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At least 1,747 records · Page 97
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Cu/Al Through Au Diffusion Characterized by KPFM

The nanometer scale characterization technique of Frequency Modulated Kelvin Probe Force Microscopy (FM-KPFM) will be used to assess a diffusion study on thin metal films that undergo accelerated aging. The KPFM technique provides a relatively easy, non-destructive methodology that does not require high-vacuum facilities to obtain nanometer-scale spatial resolution of surface chemistry changes. The KPFM technique will be exercised in an effort to explore its capacity to map surface potential contrast caused by diffusion in a manner that allows for a qualitative assessment of diffusion of Cu or Al through Au. Supporting data will be obtained from traditional techniques: AES, XPS and UPS. An aging study was conducted on thin metal test specimens comprised of 500nm Cu or Al then 500nm Au on Si. The accelerated aging process was performed under inert conditions at aging temperatures of 100°C for Cu/Au film stack and 175°C for Al/Au film stack at aging times of 8 hours, 24 hours, 96 hours (4 days), and 216 hours (9 days).A calibration method was developed using Au, Al and Cu standards to establish precision and repeatability of the KPFM technique. The average Contact Potential Difference (CPD)s and standard deviations for each metal were found and summarized. Averages from surface roughness of the AFM topography images and roughness analysis of KPFM potential images which yield an average CPD of each area of unaged vs aged coupon surfaces were compared and show trends that indicate surface chemistry.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Time-Resolved PIV Measurements of Flow Field at the Exit of a Diffuser Mounted on a Rotating Detonation Combustor

Time-resolved particle image velocimetry at 30 kHz is applied to the exhaust flow field of a RDC system configured with a diffuser geometry and a converging-diverging (CD) nozzle. The RDC is operated with methane fuel and oxygen-enriched air, motivated by power generation applications. The measured flow-field is discretized into axial and non-axial velocity components. Results are shown to characterize the flow-field across a cord length at the exit plane. Instantaneous contours of the two velocity components, and 1D histograms of velocity components, and 2D histograms of total velocity and tilt angle are shown at multiple locations in the flow. Repeatability of the data is evaluated by comparing tests conducted at similar flow rates. Results are presented for a baseline test case, and test cases showing the effect of reactant higher flow rate and CD nozzle geometry. Finally, the results are compared to a previous RDC study without the diffuser to compare the effectiveness of the diffuser and CD nozzle designs in mitigating the flow periodicity at the combustor exit.

particle image velocimetry, pressure gain combusti↗
DOE OSTI2018

Fundamental Studies of Tritium Diffusivity and Formation on the Surfaces of Defective LiAlO 2 Pellets

One of the important components of tritium-producing burnable absorber rods (TPBAR) is the γLiAlO 2 layer used in the form of an annular ceramic pellet enriched with the 6 Li isotope. Within TPBAR architecture, this oxide is located between the zircaloy-4 liner and nickel-plated zircaloy-4 tritium getter. When irradiated in a pressurized water reactor (PWR), the 6 Li pellets absorb neutrons, simulating the nuclear characteristics of a burnable absorber rod, and produce tritium ( 3 H, T). The 3 H specie chemically reacts with the metal getter, leading to the formation of a metal hydride. Accurate analysis of the 3 H transport through the ceramic pellets and the barrier/cladding system is hampered by the lack of fundamental data about the hydrogen isotope solubility and diffusivity. In TPBAR architecture, between the γ-LiAlO 2 pellet and the Ni-plated Zircaloy-4 getter (or liner), there is a gas gap. After 3 H diffuses from bulk γ-LiAlO 2 to its surface, it is possible that different 3 H species (such as T, T 2 , T 2 O, and OT, etc.) are formed on this surface which can then get into the gas gap region. Furthermore, upon recombination and desorption molecular tritium can be formed on the γ-LiAlO 2 surface. Therefore, exploring the formation of 3 H-related species on γ-LiAlO 2 surface becomes very important as an integral part of the study of 3H solubility and diffusivity in the pellets.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2020

Hydrogen Diffusivity Measurements of YH 1.87 Moderator Material with Incoherent Quasielastic Neutron Scattering

Yttrium hydride is an optimal choice of moderator material for thermal neutron spectrum reactors requiring small core volumes. The Transformation Challenge Reactor (TCR) program at Oak Ridge National Laboratory (ORNL) aims to develop an additively manufactured microreactor core by leveraging recent advances in materials, manufacturing, data analytics, and high-fidelity modeling and simulation. Yttrium hydride was selected as the moderator material for TCR due to its high moderating power, which enables the spatially efficient reactor design and thermal stability that is superior when compared with zirconium hydride. Hydrogen desorption from yttrium hydride is expected at elevated temperatures, posing safety and performance concerns. As a lifetime component in advanced reactors, appropriate strategies to mitigate hydrogen release from yttrium hydride over long-term reactor operation are required. Understanding hydrogen mobility in yttrium hydride provides the scientific basis for developing a hydrogen barrier for hydride moderator and predicting the hydrogen redistribution within the material during various operational conditions. This study investigated the hydrogen motion in YH 1.87 as a function of temperature ranging from 200 to 1,173 K using incoherent quasielastic neutron scattering (IQNS) at the Spallation Neutron Source (SNS). The results are presented and discussed in terms of hydrogen self-diffusion coefficients, activation energy for translational hydrogen motion, and hydrogen jump distances. Based on the IQNS data, YH 1.87 is found to be stable at high temperatures with no translational H motion below 1,023 K within the IQNS detection limits. In the temperature range of 1,073–1,173 K, hydrogen diffusivity in YH 1.87 is $D = 4.57 X 10^{⁻3} [m^2/s] exp (⁻\frac{1.73[eV]}{RT}$). The hydrogen concentration’s dependence on hydrogen diffusivity is also discussed in this report.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2021

A New Route to Quantum-Scale Structures through a Novel Enhanced Germanium Diffusion Mechanism

This project sought to develop a fundamental understanding of the mechanisms underlying a newly observed enhanced germanium (Ge) diffusion process in silicon germanium (SiGe) semiconductor nanostructures during thermal oxidation. Using a combination of oxidationdiffusion experiments, high resolution imaging, and theoretical modeling, a model for the enhanced Ge diffusion mechanism was proposed. Additionally, a nanofabrication approach utilizing this enhanced Ge diffusion mechanism was shown to be applicable to arbitrary 3D shapes, leading to the fabrication of stacked silicon quantum dots embedded in SiGe nanopillars. A new wet etch-based method for preparing 3D nanostructures for highresolution imaging free of obscuring material or damage was also developed. These results enable a new method for the controlled and scalable fabrication of on-chip silicon nanostructures with sub-10 nm dimensions needed for next generation microelectronics, including low energy electronics, quantum computing, sensors, and integrated photonics.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Nb3Sn growth on niobium in vapor diffusion process and its application to large RF surface

Nb3Sn-coated SRF cavities can potentially achieve superior performance in terms of quality factor, accelerating gradient, and operating temperature. Tin vapor diffusion process of Nb3Sn coating on Nb is a simple, yet efficacious technique to fabricate Nb3Sn-coated SRF cavities. The process comprises two¬ steps: ?nucleation? followed by ?deposition?. The crucible with Sn/SnCl2 and the substrate at a constant temperature of about 500 °C for several hours is the ?nucleation?; the crucible and the substrate at a constant temperature, typically, above 1000 °C is the ?deposition? step. Using custom-built sample coating chamber, we have coated over a hundred samples to systematically study the vapor diffusion process under varying process conditions and at different stages of the coating process. The surfaces, thus obtained, were investigated with surface studies techniques, such as SEM/EDS, AFM, XPS, SAM, SIMS, EBSD and TEM. Based on the experimental results, we will discuss nucleation and growth of Nb3Sn coating during vapor diffusion process.

Pudasaini, Uttar↗
DOE OSTI2024

Quantification of the Crack Evolution Process by Extracting Relevant Signal Components from Wave Propagation and Diffusive Transport Front Measurements

Wave propagation and diffusive transport phenomena in a geological rock sample undergoing crack evolution process are expected to interact with the mechanical discontinuities in the medium. The measurements of the signals associated with these phenomena can be used to assess and monitor the crack-driven micromechanical alterations in the rock. Different wave/diffusion phenomena, such as sonic propagation, pressure diffusion, and acoustic emission (AE), are sensitive to different elements of the mechanical discontinuities generated during the evolution of the crack clusters from initiation to coalescence. Sonic propagation, AE, and pressure diffusion monitoring have the potential to map the crack evolution because the transmitter-receiver arrays can be designed, arranged and tuned to (1) achieve maximum recovery of the scattered waveforms and travel times, (2) capture the later arrivals and multiple reflections, and (3) illuminate large rock volume. However, the structural/topological complexities of the mechanical discontinuities, complex distribution of the stress fields, complex mechanical alterations in media, and fluid redistribution in the crack system pose serious challenges for the detection and modeling of the crack evolution process (from here on, we will use the term ‘crack evolution process’ to mean that the crack evolution occurred under shallow crustal conditions). For purposes of accurately accounting such complexities and heterogeneities in the absence of reliable physical laws, simulation methods, and signal processing techniques, my early-career research proposal will develop and apply novel data-driven machine learning methods to: (1) extract signal components relevant to the various phases of crack evolution and (2) generate a 2D visual map of the crack evolution process.

58 GEOSCIENCES↗
DOE OSTI2024

Reference Materials for Laser and Xenon Flash Thermal Diffusivity Systems

This report presents the status of current standard reference materials for thermal diffusivity testing. In most thermal properties testing systems, it is common to use standard reference materials to show the reproducibility of the tests and certify the instrument is producing results within expected uncertainty range. The definition of reference materials or standards vary depending on the quality control protocols of the project. For thermal diffusivity, although there were standards available by the National Bureau of Standards (NBS) 40 years ago, there has been no recent NIST standard reference material (SRM). There are only a few sources that produce and provide traceable reference standards for thermal diffusivity. The certificate typically expires in 5 years and only applies to the specific lot of materials. Other reference materials are provided by the testing systems manufacturers. They are mostly well-known reference materials or materials used by the equipment manufacturers with historic data. However, most these reference materials no longer carry valid certificates. Various reference materials will be discussed along with the industry practice when only secondary standards are available.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Probing Particle Diffusion around Two Nearby Pulsars with TeV Gamma-Ray Observations from HAWC

Nearby cosmic-ray accelerators, especially pulsar wind nebulae, are possible origins of the local multi-GeV positron excess. Pulsar Geminga and B0656+14, less than 300 pc from the Earth, have been postulated as the main sources of the positron excess. With one and half years of data, the HAWC gamma-ray observatory discovered very extended TeV gamma-ray structures produced from high-energy electrons and positrons around these two nearby middle-aged pulsars. Morphology studies suggest that the diffusion in the vicinity of these two pulsars is 100 times slower than the average in our Galaxy. This result provides important constraints on the origin of positron excess, but raises questions like why diffusion is so slow at high energies near these pulsars. With more than twice of data from the previous results, we now explore the possibility of energy-dependent diffusion at these sources.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Measurements of diffuse sky emission components in high galactic latitudes at 3.5 and 4.9 μm using DIRBE and WISE data

Using all-sky maps obtained from the Cosmic Background Explorer/Diffuse Infrared Background Experiment (DIRBE) at 3.5 and 4.9 μm, we present a reanalysis of diffuse sky emissions such as zodiacal light (ZL), diffuse Galactic light (DGL), integrated starlight (ISL), and isotropic residual emission including the extragalactic background light (EBL). Our new analysis, which includes an improved estimate of ISL using the Wide-field Infrared Survey Explorer data, enabled us to find the DGL signal in a direct linear correlation between diffuse near-infrared and 100 μm emission at high Galactic latitudes (|b|>35{sup ∘}). At 3.5 μm, the high-latitude DGL result is comparable to the low-latitude value derived from the previous DIRBE analysis. In comparison with models of the DGL spectrum assuming a size distribution of dust grains composed of amorphous silicate, graphite, and polycyclic aromatic hydrocarbon (PAH), the measured DGL values at 3.5 and 4.9 μm constrain the mass fraction of PAH particles in the total dust species to be more than ∼2%. This was consistent with the results of Spitzer/IRAC toward the lower Galactic latitude regions. The derived residual emission of 8.9 ± 3.4 nWm{sup −2} sr{sup −1} at 3.5 μm is marginally consistent with the level of integrated galaxy light and the EBL constraints from the γ-ray observations. The residual emission at 4.9 μm is not significantly detected due to the large uncertainty in the ZL subtraction, the same as in previous studies. Combined with our reanalysis of the DIRBE data at 1.25 and 2.2 μm, the residual emission in the near-infrared exhibits the Rayleigh–Jeans spectrum.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Revisiting the contributions of supernova and hypernova remnants to the diffuse high-energy backgrounds: constraints on very high redshift injection

Star-forming and starburst galaxies are considered one of the viable candidate sources of the high-energy cosmic neutrino background detected in IceCube. We revisit contributions of supernova remnants (SNRs) and hypernova remnants (HNRs) in such galaxies to the diffuse high-energy neutrino and gamma-ray backgrounds, in light of the latest Fermi data above 50 GeV. We also take into account possible time-dependent effects of the cosmic-ray (CR) acceleration during the SNR evolution. CRs accelerated by the SNR shocks can produce high-energy neutrinos up to ∼100 TeV energies, but CRs from HNRs can extend the spectrum up to PeV energies. We show that, only if HNRs are dominant over SNRs, the diffuse neutrino background above 100 TeV can be explained without contradicting the gamma-ray data. However, the neutrino data around 30 TeV remain unexplained, which might suggest a different population of gamma-ray dark CR sources. We also consider possible contributions of Pop-III HNRs up to z ≲ 10 and show that they are not constrained by the gamma-ray data and thus could contribute to the diffuse high-energy backgrounds if their explosion energy reaches E{sub POP-III}∼a few×10{sup 53} erg. More conservatively, our results suggest that the explosion energy of Pop-III HNRs is E{sub POP-III}≲7×10{sup 53} erg.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Nature of the Diffuse Source and Its Central Point-like Source in SNR 0509–67.5

We examine a diffuse emission region near the center of SNR 0509−67.5 to determine its nature. Within this diffuse region we observe a point-like source that is bright in the near-IR, but is not visible in the B and V bands. We consider an emission line observed at 6766 Å and the possibilities that it is Ly α , H α , and [O ii] λ 3727. We examine the spectral energy distribution (SED) of the source, comprised of Hubble Space Telescope B , V , I , J , and H bands in addition to Spitzer /IRAC 3.6, 4.5, 5.8, and 8 μ m bands. The peak of the SED is consistent with a background galaxy at z ≈ 0.8 ± 0.2 and a possible Balmer jump places the galaxy at z ≈ 0.9 ± 0.3. These SED considerations support the emission line’s identification as [O ii] λ 3727. We conclude that the diffuse source in SNR 0509−67.5 is a background galaxy at z ≈ 0.82. Furthermore, we identify the point-like source superposed near the center of the galaxy as its central bulge. Finally, we find no evidence for a surviving companion star, indicating a double-degenerate origin for SNR 0509−67.5.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Thermal Pressure in Diffuse H{sub 2} Gas Measured by Herschel[C II] Emission and FUSE UV H{sub 2} Absorption

UV absorption studies with the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite have made important observations of H{sub 2} molecular gas in Galactic interstellar translucent and diffuse clouds. Observations of the 158 μm [C ii] fine-structure line with Herschel trace the same H{sub 2} molecular gas in emission. We present [C ii] observations along 27 lines of sight (LOSs) toward target stars of which 25 have FUSE H{sub 2} UV absorption. Two stars have only HST STIS C ii λ2325 absorption data. We detect [C ii] 158 μm emission features in all but one target LOS. For three target LOSs that are close to the Galactic plane, |b| < 1°, we also present position–velocity maps of [C ii] emission observed by Herschel Heterodyne Instrument in the Far Infrared (HIFI) in on-the-fly spectral-line mapping. We use the velocity-resolved [C ii] spectra observed by the HIFI instrument toward the target LOSs observed by FUSE to identify [C ii] velocity components associated with the H{sub 2} clouds. We analyze the observed velocity integrated [C ii] spectral-line intensities in terms of the densities and thermal pressures in the H{sub 2} gas using the H{sub 2} column densities and temperatures measured by the UV absorption data. We present the H{sub 2} gas densities and thermal pressures for 26 target LOSs and from the [C ii] intensities derive a mean thermal pressure in the range of ∼6100–7700 K cm{sup −3} in diffuse H{sub 2} clouds. We discuss the thermal pressures and densities toward 14 targets, comparing them to results obtained using the UV absorption data for two other tracers C i and CO. Our results demonstrate the richness of the far-IR [C ii] spectral data which is a valuable complement to the UV H{sub 2} absorption data for studying diffuse H{sub 2} molecular clouds. While the UV absorption is restricted to the directions of the target star, far-IR [C ii] line emission offers an opportunity to employ velocity-resolved spectral-line mapping capability to study in detail the clouds’ spatial and velocity structures.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Helium Ionization in the Diffuse Ionized Gas Surrounding UCH ii Regions

We present measurements of the singly ionized helium-to-hydrogen ratio (n{sub He{sup +}}/n{sub H{sup +}}) toward diffuse gas surrounding three ultracompact H ii (UCH ii) regions: G10.15-0.34, G23.46-0.20, and G29.96-0.02. We observe radio recombination lines of hydrogen and helium near 5 GHz using the GBT to measure the n{sub He{sup +}}/n{sub H{sup +}} ratio. The measurements are motivated by the low helium ionization observed in the warm ionized medium and in the inner Galaxy diffuse ionized regions. Our data indicate that the helium is not uniformly ionized in the three observed sources. Helium lines are not detected toward a few observed positions in sources G10.15-0.34 and G23.46-0.20, and the upper limits of the n{sub He{sup +}}/n{sub H{sup +}} ratio obtained are 0.03 and 0.05, respectively. The selected sources harbor stars of type O6 or hotter as indicated by helium line detection toward the bright radio continuum emission from the sources with mean n{sub He{sup +}}/n{sub H{sup +}} value 0.06 ± 0.02. Our data thus show that helium in diffuse gas located a few parsecs away from the young massive stars embedded in the observed regions is not fully ionized. We investigate the origin of the nonuniform helium ionization and rule out the possibilities (a) that the helium is doubly ionized in the observed regions and (b) that the low n{sub He{sup +}}/n{sub H{sup +}} values are due to additional hydrogen ionizing radiation produced by accreting low-mass stars. We find that selective absorption of ionizing photons by dust can result in low helium ionization but needs further investigation to develop a self-consistent model for dust in H ii regions.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Galactic Bar Resonances with Diffusion: An Analytic Model with Implications for Bar–Dark Matter Halo Dynamical Friction

Abstract The secular evolution of disk galaxies is largely driven by resonances between the orbits of “particles” (stars or dark matter) and the rotation of non-axisymmetric features (spiral arms or a bar). Such resonances may also explain kinematic and photometric features observed in the Milky Way and external galaxies. In simplified cases, these resonant interactions are well understood: for instance, the dynamics of a test particle trapped near a resonance of a steadily rotating bar is easily analyzed using the angle-action tools pioneered by Binney, Monari, and others. However, such treatments do not address the stochasticity and messiness inherent to real galaxies—effects that have, with few exceptions, been previously explored only with complex N -body simulations. In this paper, we propose a simple kinetic equation describing the distribution function of particles near an orbital resonance with a rigidly rotating bar, allowing for diffusion of the particles’ slow actions. We solve this equation for various values of the dimensionless diffusion strength Δ, and then apply our theory to the calculation of bar–halo dynamical friction. For Δ = 0, we recover the classic result of Tremaine and Weinberg that friction ultimately vanishes, owing to the phase mixing of resonant orbits. However, for Δ > 0, we find that diffusion suppresses phase mixing, leading to a finite torque. Our results suggest that stochasticity—be it physical or numerical—tends to increase bar–halo friction, and that bars in cosmological simulations might experience significant artificial slowdown, even if the numerical two-body relaxation time is much longer than a Hubble time.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

Debiasing with Diffusion: Probabilistic Reconstruction of Dark Matter Fields from Galaxies with CAMELS

Abstract Galaxies are biased tracers of the underlying cosmic web, which is dominated by dark matter (DM) components that cannot be directly observed. Galaxy formation simulations can be used to study the relationship between DM density fields and galaxy distributions. However, this relationship can be sensitive to assumptions in cosmology and astrophysical processes embedded in galaxy formation models, which remain uncertain in many aspects. In this work, we develop a diffusion generative model to reconstruct DM fields from galaxies. The diffusion model is trained on the CAMELS simulation suite that contains thousands of state-of-the-art galaxy formation simulations with varying cosmological parameters and subgrid astrophysics. We demonstrate that the diffusion model can predict the unbiased posterior distribution of the underlying DM fields from the given stellar density fields while being able to marginalize over uncertainties in cosmological and astrophysical models. Interestingly, the model generalizes to simulation volumes ≈500 times larger than those it was trained on and across different galaxy formation models. The code for reproducing these results can be found athttps://github.com/victoriaono/variational-diffusion-cdm✎.

Astronomy & Astrophysics↗
DOE OSTI2021

Random sub-diffusion and capture of genes by the nuclear pore reduces dynamics and coordinates inter-chromosomal movement

Hundreds of genes interact with the yeast nuclear pore complex (NPC), localizing at the nuclear periphery and clustering with co-regulated genes. Dynamic tracking of peripheral genes shows that they cycle on and off the NPC and that interaction with the NPC slows their sub-diffusive movement. Furthermore, NPC-dependent inter-chromosomal clustering leads to coordinated movement of pairs of loci separated by hundreds of nanometers. We developed fractional Brownian motion simulations for chromosomal loci in the nucleoplasm and interacting with NPCs. These simulations predict the rate and nature of random sub-diffusion during repositioning from nucleoplasm to periphery and match measurements from two different experimental models, arguing that recruitment to the nuclear periphery is due to random sub-diffusion and transient capture by NPCs. Finally, the simulations do not lead to inter-chromosomal clustering or coordinated movement, suggesting that interaction with the NPC is necessary, but not sufficient, to cause clustering.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2021

Magnetic Diffuse Scattering of YBaCo 4 O 7 and LuBaCo 4 O 7 on Kagome and Triangular Lattices

In this work, the magnetic diffuse scattering of YBaCo 4 O 7 and LuBaCo 4 O 7 with alternating kagome and triangular lattices has been studied through neutron scattering measurements on single-crystal samples. In YBaCo 4 O 7 , the line-shape-magnetic-diffuse scattering is observed around the magnetic transition temperature, which is consistent with the Monte Carlo simulation. In LuBaCo 4 O 7 , the line-shape-magnetic-diffuse scattering has not been detected. At low temperature, the propagation vectors of the observed magnetic reflections are different between YBaCo 4 O 7 and LuBaCo 4 O 7 . The structural distortion in RBaCo 4 O 7 is suggested to play a critical role for the magnetic properties.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗