Kinetics of Processes Distributed in Activation Energy
TL;DR: In this article, the effects produced by the initial distribution, the order of reaction, and the frequency factor are discussed, as well as the complications which can result from successive reactions and varying frequency factors.
Abstract: The kinetic behavior of systems in which processes occur which are distributed over a range of activation energies is considered. The effects produced by the initial distribution, the order of reaction, and the frequency factor are discussed. Imaginary and actual experimental situations are used to illustrate the large errors which can result when the distribution of the processes in activation energy is neglected. Some of the complications which can result from successive reactions and varying frequency factors are mentioned.
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TL;DR: The nonexponential rebinding observed at low temperatures and in solid samples implies that the innermost barrier has a spectrum of activation energies, similar to how myoglobin achieves specificity and order.
Abstract: Myoglobin rebinding of carbon monoxide and dioxygen after photodissociation has been observed in the temperature range between 40 and 350 K. A system was constructed that records the change in optical absorption at 436 nm smoothly and without break between 2 musec and 1 ksec. Four different rebinding processes have been found. Between 40 and 160 K, a single process is observed. It is not exponential in time, but approximately given by N(t) = (1 + t/to)-n, where to and n are temperature-dependent, ligand-concentration independent, parameters. At about 170 K, a second and at 200 K, a third concentration-independent process emerge. At 210 K, a concentration-dependent process sets in. If myoglobin is embedded in a solid, only the first three can be seen, and they are all nonexponential. In a liquid glycerol-water solvent, rebinding is exponential. To interpret the data, a model is proposed in which the ligand molecule, on its way from the solvent to the binding site at the ferrous heme iron, encounters four barriers in succession. The barriers are tentatively identified with known features of myoglobin. By computer-solving the differential equation for the motion of a ligand molecule over four barriers, the rates for all important steps are obtained. The temperature dependences of the rates yield enthalpy, entropy, and free-energy changes at all barriers. The free-energy barriers at 310 K indicate how myoglobin achieves specificity and order. For carbon monoxide, the heights of these barriers increase toward the inside; carbon monoxide consequently is partially rejected at each of the four barriers. Dioxygen, in contrast, sees barriers of about equal height and moves smoothly toward the binding site. The entropy increases over the first two barriers, indicating a rupturing of bonds or displacement of residues, and then smoothly decreases, reaching a minimum at the binding site. The magnitude of the decrease over the innermost barrier implies participation of heme and/or protein. The nonexponential rebinding observed at low temperatures and in solid samples implies that the innermost barrier has a spectrum of activation energies. The shape of the spectrum has been determined; its existence can be explained by assuming the presence of many conformational states for myoglobin. In a liquid at temperatures above about 230 K, relaxation among conformational states occurs and rebinding becomes exponential.
1,448 citations
TL;DR: In this article, a comprehensive review of the current state of the art of the study of elastic properties, the establishments of correlations between elastic moduli and properties/features, and the elastic models and elastic perspectives of metallic glasses is presented.
1,070 citations
TL;DR: In this paper, a theoretical model for relaxation in glassy materials, in particular metallic glasses, based on a spectrum of available processes distributed in activation energy is presented, where direct comparison is possible between the theory and experiment the agreement is good over all these various observed phenomena.
Abstract: A theoretical model for relaxation in glassy materials, in particular metallic glasses, based on a spectrum of available processes distributed in activation energy is presented. The model is used to discuss “In t” kinetics, “reversibility” and “crossover” effects which have been observed experimentally. Where direct comparison is possible between the theory and experiment the agreement is good over all these various observed phenomena.
400 citations
01 Jun 1990
TL;DR: In this article, the state of the art with an emphasis on recent research results and a view to identifying areas in which further developments in materials and processing might lead to even better properties and greater application of novel technology.
Abstract: The state of art is reviewed with an emphasis on recent research results and a view to identifying areas in which further developments in materials and processing might lead to even better properties and greater application of novel technology. Basis magnetic considerations are discussed, namely, B-H loop shape, core loss, magnetic anisotropy and annealing, and magnetostriction and stress effects. Materials and applications for power frequency devices are examined, covering core loss considerations, silicon steel development, metallic glasses, and high silicon materials. High-frequency and pulse applications, magnetic recording heads, and sensor and transducer applications are also discussed. Basic research questions and future directions with respect to core loss, magnetization, and stability are examined. >
212 citations
TL;DR: A review of recent progress in research on magnetic amorphous alloys and critically assesses the present level of understanding of this new class of magnetic materials, focusing mostly on the transition metal-metalloid glasses as mentioned in this paper.
Abstract: Magnetic amorphous alloys obtained by rapid quenching of the melt are excellent soft magnetic materials with a wide range of technological applications. They also represent a significant challenge to the scientific understanding of magnetic materials, since most of the existing theories of solids assume lattice periodicity. For these reasons, the magnetic and other properties of amorphous alloys have been very actively studied over the last decade. In recent years increasing attention has been directed to the fundamental understanding of the structural, thermal and magnetic properties of the amorphous alloys. It is not only scientifically but also technologically important to achieve such an understanding, since the amorphous alloys are, in many respects, so different from conventional crystalline magnetic materials. The author attempts to summarise recent progress in research on magnetic amorphous alloys and critically assesses the present level of understanding of this new class of magnetic materials, focusing mostly on the transition-metal-metalloid glasses. He starts with a review of early developments and a discussion on the nature of glasses and glass formation and proceeds to an extensive discussion of their atomic structure, both from the experimental and theoretical points of view.
203 citations