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Thermal Activation (thermal + activation)

Distribution by Scientific Domains

Physics and Astronomy	41%
Chemistry	33%

Selected Abstracts

Liquid-Phase Hydrogenation of Unsaturated Aldehydes: Enhancing Selectivity of Multiwalled Carbon Nanotube-Supported Catalysts by Thermal Activation

CHEMCATCHEM, Issue 2 2010
Bruno
Abstract Platinum and iridium organometallic precursors are used to prepare nanosized, thermally stable multiwalled carbon nanotube-supported catalysts. The materials are characterized by N2 adsorption at 77,K, temperature-programmed desorption coupled with mass spectrometry, H2 chemisorption, transmission electron microscopy and thermogravimetric analysis; they are tested in the selective hydrogenation of cinnamaldehyde to cinnamyl alcohol under mild conditions (363,K and 1,MPa). A thermal activation at 973,K is found to have a very positive effect over both activity and selectivity, leading to selectivities of approximately 70,%, at 50,% conversion, regardless of the active metal phase (Pt or Ir). Since no noticeable differences in the metal particle sizes are detected, the results are interpreted in light of an enhanced metal/support interaction. This effect, induced by the removal of oxygenated surface groups, is thought to change the adsorption mechanism of the cinnamaldehyde molecule. [source]

Insertion of Molecular Oxygen in Transition-Metal Hydride Bonds, Oxygen-Bond Activation, and Unimolecular Dissociation of Metal Hydroperoxide Intermediates.

HELVETICA CHIMICA ACTA, Issue 3 2008
Short Communication
Abstract Thermal activation of molecular oxygen is observed for the late-transition-metal cationic complexes [M(H)(OH)]+ with M=Fe, Co, and Ni. Most of the reactions proceed via insertion in a metalhydride bond followed by the dissociation of the resulting metal hydroperoxide intermediate(s) upon losses of O, OH, and H2O. As indicated by labeling studies, the processes for the Ni complex are very specific such that the O-atoms of the neutrals expelled originate almost exclusively from the substrate O2. In comparison to the [M(H)(OH)]+ cations, the ionmolecule reactions of the metal hydride systems [MH]+ (M=Fe, Co, Ni, Pd, and Pt) with dioxygen are rather inefficient, if they occur at all. However, for the solvated complexes [M(H)(H2O)]+ (M=Fe, Co, Ni), the reaction with O2 involving OO bond activation show higher reactivity depending on the transition metal: 60% for the Ni, 16% for the Co, and only 4% for the Fe complex relative to the [Ni(H)(OH)]+/O2 couple. [source]

Thermal activation of beryllium-related photoluminescence by annealing of GaN grown by molecular beam epitaxy

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7 2005
Kyoungnae Lee
Abstract Surface polarity related differences in the optical activity of Be in GaN epilayers grown by rf-plasma molecular beam epitaxy are investigated. N-polar GaN doped with Be exhibits a significantly higher intensity of donor-acceptor pair (DAP) photoluminescence (PL) than similarly doped Ga-polar GaN, indicating the Be is incorporating at microscopically different sites, or possibly is forming different compensating complexes. Highly Be-doped Ga-polar GaN forms isolated polarity-inverted regions which then incorporate Be via the N-polar mechanism resulting in the DAP PL. High temperature annealing of the Ga-polar layers both under nitrogen/hydrogen mixtures and under pure nitrogen atmospheres activates the DAP PL. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Molecular interaction of neutral trehalase with other enzymes of trehalose metabolism in the fission yeast Schizosaccharomyces pombe

FEBS JOURNAL, Issue 15 2002
Teresa Soto
Trehalose metabolism is an essential component of the stress response in yeast cells. In this work we show that the products of the principal genes involved in trehalose metabolism in Schizosaccharomyces pombe, tps1+ (coding for trehalose-6- P synthase, Tps1p), ntp1+ (encoding neutral trehalase, Ntp1p) and tpp1+ (that codes for trehalose-6- P phosphatase, Tpp1p), interact in vitro with each other and with themselves to form protein complexes. Disruption of the gene tps1+ blocks the activation of the neutral trehalase induced by heat shock but not by osmotic stress. We propose that this association may reflect the Tps1p-dependent requirement for thermal activation of trehalase. Data reported here indicate that following a heat shock the enzyme activity of trehalase is associated with Ntp1p dimers or trimers but not with either Ntp1p monomers or with complexes involving Tps1p. These results raise the possibility that heat shock and osmotic stress activate trehalase differentially by acting in the first case through an specific mechanism involving Tps1p,Ntp1p complexes. This study provides the first evidence for the participation of the catabolic enzyme trehalase in the structural framework of a regulatory macromolecular complex containing trehalose-6- P synthase in the fission yeast. [source]

Vertical Nanostructures: Vertical (La,Sr)MnO3 Nanorods from Track-Etched Polymers Directly Buffering Substrates (Adv. Funct.

ADVANCED FUNCTIONAL MATERIALS, Issue 6 2010
Mater.
The inside cover image shows a tilted SEMFE image of La0.7Sr0.3MnO3 vertical nanorods. A. Carretero-Genevrier and co-workers have established a new method to generate vertical complex oxides nanostructures based on a sol,gel precursor solution and using track-etched polymers that act to buffer the substrate, which is described on page 892. These vertical nanorods grown at mild temperature undergo a complete structural, morphological, and chemical transformation into single crystalline (LaSr)xOy nanopyramids upon thermal activation at 1000°C. [source]

Vertical (La,Sr)MnO3 Nanorods from Track-Etched Polymers Directly Buffering Substrates

ADVANCED FUNCTIONAL MATERIALS, Issue 6 2010
Adrián Carretero-Genevrier
Abstract A novel and general methodology for preparing vertical, complex-oxide nanostructures from a sol,gel-based polymer-precursor solutions is developed using track-etched polymers directly buffering substrates. This method is able to develop a nanostructure over the entire substrate, the dimensions and localization of the vertical nanostructures being preset by the polymeric nanotemplate. Thereby, nanostructures with lateral sizes in the range of 100 to 300,nm and up to 500,nm in height have been grown. Two examples are presented herein, the latter being the evolution of the initial, metastable nanostructure. Specifically, La0.7Sr0.3MnO3 polycrystalline rods are grown at mild temperatures (800,°C); upon strong thermal activation (1000,°C) they suffer a profound transformation into vertical, single-crystalline (La,Sr)xOy nanopyramids sitting on a La0.7Sr0.3MnO3 epitaxial wetting layer. The driving force for this outstanding nanostructural evolution is the minimization of the total energy of the system, which is reached by reducing the grain-boundary, total-surface, and strain-relaxation energies. Finally, advanced electron-microscopy techniques are used to highlight the complex phase separation and structural transformations occurring when the metastable state is overcome. [source]

Laser-based in situ techniques: Novel methods for generating extreme conditions in TEM samples,

MICROSCOPY RESEARCH AND TECHNIQUE, Issue 3 2009
Mitra L. Taheri
Abstract The dynamic transmission electron microscope (DTEM) is introduced as a novel tool for in situ processing of materials. Examples of various types of dynamic studies outline the advantages and differences of laser-based heating in the DTEM in comparison to conventional (resistive) heating in situ TEM methods. We demonstrate various unique capabilities of the drive laser, namely, in situ processing of nanoscale materials, rapid and high temperature phase transformations, and controlled thermal activation of materials. These experiments would otherwise be impossible without the use of the DTEM drive laser. Thus, the potential of the DTEM as a new technique to process and characterize the growth of a myriad of micro and nanostructures is demonstrated. Microsc. Res. Tech., 2009. Published 2009 Wiley-Liss, Inc. [source]

Highly conductive and optically transparent GZO films grown under metal-rich conditions by plasma assisted MBE

PHYSICA STATUS SOLIDI - RAPID RESEARCH LETTERS, Issue 3-4 2010
H. Y. Liu
Abstract We demonstrate a critical effect of a metal-to-oxygen ratio on the electrical, optical, and structural properties of ZnO films heavily doped with Ga (carrier concentration in the range of 1020,1021 cm,3) grown by plasma-assisted molecular beam epitaxy. The as-grown layers prepared under the metal-rich conditions exhibited resistivities below 3 × 10,4 , cm and an optical transparency exceeding 90% in the visible spectral range as well as a large blue shift of the transmission/absorption edge attributed to the Burstein,Moss shift of the Fermi level deep into the conduction band, indicating high donor concentration. In contrast, the films grown under the oxygen-rich conditions required thermal activation and showed inferior properties. Furthermore, electrical measurements point to the nonuniform depth distribution of free carriers. An oxygen-pressure-dependent surface disordering is suggested to be responsible for the drastic effect of the metal-to-oxygen ratio on the film properties. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Effects of correlated disorder on the magneto-transport in colossal magnetoresistance manganites

PHYSICA STATUS SOLIDI - RAPID RESEARCH LETTERS, Issue 4 2009
M. Egilmez
Abstract Monte-Carlo simulations predict that a local correlated disorder is responsible for many of the novel transport and magnetic properties of colossal magnetoresistance (CMR) materials such as manganites. One important prediction of these models is that the resistivity at the metal,insulator transition (MIT) in manganites depends strongly on the correlated quenched disorder. However, experimental confirmation has been challenging since it is difficult to control the amount of disorder in these compounds. We carried out experiments on Sm0.55Sr0.45MnO3, a prototypical CMR manganite with a sharp MIT, whereby the oxygen-related disorder is systematically enhanced by low temperature thermal activation. We observe dramatic changes in the temperature dependence of resistivity at the MIT as the amount of quenched disorder is increased, occurring in a manner that is in agreement with theoretical predictions. Temperature dependence of resistivity of Sm0.55Sr0.45MnO3 for different annealing times at 350 °C in vacuum. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Temperature dependence effective activation energy in Tl2Ba2CaCu2O8 thin film

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 6 2006
S. L. Liu
Abstract By considering the competition of thermal activation and effective pinning, a general description of temperature dependence resistivity under magnetic fields has been deduced on high temperature superconductors. The temperature dependence resistivity is a function of the energy ratio of thermal actviation and pinning. Based on vortex glass phase transition analysis, the temperature dependence activation energy can be obtained from the resistivity transition under magnetic fields. It is found that the activation energy can be scaled onto a single parabola. The experiments on Tl-2212 thin film and other superconductors support our model. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Temperature dependence of the photoluminescence of single GaAs/AlGaAs concentric quantum ring structure

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 11 2006
M. Bonfanti
Abstract We present the temperature dependence of the photoluminescence emission of single concentric quantum rings. The two rings which form the nanostructure show a decoupled recombination kinetics in the whole temperature range. The emission is characterized by a doublet. The emission linewidth, which is already larger than 1 meV at low temperatures and increases as the temperature raises, is dominated by the thermal activation of carriers to higher excited states. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]