1,720,983 research outputs found
Acid/base and surface features of pure phase magnesia powders
No full textThis paper reports data concerning the bulk and surface features of magnesia powders prepared by calcination of different precursor salts at increasing temperatures (873-1253K). The bulk characterization of the oxides has been obtained by X-ray diffraction, while the surface features have been investigated by means of N2 adsorption at subcritical temperatures and X-ray photoelectron spectroscopy. The acid/base features have been analysed by adsorption and step titration of dye indicators with known pK(b) values, in benzene.The powders appear to be composed of pure periclase, with a degree of crystallinity which is influenced by the nature of the precursor salt. The specific surface area and surface acid/base features of the samples are strongly dependent on both the nature of the precursor and the temperature of calcination. On the other hand, XPS data seem to be totally independent of the preparation route of the oxides, and acquire a general valence. Copyright (C) 1998 Elsevier Science B.V
MgO powders: Interplay between adsorbed species and localisation of basic sites
No full textMgO powders have been prepared from different precursor compounds and calcined at temperatures in the range 873-1253 K. Sequences of basic strength of the powders have been obtained, in benzene, by the Hammett-Bertolacini method, as a function of the preparation conditions, ageing and reactivating treatments. All the samples have been submitted to XPS (X-ray photoelectron spectroscopy) determinations to assess their actual chemical state. Data relative to the binding energy of the C1s and O1s peak components are elaborated and cross-compared with the outcome of the acid-base characterisations also in the light of TPD (temperature-programmed desorption) determinations. The factors affecting the surface reactivity and the localisation of the basic sites, at the different temperatures, are discussed. © 1998 Elsevier Science B.V
Structural and morphological features of MgO powders. The key role of the preparative starting compound
No full textThe present paper reports data concerning magnesia samples obtained by calcination of different precursor salts at different increasing temperatures (873-1253 K). The oxides are characterized by x-ray diffraction, scanning electron microscopy, and N2 adsorption at subcritical temperatures. The samples appear to be composed, at any temperature, of pure periclase with a degree of crystallinity which increases with the temperature of calcination. Morphologically, the products have the shape either of lamellas or of cubes of variable dimensions, depending on the nature and route of preparation of the precursor salts. The variation of the specific surface area and the degree of porosity with the nature of the precursors and the temperature is discussed
SO4-ZrO2 catalysts for the esterification of benzoic acid to methylbenzoate
No full textSO4-ZrO2 catalysts, prepared by varying the conditions of oxide preparation and H2SO4 impregnation, have been characterized by means of different techniques (XRD, BET, Hammett-Bertolacini technique, XPS). The esterification of benzoic acid to methylbenzoate has been used to test the different catalysts and their catalytic performance has been discussed in the light of their bulk and surface properties
Magnesium salts and oxide: An XPS overview
No full textXPS measurements have been performed on a series of MgII salts (NO-3, CO2-3, SO2-4, CI-, CH3COO-), hydroxides and oxides, these latter compounds are both commercial and prepared in laboratory. The binding energy (BE) of the anionic partner in MgII salts compares well in any case with literature data for the same anions in other alkaline and alkaline earth compounds. The BE of MgII appears to be affected by the electroattractive nature of the parent anion. The MgII spectra, in the case of MgO samples, are not influenced either by the nature of the oxide precursor salt or by the temperature of the oxide preparation (873 K, 1073 K, 1253 K). The BE of Mg 2p ranges between 49.9 eV and 50.2 eV. Oxygen spectra are regular and show the presence of the surface chemisorbed -OH component even at heating temperatures of 1473 K. The choice, as internal reference, of the hydrocarbon contaminant carbon peak is discussed, specifically, in the case of MgO samples with reference to specific lattice and surface properties of the oxide itself. © 1997 Elsevier Science B.V
Microcrystalline cellulose suspensions : Effects on the surface tension at the air-water boundary
No full textIn the light of the recent clinical employment of microcrystalline cellulose (MCC) as a pharmaceutical device, in the present work the changes in surface tension caused by the addition of MCC powders to aqueous solutions have been studied. Further, analyses to give a preliminary characterization the soluble solute, considered the responsible of the surface tension changes, have also been performed. The Wilhelmy plate technique has been employed for the surface tension determinations, while for the characterization of the solute polymerization degree, elemental analysis, X-ray photoelectron spectroscopy and conductivity measurements have been performed. The obtained results are preliminary discussed in the light of the clinical evidence. (C) 2001 Elsevier Science B.V.
In the light of the recent clinical employment of microcrystalline cellulose (MCC) as a pharmaceutical device, in the present work the changes in surface tension caused by the addition of MCC powders to aqueous solutions have been studied. Further, analyses to give a preliminary characterization the soluble solute, considered the responsible of the surface tension changes, have also been performed. The Wilhelmy plate technique has been employed for the surface tension determinations, while for the characterization of the solute polymerization degree, elemental analysis, X-ray photoelectron spectroscopy and conductivity measurements have been performed. The obtained results are preliminary discussed in the light of the clinical evidence
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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