1,720,985 research outputs found
PAHs and fullerenes as structural and compositional motifs tracing and distinguishing organic carbon from soot
Full text linkExamining the features distinguishing organic carbon from soot is crucial for understanding the source, the effect on the environment and their respective role in aerosol chemistry and soot formation. Beside to the obvious PAH picking-out in the low-mass mode (C number 40) of organic carbon, separated by carbon particulate matter extraction from young and mature soot thermophoretically sampled in premixed flames, was done by laser-desorption-time-of-flight mass spectrometry, exploiting the laser power increase. The perusal of organic carbon mass spectra through mathematical tools in comparison to aromatic and alkyl-substituted PAH-laden samples and the persistence of high-mass mode at high laser power led to exclude the contribution of dimers and alkyl-bridged PAHs attributing the second mode to both fully-benzenoid and cyclopenta-PAHs. Profound differences between mass spectra of organic carbon and soot were noticed as neither molecules nor radicals of PAHs could be drawn out from soot, even at high laser power, and only small radicals and carbon clusters like fullerenes were observed, especially for young soot. These inferences evidenced the importance of analysing separately organic carbon and soot especially if insights into soot particles nucleation are to be obtained. In the case of benzene flame, already at the inception, soot consists of strongly tangled aromatic motifs crosslinked each other, presumably deriving from reactive coagulation/clustering of relatively small aromatic hydrocarbons/radicals early formed. In methane and ethylene flames, coalesced liquid-like material composed of soot and PAHs is formed and transformed later on undergoing some carbonization and molecular growth, respectively
Blue, green and yellow carbon dots derived from pyrogenic carbon: Structure and fluorescence behaviour
Full text linkFluorescence lifetimes and quantum yields featuring polycyclic aromatic hydrocarbons (PAHs) and other organics constituting pyrogenic carbon particulate matter (PM) are seldom measured. In this work, PM sampled in a fuel-rich ethylene flame was firstly separated in organic carbon (OC), soluble in dichloromethane, and refractory organic carbon (ROC), soluble in N-methyl pyrrolidinone, and then analyzed by size exclusion chromatography (SEC) coupled with online UV and fluorescence detection, and by offline fluorescence spectroscopy and mass spectrometry. It was found that three classes of differently light emitting carbon dots (CDs) could be bottom-up synthesized in the same flame system by selecting appropriately the residence time. Actually, OC presented blue fluorescence regardless the residence time, whereas ROC sampled at low and high residence time emitted fluorescence in the green (green CDs) and in the yellow (yellow CDs) region, respectively. The SEC molecular weight of all CDs presented similar trimodal distributions, centered around 300, 1000 and 10,000 u. For the first time fluorescence lifetimes and quantum yields of pyrogenic CD fractions were measured as additional parameters useful for discriminating the fluorescent components and inferring their structural properties, with the support of mass spectrometry. The different spectroscopic features of CDs could be associated to different compositional characteristics as the polydispersity of molecular components featuring blue CDs, opposed to the oligomer-like nature of green and yellow CDs. Pyrogenic CDs showed different fluorescence emission ranges, quantum yield and lifetimes, appealing for their possible applications in the fields of imaging, electronics and sensors
Effect of Fuel/Air Ratio and Aromaticity on Sooting Behavior of Premixed Heptane Flames
No full textOxidation and pyrolysis products were sampled and quantified along the axis of premixed laminar flames burning n-heptane in slightly sooting (C/O=0.7) and heavily sooting conditions (C/O=0.8) by means of gas chromatographic analysis. Total particulate was extracted with dichloromethane (DCM) in order to separate condensed species (CS) including polycyclic aromatic hydrocarbons from soot. The effect of aromatic addition (10 vol % of n-propylbenzene) to n-heptane fuel in heavily sooting conditions on the distribution of light hydrocarbons was also studied. Detailed modeling extended to the formation of high-molecular-weight species and soot was performed to verify the effect of the C/O ratio and of the fuel aromatic content on soot formation. The C/O ratio was found to affect benzene formation which in turn caused an increase of condensed species and soot formation rates. The fuel aromaticity was found to shift soot inception upstream in the flame increasing soot in the oxidation region of the flame. However, aromaticity was not found to influence the ultimate soot loading because of the reduced soot growth rate due to the slightly lower temperature and lower acetylene formation in the aromatic-doped flame. The model was used to explore the effect of larger amounts of aromatics on soot formation. Increased benzene and condensed species, including polycyclic aromatic hydrocarbons, are obtained from the addition of more aromatics (>10%) to n-heptane resulting in higher ultimate soot loading, confirming the direct relation between gas-phase aromatics and final soot loading
Fluorescent carbon dots synthesis in premixed flames: Influence of the equivalence ratio
Full text linkFlame synthesis could represent a controllable method for production of carbon dots (CDs) with tunable emission properties, alternative to process involving oxidation agents and wet chemistry processing. To control final properties of CDs, a thorough analysis of different reactor parameters has to be conducted. Here we investigate the effect of combustion operative conditions, namely equivalence ratio, for CD production in a premixed flame reactor. CDs with specific optical properties have been synthesized at different equivalence ratios, burning both pure ethylene and ethylene/benzene mixtures. Carbon particulate matter was thermophoretically collected at the exhaust of the flame reactor. Successively, after dichloromethane washing, CDs were simply obtained by N-methyl-pyrrolidone extraction and filtration (<20 nm). In ethylene flames, CDs with fluorescence ranging from green to yellow, as equivalence ratio increases, were synthesized. In ethylene/benzene flames, produced CDs emit fluorescence from blue to green and equivalence ratio seems to play even a more marked role, inducing significant change in the emission spectra with small changes of equivalence ratio. Globally, their quantum yield ranges between 2 and 11%. CDs have been found generally stable for mild change of temperature (up to 80 °C), mild basic pH (pH = 8–10) and over long time at room temperature (up to 240 days). Overall, equivalence ratio demonstrates to be a powerful and simple tool to produce tunable CDs by flame synthesis on large scale
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
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