Crystallization Kinetics of α-Hemihydrate Gypsum Prepared by Hydrothermal Method in Atmospheric Salt Solution Medium

α-Calcium sulfate hemihydrate (α-HH) is an important cementitious material, which can be prepared by hydrothermal method from calcium sulfate dihydrate (DH) in an electrolyte solution. Study of the conversion kinetics of DH to α-HH in NaCl solution is helpful for understanding the control process. In this paper, X-ray diffraction (XRD) patterns were applied to study the effect of temperature on the crystallization kinetics of α-HH to determine the kinetic parameters. The research results show that the sigmoidal shape of the α-HH crystallization curve follows the Avrami-Erofeev model, which describes the crystallization kinetics of α-HH formation. Applying Arrhenius law in experimental data and model calculations, an apparent activation energy of 124 kJ/mol for nucleation and an apparent activation energy of 810 kJ/mol for growth were obtained. By adjusting the temperature of the solution, the number of α-HH nucleation and growth steps increases, which can effectively increase the DH-α-HH conversion rate in the NaCI solution.

Influence of Cooling Rate on the Size of the Precipitates and Thermal Characteristic of Al-Si Cast Alloys

In this work effect of cooling rate on the size of the grains, SDAS, β phases and thermal characteristic results of Al-Si cast alloys have been described. The solidification process was studied using the cooling and crystallization curve at cooling rate ranging from 0,1 °Cs-1 up to 1 °Cs-1. The main observation made from this work was that when cooling rate is increased the aluminum dendrites nucleation temperature and solid fraction at the dendrite coherency point increases, which implies that mass feeding is extended. In addition to that, it was observed that solidus temperature and size of the β phases decreases when cooling rate increases. Investigations were showed, that the thermal modification could be quantitatively assessed by analysis of the crystallization curve.

Le potentiel chimique d'excès dans des systèmes binaires: acides dicarboxyliques –acides monocarboxyliques ou esters. L'énergie libre d'interéchange entre groupes aliphatiques et carboxyliques

We report experimental data concerning the solid–liquid and liquid–liquid equilibrium temperatures for some binary systems composed of ethyl hexadecanoate and some bicarboxylic aliphatic acids. The crystallization curves of these systems and of others, previously published only in a graphical form, are used in order to obtain the excess chemical potential of the bicarboxylic acid along its crystallization curve. This thermodynamic quantity is compared with that calculated from the theory of group interaction, by means of the areas of the contact surfaces and of three interchange energy parameters.