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Results for “synthesis (scalable processing)”

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 955 records · Page 53
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Tandem catalysis for the production of alkyl lactates from ketohexoses at moderate temperatures

Significance Retro-aldol reactions of biomass-derived monosaccharides enable the production of a number of industrially relevant, renewable platform chemicals (e.g., C 2 –C 4 α-hydroxy carboxylic acids, acrylic acid, and esters thereof; and ethylene and propylene glycol). Here, we show a set of retro-aldol catalysts that can perform these reactions at significantly lower temperatures (∼100 °C) than previously reported, and demonstrate that these catalysts enable the key retro-aldol step when used in combination with a 1,2-hydride shift catalyst to create a reaction cascade that converts ketohexoses to lactic acid. The use of moderate temperatures enables numerous desirable features such as lower pressure and significantly less catalyst deactivation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Invisible metallic mesh

We introduce and demonstrate an invisible material––a solid composite possessing identical electromagnetic properties as air so that its arbitrarily shaped object neither reflects nor refracts light at any angle of incidence in free space. Such a material is self-invisible, unlike the cloaks for minimizing the scattering of other items. Invisible materials could provide improved mechanical stability, electrical conduction, and heat dissipation to a system, without disturbing the original electromagnetic design. As a result, one immediate application would be toward perfect antenna radomes.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Local protein solvation drives direct down-conversion in phycobiliprotein PC645 via incoherent vibronic transport

Significance To harness sunlight for growth, plants and algae rapidly convey absorbed excitation energy from antennae pigments to a reaction center, where the excitations convert to chemical energy. In deep water environments, cryptophyte algae survive by using the molecular motion of phycobiliprotein antennae to funnel excitations to low-energy pigments. The mechanism of down-conversion in phycobiliproteins remains controversial: Could specific vibrations resonant with pigment energy gaps support coherence between excited states and thereby enhance the rate of transport by transient delocalization? Here, we demonstrate that down-conversion in a specific phycobiliprotein, PC645, is both incoherent and enhanced by a broad range of high-frequency vibrations. We suggest that a similar incoherent mechanism is appropriate across phycobiliproteins.

09 BIOMASS FUELS↗
DOE OSTI2015

Effects of Aperiodicity and Roughness on Coherent Heat Conduction in Superlattices

Coherent phonon heat conduction has recently been confirmed experimentally in superlattice structures. Such traveling coherent phonon waves in superlattices lead to a linear increase in thermal conductivity as the number of periods increases. For applications such as thermal insulation or thermoelectrics, minimization of the phonon coherent effect is desirable. In this paper, we use molecular dynamics simulations to study how to control coherent heat conduction in superlattices (SLs). It is found that either aperiodic SLs or SLs with rough interfaces can significantly disrupt coherent heat conduction when the interface densities are high. For sample thickness less than 125 nm, aperiodic SLs with perfect interfaces are found to have the lowest thermal conductivity. We use the atomic Green’s function method to examine the phonon dynamics. The impact of either aperiodicity or interface roughness is attributed to reduced transmittance. Such impact diminishes as the interface density reduces.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Intrinsic to extrinsic phonon lifetime transition in a GaAs–AlAs superlattice

We have measured here the lifetimes of two zone-center longitudinal acoustic phonon modes, at 320 and 640 GHz, in a 14 nm GaAs/2 nm AlAs superlattice structure. By comparing measurements at 296 and 79 K we separate the intrinsic contribution to phonon lifetime determined by phonon–phonon scattering from the extrinsic contribution due to defects and interface roughness. At 296 K, the 320 GHz phonon lifetime has approximately equal contributions from intrinsic and extrinsic scattering, whilst at 640 GHz it is dominated by extrinsic effects. These measurements are compared with intrinsic and extrinsic scattering rates in the superlattice obtained from first-principles lattice dynamics calculations. The calculated room-temperature intrinsic lifetime of longitudinal phonons at 320 GHz is in agreement with the experimentally measured value of 0.9 ns. The model correctly predicts the transition from predominantly intrinsic to predominantly extrinsic scattering; however the predicted transition occurs at higher frequencies. Our analysis indicates that the 'interfacial atomic disorder' model is not entirely adequate and that the observed frequency dependence of the extrinsic scattering rate is likely to be determined by a finite correlation length of interface roughness.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

A novel cage for actinides: A 6 W 4 Al 43 ( A = U and Pu)

We report on synthesis and characterization of the compounds A 6 W 4 Al 43 ( A = U and Pu), that form in the hexagonal Ho 6 Mo 4 Al 43 caged-structure family. The A ions reside within W/Al cages where the A – A nearest neighbors form dimers between adjacent W/Al cages, with U–U and Pu–Pu distances of 3.3892 and 3.4080 , respectively. While the W/Al networks provide environments similar to those of other cage-like materials (e.g. filled skutterudites), the atomic displacement parameters from single crystal x-ray diffraction measurements show that the A -ions do not exhibit rattling behavior. We find that there is site interchange disorder on one of the W/Al sites. Magnetic susceptibility measurements show that U 6 W 4 Al 43 displays anisotropic Curie–Weiss behavior where it fits to the data yield an effective magnetic moment near 2.0 /U. At low temperatures the magnetic susceptibility deviates from the Curie–Weiss temperature dependence and eventually saturates to a constant value. In contrast, Pu 6 W 4 Al 43 displays nearly temperature independent Pauli paramagnetism for all temperatures, as would be expected if the 5 f -electrons are delocalized. The electrical resistivity for U 6 W 4 Al 43 increases slightly with the decreasing temperature, suggesting that it is dominated by f -electronic hybridization effects and disorder scattering that originates from the W/Al site interchange. Specific heat measurements for U 6 W 4 Al 43 further reveal an enhanced electronic Sommerfeld coefficient that is consistent with a moderately enhanced charge carrier effective mass. Together these measurements expose these materials as hosts for unstable f -electron magnetism, where the novel cage-like structures control the phenomena through the spacing between the A ions. Through this combination of mild magnetism, the low cost elements of the Al–W cages, and chemical tunability that has been shown for related materials in the same structure, the A 6 W 4 Al 43 compounds emerge as promising nuclear waste-forms for transuranics, while the wider family of materials makes an appealing environment for studying f

charge transport↗
DOE OSTI2019

The structure of graphene on graphene/C60/Cu interfaces: a molecular dynamics study

Two experimental studies reported the spontaneous formation of amorphous and crystalline structures of C 60 molecules intercalated between graphene and a surface. The findings observed included interesting phenomena ranging from reaction between fullerene C60s ('C 60 s' stands for plural of C 60 ) under graphene to graphene sheets sagging between C 60 s and control of strain in these sheets. Motivated by this work, we performed fully atomistic reactive molecular dynamics simulations to investigate the formation and thermal stability of graphene sheet wrinkles as well as graphene attachment to and detachment from a surface when the sheet is laid over a previously distributed array of C 60 molecules on a copper surface at different temperatures. As graphene compresses the C 60 s against the surface, and graphene attachment to the surface in between C 60 s depends on the height of the wrinkles in the graphene sheet, configurations with both frozen and non-frozen fullerenes were investigated in the simulations in order to examine the experimental result of stable, sagged graphene sheets when the distance between C 60 s is about 4 nm and the height of the wrinkles in the sheet is about 0.8 nm. Below a distance of 4 nm between fullerenes, the graphene is predicted to become locally suspended and less strained. The simulations predict that this happens when the fullerenes can deform under the compressive action of the graphene sheet. If the fullerenes are kept frozen, spontaneous 'blanketing' of graphene is predicted only when the distance between neighbouring C 60 s is equal to or great than about 7 nm. These predictions agree with a mechanical model relating the rigidity of a graphene sheet to the energy of graphene-surface adhesion. This work further reveals the structure of intercalated molecules and the role of stability and sheet wrinkling on the preferred configuration of graphene. This study thus might assist in the development of two-dimensional confined nanoreactors for chemical reactions.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Apparent critical phenomena in the superionic phase transition of Cu 2- x Se

The superionic phase transition of ${\mathrm{Cu}}_{2-x}\mathrm{Se}$ accompanies drastic changes in transport properties. The Seebeck coefficient increases sharply while the electrical conductivity and thermal diffusivity drops. Such behavior has previously been attributed to critical phenomena under the assumption of a continuous phase transition. However, applying Landau's criteria suggests that the transition should be first order. Using the phase diagram that is consistent with a first order transition, we show that the observed transport properties and heat capacity curves can be accounted for and modeled with good agreement. The apparent critical phenomena is shown to be a result of compositional degree-of-freedom. In conclusion, understanding of the phase transition allows to explain the enhancement in the thermoelectric figure-of-merit that is accompanied with the transition.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗