Showing papers by "Pedro José Sánchez-Soto published in 1997"

Influence of Grinding in Pyrophyllite−Mullite Thermal Transformation Assessed by 29Si and 27Al MAS NMR Spectroscopies

Abstract: The influence of grinding on the thermal transformation of pyrophyllite to mullite is analyzed by 27Al and 29Si MAS NMR spectroscopies, using X-ray powder diffraction and thermal analysis (DTA-TG) as complementary techniques. Mechanical treatments by dry grinding produce partial destruction of pyrophyllite and rearrangement of Al ions in tetra- and pentahedral environments. Tetrahedral sheet breakdown favors Al atomic diffusion and formation of Si-poor nuclei of mullite at 1000 °C. In particular, the exothermic peak detected at 1000 °C in DTA curves is ascribed to transformation of pentahedral coordination of Al in ground samples into tetra- and octahedral aluminum of mullite. During thermal decomposition of pyrophyllite, segregation of amorphous silica is produced. These results are compared with those obtained in kaolinite−mullite transformation, which were previously analyzed.

Effects of H2O2 and SO42- Species on the Crystalline Structure and Surface Properties of ZrO2 Processed by Alkaline Precipitation

Abstract: The preparation of ZrO2 by alkaline hydrolysis of ZrOCl2·8H2O, leads to an amorphous oxohydroxide precursor that upon calcination at temperatures higher than 350 °C produces a system with mixed cry...

Preparation and Characterization of Amorphous ZrO2-SiO2 Composite Powders Processed by Sol-Gel Chemistry

Abstract: Composite ZrO2-SiO2 powders, with different ZrO2 contents, including pure ZrO2 powders, were prepared by precipitation in SiO2 suspensions, of zirconia gels from solutions of zirconyl chloride at pH = 11. These products were investigated in connection with the phase changes in ZrO2 caused by heat-treatments. ZrO2-SiO2 mixtures containing 0–100% mol ZrO2, were studied by differential thermal analysis (DTA), X-ray powder diffraction (XRD), temperature programmed desorption combined with mass spectroscopy (TPD-MS), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX), to obtain information on the morphological and structural features of the particles before and during the heat treatment up to 1200°C. Specific surface areas were determined using nitrogen adsorption by the BET method. The results offer an explanation about some of the factors which can be influencing on the stabilization of metastable-cubic/tetragonal (C/T) phase of ZrO2 and the evolution of surface areas (“vulcano” profile) observed in the composites.

Thermal and spectroscopic characterization of several derivatives containing a new organic ring system

Abstract: Several derivatives containing a new organic ring system, the tropane-6-spiro-5′-hydantoin structure (namely 8-alkyl-8-azabicyclo [3.2.1.] octane-6-spiro-5′-imidazoline-2′,4′-diones) have been characterized by thermal (DSC and simultaneous DTA-TG-DTG) and spectroscopic techniques (IR,1 H-NMR,13 C-NMR). X-ray powder diffraction and elemental analysis were applied for structural and molecular characterization. All the compounds melt in the range 160–250°C and undergo decomposition with progressive mass loss after the solid-liquid thermal transition with molecular degradation. It was found that tropane-6-spiro-5′-hydantoin derivatives with the hydantoin ring in β position are thermally less stable than those containing this ring in α position.

Thermal study of an aged dredged sludge

Abstract: A dredged sludge was studied to investigate the influence of ageing and of pretreatment on its drying rate, water sorption/retention, thermal mass loss, XRD and microstructure (SEM). Ageing caused change in particle thickness and specific surface, a gradual aggregation to form units of the size 10–50 μm, formation of macropores of similar size, unhomogeneity and fissures between aggregates and “super-aggregates”. Macropores were detectable by the initial drying rate especially at 45°C. They indicated a tendency of collapsing at a lower drying rate at 30°C. This is consistent with SEM observations. With ageing the aggregates were more compact and less sensitive to drying. The aggregated system indicated a higher initial drying rate (higher permeability), whereas stirring induced a lower drying rate, favouring the formation of compact laminar structure. XRD peak intensity was lowered with ageing due to decrease in crystallinity (stacking faults and/or decrease in crystallite size). The content of amorphous material was lowered as well, reducing water sorption/desorption, which indicated that the specific surface is lower. From the suitable microstructure induced by ageing some new phases may form (feldspar, zeolites), preferably in the coarser fraction of the sludge. This is disturbed by stirring which operation expels also carbonates from the particle edges and this may reduce the structural strength of the sludge. In aged bentonite suspension a similar tendency was observed of formation of specific microstructures capable of phase transformation, e.g. to feldspar.