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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,387 records · Page 77
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

In situ nanoindentation study of plastic Co-deformation in Al-TiN nanocomposites

We performed in situ indentation in a transmission electron microscope on Al-TiN multilayers with individual layer thicknesses of 50 nm, 5 nm and 2.7 nm to explore the effect of length scales on the plastic co-deformability of a metal and a ceramic. At 50 nm, plasticity was confined to the Al layers with easy initiation of cracks in the TiN layers. At 5 nm and below, cracking in TiN was suppressed and post mortem measurements indicated a reduction in layer thickness in both layers. Our results demonstrate the profound size effect in enhancing plastic co-deformability in nanoscale metal-ceramic multilayers.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Development of very high J c in Ba(Fe 1-x Co x ) 2 As 2 thin films grown on CaF 2

Ba(Fe 1-x Co x ) 2 As 2 is the most tunable of the Fe-based superconductors (FBS) in terms of acceptance of high densities of self-assembled and artificially introduced pinning centres which are effective in significantly increasing the critical current density, J c . Moreover, FBS are very sensitive to strain, which induces an important enhancement in critical temperature,T c , of the material. In this study we demonstrate that strain induced by the substrate can further improve J c of both single and multilayer films by more than that expected simply due to the increase in T c . The multilayer deposition of Ba(Fe 1-x Co x ) 2 As 2 on CaF 2 increases the pinning force density (F p =J c x μ₀H) by more than 60% compared to a single layer film, reaching a maximum of 84 GN/m 3 at 22.5 T and 4.2 K, the highest value ever reported in any 122 phase.

36 MATERIALS SCIENCE↗
DOE OSTI2015

A new method for achieving enhanced dielectric response over a wide temperature range

We report a novel approach for achieving high dielectric response over a wide temperature range. In this approach, multilayer ceramic heterostructures with constituent compositions having strategically tuned Curie points (TC) were designed and integrated with varying electrical connectivity. Interestingly, these multilayer structures exhibited different dielectric behavior in series and parallel configuration due to variations in electrical boundary conditions resulting in the differences in the strength of the electrostatic coupling. The results are explained using nonlinear thermodynamic model taking into account electrostatic interlayer interaction. We believe that present work will have huge significance in design of high performance ceramic capacitors.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Large-Area Growth of Turbostratic Graphene on Ni(111) via Physical Vapor Deposition

Single-layer graphene has demonstrated remarkable electronic properties that are strongly influenced by interfacial bonding and break down for the lowest energy configuration of stacked graphene layers (AB Bernal). Multilayer graphene with relative rotations between carbon layers, known as turbostratic graphene, can effectively decouple the electronic states of adjacent layers, preserving properties similar to that of SLG. While the growth of AB Bernal graphene through chemical vapor deposition has been widely reported, we investigate the growth of turbostratic graphene on heteroepitaxial Ni(111) thin films utilizing physical vapor deposition. By varying the carbon deposition temperature between 800–1100°C, we report an increase in the graphene quality concomitant with a transition in the size of uniform thickness graphene, ranging from nanocrystallites to thousands of square microns. Combination Raman modes of as-grown graphene within the frequency range of 1650 cm ₋1 to 2300 cm ₋1 , along with features of the Raman 2D mode, were employed as signatures of turbostratic graphene. Bilayer and multilayer graphene were directly identified from areas that exhibited Raman characteristics of turbostratic graphene using high-resolution TEM imaging. Lastly, Raman maps of the pertinent modes reveal large regions of turbostratic graphene on Ni(111) thin films at a deposition temperature of 1100°C.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Magnetic coupling at rare earth ferromagnet/transition metal ferromagnet interfaces: A comprehensive study of Gd/Ni

Thin film magnetic heterostructures with competing interfacial coupling and Zeeman energy provide a fertile ground to study phase transition between different equilibrium states as a function of external magnetic field and temperature. A rare-earth (RE)/transition metal (TM) ferromagnetic multilayer is a classic example where the magnetic state is determined by a competition between the Zeeman energy and antiferromagnetic interfacial exchange coupling energy. Technologically, such structures offer the possibility to engineer the macroscopic magnetic response by tuning the microscopic interactions between the layers. We have performed an exhaustive study of nickel/gadolinium as a model system for understanding RE/TM multilayers using the element-specific measurement technique x-ray magnetic circular dichroism, and determined the full magnetic state diagrams as a function of temperature and magnetic layer thickness. We compare our results to a modified Stoner-Wohlfarth-based model and provide evidence of a thickness-dependent transition to a magnetic fan state which is critical in understanding magnetoresistance effects in RE/TM systems. In conclusion, the results provide important insight for spintronics and superconducting spintronics where engineering tunable magnetic inhomogeneity is key for certain applications.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Rapid prototyping of microchannels with surface patterns for fabrication of polymer fibers

Microfluidic technology has provided innovative solutions to numerous problems, but the cost of designing and fabricating microfluidic channels is impeding its expansion. In this study, Shrinky-Dink thermoplastic sheets are used to create multilayered complex templates for microfluidic channels. We also used inkjet and laserjet printers to raise a predetermined microchannel geometry by depositing several layers of ink for each feature consecutively. We achieved feature heights over 100 μm, which were measured and compared with surface profilometry. Templates closest to the target geometry were then used to create microfluidic devices from soft-lithography with the molds as a template. These microfluidic devices were, futhermore used to fabricate polymer microfibers using the microfluidic focusing approach to demonstrate the potential that this process has for microfluidic applications. Finally, an economic analysis was conducted to compare the price of common microfluidic template manufacturing methods. We showed that multilayer microchannels can be created significantly quicker and cheaper than current methods for design prototyping and point-of-care applications in the biomedical area.

42 ENGINEERING↗
DOE OSTI2019

Molecular interactions of phospholipid monolayers with a model phospholipase

The intrinsic overexpression of secretory phospholipase A2 (sPLA 2 ) in various pro-inflammatory diseases and cancers has the potential to be exploited as a therapeutic strategy for diagnostics and treatment. To explore this potential and advance our knowledge of the role of sPLA 2 in related diseases, it is necessary to systematically investigate the molecular interaction of the enzyme with lipids. By employing a Langmuir trough integrated with X-ray reflectivity and grazing incidence X-ray diffraction techniques, this study examined the molecular packing structure of 1,2-palmitoyl- sn-glycero -3-phosphocholine (DPPC) films before and after enzyme adsorption and enzyme-catalyzed degradation. Molecular interaction of sPLA 2 (from bee venom) with the DPPC monolayer exhibited Ca 2+ dependence. DPPC molecules at the interface without Ca 2+ retained a monolayer organization; upon adsorption of sPLA 2 to the monolayer the packing became tighter. In contrast, sPLA 2 -catalyzed degradation of DPPC occurred in the presence of Ca 2+ , leading to disruption of the ordered monolayer structure of DPPC. Further, the interfacial film became a mixture of highly ordered multilayer domains of palmitic acid (PA) and loosely packed monolayer phase of 1-palmitoyl-2-hydroxy- sn-glycero -3-phosphocholine (lysoPC) that potentially contained the remaining un-degraded DPPC. The redistribution of lipid degradation products into the third dimension, which produced multilayer PA domains, damaged the structural integrity of the original lipid layer and may explain the bursting of liposomes observed in other studies after a latency period of mixing liposomes with sPLA 2 . A quantitative understanding of the lipid packing and lipid-enzyme interaction provides an intuitive means of designing and optimizing lipid-related drug delivery systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Layer-by-layer assembly of a polymer of intrinsic microporosity: targeting the CO 2 /N 2 separation problem

A polymer of intrinsic microporosity, 1, has been incorporated into a series of polyelectrolyte multilayers via the layer-by-layer deposition method. One of these assemblies [a 6 nm multilayer derived from 1 plus poly(diallyldimethylammonium chloride)] showed exceptional permeation properties with respect to CO 2 and N 2 . Specifically, this film exhibited a CO 2 permeance of 130 GPU and a CO 2 /N 2 selectivity of 33.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Phonon confinement and interface lattice dynamics of ultrathin high- k rare earth sesquioxide films: the case of Eu 2 O 3 on YSZ(001)

The spatial confinement of atoms at surfaces and interfaces significantly alters the lattice dynamics of thin films, heterostructures and multilayers. Ultrathin films with high dielectric constants (high-k) are of paramount interest for applications as gate layers in current and future integrated circuits. Here we report a lattice dynamics study of high-k Eu 2 O 3 films with thicknesses of 21.3, 2.2, 1.3, and 0.8 nm deposited on YSZ(001). The Eu-partial phonon density of states (PDOS), obtained from nuclear inelastic scattering, exhibits broadening of the phonon peaks accompanied by up to a four-fold enhancement of the number of low-energy states compared to the ab initio calculated PDOS of a perfect Eu 2 O 3 crystal. Our analysis demonstrates that while the former effect reflects the reduced phonon lifetimes observed in thin films due to scattering from lattice defects, the latter phenomenon arises from an ultrathin EuO layer formed between the thin Eu 2 O 3 film and the YSZ(001) substrate. Thus, our work uncovers another potential source of vibrational anomalies in thin films and multilayers, which has to be cautiously considered.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Investigation into the crystal structure–dielectric property correlation in barium titanate nanocrystals of different sizes

For high capacitance multilayer ceramic capacitors, high dielectric constant and lead-free ceramic nanoparticles are highly desired. However, as the particle size decreases to a few tens of nanometers, their dielectric constant significantly decreases, and the underlying mechanism has yet to be fully elucidated. Herein, we report a systematic investigation into the crystal structure–dielectric property relationship of combustion-made BaTiO 3 (BTO) nanocrystals. When the nanocrystal size was 100 nm and below, a metastable paraelectric cubic phase was found in the as-received BTO (denoted as arBTO) nanocrystals based on an X-ray diffraction (XRD) study. A stable ferroelectric tetragonal phase was present when the nanocrystal size was above 200 nm. Notably, the cubic arBTO (particle size ≤100 nm) exhibited tetragonal fluctuations as revealed by Raman spectroscopy, whereas the tetragonal arBTO (particle size ≥200 nm) contained ~10% cubic fraction according to the Rietveld fitting of the XRD profiles. Thermal annealing of the multi-grain tetragonal arBTO at 950 °C yielded single crystals of annealed BTO (denoted as anBTO), whose dielectric constants were higher than those of arBTO. However, the single crystalline anBTO prevented the formation of 90° domains; therefore, they exhibited a low dielectric constant of ~300. Although X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy could not identify the exact structural defects, our study revealed that surface and bulk defects formed during synthesis affect the final crystal structures and thus the dielectric properties of BTO nanocrystals with different sizes. Finally, the understanding obtained from this study will help us design high dielectric constant perovskite nanocrystals for next-generation multilayer ceramic capacitor applications.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Mechanisms for the near-UV photodissociation of CH{sub 3}I on D{sub 2}O/Cu(110)

The system of CH{sub 3}I adsorbed on submonolayer, monolayer, and multilayer thin films of D{sub 2}O on Cu(110) has been studied by measuring the time of flight (TOF) distributions of the desorbing CH{sub 3} fragments after photodissociation using linearly polarized {lambda} = 248 nm light. For multilayer D{sub 2}O films (2-120 ML), the photodissociation is dominated by neutral photodissociation via the 'A-band' absorption of CH{sub 3}I. The polarization and angle dependent variation in the observed TOF spectra of the CH{sub 3} photofragments find that dissociation is largely via the {sup 3}Q{sub 0} excited state, but that also a contribution via the {sup 1}Q{sub 1} excitation can be identified. The photodissociation results also indicate that the CH{sub 3}I adsorbed on D{sub 2}O forms close-packed islands at submonolayer coverages, with a mixture of C-I bond axis orientations. For monolayer and submonolayer quantities of D{sub 2}O we have observed a contribution to CH{sub 3}I photodissociation via dissociative electron attachment (DEA) by photoelectrons. The observed DEA is consistent with delocalized photoelectrons from the substrate causing the observed dissociation- we do not find evidence for an enhanced DEA mechanism via the temporary solvation of photoelectrons in localized states of the D{sub 2}O ice.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Making use of x-ray optical effects in photoelectron-, Auger electron-, and x-ray emission spectroscopies: Total reflection, standing-wave excitation, and resonant effects

We present a general theoretical methodology and related open-access computer program for carrying out the calculation of photoelectron, Auger electron, and x-ray emission intensities in the presence of several x-ray optical effects, including total reflection at grazing incidence, excitation with standing-waves produced by reflection from synthetic multilayers and at core-level resonance conditions, and the use of variable polarization to produce magnetic circular dichroism. Calculations illustrating all of these effects are presented, including in some cases comparisons to experimental results. Sample types include both semi-infinite flat surfaces and arbitrary multilayer configurations, with interdiffusion/roughness at their interfaces. These x-ray optical effects can significantly alter observed photoelectron, Auger, and x-ray intensities, and in fact lead to several generally useful techniques for enhancing surface and buried-layer sensitivity, including layer-resolved densities of states and depth profiles of element-specific magnetization. The computer program used in this study should thus be useful for a broad range of studies in which x-ray optical effects are involved or are to be exploited in next-generation surface and interface studies of nanoscale systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Adsorption geometry, conformation, and electronic structure of 2H-octaethylporphyrin on Ag(111) and Fe metalation in ultra high vacuum

Due to the growing interest in the ferromagnetic properties of Fe-octaethylporphyrins (Fe-OEP) for applications in spintronics, methods to produce stable Fe-porphyrins with no Cl atoms are highly demanded. Here, we demonstrate the formation of Fe-OEP layers on Ag(111) single crystal by the ultra high vacuum in situ metalation of the free-base 2H-2,3,7,8,12,13,17,18-octaethylporphyrin (2H-OEP) molecules. The metalation proceeds exactly as in the case of 2H-5,10,15,20-tetraphenylporphyrin (2H-TPP) on the same substrate. An extensive surface characterization by means of X-ray photoemission spectroscopy, valence band photoemission, and NEXAFS with synchrotron radiation light provides information on molecular conformation and electronic structure in the monolayer and multilayer cases. We demonstrate that the presence of the ethyl groups affects the tilt of the adsorbed molecules, the conformation of the macrocycle, and the polarization screening in multilayers, but has only a minor effect in the metalation process with respect to 2H-TPP.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Tuning of the nucleation field in nanowires with perpendicular magnetic anisotropy

We report on domain nucleation in nanowires consisting of Co/Pt multilayers with perpendicular magnetic anisotropy that are patterned by electron-beam lithography, sputter deposition, and lift-off processing. It is found that the nucleation field can be tuned by changing the geometry of the wire ends. A reduction of the nucleation field by up to 60% is achieved when the wire ends are designed as tips. This contrasts with the behavior of wires with in-plane anisotropy where the nucleation field increases when triangular-pointed ends are used. In order to clarify the origin of the reduction of the nucleation field, micromagnetic simulations are employed. The effect cannot be explained by the lateral geometrical variation but is attributable to a local reduction of the perpendicular anisotropy caused by shadowing effects due to the resist mask during sputter deposition of the multilayer.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2013

Probing the local surface potential and quantum capacitance in single and multi-layer graphene

We report on local measurements of the surface potential and quantum capacitance in single layer graphene as well as multilayers thereof as a function of the carrier density by using frequency-modulated Kelvin probe force microscopy. We find excellent agreement to tight-binding calculations reported for graphene monolayers and extract the minimum quantum capacitance from density sweeps at room temperature. The surface potential of graphene multilayers is found to depend linearly on the carrier density, which suggests treating them as two-dimensional electron gases. In addition, we demonstrate that the simultaneously detected second harmonic of the Kelvin modulation, proportional to |∂{sup 2}C/∂z{sup 2}|, is directly sensitive to local changes in the quantum capacitance of graphene.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2013

Morphological stability of Cu-Nb nanocomposites under high-energy collision cascades

We use molecular dynamics and phase field simulations to demonstrate that Cu-Nb multilayered nanocomposites with individual layer thicknesses above 2–4 nm remain morphologically stable when subjected to 100 keV collision cascades, characteristic of neutron or heavy ion irradiation. The probability of morphological instability rapidly increases as the layer thickness decreases to 1 nm, which we attribute to overlap of zones of liquid-like interdiffusion inside radiation-induced thermal spikes at neighboring interfaces in the multilayer.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Surfaces of colloidal PbSe nanocrystals probed by thin-film positron annihilation spectroscopy

Positron annihilation lifetime spectroscopy and positron-electron momentum density (PEMD) studies on multilayers of PbSe nanocrystals (NCs), supported by transmission electron microscopy, show that positrons are strongly trapped at NC surfaces, where they provide insight into the surface composition and electronic structure of PbSe NCs. Our analysis indicates abundant annihilation of positrons with Se electrons at the NC surfaces and with O electrons of the oleic ligands bound to Pb ad-atoms at the NC surfaces, which demonstrates that positrons can be used as a sensitive probe to investigate the surface physics and chemistry of nanocrystals inside multilayers. Ab initio electronic structure calculations provide detailed insight in the valence and semi-core electron contributions to the positron-electron momentum density of PbSe. Both lifetime and PEMD are found to correlate with changes in the particle morphology characteristic of partial ligand removal.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Basic criteria for formation of growth twins in high stacking fault energy metals

Nanotwinned metals received significant interest lately as twin boundaries may enable simultaneous enhancement of strength, ductility, thermal stability, and radiation tolerance. However, nanotwins have been the privilege of metals with low-to-intermediate stacking fault energy (SFE). Recent scattered studies show that nanotwins could be introduced into high SFE metals, such as Al. In this paper, we examine several sputter-deposited, (111) textured Ag/Al, Cu/Ni, and Cu/Fe multilayers, wherein growth twins were observed in Al, Ni, and face-centered cubic (fcc) Fe. The comparisons lead to two important design criteria that dictate the introduction of growth twins in high SFE metals. The validity of these criteria was then examined in Ag/Ni multilayers. Furthermore, another twin formation mechanism in high SFE metals was discovered in Ag/Ni system.

36 MATERIALS SCIENCE↗