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Results for “anisotropic pressure”

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 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Evidence for two major changes in the Venus aerosol distribution - 1972-1975

High-spatial-and-spectral-resolution observations of Venus CO2 line profiles taken over a three-year interval are analyzed using inhomogeneous atmospheric models with anisotropic scattering. The data exhibit two sudden significant changes in the structure of the atmosphere, one occurring near April 1973, and the other near November 1974. Two models are developed to describe the vertical cloud structure of the atmosphere of Venus in the more quiescent periods after these changes. For each model, the CO2 specific abundance must decrease with increasing atmospheric pressure; i.e., the clouds are thinnest high in the atmosphere and become denser with depth. No evidence is found that the cloud particles must change their scattering phase function with altitude.

Cochran, W. D.↗
NASA NTRS1994

The Materials Chemistry of Atomic Oxygen with Applications to Anisotropic Etching of Submicron Structures in Microelectronics and the Surface Chemistry Engineering of Porous Solids

Neutral atomic oxygen is the most abundant component of the ionospheric plasma in the low Earth orbit environment (LEO; 200 to 700 kilometers altitude) and can produce significant degradation of some spacecraft materials. In order to produce a more complete understanding of the materials chemistry of atomic oxygen, the chemistry and physics of O-atom interactions with materials were determined in three radically different environments: (1) The Space Shuttle cargo bay in low Earth orbit (the EOIM-3 space flight experiment), (2) a high-velocity neutral atom beam system (HVAB) at Los Alamos National Laboratory (LANL), and (3) a microwave-plasma flowing-discharge system at JSC. The Space Shuttle and the high velocity atom beam systems produce atom-surface collision energies ranging from 0.1 to 7 eV (hyperthermal atoms) under high-vacuum conditions, while the flowing discharge system produces a 0.065 eV surface collision energy at a total pressure of 2 Torr. Data obtained in the three different O-atom environments referred to above show that the rate of O-atom reaction with polymeric materials is strongly dependent on atom kinetic energy, obeying a reactive scattering law which suggests that atom kinetic energy is directly available for overcoming activation barriers in the reaction. General relationships between polymer reactivity with O atoms and polymer composition and molecular structure have been determined. In addition, vacuum ultraviolet photochemical effects have been shown to dominate the reaction of O atoms with fluorocarbon polymers. Finally, studies of the materials chemistry of O atoms have produced results which may be of interest to technologists outside the aerospace industry. Atomic oxygen 'spin-off' or 'dual use' technologies in the areas of anisotropic etching in microelectronic materials and device processing, as well as surface chemistry engineering of porous solid materials are described.

Koontz, Steve L.↗
DOE OSTI2023

Helium bubble size effects on the surface morphological response of plasma-facing tungsten

Abstract We report a simulation study on the effects of helium (He) bubble size on the morphological evolution and pattern formation on the surface of tungsten used as a plasma-facing component (PFC) in nuclear fusion devices. We have carried out a systematic investigation based on self-consistent dynamical simulations of surface morphological evolution according to an atomistically-informed, 3D continuum-scale model that captures well the relevant length and time scales of surface nanostructure formation in PFC tungsten. The model accounts for PFC surface diffusion, driven by the biaxial compressive stress originating from the over-pressurized He bubbles in the near-surface region of PFC tungsten as a result of He plasma exposure, combined with the formation of self-interstitial atoms in tungsten that diffuse toward the PFC surface and the flux of surface adatoms generated as a result of surface vacancy-adatom pair formation upon He implantation; this transport of surface adatoms contributes to the anisotropic growth of surface nanostructural features due to the different rates of adatom diffusion along and across step edges of islands on the tungsten surface. Our detailed analysis reveals that varying the average He bubble size plays an important role in the PFC surface growth kinetics as well as the resulting surface topography. Specifically, we find that an increase in the He bubble size leads to a deceleration in the growth rate of the tungsten nanotendrils that emanate from the PFC surface. We also find that the separation distance between the resulting surface features increases with increasing He bubble size, as well as over time. This coarsening effect is a thermally activated process resulting in an accurate description of the temperature dependence of the average surface feature separation by an Arrhenius relation.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Three interaction energy scales in the single-layer high-$T_c$ cuprate HgBa 2 CuO 4+δ

The lamellar cuprate superconductors exhibit the highest ambient-pressure superconducting transition temperatures ($T_c$), and after more than three decades of extraordinary research activity, continue to pose formidable scientific challenges. A major experimental obstacle has been to distinguish universal phenomena from materials- or technique-dependent ones. Angle-resolved photoemission spectroscopy (ARPES) measures momentum-dependent single-particle electronic excitations and has been invaluable in the endeavor to determine the anisotropic momentum-space properties of the cuprates. HgBa 2 CuO 4+δ (Hg1201) is a single-CuO 2 -layer cuprate with a particularly high optimal Tc and a simple crystal structure, yet there exists little information from ARPES about the electronic properties of this model system. In this paper we present an ARPES study of doping-, temperature-, and momentum-dependent systematics of near-nodal dispersion anomalies in Hg1201. The data reveal a hierarchy of three distinct energy scales: a subgap low-energy kink, an intermediate-energy kink near 55 meV, and a peak-dip-hump structure. The first two features are attributed to the coupling of electrons to Ba-derived optical phonons and in-plane bond-stretching phonons, respectively. The nodal peak-dip-hump structure appears to have a common doping dependence in several single-layer cuprates and is interpreted as a manifestation of pseudogap physics at the node. These results establish several universal phenomena, both in terms of connecting multiple experimental techniques for a single material and in terms of connecting comparable spectral features in multiple structurally similar cuprates.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Shear strength measurements and hydrostatic compression of rhenium diboride under high pressures

Shear strength measurements have been carried out on rhenium diboride, ReB 2 , to a pressure of 74 GPa using a Radial X-ray Diffraction (R-XRD) technique in a diamond anvil cell using platinum as an internal x-ray pressure standard. The R-XRD result has provided a unique insight into the deformation of hexagonal crystal lattice under non-hydrostatic compression and variation of shear strength with increasing pressure. From R-XRD data, we have estimated hydrostatic component of compression to determine an equation of state of rhenium diboride yielding a bulk modulus of K 0 = 366 ± 25 GPa with a pressure derivative $K^{'}_{0}$ = 4.3 ± 0.5 in good agreement with hydrostatic density functional theory calculations. Here, the average lower bound of shear strength (τ) from various diffraction planes was then calculated using the measured interplanar d-spacing (d m ) and hydrostatic component of d-spacing (d p ) to be shown to approach 6.7 ± 0.4 GPa at 70 GPa. Our results show that the anisotropic compression effects observed in ReB 2 under hydrostatic compression are correlated to electronic structure changes under compression as predicted by theoretical calculations.

36 MATERIALS SCIENCE↗
NASA NTRSDate not supplied

The electron diffusion region and its relation to the larger-scale environment

The electron diffusion region forms the inner core of the magnetic reconnection machine. In this region, physical processes act to sustain a reconnection electric field, they accelerate to provide current density, and the heat to provide particle pressure in the current layer. The relative roles of these processes depend on whether the overall geometry is anti-parallel or asymmetric, and whether a guide field is present of not, but the overall concept applies in each case. The EDR is embedded in successively larger regions, starting with the ion diffusion region to even larger scales, where the plasma increasingly behaves like an anisotropic MHD plasma. With our new understanding of how the EDR works, it is of great interest to study how EDR processes couple to the larger scale environment. The ultimate efficacy of the magnetic reconnection is shaped by this interesting example of micro-macro coupling. In this presentation, we present some thoughts pertaining to this topic. In particular, we will discuss how information transport between scales is mediated, and how the very small scales can reach balance with the larger, overall system. We will review any pertinent research and present a set of questions inviting future research.

Michael Hesse↗
DOE OSTI2022

Creating two-dimensional solid helium via diamond lattice confinement

The universe abounds with solid helium in polymorphic forms. Therefore, exploring the allotropes of helium remains vital to our understanding of nature. However, it is challenging to produce, observe and utilize solid helium on the earth because high-pressure techniques are required to solidify helium. Here we report the discovery of room-temperature two-dimensional solid helium through the diamond lattice confinement effect. Controllable ion implantation enables the self-assembly of monolayer helium atoms between {100} diamond lattice planes. Using state-of-the-art integrated differential phase contrast microscopy, we decipher the buckled tetragonal arrangement of solid helium monolayers with an anisotropic nature compressed by the robust diamond lattice. These distinctive helium monolayers, in turn, produce substantial compressive strains to the surrounded diamond lattice, resulting in a large-scale bandgap narrowing up to ~2.2 electron volts. This approach opens up new avenues for steerable manipulation of solid helium for achieving intrinsic strain doping with profound applications.

42 ENGINEERING↗
DOE OSTI2021

Self‐Assembled Metal–Dielectric Hybrid Metamaterials in Vertically Aligned Nanocomposite Form with Tailorable Optical Properties and Coupled Multifunctionalities

Metal–dielectric hybrid metamaterials have attracted increasing research interest in recent years because of their novel optical properties and promising applications in the fields of electronic and photonic devices. Dielectric permittivity is a key parameter that strongly influences the optical properties of materials. By self‐assembling the metallic and dielectric components in a pillar‐in‐matrix structure, a strong anisotropic structure forms and results in opposite signs of permittivity components (i.e., ε || > 0 and ε ⊥ < 0, or vice versa) and exotic optical responses including hyperbolic dispersion in the visible to near‐infrared region. Herein, the main approaches of tuning the permittivity in self‐assembled metal–dielectric vertically aligned nanocomposite (VAN) thin films are reviewed, including tuning the metal pillar density and geometry, film strain state and background pressure during growth, and seeking other metal‐free and complex structure designs. Future research directions are also proposed, including unique approaches to improve their thermal stability, integrate on flexible substrates toward wearable device fabrication, and explore real‐time tunable metamaterials.

Zhang, Di↗
DOE OSTI2022

Giant Thermoelectric Power Factor Anisotropy in PtSb 1.4 Sn 0.6

In this work, we report on the giant anisotropy found in the thermoelectric power factor (S 2 σ) of marcasite structure-type PtSb 1.4 Sn 0.6 single crystal. PtSb 1.4 Sn 0.6 , synthesized using an ambient pressure flux growth method upon mixing Sb and Sn on the same atomic site, is a new phase different from both PtSb 2 and PtSn 2 , which crystallize in the cubic Pa$\bar{3}$ pyrite and Fm$\bar{3}$m fluorite unit cell symmetry, respectively. The large difference in S 2 σ for heat flow applied along different principal directions of the orthorhombic unit cell stems mostly from anisotropic Seebeck coefficients.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Estimating perturbed stress from 3-D borehole displacements induced by fluid injection in fractured or faulted shales

SUMMARY Hydrofracturing stress measurements in fractured and anisotropic shales are notoriously difficult, because opening of existing geological features tends to prevent the creation of a pure hydraulic fracture perpendicular to the least compressive principal stress. Here we show how adding 3-D borehole-displacement measurements while conducting the hydraulic injection test helps to better constrain the principal stress orientations and magnitudes. We developed a 3-D fully coupled hydromechanical numerical model to analyse the displacement, fluid pressure and injection flow-rate data measured during an injection pressure-step-rate test conducted to activate a faulted borehole interval in the Mont Terri Opalinus Clay (Switzerland). We find that injected fluids can only penetrate the fault when it is at or above the Coulomb failure pressure. Borehole displacement orientations are sensitive to a ∼15° variation in the stress–tensor orientation and a 1 MPa stress magnitude variation. Although some dispersion occurs while rupture is propagating along the fault plane ∼4 m away from the borehole, the maximum density of displacement orientations consistently informs about the stress orientation. Thus, an extended injection step-rate approach coupled with an accurate in situ measurement of the borehole wall displacements can be used to better constrain the local stress field perturbations in fractured shales and in heterogeneous rock in general.

Guglielmi, Yves↗
NASA NTRS2006

Heat Transfer In High-Temperature Multilayer Insulation

The combined radiation/conduction heat transfer in high-temperature multilayer insulations for typical reentry of reusable launch vehicles from low Earth orbit was investigated experimentally and numerically. The high-temperature multilayer insulation investigated consisted of gold-coated reflective foils separated by alumina fibrous insulation spacers. The steady-state heat transfer through four multilayer insulation configurations was investigated experimentally over the temperature range of 300-1300 K and environmental pressure range of 1.33 10(exp -5)-101.32 kPa. It was shown that including the reflective foils reduced the effective thermal conductivity compared to fibrous insulation sample at 1.5 times the density of the multilayer sample. A finite volume numerical model was developed to solve the governing combined radiation/conduction heat transfer equations. The radiation heat transfer in the fibrous insulation spacers was modeled using the modified two-flux approximation assuming anisotropic scattering and gray medium. The numerical model was validated by comparison with steady-state experimental data. The root mean square deviation between the predicted and measured effective thermal conductivity of the samples was 9.5%.

Daryabeigi, Kamran↗
NASA NTRS1977

Development of a plasma sprayed ceramic gas path seal for high pressure turbine applications

The plasma sprayed graded layered yittria stabilized zirconia (ZrO2)/metal(CoCrAlY) seal system for gas turbine blade tip applications up to 1589 K (2400 F) seal temperatures was studied. Abradability, erosion, and thermal fatigue characteristics of the graded layered system were evaluated by rig tests. Satisfactory abradability and erosion resistance was demonstrated. Encouraging thermal fatigue tolerance was shown. Initial properties for the plasma sprayed materials in the graded, layered seal system was obtained, and thermal stress analyses were performed. Sprayed residual stresses were determined. Thermal stability of the sprayed layer materials was evaluated at estimated maximum operating temperatures in each layer. Anisotropic behavior in the layer thickness direction was demonstrated by all layers. Residual stresses and thermal stability effects were not included in the analyses. Analytical results correlated reasonably well with results of the thermal fatigue tests. Analytical application of the seal system to a typical gas turbine engine application predicted performance similar to rig specimen thermal fatigue performance. A model for predicting crack propagation in the sprayed ZrO2/CoCrAlY seal system was proposed, and recommendations for improving thermal fatigue resistance were made. Seal system layer thicknesses were analytically optimized to minimize thermal stresses in the abradability specimen during thermal fatigue testing. Rig tests on the optimized seal configuration demonstrated some improvement in thermal fatigue characteristics.

Shiembob, L. T.↗
DOE OSTI2021

Pressure, temperature, and orientation dependent thermal conductivity of pentaerythritol tetranitrate (PETN)

We use reverse non-equilibrium molecular dynamics to determine the thermal conductivity tensor, κ, of tetragonal pentaerythritol tetranitrate (PETN). The most stable form under standard ambient conditions (PETN I, with space group P4¯2 1 c) is considered in the temperature and pressure intervals 200–500 K and 0–8 GPa, which covers approximately the stability range for this polymorph. We compute κ along the [100] and [001] directions, which is enough to construct the full thermal conductivity tensor for a system with tetragonal symmetry. In addition, we also determine κ along the [010] direction and confirm that κ 100 ~ κ 010 , with less than 4% average absolute error between the two quantities. We observe an anisotropic response for κ, with κ 100 > κ 001 across the whole (T,P) interval, and 37% difference at 300 K, 0 GPa. Furthermore, we provide analytical functions to interpolate κ(P,T) within the fitting interval and serve as input for continuum-scale simulations.

36 MATERIALS SCIENCE↗
NASA NTRS1992

Reduced basis technique for evaluating the sensitivity coefficients of the nonlinear tire response

An efficient reduced-basis technique is proposed for calculating the sensitivity of nonlinear tire response to variations in the design variables. The tire is modeled using a 2-D, moderate rotation, laminated anisotropic shell theory, including the effects of variation in material and geometric parameters. The vector of structural response and its first-order and second-order sensitivity coefficients are each expressed as a linear combination of a small number of basis vectors. The effectiveness of the basis vectors used in approximating the sensitivity coefficients is demonstrated by a numerical example involving the Space Shuttle nose-gear tire, which is subjected to uniform inflation pressure.

Noor, Ahmed K.↗
DOE OSTI2024

Single-crystal X-ray diffraction on the structure of (Al,Fe)-bearing bridgmanite in the lower mantle

Abstract Here we have performed single-crystal X-ray diffraction (SCXRD) experiments on two high-quality crystal platelets of (Al,Fe)-bearing bridgmanite (Mg0.88Fe0.0653+Fe0.0352+Al0.03)(Al0.11Si0.90)O3 (Fe10-Al14-Bgm) up to 64.6(6) GPa at room temperature in a Boehler-Almax type diamond-anvil cell. Refinements on the collected SCXRD patterns reveal reliable structural information of single-crystal Fe10-Al14-Bgm, including unit-cell parameters, atomic coordinates, and anisotropic displacement parameters. Together with Mössbauer and electron microprobe analyses, our best single-crystal refinement model indicates that the sample contains ~6.5 mol% Fe3+, 3.5 mol% Fe2+, and 3 mol% Al3+ in the large pseudo-dodecahedral site (A site), and ~11 mol% Al3+ in the small octahedral site (B site). This may indicate that Al3+ in bridgmanite preferentially occupies the B site. Our results show that the compression of Fe10-Al14-Bgm with pressure causes monotonical decreases in the volumes of AO12 pseudo-dodecahedron and BO6 octahedron (VA and VB, respectively) as well as the associated A-O and B-O bond lengths. The interatomic angles of B-O1-B and B-O2-B decrease from 145.2–145.8° at 4.2(1) GPa to 143.3–143.5° at 64.6(6) GPa. Quantitative calculations of octahedral tilting angles (Ф) show that Ф increases smoothly with pressure. We found a linear relationship between the polyhedral volume ratio and the Ф in the bridgmanite with different compositions: VA/VB = –0.049Φ + 5.549. Our results indicate an increased distortion of the Fe10-Al14-Bgm structure with pressure, which might be related to the distortion of A-site Fe2+. The local environmental changes of A-site Fe2+ in bridgmanite could explain previous results on the hyperfine parameters, abnormal lattice thermal conductivity, mean force constant of iron bonds and other physical properties, which in turn provide insights into our understanding on the geophysics and geochemistry of the planet.

Geochemistry & Geophysics↗
DOE OSTI2021

Anisotropic strength behavior of single-crystal TATB

High-rate strength behavior plays an important role in the shock initiation of high explosives, with plastic deformation serving to localize heat into hot spots and as a mechanochemical means to enhance reactivity. Recent simulations predict that detonation-like shocks produce highly reactive nanoscale shear bands in the layered crystalline explosive TATB (1,3,5-triamino-2,4,6-trinitrobenzene), but the thresholds leading to this response are poorly understood. We utilize molecular dynamics to simulate the high-rate compressive stress–strain response of TATB, with a focus on understanding flow behavior. Here, the dependence of strength on pressure and loading axis (crystal orientation) is explored. The deformation mechanisms fall broadly into two categories, with compression along crystal layers activating a buckling/twinning mode and compression normal to the layers producing nanoscale shear bands. Despite the complexity of the underlying mechanisms, the crystal exhibits relatively straightforward stress–strain curves. Most of the crystal orientations studied show rapid strain softening following the onset of yielding, which settles to a steady flow state. Trajectories are analyzed using five metrics for local states and structural order, but most of these metrics yield similar distributions for these deformation mechanisms. On the other hand, a recently proposed measure of intramolecular strain energy is found to most cleanly distinguish between these mechanisms, while also providing a plausible connection with mechanochemically accelerated decomposition kinetics. Localization of intramolecular strain energy is found to depend strongly on crystal orientation and pressure.

36 MATERIALS SCIENCE↗
NASA NTRS1998

Profile Evolution Simulation in Etching Systems Using Level Set Methods

Semiconductor device profiles are determined by the characteristics of both etching and deposition processes. In particular, a highly anisotropic etch is required to achieve vertical sidewalls. However, etching is comprised of both anisotropic and isotropic components, due to ion and neutral fluxes, respectively. In Ar/Cl2 plasmas, for example, neutral chlorine reacts with the Si surfaces to form silicon chlorides. These compounds are then removed by the impinging ion fluxes. Hence the directionality of the ions (and thus the ion angular distribution function, or IAD), as well as the relative fluxes of neutrals and ions determines the amount of undercutting. One method of modeling device profile evolution is to simulate the moving solid-gas interface between the semiconductor and the plasma as a string of nodes. The velocity of each node is calculated and then the nodes are advanced accordingly. Although this technique appears to be relatively straightforward, extensive looping schemes are required at the profile corners. An alternate method is to use level set theory, which involves embedding the location of the interface in a field variable. The normal speed is calculated at each mesh point, and the field variable is updated. The profile comers are more accurately modeled as the need for looping algorithms is eliminated. The model we have developed is a 2-D Level Set Profile Evolution Simulation (LSPES). The LSPES calculates etch rates of a substrate in low pressure plasmas due to the incident ion and neutral fluxes. For a Si substrate in an Ar/C12 gas mixture, for example, the predictions of the LSPES are identical to those from a string evolution model for high neutral fluxes and two different ion angular distributions.(2) In the figure shown, the relative neutral to ion flux in the bulk plasma is 100 to 1. For a moderately isotropic ion angular distribution function as shown in the cases in the left hand column, both the LSPES (top row) and rude's string method (bottom row) predict tapered profiles. The LSPES uses an AD with a FWHM = 13.5 degrees, and rude's model uses a ratio of sheath voltage to ion temperature of 50. The more anisotropic IADs produce profiles with more vertical sidewalls and a wider bottom surface, as shown in the right hand column. Here, the LSPES has an AD with a FWHM = 2 degrees, and rude's model uses a ratio of 500 of sheath voltage to ion temperature. The agreement between the LSPES and rude's model is excellent in both cases. We will present etching profiles generated by the LSPES, including calculations of the re-emitted fluxes of both neutrals and ions off of the profile walls. In addition, we will show the effect of geometric structures (overhangs, etc.) on the etching profiles. Other physical aspects, such as surface diffusion, will also be included in the model.

Hwang, Helen H.↗
NASA NTRS1993

Micromachined silicon cantilever beam accelerometer incorporating an integrated optical waveguide

A micromachined cantilever beam accelerometer is described in which beam deflection is determined optically. A diving board structure is anisotropically etched into a silicon wafer. This diving board structure is patterned from the wafer backside so as to leave a small gap between the tip of the diving board and the opposite fixed edge on the front side of the wafer. In order to sense a realistic range of accelerations, a foot mass incorporated onto the end of the beam is found to provide design flexibility. A silicon nitride optical waveguide is then deposited by low pressure chemical vapor deposition (LPCVD) onto the sample. Beam deflection is measured by the decrease of light coupled across the gap between the waveguide sections. In order to investigate sensor response and simulate deflection of the beam, we utilized a separate beam and waveguide section which could be displaced from one another in a precisely controlled manner. Measurements were performed on samples with gaps of 4.0, 6.0, and 8.0 micron and the variation of the fraction of light coupled across the gap as a function of displacement and gap spacing was found to agree with overlap integral calculations.

Burcham, Kevin E.↗