Nano Tio <sub> 2 </ Sub> On The Improvement Of Hydrogen Storage Properties Of Mg-based Composite Materials

In this thesis, previous research works on Mg-based hydrogen storage materials have been reviewed. On this basis, the following works have been carried out: Firstly, The part of Ni in Mg2Ni alloys was partial substituted by the transition element Cr. The effect of element Cr and mechanical alloying on the phase structure and hydrogen storage properties of Mg2Nii.xCrx (x=0,0. 1,0.2.0. 3) alloys were investigated. The Mg2Nio.7sCro.25 alloy was selected to be ball-milled with different amount of (0.5 wt %, 1.5 wt %, 2.5 wt %)TiO2 nano-particles. The effects of TiO2 nano-particles on crystalline structure characteristics, kinetics and thermodynamics properties of the composites were also discussed. Secondly, for the purpose of improving the kinetics properties of hydriding/dehydriding and maintaining the high hydrogen storage capacity, Mg-15 wt %Mg2Ni composite was modified by ball-milling with different amount of TiC^ nano-particles, and the microstructure, phase characteristics and hydriding/dehydriding properties of modified composites were investigated.The XRD results show that the Mg2Nii.xCrx ternary alloys can be prepared by sintering and ball-milling. The main phase of the ternary alloy is Mg2Ni and transition element Cr was in solid solution. The ball-milling and the substitution made some distortion with crystalline structure of the alloys and some of Ni was precipitated from the Mg-Ni-Cr ternary alloy. The crystalline size of the alloy decreased with the ball-milling time and an increase in the amount of Cr. The hydriding results show that the substitution of Ni by Cr could decrease the absorption temperature and improve the kinetics of hydriding.The experimental results show that the TiC>2 nano-particles did not change the main phase of Mg2Nio.7sCro.25 alloy. With increasing the amount of TiO2 nano-particles, the nano crystalline size became smaller. On the basis of the kinetics results, it was shown that TiO2 nano-particles reduced the absorption temperature and increasing the hydriding speed, but the hydriding capacity was reduced. One type of Mg2Nio.75Cro.25+1.5 wt %TiO2 nano composites can reach completely hydrogenationAbstractin about 5 min at 373 K under the hydrogen pressure of 4 MPa and desorbed hydrogen in about 20 min at 473 K under the pressure of 0. 1 MPa. The dehydriding capacity was 2.14 wt %. When the absorption temperature was increased to 453 K, the dehydriding capacity of the composite was increased to 2.67 wt %. The thermodynamics results also show that the enthalpy ( A/f ?) of composites was reduced to -58 kJ (mol H2)~' (Mg2Ni: -64.5 kJ (mol H2)~'). The enthalpy of the composites had the tendency to decrease with increasing the contents of the TiO2 nano-particles.According to the experimental results of the Mg+15 wt %Mg2Ni+x wt % TiO2 nano composites, it was shown that the composite containing 0.5 wt % TiO2 nano-particles can absorb hydrogen at 393 K under the pressure of 4 MPa in 4 min and can desorb hydrogen at 493 K under 0. 1 MPa in 20 min. The capacity of the de-hydro genation is 4.1 wt %. With increasing the temperature, the de-hydrogenation capacity and the kinetics of the dehydriding were increased. After hydrogenation at 473 K, and dehydrogenation at 523 K, the maximum desorption capacity of Mg-15 wt %Mg2Ni-0.5 wt %TiO2 composites can reach 5.8 wt %. The TiO2 nano-particles can improve the hydriding and dehydriding properties of the composites. As the content of the TiO2 nano-particles increased, the kinetics of the dehydriding was increased and the decrease in de-hydrogenation capacity was small.