Papers by Keyword: Activation Enthalpy

Abstract: Ferroelectric ceramic solid solutions Li_xNa_1-xTa_yNb_1-yO₃ (х = 0.17; у = 0 – 0.5) with the perovskite structure have been obtained by the thermobaric synthesis method. Particularities of their microstructure, elastic properties, electrical conductivity and permittivity have been researched. It has been established that an increase in the thermobaric synthesis temperature leads to a decrease in the Young’s modulus value. Specific static conductivity values have been determined; charge carrier activation enthalpies Н_а have been calculated. The Curie temperature of the samples has been determined to decrease with an increase in tantalum content. A Ferroelectric ceramic solid solution Li_0.17Na_0.83Ta_0.1Nb_0.9O₃ was shown to undergo four structure phase transitions in the temperature range 300-820 К. A Li_0.17Na_0.83Ta_0.1Nb_0.9O₃ has been shown to be a high temperature superionic. Possible mechanisms of the detected phenomena are discussed.

Abstract: In this Chapter, we review knowledge about diffusion in quasi-crystalline alloys (quasicrystals). In Section 1 we first remind the reader of some major aspects of the quasi-crystalline state and in Section 2 we introduce phase diagrams with quasi-crystalline phases, for which detailed diffusion studies are available. We mention in Section 3 the more common experimental methods for diffusion studies. The diffusive motion of atoms in quasi-crystalline alloys can be studied by the same techniques used for crystalline metallic alloys and intermetallics – measurements of radiotracer diffusion and diffusion of stable isotopes and solute atoms by SIMS profiling. The best-studied quasi-crystalline alloys are icosahedral AlPdMn, icosahedral ZnMgRE (RE = rare earth metal), and decagonal AlNiCo. The major diffusion results for these quasicrystals are reviewed in Sections 4, 5, and 6. Section 7 is devoted to the pressure dependence of diffusion in quasicrystals and to a comparison of the activation volumes with those of crystalline metals. Positron annihilation studies are also mentioned, which together with activation volumes for diffusion strongly favour a vacancy mechanism in quasicrystals. The major results and conclusions are summarized in Section 8.

Abstract: In this paper, the deformation behaviour of a high-nitrogen grade 316LN stainless steel (with 0.14%N) has been studied over a wide range of temperatures (1123-1423K) and strain rates (0.001-10 s^-1). The key deformation controlling mechanisms have been investigated using thermal activation parameters, such as activation volume and activation enthalpy. The chromium nitride precipitates, dislocation intersections, both conservative and recovery, are found to be the key deformation controlling mechanism at different temperature–strain-rate domain during hot deformation of this material.

Abstract: Interdiffusion coefficients of Al replacing elements in Ni-Al-X (X=Ti, V and Nb) were estimated by a series of experiments using diffusion couples of Al rich pseudo-binary systems at three different temperatures of 1423, 1473 and 1523K. In order to obtain interdiffusion coefficients of the pseudo-binary systems, the experimental data was analyzed by the Sauer and Freise method, and also impurity diffusion coefficients of Ti, V and Nb in Ni3Al were estimated by applying the Darken-Manning equation. The magnitude of interdiffusion coefficient decreased in order of V, Ti and Nb at all three temperatures. Impurity diffusion coefficients were described by the expressions: , , . The activation enthalpies obtained from the experimental data confirmed the retardation of Ti, V and Nb diffusion in Ni3Al by the anti-site diffusion mechanism. These results are consistent with our previous work on diffusion of Re and Ru in Ni3Al .

Abstract: The first part of this paper presents a brief historical account of the Arrhenius law, starting from the seminal paper by Arrhenius (1889) up to theoretical developments mainly based on the rate theory. The second part describes the so called compensation rule (Meyer-Neldel rule), a correlation between activation enthalpy and entropy (or between pre-exponential factor and activation energy) , and discusses whether this correlation is trivial or bears some physical meaning.