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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 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Giant piezoelectricity in fluoropolymer fiber mats achieved by corona poling in water

Polymer piezoelectrics hold great potential for energy harvesting and wearable electronics. Efforts have been dedicated to enhancing piezoelectric coefficients and thermostability for several decades, but most of these have not been successful. In this report, we demonstrate a straightforward way to achieve high piezoelectric coefficients and output voltages while maintaining high thermostability at temperatures over 110 °C. Poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-TrFE)] 80/20 mol.% nanofiber mats (made by electrospinning) with extremely high crystallinity and Curie temperatures were obtained via a two-step annealing process, from which large ferroelectric domains were formed in extended-chain crystals. After corona poling using water, which is a high dielectric constant medium, giant piezoelectricity (apparent d 33 = 1045 ± 20 pC/N) and high output voltages (29.9 ± 0.5 V) were achieved. It is found that the dimensional effect induced significant polarization changes, which is the key requirement for piezoelectricity. Our finding in this work paves a way to further improve high-performance polymer piezoelectrics.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Piezo-electrocatalytic oxidation of methanol with UV-ozone treated wurtzite zinc oxide nanostructures

Leveraging mechanically-induced piezoelectric polarization, piezocatalysis emerges as a viable mechanism for enhancing the efficiency of catalytic processes. Nanostructured, catalytically active, rationally designed piezoelectric semiconductors can achieve high-performance catalysts for various applications using cost-effective electrocatalytic pathways, such as mechanical stimuli. We design and demonstrate for the first time a cost-effective, high-performance piezo-electrocatalyst for anodic methanol oxidation, which is crucial for the practical application and deployment of direct methanol fuel cells in a variety of emerging clean energy technologies. We synthesized wurtzite ZnO nanorods and nanosheets treated with UV-O 3 to characterize and compare their efficacy for piezo-electrocatalytic methanol oxidation. The generation of piezoelectric polarization charges in nanostructured semiconducting ZnO catalysts significantly increased their electrocatalytic performance. By elucidating the charge transfer between mechanically-deformed ZnO nanostructures and methanol molecules, we identified the underlying mechanism for the piezo-electrocatalytic process for methanol oxidation. The facile synthesis of high-quality ZnO nanostructures enables low-cost, scalable manufacture and direct integration into electrocatalysts whose performance could be enhanced by harvesting mechanical energy that would otherwise be wasted in the working environment.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2021

Coexistence of Magnetoelectric and Antiferroelectric-like Orders in Mn 3 Ta 2 O 8

We report in materials showing a linear magnetoelectric (ME) effect, unconventional functionalities can be anticipated such as electric control of magnetism and nonreciprocal optical responses. Thus, the search for new linear ME materials is of interest in materials science. In this study, using a recently proposed design principle of linear ME materials, which is based on the combination of local structural asymmetry and collinear antiferromagnetism, we demonstrate that an anion-deficient fluorite derivative, Mn 3 Ta 2 O 8 , is a new linear ME material. This is evidenced by the onset of magnetic-field-induced electric polarization in its collinear antiferromagnetic phase below T N = 24 K. Furthermore, we also find an antiferroelectric-like phase transition at T S = 55 K, which is attributable to an off-center displacement of magnetic Mn 2+ ions. The present study shows that Mn 3 Ta 2 O 8 is a rare material that exhibits both ME and antiferroelectric-like transitions. Thus, Mn 3 Ta 2 O 8 may provide an opportunity to investigate the physics associated with complicated interactions between magnetic (spin) and electric dipole degrees of freedom.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Symmetrized Drude Oscillator Force Fields Improve Numerical Performance of Polarizable Molecular Dynamics

Drude oscillator potentials are a popular and computationally efficient class of polarizable models that represent each polarizable atom as a positively charged Drude core harmonically bound to a negatively charged Drude shell. Here we show that existing force fields that place all non-Coulomb forces on the Drude core and none on the shell inadvertently couple the dipole to non-Coulombic forces. This introduces errors where interactions with neutral particles can erroneously induce atomic polarization, leading to spurious polarizations in the absence of an electric field, exacerbating violations of equipartition in the employed Carr–Parinello scheme. A suitable symmetrization of the interaction potential that correctly splits the force between the Drude core and shell can correct this shortcoming, improving the stability and numerical performance of Drude oscillator-based simulations. The symmetrization procedure is straightforward and only requires the rescaling of a few force field parameters.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Operando Nanoscale Imaging of Electrochemically Induced Strain in a Locally Polarized Pt Grain

Developing new methods that reveal the structure of electrode materials under polarization is key to constructing robust structure-property relationships. However, many existing methods lack the spatial resolution in structural changes and fidelity to electrochemical operating conditions that are needed to probe catalytically relevant structures. Here, we combine a nanopipette electrochemical cell with three-dimensional X-ray Bragg coherent diffractive imaging to study how strain in a single Pt grain evolves in response to applied potential. During polarization, marked changes in surface strain arise from the Coulombic attraction between the surface charge on the electrode and the electrolyte ions in the electrochemical double layers, while the strain in the bulk of the crystal remains unchanged. The concurrent surface redox reactions have a strong influence on the magnitude and nature of the strain changes under polarization. In conclusion, our studies provide a powerful blueprint to understand how structural evolution influences electrochemical performance at the nanoscale.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Solvent–Solvent Correlations across Graphene: The Effect of Image Charges

Wetting experiments show pure graphene to be weakly hydrophilic, but its contact angle (CA) also reflects the character of the supporting material. Measurements and Molecular Dynamics simulations on suspended and supported graphene often reveal a CA reduction due to the presence of the supporting substrate. A similar reduction is consistently observed when graphene is wetted from both sides. The effect has been attributed to transparency to molecular interactions across the graphene sheet, however, the possibility of substrate-induced graphene polarization has also been considered. Computer simulations of CA on graphene have so far been determined by ignoring the material’s conducting properties. We improve the graphene model by incorporating its conductivity according to the Constant Applied Potential Molecular Dynamics. Using this method, we compare the wettabilities of suspended graphene and graphene supported by water by measuring the CA of cylindrical water drops on the sheets. The inclusion of graphene conductivity and concomitant polarization effects lead to a lower CA on suspended graphene but the CA reduction is significantly bigger when the sheets are also wetted from the opposite side. The stronger adhesion is accompanied by a profound change in the correlations among water molecules across the sheet. While partial charges on water molecules interacting across an insulator sheet attract charges of the opposite sign, apparent attraction among like charges is manifested across the conducting graphene. The change is associated with graphene polarization, as the image charges inside the conductor attract equally signed partial charges of water molecules on both sides of the sheet. Additionally, by using a non-polar liquid (diiodomethane), we affirm a detectable wetting translucency when liquid-liquid forces are dominated by dispersive interactions. Furthermore, our findings are important for predictive modeling toward a variety of applications including sensors, fuel cell membranes, water filtration, and graphene-based electrode materials in high-performance supercapacitors.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2024

Giant optical nonlinearity of Fermi polarons in atomically thin semiconductors

In this study, realizing strong nonlinear optical responses is a long-standing goal of both fundamental and technological importance. Recently, substantial efforts have been focused on exploring excitons in solids to achieve nonlinearities even down to few-photon levels. However, a crucial tradeoff arises as strong light–matter interactions require large oscillator strength and short radiative lifetime of excitons, which limits their nonlinearity. Here we experimentally demonstrate strong nonlinear optical responses with large oscillator strength by exploiting the coupling between excitons and carriers in an atomically thin semiconductor. By controlling the electric field and electrostatic doping of trilayer WSe 2 , we observe the hybridization between intralayer and interlayer excitons and the formation of Fermi polarons. Substantial optical nonlinearity is observed under continuous-wave and pulsed laser excitation, where the Fermi polaron resonance blueshifts by as much as ~10 meV. Intriguingly, we observe a remarkable asymmetry in the optical nonlinearity between electron and hole doping, which is tunable by the applied electric field. We attribute these features to the optically induced valley polarization due to the interactions between excitons and free charges. Our results establish atomically thin heterostructures as a highly versatile platform for engineering nonlinear optical response with applications to classical and quantum optoelectronics.

2d materials↗
DOE OSTI2021

First principles study of nearly strain-free Ni/WSe 2 and Ni/MoS 2 interfaces

Metal/transition metal dichalcogenide interfaces are the subject of active research, in part because they provide various possibilities for interplay of electronic and magnetic properties with potential device applications. Here, we present results of our first principles calculations of nearly strain-free Ni/WSe 2 and Ni/MoS 2 interfaces in thin-film geometry. It is shown that while both the WSe 2 and MoS 2 layers adjacent to Ni undergo metallic transition, the layers farther from the interface remain semiconducting. In addition, a moderate value of spin-polarization is induced on interfacial WSe 2 and MoS 2 layers. At the same time, the electronic and magnetic properties of Ni are nearly unaffected by the presence of WSe 2 and MoS 2 , except a small reduction of magnetic moment at the interfacial Ni atoms. Furthermore, these results can be used as a reference for experimental efforts on epitaxial metal/transition metal dichalcogenide heterostructures, with potential application in modern magnetic storage devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Generation of E × B flow shear by finite orbit width effects from heat sources in tokamaks

Here we present a possible mechanism for the generation of strong E × B flow shear relevant to internal transport barrier formation in tokamak plasmas. From gyrokinetic calculations, we show that strong E × B flow shear can be generated by finite orbit width (FOW) effects associated with a non-uniform heat source and is sufficient to lead to transport barrier formation in the core region with a moderate power level. Two FOW effects inducing neoclassical polarization are shown to be responsible for this: (1) the radial drift of particle orbit center due to the variation of the heat source within orbit width and (2) the non-uniformly evolved orbit width by the non-uniform heating.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Octahedral rotations in Ruddlesden-Popper layered oxides under pressure from first principles

The combination of reduced dimensionality and tunable structural distortions in layered perovskite oxides makes these materials ideal platforms for designing novel properties and functionalities. One example is hybrid improper ferroelectricity in n = 2 Ruddlesden-Popper oxides, where the combination of a layered crystal structure and rotations of the metal-oxide octahedra break symmetry and induce a polarization. Precisely controlling the octahedral rotation distortions, for example by the application of hydrostatic pressure, provides a pathway to tune and optimize the properties of these materials. Here we combine group theoretic methods, density functional theory calculations, and Landau theory analysis to investigate how octahedral rotations respond to pressure in the hybrid improper ferroelectrics Sr 3 Zr 2 O 7 , Ca 3 Ti 2 O 7 , and Sr 3 Sn 2 O 7 . We find that factors that are known to control the pressure response of ABO 3 perovskites—the formal charge of the A- and B-site cations, tolerance factor, and B-site chemistry—also impact the pressure response of these layered perovskites. We also show that coupling between the octahedral rotation and strain order parameters plays a key role in determining the overall pressure response. Despite some similarities, we find that these layered perovskites display a distinct pressure response compared to their ABO 3 perovskite analogs. By identifying trends and underlying mechanisms that control octahedral rotations in Ruddlesden-Popper oxides under pressure, this work lays the foundation for tailoring the structure and properties of these materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Searching for axionlike time-dependent cosmic birefringence with data from SPT-3G

Ultralight axionlike particles (ALPs) are compelling dark matter candidates because of their potential to resolve small-scale discrepancies between Λ CDM predictions and cosmological observations. Axion-photon coupling induces a polarization rotation in linearly polarized photons traveling through an ALP field; thus, as the local ALP dark matter field oscillates in time, distant static polarized sources will appear to oscillate with a frequency proportional to the ALP mass. We use observations of the cosmic microwave background from SPT-3G, the current receiver on the South Pole Telescope, to set upper limits on the value of the axion-photon coupling constant g φ γ over the approximate mass range 10 - 22 – 10 - 19 eV , corresponding to oscillation periods from 12 hours to 100 days. For periods between 1 and 100 days ( 4.7 × 10 - 22 eV ≤ m φ ≤ 4.7 × 10 - 20 eV ), where the limit is approximately constant, we set a median 95% C.L. upper limit on the amplitude of on-sky polarization rotation of 0.071 deg. Assuming that dark matter comprises a single ALP species with a local dark matter density of 0.3 GeV / cm 3 , this corresponds to g φ γ < 1.18 × 10 - 12 GeV - 1 × ( m φ 1.0 × 10 - 21 eV ) . These new limits represent an improvement over the previous strongest limits set using the same effect by a factor of ~ 3.8 .

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Optically Active Spin Defects in Few-Layer Thick Hexagonal Boron Nitride

Optically active spin defects in hexagonal boron nitride (hBN) are promising quantum systems for the design of two-dimensional quantum sensing units offering optimal proximity to the sample being probed. In this Letter, we first demonstrate that the electron spin resonance frequencies of boron vacancy centers ($V$$^{–}_{B}$) can be detected optically in the limit of few-atomic-layer thick hBN flakes despite the nanoscale proximity of the crystal surface that often leads to a degradation of the stability of solid-state spin defects. We then analyze the variations of the electronic spin properties of $V$$^{–}_{B}$ centers with the hBN thickness with a focus on (i) the zero-field splitting parameters, (ii) the optically induced spin polarization rate and (iii) the longitudinal spin relaxation time. Furthermore, this Letter provides important insights into the properties of $V$$^{–}_{B}$ centers embedded in ultrathin hBN flakes, which are valuable for future developments of foil-based quantum sensing technologies.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Heteroanionic Ruddlesden-Popper ferroelectrics from anion order and octahedral tilts

We describe a strategy to design ferroelectric heteroanionic materials based on the coupling of anion order and octahedral tilts in n = 1 Ruddlesden-Popper structures, which allows noncentrosymmetric and polar compounds to arise from centrosymmetric anion-ordered structures. Here we investigate the relative phase stabilities of the polymorphs of Sr 2 ScO 3 F and Ca 2 ScO 3 F derived from this coupling using electronic structure calculations. We find that large degrees of octahedral tilting can stabilize different anion orders derived from assembly of [ScO 5 F] 8– octahedra. To further understand the link between octahedral tilting and anion order, we quantitatively separate the contributions of electrostatics and covalent interactions to the stability of tilts. We find that the tilts are driven primarily by covalent interactions and that local out-of-plane polar displacements, induced from the anion order, further stabilize the octahedral tilts through the pseudo-Jahn-Teller effect.

36 MATERIALS SCIENCE↗
DOE OSTI2023

High phase resolution: Probing interactions in complex interfaces with sum frequency generation

An often-quoted statement attributed to Wolfgang Pauli is that God made the bulk, but the surface was invented by the devil. Although humorous, the statement really reflects frustration in developing a detailed picture of a surface. In the last several decades, that frustration has begun to abate with numerous techniques providing clues to interactions and reactions at surfaces. Often these techniques require considerable prior knowledge. Complex mixtures on irregular or soft surfaces—complex interfaces—thus represent the last frontier. Two optical techniques: sum frequency generation (SFG) and second harmonic generation (SHG) are beginning to lift the veil on complex interfaces. Of these techniques, SFG with one excitation in the infrared has the potential to provide exquisite molecular- and moiety-specific vibrational data. This Perspective is intended both to aid newcomers in gaining traction in this field and to demonstrate the impact of high-phase resolution. It starts with a basic description of light-induced surface polarization that is at the heart of SFG. The sum frequency is generated when the input fields are sufficiently intense that the interaction is nonlinear. This nonlinearity represents a challenge for disentangling data to reveal the molecular-level picture. Three, high-phase-resolution methods that reveal interactions at the surface are described.

Biophysics↗
DOE OSTI2021

Electrical and Seismic Response of Saline Permafrost Soil during Freeze - Thaw Transition: Supporting Data

These data were collected during a repeat freeze-thaw cycle on a saline permafrost portion of a core to understand mechanical and geophysical property changes over time. The core used for the experiment was collected from the NGEE-Arctic Site 0 transect site located in the Barrow Environmental Observatory (BEO) in Barrow, Alaska. This region features polygonal ground, and the core was collected in the center of a flat centered polygon. The saline permafrost portion of the core was at the bottom 260-280 cm below ground surface. Data files (*.csv) include: Electrical signal change during freeze-thaw-refreeze cycle at 1 Hz; Spectral data - Complex resistivity data during Freeze-thaw of the saline permafrost core; Seismic data; and CT cross sections (*.jpg) of the core sample from approximately the same location: (A, D) before the test (thawed), (B, E) after the test (frozen), and (C, F) after deep freeze (frozen).The Next-Generation Ecosystem Experiments: Arctic (NGEE Arctic), was a research effort to reduce uncertainty in Earth System Models by developing a predictive understanding of carbon-rich Arctic ecosystems and feedbacks to climate. NGEE Arctic was supported by the Department of Energy's Office of Biological and Environmental Research. The NGEE Arctic project had two field research sites: 1) located within the Arctic polygonal tundra coastal region on the Barrow Environmental Observatory (BEO) and the North Slope near Utqiagvik (Barrow), Alaska and 2) multiple areas on the discontinuous permafrost region of the Seward Peninsula north of Nome, Alaska. Through observations, experiments, and synthesis with existing datasets, NGEE Arctic provided an enhanced knowledge base for multi-scale modeling and contributed to improved process representation at global pan-Arctic scales within the Department of Energy's Earth system Model (the Energy Exascale Earth System Model, or E3SM), and specifically within the E3SM Land Model component (ELM).

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2022

Second-order magnetic responses in quantum magnets: Magnetization under ac magnetic fields

Here we investigate second-order magnetic responses of quantum magnets against ac magnetic fields. We focus on the case where the z component of the spin is conserved in the unperturbed Hamiltonian and the driving field is applied in the xy plane. We find that linearly polarized driving fields induce a second-harmonic response, while circularly polarized fields generate only a zero-frequency response, leading to a magnetization with a direction determined by the helicity. Employing an unbiased numerical method, we demonstrate the nonlinear magnetic effect driven by the circularly polarized field in the XXZ model and show that the magnitude of the magnetization can be predicted by the dynamical spin structure factor in the linear response regime.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Laser-induced-fluorescence imaging of a spin-polarized ultracold neutral plasma in a magnetic field

In this work, we report Doppler-sensitive laser-induced-fluorescence (LIF) imaging of an ultracold neutral plasma in a magnetic field. Local values of ion density, hydrodynamic fluid velocity, temperature, and spin polarization are obtained using a fluorescence model based on velocity-resolved rate equations (REs) including the transfer of ions between states due to laser coupling and spontaneous emission. The RE approach captures optical pumping of ions into states that are not driven by the LIF excitation laser, and this is validated with experimental data. Combined molecular-dynamics and quantum-trajectories simulations verify that velocity-changing collisions have a negligible impact on the state population evolution for typical experimental conditions. Relative intensities of Zeeman components of the LIF spectra provide clear evidence that the ions are electron-spin-polarized when created by photoionization of magnetically trapped 88 Sr atoms. This probe opens many possibilities for studying thermal transport and the equilibration of neutral plasmas in overlapping regimes of strong coupling and magnetization.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

Tunable Layer Circular Photogalvanic Effect in Twisted Bilayers

In this work, we develop a general theory of the layer circular photogalvanic effect (LCPGE) in quasi-two-dimensional chiral bilayers, which refers to the appearance of a polarization-dependent, out-of-plane static dipole moment induced by circularly polarized light. We elucidate the geometric origin of the LCPGE as two types of interlayer coordinate shift weighted by the quantum metric tensor and the Berry curvature, respectively. As a concrete example, we calculate the LCPGE in twisted bilayer graphene, and find that it exhibits a resonance peak whose frequency can be tuned from visible to infrared as the twisting angle varies. The LCPGE thus provides a promising route toward frequency-sensitive, circularly polarized light detection, particularly in the infrared range.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗