1,721,434 research outputs found
Shapes of the free volume holes in amorphous polymers as estimated by positron annihilation lifetime spectroscopy
Full text linkIntermolecular spaces in polymer chains form the free volume, a useful concept to understand mechanical and transport properties of polymers. Quantification of the free volume can be obtained theoretically, using appropriate lattice models, as well as experimentally, through suitable probes. Among these, positronium (Ps) has become widespread due to the nondestructive character of the technique and the correlation between the Ps lifetime and the size of the holes. In most of the investigations, the cavity is approximated to a sphere. However, this may bias the evaluation of the free volume fraction. We show that by coupling results from Ps lifetime and specific volume measurement for amorphous polymers at equilibrium and the predictions of the Simha–Somcynsky equation of state, it is possible to shed light on dimensions of the holes as well as on their morphology
Weighing antimatter: AEg over bar IS Phase 2, upgrades and first data
No full textAEg over bar IS (Antimatter Experiment: Gravity, Interferometry, Spec-troscopy) is an experiment at the Antiproton Decelerator (AD) facility at CERN whose goal is to study the asymmetry between matter and antimatter and, in particular, the effect of the Earth's gravitational field on antihydrogen (H over bar ). During the 2018 run, H over bar was formed, leading into considerable gain of knowledge on the processes involved. Therefore, in the last two years, during CERN LS2, multiple upgrades to the experiment have been carried out, spanning from new degraders, a new H over bar formation trap scheme, an entirely new control system, a more efficient positronium (Ps) converter, and more efficient sensors. All this work is necessary towards the goal of creating the first pulsed beam of neutral H over bar , which will enable inertial studies on antihydrogen with high degrees of precision. This contribution presents different upgrades, their validation with the antiprotons beam, and the further developments foreseen in the future
Microsegregating blends of ethyl cellulose and poly(vinyl pyrrolidone): a combined thermo-mechanical and positron annihilation spectroscopy study
No full textPolymer blends are a versatile playground for studying phase separation and its effect on the development of morphology and mechanical properties of the resulting materials. Blends obtained from two immiscible polymers, ethyl cellulose (EC) and poly(vinyl pyrrolidone) (PVP), are especially relevant for their practical applications as coatings for pharmaceutical preparations and controlled drug release. Here, films of EC-PVP blends are studied by means of thermal analysis, dynamic-mechanical analysis as well as positron annihilation lifetime spectroscopy. The morphology of the microstructures generated by phase separation is investigated by means of scanning electron microscopy. The effect of both components' ratio and of molecular mass of PVP on the final properties of the blends is determined in a systematical way. The results obtained are not only of theoretical interest but will prove useful for the optimization of industrial formulations based on these polymers
Gamma-irradiation effects on polyethylene terephthalate studied by positron annihilation lifetime spectroscopy
No full textFormal calculation of exchange correlation effects on annihilation lifetimes of positronium confined in small cavities
Full text linkPositronium atoms (Ps) are widely used as a probe to characterize voids or vacancies in non-metallic materials. The annihilation lifetime of trapped Ps is strongly modified by pickoff, depending on the cavity size and on the appropriate external electron density. The connection between these material characteristics and Ps annihilation lifetimes is usually based on models that do not consider the requirements of full electron indistinguishability, that must be taken into account for a correct description of pickoff annihilation processes. In this report, we provide a formal theoretical framework in which exchange correlation effects between confined Ps and surrounding electrons are introduced in a natural way, giving a clear and versatile picture of the various contributions to pickoff annihilation. Moreover, our results provide a simple explanation of the lowering of the contact density (the Ps-electron density at the positron position) as a direct consequence of the electrons indistinguishability, at variance with previous interpretation based on spatial deformations of Ps wavefunction. Calculations are performed within the 'symmetry adapted perturbation theory' approach, and the results are compared with available experimental data on Ps lifetimes for polymers and molecular solids. Finally, our analysis gives a formal justification to the approximations involved in early models based on the well known Tao-Eldrup approach, and gives a simple interpretation of Ps properties in subnanometric voids
Modelling and Simulation of an On-Orbit Experiment for Testing a Novel Engine Technology
Full text linkThe paper describes the design and modelling of an on-orbit mission experiment for testing a novel engine technology in Space. The goal is to assess the novel engine thrust profile through an on-orbit reverse engineering approach. The simulation tool developed at Politecnico di Milano is initially designed to propagate the free attitude dynamics of a spacecraft under the effect of main environment perturbations. Each element of the simulator is validated, prior to the on-orbit experiment, by matching orbit and attitude data from a previous similar mission of the ION spacecraft CubeSat carrier. The novel engine technology is located in one of the ION CubeSat slots and activated in orbit. The simulation tool is adapted to simulate the on-orbit experiments given the information on the attitude engine action and the mass and structure properties of the satellite. The preliminary analysis performed on the in-flight results of the tests highlighted the need of the development of smoothing techniques prior applying the reverse engineering approach. Some uncertain parameters, such as the carrier spacecraft inertias, showed a significant influence on the simulator's performance. Different methods have been applied to characterise the unmodelled dynamics and approximations' effects on the results. Future updates and corrections to the simulator are suggested to deal with the observed problems and improve the detection of the engine thrust magnitude. No end-results of the on-orbit mission experiment are presented in this paper, since the in-space operations are still on-going
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
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