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At least 361 records · Page 20
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Blob velocities and sizes in the Alcator C-Mod scrape-off layer for ohmic and high confinement mode plasmas

An improved time delay estimation method is used to calculate the velocity of cross-field blob motion in the scrape-off layer of Alcator C-Mod for an ohmic and two high confinement (H-mode) plasmas; an edge localized mode free and an enhanced D-alpha H-mode. The gas puff imaging data analysis results are interpreted in the framework of a stochastic model that describes the fluctuations as a super-position of uncorrelated blob-like structures. In all confinement modes investigated, the scrape-off layer is dominated by large amplitude, blob-like filaments moving radially outwards with velocities in the range from 400 to 1000 m/s. Blobs in high confinement mode plasmas have similar velocities and sizes as in ohmic plasma, which is consistent with the close similarity of conditionally averaged burst shapes and frequency spectra for the confinement modes investigated.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

The dependence of Ammonium-Nitrate Fuel-Oil (ANFO) detonation on confinement

As detonation is a coupled fluid-chemical process, flow divergence inside the detonation reaction zone can strongly influence detonation velocity and energy release. Such divergence is responsible for the diameter-effect and failure-diameter phenomena in condensed-phase explosives and particularly dominant in detonation of nonideal explosives such as Ammonium Nitrate and Fuel Oil (ANFO). In this study, the effect of reaction zone flow divergence on ANFO detonation was explored through variation of the inert confinement and explosive diameter in the rate-stick geometry with cylinder expansion experiments. New tests are discussed and compared to prior experiments. Presented results include the detonation velocity as a function of diameter and confinement, reaction zone times, detonation product isentropes and energies, as well as sonic surface pressures and velocities. Product energy densities and isentropes were found to increase with detonation velocity, indicating more complete chemical reaction with increased detonation velocity. In addition, detonation reaction zone times were found to scale with the acoustic transit time of the confiner wall and used to show that the ANFO diameter effect scaled with the reaction zone time for a particle along the flow centerline, regardless of the confinement. Such a result indicates that the ANFO reaction mechanisms are sufficiently slow that the centerline fluid expansion timescale is a limiting factor controlling detonation velocity and energy release.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Detonation propagation for shock-driven, subsonic and supersonic confiner flow

In this work, we study the compressible flow dynamics of two-dimensional, steady detonation wave propagation in a high explosive (HE) confined by aluminium (Al) or stainless steel (SS), outside of which is an air layer. We examine how the thickness of the confinement affects the subsonic detonation driving zone structure (DDZ) and the detonation speed (D 0 ), demonstrating a strong dependence on whether the oblique shock-driven flow in the confiner is supersonic, as for SS, or subsonic, as for Al. A characteristic path analysis is used to examine the information flow from the material boundaries through the supersonic flow regions in both the HE and confiner that can impact the sonic surfaces bounding the subsonic flow regions. It is shown that the nature of gas-dynamic wave reflection off the SS–air or Al–air boundary can significantly influence the DDZ and D 0 .

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2023

Hydrodynamic irreversibility of non-Brownian suspensions in highly confined duct flow

The irreversible behaviour of a highly confined non-Brownian suspension of spherical particles at low Reynolds number in a Newtonian fluid is studied experimentally and numerically. In the experiment, the suspension is confined in a thin rectangular channel that prevents complete particle overlap in the narrow dimension and is subjected to an oscillatory pressure-driven flow. In the small cross-sectional dimension, particles rapidly separate to the walls, whereas in the large dimension, features reminiscent of shear-induced migration in bulk suspensions are recovered. Furthermore, as a consequence of the channel geometry and the development and application of a single-camera particle tracking method, three-dimensional particle trajectories are obtained that allow us to directly associate relative particle proximity with the observed migration. Companion simulations of a steadily flowing suspension highly confined between parallel plates are conducted using the force coupling method, which also show rapid migration to the walls as well as other salient features observed in the experiment. While we consider relatively low volume fractions compared to most prior work in the area, we nevertheless observe significant and rapid migration, which we attribute to the high degree of confinement.

42 ENGINEERING↗
DOE OSTI2018

Anomalous H 2 Desorption Rate of NaAlH 4 Confined in Nitrogen-Doped Nanoporous Carbon Frameworks

Confining NaAlH 4 in nanoporous carbon scaffolds is known to alter the sorption kinetics and/or pathways of the characteristic bulk hydride reactions through interaction with the framework at the interface, increased specific surface area of the resulting nanoparticles, decreased hydrogen diffusion distances, and prevention of phase segregation. Although the nanosize effects have been well studied, the influence of the carbon scaffold surface chemistry remains unclear. Here we compare the hydrogen sorption characteristics of NaAlH 4 confined by melt infiltration in nitrogen-doped/undoped ordered nanoporous carbon of two different geometries. 23 Na and 27 Al MAS NMR, N 2 sorption, and PXRD verify NaAlH 4 was successfully confined and remains intact in the carbon nanopores after infiltration. Both the N-doped/undoped nanoconfined systems demonstrate improved reversibility in relation to the bulk hydride during hydrogen desorption/absorption cycling. Isothermal kinetic measurements indicate a lowering of the activation energy for H 2 desorption by as much as 70 kJ/mol in N-doped frameworks, far larger than the reduction in carbon-only frameworks. Most interestingly, this dramatic lowering of the activation energy is accompanied by an unexpected and anomalously low NaAlH 4 desorption rate in the N-doped frameworks. This suggests that the framework surface chemistry plays an important role in the desorption process and that the rate limiting step for desorption may be associated with interactions of the hydride and host surface. Our results indicate that functionalization of carbon scaffold surface chemistry with heteroatoms provides a powerful method of altering the characteristic hydrogen sorption properties of confined metal hydride systems. Furthermore, this technique may prove beneficial in the path to a viable metal hydride-based hydrogen storage system.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Nafion induced surface confinement of oxygen in carbon-supported oxygen reduction catalysts

We studied the surface confinement of oxygen inside layers of Nafion self-assembled on carbon-supported oxygen reduction reaction (ORR) catalysts. It is demonstrated that oxygen accumulates in the hydrophobic component of the polymer remaining in contact with the carbon surface. Furthermore, the amount of surface confined oxygen increases with the degree of carbon surface graphitization, which promotes the self-assembly of the polymer. Planar macrocyclic ORR catalysts possessing a delocalized system of π electrons such as Co and Fe porphyrins and phthalocyanines have virtually no effect on the surface confinement of oxygen, in accordance with their structural similarity to graphitic carbon surfaces where they adsorb. Platinum particles in carbon-supported ORR catalysts with high metal contents (20%) disrupt the self-assembly of Nafion and virtually eliminate the oxygen confinement, but the phenomenon is still observed for low Pt loading (4.8%) catalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Grain Growth in Nanosized Nickel Deformed in a Confining Environment

Stress-induced grain growths were observed in many nanosized metals and alloys. Correctly understanding this unique feature is critical in designing and tailoring proper nanostructures for particular applications in mechanically harsh environments, e.g., high-stress concentration, mechanical vibrations. Though extensive researches have been carried out on metals or alloys subjected to deformation, experimental studies on metals deformed in a confining environment are severely lacking. Nearly all of the previous investigations on this theme were carried out either through theoretical simulations or by using transmission electron microscopy to probe grain variation in vacuo. In the present study, we employed a powerful high-pressure technique along with high-energy synchrotron X-rays to monitor the grain size evolution in situ, in nanosized nickel deformed in a confining environment. Our experimental data demonstrate, for the first time, that grain sizes grow with the increment of the confining pressure. Finally, since plasticity strongly depends on the grain size of a given material, understanding and controlling fundamental mechanisms leading to stress-assisted grain growth may pave a new route to improve the mechanical properties of nanosized materials by tuning confining environment within which the deformation is operated.

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

Role of Undercoordinated Sites for the Catalysis in Confined Spaces Formed by Two-Dimensional Material Overlayers

Adding a two-dimensional (2D) overlayer on a metal surface is a promising route for activating reactants confined in the interfacial space. However, an atomistic understanding of the role played by undercoordinated sites of the 2D overlayer in the activation of molecules in this nanoscaled confined space is yet to be developed. In this paper, we study CO dissociation as a prototypical reaction to investigate CO activation in the confined space enclosed by Rh(111) and a monolayer of hexagonal boron nitride (h-BN). The effect of the space size (i.e., the distance between h-BN and the metal surface), the type of undercoordinated sites, and the size of the defect are explicitly studied by density functional theory with dispersion correction. The following temperature-programmed X-ray photoelectron spectroscopy measurement suggests that a small portion of the CO dissociated during the desorption, leaving the residual atomic oxygen incorporated into the h-BN lattice, which validates the theoretical prediction. Finally, the combination of theory and experiment calls for further attention to be paid to the role of undercoordinated sites in the 2D overlayers in confined systems forming potential new catalytic environments.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Delineating Confinement Regimes for Nafion Thin Films via Simultaneous QCM-D and Spectroscopic Ellipsometry

Proton-conducting ionomers used in electrodes of electrochemical devices form nanometer-scale films covering metallic catalyst surfaces, wherein they experience confinement and interfacial effects absent in the bulk polymer. This confinement alters the physical properties of the ionomer film, which is postulated to increase the density and is attributed to the transport limitations observed in fuel-cell electrodes. Despite studies showing reduced swelling, no systematic measurement has validated this phenomenon by demonstrating both densification and stiffening as the film gets thinner. Here, this study aims to fill this gap by reporting the humidity-driven changes in swelling, mass uptake, and density of Nafion ionomer films cast at varying thicknesses (10–320 nm) onto a gold-plated sensor. The films were simultaneously probed during humidification using spectroscopic ellipsometry (SE) and a quartz crystal microbalance with dissipation (QCM-D), which allowed for determination of the density and shear stiffness. The effects of confinement were prominently observed below ∼30 nm, with films showing increased density along with decreased swelling and mass uptake during hydration. In addition, the confined films show a negative excess volume upon initial water sorption, implying a larger proportion of absorbed water might bound to ionic groups in accord with notion of localized densification.

Espinet, Kevin B. [University of California, Berke↗
DOE OSTI2019

Confined Interlayer Water Promotes Structural Stability for High-Rate Electrochemical Proton Intercalation in Tungsten Oxide Hydrates

There is widespread interest in determining the structural features of redox-active electrochemical energy storage materials that enable simultaneous high power and high energy density. Here, we present the discovery that confined interlayer water in crystalline tungsten oxide hydrates, WO 3 · n H 2 O, enables highly reversible proton intercalation at subsecond time scales. By comparing the structural transformation kinetics and confined water dynamics of the hydrates with anhydrous WO 3 , we determine that the rapid electrochemical proton intercalation is due to the ability of the confined water layers to isolate structural transformations to two dimensions while stabilizing the structure along the third dimension. As a result, these water layers provide both structural flexibility and stability to accommodate intercalation-driven bonding changes. Furthermore, this provides an alternative explanation for the fast energy storage kinetics of materials that incorporate structural water and provides a new strategy for enabling high power and high energy density with redox-active layered materials containing confined fluids.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Anomalous Confinement Slows Surface Fluctuations of Star Polymer Melt Films

The unusually large film thickness at which confinement effects manifest themselves in surface fluctuations of unentangled four-arm star polymers has been defined using film thicknesses from 10 R g to 107 R g . For 15k four-arm star polystyrene (SPS) confinement appears at a thickness between 112 nm (40 R g ) and 72 nm (26 R g ), which is remarkably larger than the thicknesses at which confinement appears for unentangled 6k linear (<15 nm, <7 R g ) and 6k and 14k cyclic (24 nm and 22 nm, respectively) polystyrenes. Data for 15k star films can be rationalized using a two-layer model with a 17 nm (6 R g ) thick highly viscous layer at the substrate, which is significantly thicker than the 1 R g thick “irreversibly adsorbed” layer. For a 29 nm (10 R g ) thick film more striking confinement occurs due to the combined influence of both interfaces. 2 Here, these results underscore the extraordinary role long-chain branching plays in dictating surface fluctuations of thin films.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Oxide-Free Three-Dimensional Germanium/Silicon Core–Shell Metalattice Made by High-Pressure Confined Chemical Vapor Deposition

Metalattices are crystalline arrays of uniform particles in which the period of the crystal is close to some characteristic physical length scale of the material. In this work, we explore the synthesis and properties of a germanium metalattice in which the similar to 70 nm periodicity of a silica colloidal crystal template is close to the similar to 24 nm Bohr exciton radius of the nanocrystalline Ge replica. The problem of Ge surface oxidation can be significant when exploring quantum confinement effects or designing electronically coupled nanostructures because of the high surface area to volume ratio at the nanoscale. To eliminate surface oxidation, we developed a coreshell synthesis in which the Ge metalattice is protected by an oxide-free Si interfacial layer, and we explore its properties by transmission electron microscopy (TEM), Raman spectroscopy, and electron energy loss spectroscopy (EELS). The interstices of a colloidal crystal film grown from 69 nm diameter spherical silica particles were filled with polycrystalline Ge by high-pressure confined chemical vapor deposition (HPcCVD) from GeH4. After the SiO2 template was etched away with aqueous HF, the Ge replica was uniformly coated with an amorphous Si shell by HPcCVD as confirmed by TEM-EDS (energy-dispersive X-ray spectroscopy) and Raman spectroscopy. Formation of the shell prevents oxidation of the Ge core within the detection limit of XPS. The electronic properties of the core-shell structure were studied by accessing the Ge 3d edge onset using STEM-EELS. A blue shift in the edge onset with decreasing size of Ge sites in the metalattices suggests quantum confinement of the Ge core. The degree of quantum confinement of the Ge core depends on the void sizes in the template, which is tunable by using silica particles of varying size. The edge onset also shows a shift to higher energy near the shell in comparison with the Ge core. This shift along with the observation of Ge-Si vibrational modes in the Raman spectrum indicate interdiffusion of Ge and Si. Both the size of the voids in the template and core-shell interdiffusion of Si and Ge can in principle be tuned to modify the electronic properties of the Ge metalattice.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Pyrolysis of Phenethyl Phenyl Ether Tethered in Mesoporous Silica. Effects of Confinement and Surface Spacer Molecules on Product Selectivity

There has been expanding interest in exploring porous metal oxides as a confining environment for organic molecules resulting in altered chemical and physical properties including chemical transformations. In this paper, we examine the pyrolysis behavior of phenethyl phenyl ether (PPE) confined in mesoporous silica by covalent tethers to the pore walls as a function of tether density and the presence of co-tethered surface spacer molecules of varying structure (biphenyl, naphthyl, octyl, and hexadecyl). The PPE pyrolysis product selectivity, which is determined by two competitive free-radical pathways cycling through the two aliphatic radical intermediates (PhCH CH2OPh and PhCH2CH OPh), is shown to be dramatically different from that measured in the liquid phase as well as for PPE tethered to the exterior surface of nonporous silica nanoparticles. Tailoring the pore surface with spacer molecules further alters the selectivity such that the PPE reaction channel involving a molecular rearrangement (O-C phenyl shift in PhCH2CH OPh), which accounts for 25 % of the products in the liquid phase, can be virtually eliminated under pore confinement conditions. The origin of this change in selectivity is discussed in the context of steric constraints on the rearrangement path, confinement effects, pore surface curvature, and hydrogen bonding of PPE with residual surface silanols supplemented by nitrogen physisorption data and molecular dynamics simulations.

Kidder, Michelle↗
DOE OSTI2019

Spinon Confinement and a Sharp Longitudinal Mode in Yb 2 Pt 2 Pb in Magnetic Field

Here, the fundamental excitations in an antiferromagnetic chain of spins-1/2 are spinons, de-confined fractional quasiparticles that when combined in pairs, form a triplet excitation continuum. In an Ising-like spin chain the continuum is gapped and the ground state is Neel ordered. Here, we report high resolution neutron scattering experiments, which reveal how a magnetic field closes this gap and drives the spin chains in Yb 2 Pt 2 Pb to a critical, disordered Luttinger-liquid state. In Yb 2 Pt 2 Pb the effective spins-1/2 describe the dynamics of large, Ising-like Yb magnetic moments, ensuring that the measured excitations are exclusively longitudinal, which we find to be well described by time-dependent density matrix renormalization group calculations. The inter-chain coupling leads to the confinement of spinons, a condensed matter analog of quark confinement in quantum chromodynamics. Insensitive to transverse fluctuations, our measurements show how a gapless, dispersive longitudinal mode arises from confinement and evolves with magnetic order.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Asymmetric rotations slow down diffusion under confinement

Translation and rotation are the two most fundamental forms of diffusion, yet their coupling mechanism is not clear, especially under confinement. Here, we provided evidence of the coupling between rotation and translation using a substituted benzene molecule as an example. A counterintuitive behavior was observed where the movement of the smaller molecule with an asymmetric shape was unexpectedly slower than the larger one with a symmetric shape in confined channels of zeolite. We showed that this diffusion behavior was caused by the presence of the specific and selective interaction of the asymmetric guest with the pores, which increased the local restricted residence time, thus inhibiting the translation under confinement, as further confirmed by dynamic breakthrough curves, uptake measurements, quasi-elastic neutron scattering, and 2 H solid-state NMR techniques. Our work correlated asymmetric rotation and diffusion under a confined environment, which enriched our understanding of the coupling between rotation and translation and could shed light on a fundamental understanding of the diffusion process.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Measuring spin correlation between quarks during QCD confinement

The vacuum is now understood to have a rich and complex structure, characterized by fluctuating energy fields and a condensate of virtual quark–antiquark pairs. The spontaneous breaking of the approximate chiral symmetry, signalled by the nonvanishing quark condensate $\langle$$q\bar{q}$$\rangle$, is dynamically generated through topologically nontrivial gauge configurations such as instantons. The precise mechanism linking the chiral symmetry breaking to the mass generation associated with quark confinement remains a profound open question in quantum chromodynamics (QCD)—the fundamental theory of strong interaction. High-energy proton–proton collisions could liberate virtual quark–antiquark pairs from the vacuum that subsequently undergo confinement to form hadrons, whose properties could serve as probes into QCD confinement and the quark condensate. Here we report evidence of spin correlations in $Λ\bar{Λ}$ hyperon pairs inherited from spin-correlated strange quark–antiquark virtual pairs. Measurements by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory reveal a relative polarization signal of (18 ± 4)% that links the virtual spin-correlated quark pairs from the QCD vacuum to their final-state hadron counterparts. Crucially, this correlation vanishes when the hyperon pairs are widely separated in angle, consistent with the decoherence of the quantum system. Our findings provide a new experimental model for exploring the dynamics and interplay of quark confinement and entanglement.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2018

Confinement and asymptotic freedom with Cooper pairs

One of the most profound aspects of the standard model of particle physics, the mechanism of confinement binding quarks into hadrons, is not sufficiently understood. The only known semiclassical mechanism of confinement, mediated by chromo-electric strings in a condensate of magnetic monopoles still lacks experimental evidence. Here we show that the infinite resistance superinsulating state, which emerges on the insulating side of the superconductor insulator transition in superconducting films offers a realization of confinement that allows for a direct experimental access. We find that superinsulators realize a single-color version of quantum chromodynamics and establish the mapping of quarks onto Cooper pairs. We reveal that the mechanism of superinsulation is the linear binding of Cooper pairs into neutral “mesons” by electric strings. Here, our findings offer a powerful laboratory for exploring and testing the fundamental implications of confinement, asymptotic freedom, and related quantum chromodynamics phenomena via the desktop experiments on superconductors.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Extremely reduced dielectric confinement in two-dimensional hybrid perovskites with large polar organics

Two dimensional inorganic–organic hybrid perovskites (2D perovskites) suffer from not only quantum confinement, but also dielectric confinement, hindering their application perspective in devices involving the conversion of an optical input into current. In this report, we theoretically predict that an extremely low exciton binding energy can be achieved in 2D perovskites by using high dielectric-constant organic components. We demonstrate that in (HOCH 2 CH 2 NH 3 ) 2 PbI 4 , whose organic material has a high dielectric constant of 37, the dielectric confinement is largely reduced, and the exciton binding energy is 20-times smaller than that in conventional 2D perovskites. As a result, the photo-induced excitons can be thermally dissociated efficiently at room temperature, as clearly indicated from femtosecond transient absorption measurements. In addition, the mobility is largely improved due to the strong screening effect on charge impurities. Such low dielectric-confined 2D perovskites show excellent carrier extraction efficiency, and outstanding humidity resistance compared to conventional 2D perovskites.

42 ENGINEERING↗