Recent Scientific Results

Gas-Storage Materials

Clathrates for Hydrogen Storage

Hydrogen clathrates have potential as an alternative hydrogen gas storage material. It is important to understand how the hydrogen interacts with the clathrate structure at a molecular level. Previous models of this interaction assumed the clathrate cage to be a rigid structure (fixed cage). Ab initio molecular dynamics simulations were performed in VASP to determine the nature of these interactions and assess if they depend on whether the cage is fixed or dynamic (flexible). Interesting results were found which suggest significant coupling between some dynamical modes of the hydrogen molecule and those of the clathrate cage. These results contradict the fixed cage model. Work is underway to verify these findings from experiment.
 

Gas Hydrates

Neutron powder diffraction (Wombat) was successfully used to analyse the in-situ crystallization of methane clathrate hydrate from a D2O ice powder at a gas pressure of 45 bar and a temperature of 273 K. The in-situ experiment investigated the kinetics of clathrate formation, which shows a two stage mechanism with a fast nucleation and growth phase followed by a diffusion limited phase. Data gained at 60 second intervals were successful in studying the slow diffusion limited phase. To study the more rapid early phase of formation we intend to run future Wombat experiments at shorter time intervals.

Fuel Cell Research

Solid-state Ionic Conductors for Solid-Oxide Fuel Cells

Such work to date has revealed that in the simple Cu2-xSe system the fraction of Cu atoms which takes part in the ionic transport decreases with x, but not vanishes at x = 0.23, and that the self-diffusion coefficient depends on composition. We find a strong contribution from inelastic scattering coming from correlated thermal displacements of the ions in the super-ionic phase, and we have clarified the role of static disorder versus low-energy phonons, confirming that diffuse scattering arising from these materials is mainly inelastic and most probably comes from the low-energy phonons.

The first phonon dispersion curve for Taipan was measured using this system.We revisit a previously studied compound in Cu1.85Se superionic conductor having structure of the superionic a-phase at room temperature. Measurements of the first phonon dispersion curves were performed with new triple-axis spectrometer TAIPAN at OPAL reactor [1] using this system. A fixed final neutron energy of 14.87 meV; vertically focused HOPG (002) monochromator and 40¢ collimations before and after the sample were used in the experiment. We found that transverse phonon branch TA1 with the polarization vector along [1-10] direction demonstrates a decrease in frequency at q > 0.5 rather than the flattening observed before [2].

Transverse acoustic branches with wavevectors in [100] and [111] have a similar to TA1 [110] shape with maximum frequency at q » 0.5. The possible reason for such behaviour is ordering in Cu sublattice. Superstructure reflections are observed at the G ± 1/2 <111> and G ± 1/3 <220> positions of reciprocal space, which can cause effects similar to the folding of the Brillouin zone, although phonon intensities at new Brillouin zone centres are weak. Note that “softened” branches have at least one polarisation vector in direction <110>, orthogonal to the <111>; the <111> direction is assumed to be diffusion path for Cu atoms [5]. We speculate that the coupling of these phonon modes with displacement of mobile ions can explain the strong dumping of phonons observed in experiment.

References:

[1] S. Danilkin, G. Horton, R. Moore, G. Braoudakis, M. Hagen: J. Neutron Research, 15, (2007) 55.

[2] S.A. Danilkin, A.N. Skomorokhov, A. Hoser, H. Fuess, V. Rajevac, N.N. Bickulova: J. Alloys and Compounds 361(2003) 57.

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Semiconductor Materials for Dye-Sensitised Solar Cell Applications