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Activation Volume (activation + volume)

Distribution by Scientific Domains

Chemistry	38%
Polymers and Materials Science	23%

Selected Abstracts

Determination of Activation Volume in Nanocrystalline Cu Using the Shear Punch Test,

ADVANCED ENGINEERING MATERIALS, Issue 10 2007
K. Guduru
Stress relaxation test (SRT) is very useful to study the dislocation dynamics and thus the deformation behavior. It becomes quite difficult to use conventional testing methods when the material availability is limited. In such instances, miniaturized specimen testing procedures such as shear punch test (SPT) becomes significantly useful for studying the mechanical behavior of materials. Current research deals with a novel SRT method employed on nanocrystalline Cu using SPT to study the deformation mechanism. [source]

Determination of the Activation Volume for CO Oxidation on Platinum by Pressure Modulation

CHEMPHYSCHEM, Issue 13 2010
Dr. Han-Chun Wang
Highly charged! A pressure-modulation technique is applied to determine the activation volume for CO oxidation on platinum. The negative value obtained suggests the formation of the positively charged transition state [H+,,,O,,,CO], (see figure; grey: Pt; red: O; black: C; blue: H) for which the volume is largely reduced with respect to that of the reactants. [source]

fMRI Activation in Continuous and Spike-triggered EEG,fMRI Studies of Epileptic Spikes

EPILEPSIA, Issue 10 2003
Abdulla Al-Asmi
Summary:,Purpose: To evaluate functional magnetic resonance imaging (fMRI) with simultaneous EEG for finding metabolic sources of epileptic spikes. To find the localizing value of activated regions and factors influencing fMRI responses. Methods: Patients with focal epilepsy and frequent spikes were subjected to spike-triggered or continuous fMRI with simultaneous EEG. Results were analyzed in terms of fMRI activation, concordance with the location of EEG spiking and anatomic MRI abnormalities, and other EEG and clinical variables. In four patients, results also were compared with those of intracerebral EEG. Results: Forty-eight studies were performed on 38 patients. Seventeen studies were not analyzed, primarily because no spikes occurred during scanning. Activation was obtained in 39% of 31 studies, with an activation volume of 2.55 ± 4.84 cc. Activated regions were concordant with EEG localization in almost all studies and confirmed by intracerebral EEG in four patients. Forty percent of patients without an MRI lesion showed activation; 37.5% of patients with a lesion had an activation; the activation was near or inside the lesion. Bursts of spikes were more likely to generate an fMRI response than were isolated spikes (76 vs. 11%; p < 0.05). Conclusions: Combining EEG and fMRI in focal epilepsy yields regions of activation that are presumably the source of spiking activity. These regions are highly linked with epileptic foci and epileptogenic lesions in a significant number of patients. Activation also is found in patients with no visible MRI lesion. Intracerebral recordings largely confirm that these activation regions represent epileptogenic areas. It is still unclear why many patients show no activation. [source]

Radial profiles of seismic attenuation in the upper mantle based on physical models

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2008
Fabio Cammarano
SUMMARY Thermally activated, viscoelastic relaxation of the Earth's materials is responsible for intrinsic attenuation of seismic waves. Seismic observations have been used to define layered radially symmetric attenuation models, independent of any constraints on temperature and composition. Here, we interpret free-oscillation and surface wave attenuation measurements in terms of physical structures, by using the available knowledge on the physical mechanisms that govern attenuation at upper-mantle (<400 km) conditions. We find that observations can be explained by relatively simple thermal and grain-size structures. The 1-D attenuation models obtained do not have any sharp gradients below 100 km, but fit the data equally well as the seismic models. The sharp gradients which characterize these models are therefore not required by the data. In spite of the large sensitivity of seismic observations to temperature, a definitive interpretation is limited by the unknown effects of pressure on anelasticity. Frequency dependence of anelasticity, as well as trade-offs with deeper attenuation structure and dependence on the elastic background model, are less important. Effects of water and dislocations can play an important role as well and further complicate the interpretation. Independent constraints on temperature and grain size expected around 100 km depth, help to constrain better the thermal and grain-size profiles at greater depth. For example, starting from a temperature of 1550 K at 100 km and assuming that the seismic attenuation is governed by the Faul & Jackson's (2005) mechanism, we found that negative thermal gradients associated with several cm grain sizes (assuming low activation volume) or an adiabatic gradient associated with ,1 cm grain size, can explain the data. A full waveform analysis, combining the effects on phase and amplitude of, respectively, elasticity and anelasticity, holds promise for further improving our knowledge on the average composition and thermal structure of the upper mantle. [source]

Magnetic viscosity and activation volume in chromium substituted Pb,M hexaferrite

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 9 2005
J. C. Faloh-Gandarilla
Abstract PbFe11CrO19 polycrystalline samples were prepared by the chemical coprecipitation method. X-ray diffraction and Mössbauer spectroscopy confirm the formation of the M-type hexagonal structure. Time dependence of the magnetization was recorded on the demagnetization curve of the hysteresis loop. It is well described by a simple logarithmic law but also non-logarithmic behavior was detected. A two-peak dependence of the viscosity coefficient S with the applied field was encountered. Considering reversible susceptibility measurements and a M(H) curve at constant dM/dt in the irreversible susceptibility determination, the calculated activation volume as a function of the applied field shows two well defined zones. The zones observed in the activation volumes are related to the two local maxima that the total susceptibility exhibits. It points to the fact that the viscosity study is sensible enough to characterize this system, encountering two apparent activation volumes. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Effects of temperature and strain rate on the tensile behavior of unfilled and talc-filled polypropylene.

POLYMER ENGINEERING & SCIENCE, Issue 12 2002
Part I: Experiments
The tensile behavior of unfilled and 40 w% talc-filled polypropylene has been determined at four different temperatures (21.5, 50, 75 and 100°C) and three different strain rates (0.05, 0.5 and 5 min,1). Experimental results showed that both unfilled and talc-filled polypropylenes were sensitive to strain rate and temperature. Stressstrain curves of both materials were nonlinear even at relatively low strains. The addition of talc to polypropylene increased the elastic modulus, but the yield strength and yield strain were reduced. The temperature and strain rate sensitivities of these materials were also different. An energy-activated, rate sensitive Eyring equation was used to predict the yield strength of both materials. It is shown that both activation volume and activation of energy increased with the addition of talc in polypropylene. [source]

Thermomechanical behaviour of poly[acrylonitrile- co -(methyl acrylate)] fibres oxidatively treated at temperatures up to 180 °C

POLYMER INTERNATIONAL, Issue 11 2005
Spyridon Soulis
Abstract PAN fibres, consisting of poly[acrylonitrile- co -(methyl acrylate)], were oxidatively heat treated at low temperatures (up to 180 °C), during which the basic macromolecular backbone was not cyclized. The change of length of the fibres was determined under various treatment conditions (ie temperature, time, stress applied). Prolonged heat treatment resulted in lower tensile strength of the fibres. The pristine and treated fibres were characterized by Fourier-transform infrared (FTIR), NMR and UV-visible spectroscopy and by TGA, and the results were used for representing the different regions according to chemical aspects in a plot of temperature versus time; this is very important for the whole treatment process. A shrinkage model was proposed, having both scientific and technical importance. The change in activation volume of shrinkage of fibres with temperature, calculated from this model, is indicative of the physical transitions taking place at the molecular scale. Copyright © 2005 Society of Chemical Industry [source]

Kinetic, Thermodynamic, and Mechanistic Patterns for Free (Unbound) Cytochrome c at Au/SAM Junctions: Impact of Electronic Coupling, Hydrostatic Pressure, and Stabilizing/Denaturing Additives

CHEMISTRY - A EUROPEAN JOURNAL, Issue 27 2006
Dimitri E. Khoshtariya Prof. Dr.
Abstract Combined kinetic (electrochemical) and thermodynamic (calorimetric) investigations were performed for an unbound (intact native-like) cytochrome c (CytC) freely diffusing to and from gold electrodes modified by hydroxyl-terminated self-assembled monolayer films (SAMs), under a unique broad range of experimental conditions. Our approach included: 1) fine-tuning of the charge-transfer (CT) distance by using the extended set of Au-deposited hydroxyl-terminated alkanethiol SAMs [-S-(CH2)n -OH] of variable thickness (n=2, 3, 4, 6, 11); 2) application of a high-pressure (up to 150,MPa) kinetic strategy toward the representative Au/SAM/CytC assemblies (n=3, 4, 6); 3) complementary electrochemical and microcalorimetric studies on the impact of some stabilizing and denaturing additives. We report for the first time a mechanistic changeover detected for "free" CytC by three independent kinetic methods, manifested through 1) the abrupt change in the dependence of the shape of the electron exchange standard rate constant (ko) versus the SAM thickness (resulting in a variation of estimated actual CT range within ca. 15 to 25 Å including ca. 11 Å of an "effective" heme-to-,-hydroxyl distance). The corresponding values of the electronic coupling matrix element vary within the range from ca. 3 to 0.02 cm,1; 2) the change in activation volume from +6.7 (n=3), to ,0 (n=4), and ,5.5 (n=6) cm3,mol,1 (disclosing at n=3 a direct pressure effect on the protein's internal viscosity); 3) a "full" Kramers-type viscosity dependence for ko at n=2 and 3 (demonstrating control of an intraglobular friction through the external dynamic properties), and its gradual transformation to the viscosity independent (nonadiabatic) regime at n=6 and 11. Multilateral cross-testing of "free" CytC in a native-like, glucose-stabilized and urea-destabilized (molten-globule-like) states revealed novel intrinsic links between local/global structural and functional characteristics. Importantly, our results on the high-pressure and solution-viscosity effects, together with matching literature data, strongly support the concept of "dynamic slaving", which implies that fluctuations involving "small" solution components control the proteins' intrinsic dynamics and function in a highly cooperative manner as far as CT processes under adiabatic conditions are concerned. [source]

Determination of the Activation Volume for CO Oxidation on Platinum by Pressure Modulation

Discrete Cyanide-Bridged Mixed-Valence Co/Fe Complexes: Outer-Sphere Redox Behaviour

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 13 2003
Paul V. Bernhardt
Abstract The outer-sphere redox behaviour of a series of [LnCoIIINCFeII(CN)5], (Ln = n -membered pentadentate aza-macrocycle) complexes have been studied as a function of pH and oxidising agent. All the dinuclear complexes show a double protonation process at pH , 2 that produces a shift in their UV/Vis spectra. Oxidation of the different non-protonated and diprotonated complexes has been carried out with peroxodisulfate, and of the non-protonated complexes also with trisoxalatocobaltate(III). The results are in agreement with predictions from the Marcus theory. The oxidation of [Fe(phen)3]3+ and [IrCl6]2, is too fast to be measured, although for the latter the transient observation of the process has been achieved at pH = 0. The study of the kinetics of the outer-sphere redox process, with the S2O82, and [Co(ox)3]3, oxidants, has been carried out as a function of pH, temperature, and pressure. As a whole, the values found for the activation volumes, entropies, and enthalpies are in the following margins, for the diprotonated and non-protonated dinuclear complexes, respectively: ,V, from 11 to 13 and 15 to 20 cm3 mol,1; ,S, from 110 to 30 and ,60 to ,90 J K,1 mol,1; ,H, from 115 to 80 and 50 to 65 kJ·mol,1. The thermal activation parameters are clearly dominated by the electrostriction occurring on outer-sphere precursor formation, while the trends found for the values of the volume of activation indicate an important degree of tuning due to the charge distribution during the electron transfer process. The special arrangement on the amine ligands in the isomer trans -[L14CoIIINCFeII(CN)5], accounts for important differences in solvent-assisted hydrogen bonding occurring within the outer-sphere redox process, as has been established in redox reactions of similar compounds. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source]

Magnetic viscosity and activation volume in chromium substituted Pb,M hexaferrite

Enantioselective [4+2]-Cycloaddition Reaction of a Photochemically Generated o -Quinodimethane: Mechanistic Details, Association Studies, and Pressure Effects

CHEMISTRY - A EUROPEAN JOURNAL, Issue 9 2004
Benjamin Grosch Dipl.
Abstract 1,2,3,4-Tetrahydro-2-oxoquinoline-5-aldehyde (2) was prepared from m -aminobenzoic acid and 3-ethoxyacryloyl chloride (4) in 19,% overall yield. Compound 2 underwent a photochemically induced [4+2]-cycloaddition reaction with various dienophiles upon irradiation in toluene solution. The exo product 10,a was obtained with acrylonitrile (9,a) as the dienophile, whereas methyl acrylate (9,b) and dimethyl fumarate (9,c) furnished the endo products 11,b and 11,c (69,77,% yield). The reactions proceeded at ,60,°C in the presence of the chiral complexing agent 1 (1.2 equiv) with excellent enantioselectivity (91,94,% ee). The enantiomeric excess increases in the course of the photocycloaddition as a result of the lower product association to 1. The intermediate (E)-dienol 8 was spectroscopically detected at ,196,°C in an EPA (diethyl ether/isopentane/ethanol) glass matrix. The association of the substrate 2 to the complexing agent 1 was studied by circular dichroism (CD) titration. The measured association constant (KA) was 589,M,1 at room temperature (25,°C) and normal pressure (0.1 MPa). An increase in pressure led to an increased association. At 400 MPa the measured value of KA was 703,M,1. Despite the stronger association the enantioselectivity of the reaction decreased with increasing pressure. At 25,°C the enantiomeric excess for the enantioselective reaction 2 + 9,a,10,a decreased from 68,% ee at 0.1 MPa to 58,% ee at 350 MPa. This surprising behavior is explained by different activation volumes for the diastereomeric transition states leading to 10,a and ent - 10,a. 1,2,3,4-Tetrahydro-2-oxochinolin-5-aldehyd (2) wurde ausgehend von m -Aminobenzoesäure und 3-Ethoxyacryloylchlorid (4) in fünf Schritten und einer Gesamtausbeute von 19,% hergestellt. Die Verbindung ließ sich in Toluol als Lösungsmittel mit verschiedenen Dienophilen in einer photochemisch induzierten [4+2]-Cycloaddition umsetzen (69,77,% Ausbeute), wobei als Hauptprodukt mit Acrylnitril (9,a) das exo -Produkt 10,a entstand. Methylacrylat (9,b) und Dimethylfumarat (9,c) lieferten die endo -Produkte 11,b and 11,c. In Gegenwart des chiralen Komplexierungsreagenz, 1 (1.2 Äquiv.) verliefen die Reaktionen mit exzellenter Enantioselektivität (91,94,% ee). Der Enantiomerenüberschuß nahm im Verlauf der photochemischen Umsetzung zu, was man auf die relativ zum Substrat 2 niedrigere Assoziation des Produkts zurückführen kann. Das intermediär gebildete (E)-Dienol 8 wurde spektroskopisch in einer EPA (Ether/i -Pentan/Ethanol) Glasmatrix bei ,196,°C nachgewiesen. Die Assoziation des Substrats 2 an das Komplexierungsreagenz 1 wurde durch CD-Titration genauer untersucht. Die Assoziationskonstante (KA) wurde bei Zimmertemperatur (25,°C) und Normaldruck (0.1 MPa) zu 589,M,1bestimmt. Bei höherem Druck beobachtete man eine verstärkte Assoziation und bei 400 MPa wurde eine Assoziationskonstante von KA=703,M,1bestimmt. Trotz der stärkeren Assoziaion nahm die Enantioselektivität mit wachsendem Druck ab. Bei 25,°C sank der Enantiomerenüberschuß der enantioselektiven Reation 2 + 9,a,10,a von 68,% ee bei 0.1 MPa auf 58,% ee bei 350 MPa. Dieses überraschende Verhalten läßt sich möglicherweise durch die unterschiedlichen Aktivierungsvolumina für die Übergangszustände erklären, die zu 10,a und ent - 10,a führen. [source]