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At least 1,045 records · Page 58
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Symmetric Kondo Lattice States in Doped Strained Twisted Bilayer Graphene

We use the topological heavy fermion (THF) model and its Kondo lattice (KL) formulation to study the possibility of a symmetric Kondo (SK) state in twisted bilayer graphene. Via a large-N approximation, we find a SK state in the KL model at fillings v = 0,±1,±2 where a KL model can be constructed. In the SK state, all symmetries are preserved and the local moments are Kondo screened by the conduction electrons. At the mean-field level of the THF model at v = 0,±1,±2,±3 we also find a similar symmetric state that is adiabatically connected to the symmetric Kondo state. We study the stability of the symmetric state by comparing its energy with the ordered (symmetry-breaking) states found in [H. Hu et al., Phys. Rev. Lett. 131, 026502 (2023)., Z.-D. Song and B. A. Bernevig, Phys. Rev. Lett. 129, 047601 (2022).] and find the ordered states to have lower energy at v = 0,±1,±2. However, moving away from integer fillings by doping the light bands, our mean-field calculations find the energy difference between the ordered state and the symmetric state to be reduced, which suggests the loss of ordering and a tendency toward Kondo screening. In order to include many-body effects beyond the mean-field approximation, we also performed dynamical mean-field theory calculations on the THF model in the nonordered phase. The spin susceptibility follows a Curie behavior at v = 0,±1,±2 down to ~2 K where the onset of screening of the local moment becomes visible. This hints to very low Kondo temperatures at these fillings, in agreement with the outcome of our mean-field calculations. At noninteger filling v = ±0.5,±0.8,±1.2 dynamical mean-field theory shows deviations from a 1/T susceptibility at much higher temperatures, suggesting a more effective screening of local moments with doping. Lastly, we study the effect of a C 3⁢z -rotational-symmetry-breaking strain via mean-field approaches and find that a symmetric phase (that only breaks C 3⁢z symmetry) can be stabilized at sufficiently large strain at v = 0,±1,±2. Our results suggest that a symmetric Kondo phase is strongly suppressed at integer fillings, but could be stabilized either at noninteger fillings or by applying strain.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

In-situ effect in cross-ply laminates under various loading conditions analyzed with hybrid macro/micro-scale computational models

In this article, multi-scale finite element analyses based on three-dimensional (3D) hybrid macro/micro-scale computational models subjected to various loading conditions are carried out to examine the in-situ effect imposed by the neighboring plies on the failure initiation and propagation of cross-ply laminates. A detailed comparative study on crack suppression mechanisms due to the effect of embedded laminar thickness and adjacent ply orientation is presented. Furthermore, we compare the results of in-situ transverse failure strain and strength between the computational models and analytical predictions. Good agreements are generally observed, indicating the constructed computational models are highly accurate to quantify the in-situ effect. Subsequently, empirical formulas for calculating the in-situ strengths as a function of embedded ply thickness and different ply angle between embedded and adjacent plies are developed, during which several material parameters are obtained using a reverse fitting method. Finally, a new set of failure criteria for σ 22 -τ 12 , σ 22 -τ 23 , and σ 11 -τ 12 accounting for the in-situ strengths are proposed to predict laminated composites failure under multi-axial stress states. This study demonstrates an effective and efficient computational technique towards the accurate prediction of the failure behaviors and strengths of cross-ply laminates by including the in-situ effects.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Acoustic mapping by picosecond ultrasonics for elastic property measurement: Experimental demonstration on a TRISO fuel compact

Picosecond ultrasonics has been demonstrated on a tristructural isotropic (TRISO) fuel compact to measure the elastic properties of each compact layer. This technique utilizes an ultrashort pump laser pulse to excite vibrations in a gold transducer film covering the surface of each component and a second probe laser pulse to record the resulting acoustic strain induced change in optical reflectance. From the damping of this film vibration, the acoustic reflection coefficient, which couples the elastic properties of the transducer film and the sample, can be obtained, enabling a calculation of the sample’s acoustic velocity and elastic modulus. Overall, results obtained from this method are consistent with known values of elastic moduli, namely that the SiC coating is the stiffest component of the compact while the carbonaceous matrix is the most compliant. Nanoindentation was conducted as a benchmark technique on the same sample and shows satisfactory agreement with the results of picosecond ultrasonics. Compared to other methods like nanoindentation, picosecond ultrasonics is multimodal with a capability of measuring several key properties simultaneously and has potentials to be coupled into optical fibers for remote sensing. Thus, these demonstration measurements reveal the methodology to be a promising candidate for in-situ and high-throughput optical characterizations of nuclear materials.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2020

Correlating the three-dimensional atomic defects and electronic properties of two-dimensional transition metal dichalcogenides

The electronic, optical and chemical properties of two-dimensional transition metal dichalcogenides strongly depend on their three-dimensional atomic structure and crystal defects. Using Re-doped MoS 2 as a model system, here we present scanning atomic electron tomography as a method to determine three-dimensional atomic positions as well as positions of crystal defects such as dopants, vacancies and ripples with a precision down to 4 pm. We further measure the three-dimensional bond distortion and local strain tensor induced by single dopants. By directly offering these experimental three-dimensional atomic coordinates to density functional theory, we obtain more accurate electronic band structures than derived from conventional density functional theory calculations that relies on relaxed three-dimensional atomic coordinates. We anticipate that scanning atomic electron tomography not only will be generally applicable to determine the three-dimensional atomic coordinates of two-dimensional materials, but also will enable ab initio calculations to better predict the physical, chemical and electronic properties of these materials.

36 MATERIALS SCIENCE↗
NASA NTRS1974

Experimental evaluation of stresses in spherically hollow balls

An analysis was undertaken to evaluate stresses within spherically hollow ball bearings proportioned for 40, 50, and 60% mass reductions. Strain gage rosettes were used to determine principal strains and stresses in the steel ball models statically loaded in various orientations. Dimensionless results are reported for the balls under flate plate contact loads. Similitude considerations permit these results to be applied to calculate stresses in hollow ball bearings proportioned to these mass reductions.

Nypan, L. J.↗
NASA NTRS1994

Bolt cutter functional evaluation

The Aerospace Corporation has been implementing finite difference and finite element codes for the analysis of a variety of explosive ordnance devices. Both MESA-2D and DYNA3D have been used to evaluate the role of several design parameters on the performance of a satellite separation system bolt cutter. Due to a lack of high strain rate response data for the materials involved, the properties for the bolt cutter and the bolt were selected to achieve agreement between computer simulation and observed characteristics of the recovered test hardware. The calculations provided insight into design parameters such as the cutter blade kinetic energy, the preload on the bolt, the relative position of the anvil, and the anvil shape. Modeling of the cutting process clarifies metallographic observation of both cut and uncut bolts obtained from several tests. Understanding the physical processes involved in bolt cutter operation may suggest certain design modifications that could improve performance margin without increasing environmental shock response levels.

S. Goldstein↗
NASA NTRS1995

Program For Optimization Of Antenna Structures

JPL-ANTOPT computer program designed to help engineers optimize microwave antenna structures by use of path-length- and pointing-error-analysis calculations. Program is package of codes and subroutines added to MSC/NASTRAN program. Written in MSC/NASTRAN's(TM) Direct Matrix Abstraction Programming (DMAP) language.

Strain, Douglas↗
NASA NTRS2006

Anisotropic Laminar Piezocomposite Actuator Incorporating Machined PMN-PT Single Crystal Fibers

The design, fabrication, and testing of a flexible, laminar, anisotropic piezoelectric composite actuator utilizing machined PMN-32%PT single crystal fibers is presented. The device consists of a layer of rectangular single crystal piezoelectric fibers in an epoxy matrix, packaged between interdigitated electrode polyimide films. Quasistatic free-strain measurements of the single crystal device are compared with measurements from geometrically identical specimens incorporating polycrystalline PZT-5A and PZT-5H piezoceramic fibers. Free-strain actuation of the single crystal actuator at low bipolar electric fields (+/- 250 V/mm) is approximately 400% greater than that of the baseline PZT-5A piezoceramic device, and 200% greater than that of the PZT-5H device. Free-strain actuation under high unipolar electric fields (0-4kV/mm) is approximately 200% of the PZT-5A baseline device, and 150% of the PZT-5H alternate piezoceramic device. Performance increases at low field are qualitatively consistent with predicted increases based on scaling the low-field d33 piezoelectric constants of the respective piezoelectric materials. High-field increases are much less than scaled d33 estimates, but appear consistent with high-field freestrain measurements reported for similar bulk single-crystal and piezoceramic compositions. Measurements of single crystal actuator capacitance and coupling coefficient are also provided. These properties were poorly predicted using scaled bulk material dielectric and coupling coefficient data. Rules-of-mixtures calculations of the effective elastic properties of the single crystal device and estimated actuation work energy densities are also presented. Results indicate longitudinal stiffnesses significantly lower (50% less) than either piezoceramic device. This suggests that single-crystal piezocomposite actuators will be best suited to low induced-stress, high strain and deflection applications.

Wilkie, W. Keats↗
DOE OSTI2022

Fatigue performance of laser powder bed fusion hydride-dehydride Ti-6Al-4V powder

We report hydride-dehydride (HDH) Ti-6Al-4V alloy with particle size distribution of 50–120 µm is laser powder bed fusion (L-PBF) processed using optimum processing parameters and a near-fully dense structure with a density of 99.9 % is achieved. Microstructural observations and phase analyses indicate formation of columnar β grains with acicular α/α' phases in as-built condition. The roughness of the as-fabricated samples is significant with an average roughness of R a = 15.71 ± 3.96 µm and a root mean square roughness of R rms = 108.4 ± 24.9 µm, however, both values are reduced to R a = 0.19 ± 0.04 µm and R rms = 4.9 ± 0.6 µm after mechanical grinding. Mechanical tests are carried out on as-fabricated specimens followed by stress relief treatment. All samples are tested to failure in fatigue, under fully-reversed tension-compression conditions of R = –1. The as-built samples failed from the surface with crack initiation mainly at micro-notches, whereas after mechanically grinding, crack initiation changed to subsurface defects such as pores. Minimizing surface roughness by mechanically grinding eliminates surface micro-notches which improves fatigue strength in the high cycle fatigue region. Fatigue notch factor calculations showed that the effect of surface roughness was significantly lower when HDH powder is used compared to standard spherical powder. X-ray diffraction analysis revealed an in-plane compressive stress, micro-strain and grain refinement on the surface of the mechanically ground samples. Fractography observations (macroscale) revealed a fully brittle fracture in the first stage of crack growth with a transition to a dominantly ductile fracture in the third stage of crack growth. On the other hand, at the micro scale, even the brittle fracture regions showed evidence of ductile fracture within the α' martensite laths.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Quantitative nanoscale tracking of oxygen vacancy diffusion inside single ceria grains by in situ transmission electron microscopy

Oxygen vacancy formation and migration in ceria is critical to its electrochemical and catalytic properties in systems for chemical and energy transformation, but its quantification is rather challenging especially at atomic-scale because of disordered distribution. Here we report a rational approach to track oxygen vacancy diffusion in single grains of pure and Sm-doped ceria at -20 °C to 160 °C using in situ (scanning) transmission electron microscopy ((S)TEM). To create a gradient in oxygen vacancy concentration, a small region (~30 nm in diameter) inside a ceria grain is reduced to the C-type CeO 1.68 phase by the ionization or radiolysis effect of a high-energy electron beam. The evolution in oxygen vacancy concentration is then mapped through lattice expansion measurement using scanning nano-beam diffraction or 4D STEM at a spatial resolution better than 2 nm; this allows direct determination of local oxygen vacancy diffusion coefficients in a very small domain inside pure and Sm-doped ceria at different temperatures. Further, the activation energies for oxygen transport are determined to be 0.59, 0.66, 1.12, and 1.27 eV for pure CeO 2 , Ce 0.94 Sm 0.06 O 1.97 , Ce 0.89 Sm 0.11 O 1.945 , and Ce 0.8 Sm 0.2 O 1.9 , respectively, implying that activation energy increases due to impurity scattering. The results are qualitatively supported by density functional theory (DFT) calculations. In addition, our in situ TEM investigation reveals that dislocations impede oxygen vacancy diffusion by absorbing oxygen vacancies from the surrounding areas and pinning them locally. With more oxygen vacancies absorbed, dislocations show extended strain fields with local tensile zone sandwiched between the compressed ones. Therefore, dislocation density should be reduced in order to minimize the resistance to oxygen vacancy diffusion at low temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Intrinsic and extrinsic effects on the fracture toughness of ductile metallic glasses

The potential application of metallic glasses (MGs) as structural materials demands accurate measurements of their toughness and the understanding of the underlying factors affecting it. Currently, it is challenging to precisely measure the toughness of MGs, especially ductile MGs. Measured toughness values are widely scattered, even for MGs with identical compositions. That is attributed to the combined effect of intrinsic and extrinsic factors including processing, sample geometries, and loading conditions. A fundamental understanding of the influences of these elements is thus of great significance. In the present study, molecular dynamics simulations are performed to investigate the influence of intrinsic and extrinsic effects on the fracture toughness of CuZr MGs. In particular, focused is placed on the effects of cooling rate, notch shape and size, strain rate, and temperature. The results indicate that the fracture toughness of a MG scales with the cooling rate used to prepare it. This is attributed to increased free volume content generated at high cooling rates, which enhances plastic deformation and amplifies the associated energy dissipation during plastic deformation events. Fracture toughness also increases with the strain rate, arguably due to strain rate-induced crack extension delay. Overall, the results demonstrate that the largest fracture toughness are achieved when MG samples are fabricated at high cooling rates and subjected to high strain rate deformation. In addition, results suggest that the fracture toughness decreases with increasing temperature, due to the significant decrease in strength. Here, the correlations revealed between these crucial intrinsic and extrinsic parameters and the calculated MG fracture toughness support the development of a framework to understand the root of the discrepancies in the measurement of the toughness of MGs and provide insights into the design of tough MGs for structural applications.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Bismuthene for highly efficient carbon dioxide electroreduction reaction

Bismuth (Bi) has been known as a highly efficient electrocatalyst for CO 2 reduction reaction. Stable free-standing two-dimensional Bi monolayer (Bismuthene) structures have been predicted theoretically, but never realized experimentally. Here, we show the first simple large-scale synthesis of free-standing Bismuthene, to our knowledge, and demonstrate its high electrocatalytic efficiency for formate (HCOO - ) formation from CO 2 reduction reaction. The catalytic performance is evident by the high Faradaic efficiency (99% at -580 mV vs. Reversible Hydrogen Electrode (RHE)), small onset overpotential (<90 mV) and high durability (no performance decay after 75 h and annealing at 400 °C). Density functional theory calculations show the structure-sensitivity of the CO2 reduction reaction over Bismuthene and thicker nanosheets, suggesting that selective formation of HCOO - indeed can proceed easily on Bismuthene (111) facet due to the unique compressive strain. This work paves the way for the extensive experimental investigation of Bismuthene in many different fields.

Science & Technology - Other Topics↗
DOE OSTI2021

The Zintl phases A In 2 As 2 (A = Ca, Sr, Ba): new topological insulators and thermoelectric material candidates

Recently, there has been a lot of interest in topological insulators (TIs), being electronic materials, which are insulating in their bulk but with the gapless exotic metallic state on their surface. The surface states observed in such materials behave as a perfect conductor thereby making them more suited for several cutting-edge technological applications such as spintronic devices. Here, we report the synthesis and structural characterization of the Zintl phases AIn 2 As 2 (A = Ca, Sr, Ba), which could become a new class of TIs. Crystal structure elucidation by single-crystal X-ray diffraction reveals that CaIn 2 As 2 and SrIn 2 As 2 are isostructural and crystallize in the EuIn 2 P 2 structure type (space group P6 3 /mmc, no. 194, Z = 2) with unit cell parameters a = 4.1482(6) Å, c = 17.726(4) Å; and a = 4.2222(6) Å, c = 18.110(3) Å, respectively. Their hexagonal structure is made up of alternating [In 2 As 2 ] 2– layers separated by slabs of A 2+ cations. BaIn 2 As 2 on the other hand crystallizes in the monoclinic EuGa 2 P 2 structure type (space group P2/m, no. 10, Z = 4) with unit cell parameters a = 10.2746(11) Å, b = 4.3005(5) Å, c = 13.3317(14) Å and β = 95.569(2)°. This structure is also layered, and it is made up of different type of polyanionic [In 2 As 2 ] 2– units and Ba 2+ cations. The valence electron count for all three compounds adheres to the Zintl-Klemm formalism, and all elements achieve closed-shell electronic configurations. Bulk electronic structure calculations indicate the opening of a bandgap E g ~ 0.03 eV (CaIn 2 As 2 and Sr 2 In 2 As 2 ), and E g ~0.21 eV (BaIn 2 As 2 ) in the absence of strain and spin–orbit coupling (SOC). Furthermore, these results argue in favor of the realization of a nontrivial topological insulator state under the influence of tensile strain and SOC. Preliminary transport properties on BaIn 2 As 2 are suggestive of a degenerate p-type semiconductor—a behavior which is sought after in thermoelectric (TE) materials. Since both TIs and excellent TE materials are known to favor the same material properties such as narrow bandgap, heavy elements, and strong SOC, these three Zintl phases are also projected as candidates TE materials.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Apparatus for skidding sensitivity testing of energetic materials

A remote-operated apparatus for testing the detonation sensitivity of energetic materials is detailed. Using an air ram and rotating disk, the normal force and transverse velocity of the impact plane are controlled independently, enabling the exploration of varying impact conditions over a wide parameter space. A microcontroller local to the apparatus is used to automate apparatus operation and ensure temporal alignment of the impacting ram head with the rotating disk. Calculation of the firing parameters and issuing of operational commands are handled by a remote computer and relayed to the local microcontoller for execution at the hardware level. Finally, impact forces are taken from fast strain measurements obtained from gauges incorporated into the ram head. Infrared imaging of explosive samples provides insight to the peak thermal temperatures experienced at the sample surface during the impact event.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
NASA NTRS1974

Experimental evaluation of stresses in cylindrically hollow (drilled) balls

An experimental stress analysis was undertaken to evaluate stresses within cylindrically hollow (drilled) bearing balls proportioned for 40, 50, and 60 percent mass reductions. Strain gage rosettes were used to determine principal strains and stresses in the steel ball models statically loaded in various orientations. Results are reported for 127 mm OD balls under 44,500 N loads. Similitude considerations permit these results to be applied to calculate stresses in actual size drilled bearing balls proportioned to these mass reductions.

Nypan, L. J.↗
NASA NTRS1990

The development of physically based micro-mechanics - Based on the characteristic tip field of dislocation pileup

Based on the analytical solution by Eshelby et al. (1951) and the numerical calculations by Chou and Li (1969) and Armstrong et al. (1962), it is shown that the force, F, on the locked leading dislocation of a discrete dislocation pileup is capable of characterizing uniquely the stress, strain, and displacement field at the tip of the pileup, including the positions of the discrete mobile dislocations next to the leading dislocation. Conversely, the positions of the mobile dislocations can be used to measure F. The F thus measured can be used to study micro-plastic deformation and micro-fractures at grain boundaries or any other dislocation barriers.

Gao, Q.↗
NASA NTRS1985

A catalogue of potentially bright close binary gravitational wave sources

This is a current print-out of results of a survey, undertaken in the spring of 1985, to identify those known binary stars which might produce significant gravitational wave amplitudes at earth, either dimensionless strain amplitudes exceeding a threshold h = 10(exp -21), or energy fluxes exceeding F = 10(exp -12) erg cm(exp -2) s(exp -1). All real or putative binaries brighter than a certain limiting magnitude (calculated as a function of primary spectral type, orbital period, orbital eccentricity, and bandpass) are included. All double degenerate binaries and Wolf-Rayet binaries with known or suspected orbital periods have also been included. The catalog consists of two parts: a listing of objects in ascending order of Right Ascension (Equinox B1950), followed by an index, listing of objects by identification number according to all major stellar catalogs. The object listing is a print-out of the spreadsheets on which the catalog is currently maintained. It should be noted that the use of this spreadsheet program imposes some limitations on the display of entries. Text entries which exceed the cell size may appear in truncated form, or may run into adjacent columns. Greek characters are not available; they are represented here by the first two or three letters of their Roman names, the first letter appearing as a capital or lower-case letter according to whether the capital or lower-case Greek character is represented. Neither superscripts nor subscripts are available; they appear here in normal position and type-face. The index provides the Right Ascension and Declination of objects sorted by catalogue number.

Webbink, Ronald F.↗
NASA NTRS1996

Study of the velocity gradient tensor in turbulent flow

The behavior of the velocity gradient tensor, A(ij)=delta u(i)/delta x(j), was studied using three turbulent flows obtained from direct numerical simulation The flows studies were: an inviscid calculation of the interaction between two vortex tubes, a homogeneous isotropic flow, and a temporally evolving planar wake. Self-similar behavior for each flow was obtained when A(ij) was normalized with the mean strain rate. The case of the interaction between two vortex tubes revealed a finite sized coherent structure with topological characteristics predictable by a restricted Euler model. This structure was found to evolve with the peak vorticity as the flow approached singularity. Invariants of A(ij) within this structure followed a straight line relationship of the form: gamma(sup 3)+gammaQ+R=0, where Q and R are the second and third invariants of A(ij), and the eigenvalue gamma is nearly constant over the volume of this structure. Data within this structure have local strain topology of unstable-node/saddle/saddle. The characteristics of the velocity gradient tensor and the anisotropic part of a related acceleration gradient tensor H(ij) were also studied for a homogeneous isotropic flow and a temporally evolving planar wake. It was found that the intermediate principal eigenvalue of the rate-of-strain tensor of H(ij) tended to be negative, with local strain topology of the type stable-node/saddle/saddle. There was also a preferential eigenvalue direction. The magnitude of H(ij) in the wake flow was found to be very small when data were conditioned at high local dissipation regions. This result was not observed in the relatively low Reynolds number simulation of homogeneous isotropic flow. A restricted Euler model of the evolution of A(ij) was found to reproduce many of the topological features identified in the simulations.

Cheng, Wei-Ping↗