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170 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS2000

Freja Studies of the Current-Voltage Relation in Substorm-Related Events

Field-aligned currents and electrostatic potentials play important roles in the coupling between the magnetosphere and the ionosphere. If one assumes that the ionosphere-magnetosphere potential difference is mainly due to the mirror force, one can use the single particle adiabatic kinetic theory to describe the system. From this theory, a linear relationship j(sub II) = KV between field-aligned current density j(sub II) and potential drop V along the same field line can be derived, provided that the potential drop is not too large and not too small. With rare exceptions, observational tests of this relation have mainly concentrated on quiet magnetospheric situations, with acceleration voltages V approx. less than 5 kV. Here we use observations from the Freja satellite of precipitating auroral electrons at 1.700 km altitude to study substorm related events, with acceleration voltages up to 20 keV. The observations are found to be consistent with a linear current-voltage relation even i n these conditions, although with values of the field aligned K lower than previously reported (1-5 x 10(exp 11 S/sq m). This can be explained by lower densities and higher characteristic electron energies in the magnetospheric source region of the precipitating electrons. We analyze the data by three different methods, which are all found to be in general agreement. The results are in agreement with a previous study, where the spectra of precipitating electrons --were indirectly inferred by inversion of data from the EISCAT incoherent scatter radar, thereby validating the use of radar data for studies of auroral electrons. Comparisons with previous studies are made, emphasizing the dependence of the results on the type of auroral structure and magnetospheric conditions.

Olsson, A.↗
NASA NTRS1996

Comparison of Non-Parabolic Hydrodynamic Simulations for Semiconductor Devices

Parabolic drift-diffusion simulators are common engineering level design tools for semiconductor devices. Hydrodynamic simulators, based on the parabolic band approximation, are becoming more prevalent as device dimensions shrink and energy transport effects begin to dominate device characteristic. However, band structure effects present in state-of-the-art devices necessitate relaxing the parabolic band approximation. This paper presents simulations of ballistic diodes, a benchmark device, of Si and GaAs using two different non-parabolic hydrodynamic formulations. The first formulation uses the Kane dispersion relationship in the derivation of the conservation equations. The second model uses a power law dispersion relation {(hk)(exp 2)/2m = xW(exp Y)}. Current-voltage relations show that for the ballistic diodes considered. the non-parabolic formulations predict less current than the parabolic case. Explanations of this will be provided by examination of velocity and energy profiles. At low bias, the simulations based on the Kane formulation predict greater current flow than the power law formulation. As the bias is increased this trend changes and the power law predicts greater current than the Kane formulation. It will be shown that the non-parabolicity and energy range of the hydrodynamic model based on the Kane dispersion relation are limited due to the binomial approximation which was utilized in the derivation.

Smith, A. W.↗
NASA NTRS2018

Electrical Characteristics of the Mars Electrostatic Precipitator

NASA's next generation Mars missions will include chemical processing plants to convert the Martian atmosphere into consumable products needed to support astronaut activities. The thin, mostly carbon dioxide atmosphere of Mars is estimated to have 5-10 particles/cu. cm which have a radius of 1.6-2.27 microns on average. These dust particles could potentially foul the chemical process or reduce the purity of the products. Electrostatic precipitation is one possible solution to remove dust particles from the ingested Mars atmosphere. The Electrostatics and Surface Physics Laboratory at NASA's Kennedy Space Center has developed an electrostatic precipitator testbed to understand the intricacies of corona discharges in dusty flows simulating Mars atmospheric conditions. Current-voltage trends have been established for a number of precipitator flow conditions. Corona onset voltage and streamer onset voltage trends versus pressure are also established.

Johansen, Michael R.↗
NASA NTRS2018

Electrical Characteristics of the Mars Electrostatic Precipitator

NASA's next generation Mars missions will include chemical processing plants to convert the Martian atmosphere into consumable products needed to support astronaut activities. The thin, mostly carbon dioxide atmosphere of Mars is estimated to have 5-10 particles/cu. cm which have a radius of 1.6-2.27 microns on average. These dust particles could potentially foul the chemical process or reduce the purity of the products. Electrostatic precipitation is one possible solution to remove dust particles from the ingested Mars atmosphere. The Electro-statics and Surface Physics Laboratory at NASA's Kennedy Space Center has developed an electrostatic precipitator testbed to understand the intricacies of corona discharges in dusty flows simulating Mars atmospheric conditions. Current-voltage trends have been estab-lished for a number of precipitator flow conditions. Corona onset voltage and streamer onset voltage trends versus pressure are also established.

Johansen, Michael R.↗
NASA NTRS2007

Carbon Nanotubes for Space Photovoltaic Applications

Carbon nanotubes (CNTs) can be envisioned as an individual graphene sheet rolled into a seamless cylinder (single-walled, SWNT), or concentric sheets as in the case of a multi-walled carbon nanotube (MWNT) (1). The role-up vector will determine the hexagonal arrangement and "chirality" of the graphene sheet, which will establish the nanotube to be metallic or semiconducting. The optoelectronic properties will depend directly on this chiral angle and the diameter of the SWNT, with semiconductor types exhibiting a band gap energy (2). Characteristic of MWNTs are the concentric graphene layers spaced 0.34 nm apart, with diameters from 10-200 nm and lengths up to hundreds of microns (2). In the case of SWNTs, the diameters range from 0.4 - 2 nm and lengths have been reported up to 1.5 cm (3). SWNTs have the distinguishable property of "bundling" together due to van der Waal's attractions to form "ropes." A comparison of these different structural types is shown in Figure 1. The use of SWNTS in space photovoltaic (PV) applications is attractive for a variety of reasons. Carbon nanotubes as a class of materials exhibit unprecedented optical, electrical, mechanical properties, with the added benefit of being nanoscale in size which fosters ideal interaction in nanomaterial-based devices like polymeric solar cells. The optical bandgap of semiconducting SWNTs can be varied from approx. 0.4 - 1.5 eV, with this property being inversely proportional to the nanotube diameter. Recent work at GE Global Research has shown where a single nanotube device can behave as an "ideal" pn diode (5). The SWNT was bridged over a SiO2 channel between Mo contacts and exhibited an ideality factor of 1, based on a fit of the current-voltage data using the diode equation. The measured PV efficiency under a 0.8 eV monochromatic illumination showed a power conversion efficiency of 0.2 %. However, the projected efficiency of these junctions is estimated to be > 5 %, especially when one considers the enhanced absorption (from nanotubes whose bandgap is tailored to illumination) and electromagnetic coupling in a network of nanotubes.

Efstathiadis, Harry↗
NASA NTRS1979

Low cost cylindrical converter for measuring lead efficiency

A low cost cylindrical thermionic converter has been designed, fabricated, and tested for use in a NASA-JPL program to directly measure converter lead efficiencies using various electrode materials and surfaces. Efficiency measurements are made using input emitter heater power, output power at the leads, and calculated values of the parasitic losses that would not be present in the application configuration. This information can be used to predict the performance characteristics of the advanced converters currently under development. A series of five converters has been tested. Both structured and smooth molybdenum collectors have been used. Emitters included smooth molybdenum, smooth rhenium, and microstructured rhenium. Cesium pressure families of current-voltage curves are presented for emitter temperatures of 1600, 1700, and 1800 K, along with measured efficiencies as a function of converter current densities for each temperature.

Hatch, G. L.↗
NASA NTRS1987

Characteristics of annealed p/n junctions between GaAs and Si(100)

Electrical characteristics of a GaAs(p)/Si(n) interface were determined from capacitance-voltage, current-voltage, and secondary ion mass spectroscopy (SIMS) measurements and compared to those on GaAs(p) epitaxial layers on GaAs(n) substrates. The comparison was made between the junctions as grown, and after an anneal at 850 C for 20 min in 10 percent forming gas under an As overpressure. For the GaAs/Si junction the ideality factor changed from 2 or larger to 1.5 and the apparent intercept voltage changed from 2.5 to 1.3 V after annealing. For the GaAs homojunction, the intercept voltage increased from 1.1 to 1.3 V. In addition, the excess current in the forward and reverse bias conditions dropped drastically in the heterojunction. No movement of the metallurgical junction was discernible to within the resolution capability of SIMS. The junction properties obtained by annealing suggest an atomic restructuring of the Si(100) interface during growth or annealing. These new results raise the possibility that the GaAs/Si interface can be made into an electrically viable junction and incorporated into active devices.

Unlu, M. S.↗
NASA NTRS2001

Metallic Electrode: Semiconducting Nanotube Junction Model

A model is proposed for two observed current-voltage (I-V) patterns in an experiment with a scanning tunneling microscope tip and a carbon nanotube [Collins et al., Science 278, 100 ('97)]. We claim that there are two contact modes for a tip (metal) -nanotube semi conductor) junction depending whether the alignment of the metal and semiconductor band structure is (1) variable (vacuum-gap) or (2) fixed (touching) with V. With the tip grounded, the tunneling case in (1) would produce large dI/dV with V > 0, small dI/dV with V < 0, and I = 0 near V = 0 for an either n- or p-nanotube. However, the Schottky mechanism in (2) would result in forward current with V < 0 for an n-nanotube, while with V > 0 for an p-nanotube. The two observed I-V patterns are thus entirely explained by a tip-nanotube contact of the two types, where the nanotube must be n-type. We apply this picture to the source-drain I-V characteristics in a long nanotube-channel field-effect-transistor (Zhou et al., Appl. Phys. Lett. 76, 1597 ('00)], and show that two independent metal-semiconductor junctions connected in series are responsible for the observed behavior.

Yamada, Toshishige↗