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At least 235 records · Page 13

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NASA NTRS1984

Long life nickel electrodes for a nickel-hydrogen cell. III - Results of an accelerated test and failure analyses

Nineteen different designs of nickel electrodes were tested in Ni/H2 boiler plate cells in an accelerated low earth orbit cycle regime to the end of their life. The failure analyses of these cells showed that the major performance changes due to the cycling was a severe reduction of their high rate discharge capability rather than an absolute capacity reduction. Many physical changes of the nickel electrodes were observed after the cycling test. These changes include dimensional expansion, sinter rupture, loose black powdering of the active material, morphology changes, active material migration, increase of pore volume, change of pore distribution, and increase of surface area. All of these were caused by active material expansion with cycling. Among these changes, the morphology change which involves migration of active material away from the current collecting nickel sinter appears to be that most responsible for the reduction of the rate capability.

Lim, H. S.↗

NASA NTRS1986

Improved consolidation of silicon carbide

Alpha silicon carbide powder was consolidated by both dry and wet methods. Dry pressing in a double acting steel die yielded sintered test bars with an average flexural strength of 235.6 MPa with a critical flaw size of approximately 100 micro m. An aqueous slurry pressing technique produced sintered test bars with an average flexural strength of 440.8 MPa with a critical flaw size of approximately 25 micro m. Image analysis revealed a reduction in both pore area and pore size distribution in the slurry pressed sintered test bars. The improvements in the slurry pressed material properties are discussed in terms of reduced agglomeration and improved particle packing during consolidation.

Freedman, M. R.↗

NASA NTRS1985

Long life nickel electrodes for a nickel-hydrogen cell: Cycle life tests

In order to develop a long life nickel electrode for a Ni/H2 cell, the cycle life of nickel electrodes was tested in Ni/H2 boiler plate cells. A 19 test cell matrix was made of various nickel electrode designs including three levels each of plaque mechanical strength, median pore size of the plaque, and active material loading. Test cells were cycled to the end of their life (0.5v) in a 45 minute low Earth orbit cycle regime at 80% depth-of-discharge. It is shown that the active material loading level affects the cycle life the most with the optimum loading at 1.6 g/cc void. Mechanical strength does not affect the cycle life noticeably in the bend strength range of 400 to 700 psi. It is found that the best plaque is made of INCO nickel powder type 287 and has median pore size of 13 micron.

Lim, H. S.↗

NASA NTRS1985

Amorphous and porous ices in cometary nuclei

The heat flux into the cometary nucleus before and after perihelion and its influence on the nucleus are discussed. The surface temperature and the associated rate of evaporation are lower than usually calculated, and the effect is strongly dependent on the porosity of the nucleus. The surface temperature reaches a maximum after perihelion, in agreement with statistical studies of comae and tails. Delayed heating of the CO2-rich inclusions deep in the nucleus leads to outbursts and splittings as far as 9 AU after perihelion, also in agreement with observation. At sufficiently high temperatures, heat transport in porous cometary nuclei is controlled by vapor diffusion. The presence of CO2 vapor in the pores significantly increases the heat flow in mixed nuclei at temperatures above 140-150 C. Closed pores move up the radial thermal gradient so that the outside layer of a cometary nucleus should become denser.

Smoluchowski, R.↗

NASA NTRS1985

Design principles for nickel-hydrogen cells and batteries

Nickel-hydrogen cells and, more recently, bipolar batteries have been built by a variety of organizations. The design principles that have been used by the technology group at the NASA Lewis Research Center draw upon their extensive background in separator technology, alkaline fuel cell technology, and several alkaline cell technology areas. These design principles have been incorporated into both the more contemporary individual pressure vessel (IPV) designs that were pioneered by other groups, as well as the more recent bipolar battery designs using active cooling that are being developed at NASA Lewis Research Center and under contract. These principles are rather straightforward applications of capillary force formalisms, coupled with the slowly developing data base resulting from careful post test analyses. The objective of this overall effort is directed towards the low-earth-orbit (LEO) application where the cycle life requirements are much more severe than the geosynchronous-orbit (GEO) application. A summary of the design principles employed is presented along with a discussion of the recommendations for component pore sizes and pore size distributions, as well as suggested materials of construction. These will be made based on our experience in these areas to show how these design principles have been translated into operating hardware.

Thaller, L. H.↗

NASA NTRS1986

Filament eruption connected to protospheric activity

Two cases of activation of filaments that occured in regions of intense magnetic activity was studied. The simultaneous observations from Debrecen Observatory (white light and H alpha filtergram), and from Meudon Observatory (magnetogram, MSDP dopplergram and intensity maps in H alpha) gave a complementary set of data from which can be produced evidence of the influence of the photospheric magnetic field on the destabilization process of the filaments. On June 22, 1980, the eruption of the filament is associated with the motion of pores, which are manifestations of emerging flux knots. On September 3, 1980, the twisting motions in the filament are associated to the birth of a pore in its neighborhood. These observations are discussed.

Simon, G.↗

NASA NTRS1987

Fatigue damage interaction behavior of PWA 1480

The fatigue damage interaction behavior of PWA 1480 single crystal alloy has been experimentally established for the two-level loading case in which a block of low-cycle fatigue loading is followed by high-cycle fatigue loading to failure. A relative life ratio N1/N2 (where N1 and N2 are the low- and high-cycle fatigue baseline lives, respectively) of approximately 0.002 was explored to assess the interaction behavior. The experimental results thus far show evidence of a loading order interaction effect to a similar degree of detriment as has been observed in polycrystalline materials. Current generation single crystal alloys in general, and PWA 1480 in particular, contain pores; indeed, it was observed in all cases that specimen failure initiated from pores connected with or immediately below the surface. Detailed fractographic and metallographic studies are currently being made to assess the nature of the porosity in terms of its effect on fatigue life.

Mcgaw, Michael A.↗

NASA NTRS1987

AC impedance analysis of polypyrrole thin films

The AC impedance spectra of thin polypyrrole films were obtained at open circuit potentials from -0.4 to 0.4 V vs SCE. Two limiting cases are discussed for which simplified equivalent circuits are applicable. At very positive potentials, the predominantly nonfaradaic AC impedance of polypyrrole is very similar to that observed previously for finite porous metallic films. Modeling of the data with the appropriate equivalent circuit permits effective pore diameter and pore number densities of the oxidized film to be estimated. At potentials from -0.4 to -0.3 V, the polypyrrole film is essentially nonelectronically conductive and diffusion of polymer oxidized sites with their associated counterions can be assumed to be linear from the film/substrate electrode interface. The equivalent circuit for the polypyrrole film at these potentials is that previously described for metal oxide, lithium intercalation thin films. Using this model, counterion diffusion coefficients are determined for both semi-infinite and finite diffusion domains. In addition, the limiting low frequency resistance and capacitance of the polypyrrole thin fims was determined and compared to that obtained previously for thicker films of the polymer. The origin of the observed potential dependence of these low frequency circuit components is discussed.

Penner, Reginald M.↗

NASA NTRS1986

Improved consolidation of silicon carbide

Freedman, M. R.↗

NASA NTRS1987

Nondestructive evaluation of structural ceramics by photoacoustic microscopy

A photoacoustic microscopy (PAM) digital imaging system was developed and utilized to characterize silicon nitride material at the various stages of the ceramic fabrication process. Correlation studies revealed that photoacoustic microscopy detected failure initiating defects in substantially more specimens than microradiography and ultrasonic techniques. Photoacoustic microscopy detected 10 to 100 micron size surface and subsurface pores and inclusions, respectively, up to 80 microns below the interrogating surface in machined sintered silicon nitride. Microradiography detected 50 micron diameter fracture controlling pores and inclusions. Subsurface holes were detected up to a depth of 570 microns and 1.00 mm in sintered silicon nitride and silicon carbide, respectively. Seeded voids of 20 to 30 micron diameters at the surface and 50 microns below the interrogating surface were detected by photoacoustic microscopy and microradiography with 1 percent X-ray thickness sensitivity. Tight surface cracks of 96 micron length x 48 micron depth were detected by photoacoustic microscopy. PAM volatilized and removed material in the green state which resulted in linear shallow microcracks after sintering. This significantly limits the use of PAM as an in-process NDE technique.

Khandelwal, Pramod K.↗

NASA NTRS1987

Channel size influence on the heat flux density at zero net mass flow in the non-linear transport regime between 1.2 and 2.1 K

Porous media and narrow ducts of simple shape at zero net mass flow (ZNMF) are used to investigate the influence of pore size on the entropy/heat convection rate at ZNMF. The study is relevant to the development of specific types of phase separators. Previous work on heat transport by convection is extended to porous media without mass loss. The experimental results show the influence of pore size on heat flux for permeabilities between 10 to the -8th and 10 to the -6th sq cm. ZNMF plug data are found to be similar to results obtained for vapor liquid phase separation.

Frederking, T. H. K.↗

NASA NTRS1989

Acoustical Imaging Of Defects In Ceramics

Because of their brittle nature, ceramics are extremely sensitive to even slight variations in microstructure. Direct observations show importance of kinds of defects by identifying them as sites of localized failures. Sites cannot be determined priori by optical and x-ray methods. Color images obtained from precise acoustic measurements reveal subtle variations in porosity fractions and mean sizes of pores in ceramics. Velocity and attenuation of ultrasound found related directly to density and mean size of pores, respectively.

Generazio, Edward R.↗

NASA NTRS1989

Glass Coats For Hot Isostatic Pressing

Surface voids sealed from pressurizing gas. Coating technique enables healing of surface defects by hot isostatic pressing (HIP). Internal pores readily closed by HIP, but surface voids like cracks and pores in contact with pressurizing gas not healed. Applied to casting or weldment as thick slurry of two glass powders: one melts at temperature slightly lower than used for HIP, and another melts at higher temperature. For example, powder is glass of 75 percent SiO2 and 25 percent Na2O, while other powder SiO2. Liquid component of slurry fugitive organic binder; for example, mixture of cellulose acetate and acetone. Easy to apply, separates voids from surrounding gas, would not react with metal part under treatment, and easy to remove after pressing.

Ecer, Gunes M.↗

NASA NTRS1989

Reactive iron in marine sediments

The influence of reactive iron oxides on sediment pore-water chemistry is considered in detail. A carefully calibrated extraction scheme is used to determine the depth distributions of reactive iron phases at two very different localities: the relatively iron-rich Mississippi Delta and the relatively iron-poor FOAM site in Long Island Sound. Closed system incubations are used to characterize the rates of reaction between sulfide and both naturally occurring and pure iron mineral phases. Rates of iron liberation to pore solution are measured in the presence and absence of sulfate reduction, and the origin of dissolved iron in organic-rich sediments is speculated upon.

Canfield, Donald E.↗

NASA NTRS1989

Laboratory simulation of meteoritic noble gases. III - Sorption of neon, argon, krypton, and xenon on carbon - Elemental fractionation

The sorption of Ne, Ar, Kr, and Xe was studied in carbon black, acridine carbon, and diamond in an attempt to understand the origin of trapped noble gases in meteorites. The results support a model in which gases are physically adsorbed on interior surfaces formed by a pore labyrinth within amorphous carbons. The data show that: (1) the adsorption/desorption times are controlled by choke points that restrict the movement of noble gas atoms within the pore labyrinth, and (2) the physical adsorption controls the temperature behavior and elemental fractionation patterns.

Wacker, John F.↗

NASA NTRS1989

Inhomogeneous diffusion-limited aggregation

It is demonstrated here that inhomogeneous diffusion-limited aggregation (DLA) model can be used to simulate viscous fingering in a medium with inhomogeneous permeability and homogeneous porosity. The medium consists of a pipe-pore square-lattice network in which all pores have equal volume and the pipes have negligible volume. It is shown that fluctuations in a DLA-based growth process may be tuned by noise reduction, and that fluctuations in the velocity of the moving interface are multiplicative in form.

Selinger, Robin Blumberg↗

NASA NTRS1990

Filling Porous Microspheres With Magnetic Material

New process produces magnetic microspheres with controllable sizes, compositions, and properties for use in medical diagnostic tests, biological research, and chemical processes. Paramagnetic microspheres also made with process. Porous plastic microspheres prepared by polymerization of monomer in diluent by cross-linking agent. When diluent removed, it leaves tiny pores throughout polymerized spheres. Size and distribution of pores determined by amount and type of diluent and cross-linking agent.

Chang, Manchium↗

NASA NTRS1989

Thermal histories of the samples of two KOSI comet nucleus simulation experiments

Temperatures recorded during two KOSI comet nucleus simulation experiments strongly suggest that heat transport by vapor flow into the interior of the sample is very important. Two comet nucleus simulation experiments have been done by the KOSI team in a big space simulator. The thermal evolution of the sample during insolation and the results of simplified thermal evolution calculations are discussed. The observed thermal histories cannot be explained by a simple model with heat transferred by heat conduction at a constant conductivity, so a coupled heat and mass transfer problem was considered. The porous ice matrix was assumed to have a constant thermal conductivity and to be in thermal equilibrium with vapor in the pores, the internal pressure being the vapor pressure. The vapor was modelled as an ideal gas because, at the temperatures relevant to the problem, the mean free path length of the vapor molecules is large in comparison with the pore dimensions. The heat capacity at constant volume per unit mass of the two phase mixture was also assumed constant. The vapor was allowed to flow and transfer heat in response to an internal pressure gradient.

Spohn, T.↗