1,721,134 research outputs found
On nanocomposite fabrication: using rheology to characterize filler/polymer interactions in epoxy-based nanocomposites
No full textThe interactions that occur between an amorphous silicon nitride (Si3N4) nanofiller and an epoxy matrix are examined, as revealed by rheological changes in a diglycidyl ether of bisphenol-A (DGEBA)-based epoxy resin prior to curing and thermal analysis, scanning electron microscopy, and dielectric spectroscopy of the resulting amine-cured systems. The results show that isothermally heating the as-received Si3N4 in DGEBA at 100 °C leads to increases in the viscosity of the mixture. Analysis of rheological data obtained from unfilled, as-received Si3N4 -filled, and calcined Si3N4 -filled epoxy systems leads us to interpret this increase in viscosity as arising from reactions between epoxide groups of the DGEBA and nanoparticle surface groups, notably involving surface amines, which are stimulated by the elevated temperature. The extent of this filler/resin reaction depends on the material processing protocol used, particularly prior calcination of the Si3N4 and the temperature and duration of nanoparticle/DGEBA mixing. Glass transition temperature data show that cured samples prepared using different methods have significantly different glass transition temperatures, which is a consequence of the epoxide/amine stoichiometric imbalances that result from prior reactions between the Si3N4 and the DGEBA. Consistent behavior was observed in the dielectric response. These results demonstrate that ultimate macroscopic properties of Si3N4 /epoxy nanocomposites are critically affected by details of the processing protocol. Furthermore, we infer that, by using controlled prior calcination of the Si3N4 , it is may be possible to vary the initial surface chemistry of the nanoparticles so as to adjust their reactivity with epoxy-containing moieties. Here, this is exemplified using only two somewhat extreme thermal treatments and a bifunctional DGEBA-type compound but, we suggest, that the concept may be extended to many other mono- and polyfunctional epoxy-containing compounds in order to generate a wide range of different grafted nanoparticle systems. This strategy may provide a versatile means of adjusting the surface chemistry of inorganic nitride nanoparticles, in order to tailor their surface chemistry and thereby modify resulting nanocomposite properties
Effect of resin/hardener stoichiometry on electrical behavior of epoxy networks
No full textBy changing the ratio of resin to hardener, a series of epoxy resin samples has been produced with differing network structures and different retained chemical functionalities. The resulting materials were characterized by thermal analysis, dielectric spectroscopy, DC conductivity, and DC and AC breakdown strength measurements, to explore the effect of network structure and chemical composition on molecular dynamics and electrical properties. Differential scanning calorimetry showed that the glass transition temperature is primarily determined by the crosslinking density and indicates that, under the range of conditions employed here, side reactions, such as etherification or homopolarization, are negligible. Conversely, changes in DC conductivity with resin stoichiometry appear to occur as a result of changes in the chemical content of the system, rather than variations in network structure or dynamics. Specifically, we suggest that the DC conductivity is markedly affected by the residual amine group concentration in the system. While DC conductivity and DC breakdown appear broadly to be correlated, AC breakdown results indicated that this parameter does not vary with changing stoichiometry, which suggests that the AC and DC breakdown strengths are controlled by different mechanisms
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
Introducing particle interphase model for describing the electrical behaviour of nanodielectrics
Get PDFThis study proposes a new model for describing the electrical behaviour of nanocomposites. Unlike other models in the literature, this model has concentrated on the role of an interphase layer within the boundaries of nanoparticles. The experimental part investigates this role by filling an epoxy matrix with two types of surface-modified silicon nitride nanofiller: (a) the particles were dried at 200 °C, and (b) the particles were calcinated at 1050 °C. Electrical characterization showed that the epoxy which was filled with the calcinated particles has considerably better dielectric performance. Given that thermal and dielectric spectroscopy results demonstrate that the matrix molecular dynamics and polar content are comparable for all the investigated samples, the variations in the dielectric performance point to the particle interphase as an essential reason. As shown by infrared spectroscopy, the complex surface chemistry of the dried particles suggests a particle interphase with a high concentration of localized electronic states, which may enhance charge transport through hopping/tunnelling conduction. On the other hand, calcinating the particles results in a particle interphase with wider band gap, which may work as an energy barrier for charge movement. Consequently, this study highlights the paramount importance of particle interphase for designing dielectric properties of nanodielectrics.</p
Enhanced dielectric properties of polyethylene/hexagonal boron nitride nanocomposites
Get PDFA range of nanocomposites based on a polyethylene polymer and hexagonal boron nitride (hBN) filler has been explored in this study. The dielectric properties of the nanocomposites, which consisted of 2 wt %, 5 wt %, 10 wt %, 20 wt %, and 30 wt % of hBN, have been compared to the dielectric properties of the unfilled polyethylene blend. Scanning electron microscopy (SEM) revealed that the hBN was uniformly distributed in the polyethylene matrix, although large amounts of agglomerates were present in the nanocomposites containing more than 10 wt % of hBN. The incorporation of hBN into polyethylene resulted in a highly disordered morphology in comparison to the unfilled polyethylene, in which this effect was more pronounced with increasing hBN content. This is consistent with the increasing crystallisation temperature as the hBN content increases, as shown by differential scanning calorimetry (DSC), where the hBN acted as a highly effective nucleating due to the strong interactions between the polyethylene and the hBN. This strong interaction is again reflected in the thermal decomposition temperature which similarly increases with increasing hBN content. The study demonstrates the remarkable electrical properties of the prepared nanocomposites, where the breakdown strength monotonically increased as a function of hBN content, even with a very high 30 wt % of hBN. The improvement in electrical properties, even at high hBN concentrations, is contradictory to the reported results in the literature and are mainly attributed to the hydrophobic surface of the hBN particles
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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