P. J. Sánchez-Soto

Formation of zirconium titanate powder from a sol-gel prepared reactive precursor

A hydroxoperoxo compound of Zr and Ti (HXPZT) was prepared as a reactive powder by a sol-gel method. X-ray diffraction and SEM studies showed it to be formed from small (ca. 10–15 μm) amorphous particles. The effects of progressive thermal treatment were determined by differential thermal analysis, thermogravimetric analysis, temperature programmed desorption, X-ray diffraction, energy-dispersive spectrometry and SEM. The features observed have been attributed to the removal of physically and chemically bonded water and of some nitrogen-containing compound. Crystallization of zirconium titanate occurs at low temperature (650 °C), giving rise to spherical (ca. 2 μm dia.) regular particles.

Thermal investigation of crystallization of polyethylene glycols in solid dispersions containing oxazepam

A critical issue in the processing of solid dispersions is to elucidate the microstructure of the resulting product. Morphological features such as crystallinity degree of both carrier and drug, and particle size of the latter, have a deep effect on the properties of the drug dissolution. In the present paper, Hot Stage Microscopy (HSM) has been employed to investigate the crystallization of polyethylene glycols (PEG) of different molecular weights used in the processing of a benzodiazepine (oxazepam). The results have shown that the crystalline morphology and the radial growth rate were dependent on the polymer molecular weight, crystallization temperature, and also on the molecular state of the drug incorporated into the polymer, forming a solid dispersion and strongly influencing the drug dissolution rate.

Effect of dry grinding and leaching on polytypic structure of pyrophyllite

Abstract The polytypic structure of a purified pyrophyllite sample from Zalamea (Badajoz, Spain) and the structural alteration induced after dry grinding and leaching were studied by means of infrared spectroscopy, nitrogen adsorption, thermal analysis and by X-ray diffraction methods (reflection and transmission). Simulated X-ray powder diffraction patterns, using preparations with random and preferred orientations with coefficients of orientation g=1 and g=3, were compared with the experimental patterns. It was found that the pyrophyllite is in the polytypic modification 1Tc (1AA-II, 1). As compared to those in simulated diffractograms, the diminished intensity of 11l and 02l reflections is due to random translations along the b axis. A short grinding time caused further degradation of the 11l and 02l diffractions. This leads to disordered forms and not to the formation of a monoclinic polytype as has been claimed previously. DTA showed a temperature shift in the main endothermic peak after grinding. This shift was fastest during the first few minutes of grinding. In addition, DTA peak areas decreased with increasing grinding time and with increasing surface area of the ground silicate. These changes are related to a progressive delamination and structural breakdown during grinding. Prolonged grinding, up to 30 min, created a turbostratic-type structure and a drastic reduction in particle size to about 0.04 μm in diameter and about 0.02 μm in thickness, this size reduction being related to the thermal features. After 30 minutes of grinding the mechanochemical reduction of the original particles appears to have reached a limit. At longer times there is an increasing degree of amorphization and a decrease in the surface area by reaggregation of the ground powder. Thus, the thermal behaviour is strongly dependent on the structural state of the material. The polytypism of pyrophyllite is discussed. The X-ray results are compared with those for other pyrophyllite samples (Honami, Japan, the former Yugoslavia, Venezuela and the Ukraine). It is concluded that the formation of a one-layer triclinic polytype of pyrophyllite or disordered forms is more likely than the formation of the two-layer monoclinic variety.

Grinding effect on kaolinite-pyrophyllite-illite natural mixtures and its influence on mullite formation

The effect of dry grinding (mechanochemical activation) on a natural mixture of aluminium silicates composed of kaolinite, pyrophyllite and potassium mica (illite) was studied. The evolution of the ground material was followed by X-ray diffraction, differential thermal analysis, thermogravimetric analysis, scanning electron microscopy and nitrogen adsorption methods. Grinding produces a strong structural alteration of the silicates that constitute the mixture, increasing the surface area and decreasing particle size, with progressive amorphization and formation of agglomerates as detected by X-ray diffraction and scanning electron microscopy. Marked changes were observed in the thermogravimetric analysis and differential thermal analysis curves as influenced by grinding, with weight losses at lower temperatures and the appearance of an exothermic differential thermal analysis effect at 985‡C, which increases in intensity and sharpness as grinding time proceeds. After grinding using the pure silicates, kaolinite, pyrophyllite and illite, it was found that the increase in intensity and sharpness of this exothermic effect was only associated with the presence of pyrophyllite in the natural mixture irrespective of the presence of kaolinite and/or illite, at least up to 325 min grinding. This effect is produced in the ground mixture by mullite formation mainly from pyrophyllite by combined mechanical (by grinding) and thermal treatments.

On the influence of chemical processing in the crystallization behaviour of zirconium titanate materials

The aim of this study is to relate the crystallization phenomena of Zr TiO 4 powders starting from different forming gels as precursors. The presence of hydrogen peroxide in the chemical processing of amorphous reactive precursors plays a key role in controlling the crystallization temperature. Thus, the temperature can be as low as 640°C

Effect of grinding and water vapour on the particle size of kaolinite and pyrophyllite

Abstract The increase in specific surface of kaolinite (K) and pyrophyllite (P) induced by dry grinding in an oscillatory mill, proceeded during storage in water vapour. The average particle thickness, δ, changed from 42 nm (K) and 66 nm (P) in the original materials to 12 nm (K) and 20 nm (P) after water sorption (20 days at relative humidity RH = 1.0, at room temperature and pressure) and to similar values of 13 nm (K) and 16 nm (P) after grinding for 10 min (in agreement with some published data). The action of water molecules on ground clays (at the conditions indicated) resulted in a further decrease in δ. In pyrophyllite, prolonged grinding (30 min) and prolonged action of water molecules (36 days) caused a particle collapse. After pre-storing at RH = 0.5 the successive decrease in δ at RH = 1.0 was smaller.

ZrO2SiO2 mixed oxides: surface aspects, photophysical properties and photoreactivity for 4-nitrophenol oxidation in aqueous phase

Mixed oxides of ZrO2 and SiO2 were synthesized by a sol-gel technique by using zirconyl chloride and commercial silica gel as starting materials. The paper reports some surface features of the powders and the photoreactivity results obtained in the oxidation of 4-nitrophenol in aqueous suspensions of powdered photocatalyst. The optical characterization of the suspension was also performed by measuring the photon flows reflected and absorbed by the reacting system. The photoactivity exhibited by the ZrO2SiO2 samples is very scarce especially if compared with that shown by a TiO2 sample tested at equal experimental conditions. The low photoreactivity can be justified on the basis of two main reasons: (i) the unfavourable optical features of ZrO2SiO2 suspensions in comparison with those of TiO2 ones; and (ii) the lower degree of hydroxylation of ZrO2SiO2 samples with respect to that of TiO2 and, consequently, the less significant oxygen adsorption.

Thermal study of vermiculites and mica-vermiculite interstratifications

Simultaneous DTA-TG has been carried out on a set of natural vermiculite samples. Based on their dehydration behaviour the samples can be divided in two groups: (a) those with DTA endothermic peak temperatures at 140°–150°C and 240°–270°C (pure vermiculties) and (b) those with peak temperatures at 95°–115°C (vermiculite with mica or mica-vermiculite interstratifications). The low temperature at which the endothermic effect in group (b) appears is discussed on the basis of dilution due to the inert layers of mica, differences in chemical composition, and lowering of interlamellar water bond energy.ZusammenfassungAn einer Reihe von natürlichen Vermikulitproben wurde simultane DTA-TG durchgeführt. Anhand ihres Dehydrationsverhaltens können diese Proben zwei Gruppen zugeordnet werden: (a) jene mit endothermen DTA-Peaktemperaturen von 140°–150°C und 240°–270°C (reine Vermikulite) und (b) jene mit Peaktemperaturen von 95°–115°C (Vermikulit mit Glimmer oder Glimmer-Vermikulit Zwischenlagerungen.). Die niedrige Temperatur, bei der in Gruppe (b) der endotherme Effekt auftritt, wird anhand der Verdünnung durch inerte Glimmerschichten, den Unterschieden der chemischen Zusammensetzung und der Erniedrigung der interlamellaren Wasserbindungsenergic besprochen.

Thermal characterization of polyethylene glycols applied in the pharmaceutical technology using differential scanning calorimetry and hot stage microscopy

In the present study, the effect of the molecular weight and thermal treatments on commercial polyethylene glycols (PEG) samples used in the pharmaceutical processing technology, has been analyzed using DSC and HSM. The molecular weight of these polymers range from 1500 to 200000. Thermal investigations on the melting behavior of original PEG samples (as received from the manufacturer) showed only one single melting DSC endotherm effect before 373 K. This fact was associated to the presence of only one type of polymeric chain. Using standard conditions, PEG samples were solidified from the melt at 373 K, either by flash cooling (using liquid nitrogen and an ice bath) and by slow cooling, soaked and by slow cooling at room temperature. They were further studied by DSC. It was found that after cooling, PEG with molecular weight 1500 and 15000 showed DSC thermograms with a single endothermic peak. However, thermograms for PEG 4000 and 6000 produced a splitted melting endotherm. This fact was attributed to the presence of two types of chains, that are the folded and extended chains.Ageing time influences also the shape of the DSC endothermal effects. It was concluded that the endotherms obtained after heating these PEG indicate that the thermal history determine the structure (extended or folded chain type forms) and the degree of crystallinity, as evidenced by changes in heat of fusion values, melting points and structures after crystallization. The relationships between melting enthalpies and melting points, as deduced from DSC diagrams, with molecular weight of the polymers are also presented.

Thermoanalytical study of the pesticide 3-amino-1,2,4-triazole

Thermoanalytical study of the pesticide 3-amino-1,2,4-triazole (ATA) has been carried out, using simultaneous DTA-TG in nitrogen flow, in order to know its thermal behaviour and stability. These techniques have been further complemented using evolved gas analysis and mass spectroscopy (EGA-MS). Two different stages of ATA decomposition were observed: after the first decomposition step, a mixture of compounds is obtained, according to MS data, being the principal component a compound of molecular weight 126. It is formed by a first order reaction mechanism, according to the kinetic study, withEa=124±8 kJ·mol−1. The second decomposition step takes place about 735°C, with evolution of HCN and NH3, being the final weight loss 96%.ZusammenfassungVorliegend wird unter Einsatz simultaner DTA-TG im Stickstoffluß eine thermoanalytische Untersuchung des Pestizides 3-Amino-1,2,4-triazol (ATA) durchgeführt, um dessen thermisches Verhalten und Stabilität zu bestimmen. Diese Techniken wurden weiterhin durch Analyse der freigesetzten Gase und durch Massenspektroskopie (EGA-MS) ergänzt. Es wurden zwei verschiedene Schritte der Zersetzung von ATA beobachtet: nach dem ersten Zersetzungsschritt erhält man ein Verbindungsgemisch, wobei die Hauptkomponente laut MS-Angaben das Molekülgewicht 126 besitzt. Sie wird in einer Reaktion erster Ordnung gebildet,Ea beträgt laut kinetischer Untersuchung 1248 kJ/mol. Der zweite Zersetzungsschritt findet unter Freisetzung von HCN und NH3 bei etwa 735°C statt, der endgültige Gewichtsverlust beträgt 96%.