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XRD and EXAFS studies on the structure of Er3+-doped SiO2-HfO2 glass-ceramic waveguides: Er3+-activated HfO2 nanocrystals
No full textX-ray absorption spectroscopy studies of glass structure
No full textIn the last few decades, X-ray Absorption Spectroscopy (XAS) has recorded a remarkable amount of interest in the study of glass structure for its sensitivity to the Short Range Order (SRO) around a selected atomic species. XAS greatest evolution started in the seventies of the last century thanks to an adequate and structurally useful theoretical interpretation of the Extended X-ray Absorption Fine Structure (EXAFS) beyond the absorption edge and to the contemporaneous advent of synchrotron radiation facilities. EXAFS probes the local structure of liquid, vitreous, amorphous, and crystalline materials; it is particularly important for noncrystalline systems since they do not give rise to the Bragg diffraction. EXAFS is also used to determine the local environment of impurities contained in glasses, even if they are very diluted. Today, a reliable theoretical description of glassy system structure through modern modelling techniques should be carried out on the basis of x-ray and/or neutron scattering patterns, along with a careful use of the information derived from SRO techniques like EXAFS, NMR, etc. The use of EXAFS in the case of nanocrystalline glass-ceramic systems is very important, since the obtained information are complemental to the ones determined by the Bragg diffraction. The paper gives a brief description of EXAFS theory and analysis methods in the case of disordered systems. Examples of state-of-the-art studies in different glassy systems will be presented along with a critical assessment of EXAFS potential for such systems
Er3+-activated SiO2-based glasses and glass-ceramics: from structure to optimisation
No full textThis paper describes and compares the local environments of Er3+ ions in pure SiO2, SiO2-Al2O3, SiO2-TiO2, and SiO2-HfO2, glasses, and in SiO2-HfO2 glass-ceramic waveguiding systems. This structural information is useful for selecting waveguides with optimised functional properties. This comparison has emphasised the peculiarity of the SiO2-HfO2 glass, in which the Er3+ ions are incorporated in HfO2-rich regions. By appropriate heat treatment, a transformation of SiO2-HfO2 system from glass to glass-ceramic occurs, where Er3+ remains incorporated in HfO2 nanocrystals as substituent of Hf4+. The spectroscopic interpretations are coherent with the present structural results
Local structure around Er3+ in SiO2-HfO2 glassy waveguides using EXAFS
No full textEr3+-doped SiO2-HfO2 glassy waveguides with HfO2 concentrations ranging from 10 to 50 mol % were prepared using the sol-gel route and deposited on v-SiO2 substrates using the dip-coating technique. The local environment around Er3+ ions was determined from Er L-3-edge extended x-ray-absorption fine-structure (EXAFS) measurements. The first coordination shell around Er3+ ions is composed of oxygen atoms. Hafnium is the main constituent of the second coordination shell of Er3+, differing from the cases of pure SiO2 and SiO2-TiO2 glassy hosts, in which silicon is the main atomic species. The local structure around Er3+ ions has been found to be independent on HfO2 concentration within the studied composition range. This fact implies that Er3+ ions are preferentially dispersed in HfO2-rich regions of the glassy waveguide, even at the lowest HfO2 concentration. For all samples, no Er3+-Er3+ coordination shell has been detected by EXAFS. The presented structural results allow us to understand some spectroscopic properties typical of Er3+-doped SiO2 glassy waveguides co-doped with HfO2
XRD and EXAFS studies of HfO2 crystallisation in SiO2-HfO2 films
No full textThis paper reports a detailed structural study on the nucleation of t-HfO2 nanocrystals in thin films of 70SiO(2)-30HfO(2) prepared by sol-gel route on v-SiO2 substrates. Thermal treatment was performed at different temperatures ranging from 900 to 1200 degrees C for short (30 min) or long (24 h) time periods. Crystallisation and microstructure evolutions were traced by X-ray diffraction (XRD). The local structure around hafnium ions was determined from Hf L-3-edge extended X-ray absorption fine structure (EXAFS) measurements carried out at the BM08-GILDA Beamline of ESRF (France). XRD shows the nucleation of HfO2 nanocrystals in the tetragonal phase after heat treatment at 1000 degrees C for 30 min, and a partial phase transformation to the monoclinic phase (m-HfO2) starts after heat treatment at 1200 degrees C for 30 min. The lattice parameters as well as the average crystallites size and their distributions were determined as a function of the heat treatment. EXAFS results are in agreement with the XRD ones, with hafnium ions in the film heat treated at 1100 degrees C for 24h are present in mixed phases. (c) 2006 Elsevier Ltd. All rights reserved
Short-range order around Er3+ in silica waveguides containing aluminium, titanium and hafnium
No full textEr3+-doped silica waveguides, co-doped with aluminium, titanium, and hafnium oxides, were prepared using the sol-gel method and dip-coating processing. Here, we present a characterisation of the local environment around Er3+ ions, as determined from the Er L-3-edge extended X-ray absorption fine structure (EXAFS) measurements performed at ESRF (France). The first coordination shell is composed of 5-6 oxygen atoms at distances -2.32-2.35 angstrom, slightly varying as a function of the modifier oxide. Er3+ nearest neighbors distance does not show a significant compositional dependence. On the contrary, outer shells analysis shows significant differences: Al2O3 doping (less than 2% mol) induces an ordering around Er and increases the distance of the second shell, probably due to the substitution of Si by Al atoms; for TiO2 doping (7-15% mol), it is most probable that the second shell is composed from Si atoms as the case of pure SiO2; for HfO2 doping (from 10 to 50 mol%) there is a very clear evidence of Er-Hf coordination already at the lowest Hf content, but still in amorphous environment. (c) 2005 Elsevier B.V. All rights reserved
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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