Ultrasmall-angle X-ray scattering (USAXS) studies of morphological trends in high energy milled NaAlH 4 powders

Abstract

Transition metal dopants added to complex metal hydrides, specifically to sodium aluminum hydride (NaAlH 4), by high energy ball milling enhances dehydrogenation kinetics and induces dehydrogenation reaction reversibility. This study uses the power-law scattering parameter, p, gained from ultrasmall-angle X-ray scattering (USAXS) data to elucidate differences in NaAlH 4 particle morphology as dopant type and mill time is varied. Four dopant types were used. Two dopant types were used to represent the best kinetic enhancements having high desorption rates (e.g. TiCl 2, TiCl 3) and were compared with two dopant types which do not perform as well (e.g. ZrCl 3 and VCl 3). USAXS data for the doped hydrides were compared with undoped and milled NaAlH 4 powders. Mill times used were 0 min (blended), 1, 5, and 25 min. As indicated by the USAXS power-law scattering data, the undoped NaAlH 4 powders are comprised of primary particles each having a high surface area. The high particle surface area in the undoped NaAlH 4 particle system persists as mill time increases—with only the 25 min sample undergoing a marked a decrease in primary particle surface area. Alternatively, the doped powders milled for 1, 5, and 25 min show uniformly decreasing hydride particle surface area. These decreases in particle surface area may be explained by either the colloidal particles increasing in surface smoothness or decreasing internal void space. TiCl 3-doped NaAlH 4 powders show the trend of maintaining particles having a morphology comprised of higher particle surface area during the high energy milling stage of powder processing compared with other dopants.