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At least 343 records · Page 19

Strain-biased PLZT input devices /page composers/ for holographic memories and optical data processing.

The features of a new class of transparent ferroelectric ceramic materials, called PLZT, are described. PLZT are shown to have very strong electrooptic properties and memory, and are switched through transparent electrodes when mechanically strained. Because of these properties, they can be used to fabricate two-dimensional page composers for coherent optical systems. Techniques for fabricating such devices are outlined along with preliminary test results. Applications in displays are shown to be also feasible.

Roberts, H. N.↗

Thin film fuel cell electrodes.

Earlier work shows that fuel cell electrodes prepared by sputtering thin films of platinum on porous vycor substrates avoid diffusion limitations even at high current densities. The presented study shows that the specific activity of sputtered platinum is not unusually high. Performance limitations are found to be controlled by physical processes, even at low loadings. Catalyst activity is strongly influenced by platinum sputtering parameters, which seemingly change the surface area of the catalyst layer. The use of porous nickel as a substrate shows that pore size of the substrate is an important parameter. It is noted that electrode performance increases with increasing loading for catalyst layers up to two microns thick, thus showing the physical properties of the sputtered layer to be different from platinum foil. Electrode performance is also sensitive to changing differential pressure across the electrode. The application of sputtered catalyst layers to fuel cell matrices for the purpose of obtaining thin total cells appears feasible.

Asher, W. J.↗

Sodium-metal chloride battery research at JPL

Sodium - metal chloride batteries have certain distinct advantages over sodium - sulfur batteries such as increased safety, inherent overcharge capability and lower operating temperatures. Two systems, i.e., Na/FeCl2 and Na/NiCl2 were developed extensively elsewhere and evaluated for various applications including electric vehicles and space. Their performance has been very encouraging and prompted a detailed fundamental study on these cathodes here at the Jet Propulsion Laboratory. A brief review of our studies on these new cathode materials is presented here. The initial efforts focused on the methods of fabrication of the electrodes and their electrochemical characterization." Subsequent studies were aimed at establishing the reaction mechanism, determining the kinetics and identifying the rate-limiting processes in the reduction of metal chloride cathodes. Nickel chloride emerged from these studies as the most promising candidate material and was taken up for further detailed study on its passivation - a rate limiting process - under different experimental conditions. Also, the feasibility of using copper chloride, which is expected to have higher energy density, has been assessed. Based on the criteria established from the voltametric response of FeCl2, NiCl2 and CuCI2, several other transition metal chlorides were screened. Of these, molybdenum and cobalt chlorides appear promising.

B V Ratnakumar↗

Hybrid air revitalization system for a closed ecosystem

An air-revitalization concept is presented with experimental results to assess the practicality and applicability of the proposed system to extended-duration manned missions. The Hybrid Air Revitalization System (HARS) uses plants in a habitat to remove metabolic CO2 and moisture and produce oxygen and food. CO2 and O2 partial pressures, temperature, and humidity are regulated by means of electrochemical CO2 and O2 chemical separators and a moisture condenser separator. A cell-test facility is described in which the electrochemical CO2 removal processes are investigated with and without using H2. Performance is optimized by using 25-30 percent Teflon in the gas-diffusion-type electrode, employing a thin electrolyte matrix, operating at higher temperatures and lower dew points. The HARS concept is found to be a feasible approach to the electrochemical separation of CO2 and O2.

Lee, M. G.↗

Sol-gel Technology and Advanced Electrochemical Energy Storage Materials

Advanced materials play an important role in the development of electrochemical energy devices such as batteries, fuel cells, and electrochemical capacitors. The sol-gel process is a versatile solution for use in the fabrication of ceramic materials with tailored stoichiometry, microstructure, and properties. This processing technique is particularly useful in producing porous materials with high surface area and low density, two of the most desirable characteristics for electrode materials. In addition,the porous surface of gels can be modified chemically to create tailored surface properties, and inorganic/organic micro-composites can be prepared for improved material performance device fabrication. Applications of several sol-gel derived electrode materials in different energy storage devices are illustrated in this paper. V2O5 gels are shown to be a promising cathode material for solid state lithium batteries. Carbon aerogels, amorphous RuO2 gels and sol-gel derived hafnium compounds have been studied as electrode materials for high energy density and high power density electrochemical capacitors.

Chung-tse Chu↗

A Carbon Arc Apparatus For Production Of Nanotubes In Microgravity

Although many methods are available for production of single-walled carbon nanotubes (SWNTs), the conventional carbon arc process remains the most popular due to its simplicity and large production rate. However, high temperatures inside the carbon arc generate strong buoyancy driven convection, and it is hypothesized that the non-uniform environment created by this flow will have large effects on the growth and morphology of SWNTs produced by the arc process. Indeed, using normal gravity experiments, Marin et al. have demonstrated that changes in the buoyant convection plume produced by altering the arc electrode orientation can be used to change the diameter distribution of the SWNTs produced; an effect they attribute to changes in the temperature of the local nanotube growth environment. While these experiments present convincing evidence that buoyant convection has a strong effect on nanotube growth, normal gravity experiments are severely limited in scope. The ideal way to study the effect of buoyancy on SWNT production is to remove it completely. Toward this goal, a microgravity carbon arc reactor has been designed for use in the NASA Glenn 2.2 and 5 second drop towers. Although simple in principle, conventional carbon arc machines, which generally employ large reaction chambers and require heavy duty welding power supplies capable of supplying kilowatts of power, are not suitable for microgravity experiments. Here we describe a miniature carbon arc machine for SWNT production that fits into a conventional drop rig for use on the NASA Glenn 2.2 and 5 second drop towers, but that has a performance (production rate) that is better than most large ground-based machines.

Alford, J. M.↗

A Review of Sodium-Metal Chloride Battery Activity At JPL

Following the disclosures by Coetzer et al. on the use of transition metal chlorides in chloroaluminates as alternate cathodes to sulfur in rechargeable sodium batteries, several laboratories, including the Jet Propulsion Laboratory, focused their attention on these systems. These systems have certain distinct advantages over sodium-sulfur batteries such as increased safety, inherent overcharge capability, and lower operating temperatures. Two systems, i.e., Na/FeCl2 and NaNiCl2, were developed extensively and evaluated in various applications including electric vehicles and space. Their performance has been very encouraging and warrants a detailed fundamental study on these cathodes. At the Jet Propulsion Laboratory a program was initiated two years back to understand the electrochemical behavior of FeCl2 and NiCl2, and to identify and evaluate other transition metal chlorides of promise. The initial efforts focused on the methods of fabrication of the electrodes and their electrochemical characterization. Subsequent studies were aimed at establishing the reaction mechanism, determining the kinetics, and identifying the rate-limiting processes in te reduction of metal chloride cathodes. Nickel chloride emerged form these studies as the most promising candidate material and was taken up for further detailed study on its passivation- a rate limiting process-under different experimental conditions. Also, the feasibility of using copper chloride, which is expected to have a higher energy density, has been assessed. On the basis of the criteria established from the voltammetric response of FeCl2, NiCl2, and CuCl2, several other transition metal chlorides were screened. Of these, molybdenum and cobalt appear promising.

Ratnakumar, B. V.↗

Making AlN(x) Tunnel Barriers Using a Low-Energy Nitrogen-Ion Beam

A technique based on accelerating positive nitrogen ions onto an aluminum layer has been demonstrated to be effective in forming thin (<2 nm thick) layers of aluminum nitride (AlN(x)) for use as tunnel barriers in Nb/Al-AlN(x)/Nb superconductor/insulator/ superconductor (SIS) Josephson junctions. AlN(x) is the present material of choice for tunnel barriers because, to a degree greater than that of any other suitable material, it offers the required combination of low leakage current at high current density and greater thermal stability. While ultra-thin AlN films with good thickness and stoichiometry control are easily formed using techniques such as reactive molecular beam epitaxy and chemical vapor deposition, growth temperatures of 900 C are necessary for the dissociative adsorption of nitrogen from either nitrogen (N2) or ammonia (NH3). These growth temperatures are prohibitively high for the formation of tunnel barriers on Nb films because interfacial reactions at temperatures as low as 200 to 300 C degrade device properties. Heretofore, deposition by reactive sputtering and nitridation of thin Al layers with DC and RF nitrogen plasmas have been successfully used to form AlN barriers in SIS junctions. However, precise control over critical current density Jc has proven to be a challenge, as is attaining adequate process reproducibility from system to system. The present ion-beam technique is an alternative to the plasma or reactive sputtering techniques as it provides a highly controlled arrival of reactive species, independent of the electrical conditions of the substrate or vacuum chamber. Independent and accurate control of parameters such as ion energy, flux, species, and direction promises more precise control of film characteristics such as stoichiometry and thickness than is the case with typical plasma processes. In particular, the background pressure during ion-beam nitride growth is 2 or 3 orders of magnitude lower, minimizing the formation of compounds with contaminants, which is critical in devices the performance of which is dictated by interfacial characteristics. In addition, the flux of incoming species can be measured in situ using ion probes so that the dose can be controlled accurately. The apparatus used in the present ion-beam technique includes a vacuum chamber containing a commercial collimated- ion-beam source, a supply of nitrogen and argon, and an ion probe for measuring the ion dose. Either argon or nitrogen can be used as the feed gases for the ion source, depending on whether cleaning of the substrate or growth of the nitride, respectively, is desired. Once the Nb base electrode and Al proximity layer have been deposited, the N2 gas line to the ion beam is vented and purged, and the ion-source is turned on until a stable discharge is obtained. The substrate is moved over the ion-beam source to expose the Al surface layer to the ion beam (see figure) for a specified duration for the formation of the nitride tunnel barrier. Next, the Nb counter-electrode layer is deposited on the nitride surface layer. The Nb/Al- AlN(x)/Nb-trilayer-covered substrate is then patterned into individual devices by use of conventional integrated-circuit processing techniques.

Kaul, Anupama↗

Electrodynamic Dust Shield (EDS) Preparation for MISSE-11 Launch

The dusty surfaces of the Moon, Mars, and various asteroids present a significant challenge to NASA’s manned and unmanned space exploration efforts. The fine, electrostatically charged dust is difficult to remove and has resulted in vision obscuration, false instrument readings, contamination, elevated temperatures, performance reduction, and equipment failure. To alleviate these problems, NASA KSC’s Electrostatics and Surface Physics Laboratory (ESPL) is developing active dust mitigation systems for solar system exploration and in situ resource utilization (ISRU). Part of KSC’s Swamp Works system, the lab emphasizes fast-paced, hands-on, cost-effective, and collaborative innovation. The Electrodynamic Dust Shield (EDS) is an ESPL technology being developed to prevent dust accumulation on space components. It consists of a dielectric substrate embedded with parallel electrodes which, when applied with high voltage out-of-phase waveforms, produce a traveling electric field wave that scatters dust particles from its surface via dielectrophoretic and electrostatic forces. The EDS is currently fully functional and has been tested for scalability and endurance in reduced gravity/pressure flight. Next, it will be part of the May 2019 Materials International Space Station Experiment-11 (MISSE-11) payload, which will test small shield samples for performance durability with exposure to the harsh space environment. The summer objective was to test EDS functionality before active and passive dust shield samples launch on MISSE-11, in order to screen for good flight/control samples as well as to compare pre-flight and post-flight clearing efficiencies. I imaged glass and Kapton shields of 2-phase and 3-phase electrode configurations with the Keyence VHX-5000 digital microscope to check for damage before and after all testing for a qualitative pre-flight baseline; helped with vacuum testing of high-voltage shield breakdown; assembled shields and wires using Electrobond 004 silver epoxy; and helped with vacuum and cryogenic dust clearance testing of assembled EDS systems. I learned CAD (computer-aided design) and 3D printing skills to create parts for a shaker table/motor assembly, which will allow for repeatable experiments by evening out EDS dust distribution in a consistent manner prior to dust scattering testing. I will also be improving the interfacing with Alpha Space’s prototype Data Communications Unit (DCU), a payload component to control power to the EDS and store data, by mapping the DCU structure upon power-up and writing shell scripts to improve various processes. Finally, I also CADed and 3D-printed plastic supports for three different geometries of the Electrostatic Precipitator (ESP), a project that aims to use high voltage electrodes to charge and filter out dust from the Martian atmosphere drawn for ISRU devices. This work on dust mitigation technologies will help to support future robotic and human space exploration missions, including NASA’s Journey to Mars; the EDS has especially significant applications to spacecraft solar panels, thermal radiators, viewports, and astronaut visors. This opportunity has also allowed me to become more familiar with various lab equipment and hardware testing, develop key modeling skills in SolidWorks, explore past and present components critical to America’s space program, work with inspiring peers and mentors, and ascertain my dream of working permanently for NASA.

space↗

Manufacturing High-Quality Carbon Nanotubes at Lower Cost

A modified electric-arc welding process has been developed for manufacturing high-quality batches of carbon nanotubes at relatively low cost. Unlike in some other processes for making carbon nanotubes, metal catalysts are not used and, consequently, it is not necessary to perform extensive cleaning and purification. Also, unlike some other processes, this process is carried out at atmospheric pressure under a hood instead of in a closed, pressurized chamber; as a result, the present process can be implemented more easily. Although the present welding-based process includes an electric arc, it differs from a prior electric-arc nanotube-production process. The welding equipment used in this process includes an AC/DC welding power source with an integral helium-gas delivery system and circulating water for cooling an assembly that holds one of the welding electrodes (in this case, the anode). The cathode is a hollow carbon (optionally, graphite) rod having an outside diameter of 2 in. (approximately equal to 5.1 cm) and an inside diameter of 5/8 in. (approximately equal to 1.6 cm). The cathode is partly immersed in a water bath, such that it protrudes about 2 in. (about 5.1 cm) above the surface of the water. The bottom end of the cathode is held underwater by a clamp, to which is connected the grounding cable of the welding power source. The anode is a carbon rod 1/8 in. (approximately equal to 0.3 cm) in diameter. The assembly that holds the anode includes a thumbknob- driven mechanism for controlling the height of the anode. A small hood is placed over the anode to direct a flow of helium downward from the anode to the cathode during the welding process. A bell-shaped exhaust hood collects the helium and other gases from the process. During the process, as the anode is consumed, the height of the anode is adjusted to maintain an anode-to-cathode gap of 1 mm. The arc-welding process is continued until the upper end of the anode has been lowered to a specified height above the surface of the water bath. The process causes carbon nanotubes to form in the lowest 2.5 cm of the anode. It also causes a deposit reminiscent of a sandcastle to form on the cathode. The nanotube-containing material is harvested. The cathode and anode can then be cleaned (or the anode is replaced, if necessary) and the process repeated to produce more nanotubes. Tests have shown that the process results in approximately equal to 50-percent yield of carbon nanotubes (mostly of the single-wall type) of various sizes. Whereas the unit cost of purified single-wall carbon nanotubes produced by other process is about $1,000/g in the year 2000, it has been estimated that for the present process, the corresponding cost would be about $10/g.

Benavides, Jeanette M.↗

Stabilization of n-type semiconductors to photoanodic dissolution by competitive electron transfer processes

The behavior of n-type CdX (X = S, Se, Te) and GaP, GaAs, and InP in alkaline electrolytes containing X(--) and X sub n(--) ions is reviewed. Of the 18 combinations of electrode and electrolyte, 12 alliances are completely stable to photoanodic dissolution of the n-type photoanode. In each case the oxidation of the chalcogenide species at the photoanode is reversed at the cathode to complete an electrochemical cycle involving no net chemical change. The best system in terms of light-to-electric energy conversion seems to be the CdTe-based cell employing the Te(--)/Te2(--) electrolyte, with roughly 10% efficiency at an output voltage of 0.35 V for monochromatic 633 nm input optical energy at about 25 mW/sq cm.

Wrighton, M. S.↗

A Thermal Runaway Failure Model for Low-Voltage BME Ceramic Capacitors with Defects

Reliability of base metal electrode (BME) multilayer ceramic capacitors (MLCCs) that until recently were used mostly in commercial applications, have been improved substantially by using new materials and processes. Currently, the inception of intrinsic wear-out failures in high quality capacitors became much greater than the mission duration in most high-reliability applications. However, in capacitors with defects degradation processes might accelerate substantially and cause infant mortality failures. In this work, a physical model that relates the presence of defects to reduction of breakdown voltages and decreasing times to failure has been suggested. The effect of the defect size has been analyzed using a thermal runaway model of failures. Adequacy of highly accelerated life testing (HALT) to predict reliability at normal operating conditions and limitations of voltage acceleration are considered. The applicability of the model to BME capacitors with cracks is discussed and validated experimentally.

reliability↗

Lithium-ion Battery Strain Gauge Monitoring and Depth of Discharge Estimation

Lithium-ion battery diagnostics and prognostics rely on measurements of electrical impedance, capacity, and voltage to infer the internal state of the battery. Mechanical changes to the cell structure represent an additional measure of the battery’s state because these changes are related to the overall battery health. As lithium-ion batteries are charged and discharged, lithium ions are inserted or removed from the anode and cathode, a process called intercalation and de-intercalation. As lithium ions intercalate and de-intercalate, they can cause changes to the lattice of the electrode particles, resulting in volumetric changes. These volumetric changes cause mechanical stresses and strains on the lithium-ion battery electrodes, and subsequently, the whole cell’s thickness varies as it is charged and discharged. This paper presents a study on the use of surface-mounted strain gauges for in-situ measurement of structural changes to lithium-ion batteries, along with a characterization of the unit-to-unit differences in strain response. A neural network modeling structure is then used to predict the battery’s depth of discharge under dynamic discharge conditions.

Lithium-ion batteries, depth of discharge, strain ↗

Modified Penning-Malmberg Trap for Storing Antiprotons

A modified Penning-Malmberg trap that could store a small cloud of antiprotons for a relatively long time (weeks) has been developed. This trap is intended for use in research on the feasibility of contemplated future matter/antimatter-annihilation systems as propulsion sources for spacecraft on long missions. This trap is also of interest in its own right as a means of storing and manipulating antiprotons for terrestrial scientific experimentation. The use of Penning-Malmberg traps to store antiprotons is not new. What is new here is the modified trap design, which utilizes state-of-the-art radiofrequency (RF) techniques, including ones that, heretofore, have been used in radio-communication applications but not in iontrap applications. A basic Penning-Malmberg trap includes an evacuated round tube that contains or is surrounded by three or more collinear tube electrodes. A steady axial magnetic field that reaches a maximum at the geometric center of the tube is applied by an external source, and DC bias voltages that give rise to an electrostatic potential that reaches a minimum at the center are applied to the electrodes. The combination of electric and magnetic fields confines the charged particles (ions or electrons) for which it was designed to a prolate spheroidal central region. However, geometric misalignments and the diffusive cooling process prevent the steady fields of a basic Penning- Malmberg trap from confining the particles indefinitely. In the modified Penning-Malmberg trap, the loss of antiprotons is reduced or eliminated by use of a "rotating-wall" RF stabilization scheme that also heats the antiproton cloud to minimize loss by matter/antimatter annihilation. The scheme involves the superposition of a quadrupole electric field that rotates about the cylindrical axis at a suitably chosen radio frequency. The modified Penning-Malmberg trap (see Figure 1) includes several collinear sets of electrodes inside a tubular vacuum chamber. Each set comprises either a single metal tube or else a tube that is segmented into four electrodes that subtend equal angles about the cylindrical axis. The output of an RF signal generator is fed through a 90 hybrid coupler and then through two baluns to generate four replicas of the signal at relative phase shifts of 0 , 90 , 180 , and 270 (see Figure 2). These signal replicas are fed through 6-dB directional couplers, then via coaxial cables to the vacuum chamber. The signal is then routed to a phase cancellation network, which filters out the drive signal with the difference representing the plasma interaction. Inside the vacuum chamber, twisted-pair wires feed the signals from the coaxial cables to the four electrodes of each segmented electrode tube.

Sims, William H.↗

Screenable silver and base metal solar cell contacts

The metallurgical soundness of the all-metal screenable thick film electrode system is established for silver and copper electrodes. Silver fluoride was identified as a successful etchant material and is found most effective in the liquid phase (435-460 C). Best results were achieved with the eutectic alloys of dopants and semiconductors. The air-fired silver inks were strongly adherent, rugged, and solderable, whereas the hydrogen-fired silver inks had very poor adhesion. A two-step firing process was devised in which copper inks containing silver fluoride were activated in a nitrogen atmosphere, with sintering done at the same or higher temperatures in hydrogen. Good solar cells were made using the copper paste back contacts demonstrating that the electrodes are not the limiting factors in efficiency.

Ross, B.↗

Study of oxygen gas production phenomenon during stand and discharge in silver-zinc batteries

The effects of a number of cell process and performance variables upon the oxygen evolution rate of silver/silver oxide cathodes are studied to predict and measure the conditions which would result in the production of a minimum of oxygen. The following five tasks comprise the study: the design and fabrication of two pilot test cells to be used for electrode testing; the determination of the sensitivity and accuracy of the test cell; the determination of total volumes and rates of generation by cathodes of standard production procedures; the construction of a sequential test plan; and the construction of a series of positive formation cells in which formation process factors can be controlled.

Source record↗

Thermoelectronic laser energy conversion for power transmission in space

Long distance transmission of power in space by means of laser beams is an attractive concept because of the very narrow beam divergence. Such a system requires efficient means to both generate the laser beam and to convert the light energy in the beam into useful electric output at the receiver. A plasma-type device known as a Thermo-Electronic Laser Energy Converter (TELEC) has been studied as a method of converting a 10.6 micron CO2 laser beam into electric power. In the TELEC process, electromagnetic radiation is absorbed directly in the plasma electrons producing a high electron temperature. The energetic electrons diffuse out of the plasma striking two electrodes with different areas. Since more electrons are collected by the larger electrode there is a net transport of current, and an EMF is generated in the external circuit. The smaller electrode functions as an electron emitter to provide continuity of the current. Waste heat is rejected from the large electrode. A design for a TELEC system with an input 1 MW laser beam was developed as part of the study. The calculated performance of the system showed an overall efficiency of about 42%.

Britt, E. J.↗

Preparation of dielectric coating of variable dielectric constant by plasma polymerization

A plasma polymerization process for the deposition of a dielectric polymer coating on a substrate comprising disposing of the substrate in a closed reactor between two temperature controlled electrodes connected to a power supply is presented. A vacuum is maintained within the closed reactor, causing a monomer gas or gas mixture of a monomer and diluent to flow into the reactor, generating a plasma between the electrodes. The vacuum varies and controls the dielectric constant of the polymer coating being deposited by regulating the gas total and partial pressure, the electric field strength and frequency, and the current density.

Hudis, M.↗