1,721,027 research outputs found
Dynamics of Yb2+ to Yb3+ ion valence transformations in Yb:YAG ceramics used for high-power lasers
Full text linkDue to the good thermal-mechanical and luminescence properties, Yb:YAG ceramics are suitable as thin-disk lasers; however, their efficiency is limited by the presence of Yb2+ ions, which entail parasitic energy transfer from Yb3+ to Yb2+. This article focuses on the Yb2+ formation in Yb:YAG ceramics prepared by solid-state reaction sintering. The samples were prepared by air annealing, the oxidation of the material leads to recharging Yb2+ ion to its trivalent state. The activation energy was determined by Jander to be Ea(D) = 2.7 ± 0.2 eV, which is in good agreement with the activation energy for oxygen diffusion in the YAG lattice. It was concluded that the recharging of Yb2+ ion to its trivalent state in YAG ceramics is limited by the oxygen self-diffusion through the grain volume, and the oxygen vacancy alone and/or together with the presence of antisites can be proposed as Yb2+ charge compensation mechanism in the YAG ceramics, unlike the YbAG single crystals, where tetravalent impurities are responsible for charge compensation
Spectroscopic investigations of nanostructured LiNbO3 doped with Eu3+
No full textStructural and optical properties of the sol-gel derived nanocrystalline lithium niobate (LiNbO3) powders doped with Eu3+ ions have been studied. In particular, the influence of the sizes of nanoparticles controlled by temperature on the structural and luminescence properties has been investigated. Emission bands corresponding to 5D0 emission became more resolved with increasing nanocrystal size and changed to a typical Eu3+:LiNbO3 single crystal spectrum for nanocrystals having an average size of more than 40 nm. Nonlinear optical properties of nanostructured LiNbO3 have been confirmed by simple observation of second harmonic generation effect (SHG). The possibility of using nanostructured LiNbO3 doped with rare-earth ions as self-doubling elements in integrated optoelectronic devices has been discussed
Comment on "Colossal dielectric and magnetodielectric effect in Er(2)O(3) nanoparticles embedded in a SiO(2) glass matrix"
No full textIn a recent paper, Mukherjee et al. [Phys. Rev. B 82, 104107 (2010)] reported a colossal enhancement of dielectric constant along with magnetodielectric (MD) behavior in a SiO2-Er2O3 system synthesized via the sol-gel route. In the opinion of the authors of the criticized paper, such a colossal MD effect in the measured samples is a direct consequence of exploiting magnetoresistance changes associated with the size and concentration of Er2O3 nanoparticles. We point out that the disappearing MD behavior depends on the crystallite size and dopant concentration, as shown by the authors of the criticized paper, but also on the nature and the crystal structure of the compound
Structural and luminescence properties of Eu3+ doped BaxSr1−xTiO3 (BST) nanocrystalline powders prepared by different methods
No full textEu3+ doped BaxSr1-xTiO3 (BST) nanocrystalline powders have been prepared using molten salts, sol-gel and hydrothermal techniques in order to compare their structural and luminescence properties. The sizes of the particles are around 50 nm for the BST powders obtained by the sol-gel and hydrothermal methods while they are around 400 nm for the samples prepared by the molten salts method. The Eu3+ doped BST samples show strong emission in the visible range. The luminescence spectra of the Eu3+ ions for the investigated samples depend strongly upon the preparation method. The luminescence bands are affected by a notable amount of inhomogeneous broadening. The broadening of the emission bands and the non-exponential behavior of the emission decay curves point to the presence of significant structural disorder
Preparation and upconversion properties of Er3+,Yb3+:Y2Si2O7 nanocrystallites embedded in PVA polymer nanocomposites
No full textThe preparation of polymer nanocomposites consisting of a poly(vinyl) alcohol (PVA) network and RE3+: Y2Si2O7 nanocrystalline particles (RE = Yb, Er) is presented. Optical properties of the nanocomposites were preliminarily studied. In particular, efficient upconversion was observed in Er3+ and Yb3+, co-doped Y2Si2O7 nanoparticles embedded in the polymeric PVA host. It was found that the luminescence features of the RE3+: Y2Si2O7 nanoparticles were affected by the polymeric host, resulting in a shortening of their luminescence lifetimes. This effect is discussed in terms of the effective refractive index
Infrared induced red luminescence of Eu3+-doped polycrystalline LiNbO3
No full textPolycrystalline lithium niobate powders were prepared by the sol-gel method. Different grain sizes were obtained by controlling the annealing temperature. The Eu3+ ion was used as a luminescent probe of the nonlinear optical properties of the grains. Strong red emission deriving from Eu3+ ions was observed under pulsed infrared laser irradiation at 936.0 nm due to absorption of the light produced in the second harmonic generation process. A measurable emission was observed only for grains having a size of the least 200 nm, depending on the annealing temperatur
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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