1,720,956 research outputs found
Low-energy impact testing on epoxy composites reinforced with DNA-functionalized carbon nanotubes
No full textThe discovery of the exceptional electrical, thermal and mechanical properties of carbon nanotubes (CNTs) has initiated many investigations into the possibility to embed them in polymer matrices with the main objective of creating advanced multifunctional materials. This possibility is particularly relevant in the aerospace field, where the use of such materials can satisfy the lightweight requirement by replacing complex and heavier subsystems of a spacecraft. For these reasons, CNTs are largely investigated as reinforcements for epoxy matrix, which is the predominant polymer for structural applications in spacecraft and aircraft vehicles.
Despite their high elastic modulus and tensile strength, the enhancing of the mechanical properties of epoxy resins reinforced with CNT, either single-walled or multi-walled, are largely unpredictable due to their strong dependence on the manufacturing process. In particular, the mechanical performance of epoxy nanocomposites is greatly affected by the dispersion efficiency of the nanotubes inside the polymer matrix. To improve their dispersion in epoxy resins, CNTs are commonly functionalized following covalent strategies. Chemical treatments, by which functional groups are attached to the nanotube walls, make the CNTs more compatible with the matrix improving the homogenization of the dispersion. However, surface modifications cause a degradation of the overall properties of the nanotubes, so that the benefits added by the more homogenous dispersion are in fact lost in the final composite material.
In previous work, we demonstrated the possibility to obtain a homogeneous dispersion of multi-walled carbon nanotubes (MWCNTs) in an epoxy resin using a non-covalent functionalization based on DNA wrapping. The advantage of this approach is that MWCNTs maintain the intact features of pristine nanotubes. Here, we study the effects of the use of such functionalization on the fracture toughness of the composite material. In addition, we investigate the low impact behavior using a dropped weight test, in order to assess on a whole the impact resistance of such materials. For the tests we used an experimental set-up consisting of an in-house drop tower system capable to simulate low velocity impact conditions with energy of 9.8 J. The performance evaluation is linked to the observation of the samples behavior during the test, followed by microscopy inspection of the fracture surfaces
UV-C effects on carbon nanostructured films fabricated on Mylar substrate
No full textFlexible membranes are the basic elements of different spacecraft subsystems, such as thermal blankets in satellites and propulsion systems in solar sails. One of the major technical challenges for a spacecraft engineer is to maintain different equipment working in different bands uncontaminated by neighboring antennas, feeds and the thermal hardware such as the multi-layer insulation (MLI). It is a known fact that MLI causes unwanted interference and even passive intermodulation products problems due to its high radio frequency (RF) reflectivity in the specular direction. On the other hand, solar phonon sailing is expected to be a revolution in space propulsion, as a continuous low-thrust propellant-less thrusting mode that can act for any mission time length. For these applications, the membrane general performance, at both optical and electromagnetic level, can be improved by coating the membranes with carbon nanoparticles. However, polymer-based materials in space exhibit larger degradation effects due to the combination of energetic UV radiations, vacuum, atomic oxygen, as well as large temperature gradients, which generate thermal stresses inside the structures. In this work, we investigated the effects of UV-C radiations on carbon nanostructured films realized on Mylar substrate. The carbon nanostructured films were fabricated at different nanoparticle loadings using spin coating and spraying processes. The film deposition process was optimized based on the results from electrical conductivity measurements. The specimens were irradiated with different UV-C doses, and their surface modification was studied in terms of morphology, wettability and electrical properties. In particular, scanning electron microscopy (SEM) and contact angle analysis were performed on untreated and irradiated samples, and showed the different effects of highly energetic UV-C radiation on several types of films
DNA-assisted dispersion of multi-walled CNTs in epoxy polymer matrix
No full textThe main problem that is encountered in reinforcing a polymer matrix with CNTs is to obtain a homogeneous dispersion avoiding that the CNTs aggregate and turn into localized defects for the nano-reinforced composites. In order to overcome these problems, different approaches have been developed based on surface modification such as covalent and non-covalent functionalization methods. The covalent functionalization of CNTs reduces their exceptional properties because chemical treatment breaks the CNT structure introducing defects on its surface. Non-covalent functionalization mainly based on the use of dispersants, such as surfactants and polymers, are often not effective and the CNTs remain predominantly entangled. In this work, we present the effect of a recently designed dispersion approach, based on non-covalent functionalization of CNTs by DNA molecules, on the preparation of CNT/PRIME 20 LV composites. Multiwalled CNTs were first functionalized with DNA, and then used to fabricate nanocomposites at CNTs loading of 0.5 wt.%. Different techniques were used to test the dispersion homogeneity of the composites. In particular, a set-up for electrical measurements was developed to assess the stability of the CNT/PRIME 20 LV mixture in time
Nanomaterial-based biosensors for a real-time detection of biological damage by UV light
No full textIn this work, the design and fabrication of a miniaturized and light-weight biosensor that can be used to monitor the biological effects of hostile ultraviolet radiation in earth and space are presented. The biosensor is generated by embedding a sensitive element to UV radiation, DNA, in a hybrid carbon-based nanomaterial. In particular, we present results on the fabrication and characterization of hybrid nanostructured films containing graphene nanoplatelets (GNPs) and double-stranded DNA for the in situ and real-time detection of UV radiation damaging effects from the changes of the film electrical properties induced by exposure to UV-C radiation. The biosensor is realized by the deposition of the sensitive unit GNP/DNA on a supporting substrate made of flexible polymers or glass
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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