Search

This paper considers the metastable zone width (MSZW) of aluminium hydroxyfluoride hydrate (AHF, AlF2OH.H2O) crystallizing solution, with a view to identifying suitable conditions for crystal growth. Speciation calculations combined with experimental validations have been used to estimate the degree of saturation at varying pHs leading to the identification of the metastable zone width (MSZW). From the results obtained, the solutions are saturated at pH 3.0 ± 0.3 and remain metastable until the nucleation threshold is reached at pH 5.0 ± 0.3. A narrowing of the MSZW at elevated temperatures has been observed and is attributed to the decrease in AHF solubility with increasing temperatures. This phenomenon was applied to obtain a tripling of the crystal particles in 30 minutes. The pH profile of the crystallizing solution is similar to that of acid/base titration curves and is found to be a quick and accurate method of estimating the MSZW. Using similar terminologies, the equivalent point corresponds to solution saturation while the mid-point corresponds to the nucleation threshold. These results are applicable in the recovery of fluoride values of spent pot lining. [ABSTRACT FROM AUTHOR]

Abstract: The reactions of ozone with a series of monoterpenes (α-pinene, sabinene, limonene and myrcene) were investigated in a novel flow reactor dedicated to the investigation of secondary organic aerosol (SOA) formation. Rate constants for the gas phase reactions and nucleation thresholds were determined at T∼296K, P∼764Torr under dry conditions (dew point ≤−33°C) and in absence of OH radicals scavenger and seed particles. Comparison with the literature as well as data from a simulation chamber showed good agreement. The experiments also show that the novel flow reactor improves the accuracy in evaluating the nucleation thresholds during the ozonolysis of monoterpenes and show that aerosol flow reactor is a useful tool to study the SOA nucleation step. Given as an upper limit, the nucleation thresholds obtained are (in molecule cm−3/ppb): α-pinene, 3.9×1010/1.56; sabinene, 6.2×109/0.26; limonene, 1.1×1010/0.43 and myrcene 2.1×1010/0.83. [Copyright &y& Elsevier]

We develop the theory of homogeneous nucleation of dislocations, which predicts plasticity incipience in FCC single crystal using material properties at elastic stage. In contrast to previous nucleation theories, we show that the work of shear stress is compensated by variation of the generalized stacking fault (GSF) energy during the loop formation. As a result, the energy barrier of nucleation includes only the line defect energy of the critical loop, which radius is defined by balance between all three contributions including the work of shear stress, the surface defect energy of the GSF and the line defect energy. This distinction is consequence of the difference between the mechanisms of the initial loop formation consisting in gradual shear along the proto-loop area and the following growth by slip of bordering dislocation line. Calculation of the energy barrier requires material properties at elastic stage such as stress-strain relationship, the GSF and shear modulus. These data are determined from molecular dynamics (MD) simulations and approximated in the form of artificial neural networks (ANNs). We perform direct MD simulations of the dislocation nucleation for simple shear in different directions parallel to (111) plane in Al single crystal at initial pressures from −3 to +3 GPa. MD simulations show complex and nonlinear evolution of stresses at simple shear deformation even at elastic stage before the dislocation nucleation. Predictions of ANN-informed theory are in accordance with our MD simulations and with the literature data. The calculated nucleation threshold for shear along the Burgers vector of partial Shockley dislocation in [ 11 2 ¯ ] direction at zero initial pressure varies from 2.03 to 2.16 GPa at the strain rate increase from 1 to 109 s−1. The conditions for homogeneous nucleation are close to the states of matter in thin metal films under powerful sub-picosecond laser irradiation. It is shown that twins are nucleated at angles less or equal to 7.5° between the shear direction and [ 11 2 ¯ ] direction. Perfect dislocation loops formed by leading and trailing partial dislocations arise at larger angles. In spite of change in the final form of nucleated defect, the energy barrier of nucleation is well-described by the same theoretical expression. It is explained by the fact that the initial defect in both cases is the loop of leading partial dislocation with the stacking fault over the entire loop area. The formation of twin or perfect dislocation occurs after the nucleation and does not influence the nucleation threshold. The ANNs reveal themselves as an efficient tool for approximation of the complex and non-obvious dependencies, while one should be careful with using them for data extrapolation. The paper is supplemented by files with the parameters of the trained ANNs and description of their structure so that one can use them within the nucleation model or separately. • Elastic deformation leads to complex stress evolution before plasticity incipience. • Work of stress is compensated by surface defect growth at dislocation loop nucleation. • Nucleation theory is supplemented by ANNs trained on MD data for elastic stage. • Numerical correspondence between theory and MD for nucleation threshold is achieved. [ABSTRACT FROM AUTHOR]