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Quenching Rates (quenching + rate)

Distribution by Scientific Domains

Physics and Astronomy	50%
Chemistry	50%

Selected Abstracts

An optimized kinetics model for OH chemiluminescence at high temperatures and atmospheric pressures

INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 12 2006
Joel M. Hall
Chemiluminescence from the OH(A , X) transition near 307 nm is a commonly used diagnostic in combustion applications such as flame chemistry, shock-tube experiments, and reacting-flow visualization. Although absolute measurements of OH(X) concentrations are well defined, there is no elementary relation between emission from the electronically excited state (OH*) and its absolute concentration. Thus, to enable quantitative emission measurements, a kinetics model has been assembled and optimized to predict OH* formation and quenching at combustion conditions. Shock-tube experiments were conducted in mixtures of H2/O2/Ar, CH4/O2/Ar, and CH4/H2/O2/Ar with high levels of argon dilution (>98%). Elementary reactions to model OH*, along with initial estimates of their rate coefficients, were taken from the literature. The important formation steps follow: (R0) (R1) Sensitivity analyses were performed to identify experimental conditions under which the shape of the measured OH* profiles and the magnitude of the OH* emission would be sensitive to the formation reactions. A fitting routine was developed to express the formation rate parameters as a function of a single rate, k1 at the reference temperature (1490 K). With all rates so expressed, H2/CH4 mixtures were designed to uniquely determine the value of k1 at the reference temperature, from which the remaining rate parameters were calculated. Quenching rates were fixed at their literature values. The new model predicts the experimental data over the range of conditions studied and can be used to calibrate the emission diagnostic for other applications, such as measurements in real combustion environments, containing higher order hydrocarbon fuels and lower levels of dilution in air. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 714,724, 2006 [source]

Thermal vacancies and self-diffusion energy in 2024 Al-alloy by positron annihilation lifetime technique

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 11 2009
Emad A. Badawi
Abstract Positron annihilation lifetime technique (PALT) is one of the most important nuclear non-destructive techniques. It was used to study the thermal vacancies in one of the most important engineering aluminum alloys , the 2024 Al-alloy. Quenching experiments were usually performed on thin specimens to ensure a uniform quenching rate throughout the specimen. The specimens were prepared with dimensions of 0.15 × 1.5 × 1.5 cm3. After grinding, polishing and etching, samples of 2024 were homogenized for 12 h at 673 K and annealed for 90-min., before being quenched in water (277 K). Positron lifetime measurements followed. From such measurements, it is possible to deduce the vacancy formation enthalpy, which in combination with the results of self-diffusion measurements, gives a value for migration enthalpy of the vacancy. These are very important quantities in the study of the annealing of irradiation induced defects. The use of the quenching technique in the positron annihilation study has the advantage that it allows a distinction between vacancy and dislocation. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Photochemical reaction mechanisms and kinetics with molecular nanocrystals: surface quenching of triplet benzophenone nanocrystals

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 4 2010
Sabrina Simoncelli
Abstract Organic molecular nanocrystals suspended in water are useful when studying reactions that occur in the solid state because they retain not only the reactive and supramolecular properties of bulk crystals, but are also amenable to transmission spectroscopy. Having previously studied the triplet state of benzophenone nanocrystals by laser flash photolysis transmission spectroscopy, we now report nanosecond experiments in the presence of several possible quenchers: anionic and cationic surfactants, dissolved oxygen, and as a function of solvent deuteration (H2O and D2O). After finding these to have no effect, several anionic quenchers (I,, Br,, and N) were tested by Stern,Volmer analysis. Significant correlation between the quenching rates in solution and in nanocrystals suggests that the electronic excitation is accessible to quenchers at the surface. Copyright © 2010 John Wiley & Sons, Ltd. [source]

The recovery of the shear viscosity of thermally aged bulk and ribbon glassy Pd40Cu30Ni10P20 by rapid quenching from the supercooled liquid state

PHYSICA STATUS SOLIDI - RAPID RESEARCH LETTERS, Issue 2-3 2009
V. A. Khonik
Abstract Isochronal (= linear heating) measurements of the shear viscosity below the glass transition of bulk and ribbon glassy Pd40Cu30Ni10P20 samples differing ,104 -fold in the production quenching rates have been performed. It has been found that heating up into the supercooled liquid region followed by slow cooling leads to a significant structural relaxation-induced viscosity increase upon subsequent testing. This increase, however, is not truly irreversible and the viscosity can be fully recovered (= decreased) by fast quenching from the supercooled liquid state. The effect is nearly independent of whether ribbon or bulk samples are used despite the fact that those latter are notably denser. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]