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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 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

New scaling law for turbulent boundary layers with high surface mass transfer

A fundamental understanding of the phenomena occurring in the turbulent boundary layer in the presence of surface blowing is limited, and considerable disagreements persist even in describing primary quantities, such as the boundary layer profile. The theories based on the linear boundary layer equations show that the thickness of the sublayer increases in the presence of surface blowing; therefore, the viscous sublayer and law of the wall modify. In this study, direct numerical simulations (DNS) of turbulent boundary layers with uniform surface mass transfers are carried out in order to scale the velocity profile. Emphasis is placed on moderate to high mass transfer rates, which are relevant to the most common hybrid rockets configuration. DNS data are used to establish a functional law of the wall and a law of wake by means of the relation between the wall shear stress and surface mass transfer. Analysis of the mean kinetic energy budget shows that the magnitude of turbulent kinetic energy increases by surface mass transfer, and the production rate extends significantly in the inner layer as the injection rate increases. DNS data of various surface blowing are used to complete the closure of turbulence kinetic energy equation and develop an eddy viscosity model. The predicted turbulent kinetic energy and eddy viscosity agree with DNS data for moderate to high blowing rates.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

The Surface Chemistry of Methanol on Pd(111) and H–Pd(111) Surfaces: C–O Bond Cleavage and the Effects of Metal Hydride Formation

Palladium catalysts are frequently employed in processes where methanol is an energy vector or carrier, being useful for the synthesis of methanol from mixtures of carbon dioxide and hydrogen (CO 2 /H 2 ) or its steam reforming on demand. Results of synchrotron-based ambient pressure X-ray photoelectron spectroscopy for the adsorption of methanol on a Pd(111) model catalyst show a rich surface chemistry and complex phenomena that strongly depend on pressure and temperature. At low pressures (< 10 -6 Torr) and temperatures (< 300 K), CO is the dominant decomposition product. Further, as the pressure increases, cleavage of C-H, O-H and C-O bonds is observed, and at elevated temperatures (400-600 K) the formation of CO and CH x /C fragments compete on the surface. Thus, existing reaction networks for methanol decomposition must be modified. Furthermore, surface and subsurface hydrogen (coming from PdH x ) play a significant role in the stability and removal of CH x and C species.

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

Enhanced Interface-Driven Perpendicular Magnetic Anisotropy by Symmetry Control in Oxide Superlattices

Perpendicular magnetic anisotropy (PMA) has recently been shown to emerge at interfaces of 3d and 5d transition-metal oxides (TMOs). However, strategies to systematically stabilize such interface-driven PMA still remains elusive, hindering further applications of this design approach. Here, tuning crystal symmetry is shown to be an effective means to engineer this interfacial phenomenon. The evolution of PMA strength as a function of ferromagnetic oxide thickness quantitatively reveals the competition between volume- and interface-specific contributions that determine the magnetic anisotropy. By applying different degrees of epitaxial strain, the relative contributions to PMA are modulated, clearly revealing their correlations with crystal symmetries. To be more specific, the volume anisotropy energy is found to be correlated with the tetragonal distortion of the ferromagnetic layer, while the interface anisotropy energy is mainly modulated by the octahedral tilting at the interface. With these insights, superlattices with enhanced interface-driven PMA and higher Curie temperature are realized. These findings reveal a route to engineering interface-driven PMA and associated magnetic phenomena in TMO heterostructures for future spintronic applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Strain-driven surface reconstruction and cation segregation in layered Li(Ni 1– x – y Mn x Co y )O 2 (NMC) cathode materials

The composition, structure and phase transformations occurring on cathode surfaces greatly affect the performance of Li-ion batteries. Li-Ion diffusion and surface–electrolyte interaction are two major phenomena that impact the capacity and cell impedance. The effects of surface reconstruction (SR) of cathode materials on the performance of Li-ion batteries are of current interest. However, the origin and evolution of the SR are still not well understood. In this work, density functional theory (DFT) calculations are used to investigate the processes taking place during surface segregation and reconstruction. Facet dependent segregation was found in Li(Ni 1– x – y Mn x Co y )O 2 (NMC) cathodes. Specifically, Co tends to segregate to the (104) surface of the primary particles within the transition metal layer, while Ni ions tend to segregate to the (012) surface in the Li layer, forming a SR. Experimental evidence shows the SR to be epitaxial with the bulk of the as-synthesized material, and the new SR phase is pinned to the NMC unit cell leading to a strained SR. Here, we demonstrate that strain can stabilize a spinel structure of the SR layers. Understanding the effects of surface strain opens a new avenue for the design of cathode materials with enhanced surface properties.

25 ENERGY STORAGE↗
DOE OSTI2022

Core-level binding energy shifts between interior, terrace and edge atoms in MnO(001) thin films

Understanding the physical origin of core-level photoemission line shapes can offer valuable information about the chemical and physical properties of surfaces. For instance, in a large number of transition metals oxides, changes in the line shape allow the accurate determination of their oxidation states and cation site symmetry. Yet, despite this importance, experimental investigations on core-level shifts have been much neglected in recent years. In order to provide further evidence of the physical relevance, we have, in this contribution, introduced a new aspect of interior-, terrace- and edge- atom core-level binding energy shifts to describe monolayers of MnO(001) films grown on an Au(111) substrate. By this means we were able to distinguish the line shape contributions related to different types of atomic sites. Here we show that their relative intensities and energy shifts are able to provide information about the relative amount of under coordinated atoms on the surface and, thus give insights into their catalytic properties. Our findings reveal the importance of a detailed surface science characterization to provide the correct interpretation of distinct photoemission line shapes when considering thin film samples as well as nanostructures in general.

36 MATERIALS SCIENCE↗
DOE OSTI2021

A perspective on two-dimensional van der Waals opto-spin-caloritronics

Two-dimensional (2D) van der Waals magnetic semiconductors displaying controllable ferromagnetism at room temperature form atomically sharp interfaces with various substrates. Such heterostructures create platforms for understanding spin-dependent phenomena across interfaces and surfaces for high-performance technological applications. Combining these 2D magnets with light and the spin Seebeck effect (SSE) in-state-of-the-art thermo-opto-spin studies forms a new paradigm in the field of spin-caloritronics that harnesses light as the new heat. In this Perspective, we detail how to exploit recent advances in 2D van der Waals materials to boost the SSE and propose a new strategy for optically controlled SSE in 2D magnetic semiconductor-based heterostructures with the intent to establish the research thrust of “opto-spin-caloritronics.”

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Embedding thermocouples in SS316 with laser powder bed fusion

Recent advances in manufacturing technologies have enabled the fabrication of complex geometries for a wide range of applications, including the energy, aerospace, and civil sectors. The ability to integrate sensors at critical locations within these complex components during the manufacturing process could benefit process monitoring and control by reducing reliance on models to relate surface measurements to internal phenomena. This study investigated embedding thermocouples in a SS316 matrix using laser powder bed fusion. Under optimal processing conditions, embedded thermocouples were characterized post-building, finding good bonding to the matrix with no melt pool penetration to the sensing elements. Futher, the embedded thermocouple performed similarly to an identical non-embedded thermocouple during thermal testing to 500 °C with only a slight difference in response time, which was attributed to the differences in mass and the associated thermal time constants.

image analysis↗
DOE OSTI2025

Impact of external screening on the valence and core level photoelectron spectra of monolayer W⁢S 2

Transition metal dichalcogenides (TMDs) represent an emerging class of layered materials with applications in microelectronics, optoelectronics, photonics, and catalysis. The confinement of charge carriers within the highly anisotropic, quasi-two-dimensional geometry of one-layer (1L) TMDs leads to reduced, variable dielectric screening, giving rise to a quasiparticle band gap that is highly susceptible to the surrounding dielectric environment. Here, exploiting the contrasting external dielectric environments of gold-supported and suspended 1L W⁢S 2 , we show how the electronic states of W⁢S 2 under the effective and ineffective screening environments align at a junction made within the same sheet of material. Photoelectron spectra point to the close alignment of the charge neutrality levels of W⁢S 2 in both environments, and the breakdown of rigid shifts between the valence states and core levels with the core levels shifting more than twice as much as the valence states. Furthermore, the effectively screened W⁢S 2 exhibits a valence state with the photoemission linewidth twice as large as the ineffectively screened suspended W⁢S 2 , presumably originated from the locally varying W⁢S 2 -Au distance and substrate disorders. Collectively, these findings provide key insights into the electronic behavior of W⁢S 2 and its photoemission process with the electronic states renormalized according to the external screening environments.

Dielectric properties↗
DOE OSTI2025

Tuning magneto-optical zero reflection via dual-channel hybrid magnonics

Multichannel coupling in hybrid systems makes an attractive testbed not only because of the distinct advantages entailed by each constituent mode but also because the opportunity to leverage interference among the various excitation pathways. Here, via combined analytical calculation and experiment, we demonstrate that the phase of the magnetization precession at the interface of a coupled yttrium iron garnet (YIG)/permalloy (Py) bilayer is collectively controlled by the microwave photon field torque and the interlayer exchange torque, manifesting a coherent, dual-channel excitation scheme that effectively tunes the magneto-optical spectrum. Furthermore, the different torque contributions vary with frequency, external bias field, and type of interlayer coupling between YIG and Py, which further results in destructive or constructive interferences between the two excitation channels, and hence selective suppression or amplification of the hybridized magnon modes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Characterization of light-induced potentials in the strong-field dissociation of O 2 +

In this work, we investigate theoretically the imprints of light-induced potentials (LIPs) on the dissociation dynamics of O 2 + molecular ions, as observable in angle-resolved fragment kinetic-energy-release (KER) spectra. Following the vibrational and rotational dynamics of the initial pump-laser-excited cationic nuclear wave packet, while accounting for the dipole coupling between the O 2 + ( a 4 Π u ) and O 2 + ( f 4 Π g ) electronic states in 800-nm 40-fs probe-laser pulses with peak intensities between 10 13 and 10 14 W / cm 2 , we calculate angle-resolved KER spectra which reveal characteristic energy- and angle-dependent fringe structures. These fringes shift downward in energy as the molecular alignment angle θ relative to the probe-pulse polarization direction increases from 0 to π / 2 . The angle-dependent shifts in the KER fringes increase for larger probe-pulse peak intensities and follow the angle and light-wave-intensity dependence of the vibrational spectrum in the associated Floquet bond-hardening well, which is a manifestation of transient O 2 + nuclear-probability trapping in the LIP during dissociation. By examining the rovibrational dynamics of the dissociating molecular cation near the light-induced conical intersection (LICI) in the cationic LIP surface at θ = π / 2 , we identify related angle-dependent structures in the KER spectra, suggesting a means for assessing the significance of LICIs in molecular dissociation pathways.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

Resolving Discrepancies in Spin-Torque Ferromagnetic Resonance Measurements: Lineshape versus Linewidth Analyses

When spin-orbit torques are measured using spin-torque ferromagnetic resonance, two alternative ways of analyzing the results to extract the torque efficiencies—lineshape analysis and analysis of the change in linewidth versus direct current—often give inconsistent results. We identify a source for these inconsistencies. We show that fits of spin-torque ferromagnetic resonance data to the standard analysis framework leave significant residuals that we identify as due to (i) current-induced excitations of a small volume of magnetic material with magnetic damping much larger than that of the bulk of the magnetic layer, which we speculate is associated with the heavy-metal–magnet interface and (ii) oscillations of the sample magnetization at the modulation frequency due to heating. Here, the dependence of the residual signals on direct current can interfere with an accurate extraction of spin-torque efficiencies by the linewidth method. We show that the discrepancies between the two types of analysis can be largely eliminated by extrapolation of the window of magnetic fields used in the linewidth fits to small values so as to minimize the influence of the residual signals.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Effect of Surface Ionic Screening on Polarization Reversal and Phase Diagrams in Thin Antiferroelectric Films for Information and Energy Storage

Emergent behaviors in antiferroelectric thin films due to a coupling between surface electrochemistry and intrinsic polar instabilities are explored within the framework of the modified 2-4-6 Landau-Ginzburg-Devonshire (LGD) thermodynamic approach. By using phenomenological parameters of the LGD potential for a bulk antiferroelectric and a Stephenson-Highland (SH) approach, we study the role of surface ions with a charge density proportional to the relative partial oxygen pressure on the dipole states and their reversal mechanisms in antiferroelectric thin films. The combined LGDSH approach allows the boundaries of antiferroelectric, ferroelectriclike antiferroionic, and electretlike paraelectric states as a function of temperature, oxygen pressure, surface-ion formation energy and concentration, and film thickness to be delineated. This approach also allows the characterization of the polar and antipolar orderings dependence on the voltage applied to the antiferroelectric film, as well as the analysis of their static and dynamic hysteresis loops. Furthermore, the applications of the antiferroelectric films covered with a surface-ion layer for energy and information storage are explored and discussed.

25 ENERGY STORAGE↗
DOE OSTI2022

Spontaneous Polarization in an Ultrathin Improper-Ferroelectric/Dielectric Bilayer in a Capacitor Structure at Cryogenic Temperatures

To determine the effect of depolarization and the critical thickness in improper-ferroelectric hexagonal-ferrite thin films, we investigate the polarization switching of a ferroelectric/dielectric bilayer in capacitor structures at 20 K. Experimentally, we show that the spontaneous polarization persists throughout the studied thickness range (3 to 80 unit cell), even with a thick (10-nm) dielectric layer, suggesting no practical thickness limit for applications. By fitting the effect of depolarization using the phenomenological theory, we show that the spontaneous polarization remains finite when the thickness of the ferroelectric layer approaches zero, providing a hint for the absence of critical thickness. We also find that the interfacial effects limit the multidomain formation and govern the polarization switching mechanisms.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Signatures of enhanced superconducting properties in niobium cavities

Superconducting radio-frequency (SRF) niobium cavities are critical for modern particle accelerators, as well as for advancing superconducting quantum systems and enabling ultrasensitive searches for new physics. In this work, we report a systematic observation of an anomalous frequency dip in Nb cavities, which occurs at temperatures just below the critical temperature (𝑇𝑐 ), indicative of enhanced superconducting properties at 𝑇 ≪ 𝑇 𝑐 . The magnitude of this dip is strongly correlated with the rf surface resistance, impurity distribution near the surface, and 𝑇𝑐 . Additionally, we report measurements of the coherence peak in the ac conductivity of two Nb SRF cavities processed using distinct methods. By comparing recent theories developed to model this experimental data, we show that the frequency-dip feature, larger coherence peak height, and reduction in the temperature-dependent surface resistance with rf current occur at minimal but finite levels of disorder.

43 PARTICLE ACCELERATORS↗
DOE OSTI2020

Strain modulation using defects in two-dimensional MoS 2

We investigate the nature of strain in MoS 2 and correlate it to defect types and densities, while systematically assessing the tolerance of this low dimensional material to He and Au ion irradiations. Through a series of theoretical predictions and experimental observations, we establish the onset of the crystalline-to-amorphous transition in MoS 2 and identify sulfur vacancies as the most favorable defects introduced during irradiation. We note the presence of both tensile and compressive strains, which depend on the types of defects introduced into the lattice and vary with increasing fluence. In conclusion, the results show that defects can be used to tune strain in two-dimensional materials and provide an exciting pathway for using external stimuli to control properties of low dimensional materials.

2-dimensional systems↗
DOE OSTI2020

Image-enhanced bipolaron formation at organic semiconductor/electrode interfaces

We explore the image charge interaction for organic semiconductor bipolarons near a conducting interface and find that the cross term between one of the constituent charges and the image of its neighbor stabilizes the bipolaron by up to ~0.3 eV, dramatically increasing the concentration of this species near the interface. Using density functional theory calculations for the common hole transport molecule N,N'-bis(3-methylphenyl)- N,N'-diphenylbenzidine, we validate a simple point charge description of this effect and incorporate it within an interface energy level alignment model to predict the density of polarons and bipolarons near the interface. We find that the image effect greatly enhances bipolaron formation in the first few monolayers, leading to the expectation that bipolarons account for more than 1% of the total interface charge in many cases of practical interest. Furthermore, this result reinforces the notion that bipolarons are robust near the contacts of many organic semiconductor devices and thus helps to rationalize their involvement in the phenomenon of unipolar organic magnetoresistance.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Vector potential and surface magnetic field in magnetoelectric antiferromagnetic materials

A general formula for the average vector potential of bulk periodic systems is proposed and shown to set the boundary conditions at magnetic interfaces. Here, for antiferromagnetic materials, the study reveals a unique relation between the macroscopic potential and the orientation-dependent magnetic quadrupole as a result of the different crystalline and magnetic symmetries. In particular, at surfaces and interfaces of a truncated bulk without inversion and time-reversal symmetries, the average vector potential exhibits a discontinuity which results in an interfacial magnetic field. In general, however, due to the surface and interface electronic and atomic relaxations, additional magnetization may result. For the experimentally observed magnetoelectric antiferromagnets, in particular, our symmetry analysis suggests that the relaxation effects could well be a system response to the presence of such a potential discontinuity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗