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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 451 records · Page 25
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Theoretical Model of Helium Bubble Growth and Density in Plasma-Facing Metals

We present a theoretically-motivated model of helium bubble density as a function of volume for high-pressure helium bubbles in plasma-facing tungsten. The model is a good match to the empirical correlation we published previously [Hammond et al., Acta Mater. 144, 561–578 (2018)] for small bubbles, but the current model uses no adjustable parameters. The model is likely applicable to significantly larger bubbles than the ones examined here, and its assumptions can be extended trivially to other metals and gases. We expect the model to be broadly applicable and useful in coarse-grained models of gas transport in metals.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2024

Discovering tungsten-based composites as plasma facing materials for future high-duty cycle nuclear fusion reactors

Abstract Despite of excellent thermal properties and high sputtering resistance, pure tungsten cannot fully satisfy the requirements for plasma facing materials in future high-duty cycle nuclear fusion reactions due to the coupled extreme environments, including the high thermal loads, plasma exposure, and radiation damage. Here, we demonstrated that tungsten-based composite materials fabricated using spark-plasma sintering (SPS) present promising solutions to these challenges. Through the examination of two model systems, i.e., tungsten-zirconium composite for producing porous tungsten near the surface and dispersoid-strengthened tungsten, we discussed both the strengths and limitations of the SPS-fabricated materials. Our findings point towards the need for future studies aimed at optimizing the SPS process to achieve desired microstructures and effective control of oxygen impurities in the tungsten-based composite materials.

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2025

First-principles theory for cerium predicts three distinct face-centered cubic phases

We show results from first-principles calculations for cerium at very high compressions. These reveal a most remarkable behavior in a material; depending on atomic volume, cerium adopts three distinct face-centered cubic (fcc) phases driven by different physical mechanisms. The two well-known a and phases are vigorously debated in the literature, but we focus on the a phase as a metal with delocalized character of the 4f electron. The ultimate high compression fcc phase, here named ω, is driven partly by electrostatics. Our density-functional theory (DFT) study excellently reproduces the experimentally known compression behavior of cerium up to a few Mbar but goes beyond those pressures with structural transitions to tetragonal, hexagonal, and cubic (fcc) phases occurring before 100 Mbar (10000 GPa or 10 TPa). The 4f-electron contribution to the chemical bonding is shown to rule phase transitions and compressibility. The change of 4f occupation nicely explains the pressure dependence of the structural axial ratio in the tetragonal phase. At very high pressure, structures known at low pressures return because of band broadening, electrostatic ion repulsion, and an increase in hybridization between states that under normal conditions can be considered core (atomic like) states and the valence-band states.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Rolling-induced Face Centered Cubic Titanium in Hexagonal Close Packed Titanium at Room Temperature

Combining transmission electron microscopes and density functional theory calculations, we report the nucleation and growth mechanisms of room temperature rolling induced face-centered cubic titanium (fcc-Ti) in polycrystalline hexagonal close packed titanium (hcp-Ti). Fcc-Ti and hcp-Ti take the orientation relation: $\langle$0001$\rangle$ hcp|| $\langle$001$\rangle$ fcc and {$10\bar{1}0$} hcp|| {110} fcc , different from the conventional one. The nucleation of fcc-Ti is accomplished via pure-shuffle mechanism with a minimum stable thickness of three atomic layers, and the growth via shear-shuffle mechanisms through gliding two-layer disconnections or pureshuffle mechanisms through gliding four-layer disconnections. Such phase transformation offers an additional plastic deformation mode comparable to twinning.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Controllable growth of vertically aligned graphene on C-face SiC

We investigated how to control the growth of vertically aligned graphene on C-face SiC by varying the processing conditions. It is found that, the growth rate scales with the annealing temperature and the graphene height is proportional to the annealing time. Temperature gradient and crystalline quality of the SiC substrates influence their vaporization. The partial vapor pressure is crucial as it can interfere with further vaporization. A growth mechanism is proposed in terms of physical vapor transport. The monolayer character of vertically aligned graphene is verified by Raman and X-ray absorption spectroscopy. With the processed samples, d 0 magnetism is realized and negative magnetoresistance is observed after Cu implantation. We also prove that multiple carriers exist in vertically aligned graphene.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Unusual Strain Effect of Pt-Based L10 Face-Centered Tetragonal Core in Core/Shell Nanoparticles for Oxygen Reduction Reaction

Here, nanoparticles with low-Pt content core and a few-layer Pt skin are attractive catalysts toward the oxygen reduction reaction (ORR) not only for its low cost, but also because its activity can be enhanced by judiciously choosing the core alloy. Achieving optimal ORR performance would require fine tuning of the core composition and structure. Previous work studying the enhancement effects has primarily focused on core alloys with a cubic structure, (i.e. disordered alloy or L12 ordered structure) which limits the tuning to composition alone. In this work, using ab initio calculations, we have systemically investigated a new class of Pt0.5M0.5 (M=V, Cr, Fe, Co, Ni and Cu) core alloy that has a face-centered tetragonal L10 intermetallic structure. We have calculated the adsorption energies of O, OH, and OOH on various Pt skins and the underlying tetragonal structured alloys, which allows us to not only predict the optimal number of pure Pt skin layers but also tune the activity of the catalysts toward the peak of the ORR volcano plot. More importantly, using adsorption energies on intermediate structures, we are able to decompose the enhancement factor into ligand, normal and shear strain effects, and reveal the significant contribution of the shear strain that is only possible with a tetragonal core but not a cubic one. Our results point to a new direction in designing tetragonal structured intermetallic core-shell nanoparticles for ORR applications.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2011

Direct writing of ferroelectric domains on the x- and y-faces of lithium niobate using a continuous wave ultraviolet laser

Ferroelectric domain reversal has been achieved by scanning a tightly focused, strongly absorbed ultraviolet laser beam across the x- and y-faces of lithium niobate crystals. The domains were investigated by piezoresponse force microscopy. The emergence and width of any domain was found to depend on the scanning direction of the irradiating laser beam with respect to the polar z-axis. Full width and half width domains or no domain formation at all could be achieved for scanning along specific directions. We interpret the results by a direct correlation between the local temperature gradient and the resulting polarization direction.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Epitaxial high saturation magnetization FeN thin films on Fe(001) seeded GaAs(001) single crystal wafer using facing target sputterings

It was demonstrated that Fe-N martensite ({alpha}') films were grown epitaxially on Fe(001) seeded GaAs(001) single crystal wafer by using a facing target sputtering method. X-ray diffraction pattern implies an increasing c lattice constant as the N concentration increases in the films. Partially ordered Fe{sub 16}N{sub 2} films were synthesized after in situ post-annealing the as-sputtered samples with pure Fe{sub 8}N phase. Multiple characterization techniques including XRD, XRR, TEM, and AES were used to determine the sample structure. The saturation magnetization of films with pure Fe{sub 8}N phase measured by VSM was evaluated in the range of 2.0-2.2 T. The post annealed films show systematic and dramatic increase on the saturation magnetization, which possess an average value of 2.6 T. These observations support the existence of giant saturation magnetization in {alpha}''-Fe{sub 16}N{sub 2} phase that is consistent with a recent proposed cluster-atom model and the first principles calculation [N. Ji, X. Q. Liu, and J. P. Wang, New J. Phys. 12 063032 (2010)].

36 MATERIALS SCIENCE↗
DOE OSTI2011

Edge transport and turbulence reduction with lithium coated plasma facing components in the National Spherical Torus Experiment

The coating of plasma facing components (PFCs) with lithium improves energy confinement and eliminates ELMs in the National Spherical Torus Experiment, the latter due to a relaxation of the density and pressure profiles that reduces the drive for peeling-ballooning modes. 2-D interpretive transport modeling of discharges without and with lithium shows that a reduction in the PFC recycling coefficient from R {approx} 0.98 to R {approx} 0.90 is required to match the drop in D{sub {alpha}} emission with lithium coatings. A broadening of the edge barrier region showing reduced transport coefficients is observed, with a {approx}75% drop of the D and {chi}{sub e} from 0.8 < {psi}{sub N} < 0.93 needed to match the profile relaxation with lithium coatings. Turbulence measurements using an edge reflectometry system as well as high-k microwave scattering show a decrease in density fluctuations with lithium coatings. These transport changes allow the realization of very wide pedestals, with a {approx}100% width increase relative to the reference discharges.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Amorphous Interface Layer in Thin Graphite Films Grown on the Carbon Face of SiC

Cross-sectional transmission electron microscopy (TEM) is used to characterize an amorphous layer observed at the interface in graphite and graphene films grown via thermal decomposition of C-face 4H-SiC. The amorphous layer does not cover the entire interface, but uniform contiguous regions span microns of cross-sectional interface. Scanning transmission electron microscopy (STEM) images and electron energy loss spectroscopy (EELS) demonstrate that the amorphous layer is a carbon-rich composition of Si/C. The amorphous layer is clearly observed in samples grown at 1600 C for a range of growth pressures in argon, but not at 1500 C, suggesting a temperature-dependent formation mechanism.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2012

Structural properties of InN films grown on O-face ZnO(0001) by plasma-assisted molecular beam epitaxy

We study the impact of substrate temperature and layer thickness on the morphological and structural properties of InN films directly grown on O-face ZnO(0001) substrates by plasma-assisted molecular beam epitaxy. With increasing substrate temperature, an interfacial reaction between InN and ZnO takes place that eventually results in the formation of cubic In{sub 2}O{sub 3} and voids. The properties of the InN films, however, are found to be unaffected by this reaction for substrate temperatures less than 550 deg. C. In fact, both the morphological and the structural quality of InN improve with increasing substrate temperature in the range from 350 to 500 deg. C. High quality films with low threading dislocation densities are demonstrated.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Effects of Ga on the growth of InN on O-face ZnO(0001) by plasma-assisted molecular beam epitaxy

We compare the structural properties of InN and In{sub 0.95}Ga{sub 0.05}N films grown on O-face ZnO(0001) substrates at different temperatures. The small amount of Ga results in dramatic changes in the morphology and structural properties of InN. In particular, inversion domains start to appear at higher temperatures in the In{sub 0.95}Ga{sub 0.05}N film. This process is a consequence of the chemical reaction of ZnO with Ga which can be prevented by choosing the substrate temperature to be 450{sup Degree-Sign }C or below.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Ion-beam-induced magnetic and structural phase transformation of Ni-stabilized face-centered-cubic Fe films on Cu(100)

Metastable face-centered cubic (fcc) Fe/Cu(100) thin films are good candidates for ion-beam magnetic patterning due to their magnetic transformation upon ion-beam irradiation. However, pure fcc Fe films undergo spontaneous transformation when their thickness exceeds 10 ML. This limit can be extended to approximately 22 ML by deposition of Fe at increased CO background pressures. We show that much thicker films can be grown by alloying with Ni for stabilizing the fcc γ phase. The amount of Ni necessary to stabilize nonmagnetic, transformable fcc Fe films in dependence on the residual background pressure during the deposition is determined and a phase diagram revealing the transformable region is presented.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Growth diagram of N-face GaN (0001{sup ¯}) grown at high rate by plasma-assisted molecular beam epitaxy

N-face GaN was grown on free-standing GaN (0001{sup ¯}) substrates at a growth rate of 1.5 μm/h using plasma-assisted molecular beam epitaxy. Difference in growth rate between (0001{sup ¯}) and (0001) oriented GaN depends on nitrogen plasma power, and the (0001{sup ¯}) oriented GaN had only 70% of the growth rate of the (0001) oriented GaN at 300 W. Unintentional impurity concentrations of silicon, carbon, and oxygen were 2 × 10{sup 15}, 2 × 10{sup 16}, and 7 × 10{sup 16} cm{sup −3}, respectively. A growth diagram was constructed that shows the dependence of the growth modes on the difference in the Ga and active nitrogen flux, Φ{sub Ga} − Φ{sub N*}, and the growth temperature. At high Φ{sub Ga} − Φ{sub N*} (Φ{sub Ga} ≫ Φ{sub N*}), two-dimensional (step-flow and layer-by-layer) growth modes were realized. High growth temperature (780 °C) expanded the growth window of the two-dimensional growth modes, achieving a surface with rms roughness of 0.48 nm without Ga droplets.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Digital Holography for in Situ Real-Time Measurement of Plasma-Facing-Component Erosion

In situ, real time measurement of net plasma-facing-component (PFC) erosion/deposition in a real plasma device is challenging due to the need for good spatial and temporal resolution, sufficient sensitivity, and immunity to fringe-jump errors. Design of a high-sensitivity, potentially high-speed, dual-wavelength CO2 laser digital holography system (nominally immune to fringe jumps) for PFC erosion measurement is discussed.

Thomas, C. E.↗
DOE OSTI2014

Generalizing the flash technique in the front-face configuration to measure the thermal diffusivity of semitransparent solids

In this work, we have extended the front-face flash method to retrieve simultaneously the thermal diffusivity and the optical absorption coefficient of semitransparent plates. A complete theoretical model that allows calculating the front surface temperature rise of the sample has been developed. It takes into consideration additional effects, such as multiple reflections of the heating light beam inside the sample, heat losses by convection and radiation, transparency of the sample to infrared wavelengths, and heating pulse duration. Measurements performed on calibrated solids, covering a wide range of absorption coefficients (from transparent to opaque) and thermal diffusivities, validate the proposed method.

42 ENGINEERING↗
DOE OSTI2014

2D surface temperature measurement of plasma facing components with modulated active pyrometry

In nuclear fusion devices, such as Tore Supra, the plasma facing components (PFC) are in carbon. Such components are exposed to very high heat flux and the surface temperature measurement is mandatory for the safety of the device and also for efficient plasma scenario development. Besides this measurement is essential to evaluate these heat fluxes for a better knowledge of the physics of plasma-wall interaction, it is also required to monitor the fatigue of PFCs. Infrared system (IR) is used to manage to measure surface temperature in real time. For carbon PFCs, the emissivity is high and known (ε ~ 0.8), therefore the contribution of the reflected flux from environment and collected by the IR cameras can be neglected. However, the future tokamaks such as WEST and ITER will be equipped with PFCs in metal (W and Be/W, respectively) with low and variable emissivities (ε ~ 0.1–0.4). Consequently, the reflected flux will contribute significantly in the collected flux by IR camera. The modulated active pyrometry, using a bicolor camera, proposed in this paper allows a 2D surface temperature measurement independently of the reflected fluxes and the emissivity. Experimental results with Tungsten sample are reported and compared with simultaneous measurement performed with classical pyrometry (monochromatic and bichromatic) with and without reflective flux demonstrating the efficiency of this method for surface temperature measurement independently of the reflected flux and the emissivity.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗