1,720,977 research outputs found
Relaxor ferroeletric behavior in S r1-x P rxTi O3: Cooperation between polar and antiferrodistortive instabilities
No full textChemical doping at the Sr and Ti sites is a feasible way to alter the quantum paraelectric state of SrTiO 3 perovskite. Doping with Pr is known to induce relaxor ferroelectricity at room temperature in the Sr1-xPrxTiO 3 solid solution. The relationship between its dielectric properties and structural phase transition has been debated, but no definitive structural argument has been proposed. Here we present a systematic structural study of Sr1-xPrxTiO 3 (0.020≤x≤0.150). We establish the structural phase diagram using high-resolution x-ray powder diffraction by finding the antiferrodistortive structural phase transitions for all the compositions studied. By using pair distribution function analysis, we show the mismatch between local and long-range structures in terms of increased local order parameters. Finally, we propose a correlation between the local structural order parameters and the emergence of hard polar modes as found by Raman spectroscopy. Our results are quantitatively consistent with recent theoretical calculations showing that the increase of local tetragonality and local octahedral tilting above a critical value in fact underlie the polar instability. This confirms that structural orders involving both polar and antiferrodistortive characters compete and cooperate at different levels, promoting ferroelectricity in Sr1-xPrxTiO 3
Insight into the effect of different thermal treatment routes on the microstructure of AlSi7Mg produced by laser powder bed fusion
Full text linkThe present work gives an insight into the microstructure evolution of AlSi7Mg produced by laser powder bed fusion and subjected to different heat treatment routes. Synchrotron powder diffraction and diffraction contrast tomography coupled with scanning and transmission electron microscopy investigations allowed an in-depth understanding of the effect of rapid solidification induced by laser powder bed fusion on material structure. In addition, the effects of solution treatment followed by water quenching and artificial aging and that of direct aging from the as built condition was investigated. The as built material exhibits the most important lattice contraction and the lower amount of Si phase, thus suggesting a higher content of solute Si, which decreases after artificial aging and after solution treatment. The eutectic Si is found in nanometric form, with crystallites of about 10 nm in size, which grow significantly after solution treatment. A similar behavior is also observed for the second phase β-Al5FeSi. Traces of the π-Al8FeMg3Si6 were observed after solution treatment. Direct aging induced the formation of a fine dispersion of β” precipitates in the primary Al solidification cells, which is responsible for material hardness and strength increase. The thermal treatments strongly affect the residual macrostrain, which is higher for the as built material and decreases after aging and solution treatment. This was confirmed by diffraction contrast tomography, which provided a spatial resolved structural information and was used as alternative tool to quantify the distribution of Si and Al phases
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
Local and average structure of Yb-doped ceria through synchrotron and neutron pair distribution function
No full textAs transport properties of doped ceria electrolytes depend significantly on the nature of the dopant and the defectivity, the design of new materials and devices requires proper understanding of the defect structure. Among lanthanide dopants, Yb shows some peculiar characteristics that call for a possible different defect structure compared to Gd and Sm conventional dopants, which could be linked to its poorer performance. For this purpose, we combine synchrotron and neutron powder diffraction exploiting the Rietveld and Pair distribution Function. By increasing its concentration, Yb produces qualitatively the same structural distortions as other dopants, leading to a domain structure involving the progressive nucleation and growth of nanodomains with a Yb2O3-like (C-type) structure hosted in a fluorite CeO2 matrix. However, when it comes to growing the C-type nanodomains into a long-range phase, the transformation is less pronounced. At the same time, a stronger structural distortion occurs at the local scale, which is consistent with the segregation of a large amount of oxygen vacancies. The strong trapping of VOs by Yb3+ explains the poor performance of Yb-doped ceria with respect to conventional Sm-, Gd-, and Y-doped samples at equal temperature and dopant amount
Rare earth doped ceria: The complex connection between structure and properties
No full textThe need for high efficiency energy production, conversion, storage and transport is serving as a robust guide for the development of new materials. Materials with physical-chemical properties matching specific functions in devices are produced by suitably tuning the crystallographic- defect- and micro-structure of the involved phases. In this review, we discuss the case of Rare Earth doped Ceria. Due to their high oxygen diffusion coefficient at temperatures higher than ~500°C, they are very promising materials for several applications such as electrolytes for Solid Oxide Fuel and Electrolytic Cells (SOFC and SOEC, respectively). Defects are integral part of the conduction process, hence of the final application. As the fluorite structure of ceria is capable of accommodating a high concentration of lattice defects, the characterization and comprehension of such complex and highly defective materials involve expertise spanning from computational chemistry, physical chemistry, catalysis, electrochemistry, microscopy, spectroscopy, and crystallography. Results coming from different experimental and computational techniques will be reviewed, showing that structure determination (at different scale length) plays a pivotal role bridging theoretical calculation and physical properties of these complex materials
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
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