Study Of The Crystallization And Hydrogen Storage Properties Of The Mg-Ni-La Bulk Metallic Glass System

Mg-based amorphous alloys are very promising as hydrogen storage materials because of the relatively low dehydrogenation temperature and good dehydrogenation kinetics compared with other Mg-based alloys. In this thesis, a brief review of the research and development of the Mg-Ni-La based hydrogen storage alloys, especially the Mg-Ni-La based amorphous alloys, was introduced. On this basis, some composites of bulk metallic glass system with high glass forming ability (GFA) were selected for systematical study. The Mg-Ni-La metallic glasses were prepared and the microstructure and thermodynamic properties were analyzed using X-ray diffraction (XRD) and differential scanning calorimetry (DSC), as well as the hydrogenation effect on the microstructure and thermodynamic properties. The hydrogen storage and electrochemical properties were also well researched. The conclusions are as following:(l)The alloys with the composites of Mg50Ni30La20, Mg60Ni23.6La16.4 and Mg71Ni18La11 respectively has very high GFA and the alloys with a homogeneous amorphous phase can be prepared using single roller melt-spun technique. All the three metallic glasses exhibits a wide supercooled liquid region and significant kinetic nature. The activation energies of the first crystallization transformations reaches 221.73 KJ/mol,231.79 KJ/mol and 248.58KJ/mol respectively. The Mg50Ni30La20 glass keeps the single amorphous phase but the characteristic peak becomes much broader after hydrogenation. The crystallization procedure also becomes more complex and changes from 2 steps for the as-quenched ribbons to 3 steps for the hydrogenated sample. The kinetic properties of the crystallization procedure change and the activation energy for the first crystallization transformation reaches 725.56 KJ/mol after hydrogenation.(2)The metallic glass has a relatively good hydrogen absorption kinetics. The hydrogen absorption is over 2.5wt.% at 423K under the hydrogen pressure of 4.5MPa for 4h. But the desorption kinetics are quiet poor. The hydrogen desorption is very little below 523K and reaches 0.41wt.% at 583k under the hydrogen pressure of 10"3MPa for 4h. The P-C-T curve of the Mg5oNi3oLa2o metallic glass shows a wide plateau pressure at 0.07MPa.(3) All the three metallic glasses appear a passivation phenomenon while it is not very obvious for the as-cast samples according to the tafel curves. The numbers of electron transferred in the oxidation and reduction reaction near corrosion potential are both 1. The corrosion potentials of metallic glasses are higher than the as-cast alloys, indicating that the metallic glasses have better corrosion resistance properties in the electrolyte. The cyclic voltammetry tests show that the reaction rates of the amorphous ribbons depend on hydrogen diffusion. The hydrogen diffusion coefficient is effected slightly by the hydrogen concentration in the electrode and is 2.55×10-0,3.12×10-10,3.21×10-10and 3.28×10-10 cm2/s when the charge capacity is 90%,70%,40% and 10%, respectively, indicating that the hydrogen atoms transfer faster at a low hydrogen concentration. All the metallic glasses show good activation characteristics. The discharge capacity is lower than those as-cast samples but show a relatively good cycle performance. The cycle stability is 76.2% after 55 cycles at 298K.