1,720,996 research outputs found
Nanowear of polymers
No full textThe use of viscoelastic materials, such as polymers, constantly increases in the field of nanotechnology. These materials are softer than metallic and inorganic ones, and, because of that, they are easier to deform and wear off. The wear mechanisms occurring for viscoelastic materials are rather complex, and, generally, present more complications for a direct investigation with respect to metals or ceramics materials. With the advent of Scanning Probe Microscopy (SPM), well characterized forces can be applied to a surface with a nanometer-scale spatial resolution. In particular Atomic ForceMicroscopy (AFM), working at high contact forces, can significantly modify many surfaces. Polymers are soft enough to be modified by hard AFM tips, such as those of silicon, silicon nitride or diamond. For these reasons, the AFM is today the main tool employed to investigate wear occurrence on polymer surfaces. The wear of a polymer surface caused by an AFM tip in a regime of single asperity contact is an articulate process that depends on conditions such as, namely, the applied forces, the tip shape, size and the relative velocity. Since the influence of all these parameters is in close connection with the sample properties, one can expect a dependence of the wearing process on the mechanical properties of the sample surfaces. These properties can vary significantly from the bulk properties, if cross linking is made or, on contrary, residual solvents are present in the specimens. This chapter is divided in three sections following a general introduction. Specifically, the first section deals with wear induced by means of AFM tips to study the mechanical properties of films at the nanoscale; the second one regards the exploitation of wear for the creation of nanolithographic patterns; the last one is finally dedicated to an applicative field such as the characterization ofwear of polymers for biomedical applications at the meso- and nanoscales
Nanoscale rippling on polymer surfaces induced by AFM manipulation
Full text linkNanoscale rippling induced by an atomic force microscope (AFM) tip can be observed after performing one or many scans over the same area on a range of materials, namely ionic salts, metals, and semiconductors. However, it is for the case of polymer films that this phenomenon has been widely explored and studied. Due to the possibility of varying and controlling various parameters, this phenomenon has recently gained a great interest for some technological applications. The advent of AFM cantilevers with integrated heaters has promoted further advances in the field. An alternative method to heating up the tip is based on solvent-assisted viscoplastic deformations, where the ripples develop upon the application of a relatively low force to a solvent-rich film. An ensemble of AFM-based procedures can thus produce nanoripples on polymeric surfaces quickly, efficiently, and with an unprecedented order and control. However, even if nanorippling has been observed in various distinct modes and many theoretical models have been since proposed, a full understanding of this phenomenon is still far from being achieved. This review aims at summarizing the current state of the art in the perspective of achieving control over the rippling process on polymers at a nanoscale level
Subsurface imaging of two-dimensional materials at the nanoscale
Full text linkScanning probe microscopy (SPM) represents a powerful tool that, in the past 30 years, has allowed for the investigation of material surfaces in unprecedented ways at the nanoscale level. However, SPM has shown very little capability for depth penetration, which several nanotechnology applications require. Subsurface imaging has been achieved only in a few cases, when subsurface features influence the physical properties of the surface, such as the electronic states or the heat transfer. Ultrasonic force microscopy (UFM), an adaption of the contact mode atomic force microscopy, can dynamically measure the stiffness of the elastic contact between the probing tip and the sample surface. In particular, UFM has proven highly sensitive to the near-surface elastic field in non-homogeneous samples. In this paper, we present an investigation of two-dimensional (2D) materials, namely flakes of graphite and molybdenum disulphide placed on structured polymeric substrates. We show that UFM can non-destructively distinguish suspended and supported areas and localise defects, such as buckling or delamination of adjacent monolayers, generated by residual stress. Specifically, UFM can probe small variations in the local indentation induced by the mechanical interaction between the tip and the sample. Therefore, any change in the elastic modulus within the volume perturbed by the applied load or the flexural bending of the suspended areas can be detected and imaged. These investigation capabilities are very promising in order to study the buried interfaces of nanostructured 2D materials such as in graphene-based devices
Electrical conduction and noise spectroscopy of sodium-alginate gold-covered ultrathin films for flexible green electronics
Full text linkGreen electronics is an emerging topic that requires the exploration of new methodologies for the integration of green components into electronic devices. Therefore, the development of alternative and eco-friendly raw materials, biocompatible and biodegradable, is of great importance. Among these, sodium-alginate is a natural biopolymer extracted from marine algae having a great potential in terms of transparency, flexibility, and conductivity, when functionalized with a thin gold (Au) layer. The electrical transport of these flexible and conducting substrates has been studied, by DC measurements, from 300 to 10 K, to understand the interplay between the organic substrate and the metallic layer. The results were compared to reference bilayers based on polymethyl-methacrylate, a well-known polymer used in electronics. In addition, a detailed investigation of the electric noise properties was also performed. This analysis allows to study the effect of charge carriers fluctuations, providing important information to quantify the minimum metallic thickness required for electronic applications. In particular, the typical noise behavior of metallic compounds was observed in samples covered with 5 nm of Au, while noise levels related to a non-metallic conduction were found for a thickness of 4.5 nm, despite of the relatively good DC conductance of the bilayer
Surface rippling induced by periodic instabilities on a polymer surface
Full text linkWhen the shear stress on a compliant surface exceeds the yield strength of the material, a periodic wrinkle pattern is often observed. This phenomenon has been also recognized at the nanometer scale on polymers, metals, ionic crystals and semiconductors. In those cases, the mechanical stress can be efficiently provided by a sharp indenter elastically driven at constant velocity along the surface. Here we suggest that the formation of such surface ripples can be explained by the competition between the driving spring force and the plastic response of the substrate. In particular, we show how the ripples are expected to disappear when the indentation rate is below a critical value or, alternatively, when the sliding velocity or the lateral stiffness of the contact are too high. The model results are compared to atomic force microscopy experiments on a solvent-enriched polystyrene surface, where the rippling formation is enhanced at room temperature, compared to bulk melts. A similar approach could be employed to describe rippling phenomena on larger scales.SCOPUS: ar.jinfo:eu-repo/semantics/publishe
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
Archivio Disarmo: essere un messaggero di pace come unica scelta possibile per il futuro dell'umanità
No full textL’autore ripercorre la sua esperienza di tirocinio curricolare svolta presso l'Istituto di Ricerche Internazionali Archivio Disarmo. Dalla partecipazione ad importanti eventi organizzati dall'associazione fino all'approfondimento della principale mansione realizzata. La stesura di tre pubblicazioni incentrate sul fenomeno del cosiddetto water grabbing e sull'impatto del climate change. Lo stress idrico di origine antropico correlato allo scioglimento dell’Antartide rivela l’estrema drammaticità della sfida ambientale, che si impone dinanzi alla comunità internazionale nel suo insieme. Infine, l'autore ha voluto divulgare la conoscenza riguardo al premio annuale organizzato esclusivamente da Archivio Disarmo: il premio giornalistico delle Colombe d'oro per la pace.
The author retraces his experience of curricular training held at the Institute of International Research Archivio Disarmo. From the participation to important events organized by the association until the deepening of the main task realized. The writing of three publications focused on the phenomenon of so-called water grabbing and on the impact of climate change. The anthropogenic water stress associated with the dissolution of Antarctica reveals the extreme drama of the environmental challenge, which is imposed before the international community as a whole. Finally, the author wanted to disseminate knowledge about the annual award organized exclusively by Archivio Disarmo: the journalistic prize of the Colombe d'oro for peace
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