1,720,973 research outputs found
Dissolution rates of talc as a function of solution composition, pH and temperature
No full textSteady-state talc dissolution rates, at far-from-equilibrium conditions, were measured as a function of aqueous silica and magnesium activity, pH from 1 to 10.6, and temperature from 25 to 150 degrees C. All rates were measured in mixed flow reactors and exhibited stoichiometric or close to stoichiometric dissolution. All measured rates at pH > 2 obtained at a fixed ionic strength of 0.02 M can be described to within experimental uncertainty usingr(+) = S-BET (A(A) (a(H+)(2)/aMg(2+))(1/4) + A(B)) exp(-E-A/RT)where r+ signifies the BET surface area normalized forward talc steady-state dissolution rate, S-BET denotes the BET surface area of talc present in the reactor, A(A) and A(B) refer to pre-exponential factors equal to 5.0 x 10(-9) and 0.8 x 10(-9) mol/cm(2)/s, respectively, E-A designates an activation energy equal to 45 kJ mol(-1), R represents the gas constant, T denotes absolute temperature, and ai refers to the activity of the subscripted aqueous species. The first term of this rate expression is consistent with talc dissolution rates at acidic pH being controlled by the detachment of partially liberated silica tetrahedral formed at talc edge surfaces from the exchange of Mg2+ for two protons. Corresponding atomic force microscopic observations confirms that dissolution proceeds by the removal of T-O-T sheets from talc edges. At pH <=, 2, the Mg2+ for proton exchange is so extensive that talc T-O-T sheets break apart leading to increased surface area and accelerated rates, whereas rates appear to be pH independent at pH >= 7. (C) 2007 Published by Elsevier Ltd
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
Boron isotopic fractionation during adsorption by calcite – Implication for the seawater pH proxy
No full textAlthough adsorption at the solid surface is the first step controlling boron incorporation in the crystal lattice during the standard growth mechanism of calcite and aragonite, little is known about the identity, structure and isotopic composition of the boron complexes formed at the CaCO3-solution interface. To generate this important information, we investigated experimentally the boron chemical and isotopic fractionation during adsorption at the calcite-water interface as a function of pH (6.5-11.7) at 4 and 20 degrees C in 0.01 and 0.1 M NaCl aqueous solutions. The surface complexation modeling of B adsorption and isotopic composition data showed that boron is sorbed at the calcite surface as a tetrahedral complex (>CaB(OH)(4)(0)) formed by reaction of borate ions with Ca-protonated surface sites (log K-int(0)= 1.54 +/- 0.37 at 20 degrees C) and excluded the formation of trigonal B surface complexes (>CaB(OH)(3)(+)). The B isotopic composition of >CaB(OH)(4)(0) is similar to 5 parts per thousand and 2 parts per thousand heavier than that of aqueous B(OH)(4)(-) in 0.01 and 0.1 M NaCl solution, respectively. Consistently, these values suggest that adsorbed borate ions have isotopic compositions intermediate between those of aqueous borate and structural tetrahedral species in calcite, which have been recently predicted to be similar to 12 parts per thousand heavier than aqueous borate using quantum mechanical calculations ( 2018). The good agreement between the isotopic composition of adsorbed boron measured in this study and boron experimentally co-precipitated with calcite in the 8-9 pH range at close to equilibrium conditions (i.e. via ion-by-ion attachment at advancing steps) (Noireax et al., 2015) indicate that the isotopic composition of structural boron can be inherited from the boron surface complexes formed at the calcite/water interface. The results of this study contradict the assumption of no isotopic fractionation between tetrahedral boron in calcite and the borate ion that sustains the boron paleo-pH proxy, but confirm that trigonal B cannot be directly incorporated in the crystal structure during near equilibrium growth of calcite. (C) 2018 Elsevier Ltd. All rights reserved
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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