We have investigated the coarsening kinetics and the morphology of precipitates after quenching a dilute binary alloy into its miscibility gap. Three-dimensional kinetic Monte Carlo simulations with a vacancydiffusion mechanism are performed. The atomic diffusion model accounts for the asymmetry of the two terminal phases with respect to the vacancy concentration and diffusivity. It is shown that, at a fixed low temperature of about 0.25T c , this asymmetry has a profound effect on the mechanism responsible for precipitate coarsening and on precipitate morphology. For positive asymmetry ͑i.e., when the vacancy is mostly diffusing inside the precipitates͒, precipitate diffusion and coagulation are favored. Nearly pure solute precipitates with atomically sharp interfaces are formed in a persistently supersaturated matrix. For negative asymmetry ͑i.e., when the vacancy is mostly diffusing in the matrix͒, the evaporation condensation of solute atoms becomes dominant even at early stages. The lack of interfacial mobility produces disordered, diffuse interfaces, which then result in highly supersaturated precipitates. These last results offer an explanation to recent atomic observations of the precipitate morphology in the Cu-Co system. A mean-field model is introduced to rationalize how the asymmetry parameter controls the distribution of vacancies in a two-phase alloy. This model predicts that cluster mobility increases when the asymmetry parameter is increased, resulting in an increase of the coagulation exponent in agreement with the simulations. Furthermore, this model offers a rationalization for persistent supersaturations of the matrix or the precipitates, and this provides some insight into the formation of diffuse interfaces.
Membrane-assisted solvent extraction was applied for the determination of different classes of compounds in water, having K(o/w) (octanol-water partition coefficient) values between 10(1) (aniline) and 10(8) (methyl stearate), by means of experimental designs. Four solvents were investigated--propan-2-ol, ethyl acetate, diisopropyl ether and cyclohexane--as well as extraction time, temperature, salt impact, pH and methanol addition. The best choice was diisopropyl ether, 50 degrees C, 30 min and an addition of 3 g of sodium chloride at pH 2 for polar compounds. The relative standard deviation (n = 3) was found in the range from 5 to 17%. Recoveries ranged between 34 and 100%. Membrane-assisted solvent extraction was successfully applied to a fast screening method dedicated to an unknown wastewater sample.
Surface segregation in transition metals can be analysed within a generalised Ising model,derived from Tight-Binding electronic structure calculations, which identifies three driving forces:the difference in surface energy and atomic volume between the two components and their tendencyto order or phase separate in the bulk. Using this ”three effects” rule, we present here general mapswhich predict the tendency of the solute metal element to segregate (or not) at the surface of a metalmatrix, for the 702 solute/matrix systems that can be formed with transition metal elements. Ourpredictions compare fairly well to the existing ab initio calculations and experimental data availableon these systems. The few exceptions, which mainly concern given matrix elements are discussed indetails.
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