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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 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Effects of pyrolysis conditions on dielectric properties of PLZT films derived from a polyvinylpyrrolidone-modified sol-gel process

Graphical abstract: SEM images showing the surface morphology of PLZT films derived from solutions with various PVP content and treated with either RTA (rapid thermal annealing) or SPT (step-wise preheat treatment) process. Highlights: Black-Right-Pointing-Pointer High-quality PLZT films were fabricated by using a polyvinylpyrrolidone-modified sol-gel method. Black-Right-Pointing-Pointer Improved film density and integrity was achieved by employing a novel step-wise preheat treatment (SPT) process. Black-Right-Pointing-Pointer Enhanced dielectric properties were correlated to the improved microstructures as a result of the SPT process. -- Abstract: Pb{sub 0.92}La{sub 0.08}Zr{sub 0.52}Ti{sub 0.48}O{sub 3} (PLZT) films were deposited on platinized silicon substrates (Pt/Si) using a polyvinylpyrrolidone (PVP) modified sol-gel method. Pyrolysis of the green films was conducted via two methods: rapid thermal annealing (RTA) and a step-wise preheat treatment (SPT). Microstructure analysis and dielectric property characterization were performed on samples treated by these two methods. Results showed that the SPT-pyrolyzed films exhibited much better dielectric properties when compared with the RTA-pyrolyzed films. The differences in dielectric properties were correlated to microstructural features caused by the different pyrolysis conditions. High-quality PLZT films with high dielectric constant ( Almost-Equal-To 860 at zero bias) and high breakdown strength ( Almost-Equal-To 2.1 MV/cm) were fabricated under controlled pyrolysis conditions. This work demonstrated the potential application of this material for power electronics in electric drive vehicles.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Silver nanowire based flexible electrodes with improved properties: High conductivity, transparency, adhesion and low haze

Graphical abstract: This graphical abstract illustrates the schematic representation of the main drawbacks and rectifications for AgNWs based transparent electrodes. - Highlights: • Films exhibited low sheet resistance and optical properties with R{sub s} ≤ 30 Ω/□ and T ≥ 90%. • We decreased haze to 2% by controlling AgNWs length, diameter, and concentration. • We achieved good adhesion for AgNWs on PET film. • There is no significant change in resistance in the bending angle from 0° to 180°, and on twisting. - Abstract: Recent work has been focusing on solution processable transparent electrodes for various applications including solar cells and displays. As well as, the research aims majorly at silver nanowires (AgNWs) to replace ITO. We enhance the transparent electrode performance as a function of optical and mechanical properties with low sheet resistance, by controlling the AgNWs accept ratios, ink composition, and processing conditions. The nanowire network of transparent films agrees with the 2D percolation law. The film transmittance values at 550 nm are coping with a reference ITO film. Sheet resistance and haze values are suitable for flexible electronic applications. We fabricate transparent flexible film using a low-cost processing technique.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Large scale and fast synthesis of multiferroic TbMn{sub 2}O{sub 5} single-crystalline nanorods

Graphical abstract: - Highlights: • Pure TbMn{sub 2}O{sub 5} phase was synthesized in a wide temperature range for a short time. • Smooth and uniform single-crystalline nanorods were obtained. • The element Mn in TbMn{sub 2}O{sub 5} exhibits a mix Mn{sup 3+/4+} valence state with an average value of 3.5+, and the element Tb exhibits Tb{sup 3+}. - Abstract: Crystalline multiferroic TbMn{sub 2}O{sub 5} nanorods were successfully synthesized by a facile and scalable molten salt synthesis with NaCl–Na{sub 2}SO{sub 4} eutectic salts used in the temperature range of 750–900 °C for only 2 h. The obtained TbMn{sub 2}O{sub 5} could be indexed to an orthorhombic phase by the results of XRD patterns. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), combined with high-resolution TEM (HRTEM), selected area electron diffraction (SAED), and EDS, have been employed to investigate the morphology, size, crystallinity, structure, and composition of the as-synthesized TbMn{sub 2}O{sub 5} nanorods. The TbMn{sub 2}O{sub 5} product exhibits smooth, uniform and crystalline nanorods with diameters of 30–70 nm, and length of varying from 3 to 6 μm. The results of X-ray photoelectron spectroscopy (XPS) analysis indicate that the elements Mn and Tb in TbMn{sub 2}O{sub 5} exhibit Mn{sup 3+/4+} and Tb{sup 3+}, respectively. The magnetic properties of the as-prepared TbMn{sub 2}O{sub 5} nanorods have been also characterized by physical properties measurement system (PPMS)

36 MATERIALS SCIENCE↗
DOE OSTI2025

Understanding the impact of the gas diffusion layer structure on catalyst utilization in the PEM water electrolyzer

A multiphysics half-cell model of a polymer electrolyte membrane water electrolyzer (PEMWE) was developed to probe impacts of the detailed 3-dimensional pore structure of the gas diffusion layer (GDL) on performance characteristics. We show that pores in the titanium GDL mesh led to significant underutilization of the catalyst layer (CL), with only 45% of the catalyst effectively utilized. This contradicts the assumption of uniform electron flow across the CL, as shown in graphical abstract (a), as near-zero current was observed near GDL pore regions and the current distribution in CL was influenced by GDL structure, as shown in graphical abstract (b). Instead, oxygen generation was primarily concentrated under the solid titanium regions, diffusing out around the pore walls. High current density peaks were also noted at the GDL-catalyst contact, correlating with degradation hotspots that were directly observed in companion experiments, as shown in graphical abstract (c). Collectively, these findings point to the critical importance of the heterogeneous GDL porous architecture not only for PEMWE efficiency but also for uneven degradation of the CL.

COMSOL↗
DOE OSTI2020

On the nature of macroradicals formed upon radiolysis of aqueous poly(N-vinylpyrrolidone) solutions

Herein we have explored the nature of macroradicals formed upon radiolysis of aqueous poly(N-vinylpyrrolidone) (PVP) solutions using pulse radiolysis, density functional theory (DFT) and literature data. On the basis of literature data on site-specific kinetics of hydrogen abstraction from simple amides and spectra corresponding to specific radical sites on the same amides we have assessed the distribution of H-atom abstraction by • OH radicals from different positions on the pyrrolidone ring and the polymer backbone. Pulse radiolysis experiments performed at different doses per pulse and different PVP concentrations demonstrate that the H-abstracting radiolysis products are not quantitatively scavenged by the polymer when the dose per pulse exceeds ≈40 Gy. The implications of this are discussed in the context of radical-initiated crosslinking reactions. At a mass fraction of 0.1% PVP and doses per pulse ranging from 7 Gy to 117 Gy, the overall radical decay observed at 390 nm follows second order kinetics with rate constants on the order of 10 9 dm 3 mol -1 s -1 .

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2017

Theoretical Kinetics Analysis for $\dot{H}$ Atom Addition to 1,3-Butadiene and Related Reactions on the $\dot{C}$ 4 H 7 Potential Energy Surface

The oxidation chemistry of the simplest conjugated hydrocarbon, 1,3-butadiene, can provide a first step in understanding the role of poly-unsaturated hydrocarbons in combustion and, in particular, an understanding of their contribution towards soot formation. Based on our previous work on propene and the butene isomers (1-, 2- and isobutene), it was found that the reaction kinetics of H-atom addition to the C=C double bond plays a significant role in fuel consumption kinetics and influences the predictions of high-temperature ignition delay times, product species concentrations and flame speed measurements. In this study, the rate constants and thermodynamic properties for $\dot{H}$-atom addition to 1,3-butadiene and related reactions on the $\dot{C}$ 4 H 7 potential energy surface have been calculated using two different series of quantum chemical methods and two different kinetic codes. Excellent agreement is obtained between the two different kinetics codes. The calculated results including zero point energies, single point energies, rate constants, barrier heights and thermochemistry are systematically compared among the two quantum chemical methods. 1-methylallyl ($\dot{C}$ 4 H 7 1-3) and 3-buten-1- yl ($\dot{C}$ 4 H 7 1-4) radicals and C 2 H 4 + $\dot{C}$2H3 are found to be the most important channels and reactivity promoting products, respectively. We calculated that terminal addition is dominant (> 80%) compared to internal $\dot{H}$-atom addition at all temperatures in the range 298 – 2000 K. However, this dominance decreases with increasing temperature. The calculated rate constants for the bimolecular reaction C 4 H 6 + $\dot{H}$ → products and C 2 H 4 + $\dot{C}$ 2 H 3 → products are in excellent agreement with both experimental and theoretical results from the literature. For selected C 4 species the calculated thermochemical values are also in good agreement with literature data. In addition, the rate constants for H-atom abstraction by $\dot{H}$ atoms have also been calculated, and it is found that abstraction from the central carbon atoms is the dominant channel (> 70%) at temperatures in the range 298 – 2000 K. Lastly, by incorporating our calculated rate constants for both H-atom addition and abstraction into our recently developed 1,3-butadiene model, we show that laminar flame speed predictions are significantly improved, emphasizing the value of this study.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Surface-Mediated Solvent Decomposition in Li–Air Batteries: Impact of Peroxide and Superoxide Surface Terminations

A viable Li/O 2 battery will require the development of stable electrolytes that do not continuously decompose during cell operation. In some recent experiments it is suggested that reactions occurring at the interface between the liquid electrolyte and the solid lithium peroxide (Li 2 O 2 ) discharge phase are a major contributor to these instabilities. To clarify the mechanisms associated with these reactions, a variety of atomistic simulation techniques, classical Monte Carlo, van der Waals-augmented density functional theory, ab initio molecular dynamics, and various solvation models, are used to study the initial decomposition of the common electrolyte solvent, dimethoxyethane (DME), on surfaces of Li 2 O 2 . Comparisons are made between the two predominant Li 2 O 2 surface charge states by calculating decomposition pathways on peroxide-terminated (O 2 2– ) and superoxide-terminated (O 2 1– ) facets. For both terminations, DME decomposition proceeds exothermically via a two-step process comprised of hydrogen abstraction (H-abstraction) followed by nucleophilic attack. In the first step, abstracted H dissociates a surface O 2 dimer, and combines with a dissociated oxygen to form a hydroxide ion (OH – ). In the remaining surface oxygen then attacks the DME, resulting in a DME fragment that is strongly bound to the Li 2 O 2 surface. DME decomposition is predicted to be more exothermic on the peroxide facet; nevertheless, the rate of DME decomposition is faster on the superoxide termination. The impact of solvation (explicit vs implicit) and an applied electric field on the reaction energetics are investigated. Finally, our calculations suggest that surface-mediated electrolyte decomposition should out-pace liquid-phase processes such as solvent auto-oxidation by dissolved O 2 .

25 ENERGY STORAGE↗
DOE OSTI2020

Structural Rearrangements of Subnanometer Cu Oxide Clusters Govern Catalytic Oxidation

Sub-nanometer metal oxide clusters are very important materials that are widely used, for example, in catalysis or electronic devices such as sensors. Hence, it is critical to understand the atomic structures and properties of sub-nanometer metal oxide clusters under a reactive gas environment, such as O 2 . We consider here experimentally accessible precise-size Cu clusters (Cu 4 ) supported on partially hydroxylated amorphous alumina and show that such clusters can access, in catalytic conditions at high temperature under a pressure of O 2 , a large ensemble of oxidized structures, representing a large variety of oxygen content, of geometries in link with the support, and of catalytic activities for oxidation reactions, as seen from their reducibilities. A grand canonical basin hopping method based on first-principles energy reveals an ensemble of 24 configurations for the Cu 4 O x cluster of low free energy, less than 0.8 eV above the global minimum. The low free energy ensemble consists of clusters of different stoichiometries, which are mainly Cu 4 O 3 and Cu 4 O 4 in the temperature range of 200–400 °C and under a pressure of 0.5 bar of O 2 . The presence of several competitive isomers at each composition implies that cluster fluxionality impacts the phase diagram, which should be ensemble-averaged. In terms of catalytic oxidation activity, Cu 4 O 3 isomers present highly variable O abstraction energy: the most stable isomers are inactive for alkane oxidative dehydrogenation, but isomerization to metastable isomers, that proceed with low barrier, enable to create active configurations with low O abstraction energy. O atoms with the lowest anionic character, and thus of more electrophilic nature, present the best oxidation capability. In contrast, all Cu 4 O 4 isomers show a low O abstraction energy and a high potential catalytic activity. Here, this manuscript demonstrates the unique structural and electronic properties of sub-nano Cu oxide clusters and illustrates the critical roles of configuration ensembles and rearrangement to highly reactive metastable cluster isomers in nanocatalysis.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Ethanol Conversion over La 0.7 Sr 0.3 MnO 3–x (100): Autocatalysis, Adjacent O-Vacancies, Disproportionation, and Dehydrogenation

The mechanism for catalytic conversion of ethanol over La 0.7 Sr 0.3 MnO 3– x (100) surface to acetaldehyde and ethene was investigated. Pre-exposure temperature-programmed reaction (PE-TPR) experiments were performed in which ethanol was introduced to oxidized or reduced surfaces followed by heating. In particular, sequential PE-TPR experiments were conducted to incrementally and gradually reduce the surface. The products and their ratios were investigated as a function of surface reduction. The data show that acetaldehyde and ethene production is catalyzed with hydrogen abstraction and oxygen abstraction reactions occurring by intermediates in vacancies at various temperatures >400 K. Adsorption of acetaldehyde followed by a temperature-programmed reaction does not produce ethene, indicating that acetaldehyde is not an intermediate to ethene and that the hydrogen and oxygen abstraction from ethanol to ethene are decoupled steps. Further evidence for this mechanistic nuance was obtained using isotopically labeled ethanol (CD 3 CH 2 OH), which produces CD 3 CHO and CD 2 CH 2 . Additionally, the ratio of aldehyde production to alkene production increases with reduction, suggesting that aldehyde is produced from a disproportionation reaction between ethoxy species in adjacent O-vacancies, while ethene is produced from a dehydrogenation reaction with ethoxy species in vacancies without requiring adjacent O-vacancies. Counterintuitively, this finding indicates that the more oxygenated product (aldehyde vs ethene) is favored with more vacancies and that the net alcohol conversion is autocatalytic. Density functional theory calculations were able to find the previously unknown disproportionation pathway between ethoxies in adjacent O-vacancies, and kinetic Monte Carlo simulations support this interpretation by reproducing experimental selectivities. The activation energies for these pathways are estimated as 132 ± 10 kJ/mol for the disproportionation reaction (when occurring between ethoxies in adjacent vacancies) and as 148 ± 11 kJ/mol for the direct dehydrogenation reaction of an ethoxy in a vacancy. Based on these results, a mechanism with operative pathways based on elementary steps in O-vacancies is reported.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Iridium Polypyridyl Carboxylates as Excited-State PCET Catalysts for the Functionalization of Unactivated C–H Bonds

The design of catalysts capable of functionalizing unactivated C(sp 3 )–H bonds remains a significant goal in synthetic organic chemistry. Herein, we present a novel set of iridium polypyridyl complexes bearing pendent Brønsted basic carboxylates that become potent hydrogen atom abstraction catalysts upon visible light irradiation. Thermochemical and spectroscopic characterization reveal that these excited-state complexes exhibit bond dissociation free energies (BDFEs) of up to 105 kcal mol –1 with long excited-state lifetimes. We demonstrate that these complexes can catalyze C–H alkylation reactions in which the Ir carboxylate mediates both C–H abstraction and formation steps. Mechanistic, spectroscopic, and computational studies are consistent with C−H abstraction proceeding through an excited-state proton-coupled electron transfer (PCET) step. Here, the modular nature of these Ir polypyridyl complexes establishes a foundation for designing tunable and efficient C–H functionalization catalysts based on covalent tethering of excited-state oxidants and bases.

Alcohols↗
DOE OSTI2026

Photochemical Tungsten–Carbon Bond Cleavage in Bicyclic Tungstacyclopentanes

W(NCPh 3 )(OSiPh 3 ) 2 (C7H 12 ) (1) and W(NAd)(OSiPh 3 ) 2 (C 7 H 12 ) (2) undergo photochemical α-hydrogen abstraction to form alkylidenes. Transient absorption studies suggest that fast W–C homolysis precedes thermal α-hydrogen abstraction. Radical trapping experiments in the presence of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) support formation of an alkyl radical in these reactions. DFT calculations of models of 1 and 2 are also consistent with formation of W(V)-alkyl-radical intermediates that ultimately abstract an α-hydrogen to form the alkylidene. This article provides evidence for W–C bond cleavage in reac-tions that convert catalytically inactive d 0 tungstacyclopentanes into metathesis-active alkylidenes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Methanol Oxidative Dehydrogenation on Oxide Catalysts: Molecular and Dissociative Routes and Hydrogen Addition Energies as Descriptors of Reactivity

The oxidative dehydrogenation (ODH) of alkanols on oxide catalysts is generally described as involving H-abstraction from alkoxy species formed via O–H dissociation. Kinetic and isotopic data cannot discern between such routes and those involving kinetically-relevant H-abstraction from undissociated alkanols. Here, we combine such experiments with theoretical estimates of activation energies and entropies to show that the latter molecular routes prevail over dissociative routes for methanol reactions on polyoxometalate (POM) clusters at all practical reaction temperatures. The stability of the late transition states that mediate H-abstraction depend predominantly on the stability of the O–H bond formed, making H-addition energies (HAE) accurate and single-valued descriptors of reactivity. Density functional theory-derived activation energies depend linearly on HAE values at each O-atom location on clusters with a range of composition (H3PMo12, H4SiMo12, H3PW12, H4PV1Mo11, and H4PV1W11); both barriers and HAE values reflect the lowest unoccupied molecular orbital energy of metal centers that accept the electron and the protonation energy of O-atoms that accept the proton involved in the H-atom transfer. Bridging O-atoms form O–H bonds that are stronger than those of terminal atoms and therefore exhibit more negative HAE values and higher ODH reactivity on all POM clusters. For each cluster composition, ODH turnover rates reflect the reactivity-averaged HAE of all accessible O-atoms, which can be evaluated for each cluster composition to provide a rigorous and accurate predictor of ODH reactivity for catalysts with known structure. These relations together with oxidation reactivity measurements can then be used to estimate HAE values and to infer plausible structures for catalysts with uncertain active site structures.

Environmental Molecular Sciences Laboratory↗