טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentBuchman Eli
SubjectThe Effect of Nanostructuring on Hydrogen Storage
of Mg-Based Alloys Properties
DepartmentDepartment of Materials Science and Engineering
Supervisors Professor Eugen Rabkin
Professor Elazar Gutmanas


Abstract

Future hydrogen economy can become more promising by introducing the hydrogen  in transportation industry as a fuel instead of fossil ones. The major problem is the on board storage of hydrogen in the vehicle. On board storage demands reliable, light , safe and inexpensive material system. Metal hydrides are most promising candidates for this purpose and the Mg-based alloys among them

In this research high energy ball milling(HEBM) and a multipass equal channel angular pressing(ECAP) methods have been applied to Mg-Ni alloys[Mg4.3Ni (81%at.Mg ,19%at.Ni), Mg6.2Ni (86%at.Mg ,14%at.Ni) , (Mg-Ni)eu (89%at.Mg ,11%at.Ni)]  to refine its microstructure down to sub-micrometer size of Mg and Mg2Ni grains. The nanocrystalline particles of Mg and Mg2Ni has been achieved in the case of HEBM and the sub-micrometer size of Mg and Mg2Ni grains in the case of ECAP  after 10 ECAP passes. The microstructures of all samples were investigated using XRD, TEM and SEM. The pressure-composition isotherms (PCT) for the hydrogenation and hydrogenation kinetics of ball milled and ECAP-ed samples have been determined.

In the case of HEBM samples a gravimetric hydrogen storage capacities were   4 wt%  of hydrogen for Mg4.3Ni  and about 6 wt% for Mg6.2Ni. It has been shown that the equilibrium desorption hydrogen pressure is higher for the processed samples than the one of pure Mg. Moreover both kind of samples exhibit excellent hydrogen desorption kinetics which outperform the alloys of close composition known in literature.

For ECAP-ed samples Mg grains exhibited a supersaturation in Ni, which was non-homogeneously distributed across the grains. All studies alloys of ECAP-ed samples exhibited a gravimetric hydrogen storage capacity of about 6 wt%.   The ECAP-processed alloy exhibited an excellent hydrogen desorption kinetics, desorbing 5 wt% of hydrogen in less than 5 min at the temperature lower than 573 K. It has been also shown that in terms of hydrogen desorption pressure the ECAP treated Mg-Ni alloy outperforms the alloys of close composition nanostructured by alternative processing techniques.

The equilibrium pressures and kinetics of the hydrogenation in both cases showed significant improvement at 300ºC with great potential for lower temperatures. However according to Van’t Hoff plots , nanostructuring of Mg-Ni alloys using these methods have mainly  effected on the entropy factor of the hydrogenation.

On the basis of the results obtained ECAP and HEBM methods have a great potential for improving the hydrogen storage properties of Mg-Ni alloys.