51 research outputs found
NMR investigation of the pressure induced Mott transition to superconductivity in Cs3C60 isomeric compounds
The discovery in 1991 of high temperature superconductivity (SC) in A3C60
compounds, where A is an alkali ion, has been initially ascribed to a BCS
mechanism, with a weak incidence of electron correlations. However various
experimental evidences taken for compounds with distinct alkali content
established the interplay of strong correlations and Jahn Teller distortions of
the C60 ball. The importance of electronic correlations even in A3C60 has been
highlighted by the recent discovery of two expanded fulleride Cs3C60 isomeric
phases that are Mott insulators at ambient pressure. Both phases undergo a
pressure induced first order Mott transition to SC with a (p, T) phase diagram
displaying a dome shaped SC, a common situation encountered nowadays in
correlated electron systems. NMR experiments allowed us to establish that the
bipartite A15 phase displays N\'eel order at 47K, while magnetic freezing only
occurs at lower temperature in the fcc phase. NMR data do permit us to conclude
that well above the critical pressure, the singlet superconductivity found for
light alkalis is recovered. However deviations from BCS expectations linked
with electronic correlations are found near the Mott transition. So, although
SC involves an electron-phonon mechanism, correlations have a significant
incidence on the electronic properties, as had been anticipated from DMFT
calculations.Comment: 9 pages, 6 figures, invited at M2SHTSC Conference Washington (2012
Optimal hydrogen storage in sodium substituted lithium fullerides
A relevant improvement in the hydrogen storage capability of lithium fullerides is obtained by the co-intercalation of a small amount of sodium
Probing the thermal stability and the decomposition mechanism of a magnesium–fullerene polymer via X-ray Raman spectroscopy, X-ray diffraction and molecular dynamics simulations
Extending the hydrogen storage limit in fullerene
Li6C60 has been chosen as the most representative system to study the hydrogenation mechanism in alkali-cluster intercalated fullerides. We present here a muon spin relaxation (mu SR) experiment that hints the chance to achieve a higher storage capacity on fullerene with respect to the values suggested in literature. Moreover, a linear relationship between the muonium adduct radical hyperfine frequency and the level of C-60 hydrogenation was found and it can be exploited to probe the C-60 hydrogenation level, giving more credit to this technique in the field of hydrogen storage materials. (C) 2017 Elsevier Ltd. All rights reserved.Peer reviewe
Fundamentals of hydrogen storage in nanoporous materials
Physisorption of hydrogen in nanoporous materials offers an efficient and competitive alternative for hydrogen storage. At low temperatures (e.g. 77 K) and moderate pressures (below 100 bar) molecular H2 adsorbs reversibly, with very fast kinetics, at high density on the inner surfaces of materials such as zeolites, activated carbons and metal–organic frameworks (MOFs). This review, by experts of Task 40 ‘Energy Storage and Conversion based on Hydrogen’ of the Hydrogen Technology Collaboration Programme of the International Energy Agency, covers the fundamentals of H2 adsorption in nanoporous materials and assessment of their storage performance. The discussion includes recent work on H2 adsorption at both low temperature and high pressure, new findings on the assessment of the hydrogen storage performance of materials, the correlation of volumetric and gravimetric H2 storage capacities, usable capacity, and optimum operating temperature. The application of neutron scattering as an ideal tool for characterising H2 adsorption is summarised and state-of-the-art computational methods, such as machine learning, are considered for the discovery of new MOFs for H2 storage applications, as well as the modelling of flexible porous networks for optimised H2 delivery. The discussion focuses moreover on additional important issues, such as sustainable materials synthesis and improved reproducibility of experimental H2 adsorption isotherm data by interlaboratory exercises and reference materials
Procedimento di separazione di Fullereni da una miscela solida di Fullereni a diverso peso molecolare di carbonio amorfo
Ambito C - Un percorso di formazione insegnanti: Il moto circolare - Section C - A teacher training course: The circular motion
Probing the thermal stability and the decomposition mechanism of a magnesium-fullerene polymer via X-ray Raman spectroscopy, X-ray diffraction and molecular dynamics simulations
We report the microscopic view of the thermal structural stability of the magnesium intercalated fullerene polymer Mg2C60. With the application of X-ray Raman spectroscopy and X-ray diffraction, we study in detail the decomposition pathways of the polymer system upon annealing at temperatures between 300 and 700 degrees C. We show that there are at least two energy scales involved in the decomposition reaction. Intermolecular carbon bonds, which are responsible for the formation of a 2D fullerene polymer, are broken with a relatively modest thermal energy, while the long-range order of the original polymer remains intact. With an increased thermal energy, the crystal structure in turn is found to undergo a transition to a novel intercalated cubic phase that is stable up to the highest temperature studied here. The local structure surrounding magnesium ions gets severely modified close to, possibly at, the phase transition. We used density functional theory based calculations to study the thermodynamic and kinetic aspects of the collapse of the fullerene network, and to explain the intermediate steps as well as the reaction pathways in the break-up of this peculiar C-60 intermolecular bonding architecture.Peer reviewe
Hydrogen Desorption Kinetics in Metal Intercalated Fullerides
For 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
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