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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 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
NASA NTRS2013

Large Abundances of Polycyclic Aromatic Hydrocarbons in Titan's Upper Atmosphere

In this paper, we analyze the strong unidentified emission near 3.28 micron in Titan's upper daytime atmosphere recently discovered by Dinelli et al.We have studied it by using the NASA Ames PAH IR Spectroscopic Database. The polycyclic aromatic hydrocarbons (PAHs), after absorbing UV solar radiation, are able to emit strongly near 3.3 micron. By using current models for the redistribution of the absorbed UV energy, we have explained the observed spectral feature and have derived the vertical distribution of PAH abundances in Titan's upper atmosphere. PAHs have been found to be present in large concentrations, about (2-3) × 10(exp 4) particles / cubic cm. The identified PAHs have 9-96 carbons, with a concentration-weighted average of 34 carbons. The mean mass is approx 430 u; the mean area is about 0.53 sq. nm; they are formed by 10-11 rings on average, and about one-third of them contain nitrogen atoms. Recently, benzene together with light aromatic species as well as small concentrations of heavy positive and negative ions have been detected in Titan's upper atmosphere. We suggest that the large concentrations of PAHs found here are the neutral counterpart of those positive and negative ions, which hence supports the theory that the origin of Titan main haze layer is located in the upper atmosphere.

PAH↗
DOE OSTI2026

Viability of perturbative expansion for quantum field theories on neurons

Neural Network (NN) architectures that break statistical independence of parameters have been proposed as a new approach for simulating local quantum field theories (QFTs) [1]. In the infinite neuron number limit, single-layer NNs can exactly reproduce QFT results. This paper examines the viability of this architecture for perturbative calculations of local QFTs for finite neuron number N using scalar ϕ 4 theory in d Euclidean dimensions as an example. We find that the renormalized O(1/N ) corrections to two-and four-point correlators yield perturbative series which are sensitive to the UV cut-off and therefore have a weak convergence. We propose a modification to the architecture to improve this convergence and discuss constraints on the parameters of the theory and the scaling of N which allow us to extract accurate field theory results.

Sen, Srimoyee [Iowa State University, Ames, IA (Un↗
NASA NTRS1981

An experimental study of transmission, reflection and scattering of sound in a free jet flight simulation facility and comparison with theory

When a free jet (or open jet) is used as a wind tunnel to simulate the effects of flight on model noise sources, it is necessary to calibrate out the effects of the free jet shear layer on the transmitted sound, since the shear layer is absent in the real flight case. In this paper, a theoretical calibration procedure for this purpose is first summarized; following this, the results of an experimental program, designed to test the validity of the various components of the calibration procedure, are described. The experiments are conducted by using a point sound source located at various axial positions within the free jet potential core. By using broadband excitation and cross-correlation methods, the angle changes associated with ray paths across the shear layer are first established. Measurements are then made simultaneously inside and outside the free jet along the proper ray paths to determine the amplitude changes across the shear layer. It is shown that both the angle and amplitude changes can be predicted accurately by theory. It is also found that internal reflection at the shear layer is significant only for large ray angles in the forward quadrant where total internal reflection occurs. Finally, the effects of sound absorption and scattering by the shear layer turbulence are also examined experimentally.

Ahuja, K. K.↗
NASA NTRS1982

The room-temperature shapes of four-layer unsymmetric cross-ply laminates

A geometrically nonlinear extension of classical lamination theory developed by Hyer (1981) for predicting the room-temperature shapes of unsymmetric laminates is reformulated using relaxed restrictions regarding the inplane strains. The inplane residual strains of unsymmetric laminates which have cooled from curing into a cylindrical room-temperature shape are examined numerically. Results show that the residual strains are compressive and practically independent of spatial location on the laminate. In addition, the room temperature shapes of the four-layer unsymmetric cross-ply laminates are predicted, and it is shown that the temperature shapes are a strong function of their size and their stacking arrangement. It is demonstrated that, depending on the parameters selected, the room-temperature shape of a four-layer cross-ply unsymmetric laminate can be a unique saddle shape, a unique cylindrical shape, or a cylindrical shape that can be snapped through to another cylindrical shape.

Hyer, M. W.↗
DOE OSTI2026

Enhanced Water Interaction at Dual Cu Sites Within the Defects on a Copper Sulfide Layer

Electrochemical transformations of stable molecules and water into fuels and value-added chemicals require efficient catalyst surfaces. Introducing controlled defects at atomic scales can offer promising routes to enhance catalyst performance. In this study, we found novel dual copper site (-Cu-Cu-) defects within a copper sulfide (Cu-S) layer supported on Cu(111). Using scanning tunneling microscopy (STM) and density functional theory (DFT), we found these dual copper sites enhance molecular adsorption strength, specifically for water molecules, compared to intact Cu-S layer or pristine copper surfaces. This discovery highlights the potential of engineered dual-site copper defects to advance electrochemical catalytic materials, particularly for reactions involving water activation.

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

Dynamic Molecular Investigation of the Solid-Electrolyte Interphase of an Anode-Free Lithium Metal Battery Using In Situ Liquid SIMS and Cryo-TEM

Solid electrolyte interphase (SEI) has been widely perceived to play a critical role in the stable cycling of rechargeable batteries. However, associated with the fragile and air-sensitive nature of the SEI layer, delineation of the formation process and the nature of SEI remains a big challenge. Here, we use in situ liquid time-of-flight secondary ion mass spectroscopy (TOF-SIMS), cryo- transmission electron microscope (TEM) and density functional theory (DFT) calculation to delineate molecular process on the formation of SEI layer under the dynamic operating condition. We discover that the onset potential for SEI layer formation and the thickness of the SEI show dependence on the solvation shell structure. Using LiCoO 2 as a cathode and Cu film as an anode, the SEI is noticed to start to form at around 2.0 V and reach its final thickness (irreversible part, ~ 40-50 nm) at about 3.0 V in the 1 M LiPF 6 –EC/DMC electrolyte, while for the case of 1 M LiFSI–DME, the SEI starts to form at around 1.5 V and reaches its final thickness (~ 20 nm) at about 2.0 V. The in situ TOF-SIMS clearly indicates the outer SEI layer formation and dissipation upon charging and discharging, implying a continued evolution of electrolyte structure with extended cycling. In conclusion, the present work establishes a direct correlation between the molecular signature of SEI layer with solvation feature of electrolytes in lithium batteries, providing insights for tailoring SEI layer toward improved electrochemical properties of lithium batteries.

25 ENERGY STORAGE↗
DOE OSTI2022

Highly Asymmetric Graphene Layer Doping and Band Structure Manipulation in Rare Earth–Graphene Heterostructure by Targeted Bonding of the Intercalated Gadolinium

Heterostructures consisting of vertically stacked two-dimensional (2D) materials have recently gained large attention due to their highly controllable electronic properties and resulting quantum phases. In contrast to the mechanically stacked multilayered systems, which offer exceptional control over a stacking sequence or interlayer twist angles, the epitaxially grown 2D materials express unprecedented quality and stability over wafer-scale lengths. However, controlling the growth conditions remains a major obstacle toward the formation of complex, epitaxial heterostructures with well-defined electronic properties. Here, we synthesized a trilayer graphene heterostructure on the SiC(0001) substrate with two specific interlayer locations occupied by gadolinium. We applied multitechnique methodology based on low-temperature scanning tunneling microscopy/spectroscopy (STM/S) and angle-resolved photoelectron spectroscopy (ARPES) to determine the intercalant’s locations in the complex, epitaxial graphene heterostructure. Our approach relies on very high quality and large, micrometer-scale homogeneity of the synthesized system. The experimentally determined electronic structure is dominated by the two topmost graphene layers. Overall, our spectroscopic results show quantitative agreement between global ARPES, local STM/S, and density functional theory predictions. The characterized electronic properties primarily reflect highly anisotropic doping levels between the two corresponding graphene layers, which significantly affect the band structure topology. Two pairs of hybridized massive Dirac bands from our initial synthesis–the bilayer graphene on the SiC(0001) substrate–are transformed upon Gd intercalation into two pairs of massless Dirac bands with a new hybridization region in between. Our results open perspectives in the realization of exotic 2D quantum materials via atomically precise synthesis of epitaxial, multilayered graphene–rare earth heterostructures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS2002

Analysis of Graphite-Reinforced Cementitious Composites

Strategically embedding graphite meshes in a compliant cementitious matrix produces a composite material with relatively high tension and compressive properties as compared to steel-reinforced structures fabricated from a standard concrete mix. Although these composite systems are somewhat similar, the methods used to analyze steel-reinforced composites often fail to characterize the behavior of their more advanced graphite-reinforced counterparts. This Technical Memorandum describes some of the analytical methods being developed to determine the deflections and stresses in graphite-reinforced cementitious composites. It is initially demonstrated that the standard transform section method fails to provide accurate results when the elastic moduli ratio exceeds 20. An alternate approach is formulated by using the rule of mixtures to determine a set of effective material properties for the composite. Tensile tests are conducted on composite samples to verify this approach. When the effective material properties are used to characterize the deflections of composite beams subjected to pure bending, an excellent agreement is obtained. Laminated composite plate theory is investigated as a means for analyzing even more complex composites, consisting of multiple graphite layers oriented in different directions. In this case, composite beams are analyzed using the laminated composite plate theory with material properties established from tensile tests. Then, finite element modeling is used to verify the results. Considering the complexity of the samples, a very good agreement is obtained.

Vaughan, R. E.↗
DOE OSTI2020

Accelerated Cu2O Reduction by Single Pt Atoms at the Metal-Oxide Interface§

The reducibility of metal oxides, when serving as the catalyst support or the active sites themselves, plays an important role in heterogeneous catalytic reactions. Here we present an integrated experimental and theoretical study that reveals how the addition of small amounts of atomically dispersed Pt at the metal/oxide interface dramatically enhances the reducibility of a Cu2O thin film by H2. X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) results reveal that upon oxidation, a PtCu single-atom alloy (SAA) surface is covered by a thin Cu2O film and is therefore unable to dissociate H2. Despite this, in situ studies using ambient pressure (AP) XPS reveal that the presence of a small amount of Pt under the oxide layer can, at the single-atom limit, promote the reduction of Cu2O by H2 at room temperature. We build two density functional theory based surface models to better understand these experimental findings: A Cu2O/Cu(111)-like surface oxide layer, known as the ‘29’ oxide, in which Pt is alloyed into the Cu(111) surface as well as a PtCu SAA. Our calculations suggest that the increased activity is due to the presence of atomically dispersed Pt under the surface oxide layer, which weakens the Cu-O bonds in its immediate vicinity, thus making the interface between subsurface Pt and the surface oxide a nucleation site for the formation of metallic Cu. This initial step in the reduction process results in the presence of surface Pt atoms surrounded by metallic Cu patches, and the Pt atoms become active in H2 dissociation, which consequently accelerates the reduction of the oxide layer. This work demonstrates how isolated Pt atoms at the metal/oxide interface of a Cu-based catalyst accelerate the reduction of the oxide and, therefore, helps maintain the active, reduced state of the catalyst under reaction conditions, as is required for the synthesis of methanol from CO2 hydrogenation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1986

Radar scene generation for tactical decision aids

The Mueller matrix and polarization covariance matrix for polarimetric radar systems was studied. The clutter was modeled by a layer of random permittivity, described by a three-dimensional correlation function, with variance, and horizontal and vertical correlation lengths. This model was applied, using the wave theory with Born approximations carried to the second order, to find the backscattering elements of the polarimetric matrices. Theoretical predictions are matched with experimental data for vegetation fields. The strong fluctuation theory was used to derive the backscattering cross sections. A two-layer anisotropic random medium model was developed for the active and passive microwave remote sensing of ice fields. A three-layer random medium model was adopted to study the volume scattering effects for the active and passive microwave remote sensing of snow-covered ice fields.

Kong, J. A.↗
DOE OSTI2022

Stacking Fault Induced Symmetry Breaking in van der Waals Nanowires

While traditional ferroelectrics are based on polar crystals in bulk or thin film form, two-dimensional and layered materials can support mechanisms for symmetry breaking between centrosymmetric building blocks, e.g., by creating low-symmetry interfaces in van der Waals stacks. Here, we introduce an approach toward symmetry breaking in van der Waals crystals that relies on the spontaneous incorporation of stacking faults in a nonpolar bulk layer sequence. The concept is realized in nanowires consisting of Se-rich group IV monochalcogenide (GeSe 1–x S x ) alloys, obtained by vapor–liquid–solid growth. The single crystalline wires adopt a layered structure in which the nonpolar A-B bulk stacking along the nanowire axis is interrupted by single-layer stacking faults with local A-A' stacking. Density functional theory explains this behavior by a reduced stacking fault formation energy in GeSe (or Se-rich GeSe 1–x S x alloys). Computations demonstrate that, similar to monochalcogenide monolayers, the inserted A-layers should show a spontaneous electric polarization with a switching barrier consistent with a Curie temperature above room temperature. Second-harmonic generation signals are consistent with a variable density of stacking faults along the wires. Here, our results point to possible routes for designing ferroelectrics via the layer stacking in van der Waals crystals.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS2000

"Genetically Engineered" Nanoelectronics

The quantum mechanical functionality of nanoelectronic devices such as resonant tunneling diodes (RTDs), quantum well infrared-photodetectors (QWIPs), quantum well lasers, and heterostructure field effect transistors (HFETs) is enabled by material variations on an atomic scale. The design and optimization of such devices requires a fundamental understanding of electron transport in such dimensions. The Nanoelectronic Modeling Tool (NEMO) is a general-purpose quantum device design and analysis tool based on a fundamental non-equilibrium electron transport theory. NEW was combined with a parallelized genetic algorithm package (PGAPACK) to evolve structural and material parameters to match a desired set of experimental data. A numerical experiment that evolves structural variations such as layer widths and doping concentrations is performed to analyze an experimental current voltage characteristic. The genetic algorithm is found to drive the NEMO simulation parameters close to the experimentally prescribed layer thicknesses and doping profiles. With such a quantitative agreement between theory and experiment design synthesis can be performed.

Klimeck, Gerhard↗
NASA NTRS1984

Antisoiling technology: Theories of surface soiling and performance of antisoiling surface coatings

Physical examination of surfaces undergoing natural outdoor soiling suggests that soil matter accumulates in up to three distinct layers. The first layer involves strong chemical attachment or strong chemisorption of soil matter on the primary surface. The second layer is physical, consisting of a highly organized arrangement of soil creating a gradation in surface energy from a high associated with the energetic first layer to the lowest possible state on the outer surfce of the second layer. The lowest possible energy state is dictated by the physical nature of the regional atmospheric soiling materials. These first two layers are resistant to removal by rain. The third layer constitutes a settling of loose soil matter, accumulating in dry periods and being removed during rainy periods. Theories and evidence suggest that surfaces that should be naturally resistant to the formation of the first two-resistant layers should be hard, smooth, hydrophobic, free of first-period elements, and have the lowest possible surface energy. These characteristics, evolving as requirements for low-soiling surfaces, suggest that surfaces or surface coatings should be of fluorocarbon chemistry. Evidence for the three-soil-layer concept, and data on the positive performance of candidate fluorocarbon coatings on glass and transparent plastic films after 28 months of outdoor exposure, are presented.

Cuddihy, E. F.↗
NASA NTRS1971

Application of the method of steepest descent to laminated shield weight optimization with several constraints: Theory

The method of steepest descent used in optimizing one-dimensional layered radiation shields is extended to multidimensional, multiconstraint situations. The multidimensional optimization algorithm and equations are developed for the case of a dose constraint in any one direction being dependent only on the shield thicknesses in that direction and independent of shield thicknesses in other directions. Expressions are derived for one-, two-, and three-dimensional cases (one, two, and three constraints). The precedure is applicable to the optimization of shields where there are different dose constraints and layering arrangements in the principal directions.

Lahti, G. P.↗
NASA NTRS1994

Navier-Stokes and potential theory solutions for ahelicopter fuselage and comparison with experiment

A thin-layer Navier-Stokes code and a panel method code are used to predict the flow over a generic helicopter fuselage. The computational results are compared with pressure data at four experimental conditions. Both methods produce results that agree with the experimental pressure data. However, separation patterns and other viscous flow features from the Navier-Stokes code solution are shown that cannot be easily modeled with the panel method.

Chaffin, Mark S.↗
DOE OSTI2023

Bimolecular Reaction Mechanism in the Amido Complex-Based Atomic Layer Deposition of HfO 2

The surface chemistry of the initial growth during the first or first few precursor cycles in atomic layer deposition is decisive for how the growth proceeds later on and thus for the quality of the thin films grown. Yet, although general schemes of the surface chemistry of atomic layer deposition have been developed for many processes and precursors, in many cases, knowledge of this surface chemistry remains far from complete. For the particular case of HfO 2 atomic layer deposition on a SiO 2 surface from an alkylamido-hafnium precursor and water, we address this lack by carrying out an operando atomic layer deposition experiment during the first cycle of atomic layer deposition. Ambient-pressure X-ray photoelectron spectroscopy and density functional theory together show that the decomposition of the metal precursor on the stoichiometric SiO 2 surface in the first half-cycle of atomic layer deposition proceeds via a bimolecular reaction mechanism. The reaction leads to the formation of Hf-bonded methyl methylene imine and free dimethylamine. In addition, ligand exchange takes place involving the surface hydroxyls adsorbed at defect sites of the SiO 2 surface.

36 MATERIALS SCIENCE↗
NASA NTRS1992

Large-scale flows and solar luminosity variations

The influence of a variation of the large-scale flows present in the solar convection zone on solar luminosity is examined. The convection zone is modeled as a plane-parallel fluid layer through which heat is transported by small-scale convection as described by the mixing-length theory, and the large-scale flow is treated as a small perturbation superposed on the layer. The excess energy transported by the flow is calculated and is found to depend critically on the mixing length. The excess luminosity is found to be carried as small-scale convective heat flow rather than bulk enthalpy transport throughout all but the top few percent of the convection zone. The results are applied to the specific examples of supergranules and giant cells.

Arendt, S.↗
DOE OSTI2021

A record high average ZT over a wide temperature range in a Single-layer Sb 2 Si 2 Te 6

Conversion of waste heat into usable energy requires development of thermoelectric materials with high efficiency in a wide temperature range. Here, using first principles theory and Boltzmann transport theory, we show that the thermoelectric performance of the p-type single-layer Sb 2 Si 2 Te 6 has a high figure of merit ZT of 2.62 at 900 K and a record high average ZT of ~1.93 (corresponding the conversion efficiency of ~23.2%) in the temperature range of 300-900 K. These values are significantly higher than the recently measured average ZT of ~0.57 in the temperature range of 310–823 K [Luo et al., Joule 4, 159–175 (2020)] in layered bulk Sb 2 Si 2 Te 6 . We attribute the large enhancement of ZT in the single layer material to the increase in the thermoelectric power factor resulting from the complex Fermi surface. Our work reveals the great potential of a single-layer Sb 2 Si 2 Te 6 for wide-temperature-range thermoelectric applications.

36 MATERIALS SCIENCE↗