Publications

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Sodium aluminum hydride, NaAlH{sub 4}, has been studied for use as a hydrogen storage material. The effect of Ti, as a few mol. % dopant in the system to increase kinetics of hydrogen sorption, is studied with respect to changes in lattice structure of the crystal. No Ti substitution is found in the crystal lattice. Electronic structure calculations indicate that the NaAlH{sub 4} and Na{sub 3}AlH{sub 6} structures are complex-ionic hydrides with Na{sup +} cations and AlH{sub 4}{sup -} and AlH{sub 6}{sup 3-} anions, respectively. Compound formation studies indicate the primary Ti-compound formed when doping the material at 33 at. % is TiAl{sub 3} , and likely Ti-Al compounds at lower doping rates. A general study of sorption kinetics of NaAlH{sub 4}, when doped with a variety of Ti-halide compounds, indicates a uniform response with the kinetics similar for all dopants. NMR multiple quantum studies of solution-doped samples indicate solvent interaction with the doped alanate. Raman spectroscopy was used to study the lattice dynamics of NaAlH{sub 4}, and illustrated the molecular ionic nature of the lattice as a separation of vibrational modes between the AlH{sub 4}{sup -} anion-modes and lattice-modes. In-situ Raman measurements indicate a stable AlH{sub 4}{sup -} anion that is stable at the melting temperature of NaAlH{sub 4}, indicating that Ti-dopants must affect the Al-H bond strength.

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We observe the spontaneous formation of parallel oxide rods upon exposing a clean NiAl(110) surface to oxygen at elevated temperatures (850-1350 K). By following the self-assembly of individual nanorods in real time with low-energy electron microscopy (LEEM), we are able to investigate the processes by which the rods lengthen along their axes and thicken normal to the surface of the substrate. At a fixed temperature and O{sub 2} pressure, the rods lengthen along their axes at a constant rate. The exponential temperature dependence of this rate yields an activation energy for growth of 1.2 {+-} 0.1 eV. The rod growth rates do not change as their ends pass in close proximity (<40 nm) to each other, which suggests that they do not compete for diffusing flux in order to elongate. Both LEEM and scanning tunneling microscopy (STM) studies show that the rods can grow vertically in layer-by-layer fashion. The heights of the rods are extremely bias dependent in STM images, but occur in integer multiples of approximately 2-{angstrom}-thick oxygen-cation layers. As the rods elongate from one substrate terrace to the next, we commonly see sharp changes in their rates of elongation that result from their tendency to gain (lose) atomic layers as they descend (climb) substrate steps. Diffraction analysis and dark-field imaging with LEEM indicate that the rods are crystalline, with a lattice constant that is well matched to that of the substrate along their length. We discuss the factors that lead to the formation of these highly anisotropic structures.

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