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Abstract  

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.

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Abstract  

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.

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Abstract  

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 (S res ) 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.

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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.

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Abstract  

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.

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Partial thermal reduction of ammonium paratungstate tetrahydrate

Evolved gas analysis (TG/DTA-MS) and solid state studies (XRD, FTIR)

Journal of Thermal Analysis and Calorimetry
Authors:
I. Szilágyi
,
J. Madarász
,
F. Hange
, and
G. Pokol

Abstract  

Thermal decomposition of ammonium paratungstate tetrahydrate, (NH4)10[H2W12O42]4H2O has been followed by simultaneous TG/DTA and online evolved gas analysis (TG/DTA-MS) in flowing 10% H2/Ar directly up to 900C. Solid intermediate products have been structurally evaluated by FTIR spectroscopy and powder X-ray diffraction (XRD). A previously unexplained exothermic heat effect has been detected at 700–750C. On the basis of TG/DTA as well as H2O and NH3 evolution curves and XRD patterns, it has been assigned to the formation and crystallization heat of γ-tungsten-oxide (WO2.72/W18O49) from β-tungsten-oxide (WO2.9/W20O58) and residual ammonium tungsten bronze.

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Abstract  

Identification and monitoring of gaseous species released during thermal decomposition of the title compound 1, Zn(tu)2Cl2, (tu=thiourea, (NH2)2C=S) have been carried out in flowing air atmosphere up to 800°C by both online coupled TG-EGA-FTIR and simultaneous TG/DTA-EGA-MS. The first gaseous products of 1, between 200 and 240°C, are carbon disulfide (CS2) and ammonia (NH3). At 240°C, an exothermic oxidation of CS2 vapors occurs resulting in a sudden release of sulphur dioxide (SO2) and carbonyl sulphide (COS). An intense evolution of hydrogen cyanide (HCN) and beginning of the evolution of cyanamide (H2NCN) and isothiocyanic acid (HNCS) are also observed just above 240°C. Probably because of condensation and/or polymerization of cyanamide vapors on the windows and mirrors of the FTIR gas cell optics, some strange baseline shape changes are also occurring above 330°C. Above 500°C the oxidation process of organic residues appears to accelerate which is indicated by the increasing concentration of CO2, while above 600°C zinc sulfide starts to oxidize resulting in the evolution of SO2. All species identified by FTIR gas cell were also confirmed by mass spectrometry, except for HNCS.

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Thermally induced changes in the oxidation state of cerium(IV)

A study of carbonate and chloro complexes

Journal of Thermal Analysis and Calorimetry
Authors:
J. Madarász
,
T. Leskelä
,
G. Pokol
, and
L. Niinistö

Abstract

Cesium hexachlorocerate(IV), Cs2CeCl6 (I) and sodium pentakis(carbonato)cerate(IV), Na6Ce(CO3)5·12H2O (II) have been investigated in air by simultaneous TG/DTA, FTIR and XRD in order to follow the oxidation state of cerium during their thermal treatment. The thermal decomposition of the hexachloro compound (I) is accompanied by a double change in the oxidation state of cerium. First, in an inner reduction-oxidation reaction, chlorine is evolved and a Cs2CeCl5 phase is obtained. The immediately starting oxidation of this Ce(III) species caused various phase transitions in the CeCl3-CsCl system formed. The presence of Cs3CeCl6 above 400°C can also be assumed and finally this phase also oxidizes into CeO2 with the formation of CsCl as by-product. In the case of the pentacarbonato complex (II), no Ce(III) species were detected. The final products of its decomposition were CeO2 and Na2CO3.

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Abstract  

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.

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Summary Oxide materials belonging to the Sn-Ce-O system are very interesting due to their use as solid electrolytes in fuel cells, catalysts, sensors and photoanodes in solar cells. The aim of the present work is to investigate the thermal behaviour of some tin and cerium salt mixtures. Mixtures with different representative Sn:Ce atomic ratio were prepared by classical ceramic method using SnC2O4, Ce(SO4)2·4H2O and (NH4)2Ce(NO3)6 as starting compounds. The samples were investigated by means of TG/DTA methods in flowing and static air atmosphere. SnO2 and/or CeO2 were identified by X-ray diffraction and IR spectroscopy in the final decomposition products, depending on the initial composition of the both series. A different crystallinity degree of the solid products was observed depending on the Ce precursor.

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