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The large difference between the calculated strain and defect energies and the driving force for grain growth (21,100Â kJ/m 3 ) casts doubt on the applicability of a simple thermodynamic model of texture transformation.Ĭhemical Equilibrium as Balance of the Thermodynamic Forces
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Nanotwins in reported densities are shown to provide greater driving force (≈1000 kJ/m 3 ) and may account for orientation selection. However, vacancy and dislocation densities must be exceptionally high to significantly exceed the strain energy and do not provide obvious orientation selection mechanisms.
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Reduction in defect energy has been proposed as an alternative. Stress analysis shows that differences in driving forces for texture transformation due to applied bulge pressure were significant (≈200 kJ/m 3 ), suggesting that a different, much larger driving force must be responsible.
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In situ synchrotron XRD measurements show the change in texture during annealing, and reveal that applied stresses have no effect on the transformation. We investigated the driving forces involved in this transformation by using a bulge test apparatus to induce different stresses in thin Ag films under identical annealing conditions. However, recent work has called this model into question, suggesting that neither the stress nor the interface energy play a dominant role in texture transformation. In this model, thin films retain their (111) texture due to the lower energy of the (111) interface, while thick films transform to (100) due to the lower stiffness and thus strain energy of a (100) film. The well-known thickness-dependent (111)-to-(100) texture transformation in thin FCC films is usually attributed to a competition between interface and strain energies. Chmielus, Markus Lin, Ming-Tzer Joress, Howie Visser, Kyle Woll, Arthur Vinci, Richard P. The modeling studies showed reaction at the phase boundary to be likely process controlling the reaction rates of all the systems studied.ĭriving forces for texture transformation in thin Ag filmsĮllis, Elizabeth A. The results showed Nb2O5 to be vastly superior to other catalysts for improving the thermodynamics and kinetics of MgH2. It is relatively fast for the phases to separate and select a uniform size, but exceedingly slow to order over a long distance, unless the symmetry is suitably brokenĭehydrogenation Kinetics and Modeling Studies of MgH2 Enhanced by Transition Metal Oxide Catalysts Using Constant Pressure Thermodynamic Driving Forcesĭirectory of Open Access Journals (Sweden)įull Text Available The influence of transition metal oxide catalysts (ZrO2, CeO2, Fe3O4 and Nb2O5 on the hydrogen desorption kinetics of MgH2 was investigated using constant pressure thermodynamic driving forces in which the ratio of the equilibrium plateau pressure (pm to the opposing plateau (pop was the same in all the reactions studied. Numerical simulations reveal rich dynamics of the pattern formation process. These thermodynamic forces are embodied in a nonlinear diffusion equation. It is the competition between the coarsening and the refining that leads to size selection and spatial ordering. The concentration-dependent surface stress drives phase refining. The phase boundary energy drives phase coarsening.
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A double-welled, composition-dependent free energy drives phase separation. Attention is focused on thermodynamic forces that drive the self-assembly. This paper reviews a model that accounts for these behaviors. Sometimes the phases select sizes about 10 nm, and order into an array of stripes or disks. A two-component monolayer grown on a solid surface may separate into distinct phases. International Nuclear Information System (INIS)Įxperimental evidence has accumulated in the recent decade that nanoscale patterns can self-assemble on solid surfaces. Forces that Drive Nanoscale Self-assembly on Solid Surfaces