Neutron scattering study of the proton dynamics in NH+4 and OH+3 β alumina

Quasielastic neutron scattering experiments have been performed at different resolutions on powdered samples of β alumina containing either NH 4+ or more or less hydrated OH3+ cations. The existence of reorientations with a proton jump rate of 1.4 x 1012 s-1 at 395 K is confirmed for the ammonium derivative. Furthermore, it is shown that only the cations occupying Beevers-Ross sites (∼ 75 %) are concerned by this fast motion, in agreement with the predictions of previous X-Ray and spectroscopic studies. No reorientational motion is detected for the fully hydrated β alumina but a dehydrated sample obtained by heating the latter above 670 K presents a quasielastic broadening which can be associated with jump rotations of OH3+ cations around their threefold axis. The frequency of this motion is 4.1 x 10 11 s-1 at 395 K and 4.9 x 1011 s-1 at 485 K. At these two temperatures the percentage of rotating protons, 44 %, is constant and corresponds rather well to the expected non stoichiometric composition 1.33(OH3+)2O, 11 Al1O 3 in which one third of the unit cell planes are occupied by two hydrogen bonded cations in middle-oxygen sites, the other two planes being occupied by only one cation relatively free to rotate. The same sample undergoes a partial rehydration at room temperature. Only 24 % of the protons are then found to be involved in a reorientational motion with a frequency of 2 x 1011 s-1. With a resolution of the order of 1 μeV, another kind of broadening seems to occur in the case of the ammonium derivative. It could be due to translational jump diffusion characterized by a self-diffusion constant of the order of 10-8 cm2 s-1. This result is consistent with the conductivity measurements.Des expériences de diffusion quasiélastique des neutrons ont été effectuées à différentes résolutions sur des échantillons en poudre d'alumine βcontenant soit des cations NH+4 soit des cations OH+ 3 plus ou moins hydratés. On confirme l'existence de réorientations des ions NH+4 avec une vitesse de saut du proton de 1,4 x 1012 s-1 à 395 K. On montre en outre que seuls les cations occupant des sites Beevers-Ross (∼ 75 %) sont concernés par ce mouvement rapide, en accord avec les prédictions d'études cristallographiques et spectroscopiques antérieures. Aucun mouvement de réorientation n'est détecté dans l' alumine β complètement hydratée. Par contre, un échantillon deshydraté, obtenu par chauffage du précédent au-dessus de 670 K, présente un élargissement quasiélastique qui peut être associé à des rotations par sauts des cations OH+ 3 autour de leur axe ternaire. La fréquence de ce mouvement est 4,1 x 1011 s-1 à 395 K et 4,9 x 1011 s-1 à 485 K. A ces deux températures le pourcentage de cations en rotation (44 %) reste constant et correspond assez bien à la composition non stoechiométrique attendue : 1,33(OH3+)2O, 11 Al2O 3, pour laquelle un tiers des plans-maille sont occupés par deux cations liés par liaison hydrogène tandis que les deux autres plans sont occupés par un seul cation en interaction faible avec l'environnement. Le même échantillon se ré-hydrate partiellement à température ambiante ; 24 % seulement des cations sont en effet impliqués dans un mouvement de réorientation avec une fréquence de 2 x 1011 s-1. Si l'on utilise une résolution de 1 μeV, un autre type d'élargissement semble apparaitre dans le cas du dérivé NH+4 Il pourrait être dû à une diffusion translationnelle par sauts et on peut alors déduire une constante d'auto-diffusion de l'ordre de 10-8 cm2 s-1. Les mesures de conductivité antérieures rendent ce résultat vraisemblable

Physical characterization of carbonaceous materials prepared by mechanical grinding

By means of mechanical grinding, we recently reported the ability to prepare tailor-made carbon materials able to reversibly intercalate two lithiums per six carbons (e.g., Li2C6) while having irreversible capacities of 320 mA h/g. A schematic model involving two different types of surface area was previously proposed to account for the reversible and irreversible capacities measured vs. Li with these powders. We experimentally verified this model by means of differential scanning calorimetry (DSC) measurements. Transmission Electronic Microscopy (TEM), which is a powerful tool for the direct imaging of poorly organized materials at the atomic scale has been used, together with Raman Spectroscopy, to follow the disorganization generated by mechanical grinding. © 1999 Elsevier Science S.A. All rights reserved

Raman study on carbonaceous materials prepared by mechanical milling

Through Raman spectroscopy studies, we show that mechanical grinding generates an increasing amount of unorganized carbon at a rate depending on the type of grinding mode used (Shear and Shock-type grinding). The first-order Raman spectrum for pure unground graphite has a well-known G sharp band at 1579 cm -1, which corresponds to the E 2g vibration while the ground samples present a broadened G band accompanied by new components at about 1610 cm -1 (D′), 1510 cm -1 (D″) and 1348 cm -1 (D), usually explained as arising from disorder and defects 1. Shock-type grinding produces a faster disorder increase than shear-type grinding. The latter preserves part of the graphitic character. The general effect of mechanical milling remains however opposite to that of Thermal Treatment (Graphitization). © 1998 OPA (Overseas Publishers Association) Amsterdam B.V. Published under license under the Gordon and Breach Science Publishers imprint