1,721,009 research outputs found
Graphene and Selected Derivatives as Negative Electrodes in Sodium- and Lithium-Ion Batteries
Full text linkThe performance of graphene, and a few selected derivatives,
was investigated as a negative electrode material in sodium and
lithium-ion batteries. Hydrogenated graphene shows significant
improvement in battery performance compared with
as-prepared graphene, with reversible capacities of
488 mA h g-1 for lithium-ion batteries after 50 cycles and
491 mA h g-1 for sodium-ion batteries after 20 cycles. Notably,
high rates of 1 A g-1 for graphene and 5 A g-1 for hydrogenated
graphene indicate higher capacities in sodium-ion batteries
than in lithium-ion batteries. Alternatively, nickel-nanoparticle decorated
graphene performed relatively poorly in lithium-ion
batteries. However, in sodium-ion batteries they showed the
highest reversible capacities of all studied batteries and graphene
derivatives, with 826 mAhg-1 after 25 cycles with
97% coulombic efficiency. Overall, minor modifications to
graphene can dramatically improve electrochemical performance
in both lithium-ion and sodium-ion batteries
Hydrogen storage mechanism and lithium dynamics in Li12C60 investigated by mu SR
No full textThe lithium cluster intercalated fulleride Li12C60 was investigated by means of Muon
Spin Relaxation (μSR) spectroscopy with the intent of unveil its hydrogen storage mechanism.
Thanks to the well-known propensity of positive muons to form Muonium, a light isotope of the
hydrogen atom, the final stages of the absorption process can be probed. The appearance of a slow
oscillating signal in the time evolution of the muon polarization indicates the presence of Li-Mu
covalent pair, never observed before in lower doped Li fullerides, which mimics the formation of
LiH at the first stage of hydrogen chemisorption in the material. In addition, the μSR signal shows a
clear transition above 150 K, compatible with a thermally activated Li cluster rearrangement. The
combined Inelastic Neutron Scattering analysis suggests that tetrahedral Li clusters may undergo a
progressive melting upon heating, which could favour room temperature ionic diffusion
Hydrogen Desorption Kinetics in Metal Intercalated Fullerides
No full textFor different hydrogenated metal intercalated fullerides (Na10C60-H, Li12C60-H, and Li28C60-H) the activation energies for hydrogen desorption were determined by DSC. The Vyazovkin advanced method (VA) was used for the calculation of the reaction model free activation energy as a function of the extent of conversion a. Activation energies are highest for low a and decrease for increasing alpha, between around 200-145 and 245-175 kJ/mol for the Na and Li compounds, respectively. The decrease of activation energy as a function of the extent of conversion can be explained by an increasing charge transfer to the C60H36+y cage during desorption. Na intercalation leads to a significant thermodynamic destabilization for hydrogen desorption. Dehydrogenation enthalpies of 52 (Na10C60-H), 66 (Li12C60-H), and 69 kJ/mol H-2 (Li28C60-H) were determined. These values are lower compared to literature values for desorption of pure C60H36 (74 kJ/mol H-2). The onsets of hydrogen desorption are 185 degrees C (Na10C60-H), 260 degrees C (Li12C60-H), and 250 degrees C (Li28C60-H) compared to >400 degrees C for pure C60H36.LME
Ionic conductivity in the Mg intercalated fullerene polymer Mg2C60
No full textWe report the identification of the new intercalated fullerene polymer Mg2C60. Mg intercalation
was obtained either by solid state reaction between C60 and Mg, or by thermal
decomposition of the metallorganic precursor Mg–Anthracene–(THF)3. High resolution
powder synchrotron and neutron diffraction data have clearly shown that Mg2C60 is isostructural
to the superionic conductor Li4C60, where fullerenes form a two-dimensional
network connected either by four-membered carbon rings, or single C–C bonds. Because
of its peculiar structural arrangement Mg2C60 behaves as a good ionic conductor by means
of uncorrelated ionic hopping across very small energy barriers (DE less than 100 meV), as
found from DC and AC conductivity measurements, thus suggesting its possible use in
future Mg-ion batteries
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
Li12C60: A lithium clusters intercalated fulleride
No full textWe report the structural analysis of the highly-doped lithium fulleride Li12C60, performed using low temperature neutron powder diffraction. Although the main reflections could be initially indexed with
a fcc cell, Monte Carlo Simulated Annealing suggests an unusual monoclinic arrangement for the fullerene molecules. In this structure, C60 units with the same orientation are alternatively stacked to form layers, thus maximising their sterical crowding. Rietveld refinement allowed to localise Li ions, which are organised in clusters belonging to the pseudo-tetrahedral voids with a non-complete charge transfer as confirmed by Raman spectroscopy
Non-interacting hard ferromagnetic L10 FePt nanoparticles embedded in a carbon matrix
No full textMonodispersed ferromagnetic FePt nanoparticles, partially ordered in the L10 phase, were directly
prepared without further annealing by high temperature synthesis (z300 C) involving poly(N-vinyl-2-
pyrrolidone) and Triton X-100 as protective agent and reaction solvent respectively. Depending on the
synthesis conditions, nanoparticles with average sizes ranging from 5 to 7 nm and coercive fields
reaching 0.1 T at 300 K were obtained, but they invariably aggregate by magnetic dipolar interaction.
By increasing the solvent viscosity (using PEG 600), 5 nm superparamagnetic nanoparticles are
embedded in an amorphous matrix derived from solvent condensation/decomposition, thus avoiding
aggregation. Nanoparticles are then completely converted to the hard tetragonal L10 phase, preserving
the original size, by annealing in a vacuum at higher temperatures that, at the same time, transform the
matrix into amorphous carbon. Annealing at 650 C for 3 h leads to coercive fields of about 0.25 T at
RT and 1.3 T at 5 K (without reaching the saturation magnetization) and to a peculiar squeezing of the
hysteresis loops. Subsequent treatments at higher temperatures induce a further shrinking of the loop
and a reduction of the coercive field. The possible explanation takes into account that, by raising the
annealing temperature, an increasing number of nanoparticles becomes free to rotate inside the matrix,
aligning like ‘‘nano-compasses’’ with the applied magnetic field. However a fraction of nanoparticles
remains still locked to the matrix, generating a superimposed magnetically hard contribution
Muons probe magnetism and hydrogen interaction in graphene
No full textMuon spin resonance (mu SR) is a powerful technique for investigating the local magnetic fields in materials through implanted muons. Here we report a mu SR study of chemically produced thermally exfoliated graphene. Our results provide an experimental answer to the many theoretical investigations of magnetic properties of graphene. The observed muon spin precession is attributed to a localized muon-hydrogen nuclear dipolar interaction rather than to a hyperfine interaction with magnetic electrons. This proves the absence of magnetism in chemically produced thermally exfoliated graphene.C3M
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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