Search Results
Abstract
TlSbSe2 monocrystals were grown using the modified Bridgman–Stockbarger method and were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). Reflectivity spectra have been registered in the range 50 to 4000 cm–1 for E parallel to a and E parallel to b polarizations, on the cleavage plane. A remarkable anisotropy at two directions was verified. With regard to previous observations, additional peaks were discriminated and the fundamental phonon parameters were determined using classical dispersion relations. The material presents a complex phase transformation – with two thermal effects – that was examined using differential scanning calorimetry (DSC). Non-isothermal measurements, at different heating and cooling rates (β), were used to study the thermal phenomena. The main effect is attributed to a structural displacement and the second one to a cation exchange procedure. The phase transformation temperature depends strongly on the cooling rate and the peaks are shifted by 30 K with the increase of this rate, on the contrary to the increase of the heating rate that has a smaller effect. Phenomena related with the influence of the previous, repeated heating and cooling cycles on the transformation are also examined and analytically discussed.
Abstract
Phase transformation studies carried out on Mn−Al−C permanent magnet alloy employing DSC and electrical resistivity measurements, are reported and discussed. The transformation of the hexagonal Mn−Al phase (disordered and non-magnetic) to the ferromagnetic fct phase proceeds via the formation, in at least two stages, of the ordered orthorhombic phase. The activation energy for the formation of the fct phase is ≈34.65 kcal/mol. Microstructural changes occurring at various stages of the transformation are also studied.
Abstract
The thermal interactions of dolomite and calcite with calcium polyphosphates up to 1000C were studied by thermal (dynamic heating and isobaric conditions) and X-ray diffraction methods. It was found that in mixtures with Ca-polyphosphates less than half of the carbonates reacted before decomposition to oxides with the phosphates. The rates of the reactions of phosphates with MgO were much lower than those with CaO. The scheme of the main crystalline phase transformations was constructed.
Abstract
A method of calculation of average heat capacities of phase transformation products of complex oxides is suggested. The method takes into account the physical state of products and the increase in the heat capacities of products due to the change of entropy at a phase transformation. Average heat capacities of products formed in a congruous melting of compounds (YCuO2 and Y4Ba3O9), in an incongruous melting of compounds (Y2Cu2O5, BaCuO2, BaCu2O2, Y2BaCuO5, YBa2Cu3O7, YBa2Cu3O6) and in a decomposition in a crystalline state of compounds (Y2BaO4, Y2Ba2O5, Y2Ba4O7, Ba2CuO3, Ba3Cu5O8, YBa2Cu3.5O7.5, YBa2Cu4O8, YBa2Cu5O9) was estimated by using three methods.
Abstract
An increase of the specific surface area of solid phases is often desirable e.g. for the bioavailability of pharmaceuticals or in chemical processes. Such an increase can a.o. be achieved by suspending crystalline substances in a solvent that induces phase transformations. Hence, the original substance has to be in a metastable state in the solvent. If the stable phase after transformation has in addition a very low solubility in the solvent, a dendritic growth is forced to occur because of the high local supersaturations during the phase change. This dendritic growth of the stable phase in term leads to needle- or whisker-like crystals, which have the desired larger specific surface area in comparison to the initial crystalline substance.In order to investigate this phenomenon several hydrates of salts were chosen, which undergo phase transformations to their anhydrates accompanied by a corresponding loss of crystal water when suspended in excess in lower alcohols. Consequently, anhydrous forms were created by dehydrating these hydrates. The transformation rate or in this case the dehydration level can thus be indirectly measured by Karl-Fischer titration. The thermodynamic background of the dehydration phenomena can be clarified by solubility studies of the hydrates and anhydrates in water/alcohol-mixtures.
, interesting to monitor the evolution of goethite first and, then, of hematite. Fig. 2 Synchrotron radiation diffraction patterns displaying the evolution of the phase transformation of goethite ( G-peaks ) into
Phase transformations in Cu-12.4% Al and Cu-14.4% Zn-8.4% Al alloys were examined by DTA. The influence of the rate of temperature change on the sequence of phase transformations was studied. It was found that the rates of heating and cooling were the major factors determining the transformations which take place in these alloys.
Abstract
Syrian phosphorite is subjected to mechanochemical activation carried out in planetary mill. Some phase transformations are ascertained by means of powder XRD and thermal analyses. They reveal as partial transformation of carbonate fluorine apatite into carbonate hydroxyl fluorine apatite and formation of Ca(PO3)2, as well. The solubility of the activated sample in 2% citric acid is increased as a result of these changes.
The influence of the alloying elements magnesium, copper and silicon on phase transformations in Al-60 wt% Zn alloy solidified at rates from 0.4 up to 65 deg/s has been investigated by means of DTA method.
The purpose of this study is to elucidate the nature of the phase transformations of lead monoxide powder. Lead monoxide is prepared by calcination of a lead oxalate precursor salt, and its phase transformations are studied using X-ray diffraction (XRD), differential scanning calorimetry (DSC) and thermal gravimetric analysis (TG). Analysis reveals that the phase transformations observed for oxalate-derived lead monoxide powder are highly dependent on the firing atmosphere. In nitrogen, as the temperature is increased 1 deg/min from room temperature, lead monoxide undergoes a reconstructive litharge-to-massicot phase transformation in a temperature range of 525–575°C. In air, litharge, metastable at room temperature, slowly oxidizes to the Pb3O4 phase at a temperature of 350°C and rapidly reduces to litharge at 560°C. At temperatures greater than 560°C, litharge converts to massicot. With heating rates of 10 deg/min or higher, formation of Pb3O4 is not observed.
