1,720,963 research outputs found
Dynamics in Cu46Zr42Al7Y5 melts: Interplay between packing density and viscosity
No full textWe have investigated microscopic dynamics in the bulk glass-forming Cu46Zr42Al7Y5 melts using quasielastic neutron scattering (QENS). Self-correlation functions show a fast beta and a slow alpha-relaxation process. alpha relaxation exhibits stretching and is independent of momentum transfer and temperature. The melt is characterized by low packing density and high viscosity. The dynamics observed and the critical packing density derived from the QENS data are in good agreement with mode-coupling theory predictions for hard-sphere liquids. However, we can show that viscosity dominates over packing density in determining the atomic dynamics and glass-forming ability of the metallic melt.EU-NMI3 program (FP6); DLR [50WM0541, DFG SFB 602
Equilibrium Structure, Hydrogen Bonding, and Proton Conductivity in Half-Neutralized Diamine Ionic Liquids
No full textRecent
experiments on proton conducting ionic liquids point to
half-neutralized diamine-triflate salts as promising candidates for
applications in power generation and energy conversion electrochemical
devices. Structural and dynamical properties of the simplest among
these compounds are investigated by a combination of density functional
theory (DFT) and molecular dynamics (MD) simulations based on an empirical
force field. Three different cations have been considered, consisting
of a pair of amine-ammonium terminations joined by a short aliphatic
segment −(CH2)n–
with n = 2, 3, and 4. First, the ground state structure,
vibrational eigenstates, and hydrogen-bonding properties of single
ions, neutral ion pairs, small neutral aggregates of up to eight ions,
and molecularly thin hydrogen bonded wires have been investigated
by DFT computations. Second, structural and dynamical properties of
homogeneous liquid and amorphous phases are investigated by MD simulations
over the temperature range of 200 ≤ T ≤
440 K. Structure factors, radial distribution functions, diffusion
coefficient, and electrical conductivity are computed and discussed,
highlighting the inherent structural heterogeneity of these compounds.
The core investigation, however, is the characterization of connected
paths consisting of cation chains that could support proton transport
via a Grotthuss-type mechanism. Since simulations are carried out
using a force field of fixed bonding topology, this analysis is based
on the equilibrium structure only, using geometrical criteria to identify
potential paths for proton conduction. Paths of connected cations
can reach a length of 80 cations and 30 Å, provided that bridging
oxygen atoms from triflate anions are taken into account. The effects
of water contamination at 1% weight concentration on the structure,
dynamics, and paths for proton transport are discussed
Giant changes in atomic dynamics on microalloying metallic melt
No full textWe report unexpectedly strong variations in the relaxational dynamics in a glass-forming metallic melt while microalloying. Analysis of quasielastic neutron scattering data revealed that changes in the values of stretching of the self-correlation function and the temperature dependence of self-diffusivity showed an Arrhenius to non-Arrhenius transition. The intermediate Structure of the melts did not show any prepeak or indication of short range order. These observations are correlated with the enhanced glass-forming ability of metallic melts oil microalloying. (C) 2009 American Institute of Physics [doi: 10.1063/1.3263950]EU; DLR [50WM0541]; DFG [SFB 602]; Leibniz progra
Density Functional Computations and Molecular Dynamics Simulations of the Triethylammonium Triflate Protic Ionic Liquid
No full textSystematic molecular dynamics simulations based on an empirical force field have been carried out for samples of triethylammonium trifluoromethanesulfonate (triethylammonium triflate, [TEA][Tf]), covering a wide temperature range 200 K ≤ T ≤ 400 K and analyzing a broad set of properties, from self-diffusion and electrical conductivity to rotational relaxation and hydrogen-bond dynamics. The study is motivated by recent quasi-elastic neutron scattering and differential scanning calorimetry measurements on the same system, revealing two successive first order transitions at T ≈ 230 and 310 K (on heating), as well as an intriguing and partly unexplained variety of subdiffusive motions of the acidic proton. Simulations show a weakly discontinuous transition at T = 310 K and highlight an anomaly at T = 260 K in the rotational relaxation of ions that we identify with the simulation analogue of the experimental transition at T = 230 K. Thus, simulations help identifying the nature of the experimental transitions, confirming that the highest temperature one corresponds to melting, while the one taking place at lower T is a transition from the crystal, stable at T ≤ 260 K, to a plastic phase (260 ≤ T ≤ 310 K), in which molecules are able to rotate without diffusing. Rotations, in particular, account for the subdiffusive motion seen at intermediate T both in the experiments and in the simulation. The structure, distribution, and strength of hydrogen bonds are investigated by molecular dynamics and by density functional computations. Clustering of ions of the same sign and the effect of contamination by water at 1% wgt concentration are discussed as well
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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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