(7f) Ultrasmall-Angle X-Ray Scattering (USAXS) Studies of the Effect of Transition Metal (Ti2+, Ti3+, V3+, Fe3+, and Zr4+) Dopants in NaAlH4 Powders

Transition metal dopants added to complex metal hydrides, specifically to sodium aluminum hydride (NaAlH4), by high energy ball milling enhances dehydrogenation kinetics and induces dehydrogenation reaction reversibility. This study uses ultrasmall angle x-ray scattering (USAXS) to elucidate differences in NaAlH4 particle morphology as dopant type and mill time is varied. Five dopant types were used. Two dopant types were used to represent the best kinetic enhancements having high desorption rates (e.g. TiCl2, TiCl3) and were compared with two dopant types which do not perform as well (e.g. ZrCl4, FeCl3 and VCl3). USAXS data for the doped hydrides were compared with undoped and milled NaAlH4 powders. Mill times used were 0 minutes (blended), 1 minute, 5 minutes, and 25 minutes. As indicated by USAXS power-law scattering data, the undoped NaAlH4 powders are comprised of primary particles having a high surface area (surface fractal geometries) and of smooth colloidal particles. The powders, when catalyzed with a single transition metal cation, and milled for 1 minute, 5 minutes, and 25 minutes show uniformly decreasing hydride particle surface area with mill time.

surface area using measured particle sizes and volume fractions. The variation in desorption rates in those co-doped systems were correlated with changes in powder surface area.