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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 379 records · Page 21
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Transverse-electric Brewster effect enabled by nonmagnetic two-dimensional materials

It was discovered in the 19th century, the Brewster effect is known to occur for transverse-magnetic waves in regular optical dielectrics; however, it is believed to arise for transverse-electric (TE) waves only in systems with magnetic responses, i.e., nonunity effective relative permeability. This paper introduces a scheme to realize the TE Brewster effect in a homogeneous dielectric interface without magnetic responses, by adding ultrathin two-dimensional (2D) materials such as graphene. In particular, the effect remains even for waves approaching normal incidence, spanning from terahertz to visible frequencies. In contrast to the conventional Brewster effect, the graphene-assisted TE Brewster effect is asymmetric, and can be achieved only when the incidence is from the higher-refractive-index side. Moreover, graphene layers can tailor a total-internal-reflection dielectric interface into zero reflection, accompanied by perfect absorption. This control over TE waves enabled by ultrathin 2D materials may lead to a variety of applications, such as atomically thin absorbers, polarizers, and antireflection coating.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Heat transport in silicon from first-principles calculations

Using harmonic and anharmonic force constants extracted from density functional calculations within a supercell, we have developed a relatively simple but general method to compute thermodynamic and thermal properties of any crystal. First, from the harmonic, cubic, and quartic force constants, we construct a force field for molecular dynamics. It is exact in the limit of small atomic displacements and thus does not suffer from inaccuracies inherent in semiempirical potentials such as Stillinger-Weber's. By using the Green-Kubo formula and molecular dynamics simulations, we extract the bulk thermal conductivity. This method is accurate at high temperatures where three-phonon processes need to be included to higher orders, but may suffer from size scaling issues. Next, we use perturbation theory (Fermi golden rule) to extract the phonon lifetimes and compute the thermal conductivity κ from the relaxation-time approximation. This method is valid at most temperatures, but will overestimate κ at very high temperatures, where higher-order processes neglected in our calculations also contribute. As a test, these methods are applied to bulk crystalline silicon, and the results are compared and differences are discussed in more detail. The presented methodology paves the way for a systematic approach to model heat transport in solids using multiscale modeling, in which the relaxation time due to anharmonic three-phonon processes is calculated quantitatively, in addition to the usual harmonic properties such as phonon frequencies and group velocities. As a result it also allows the construction of an accurate bulk interatomic potentials database.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Metamaterial broadband angular selectivity

We reveal how broadband angular selectivity can be achieved with stacks of one-dimensionally periodic photonic crystals, each consisting of alternating isotropic layers and effective anisotropic layers, where each effective anisotropic layer is constructed from a multilayered metamaterial. We demonstrate that by simply changing the structure of the metamaterials, the selective angle can be tuned to a broad range of angles; and, by increasing the number of stacks, the angular transmission window can be made as narrow as desired. As a proof of principle, we realize the idea experimentally in the microwave regime. The angular selectivity and tunability we report here can have various applications such as in directional control of electromagnetic emitters and detectors.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Intrinsic ultralow lattice thermal conductivity of the unfilled skutterudite FeSb 3

It is generally accepted that unfilled skutterudites process high lattice thermal conductivity $κ_l$ that can be efficiently reduced upon filling. Here by using first-principles Boltzmann-Peierls transport calculations, we find pure skutterudite of FeSb 3 with no filler in fact has an intrinsic ultralow $κ_l$ smaller than that of CoSb 3 by one order of magnitude. The value is even smaller than those of most of the fully filled skutterudites. This finding means that with FeSb 3 as a reference, filling does not necessarily lower $κ_l$. The ultralow $κ_l$ of FeSb 3 is a consequence of the overall softening of phonon spectrum, especially the lowering in frequency of optical phonon branches associated with the weakly bonded Sb 4 rings. They overlap more with heat-carrying acoustic phonons and significantly increase the phase space for three-phonon anharmonic scattering processes. This provides an alternative non-filling-related mechanism for lowering the $κ_l$ of skutterudites.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Frequency-Selective Near-Field Radiative Heat Transfer between Photonic Crystal Slabs: A Computational Approach for Arbitrary Geometries and Materials

Here, we demonstrate the possibility of achieving enhanced frequency-selective near-field radiative heat transfer between patterned (photonic-crystal) slabs at designable frequencies and separations, exploiting a general numerical approach for computing heat transfer in arbitrary geometries and materials based on the finite-difference time-domain method. Our simulations reveal a tradeoff between selectivity and near-field enhancement as the slab-slab separation decreases, with the patterned heat transfer eventually reducing to the unpatterned result multiplied by a fill factor (described by a standard proximity approximation). We also find that heat transfer can be further enhanced at selective frequencies when the slabs are brought into a glide-symmetric configuration, a consequence of the degeneracies associated with the nonsymmorphic symmetry group.

36 MATERIALS SCIENCE↗
DOE OSTI2024

STEM Imaging of Materials: Final Report for DE-FG02-08ER46547

This project supported research advancing the state of the art in scanning transmission electron microscopy (STEM) imaging and characterization of materials, especially electronic materials. The project began with studies of point defects in materials using high-resolution Z-contrast STEM, then evolved to including methods in four-dimensional (4D) STEM and machine learning. Highlights include: (1) the discovery of stable, p-type ZnO via Sb doping, (2) demonstration that the high performance of InGaN LEDs is not due to In composition fluctuations as previously proposed, (3) the demonstration of the first sub-pm precision high-resolution STEM images using non-rigid registration of a series of STEM images, (4) the extension of high-precision STEM to low dose imaging and spectrum imaging, (5) discovery of a synthesis method for twisted spiral growth of transition-metal dichalcogenide materials via chemical vapor deposition, and (6) development of machine-learning methods for analysis and denoising of enormous 4D STEM data sets.

36 MATERIALS SCIENCE↗
NASA NTRS1981

Environmental effects on defect growth in composite materials

Data for evaluating the effects of moisture and temperature on the integrity of fiber composite components was gathered. In particular, the static and cyclic performance of three composite laminates containing flaws was investigated at room temperature and at 422 K (300 F) in wet and dry conditions.

Porter, T. R.↗