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At least 235 records · Page 13

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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

Electrode erosion in arc discharges at atmospheric pressure

An experimental investigation was performed in an effort to measure and increase lifetime of electrodes in an arcjet thruster. The electrode erosion of various anode and cathode materials was measured after tests in an atmospheric pressure nitrogen arc discharge at powers less than 1 kW. A free-burning arc configuration and a constricted arc configuration were used to test the materials. Lanthanum hexboride and thoriated tungsten had low cathode erosion rates while thoriated tungsten and pure tungsten had the lowest anode erosion rates of the materials tested. Anode cooling, reverse gas flow, an external magnetic fields were all found to reduce electrode mass loss.

Hardy, T. L.↗

NASA NTRS1985

TWT efficiency enhancement with textured carbon surfaces on copper MDC electrodes

A method of improving the efficiency of multistage depressed collectors (MDC's) for traveling-wave tubes (TWT's) has been demonstrated at the NASA Lewis Research Center. A significant increase in MDC efficiency was brought about by the application of a thin layer of highly-textured carbon to the surfaces of oxygen-free, high-conductivity (OFHC) copper collector electrodes in an experimental TWT. The textured carbon layer was applied by means of a NASA developed sputtering procedure. In an experimental investigation recently completed, this electrode surface modification resulted in an increase in MDC efficiency by as much as 8.6 percentage points relative to that of the same MDC with untreated copper electrode surfaces. This increase in MDC efficiency was reflected by an increase in overall TWT efficiency by as much as 5.4 percentage points.

Curren, A. N.↗

NASA NTRS1987

Negative-Electrode Catalysts for Fe/Cr Redox Cells

Electrodes perform more consistently and less expensive. Surfaces catalyzed by bismuth and bismuth/lead developed for application on chromium electrode in iron/chromium redox electrochemical energy storage system. NASA Fe/Cr storage system incorporates two soluble electrodes consisting of acidified solutions of iron chloride (FeC13 and FeC12) and chromium chloride (CrC13 and CrC12) oxidized and reduced in power-conversion unit to store and produce electricity. Electrolytes circulated with pumps and stored in external tanks.

Gahn, R. F.↗

NASA NTRS1987

Polyphase alloys as rechargeable electrodes in advanced battery systems

The rechargeability of electrochemical cells is often limited by negative electrode problems. These may include loss of capacity, increased impedance, macroscopic shape change, dendrite growth, or a tendency for filamentary or whisker growth. In principle, these problems can be reduced or eliminated by the use of alloys that undergo either displacement or insertion reactions at reactant species activities less than unity, rather than pure elements. The fundamental reasons for some of these problems with elemental electrodes, as well as the basic principles involved in the different behavior of alloys, are briefly discussed. More information is now available concerning the thermodynamic and kinetic properties of a number of alloys of potential interest for use as electrodes in elevated temperature lithium battery systems. Recent results have extended these results down to ambient temperatures, indicating that some such materials may be of interest for use with new low temperature molten salt electrolytes, or with organic solvent electrolytes. The all solid mixed conductor matrix concept is also reviewed.

Huggins, Robert A.↗

NASA NTRS1987

Electrochemical impregnation of nickel hydroxide in porous electrodes

The electrochemical impregnation of nickel hydroxide in porous electrode was investigated both experimentally and theoretically. The loading level and plaque expansion were the most important parameters to be considered. The effects of applied current density, stirring, ratio of solution to electrode volume and pH were identified. A novel flow through electrochemical impregnation is proposed in which the electrolyte is forced through the porous nickel plaque. The thickening of the plaque can be reduced while maintaining high loading capacity. A mathematical model is presented which describes the transport of the nitrate, nickel and hydroxyl ions and the consecutive heterogeneous electrochemical reduction of nitrate and the homogeneous precipitation reaction of nickel hydroxide. The distributions of precipitation rate and active material within the porous electrode are obtained. A semiempirical model is also proposed which takes into account the plugging of the pores.

Ho, Kuo-Chuan↗

NASA NTRS1986

Improvements in MDC and TWT overall efficiency through the application of carbon electrode surfaces

The effects of secondary electron emission losses on TWT efficiency are investigated and techniques for minimizing these losses are described. The TWT-multistage depressed collector performance was optimized and measured over a wide range of operating conditions using geometrically identical collectors that utilized copper, pyrolytic graphite, and isotropic graphite electrodes. The data reveal that carbon rather than copper electrodes improve the TWT efficiency, and the ion-textured graphite is most effective in minimizing the secondary electron emission losses. It is noted that degradation of the collector efficiency can be limited to a small percentage with the proper MDC design and the use of low secondary electron yield carbon electrode surfaces.

Ramins, Peter↗

NASA NTRS1986

Voltammetric studies of porous molybdenum electrodes for the alkali metal thermoelectric converter

Voltammetry of partially oxidized porous molybdenum alkali metal thermoelectric converter (AMTEC) electrodes from about 600 to 1000 K revealed a series of redox processes within the AMTEC operational voltage range which can be used to establish the electronic and ionic conductivities of these electrodes. Improved estimates of the free energies of formation of Na2Mo3O6, NaMoO2, and Na3MoO4 are obtained. Evidence is provided for the slow corrosive attack by Na2MoO4 on molybdenum. The ionic conductivity of Na2MoO4 is found to be sufficiently large at temperatures of greater than 700 K to explain the observed electrochemical phenomena in addition to the enhanced sodium transport in AMTEC electrodes below the freezing point of Na2MoO4.

Williams, R. M.↗

NASA NTRS1987

TWT efficiency improvement by a low-cost technique for deposition of carbon on MDC electrodes

A simple method of improving the TWT and multistage depressed collector (MDC) efficiency has been demonstrated. The efficiency improvement was produced by the application of a thin layer of carbon to the copper electrodes of the MDC by means of a rapid low-cost technique involving the pyrolysis of hydrocarbon oil in electric arc discharges. Experimental results with a representative TWT and MDC showed an 11 percent improvement in both the TWT and MDC efficiencies as compared to those of the same TWT and MDC with machined copper electrode surfaces. An extended test with a 550-W CW TWT indicated good durability of the carbon-coated electrode surfaces.

Ebihara, Ben T.↗

NASA NTRS1987

AMTEC electrode development

The Alkali Metal Thermoelectric Converter (AMTEC) is a direct energy conversion device, utilizing a high sodium vapor pressure or activity ratio across a beta-alumina solid electrolyte. Progress is reported on a long life, high power, porous electrode. Two electrode compositions were identified which have the potential for long life operation at power densities above 0.5 W/sq cm. Longer lifetime testing is being initiated. Successful optimization and demonstration of very long lifetimes for these electrodes will be a major step toward establishing the feasibility of AMTEC space power systems.

Bankston, C. P.↗

NASA NTRS1988

Analysis of a used pair of arcjet electrodes

The anode and cathode of a 30-kW class arcjet engine have been sectioned and analyzed. This arcjet was operated for a total time of 573 hours at power levels between 25 and 30 kW with ammonia at flow rates of 0.25 and 0.27 gm/s. The accumulated run time was sufficient to clearly establish erosion patterns and their causes. The type of electron emission from various parts of the cathode surface was made clear by Scanning Electron Microscope analysis. A Scanning Electron Microscope was used to study recrystallization on the hot anode surface. These electrodes were made of 2 percent thoriated tungsten and the surface thorium content and gradient perpendicular to the surfaces was determined by quantitative microprobe analysis. The results of this material analysis on the electrodes and recommendations for improving electrode operational life time are presented.

Pivirotto, Thomas J.↗

NASA NTRS1989

Single Electrode Would Control Charge-Coupled Device

Space saved and interelectrode short circuits eliminated. Proposed virtual-phase, interline-transfer, charge-couple device uses single gate electrode to control both interline and intraline transfer of charge. Voltage on single control electrode varied to transfer charge from photosites to shift register, or else along sift register. Two kinds of transfer independent of each other. Charge transferred along shift register when voltage on control electrode alternated between gate potentials.

Wadsworth, Mark↗

NASA NTRS1988

Electrode erosion in steady-state electric propulsion engines

The anode and cathode of a 30 kW class arcjet engine were sectioned and analyzed. This arcjet was operated for a total time of 573 hr at power levels between 25 and 30 kW with ammonia at flow rates of 0.25 and 0.27 gm/s. The accumulated run time was sufficient to clearly establish erosion patterns and their causes. The type of electron emission from various parts of the cathode surface was made clear by scanning electron microscope analysis. A scanning electron microscope was used to study recrystallization on the hot anode surface. These electrodes were made of 2 percent thoriated tungsten and the surface thorium content and gradient perpendicular to the surfaces was determined by quantitative microprobe analysis. The results of this material analysis on the electrodes and recommendations for improving electrode operational life time are presented.

Pivirotto, Thomas J.↗

NASA NTRS1988

Experimental observation of electrochemical rate limitations affecting sodium ion-electron recombination at electrodes of the alkali metal thermoelectric converter at T about 1200 K

This paper considers a model of the internal impedances of thin porous Mo and W alkali metal thermoelectric converter (AMTEC), in which the kinetic parameters associated with the reaction of the beta-double-prime alumina solid electrolite (BASE)/porous metal/gas three-phase boundary can be evaluated. Impedance data in the frequency range 0.01-100,000 Hz were collected over a range of AMTEC cell operating voltages for small-area thin porous Mo and W electrodes, yielding apparent charge transfer resistances at a series of cell potentials/currents. The ohmic resistance in the AMTEC cell could be broken down and characterized with three parameters: the BASE ionic resistance, the electrode film sheet resistance, and the contact/lead resistance, all of which could be calculated or measured independently and used to calculate power curves in good agreement with observed power curves. It is shown that these calculations can be used to predict the properties of electrodes with optimized parameters or to detect enhanced transport modes.

Williams, R. M.↗

NASA NTRS1988

Progress in AMTEC electrode experiments and modeling

Four electrode compositions have demonstrated power densities high enough to make practical alkali metal thermoelectric converter (AMTEC) systems feasible. These electrode compositions are very thin molybdenum, platinum/tungsten trilayer, rhodium/tungsten bilayer, and titanium nitride. These electrodes have produced stable maximum power densities of 0.4 to 0.8 W/sq cm at 1100-1250 K for 90 to 160 hours.

Underwood, M. L.↗

NASA NTRS1990

Implantable Electrode For Critical Locations

Implantable electrode holds itself in place until body tissue grows around it and retains it. Sheath covers electrode during implantation. Partially retracted after implantation and completely retracted after growth of tissue onto electrode. Intended to provide electrical stimuli to muscles having become inactive through accident or disease. Helps paraplegics and others to exercise voluntary control over arms and legs.

Collins, Earl R., Jr.↗

NASA NTRS1990

Analysis Of Used Arc-Jet Electrodes

Report discusses conditions of electrodes used in arc-jet engine. Electrodes examined in effort to determine causes of erosion and to develop recommendations for improved electrode designs yielding longer operating lives.

Pivirotto, Thomas J.↗

NASA NTRS1990

Heat- And Oxidation-Resistant Electrodes

Alloys coated with electrically conductive ceramics used to make strong, oxidation-resistant electrodes for electrochemical cells operating at temperatures of 1,000 to 1,300 degrees C. Fe3Al or Ni3Al coated with strontium-doped lanthanum manganite more resistant to chemical attack than all-metal electrode, less brittle than all-ceramic electrode, and less costly than either alternative.

Schroeder, James E.↗