Isothermal Thermogravimetric Data Research Articles

Oxidation kinetics of magnesium particles determined by isothermal and non-isothermal methods of thermogravimetric analysis

Knowledge of the oxidation kinetics of magnesium (Mg) particles is important for the development of advanced propulsion and power systems based on combustion of metals. However, reports on the high-temperature oxidation of Mg powders in oxygen are contradictory. In the present work, the oxidation of spherical and non-spherical (flakes) Mg particles in O2 flow was investigated using isothermal and non-isothermal thermogravimetry. Three fractions of the spherical powder were tested, with average sizes about 30, 60, and 300 μm. The oxidation of all powders was complete at temperatures lower than the melting point of Mg, 650 °C. The Friedman analysis of non-isothermal and isothermal thermogravimetric (TG) data has shown that the activation energy exhibits only small fluctuations during a large part of the oxidation process, which indicates that the process can be modeled as a single-step reaction. Model-based analysis has shown that the Avrami-Erofeev model of simultaneous nucleation and growth provides the best fit with the experimental isothermal and non-isothermal TG curves. Scanning electron microscopy of the oxidized spherical particles has shown submicron grains, which is consistent with the Avrami-Erofeev model. The Mampel-Delmon analysis of the isothermal TG data has shown that for the 30 and 60 μm particles, the nucleation is relatively slow, leading to the sigmoidal TG curve, described by the three-dimensional Avrami-Erofeev equation. An increase in the particle size decreases the dimension of this equation, and for the 300 μm particles and flakes, the entire process can be considered as a growth of a non-protective oxide layer. The apparent activation energy of Mg oxidation is likely in the range of 200 – 230 kJ/mol, independently of the particle size and shape.

A Logistic Approach for Kinetics of Isothermal Pyrolysis of Cellulose

A kinetic model is proposed to fit isothermal thermogravimetric data obtained from cellulose in an inert atmosphere at different temperatures. The method used here to evaluate the model involves two steps: (1) fitting of single time-derivative thermogravimetric curves (DTG) obtained at different temperatures versus time, and (2) fitting of the rate parameter values obtained at different temperatures versus temperature. The first step makes use of derivative of logistic functions. For the second step, the dependence of the rate factor on temperature is evaluated. That separation of the curve fitting from the analysis of the rate factor resulted to be very flexible since it proved to work for previous crystallization studies and now for thermal degradation of cellulose.

Performance and Kinetic Evaluation of Synthesized CuO/SBA‐15 Oxygen Carrier for Chemical Looping with Oxygen Uncoupling

Chemical looping oxygen uncoupling (CLOU), a variant of chemical looping combustion (CLC), is one of the most promising combustion technologies for inherent separation and feasible capture of carbon dioxide. Herein, synthesized SBA‐15, a highly ordered mesoporous silica support, is used to host CuO for developing a new variant of an oxygen‐carrier material. Based on thermoanalytical evaluations, a typical CuO/SBA‐15 composition with 36 mass% CuO loading is identified as highly suitable with an average oxygen transport capacity of 3.61%. In addition, extensive studies along with model‐dependent kinetic evaluations are carried out using isothermal thermogravimetric data for the reduction–reoxidation of CuO/SBA‐15. It is observed that the Avrami‐Erofeev random nucleation and subsequence growth model (A2) fitted well for the reduction reaction, while the 3D diffusion model (D3) described suitably the reoxidation reaction in the temperature range of 800–950 °C. Reduction–reoxidation rate data and kinetic information derived from corresponding isothermal conversion curves can contribute immensely to design and establish operational strategies for interconnected CLOU reactors.

The Effect of Menisci on Kinetic Analysis of Evaporation for Molten Alkali Metal Salts (CsNO3, CsCl, LiCl, and NaCl) in Small Cylindrical Containers

Using isothermal thermogravimetric data of alkali metal salts (CsNO3, CsCl, LiCl, and NaCl), we conducted kinetic analysis on atmospheric evaporation to investigate the effect of meniscus on determining the condensation coefficient. In the process of evaporation into an atmospheric gas, molten salt decomposed at the interface between molten salt and an atmospheric gas reacts with chemical compositions of the atmospheric gas to be an equilibrium state. In this atmospheric evaporation, the interface shape of molten salts is affected by the container diameter and the contact angle at the container wall. In the analysis results, the formed concave/convex meniscus led to underestimating the condensation coefficient of molten salts. However, whether the values of the condensation coefficient of molten salts were affected by menisci, the range of the predicted values was still low from 10−3 to 10−5. This result means that the presence of the foreign gas (air and Ar) is a dominant parameter in determining the condensation coefficient of atmospheric evaporation.

Exploiting kinetics to unravel the role of a ZnO diluent in the production of CO via oxidizing Zn particles with CO2

Direct oxidation of pure Zn particles with CO2 is inhibited by an impervious ZnO scale. The presence of a ZnO diluent surface provides a site for an additional heterogeneous reaction of sublimated Zn that allows for a fast and high conversion of Zn. This is relevant to the efficient production of CO by the oxidation of Zn particles produced by the solar thermal dissociation of ZnO that are generally contaminated by the recombined ZnO. The overall reaction mechanism thus involves the sublimation of Zn(g) from the Zn surface, its transport to the ZnO diluent surface, and its subsequent heterogeneous reaction with CO2 on this surface. To elucidate the most relevant of those elementary steps different kinetic models were tested against a broad set of isothermal thermogravimetric data acquired at different temperatures, CO2-concentrations, and initial ZnO contents. The overall rate was found to be controlled by the transport of Zn(g) to the ZnO diluent surface and the reaction of the chemisorbed CO2 either with Zn(g) or with Zn incorporated from gas phase into the ZnO lattice surface sites. Increasing the initial content of ZnO diluent increases the effectiveness of the heterogeneous reaction at the ZnO diluent surface which facilitates the sublimation of Zn and appears to render the ZnO product scale surrounding unreacted Zn more permeable.

The regression of isothermal thermogravimetric data to evaluate degradation Ea values of polymers: A comparison with literature methods and an evaluation of lifetime predictions reliability. Part II

The kinetics of the isothermal degradation in static air atmosphere of four well known polymers, polyethylene (PE), polystyrene (PS), polycarbonate (PC) and poly(methyl methacrylate) (PMMA) was studied by both a long-term (more than three years) experiment at relatively low temperature (423 K) and a set of short-term experiments at higher temperatures. The activation energy (Ea) values of degradation were determined by both the MacCallum and Wilkinson literature methods, and were compared with those obtained through a new very simple method we set up, based on the direct regression of TG mass loss data. About two years ago we published the results concerning PE and PS because their mass losses during long-term experiments were sufficiently high. The long-term degradation experiments were continued until now and in this second part we report the results concerning PC and PMMA. The degradation Ea values calculated from short-term experimental data through the three different methods were in good agreement with each other for both PC and PMMA, thus confirming the general applicability of our simple method for the determination of Ea. The experimental data at lower temperature of PC were not in agreement with those at higher temperatures, thus confirming the low reliability of the kinetic parameters (and then of lifetime predictions) at low temperature determined by experiments at higher temperatures. Partially disagreeing results were obtained for PMMA, which were discussed and interpreted.

The regression of isothermal thermogravimetric data to evaluate degradation Ea values of polymers: A comparison with literature methods and an evaluation of lifetime prediction reliability

The thermooxidative degradation of four well known polymers, polyethylene (PE), polystyrene (PS), polycarbonate (PC) and poly(methyl methacrylate) (PMMA), was carried out in a thermogravimetric (TG) analyser, at various temperatures (in the 473–533 K range), in isothermal heating conditions. The resulting set of experimental TG data was used to determine the apparent activation energy ( E a) of degradation through two isothermal literature methods, as well as through a very simple method we set up, based on the direct regression of the experimental mass loss data, in order to verify the general applicability of our method to various polymers. The results from different methods were in good agreement. Degradation experiments in dynamic heating conditions, which were also performed, gave E a values in good agreement with those in isothermal heating conditions for PS, PC and PMMA, while for PE a large discrepancy was observed, which was discussed and interpreted. The results suggested the general applicability of our method to all polymers, independently on their structure and degradation mechanism. A long-term (about 13 months) isothermal degradation experiment was also carried out with the same polymers at relatively low temperature (423 K). Only PE and PS evidenced appreciable mass loss in the investigated period, but the experimental data were not in agreement with those from the short-term degradations at higher temperatures, thus suggesting different degradation kinetics, and a low reliability of the lifetime predictions for polymers in service based on experiments at higher temperatures.

Identification of the effective distribution function for determination of the distributed activation energy models using Bayesian statistics: Application of isothermal thermogravimetric data

The new procedure for identification of the effective distribution function for determination of the distributed activation energy models, which is based on the Bayesian statistics, has been established. The five different continuous probability functions (exponential, logistic, normal, gamma and Weibull probability functions (the extended set of distributions)) were used for searching the most appropriate reactivity model for two heterogeneous processes: (a) the isothermal reduction process of nickel oxide under hydrogen atmosphere and (b) the isothermal degradation process of bisphenol-A polycarbonate (Lexan) under nitrogen atmosphere. Using the Bayes weights, it was shown that for both processes, the most suitable distributed reactivity model is the Weibull distribution model. The kinetic parameters (ln A, Ea) attached with the Weibull distribution model were calculated for both investigated processes, using three different computational methods (the maximum likelihood method (MLM), the nonlinear regression analysis (NRA), and the posterior mean (the expected value of scale parameter η, E(η)). It was shown that there is an excellent agreement between the values of kinetic parameters calculated by the MLM, NRA, and E(η) approaches. Using Bayes weights, it is possible to discriminate between different probability models and to quantify how well a distribution fits the experimental data. For the formal reactivity model comparison, the use of the (nonnormalized) Jeffreys prior is recommended. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 641–658, 2010

Synthesis, characterization, and kinetic study of Mn(DPM)3 used as precursor for MOCVD

Highly pure Mn(DPM)3 (DPM-2,2,6,6-tetramethyl-3, 5-heptanedionato) complex, usually used as precursor for metal-organic chemical vapor deposition, was synthesized and characterized by elemental analyses, 1H-NMR spectroscopy, mass spectroscopic analysis, thermogravimetry, and differential scanning calorimetry. The thermal decomposition behavior of the complex is sensitive to the ambient gases, and the oxygen atmosphere will accelerate the decomposition and oxidation of the complex. According to mass spectroscopic analysis at elevated temperature, one of the three DPM groups in Mn(DPM)3 will dissociate primarily, following with dissociation of +C(CH3)3 and +OCCH2COC(CH3)3 groups in sequence. It can be interpreted by the difference of metal ion radius. The kinetic parameters of activation energy and frequency factor were computed using different models and thereinto D2 model best adjusted the experimental isothermal thermogravimetric data.

Kinetics of Isothermal Degradation Studies in Adhesives by Thermogravimetric Data: Effect of Hydrophilic Nanosilica Fillers on the Thermal Properties of Thermoplastic Polyurethane-Silica Nanocomposites

The kinetics of thermal degradation process on thermoplastic polyurethanes (TPUs) adhesives with hydrophilic nanosilicas was studied using isothermal thermogravimetric analysis within the temperature range from 360 degrees C to 460 degrees C in nitrogen. The effect of nanosilicas with different hydrophilic degree in the thermoplastic polyurethanes was investigated. It was found that the thermoxidative degradation of polyurethane-silica nanocomposites takes place in one step. An analysis of the isothermal methods to evaluate kinetic parameters of decomposition of solids from isothermal thermogravimetric data is presented. Patents WO07146353A2 and US20070292623A1 have some relevant information about the topic develop in this study, because the principle in both cases relies on the interactions between reactive groups in the polymer (TPUs) and the silanol in the silica nano-particles.

Identification of the effective distribution function for determination of the distributed activation energy models using the maximum likelihood method: Isothermal thermogravimetric data

The new procedure for identification of the effective distribution function for determination of the distributed activation energy models, which is based on use the maximum likelihood method (MLM), was established. The five different continuous probability functions (exponential, logistic, normal, gamma, and Weibull probability functions (the extended set of distributions)) were used for searching the best reactivity model for two heterogeneous processes: (a) the isothermal reduction process of nickel oxide under hydrogen atmosphere and (b) the isothermal degradation process of bisphenol-A polycarbonate (Lexan) under nitrogen atmosphere. The MLM showed that for both processes, the most suitable reactivity model represents the Weibull distribution model. It was concluded that the values of Arrhenius parameters (ln A and Ea), evaluated from the Weibull distribution model, represent the effective kinetic values for both considered processes. This procedure enables identification the suitable distribution model for considered process only from the experimental data (based on the shapes of obtained integral kinetic curves), and this fact represents the advantage of established analysis. The established mathematical procedure, which is based on the MLM, can be applied as the preliminary analysis for evaluating the distribution of activation energies for complex heterogeneous processes. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 27–44, 2009

Thermogravimetry as a screening tool for the estimation of the vapor pressures of pure compounds

A simplified approach was developed to estimate the vapor pressure of pure compounds from experimental data obtained by isothermal thermogravimetric (TG) analysis. A numerical procedure was developed to estimate the Antoine parameters of the substance by the analysis of isothermal TG data. The results of the experimental validations carried out evidenced that at least a preliminary estimation of vapour pressures of pure substances by the analysis of TG data is possible. The limited time and the reduced amounts of sample required for the experimental runs make the technique attractive with respect to the conventional and more accurate techniques for vapor pressure assessment.

Kinetic study of dipivaloylmethane by ozawa method

The lanthanidic complexes of general formula Ln(C11H19O2)3 were synthesized and characterized by elementary analysis, infrared absorption espectroscopy, thermogravimetry (TG) and differential scanning calorimetry (DSC). The reaction of thermal decomposition of complexes has been studied by non-isothermal and isothermal TG. The thermal decomposition reaction of complexes began in the solid phase for Tb(thd)3, Tm(thd)3 and Yb(thd)3 and in the liquid phase for Er(thd)3 and Lu(thd)3, as it was observed by TG/DTG/DSC superimposed curves. The kinetic model that best adjusted the experimental isothermal thermogravimetric data was the R1 model. Through the Ozawa method it was possible to find coherent results in the kinetic parameters and according to the activation energy the following stability order was obtained: Tb(thd)3>Lu(thd)3>Yb(thd)3>Tm(thd)3>Er(thd)3

Some remarks on the use of isothermal thermogravimetric data for the evaluation of kinetic parameters. Applications for degradation of polymeric materials

AbstractA critical analysis of the isoconversional methods for evaluating kinetic parameters of decomposition of solids from isothermal thermogravimetric data is presented. An isoconversional integral method to evaluate the activation energy is suggested. This method allows removing the errors due the correction of the degradation time by subtracting the induction period to onset of the main reaction (also including the time required to heat the sample to the temperature at which the isotherm is recorded). This procedure was used to study the degradation of two series of polymeric materials, (a) poly(vinyl chloride)/acrilonitrile–butadiene–styrene (PVC/ABS) blends and (b) poly(vinyl chloride)/chlorinated poly(ethylene) (PVC/CPE) blends. The values obtained for the activation energy are in fairly good agreement with those obtained from the Prout–Tompkins model in case (a) and from nonisothermal data in case (b). © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 355–360, 2003

Kinetics of calcium carbonate decomposition

AbstractExperiments on thermal decomposition of calcium carbonate were carried out in a thermogravimetric analyser under non‐isothermal conditions of different heating rates (10 to 100°C/ min). A new technique for determining the kinetic parameters from non‐ isothermal thermogravimetric data was described. The activation energy and frequency factors were determined from the proposed method and also by the widely used Coats and Redfern method. The kinetic compensation effect between the activation energy and frequency factors obtained from both the methods were found to be very consistent and are in very good agreement with the literature values. The activation energy and frequency factors were also determined from isothermal experiments in the temperature range from 680 to 875°C. The activation energy and frequency factors determined from isothermal data using initial rate method were also found to be in very good agreement with the above results. It is also found that the kinetic parameters determined by isothermal analysis were consistent with the values determined by non‐isothermal analysis.

Use of databases in thermal analysis

There have recently become available many commercial programs for microcomputers which can be utilized to manage databases (DBs). Such relational DB managers provide various mathematical, geometric, statistical, financial, string and error-trapping functions. Many of these functions are also available in spreadsheets which have been employed by the present author to determine successfully various kinetic parameters from thermogravimetric (TG) data, such as rate constants ( k) and reaction order ( n). In view of the preceding, the aim of the present study was to employ a DB script (just as macros were utilized in spreadsheets) to estimate kinetic parameters from TG data. Thus, a DB script was used which contains an algorithm that employs an iterative procedure along with isothermal TG data to determine values of k and n. The powerful Paradox 3 DB was utilized since it possesses the extensive paradox application language (PAL) which can be used to write scripts pertaining to the determination of kinetic parameters from TG data.

Interpreting isothermal thermogravimetric data of complex reactions: Application to cellulose pyrolysis at low temperatures

Isothermal thermogravimetric data are most informative when plotted in an unusual way, namely, as -d M′/dt vs. M′, where M′ is dimensionless mass and t is time. The resulting ‘DMM plots’ are particularly useful in the case of complex reaction schemes, as in the pyrolysis of lignocellulosic materials. DMM curves have been used previously, together with multistage isothermal thermogravimetry, to show that the pyrolysis of woods and barks can be interpreted as the independent volatilization of several fractions. The volatilization of most of these fractions follows first-order kinetics. The fraction identified as cellulose, however, shows a complex DMM pattern which recurs in various materials, including pure cellulose (filter paper, cotton wool). In this work, various reaction schemes were tried to model the DMM curves generated by cellulose pyrolysis. The simplest one consists of three consecutive first-order reactions. The first reaction produces about 30% of the total volatiles. The second reaction produces little or no volatiles: it is probably a rearrangement or activation of the solid, before further reaction. The third reaction produces most of the total volatiles.

Evaluation of photodegradability of low density polyethylene by thermoanalytical technique

Accelerated photoageing of low density polyethylene film has been studied using high energy UV radiation in the presence of air. The change in molecular structure by chain scission and the formation of oxygenated and unsaturated groups due to photooxidative degradation is followed by a decrease in viscosity average molecular weight (M̄v) and an increase in density. The gel content is formed by recombination of macroalkyl and alkoxy radicals. Degradability of aged samples has been evaluated by TG and DSC measurements. Dynamic and isothermal thermogravimetric data show the destabilization behaviour of aged samples. DSC data show that the area of the melt endotherm increases with crystallinity increase due to the formation of a more ordered structure by rearrangement of smaller molecular chains and also due to chemicrystallization. Internal plasticization due to smaller molecules causes broadening of the endotherm towards lower temperatures. The change in exotherms at 503 and 565 K shows the increase in degradability of polyethylene during UV ageing.

Difference equation method of analysing isothermal thermogravimetric data

Difference equation method has been suggested to evaluate the kinetic parameter of the degradation of polymers under isothermal condition. The method is simple and applicable to degradation process for any value ofn, the reaction order. By using one simple method all the kinetic parameters cannot be obtained in other methods only by difference equation.

The kinetics of olation of some hydroxoaquotetrammine-cobalt(III) complexes in the solid state

The solid-state kinetics for the olation reactions of [Co(NH 3 4(OH)(H 2O)]X 2 (where X  Cl −, Br −, or 1 2 SO 2− 4 were determined by several different methods using dynamic and isothermal thermogravimetric data. For the reduced-time plot method, E values were 20, 43, and 25 kcal mol −1 for the chloride-bromide, and sulfate complexes, respectively. For the Jacobs and Kureishy method, E values of 21, 37, and 17 kcal mol −1, were obtained for the above three complexes, respectively. A possible reaction pathway is suggested for the olation reaction.