Constant Rate Thermal Analysis Research Articles

Constant rate thermal analysis study of a trihydrate calcium atorvastatin and effect of grinding on its thermal stability

Constant rate thermal analysis study of a trihydrate calcium atorvastatin and effect of grinding on its thermal stability

Burn Them Right! Determining the Optimal Temperature for the Purification of Carbon Materials by Combustion

A new purification procedure for carbon nanoforms is proposed. It was tested on multiwall carbon nanotubes (MWCNTs) prepared by arc discharge, which is among the most challenging of cases due to the chemical and structural similarity between the MWCNTs and most of the impurities to be removed. Indeed, the various methods for synthesizing carbon nanoforms lead to a distribution of carbonaceous products, such as carbon shells, carbon spheres, fullerenes, and a variety of other species. Thus, many strategies to purify the desired products have been developed. Among the most successful ones, thermal oxidation (combustion) seems particularly efficient. To be successful while preserving a reasonable amount of MWCNTs, the combustion temperature has to be carefully selected. Moreover, the ideal combustion temperature does not only depend on the material to be treated but also on the overall system used to perform the reaction, including the reactor type and the parameters of the gaseous reactant. Typically, the optimization of the purification relies on multiple experiments and analysis of the products. However, to the best of our knowledge, a strategy to determine a priori the most suitable temperature has not been reported yet. We demonstrate here that a thermogravimetric method, namely the constant decomposition rate thermal analysis (CRTA), is particularly well adapted to answer this question. An isothermal treatment based on the results obtained from a CRTA program allowed arc-MWCNTs to be successfully purified from graphenic shells while optimizing the yield of the MWCNTs. This strategy is believed to be valuable not only for purifying MWCNTs but also for the purification of other carbonaceous forms, including new carbon nanoforms.

Development of a high-pressure thermobalance working under constant rate thermal analysis

A thermogravimetric instrument that works at high pressure of different gases has been designed and assembled. The instrument has been devised to work in a temperature range from room temperature to 1000 °C in various controlled pressures of selected gas up to 15 bar, and under conventional rising temperature and constant rate thermal analysis (CRTA) modes. CRTA method allows an intelligent control of the reaction temperature using a feedback system that monitors the mass gain or mass loss of the sample in such a way that the reaction rate is maintained constant all over the process at a preselected value. CRTA method provides a significant advantage for studying processes under high pressure as it reduces heat and mass transfer phenomena that are very relevant under these high-pressure experimental conditions. The thermal oxidation of Ni at 8 bar of pure oxygen has been used for testing the performance of the instrument under both linear heating rate and CRTA conditions.

Constant rate thermal analysis of cobaltoblödite and cupper–kröhnkite at low water vapor pressure 5 hPa

Thermal decomposition mechanisms for cobaltoblodite Na2Co(SO4)2·4H2O and cupper–krohnkite Na2Cu(SO4)2·2H2O have been developed using constant rate thermal analysis technique under controlled residual water vapor pressure above the sample $$ P_{{{\text{H}}_{2} {\text{O}}}} = \, 5\;{\text{hPa}} $$ . The apparent activation energy of each dehydration process was measured by means of two CRTA curves without any presumption to the kinetic low. Correlation between structure and thermal behavior was highlighted. It has shown that dehydration of cobaltoblodite mineral occurs in two steps with formation of Na2Co(SO4)2·1.5H2O and Na2Co(SO4)2 as intermediate and final phases, respectively. The activation energies of both dehydration steps are 167 kJ mol−1 and 107 kJ mol−1, respectively. For cupper–krohnkite mineral, its dehydration occurs in one step leading to Na2Cu(SO4)2. The corresponding activation energy was found equal to 71 kJ mol−1.

A Promising approach to the kinetics of crystallization processes: The sample controlled thermal analysis

Constant Rate Thermal Analysis (CRTA) method implies controlling the temperature in such a way that the reaction rate is maintained constant all over the process. This method allows determining simultaneously both the kinetic parameters and the kinetic model from a single experiment as the shape of the CRTA α-T curves strongly depends on the kinetic model. CRTA method has been developed in the market only for thermogravimetric and thermodilatometric systems and, therefore, its use has been limited until now to the kinetic study of processes involving changes in mass or size of the samples, respectively. To overcome this obstacle, a method has been developed in this work for using the DSC signal for controlling the process rate in such a way that CRTA would be applied to the kinetic analysis of either phase transformations or crystallizations. The advantages of CRTA for performing the kinetics of crystallization processes have been here successfully demonstrated for the first time after selecting the crystallization of zirconia gel as test reaction.

New concept of thermokinetic analysis with artificial neural networks

Inspired by the recent victory of the computer program Alpha Go [1] over the world strongest human Go player and by the ability of this program based on artificial neural networks (ANN) for self-learning, we present the attempt to perform the thermokinetic analysis using ANN models. Various virtual combinations of kinetic parameters and reaction model types were used to generate thermoanalytical signals similar to experimental ones for neural network training. Results prove that trained artificial neural network models can be successfully used for the kinetic analysis. For methodological purposes, various types of input parameters have been considered, showing the weakness of analysis based on the single heating runs along with the high resolution ability of constant rate thermal analysis. The principal capability of neural networks to perform kinetic analysis within wide ranges of activation energy and pre-exponential factor for a number of reaction models has been demonstrated.

Constant rate thermal analysis of a dehydrogenation reaction

The kinetic analysis of constant rate thermal analysis (CRTA) curves obtained for MgH2 dehydrogenation under high vacuum reveals that the reaction obeys first-order kinetics.

Synthesis and thermal behaviour of Na2SO4·MgSO4·4H2O

The thermal behaviour of astrakanite was investigated by conventional thermal analysis techniques in air and by constant rate thermal analysis (CRTA) under residual water vapour pressure above the sample $$ P_{{{\text{H}}_{ 2} {\text{O}}}} = 5 $$ hPa. It has been shown that the pathway of the thermal dehydration depends strongly on the nature of atmosphere above the sample. A correlation between structure evolution and thermal behaviour was developed. The apparent activation energies corresponding to the two dehydration steps of Na2Mg(SO4)2·4H2O were also measured experimentally by means of two CRTA curves and were found equal to 83 ± 7 and 111 ± 8 kJ mol−1, respectively.

Kinetic study of hydromagnesite thermolysis by constant rate thermal analysis

The present work is focused on the kinetic study of hydromagnesite thermal decomposition carried out by constant rate thermal analysis technique at 5 hPa partial pressure. The apparent activation energies were measured experimentally all along the decomposition without any assumption about the rate law of the determining step. Under these conditions the decomposition of hydromagnesite occurs in two steps. The first step is a dehydration which occurs with apparent activation energy of 60 kJ.mol-1 and D4 kinetic model. The second step is essentially decarbonatation, which occurs according to an F1 kinetic model and activation energy equal to 95 kJ.mol-1.

Role of Looping-Calcination Conditions on Self-Reactivation of Thermally Pretreated CO2 Sorbents Based on CaO

The conversion of thermally pretreated CaO along successive carbonation/calcination cycles has been investigated, as affected by looping-calcination conditions, by means of Thermogravimetric Analysis (TGA). Sorbent samples have been subjected in situ to a thermal preheating program based on Constant Rate Thermal Analysis (CRTA) by virtue of which decarbonation is carried out at a low controlled rate, which is able to promote self-reactivation in the first carbonation/calcination cycles. Our observations support a pore-skeleton model according to which solid-state diffusion in the first carbonation stages, which is enhanced by thermal pretreatment, gives rise to a soft skeleton with increased surface area. Yet, the results show that self-reactivation is hindered as looping-calcination conditions are harshened. Increasing the looping-calcination temperature and/or the looping calcination time period favors sintering of the soft skeleton and eventually self-reactivation is precluded. A model is developed that retrieves the main features of multicyclic conversion of thermally pretreated sorbents in the first cycles based on the balance between surface area gain due to promoted solid-state diffusion carbonation and surface area loss due to sintering of the soft skeleton in the looping-calcination stage, which can be useful to investigate the critical looping-calcination conditions that nullify self-reactivation. The proposed model allows envisaging the behavior of the sorbent performance as a function of the pretreatment conditions.

Kinetic studies in solid state reactions by sample-controlled methods and advanced analysis procedures

A comparative study of both conventional rising temperature and sample-controlled methods, like constant rate thermal analysis (CRTA), is carried out after analyzing a set of solid state reactions using both methods. It is shown that CRTA avoids the influence of heat and mass transfer phenomena for a wide range of sample sizes leading to reliable kinetic parameters. On the other hand, conventional rising temperature methods yield α–T plots dependent on experimental conditions, even when using samples sizes smaller than 2 mg. Moreover, it is shown that the discrimination of overlapping processes is dramatically improved using sample-controlled methods instead of conventional heating procedures. An advanced method for performing the kinetic analysis of complex processes from a single CRTA experiment is proposed.

Constant rate thermal analysis for enhancing the long-term CO2 capture of CaO at Ca-looping conditions

Experimental results are reported on the (Ca-looping) multicyclic CO2 capture of CaO and nanosilica/CaO composites derived from Ca(OH)2 and nanosilica/Ca(OH)2 dry mixtures subjected in situ to linear and constant rate thermal analysis (CRTA) preheating programs in either air or air/CO2 atmospheres. By means of CRTA preheating the rates of the reactions taking place during pretreatment are kept at a constant and small value along the entire process. In agreement with a pore skeleton model, previously proposed in the literature for explaining the behavior of natural limestones thermally pretreated, our results suggest that air/CO2-CRTA pretreatment yields a thermally stable hard skeleton of poorly reactive CaO on which a soft skeleton of reactive CaO would be supported. The sorbent subjected to this preheating program exhibits a reactivation in the very first carbonation/calcination cycles, after which CaO conversion decays slowly with the cycle number. In contrast, linearly or air-CRTA preheated sorbents show a significant decrease of CaO conversion within the first cycles. In the latter case, CaO multicyclic conversion fits well to a model where it is assumed that the progressive reduction of surface area as the number of carbonation/calcination cycles is increased obeys to sintering of the preheated sorbent skeleton as it is subjected to repeated calcinations during cycling. In the former case, CaO conversion data conforms to the prediction by a model in which the loss of surface area is mainly due to sintering of a nascent CaO soft skeleton regenerated in the diffusive carbonation phase, which is enhanced by the air/CO2-CRTA pretreatment. As regards the effect of nanosilica, the results indicate that it slows down CaO sintering during pretreatment, which hinders the development of a stable CaO skeleton thus hampering reactivation and stabilization of conversion. On the other hand, as CaO sintering is also lessened during looping calcination, nanosilica is useful to increase the absolute values of CaO conversion.

Generalized master plots as a straightforward approach for determining the kinetic model: The case of cellulose pyrolysis

The thermal degradation of cellulose is a complex reaction and, despite the large amount of work by many investigators during the last decades, the actual understanding of the thermal decomposition kinetics is still very limited. Thus, while several mechanisms have been proposed to describe the process, the real model has not yet been clearly identified. In this paper, a set of experimental curves recorded under different heating schedules, i.e., linear heating rate, isothermal and constant rate thermal analysis (CRTA), has been analyzed using isoconversional and master plots methodology to discriminate the kinetic model followed by the reaction.

Constant rate thermal analysis for thermal stability studies of polymers

This paper explores the relationship between the shapes of temperature–time curves obtained from experimental data recorded by means of constant rate thermal analysis (CRTA) and the kinetic model followed by the thermal degradation reaction. A detailed shape analysis of CRTA curves has been performed as a function of the most common kinetic models. The analysis has been validated with simulated data, and with experimental data recorded from the thermal degradation of polytetrafluoroethylene (PTFE), poly(1,4-butylene terephthalate) (PBT), polyethylene (PE) and poly(vinyl chloride) (PVC). The resulting temperature–time profiles indicate that the studied polymers decompose through phase boundary, random scission, diffusion and nucleation mechanisms respectively. The results here presented demonstrate that the strong dependence of the temperature–time profile on the reaction mechanism would allow the real kinetic model obeyed by a reaction to be discerned from a single CRTA curve.

Quantitative Characterization of Multicomponent Polymers by Sample-Controlled Thermal Analysis

This paper explores the potential of sample-controlled thermal analysis (SCTA) in order to perform compositional analysis of multicomponent polymeric materials by means of thermogravimetric experiments. In SCTA experiments, the response of the sample to the temperature determines the evolution of the temperature by means of a feedback system; thus, what is controlled is not the temperature-time profile, as in conventional analysis, but rather the evolution of the reaction rate with time. The higher resolving power provided by the technique has been used for determining the composition of polymer blends composed of polyvinyl chloride (PVC) and different commercial plasticizers, a system where the individual components have very similar thermal stabilities, thereby rendering useless thermogravimetric experiments run under conventional conditions. Different SCTA procedures, such as constant rate thermal analysis (CRTA), which has received special attention, and high-resolution and stepwise isothermal analysis have been tested, and the results obtained have been compared with linear heating rate technique. It has been proven that CRTA can be used to effectively determine the exact composition of the blend.

ChemInform Abstract: Crystalline Structure and Thermal Stability of Double Strontium and Barium Carbonates.

Abstract The crystalline structures of double salts of composition SrxBa1-xCO3 have been analysed. It has been shown that this series of compounds crystallizes in the orthorhombic system over the entire composition range. Moreover, they conform to Veghard's law. The thermal decomposition of a series of samples of composition SrxBa1-xCO3 (x = 0.2, 0.4, 0.5, 0.6 and 0.8) has been studied by means of the constant rate thermal analysis (CRTA) technique. This method permits monitoring of the vacuum and the sample temperature in such a way that both the pressure in the close vicinity of the sample and the total decomposition rate remain constant over the whole reaction range. The results obtained at a CO2 residual pressure of 4 ×10−5 Torr show that the thermal decomposition of BaSr double salts takes place in a single step, instead of the two step process previously claimed in the literature. The comparative kinetic analysis of both CRTA and TG curves at high vacuum has allowed the reaction mechanism and the activation energy for the process to be established.

Development of a universal constant rate thermal analysis system for being used with any thermoanalytical instrument

The SCTA method implies to control the temperature in such a way that the reaction rate changes with the time according to a function previously defined by the user. Constant Rate Thermal Analysis (CRTA) is one of the most commonly used SCTA methods and implies achieving a temperature profile at which the reaction rate remains constant all over the process at a value previously selected by the user. This method permits to minimize the influence of heat and mass transfer phenomena on the forward reaction. The scope of this work is to develop a universal CRTA temperature controller that could be adapted to any thermoanalytical device. The thermoanalytical signal is programmed to follow a preset linear trend by means of a conventional controller that at the time controls a second conventional temperature programmer that forces the temperature to change for achieving the trend programmed for the thermoanalytical signal. Examples of the performance of this control system with a Thermobalance and a Thermomechanical Analyser (TMA) are given.

Constant transformation rate thermal analysis of HGdP2O7·3H2O

The aim of this work is the optimization of the preparation of anhydrous gadolinium hydrogen phosphate with good fluorescence properties. The products obtained by dehydration of gadolinium hydrogen phosphate trihydrate depend on the conditions chosen for the thermal treatment. For this reason, the dehydration was followed by Constant Rate Thermal Analysis whilst strictly controlling the water vapour pressure above the sample. Intermediate samples, obtained during dehydration were characterised by IR spectroscopy and X-ray diffraction. It has thus been shown that the thermal pathway taken for the dehydration depends on the water vapour pressure above the sample in the region from 10-2to 5 mbar. Under the lowest water vapour pressure (5·10-3mbar), the elimination of the crystallization water is carried out in a continuous way and produces a quasi-amorphous intermediate. Under higher water vapour pressure (5 mbar), well crystallized intermediate products are obtained. The results obtained suggest that the trihydrate contains zeolitic water which confirms a prior structural study.

Direct use of the mass output of a thermobalance for controlling the reaction rate of solid-state reactions

Sample controlled thermal analysis equipment has been developed constituted by an electrobalance in which the mass output (TG signal) is directly used for monitoring the temperature of thermal decomposition reactions under constant rate thermal analysis (CRTA) or stepwise isothermal analysis (SIA) control. The sample weight is programmed to follow a preset linear decrease as a function of the time by means of a conventional controller, that at the time control a second conventional temperature programmer. The CRTA control is achieved by controlling the temperature is such a way that if the mass input is higher than the setpoint, the temperature increases at a predefined heating rate; while if the mass input is lower than the setpoint, the temperature decreases at a predefined cooling rate. The SIA control is achieved by selecting the run–hold command from the menu of the digital input of the temperature programmer. In such a case, the programmed linear heating schedule is in progress while the sample weight is higher than the setpoint and an isothermal dwell is maintained as soon as the weight becomes lower than the setpoint. The direct use of the mass output for the control provides a higher sensitivity for selecting very low values of constant reaction rates than the more conventional methods using the DTG output as control parameter. The thermal degradation of polyvinye chloride (PVC) has been used for checking the behavior of the equipment here developed, showing that the dehydrochlorination of PVC is controlled either by a nucleation and growth of nuclei or by a random scission of the main chain of the polymer.

Synthesis of Silicon Nitride from Carbothermal Reduction of Rice Husks by the Constant‐Rate‐Thermal‐Analysis (CRTA) Method

Silicon nitride (Si3N4) has been obtained from the carbothermal reduction of rice husks by applying the constant‐rate‐thermal‐analysis (CRTA) method. Under this method, the reaction rate of carbothermal reduction can be controlled and at the same time the CO concentration generated is maintained constant at a previously selected value by the user. By using this synthesis technique it has been possible to obtain ceramic powder from rice husks with a determined phase composition and a controlled microstructure. Therefore, rice husks have great potential as a raw material for producing Si3N4 by the CRTA method.