ULTRASMALL ANGLE X-RAY SCATTERING (USAXS) STUDIES OF MORPHOLOGICALCHANGES IN NaAlH4

摘要

The ultra-small Angle X-Ray Scattering (USAXS) technique has been explored to study morphological changes that occur in the hydrides during various stages of hydrogen release. The power law scattering data from USAXS measurements yield the power law slope (p) which is used as a definitive parameter to study changes in surface fractal dimensions (Ds). Changes in surface area occur due to densification during desorption at elevated temperatures and the rate of those changes are mitigated by the addition of transition metal dopants. For the present study, NaAlH* was doped with 4 mol% TiCU by high energy ball milling and subjected to USAXS measurements to determine the effect of the catalyst. USAXS measurements were also done on NaAlH4 nano-confined within porous alumina membranes. Results showed that the power law slope from USAXS analysis increased from p=-3.6 to p=-3.46 after high energy ball milling; to p=-3.26 after catalyst addition and to p=-3 after heat treatment, indicating an increase in specific surface area accompanying hydrogen release. The particle sizes before and after heat treatment were also evaluated to quantify the extent of densification occurring due to heat treatment. The radius of gyration (Rg) for unmilled NaAlH4 increased from 9.7nm to 26.4nm and a 5 min milled NaAlH4 particle size increased from Rg=6nm to Rg=14.5nm. USAXS on nanostructured NaAlH4 yielded power law slopes of p=-1.41 (rod shaped particles) and p = -2.7 (disc shaped particles) which proved USAXS to be an effective tool to identify the confined particles in porous matrices by virtue of shape.