Preparation And Hydrogen Storage Properties Of Mg/TMO_x(TM=Ti,NB) Thin Films By Magnetron Sputtering

Hydrogen is a type of renewable energy and clean energy carrier. So far, hydrogen storage technology still limits hydrogen energy development. Solid state hydrogen storage technology is the most important hydrogen storage methods. Magnesium-based hydrogen storage materials are of the most promising for practical application because of their high hydrogen storage capacity, low cost and environmental characteristics. However, magnesium hydride(Mg H2) which has rather high thermodynamic stability, slow de/sorption kinetics Mg H2/Mg and high dehydrogenation temperature, limits its practical application. Adding catalysts or nano-structuring of Mg could improve de/sorption kinetics of Mg-based hydrogen storage alloy. The transition metals(TM) and their oxides can catalyze the Mg/Mg H2 reaction effectively. However the direct evidence and catalytic mechanism need to be provided and to be studied in depth. Therefore, in this thesis we intend to design and fabricate the different Mg/TMs or their oxides(TMOx,TM=Ti, Nb) multilayer Mg/TMOx films by means of magnetron sputtering, and to study the effects of the TMOx catalytic layer,and TMOx multi- valence on the desorption kinetics of Mg-based film, then to discuss TMOx multi-valence the catalytic mechanism.Firstly, we studied the magnetron sputtering deposition processes of Mg/Ti Ox(X=0~2) and Mg/Nb Ox(X=0~2.5) films, particularly the impacts of different TMOx sputtering powers on the deposition rates and on the microstructures. The results indicate that the deposition rate increases with the sputtering power increasing, and the thickness, surface morphology, grain size and density of Mg/TMOx multilayer also varies with sputtering power. It was found that the preferred growth orientations of Mg and Ti were [0 0 1] and <1 0 0>, respectively. However, the Ti O2,(Ti O2+Ti) hybrid, Pd, Nb, Nb2O5, and(Nb2O5+Nb) hybrid films grew isotopically.Secondly, we studied the de/hydrogenation properties of the Mg/TMOx multilayer with different types, distribution and thicknesses of Ti Ox catalyst layer. The results show that Mg/Ti O x films can absorb hydrogen under 3MPa hydrogen pressure at room temperature, and their desorption temperatures are below 170°C. We also found that hydrogen induced optical switching phenomenon and the catalyst phases kept untransformed during the Mg/Ti Ox de/hydrogenation process. The results also show that Ti/Mg/Ti O2 multi- valence films has better dehydrogenation properties than Ti O2/Mg/Ti O2 and Ti/Mg/Ti single valence films, and the initial desorption temperature and peak temperature of the former films are lower than that of the latter films. For the same type catalyst layer deposited in the Mg/Ti Ox multilayer films, the multi- valence(Ti O2+Ti)/Mg/(Ti O2+Ti) film prepared by(Ti O2+Ti) hybrid target shows more excellent dehydrogenation kinetics and lower dehydrogenation temperature than the only pure Ti or pure Ti O2 catalyst layer. The different Mg(or Ti O x) thicknesses impacts on the dehydrogenation temperature of Mg/Ti Ox multilayer films are also studied with the uniform thicknesses of catalysts Ti Ox(or Mg).Finally, we studied the hydrogen storage properties o f Mg/Nb Ox multilayer thin films, with different types of Nb Ox, different catalyst distributions and film thicknesses. The results show that the de/hydrogen kinetics properties of Mg/Nb Ox multilayer films are similar to that of Mg/Ti Ox films. On the condition of keeping the unique thickness of Mg(or Nb Ox) catalyst layer, the dehydrogenation kinetics of Mg/Nb Ox multilayer thin films with sandwich type Nb/Mg/Nb2O5 or with hybrid type(Nb2O5+Nb)/Mg/(Nb2O5+Nb) catalyst layers are better than that of multilayer films with pure Nb or with pure Nb2O5 catalyst layer. The former dehydrogenation temperatures are lower than the latter’s. We also discussed the effect of the structures of Mg/Nb Ox films on the hydrogen storage properties and the multi- valence catalytic mechanism.