Preparation Of Nanocrystalline / Amorphous Mg-ni Alloy And Its Electrochemical Performance

Mg-based hydrogen storage alloys are regard as one of the most promising alloys because of their high hydrogen storage capacity, abundant resource and low price. However, their hydriding and dehydriding kinetics are slow and the working temperatures are too high for practical applications. This paper details the preparation of three Mg-based hydrogen storage alloys.First, a single phase Mg2Ni hydrogen storage alloy was synthesized by self-propagating high-temperature synthesis (SHS) using magnesium and nickel powder. The influences of change in diameter of green compact, pressure, and hold condition after combustion reaction (i.e. 550 for 30 min or not) were studied. The results show that SHS is an ideal method to fabricate Mg2Ni. The product was pure. However, the electrochemical properties of the Mg2Ni prepared by SHS was poor. For example, the initial discharge capacity was only 50 mAh/g.Second, to improve its electrochemical properties, a nanocrystalline Mg2Ni was prepared using mechanical milling. The decrease in crystalline-size of the alloy after mechanical treatment were calculated from the peak width of the X-ray diffraction (XRD) data using Scherrer Formula. Nanocystalline Mg2Ni (5.90 nm) was obtained after only 3 hours of milling. With further milling the size reduced even more. After 20 hours milling, it was 4.96 nm. The initial discharge capacity of the alloy after 5 hours of milling was improved to 150 mAh/g.Third, an amorphous MgNi hydrogen storage alloy was synthesized by mechanical alloying. The Mg2Ni alloy prepared by SHS was combined with Nickel (powder) as starting materials. Changes in product were achieved by: 1) milling speed, 2) ball-to-powder mass ratio (R), 3) milling duration. The milling period extremely increased when the milling speed was slow. A low ball-to-powder mass ratio was counterproductive to the formation of amorphous MgNi. There was an optimal duration in milling process, if prolonged, a nanocrystalline nickel formed from the amorphous MgNi. The electrochemical performance improved with the amorphous structure. A maximum initial discharge capacity of 202mAh/g was obtained. The initial discharge capacity was in accordance with the amount of amorphous alloy. The influence of the 3 milling parameters to the productivity of amorphous followed the same trend to the electrochemical properties.