The utilisation of clean energy sources is paramount in ensuring the future preservation of our environment. Hydrogen has been identified as an efficient and inexpensive alternative to fossil fuels, however it is difficult to store safely. A promising solution to this problem is storing the hydrogen in the form of metal hydrides. Magnesium can reversible store as much as 7.6wt% hydrogen, however it has a low hydrogen dissociation pressure and its’ hydriding/dehydriding reaction kinetics are too slow. Fortunately, studies have shown that this reaction can be catalysed through alloying of magnesium with copper.
The present study has investigated ways of improving the hydriding/dehydriding reaction kinetics in Mg-Cu alloys by decreasing the size of the catalysing phase (in this case Mg2Cu). As this phase exists in the eutectic region of Mg-Cu alloys, the intended method of size reduction involved modification of the eutectic microstructure. Minor additions of a range of elements (zirconium, sodium, barium, and strontium) were therefore tested to determine their effect on the development of the eutectic microstructure. Also of interest was whether these additions could refine the primary Mg phase, as this may be desirable for of improving the hydriding reaction kinetics. Thermal, chemical, and microstructural analysis techniques were used to characterise the resultant alloys.
According to the results of this analysis, the most successful modifying addition was sodium, providing over 60% refinement for an alloy of 2wt% sodium. Zirconium was found to considerably refine both the primary and eutectic phase microstructures. Barium did not have a refining effect on the eutectic, and may even have coarsened it. Strontium, added in the form of an Al-10Sr alloy, was found to introduce new phases into the alloys; however the considerable proportions of Al involved prevented determination of the ability of strontium to modify the eutectic.
The results of this investigation show that it is possible to control the eutectic microstructure of Mg-Cu alloys. Such microstructural control may one day enable the production of alloys with markedly improved hydrogen storage properties.