Authors:Imre Szilágyi, István Sajó, Péter Király, Gábor Tárkányi, Attila Tóth, András Szabó, Katalin Varga-Josepovits, János Madarász, and György Pokol
This article discusses the formation and structure of ammonium tungsten bronzes, (NH4)xWO3−y. As analytical tools, TG/DTA-MS, XRD, SEM, Raman, XPS, and 1H-MAS NMR were used. The well-known α-hexagonal ammonium tungsten bronze (α-HATB, ICDD 42-0452) was thermally reduced and
around 550 °C a hexagonal ammonium tungsten bronze formed, whose structure was similar to α-HATB, but the hexagonal channels
were almost completely empty; thus, this phase was called reduced hexagonal (h-) WO3. In contrast with earlier considerations, it was found that the oxidation state of W atoms influenced at least as much the
cell parameters of α-HATB and h-WO3, as the packing of the hexagonal channels. Between 600 and 650 °C reduced h-WO3 transformed into another ammonium tungsten bronze, whose structure was disputed in the literature. It was found that the
structure of this phase—called β-HATB, (NH4)0.001WO2.79—was hexagonal.
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.
Authors:Stefano Gialanella, Fabrizio Girardi, Gloria Ischia, Ivan Lonardelli, Maurizio Mattarelli, and Maurizio Montagna
, interesting to monitor the evolution of goethite first and, then, of hematite.
Synchrotron radiation diffraction patterns displaying the evolution of the phasetransformation 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.
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.
Authors:K. Chrissafis, M. Ozer, E. Vinga, E. Polychroniadis, X. Chatzistavrou, and K. M. Paraskevopoulos
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.
The phase transformations of Syrian phosphorite upon mechanochemical activation are examined in the present work. The latter
is carried out in planetary mill equipped with 20 mm steel milling bodies and duration from 30 to 300 min. The established
by means of DTA, DTG, TG analyses transformation of non-activated carbonate fluorine apatite type B into the carbonate hydroxyl
fluorine apatite (COHFAp) mixed type A2-B leads to substantial changes in the properties of the activated samples expressed
in lowering the degree of crystallinity, strong defectiveness of the structure, and increase of the citric solubility. The
thermal analysis gives evidence for the decomposition of the carbonate-containing component within the phosphorite, as from
the positions placed in the vicinity of the hexagonal 63 axis (type A2), as well as from the positions of the phosphate ion (type B), and from the free carbonates. The data from
the thermal analysis, the powder X-ray analysis and the infrared spectroscopy give also evidence for phase transformations
of the activated apatite (with admixtures of quartz and calcite) into Ca10FOH(PO4)6, β-Ca3(PO4)2, Ca4P2O9, Ca3(PO4)2 · Ca2SiO4 and for that one of the quartz—into larnite and wollastonite. The influence of the α-quartz as a concomitant mineral is considered
to be positive. The α-quartz forms Si–O–Si–OH bonds retaining humidity in the solid phase thus facilitating the isomorphous
substitution OH− → F− with the subsequent formation of partially substituted COHFAp. Calcium silicophosphate and Ca4P2O9 are obtained upon its further heating. The presented here results settle a perspective route for processing of low-grade
phosphate raw materials by means of tribothermal treatment aiming at preparation of condensed phosphates suitable for application
as slowly acting fertilizer components.
The reason for the special thermal behaviour of ammonium nitrate (AN) has been examined. Under certain experimental conditions more transition temperatures were obtained than hitherto found (37–42°, 50° and 86°). With Du Pont DSC curves several exothermic peaks or exothermic oscillations were shown after the endothermic peak at 51°, indicating that phase IV had been transformed to metastable phase III, as a consequence of which the III→II transformation at 86° also became possible. On repeated cycling the exothermic peak decreased or disappeared if the III→II transformation had developed to a greater extent. A successful IV→III transformation was induced by inoculation of AN with phase III, an unusual procedure in investigating the phase transformation of AN. The use of the method is obvious with regard to the fact that all transformations are controlled by the rate of nucleation.