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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 379 records · Page 21
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

The platinum microelectrode/Nafion interface - An electrochemical impedance spectroscopic analysis of oxygen reduction kinetics and Nafion characteristics

The objectives of this study were to use electrochemical impedance spectroscopy (EIS) to study the oxygen-reduction reaction under lower humidification conditions than previously studied. The EIS technique permits the discrimination of electrode kinetics of oxygen reduction, mass transport of O2 in the membrane, and the electrical characteristics of the membrane. Electrode-kinetic parameters for the oxygen-reduction reaction, corrosion current densities for Pt, and double-layer capacitances were calculated. The production of water due to electrochemical reduction of oxygen greatly influenced the EIS response and the electrode kinetics at the Pt/Nafion interface. From the finite-length Warburg behavior, a measure of the diffusion coefficient of oxygen in Nafion and diffusion-layer thickness was obtained. An analysis of the EIS data in the high-frequency domain yielded membrane and interfacial characteristics such as ionic conductivity of the membrane, membrane grain-boundary capacitance and resistance, and uncompensated resistance.

Parthasarathy, Arvind↗
NASA NTRS1992

A Schottky/2-DEG varactor diode for millimeter and submillimeter wave multiplier applications

A new Schottky diode is investigated for use as a multiplier element in the millimeter and submillimeter wavelength regions. The new diode is based on the Schottky contact at the edge of a 2-dimensional electron gas (2-DEG). As a negative voltage is applied to the Schottky contact, the depletion layer between the Schottky contact and the 2-DEG expands and the junction capacitance decreases, resulting in a nonlinear capacitance-voltage characteristic. In this paper, we outline the theory, design, fabrication, and evaluation of the new device. Recent results include devices having cutoff frequencies of 1 THz and above. Preliminary multiplier results are also presented.

Peatman, W. C. B.↗
NASA NTRS1992

A new fabrication technique for back-to-back varactor diodes

A new varactor diode process has been developed in which much of the processing is done from the back of an extremely thin semiconductor wafer laminated to a low-dielectric substrate. Back-to-back BNN diodes were fabricated with this technique; excellent DC and low-frequency capacitance measurements were obtained. Advantages of the new technique relative to other techniques include greatly reduced frontside wafer damage from exposure to process chemicals, improved capability to integrate devices (e.g. for antenna patterns, transmission lines, or wafer-scale grids), and higher line yield. BNN diodes fabricated with this technique exhibit approximately the expected capacitance-voltage characteristics while showing leakage currents under 10 mA at voltages three times that needed to deplete the varactor. This leakage is many orders of magnitude better than comparable Schottky diodes.

Smith, R. Peter↗
NASA NTRS1992

Planar doped barrier subharmonic mixers

The Planar Doped Barrier (PDB) diode is a device consisting of a p(+) doping spike between two intrinsic layers and n(+) ohmic contacts. This device has the advantages of controllable barrier height, diode capacitance and forward to reverse current ratio. A symmetrically designed PDB has an anti-symmetric current vs. voltage characteristic and is ideal for use as millimeter wave subharmonic mixers. We have fabricated such devices with barrier heights of 0.3, 0.5 and 0.7 volts from GaAs and InGaAs using a multijunction honeycomb structure with junction diameters between one and ten microns. Initial RF measurements are encouraging. The 0.7 volt barrier height 4 micron GaAs devices were tested as subharmonic mixers at 202 GHz with an IF frequency of 1 GHz and had 18 dB of conversion loss. The estimated mismatch loss was 7 dB and was due to higher diode capacitance. The LO frequency was 100.5 GHz and the pump power was 8 mW.

Lee, T. H.↗
NASA NTRS1992

Novel heterojunction varactors

An account is given of new varactor multiplier designs, for mm-wave and sub-mm-wave receiver systems, which have their bases in heterostructure layers and exhibit either stronger capacitance nonlinearity or a special symmetry of characteristics. While barrier-n-layer-n(+) C-V characteristics yield superior performance at low pump powers, the symmetric C-V characteristics of the single-barrier varactor devices yield efficient low-order harmonic generation. Capacitance nonlinearity in these devices is due to depletion of a semiconductor drift region.

Frerking, Margaret A.↗
NASA NTRS1992

Design and test hardware for a solar array switching unit

This paper describes the control of a pulse width modulated (PWM) type sequential shunt switching unit (SSU) for spacecraft applications. It is found that the solar cell output capacitance has a significant impact on SSU design. Shorting of this cell capacitance by the PWM switch causes input current surges. These surges are minimized by the use of a series filter inductor. The system with a filter is analyzed for ripple and the control to output-voltage transfer function. Stable closed loop design considerations are discussed. The results are supported by modeling and measurements of loop gain and of closed-loop bus impedance on test hardware for NASA's 120 V Earth Observation System (EOS). The analysis and modeling are also applicable to NASA's 160 V Space Station power system.

Patil, A. R.↗
NASA NTRS1992

Broadside-coupled coplanar waveguides and their end-coupled band-pass filter applications

New broadside-coupled coplanar waveguides, suitable for applications requiring wide bandwidths, tight couplings, and large mode effective-dielectric-constant ratios, are presented. Their analysis and investigation are described. Simple equations relating the per-unit-length capacitances of the c- and pi-modes and the per-unit-length capacitance matrices of the coupled structures to those obtained when applying even- and odd-symmetric voltages are also derived. New broadside end-coupled band-pass filters have been developed at X-band (8-12 GHz) with less than 1.5 and 1.0 dB passband insertion losses using the proposed four-ground-plane and two-ground-plane coplanar waveguide structures, respectively. Good agreement between the experimental results and those predicted theoretically was also observed.

Nguyen, Cam↗
NASA NTRS1992

Dielectric measurement using an open ended coaxial line with an extended center conductor

Permittivity may be determined by measuring the complex reflection coefficient of an open-ended coaxial line placed in contact with a test sample. This method works best for liquid samples. For the measurement of non-liquid materials problems can arise. A perfect preparation is needed to provide a good contact between the tip and the dielectric medium. At times, the dimension of the sensor cannot be freely chosen, as is the case for the measurement of the dielectric constant of the xylem layer of a living tree. The influence of the installation of the sensor on a tree's water status must be minimized by using a small sensor. In such cases the capacitance cannot be optimized. By extending the center conductor of the open-ended coaxial line, some of these problems can be avoided. This provides an additional tool to tune the capacitance of the sensor by adjusting the length of the extension. Therefore the measurement accuracy can be optimized. The sensor also becomes sensitive to a larger volume. A comparative study of a flush and extended tipped probes shows that the ability to measure the dielectric constant of trees has been notably increased due to the extension of the center conductor.

Wegmueller, Urs↗
NASA NTRS1993

A piezoelectric transformer

This work describes a modeling and design method whereby a piezoelectric system is formulated by two sets of second-order equations, one for the mechanical system, and the other for the electrical system, coupled through the piezoelectric effect. The solution to this electromechanical coupled system gives a physical interpretation of the piezoelectric effect as a piezoelectric transformer that is a part of the piezoelectric system, which transfers the applied mechanical force into a force-controlled current source, and short circuit mechanical compliance into capacitance. It also transfers the voltage source into a voltage-controlled relative velocity input, and free motional capacitance into mechanical compliance. The formulation and interpretation simplify the modeling of smart structures and lead to physical insight that aids the designer. Due to its physical realization, the smart structural system can be unconditional stable and effectively control responses. This new concept has been demonstrated in three numerical examples for a simple piezoelectric system.

Won, C. C.↗
NASA NTRS1992

Low power, highly linear output buffer

A class AB CMOS output buffer has been designed for use on an IR focal plane array. Given the requirements for power dissipation and load capacitance a class A output, such as a source follower, would be unsuitable. The approach taken uses a class AB amplifier configured as a charge integrator. Thus it converts a charge packet in the focal plane multiplexer to a voltage which is then the output of the focal plane. With a quiescent current of 18 micro-a and a load capacitance of 100 pf, the amplifier has an open loop unity gain bandwidth of 900 khz. Integral nonlinearity is better than .03 percent over 5.5 volts when run with VDD-VSS = 6v.

Foley, D.↗
NASA NTRS1931

An Investigation of the Effectiveness of Ignition Sparks

The effectiveness of ignition sparks was determined by measuring the volume (or mass) of hydrogen and of oxygen which combines at low pressures. The sparks were generated by a magneto and an ignition spark coil. It was found that with constant energy the amount of reaction increases as the capacitance component of the spark increases. The use of a series spark gap may decrease or increase the amount of reaction, the effect depending upon the amount and distribution of capacitance in the circuit. So far as the work has progressed, it has been found that sparks reported by other investigations as being most efficient for igniting lean mixtures cause the largest amount of reaction. Differences between the amount of reaction with a magneto spark and an ignition spark coil were noted. The method appears to offer a means of determining the most efficient spark generator for internal-combustion engines as well as determining a relation between the character of spark, energy, and effectiveness in igniting inflammable mixtures.

Peters, Melville F↗
NASA NTRS1991

Single phase dynamic CMOS PLA using charge sharing technique

A single phase dynamic CMOS NOR-NOR programmable logic array (PLA) using triggered decoders and charge sharing techniques for high speed and low power is presented. By using the triggered decoder technique, the ground switches are eliminated, thereby, making this new design much faster and lower power dissipation than conventional PLA's. By using the charge-sharing technique in a dynamic CMOS NOR structure, a cascading AND gate can be implemented. The proposed PLA's are presented with a delay-time of 15.95 and 18.05 nsec, respectively, which compare with a conventional single phase PLA with 35.5 nsec delay-time. For a typical example of PLA like the Signetics 82S100 with 16 inputs, 48 input minterms (m) and 8 output minterms (n), the 2-SOP PLA using the triggered 2-bit decoder is 2.23 times faster and has 2.1 times less power dissipation than the conventional PLA. These results are simulated using maximum drain current of 600 micro-A, gate length of 2.0 micron, V sub DD of 5 V, the capacitance of an input miniterm of 1600 fF, and the capacitance of an output minterm of 1500 fF.

Dhong, Y. B.↗
NASA NTRS1993

Analysis of high vacuum systems using SINDA'85

The theory, algorithms, and test data correlation analysis of a math model developed to predict performance of the Space Station Freedom Vacuum Exhaust System are presented. The theory used to predict the flow characteristics of viscous, transition, and molecular flow is presented in detail. Development of user subroutines which predict the flow characteristics in conjunction with the SINDA'85/FLUINT analysis software are discussed. The resistance-capacitance network approach with application to vacuum system analysis is demonstrated and results from the model are correlated with test data. The model was developed to predict the performance of the Space Station Freedom Vacuum Exhaust System. However, the unique use of the user subroutines developed in this model and written into the SINDA'85/FLUINT thermal analysis model provides a powerful tool that can be used to predict the transient performance of vacuum systems and gas flow in tubes of virtually any geometry. This can be accomplished using a resistance-capacitance (R-C) method very similar to the methods used to perform thermal analyses.

Spivey, R. A.↗
NASA NTRS1995

Multilayer Thin-Film Microcapacitors

Miniature capacitors containing multiple alternating thin-film dielectric and metal layers proposed, especially for use in integrated and hybrid electronic circuits. Because capacitance inversely proportional to thickness of dielectric layers, use of thin, high-quality dielectric layers affords capacitance and energy-storage densities much greater than now available. These devices much smaller and more reliable than state-of-art capacitors.

Thakoor, Sarita↗
NASA NTRS1995

Resonant Tunneling Analog-To-Digital Converter

As sampling rates continue to increase, current analog-to-digital converter (ADC) device technologies will soon reach a practical resolution limit. This limit will most profoundly effect satellite and military systems used, for example, for electronic countermeasures, electronic and signal intelligence, and phased array radar. New device and circuit concepts will be essential for continued progress. We describe a novel, folded architecture ADC which could enable a technological discontinuity in ADC performance. The converter technology is based on the integration of multiple resonant tunneling diodes (RTD) and hetero-junction transistors on an indium phosphide substrate. The RTD consists of a layered semiconductor hetero-structure AlAs/InGaAs/AlAs(2/4/2 nm) clad on either side by heavily doped InGaAs contact layers. Compact quantizers based around the RTD offer a reduction in the number of components and a reduction in the input capacitance Because the component count and capacitance scale with the number of bits N, rather than by 2 (exp n) as in the flash ADC, speed can be significantly increased, A 4-bit 2-GSps quantizer circuit is under development to evaluate the performance potential. Circuit designs for ADC conversion with a resolution of 6-bits at 25GSps may be enabled by the resonant tunneling approach.

Broekaert, T. P. E.↗
NASA NTRS1995

Fluid flow volume measurements using a capacitance/conductance probe system

A probe system has been developed to measure the flow volume of a single fluid passing through an orifice or flow line. The system employs both capacitance and a conductance probe at the orifice, together with phase detection and data acquisition circuitry to measure flow volume and salinity under low or zero gravity conditions. A wide variety of frequencies can be used for the radio frequency (RF) signal source which is chosen primarily by the capacitance of the orifice probe and the fluid passing through the orifice. Rapid measurements are made using the reflected signal from the orifice probe to determine the 'instantaneous' permittivity of the fluid/gas mixture passing through. The 'instantaneous' measurements are integrated over time to determine flow volume. Analysis reveals that a narrow orifice helps to reduce non-linearities caused by differing flow rates. The geometry of 'deflectors' and 'directors' for the flowing fluid are important in obtaining linearity. Measured data shows that a volume measurement accuracy of approximately four percent can be consistently achieved. The prototype hardware system and associated software have been optimized and are available for further applications. The system has immediate application in low or zero gravity environments for measurements of urine or other liquid volumes.

Nguyen, T. X.↗
NASA NTRS1998

Experimental Study of Load Carrying Capacity of Point Contacts at Zero Entrainment Velocity

A capacitance technique was used to monitor the film thickness separating two steel balls while subjecting the ball-ball contact to highly stressed, zero entrainment velocity conditions. Tests were performed in a nitrogen atmosphere and utilized 52100 steel balls and a polyalphaolefin lubricant. Capacitance to film thickness accuracy was verified under pure rolling conditions using established EHL theory. Zero entrainment velocity tests were performed at sliding speeds from 6.0 to 10.0 m/s and for sustained amounts of time to 28.8 min. The protective lubricant film separating the specimens at zero entrainment velocity had a film thickness between 0.10 to 0.14 microns (4 to 6 micro in.), which corresponded to a k value of 4. The formation of an immobile surface film formed by lubricant entrapment is discussed as an explanation of the load carrying capacity at zero entrainment velocity conditions, relevant to the ball-ball contacts occurring in retainerless ball bearings.

Shogin, B. A.↗
NASA NTRS1999

Experimental Determination of Load Carrying Capacity of Point Contacts at Zero Entrainment Velocity

A capacitance technique was used to monitor the film thickness separating two steel balls of a unique tribometer while subjecting the ball-ball contact to highly stressed, zero entrainment velocity (ZEV) conditions. All tests were performed under a N2 purge (R.H. < 1.0%) and utilized 52100 steel balls (R(sub a) = 0.02 mm). Tribometer operations and capacitance-to-film-thickness accuracy were verified by comparing the film thickness approximations to established theoretical predictions for test conditions involving pure rolling. Pure rolling experiments were performed under maximum contact stresses and entrainment velocities of 1.0 GPa and 1.0 m/s to 3.0 m/s, respectively. All data from these baseline tests conformed to theory. ZEV tests were initiated after calibration of the tribometer and verification of film thickness approximation accuracy. Maximum contact stresses up to 0.57 GPa were supported at zero entrainment velocity with sliding speeds from 6.0 to 10.0 m/s for sustained amounts of time up to 28.8 minutes. The protective lubricating film separating the specimens at ZEV had a thickness between 0.10 and 0.14 mm (4 to 6 min), which corresponds to an approximate L-value of 4. The film thickness did not have a strong dependence upon variations of load or speed. Decreasing the sliding speed from 10.0 m/s to 1 m/s revealed a rapid loss in load support between 3.0 and 1.0 m/s. The formation of an immobile film formed by lubricant entrapment is discussed as an explanation of the load carrying capacity at these zero entrainment velocity conditions, relevant to the ball-ball contact application in retainerless ball bearings.

Shogrin, Bradley A.↗