dimensional finite element model are used to investigate the influence of thermocapillarity, buoyancy, and temperature-dependent mass flux on flowfield, interfacial heat transfer, and meniscus morphology. Numerical investigations indicate the existence of two counter-rotating cells symmetric about the cavity center. Results also show that evaporation tends to counteract this circulation by directing flow toward the hotter sidewalls. Although thermocapillarity and evaporation yield different flowfield distributions, both effects tend to increase interfacial temperature and heat transfer.

Schmidt, G. R.↗
NASA NTRS1992

The gabbro-eclogite phase transition and the elevation of mountain belts on Venus

Among the four mountain belts surrounding Lakshmi Planum, Maxwell Montes is the highest and stands up to 11 km above the mean planetary radius and 7 km above Lakshmi Planum. The bulk composition and radioactive heat production of the crust on Venus, where measured, are similar to those of terrestrial tholeiitic basalt. Because the thickness of the low-density crust may be limited by the gabbro-garnet granulite-eclogite phase transitions, the 7-11 km maximum elevation of Maxwell Montes is difficult to understand except in the unlikely situation that the crust contains a large volume of magma. A possible explanation is that the base of the crust is not in phase equilibrium. It has been suggested that under completely dry conditions, the gabbro-eclogite phase transition takes place by solid-state diffusion and may require a geologically significant time to run to completion. Solid-state diffusion is a strongly temperature-dependent process. In this paper we solve the thermal evolution of the mountain belt to attempt to constrain the depth of the gabbro-eclogite transition and thus to assess this hypothesis quantitatively. The one-dimensional heat equation is solved numerically by a finite difference approximation. The deformation of the horizontally shortening crustal and mantle portions of the thermal boundary layer is assumed to occur by pure shear, and therefore the vertical velocity is given by the product of the horizontal strain rate and depth.

Namiki, Noriyuki↗