1,721,324 research outputs found
PARTICLE-SIZE DISTRIBUTION AND X-RAY-DIFFRACTION PEAK PROFILES IN SUPERSATURATED SOLID-SOLUTIONS
Full text linkOSCILLATIONS IN THE POROD PLOT AND PARTICLE SIZES IN DEMIXED GLASSES
No full textA new theoretical approach, based on the oscillations sometimes observed in the Porod plot, has been used to estimate particle sizes in phase separated glasses. We have studied the demixing kinetics consequent upon appropriate isothermal heating of the glasses of the quaternary system SiO2-Li2O-TiO2-Al2O3 of the mol% compositions (70,25,2,3), (70,21,6,3), and (70,17,10,3). Glasses of the first composition did not yield appreciable x-ray intensities. Oscillations were observed in the scattered intensities of glasses of the second composition, the amplitudes increasing with the duration of heating. By using a theoretical expression which we recently developed we have easily determined the particle sizes and the degree of monodispersity
PREPARATION AND STRUCTURAL CHARACTERIZATION OF ULTRAFINE ZIRCONIA POWDERS
No full textFactors concerning the stabilization of the tetragonal and cubic phases, metastable at room temperature, with respect to the monoclinic stable phase in ultrafine zirconia powders are studied. The importance of Na+ ions in the initial zirconia amorphous gel in obtaining a cubic phase has been confirmed. By an X‐ray diffraction study using a new peak profile fitting procedure, the amount of the crystalline phases and their microstructural properties (crystallite size and lattice distortions) have been obtained. Copyright © 1989, Wiley Blackwell. All rights reserve
A PROFILE-FITTING PROCEDURE FOR ANALYSIS OF BROADENED X-RAY-DIFFRACTION PEAKS .2. APPLICATION AND DISCUSSION OF THE METHODOLOGY
No full textThe convolutive X-ray peak profile-fitting methodology described in the previous paper [Enzo, Fagherazzi, Benedetti & Polizzi (1988). J. Appl. Cryst. 21, 536-542] has been applied to a series of fluorite samples milled for different times and two zirconia ultrafine powders, by using either pseudo-Voigt or Voigt (in the Kielkopf approximation) functions, in order to investigate the broadening due to microstructural factors (crystallite size and lattice distortions). In the fluorite milled powders Fourier analysis (Warren & Averbach and Vogel, Haase & Hosemann methods) has shown that lattice disorder is due to dislocations. Values for the crystallite size as well as for the lattice distortion parameters similar to those obtained with Fourier methods have been found using a suitable integral breadth plot based on an assumed Cauchyian shape for the broadened profiles. A very high computability limit was reached for the crystallite size (about 5000 A). In the ultrafine powders of zirconia the cubic, tetragonal and monoclinic polymorphs were studied. The noticeable Gaussian content present in the 222 reflections for both cubic and tetragonal forms can be explained in terms of the presence of'intercrystalline'-like distortions. This kind of lattice distortion, detected by means of the Fourier analysis, has also been confirmed by 'simplified' methods (two-line and single-line procedures). Examples of limitations of the present methodology are reported and discussed
CRYSTALLINITY DETERMINATION OF PARTIALLY CRYSTALLIZED GLASSES OF THE SYSTEMS SIO2-LI2O-ZNO AND SIO2-LI2O-TIO2-AL2O3 - A COMPARISON OF DIFFERENT METHODS
No full textINFLUENCE OF THE VALVE METAL-OXIDE ON THE PROPERTIES OF RUTHENIUM BASED MIXED-OXIDE ELECTRODES .1. TITANIUM SUPPORTED RUO2/TA2O5 LAYERS
No full textRuO2/Ta2O5 mixed oxide electrodes have been investigated by XPS-AES techniques and X-ray diffractometry and their electrochemical behaviour for the oxygen evolution reaction has been studied. The surface analysis indicated a relative insensitivity of the surface composition to the changes of the Ru/Ta ratio in the bulk of the coatings. The diffractometric study showed the existence of a RuO2 rutile phase and a Ta2O5 amorphous phase in the electrode materials. no evidence for solid solutions was found. The influence of the noble metal concentration in the coatings on their catalytic activity has been studied for different oxide loadings. Maxima of Activity for a Ru/Ta atom ratio close to 1 have been found for coating thicknesses of 1 μm. Thicker layers exhibit an activity increasing with the noble metal content. The correlation between anodic charge density, q*, and current density, i, for the oxygen evolution reaction has been tested and definite trends related to the morphology of the electrode material have been assessed. © 1986
PUMICE-SUPPORTED PALLADIUM CATALYSTS .2. SELECTIVE HYDROGENATION OF 1,3-CYCTOOCTADIENE
No full textTwo series of pumice-supported palladium catalysts (W and U) have been tested in the liquid-phase selective hydrogenation of 1,3-cyclooctadiene (1,3-COD) to cyclooctene (COE). The two series of catalysts, obtained via organometallic precursors, differ in the preparation procedure. In the W series the reduced metal derives only from Pd intermediates anchored to pumice; in the U series the metal originates also from unreacted Pd(allyl)(2) species in solution. The U catalysts present agglomerated metal particles. The hydrogenations, free of any diffusion problems, were performed at constant pressure of hydrogen (1 at-m). Analysis of the data suggests that the rate-determining step is a surface reaction involving activated 1,3-COD. Selectivity is very high since cyclooctane (COA) is detected only when all 1,3-COD is consumed. The turnover frequencies (TOF1) of the first semihydrogenation do not change with palladium dispersion, determined from the Pored diameter D-p up to D-x < 35%, but thereafter they slowly decrease. Experimental evidence, such as the absence of oxidation when exposed to air and the negative shift of binding energy of the Pd 3d level in WS measurements, indicate a different behavior of the present catalysts in comparison with other supported palladium catalysts. These differences are attributed to the presence of alkali metal ions (Na+, K+) on the pumice surface and are explained on the basis of the change in electron density and/or in the ensemble size of the supported palladium. Several interesting features and differences in activity and selectivity between the two series of catalysts can be accounted for by the presence of agglomerated palladium particles in the U series and are discussed in terms of the evolution of parameters with dispersio
STRUCTURAL INVESTIGATIONS OF TIO2-SIO2 GLASSY AND GLASS-CERAMIC MATERIALS PREPARED BY THE SOL-GEL METHOD
No full textTHERMAL EVOLUTION AND MECHANICAL-PROPERTIES OF VITREOUS FIBERS IN THE SIO2-LI2O-TIO2-AL2O3 SYSTEM
No full textA PROFILE-FITTING PROCEDURE FOR ANALYSIS OF BROADENED X-RAY-DIFFRACTION PEAKS .2. APPLICATION AND DISCUSSION OF THE METHODOLOGY
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