1,721,053 research outputs found
Interface-Dominated Time-Dependent Behavior of Poled Poly(Vinylidene Fluoride–Trifluoroethylene)/Barium Titanate Composites
Get PDFComposites in which particles of ferroelectric ceramic phase are randomly dispersed in a polymeric matrix are of interest because of flexibility, conformability, and ease of processing. However, their piezoelectric properties are rather low, unless very high volume fractions of ceramics are used. This brings agglomeration and porosity issues due to the large mismatch between the surface energies of the ceramics and of the polymer. Particle surface modification is a common approach for better dispersion; however, it may bring other effects on the properties of the composites, which are usually concealed by the huge improvement in performance due to the more homogenous microstructure. In this work, we compared poly(vinylidene fluoride–trifluoroethylene)/barium titanate composites containing 15 vol.% and 60 vol.% of pristine ceramic particles or particles modified with an aminosilane or a fluorosilane. Similar morphology, with good particle dispersion and low porosity, was achieved for all composites, owing to an efficient dispersion method. The materials were poled with two different poling procedures, and the piezoelectric coefficient d33, the relative permittivity, and the poling degree of barium titanate were followed in time. We highlighted that, although similar d33 were obtained with all types of particles, the nature of the particles surface and the poling procedure were associated with different charge trapping and influenced the evolution of d33 with time.SCI-STI-DDThis is an open access article distributed under the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/
Multilayer diffusion-barrier composites based on microfibrillated cellulose from wood, bacterial and algae sources
No full textThis work is driven by the effort towards increased environmental sustainability and aims to develop a new food packaging material based on renewable materials, and more specifically cellulose as the most abundant biomass resource on Earth. Focus was on microfibrillated cellulose (MFC) known for its exceptional diffusion barrier performance at low relative humidity, which however rapidly degrades with increasing humidity, and on strategies to reduce this sensitivity to moisture.
The first objective was to investigate alternative MFC from algae and bacteria. A commercially available wood-MFC was selected as a benchmark. The algae-MFC was extracted from Boergesenia forbesii algae. The bacterial cellulose (BC) was produced via fermentation of Komagataeibacter xylinus bacteria in two variations: a standard BC-Control and BC-Cremodan, cultivated in a foamed medium with cremodan and xanthan. Aqueous suspensions of MFCs were shear-thinning and exhibited a yield stress with increasing concentration. The BC-Cremodan and algae-MFC suspensions with high nanosized fractions and fewer physically entangled agglomerates were less stiff than the BC-Control and wood-MFC suspensions with rougher and longer macro-fibers. The analysis of self-standing nanopapers produced with these MFC revealed the complex interplay between microstructural factors such as crystallinity, nanofraction and residual water content and macroscopic properties, as the Young's modulus and the water vapor permeability. The oxygen transmission rate (OTR) of the MFC films at high humidity was comparable to the values of conventional packaging materials such as PET and their water vapor transmission rate (WVTR) was an order of magnitude higher than that of PET, but an order of magnitude lower than values for previous MFC films.
The second objective was to suppress the considerable sensitivity of MFC to moisture, and more specifically to maintain its very high barrier against the diffusion of oxygen at high relative humidity. The approach was to encapsulate the MFC films with hydrophobic polymers in the form of multilayer coatings on a paper substrate. Two different types of polymers, namely photo-curable organic-inorganic epoxy-based formulations, and an acid-modified polyolefin suspension (PO) were selected. The epoxy hybrid approach improved the OTR and WVTR of the MFC by an order of magnitude, and the addition of a sol-gel inorganic network into the epoxy network was instrumental to achieve a high mechanical integrity, with no delamination at the polymer-MFC interface. The PO approach was tested with the wood-MFC and algae-MFC and enabled to largely decrease the WVTR of the MFC, both at 50%RH and 85%RH, with a marginal effect on the OTR. The algae-MFC with an open porosity led to a high adhesion with cohesive failure within the MFC layer owing to a mechanical interlocking effect with the PO layers.
The phenomenon of water transport within the MFC/epoxy hybrid and MFC/PO multilayers was further explored using transient and steady-state permeation approaches. This analysis confirmed the absence of a strong synergy within the multilayer architectures with a limited encapsulation effect. The assumption of a Fickian diffusion moreover led to artefacts in the derivation of the diffusion and solubility coefficients for water. This was considered to reflect a diffusion-reaction type transport, where water molecules absorb on active sites at the interfaces and within the MFC network.LPA
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
Optical and Mechanical Properties of Static and Tunable Color Coatings Based on Ultrathin Semiconductors
No full textVariations 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
High diffusion barrier and piezoelectric nanocomposites based on polyvinylidene fluoride-trifluoroethylene copolymer and hydrophobized clay
No full textNanocomposites based on polyvinylidene fluoride– trifluoroethylene copolymer and up to 4 vol % of hydrophobized clay nanoparticles are investigated. The structure, piezoelectric properties, and oxygen permeability of solvent cast films are analyzed before and after annealing above the Curie temperature of the polymer. Exfoliation of the clay takes place at concentrations up to 1 vol %, beyond which it rapidly drops and is absent at a concentration of 4 vol %. The presence of clay does not change the crystallinity of the polymer, but leads to a threefold decrease of the oxygen permeability at a concentration of 0.5 vol %. Annealing at 130 8C increases the crystallinity, the proportion of b phase up to 94%, and the piezoelectric coefficient by 20–40% at clay fractions below 1 vol %. Annealing also leads to a remarkable 3- to 10-fold decrease of O2 permeability and to intriguing changes of the activation energy for O2 transport, which decreases from 56 kJ/mol for the as-cast polymer to below 10 kJ/mol for the polymer and exfoliated composite.SCI-STI-DDLPA
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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