Search Results
Abstract
A differential isoconversional non-linear procedure for evaluating activation energy from non-isothermal data is suggested. This procedure was applied to model reactions (simulations) and to the dehydration of CaC2O4⋅H2O. The results were compared with those obtained by other isoconversional methods.
Abstract
In the case of a complex mechanism of two parallel independent reactions, peak maximum evolution methods and model-fitting methods give only a mean value of the kinetic parameters, while isoconversional methods are useful to describe the complexity of the mechanism. Isothermal and non-isothermal isoconversional methods can be used to elucidate the kinetics of the process. Nevertheless, isothermal isoconversional methods can be limited by restrictions on the temperature regions experimentally available because of duration times or detection limits.
Abstract
Transformation diagrams are one of the most reliable ways to predict the thermal behavior of the materials. In this work, the crystallization process of several metallic glasses and polymer materials was analyzed and two isoconversional methods are applied to perform kinetic analysis of non-isothermal heating/cooling and isothermal processes. Moreover, the diagrams were constructed from modeling experimental data without the knowledge of the kinetic model. There was a good agreement between experimental data and the calculated curves, which verifies the reliability of the method and the validity of the rate constant model description.
Isoconversional vs. Model fitting methods
A case study of crystallization kinetics of a Fe-based metallic glass
Abstract
The crystallization of metallic glasses has been studied quite extensively using differential scanning calorimetry (DSC) technique. Most methods rely on isokinetic hypothesis for the kinetic analysis of crystallization for which the choice of a reliable model is very important. Due to inherent uncertainty in the determination of kinetic parameters, the model-free isoconversional analytical techniques were proposed. However, these isoconversional methods are scarcely used for metallic glasses. In the present work, the crystallization kinetics of Fe67Co18B14Si1 metallic glass through both isoconversional and isokinetic methods has been investigated and attention has been focused on the relative applicability of the two methods.
Abstract
The thermal behavior under isothermal conditions of some ammonia nitrate, ammonia phosphates and calcium phosphates mixtures with added micronutrients was studied. In order to establish the variation of activation energy (E) vs. conversion (α), the TG data were interpolated with spline functions, followed by numeric derivation. Using the so determined reaction rate the Friedman differential-isoconversional method was applied. A dependence of the activation energy vs. conversion was observed, meaning a many-step reaction. Therefore a procedure based on the compensation effect (and suggested by Budrugeac and Segal) was applied.
Summary The thermal behavior of KH2PO4, NaH2PO4 and Na2HPO4 under non-isothermal conditions using TG method with different heating rates was studied. The values of the reaction rate were processed by means of Friedman’s differential-isoconversional method. A dependence of the activation energy vs. conversion was observed. Therefore a procedure based on the compensation effect (suggested by Budrugeac and Segal) was applied. A less speculative data processing protocol was offered by the non-parametric kinetics method suggested by Serra, Nomen and Sempere. Three steps were observed by non-isothermal heating: a dehydration, a dimerization and a polycondensation. The differences in the intimate reaction mechanism are determined by the initial number of water molecules.
Summary Due to the criticism of the non-isothermal kinetic at a single heating rate, in the last period, data obtained at different heating rates are processed by means of elevated methods like Friedman’s (FD) differential-isoconversional method or the one suggested by Budrugeac and Segal (BS). The non-parametric kinetics (NPK) method, suggested by Serra, Nomen and Sempere offers two major advantages: the possibility of separation of two or more steps of a complex decomposition reaction; and the possibility of discrimination between the conversion, with regard to the temperature functions of a rate equation. Comparative data of FD, BS and modified version of NPK method are presented for decomposition of three compounds used as polyisocyanate stabilizer.
Summary Many years ago, thermal analysis earned its place as a current instrumentation technique in assisting/solving the analytical problems of pharmaceuticals. A relative new trend is the study of the thermal stability of food additives in connection with the molecular structure. The studied compounds were: natrium and potassium glutamate, respectively natrium, potassium and calcium benzoate. The thermogravimetric data (TG) were obtained in dynamic nitrogen atmosphere, with open Pt crucible and heating rates of 5, 7, 10 and 12 K min-1, using a Perkin-Elmer TGA7 equipment. In order to estimate the non-isothermal kinetic parameters, the Friedman's differential-isoconversional method and the method suggested by Budrugeac and Segal (based on the compensation effect) were used. A variation of the activation energy vs. conversion was observed by using Friedman's method. The discrimination between the different reaction steps was performed by the non-parametric kinetic method, suggested by Sempere, Nomen and Serra. This is due to a complex process. The thermal stability data are very important for avoiding a possible misuse by processing of the studied food additives.
