1,720,955 research outputs found
Second-order phase transitions of pure substances
No full textIn this report we are dealing with the thermodynamic theory of second-order phase transitions or continuous transitions of unary systems. The first classification of these phase transitions is due to Ehrenfest (1933), based on chemical potentials. First-order transitions are changes in which the derivatives of the chemical potentials, with respect to temperature or pressure are discontinuous. Second-order phase transitions are continuous in the first derivatives of the chemical potentials, but exibits discontinuity for at least one second derivative. However, the modern classification is based on systems, with or without latent heat, during the transitions. Landau (1937) formulated the first theory, which is based on a power series of the Gibbs free energy, including a so-called scalar order parameter. It’s a simple and powerful theory, but fails for temperatures in the vicinity of the transition line. This is because there exists long-range fluctuations in the systems. The order parameter is zero for the high temperature, high symmetric, disordered phase, whereas it is non-zero for the low temperature, low symmetric, ordered phase. The classical Landau theory can be outlined with statistical thermodynamics and with symmetry considerations in crystals, which is in fact the group/representation theory. There are two Landau symmetry rules: the space group of the low symmetric phase should be a subgroup of the high symmetric phase. The second is that the continuous transitions leads to crystal changes which corresponds to a single irreducible representation of the initial space group. The last subject contains some remarks about the study of critical phenomena and exponents and the renormalization group theory (temperatures near the transition). However a thorough discussion of these subjects are beyond the scope of this report
Theory and Examples of Spinodal Decomposition in a Variety of Materials
No full textSpinodal decomposition is observed in a variety of materials, such as metal alloys, oxide glasses, mineral solid solutions, steels, gels, ceramics and mixtures of polymers and liquids. It’s an irreversible process in which the transition of solid or liquid solutions is not triggered by nucleation and growth from a sudden concentration peak, but by local sinusoidal fluctuations in the concentration of the components. It occurs, when rapidly quenched, in the thermodynamically unstable region of the demixing area, beyond the spinodal. The spinodal which leads to spontaneous decomposition is the spinodal of the coherent free energy curve of the metastable coherent phase diagram.
The early-stage kinetics of spinodal decomposition of a binary alloy A-B can be obtained with a nonlinear diffusion equation, the Cahn-Hilliard-Cook equation, with constant mobility. The equation with variable mobility, describes the later-stage and the coarsening process.
In this report the process of spinodal decomposition is discussed phenomenological and mathematically. Also two examples of materials in which decomposition takes place are given, i.e. computer simulations in solids and mineral alkali feldspar solid solution in geology
Phase diagram measurements by Heat-flux DSC and thermodynamic calculations of the mixture of the Esters Ethyl undecanoate (C13H26O2) and Ethyl tridecanoate (C15H30O2)
No full textIn this report a phase diagram is determined by heat flux DSC of the binary mixture Ethyl undecanoate and Ethyl tridecanoate. Our hypothesis for equilibrium phase behaviour is that the components Ethyl undecanoate and Ethyl tridecanoate do have the same crystal form and they have restricted miscibility in that form. However, the results of the DSC measurements are complex and are not unambiguous. We determined a simple phase diagram and calculated it with the program Winifit. The calculated phase diagram with the program shows good similarity with the one obtained by the experiments. The excess Gibbs energy parameters of the solid state are determined with Winifit, being A = 5000 J.mol-1 and B = 0,1. The mole fraction of the critical point XC is calculated as XC =0,43. The critical temperature C T at XC =0,43 is also calculated, being C T = 307,2 K. For better understanding the crystal structures have to be examined in greater detail. On the left-hand side and on the right-hand side of the phase diagram they have to be the same
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
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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