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At least 1,711 records · Page 95
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Characterization of thin film deposits on tungsten filaments in catalytic chemical vapor deposition using 1,1-dimethylsilacyclobutane

Metal filament plays a key role in the technique of catalytic chemical vapor deposition (Cat-CVD) as it serves as a catalyst in dissociating the source gas to form reactive species. These reactive species initiate the gas-phase reaction chemistry and final thin film and nanostructure formation. At the same time, they also react with the metal itself, leading to the formation of metal alloys and other deposits. The deposits on the tungsten filaments when exposed to 1,1-dimethylsilacyclobutane (DMSCB), a single-source precursor for silicon carbide thin films, in the process of Cat-CVD were studied in this work. It has been demonstrated that a rich variety of deposits, including tungsten carbides (W{sub 2}C and WC), tungsten silicide (W{sub 5}Si{sub 3}), silicon carbide, amorphous carbon, and graphite, form on the W filament surfaces. The structural and morphological changes in the tungsten filaments depend strongly on the DMSCB pressure and filament temperature. At 1000 and 2000 °C, the formation of WC and W{sub 2}C dominates. In addition, a thin amorphous carbon layer has been found at 1500 °C with the 0.12 and 0.24 Torr of DMSCB and a lower temperature of 1200 °C with the 0.48 Torr of DMSCB. An increase in the DMSCB sample pressure gives rise to higher Si and C contents. As a result, the formation of SiC and W{sub 5}Si{sub 3} has been observed with the two high-pressure DMSCB samples (i.e., 0.24 and 0.48 Torr). The rich decomposition chemistry of DMSCB on the W surfaces is responsible for the extensive changes in the structure of the W filament, providing support for the close relationship between the gas-phase decomposition chemistry and the nature of alloy formation on the metal surface. The understanding of the structural changes obtained from this work will help guide the development of efficient methods to solve the filament aging problem in Cat-CVD and also to achieve a controllable deposition process.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Spin scattering and noncollinear spin structure-induced intrinsic anomalous Hall effect in antiferromagnetic topological insulator MnB i 2 T e 4

MnB i 2 T e 4 has recently been established as an intrinsic antiferromagnetic (AFM) topological insulator—an ideal platform to create quantum anomalous Hall insulator and axion insulator states. We performed comprehensive studies on the structure, nontrivial surface state, and magnetotransport properties of this material. Our results reveal an intrinsic anomalous Hall effect arising from a noncollinear spin structure for the magnetic field parallel to the c axis. We observed negative magnetoresistance under arbitrary field orientation below and above the Néel temperature (T N ) , providing clear evidence for strong spin fluctuation-driven spin scattering in both the AFM and paramagnetic states. Furthermore, we found that the nontrivial surface state opens a large gap (~85 meV) even far above TN. Our findings demonstrate that the bulk band structure of MnB i 2 T e 4 is strongly coupled with the magnetic property and that a net Berry curvature in momentum space can be created in the canted AFM state. In addition, our results imply that the gap opening in the surface states is intrinsic, likely caused by the strong spin fluctuations in this material.

Angle-resolved photoemission spectroscopy↗
NASA NTRS1987

Device for quick changeover between wind tunnel force and pressure testing

This device allows for expeditious and repeated changeovers between pressure and force testing and which uses a minimum internal volume of a wind tunnel test structure. A matrix configuration of holes is located on the outer surface of the structure. Pressure tubes lead through the internal cavity of the structure from test sites to this outer surface matrix configuration. A pressure tube connector with a corresponding matrix of holes is connected to the surface of the structure. Pressure tubes leading from remotely located transducers are joined to the connector, thus forming pressure passageways from the test sites to the transducers to allow for pressure testing. When force testing is required, the pressure tube connector is disconnected and a cover plate is connected. The cover plate seals the exposed internal pressure tubes. Also, the outer surface of the cover plate conforms to the exterior of the structure, providing the necessary smooth surface for force testing. If further pressure testing is required, the cover plate can be disconnected and the pressure tube connector reconnected.

Wood, Richard M.↗
DOE OSTI2022

Superior Metal‐Organic Framework Activation with Dimethyl Ether

Abstract Metal–organic frameworks (MOFs) are demonstrated to be readily activated by treatment with the low surface tension, low boiling point solvent dimethyl ether (DME). The mildness of the method enables access to high surface areas by avoiding structural changes in the framework that often plague thermal activation methods. A distinction from previous methods is that DME activation succeeds for materials with coordinatively unsaturated sites (CUS) and non‐CUS MOFs as well. DME displaces solvent molecules occupying the pores of the MOF as well as those coordinated to metal centers; reducing evacuation temperature by using a coordinating, yet highly volatile guest enables low temperature activation with structural retention as demonstrated surface area measurements that match or exceed existing activation protocols.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS2014

Geologic Mapping of Vesta

We report on a preliminary global geologic map of Vesta, based on data from the Dawn spacecraft's High- Altitude Mapping Orbit (HAMO) and informed by Low-Altitude Mapping Orbit (LAMO) data. This map is part of an iterative mapping effort; the geologic map has been refined with each improvement in resolution. Vesta has a heavily-cratered surface, with large craters evident in numerous locations. The south pole is dominated by an impact structure identified before Dawn's arrival. Two large impact structures have been resolved: the younger, larger Rheasilvia structure, and the older, more degraded Veneneia structure. The surface is also characterized by a system of deep, globe-girdling equatorial troughs and ridges, as well as an older system of troughs and ridges to the north. Troughs and ridges are also evident cutting across, and spiraling arcuately from, the Rheasilvia central mound. However, no volcanic features have been unequivocally identified. Vesta can be divided very broadly into three terrains: heavily-cratered terrain; ridge-and-trough terrain (equatorial and northern); and terrain associated with the Rheasilvia crater. Localized features include bright and dark material and ejecta (some defined specifically by color); lobate deposits; and mass-wasting materials. No obvious volcanic features are evident. Stratigraphy of Vesta's geologic units suggests a history in which formation of a primary crust was followed by the formation of impact craters, including Veneneia and the associated Saturnalia Fossae unit. Formation of Rheasilvia followed, along with associated structural deformation that shaped the Divalia Fossae ridge-and-trough unit at the equator. Subsequent impacts and mass wasting events subdued impact craters, rims and portions of ridge-and-trough sets, and formed slumps and landslides, especially within crater floors and along crater rims and scarps. Subsequent to the formation of Rheasilvia, discontinuous low-albedo deposits formed or were emplaced; these lie stratigraphically above the equatorial ridges that likely were formed by Rheasilvia. The last features to be formed were craters with bright rays and other surface mantling deposits. Executed progressively throughout data acquisition, the iterative mapping process provided the team with geologic proto-units in a timely manner. However, interpretation of the resulting map was hampered by the necessity to provide the team with a standard nomenclature and symbology early in the process. With regard to mapping and interpreting units, the mapping process was hindered by the lack of calibrated mineralogic information. Topography and shadow played an important role in discriminating features and terrains, especially in the early stages of data acquisition.

Vesta↗
NASA NTRS1990

Surface-crack growth: Models, experiments, and structures; Proceedings of the Symposium, Sparks, NV, Apr. 25, 1988

The present volume on surface-crack growth modeling, experimental methods, and structures, discusses elastoplastic behavior, the fracture analysis of three-dimensional bodies with surface cracks, optical measurements of free-surface effects on natural surfaces and through cracks, an optical and finite-element investigation of a plastically deformed surface flaw under tension, fracture behavior prediction for rapidly loaded surface-cracked specimens, and surface cracks in thick laminated fiber composite plates. Also discussed are a novel study procedure for crack initiation and growth in thermal fatigue testing, the growth of surface cracks under fatigue and monotonically increasing load, the subcritical growth of a surface flaw, surface crack propagation in notched and unnotched rods, and theoretical and experimental analyses of surface cracks in weldments.

Reuter, Walter G.↗
DOE OSTI2017

Laser Nitriding of the Newly Developed Ti-20Nb-13Zr at.% Biomaterial Alloy to Enhance Its Mechanical and Corrosion Properties in Simulated Body Fluid

Despite the widespread application of Ti alloy in the biomedical field, surface treatments are typically applied to improve its resistance to corrosion and wear. A newly developed biomedical Ti-20Nb-13Zr at.% alloy (TNZ) was laser-treated in nitrogen environment to improve its surface characteristics with corrosion protection performance. Surface modification of the alloy by laser was performed through a Nd:YAG laser. The structural and surface morphological alterations in the laser nitrided layer were investigated by XRD and a FE-SEM. The mechanical properties have been evaluated using nanoindentation for laser nitride and as-received samples. The corrosion protection behavior was estimated using electrochemical corrosion analysis in a physiological medium (SBF). The obtained results revealed the production of a dense and compact film of TiN fine grains (micro-/nanosize) with 9.1 µm below the surface. The mechanical assessment results indicated an improvement in the modulus of elasticity, hardness, and resistance of the formed TiN layer to plastic deformation. The electrochemical analysis exhibited that the surface protection performance of the laser nitrided TNZ substrates in the SBF could be considerably enhanced compared to that of the as-received alloy due to the presence of fine grains in the TiN layer resulting from laser nitriding. Furthermore, the untreated and treated Ti-20Nb-13Zr alloy exhibited higher corrosion resistance than the CpTi and Ti6Al4V commercial alloys. The improvements in the surface hardness and corrosion properties of Ti alloy in a simulated body obtained using laser nitriding make this approach a suitable candidate for enhancing the properties of biomaterials.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Surface reconstructions and modified surface states in L a 1 – x C a x Mn O 3

Here, the unique functionalities present at surfaces and interfaces of complex oxides have attracted intense research in the past decade. Yet, the fundamental mechanisms underpinning functionality are often elusive, especially in doped manganites, limiting their implementation in functional electronic devices such as memristors and spin valves. Here, we present a local probe-based study on mixed-terminated La 5/8 Ca 3/8 MnO 3 (LCMO) films, and reveal surface metallicity in a thin film grown by pulsed-laser deposition. Using first-principles density-functional theory calculations with Hubbard correction that are more accurate to capture effects of correlation in these systems, we show that for Ca-segregated (001) LCMO surfaces the (La,Ca)O-site terminated surfaces are half metallic due to delocalized Mn- d states populating the Fermi level, whereas the MnO 2 -site terminated surfaces exhibit a half-metallic or insulating character depending on the type of surface reconstruction. Computations not only explain the current measurements, but also explain other recent surface measurements on LCMO thin films, leading to a coherent picture of how the crucial link between surface segregation and Jahn-Teller couplings in the manganese oxides tune the surface electronic/magnetic structure, thereby pointing to the fine control of transport and magnetism at the conductive oxide surface independent of the bulk.

36 MATERIALS SCIENCE↗
NASA NTRS1985

Corrosion of metals by hydrazine

The mechanism of corrosion of metals by hydrazine has been studied by means of coupons in sealed ampoules and by electrochemical techniques. The variables considered were temperature, CO2 impurity level, alloy composition and microcrystalline structure. The coupon studies, to date, verify that increasng temperature and the presence of CO2 does increase the corrosion rate as expected. The presence of molybdenum in stainless steels to the 3 percent level is not necessarily deleterious, contrary to published reports. The influence of microcrystalline structure and surface characteristics are more dominant effects. However, with Ti-6Al-4V, two different microcrystalline structures showed no significant differences. Corrosion rates of CRES 304 L in hydrazine have also been measured by several electrochemical techniques such as Tafel plots, polarization resistance and A. C. Impedance. This is the first documented work to show that A. C. Impedance can be used with non-aqueous solvents. Preliminary data correlated satisfactorily with the results of the coupon studies.

Lawton, E. A.↗
DOE OSTI2018

Study of charge transfer interaction modes in the mixed Donor-Acceptor cocrystals of pyrene derivatives and TCNQ: A combined structural, thermal, spectroscopic, and hirshfeld surfaces analysis

Highlights: • Three pyrene derivatives were synthesized as donors. • Co-crystallization of donor and acceptor resulted in mixed CT cocrystals. • The crystal structures and packing of the CT mixed assemblies are studied. • CT state has been affirmed by spectroscopic measurements. • The three cocrystals exhibited strong absorptions extend to near infrared. - Abstract: The role of charge-transfer (CT) interactions in the synthesis of charge-transfer (CT) complexes is of great significance, which has led to promising materials for a range of applications. Herein, we report the synthesis of three new binary mixed stack CT systems I, II, and III, grown by solvent evaporation technique involving, 1-(naphthalene-2-yl)− 3-(pyrene-1-yl)prop-2-en-1-one (NPPO), 3-(pyrene-1-yl)− 1-(thiophene-2-yl)prop-2-en-1-one (PTPO) and 2-phenyl-3-(pyrene-2-yl)acrylonitrile (PPA) as the donors and 7,7,8,8-tetracyanoquinodimethane (TCNQ) as an acceptor material. X-ray diffraction, thermal behavior, and spectroscopic analysis affirm the formation of CT complex and material purity. Based on the structural analysis the three CT complexes exhibit mixed stack arrangement (DAD-DAD) directed by the CT π–π interactions while the auxiliary hydrogen bonds (C-H-O, and C-H-N) stabilize and contribute to the overall packing of CT complexes. Remarkably, the complexes exhibit strong CT absorptions spreading to near infrared, making the crystals appear black. Not to mention, Hirshfeld surface analysis was employed to study the different intermolecular interactions and crystal packing in these materials. These results indicate that such a cocrystal design helps to explore new D-A systems, particularly with a mixed stack, that might have potential research values for applications in materials science.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS2017

Patterns of Canopy and Surface Layer Consumption in a Boreal Forest Fire from Repeat Airborne Lidar

Fire in the boreal region is the dominant agent of forest disturbance with direct impacts on ecosystem structure, carbon cycling, and global climate. Global and biome-scale impacts are mediated by burn severity, measured as loss of forest canopy and consumption of the soil organic layer. To date, knowledge of the spatial variability in burn severity has been limited by sparse field sampling and moderate resolution satellite data. Here, we used pre- and post-fire airborne lidar data to directly estimate changes in canopy vertical structure and surface elevation for a 2005 boreal forest fire on Alaskas Kenai Peninsula. We found that both canopy and surface losses were strongly linked to pre-fire species composition and exhibited important fine-scale spatial variability at sub-30m resolution. The fractional reduction in canopy volume ranged from 0.61 in lowland black spruce stands to 0.27 in mixed white spruce and broad leaf forest. Residual structure largely reflects standing dead trees, highlighting the influence of pre-fire forest structure on delayed carbon losses from above ground biomass, post-fire albedo, and variability in understory light environments. Median loss of surface elevation was highest in lowland black spruce stands (0.18 m) but much lower in mixed stands (0.02 m), consistent with differences in pre-fire organic layer accumulation. Spatially continuous depth-of-burn estimates from repeat lidar measurements provide novel information to constrain carbon emissions from the surface organic layer and may inform related research on post-fire successional trajectories. Spectral measures of burn severity from Landsat were correlated with canopy (r = 0.76) and surface (r = -0.71) removal in black spruce stands but captured less of the spatial variability in fire effects for mixed stands (canopy r = 0.56, surface r = -0.26), underscoring the difficulty in capturing fire effects in heterogeneous boreal forest landscapes using proxy measures of burn severity from Landsat.

Alonzo, Michael↗
DOE OSTI2015

Asymmetric projections of the arcuate fasciculus to the temporal cortex underlie lateralized language function in the human brain

The arcuate fasciculus (AF) in the human brain has asymmetric structural properties. However, the topographic organization of the asymmetric AF projections to the cortex and its relevance to cortical function remain unclear. Here we mapped the posterior projections of the human AF in the inferior parietal and lateral temporal cortices using surface-based structural connectivity analysis based on diffusion MRI and investigated their hemispheric differences. We then performed the cross-modal comparison with functional connectivity based on resting-state functional MRI (fMRI) and task-related cortical activation based on fMRI using a semantic classification task of single words. Structural connectivity analysis showed that the left AF connecting to Broca's area predominantly projected in the lateral temporal cortex extending from the posterior superior temporal gyrus to the mid part of the superior temporal sulcus and the middle temporal gyrus, whereas the right AF connecting to the right homolog of Broca's area predominantly projected to the inferior parietal cortex extending from the mid part of the supramarginal gyrus to the anterior part of the angular gyrus. The left-lateralized projection regions of the AF in the left temporal cortex had asymmetric functional connectivity with Broca's area, indicating structure-function concordance through the AF. During the language task, left-lateralized cortical activation was observed. Among them, the brain responses in the temporal cortex and Broca's area that were connected through the left-lateralized AF pathway were specifically correlated across subjects. These results suggest that the human left AF, which structurally and functionally connects the mid temporal cortex and Broca's area in asymmetrical fashion, coordinates the cortical activity in these remote cortices during a semantic decision task. As a result, the unique feature of the left AF is discussed in the context of the human capacity for language.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2014

Frequency selective infrared sensors

A frequency selective infrared (IR) photodetector having a predetermined frequency band. The exemplary frequency selective photodetector includes: a dielectric IR absorber having a first surface and a second surface substantially parallel to the first surface; an electrode electrically coupled to the first surface of the dielectric IR absorber; and a frequency selective surface plasmonic (FSSP) structure formed on the second surface of the dielectric IR absorber. The FSSP structure is designed to selectively transmit radiation in the predetermined frequency band that is incident on the FSSP structure substantially independent of the angle of incidence of the incident radiation on the FSSP structure.

36 MATERIALS SCIENCE↗
DOE OSTI2020

The role of surface hydroxyls on the radiolysis of gibbsite and boehmite nanoplatelets

Understanding mechanistic pathways to radiolytic hydrogen generation by metal oxyhydroxide nanomaterials is challenging because of the difficulties of distinguishing key locations of OH bond scission, from structural interiors to hydroxylated surfaces to physi-sorbed water molecules. In this work we exploited the interface-selectivity of vibrational sum frequency generation (VSFG) to isolate surface versus bulk hydroxyl groups for gibbsite and boehmite nanoplatelets before and after 60 Co irradiation at dose levels of approximately 7.0 and 29.6 Mrad. While high-resolution microscopy revealed no effect on particle bulk and surface structures, VSFG results clearly indicated up to 83% and 94% radiation-induced surface OH bond scission for gibbsite and boehmite, respectively, a substantially higher proportion than observed for interior OH groups by IR and Raman spectroscopy. Electron paramagnetic spectroscopy revealed that the major radiolysis products bound in the mineral structures are trapped electrons, O , O 2 – and possibly F-centers in gibbsite, and H , O and O 3 – in boehmite, which persist on the time frame of several months. The entrapped radiolysis products appear to be highly stable, enduring re-hydration of particle surfaces, and likely reflect a permanent adjustment in the thermodynamic stabilities of these nanomaterials.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2021

Radiolysis of thermally dehydrated gibbsite

Aluminum hydroxide (gibbsite, Al (OH) 3 ) powders and their thermally dehydrated forms were rehydrated and irradiated with gamma rays to examine the effects of the collapse of the crystalline gibbsite structure as it is converted to alumina on the transport of precursors of molecular hydrogen to the surface. The amount of hydrogen gas, H 2 , production from irradiated gibbsite and its transition phases with adsorbed water increased substantially with increasing temperature for dehydration of the samples, up to a point. Furthermore, the production of H 2 for samples without water was considerably lower than the hydrated samples signifying the importance of surface water and the transport of precursors to the surface. EPR spectroscopy showed that the major radiolytic products are trapped electrons and related O – centers. Thermal gravimetric analysis (TGA) measurements of gibbsite dehydration established the temperatures of phase transitions from gibbsite to alumina. Changes to the surface and structure of gibbsite and its transition phases following irradiation were analyzed using nitrogen adsorption, powder X-ray diffraction (pXRD), Raman spectroscopy, electron paramagnetic resonance (EPR), and scanning electron microscopy (SEM). pXRD and Raman spectra showed amorphorization at about 300 °C, coupled to a substantial increase in specific surface area due to increasing porosity.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2016

Phase transition kinetics and surface binding states of methylammonium lead iodide perovskite

We have presented a detailed analysis of phase transition kinetics and binding energy states of solution processed methylammonium lead iodide (MAPbI3) thin films prepared at ambient conditions and annealed at different elevated temperatures. The processing temperature and environmental conditions predominantly control the crystal structure and surface morphology of MAPbI3 thin films. The structural transformation from tetragonal to cubic occurs at 60C for 30 minutes annealing time while 10 minutes annelaed films posses tetragonal structure. The transformed phase is greatly intact even at higher annealing temperature of 150 C and time of 2 hours. Existence of charge transfer interaction between Pb 4f and I 3d oxidation states is quantified by the XPS spin orbit spilitting.

Kumar, G. R.↗
DOE OSTI2015

Effect of xenon on the structural phase state of the surface layer of cemented carbide under pulsed electron-beam irradiation

A comparative analysis of the surface-layer microstructure of a tungsten-based cemented carbide modified with pulsed high-energy electron beams generated by gas-discharge plasmas and of the tool life of metal-cutting plates prepared from this alloy is performed. The choice of a plasma-forming gas providing for the emission of electrons out of the plasma-filled cathode is shown to have a profound influence both on the formation process of nano-sized structural-phase states in the surface layer of the cemented carbide and on the tool life of the metal-cutting plates prepared from this alloy.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Surface modification of carbon black nanoparticles enhances photothermal separation and release of CO 2

Here, carbon black nanoparticles (CB) were covalently modified to improve the photothermal regeneration of a CO 2 capture nanofluid through decarboxylation. The photothermal release of CO 2 addresses high energy costs associated with regenerating capture fluids. By incorporating sulfonamides on the surface of CB, we enhance the photothermal separation of CO 2 from MEA by approximately 70% more than the unmodified CB. In contrast, with an anionic sulfonate on the surface, the total CO 2 released fell by approximately 60%. We verified the chemical composition and structure of surface modification using complementary techniques including FT-IR, TGA, XPS, and Raman spectroscopy.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗