Analysis of polydisperse bubbles in the aluminium–hydrogen system using a size-dependent contrast

Abstract

The characterization of hydrogen defects in an aluminium–hydrogen system was performed previously [Buckleyet al.(2001).J. Appl. Cryst.34, 119–129] using small-angle scattering, inelastic neutron scattering and electron microscopy techniques. This analysis resulted in the determination of the relative change in lattice parameter as a result of hydrogen introduction into the Al matrix. However, this method relied on the average volume of the bubbles of hydrogen and also the pressure in a bubble of average volume. The characterization of the Al–H system has been improved by considering the size polydispersity of the hydrogen bubbles. The determination of a volume-fraction size distribution of the bubbles from small-angle scattering data has allowed a polydispersity analysis to be undertaken. A size-dependent contrast has been utilized in the modification of the volume-fraction size distribution into a more accurate form that accounts for varying concentrations of hydrogen within bubbles of different sizes. The determination of the size-dependent contrast is based upon an equation of state for molecular hydrogen which incorporates the compressibility of hydrogen under high pressures. The formation of alane (AlH3) is also investigated, as it can be formed by the chemisorption of hydrogen in aluminium under high pressures. The polydispersity analysis has allowed a more accurate description of the Al–H system and can be applied to similar scattering systems where the scattering length density is not constant over the whole scattering size regime.