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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 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Impact of baryonic physics on intrinsic alignments

We explore the effects of specific assumptions in the subgrid models of star formation and stellar and AGN feedback on intrinsic alignments of galaxies in cosmological simulations of "MassiveBlack-II" family. Using smaller volume simulations, we explored the parameter space of the subgrid star formation and feedback model and found remarkable robustness of the observable statistical measures to the details of subgrid physics. The one observational probe most sensitive to modeling details is the distribution of misalignment angles. We hypothesize that the amount of angular momentum carried away by the galactic wind is the primary physical quantity that controls the orientation of the stellar distribution. Finally, our results are also consistent with a similar study by the EAGLE simulation team.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Electronic Structure and Band Alignment of LaMnO 3 /SrTiO 3 Polar/Nonpolar Heterojunctions

The behavior of polar LaMnO3 (LMO) thin films deposited epitaxially on non-polar SrTiO3(001) (STO) is dictated by both the LMO/STO band alignment and the chemistry of the Mn cation. Using in situ x-ray photoelectron spectros-copy (XPS), we directly measure the valence band offset (VBO) of LMO/STO heterojunctions as a function of thickness, and find that the VBO is 2.5 eV for thicker (= 3 u.c.) films. We find no evidence of a built-in potential in LMO films of any thickness. Measurements of the Mn valence by Mn L-edge x-ray absorption spectroscopy (XAS) and by spatially-resolved electron energy loss spectra in scanning transmission electron microscopy images (STEM-EELS) reveal that Mn2+ is present at the LMO surface, but not at the LMO/STO interface. These results are corroborated by DFT simula-tions that confirm a VBO of approximately 2.5 eV for both ideal and intermixed interfaces. We propose a model for the behavior of polar / non-polar LMO/STO heterojunctions in which the built-in potential is alleviated by the formation of oxygen vacancies at the LMO surface.

Kaspar, Tiffany C.↗
DOE OSTI2015

Microwave conductance properties of aligned multiwall carbon nanotube textile sheets

Understanding the conductance properties of multi-walled carbon nanotube (MWNT) textile sheets in the microwave regime is essential for their potential use in high-speed and high-frequency applications. To expand current knowledge, complex high-frequency conductance measurements from 0.01 to 50 GHz and across temperatures from 4.2 K to 300 K and magnetic fields up to 2 T were made on textile sheets of highly aligned MWNTs with strand alignment oriented both parallel and perpendicular to the microwave electric field polarization. Sheets were drawn from 329 and 520 μm high MWNT forests that resulted in different DC resistance anisotropy. For all samples, the microwave conductance can be modeled approximately by a shunt capacitance in parallel with a frequency-independent conductance, but with no inductive contribution. Finally, this is consistent with diffusive Drude conduction as the primary transport mechanism up to 50 GHz. Further, it is found that the microwave conductance is essentially independent of both temperature and magnetic field.

Brown, Brian L.↗
DOE OSTI2015

Determining the band alignment of TbAs:GaAs and TbAs:In 0.53 Ga 0.47 As

Here, we propose and systematically justify a band structure for TbAs nanoparticles in GaAs and In 0.53 Ga 0.47 As host matrices. Moreover, fluence-dependent optical-pump terahertz-probe measurements suggest the TbAs nanoparticles have a band gap and provide information on the carrier dynamics, which are determined by the band alignment. Spectrophotometry measurements provide the energy of optical transitions in the nanocomposite systems and reveal a large blue shift in the absorption energy when the host matrix is changed from In 0.53 Ga 0.47 As to GaAs. Finally, Hall data provides the approximate Fermi level in each system. From this data, we deduce that the TbAs:GaAs system forms a type I (straddling) heterojunction and the TbAs:In 0.53 Ga 0.47 As system forms a type II (staggered) heterojunction.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Band alignment of epitaxial SrTiO{sub 3} thin films with (LaAlO{sub 3}){sub 0.3}-(Sr{sub 2}AlTaO{sub 6}){sub 0.7} (001)

SrTiO{sub 3} (STO) epitaxial thin films and heterostructures are of considerable interest due to the wide range of functionalities they exhibit. The alloy perovskite (LaAlO{sub 3}){sub 0.3}-(Sr{sub 2}AlTaO{sub 6}){sub 0.7} (LSAT) is commonly used as a substrate for these material structures due to its structural compatibility with STO and the strain-induced ferroelectric response in STO films grown on LSAT. However, surprisingly little is known about the electronic properties of the STO/LSAT interface despite its potentially important role in affecting the overall electronic structure of system. We examine the band alignment of STO/LSAT heterostructures using x-ray photoelectron spectroscopy for epitaxial STO films deposited using two different molecular beam epitaxy approaches. We find that the valence band offset ranges from +0.2(1) eV to −0.2(1) eV depending on the film surface termination. From these results, we extract a conduction band offset from −2.4(1) eV to −2.8(1) eV, indicating that the conduction band edge is more deeply bound in STO and that LSAT will not act as a sink or trap for electrons in the supported film or multilayer.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Cooperative Behavior in the Evolution of Alignment and Structure in Vertically Aligned Carbon-Nanotube Arrays Grown using Chemical Vapor Deposition

We discuss the origin and the influence of autocatalytic kinetics on the structure-property relationship in the evolution of macromolecular structure during self-organized growth of vertically aligned carbon-nanotube arrays by chemical vapor deposition of acetylene. Real-time thickness measurements consistent with postgrowth structural characterization show that the rate of carbon incorporation governs not just the length of individual nanotubes, but also cooperative properties such as packing density, alignment, and order in the arrays, which profoundly affect the mechanical strength and the thermal and electrical conductivity of the assembly. Our analysis shows that the autocatalytic kinetics points to radical chain polymerization of acetylene as the most likely mechanism capable of producing the large variations in carbon-addition rates revealed by real-time data. Furthermore this fresh insight provides alternative approaches for targeting specific array properties using the polymerization reaction framework for creating novel applications in high-density energy storage, advanced interconnects, and high-efficiency heat dissipation.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Fabrication of Low-Tortuosity Ultrahigh-Area-Capacity Battery Electrodes through Magnetic Alignment of Emulsion-Based Slurries

High energy–density, low–cost batteries are critically important to a variety of applications ranging from portable electronics to electric vehicles (EVs) and grid–scale storage. While tremendous research effort has been focused on new materials or chemistries with high energy–density potential, design innovations such as low–tortuosity thick electrodes are another promising path toward higher energy density and lower cost. Growing demand for fast–charging batteries has also highlighted the need for negative electrodes that can accept high rate charging without metal deposition; low tortuosity can be a benefit in this regard. However, a general and scalable fabrication method for low–tortuosity electrodes is currently lacking. Here an emulsion–based, magnetic–alignment approach to producing thick electrodes (>400 µm thickness) with ultrahigh areal capacity (up to ≈14 mAh cm –2 vs 2–4 mAh cm –2 for conventional lithium ion) is reported. The process is demonstrated for LiCoO 2 and meso–carbon microbead graphite. The LiCoO 2 cathodes are confirmed to have low tortuosity via DC–depolarization experiments and deliver high areal capacity (>10 mAh cm –2 ) in galvanostatic discharge tests at practical C–rates and model EV drive–cycle tests. Lastly, this simple fabrication method can potentially be applied to many other active materials to enable thick, low–tortuosity electrodes.

25 ENERGY STORAGE↗
DOE OSTI2015

Thermoresponsive PNIPAM Coatings on Nanostructured Gratings for Cell Alignment and Release

Thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) has been widely used as a surface coating to thermally control the detachment of adsorbed cells without the need for extreme stimuli such as enzyme treatment. Recently, the use of 2D and 3D scaffolds in controlling cell positioning, growth, spreading, and migration has been of a great interest in tissue engineering and cell biology. We use a PNIPAM polymer surface coating atop a nanostructured linear diffraction grating to controllably change the surface topography of 2D linear structures using temperature stimuli. Neutron reflectometry and surface diffraction are utilized to examine the conformity of the polymer coating to the grating surface, its hydration profile, and its evolution in response to temperature variations. Our results show that, in the collapsed state, the PNIPAM coating conforms to the grating structures and retains a uniform hydration of 63%. In the swollen state, the polymer expands beyond the grating channels and absorbs up to 87% water. Such properties are particularly desirable for 2D cell growth scaffolds with a built-in nonextreme tissue-release mechanism. Indeed, the current system demonstrates advanced performance in the effective alignment of cultured fibroblast cells and the easy release of the cells upon temperature change.

36 MATERIALS SCIENCE↗