1,721,086 research outputs found
Cation migration in zeolites: An in situ powder diffraction and MAS NMR study of the structure of zeolite Cs(Na)-Y during dehydration
No full textIn situ synchrotron X-ray powder diffraction and Cs-133 and Na-23 MAS NMR have been used to investigate the cation migration and ordering in samples of cesium-exchanged zeolite NaY as a function of temperature and cesium cation-exchange level and during dehydration. Samples were prepared with cesium-exchange levels varying from 68 to 83% of the total cation content by carrying out repeated ion-exchange and calcination steps. Lower cesium content samples contain sodium cations in the sodalite cages and cesium cations in the supercages (SII and SIII) directly after the ion-exchange process. After dehydration at 350 degrees C and above, sodium cations are observed in the supercages (in SII) and in the double B-rings (SI). A maximum of 8 cesium cations/unit cell (1/sodalite cage) migrate from the super to the sodalite cages, occupying SII' and SI' positions. The supercage sodium cations are then exchanged for cesium in the subsequent ion-exchange steps, increasing the cesium content. In situ X-ray data, collected during dehydration, showed that the sodium migration from SI' to SI occurs initially (at > 180 degrees C). This migration appears to be accompanied by, or drives, the migration of Cs+ from the supercages to the sodalite cages, which occurs at approximately the same temperature (> 200 degrees C). It is not until > 300 degrees C that the migration of SI' sodium cations to SII is observed. Significant variations in the cation occupancies within the cages are seen at different temperatures in the dehydrated samples. For example, at 500 degrees C, there are an equal number of SII and SIII' cesium cations in the lower cesium content sample. On cooling, the cesium cations order in the SII position, the SIII' occupancy dropping from 12.5 to 7. An ordering scheme for the cations in the supercage is suggested to explain these observations. A number of resonances are seen in the Cs-133 MAS NMR, which are assigned, making use of the occupancies obtained from the Rietveld refinements, to the various cesium positions. The lack of spinning sidebands associated with some of the resonances indicates that the some cations in the supercages are mobile, even at room temperature. When the temperature is raised, a number of SII and Sm resonances coalesce, as the cation mobility in the supercages increases
Sorbate rearrangement and cation migration in HFC-134 loaded NaY zeolite: A temperature dependent synchrotron powder diffraction study
No full textThe HFC-134/NaY binding has been monitored by temperature dependent X-ray powder diffraction. Diffraction patterns were obtained as a function of different loading levels to investigate the rearrangement of the cations and of the HFC-134 molecules, in the temperature range 100-230 K. In situ real-time data were collected using a Translating Imaging Plate System (TIPS) at the NSLS (USA). Rietveld structure refinements were performed for the HFC-134 (32 molecules per unit cell [m/uc = molecules per unit cell] or 4 molecules per supercage [m/sc = molecules per supercage]) using data collected from 100 to 230 K. A phase transition is observed at about 180 K (A to B phase transition) where a disordering of the HFC-134 molecule takes place and modification of its surrounding due to Na+ migrations is observed. These results are preliminary and the experiment with the 32 m/ue should be repeated in order to assess whether the observed effect is reproducible
Combined MAS NMR and X-ray powder diffraction structural characterization of hydrofluorocarbon-134 adsorbed on zeolite NaY: Observation of cation migration and strong sorbate-cation interactions
No full textNa-23 MAS NMR and synchrotron X-ray powder diffraction methods have been used to study the binding of hydrofluorocarbon-134 (HFC-154, CF2HCF2H) in zeolite NaY. A contraction of the volume of the unit cell is observed on gas adsorption, and the interaction of HFC-134 with the extraframework sodium cations is so strong that extraframework sodium cations in the sodalite cages (site I') migrate into the supercages. These sodium cations are found on positions close to the site III' positions of zeolite NaX. Both ends of the HFC molecules are bound to sodium cations, the HFC molecule bridging the site II and III' cations in the supercages. The strong cation-HFC interaction results in a considerable displacement of the sodium site II cation along the [111] direction into the supercage and an increase in the T-O-T bond angle for the three oxygen atoms coordinated to this cation. A decrease in the Na-23 quadrupole coupling constant on HFC adsorption from 4.4 to less than 2.8 MHz, for the sodium cations originally located in the sodalite cages (site I'), is consistent with the sodium cation migrations
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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