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  • Author or Editor: B. Tiţa x
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Abstract  

Thermal analysis is one of the most widely used methods for studying the solid state of pharmaceutical substances. TG/DTG and DSC curves provide important information regarding the physical properties of the pharmaceutical compounds (stability, compatibility, polymorphism, kinetic analysis, phase transitions etc.). The purpose of a kinetic investigation is to calculate the kinetic parameters and the kinetic model for the studied process. The results are further used to predict the system’s behaviour in various circumstances. A kinetic study regarding the diazepam, nitrazepam and oxazepam thermal decomposition was performed, under non-isothermal and isothermal conditions and in a nitrogen atmosphere, for the temperature steps: 483, 498, 523, 538 and 553 K. The TG/DTG data were processed by three methods: isothermal model-fitting, Friedman’s isothermal-isoconversional and Nomen-Sempere non-parametric kinetics. In the model-fitting methods the kinetic triplets (f(α), A and E a) that defines a single reaction step resulted in being at variance with the multi-step nature of diazepines decomposition. The model-free approach represented by isothermal and non-isothermal isoconversional methods, gave dependences of the activation energies on the extent of conversion. It is very difficult to obtain an accord with the similar data which resulted under non-isothermal conditions from a previous work. The careful treatment of the kinetic parameters obtained in different thermal conditions was confirmed to be necessary, as well as a different strategy of experimental data processing.

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Abstract  

The crystallization mechanism of the glass-ceramics obtained from Romanian (Şanoviţa) basalt in the presence of 3 and 5% CaF2 as nucleation agent has been investigated under non-isothermal conditions using DTA technique. The activation energies of the crystallization processes were calculated using the Kissinger-Akahira-Sunose, Ozawa-Flynn-Wall, Starink and Tang isoconversional methods. The monotonous decreases in the activation energy (E a) with the crystallized fraction (α) confirms the complex mechanism of the glass-ceramics crystallization process. It has been proved that the Johnson-Mehl-Avrami model cannot be applied for the studied glass-ceramics crystallization process.

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