Ev Equation Research Articles

Studies on kinetics and mechanism of initial thermal decomposition of nitrocellulose

The thermal decomposition of nitrocellulose (NC) 12.1% N, has been studied with regard to kinetics, mechanism, morphology and the gaseous products thereof, using thermogravimetry (TG), differential thermal analysis (DTA), IR spectroscopy, differential scanning calorimetry (DSC) and hot stage microscopy.The kinetics of the initial stage of thermolysis ofNC in condensed state has been investigated by isothermal high temperature infrared spectroscopy (IR). The decomposition ofNC in KBr matrix in the temperature range of 142–151°C shows rapid decrease in O−NO2 band intensity, suggesting that the decomposition of NC occurs by the rupture of O−NO2 bond. The energy of activation for this process has been determined with the help of Avrami-Erofe'ev equation (n=1) and is ≈188.35 kJ·mol−1. Further, the IR spectra of the decomposition products in the initial stage of thermal decomposition ofNC, indicates the presence of mainly NO2 gas and aldehyde.

Kinetic studies of the formation of nanocrystalline chromium silicide-silicon carbide from homogeneous amorphous precursor powders

The kinetics of formation of nanocrystalline chromium silicide and silicon carbide composites from homogeneous amorphous precursor powders has been investigated. This study was conducted in a dynamic argon atmosphere at 1 atm in the temperature range 900–1300 °C for time periods ranging from 3 min to 62 h. Two precursors with different concentration ratios of Cr, Si and C were examined. The phase percentages of the final composites, (1) hexagonal Cr 5Si 3C X and cubic β-SiC and (2) tetragonal Cr 5Si 3 and cubic β-SiC, vs. temperature and time were determined using X-ray diffraction. The relationship between the phase percentages of the crystalline material, the processing temperature and time follow the Avrami-Erofe'ev equation (J.D. Verhoeven, Fundamentals of Physical Metallurgy, Wiley, 1975). The activation energies have also been determined, and the formation of nanocrystalline chromium silicide and silicon carbide has been discussed.

Pyrolytic process of La(III)Cr(VI) precursor for the perovskite type lanthanum chromium oxide

The kinetics of the conversion of monazite type LaCrO 4 to perovskite type LaCrO 3 have been investigated by isothermal TG. The kinetics are dependent on the partial oxygen pressure and the conversion reaction is clearly suppressed with increase of pressure. The appropriate model functions for the kinetics are determined based on the Avrami-Erofe'ev equation. In nitrogen, the rate determining step is a three-dimensional phase-boundary reaction ( R 3( α) function) and the activation energy E a is estimated to be about 198 kJ mol −1. In oxygen, three-dimensional diffusion of oxygen through the LaCrO 3 layer is the rate determining step ( D 3( α) Jander function) with E a of about 506 kJ mol −1. For the latter case deoxygenation might proceed via an intermediate state in which oxygen adsorbs on the LaCrO 3 surface.

Thermal Analysis of Crystallization of Monoacylglycerol from Ethanol Solution Containing Sorbitan Laurate

Calorific values of glyceryl monopalmitate (GMP) during crystallization from ethanol solution containing sorbitan laurate (SL) were determined with a differential scanning calorimeter (DSC). Molecular interactions between GMP and SL in erthanol are discussed. This parameter was taken as the enthalpy change (ΔH) in GMP crystallization. ΔH of GMP crystallization from 0.2 mol/kg of GMP in the absence and presence of SL (0.38 mol/kg) in ethanol was 74 and 62 kJ/mol, respectively. Crystallization temperature in the latter case decreased by about 9°C. The same conditions the apparent activation energy of GMP crystallization was determined from Avrami-Erofe'ev equation. It was found to decreased with increase in SL concentration in GMP solution in ethanol. This consistent with crystallization GMP from ethanol. The results are discussed in relation to findings presented in the previous paper.

XRD, spectroscopic and thermal analysis studies on 3-nitro-1,2,4-triazole-5-one (NTO)

The kinetics and mechanism of the thermal decomposition of 3-nitro-1,2,4-triazole-5-one (NTO), a heterocyclic nitro triazole, have been studied, together with its morphology and evolved gaseous products, using thermogravimetry (TG), differential thermal analysis (DTA), infrared spectroscopy (IR), differential scanning calorimetry (DSC), X-ray diffractometry (XRD) and hot-stage microscopy. The crystal structure parameters obtained from the XRD pattern are a = 20.000 ± 0.005 Å, b = 20.000 ± 0.005 Å, and c = 6.573 ± 0.005 Å. The crystal belongs to the tetragonal system, with a c/a ratio of 0.329. The IR spectrum of NTO has also been recorded and the bands assigned. The kinetics of the thermolysis has been followed by both isothermal TG and IR. The best linearity, with a correlation coefficient of 0.9937, was obtained for the Avrami-Erofe'ev equation, n = 3, in the range 0–83% α, using isothermal TG. The activation energy was found to be 186 kJ mol −1, with log( A/s −1) being 16.64. The effect of a series of additives, incorporated to an extent of 5%, on the initial thermolysis of NTO has also been studied. Evolved gas analysis by IR showed that CO 2, NO 2, NO and N 2O are produced in larger amounts than CO and HCN. The cleavage of the CN bond appears to be the primary step in the thermolysis of NTO.

Crystallization kinetics of polysilazane-derived amorphous silicon nitride

The crystallization behaviour of amorphous silicon nitride (Si 3N 4) derived from chlorine-free polysilazanes was investigated by quantitative optical microscopy. The kinetic parameters, i.e. rate constants and activation energy, were calculated by a modified Avrami-Erofe'ev equation. In contrast to former investigations on amorphous Si 3N 4 synthesized from a chlorine-containing polysilazane, the crystallization temperature is shifted to lower (about 200 K) temperatures, while the activation energy remains constant.

A kinetic and mechanistic study of the isothermal decomposition of ferric oxalate hydrate

We report a kinetic and mechanistic study of the isothermal decomposition of ferric oxalate hydrate [Fe 2(C 2O 4) 3·4.6H 2O]. The principal products of reaction between 410 and 450 K are carbon dioxide and ferrous oxalate which retains a small proportion (approx. 10%) of the ferric salt. From microscopic studies, it was concluded that reaction was not accompanied by melting. Yield-time curves were sigmoid shaped providing evidence that this is a nucleation and growth process occurring in the solid state. Data did not fit a single-rate equation but was satisfactorily expressed in two stages: the acceleratory first half fitted the Avrami-Erofe'ev equation ( n = 2) and the second half fitted the contracting cube equation. The activation energies were relatively low for this type of reaction, 107 ± 8 and 120 ± 10 kJ mol −1 respectively. The reaction mechanism is discussed and it is concluded that the rate is controlled either by electron transfer or rupture of the C-C bond in the anion. The participation of water retained in the solid through intermediate formation of oxalic acid is discussed.

A kinetic and mechanistic study of the isothermal decomposition of ferrous oxalate dihydrate

A kinetic study has been made of the isothermal decomposition of ferrous oxalate between 596 and 638 K. The principal products of reaction are FeO (which disproportionates to Fe 3O 4 and Fe) and CO 2 + CO (3:2 ratio). Yield-time data for the evolution of the gaseous products obeyed the Avrami-Erofe'ev ( n = 2) equation, [-1n(1 − α)] 1 2 = kt, characteristic of solid state nucleation and growth reactions. The activation energy calculated for this reaction was 175 ± 7 kJmol −1. Scanning electron microscopic examinations of partially decomposed salt showed that anion breakdown was not accompanied by reactant melting. The reaction mechanism is discussed. It is concluded that decomposition is initiated rapidly and proceeds as an interface advance process thereafter. The activation energy is comparable with values for decomposition of other oxalates and it is concluded that similar controls on the rate-limiting step apply.

A kinetic and mechanistic study of the thermal decomposition of nickel permanganate

The isothermal anion decomposition reaction of nickel permanganate ((Ni(MnO 4) 2→ [NiMn 2] + 1.50 2) has been studied in the range 356–400 K and the phases present in the [residue] have not been fully characterized. Fractional reaction α-time plots are sigmoid shaped, indicative of solid state nucleation and growth reactions, but the possible participation of local and temporary melt formation has not been excluded. Data obey the Avrami-Erofe'ev equation ( n = 2) and the activation energy for salt breakdown is 100 ± 5 kJ mol −1. The first half of the reaction (α < 0.5) proceeds relatively more rapidly ( X 2) under dry conditions compared with the rate in the presence of 4-5 Torr water vapour. Decomposition of nickel permanganate is more rapid than the breakdown of MnO 4 − in aqueous solution, which is not influenced by the presence of dissolved nickel ions. This is an example of an autocatalytic reaction proceeding more rapidly in the solid state than the comparable homogeneous reaction in solution.

Diffusional limitations on the kinetics of dehydration reactions of hydrated barium chloride

The dehydration kinetics of BaCl 2·2H 2O powders were examined as a function of water vapor pressure, sample bed depth, particle diameter and temperature. The conversion behavior was modeled in both the kinetic and the combined diffusion and kinetic limited regimes with modified forms of the Avrami-Erofe'ev equation. Kinetic data were obtained by thermogravimetric techniques and by calorimetry. Structural data for intermediates and products were obtained by X-ray diffraction and optical microscopy. Dehydrations were conducted isothermally between 317 and 335 K either under vacuum or in controlled pure water vapor atmospheres ranging from 40 to 6.67 × 10 2 Pa. The dehydration rate decreased as the water vapor pressure, sample bed depth and particle diameter increased. The estimated effective bed diffusivity is 1.0×10 −6 cm 2 s −1. After a critical pressure was exceeded, severe reductions in rate were observed. The dehydration of BaCl 2·2H 2O proceeded stepwise via the crystalline monohydrate above about 80 Pa at 317 K; below this pressure the crystalline monohydrate was not observed. Transient increases in water vapor pressure in the sample bed caused crystallization of the monohydrate under conditions where it did not otherwise form.

Effect of thermal treatment on the kinetics and sintering characteristics of nickel hydroxide as a precursor for the thermal genesis of nickel oxide catalyst

The thermal events encountered throughout the calcination course of the parent nickel hydroxide precursor were investigated using a number of physicochemical techniques. The thermal decomposition products obtained by calcination of this precursor were investigated by infrared and X-ray techniques. Analysis of the surface excess oxygen and N 2 adsorption data revealed the existence of a considerable disparity in their degrees of oxidation, surface area and pore structure. The variation of the surface area with time was correlated with the differences in the concentration of the created cationic vacancies during the calcination. The presence of such vacancies leads to an increase in the growth rate of the fine pores compared with the large ones. A reduction in the volume of the pores which are intercommunicating and which communicate with the surface of the catalyst, was observed at a certain moment during the sintering process of the products obtained. The thermal decomposition of the parent hydroxide was studied non-isothermally and isothermally. The kinetic analysis of the α- t data obtained under isothermal conditions in the temperature range 250-350°C was performed. From the complementary consideration of the sintering study and the kinetic evidence we can conclude that the thermal decomposition of the hydroxide occurs by a nucleation-and-growth reaction and that the main decomposition process obeys the Avrami-Erofe'ev equation. During the decomposition course, the nucleation reaction is followed by an advancing interface mechanism in the brucite-type structure of the reactant, and both proton and electron transfer steps are involved.

A mechanistic investigation and kinetic analysis of the thermal decomposition of diammonium hexanitrato cerate catalysed by Cd xCo 1− xFe 2O 4 catalysts

The isothermal and non-isothermal decomposition of pure diammonium hexanitrato cerate(IV) and its mixtures with 10% (wt/wt) of Cd xCo 1− x Fe 2O 4 catalysts (x = 0.0, 0.5 and 1.0) were investigated. The effect of catalyst additives on the kinetics of isothermal decomposition of the parent salt was also studied. Their isothermal curves of fraction decomposed (α) against time were sigmoid in nature and the decomposition reaction in the presence of these catalyst additives obeys the Avrami-Erofe'ev equation. Such catalytic pretreatment reduced the induction period to the onset of the reaction decomposition. From the complementary consideration of microscopic observations and kinetic evidence, we conclude that the thermal decomposition of the parent salt in the presence of these catalyst additives, in the temperature range 250–325 ° C, proceeds by the nucleation-and-growth mechanism. The promotional effect of the catalysts used was also described. It can be stated that the activity of these catalysts in promoting salt breakdown is a consequence of their appreciable solubility in the parent salt and that the chemistry of the reaction in such conditions is complicated. It involves several participants, including the unstable ammine complexes of both Cd and Co ions, located in the octahedral sites of the catalysts, formed as intermediates of the type [M(NH 3)] 6 2+.

A new temperature versus heating rate transformation (T-HR-T) diagram: Application to study the crystallization behaviour of Fe 67.5Co 15Nb 1.5B 16 metallic glass

A new approach based on a temperature-heating rate-transformation (T-HR-T) curves, useful when heating an amorphous sample under continuous heating conditions, together with the traditional time-temperature-transformation (T-T-T) diagram is presented in the study of the crystallization behaviour of Fe 67.5Co 15Nb 1.5B 16 metallic glass, prepared planar flow casting. The study has been carried out by differential scanning calorimetry, X-ray diffraction and transmission electron microscopy. The calorimetric results show that there are two separate stages of crystallization. The second stage of crystallization is described correctly by the Johnson-Mehl-Avrami-Erofe'ev equation, with an Arrhenius form of the rate constant, under both isothermal and continuous heating. A more complicated kinetic behaviour is obtained for the first stage of crystallization. Transmission electron microscopy of the specimens after controlled heat treatment shows that the primary crystallization of α-(Fe,Co) dendrites in the amorphous matrix occurs in the first stage of crystallization with, in addition, sporadically secondary precipitation of (Fe,Co) 3B crystals. X-ray diffraction experiments confirm these results and show that a progressive enrichment in Fe in the α-(Fe,Co) phase occurs on the first stage of crystallization. From the crystallization kinetic results the low temperature part of the T-T-T and T-HR-T curves of the two exothermic crystalization processes are deduced.

A crystallization study of the Ge25Sb20S55 glassy alloy

Differential scanning calorimetry analysis was used to gain insight on the thermal stability and calorimetric behaviour on heating of the Ge25Sb20S55 glassy alloy. The as-quenched glass shows a glass transition atTg=570 K. From the variation ofTg with the heating rate a value of Δh=240 kJ mol−1 was obtained for the activation enthalpy of the mean relaxation time. On further heating an exothermic process appears ranging from 725 to 800 K giving rise to glassy GeS2 and needle-like crystals of antimony. The microstructure of the crystallization product was obtained from X-ray diffraction and scanning electron microscopy. The resulting material shows two glass transitions. The crystallization process is explained correctly by the Johnson-Mehl-Avrami-Erofe'ev equation with a kinetic exponent ofn=1.6. The apparent activation energy of crystallization is evaluated asE=286 kJ mol−1.

Kinetic studies on ?-SiC formation from homogeneous precursors

The kinetics of the carbothermic reduction of SiO2 by carbon to produce β-SiC from a homogeneous organic precursor has been investigated over the temperature range 1500 to 1800 °C in nitrogen by the use of a high-temperature thermobalance. The kinetic behaviour differed significantly from that of the heterogeneous reaction of SiO2 and carbon particles. The weight-loss curves could be fitted well by the Avrami-Erofe'ev equation with an exponent of 1.5. The result was interpreted as showing instantaneous nucleation in a homogeneous matrix followed by the diffusion-controlled growth of β-SiC. The obtained activation energy of 391 kJ mol−1 was consistent with the assumption that the reaction is controlled by the diffusion of carbon through the amorphous matrix to the growing surface of β-SiC.

Crystallization behaviour of amorphous silicon nitride

The crystallization of amorphous polymer-derived Si3N4 was investigated. The transformation of the amorphous into the crystalline α-phase was determined to be diffusion-controlled. In this report, the rate constants and the activation energy as well as the temperature-dependent induction period τ of the crystallization reaction were calculated by using a modified Avrami-Erofe'ev equation.

The reduction of V 2O 5: Part I. Reduction by carbon monoxide

The reduction of V 2O 5 by CO was studied using thermal analysis. X-ray powder diffraction, and electron and optical microscopy. It was shown that the reduction reaction passes through the oxide phases V 6O 13 and VO 2 with V 2O 3 being the final product. The kinetics of the reduction seems to be described by the Avrami-Erofe'ev equation with n = 4 f or V 2O 5 powder and n = 2 for the platelets. The reaction starts on the surface of the crystal and proceeds towards the centre.

The reduction of V 2O 5: Part II. Reduction by hydrogen

The reduction of V 2O 5 by H 2 was studied and compared with the results obtained for the reaction of V 2O 5 with CO. The reaction was found to be exothermic and various intermediate oxides were formed during the course of the reaction. The oxides with general composition V n O 2n−1 with 4 ⩽ n ⩽ 8 seem to be the intermediates formed. The rate of reduction was found to be faster in the H 2 than in the CO atmosphere. The sample morphology seems to influence the rate of reduction as well: the powder sample was found to react more slowly than the platelets at temperatures below 460 ° C ; above 460 °C, the platelets reacted more slowly than the powder samples. The Avrami-Erove'ev equation with n = 4 seems to describe the reduction of the powder while the reaction of the platelets is described by a first-order kinetic equation.

Glass formation and crystallization kinetics of some GeSbSe glasses

The glass formation and crystallization kinetics of alloys in the GeSe 2Sb 2Se 3Se system have been investigated by means of differential scanning calorimetry and X-ray diffraction. Minimal quenching rates in the range 10 −3 to 10 1 K/s were determined. The crystallization kinetics of some air-quenched glasses were studied calorimetrically with both isothermal and continuous heating experiments. The results obtained have been successfully interpreted with a unique set of kinetic parameters. The Johnson-Mehl-Avrami-Erofe'ev equation with a rate constant following the Arrhenius behaviour gives the best fit to the experimental data. The values obtained for the effective activation energy increase from 74 to 227 kJ/mol with antimony content, except for pure selenium (81 kJ/mol). The pre-exponential factor of the rate constant increases at the same time from 1 × 10 6 to 1 × 10 16 s −1 except for pure Se (4 × 10 8 s −1). The kinetic exponent ranges between 1.1 and 2.5 without any clear trend against composition. These results were analyzed assuming that a compensation effect occurs that can be attributed to the primary crystallization of Sb 2Se 3 in most of the studied glasses.

Kinetic study of isothermal and continuous heating crystallization in Se 100− xBi x ( x ⩽ 8) glasses

Calorimetric studies of chalcogenide glasses Se 100- x Bi x obtained by vacuum co-evaporation have been performed by isothermal and continuous heating experiments in a Perkin Elmer DSC-2C calorimeter. The Se-Bi system displays stable immiscibility and this could be an explanation for the small number of papers in the literature. Results obtained in both isothermal and non-isothermal experiments are interpreted with the same set of kinetic parameters. Our data are in agreement with the kinetic Johnson-Mehl-Avrami-Erofe'ev equation. The interpretation of the Se-Bi crystallization can best be understood by reference to phase separation phenomena.