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Isoconversional vs. Model fitting methods

A case study of crystallization kinetics of a Fe-based metallic glass

Journal of Thermal Analysis and Calorimetry
Authors: A. Pratap, T. Lilly Shanker Rao, K. Lad, and Heena Dhurandhar

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.

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certain conversion rate, the slope − E a / R indicate the activation energy. Model-fitting method The model-based kinetic analysis depends on the reaction type and reaction model. The reaction model may take various

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-Elmer diamond model thermal analyzer with different heating rates 1, 3, and 5 K/min ranging from room temperature to 1,173 K using nitrogen gas atmosphere. Model-free methods, Arrhenius, Kissinger, and Flynn–wall methods and model fitting methods, Coats

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Journal of Thermal Analysis and Calorimetry
Authors: Gurdip Singh, A. K. Shrimal, Inder Pal Singh Kapoor, Chandra Prakash Singh, Dinesh Kumar, and Manan S. Mudi

allows the estimation of apparent activation energy independent of the model. This approach indicates that the decomposition of these complexes is not as simple as indicated by model-fitting method. As Fig. 5 shows that the E value changes with extent

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Model-fitting methods are among the first and most popular methods for kinetic description of thermal decomposition of solid. It requires only a single heating rate experiments to calculate the kinetic parameters. Several kinetic models are available in

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strongly emphasized on the fact that kinetics of thermal degradation of a solid are most often multi-step reaction process and should not be described by a single kinetic triplet and therefore, single heating rate based model-fitting methods should be

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methods based on the single-step kinetics approximation, which use analytical forms of the temperature and/or conversion functions, i.e., the model-free and model-fitting methods [ 4 ]. Despite this ability of the NPK method, it has not yet become

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Kinetics of thermal degradation of polymers

Complementary use of isoconversional and model-fitting methods

Journal of Thermal Analysis and Calorimetry
Author: K. Chrissafis

Abstract  

The thermal degradation of polymers has been studied quite extensively using thermogravimetric measurements. For the kinetic description, most of the times single rate heating data and model-fitting methods have been used. Since the thermal degradation of the polymers is a very complex reaction, the choice of a reliable model or a combination of kinetic models is very important. The advantages or the disadvantages of using a single heating rate or multiple heating rates data for the determination of the kinetic triplet have been investigated. Also, the activation energy has been calculated with the isoconversional and model-fitting methods. The reaction model was determined with the model-fitting method. The limits of all these procedures were investigated with experimental data of the thermal degradation of the poly(ethylene adipate) (PEAd).

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Abstract

Pyrolytic characteristics and kinetics of pistachio shell were studied using a thermogravimetric analyzer in 50–800 °C temperature range under nitrogen atmosphere at 2, 10, and 15 °C min−1 heating rates. Pyrolysis process was accomplished at four distinct stages which can mainly be attributed to removal of water, decomposition of hemicellulose, decomposition of cellulose, and decomposition of lignin, respectively. The activation energies, pre-exponential factors, and reaction orders of active pyrolysis stages were calculated by Arrhenius, Coats–Redfern, and Horowitz–Metzger model-fitting methods, while activation energies were additionaly determined by Flynn–Wall–Ozawa model-free method. Average activation energies of the second and third stages calculated from model-fitting methods were in the range of 121–187 and 320–353 kJ mol−1, respectively. The FWO method yielded a compatible result (153 kJ mol−1) for the second stage but a lower result (187 kJ mol−1) for the third stage. The existence of kinetic compensation effect was evident.

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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.

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