Dynamic thermal analysis of solid-state reactions

There are many reactions of interest in which one or more of the reactants belong to some solid phases. Modern thermoanalytical instruments can conveniently provide reaction kinetic data of high precision and accuracy, from which the underlying activation energyE may be derived in principle. Unfortunately, no best method yet exists for the derivation when the data have been collected with a programmed linear increase in sample temperature, unlike the case of isothermal measurements, which however suffer from experimental limitations [1]. Here we propose a method for extractingE from non-isothermal data, that promises general validity.     

Applications of thermal analysis in some solid-state reactions

TG-DTG-DTA has been used to characterize various isomers of CoCl₂·2(CH₃C₆H₄NH₂). Thermal analysis is further used to analyse the binary mixtures of these isomers. DTA recorded after different elapsed times follows the progress of reaction between cobalt chloride and benzocaine where progressively small endotherms are associated with starting materials. The application of thermal analysis to following the solid-solid reactions between metal acetates and 8-hydroxyquinoline was highlighted. The stoichiometry of such reactions was confirmed from the decrease in intensity of an endotherm as one increases the stoichiometry.Thanks are due to Drs. Keshav Chander, K. Kaur, Rajinder Singh and G. S. Chopra for helpful discussions and experimental work.     

Thermal studies on some reactions in the solid-state

The solid-state thermal reactions of 2,6-diamino-4-chloropyrimidine, 3,3-dimethoxybenzidine and of thiosemicarbazide with each of 2-hydroxy-1-naphthaldehyde and 2,4-dihydroxybenzaldehyde in a mole ratio of 1∶1 produced (1+1) condensation products (‘half units’). These solid state reactions have been studied by means of differential thermal analysis (DTA). The products of the reactions were identified using elemental analysis and IR spectroscopy. Some kinetic parameters such as orders and activation energies of the reactions were evaluated form the DTA curves.     

What theoretical and/or chemical significance is to be attached to the magnitude of an activation energy determined for a solid-state decomposition?

This critical survey argues that the theory, conventionally used to interpret kinetic data measured for thermal reactions of initially solid reactants, is not always suitable for elucidating reaction chemistry and mechanisms or for identifying reactivity controls. Studies of solid-state decompositions published before the 1960s usually portrayed the reaction rate as determined by Arrhenius type models closely related to those formulated for homogeneous rate processes, though scientific justifications for these parallels remained incompletely established. Since the 1960s, when thermal analysis techniques were developed, studies of solid-state decompositions contributed to establishment of the new experimental techniques, but research interest became redirected towards increasing the capabilities of automated equipment to collect, to store and later to analyze rate changes for selected reactions. Subsequently, much less attention has been directed towards chemical features of the rate processes studied, which have included a range of reactants that is much more diverse than the simple solid-state reactions with which early thermokinetic studies were principally concerned. Moreover, the theory applied to these various reactants does not recognize the possible complexities of behaviour that may include mechanisms involving melting and/or concurrent/consecutive reactions, etc. The situation that has arisen following, and attributable to, the eclipse of solid-state decomposition studies by thermal analysis, is presented here and the consequences critically discussed in a historical context. It is concluded that methods currently used for kinetic and mechanistic investigations of all types of thermal reactions indiscriminately considered by the same, but inadequate theory, are unsatisfactory. Urgent and fundamental reappraisal of the theoretical foundations of thermokinetic chemical studies is now necessary and overdue. While there are important, but hitherto unrecognized, delusions in thermokinetic methods and theories, an alternative theoretical explanation that accounts for many physical and chemical features of crystolysis reactions has been proposed. However, this novel but general model for the thermal behaviour and properties of solids has similarly remained ignored by the thermoanalytical community. The objective of this article is to emphasize the now pressing necessity for an open debate between these unreconciled opinions of different groups of researchers. The ethos of science is that disagreement between rival theories can be resolved by experiment and/or discussion, which may also strengthen the foundations of the subject in the process. As pointed out below, during recent years there has been no movement towards attempting to resolve some fundamental differences of opinion in a field that lacks an adequate theory. This should be unacceptable to all concerned. Here some criticisms are made of specific features of the alternative reaction models available with the stated intention of provoking a debate that might lead to identification of the significant differences between the currently irreconciled views. This could, of course, attract the displeasure of both sides, who will probably criticise me severely. Because I intend to retire completely from this field soon, it does not matter to me if I am considered to be ‘wrong’, if it contributes to us all eventually agreeing to get the science ‘right’.     

使固态化学反应100%完成的方法

无溶液形成的固态化学反应具有“一旦发生,就必完全”的特质,但因为固体的非流动性,它常常会因严重的传质困难而有始无终。为了克服这个困难,作者提出了少溶剂固态化学反应(LSR)的概念,即通过使用少量的溶剂赋予固体流动性,使反应物固体部分溶解,再使用搅拌反应器解决其“三传”问题,使之完全反应,从而达到使一些化学工业过程绿色化的目的。本文用平衡热力学数值计算的方法图解了几种代表性LSR的Gibbs自由能随反应进度的关系,进而指出：(ⅰ) LSR的中间阶段如同没有溶液形成的固态化学反应,两端像溶液化学反应,因此,LSR虽继承了混合自由能导致的化学平衡,但其平衡点比溶液化学反应更接近100%完全。特别地,在LSR即将结束时,通过逐步移除溶剂以使反应物和产物固体持续存在而延长中段,能推动反应达到100%完全;(ⅱ)对于连串反应,LSR仍有可能通过控制反应物的物质的量之比来获得中间产物;(ⅲ)对于Δ_rG_m^Θ略小于零的反应,可以通过使用较多的溶剂跨越可能的早期平衡态,或者使用大量的产物固体“种子”使所有的产物一直与其饱和溶液平衡,最终通过逐渐蒸发溶剂推动反应进行到底;(ⅳ) LSR的反应速度与反应物或产物的溶解度关系不大,但与反应物的溶解速度、化学反应速度和产物的结晶速度紧密相关。本文还讨论了使一些竞争反应、非自发反应进行到底的方法,获得高度的反应控制能力。     

The problem of feed-back in solid-state chemistry

The paper deals with some of the assumtions and limitations of classical models used in thermal analysis kinetic studies of solid-state rections.

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Zusammenfassung Vorliegende Arbeit beschäftigt sich mit einigen Annahmen und Grenzen von klassischen≓ Modellen, die in thermoanalytischen kinetischen Studien von Feststoffreaktionen verwendet werden.

     

On the possibilities of theoretical analysis of kinetics of the thermal decomposition of solids

The main reasons for changes in the environment surrounding us are discussed on the basis of thermodynamics of irreversible processes. Subsequently, relations between thermodynamics of irreversible processes and chemical kinetics are shown, then the possibilities of theoretical determination of rate constants on the framework of the modified RRKM theory are presented. These latter considerations are supplemented by a discussion concerning the possibilities of determining the activation barriers and structural changes (necessary to account for entropy changes upon reaction) in molecules kept on the surface of crystalline phases by combination of quantum chemistry methods for isolated molecules with those reflecting the influence of the environment (i.e. interaction within the lattice). Finally, the future of theoretical methods in examining the reactivity of solid state systems is briefly discussed.     

Application of thermal analysis to study smelting reduction kinetics

A thermal analysis study on the reduction of iron oxide rich slags under different conditions is presented in this paper. The effects of important process variables such as time, temperature, lime-silica ratio, FeO level in slag etc. are discussed. It is shown that the mechanism of reduction by externally added graphite is different from that by a carbon saturated bath although the activation energy values are similar.     

On the thermal stability and thermal decomposition kinetics of some mono- and disubstituted furan-2,3-dions

The thermal behaviour of some compounds derived from 5-phenylfuran-2,3-dione was studied. The thermoanalytical data relating to the decomposition steps and intermediates were completed with mass spectrometric analysis and infrared spectroscopy results. For some of the investigated reactions, the kinetic and structural data correlated satisfactorily.     

Kinetic study on thermal decomposition of the high-temperature superconductor in vacuum microbalance

In this paper the theoretical approach and applications of Cahn ultramicrobalance to kinetic study on the thermal decomposition of the high-temperature Y₁Ba₂Cu₃O_7-x superconductor are presented. Thermogravimetric in situ measurements of oxygen loss from Y₁Ba₂Cu₃O₆ samples heated isothermally in a relatively high dynamic vacuum were performed with a Cahn RG electrobalance. Single-phase orthorhombic samples of composition Y₁Ba₂Cu₃O_7-x (highest oxygen content) were synthesized from stoichiometric (1:2:3) mixtures of high-purity Y₂O₃, BaCO₃ and CuO. The original 1:2:3 mixture was prepared by the two-stage procedure described earlier. The crystal structure of the sample in the original orthorhombic phase was controlled by the X-ray powder method (CuK_α radiation) using a Stadi P Stoe diffractometer with a position-sensitive detector. Activation energy is estimated from appropriate Arrhenius plots. This revised version was published online in July 2006 with corrections to the Cover Date.     

Improved version of Doyle integral method for nonisothermal kinetics of solid-state reactions

An improved version of Doyle integral method for the determination of the kinetic parameters from nonisothermal thermoanalytical data has been presented. The relative errors involved in the activation energy and frequency factor determined from Doyle integral method and its improved integral method have been estimated. The results have shown that the precision of the improved version of Doyle integral method for the determination the kinetic parameters (including the activation energy and frequency factor) is much higher than that of Doyle integral method.     

Effect of mechanical grinding on the reaction pathway and kinetics of the thermal decomposition of hydromagnesite

Effect of mechanical grinding of hydromagnesite on the reaction pathway and kinetic behaviors of the thermal decomposition process was investigated by means of thermoanalytical techniques, together with crystallographic and morphological measurements. A crystalline hydromagnesite, the as-received sample, was decomposed in two distinguished mass loss steps of overlapped dehydration-dehydroxylation and dehydroxylation-decarbonation via an amorphous intermediate of carbonate compound. Thermal decomposition of an amorphous hydromagnesite, obtained by mechanical grinding of the as-received sample, was characterized by three well-separated decomposition processes of dehydration, dehydroxylation and decarbonation. The kinetic behaviors of the respective decomposition steps were estimated separately using a mathematical deconvolution of the partially overlapped reaction steps. From the formal kinetic analyses of the respective reaction processes, it was revealed that the dehydration and dehydroxylation processes indicate the decelerate rate behaviors controlled by diffusion, while the rate behavior of nucleation limited type is predominant for the decarbonation process.     

Accommodation of the actual solid-state process in the kinetic model function

The degree of coordination between the kinetic information from the thermonalytical measurements and the kinetic theory of the solid-state reactions was investigated through the microscopic study of the thermal dehydration of several inorganic salt hydrates. An accommodation function was applied to the conventional kinetic model functionsf(α), in an attempt to reduce the disagreement between the actual process and the idealized one assumed in formulatingf(α). The significance of the non-integral kinetic exponent in the kinetic model function was discussed with its physico-chemical meanings. In celebration of the 60^th birthday of Dr. Andrew K. Galwey     

The thermal degradation kinetics of dextran and pullulan

The effect of molar mass and, in the case of dextran, of the degree of branching on the thermal degradation kinetics of dextran and pullulan was studied in the presence and absence of oxygen. Although the initial mass loss of the dextran samples occurred at higher temperatures than that of the pullulan samples, the overall thermal degradation activation energies were lower for dextran than for pullulan. In the case of dextran the thermal stability was found to decrease with molar mass and degree of branching. The molar mass of pullulan, in the range of 10⁴ to 10⁵ g/mol, appeared to have no significant influence on the thermal characteristics of the samples.     

New approximations of the temperature integral for nonisothermal kinetics

The accuracy and scope of application of previously reported approximations of the temperature integral were evaluated. The exact solution was obtained independently by solving the temperature integral numerically be Simpson's rule, the trapezoidal rule and the Gaussian rule. Two new approximations have been proposed: $$\begin{gathered} P(X) = e^{ - x} (1/X^2 )(1 - 2/X)/(1 - 5.2/X^2 ) \hfill \\ P(X) = e^{ - x} (1/X^2 )(1 - 2/X)/(1 - 4.6/X^2 ) \hfill \\ \end{gathered} $$ whereX=E/RT. The first equation gives higher accuracy, with a deviation of less than 1% and 0.1% from the exact solution forX≥7 andX≥10, respectively. The second equation has a wider scope of application, with a deviation of less than 1% forX≥4 and of less than 0.1% forX≥35.     

Simultaneous thermal analyses for thermal dissociation studies on ZrO2

The behaviour of zirconium oxide ZrO₂ during its heating in the temperature range 300–2670 K under high-vacuum conditions and at different heating rates has been studied.The experiments were performed with a Swiss made Universal Mettler thermoanalyser equipped with a super-high-temperature furnace SHT. Volatile products were analyzed simultaneously by means of a Balzers QMG-101 mass spectrometer.An analysis of the TG, DTG, DTA and EGA curves recorded simultaneously has shown the complex character of the studied process. The elementary steps of the overall process were defined and taken into consideration. The mechanism of the thermal dissociation reaction of ZrO₂ as function of the hearing rate was discussed too.

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Zusammenfassung Im Temperaturintervall von 300 K bis 2670 K wurde im Hochvakuum und bei verschiedenen Aufheizgeschwindigkeiten das thermische Verhalten von Zirkoniumoxid ZrO₂ untersucht.Die Experimente wurden mit Hilfe eines schweizer Universal Mettler Thermoanalysators, ausgerüstet mit einem SHT Super-Hochtemperatur-Ofen, ausgeführt. Flüchtige Produkte wurden simultan dazu mit einem Massenspektrometer QMG-101 der Balzers-Company analysiert.Die Auswertung der durch aufgezeichneten TG-, DTG-, DTA- und EGA-Kurven zeigt den komplexen Charakter des untersuchten Prozesses. Die Elementarschritte der Gesamtreaktion wurden definiert und berücksichtigt. Weiterhin wurde der Mechanismus der Thermodissoziation von ZrO₂ auch in Abhängigkeit von der Aufheizgeschwindigkeit diskutiert.

     

Preparation of cobalt and nickel complexes of 8-hydroxyquinoline with nanobelt structure via one-step,low-heating,solid-state reactions

Two nano-structural complexes, bis-(8-hydroxyquinoline) cobalt and bis-(8-hydroxyquinoline) nickel, have been prepared by one-step, low-heating, solid-state reaction, a simple, safe, economical and environmentally-friendly method. TEM and SEM images show that the complexes are composed of nanobelts with width ranging from 100 to 300?nm and a length of up to 1?µm. The technique offers a new way for fabricating coordination compounds with one-dimensional nanostructure.     

Thermal reactions of solid-state amorphized borates and borate glasses

The properties and course of phase transition of amorphous borates obtained by solid-state thermal and mechanical treatment of hydrated borates and melt quenched glasses with the compositions of these borates were studied. Processes of structural relaxation and crystallization were considered.It was found that different methods of preparation of amorphous borates and the differences in their structural ordering resulting from these methods affect the rate and the mechanism of transformations occurring during their heating.Support for this work was provided by the Polish Committee of Scientific Research (KBN) — grant No. 3 P407 034 06.     

Fitting isoperibolic calorimeter data for reactions with pseudo-first order chemical kinetics

The non-linear least squares fitting of the chemical heat flow and the reactor temperature are compared for reactions with pseudo-first order chemical kinetics carried out in an isoperibolic calorimeter operating quasi-isothermally. Both methods give very similar results for the reaction rate constant and enthalpy of reaction but fitting the reactor temperature appears to have some advantages especially when there is an enthalpy of mixing of the reagents.     

氢氧化镁分解动力学的研究

以硼泥为原料与硫酸反应制备出七水硫酸镁,以氢氧化钠为沉淀剂制备出符合标准HG/T 3607-2000的氢氧化镁.利用XRD,SEM和TEM对氢氧化镁进行了表征,DTA-TG对氢氧化镁的热分解动力学进行了研究.XRD结果表明:制备粉体为单一Mg(OH)2.SEM和TEM结果表明:样品为片状或针状纤维,片直径大小不一,在20～50 nm之间,针状纤维形状不规则,大小不一致,长度在20～100 nm之间.利用Kissinger法和Ozawa法计算出的氢氧化镁热分解反应活化能分别为135.14和141.61 kJ·mol-1.利用Coats-Redfern法和Dolye法判断氢氧化镁热分解反应机理函数为A1.5.