RESUMEN
The demand for hydrogen is increasing due to commercialization of fuel cells. Palladium (Pd)-based crystalline membranes have been used for separation of hydrogen from a mixture of gases in coal-based power generation process. However, very high cost of Pd has prompted to explore inexpensive alternative alloys. Amorphous Ni-Nb-Zr alloy membranes are promising cheaper alternatives which exhibit comparable hydrogen permeability to Pd membranes at nominal temperature of ~ 400 °C. Constant exposure to high temperature and hydrogen pressure may lead to changes in the local atomic structure and possible devitrification of membrane. It is critical to understand short-range order of these membranes in order to improve their hydrogen permeability and durability. Icosahedral clusters are the building blocks of amorphous material and hydrogen is expected to interact with them in various different ways. The density functional theory-based molecular dynamics (DFT-MD) approach is the best suited approach to study the local atomic structures for (Ni0.6Nb0.4)90Zr10 and (Ni0.6Nb0.4)70Zr30 amorphous membranes with the help of nearest neighbor distances and icosahedral cluster analysis. It can help predict the behavior of the membrane under extreme operating conditions. Three types of icosahedra (so called Ni-centered, Zr-centered, and Nb-centered) were identified in six different compositions in these amorphous alloys. Evolution of these icosahedra with temperature and in the presence of hydrogen gave an insight into the local structure of the membrane. Zr plays an important role in the formation of icosahedra. Hydrogen atoms interact with the icosahedra in three different ways. It is observed that H atoms did not show tendency to enter Ni-centered icosahedra leading to easier hydrogen diffusion outside the icosahedra. Hence, the more the number of Ni-centered icosahedra, the better the permeation properties of the alloy.
RESUMEN
Ni-Nb-Zr amorphous membranes, prepared by melt-spinning, show great potential for replacing crystalline Pd-based materials in the field of hydrogen purification to an ultrapure grade (>99.999%). In this study, we investigate the temperature evolution of the structure of an amorphous ribbon with the composition Ni32Nb28Zr30Cu10 (expressed in atom %) by means of XRD and DTA measurements. An abrupt structural expansion is induced between 240 and 300 °C by hydrogenation. This structural modification deeply modifies the hydrogen sorption properties of the membrane, which indeed shows a strong reduction of the hydrogen capacity above 270 °C.
RESUMEN
We present a study of the high-pressure behavior of BaReH9, a novel hydrogen-rich compound, using optical, Raman, and infrared spectroscopy as well as synchrotron x-ray diffraction. The x-ray diffraction measurements demonstrate that BaReH9 retains its hexagonal structure on room temperature compression up to 40 GPa. Optical absorption shows the absence of a gap closure to 80 GPa. Raman and IR spectra reveal the pressure evolution of a newly observed phonon peak, and large peak broadening with increasing pressure. These data constrain the disorder present in the material following the P-T paths explored.
RESUMEN
High-pressure Raman spectroscopy has been used to study tris(hydroxymethyl)aminomethane (C(CH(2)OH)(3)NH(2), Tris). Molecules with globular shapes such as Tris have been studied thoroughly as a function of temperature and are of fundamental interest because of the presence of thermal transitions from orientational order to disorder. In contrast, relatively little is known about their high-pressure behavior. Diamond anvil cell techniques were used to generate pressures in Tris samples up to approximately 10 GPa. A phase transition was observed at a pressure of approximately 2 GPa that exhibited relatively slow kinetics and considerable hysteresis, indicative of a first-order transition. The Raman spectrum becomes significantly more complex in the high-pressure phase, indicating increased correlation splitting and significant enhancement in the intensity of some weak, low-pressure phase Raman-active modes.
RESUMEN
In situ high-pressure Raman spectroscopy studies on LiNH2 (lithium amide) have been performed at pressures up to 25 GPa. The pressure-induced changes in the Raman spectra of LiNH2 indicates a phase transition that begins at approximately 12 GPa is complete at approximately 14 GPa from ambient-pressure alpha-LiNH2 (tetragonal, I) to a high-pressure phase denoted here as beta-LiNH2. This phase transition is reversible upon decompression with the recovery of the alpha-LiNH2 phase at approximately 8 GPa. The N-H internal stretching modes (nu([NH2]-)) display an increase in frequency with pressure, and a new stretching mode corresponding to high-pressure beta-LiNH2 phase appears at approximately 12.5 GPa. Beyond approximately 14 GPa, the N-H stretching modes settle into two shouldered peaks at lower frequencies. The lattice modes show rich pressure dependence exhibiting multiple splitting and become well-resolved at pressures above approximately 14 GPa. This is indicative of orientational ordering [NH2]- ions in the lattice of the high-pressure beta-LiNH2 phase.
RESUMEN
The pressure-induced phase transformations in pure LiAlH4 have been studied using in situ Raman spectroscopy up to 7 GPa. The analyses of Raman spectra reveal a phase transition at approximately 3 GPa from the ambient pressure monoclinic alpha-LiAlH4 phase (P2(1)/c) to a high pressure phase (beta-LiAlH4, reported recently to be monoclinic with space group I4(1)/b) having a distorted [AlH4]- tetrahedron. The Al-H stretching mode softens and shifts dramatically to lower frequencies beyond the phase transformation pressure. The high pressure beta-LiAlH4 phase was pressure quenchable and can be recovered at lower pressures ( approximately 1.2 GPa). The Al-H stretching mode in the quenched state further shifts to lower frequencies, suggesting a weakening of the Al-H bond.
