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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 OSTI2026

Photoinduced Electron-Transfer Distance Is Controlled by the Driving Force in Solid-State Organic Donor-Acceptor Systems

Understanding how short-range electron transfer generates photocurrent in organic solar cells and what photochemical driving force is needed to maximize efficiencies has been an immensely difficult problem. Here we show how these questions are intertwined: the driving force controls the average charge-transfer distance, skipping the shortest-range states in the optimal case. Using photoinduced absorption-detected magnetic resonance, we measure average charge-separation distances in dilute donor–acceptor blends as a function of driving force, and compare these with free charge yields measured by time-resolved microwave conductivity. We find that the largest driving forces show the shortest charge-separation distances, explaining their suppressed free-carrier generation. These results support a long-range electron-transfer model, wherein the driving force controls the initial delocalization of the electron–hole pair. The minimum driving force required for efficient charge separation (and minimal voltage losses) is ultimately set by the dielectric constant of the material.

14 SOLAR ENERGY↗
DOE OSTI2021

Directional dependency of electronic stopping in nickel, projectile’s excited charge state and momentum transfer

Abstract We studied the directional dependency of electronic stopping power of swift light ions in nickel using real-time time-dependent density functional theory. We report a variation of electronic stopping for moving ions as the projectile probes different electronic densities of the host material. These results show that while the predicted magnitude stays in reasonable agreement with experiment, for $$v > 2$$ v > 2 . a.u. simulating only low index crystallographic directions is not enough to sample the experimental average values. The ab initio simulations give us access to microscopic quantities, such as non-adiabatic forces, momentum transfer and transient excited state charges of the projectile and host ions, which are not available through other methods. We report these quantities for the first time.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2021

Proton-Coupled Defects Impact O–H Bond Dissociation Free Energies on Metal Oxide Surfaces

Proton-coupled electron transfer (PCET) reactions on metal oxides require coupling between proton transfer at the solid-liquid interface and electron transfer involving defects at or near the band edge. Herein, hybrid functional periodic density functional theory is used to elucidate the impact of proton-coupled defects on the bond dissociation free energies (BDFEs) of O-H bonds on anatase TiO 2 surfaces. These O-H BDFEs are directly related to interfacial PCET thermochemistry. Comparison between geometrically similar O-H bonds associated with different defect types, namely conduction d-band electrons or valence p-band holes, reveals that the BDFEs differ by ~81 kcal/mol (3.50 eV), comparable to the wide TiO 2 band gap. These differences are shown to be determined primarily by differences in electron transfer driving forces, which are analyzed by using band energies and inner-sphere reorganization energies within a Marcus theory framework. Furthermore, these fundamental insights about the impact of proton-coupled defects on PCET thermochemistry at semiconductor surfaces have broad implications for electrocatalysis.

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

Deciphering Charge Transfer Processes in Transition Metal Complexes from the Perspective of Ultrafast Electronic and Nuclear Motions

Chemical transformations in charge transfer states result from the interplay between electronic dynamics and nuclear reorganization along excited-state trajectories. Here, in this study, we investigate the ultrafast structural dynamics following photoinduced electron transfer from the metal-metal-to-ligand charge transfer state of an electron donor, a Pt dimer complex, to a covalently linked electron acceptor group using ultrafast time-resolved wide-angle X-ray scattering and optical transient absorption spectroscopy methods to disentangle the interdependence of the excited-state electronic and nuclear dynamics. Following photoexcitation, Pt-Pt bond formation and contraction takes up to 1 ps, much slower than the corresponding process in analogous complexes without electron acceptor groups. Because the Pt-Pt distance change is slow with respect to excited-state electron transfer, it can affect the rate of electron transfer. These results have potential impacts on controlling electron transfer rates via structural alterations to the electron donor group, tuning the charge transfer driving force.

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

NSTXU Diagnostic Disruption Dynamic Loading Represented by Response Spectra

This article presents the results of transient dynamic simulations of loads due to disruption eddy currents on the NSTXU vacuum vessel. Dynamic loading at diagnostic mounting locations is expressed as response spectra derived from the time history results of the dynamic structural simulations of a variety of disruption scenarios. The disruption simulations draw on a history of the project assessments of worst case disruptions for specific components. Major efforts to assess disruption loading have included the vacuum vessel which is the major structural support for the machine, as well as the passive plates (PPs), high harmonic fast wave (HHFW) antenna, and centerstack casing. Each one of these efforts included transient electromagnetic simulations producing time-dependent eddy current Lorentz loads (and in some cases halo loads) which then were applied to time-dependent structural dynamic analyses intended to obtain the proper dynamic amplification factors. In some instances, the EM model and structural model were identical allowing direct transfer of EM forces to the structural model. In other cases, the EM and structural model were not identical and the vector potential (VP) transfer method was used. The results files from these analyses were available (or re-run) to post process in ANSYS Classic time history postprocessor. In conclusion, the ANSYS command is used to create response spectra from time history data at desired points on the vessel.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

Self-aligning swaging punch and method for swaging

A self-aligning swaging punch is provided for adjoining together a pin and a workpiece having a pinhole. The swaging punch comprises a shaft and a tip having an open-ended cavity. The shaft transfers an axial force from a press or hammer to the tip. The tip transfers the axial force from the shaft to a top surface of the workpiece near the pinhole. The tip may be spherical in shape for evenly deforming the workpiece near the upper edge of the pinhole. The open-ended cavity receives the pin therein and allows the pin to extend at least partially into the open-ended cavity while the tip contacts the workpiece. The open-ended cavity also ensures that the swaging punch is aligned with the pinhole and perpendicular to the surface of the workpiece when the pin is inserted into the pinhole such that the workpiece evenly deforms and presses against the pin.

42 ENGINEERING↗
DOE OSTI2021

QXF Cold Mass Friction Test

The purpose of this test is to determine the longitudinal force on the stainless steel shell that is required to break the friction between the stainless steel shell and the aluminum shell underneath. This is achieved by suspending the short magnet between two backing plates on the eight grade 8 threaded rods which replace the 8 stainless steel toke tie rods in the magnet assembly. Eight hydraulic cylinders are mounted to each backing plate. Six, 22" sections of the stainless steel shell are welded in place on the magnet, each of which has a slightly different developed length, starting from the baseline design, and becoming progressively looser. A pushing flange is mounted to the stainless steel shells via small tack welds. Four load cells are then attached to the flange at designated points, while additional tooling is attached to each of the cylinders to facilitate the transfer of force from the cylinders to the stainless steel shell During the test, the cylinders will be slowly energized to no more than 7000 psi, with a resultant, longitudinal force on the flange of no more than 125,400 lbf. For each set of shells, the test ends when the load cell monitoring software detects a sharp decrease in force. This will indicate that the friction has broken. The cullenders will then be de-energized and the cycle will repeat for the next test. Welding operations outlined in the procedure (US-HiLumi-doc-4200) are outside the scope of the HA as the work is occurring under a previously-issued burn permit by qualified welders.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

In situ Stress Monitoring Reveals Tension and Wrinkling Evolutions during Halide Perovskite Film Formation

Metal halide perovskites (MHPs) are promising candidates for next-generation thin film photovoltaics and high-performance tandems. Solution-processed MHP films are susceptible to residual stress that can induce undesirable surface wrinkles. However, the origins and evolution of stress during solution processing remain elusive. Here, in this work, we utilize multimodal in situ characterizations, including substrate curvature, reflectance, absorbance, and photoluminescence, to monitor stress and morphology evolution during MHP film formation. A film formation model emerges, consisting of a perovskite top crust on a semirigid sol with the ability to transfer mechanical forces. The phase transformation induces tension in the MHP crust, while shrinkage of the sol causes additional compression and surface wrinkles. Wrinkle-free films are formed through dynamically balancing forces between the crust and the sol. This study provides a powerful toolkit for the fast-growing area of stress engineering in MHP photovoltaics to achieve dynamic control of film stress and surface morphology.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Elucidation of Marcus Relationships for Hydride Transfer Reactions Involving Transition Metal Hydrides

The rate of hydride transfer from three Ir hydride complexes of the type Cp*Ir( R bpy)H + (Cp* = C 5 Me 5 ; R bpy = 4,4′-R-2,2′-bipyridine, R = OMe, H, CO 2 Me) to six N-methylacridinium ( R Acr + ) acceptors with electronically different substituents in the 2- or 2,7-positions were measured. Using the thermodynamic hydricity of the donors and the hydride affinity of the acceptors the thermodynamic driving forces for hydride transfer were determined. Brønsted plots, which correlate kinetic and thermodynamic hydricity, demonstrate distinct linear free energy relationships for each complex, with different Brønsted α values. Thus, at the same driving force hydride transfer from Cp*Ir( OMe bpy)H + is faster than for Cp*Ir(bpy)H + or Cp*Ir( CO2Me bpy)H + . Experimental and computational analyses are consistent with a concerted hydride transfer mechanism for all Ir complexes. As the thermodynamic driving force increases an earlier transition state is observed and all transition states also include π-stacking interactions between the donor and acceptor, which likely contribute to the different α values. The experimental data fits well to the Marcus model, enabling the determination of reorganization energies (λ) that range from 58 to 69 kcal mol -1 . These are lower than λ values for hydride transfer reactions involving organic donors and acceptors. This work provides a rare example of the correlation of kinetic and thermodynamic hydricity using only experimental data and shows that hydride transfer reactions involving metal hydrides can follow Marcus theory. Furthermore, the findings offer insight into controlling metal-catalyzed hydride transfer reactions, which is valuable for designing improved systems for a range of transformations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Controlling and Optimizing Photoinduced Charge Transfer across Ultrathin Silica Separation Membrane with Embedded Molecular Wires for Artificial Photosynthesis

Ultrathin amorphous silica membranes with embedded organic molecular wires (oligo( p -phenylenevinylene), three aryl units) provide chemical separation of incompatible catalytic environments of CO 2 reduction and H 2 O oxidation while maintaining electronic and protonic coupling between them. For an efficient nanoscale artificial photosystem, important performance criteria are high rate and directionality of charge flow. Here, the visible-light-induced charge flow from an anchored Ru bipyridyl light absorber across the silica nanomembrane to Co 3 O 4 water oxidation catalyst is quantitatively evaluated by photocurrent measurements. Charge transfer rates increase linearly with wire density, with 5 nm -2 identified as an optimal target. Accurate measurement of wire and light absorber densities is accomplished by the polarized FT-IRRAS method. Guided by density functional theory (DFT) calculations, four wire derivatives featuring electron-donating (methoxy) and -withdrawing groups (sulfonate, perfluorophenyl) with highest occupied molecular orbital (HOMO) potentials ranging from 1.48 to 0.64 V vs NHE were synthesized and photocurrents evaluated. Charge transfer rates increase sharply with increasing driving force for hole transfer from the excited light absorber to the embedded wire, followed by a decrease as the HOMO potential of the wire moves beyond the Co 3 O 4 valence band level toward more negative values, pointing to an optimal wire HOMO potential around 1.3 V vs NHE. Comparison with photocurrents of samples without nanomembrane indicates that silica layers with optimized wires are able to approach undiminished electron flux at typical solar intensities. Combined with the established high proton conductivity and small-molecule blocking property, the charge transfer measurements demonstrate that oxidation and reduction catalysis can be efficiently integrated on the nanoscale under separation by an ultrathin silica membrane.

photocurrent measurements↗
DOE OSTI2022

Compositionally Tuning Electron Transfer from Photoexcited Core/Shell Quantum Dots via Cation Exchange

It is critical to find methods to control the thermodynamic driving force for photoexcited charge transfer from quantum dots (QDs) and explore how this affects charge transfer rates, since the efficiency of QD-based photovoltaic and photocatalysis technologies depends on both this rate and the associated energetic losses. In this work, we introduce a single-pot shell growth and Cu-catalyzed cation exchange method to synthesize Cd x Zn 1-x Se/Cd y Zn 1-y S QDs with tunable driving forces for electron transfer. Functionalizing them with two molecular electron acceptors—naphthalenediimide (NDI) and anthraquinone (AQ)—allowed us to probe nearly 1 eV of driving forces. For AQ, at lower driving forces, we find that higher Zn content results in a 130-fold increase of electron transfer rate constants. However, at higher driving forces electron transfer dynamics are unaltered. Here, the data are understood using an Auger-assisted electron transfer model and analyzed with computational work to determine approximate binding geometries of these electron acceptors. Our work provides a method to tune QD reducing power and produces useful metrics for optimizing QD charge transfer systems that maximize rates of electron transfer while minimizing energetic losses.

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

Conjugate Heat Transfer Analysis of the Supercritical CO2 Based Counter Flow Compact 3D Heat Exchangers

Compact heat exchangers using supercritical fluids such as CO2 are preferred due to their high heat transfer capacity and smaller footprint. Three-dimensional conjugate forced convection heat transfer analysis was performed on several shell-and-tube counter-flow microchannel heat exchangers. Numerical simulations were conducted to test effect of change in mass flow rate, hydraulic diameter and various cross sections on the heat transfer. Increasing mass flow rate improved heat transfer up to a maximum value and then decreased downstream with increasing turbulence. Maximum heat transfer was obtained for the micro channel with the smallest hydraulic diameter. Amongst the cross sections analyzed (circular, square, circular with radial ribs, and square with radial ribs), the most uniform distribution of temperature and maximum heat transfer were obtained for circular cross section with radial ribs. An optimally efficient operation of such a heat exchanger can be attained by considering these factors during multi-objective constrained optimization of geometric parameters and requirements for additive manufacturing of such compact heat exchangers.

Chitale, Janhavi↗
DOE OSTI2020

Conjugate Heat Transfer Analysis of the Supercritical CO2 Based Counter Flow Compact 3D Heat Exchangers

Compact heat exchangers using supercritical fluids such as CO2 are preferred due to their high heat transfer capacity and smaller footprint. Three-dimensional conjugate forced convection heat transfer analysis was performed on several shell-and-tube counter-flow microchannel heat exchangers. Numerical simulations were conducted to test effect of change in mass flow rate, hydraulic diameter and various cross sections on the heat transfer. Increasing mass flow rate improved heat transfer up to a maximum value and then decreased downstream with increasing turbulence. Maximum heat transfer was obtained for the micro channel with the smallest hydraulic diameter. Amongst the cross sections analyzed (circular, square, circular with radial ribs, and square with radial ribs), the most uniform distribution of temperature and maximum heat transfer were obtained for circular cross section with radial ribs. An optimally efficient operation of such a heat exchanger can be attained by considering these factors during multi-objective constrained optimization of geometric parameters and requirements for additive manufacturing of such compact heat exchangers.

Chitale, Janhavi↗
DOE OSTI2022

Why the Forcing from Carbon Dioxide Scales as the Logarithm of Its Concentration

Abstract The radiative forcing from carbon dioxide is approximately logarithmic in its concentration, producing about 4 W m −2 of global-mean forcing for each doubling. Although these are basic facts of climate science, competing explanations for them have been given in the literature. Here, the reasons for the logarithmic forcing of carbon dioxide are explored in detail and a simplified model for the forcing is constructed. An essential component is the particular distribution of absorption coefficients within the 15- μ m band of carbon dioxide. An alternative explanation, which does not depend on the spectrum of carbon dioxide but instead hinges on the tropospheric lapse rate, is shown to be neither necessary nor sufficient to explain the logarithmic forcing of carbon dioxide and to be generally inapplicable to well-mixed greenhouse gases in Earth’s atmosphere.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2024

Toward transferable empirical valence bonds: Making classical force fields reactive

The empirical valence bond technique allows classical force fields to model reactive processes. However, parametrization from experimental data or quantum mechanical calculations is required for each reaction present in the simulation. We show that the parameters present in the empirical valence bond method can be predicted using a neural network model and the SMILES strings describing a reaction. This removes the need for quantum calculations in the parametrization of the empirical valence bond technique. In doing so, we have taken the first steps toward defining a new procedure for enabling reactive atomistic simulations. This procedure would allow researchers to use existing classical force fields for reactive simulations, without performing additional quantum mechanical calculations.

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

Radiative heat transfer in FLiBe molten salt participating medium in a vertical heated tube under forced and mixed convection laminar flows

The contribution of radiative heat transfer (RHT) to convective heat transfer in a heated vertical pipe with laminar flow and constant wall temperature, for FLiBe molten salt, is investigated computationally. The combined effects of conduction in the fluid, forced convection, buoyancy, temperature-dependent physical properties, and thermal radiation are investigated. The P1 approximation is employed in the discretization of the Radiative Transfer Equation (RTE). The COMSOL Multiphysics software is used to generate the numerical solutions. The theoretical analysis developed here demonstrates that only for intermediate values of the optical thickness, i.e. τ D ~ O 1 , the participating media effects are expected to be important. This analysis is confirmed computationally and a value of τ D ≈ 4 is shown to lead to the highest increase in overall heat transfer behavior. Under forced convection, the Nusselt ratio N u total / N u no - rad reaches a peak value of 1.76 at τ D = 4 and z/D = 200 , and is less than 1.1 for τ D < 0 . 1 and τ D > 60 . Under aiding-flow mixed convection, N u total / N u no - rad reaches a peak of 1.34 at τ D = 4 . 2 and is less than 1.1 for τ D < 0 . 4 and τ D > 36 . Under opposing-flow mixed convection, Nu total / Nu no - rad reaches a peak of 1.81 at τ D = 4 . 2 and is less than 1.1 for τ D < 0 . 2 and τ D > 50 . RHT effects on the overall heat transfer are most pronounced in opposing mixed convection, where N u total / N u no - rad is 2.26 observed at z/D = 50 . The sensitivity to wall emissivity is evaluated; in forced convection, the peak N u total / N u no - rad is 1.66 at ε = 0 (reflective wall) and 2.04 at ε = 1 (absorptive wall). Finally, the sensitivity to pipe diameter is also discussed. Overall, for a vertical heated tube, radiative heat transfer effects lead to enhancement of heat transfer by as high as a factor of two, and they depend on the optical thickness of the flow, mixed convection environment ( Gr / R e 2 and direction of flow relative to the gravitational force), surface emissivity, and entrance effects.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2024

The Driving Force Controls Charge Separation Distance in Solid-State Organic Donor/Acceptors

In this work, charge pairs of varying separation distance are interpreted from photoinduced absorption detected magnetic resonance (PADMR) spectroscopy of organic, small molecule dilute donor/acceptor thin films. We report that a donor/acceptor film that generates few free charges at room temperature yet has a relatively large, calculated driving force for electron transfer generates a large concentration of tightly bound CT states when measured with PADMR. These states are markedly absent in films with smaller driving forces yet higher free charge yields which instead only show charge-separated state signals with weaker spin coupling. We interpret this result to be in support of a hypothesis where a larger reorganization energy associated with charge transfer to tightly bound CT states means that they are primarily generated in systems far from the Marcus optimum for free charge yield. And the highest free charge yielding systems instead predominantly undergo long-range charge separation into the acceptor host.

charge separation↗