Microstructural evolution during low temperature sorption cycling of Mg-AlTi multilayer nanocomposites

We studied the hydrogen storage behavior of sputtered Mg-AlTi multilayers where nanometric Mg or Mg–Al–Ti layers were confined by 2 nm thick layers of AlTi. By decreasing the thickness of Mg layers, we were able to achieve 5.1 wt.% H capacity without significant degradation in over 200 cycles at 473 K. However, for the samples with pure Mg layers degradation eventually occurred at higher cycle numbers. In multilayers of 34 nm thick Mg degradation was followed by disintegration of the films into sponge-like flakes. Alloying Mg layers with Al and Ti through cosputtering improved the performance of the multilayer composites. Through cycling, Al and Ti segregated out of Mg matrix and formed a nanocrystalline/amorphous AlTi phase as observed by X-ray diffraction and electron microscopy. This improved resistance of the microstructure against coarsening while a well dispersion of AlTi particles was achieved. Moreover, the stability of multilayers enhanced to an extent that they not only preserved their physical integrity, also did they maintain their superior kinetics up to over 250 cycles. Pressure - composition isotherms showed no significance change in thermodynamics of MgH2 formation.