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At least 1,567 records · Page 87
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Effect of thermal annealing on microstructure evolution and mechanical behavior of an additive manufactured AlSi10Mg part

The powder-bed laser additive manufacturing (AM) process is widely used in the fabrication of three-dimensional metallic parts with intricate structures, where kinetically controlled diffusion and microstructure ripening can be hindered by fast melting and rapid solidification. Therefore, the microstructure and physical properties of parts made by this process will be significantly different from their counterparts produced by conventional methods. This work investigates the microstructure evolution for an AM fabricated AlSi10Mg part from its nonequilibrium state toward equilibrium state. Special attention is placed on silicon dissolution, precipitate formation, collapsing of a divorced eutectic cellular structure, and microstructure ripening in the thermal annealing process. These events alter the size, morphology, length scale, and distribution of the beta silicon phase in the primary aluminum, and changes associated with elastic properties and microhardness are reported. In conclusion, the relationship between residual stress and silicon dissolution due to changes in lattice spacing is also investigated and discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Effect of the wire width on the intrinsic detection efficiency of superconducting-nanowire single-photon detectors

A thorough spectral study of the intrinsic single-photon detection efficiency in superconducting TaN and NbN nanowires with different widths has been performed. The experiment shows that the cut-off of the intrinsic detection efficiency at near-infrared wavelengths is most likely controlled by the local suppression of the barrier for vortex nucleation around the absorption site. Beyond the cut-off quasi-particle diffusion in combination with spontaneous, thermally activated vortex crossing explains the detection process. For both materials, the reciprocal cut-off wavelength scales linearly with the wire width where the scaling factor agrees with the hot-spot detection model.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

From stochastic processes to numerical methods: A new scheme for solving reaction subdiffusion fractional partial differential equations

We have introduced a new explicit numerical method, based on a discrete stochastic process, for solving a class of fractional partial differential equations that model reaction subdiffusion. The scheme is derived from the master equations for the evolution of the probability density of a sum of discrete time random walks. We show that the diffusion limit of the master equations recovers the fractional partial differential equation of interest. This limiting procedure guarantees the consistency of the numerical scheme. The positivity of the solution and stability results are simply obtained, provided that the underlying process is well posed. We also show that the method can be applied to standard reaction–diffusion equations. This work highlights the broader applicability of using discrete stochastic processes to provide numerical schemes for partial differential equations, including fractional partial differential equations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Atmospheric pressure synthesis of photoluminescent hybrid materials by sequential organometallic vapor infiltration into polyethylene terephthalate fibers

Exposing a polymer to sequential organometallic vapor infiltration (SVI) under low pressure conditions can significantly modify the polymer's chemical, mechanical, and optical properties. We demonstrate that SVI of trimethylaluminum into polyethylene terephthalate (PET) can also proceed readily at atmospheric pressure, and at 60 °C the extent of reaction determined by mass uptake is independent of pressure between 2.5 Torr and 760 Torr. At 120 °C, however, the mass gain is 50% larger at 2.5 Torr relative to that at 760 Torr, indicating that the precursor diffusion in the chamber and fiber matrix decreases at higher source pressure. Mass gain decreases, in general, as the SVI process temperature increases both at 2.5 Torr and 760 Torr attributed to the faster reaction kinetics forming a barrier layer, which prevents further diffusion of the reactive species. The resulting PET/Al-O{sub x} product shows high photoluminescence compared to untreated fibers. A physical mask on the polymer during infiltration at 760 Torr is replicated in the underlying polymer, producing an image in the polymer that is visible under UV illumination. Because of the reduced precursor diffusivity during exposure at 760 Torr, the image shows improved resolution compared to SVI performed under typical 2.5 Torr conditions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Deconvolution of Water-Splitting on the Triple-Conducting Ruddlesden–Popper-Phase Anode for Protonic Ceramic Electrolysis Cells

Triple-conducting materials have been proved to improve the performance of popular protonic ceramic electrolysis cells. However, partially because of the complexity of the water splitting reaction involving three charge carriers, that is, oxygen (O 2– ), proton (H + ), and electron (e – ), the triple-conducting reaction mechanism was not clear, and the reaction conducting pathways have seldom been addressed. In this study, the triple conducting Ruddlesden–Popper phase Pr 1.75 Ba 0.25 NiO 4+δ as an anode on the BaCe 0.7 Zr 0.1 Y 0.1 Yb 0.1 O 3–δ electrolyte was fabricated and its electroresponses were characterized by electrochemical impedance spectroscopy with various atmospheres and temperatures. The impedance spectra are deconvoluted by means of the distribution of the relaxation time method. The surface exchange rate and chemical diffusivity of H + and O 2– are characterized by electrical conductivity relaxation. The physical locations of electrochemical processes are also identified by atomic layer deposition with a surface inhibitor. A microkinetics model is proposed toward conductivities, triple-conducting pathways, reactant dependency, surface exchange and bulk diffusion capabilities, and other relevant properties. Lastly, the rate-limiting steps and suggestions for further improvement of electrode performance are presented.

25 ENERGY STORAGE↗
DOE OSTI2025

Tritium adsorption and absorption on (100) and (001) surfaces of pure and tin defective zirconium

Zirconium alloys such as zircaloy-4 are used as tritium (T) getter materials in tritium-producing burnable absorber rods (TPBARs) due to their ability to capture T, thereby forming metal hydrides. Developing an understanding of T adsorption onto zircaloy prior to diffusion into the subsurface is relevant for rational tritium getter and TPBAR design, to improve material properties for nuclear applications. Herein, density functional theory calculations revealed the preferred binding sites for T adsorption on Zr(001) and Zr(100). The energy barriers of T transfer, along the surface and from the surface to the subsurface were computed. The adsorption properties of Zr(001) were found to be superior to those of Zr(100). Surface tin impurities were found to strongly repel T. The presence of subsurface and surface tin resulted in higher absorption energy barriers for both the forward and reverse processes. Based on the calculated energy barriers, a surface to surface T diffusion coefficient of 9.53 × 10 -10 m 2 s -1 is expected for pristine Zr(001). Finally, a surface to subsurface T diffusion coefficient on the order of 10 -13 m 2 s -1 is predicted in pristine Zr, decreasing to 10 -19 m 2 s -1 for the transfer with a subsurface tin impurity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Transformation of divacancies to divacancy-oxygen pairs in p-type Czochralski-silicon; mechanism of divacancy diffusion

In this work, a comprehensive study on the transition of divacancy (V{sub 2}) to divacancy-oxygen (V{sub 2}O) pairs in p-type silicon has been performed with deep level transient spectroscopy (DLTS). Czochralski grown, boron doped p-type, silicon samples, with a doping concentration of 2 × 10{sup 15} cm{sup −3} and oxygen content of 7.0 ± 1.5 × 10{sup 17} cm{sup −3}, have been irradiated with 1.8 MeV protons. Isothermal annealing at temperatures in the range of 200 °C–300 °C shows a close to one-to-one correlation between the loss in the donor state of V{sub 2} and the formation of the donor state of V{sub 2}O, located at 0.23 eV above the valence band edge. A concurrent transition takes place between the single acceptor states of V{sub 2} and V{sub 2}O, as unveiled by injection of electrons through optical excitation during the trap filling sequence of the DLTS measurements. Applying the theory for diffusion limited reactions, the diffusivity of V{sub 2} in the studied p-type samples is determined to be (1.5 ± 0.7) × 10{sup −3}exp[−(1.31 ± 0.03) eV/kT] cm{sup 2}/s, and this represents the neutral charge state of V{sub 2}. Further, the data seem to favor a two-stage diffusion mechanism involving partial dissociation of V{sub 2}, although a one-stage process cannot be fully excluded.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Cation exchange kinetics of pyridinecarboxylic acids

The sorption—desorption kinetics of nicotinic and isonicotinic acids on strong acid cationites (sulfocationite CU-2-8, sulfonated polycalixarene) were considered. The rate of the processes, following both the cation exchange mechanism H{sup +} ⇄ Cat{sup +} and the ion exchange mechanism accompanied by the chemical reaction of protonation of pyridinecarboxylic acid zwitterions, is determined by a slow diffusion of components in the polymeric phase. The interdiffusion coefficient in sulfocationite CU-2-8 for the process H{sup +} ⇄ Cat{sup +} was equal to ~1•10{sup –11} m{sup 2} s{sup –1}, in sulfonated polycalixarene the corresponding value was in the range of (1—4)•10{sup –12} m{sup 2} s{sup –1}.

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

Strongly extended diffusion length for the nonequilibrium magnons in Y 3 F e 5 O 12 by photoexcitation

Y 3 Fe 5 O 12 (YIG) is known for its long magnon diffusion length. Although it has the known lowest damping rate, an even longer diffusion distance is still highly desired since it may lead to a much more efficient information transmission and processing. While most of previous works focused on the generation and detection of magnons in YIG, here we demonstrate how to depress the damping rate during the diffusion of magnon. By selectively exciting the spin state transition of the Fe ions in YIG, we successfully increase magnon diffusion length by one order of magnitude, i.e., from the previous reported ~10 μm up to ~156 μm (for the sample prepared by liquid phase epitaxy) and ~180 μm (for the sample prepared by pulsed laser deposition) at room temperature. The diffusion length, determined by nonlocal geometry, is ~30 μm for the magnons induced by visible light and above 150 μm for the laser of 980 nm. In addition to thermal gradient, light excitation affects the electron configuration of the Fe 3+ ion in YIG. Long-wavelength laser is more effective since it causes a transition of the Fe 3+ ions in FeO 6 octahedron from a high spin to a low spin state and thus causes a magnon softening which favors a long-distance diffusion. Furthermore, the present work paves the way toward an efficient tuning of magnon transport which is crucially important for magnon spintronics.

36 MATERIALS SCIENCE↗
DOE OSTI2019

The Hot Bending and Diffusion Bonding of TiAl-Based Alloy for Corrugated-Core Sandwich Structure

The multi-layer structure of TiAl-based alloys can meet the demand of having lightweight and high-strength properties at elevated temperature. In this paper, the corrugated-core sandwich structure of a TiAl-based alloy was fabricated by the hot bending/diffusion bonding process. In order to fabricate this structure, the high-temperature tensile property, hot bending behavior, and vacuum diffusion bonding of a Ti-47.5Al-Cr-V alloy were investigated. The flow behavior was evaluated by uniaxial tensile experiment, and the results showed that the alloy exhibited good plasticity at the given conditions, and the maximum elongation reached 200% at 1000 °C at strain rate of 3 × 10{sup −4} s{sup −1}. The bending experiments were also performed at temperatures of 950, 1000, and 1050 °C with fillet radii of R = 0, 0.5, and 1 mm, respectively. No micro-/macro-defects developed. The vacuum diffusion bonding of this alloy was also carried out, and the shear bonding strength of bonding joints was measured at room temperature. The maximum shear strength was 398 MPa, and the joint was obtained at 1000 °C/30 MPa/3 h. Combining the bonding interface with the shear strength of different joints, the appropriate bonding pressure and bonding time for diffusion bonding of the TiAl alloy at 1000 °C were 20 MPa and 2 h, respectively. Then, the forming process of the corrugated-core sheet was conducted at 1000 °C with a fillet radius of 1 mm. Finally, the corrugated-core sandwich structure was manufactured under vacuum atmosphere, and the mechanical properties and the interface microstructure were investigated.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Thermoacoustic Soret separation

In a fluid mixture in a channel with an axial time-averaged temperature gradient, high-amplitude oscillating flow can greatly increase the axial flux of thermal diffusion (Soret) separation of the components of the mixture. The enhancement occurs when the oscillating lateral temperature gradient greatly exceeds the axial gradient, causing a large oscillating concentration that can be favorably time-phased with the oscillating flow. This process can occur even with a negligible pressure oscillation or with a negligible temperature response to pressure, as is the case in most liquid solutions. The thermal boundary condition imposed by realistic solids on thermoacoustic liquids is imperfect, adding mathematical complications that are absent for typical gases, for which the solid surface is temporally isothermal. Compared with gas mixtures, the high Lewis number in typical liquid solutions reduces the separation flux associated with the time-averaged temperature gradient, but it also reduces the remixing associated with the time-averaged mole-fraction gradient. For large enough channels, the second-law separation efficiency is only slightly reduced from that of steady liquid Soret separation.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Role of surface diffusion in formation of unique reactivity for graphite oxidation: Time-resolved measurements in a pulsed diffusion reactor

Quantification of oxidation kinetics is essential to develop graphitic materials for diverse applications: from refractories found in gas-cooled nuclear reactors to catalysts needed for chemical manufacturing. In this work, using well-defined highly oriented pyrolytic graphite, low-pressure isotopic transient experiments combined with controlled annealing periods, we resolve the role of surface diffusion and quantify oxidation kinetics with nanomole-precision. We observe an unexpected increase in reactivity following annealing which is explained by the role of surface diffusion increasing the probability for trapping mobile oxygen at more reactive edge sites. Here, the locus of adsorption and spillover to the basal plane is distinct from the trapping location creating a more active oxygen species. Isotopic products reflect the population dynamics of oxygen added at the edge and surface diffusion that relocates basal plane oxygen to more reactive edge sites. Since this process proceeds in parallel with direct oxidation reactions, it is not likely to be observed using steady-state or conventional ‘bulk’ characterization techniques. Our unique time-resolved non-equilibrium measurement in a well-defined transport regime, enables observation of three distinct behaviors: short-term deactivation due to the balance of rates in oxygen supply/product formation, reactivity increases due to surface diffusion and longer-term reactivity increase with oxygen accumulation.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Altered white matter connectivity in patients with schizophrenia: An investigation using public neuroimaging data from SchizConnect

Several studies have produced extensive evidence on white matter abnormalities in schizophrenia (SZ). However, optimum consistency and reproducibility have not been achieved, and reported low white matter tract integrity in patients with SZ varies between studies. A whole-brain imaging study with a large sample size is needed. This study aimed to investigate white matter integrity in the corpus callosum and connections between regions of interests (ROIs) in the same hemisphere in 122 patients with SZ and 129 healthy controls with public neuroimaging data from SchizConnect. For each diffusion-weighted image (DWI), two-tensor full-brain tractography was performed; DWIs were parcellated by processing and registering T1 images with FreeSurfer and Advanced Normalization Tools. White matter query language was used to extract white matter fiber tracts. We evaluated group differences in means of diffusion measures between the patients and controls, and correlations of diffusion measures with the severity of clinical symptoms and cognitive impairment in the patients using the Positive and Negative Syndrome Scale (PANSS), a letter-number sequencing (LNS) test, vocabulary test, letter fluency test, category fluency test, and trail-making test, part A. To correct for multiple comparisons, a false discovery rate of q < 0.05 was applied. In patients with SZ, we observed significant radial diffusivity (RD) and trace (TR) increases in left thalamo-occipital tracts and the right uncinate fascicle, and a significant RD increase in the right middle longitudinal fascicle (MDLF) and the right superior longitudinal fascicle ii. Correlations were present between TR of left thalamo-occipital tracts, and the letter fluency test and the LNS test, and RD in the right MDLF and PANSS positive subscale score. However, these correlations were not significant after correction for multiple comparisons. These results indicated widespread white matter fiber tract abnormalities in patients with SZ, contributing to SZ pathophysiology.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2020

Broadband and omnidirectional polymer antireflection coatings

A method for generating antireflective coatings for polymeric substrates using a deposition process and/or a dissolving process can provide a coating onto the outer surface of the substrate. Some embodiments can include a GLAD generated fluoropolymer coating or a co-evaporated fluoropolymer coating on a substrate that may achieve ultralow refractive index as well as improved adhesion and durability properties on polymeric substrates. In some embodiments, the deposition process is performed such that a fluoropolymer can be evaporated to form chain fragments of the fluoropolymer. The chain fragments diffused into the substrate can subsequently re-polymerize, interlocking with the polymer chains of the substrate. In some embodiments, the co-evaporation process can form a nanoporous polymer chain scaffold of the fluoropolymer, from which a sacrificial material can be dissolved out. The formed coating can be a multilayer or continuously-graded antireflective coating that has strong adhesion with the substrate.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Broadband and omnidirectional polymer antireflection coatings

A method for generating antireflective coatings for polymeric substrates using a deposition process and/or a dissolving process can provide a coating onto the outer surface of the substrate. Some embodiments can include a GLAD generated fluoropolymer coating or a co-evaporated fluoropolymer coating on a substrate that may achieve ultralow refractive index as well as improved adhesion and durability properties on polymeric substrates. In some embodiments, the deposition process is performed such that a fluoropolymer can be evaporated to form chain fragments of the fluoropolymer. The chain fragments diffused into the substrate can subsequently re-polymerize, interlocking with the polymer chains of the substrate. In some embodiments, the co-evaporation process can form a nanoporous polymer chain scaffold of the fluoropolymer, from which a sacrificial material can be dissolved out. The formed coating can be a multilayer or continuously-graded antireflective coating that has strong adhesion with the substrate.

Wang, Baomin↗
DOE OSTI2016

High electrochemical performances of hierarchical hydrangea macrophylla like NiCo{sub 2}O{sub 4} and NiCo{sub 2}S{sub 4} as anode materials for Li-ion batteries

Graphical abstract: Mesoporous hydrangea macrophylla like NiCo{sub 2}O{sub 4} and NiCo{sub 2}S{sub 4} have been fabricated, which present excellent electrochemical performances as anode materials for Li-ion batteries. - Highlights: • Hierarchical NiCo{sub 2}O{sub 4} is successfully fabricated. • Hierarchical NiCo{sub 2}S{sub 4} is prepared via sulfide anion exchange. • The hierarchical NiCo{sub 2}O{sub 4} and NiCo{sub 2}S{sub 4} exhibit good electrochemical properties. - Abstract: In this work, hierarchical hydrangea macrophylla like NiCo{sub 2}O{sub 4} has been synthesized by solvothermal method followed by calcination treatment in air. By using Na{sub 2}S as sulfur source, the NiCo{sub 2}O{sub 4} is converted into NiCo{sub 2}S{sub 4}. Such hierarchical NiCo{sub 2}O{sub 4} exhibits a high specific capacity and excellent cycling stability (928 mAh g{sup −1} at a current density of 100 mA g{sup −1} after 100 cycles). Even at high current density of 2000 mA g{sup −1}, the electrode still delivers a specific capacity of 371 mAh g{sup −1} after 50 cycles. When the NiCo{sub 2}S{sub 4} is used as anode materials for lithium-ion batteries, a high discharge capacity of 1204 mAh g{sup −1} can be achieved. Meanwhile, the NiCo{sub 2}S{sub 4} electrode displays good cycling stability and rate capability. The excellent electrochemical performances can be attributed to the unique porous structure, which can effectively reduce the diffusion length for lithium ions and electrons, and alleviate volume expansion during the charge-discharge processes.

36 MATERIALS SCIENCE↗
DOE OSTI2022

The effect of stress on the migration of He gas bubbles under a thermal gradient in Fe by phase-field modeling

Here a phase-field model is parameterized to study the effect of elastic stresses on the migration of He gas bubbles in Fe under a temperature gradient. Stresses caused by the gas bubble pressure and residual stress in the Fe matrix are considered. The dependence of He bubble migration velocity on the magnitude of the residual stress, average temperature, temperature gradient, and bubble size is measured. In agreement with a theoretical model based on surface diffusion, simulation results demonstrate that He bubbles move towards the high temperature region with velocities in Fe that are orders of magnitude faster than previously reported in UO 2 . It is found that local stresses in the matrix caused by the He bubble have negligible effect on the bubble migration process; however, residual stresses in the Fe matrix, potentially caused by processing or irradiation, can modestly modify bubble kinetics through pressure dependence of the He diffusion coefficients. Compressive residual stress decreases diffusion coefficients for bulk and surface diffusion mechanisms, thus reducing the migration velocity of the gas bubble. In contrast, tensile residual stress increases the diffusion coefficients, resulting in an increase in the gas bubble migration velocity. This pressure dependence is also consistent with a theoretical model. This phase field model lays the foundation for analysis of bubble coalescence-induced fracture in He bubble-containing steels.

36 MATERIALS SCIENCE↗