Your browser doesn't support javascript.
loading
Mostrar: 20 | 50 | 100
Resultados 1 - 5 de 5
Filtrar
Mais filtros

Base de dados
Tipo de documento
Assunto da revista
País de afiliação
Intervalo de ano de publicação
1.
J Phys Chem A ; 116(35): 8827-32, 2012 Sep 06.
Artigo em Inglês | MEDLINE | ID: mdl-22794328

RESUMO

We have measured the Raman spectrum of ammonia borane at low temperature (T = 15 K) and across the orthorhombic-to-tetragonal phase transition at T = 225 K. A comprehensive study of the low frequency lattice modes using Raman spectroscopy has been carried out. Data analysis has been complemented by a density functional theory calculation of which the results have been used for a detailed assignment of the Raman active modes. The analysis of the spectroscopic measurements taken across the phase transition seems to be consistent with the increasing orientational disorder of the molecular components and seems to be compatible with the equalization of the a and b lattice constants characteristic of the tetragonal phase.

2.
J Phys Chem A ; 115(5): 691-9, 2011 Feb 10.
Artigo em Inglês | MEDLINE | ID: mdl-21235227

RESUMO

A Raman investigation of the AlH(3) polymorph has been carried out at a low temperature (20 K) under helium atmosphere (2 bar). The pristine material was composed of three polymorphs, namely, the α, ß, and γ phases. The ß phase has been removed by warming the sample to 70 °C, while further heating at 100 °C was used to remove the γ phase. This allowed us to evidence, on a purely experimental basis, the characteristic Raman spectrum for each phase. Raman spectra, for the three phases, have been also calculated using density functional theory, and the results have been compared to the present experimental data, allowing for a univocal assignment, to each phase, of its characteristic spectral features.

3.
J Phys Chem A ; 115(26): 7503-10, 2011 Jul 07.
Artigo em Inglês | MEDLINE | ID: mdl-21675722

RESUMO

Inelastic neutron scattering and Raman scattering spectra of a melt-infiltrated composite of NaAlH(4) and active carbon fibers have been measured at low temperature for two sample conditions: as prepared and subjected to hydrogen desorption-absorption cycling. After a careful data analysis, the present experimental results have been compared to the corresponding spectroscopic data taken from bulk NaAlH(4) and Na(3)AlH(6). Evident signatures induced by infiltration process onto the NaAlH(4) phonon bands have been detected, showing up as a strong peak broadening and smoothing together with, in some cases, an energy shift. Traces of Na(3)AlH(6), appearing as an extra intensity between 130 and 200 meV, seem also confirmed. A substantial agreement between neutron and Raman measurements has been found for the as-prepared melt-infiltrated sample, while for the cycled sample the two techniques produced rather dissimilar results. However, this apparent discrepancy can be explained by considering the different penetration depths of the two spectroscopic probes. Further work, both experimental and based on ab initio simulations, is surely needed in order to rationalize the finding of the present measurements.

4.
J Phys Chem A ; 114(8): 2788-93, 2010 Mar 04.
Artigo em Inglês | MEDLINE | ID: mdl-20141188

RESUMO

Raman spectra of Mg(BH(4))(2) have been measured in an extensive temperature range, from 15 to 473 K. Taking into account the high temperature conversion from the alpha to the beta phase, we have observed evident signatures of this phase transition and determined the Raman vibrational spectrum of each phase. The neutron scattering spectra of the beta phase sample were also recorded. The present experimental results have been compared to the density functional theory calculations available in the literature, and a substantial agreement has been found.

5.
Rev Sci Instrum ; 79(1): 013105, 2008 Jan.
Artigo em Inglês | MEDLINE | ID: mdl-18248020

RESUMO

We report the design and realization of an integrated system for measuring, at the same time, the thermodynamic and spectroscopic features of nanoporous materials interesting for hydrogen storage purposes. The whole investigation cycle, from thermal activation to the actual investigation of uptake and release of hydrogen, is carried out in the same vacuum tight vessel, equipped with an optical window, whose temperature can range between 10 and 750 K, up to a maximum pressure of 50 bars. The system has been designed to investigate properties of carbon nanotubes but its use can be extended to any kind of nanoporous sample such as, for example, carbon nanofibers, zeolytes, metal organic frameworks, and similar materials.

SELEÇÃO DE REFERÊNCIAS
DETALHE DA PESQUISA