A technique of trace element doping

A technique of trace element doping to modify the solidification mechanisms of the faceted/non-faceted eutectic Mg–Mg2 Ni alloy system has been developed [2] It was demonstrated that the microand
nanostructure of cast hypoeutectic Mg–Mg2 Ni alloys can be varied by trace additions of Na, Ca or Eu to the liquid prior to solidification. As a result, a reversible hydrogen absorption capability in excess of 90% of the theoretical value of 6.8 wt% under the absorption parameters of 350 C and 1 MPa for 24 min and subsequent desorption at 0.2 MPa for 24 min after activation has been achieved [3] The hydrogen absorption kinetics have been dramatically improved under realistic industrial conditions, and the alloys
also show no sign of reduced capacity even after 200 hydride/dehydride cycles [4] The solidification processing route results in a non-pyrophoric material that may be produced in large quantities at comparatively low cost [4] The exact mechanisms of improved kinetics are still not understood, however, it is expected that the combination of the morphological change and trace element distribution along with possible changes in the density of crystallographic defects and the chemical nature of the modifying additions are all likely to contribute to the unique hydrogen storage properties of modified magnesium based hydrogen storage alloys [5] It has been found that Mg–Ni alloys in machined ‘chips’ react with the external atmosphere (air/oxygen) at a slower rate than samples of higher specific surface area such as powders which are produced by high energy ball milling under inert atmospheres