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.
We characterize the finite-dimensional elements of a free cylindric algebra. This solves Problem 2.10 in [Henkin, Monk, Tarski: Cylindric Algebras, North-Holland, 1971 and 1985]. We generalize the characterization to quasi-varieties of Boolean algebras with op- erators in place of cylindric algebras.
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.
Authors:Margit Bán, Petra Bombicz, and J. Madarász
A new co-crystal of theophylline and phthalic acid with 1:1 molar ratio has been prepared. It crystallises in the monoclinic
crystal system, space group P21/c, a=11.5258(9), b=10.1405(6), c=13.9066(12) Å, β=106.827(4)°. The structure of the co-crystal has been revealed by single crystal X-ray diffraction. An infinite
helical polymeric chain is formed by intermolecular hydrogen bonds of the two neutral constituents. The hydroxyl group and
carbonyl oxygen atom in one of the carboxyl groups of phthalic acid form hydrogen bonds to O6 and to N(7)H atoms of theophylline,
respectively, while the other carboxyl OH group of phthalic acid is in hydrogen bond to N9 atom of theophylline by very strong
intermolecular interactions proven by 1883 cm−1 centred peak in FTIR spectrum.
Thermal degradation of this new supramolecular compound is a two-step process in air. At first phthalic acid (47.4%) released
up to 230°C, meanwhile it loses water and transforms into phthalic anhydride. In EGA-MS spectra, the characteristic fragments
of water (m/z=17, 18) appear from about 180°C, while absorption bands of phthalic anhydride are shown in EGA-FTIR spectrum at about 210°C.
In the second step theophylline begins to sublime, melts at 276°C, and then evaporates up to 315°C with minute residues.
Authors:J. Madarász, E. Beregi, J. Sztatisz, I. Földvári, and G. Pokol
Sintering processes in the Y2O3–Al2O3–B2 O3 system and its subsystems (Y2O3–B2O3 and Al2 O3–B2O3) have been investigated by using combined DTA and XRD measurements to get a better understanding of solid state chemical
changes resulting in the formation of yttrium aluminum borate (YAl3(BO3)4, YAB) phase and to study the possible role and contribution of various simple borates formed also in the former processes.
Two new exothermic heat effects of YBO3 formation have been detected by DTA in the Y2O3–B2O3 system between 720 and 980C. In the Al2O3–B2O3 system a new experimental XRD profile of Al4B2O9 was observed. Formation of these borates seems to promote the nucleation of double borate YAB below 1000C. Conversion of
Al4B2O9 to Al18B4 O33 was observed after a long term (10 h) sintering at 1050C. Similarly, an increased formation of YAB has been observed as
a product of the sintering reaction between YBO3 and Al18B4O33 at 1150C. The two latter single borates are found to be identical with the high temperature decomposition products of YAB.
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
Authors:D. Janke, J. Madarász, S. Lukács, and G. Pokol
Differential scanning calorimetric (DSC) measurements have been carried out on Bi-Sn based amalgam precursors to be used in
compact fluorescent lamps (CFLs) to study the changes in melting and solidifying behaviour caused by In dopant. The phase
and elemental compositions of the samples have been characterized by using X-ray diffraction (XRD) and scanning electron microscopy-energy
dispersive X-ray analysis (SEM-EDX), respectively. One of the endothermic peaks of the liquid amalgam formation shifted from
121°C to 112 and 105°C, with increasing content of 2.5 and 4.8 mass% In of samples, respectively.