The thermal decomposition of four commercial powders and of differently stored single crystals of sodium hydrogen carbonate
is studied by power compensation DSC and by optical and FT-IR microscopy. Independently of manufacturer, specified purity
and price, the thermal curves of all the commercial powders show a more or less pronounced low temperature peak preceding
the one due to the main decomposition. Such small peak is not observed when samples of laboratory recrystallized material
are used. However the thermal behaviour of the latter preparation differs remarkably depending on storage conditions: the
material kept in closed glass containers decomposes at temperatures higher than those of the material stored in a dessiccator
in the presence of concentrated H2SO4.
The observation by optical microscopy of the behaviour of the surfaces of single crystals coming from different storage conditions
when the temperature is raised in a Kofler heater helps the interpretation of the data collected.
The mechanism of the decomposition is discussed and the relevant kinetic parameters reported.
Authors:A. Raemy, C. Kaabi, E. Ernst, and G. Vuataz
The DSC curve of freeze-dried amorphous sucrose shows the glass transition, the crystallization and the melting (just before
decomposition) of the sample. Sucrose crystallization occurs below 100°C: this phenomenon can therefore be observed with the
microcalorimeter Setaram Micro-DSC used in the scanning mode. Mixtures of amorphous and crystalline sucrose in known proportions
were used to calibrate the instrument. Low level amorphism (down to about 0.5%) could be detected and quantitatively evaluated
on the basis of the crystallization enthalpies determined. The calibration curve obtained can be applied to determine the
degree of amorphism in milled sucrose. A simple gravimetric method, based on the desorption of water induced by recrystallization
of the amorphous layer can be used to obtain similar data more rapidly. This simple method is particularly useful for controlling
the amorphism on line during a process, and is also briefly described.
Authors:A. S. Abdel-Halim, Y. K. Afifi, and N. Afify
The effects of the drying conditions on the thermal behaviour of UO3 gel microspheres were studied by TG, DTA and X-ray examination. The effects of drying with air, steam or alcohol on the crystal structure and crystallite size were also studied. The results indicate that the thermal decomposition of UO3 gel microspheres involves five steps: the first two for dehydration, the third for ammonia release, the fourth for ammonia oxidation, and the last one for UO3 recrystallization. It was also found that the crystal growth varied from 110 Å after air drying to 512 Å and 496 Å after steam and alcohol treatment, respectively.
Authors:G. P. Bettinetti, C. Caramella, F. Giordano, A. La Manna, C. Margheritis, and C. Sinistri
Thermal analysis of the binary system benzoic acid (BA) and trimethoprim (TMP) provided evidence of the formation of two molecular compounds. BA-TMP and two crystalline forms of (BA)2-TMP were characterized on the basis of their thermodynamic parameters as well as of crystallographic and spectroscopic properties. The availability of these compounds (by recrystallization) allowed interpretation of thermal effects in the DSC curves of the mixtures and the theoretical phase diagrams could be drawn. The results are consistent with the model of a very slight dissociation of the molecular compounds in the melt.
Authors:Dariusz Bochenek, Małgorzata Płońska, Julian Dudek, and Zygmunt Surowiak
Results of investigations on selected ceramic ferroics and multiferroics by TA method were presented. The authors of the work
used the thermal analysis both to optimize a process of producing the ceramic ferroics and multiferroics and to examine phase
transitions in that type of materials. In the case of synthesis of the ferroics and multiferroics as a result of sintering
of a mixture of simple oxides, the TA method enables to determine the optimum synthesis temperature and temperatures of re-crystallization
and disintegration of compounds and solid solutions. In the case of the sol–gel method, temperatures of dehydratization, burning
of an organic phase, and crystallization of an amorphous powder formed from the residual gel were determined by the TA method.
The TA method was also used to control a process of compacting and sintering the powders at high temperatures (Ts > 1,200 K), thus in a process of ceramic specimen formation. During rapid phase transitions, the ferroelectric specimens
of a first type emit (in the cooling process) or absorb (in the heating process) so called latent heat of the phase transitions.
On the DTA courses, it may be manifested in a form of exo- or endothermic peaks in the Curie temperature area (TC). The test materials included the ferroelectric ceramics of composition x/65/35 PLZT (ferroic for x < 9 at%) and mixed bismuth oxide layered perovskites (M-BOLP) of composition Bi5TiNbWO15 with <m> = 1.5 and the mutliferroic Pb(Fe1−xNbx)O3 ceramics (PFN) and Bi5TiFeO15 (BTF).
A three-phase model, comprising crystalline, mobile amorphous, and rigid amorphous fractions (χc, χMA, χRA, respectively) has been applied in the study of semicrystalline Nylon-6. The samples studied were Nylon-6 alpha phase prepared
by subsequent annealing of a parent sample slowly cooled from the melt. The treated samples were annealed at 110°C, then briefly
heated to 136°C, then re-annealed at 110°C. Temperature-modulated differential scanning calorimetry (TMDSC) measurements allow
the devitrification of the rigid amorphous fraction to be examined.
We observe a lower endotherm, termed the ‘annealing’ peak in the non-reversing heat flow after annealing at 110°C. By brief
heating above this lower endotherm and immediately quenching in LN2-cooled glass beads, the glass transition temperature and χRA decrease substantially, χMA increases, and the annealing peak disappears. The annealing peak corresponds to the point at which partial de-vitrification
of the rigid amorphous fraction (RAF) occurs. Re-annealing at 110°C causes the glass transition and χRA to increase, and χMA to decrease. None of these treatments affected the measured degree of crystallinity, but it cannot be excluded that crystal
reorganization or recrystallization may also occur at the annealing peak, contributing to the de-vitrification of the rigid
Using a combined approach of thermal analysis with wide and small angle X-ray scattering, we analyze the location of the rigid
amorphous and mobile amorphous fractions within the context of the Heterogeneous and Homogeneous Stack Models. Results show
the homogeneous stack model is the correct one for Nylon-6. The cooperativity length (ξA) increases with a decrease of rigid amorphous fraction, or, increase of the mobile amorphous fraction. Devitrification of
some of the RAF leads to the broadening of the glass transition region and shift of Tg.
Thermogravimetry, differential thermal, X-ray diffraction and infrared spectroscopy analyses showed La(CH3COO)3·1.5H2O to decompose completely at 700°C yielding La2O3. The results revealed that the compound dehydrates in two steps at 130 and 180°C, and recrystallizes at 210°C. Water thus
produced hydrolyzes surface acetates (at 310°C), releasing acetic acid into the gas phase. At 334°C, the anhydrous acetate
releases gas phase CH3COCH3 to give La2(CO3)3 residue, which decomposes to La2O2(CO3) via the intermediate La2O(CO3)2. On further heating up to 700°C, La2O3 is formed. IR spectroscopy of the gaseous products indicated a chemical reactivity at gas/solid interfaces formed throughout
the decomposition course. As a result, CH3COCH3 was involved in a surface-mediated, bimolecular reaction, releasing CH4 and C4H8 (isobutene) into the gas phase.
Non-isothermal kinetic parameters, the rate constantk, frequency factorA, and activation energy ΔE, were calculated on the basis of temperature shifts experienced in the thermal processes encountered, at various heating
rates (2–20 deg·min−1).
The special position occupied by glasses amongst solids is again underlined by their thermal behaviour. This feature was studied using As2Se3 as model glass. The range of transformation characteristic of glass is less sharply defined than the freezing point. In the thermal characterization of glass the former is highly dependent on the rate of heating and the thermal history of the glass. The recrystallization and melting temperatures are subject to corresponding modifications.
Authors:D. Seetharamacharyulu, R. M. Mallya, and V. R. Pai Verneker
A study has been made of the differential thermal analysis of (i) potassium perchlorate in powdered form, (ii) potassium perchlorate in pelletized form, (iii) potassium perchlorate recrystallized from liquid NH3, and (iv) potassium perchlorate preheated for 24 hours at 375°. Pretreatment of potassium perchlorate leads to a desensitization of both endothermic and exothermic processes. Additionally, the pretreatment tends to convert the symmetric exotherm into an asymmetric exotherm due to merging of the two exotherms. An analysis of the factors causing asymmetry in the exotherm has thrown fresh light on the mechanism of thermal decomposition of potassium perchlorate.