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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
DOE OSTI2020

Model for photothermal ionization and molecular recombination during pulsed ablation of polyethylene

The surface erosion of polyethylene is studied under conditions characteristic of extremely high-rate heating expected in laser-driven and high energy density experiments. A coupled model for photoionization and volumetric photothermal degradation of polyethylene is developed that includes finite rate chemistry and molecular recombination of radicals. First, the model is used to explore the evolution of radicals and hydrocarbons during isochoric heating, for which it is found that polyethylene is not in chemical equilibrium for rates >108K/s. Then, the model is used to explore the cooperativity between photoionization and photothermal ionization during one-dimensional ablation from a pulsed heating source, accounting for coupled energy deposition, thermokinetics, thermochemistry, hydrodynamics, mechanics, and thermal conduction in the ALEGRA multi-physics code. The ablation depth per energy pulse is found to agree with an analytical model for instantaneous energy deposition and absent molecular recombination or photoionization. Otherwise, molecular recombination of radicals reduces the depth of ablation per pulse, owing to thermal stabilization on forming branched molecules. Radical formation through photoionization can be compensated for by recombination, but it accelerates ablation at high enough photoionization rates. Finally, the influence of molecular recombination on the ablation depth is found to decrease for long pulses, owing to decreased recombination reaction rates at lower vapor densities near the ablated surface. These and other findings highlight competition between the processes of scission, recombination, and photoionization during pulsed ablation of polyethylene.

Moore, N. W. (ORCID:0000000272259674)↗
NASA NTRS1986

Review and analysis of high temperature chemical reactions and the effect of non-equilibrium conditions

Chemical reactions at high temperatures have been considered extensively because of their importance to the heating effects on re-entry of space vehicles. Data on these reactions however, are not abundant and even when found there are discrepancies in data collected by various investigators. In particular, data for recombination reactions are calculated from the dissociation reactions or vice versa through the equilibrium constant. This involves the use of the principle of detailed balancing. This principle is discussed in reference to conditions where it is valid as well as to those where it is not valid. Related topics that merit further study or for which applicable information was available are briefly mentioned in an appendix to this report.

Johnson, R. E.↗
NASA NTRS1986

Theoretical analysis of isotope effects on ozone formation in oxygen photochemistry

In situ measurements of stratospheric ozone and laboratory studies of ozone production in electric discharge through oxygen have shown previously that ozone containing heavy isotopes of oxygen (O-17, O-18) may be formed preferentially. In order to assess the relevance of thee latter experiment to the stratospheric measurements, detailed understanding of the effect of isotopic substitution on the O3 formation reaction O + O2 + M yields O3 + M and on the O atom exchange reaction O + O2 + O yields O2 + O is necessary. In this work, an estimate of the effect of isotopic substitution on the recombination rate is made by us of approximate dynamical theories and statistical mechanics. The results indicate the possibility of isotope effects on the O + O2 recombination rate of the order of several percent at stratospheric temperatures. In general, recombination reactions involving heavy (mass 49, 50) O3 formation are found to be slower than the reaction leading to normal (mass 48) O3 formation. The calculated isotope effects are sufficiently small that the uncertainties in the model input and the approximations in the dynamical theories will probably make the quantitative nature of these results subject to considerable uncertainty. This isotope effect should not be observable in the atmosphere given the precision of the current measurements but may be crucial in the understanding of the laboratory experiments, where observed enhancements are only of the order of several percent. Possible reasons for this discrepancy between the observed enhancement and predicted depletion are presented.

Kaye, J. A.↗
NASA NTRS2005

Experimental Study on Interactions Between H Atoms and Organic Haze

In Titan s atmosphere composed of N2 and CH4, irradiations of both solar ultraviolet light and charged particles induce active chemical reactions. In the processes of these reactions, a large amount of hydrogen (H) atoms are expected to be formed by dissociation of CH4 and other hydrocarbons [e.g., 1, 2]. Theoretical models suggest that these active H atoms need to be converted to stable hydrogen molecules (H2) efficiently to maintain unsaturated hydrocarbons and organic haze in Titan s atmosphere [e.g., 1]. Furthermore, molecular hydrogen is an important greenhouse effect gas in Titan s atmosphere, and small variation in its abundance strongly affects Titan s surface temperature [3]. Thus, the formation of H2 molecules from H atoms is a key reaction for both the atmospheric chemistry and the surface environment of Titan. Although several numerical calculations have been conducted to investigate the atmospheric chemistry of Titan with hypothesized recombination reactions of H atoms, such as catalytic scheme of C4H2 [e.g., 1, 2], it is still unclear what chemical reaction is responsible for the conversion of H atoms to H2 molecules in Titan s atmosphere.

Sekine, Y.↗
DOE OSTI2025

Correlated solvent coordinates accelerate multi-donor proton-coupled electron transfer

The rate of charge transfer within a discrete donor–acceptor (D/A) pair is well-described by semi-classical electron transfer theory, but the effects of multiple equivalent redox sites remain less understood. We report a series of ground-state intramolecular proton-coupled electron transfer (PCET) complexes designed to isolate the effects of donor number, N, while holding geometry, coupling, and driving forces constant. The [Ru(L) 3−N (OH) N ] 2+ complexes incorporate one, two, or three identical phenolic electron donors linked to Ru through rigid phenanthroline bridges (OH = 2,4-di-tert-butyl-6-(1-methyl-1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)phenol). Upon flash photolysis and oxidative quenching with methyl viologen (MV 2+ ), the transient Ru(III) oxidizes an appended phenol by PCET with the hydrogen-bonded imidazole nitrogen atom functioning as the base. The rate increased by 3.4-fold and 5.7-fold (1.7-fold and 1.9-fold after statistical correction) for two- and three-donor complexes compared to the single-donor system. The supra-statistical acceleration is attributed to a reduced effective outer sphere reorganization energy (λ m ) modeled by a partially shared solvent reaction coordinate, in which a subset of solvent dipoles is already oriented to stabilize charge from any donor. The final phenoxyl radical state is localized due to the transfer of a proton, and the recombination reaction with the viologen radical is not accelerated. These results demonstrate the effects of solvent dynamics on intramolecular PCET rates, offering a new strategy for the design of synthetic charge transfer systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1986

Kinetic study of the reaction CH (X 2Pi) + H2 yields CH2 (X 3B1) + H in the temperature range 372 to 675 K

The kinetics of the reversible reaction CH (X 2Pi) + H2 yields CH2 (X 3B1) + H at 372-675 K and total pressure 100 torr (mainly Ar) is investigated experimentally. The ground-state CH radicals are produced by photolysis of CHBr3 using 10-mJ 266-nm laser pulses (repetition rate 10 Hz) and monitored by measuring the fluorescence induced by a 429.8-nm dye laser, in the apparatus described by Berman et al. (1982) and Berman and Lin (1984). The results are presented in tables and graphs and characterized. The absolute rate constants for the forward and reverse reactions are determined, and their temperature dependence is given by Arrhenius expressions and formulas obtained in transition-state-theory calculations. The heat of formation of CH2 at 0 K is estimated (assuming that the recombination reaction CH2 + H has zero activation energy) as 92.6 + or - 0.5 kcal/mol.

Zabarnick, S.↗
NASA NTRS1991

Theoretical studies of important processes in planetary and comet atmospheres

This is the fifth semi-annual progress report describing research on dissociative recombination reactions in planetary and comet atmospheres. The Appendix has two papers that describe NASA supported research. Both papers have been recently accepted for publication. The first paper, 'The Generation of O(S-1) from the Dissociative Recombination of O2(+)', describes in detail the Multichannel Quantum Defect (MQDT) theory used for the calculation of dissociative recombination (DR) cross sections and rates. The application to the generation of the upper state of the atomic oxygen green line emission is of great importance to the modelling of planetary atmospheres. The second paper in the Appendix, 'Dissociative Recombination of the Ground State of N2(+)', applies the methods described in the first paper to N2(+). We find remarkable agreement with the prior microwave afterglow experiments for both the total recombination rate and for its electron temperature dependence. However, the results disagree with recent merged beams results which gave cross sections that are a factor of five below the microwave afterglow experiments and the current results. DR of N2(+) is an important mechanism for generating energetic N atoms which can escape the atmosphere of Mars. Currently we are also continuing additional work on the DR of O2(+) which is aimed at calculating both the total DR rate as a function of ion vibrational level and the rate for production of O(D-1).

Guberman, Steven L.↗
NASA NTRS1991

Finite-rate chemistry effects in a Mach 2 reacting flow

UV spontaneous vibrational Raman scattering and laser-induced predissociative fluorescence (LIPF) are combined and applied to a supersonic flame. For the first time, simultaneous measurements of temperature, major species (H2, O2, N2, H2O), and minor species (OH) concentrations are obtained with a 'single' excimer laser in a supersonic-lifted hydrogen-air diffusion flame. In the supersonic flame, a small amount of reaction occurs upstream of the lifted flame base, due to shock wave interactions and mixing with hot vitiated air. The strong turbulent mixing and high total enthalpy fluctuations lead to nonequilibrium values of temperature, and major and minor species concentrations. Combustion occurs farther downstream of the lifted region where slow three-body recombination reactions result in superequilibrium OH concentrations that depress the temperatures below their equilibrium values. Farther downstream, ambient air entrainment contaminates flame properties.

Cheng, T. S.↗
NASA NTRS1994

New sources for the hot oxygen geocorona

This paper investigates new sources of thermospheric non thermal (hot) oxygen due to exothermic reactions involving numerous minor (ion and neutral) and metastable species. Numerical calculations are performed for low altitude, daytime, winter conditions, with moderately high solar activity and low magnetic activity. Under these conditions we find that the quenching of metastable species are a significant source of hot oxygen, with kinetic energy production rates a factor of ten higher than those due to previously considered O2(+) and NO(+) dissociative recombination reactions. Some of the most significant new sources of hot oxygen are reactions involving quenching of O(+)((sup 2)D), O((sup 1)D), N((sup 2)D), O(+)((sup 2)P) and vibrationally excited N2 by atomic oxygen.

Richards, P. G.↗
NASA NTRS1988

Computed potential energy surfaces for chemical reactions

The minimum energy path for the addition of a hydrogen atom to N2 is characterized in CASSCF/CCI calculations using the (4s3p2d1f/3s2p1d) basis set, with additional single point calculations at the stationary points of the potential energy surface using the (5s4p3d2f/4s3p2d) basis set. These calculations represent the most extensive set of ab initio calculations completed to date, yielding a zero point corrected barrier for HN2 dissociation of approx. 8.5 kcal mol/1. The lifetime of the HN2 species is estimated from the calculated geometries and energetics using both conventional Transition State Theory and a method which utilizes an Eckart barrier to compute one dimensional quantum mechanical tunneling effects. It is concluded that the lifetime of the HN2 species is very short, greatly limiting its role in both termolecular recombination reactions and combustion processes.

Walch, Stephen P.↗
DOE OSTI2024

Structural Gating Enhances Long-Distance Light-Driven Interfacial Electron Transfer

Structural gating provides a molecular means to transfer electrons preferentially in one desired vectorial direction, a behavior needed for applications in artificial photosynthesis. At the interfaces utilized herein, visible-light absorption by a transition metal complex opens a “structural gate” by planarization of otherwise rotating phenyl rings in p-phenylene ethynylene (PE) bridge units. Planarization provides a conjugated pathway for electron flow toward a conductive oxide surface. Interfacial electron transfer to the oxide restores rotation and closes the gate to the unwanted recombination reaction. This structural gating results in nearly quantitative long-distance (>20 Å) interfacial electron transfer that occurs ~1000 times faster than transfer in the opposite direction. A comparative kinetic study of these complexes with those that contain ionic bridge units, without gating function, as a function of the applied potential and hence –ΔG° provided a physical basis for the structural gating. A small distance-dependent reorganization energy with weak electronic coupling underlies the success of this gate that enables efficient long-distance electron transfer and slow recombination.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Adsorption and reaction of methanol on Fe 3 O 4 (001)

The interaction of methanol with iron oxide surfaces is of interest due to its potential in hydrogen storage, and from a fundamental perspective as a chemical probe of the reactivity. We present here a study examining the adsorption and reaction of methanol on magnetite Fe 3 O 4 (001) at cryogenic temperatures using a combination of temperature programmed desorption, x-ray photoelectron spectroscopy, and scanning tunneling microscopy. We report the methanol desorption profile from Fe 3 O 4 (001) is complex, exhibiting peaks at 145, 175, 238, and 273 K, corresponding to the desorption of intact methanol, as well as peaks at 350 and 500 K due to the reaction of methoxy intermediates. The saturation of a monolayer of methanol corresponds to ~5 molecules/unit cell (u.c.), which is slightly higher than the number of surface octahedral iron atoms of 4/u.c. We probe the kinetics and thermodynamics of the desorption of molecular methanol using inversion analysis. The deconvolution of the complex desorption profile into individual peaks allows for calculations of both the desorption energy and the prefactor of each feature. The initial 0.7 methanol/u.c. reacts to form a methoxy and hydroxy intermediates by 180 K, which remains on the surface above room temperature after intact methanol has desorbed. The methoxy species react via one of two channels, a recombination reaction with surface hydroxyls to form additional methanol at ~350 K, and a disproportionation reaction to form methanol and formaldehyde at ~500 K. Only 20% of the methoxy species undergo the disproportionation reaction, with most of them reacting via the 350 K pathway.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Reaction Pathways and Energy Consumption in NH 3 Decomposition for H 2 Production by Low Temperature, Atmospheric Pressure Plasma

Pathways for NH 3 decomposition to N 2 and N 2 H 4 by atmospheric pressure nonthermal plasma are analyzed using a combination of molecular beam mass spectrometry measurements and zero-dimensional kinetic modeling. Experimental measurements show that NH 3 conversion and selectivity towards N 2 formation scale monotonically with the specific energy input into the plasma with ~ 100% selectivity to N 2 formation achieved at specific energy inputs above 0.12 J cm −3 (3.1 eV (molecule NH 3 ) −1 ). The kinetic model recovers these trends, although it underpredicts N 2 selectivity at low specific energy input. These discrepancies can be explained by the underestimation of reaction rate coefficients for reactions that consume N 2 H x species in collisions with H radicals and/or radial nonuniformities in power deposition, gas temperature, and species concentrations that are not represented by the plug flow approximation used in the model. The kinetic model shows that N 2 formation proceeds through N 2 H x decomposition pathways rather than NH x decomposition pathways in low temperature, atmospheric pressure plasma. Higher selectivity toward N 2 production can be achieved by operating at higher NH 3 conversion and with a higher gas temperature. Furthermore, the high energy cost of NH 3 decomposition by atmospheric pressure nonthermal plasma found in this work (25–50 eV (molecule NH 3 converted) −1 ; 17–33 eV (molecule H 2 formed) −1 ) is a result of the energy requirement for electron-impact dissociation of NH 3 and the significant re-formation of NH 3 by three-body recombination reactions between NH 2 and H.

Nonthermal plasma↗
DOE OSTI2023

Non-Oxidative Ethane Dehydrogenation in a Packed-Bed DBD Plasma Reactor

Abstract Plasma-assisted conversion of ethane (C 2 H 6 ) can produce value-added chemical building blocks using green electricity. Here we employ a simple packed-bed coaxial dielectric barrier discharge (DBD) reactor to convert C 2 H 6 at mild operating conditions unattainable by conventional thermocatalysis. Ethylene (C 2 H 4 ), acetylene (C 2 H 2 ), and methane (CH 4 ) are the main products along with small fractions of C 3 and C 4 hydrocarbons. Interestingly, the C 2 H 4 selectivity is primarily correlated to C 2 H 6 conversion, dominated by electron dissociation and recombination reactions irrespective of the dielectric properties of the packed bed material (SiO 2 , Al 2 O 3 , ZrO 2 , TiO 2 , and BaTiO 3 ), packing material size, supplied power, and C 2 H 6 concentration. While a distortion of the electric field and discharge propagation results in varying dissipated power as materials change, the C 2 H 4 energy yield remains constant. The particle size appears to affect conversion mainly due to pressure alterations. Pd/SiO 2 catalyst can change the selectivity, favoring saturated species by expending hydrogen.

Engineering↗
DOE OSTI2023

Nanosecond time-resolved infrared spectroscopy for the study of electron transfer in photosystem I

Abstract Microsecond time-resolved step-scan FTIR difference spectroscopy was used to study photosystem I (PSI) from Thermosynechococcus vestitus BP-1 ( T. vestitus, formerly known as T. elongatus ) at 77 K. In addition, photoaccumulated (P700 + –P700) FTIR difference spectra were obtained at both 77 and 293 K. The FTIR difference spectra are presented here for the first time. To extend upon these FTIR studies nanosecond time-resolved infrared difference spectroscopy was also used to study PSI from T. vestitus at 296 K. Nanosecond infrared spectroscopy has never been used to study PSI samples at physiological temperatures, and here it is shown that such an approach has great value as it allows a direct probe of electron transfer down both branches in PSI. In PSI at 296 K, the infrared flash-induced absorption changes indicate electron transfer down the B- and A-branches is characterized by time constants of 33 and 364 ns, respectively, in good agreement with visible spectroscopy studies. These time constants are associated with forward electron transfer from A 1 – to F X on the B- and A-branches, respectively. At several infrared wavelengths flash-induced absorption changes at 296 K recover in tens to hundreds of milliseconds. The dominant decay phase is characterized by a lifetime of 128 ms. These millisecond changes are assigned to radical pair recombination reactions, with the changes being associated primarily with P700 + rereduction. This conclusion follows from the observation that the millisecond infrared spectrum is very similar to the photoaccumulated (P700 + –P700) FTIR difference spectrum.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2022

Time-resolved measurements of HO 2 radical in a heated plasma flow reactor

Time-resolved, absolute HO 2 number density in diluted H 2 –O 2 –Ar, CH 4 –O 2 -Ar, and C 2 H 4 –O 2 –Ar mixtures excited by a repetitive ns pulse discharge in a heated plasma flow reactor is measured by Cavity Ringdown Spectroscopy (CRDS). The experimental results are obtained at $\textit{T}$ = 300-600 K and $\textit{P}$ = 130 Torr, both during the discharge pulse burst and in the afterglow. In this work, the HO 2 number density is inferred from the CRDS data using a spectral model exhibiting good agreement with previous measurements of absolute HO 2 absorption cross sections. In the room-temperature H 2 –O 2 mixture, as well as in CH 4 –O 2 and C 2 H 4 –O 2 mixtures over the entire temperature range studied, HO2 is generated only during the discharge burst and decays in the afterglow. However, in the H 2 –O 2 mixture at elevated temperatures, $\textit{T}$ = 400-600 K, HO 2 persists in the afterglow up to 10 ms after the discharge burst, comparable with the flow residence time in the reactor. Comparison with kinetic modeling shows that the sustained reactivity after the source of radicals is turned off is due to a chain propagation / hydrogen oxidation process, which dominates the radical recombination reactions. The kinetic modeling predictions are in good agreement with the relative HO 2 number density measured in all three mixtures, although the model underpredicts the absolute number densities in H 2 –O 2 at $\textit{T}$ = 400-600 K by up to a factor of two. Detection of the sustained low-temperature reactivity in H 2 –O 2 , initiated by the radical generation in the plasma, suggests that the plasma excitation may also affect kinetics of oxidation and reforming of fuels exhibiting low-temperature chemistry below hot ignition point.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Radiation-Induced Long-Lived Transients and Metal Particle Formation in Solid KCl–MgCl 2 Mixtures

Here, the electron paramagnetic resonance and diffuse reflectance-optical absorption spectra of room-temperature γ-irradiated KCl–MgCl 2 binary solid salt mixtures (98:2 mol % and 2:98 mol %) and the eutectic (68:32 mol %) are reported. Additionally, powder X-ray diffraction of the pristine salts and thermal annealing studies of the irradiated salts were performed to evaluate the radiolysis product stability, annihilation, and association in metallic particles. The main long-lived transient species detected in 98:2 mol % KCl–MgCl 2 salts were perturbed F-centers, that is, trapped electrons (e t – ) in the vicinity of Mg ions (λ max at 561 nm), and the radiolytic reduction of Mg 2+ to Mg + and Mg 0 . Thermal annealing promoted the diffusion of defects to yield polycations (Mg n + ). On the other hand, irradiation of 2:98 mol % KCl/MgCl 2 salts showed the formation of cationic and neutral Mg dimers (Mg 2 + and Mg 2 ) and trimers as well as centers with a rhombic powder pattern apparently consisting of an electron shared between three Mg 2+ nuclei associated with an anion vacancy (ν a + –Mg 3 5+ ). Trapped electrons (e t – ) (F-centers) were not observed in the irradiated eutectic mixture; instead, Mg 2 , Mg 0 , and Cl 3 – were observed. It was observed that the higher temperature for thermal ionization of radiation-reduced Mg species decreased the extent of electron recombination reactions and the disproportionation of Cl 3 – compared to the pure KCl but enhanced the aggregation of Mg into larger metallic microstructures (metallic particles).

36 MATERIALS SCIENCE↗
DOE OSTI2023

High-pressure CO 2 dissociation with nanosecond pulsed discharges

Herein we investigate the conversion of CO 2 into CO and O 2 with nanosecond repetitively pulsed (NRP) discharges in a high-pressure batch reactor. Stable discharges are obtained at up to 12 bar. By-products are measured with gas chromatography. The energy efficiency is determined for a range of processing times, pulse energy, and fill pressures. It is only weakly sensitive to the plasma operating parameters, i.e the extent of CO 2 conversion is almost linearly-dependent on the specific energy invested. A conversion rate as high as 14% is achieved with an energy efficiency of 23%. For long processing times, saturation in the yield and a drop in efficiency are observed, due to the increasing role of three-body recombination reactions, as described by zero-dimensional detailed kinetic modeling. The modeling reveals the presence of three-stage kinetics between NRP pulses, controlled by electron-impact CO 2 dissociation, vibrational relaxation, and neutral elementary kinetics. Transport effects are shown to be important for CO 2 conversion at high pressures. For fill pressures beyond 10 bar, CO 2 may locally transit into supercritical states. The supercritical plasma kinetics may bypass atomic oxygen pathways and directly convert CO 2 into O 2 . This work provides a detailed analysis of plasma-based high-pressure CO 2 conversion, which is of great relevance to future large-scale sustainable carbon capture, utilization, and storage.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗