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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 1,693 records · Page 94
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Ultrafast One-Dimensional Peierls-Distortion Dynamics in 1⁢T′−Re⁢S 2 Revealed by 4D Electron Microscopy

Rhenium disulfide (ReS 2 ), a prototypical 2D semiconductor with an anisotropic 1⁢T′ structure due to the pronounced Peierls distortion, has demonstrated great potential for polarization-dependent optoelectronics and photonics. Here, we report an ultrafast phase transition occurring within 1 ps, accompanied by a one-dimensional Peierls-distortion relaxation in 1⁢T′−ReS 2 revealed with combined 4D electron microscopy and time-dependent density functional theory calculations. Upon femtosecond laser pulse excitation, the Re-Re dimerization morphology rapidly transforms from a diamond cluster to zigzag chains and persists for several nanoseconds. Here, this ultrafast Peierls-distortion relaxation is further verified by transient changes in optical anisotropy via polarization-dependent transient absorption spectroscopy. Time-dependent density functional theory calculations attribute this novel phase transition to strong correlation between Peierls distortion and impulsive photoexcited carrier doping, predicting a transient band-gap collapse. This Letter opens up an exciting avenue for ultrafast control of Peierls-distortion-induced anisotropy and the metal-insulator transition in 1⁢T′−ReS 2 .

1T’-ReS2↗
DOE OSTI2021

Variable frequency response testbed to validate protective relays up to 20 kHz☆

Microgrids operate in islanding or utility grid mode. In islanding mode, distributed energy sources with power electronics inverters can generate fault currents over a wide range of frequencies. To gain a better understanding of the nature of these events, it is crucial to measure currents at frequencies of up to 20 kHz at transient states with protective relays. Protective relays at these frequency responses should be tested prior to being installed in microgrids. However, testing at high-frequency currents cannot be done with Relay Test Equipment (RTE) that generate signals up to 50th Harmonic for 60 Hz (3 kHz). In this study, a frequency response method and testbed for high-frequency currents of up to 20.16 kHz was developed and tested. The testbed utilized a RTE and High-Frequency Power Source/ Amplifier. Furthermore, frequency relay experiments were run at frequencies of up to 1 and 20.16 kHz at a current magnitude of 0.1 and 0.5 A, respectively. The events were collected, and the current magnitude and frequency were plotted at time and frequency domain, respectively. The study of current magnitude and frequencies of up to 20.16 kHz for two overcurrent relays was performed and validated the variable frequency testbed and methodology.

24 POWER TRANSMISSION AND DISTRIBUTION↗
DOE OSTI2023

Benchmarking magnetised three-wave coupling for laser backscattering: analytic solutions and kinetic simulations

Understanding magnetised laser–plasma interactions is important for controlling magneto-inertial fusion experiments and developing magnetically assisted radiation and particle sources. For nanosecond pulses at non-relativistic intensities, interactions are dominated by coherent three-wave interactions, whose nonlinear coupling coefficients became known only recently when waves propagate at oblique angles with the magnetic field. In this paper, backscattering coupling coefficients predicted by warm-fluid theory are benchmarked using particle-in-cell simulations in one spatial dimension, and excellent agreements are found for a wide range of plasma temperatures, magnetic field strengths and laser propagation angles, when the interactions are mediated by electron-dominant hybrid waves. Systematic comparisons between theory and simulations are made possible by a rigorous protocol. On the theory side, the initial boundary value problem of linearised three-wave equations is solved, and the transient-time solutions allow the effects of growth and damping to be distinguished. On the simulation side, parameters are carefully chosen and calibration runs are performed to ensure that comparisons are well controlled. Fitting simulation data to analytical solutions yields numerical growth rates that match theory predictions within error bars. Although warm-fluid theory is found to be valid for a wide parameter range, genuine kinetic effects have also been observed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Role of Macromolecular Structure in the Ultrafast Energy and Electron Transfer Dynamics of a Light-Harvesting Polymer

Ultrafast energy and electron transfer (EnT and ET, respectively) are characterized in a light-harvesting assembly based on a π-conjugated polymer (poly(fluorene)) functionalized with broadly absorbing pendant organic isoindigo (iI) chromophores using a combination of femtosecond transient absorption spectroscopy and large-scale computer simulation. Photoexcitation of the π-conjugated polymer leads to near-unity quenching of the excitation through a combination of EnT and ET to the iI pendants. The excited pendants formed by EnT rapidly relax within 30 ps, whereas recombination of the charge-separated state formed following ET occurs within 1200 ps. A computer model of the excited-state processes is developed by combining all-atom molecular dynamics simulations, which provides a molecular-level view of the assembly structure, with a kinetic model that accounts for the multiple excited-state quenching pathways. In conclusion, direct comparison of the simulations with experimental data reveals that the underlying structure has a dramatic effect on the partitioning between EnT and ET in the polymer assembly, where the distance and orientation of the pendants in relation to the backbone serve to direct the dominant quenching pathway.

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

Pt-Assisted Carbon Remediation of Mo2C Materials for CO Disproportionation

Using the CO disproportionation (Boudouard) reaction as a probe reaction, an in-depth analysis of temperature-programmed pulse response data shows that the addition of Pt to Mo2C mitigates deactivation of Mo active sites by acting as a carbon collector. CO2 production on Mo2C and Pt/Mo2C materials is dependent on both the activation energy and the CO surface concentration. Detailed plane-wave density functional theory calculations of the CO adsorption and disproportion reactions on Mo2C-supported Pt nanoparticles (NPs) are reported. The Mo2C was modeled by the ß-Mo2C (100) surface, and the Pt/Mo2C interface was modeled by the addition of 12 Pt atoms to the Mo2C (100) surface (12Pt@Mo2C). The potential energy surfaces of the Boudouard reaction were calculated on pure Mo2C, 12Pt@Mo2C, and Pt (111) surfaces. CO dissociation readily occurs on the Mo2C (100) surface, but not on the Pt (111) surface, with the former being exothermic and the latter being endothermic. At the Pt/Mo2C interface, CO dissociation is still exothermic, but with a larger energy barrier. The Boudouard reaction takes place on the Mo2C region, where CO2 is formed from a surface O atom dissociated from one CO molecule in reaction with another CO molecule, leaving one C atom on the surface. C adsorption is preferential on the Pt site in comparison to the Mo site. The supported Pt domains can collect the remaining C atoms, facilitating further CO2 formation on the active Mo sites. A Bader charge analysis shows that the surface metal-carbon bond is a mixture of covalent and ionic bonds, whereas the surface metal-oxygen bond is ionic. Electron localization function (ELF) and partial charge density calculations agree well with the Bader charge analysis. These computational results are consistent with experimental observations of the interaction of CO with Mo2C nanotube supported Pt domains in the transient regime under far from equilibrium conditions. The Boudouard reaction is an important side reaction, and the unexpected role found for Pt as a carbon collector, with Mo serving as a disproportionation site, provides a unique vantage point for understanding carbon and coke formation on catalytic materials.

Pt-assisted carbon remediation, Pt/Mo2C interface,↗
DOE OSTI2018

Visible Light Driven Bromide Oxidation and Ligand Substitution Photochemistry of a Ru Diimine Complex

We report the complex [Ru(deeb)(bpz) 2 ] 2+ ( RuBPZ 2+ , deeb = 4,4'-diethylester-2,2'-bipyridine, bpz = 2,2'-bipyrazine) forms a single ion pair with bromide, [ RuBPZ 2+ , Br – ] + , with K eq = 8400 ± 200 M –1 in acetone. The RuBPZ 2+ displayed photoluminescence (PL) at room temperature with a lifetime of 1.75 μs. The addition of bromide to a RuBPZ 2+ acetone solution led to significant PL quenching and Stern–Volmer plots showed upward curvature. Time-resolved PL measurements identified two excited state quenching pathways, static and dynamic, which were operative toward [ RuBPZ 2+ , Br – ] + and free RuBPZ 2+ , respectively. The single ion-pair [ RuBPZ 2+ , Br – ] + * had a lifetime of 45 ± 5 ns, consistent with an electron transfer rate constant, k et = (2.2 ± 0.3) × 10 7 s –1 . In contrast, RuBPZ 2+ * was dynamically quenched by bromide with a quenching rate constant, k q = (8.1 ± 0.1) × 10 10 M –1 s –1 . Nanosecond transient absorption revealed that both the static and dynamic pathways yielded RuBPZ + and Br 2 •– products that underwent recombination to regenerate the ground state with a second-order rate constant, k cr = (2.3 ± 0.5) × 10 10 M –1 s –1 . Kinetic analysis revealed that RuBPZ + was a primary photoproduct, while Br 2 •– was secondary product formed by the reaction of a Br • with Br – , k = (1.1 ± 0.2) × 10 10 M –1 s –1 . Marcus theory afforded an estimate of the formal reduction potential for E 0 (Br •/– ) in acetone, 1.42 V vs NHE. A 1 H NMR analysis indicated that the ion-paired bromide was preferentially situated close to the Ru II center. Prolonged steady state photolysis of RuBPZ 2+ and bromide yielded two ligand-substituted photoproducts, cis - and trans -Ru(deeb)(bpz)Br 2 . Finally, a photochemical intermediate, proposed to be [Ru(deeb)(bpz)(κ 1 -bpz)(Br)] + , was found to absorb a second photon to yield cis - and trans -Ru(deeb)(bpz)Br 2 photoproducts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Photoinduced coherent acoustic phonon dynamics inside Mott insulator Sr 2 IrO 4 films observed by femtosecond X-ray pulses

We investigate the transient photoexcited lattice dynamics in a layered perovskite Mott insulator Sr 2 IrO 4 film by femtosecond X-ray diffraction using a laser plasma-based X-ray source. The ultrafast structural dynamics of Sr 2 IrO 4 thin films are determined by observing the shift and broadening of (0012) Bragg diffraction after excitation by 1.5 eV and 3.0 eV pump photons for films with different thicknesses. The observed transient lattice response can be well interpreted as a distinct three-step dynamics due to the propagation of coherent acoustic phonons generated by photoinduced quasiparticles (QPs). Employing a normalized phonon propagation model, we found that the photoinduced angular shifts of the Bragg peak collapse into a universal curve after introducing normalizedn coordinates to account for different thicknesses and pump photon energies, pinpointing the origin of the lattice distortion and its early evolution. In addition, we report a transient photocurrent measurement indicates that the photoinduced QPs are charge neutral excitons. Mapping the phonon propagation and correlating its dynamics with the QP by ultrafast X-ray diffraction (UXRD) establish a powerful way to study electron-phonon coupling and uncover the exotic physics in strongly correlated systems under nonequilibrium conditions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2025

Detecting anomalous SRF cavity behavior with unsupervised learning

We present an unsupervised learning framework for detecting anomalous superconducting radio-frequency (SRF) cavity behavior at the Continuous Electron Beam Accelerator Facility (CEBAF), emphasizing its initial performance and effectiveness. Key to the system’s success was the development of data acquisition systems (DAQs) that capture fast-sampled, information-rich signals, essential for detecting transient effects. The approach involves creating daily cavity-specific models using principal component analysis to handle variations in rf signal behavior and mitigate performance degradation from data drift. This unsupervised method eliminates the need for expensive labeling by continuously updating models with recent data. Deployed and operational for 3 months before a scheduled shutdown, the system successfully identified several issues with DAQ signals, confirming its effectiveness. Despite access to only a fraction of CEBAF’s SRF cavity signals, the framework efficiently detected several instances requiring intervention, demonstrating a significant improvement over traditional, labor-intensive methods of manual plot inspection. Published by the American Physical Society 2025

43 PARTICLE ACCELERATORS↗
NASA NTRS1982

Qualitative differences between on-orbit and transition RCS control

Two separate reaction control system (RCS) digital autopilots (DAPs) evolved from one original Space Shuttle orbital autopilot concept. A computer overload forced this evolution. Part of the overload problem was due to unique performance requirements imposed on the RCS controller during each of several different flight regimes. The two resultant RCS DAPs yield different effector responses because they rely on different sources of sensory input and they process data differently. This paper describes the evolution of the two RCS controllers and illustrates their behavioral differences. The transition autopilot, used in orbital insertion and deorbit, is sensitive to orbiter flexure due to its feedthrough character. The on-orbit autopilot is sensitive to transient rate control degradation from large disturbances due to feed-forward rate estimation. Simulation results and flight data are used to illustrate performance differences between the two autopilots under various conditions. These include computer failures where electronic stringing and procedural reconfiguration differences affect autopilot behavior.

Hattis, P. D.↗
NASA NTRS1984

Sensitivity analysis of discrete structural systems: A survey

Methods for calculating sensitivity derivatives for discrete structural systems are surveyed, primarily covering literature published during the past two decades. Methods are described for calculating derivatives of static displacements and stresses, eigenvalues and eigenvectors, transient structural response, and derivatives of optimum structural designs with respect to problem parameters. The survey is focused on publications addressed to structural analysis, but also includes a number of methods developed in nonstructural fields such as electronics, controls, and physical chemistry which are directly applicable to structural problems. Most notable among the nonstructural-based methods are the adjoint variable technique from control theory, and the Green's function and FAST methods from physical chemistry.

Adelman, H. M.↗
NASA NTRS1985

Galileo internal electrostatic discharge program

The Galileo spacecraft which will orbit Jupiter in 1988 will encounter a very harsh environment of energetic electrons. These electrons will have sufficient energy to penetrate the spacecraft shielding, consequently depositing charges in the dielectric insulating materials or ungrounded conductors. The resulting electric field could exceed the breakdown strength of the insulating materials, producing discharges. The transients produced from these Internal Electrostatic Discharges (IESD) could, depending on their relative location, be coupled to nearby cables and circuits. These transients could change the state of logic circuits or degrade or even damage spacecraft components, consequently disrupting the operation of subsystems and systems of the Galileo spacecraft during its expected mission life. An extensive testing program was initiated for the purpose of understanding the potential threats associated with these IESD events. Data obtained from these tests were used to define design guidelines.

Leung, P. L.↗
NASA NTRS1993

Equatorial spacecraft-plasma interaction phenomenon observed with DE 2

Data from the retarding potential analyzer and ion drift meter on Dynamics Explorer 2 indicate that an unusual spacecraft-plasma interaction phenomemon occurs at times when the spacecraft velocity vector becomes nearly aligned with the local geomagnetic field lines. The primary signature of the interaction is a transient increase in the ion collection currents obtained with these instruments, up to approximately 15% in magnitude and typically a few tens of seconds duration. This signature is indicative of an increase in the net ion and electron currents to the satellite and is accompanied by a small positive increase in the spacecraft potential relative to the plasma. We present here a case study covering six of the strongest such events observed with DE 2 and discuss a possible physical mechanism. We suggest in particular that what might be called a collisional electron 'snowplow' effect may be occurring, and we derive a simple numerical model based on this scenario. Least squares fitting is employed to test the model and to derive new estimates of the ambient ion concentration at times when the measurements are being perturbed by the interaction.

Cragin, B. L.↗
NASA NTRS1995

Testing the DC-electric field model in a solar flare observed by Yohkoh and the Compton Gamma-Ray Observatory

We apply a DC-electric field model to the analysis of soft and hard X-ray observations of a solar flare observed by Yohkoh and the Compton Gamma Ray Observatory (CGRO) on 6 September 1992. The flare was observed simultaneously in the soft X-ray Ca XIX line by the Yohkoh Bragg Crystal Spectrometer (BCS) and in hard X-rays (greater than 50 keV) by the CGRO Burst and Transient Spectrometer Experiment (BATSE). A strong stationary component of Ca XIX emission was present at the start of impulsive hard X-ray emission indicating an extended phase of heating prior to the production of energetic nonthermal electrons. We interpret the preflare Ca XIX emission as a signature of Joule heating by field-aligned currents. We relate the temporal variation of impulsive hard X-ray emission to the rate of runaway electron acceleration by the DC-electric field associated with the current. We find that the initial rise in hard X-ray emission is consistent with electron acceleration by a DC-electric field that increased from a preflare value of less than approximately 10(exp -5) V/cm to approximately (9 +/- 1) x 10(exp -5) V/cm at the time of the first hard X-ray peak and then remained constant during the rest of the impulsive phase. We attribute the increase in electric field strength to the formation of a current sheet at the reconnection point of two loop structures. The decrease in hard X-ray emission after flare maximum is consistent with a reduction in the number of runaway electrons due to an increase in coronal density produced by chromospheric evaporation. The increased density quenches the runaway process by enhancing collisional thermalization of electrons. To avoid the generation of an unrealistically large magnetic field, the flaring region must be highly filamented into greater than approximately 10(exp 6) oppositely directed current channels of approximately 30 cm width with an initial preflare current of approximately 3 x 10(exp 10) A per channel.

Zarro, D. M.↗
NASA NTRS1993

Equatorial Spacecraft-Plasma Interaction Phenomenon Observed With DE 2

Data from the retarding potential analyzer and ion drift meter on Dynamics Explorer 2 indicate that an unusual spacecraft-plasma interaction phenomenon occurs at times when the spacecraft velocity vector becomes nearly aligned with the local geomagnetic field lines. The primary signature of the interaction is a transient increase in the ion collection currents obtained with these instruments, up to approximately 15% in magnitude and typically a few tens of seconds duration. This signature is indicative of an increase in the net ion and electron currents to the satellite and is accompanied by a small positive increase in the spacecraft potential relative to the plasma. We present here a case study covering six of the strongest such events observed with DE 2 and discuss a possible physical mechanism. We suggest in particular that what might be called a collisional electron snowplow effect may be occurring, and we derive a simple numerical model based on this scenario. Least squares fitting is employed to test the model and to derive new estimates of the ambient ion concentration at times when the measurements are being perturbed by the interaction.

Cragin, B. L.↗
DOE OSTI2021

Anisotropic time-domain electronic response in cuprates driven by midinfrared pulses

Superconductivity in the cuprates is characterized by an anisotropic electronic gap of d -wave symmetry. The aim of this study is to understand how this anisotropy affects the nonequilibrium electronic response of high-T c superconductors. Here we use a polarization selective time domain experiment to address the dynamics of electronic excitation of different symmetry in optimally doped Bi 2 Sr 2 Y 0.08 Ca 0.92 Cu 2 O 8+δ and measure the nodal and antinodal nonequilibrium response resulting from photoexcitations with ultrashort pulses with photon energy comparable to the superconducting gap. The response to long wavelength photoexcitation with pump polarization along the Cu-Cu axis of the sample is discussed with the support of an effective d-wave BCS model which suggests that such transient response could be ascribed to an increase of pair coherence in the antinodal region.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Rationally Engineered Vertically Aligned β ‐Ga 2− x W x O 3 Nanocomposites for Self‐Biased Solar‐Blind Ultraviolet Photodetectors with Ultrafast Response

Abstract With the astonishing advancement of present technology and increasing energy consumption, there is an ever‐increasing demand for energy‐efficient multifunctional sensors or transducers based on low‐cost, eco‐friendly material systems. In this context, self‐assembled vertically aligned β ‐Ga 2− x W x O 3 nanocomposite (GWO‐VAN) architecture‐assisted self‐biased solar‐blind UV photodetection on a silicon platform, which is the heart of traditional electronics is presented. Utilizing precisely controlled growth parameters, the formation of W‐enriched vertical β ‐Ga 2− x W x O 3 nanocolumns embedded into the W‐deficient β ‐Ga 2− x W x O 3 matrix is reached. Detailed structural and morphological analyses evidently confirm the presence of β ‐Ga 2− x W x O 3 nanocomposite with a high structural and chemical quality. Furthermore, absorption and photoluminescence spectroscopy explains photo‐absorption dynamics and the recombination through possible donor–acceptor energy states. The proposed GWO‐VAN framework facilitates evenly dispersed nanoregions with asymmetric donor energy state distribution and thus forms build‐in potential at the vertical β ‐Ga 2− x W x O 3 interfaces. As a result, the overall heterostructure evinces photovoltaic nature under the UV irradiation. A responsivity of ≈30 A/W is observed with an ultrafast response time (≈350 µs) under transient triggering conditions. Corresponding detectivity and external quantum efficiency are 7.9 × 10 12 Jones and 1.4 × 10 4 %, respectively. It is believed that, while this is the first report exploiting GWO‐VAN architecture to manifest self‐biased solar‐blind UV photodetection, the implication of the approach is enormous in designing electronics for extreme environment functionality and has immense potential to demonstrate drastic improvement in low‐cost UV photodetector technology.

Das, Debabrata↗
DOE OSTI2021

Photophysical properties of zero-dimensional perovskites studied by PBE0 and GW+BSE methods

Due to the exceptional photophysical properties and great potential applications in solid state lighting, zero-dimensional halide perovskites with broadband emitting have recently attracted much attention. In this work, to reveal the origin of the broadband absorption and emission spectra of 0D (C 4 N 2 H 14 Br) 4 SnBr x I 6-x (x = 3), the ground state and exciton dynamics properties have been studied by hybrid PBE0 functional and the GW+BSE method. Different self-trapped excitons (STEs) are obtained, and their emission energies by PBE0 are consistent well with experimental data. By considering the vibrational broadening of emission, our calculated full width at half maximum of emission accords well with the experiment. The absorption of free exciton and STEs is also calculated by considering the quasiparticle and excitonic effect. Our results show that these light-induced trapped states can be observed by experiments, as confirmed in other similar systems by the femtosecond transient absorption spectra.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Full information acquisition in piezoresponse force microscopy

The information flow from the tip-surface junction to the detector electronics during the piezoresponse force microscopy (PFM) imaging is explored using the recently developed general mode (G-mode) detection. Information-theory analysis suggests that G-mode PFM in the non-switching regime, close to the first resonance mode, contains a relatively small (100–150) number of components containing significant information. The first two primary components are similar to classical PFM images, suggesting that classical lock-in detection schemes provide high veracity information in this case. At the same time, a number of transient components exhibit contrast associated with surface topography, suggesting pathway to separate the two. The number of significant components increases considerably in the non-linear and switching regimes and approaching cantilever resonances, precluding the use of classical lock-in detection and necessitating the use of band excitation or G-mode detection schemes. The future prospects of full information imaging in scanning probe microscopy are discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗