TEM characterization of pure and transition metal enhanced NaAlH4

Possibilities and limitations in using transmission electron microscopy to characterize pure NaAlH₄ and transition metal enhanced NaAlH₄ have been investigated in detail. NaAlH₄ is extremely sensitive to O₂ and H₂O and must be handled under inert atmosphere at all times. Furthermore, it is highly unstable under the electron beam and only basic techniques such as diffraction contrast imaging and selected area diffraction that can be performed with a low flux electron beam can be used without the NaAlH₄ decomposing. By comparison, phases containing transition metal additive are very stable under the electron beam. The latter are investigated by a combination of high resolution imaging, electron diffraction and spectroscopy to determine distribution, composition, crystal structure and defect content in ball milled and hydrogen cycled, TiCl₃ and FeCl₃ enhanced NaAlH₄. It is demonstrated that a large amount of the added Ti or Fe is located at the surface of the NaAlH₄ grains as a combination of crystalline and amorphous Al_1−xTM_x (TM = Ti, Fe) nanoparticles.