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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

RDMA vs. RPC for Implementing Distributed Data Structures

Distributed data structures are key to implementing scalable applications for scientific simulations and data analysis. In this paper we look at two implementation styles for distributed data structures: remote direct memory access (RDMA) and remote procedure call (RPC). We focus on operations that require individual accesses to remote portions of a distributed data structure, e.g., accessing a hash table bucket or distributed queue, rather than global operations in which all processors collectively exchange information. We look at the trade-offs between the two styles through microbenchmarks and a performance model that approximates the cost of each. The RDMA operations have direct hardware support in the network and therefore lower latency and overhead, while the RPC operations are more expressive but higher cost and can suffer from lack of attentiveness from the remote side. We also run experiments to compare the real-world performance of RDMA- and RPC-based data structure operations with the predicted performance to evaluate the accuracy of our model, and show that while the model does not always precisely predict running time, it allows us to choose the best implementation in the examples shown. We believe this analysis will assist developers in designing data structures that will perform well on current network architectures, as well as network architects in providing better support for this class of distributed data structures.

distributed data structures↗
DOE OSTI2016

PRINCEPS: A computer-based approach to the structural description and recognition of trends within structural databases, and its application to the Ce-Ni-Si System

Intermetallic crystal structures offer an enormous structural diversity, with an endless array of structural motifs whose connection to stability and physical properties are often mysterious. Making sense of the often complex crystal structures that arise here, developing a clear structural description, and identifying connections to other phases can be laborious and require an encyclopedic knowledge of structure types. In this Article, we present PRINCEPS, an algorithm based on a new coordination environment projection scheme that facilitates the structural analysis and comparison of such crystal structures. We demonstrate the potential of this approach by applying it to the complex Ce-Ni-Si ternary system, whose 17 binary and 21 ternary phases would present a daunting challenge to one seeking to understand the system by manual inspection (but has nonetheless been well-described through the heroic efforts of previous researchers). With the help of PRINCEPS, most of the ternary phases in this system can be rationalized as intergrowths of simple structural fragments, and grouped into a handful of structural series (with some outliers). Lastly, these results illustrate how the PRINCEPS approach can be used to organize a vast collection of crystal structures into structurally meaningful families, and guide the description of complex atomic arrangements.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2019

Rare earth silicates and germanates crystallizing in the wadeite and related structure types

Single crystals of six new alkali lanthanide silicates and germanates were synthesized via the flux growth method. A2CeSi3O9 (A = Rb, Cs) and Cs2CeGe3O9 crystallize in the Wadeite structure with space group P63/m and lattice parameters a = 7.1950(2) Å and c = 10.5109(3) Å for the Rb analogue. K2CeSi3O9 crystallizes in a previously unobserved monoclinic distortion of the Wadeite structure with space group P21/n and lattice parameters a = 7.0801(2) Å, b = 10.458(2) Å, c = 12.238(2) Å and β = 90.07(3)°. Finally, Cs8Dy2Ge16O39 and Cs8Dy2Ge16O38F2 crystallize in two new structure types that are related to each other and to the Wadeite structure. Cs8Dy2Ge16O39 crystallizes in the monoclinic space group P2/c with lattice parameters a = 12.4054(3) Å, b = 7.4310(2) Å, c = 24.5761(6) Å, and β = 90.8849(16)° and Cs8Dy2Ge16O38F2 crystallizes in the orthorhombic space group Pnn2 with lattice parameters a = 12.3489(4) Å, b = 12.4805(5) Å, and c = 7.3802(3) Å. The synthesis and structure of the six compounds are reported and the crystallochemical reasons for the formation of each structure are discussed.

nuclear, materials and chemistry by design, synthe↗
DOE OSTI2016

Forging Fast Ion Conducting Nanochannels with Swift Heavy Ions: The Correlated Role of Local Electronic and Atomic Structure

Atomically disordered oxides have attracted significant attention in recent years due to the possibility of enhanced ionic conductivity. However, the correlation between atomic disorder, corresponding electronic structure, and the resulting oxygen diffusivity is not well understood. The disordered variants of the ordered pyrochlore structure in gadolinium titanate (Gd 2 Ti 2 O 7 ) are seen as a particularly interesting prospect due to intrinsic presence of a vacant oxygen site in the unit atomic structure, which could provide a channel for fast oxygen conduction. As such, in the present work, we provide insights into the subangstrom scale on the disordering-induced variations in the local atomic environment and its effect on the electronic structure in high-energy ion irradiation-induced disordered nanochannels, which can be utilized as pathways for fast oxygen ion transport. With the help of an atomic plane-by-plane-resolved analyses, the work shows how the presence of various types of TiO$_x$ polyhedral that exist in the amorphous and disordered crystalline phase modify the electronic structures relative to the ordered pyrochlore phase in Gd 2 Ti 2 O 7 . The correlated molecular dynamics simulations on the disordered structures show a remarkable enhancement in oxygen diffusivity as compared with ordered pyrochlore lattice and make that a suitable candidate for applications requiring fast oxygen conduction.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Amyloid structure exhibits polymorphism on multiple length scales in human brain tissue

Although aggregation of Aβ amyloid fibrils into plaques in the brain is a hallmark of Alzheimer's Disease (AD), the correlation between amyloid burden and severity of symptoms is weak. One possible reason is that amyloid fibrils are structurally polymorphic and different polymorphs may contribute differentially to disease. However, the occurrence and distribution of amyloid polymorphisms in human brain is poorly documented. Here we seek to fill this knowledge gap by using X-ray microdiffraction of histological sections of human tissue to map the abundance, orientation and structural heterogeneities of amyloid within individual plaques; among proximal plaques and in subjects with distinct clinical histories. A 5 µ x-ray beam was used to generate diffraction data with each pattern arising from a scattering volume of only ~ 450 µ3 , making possible collection of dozens to hundreds of diffraction patterns from a single amyloid plaque. X-ray scattering from these samples exhibited all the properties expected for scattering from amyloid. Amyloid distribution was mapped using the intensity of its signature 4.7 Å reflection which also provided information on the orientation of amyloid fibrils across plaques. Margins of plaques exhibited a greater degree of orientation than cores and orientation around blood vessels frequently appeared tangential. Variation in the structure of Aβ fibrils is reflected in the shape of the 4.7 Å peak which usually appears as a doublet. Variations in this peak correspond to differences between the structure of amyloid within cores of plaques and at their periphery. Examination of tissue from a mismatch case - an individual with high plaque burden but no overt signs of dementia at time of death - revealed a diversity of structure and spatial distribution of amyloid that is distinct from typical AD cases. As a result, we demonstrate the existence of structural polymorphisms among amyloid within and among plaques of a single individual and suggest the existence of distinct differences in the organization of amyloid in subjects with different clinical presentations.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2017

Electrochemical properties and structural evolution of O3-type layered sodium mixed transition metal oxides with trivalent nickel

The electrochemical properties of NaNi 0.5 Co 0.5 O 2 and NaNi 0.5 Fe 0.5 O 2 and their structural transitions as a function of Na extraction associated with redox reactions are investigated. Synthesized in the O3-type layered structure, both materials show reasonable electrochemical activities at room temperature, delivering approximately 0.5 Na per formula unit at C/10 discharge. More Na can be reversibly cycled in NaNi 0.5 Co 0.5 O 2 at elevated temperature and/or in an extended voltage window, while NaNi 0.5 Fe 0.5 O 2 shows significant capacity fading at a high voltage cutoff which is likely due to Fe 4+ migration. In situ X-ray diffraction shows that the structural changes in the two materials upon desodiation are very different. NaNi 0.5 Co 0.5 O 2 goes through many different two-phase reactions including three different O3-type and three different P3-type structures during cycling, producing a voltage profile with multiple plateau-like features. In contrast, NaNi 0.5 Fe 0.5 O 2 has a smooth voltage profile and shows the typical O3–P3 phase transition without lattice distortion seen in other materials. Finally, this different structural evolution upon desodiation and re-sodiation can be explained by the electronic structure of the mixed transition metals and how it perturbs the ordering between Na ions differently.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Coherent high-power RF wakefield generation by electron bunch trains in a metamaterial structure

In this paper, we present an experimental study of coherent high-power wakefield generation in a metamaterial (MTM) structure at 11.7 GHz by 65 MeV electron bunch trains at the Argonne Wakefield Accelerator (AWA), following a previous experiment, the Stage-I experiment, at the AWA. Both the Stage-II experiment, reported in this paper, and the Stage-I experiment were conducted using MTM structures, which are all-metal periodic structures with the period being much smaller than the wavelength. Differences between the two experiments include (1) structure length (Stage-I 8 cm and Stage-II 20 cm); (2) number of bunches used to excite the structure (Stage-I with two bunches, up to 85 nC of total charge; Stage-II with eight bunches, up to 224 nC of total charge); and (3) highest peak power measured (Stage-I 80 MW in a 2 ns pulse and Stage-II 380 MW in a 10 ns pulse). High-power radio frequency pulses were generated by reversed Cherenkov radiation of the electron beam due to the negative group velocity in the MTM structures. Because the radiation is coherent, a train of bunches with a proper spacing can build up to achieve a high peak power. The observed output power levels are very promising for future applications in direct collinear wakefield acceleration or in transfer to a second accelerator for two-beam acceleration.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Revealing the structure of light pseudoscalar mesons at the electron–ion collider

The questions of how the bulk of the Universe's visible mass emerges and how it is manifest in the existence and properties of hadrons are profound, and probe the heart of strongly interacting matter. Paradoxically, the lightest pseudoscalar mesons appear to be key to a further understanding of the emergent mass and structure mechanisms. These mesons, namely, the pion and kaon, are the Nambu-Goldstone boson modes of quantum chromodynamics (QCD). Unravelling their partonic structure and the interplay between emergent and Higgs-boson mass mechanisms is a common goal of three interdependent approaches - continuum QCD phenomenology, lattice-regularised QCD, and the global analysis of parton distributions - linked to experimental measurements of hadron structure. Experimentally, the anticipated electron-ion collider will enable a revolution in our ability to study pion and kaon structures, accessed by scattering from the 'meson cloud' of the proton through the Sullivan process. With the goal of enabling a suite of measurements that can address these questions, we examine key reactions that identify the critical detector-system requirements needed to map tagged pion and kaon cross-sections over a wide range of kinematics. The excellent prospects for extracting pion structural, functional, and form-factor data are outlined, and similar prospects for kaon structures are discussed in the context of a worldwide programme. Finally, the successful completion of the programme outlined herein will deliver deep, far-reaching insights into the emergence of pions and kaons, their properties, and their role as QCD's Goldstone boson modes.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2015

Structural and magnetic phase transitions in CeCu 6 - x T x ( T = Ag , Pd )

The structural and the magnetic properties of CeCu6-xAgx (0≤x≤0.85) and CeCu6-xPdx (0≤x≤0.4) have been studied using neutron diffraction, resonant ultrasound spectroscopy (RUS), x-ray diffraction measurements, and first principles calculations. The structural and magnetic phase diagrams of CeCu6-xAgx and CeCu6-xPdx as a function of Ag/Pd composition are reported. The end member, CeCu6, undergoes a structural phase transition from an orthorhombic (Pnma) to a monoclinic (P21/c) phase at 240 K. In CeCu6-xAgx, the structural phase transition temperature (Ts) decreases linearly with Ag concentration and extrapolates to zero at xS ≈0.1. The structural transition in CeCu6-xPdx remains unperturbed with Pd substitution within the range of our study. The lattice constant b slightly decreases with Ag/Pd doping, whereas a and c increase with an overall increase in the unit cell volume. Both systems, CeCu6-xAgx and CeCu6-xPdx, exhibit a magnetic quantum critical point (QCP), at x≈0.2 and x≈0.05, respectively. Near the QCP, long range antiferromagnetic ordering takes place at an incommensurate wave vector (δ10 δ2), where δ1~0.62,δ2~0.25,x=0.125 for CeCu6-xPdx and δ1~0.64,δ2~0.3,x=0.3 for CeCu6-xAgx. The magnetic structure consists of an amplitude modulation of the Ce moments which are aligned along the c axis of the orthorhombic unit cell.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Generation of High-Power, Reversed-Cherenkov Wakefield Radiation in a Metamaterial Structure

Here, we present the first demonstration of high-power, reversed-Cherenkov wakefield radiation by electron bunches passing through a metamaterial structure. The structure supports a fundamental transverse magnetic mode with a negative group velocity leading to reversed-Cherenkov radiation, which was clearly verified in the experiments. Single 45 nC electron bunches of 65 MeV traversing the structure generated up to 25 MW in 2 ns pulses at 11.4 GHz, in excellent agreement with theory. Two bunches of 85 nC with appropriate temporal spacing generated up to 80 MW by coherent wakefield superposition, the highest rf power that metamaterial structures ever experienced without damage. These results demonstrate the unique features of metamaterial structures that are very attractive for future high-gradient wakefield accelerators, including two-beam and collinear accelerators. Advantages include the high shunt impedance for high-power generation and high-gradient acceleration, the simple and rugged structure, and a large parameter space for optimization.

43 PARTICLE ACCELERATORS↗
NASA NTRS1993

Flight-vehicle materials, structures, and dynamics - Assessment and future directions. Vol. 5 - Structural dynamics and aeroelasticity

Various papers on flight vehicle materials, structures, and dynamics are presented. Individual topics addressed include: general modeling methods, component modeling techniques, time-domain computational techniques, dynamics of articulated structures, structural dynamics in rotating systems, structural dynamics in rotorcraft, damping in structures, structural acoustics, structural design for control, structural modeling for control, control strategies for structures, system identification, overall assessment of needs and benefits in structural dynamics and controlled structures. Also discussed are: experimental aeroelasticity in wind tunnels, aeroservoelasticity, nonlinear aeroelasticity, aeroelasticity problems in turbomachines, rotary-wing aeroelasticity with application to VTOL vehicles, computational aeroelasticity, structural dynamic testing and instrumentation.

Noor, Ahmed K.↗
NASA NTRS1996

A Concept for the Use and Integration of Super-Conducting Magnets in Structural Systems in General and Maglev Guideway Mega-Structures in Particular

Recent breakthroughs in several different fields now make it possible to incorporate the use of superconducting magnets in structures in ways which enhance the performance of structural members or components of structural systems in general and Maglev guideway mega-structures in particular. The building of structural systems which connect appropriately scaled superconducting magnets with the post-tensioned tensile components of beams, girders, or columns would, if coupled with 'state of the art' structure monitoring, feedback and control systems, and advanced computer software, constitute a distinct new generation of structures that would possess the unique characteristic of being heuristic and demand or live-load responsive. The holistic integration of powerful superconducting magnets in structures so that they do actual structural work, creates a class of 'technologically endowed' structures that, in part - literally substitute superconductive electric power and magnetism for concrete and steel. The research and development engineering, and architectural design issues associated with such 'technologically endowed' structural system can now be conceptualized, designed, computer simulates built and tested. The Maglev guideway mega-structure delineated herein incorporates these concepts, and is designed for operation in the median strip of U.S. Interstate Highway 5 from San Diego to Seattle an Vancouver, and possibly on to Fairbanks, Alaska. This system also fits in the median strip of U.S. Interstate Highway 55 and 95 North-South, and 80 and 10, East-West. As a Western Region 'Peace Dividend' project, it could become a National or Bi-National research, design and build, super turnkey project that would create thousands of jobs by applying superconducting, material science, electronic aerospace and other defense industry technologies to a multi-vehicle, multi-use Maglev guideway megastructure that integrates urban mass transit Lower Speed (0-100 mph), High Speed (100-200 mph), Super Speed (200-400 mph), and Hypersonic evacuated tube (400-10,000 mph) Maglev systems.

Ussery, Wilfred T.↗
DOE OSTI2018

A complex approach to assessing porous structure of structured ceramics obtained by SPS technique

Highlights: • A complex approach to microstructural analysis of nanostructured SPS ceramics was presented. • A bunch of gas, intrusion and microscopic methods were applied for open/closed porosity determination in nanoceramics. • Kinetic TPR analysis for structural changes in nanoceramic was carried out. • 3D pore visualization for nanoceramics was provided via SEM/FIB tomography • Porosity calculations were performed via BET, BJH, t-plot, Washburn equation, FEI Avizo 9.1 software results. - Abstract: Spark plasma sintering (SPS) is a highly prospective technology providing rapid consolidation of nano-disperse compositions into structured ceramics with complex structural organization. Characterizing pore structure of such systems remains a challenge due to different types, size and shape of the pores as well as structural defects occurring in the material, thus requiring special methods to quantify certain porosity type. The paper presents a complex approach to studying structure of SPS hematite-based structured ceramics. The method involves a number of gas, intrusion and electron microscopy probing techniques to characterize open and closed pores in the ceramics bulk. Low temperature nitrogen sorption has been implemented to estimate open micro- and mesoporosity in the size range 0.7–50 nm using BET, BJH and t-plot methods to process experimental data. Additionally, structural peculiarities of consolidated structured ceramics have been studied using temperature-programmed reduction (TPR) with the kinetic analysis of the results being carried out. Volume of macropores sized above 50 nm has been measured with mercury intrusion. Closed pores and structural defects have been quantified with SEM and FIB/SEM tomography. By comparing experimental and estimated results we have shown that addressing all porosity types provides a highly precise structural characterization of the material.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2021

Moving beyond the constraints of chemistry via crystal structure discovery with isotropic multiwell pair potentials

The rigid constraints of chemistry-dictated by quantum mechanics and the discrete nature of the atom-limit the set of observable atomic crystal structures. What structures are possible in the absence of these constraints? Here, we systematically crystallize one-component systems of particles interacting with isotropic multiwell pair potentials. We investigate two tunable families of pairwise interaction potentials. Our simulations self-assemble a multitude of crystal structures ranging from basic lattices to complex networks. Sixteen of the structures have natural analogs spanning all coordination numbers found in inorganic chemistry. Fifteen more are hitherto unknown and occupy the space between covalent and metallic coordination environments. The discovered crystal structures constitute targets for self-assembly and expand our understanding of what a crystal structure can look like.

crystal structures↗
DOE OSTI2018

Structural and molecular dynamics studies of a C1-oxidizing lytic polysaccharide monooxygenase from Heterobasidion irregulare reveal amino acids important for substrate recognition

Lytic polysaccharide monooxygenases (LPMOs) are a group of recently discovered enzymes that play important roles in the decomposition of recalcitrant polysaccharides. Here, we report the biochemical, structural, and computational characterization of an LPMO from the white-rot fungus Heterobasidion irregulare (HiLPMO9B). This enzyme oxidizes cellulose at the C1 carbon of glycosidic linkages. The crystal structure of HiLPMO9B was determined at 2.1 A resolution using X-ray crystallography. Unlike the majority of the currently available C1-specific LPMO structures, the HiLPMO9B structure contains an extended L2 loop, connecting ..beta..-strands ..beta..2 and ..beta..3 of the ..beta..-sandwich structure. Molecular dynamics (MD) simulations suggest roles for both aromatic and acidic residues in the substrate binding of HiLPMO9B, with the main contribution from the residues located on the extended region of the L2 loop (Tyr20) and the LC loop (Asp205, Tyr207, and Glu210). Asp205 and Glu210 were found to be involved in the hydrogen bonding with the hydroxyl group of the C6 carbon of glucose moieties directly or via a water molecule. Two different binding orientations were observed over the course of the MD simulations. In each orientation, the active-site copper of this LPMO preferentially skewed toward the pyranose C1 of the glycosidic linkage over the targeted glycosidic bond. This study provides additional insight into cellulose binding by C1-specific LPMOs, giving a molecular-level picture of active site substrate interactions.

09 BIOMASS FUELS↗
DOE OSTI2019

Electronic and Molecular Structures of the CeB 6 Monomer

The electronic and molecular structure of the CeB 6 molecular unit has been probed by anion PE spectroscopy and DFT calculations to gain insight into structural and electronic relaxation on edge and corner sites of this ionic material. While boron in bulk lanthanide hexaboride materials assumes octahedral B 6 3– units, the monomer assumes a less compact structure to delocalize the charge. Two competitive molecular structures were identified for the anion and neutral species, which include a boat-like structure and a planar or near-planar teardrop structure. Ce adopts different orbital occupancies in the two isomers; the boat-like structure has a 4f superconfiguration while the teardrop favors a 4f 6s occupancy. The B 6 ligand in these structures carries a charge of –4 and –3, respectively. The teardrop structure, which was calculated to be isoenergetic with the boat structure, was most consistent with the experimental spectrum. B 6 -local orbitals crowd the energy window between the Ce 4f and 6s (HOMO) orbitals. A low-lying transition from the B-based orbitals is observed slightly less than 1 eV above the ground state. Here, the results suggest that edge and corner conductivity involves stabilized, highly diffuse 6s orbitals or bands rather than the bulk-favored 5d band. High-spin and open-shell low-spin states were calculated to be very close in energy for both the anion and neutral, a characteristic that reflects how decoupled the 4f electron is from the B 6 2p- and Ce 6s-based molecular orbitals.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Effects of rapid thermal annealing on the structural and local atomic properties of ZnO: Ge nanocomposite thin films

We have investigated the structural and local atomic properties of Ge nanocrystals (Ge-ncs) embedded ZnO (ZnO: Ge) thin films. The films were deposited by sequential sputtering of ZnO and Ge thin film layers on z-cut quartz substrates followed by an ex-situ rapid thermal annealing (RTA) at 600 °C for 30, 60, and 90 s under forming gas atmosphere. Effects of RTA time on the evolution of Ge-ncs were investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), hard x-ray photoelectron spectroscopy (HAXPES), and extended x-ray absorption fine structure (EXAFS). XRD patterns have clearly shown that fcc diamond phase Ge-ncs of sizes ranging between 18 and 27 nm are formed upon RTA and no Ge-oxide peak has been detected. However, cross-section SEM images have clearly revealed that after RTA process, Ge layers form varying size nanoclusters composed of Ge-ncs regions. EXAFS performed at the Ge K-edge to probe the local atomic structure of the Ge-ncs has revealed that as prepared ZnO:Ge possesses Ge-oxide but subsequent RTA leads to crystalline Ge structure without the oxide layer. In order to study the occupied electronic structure, HAXPES has been utilized. The peak separation between the Zn 2p and Ge 3d shows no significant change due to RTA. This implies little change in the valence band offset due to RTA.

77 NANOSCIENCE AND NANOTECHNOLOGY↗