New Aspects on the Decomposition of Sodium Alanate Revealed by Small-Angle X-ray Scattering

Abstract

Transition metals added to sodium aluminum hydride by high-energy ball milling have been shown to significantly enhance its absorption and desorption properties. In the present study, we have used small-angle X-ray scattering to elucidate how TiCl₃ affects the nanostructure of NaAlH₄ particles. Scattering data from as-purchased and ball milled NaAlH₄ for 6 and 15 min are compared with NaAlH₄ ball milled for the same time with 4 mol % TiCl₃. Drastic differences were noticed in the two systems which cast a new light on the decomposition of NaAlH₄, in particular on the effect of ball milling and of TiCl₃ on the morphology, grain size, and distribution of the phases. The particle morphology of pure NaAlH₄ showed significant evolution/changes during heating from room temperature to 290 °C, as evidenced by the variations in the power-low scattering parameter, α. Drastic changes were noticed in the particle surface structure during the phase transformation from NaAlH₄ to Na₃AlH₆ + Al, when the system becomes less compact and the particle surface rougher. The addition of TiCl₃ induces a different effect on both surface and mass structure, at least in the nanometer length scale considered in this study: the particles retain their surface morphology at all temperatures. Furthermore, even after short ball milling times the addition of TiCl₃ increases the system compactness with reduction of internal voids