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Transformation Kinetics (transformation + kinetics)

Distribution by Scientific Domains
Polymers and Materials Science42%

Selected Abstracts

Oxidative transformation of tetrachlorophenols and trichlorophenols by manganese dioxide

ENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 6 2009
Ling Zhao
Abstract This study examined the transformation kinetics of three tetrachlorophenols (TeCPs) and three trichlorophenols (TCPs) in the presence of MnO2 under different solution chemistry conditions. The reaction rate measured for each CP decreased as a function of solution pH, and under the same solution chemistry conditions, the measured rates may depend primarily on both the adsorbability at the MnO2 surfaces and the isomeric structures of the CPs. Isomeric effects indicated that chloro substituent on ortho or para positions exhibited faster rates of transformation than on meta positions. Gas chromatography-mass spectrometry analysis with a derivatization method showed that dimers including polychlorinated phenoxyphenols and chlorinated polyhydroxybi-phenyl were among the major products for all CPs. Monomeric products were among the major products of 2,4,6-TCP, 2,3,4-TCP, and 2,3,4,6-TeCP, whereas trimeric products also were among the major products of 2,3,4-TCP and 2,4,5-TCP. It appeared that hydroxylation of CPs and formation of dimeric or trimeric products via oxidative coupling were the major reaction mechanisms involved in the oxidation of CPs by MnO2. [source]

Oxidation kinetics of pentachlorophenol by manganese dioxide

ENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 11 2006
Ling Zhao
Abstract This study examined the abiotic transformation kinetics of pentachlorophenol (PCP) by manganese dioxide (MnO2) at different solution chemistry and initial concentrations of PCP and MnO2. The measured PCP transformation rates were found to be on the order of 1.07 with respect to [PCP] and 0.91 and 0.87 with respect to [MnO2] and [H+], respectively. Dissolved Mn2+ and Ca2+ as background electrolytes considerably decreased the reaction rate because of their adsorption and hence blocking of active sites on MnO2 surfaces. The dechlorination number, 0.59 chloride ions per transformed PCP after a 1-h reaction, suggests that a fraction of the transformed PCP was not dechlorinated and may be coupled directly to dimeric products. Gas chromatography/ mass spectrometry and liquid chromatography/mass spectrometry/mass spectrometry techniques were used to identify two isomeric nonachlorohydroxybiphenylethers as major products and 2,3,5,6-tetrachloro-1,4-hydroquinone and tetrachlorocatechol as minor products. Product identification suggested that the reaction may include two parallel reactions to form either dimers or 2,3,5,6-tetrachloro-1,4-hydroquinone and tetrachlorocatechol via simultaneous dehydrochlorination and hydroxylation. [source]

Kinetics of intra- and intermolecular zymogen activation with formation of an enzyme,zymogen complex

FEBS JOURNAL, Issue 1 2005
Matilde Esther Fuentes
A mathematical description was made of an autocatalytic zymogen activation mechanism involving both intra- and intermolecular routes. The reversible formation of an active intermediary enzyme,zymogen complex was included in the intermolecular activation route, thus allowing a Michaelis,Menten constant to be defined for the activation of the zymogen towards the active enzyme. Time,concentration equations describing the evolution of the species involved in the system were obtained. In addition, we have derived the corresponding kinetic equations for particular cases of the general model studied. Experimental design and kinetic data analysis procedures to evaluate the kinetic parameters, based on the derived kinetic equations, are suggested. The validity of the results obtained were checked by using simulated progress curves of the species involved. The model is generally good enough to be applied to the experimental kinetic study of the activation of different zymogens of physiological interest. The system is illustrated by following the transformation kinetics of pepsinogen into pepsin. [source]

Aliovalent Substitutions in Olivine Lithium Iron Phosphate and Impact on Structure and Properties

ADVANCED FUNCTIONAL MATERIALS, Issue 7 2009
Nonglak Meethong
Abstract Lithium transition metal phosphate olivines are enabling a new generation of high power, thermally stable, long-life rechargeable lithium batteries that may prove instrumental in the worldwide effort to develop cleaner and more sustainable energy. Nanoscale (<100,nm) derivatives of the olivine family LiMPO4 (M,=,Fe, Mn, Co, Ni) are being adopted in applications ranging in size scale from hybrid and plug-in hybrid electric vehicles to utilities-scale power regulation. Following the previous paradigm set with intercalation oxides, most studies have focused on the pure ordered compounds and isovalent substitutions. In contrast, even the possibility for, and role of, aliovalent doping has been widely debated. Here, critical tests of plausible defect compensation mechanisms using compositions designed to accommodate Mg2+, Al3+, Zr4+, Nb5+ ions on the M1,and/or M2 sites of LiFePO4 with appropriate charge-compensating defects are carried out, and conclusive crystallographic evidence for lattice doping, e.g., up to at least 12 atomic percent added Zr, is obtained. Structural and electrochemical analyses show that doping can reduce the lithium miscibility gap, increase phase transformation kinetics during cycling, and expand Li diffusion channels in the structure. Aliovalent modifications may be effective for introducing controlled atomic disorder into the ordered olivine structure to improve battery performance. [source]

Low-Temperature Aging Behavior of Alumina-Toughened Zirconia

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2008
Jens Schneider
The corrosion of alumina-toughened zirconia (ATZ) as a consequence of hydrothermal treatment was investigated, especially the transformation kinetics from tetragonal zirconia to the monoclinic phase. For this purpose, polished ATZ specimens (Ra<5 nm) were aged in water vapor at different temperatures ranging from 70° to 134°C. The fraction of the monoclinic phase was determined using X-ray diffraction and Rietveld refinement. The isothermal transformation curves obtained were fitted to the Mehl,Johnson,Avrami equation by least squares. An Arrhenius plot of the fitted transformation rates was used to determine the activation energy and the pre-exponential factor. Following this procedure, the kinetic parameters of the phase transformation were extrapolated down to body temperature and the formation of the monoclinic phase was simulated. In addition, optical interferometry on well-polished specimens (Ra<2 nm) was alternatively used to calculate the monoclinic fraction from the histogram dataset. The results agree very well with those of the X-ray measurements. Additionally, the development of surface roughness with increasing aging time is discussed. [source]

Effect of Yttrium and Erbium Ions on Epitaxial Phase Transformations in Alumina

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2003
Deirdre D. Ragan
The effect of low concentrations of Y, Er, and Cr solutes on the amorphous-to-, transformation and on the ,-to-, transformation in aluminum oxide has been studied in situ by time-resolved reflectivity. The activation energies of the two transformations with these dopants are the same as in undoped alumina, being 4.1 ± 0.1 and 5.2 ± eV, respectively. Although not affecting the activation energies, Y, Er, and Cr do affect the transformation kinetics. Y and Cr ions decrease the ,-to-, transformation velocity and, over the limited range studied, do so in proportion to their concentration. Concentrations of Er as low as ,6 ppm retard the ,-to-, transformation and concentrations of 32 ppm essentially stop the transformation occurring within the times and temperatures accessible within the present experiment, thereby preventing quantification of the effect of Er on the ,-phase transformation. Erbium also retards the amorphous-to-, transformation relative to undoped alumina whereas yttrium and chromium accelerate it. [source]

A Multi-phase-field model including inelastic deformation for solid state transformations

PROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2008
Stefan BenkeArticle first published online: 26 FEB 200
A multi phase field model is presented in order to take the plastic deformation during a solid state transformation into account and to investigate its effect on the transformation kinetics and morphology in a multi phase material. The model is formulated consistently with the multi phase field model for diffusional and surface driven phase transformations [1]. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]