1,720,958 research outputs found
Graphene/silicon nanocomposite anode with enhanced electrochemical stability for lithium-ion battery applications
No full textA graphene/silicon nanocomposite has been synthesized using a green approach during both synthesis and electrode processing. It has been characterized and tested as anode active material for lithium-ion batteries. The synthesis was performed by dispersing silicon nanoparticles in a carbonaceous matrix, obtained by a dual step reduction process of a previously functionalized graphene oxide substrate which avoids the formation of aggregates of Si particles and partially buffers the huge volume variations associated with Li/Si alloying processes. The graphene oxide matrix functionalization was achieved using low-molecular weight polyacrylic acid and a low-cost and eco-friendly solvent like ethylene glycol. As concerns electrode processing, composite anodes were prepared using high-molecular PolyAcrylic Acid as green binder and using ethanol as non- toxic and cheap solvent, thus avoiding the standard PVDF/NMP system which is, on the other hand, toxic and highly expensive. Furthermore, Vinylene Carbonate (VC) was used as electrolyte additive. Long cycling performance was evaluated at a current of 500 mAg-1 : after 100 cycles the anode showed a discharge capacity retention of about 80%. Analyzing the impedance spectra of the tested cells, the beneficial effect of the VC additive was showed in terms of lower SEI resistance in the long term
A Silicon/Graphene Composite Anode for High-Efficiency Lithium Batteries
No full textComposite anodes based on Si and reduced graphene oxide (RGO) have been prepared using commercial Si nanopowder, graphene oxide (GO) and polyacrilic acid (PAA) as starting materials. A double reduction step, consisting in microwave irradiation at mild power followed by thermal annealing in reducing atmosphere, yielded the composite powder made of Si:reduced graphene oxide (RGO) in the approximate mass ratio 30:70. The charge/discharge properties of the anode materials are determined by the homogeneous dispersion of Si grains between RGO nanosheets, that act as structural buffer for volume changes related to Li-Si reversible alloying and as improved electrical conductor. Electrodes have been prepared using high-molecular weight PAA as binder, which promises better mechanical stability towards silicon volume changes. The electrochemical behavior of the composite anode material has been characterized by galvanostatic cyclations and electrochemical impedance spectroscopy, using LiPF61M in EC:DMC 1:1 electrolyte, also modified by the addition of 5% vinylene carbonate (VC).
Several anodes have been investigated, consistently delivering reversible capacities higher than 1000 mAhg-
1, with a mechanism that, after initial lithiation of crystalline Si, mainly involves reversible Li-Si alloying/dealloying between amorphous a-Li and a-LixSi phases. Particularly, when cycled in VC-modified
electrolyte the anode exhibits a remarkable cycle life, resulting in a residual capacity of more than 900 mAhg-
1 after 60 cycles at 500 mAg-1 and efficiency values close to unity. Several factors concur in determining this behavior, namely: (i) the efficient Si dispersion in RGO carbonaceous matrix; (ii) the good mechanical properties of PAA binder; (iii) the formation of a stabilized SEI by VC additive in the electrolyte
RICERCA DI SISTEMA ELETTRICO. Ricerca di materiali anodici per batterie litio ione operanti in elettroliti organici convenzionali di più elevata energia rispetto a quelle sul mercato
Full text linkIl rapporto descrive il lavoro svolto per la preparazione e caratterizzazione di anodi per batterie litio ione costituiti da metalli (Si, Sn, Sb) capaci di formare leghe del tipo LixM con il litio secondo la reazione elettrochimicamente reversibile
x Li+ + x e- + M LixM
dove x può assumere valori pari a 4.4 per Sn e Si e 3 per Sb con capacità teoriche pari a 993 mAh/g, 4200 mAh/g e 660 mAh/g rispettivamente. I problemi degli elettrodi costituiti da leghe metalliche sono essenzialmente dovuti alle variazioni di volume durante i processi di formazione delle leghe che possono raggiungere valori dell’ordine del 300% (Sn), 360 % (Si) e 200 % (Sb). Le variazioni di volume inducono stress meccanici sugli elettrodi che polverizzano progressivamente con rapida perdita di capacità. In letteratura si trovano moltissimi lavori in cui gli stress meccanici sono tamponati preparando elettrodi compositi nei quali le particelle metalliche sono supportate su matrici di varia natura capaci di compensare le variazioni di volume [1]. Sulla base di recenti lavori di letteratura [2-6] durante questa ricerca è stato utilizzato grafene come supporto per le particelle metalliche. Dal punto di vista strutturale il grafene (GNS) corrisponde a grafite completamente esfoliata: array bidimensionale di atomi di carbonio ibridizzati sp2 con uno spessore di un atomo. In quanto tale può essere considerato come il mattone fondamentale per tutte le forme allotropiche dei fullereni (buckyball, carbon nanotubes etc.) [7]. Proprietà fondamentali del grafene sono: (i) elevata area superficiale( 2630 m2/g), (ii) elevatissima conducibilità ( 64 mS cm-1 circa 60 volte quella dei nanotubi).
Nel caso oggetto di questa ricerca nei compositi M/grafene, il grafene serve sia come materiale ad elevata conducibilità elettronica, sia come materiale che può intercalare litio ed allo stesso tempo può agire da tampone per minimizzare gli stress meccanici durante i processi di formazione/dissoluzione delle leghe metalliche. La sintesi dei nano compositi grafene-metallo è stata effettuata mediante impregnazione di grafene ossido (GO), preparato in laboratorio, con diversi tipi di sali. I gruppi funzionali (-OH, -COOH, -OOH) presenti sul GO possono ancorare per coordinazione gli ioni metallici che per successiva riduzione danno origine a nanoparticelle [6]. La procedura di preparazione è stata compiuta aggiungendo un sale ad una sospensione di GO in glicole etilenico. La sospensione è stata successivamente trattata con microonde. Il trattamento determina la riduzione del GO a RGO (Reduced Graphene Oxide) e la formazione di metalli ed ossidi metallici. A causa della disposizione casuale dei gruppi funzionali nel GO, le particelle risultanti sono uniformemente distribuite nella matrice di RGO che funziona da tampone per le variazioni di volume. Le polveri così ottenute sono state caratterizzate mediante SEM, TEM, XRD ed utilizzate per la preparazione degli elettrodi con tecnica doctor-blade.
I test elettrochimici sono stati condotti in celle a T con Li metallico come contro-elettrodo ed elettrodo di riferimento, fibra di vetro imbevuta con una soluzione 1M di LiPF6 in etilene carbonato/dimetile carbonato 1:1 in peso, (Merck LP30) come separatore. Le capacità specifiche ottenute a velocità di carica e scarica dell’ordine dei 100 mAh/g sono risultate pari a 600, 1000 e 400 per compositi di Sn, Si e Sb, rispettivamente
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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