Thermal decomposition of different inorganic sulphates are presented. A number of techniques, but mainly TG and DTA, are used to prove the mechanism and kinetics of CaSO4, BaSO4, FeSO4·xH2O, Al2(SO4)3·xH2O under various gas atmospheres. It is shown how the partial pressure of gas components and heating rate may effect the mechanism and kinetic parameters. There are also examples on the effects of some additives and initial treatment on the thermal processes.
On the base of the results obtained some recommendations are given concerning the precautions to be taken into account in the thermal decomposition studies and the sulphur recovering.
The kinetics of solute segregation to partial dislocations in a Cu–3.4 At.% Sb alloy was studied by using a phenomenological approach with differential scanning calorimetry and isothermal calorimetry. The material, severely deformed by repeated bending, presented an excess of dissociated edge dislocations with a dislocation density amounting to about 8.5·1014 m–2, calculated using a prior model of the authors, together with calorimetric recrystallization trace analysis. The kinetics was found to be ruled by two overlapping mechanisms: diffusion of solute atoms mostly through dislocation pipes in the initial and middle stages of the reaction process, acting together with bulk solute diffusion in these stages and later. Bulk solute diffusion increases as the reaction proceeds, as shown by the increasing values of apparent activation energy in the reaction. The exponent of the Mehl-Johnson-Avrami equation used in the phenomenological description was successfully fitted to a time—temperature-dependent function, increasing in agreement with the apparent activation energy behaviour, as may be expected.
Authors:Ž. Živković, D. Živković, D. Grujičić, N. Štrbac and V. Savović
Results obtained by the comparative investigations of the natural mineral marmatite (Zn,Fe)S (Stari Trg, Yugoslavia) oxidation
process are presented in this paper.
Determination of the oxidation process mechanism was done by thermal analysis methods: DTA-TG-DTG and DSC, while Mssbauer
spectroscopy, X-ray and electronic microanalysis were used for phase composition determination of the products formed during
the oxidation process. Kinetics of the process was defined for two temperature intervals 862–973 and 1023–1173 K, under isothermal
The kinetics of direct reduction of artificial chrome iron ore was studied by isothermal and non-isothermal methods. In the initial, middle and final periods, the reaction is controlled by nucleation and growth, a phase boundary reaction, and diffusion, respectively. In the main reaction region, the kinetic equation is 1–(1–)1/3=kt and the apparent activation energy is 270 kJ mol–1. The kinetic mechanisms found with the isothermal and non-isothermal methods do not differ, and the activation energy values are approximately the same. However, the non-isothermal method can demonstrate the kinetic process completely.
Authors:J. G. Popovic, L. Katsikas and J. S. Velickovic
The non-oxidative thermal degradation kinetics of poly(di-n-alkyl itaconates), ranging from the methyl to then-octyl derivatives, were studied by non-isothermal and isothermal TG. The thermal degradation activation energy and characteristic mass loss temperatures were found to decrease with increasing substituent size. The shapes of the DTG curves were dependent on the size of the alkyl substituent. The different DTG maxima were ascribed to various modes of initiation of depolymerisation. The thermal stability of poly(di-n-hexyl itaconate) was found to be independent of the initial molar mass of the sample in the range ofMw from 104 to 107 g/mol.
Authors:L. Katsikas, K. Jeremic, S. Jovanovic, J. Velickovic and I. Popovic
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.
Kinetics of the oxidation of magnetite (Fe3O4) to hematite (a-Fe2O3) are studied in air using simultaneous TG/DSC. The mechanism is complex and the differences between the kinetic conclusions
and Arrhenius parameters based on either TG or DSC are discussed. As in our previous work on CaCO3 , the determination of a satisfactory baseline for the DSC results adds considerable uncertainty to those kinetic results.
Consequently the calculations based on the TG data are considered superior. Solid state reactivity varies from one source
of material to another and the results are compared for two different commercial samples of magnetite, both presumably prepared
by wet chemical methods. These materials are much more reactive than the material studied previously , which had been coarsened
and refined at high temperatures. In that earlier study, the metastable spinel, g-Fe2O3, was formed as an intermediate in the oxidation to the final stable form, a-Fe2O3. The exothermic reaction of the gamma to alpha form of the product during the oxidation process destroys the direct comparison
between the TG and DSC results, since the former only detects the change in mass of the sample and not the crystallographic
transformation. The TG results, however, represent the true oxidation process without superposition of the structural aspects.
The PVC/ABS blends were degradated by means of isothermal thermogravimetry at temperatures at 210...240°C in nitrogen. Applying
the stationary point method to the data obtained from thermogravimetric curves, apparent activation energy, preexponential
factor and compensation parameter for each blend were calculated. The constancy of compensation parameters points to an unchanged
mechanism of poly (vinyl-chloride) (PVC) thermal degradation in the presence of acrylonitrile butadiene-styrene (ABS). Upon
increasing the fraction of ABS in the blend up to 50% only the kinetics of the process is changed.
The physico-geometric kinetics for the solid-state reactions by thermoanalytical (TA) measurements were reexamined by focusing
some fundamental aspects: (1) the fundamental kinetic equation, (2) the kinetic model function, (3) the fractional reaction
α, and (4) the apparent kinetic parameters. It was pointed out that some pitfalls in the practical kinetic study are originated
by the disagreement between the kinetic information from the TA measurements and the theory of the physico-geometric kinetics.
In order to increase the degree of coordination between the theory and practice, several attempts were made from both the
theoretical and experimental points of views. The significance of the apparent kinetic parameters was discussed with a possible
orientation for obtaining the reliable kinetic parameters.