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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Electron impact cross-sections and cooling rates for methane

Energy transfer between electrons and methane gas by collisional processes plays an important role in the thermal balance of electrons in the atmospheres and ionospheres of planets and satellites in the outer solar system. The literature is reviewed for electron impact cross-sections for methane in this paper. Energy transfer rates are calculated for elastic and inelastic processes using a Maxwellian electron distribution. Vibrational, rotational, and electronic excitation and ionization are included. Results are presented for a wide range of electron temperatures and neutral temperatures.

Gan, L.↗
NASA NTRS1987

Ion and electron velocity distributions within flux transfer events

The detailed nature of the thermal and suprathermal ion and electron distributions within magnetic flux transfer events (FTEs) is examined. Examples of both magnetosheath FTEs and magnetospheric FTEs are discussed. The detailed distributions confirm that FTEs contain a mixture of magnetosheath and magnetospheric plasmas. To lowest order, the distributions are consistent with a simple superposition of the two interpenetrating populations, with no strong interactions between them. To first order, some interesting differences appear, especially in the electron distributions, suggesting that considerable pitch angle scattering and some electron energy diffusion are also occurring. These observations should provide a useful test of analytical and numerical studies of interpenetrating plasmas.

Thomsen, M. F.↗
NASA NTRS2005

Pd/CeO2/SiC Chemical Sensors

The incorporation of nanostructured interfacial layers of CeO2 has been proposed to enhance the performances of Pd/SiC Schottky diodes used to sense hydrogen and hydrocarbons at high temperatures. If successful, this development could prove beneficial in numerous applications in which there are requirements to sense hydrogen and hydrocarbons at high temperatures: examples include monitoring of exhaust gases from engines and detecting fires. Sensitivity and thermal stability are major considerations affecting the development of high-temperature chemical sensors. In the case of a metal/SiC Schottky diode for a number of metals, the SiC becomes more chemically active in the presence of the thin metal film on the SiC surface at high temperature. This increase in chemical reactivity causes changes in chemical composition and structure of the metal/SiC interface. The practical effect of the changes is to alter the electronic and other properties of the device in such a manner as to degrade its performance as a chemical sensor. To delay or prevent these changes, it is necessary to limit operation to a temperature <450 C for these sensor structures. The present proposal to incorporate interfacial CeO2 films is based partly on the observation that nanostructured materials in general have potentially useful electrical properties, including an ability to enhance the transfer of electrons. In particular, nanostructured CeO2, that is CeO2 with nanosized grains, has shown promise for incorporation into hightemperature electronic devices. Nanostructured CeO2 films can be formed on SiC and have been shown to exhibit high thermal stability on SiC, characterized by the ability to withstand temperatures somewhat greater than 700 C for limited times. The exchanges of oxygen between CeO2 and SiC prevent the formation of carbon and other chemical species that are unfavorable for operation of a SiC-based Schottky diode as a chemical sensor. Consequently, it is anticipated that in a Pd/CeO2/SiC Schottky diode, the nanostructured interfacial CeO2 layer would contribute to thermal stability and, by contributing to transfer of electrons, would also contribute to sensitivity.

Lu, Weijie↗
NASA NTRS1976

A scaling law for energy transfer by inelastic electron-molecule collisions in mixtures

The equation governing the electron energy distribution in the presence of a spatially uniform electric field in a weakly ionized gas was reformulated into an integral equation for the logarithmic slope of the distribution function. For gas mixtures in which the dominant electron energy loss mechanism is by vibrational excitation of the molecules, this equation is suitable for approximate analysis and exact numerical solution by iteration. Superelastic collisions are easily included in this formulation, and do not seriously effect the convergence of the numerical scheme. The approximate analytical results are only qualitatively correct, but suggest appropriate parameters which correlate the exact numerical results very well. The distribution function as well as certain gross properties such as net energy transfer into vibration, mean energy, and drift velocity depend primarily on a single nondimensional parameter involving only E/N and the cross sections.

Bienkowski, G. K.↗
NASA NTRS1974

Electronic-to-vibrational energy transfer efficiency in the O/1 D/-N2 and O/1 D/-CO systems

With the aid of a molecular resonance fluorescence technique, which utilizes optical pumping from the v = 1 level of the ground state of CO by A 1 Pi-X 1 Sigma radiation, a study is made of the efficiency of E-V transfer from O(1 D) to CO. O(1 D) is generated at a known rate by O2 photodissociation at 1470 A in an intermittent mode, and the small modulation of the fluorescent signal associated with CO (v = 1) above the normal thermal background is interpreted in terms of E-V transfer efficiency. The CO (v = 1) lifetime in this system is determined mainly by resonance trapping of the IR fundamental band, and is found to be up to ten times longer than the natural radiative lifetime. For CO, (40 plus or minus 8)% of the O(1 D) energy is converted into vibrational energy. By observing the effect of N2 on the CO (v = 1) fluorescent intensity and lifetime, it is possible to obtain the E-V transfer efficiency for the system O(1 D)-N2 relative to that for O(1 D)-CO. The results indicate that the efficiency for N2 is (83 plus or minus 10)% of that for CO.

Slanger, T. G.↗
NASA NTRS1981

Solar-pumped electronic-to-vibrational energy transfer lasers

The possibility of using solar-pumped lasers as solar energy converters is examined. The absorbing media considered are halogens or halogen compounds, which are dissociated to yield excited atoms, which then hand over energy to a molecular lasing medium. Estimates of the temperature effects for a Br2-CO2-He system with He as the cooling gas are given. High temperatures can cause the lower energy levels of the CO2 laser transition to be filled. The inverted populations are calculated and lasing should be possible. However, the efficiency is less than 0.001. Examination of other halogen-molecular lasant combinations (where the rate coefficients are known) indicate efficiencies in all cases of less than 0.005.

Harries, W. L.↗
NASA NTRS1972

Anode heat transfer for a flowing argon plasma at elevated electron temperature.

Theoretical investigation of heat transfer from a preionized gaseous plasma flowing over an anode surface at an elevated electron temperature in the presence of an electric field normal to the surface. A laminar boundary layer is considered in which only the velocity profile is locally similar and fluid properties are assumed to change uniformly in the gas flow direction. Results obtained by an approximation method show that for moderate current densities, the velocity and temperature distributions are insensitive to current. In addition, the effect of elevated electron temperature is negligible on convective heat transfer, but is significant for the overall heat transfer due to the enthalpy transport by current. Total heat flux to the anode is obtained by evaluating the Nusselt number and adding terms due to the potential drop in the sheath and the surface work function.

Bose, T. K.↗
NASA NTRS2008

Molecules for Fluorescence Detection of Specific Chemicals

A family of fluorescent dye molecules has been developed for use in on-off fluorescence detection of specific chemicals. By themselves, these molecules do not fluoresce. However, when exposed to certain chemical analytes in liquid or vapor forms, they do fluoresce (see figure). These compounds are amenable to fixation on or in a variety of substrates for use in fluorescence-based detection devices: they can be chemically modified to anchor them to porous or non-porous solid supports or can be incorporated into polymer films. Potential applications for these compounds include detection of chemical warfare agents, sensing of acidity or alkalinity, and fluorescent tagging of proteins in pharmaceutical research and development. These molecules could also be exploited for use as two-photon materials for photodynamic therapy in the treatment of certain cancers and other diseases. A molecule in this family consists of a fluorescent core (such as an anthracene or pyrene) attached to two end groups that, when the dye is excited by absorption of light, transfer an electron to the core, thereby quenching the fluorescence. The end groups can be engineered so that they react chemically with certain analytes. Upon reaction, electrons on the end groups are no longer available for transfer to the core and, consequently, the fluorescence from the core is no longer quenched. The chemoselectivity of these molecules can be changed by changing the end groups. For example, aniline end groups afford a capability for sensing acids or acid halides (including those contained in chemical warfare agents). Pyridine or bipyridyl end groups would enable sensing of metal ions. Other chemicals that can be selectively detected through suitable choice of end groups include glucose and proteins. Moreover, the fluorescent cores can be changed to alter light-absorption and -emission characteristics: anthracene cores fluoresce at wavelengths around 500 nm, whereas perylene cores absorb and emit at wavelengths of about 600 nm.

Fedor, Steve↗
NASA NTRS1977

Application of an electron beam facility for heat transfer measurements in capillary tubes

A unique method was developed for the determination of heat transfer coefficients for water flowing through capillary tubes using a rastered electron beam heater. Heat flux levels of 150 and 500 watts/sq cm were provided on the top surface of four square tubes. Temperature gradient along the tube length and mass flow rates versus pressure drop were measured.

Lunde, A. R.↗
NASA NTRS1972

Electrochemical behavior of the boron anode in aqueous solutions

The electrochemical oxidation of p-type boron in 0.2 N NaNO3 solutions of pH 0.4 to 13.1 was studied by galvanostatic techniques. The capacitance of the electrode, Tafel slopes, and a limited analysis of reaction mechanisms are reported. The anodic dissolution in acid solution is a charge-transfer-limited one-electron reaction to form a monovalent species in the over-potential region of 0.25 to 0.70 V. The dissolution in basic solution is most probably a one-electron charge-transfer reaction from a monovalent to divalent species involving three hydroxyl ions. Open circuit potentials were mixed potentials, probably due to hydrogen formation at open circuit but not contributing to the electrochemical reaction at the applied overpotential. Exchange current, estimated by extrapolation of the Tafel line to the standard oxidation potential for the boron reaction, were of the order of 10 to the minus 6th power A/cm2 in acid solution and 10 to the minus 12th power A/cm2 in basic solution. The reaction order of the rate determining acidic and basic reactions was determined with regard to (H+) and (OH-). The level of illumination had no effect on the electrochemical behavior of the electrode.

Delduca, B. S.↗
NASA NTRS1976

On the magnetic mirroring as the basic cause of parallel electric fields

Among the different proposed mechanisms for generating parallel electric fields, magnetic mirroring of charged particles seems to be the most plausible. In the present paper, it is suggested that magnetic mirroring is the basic cause of parallel electric fields in the magnetosphere and that the magnetic mirroring effect may be able to form the basis of an auroral theory that can remove a major portion of the ambiguity of observations. In the model proposed, the parallel electric field is due to a magnetic confinement of a negatively charged hot collision-free plasma. A transfer of electron gyroenergy into wave energy tends to weaken this confinement; if this energy transfer becomes too strong, the parallel potential gradient will break down. Hence, from this model, in contrast to certain other models of parallel electric fields, only a small fraction of the total auroral particle energy may be expected to be transformed into electromagnetic wave energy during the acceleration process.

Lennartsson, W.↗
NASA NTRS2002

Effect of Electronic Excitation on Hydrogen Atom Transfer (Tautomerization) Reactions for the DNA Base Adenine

Geometrical structures and energetic properties for four different tautomers of adenine are calculated in this study, using multi-configurational wave functions. Both the ground and the lowest single excited state potential energy surface are studied. The energetic order of the tautomers on the ground state potential surface is 9H less than 7H less than 3H less than 1H, while on the excited state surface this order is found to be different: 3H less than 1H less than 9H less than 7H. Minimum energy reaction paths are obtained for hydrogen atom transfer (9 yields 3 tautomerization) reactions in the ground and the lowest excited electronic state. It is found that the barrier heights and the shapes of the reaction paths are different for the ground and the excited electronic state, suggesting that the probability of such tautomerization reaction is higher on the excited state potential energy surface. The barrier for this reaction in the excited state may become very low in the presence of water or other polar solvent molecules, and therefore such tautomerization reaction may play an important role in the solution phase photochemistry of adenine.

Chaban, Galina M.↗
NASA NTRS1991

Molecules with enhanced electronic polarizabilities based on defect-like states in conjugated polymers

Highly conjugated organic polymers typically have large non-resonant electronic susceptibilities, which give the molecules unusual optical properties. To enhance these properties, defects are introduced into the polymer chain. Examples include light doping of the conjugated polymer and synthesis, conjugated polymers which incorporate either electron donating or accepting groups, and conjugated polymers which contain a photoexcitable species capable of reversibly transferring its electron to an acceptor. Such defects in the chain permit enhancement of the second hyperpolarizability by at least an order of magnitude.

Beratan, David N.↗
NASA NTRS1986

The quasi-linear relaxation of thick-target electron beams in solar flares

The effects of quasi-linear interactions on thick-target electron beams in the solar corona are investigated. Coulomb collisions produce regions of positive gradient in electron distributions which are initially monotonic decreasing functions of energy. In the resulting two-stream instability, energy and momentum are transferred from electrons to Langmuir waves and the region of positive slope in the electron distribution is replaced by a plateau. In the corona, the timescale for this quasi-linear relaxation is very short compared to the collision time. It is therefore possible to model the effects of quasi-linear relaxation by replacing any region of positive slop in the distribution by a plateau at each time step, in such a way as to conserve particle number. The X-ray bremsstrahlung and collisional heating rate produced by a relaxed beam are evaluated. Although the analysis is strictly steady state, it is relevant to the theoretical interpretation of hard X-ray bursts with durations of the order of a few seconds (i.e., the majority of such bursts).

Mcclements, K. G.↗