1,720,988 research outputs found
Local distortions in the structure of nanocrystalline gahnite
No full textGahnite is a zinc-aluminum spinel with stoichiometric formula ZnAl2O4 and compact cubic structure (fcc), belonging to the space group Fd-3m. The zinc ion shows a tetrahedral coordination with four oxygens, while aluminum is placed in the octahedral site: the degree of inversion (i.e. the fraction of trivalent atoms in the tetrahedral sites) is very small and largely temperature independent. A lot of research work has been done to show the different properties of nanospinels (see [1] for example). These technological properties seem to be enhanced as a function of decreasing grain size (see [2] for example). The primary purpose of this work is the structural study of nanocrystalline gahnite and its possible local distortions. Two samples of nanocrystalline gahnite have been synthesised with a hydrothermal method, and then calcined at different temperatures (340°C, 700°C, 800°C, 900°C). The data collected from the high-energy beamline ID15B at the European Synchrotron Radiation Facility in Grenoble (F) were used applying classical crystallography using the Rietveld method (with many limitations), and the analysis of Pair Distribution Function (PDF) in order to explore its structure, both locally and on average. Pair Distribution Function analysis shows a good agreement with the crystallographic structure in the average region; the local part (below 10Å) most influenced by the diffuse scattering, cannot be properly fit with this structure, even though the fit improved with calcination temperature. The local part was studied using two different distortion approaches, to understand the local structure better, but with the simplest possible models (i.e. the one with least refinable parameters.). The APPROACH I identifies three different zones of distortion, with increasing r: this resulted in good values of the Rw parameter, but such a model was difficult to understand from the physical point of view. The APPROACH II was applied by modelling a distortion in the octahedral site, easier to deform than the very stiff tetrahedral site. This again gave very good Rw values (lower than in the other approach), with the advantage of being easy to comprehend. The distortion obtained is similar for the two powders samples, regardless of the synthesis conditions. In addition, the TEM observations, independent from the diffraction data, confirm the good accuracy in the calculation of the diameters of nanoparticles through Pair Distribution Function methodology. The small distortion refined is compatible with the extreme ordering of the particles observed by TEM (lattice planes are clearly visible also in the sample with the grains of 2 nm). This study provides a model that, though approximate and certainly not unique, gives account of the local distortions with good accuracy, with growing similarities with the crystallographic structure as a function of calcination temperature.
[1] I. Miron, C. Enache, M. Vasile, I. Grosescu, Phys. Scr. (2012), 149, 014064
[2] T. Sirikajorn, O. Mekasuwandumrong, P. Praserthdam, J.G. Goodwin Jr. and J. Panpranot, Cat.Lett. (2010) 501, 10
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
Pair distribution function structural investigation: BaTi1-xCexO3 as a locally disordered perovskite
No full textNowadays barium titanate has become the most widely used functional ceramic material in electronics, finding wide applications in devices as microphones, ultrasonic and underwater transducers, multilayer ceramic capacitors and spark generators [1]. In order to tailor the properties for the specific applications, BaTiO3 is easily and usually doped to improve the material performance, for example modifying the long-range order and consequently the ferroelectric and dielectric properties. The solid solutions with other BaMIVO3 perovskites (M =Sn, Zr, Hf, Ce) show an almost continuous variation of the ferroelectric behaviour with composition, from conventional ferroelectric, via diffuse ferroelectric transition to a clear relaxor state and further to dipolar glass behaviour [2]. Different researches have investigated these systems, but the relation with the average crystallographic structure and the local order is still mostly unknown. Just few previous works, as for the BaTi1-xZrxO3 [3], have addressed the issue demonstrating the existence of a structural local disorder linked to the evolution of the polar behaviour. In order to improve our knowledge on the evolution of polar order in these systems more structural investigations are necessary.
In this work the case of BaTi1-xCexO3 is presented and the average and local structure variations as a function of composition and temperature are shown. Ce4+ (r =0.87 Å) is much bigger than Ti4+ (r = 0.605 Å), so this system represents a limit and interesting case also because this kind of substitution does not involve the creation of charge compensating lattice defects. In particular BaTi1-xCexO3 ceramic solid solutions with x = 0.05, 0.10, 0.20 have been investigated between 100 and 400 K. As indicated by dielectric permittivity measurements, the three samples under study correspond to a different polar behaviour: conventional ferroelectric (x = 0.05) but close to the so-called pinched transition, diffuse phase transition (x = 0.10) and non-ergodic relaxor (x = 0.20).
Total scattering data have been collected at ID22 high resolution beamline, ESRF (European Synchrotron Radiation Facility; Grenoble, France). This because there are subtle phase transitions in the studied temperature range, that need very high resolution data to be properly detected. Pair Distribution Function (PDF) refinements have been performed with the aim of understanding the induced differences between average and local structure. The results show a clear local disorder which exhibits displacements in atomic positions or even different crystallographic space group compared with the average structure. These local deviations are caused by the evolution of Ti-O and Ce-O bond distances with temperature and by the increase of the cerium amount.
[1] S. Yasmin, S. Choudhury, M.A. Hakim, A.H. Bhuiyan and M.J. Rahman J. Mater. Sci. Technol. 2011, 27(8), 759-763.
[2] V.V. Shvartsman and D.C. Lupascu J. Am. Ceram. Soc. 2012, 95 (1), 1-26.
[3] V. Buscaglia, S. Tripathi, V. Petkov, M. Dapiaggi, M. Deluca, A. Gajovic, Y.J. Ren J. Phys.: Condens. Matter 2014, 26, 065901
Average and local structural comparison of BaTi1-xCexO3 by pair distribution function
No full textBaTiO3 is considered as a typical ferroelectric material. It presents, with a temperature decrease, the classical phase transition from a paraelectric cubic structure to ferroelectric phases (progressively: tetragonal, orthorhombic and rhombohedral). The solid solutions with other BaMIVO3 perovskites (M=Sn, Zr, Hf, Ce) can strongly modify these transitions and the related polar behaviors. Some materials show, dependently from the dopant type and amounts, a variation from conventional ferroelectric, via diffuse ferroelectric transition to a clear relaxor state and further to dipolar glass behaviour [1]. Considering that BaTiO3-based solid solutions are environment-friendly dielectrics with similar performances as many toxic Pb-based electroceramics [2], the study of these compounds becomes a topic of great interest and a current matter. Different researches have investigated these systems, but the relation with the average crystallographic structure, the local order and electric properties is still mostly unknown. Just few previous studies, as for the BaTi1-xZrxO3 [3], have addressed the issue demonstrating the existence of a structural local disorder linked to the evolution of the polar behaviour. In this work BaTi1-xCexO3 ceramic solid solutions with different cerium amounts (x = 0.05, 0.10, 0.20) are presented. This system represents a limit and interesting case due to a large difference in the Ti and Ce atomic radius ( rCe4+ = 0.87 Å and rTi4+= 0.605 Å), and because this kind of substitution does not involve the creation of charge compensating lattice defects. As indicated by dielectric permittivity measurements, the three samples under study correspond to a different polar behaviour: conventional ferroelectric (x = 0.05) but close to the so-called pinched transition, diffuse phase transition (x = 0.10) and non-ergodic relaxor (x = 0.20). Total scattering data has been collected ID22 high resolution beamline, ESRF (European Synchrotron Radiation Facility; Grenoble, France) in a range between 100 and 400 K every 100 K to explore the phase transitions associated to each sample. Pair Distribution Function (PDF) refinements have been performed with the aim of understanding the induced differences between average and local structure as a function of composition and temperature. The Rietveld and average PDF results show a good agreement with the structural hypothesis suggested by the dielectric permittivity measurements. On the other hand, PDFs analysis also highlights a clear local disorder for all the samples. Often local and average structure have required different space groups to be fitted appropriately. PDFs comparison and structural refinements demonstrate in fact a clear local evolution of Ti-O and Ce-O bond distances with temperature and by the increase of the cerium amount.
[1] V.V. Shvartsman and D.C. Lupascu J. Am. Ceram. Soc. 2012, 95(1), 1-26.
[2] C. Ciomaga, M. Viviani, M.T. Buscaglia, V. Buscaglia, L. Mitoseriu, A. Stancu, P. Nanni J. Eur. Ceram. Soc. 2007, 27, 4061-4064.
[3] V. Buscaglia, S. Tripathi, V. Petkov, M. Dapiaggi, M. Deluca, A. Gajovic, Y.J. Ren J. Phys.: Condens. Matter. 2014, 26, 065901
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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