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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 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Progress toward a 30 percent-efficient, monolithic, three-junction, two-terminal concentrator solar cell for space applications

Component efficiencies of 0.2/sq cm cells at approximately 100x AMO light concentration and 80 C temperatures are not at 15.3 percent for a 1.9 eV AlGaAs top cell, 9.9 percent for a 1.4 eV GaAs middle cell under a 1.9 eV AlGaAs filter, and 2.4 percent for a bottom 1.0 eV InGaAs cell under a GaAs substrate. The goal is to continue improvement in these performance levels and to sequentially grow these devices on a single substrate to give 30 percent efficient, monolithic, two-terminal, three-junction space concentrator cells. The broad objective is a 30 percent efficient monolithic two-terminal cell that can operate under 25 to 100x AMO light concentrations and at 75 to 100 C cell temperatures. Detailed modeling predicts that this requires three junctions. Two options are being pursued, and both use a 1.9 eV AlGaAs top junction and a 1.4 eV GaAs middle junction grown by a 1 atm OMVPE on a lattice matched substrate. Option 1 uses a low-doped GaAs substrate with a lattice mismatched 1.0 eV InGaAs cell formed on the back of the substrate. Option 2 uses a Ge substrate to which the AlGaAs and GaAs top junctions are lattice matched, with a bottom 0.7 eV Ge junction formed near the substrate interface with the GaAs growth. The projected efficiency contributions are near 16, 11, and 3 percent, respectively, from the top, middle, and bottom junctions.

Partain, L. D.↗
NASA NTRS1971

Approximate optimal atmospheric entry trajectories.

For a reentry glider, approximate solutions are found in closed form for the problem of maximizing a function of the terminal velocity, altitude, flight-path angle and heading angle subject to, at most, three terminal nonlinear constraints. The results given here extend the previous results in two important ways: (1) the second-order approximation of the entry dynamics of Loh is used instead of the first-order approximation of Allen and Eggers. This approximation is found to compare extremely well with an exact numerical optimal path. (2) A three-dimensional optimization problem is solved which includes Loh's second-order approximation where the roll angle as well as the lift coefficient are determined subject to constraints which include the terminal heading angle. Furthermore, closed-form solutions can also be obtained subject to in-flight constraints on the lift coefficient and roll angle.

Speyer, J. L.↗
NASA NTRS2012

Optimal Dynamic Sub-Threshold Technique for Extreme Low Power Consumption for VLSI

For miniaturization of electronics systems, power consumption plays a key role in the realm of constraints. Considering the very large scale integration (VLSI) design aspect, as transistor feature size is decreased to 50 nm and below, there is sizable increase in the number of transistors as more functional building blocks are embedded in the same chip. However, the consequent increase in power consumption (dynamic and leakage) will serve as a key constraint to inhibit the advantages of transistor feature size reduction. Power consumption can be reduced by minimizing the voltage supply (for dynamic power consumption) and/or increasing threshold voltage (V(sub th), for reducing leakage power). When the feature size of the transistor is reduced, supply voltage (V(sub dd)) and threshold voltage (V(sub th)) are also reduced accordingly; then, the leakage current becomes a bigger factor of the total power consumption. To maintain low power consumption, operation of electronics at sub-threshold levels can be a potentially strong contender; however, there are two obstacles to be faced: more leakage current per transistor will cause more leakage power consumption, and slow response time when the transistor is operated in weak inversion region. To enable low power consumption and yet obtain high performance, the CMOS (complementary metal oxide semiconductor) transistor as a basic element is viewed and controlled as a four-terminal device: source, drain, gate, and body, as differentiated from the traditional approach with three terminals: i.e., source and body, drain, and gate. This technique features multiple voltage sources to supply the dynamic control, and uses dynamic control to enable low-threshold voltage when the channel (N or P) is active, for speed response enhancement and high threshold voltage, and when the transistor channel (N or P) is inactive, to reduce the leakage current for low-leakage power consumption.

Duong, Tuan A.↗
NASA NTRS1991

Modeled performance of monolithic, 3-terminal InP/Ga(0.47)In(0.53)As concentrator solar cells as a function of temperature and concentration ratio

Using measured device parameters from a monolithic three-terminal In/GaInAs (0.75 eV) tandem concentrator solar cell, a numerical model is constructed that calculates efficiency as a function of temperature and concentration ratio. The device measurements indicate that the series resistance in the InP top cell severely limits the maximum efficiency at high concentration ratios. Results from the model in which a single-junction InP concentrator solar cell that has a lower resistance by a factor of six was substituted for the top cell show that peak 20 C air-mass-zero (AM0) efficiencies should approach 30 percent at concentration ratios greater than 100. At 80 C, this tandem cell should exceed 24 percent AM0 efficiency.

Osterwald, C. R.↗
NASA NTRS2003

Switching and Rectification in Carbon-Nanotube Junctions

Multi-terminal carbon-nanotube junctions are under investigation as candidate components of nanoscale electronic devices and circuits. Three-terminal "Y" junctions of carbon nanotubes (see Figure 1) have proven to be especially interesting because (1) it is now possible to synthesize them in high yield in a controlled manner and (2) results of preliminary experimental and theoretical studies suggest that such junctions could exhibit switching and rectification properties. Following the preliminary studies, current-versus-voltage characteristics of a number of different "Y" junctions of single-wall carbon nanotubes connected to metal wires were computed. Both semiconducting and metallic nanotubes of various chiralities were considered. Most of the junctions considered were symmetric. These computations involved modeling of the quantum electrical conductivity of the carbon nanotubes and junctions, taking account of such complicating factors as the topological defects (pentagons, heptagons, and octagons) present in the hexagonal molecular structures at the junctions, and the effects of the nanotube/wire interfaces. A major component of the computational approach was the use of an efficient Green s function embedding scheme. The results of these computations showed that symmetric junctions could be expected to support both rectification and switching. The results also showed that rectification and switching properties of a junction could be expected to depend strongly on its symmetry and, to a lesser degree, on the chirality of the nanotubes. In particular, it was found that a zigzag nanotube branching at a symmetric "Y" junction could exhibit either perfect rectification or partial rectification (asymmetric current-versus-voltage characteristic, as in the example of Figure 2). It was also found that an asymmetric "Y" junction would not exhibit rectification.

Srivastava, Deepak↗
NASA NTRS2015

Tool-Enabled Changes in Terminal Air Traffic Controller Task and Workload Distribution

A human-in-the-loop simulation was conducted that examined an approach to adjusting airport arrivals in such a way as to enable higher departure throughput while maintaining arrival throughput. This approach, referred to as Departure-Sensitive Arrival Spacing (DSAS), leverages the capabilities of the Terminal Sequencing and Spacing (TSS) system with an additional Decision Support Tool (DST) to assign more precise arrival spacing interval. This presentation will focus on the changes observed in the task distribution and control strategies among three Terminal Radar Approach Control (TRACON) controller participants and the workload associated with those changes across three conditions: Baseline, TSS, and DSAS. Results showed that the application of the DSTs in the TSS condition and those in the DSAS condition enabled a progressive reduction in the number of clearances issued and for the clearances to be issued earlier, further away from the airport. In doing so, there was also a greater use of speed control versus vectoring and altitude for schedule conformance in the TSS and DSAS conditions respectively and relative to Baseline. Workload also shifted in conjunction with the clearance distribution with an overall reduction across conditions. The changes in task and workload distribution enabled a shift from tactical to strategic control, which allowed for a more predictable and efficient delivery of arrivals.

task↗
NASA NTRS1970

Superconducting ''transistor'' acts as high-speed switch

Cryogenic, three-terminal device yields current-voltage characteristics of ordinary transistor. Device consists of two superconducting tin rods in a crossed-wire geometry, separated by a thin gold film. New construction technique is described and possible applications are discussed.

Meissner, H.↗
NASA NTRS1972

Use of a three-layer distributed RC network to produce two pairs of complex conjugate zeros

The properties of a three layer distributed RC network consisting of two layers of resistive material separated by a dielectric are described. When the three layer network is used as a three terminal element by connecting conducting terminal strips across the ends of one of the resistive layers and the center of the other resistive layer, the network may be used to produce pairs of complex conjugate transmission zeros. The location of these zeros are determined by the parameters of the network. Design charts for determining the zero positions are included as part of the report.

Huelsman, L. P.↗
NASA NTRS1977

Overload-protector/fault-indicator circuit

Circuit incorporates three-terminal current limiter (78M24) to increase overall reliability and to eliminate transistor burnouts resulting from shorted interconnection lines and other overloads. Fact-acting light emitting diodes across the limiters show status of transistor output circuits.

Paluka, J. R.↗
NASA NTRS1984

High-efficiency solar cells fabricated by vacuum MO-CVD

High-efficiency, monolithic, two-color, three-terminal solar cells were fabricated by a novel growth technique, vacuum metal-organic chemical vapor deposition. The technique uses the expensive metal alkyls efficiently and toxic gases sparingly. The fact that the outer chamber is constructed of nonbreakable stainless steel is an attractive safety feature associated with this deposition system.

Fraas, L. M.↗
NASA NTRS1987

Computer modelling of aluminum-gallium arsenide/gallium arsenide multilayer photovoltaics

The modeled cascade cells offer an alternative to conventional series cascade designs that require a monolithic intercell ohmic contact. Selective electrodes provide a simple means of fabricating three-terminal devices, which can be configured in complementary pairs to circumvent the attendant losses and fabrication complexities of intercell ohmic contacts. Moreover, selective electrodes allow incorporation of additional layers in the upper subcell which can improve spectral response and increase radiation tolerance. Realistic simulations of such cells operating under one-sun AMO conditions show that the seven-layer structure is optimum from the standpoint of beginning-of-life efficiency and radiation tolerance. Projected efficiencies exceed 26 percent. Under higher concentration factors, it should be possible to achieve efficiencies beyond 30 percent. However, to simulate operation at high concentration will require a model for resistive losses. Overall, these devices appear to be a promising contender for future space applications.

Wagner, Michael Broderick↗
NASA NTRS1987

Modelling of multijunction cascade photovoltaics for space applications

An alternative class of photovoltaics was presented, which is designed to overcome two problem areas with conventional cascade designs: poor upper subcell performance and lossy intercell ohmic contact (IOC). It was shown that upper subcell quality can be improved by incorporating additional junctions into the upper subcell and that the problems with monolithic IOCs may be circumvented by using complementary pairs of three-terminal cells or a 1 x 2 voltage-matched configuration. Realistic simulations show that AlGaAs-GaAs and AlGaAs-InGaAs multijunction, multiband-gap solar cells (MJSC) may achieve benginning-of-life (BOL) one-sun, AMO efficiencies of 26 and 28 percent, respectively. Complementary cells made in the AlGaAs-InGaAs system can achieve BOL one-sun AMO efficiencies in excess of 27 percent. Seven-layer MJSCs are most advantageous for space applications due to their superior tolerance to radiation degradation.

Educato, James Louis↗
NASA NTRS1987

AlGaAs-InGaAs SHEP's for space applications

The authors propose a novel class of cascade photovoltaics which are intended to overcome two problems with current cascade solar cell designs: poor upper subcell performance and excessive joule losses in the intercell ohmic contact (IOC). The devices feature multijunction upper subcells which improve spectral response by reducing the mean collection distance for photogenerated carriers. They circumvent the need for a monolithic IOC by using npn and pnp three-terminal cells arranged in a complementary configuration. Emphasis is on the AlGaAs-InGaAs system, which allows optimal partitioning of the solar spectrum to attain maximum possible efficiency for a two-gap cascade design. Computer simulations of these cells show that one-sun beginning-of-life efficiencies in excess of 27 percent are possible.

Educato, J. L.↗
NASA NTRS1988

Microwave and millimeter-wave resonant-tunneling devices

Resonant-tunneling devices in microelectronic component form, whose structure is the electron analog of a Fabry-Perot resonator, encompass oscillators, self-oscillating mixers, and harmonic multipliers. The negative differential resistance characteristic of these devices has been obtained at room temperature, and resonant-tunneling transistors capable of operation in the THz-frequency range appear to be feasible. Three-terminal resonant-tunneling device development is in its infancy, as is that of devices for digital applications.

Sollner, T. C. L. Gerhard↗
NASA NTRS1989

W-band six-port network analyzer for two-port characterization of millimeter wave transistors

A W-band (75-100 GHz) six-port junction network analyzer was constructed from commercially available descrete waveguide components and was used for the direct two-port S-parameter measurement of active three-terminal devices. A comparison between the six-port and a down-converter-type frequency extender for a conventional network analyzer revealed the superior performance of the six-port. The application of the six-port to characterize a 0.1-micron gate-length HEMT at W-band is described, and representative results are presented.

Moeller, Karl J.↗