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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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Mechanism of enhanced critical fields and critical current densities of MgB2 wires with C/Dy$^{2}$O3 co-additions

A series of monofilamentary powder-in-tube MgB2 wires were fabricated with 2 mol. % C doping and co-additions of 0–3 wt. % Dy2O3. Irreversibility fields (μ0Hirr), upper critical fields (μ0Hc2), and transport critical currents were measured, and from these quantities, anisotropies (γ) and electronic diffusivities (Dπ,σ) were estimated. The addition of 1 wt. % Dy2O3 to already optimally C-doped MgB2 wires produced higher Hc2//ab, Hc2//c, and Hirr values at 4.2 K. In addition, the critical current density, Jc, increased with Dy2O3 concentration up to 1 wt. % where non-barrier Jc reached 4.35 × 104 A/cm2 at 4.2 K, 10 T. At higher temperatures, for example, 20 K and 5 T, co-additions of 2 mol. % C and 2 wt. % Dy2O3 improved non-barrier Jc by 40% and 93% compared to 2 and 3 mol. % C doping, respectively. On the other hand, measurements of Tc showed that C/Dy2O3 co-additions increase interband scattering rates at a lower rate than C doping does (assuming C doping levels giving similar levels of low-T μ0Hc2 increase as co-addition). Comparisons to a two-band model for μ0Hc2 in MgB2 allowed us to conclude that the increases in Hc2//ab, Hc2//c, and Hirr (as well as concomitant increases in high-field Jc) with Dy2O3 addition are consistent with increases primarily in intraband scattering. This suggests C/Dy2O3 co-addition to be a more promising candidate for improving non-barrier Jc of MgB2 at temperatures above 20 K.

36 MATERIALS SCIENCE↗
DOE OSTI2024

On the nature of two-photon transitions for a collection of molecules in a Fabry–Perot cavity

Here, we investigate the effect of a cavity on nonlinear two-photon transitions of a molecular system and we analyze how such an effect depends on the cavity quality factor, the field enhancement, and the possibility of dephasing. We find that the molecular response to strong light fields in a cavity with a variable quality factor can be understood as arising from a balance between (i) the ability of the cavity to enhance the field of an external probe and promote multiphoton transitions more easily and (ii) the fact that the strict selection rules on multiphoton transitions in a cavity support only one resonant frequency within the excitation range. Although our simulations use a classical level description of the radiation field (i.e., we solve Maxwell–Bloch or Maxwell–Liouville equations within the Ehrenfest approximation for the field–molecule interaction), based on experience with this level of approximation in the past studies of plasmonic and polaritonic systems, we believe that our results are valid over a wide range of external probing.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2024

Formation of field-induced breakdown precursors on metallic electrode surfaces

Understanding the underlying factors responsible for higher-than-anticipated local field enhancements required to trigger vacuum breakdown on pristine metal surfaces is crucial for the development of devices capable of withstanding intense operational fields. In this study, we investigate the behavior of nominally flat copper electrode surfaces exposed to electric fields of hundreds of MV/m. Our novel approach considers curvature-driven diffusion processes to elucidate the formation of sharp breakdown precursors. To do so, we develop a mesoscale finite element model that accounts for driving forces arising from both electrostatic and surface-tension-induced contributions to the free energy. Our findings reveal a dual influence: surface tension tends to mitigate local curvature, while the electric field drives mass transport toward regions of high local field density. This phenomenon can trigger the growth of sharper protrusions, ultimately leading to a rapid enhancement of local fields and, consequently, to a runaway growth instability. We delineate supercritical and subcritical regimes across a range of initial surface roughness. Our numerical results are in qualitative agreement with experimentally reported data, indicating the potential practical relevance of field-driven diffusion in the formation of breakdown precursors.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2026

Effects of external magnetic fields on the multi-mode Rayleigh–Taylor instability

The magneto-Rayleigh–Taylor instability (mRTI) is a key process in inertial confinement fusion and is thought to be widespread in the interstellar medium, where it can concentrate plasma into discrete structures. We present resistive MHD simulations of the nonlinear evolution of multi-mode mRTI in both two and three dimensions, examining the effects of uniform external magnetic fields oriented either parallel or perpendicular to the initial interface. In both 2D and 3D, weak parallel fields enhance mixing-zone growth, whereas stronger fields suppress it. For perpendicular fields, growth is initially inhibited but becomes enhanced at later times. These behaviors arise from magnetic tension, which modifies flow anisotropy, buoyancy, drag, and vortex dynamics. The interplay of these mechanisms governs the distinct ways in which magnetic fields influence mRTI evolution.

Flow instabilities↗
DOE OSTI2022

Contributions to Dielectric Constant Enhancement in Thin Film Metal Insulator Metal Capacitors

Thin film metal-insulator-metal capacitors were fabricated with varying dielectric and electrode thicknesses and areas. Measurement of the dielectric properties of the capacitors consistently yielded higher than predicted capacitance values, which prompted the exploration of three factors that could enhance the capacitance. Modeling of the fringing capacitance and field enhancement due to the capacitor geometry, both yield capacitance gains, albeit insufficient to fully explain the observed behavior. The possible role of changes in polarizability of the dielectric film constituents, due to the amorphous nature of the films was also evaluated, but the results of the investigation of this contribution were inconclusive. Herein, the role of accumulation/depletion layers on capacitance enhancement was also analyzed. For capacitors fabricated with thinner dielectrics, the presence of these layers can effectively reduce the dielectric thickness, resulting in contributions to capacitance that are significant. The calculations completed suggest these contributions may be more important than fringing field contributions. Additional studies of these mechanisms are in progress.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Hidden variable in the electrocaloric effect of ferroics

Caloric effects allow for temperature control through adiabatic application of external fields and are actively explored for solid-state refrigeration. The common wisdom is that the application of ultrahigh fields enhances the effects, thus providing a route to their practical applications. Here, using the ferroelectric relaxor (Ba, Ca)(Ti, Zr)O 3 , we demonstrate that in ferroics, which are the prime candidates for such application, this is not true in general and that caloric effects can be enhanced through the reduction of the applied field. The explanation of such a counterintuitive response is in the dependence of the electrocaloric effect on the effective poling field that can be regarded as a “hidden” variable of the caloric effects.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Directional finite difference method for directly solving 3D gyrokinetic field equations with enhanced accuracy

The gyrokinetic (GK) field equation is a three-dimensional (3D) elliptic equation, but it is often simplified to a set of two-dimensional (2D) equations by assuming that the field does not vary along a specific direction. However, this simplification can introduce inevitable 0th-order numerical errors, as nonlinear mode coupling in toroidal geometry can produce undesirable harmonic modes that violate the assumption. In this work, we propose a novel directional finite difference method (FDM) with a local coordinate transformation to better resolve the target field of interest. The directional FDM can accurately solve 3D GK field equations without simplifications, which can overcome the limitations of conventional methods. The accuracy and efficiency of different FDMs are analyzed in great detail for a variety of geometries, from simple 2D Cartesian coordinates to realistic 3D curvilinear coordinates. The 0th-order numerical errors of simplified 2D GK equations were found to be more problematic for low-harmonic modes and low aspect ratio geometries such as spherical tokamaks. On the other hand, the directional 3D FDM can accurately resolve a much wider range of harmonic modes aligned to the direction of interest, including the low-harmonic modes. In conclusion, we demonstrate that the directional 3D FDM is a highly effective algorithm for solving the 3D GK field equations, achieving accuracy improvements of 10 to 100 times or more, particularly for low-harmonic modes in spherical tokamaks.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Directive giant upconversion by supercritical bound states in the continuum

Photonic bound states in the continuum (BICs), embedded in the spectrum of free-space waves with diverging radiative quality factor, are topologically non-trivial dark modes in open-cavity resonators that have enabled important advances in photonics. However, it is particularly challenging to achieve maximum near-field enhancement, as this requires matching radiative and non-radiative losses. Here we propose the concept of supercritical coupling, drawing inspiration from electromagnetically induced transparency in near-field coupled resonances close to the Friedrich–Wintgen condition. Supercritical coupling occurs when the near-field coupling between dark and bright modes compensates for the negligible direct far-field coupling with the dark mode. This enables a quasi-BIC field to reach maximum enhancement imposed by non-radiative loss, even when the radiative quality factor is divergent. Our experimental design consists of a photonic-crystal nanoslab covered with upconversion nanoparticles. Near-field coupling is finely tuned at the nanostructure edge, in which a coherent upconversion luminescence enhanced by eight orders of magnitude is observed. The emission shows negligible divergence, narrow width at the microscale and controllable directivity through input focusing and polarization. This approach is relevant to various physical processes, with potential applications for light-source development, energy harvesting and photochemical catalysis.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2025

Omni‐Resonant Imaging Across the Visible

Resonant field enhancement in optical cavities is provided over only narrow linewidths and for specific spatial modes. Consequently, spectrally restrictive planar Fabry-Pérot cavities have not contributed to date to white-light imaging, which necessitates a highly multimoded broadband field to satisfy the resonance condition. It is shown that introducing judicious angular-dispersion circumvents the fundamental trade-off between cavity linewidth and finesse in a Fabry-Pérot cavity by exciting a 130-nm-bandwidth achromatic resonance across the visible spectrum, which far exceeds the finesse-limited linewidth (0.5 nm), and even exceeds the free spectral range (45 nm). This omni-resonant configuration enables broadband color-imaging over a 100-nm-bandwidth in the visible with minimal spherical and chromatic aberrations. Omni-resonant imaging is demonstrated using coherent and incoherent light, and spatially extended and localized fields comprising stationary and moving objects. This work paves the way to harnessing broadband resonant enhancements for spatially structured fields, as needed for example in solar windows.

42 ENGINEERING↗
DOE OSTI2023

Transformer Analysis with ANSYS Maxwell and COMSOL

This is an extension of work described by Rodriguez et al . (2021). It continues analyses of a generic transformer design by Wes Greenwood. In this report, we summarize that work and add comparable results using the ANSYS Maxwell software (henceforward, “ANSYS”), and with COMSOL . We found the ANSYS and COMSOL calculations of inductance agreed well with previous results for simplified coils in air, and with a ferromagnetic core. We then describe the ANSYS and COMSOL approach and show results for a full transformer model based on magnetic field analyses. Finally, we present electrostatic analyses of E field enhancement, once again resolving individual wires. The purpose is to assess the electrostatic fields in order to locate where electric breakdown is likely to originate. We found the maximum enhancement between the secondary and either the primary or the tertiary at the end of the windings with a large potential difference.

42 ENGINEERING↗
DOE OSTI2025

Forced 3D Reconnection in an Exponentially Separating Magnetic Field

We present a solvable scenario for 3D reconnection in a sheared magnetic field. We consider a localized external force that is applied slowly to a flux tube and then maintained, generating an Alfvénic perturbation that spreads along the field lines. Separation of the sheared field lines reduces the scale of the perturbation across the field, enhancing magnetic diffusion. For a fusion-motivated equilibrium with exponential field-line separation, we find a reconnection timescale proportional to 𝒮/ ln⁡ 𝒮 under magnetohydrodynamics (MHD) and to 𝒮 1/3 for semicollisional electron-only reconnection, where 𝒮 is the Lundquist number of the perturbed flux tube. We generalize these results to arbitrary magnetic geometries, showing that the semicollisional case is geometry independent. Interestingly, we find that slower field-line separation yields an increased reconnection rate in MHD.

Magnetic confinement fusion↗
DOE OSTI2022

Exploring the Basic Physical Mechanisms of Cathode- and Anode-Initiated High-Voltage Surface Flashover

Surface flashover in vacuum imposes a substantial physical limit on modern, large-scale pulsed power. One of the ramifications is a minimum size requirement for new machines, which in itself becomes a hard barrier to the modernization and improvement of existing infrastructure. Pulsed power topologies require the physical mechanisms of both anode- and cathode-initiated flashover to be considered. Originally, the geometrical implications of field emission at the cathode triple junction (CTJ) motivated the usage of configurations that avoid electrons impinging on the insulating material. This will largely suppress the cathode-initiated flashover, which is best described by the secondary electron avalanche mechanism, gas desorption, and final breakdown in the desorbed gas. It depends on the cascade growth of a conducting plasma along the length of the insulator from the cathode. Mitigating the cathode-initiated flashover typically comes at the cost of a significant field enhancement at the anode triple junction (ATJ). In a typical implementation, the anode field may be three times higher than the cathode field for a given voltage, making the corresponding anode-initiated flashover much more common than otherwise. In the case of pulsed, anode-initiated flashover, experimental evidence suggests that charge is directly extracted from the insulator resulting in the insulator taking on a net positive charge advancing the anode potential. Furthermore, along with accompanying gas desorption from the surface, the potential will then propagate from the anode toward the cathode until the effective length of the gap is sufficiently reduced to support flashover. The underlying physical mechanisms of cathode- and anode-directed flashover are discussed in light of previously gathered experimental data and recent experiments with pulsed, high-gradient, anode-initiated flashover.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2025

Effect of inversion asymmetry on the superconducting and exciton condensates of bilayer graphene

Inversion asymmetry in bilayer graphene can be tuned by the displacement field. As a result, the band dispersion in biased bilayer graphene acquires flatband regions near the Dirac points along with a nontrivial band geometry. We analyze the effect of inversion asymmetry on the critical temperature and superfluid stiffness of the superconducting state of AB-stacked graphene bilayer and the exciton condensate in double layers formed by two AB-stacked graphene bilayers. We find that the geometric superfluid stiffness in bilayer graphene superconductors is negligible due to the small superconducting gap. Furthermore, since the geometric superfluid stiffness is maximized for a constant order parameter, it can be neglected in biased bilayer graphene superconductors with any pairing symmetry. In contrast, the displacement field enhances the geometric superfluid stiffness in exciton condensates. It is most prominent at low densities and high displacement fields. Here, a consequence of the geometric superfluid stiffness is a modest enhancement of the Berezinskii-Kosterlitz-Thouless transition temperature in bilayer graphene’s exciton condensate.

BKT transition↗
DOE OSTI2025

Field-dependent carrier transport in implanted isolation region of GaN lateral power devices

Among GaN lateral power devices, with proper E-field management, the breakdown occurs in the device isolation region rather than the active region. In this paper, we investigate field-dependent carrier transport in the isolation structures. Isolation test structures with variations in buffer doping, un-intentionally doped (UID) GaN thickness, and implantation conditions were fabricated. Electrical characterization was performed over a wide range of voltage and temperature. With varying UID GaN thickness and buffer doping, the leakage current and breakdown characteristics remain the same for a specific implantation condition, indicating that the leakage conduction and breakdown are governed by the implanted GaN region. Analysis of the temperature-dependent I–V reveals that carrier transport in the implanted GaN can be well explained by hopping conduction, involving three distinctive regions of operation depending on the E-field. At low E-field, carrier transport is ohmic, consistent with variable range hopping, driven by thermal activation. At medium E-field, a field enhanced thermally activated hopping is observed, following σ ∼ σ(T)exp(E). At high E-field, activation-less hopping is visible, following σ ∼ exp(−1/E), which occurs above a critical E-field limited by localization length.

Sadek, Mansura (ORCID:0000000322588338)↗
DOE OSTI2021

Ni 3 Si 2 nanowires for efficient electron field emission and limitations of the Fowler–Nordheim model

This article reports on top-down nanofabricated Ni 3 Si 2 nanowires and tests of their electron field emission capabilities. Here, the results include low turn-on electric field, E ON , moderate work function, Φ, and the field enhancement factor, β, customizable through nanofabrication. The article also reports on the issues ahead in the field of nanowires-based electron mission, as there are quantitative limitations of the applicability of the Fowler–Nordheim model, which will become increasingly apparent as we continue to optimize the field emission of electrons. To this end, we suggest adding the studies of surface-to-volume ratio effects of the nanowires as another standard for comparison in order to lead to the input form of the density of states as quantum effects becoming more prominent.

42 ENGINEERING↗
DOE OSTI2023

Strain-switchable field-induced superconductivity

Field-induced superconductivity is a rare phenomenon where an applied magnetic field enhances or induces superconductivity. Here, we use applied stress as a control switch between a field-tunable superconducting state and a robust non–field-tunable state. This marks the first demonstration of a strain-tunable superconducting spin valve with infinite magnetoresistance. We combine tunable uniaxial stress and applied magnetic field on the ferromagnetic superconductor Eu(Fe 0.88 Co 0.12 ) 2 As 2 to shift the field-induced zero-resistance temperature between 4 K and a record-high value of 10 K. We use x-ray diffraction and spectroscopy measurements under stress and field to reveal that strain tuning of the nematic order and field tuning of the ferromagnetism act as independent control parameters of the superconductivity. Combining comprehensive measurements with DFT calculations, we propose that field-induced superconductivity arises from a novel mechanism, namely, the uniquely dominant effect of the Eu dipolar field when the exchange field splitting is nearly zero.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Lattice Resonances of Nanohole Arrays for Quantum Enhanced Sensing

We report periodic arrays of nanoholes perforated in metallic thin films interact strongly with light and produce large electromagnetic near-field enhancements in their vicinity. As a result, the optical response of these systems is very sensitive to changes in their dielectric environment, thus making them an exceptional platform for the development of compact optical sensors. Given that these systems already operate at the shot-noise limit when used as optical sensors, their sensing capabilities can be enhanced beyond this limit by probing them with quantum light, such as squeezed or entangled states. Motivated by this goal, here, we present a comparative theoretical analysis of the quantum enhanced sensing capabilities of metallic nanohole arrays with one and two holes per unit cell. Through a detailed investigation of their optical response, we find that the two-hole array supports resonances that are narrower and stronger than its one-hole counterpart, and therefore have a higher fundamental sensitivity limit as defined by the quantum Cramer-Rao bound. We validate the optical response of the analyzed arrays with experimental measurements of the reflectance of representative samples. The results of this work advance our understanding of the optical response of these systems and pave the way for developing sensing platforms capable of taking full advantage of the resources offered by quantum states of light.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2025

Experimental Measurement of Particle-to-Particle Heat Transfer in Nanoparticle Solids

Thermal conductivity in nanoparticle solids has been previously reported in the range of 0.1-1 W m -1 K -1 , which is a much smaller variation than the orders of magnitude differences achievable in electrical conductivity of similar systems. Both the low absolute magnitude of thermal conductivity and the relative insensitivity compared to electrical conductivity may be largely attributed to the poor interfacial thermal conductance of the many interfaces of the nanocrystal solid, but a direct experimental study of these interfaces is challenging. Here, in this work, we overcome this challenge via direct spectroscopic observation of heat flow within the components of a nanocrystal solid. These thermal transfers are studied by mixing two distinct types of particles: one that serves as a selectively excited antenna to inject heat and the other as the thermal acceptor to report the time-dependent change in temperature. Using transient spectroscopy, the equilibration between the heat donor and heat acceptor is observed to require similar to ~300 ps at room temperature, speeds up at reduced temperature, and has only weak sensitivity to the relative stoichiometry of the components or the intervening ligands. These results contrast strongly with the 10-20 ps time-scale of through-bond heat transfer at the ligand-particle surface and highlight the substantially lower interfacial thermal conductance of particle-to-particle transport without covalent bonding. It is also found, serendipitously, that the mixed composite films show an unexpected, substantially enhanced nonlinear absorption at the resonant wavelength of the plasmonic particles, which is tentatively attributed to a local field enhancement.

heat transfer↗