1,720,988 research outputs found
Particle tracking in horizontal ball milling
No full textBall milling is a material processing method that allows near room temperature process of elements in powder and permits to synthesize materials with peculiar properties otherwise difficult or impossible to obtain. Due to the repeated fractures and cold welding of the reactants particles, solid solutions, amorphous alloys and equilibrium nanostructured materials can be obtained. Products coming from mechanosynthesis process can be advantageously applied as high performance structural and functional materials. Different milling devices can be utilized for the powder processing: planetary mill, attritor, horizontal ball mill, ID and 3D vibrating machines. The wide differentiation in configuration of the mills obstacles the development of a unified model on powder transformation kinetics in the milling process and then most of processing procedures are still developed by using a trial and error method. The focus of this paper is on a horizontal fixed vial ball mill; in this configuration a high speed rotor launches several tumbling against the vial wall. During hits, a quantity of energy is released to the trapped powder and the solid state reactions are promoted. To characterize the powder transformation process, knowledge of the kinematics of the balls system must be inferred. For this purpose an experimental setup based on digital image acquisition was constructed and the movement of balls inside the vial was filmed. Particle trace analysis methodology permitted to obtain balls trajectories, velocity vector field and total hit frequency. The kinetic energy of the impacting balls inside the vial was quantified, so that the energy released to the powder in a milling experiment can be estimated. Copyright © 2006 by ASME
Disordered boron nitride - graphite composite for hydrogen storage
No full textMolecular hydrogen can be stored as physisorbed on low weight materials such as metal-organic frameworks, and carbon-based materials in the form of nanotubes, activated carbon and other carbon nanostructures. A fundamental requisite for every-day applications is the adsorption reversibility at practical temperature and pressure conditions. From thermodynamic calculations, this could be achieved when the molecular hydrogen binding energy to the adsorbent surface is around 300-400 meV/molecule. Actually, for a graphitic surface the binding energy results to be a factor 3-4 below. A possible solution consists in allowing the contemporary interaction of the hydrogen molecule with two or more surfaces or, in other words, to sandwich H2 between graphite layers. Hexagonal BN and graphite share very similar structural properties. Taking advantage from the possibility of exfoliating hexagonal structures, a boron nitride – graphite (BN-C) intercalated composite material with statistically distributed slit-like porosities was obtained. For the purpose, pure compounds were mixed (1:1 by weight) and subjected to a mechano-chemical treatment by high energy Ball Milling (BM). Structural and morphological characteristics were studied as a function of the milling time by using X-ray diffraction (XRD), electron microscopy (SEM) and nitrogen adsorption techniques. Milling treatment produced slit-like pores (7 Å pore diameter) even at short milling times, increasing the surface area up to 10 h milling (SA=340 m2/g). At prolonged milling times, materials reactivity induce pores collapse as evidenced by the decreasing in pore volumes and surface area at constant pore size. The material was tested with respect to its hydrogen storage capacity by adsorption measurements at 77 K as reported in the figure.
Work is in progress to enhance the composite porosity by decreasing components reactivity
Caratterizzazione di materiali porosi mediante adsorbimento di gas: evoluzione del composito nitruro di boro - grafite ottenuto mediante High Energy Ball Milling
No full textI materiali porosi sono noti per le loro proprietà dovute alla capacità di adsorbimento di specie liquide o gassose. Lo
sviluppo di nuovi composti a elevatissima superficie specifica ha dato avvio a un rinnovato interesse scientifico e
tecnologico dovuto alla possibilità di utilizzare solidi porosi quali substrati per l'accumulo di gas in fase condensata. In
particolare, lo stoccaggio sicuro d’idrogeno in materiali adsorbenti in particolare, risulta cruciale per lo sviluppo delle
tecnologie per la sostenibilità energetica. Il presente rapporto tecnico è costituito da due parti: nella parte A si danno
alcuni cenni teorici di base sulle metodologie di caratterizzazione mediante adsorbimento di azoto; nella parte B è
riportato lo sviluppo mediante High Energy Ball Milling e la caratterizzazione di un materiale composito con porosità
di tipo a fenditura, costituito da nitruro di boro esagonale e grafite.Porous materials are well known for their capacity of adsorbing both liquid and gaseous species. The development of
new classes of materials with very high specific surface promoted the interest of the scientific community in their use as
solid substrates for gas storage. In particular, the safe hydrogen storage in adsorbing materials constitutes a crucial
issue in enabling the use of new technologies for energy sustainability. This technical report is divided in two parts: in
part A some theoretical basics are described on gas adsorption fundamentals for the characterization of porous
materials; in B on the synthesis by High Energy Ball Milling and the characterization of a boron nitride-graphite
composite with slit-like porosities are reported
Ricoprimento mediante tumbling mill per la fabbricazione di nanoparticelle magnetiche biocompatibili.
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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
Advances in Water Splitting Process Promoted by Nanocrystalline MnFe2O4 Reactant, Synthetised by Mechanochemical route
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