1,721,001 research outputs found
[On the frequency of mongolism in the Clinica Ostetrica e Ginecologica of the University of Padua from 1963 to 1968]
No full textSynthesis and comparative characterization of Al, B, Ga, and Fe containing Nu-1-type zeolitic framework
No full textWe synthesized four different Nu-1-type zeolites containing, respectively, Al, B, Ga, and Fe as the trivalent element. With the exception of B-Nu-1, the synthesis is difficult because of the competitive formation of other crystalline phases as sodalite, ZSM-39-type, or glassy phases. The trivalent elements isomorphous substitution was demonstrated by X-ray powder diffraction, FTi.r., e.s.r., u.v.-Vis., and 27Al, 11B, and 71Ga-MAS-n.m.r. By the same techniques and t.g.a. measurements, we evaluated the thermal stability of the different Nu-1-type zeolites
Investigation on the porosity of zeolite NU-88 by means of positron annihilation lifetime spectroscopy
No full textSeven well characterized zeolites were investigated by positron annihilation lifetime spectroscopy. The lifetime spectra were analysed in four discrete components. The third one was associated with ortho-positronium annihilation in the channels, framed in terms of infinite cylinders. Differences between the radii determined from the positron annihilation technique and X-ray diffraction data were found and explained in terms of the physical structure of the channel. An analogous study on a high-silica NU-88 zeolite gave a value of 0.33 nm for the corresponding radius, in agreement with At and N(2) adsorption data as well as with the catalytic behaviour of this zeolite in several acid catalyzed reactions. The longest lifetime component in NU-88 reveals the existence of mesopores, with average radius of about 1.8 nm, which could explain the importance of hydrogen transfer reactions in this zeolite
DETERMINAZIONE DELLA NATURA CHIMICA DEL NICHEL NEI CATALIZZATORI DI CRACKING CATALITICO FLUIDO ESAUSTI E LORO CLASSIFICAZ IONE COME RIFIUTI
No full textSynthesis, characterization and crystal structure of new microporous bismuth silicates
No full textThe hydrothermal synthesis and structural characterization of two new microporous bismuth silicates named EMS-4 and EMS-5 are reported. The crystal structure of these materials has been elucidated by high-resolution synchrotron X-ray powder diffraction data and further characterized by chemical analysis, thermogravinietry and scanning electron microscopy. The crystal structure of EMS-4 is strictly related to that of the mineral tobermorite 11 angstrom which can be described by infinite layers formed by six- and sevenfold-coordinated metal polyhedra, held together by tetrahedral double chains. This stacking arrangement produces a relatively wide channel system able to host cations (sodium and potassium) and water molecules.
EMS-5, the silicon counterpart of Bi2Ge3O9, can be classified as a clathrate-like structure having small cages with approximate dimensions 4.0 x 4.2 x 2.2 angstrom Although the tetramethylammonium hydroxide (TMA-OH) has been used as structure directing agent in the synthesis, no organic or water molecules are localized inside the EMS-5 cages by the crystal structure refinement. This finding is also confirmed by TG and IR analyses. EMS-4 and EMS-5 materials are the first microporous bismuth silicates reported so far
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
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