Diffusion in Solids and Liquids X

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Authors: Jozef Minár
Abstract: We discuss the numerical modelling of two phase flow in porous media (see [1]). We consider a one-dimensional problem describing flow of two incompressible and immiscible fluids through a porous medium where the non-wetting phase (oil) is displaced by the wetting fluid (water). The used model is based on Darcy’s law and we consider either horizontal (neglecting the influence of gravity) or vertical flow. In case of horizontal flow, we compare our solution with analytical solution published in [2]. In case of gravity driven vertical flow, there is no known analytical solution and we propose our solution as a benchmark solution.Our numerical model is based on the modelling of interface separating zones where the water is present and where it is not. We semidiscretize the problem in space and obtain a mass preserving system of ODEs. We have moving grid points only on region where the water is present, and these move accordingly to the evolution of the interface. We can choose to have non-equidistant grid with more grid points in the neighbourhood of the interface. This guarantees very good approximation of sharp fronts during infiltration. Results obtained by our methods are compared with well-known result in [2] which was obtained by semi-analytical method and they are in perfect agreement.
Authors: Alexander S. Chaus, Matej Beznák, Miroslav Sahul
Abstract: The influence of heat treatments on the microstructure of AISI M2 type high-speed steel obtained using conventional metallurgy has been studied. The primary focus was on the effects of austenitisation temperatures on the behaviour of the initial eutectic carbides during austenitisation. In order to investigate kinetics of both the microstructure and phase transformations in eutectic carbides upon heat treatments with respect to diffusion processes, different techniques of electron scanning microscopy and energy dispersive X-ray spectrometry have been used. The effect of the austenitisation temperature on the primary austenite grain size, martensite microstructure and volume fraction, size distribution, and morphology of the primary carbides of eutectic origin as well as their behaviour during heat treatment in the wrought M2 high-speed steel was thoroughly investigated.
Authors: R.J. Brandão, M.M. Prado, L.G. Marques
Abstract: The freeze-drying rate is essentially low, since it is controlled by internal moisture diffusion. In addition, the application of vacuum and low temperature during the process presents a higher energy demand. Therefore, the search for new strategies to improve water mobility during freeze-drying constitutes a topic of relevant research. The aim of this work was to evaluate the use of power ultrasound to improve freeze-drying characteristics of açai, quantifying the influence of the applied power on both the drying and rehydration kinetics of the material. Açai (Euterpe oleracea Martius) samples were sonicated with two different frequency levels, 20 kHz and 40 kHz, and two sonication times, 3 min and 10 min. Page’s equation considering internal and external resistances to mass transfer provided a good fit of freeze-drying kinetics, while the Peleg’s equation was found to be suitable for describing the rehydration kinetics of freeze-dried açai. Pretreatment of açai with ultrasound waves was not effective. Ultrasound-induced structural disruption in the açai skin hindered the mass transfer during both freeze-drying and rehydration processes.
Authors: Ali Mahieddine, Mohammed Ouali, Amel Mazouz
Abstract: To investigate the dynamic behavior of beams with partially delaminated layers, a model is presented in this work. This model takes into account the lateral strains. The principal advantage of the element is that it allows the modeling of delamination anywhere in the structure. The region without delamination is modeled to carry constant peel and shear stresses. Numerical results of the present model are presented and its performance is evaluated.
Authors: Stepan Alexandrovich Lushnikov, Tatyana Victorovna Filippova
Abstract: Hydrides of CeNi3 intermetallic compounds were synthesized with hydrogen at a pressure of up to 50 bars at room and low temperatures. Using the X-ray diffraction method gives phase composition and lattice parameters of the hydride samples. It was revealed that one set of the hydride samples was stable in air and at room temperature, while another set was very unstable at the same conditions and rapidly desorbed hydrogen. This diverse behaviour depends on the proportion of obtained hydride phases at low and room temperatures, coexisting in the samples. A possible explanation has been proposed based on the different diffusion of hydrogen atoms in ordered and disordered hydride phases, incorporated in the samples.
Authors: Oksana Melikhova, Jakub Čížek, Marián Vlček, František Lukáč, Martin Vlach, Bohumil Smola, Ivana Stulíková, Radomír Kužel, Ivan Procházka, Hana Kudrnova, Aleš Jäger
Abstract: The Mg-Zn-Y system attracts interest of researchers due to the formation of quasicrystalline particles which are believed to improve mechanical properties of Mg-based alloys. In the Mg-Zn-Y-Nd-Zr alloy (WE43 modified by addition of Zn) studied here the formation of icosahedral phase (I-phase) with quasicrystalline structure competes with cubic W-phase. Grain boundary phases in Mg-Zn-Y-Nd-Zr alloys subjected to various heat treatments were characterized. It was found that the portion of the I-phase in Mg-Zn-Y-Nd-Zr alloy can be increased by a suitable heat treatment. Moreover, the solidification process is influenced by the cooling rate resulting in a striking difference in the morphology of grain boundary phases.
Authors: Martin Vlach, Jakub Čížek, Oksana Melikhova, Veronika Kodetova, Ivana Stulíková, Ivan Procházka
Abstract: Hydrogen interaction with vacancies and dislocations in Pd were investigated in the present work. Well annealed and plastically deformed Pd samples were electrochemically doped with hydrogen up to various hydrogen concentrations. Subsequently the samples were subjected to linear annealing (10 K/min) and hydrogen desorption was studied by differential scanning calorimetry (DSC). An endothermic peak caused by hydrogen desorption was observed in the DSC curve of well annealed sample at ∼ 178 °C. In plastically deformed samples this peak is shifted to higher temperatures since hydrogen is trapped at dislocations and its diffusivity is suppressed. Moreover, it was found that if the atmosphere surrounding the heated sample contains oxygen the endothermic hydrogen desorption peak is followed by a strong exothermal peak caused by fusion of desorbed hydrogen with oxygen into water vapour molecules. To avoid this undesired effect DSC measurements have to be done in a protective atmosphere which does not contain oxygen.
Authors: Tomáš Kekule, Hana Kudrnova, Martin Vlach, Jakub Čížek, Oksana Melikhova, Ivana Stulíková, Bohumil Smola
Abstract: The Mg-10 wt. % Gd and Mg-15 wt. % Gd alloys produced by squeeze casting were solution treated at 500 °C for 8 hours and subsequently naturally aged for more than 2 months. Electrical resistivity of both materials measured at 77 K decreases, if the alloys are kept at room temperature after quenching from the solution temperature. This change accompanied by a microhardness increase almost saturates after 2 months and is caused most probably by solute atoms clustering. Phase transformations and microhardness changes were investigated during isochronal annealing in both naturally aged alloys in comparison to just solution treated ones. Electrical resistivity changes measured at 77 K were used to characterize microstructure development. Transmission electron microscopy was performed at selected states heat treated in the identical way. The Mg15Gd supersaturated solid solution isochronally annealed up to 500 °C immediately after the solution treatment decomposes into following successive phases: β ́ ́ (D019) metastable → β ́ (cbco) metastable → β (Mg5Gd) stable. All three possible orientation relationship modes of the metastable β ́ (cbco) phase existed at lower temperatures (up to 280 °C) but only one mode persists up to 330 °C. Precipitation of the β ́ (cbco) phase has not been observed in the Mg10Gd alloy annealed isochronally immediately after the solution treatment. The natural ageing does not change the precipitation sequence but concentration of Gd atoms involved in individual precipitation processes is influenced in both alloys. Peak hardening increases after natural ageing in the Mg15Gd alloy, shifts to higher temperatures and the temperature region of peak hardening extends.
Authors: Ivan Procházka, Jakub Čížek, Jan Valenta, Vladimír Havránek, Petr Hruška, Michal Novotný, Ján Lančok
Abstract: In the present work, defects created by implantation of hydrothermally grown ZnO single crystals of high quality with H+ ions were investigated by positron annihilation lifetime (LT) spectroscopy combined with measurements of optical transmittance (OT) and photoluminescence (PL). First, zinc vacancies attached with one hydrogen impurity (VZn – 1H) atom were identified in the virgin ZnO single crystal. The ZnO single crystals were then bombarded by H+ ions with the energy of 2.5 MeV to the fluence of 1016 cm-2. It was found that VZn – VO divacancies were introduced into ZnO by H+-implantation. Effects of H+-implantation on the optical activity of defects in ZnO lattice are characterised in the light of the present OT and PL data.

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