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At least 703 records · Page 39
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Evolutionary inverse design of defects at graphene 2D lateral interfaces

Grain boundaries (GBs) in two-dimensional (2D) materials often have a profound impact on various material properties from mechanical to optical to electronic, yet predicting all possible GB formations is a challenge. Here, we introduce a workflow based on an evolutionary algorithm for exploring possible GBs formed at a lateral 2D interface. In a departure from conventional genetic algorithm based structure optimization methods, we perform genetic operations in the near interface region that allow us to be computationally efficient. We benchmark our method using graphene, which is a well-studied 2D material with a wide range of point defects. An empirical potential was used as the surrogate of the evolutionary search. More than 11.5 × 106 structures in total were evaluated for 128 GB orientations, and for each orientation, the ten best structures are recorded. A subset of low energy GBs predicted by empirical potential based search was relaxed by first-principles calculations and used to validate the energetic rank order. With the validated formation energy, we rank-ordered the best 128 GB structures and performed a detailed statistical analysis of primitive rings to find the correlation between the ring distribution and the formation energy. We found that for low energy GBs (below 0.5eV/Å), Stone–Wales defects will dominate, while structures with a higher energy (0.5–1.1eV/Å) show an increasing population of heptagons and nine-membered rings to form seven-nine defect pairs. For structures with energy higher than 1.1eV/Å, the percentage of octagons and nine-membered rings increases, which indicates that these two types of rings are not energetically favorable. Our proposed methodology is broadly applicable to explore defective low dimensional materials and represents a powerful tool that enables a systematic search of GBs of lateral interfaces for 2D materials.

Zhang, Jianan↗
DOE OSTI2023

Monte Carlo simulation of shielding designs for a cabinet form factor preclinical MV–energy photon FLASH radiotherapy system

Purpose: A preclinical MV-energy photon FLASH radiotherapy system is being designed at Stanford and SLAC National Accelerator Laboratory. Because of the higher energy and dose rate compared to conventional kV-energy photon laboratory-scale irradiators, adequate shielding in a stand-alone cabinet form factor is more challenging to achieve. We present a Monte Carlo simulation of multilayered shielding for a compact self-shielding system without the need for a radiation therapy vault. Methods: A multilayered shielding approach using multiple alternating layers of high-Z and low-Z materials is applied to the self-shielded cabinet to effectively mitigate the secondary radiation produced and to allow the device to be housed in a Controlled Radiation Area outside of a radiation vault. The multilayered shielding approach takes advantage of the properties of high-Z and low-Z radiation shielding materials such as density, cross-section, atomic number of the shielding elements, and products of radiation interactions within each layer. The Monte Carlo radiation transport code, FLUKA, is used to simulate the total effective dose produced by the operation. Results: The multilayered shielding designs proposed and simulated produced effective dose rates significantly lower than monolayer designs with the same total material thickness at the regulatory boundary; this is accomplished through the manipulation of the locations where secondary radiation is produced and reactions due to material properties such as neutron back reflection in hydrogen. Borated polyethylene at 5 wt% significantly increased the shielding performance as compared to regular polyethylene, with the magnitude of the reduction depending upon the order of the shielding material. Conclusions: The multilayered shielding provides a path for shielding preclinical FLASH systems that deliver MV-energy bremsstrahlung photons. Furthermore, this approach promises to be more efficient with respect to the shielding material mass and space claim as compared to shielded vaults typically required for clinical radiation therapy with MV photons.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2021

Analysis of defects in In 2 O 3 :H synthesized in presence of water vapor and hydrogen gas mixture

Using hybrid functional-based density functional theory calculations, we analyze the structure and kinetics of defects formed in two competing synthesis routes to prepare hydrogen-doped In 2 O 3 films, using a hydrogen and oxygen gas mixture vs synthesis in the presence of water vapor. For both of these synthesis routes, we find that H + is the dominant defect species: when the Fermi level is close to the conduction band, H + has a lower formation energy than other intrinsic or extrinsic defects. Our results also suggest that water molecules spontaneously split into H + (which occupies octahedral voids) and OH – interstitials (which occupies vacant oxygen lattice sites or oxygen vacancies). From the analysis of the binding energies between these different defects, we conclude that these defects do not cluster and are most likely to stay spatially distributed throughout the films. In addition, the sum of formation energies of an oxygen (i.e., O$^{2–}_i$) and a H + interstitial is close to the formation energy of a OH – interstitial, meaning that water molecules are completely split into 2H + and O$^{2–}_i$ at the synthesis conditions. Further, in the presence of H 2 + O 2 gas mixture, oxygen interstitials occupy oxygen vacancies while hydrogen interstitials occupy vacant oxygen lattice sites and form bonds with lattice oxygens. Our analysis of the defect equilibria suggests that the hydrogen content in films synthesized in the presence of water vapor is higher than films synthesized in the presence of a hydrogen gas mixture. At high dopant concentrations, a hydrogen bond network is formed in the system and this leads to large distortions in the lattice.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1990

Consequences of a hydrous mantle for Venus tectonics - Predictions for Magellan

The consequences of a Venus mantle enriched in water relative to the earth are considered. If the moon-forming impact hypothesis is correct, earth may have lost substantial volatiles. Venus, however, experienced no such impact and is therefore likely to be relatively more water-rich. It is demonstrated that a higher abundance of water in the Venus mantle has the effect of inhibiting the rise of crust-forming melts and volatiles to the surface, which results in a thin stable crust, a dry, stiff upper mantle, and restricted magmatism. Conduction through a shallow thermal lithosphere dominates the removal of heat from the interior, and flow in the mantle is strongly coupled to the surface deformation. Current observations of Venus are consistent with this model. A review of the consequences of this model makes it possible to make predictions for the Magellan radar images.

Williams, David R.↗
NASA NTRS1979

Martian soil stratigraphy and rock coatings observed in color-enhanced Viking Lander images

Subtle color variations of martian surface materials were enhanced in eight Viking Lander (VL) color images. Well-defined soil units recognized at each site (six at VL-1 and four at VL-2), are identified on the basis of color, texture, morphology, and contact relations. The soil units at the Viking 2 site form a well-defined stratigraphic sequence, whereas the sequence at the Viking 1 site is only partially defined. The same relative soil colors occur at the two sites, suggesting that similar soil units are widespread on Mars. Several types of rock surface materials can be recognized at the two sites; dark, relatively 'blue' rock surfaces are probably minimally weathered igneous rock, whereas bright rock surfaces, with a green/(blue + red) ratio higher than that of any other surface material, are interpreted as a weathering product formed in situ on the rock. These rock surface types are common at both sites. Soil adhering to rocks is common at VL-2, but rare at VL-1. The mechanism that produces the weathering coating on rocks probably operates planet-wide.

Strickland, E. L., III↗
DOE OSTI2026

In situ transmission electron microscopy observations of CaCO 3 crystallization onto polysaccharide-coated nanoparticles

Polysaccharides and proteoglycans are widely associated with the organic matrix at sites of CaCO 3 biomineralization, and previous studies indicate that these macromolecules may confer greater roles in mineral nucleation than previously recognized. This investigation uses in situ liquid-phase transmission electron microscopy (LP-TEM) to observe CaCO 3 nucleation onto aminated silica (SiO 2 –NH 3 + ) nanoparticles treated with a layer of chitosan (near-neutral derivative of chitin) or heparin (a carboxylated and highly sulfated glycosaminoglycan). In the absence of polysaccharides, few CaCO 3 particles formed and exhibited mobility. However, the SiO 2 –NH 3 + nanoparticles were enveloped in a region of higher mass density relative to the bulk solution, suggesting the development of a local solute-rich environment that surrounds the charged NH 3 + groups. The heparin- or chitosan-coated silica particles also exhibited regions of higher mass density around the nanoparticles. In the presence of these polysaccharide coatings, we observed the nucleation of abundant CaCO 3 particles whereby the polyanionic heparin promoted more nucleation than the weakly cationic chitosan. Many crystallites appeared to form at the polysaccharide–TEM cell membrane–solution interface, further indicating interfacial and macromolecule-specific control on crystallization. The combined results demonstrate that chitosan and heparin have an appreciable effect on the timing, size, and location of CaCO 3 nucleation compared to the polysaccharide-free nanoparticles.

15 GEOTHERMAL ENERGY↗
NASA NTRSDate not supplied

Spin Formability of High-Strength Aluminum Alloys for Aerospace Applications

Near-net-shape fabrication of launch vehicle tanks and aircraft fuselages is critical for meeting the growing demand for unitized structures in the aerospace industry. Cryogenic tank manufacturing relies on multi-piece construction requiring hundreds of meters of welds that increase both structural weight and inspection time. Currently, fuselages have an immense part count requiring tens of thousands of rivets and long assembly time. NASA is pursuing spin and flow forming technology for manufacturing such structures with minimal assembly. These processes enable domes and stiffened barrels to be fabricated using single-piece construction, offering significant manufacturing rate benefits over conventional methods. However, Al alloys with sufficient formability, such as Al 6061, tend to have insufficient strength for aerospace structural applications. In contrast, alloys with higher strength, such as Al 2139 and Al 2050, exhibit insufficient formability during ambient temperature forming. This study investigates the spin formability of competitive 5xxx and 2xxx series Al alloys, identifies defects impeding formability, and sheds light on future processing routes for successful forming.

Aluminum↗
NASA NTRSDate not supplied

Spin Formability of High-Strength Aluminum Alloys for Aerospace Applications

Near-net-shape fabrication of launch vehicle tanks and aircraft fuselages is proposed to increase manufacturing rates of aerospace structures. Currently, cryogenic tank manufacturing relies on multi-piece construction requiring hundreds of meters of welds that increase both structural weight and inspection time. Metallic aircraft fuselages involve installation of tens of thousands of rivets resulting in long assembly times. NASA is pursuing spin and flow forming technologies for manufacturing such structures with minimal assembly. These processes enable domes and stiffened barrels to be fabricated using single-piece construction, offering significant manufacturing rate benefits over conventional methods. However, aluminum alloys with sufficient formability, such as Al 6061, tend to have insufficient strength for aerospace structural applications. In contrast, alloys with higher strength, such as Al 2139 and Al 2050, exhibit insufficient formability during ambient temperature forming. This study investigates the spin formability of competitive 5xxx- and 2xxx-series Al alloys, identifies defects impeding formability, and sheds light on future processing routes for successful forming.

Aluminum↗
DOE OSTI2021

Black hole perturbations of massive and partially massless spin-2 fields in (anti) de Sitter spacetime

We provide a systematic and comprehensive derivation of the linearized dynamics of massive and partially massless spin-2 particles in a Schwarzschild (anti) de Sitter black hole background, in four and higher spacetime dimensions. In particular, we show how to obtain the quadratic actions for the propagating modes and recast the resulting equations of motion in a Schrödinger-like form. In the case of partially massless fields in Schwarzschild de Sitter spacetime, we study the isospectrality between modes of different parity. In particular, we prove isospectrality analytically for modes with multipole number L = 1 in four spacetime dimensions, providing the explicit form of the underlying symmetry. We show that isospectrality between partially massless modes of different parity is broken in higher-dimensional Schwarzschild de Sitter spacetimes.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Lowly Fused Non-Fullerene Acceptors Towards Efficient Organic Solar Cells Enabled by Isomerization

Here two lowly fused non-fullerene acceptors (NFAs) with isomeric structures, named as BTP-out-4F and BTP-in-4F, were developed by tailoring the fused 7-ring central core of Y6 into a lowly fused 5-ring linked with two octyloxythiophene bridges. BTP-out-4F with octyloxy side chains away from the central core exhibited large steric hindrance that restrained the rotational freedom between the thiophene bridge and end group but maintained free rotation between the central core and the thiophene bridge. In contrast, BTP-in-4F with octyloxy side chains close to the central core had much lower rotation freedom due to the non-covalent S … O interactions locked the central core, thiophene bridge and end group simultaneously, making BTP-in-4F have higher molecular crystallinity. On the other hand, the optical properties, energy levels and the blend morphology properties were significantly influenced, leading to distinctive photovoltaic performances. BTP-out-4F formed favorable energy level alignment and morphology when matching with PBDB-T donor, thus its device realized a much higher PCE of 13.32%, which was over 13 times than that of BTP-in-4F based device (PCE=0.97%).

36 MATERIALS SCIENCE↗
DOE OSTI2021

Properties and reaction mechanism of phosphoric acid activated metakaolin geopolymer at varied curing temperatures

Reaction mechanism of phosphoric activated metakaolin is studied by techniques of calorimetry, ICP-OES, FTIR and NMR taking curing temperature and [H{sub 3}PO{sub 4}] as variables. A two-stage curing method including pre-curing at 40 °C for 24 h and second curing at 60/80 °C for another 24 h effectively mitigates thermal-cracking of the obtained silico-aluminate phosphate (SAP) samples, whose maximum compressive strength reaches 120 MPa. Dealumination of metakaolin in H{sub 3}PO{sub 4} solution produces low aluminate-containing silicate units and subsequently condensation of silicate tetrahedrons occurs. Higher temperature and H{sub 3}PO{sub 4} concentration accelerate the two processes. After complete reaction, A{sub VI} structure from metakaolin disappears and the dissolved Al preferably forms Al{sub VI}-O-P units. During reaction, metastable P-O-P intermediate appears in SAPs with higher P/Al ratio and finally transforms into Al-O-P structure upon PO{sub 4}{sup 3−} consumption. Phosphate units as P(OAl){sub x}(H{sub 2}O){sub 4-x} are connected with Si or Al structure units. Major structural units of the SAPs include Al-O-P, Si-O-P, Si-O-Si, and Si-O-Al.

36 MATERIALS SCIENCE↗
NASA NTRS1978

Extensions of guiding center motion to higher order

In a static magnetic field, some well-known guiding-center equations maintain their form when extended to next order in gyroradius. In these cases, it is only necessary to include the next order term in the magnetic moment series. The differential equation for guiding-center motion which describes both the parallel and perpendicular velocities correctly through first order in gyroradius is given. The question of how to define the guiding center position through second order arises and is discussed, and second order drifts are derived for one usual definition. The toroidal canonical angular momentum, P-phi, of the guiding center in an axisymmetric field is shown to be conserved using the guiding center velocity correct through first order. When second-order motion is included, P-phi is no longer a constant. The above extensions of guiding-center theory help to resolve the different tokamak orbits obtained either by using the guiding-center equations of motion or by using conservation of P-phi.

Northrop, T. G.↗
NASA NTRS2023

The First Extragalactic Detection of Higher-Order Hydrogen Recombination Lines

The Large Magellanic Cloud (LMC) is the nearest (~50 kpc) star-forming galaxy characterized by a low metallicity (Z~0.3-0.5 Z⨀) similar to galaxies during the early phases of their assembly. As a result, star formation studies in the LMC provide a stepping stone to understanding star formation at earlier epochs of the universe where these processes cannot be directly observed. N113 is one of the most prominent star-forming regions in the LMC hosting one of the most massive giant molecular clouds. N113 is small enough to be imaged in its entirety, but large enough to showcase many important phenomena such as multiple generations of stars, stellar feedback, and different environments. We present our findings from an investigation of the early stages of star formation in the N113 region using the Atacama Large Millimeter/submillimeter Array (ALMA) molecular line data probing a wide density range: 12CO, 13CO, and C18O (2-1), 13CO and C18O (1-0), HCN (1-0), HCO+ (1-0), H13CN (1-0) and (3-2), H13CO+ (1-0) and (3-2), CS (2-1) and (5-4), as well as 1.3 mm and 3 mm continuum. We used the Python package quickclump to identify molecular clumps. We utilized the multiline non-LTE fitting tool based on models from RADEX developed by Finn et al. (2021, ApJ, 917, 106) to construct the CO, HCN, HCO+, and CS temperature and column density, and the H2 density maps of N113. We constructed a catalog of molecular clumps including their physical properties, chemical abundances, sizes, velocities, and velocity dispersions. To establish the evolutionary status of the clumps, their positions were compared with previously identified young stellar objects (YSOs) from the Spitzer/SAGE and Herschel/HERITAGE surveys, as well as water and OH masers. We compared the properties of the clumps in N113 to those in the Galaxy and other regions in the LMC to assess the impact of the environment (e.g., metallicity, stellar feedback) on the star formation process.

Marta M Sewilo↗
NASA NTRS1989

Origin of opaque assemblages in C3V meteorites - Implications for nebular and planetary processes

The results of analyses of meteoritic opaque assemblages (OAs) are presented and used in conjunction with literature data on phase equilibria and diffusion to further develop the hypothesis of Blum et al. (1989) for the origin of OAs in Ca,Al-rich inclusions (CAIs). Based on the results of the analyses, it is suggested that OAs formed after the crystallization of host CAIs by exsolution, sulfidation, and oxidation of precursor alloys at low temperatures (about 770 K) and higher than solar gas sulfur and oxygen fugacities. This model contrasts with previous models that call upon the formation of CAI OAs by aggregation of previously formed phases in the solar nebula prior to the crystallization of CAIs.

Blum, Joel D.↗
DOE OSTI2024

Double-Helical Tiled Chain Structure of the Twist-Bend Liquid Crystal Phase in CB7CB

The twist-bend nematic liquid crystal phase is a three-dimensional fluid in which achiral bent molecules spontaneously form an orientationally ordered, macroscopically chiral, heliconical winding of a ten nanometer-scale pitch in the absence of positional ordering. Here, the structure of the twist-bend phase of the bent dimer CB7CB and its mixtures with 5CB is characterized, revealing a hidden invariance of the self-assembly of the twist-bend structure of CB7CB, such that over a wide range of concentrations and temperatures, the helix pitch and cone angle change as if the ground state for a pitch of the TB helix is an inextensible heliconical ribbon along the contour formed by following the local molecular long axis (the director). Remarkably, the distance along the length for a single turn of this helix is given by 2πR mol , where R mol is the radius of bend curvature of a single all-trans CB7CB molecule. This relationship emerges from frustrated steric packing due to the bent molecular shape: space in the fluid that is hard to fill attracts the most flexible molecular subcomponents, a theme of nanosegregation that generates self-assembled, oligomer-like correlations of interlocking bent molecules in the form of a brickwork-like tiling of pairs of molecular strands into duplex double-helical chains. At higher temperatures in the twist-bend phase, the cone angle is small, the director contour is nearly along the helix axis z, and the duplex chains are sequences of biaxial elements formed by overlapping half-molecule pairs, with an approximately 45° rotation of the biaxis between each such element along the chain.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Spatiotemporal Changes in Trace Metal Bioavailability in the Sediment Pore water of a Constructed Wetland Using Passive Pore water Samplers

Sediments in aquatic systems often act as a major sink for contaminants. Diffusive gradient in thin films (DGTs) and in situ equilibrium dialysis samplers (peepers) are two major in situ pore water sampling devices that overcome the problems associated with conventional pore water sampling methods. In the present study, DGTs and peepers were used to study the spatial and seasonal effects (cool months, October–February; warm months, May–September) on metal bioavailability in the H‐02 constructed wetland and the sink versus source role of the sediments by calculating the metal resupply capacity. Data showed similar seasonal trends in metal concentrations using passive samplers, peepers, and DGTs. Pooled Cu and Zn concentrations measured using DGTs were lower in warm months (1.67 ± 1.50 and 2.62 ± 0.68 μg L –1 , respectively, p < 0.001) versus in cool months (2.12 ± 0.65 and 5.58 ± 1.33 μg L –1 , respectively, p < 0.001; mean ± 95% confidence interval). Sulfate (SO 4 2− ) concentrations were significantly (p = 0.0139) lower in warm months (averaged at 0.22 ± 0.05 mg L –1 ) compared to in cool months (0.16 ± 0.05 mg L –1 ). The increase in SO 4 2− concentration is an indicator of the lower activity of sulfate‐reducing bacteria, which need SO 4 2− during anaerobic respiration, in which SO 4 2− is reduced to sulfide (S 2– ) that forms insoluble salts with Cu and Zn, which could partially explain the higher bioavailability of these metals in the cool season. Metal resupply capacity of the sediments was mostly <0.2 for Cu and Zn. Taken together, the H0‐2 wetland sediments mostly acted as a sink to both Cu and Zn over the course of the present study. Environ Toxicol Chem 2023;42:2726–2736. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2023

Dissecting the structural heterogeneity of proteins by native mass spectrometry

Abstract A single gene yields many forms of proteins via combinations of posttranscriptional/posttranslational modifications. Proteins also fold into higher‐order structures and interact with other molecules. The combined molecular diversity leads to the heterogeneity of proteins that manifests as distinct phenotypes. Structural biology has generated vast amounts of data, effectively enabling accurate structural prediction by computational methods. However, structures are often obtained heterologously under homogeneous states in vitro. The lack of native heterogeneity under cellular context creates challenges in precisely connecting the structural data to phenotypes. Mass spectrometry (MS) based proteomics methods can profile proteome composition of complex biological samples. Most MS methods follow the “bottom‐up” approach, which denatures and digests proteins into short peptide fragments for ease of detection. Coupled with chemical biology approaches, higher‐order structures can be probed via incorporation of covalent labels on native proteins that are maintained at the peptide level. Alternatively, native MS follows the “top‐down” approach and directly analyzes intact proteins under nondenaturing conditions. Various tandem MS activation methods can dissect the intact proteins for in‐depth structural elucidation. Herein, we review recent native MS applications for characterizing heterogeneous samples, including proteins binding to mixtures of ligands, homo/hetero‐complexes with varying stoichiometry, intrinsically disordered proteins with dynamic conformations, glycoprotein complexes with mixed modification states, and active membrane protein complexes in near‐native membrane environments. We summarize the benefits, challenges, and ongoing developments in native MS, with the hope to demonstrate an emerging technology that complements other tools by filling the knowledge gaps in understanding the molecular heterogeneity of proteins.

59 BASIC BIOLOGICAL SCIENCES↗