Effect of γ-irradiation on the thermal decomposition of strontium bromate

The effect of irradiation on the thermal decomposition of strontium bromate has been studied by dynamic thermogravimetry. Irradiaton enhances the decomposition and decreases the energy of activation but does not change the mechanism of decomposition.     

Effect of metal oxide catalysts on thermal decomposition of potassium bromate

Kinetic studies on the effect of metal oxide catalysts, (25% w/w) such as Cr₂O₃, CuO, MnO₂, Al₂O₃ and TiO₂, on the thermal decomposition of KBrO₃ have been carried out. The kinetic parameters of the catalysed and uncatalysed decompositions were calculated by the Freeman-Carroll, Coats-Redfern and Horowitz-Metzger methods. It has been found that Al₂O₃ is almost as good a catalyst as any other oxide used unlike in the thermal decomposition of KClO₃. In the case of TiO₂ there was an increase in the activation energy of decomposition. Of the other oxides Cr₂O₃ underwent reaction and was converted to dichromate and so the parameters of the reaction were not calculated. The effect of varying amounts of Al₂O₃ on the thermal decomposition was also studied.

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Zusammenfassung Kinetische Untersuchungen bezüglich der Wirkung von Metalloxid-katalysatoren z. B. 25 Gew. %, Cr₂O₃, CuO, MnO₂, Al₂O₃ und TiO₂ auf die thermische Hersetzung von KBrO₃ wurden durchgeführt. Die kinetischen Parameter der katalysierten und nichtkatalysierten Zersetzungen wurden mit den Methoden von Freeman-Carroll, Coats-Redfern und Horowitz-Metzger errechnet. Es wurde festgestellt, dass — im Gegensatz zur thermischen Zersetzung von KClO₃-Al₂O₃ ein beinahe so guter Katalysator ist wie jedes andere angewandte Oxyd. Im Falle von TiO₂ trat eine Erhöhung der Aktivierungsenergie der Zersetzung auf. Von den anderen Oxiden trat Cr₂O₃ in Reaktion und wurde zu Dichromat umgesetzt, deswegen wurden diese Reaktionsparameter nicht berechnet. Die Wirkung verschiedener Mengen von Al₂O₃ auf die thermische Zersetzung wurde ebenfalls untersucht.

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Résumé Des études cinétiques sur l'influence d'oxydes métalliques tels que Cr₂O₃, CuO, MnO₂, Al₂O₃ et TiO₂ comme catalyseurs (25 % en poids) de la décomposition thermique de KBrO₃ ont été effectuées. Les paramÊtres cinétiques des réactions de décomposition catalysé es ou non ont été calculés par les méthodes de Freeman-Carroll, Coats-Redfern et Horowitz-Metzger. On a établi que — contrairement à la décomposition thermique de KClO₃-l'oxyde Al₂O₃ était un catalyseur presque aussi bon que n'importe quel autre oxyde utilisé. Dans le cas de TiO₂ l'énergie d'activation de la décomposition se trouve augmentée. Parmi les autres oxydes, Cr₂O₃ entre en réaction et se transforme en bichromate; c'est pourquoi les paramÊtres de la réaction n'ont pas été calculés. L'effet de quantités variables en Al₂O₃ sur la décomposition thermique a également été étudié.

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(25 .%), ₂₃, u, n₂, l₂₃ i_{2 3}. - , - . , l₂₃ , , , ClO₃. iO₂ . ₂₃ , . l₂₃ .

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The authors are thankful to Dr. S. S. Moosath, Professor of Chemistry, University of Calicut, for encouragement and to Dr. K. H. Stern, Naval Research Laboratory, Washington (DC) for graciously providind reprints from High Temperature Properties of Inorganic Salts.     

Effect of gamma irradiation on the thermal decomposition of zinc bromate by dynamic thermogravimetry

The thermal decoposition of -irradiated zinc bromate has been studied by dynamic thermogravimetry. The reaction order, activation energy, frequency factor and entropy of activation were computed using the Coats-Redfern, Freeman-Carroll and Horowitz-Metzger methods and were compared with those of the unirradiated salt. Irradiation enhances the decomposition and the effect increases with the irradiation dose. The activation energy decreases on irradiation. The mechanism for the decomposition of unirradiated and irradiated zinc bromate follows the Avrami model equation, 1-(1-)^1/3=kt, and the rate-controlling process is a phase boundary reaction assuming spherical symmetry.     

The effect of γ-irradiation on the thermal decomposition of magnesium bromate

The thermal decomposition of -irradiated magnesium bromate was studied by dynamic thermogravimetry. The reaction order, activation energy, frequency factor and entropy of activation were calculated by the Coats-Redfern equation and were compared with those of the unirradiated salt. Irradiation enhances the decomposition and the effect increases with irradiation dose. The activation energy decreases on irradiation. The mechanism for the decomposition of unirradiated and irradiated magnesium bromate follows the Avrami model equation, 1-/1-/1/3=kt, and the rate-controlling, process is a phase boundary reaction assuming spherical symmetry.     

The effect of particle size on the thermal decomposition kinetics of potassium bromate

The thermal decomposition of potassium bromate (KBrO₃) has been studied as a function of particle size, in the range 53?C150???m, by isothermal thermogravimetry at different temperatures, viz. 668, 673, 678, and 683?K in static air atmosphere. The theoretical and experimental mass loss data are in good agreement for the thermal decomposition of all samples of KBrO₃ at all temperatures studied. The isothermal decomposition of all samples of KBrO₃ was subjected to both model fitting and model-free (isoconversional) kinetic methods of analysis. Isothermal model fitting analysis shows that the thermal decomposition kinetics of all the samples of KBrO₃ studied can be best described by the contracting square equation. Contrary to the expected increase in rate followed by a decrease with decrease in particle size, KBrO₃ shows a regular increase in rate with reduction in particle size, which, we suggest, is an impact of melting of this solid during decomposition.     

Effect of gamma-irradiation on the thermal decomposition of pure and doped (Ba2+ caesium bromate

Effect of γ-irradiation on the isothermal decomposition of pure and doped (Ba²⁺, 0.50 mol%) caesium bromate has been studied in the temperature range 633–673 K. It is indicated that though the pure and pure irradiated crystals are immune to decomposition, doped and doped irradiated crystals undergo decomposition rapidly. There is initial rapid gas evolution representing 1–2% reaction, which is completely eleminated in doped irradiated crystals. The other stages exhibited by the crystals are, (1) acceleratory and (2) decay stages. Presence of two decay stages (one short and one long) is indicated in the doped substance, and the short decay diminishes with increase in temperature and virtually remains absent at 673 K. The acceleratory as well as decay periods of doped and doped irradiated crystals are analysed according to Prout-Tompkins, Avrami-Erofeev and Contracting square models. The rate constants in all the stages increase with increase in temperature. The energy of activation for the acceleratory periods of both the substances are almost same (± 10 kJ/mol) irrespective of the kinetic equation employed. Similar is the case with decay stages. But the energy of activation of the decay stages are higher than those of the acceleratory stages. Microscopic observation reveals that the reaction begins essentially on surfaces by the rapid formation of an interface and is followed by the penetration of the interface into the crystallite. The melting of a eutectic formed between the product CsBr and the parent material causes a marked increase in the rate.     

Effects of admixtures of potassium bromide on the thermal decomposition of potassium bromate

The effects of admixtures of potassium bromide (2.5% and 5%) on the thermal decomposition of potassium bromate were studied within the temperature range 653–683 K. The fraction decomposed() vs. time (t) relations revealed (i) initial gas evolution, (ii) acceleratory and (iii) decay steps. The data were analysed on the basis of the first-order law with two rate constantsk ₁ andk ₂,k ₁ being the rate constant for the initial, slow first-order process (0.02, 0.26), andk ₂ being the rate constant for the subsequent faster process, holding within the range 0.21 to 0.98. Upon increase of the concentration of added potassium bromide to 5%, the range for the slow and faster processes became 0.01 to 0.16 and 0.1 to 0.98, respectively. At a given temperature,k ₁ increased with increasing bromide concentration, whereask ₂ almost identical for pure potassium bromate and its mixtures (2.5% and 5% bromide).

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Zusammenfassung Im Temperaturbereich von 653–683 K wurde der Einfluß von Beimengen von Kaliumbromid (2,5% und 5%) auf die thermische Zersetzung von Kaliumbromat untersucht. Die Funktion der zersetzten Menge () in Abhängigkeit von der Zeit (t) weist eine anfängliche Gasentwicklung(i), eine beschleunigte(ii) und eine abklingende Stufe(iii) auf. Die Daten wurden auf der Grundlage einer Kinetik erster Ordnung mit den zwei Geschwindigkeitskonstantenk ₁ undk ₂ durchgeführt, wobeik ₁ die Geschwindigkeitskonstante für den einleitenden, langsameren Schritt erster Ordnung ist (0,02, 0,26) undk ₂ die Geschwindigkeitskonstante für den darauffolgenden schnelleren Vorgang mit Werten zwischen 0,21 und 0,98. Bei Erhöhung der Konzentration von Kaliumbromid auf 5% betrug der Bereich für den langsameren und den schnelleren Vorgang 0,01 bis 0,16 bzw. 0,10 bis 0,98. Bei gegebener Temperatur steigtk ₁ mit zunehmender Kaliumbromidkonzentration an, währendk ₂ für reines Kaliumbromat und seine Mischungen mit 2,5% und 5% Kaliumbromid fast identisch bleibt.

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653–683 (2,5 5%) . , . k ₁ k ₂, ( 0,02 0,26), - 0,21–0,98. 5% k ₁ k ₂ , , 0,01–0,16 0,1–0,98. k ₁, , k ₂ , (2,5 5%) .

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The work is dedicated to the memory of the late Prof. S. R. Mohanty.     

Thermal annealing of chemical radiation damage in sodium bromate

The kinetics of annealing of -irradiation damage in sodium bromate has been studied. The data are analyzed using the Fletcher-Brown and Waite models and also by conventional kinetic method. The energy of activation for annealing reactions obtained by these different methods are in reasonable agreement.     

Catalytic activity of Dy2O3 in the thermal decomposition of sodium bromate

The catalytic effect of Dy₂O₃ on the isothermal decomposition of sodium bromate has been investigated at 613.0 K. The data are analyzed in the light of various topochemical models. Dy₂O₃, being a p-type semiconductor, accelerates the decomposition of bromate by favoring the electron transfer reaction involved in the process. The plausible mechanism of the catalytic activity on the decomposition process is discussed.     

Study of the induction period of the thermal decomposition of irradiated barium bromate

The influence of -irradiation upto 200 Mrad on the thermal decomposition of barium bromate has been studied in the temperature range 553–583 K. The fraction decomposed //-time /t/ characteristics for irradiated barium bromate exhibit stages, viz. initial gas and decay stages. Irradiation shortens the induction period conforming to the relation I=C₁–C₂ lg dose. All experimental data for induction period are tested against the values obtained from the theoretical calculations by making use of this induction-period dose relation. The topochemical aspect of the validity of the above relation has been considered to be due to the linear growth of nuclei during irradiation and subsequent exponential growth during the induction period.     

Kinetic studies on the thermal decomposition of phosphate-doped sodium oxalate

The thermal decomposition kinetics of sodium oxalate (Na₂C₂O₄) has been studied as a function of concentration of dopant, phosphate, at five different temperatures in the range 783–803 K under isothermal conditions by thermogravimetry (TG). The TG data were subjected to both model-fitting and model-free kinetic methods of analysis. The model-fitting analysis of the TG data of all the samples shows that no single kinetic model describes the whole α versus t curve with a single rate constant throughout the decomposition reaction. Separate kinetic analysis shows that Prout–Tompkins model best describes the acceleratory stage of the decomposition, while the decay region is best fitted with the contracting cylinder model. Activation energy values were evaluated by both model-fitting and model-free kinetic methods. The observed results favour a diffusion-controlled mechanism for the thermal decomposition of sodium oxalate.     

Influence of SO 4 2− doping and Y-irradiation on the thermal decomposition of barium bromate

The effects of SO ₄ ^2– doping and Y-irradiation on the isothermal (563 K) decomposition of barium bromate have been investigated gasometrically with a vacuum apparatus. Plots of the fraction decomposed avs. timet for pure, doped and irradiated crystals exhibited characteristic stages: initial gas evolution (initial puff), an induction period, linear reaction, an acceleratory period and decay. The data are considered in the light of various kinetic models, e.g. linear rate equation, Prout-Tompkins and Avrami-Erofeev. The plausible mechanism of the reaction and the effects on this of anion vacancies and Y-irradiation are discussed briefly.

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Zusammenfassung Die Wirkung von Versetzen mit SO ₄ ^2} - und -Bestrahlung auf die isotherme (563 K) Zersetzung von Bariumbromat wurde gasometrisch untersucht. Es zeigte sich, daß die Zeitcharakteristiken für die reinen, die versetzten und bestrahlten Kristalle verschiedene Stufen durchlaufen, nämlich anfängliche Gasentwicklung (Aufblähen), Induktionsperiode, lineare Reaktion, Beschleunigung und Abklingen. Die Daten wurden unter dem Gesichtspunkt verschiedener kinetischer Modelle betrachtet, z. B. lineare Geschwindigkeitsgleichung, das Modell von Prout-Tompkins und Avrami-Erofeev. Es werden kurz ein einleuchtender Reaktionsmechanismus und der Einfluß von Anionengitterlücken und -Bestrahlung darauf diskutiert.

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- (563 ) . , ,-, «t» , , , , , . , - -. .

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The authors thank the Department of Atomic Energy, Government of India, for financial support. M.K.S. also gratefully acknowledges the award of a Research Fellowship by the same agency, which made the work possible.     

The effect of catalysts on the thermal decomposition of sodium superoxide

Simultaneous differential thermal, thermogravimetric and differential thermogravimetric analysis have been made on samples of sodium superoxide and sodium superoxide containing l% (w/w) copper(I) oxide. Decomposition of the superoxide involving oxygen production (weight loss) does not occur at a meaningful rate until temperatures approach 250°. The effects of 12 metallic oxide catalysts and one metalloorganic catalyst on the decomposition of sodium superoxide have been studied by differentialthermal analysis. Six metallic oxides had no effect while 3 oxides, (palladium oxide, titanium oxide and cadmium oxide) caused small but distinct changes in the DTA plots. A polymeric phthalocyanine, and the oxides of vanadium(III), vanadium(V) and manganese (IV) apparently reacted with the superoxide above 250°. Pretreatment of the superoxide by brief exposure to 100% humidity resulted in the formation of peroxyhydrates of sodium peroxide which upon dissociation produced water vapor in turn causing the release of oxygen from the superoxide and peroxide at lower temperatures than those experienced with untreated superoxide samples.     

Phenomenological kinetics of the thermal decomposition of sodium hydrogencarbonate

Aiming to find rigorous understanding and novel features for their potential applications, the physico-geometrical kinetics of the thermal decomposition of sodium hydrogencarbonate (SHC) was investigated by focusing on the phenomenological events taking place on a single crystalline particle during the course of the reaction. The overall kinetics evaluated by systematic measurements of the kinetic rate data by thermogravimetry under carefully controlled conditions were interpreted in association with the morphological studies on the precursory reaction, mechanism of surface reaction, structure of the surface product layer, diffusion path of evolved gases, crystal growth of the solid product, and so on. The precursory reaction was identified as the decomposition of impurity, taking place at the boundary between the surface of the SHC crystal and the adhesive small SHC particles deposited on the surface. In flowing dry N(2), the thermal decomposition of SHC proceeds by two-dimensional shrinkage of the reaction interface controlled by chemical reaction with the apparent activation energy of about 100 kJ mol(-1), after rapid completion of the surface reaction and formation of porous surface product layer. Atmospheric CO(2) and water vapor influence differently on the overall kinetics of the thermal decomposition of SHC. Added gas phase of CO(2) slightly inhibits the overall rate because of the increasing contribution of the surface reaction. Under higher water vapor pressure, the physico-geometrical mechanism of the surface reaction changes drastically, indicating the preliminary reformation of reactant surface and the formation of needle crystals of solid product on the surface. The mechanistic change and extended contribution of the surface reaction result in the deceleration of the surface reaction and acceleration of the established reaction.     

Effect of precompression on isothermal decomposition kinetics of pure and doped potassium bromate

Pure and doped samples of potassium bromate (KBrO₃) were subjected to precompression and their thermal decomposition kinetics was studied by thermogravimetry at 668 K. The samples decomposed in two stages governed by the same rate law (contracting square equation), but with different rate constants, k ₁ (for a α ≤ 0.45) and k ₂ (for α ≥ 0.45), as in the case of uncompressed samples. The rate constants k ₁ and k ₂ decreased dramatically on precompression, the decrease being higher for doped samples. Cation dopants (Ba²⁺, Al³⁺) caused more desensitization effect than the anion dopants ( \textSO₄ ^2- {\text{SO}}_{4}{}^{ 2- } , PO₄ ³⁻) of the same magnitude of charge and concentration. The results favor ionic diffusion mechanism proposed earlier on the basis of doping studies.     

Effect of thermal transport mechanisms on the thermal decomposition of CaCO3

Isothermal and dynamic techniques were employed to examine the rate of weight loss of CaCO₃. Thermogravimetric studies were conducted in atmospheres of He, N₂, Ar, and various percentage of CO₂ in Ar. Three methods for deriving kinetic parameters from thermogravimetric data were used and these results were then compared with data obtained from isothermal investigations done on identical samples. It was found that the higher the thermal conductivity of the atmosphere, the more repidly the reaction proceeded. Also, as the percentage of CO₂ in Ar increased, the temperature range of the decomposition became higher and narrower, resulting in a higher activation energy.     

Effect of hydration on the annealing of chemical radiation damage in gamma-irradiated strontium bromate

Rehydration of -irradiated anhydrous strontium bromate induces direct recovery of damage. The recovery process is unimolecular and the rehydrated salt is susceptible to thermal annealing.     

Stages of thermal decomposition of sodium oxo-salts of sulphur

Thermal behaviour of sodium oxo-salts of sulphur: Na₂SO₄, Na₂S₂O₇, Na₂S₂O₆, Na₂SO₃, Na₂S₂O₅, Na₂S₂O₄, Na₂S₂O₃, Na₂S₃O₆ and of sulphides Na₂S and Na₂S₂ was studied on heating up to 1000°C. The experiments were performed with anhydrous compounds obtained from commercial products by recrystallisation and dehydration. The stage mechanisms of decomposition of anionic sub-lattices of the salts have been proposed basing on the Górski’s morphological classification of simple species. The thermal stability and the stage decomposition mechanisms were correlated with the structure and the potential chemical properties of the salt anions. The thermal decomposition processes were studied by means of thermal analysis, and the decomposition products were identified by means of X-ray phase analysis.