1,720,982 research outputs found
Pressure and temperature dependence of the decomposition rate of aliphatic tert-butyl peroxyesters
No full textThe thermal decomposition in solution of n-heptane of several aliphatic tert-butyl peroxyesters RC(O)OOTB: TB peroxypropionate, TB peroxy-n-butyrate, TB peroxy-iso-butyrate, TB peroxy-2-methylbutyrate, TB peroxy-2-ethylhexanoate, TB peroxy-3,5,5-trimethylhexanoate, and TB peroxy-neo-decanoate has been investigated at pressures up to 2500 bar and temperatures up to 185 degrees C. The experiments were carried out in a tubular reactor at residence times up to 140 s. Peroxide concentration was monitored via quantitative FT-IR spectroscopic analysis of the pressurized medium under continuous flow conditions. For each peroxide first-order decomposition kinetics were observed over several half-lives. The type of carbon atom (in the R moiety) that is in ex-position to the carbonyl group controls decomposition kinetics. The observed first-order rate coefficient, k(obs), is smallest in situations where this particular C-atom is primary and is largest when it is tertiary. The "primary" TB peroxyesters are associated with the higher activation energies, around 145 kJ mol(-1). The activation energies, E-A, of the "tertiary" and "secondary" TB peroxyesters are around 120 und 130 kJ mol(-1), respectively, with the individual E-A's being proportional to the stability of the particular free-radical species R-.. The activation volumes, Delta V-obs(not equal) of the "secondary" and "tertiary" TB peroxyesters are in the narrow range 3.0 +/- 1.5 cm(3) mol(-1). Delta V-obs(not equal), of the "prinnary" TB peroxyesters is above 10 cm(3) mol(-1) and, in addition, is slightly pressure-dependent. Both observations demonstrate that the "primary" aliphatic TB peroxyesters decompose via single-bond scission with this result being supported by modelling dissociation dynamics in the compressed fluid state. The "secondary" and "tertiary" TB peroxyesters decompose via concerted two-bond scission associated with the immediate formation of CO2. These results are fully consistent with evidence from the literature and from our preceding detailed study into TB peroxyacetate and TB peroxypivalate dissociation kinetics. The mode of primary bond dissociation, via single-bond or via concerted two-bond scission, largely affects the initiation efficiency of TB peroxyesters in free-radical polymerization. For each TB peroxyester, rate expressions for k(obs)(p,T) are presented. This data for decomposition in compressed n-heptane should be highly suitable for simulation and optimization of technical high-pressure polymerization processes
Initiator efficiency of peroxides in high-pressure ethene polymerization
No full textThe initiator efficiency of three organic peroxides, tert-butyl peroxypivalate (TBPP), tert-butylperoxy-2-ethylhexanoate (TBPEH), and di-tert-butyl peroxide (DTBP), has been measured in a stirred tank reactor (CSTR) which is part of a continuously operated miniplant device. The experiments were carried out at 2000 bar and temperatures between 150 and 250 degreesC. Initiator efficiency f is defined as the fraction of primary (initiator-derived ) free-radical species that add to monomer molecules. The reported data refer to ideal CSTR mixing conditions. The analysis of f is based on results of separate measurements of (i) rate coefficients of peroxide decomposition carried out in dilute solution of heptane and of (ii) ethene propagation and termination rate coefficients from laser single-pulse experiments. The novel quantity "reduced peroxide-induced monomer conversion" defined as the amount of monomer polymerized per square root of the amount of decomposed initiator turns out to be very suitable for deducing initiator efficiency. At the low and moderate ethene conversions, up to 10%, which were applied in the present study, f appears to be independent of temperature. The values of f obtained for DTBP, TBPEH, and TBPP are close to 1.0, 0.64, and 0.42, respectively. The relative low efficiency in TBPP polymerization is primarily assigned to in-cage cross-disproportionation of the tert-butoxy radical with the tert-butyl radical that is instantaneously produced, together with carbon dioxide, during TBPP decomposition. The primary fragments from DTBP decomposition cannot undergo cross-disproportionation, as hydrogen atoms in beta-position to the oxygen-centered free-radical site are not available
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
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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