This paper outlines the different ways of taking the distance from thermodynamic equilibrium into account in kinetic studies
based on thermoanalytical experiments. The three main approaches are: (i) avoiding or neglecting the effect of the reverse
reaction, (ii) describing the influence of distance from equilibrium on apparent kinetic parameters, and (iii) incorporating
a driving force factor in the rate equation. Finally, the contradiction of the microscopic nature of the processes and the
macroscopic character of the usual rate equation are briefly discussed.
Identification and monitoring of gaseous
species released during thermal decomposition of pure thiourea, (NH2)2C=S
in argon, helium and air atmosphere have been carried out by both online coupled
TG-FTIR and simultaneous TG/DTA-MS apparatuses manufactured by TA Instruments
(USA). In both inert atmospheres and air between 182 and 240°C the main
gaseous products of thiourea are ammonia (NH3) and
carbon disulfide (CS2), whilst in flowing air sulphur
dioxide (SO2) and carbonyl sulphide (COS) as gas phase
oxidation products of CS2, and in addition hydrogen
cyanide (HCN) also occur, which are detected by both FTIR spectroscopic and
mass spectrometric EGA methods. Some evolution of isothiocyanic acid (HNCS)
and cyanamide (NH2CN) vapours have also observed mainly
by EGA-FTIR, and largely depending on the experimental conditions. HNCS is
hardly identified by mass spectrometry. Any evolution of H2S
has not been detected at any stage of thiourea degradation by either of the
two methods. The exothermic heat effect of gas phase oxidation process of
CS2 partially compensates the endothermicity of the
corresponding degradation step producing CS2.
Authors:Katalin Szécsényi, I. Esztelecki, and G. Pokol
In order to increase the nutrition value of bread, one of the most commonly used foodstuff all over the world, different additives
are used in bread processing. In this paper we describe the thermal changes in bread and that of with 0.5% crude soybean lecithin
additive. Their thermal stability has been investigated by TG, DSC and EGD methods. The thermal changes were also followed
of soy products, lecithin and lysine, ingredients used as bread additives in order to check if they may suffer any thermal
degradation during the baking process. The data obtained can be of use only for qualitative conclusions. According to the
obtained data at the usual bread baking temperature only the additives in crust may partly decompose while in the crumb, at
lower temperatures the additives, due to baking, are not damaged. The thermal methods give a possibility for rapid estimation
of processes induced by heat effects in additives during the baking, and they are suitable to detect the changes during the
bread-making procedure. However, they are neither suitable to provide any quantitative data on these changes nor facts affecting
the nutrition value and of the bread.
Novel methods of unified evaluation of two (or more) thermogravimetric curves have been worked out on the basis of known non-linear
parameter estimating procedures (Gauss-Newton-Marquardt-type regression and the direct integral method of Valkó and Vajda
were adapted). Their ability to provide estimate for common kinetic parameters of several TG (m−T) or DTG (dm/dt-T) curves were tested for pairs of curves of different heating rates, and for repeated curves of the same heating rate, obtained
for the decomposition of CaCO3 in open crucible. In these cases the Arrhenius terms and then-th order model functions were assumed. The fitting ability of estimations made for single curves and for pairs of curves
sharing different number of parameters, was judged on the base of residual deviations (Sres) and compared to the standard deviation of the measurements.
In the case of different heating rates, the two curves could not be described with the assumption of three common parameters,
because of the minimum residual deviation was very high. However, sharing of activation energy and preexponential term only,
and applying different exponents for the two curves, provided a satisfactory fit by our methods. Whilst in the case of repeated
curves, we could find such a common three-parameter set, which has a residual deviation comparable with the standard deviation
of the measurements.
Because of their flexibility (taking into account the variable number of common parameters and the versatile forms of model
equations), these methods seem to be promising means for unified evaluation of several related thermoanalytical curves.
Four sulphato and nitrato complexes of cerium(IV),viz. (NH4)4Ce(SO4)4·2H2O (1), (NH4)2Ce(SO4)3 (2), (NH4)2Ce(NO3)6 (3) and Cs2Ce(NO3)6 (4) were studied by simultaneous TG/DTA under various experimental conditions in order to establish their decomposition mechanism
and to compare the results with the literature data which have been reviewed. In the case of the ammonium compounds (1, 2 and3) the decompositions are accompanied by changes in the oxidation state of cerium; the presence of Ce(III) and Ce(IV) were
studied byex situ magnetic susceptibility and XPS measurements. The crystal structure of (1) was determined as well. It forms monoclinic crystals with space groupP21/c; the parameters of the unit cell are:a=12.638(18) Å,b=11.362(10) Å,c=13.607(11) Å, β=110.17(9)°,V=1834.05 Å3.
The phase diagram for the AgNO3−KNO3 system has been determined using differential scanning calorimetry (DSC). Eutectic point has been found at 391 K andXAg=0.580 mole fraction AgNO3. The DSC curves indicate the existence of an intermermediate compound (AgNO3·KNO3) in the KNO3-rich region of the phase diagram. This compound was identified in the solid phase by X-ray diffraction. The melting and the
crystallization processes were followed with the aid of a hot stage microscope, too.
Authors:J. Madarász, G. Pokol, C. Novák, H. Moselhy, and S. Gál
Residual differences after model fitting were investigated in both isothermal and non-isothermal kinetics in order to make
numerical comparisons between several models and various parameter-estimating methods. Data from two independent experimental
series were evaluated.
A large data set, collected earlier under isothermal conditions from decompositions and hydrothermal reactions of aluminium
hydroxides and oxides, was processed first. It showed that mechanical activation of the starting gibbsite affected reactivity
of samples in several subsequent reactions for all model equations tried. The relative residual deviation concept is introduced,
and statistics were applied to find a model that fits a certain reaction in most of the cases.
In the second study, the sulphate decomposition step of aluminium sulphate octadecahydrate was investigated. TG curves were
measured using a constant heating rate. Dynamic models were fitted by three mathematical methods, including a new general
purpose one. Fitting ability of the methods with various complexity were compared on the basis of residual deviations obtained
after integration of the model equations. As well as evaluating the best fit, this new parameter-estimating method provides
a statistical analysis of the reliability of the whole model fitting process.
Authors:H. Moselhy, J. Madarász, G. Pokol, S. Gál, and E. Pungor
Three different calculation methods of deriving kinetic parameters (activation energy and preexponential factor) from dynamic
TG data have been applied for the sulphate decomposition stage of the aluminum sulphate octadecahydrate. The constant rate
experiments were carried out by Derivatograph and DuPont thermobalances. The three parameters estimation methods included
a simple differential method, the classical Coats-Redfern and a new direct integral method. The fits of the curves obtained
by these procedures were compared both graphically and numerically. It was found that the direct integral method gave the
most satisfactory results. With the order type reaction models this method in each case produced the smallest residual deviation
values and the best fitting curves compared to those obtained by the other two methods. The activation parameters calculated
by the differential method were not acceptable at all, for the estimated curves were very far from the measured ones.
Authors:M. Zaharescu, A. Jitianu, A. Braileanu, J. Madarász, and G. Pokol
It is well known that SiO2 -based inorganic-organic hybrid materials present significant differences due to the organic moieties bound to the inorganic
network and to the preparation conditions.
In the present work the ageing effect on the thermal stability of the SiO2 -based inorganic-organic hybrid materials prepared using tetraethoxysilan (TEOS), triethoxymethylsilan (MTEOS), triethoxyvinylsilan
(VTEOS), tetramethoxysilan (TMOS), trimethoxymethylsilan (MTMOS) and trimethoxyvinylsilan (VTMOS) was studied.
TG/DTA-MS was used to study the decomposition process of the materials. The structural modifications of the materials during
ageing were studied using spectral methods.
The gels obtained starting with more reactive alkoxides, of methoxy-type, present more complex structures and are less stable