| Based on the research status of the Magnesium-based hydrogen storage materials,the Mg-Ni system was chosen to be experimental material. Nanocrystalline Mg2Ni alloy has been prepared by mechanical alloying, the details of milling conditions and the hydrogenation properties have been investigated.The milling conditions of mechanical alloying carried out through high energy ball mill method were investigated in detail. The effect of technical parameter such as rotation speed, process control agent etc on the process of mechanical alloying and the form of nanocrystalline have been analyzed and the hydrogenation properties of the nanocrystalline Mg2Ni powder also have been investigated.Elemental powders of Mg and Ni were mixed in stoichiometric atomic ratio of 2 :1, the ratio of powder to ball is 20 .' l,and then put into the container of SIMOLOYOR high energy ball mill equipment. The temperature inside the container was keep to 30C by using the cooling water. Stearic acid was added as process control agent. MA was carried out at rotation speed of 900rpm~1300rpm.After 40 hours' milling, nanocrystalline Mg2Ni alloy was obtained, which weight percent is about 95%.In order to prevent the oxygenation of the materials, the powders were milling under argon. The results of XRD and the Malvern particle size analyzer indicate that the powder consists of many nanocystalline which grain size is about 10nm,and the particle size of the powder is at the range of micron.Nanocrystalline Mg2Ni synthesized by mechanical alloying can absorb hydrogen rapidly at room temperature without activation. When the initial hydrogen pressure is 1.1 MPa the value of H/Mg2Ni was 1.29 in 20 minutes, after 4 hour's reaction the value was 1.75.Keeping the initial hydrogen pressure comparative, the nanocrystalline Mg2Ni showed a marked increase in the initial hydrogen absorption rate and the hydrogen capacity as the temperature increase. When the temperature was 200C and 300C, the value of H/Mg2Ni was 2.08 and 2.37 in 20 minutes respectively, after 2.5 hours reaction the value was 2.37 and 2.90.As a result, it was found that the materials prepared by mechanical alloying showed a better activation for hydrogen absorption at lower temperatures compared with conventional ploycrystalline Mg2Ni.The plateau pressure and the hydrogen capacity of nano-structured Mg2Ni increase with the increasing temperature, which contrary to the conventional materials. The values of enthalpy (H0) and entropy (S0) calculated from the linearity of van't Hoff plot were -30.4kJ/molH2 and -68.4J/KmolH2 respectively. The value of enthalpy was in good agreementwith the LT-phase Mg2NiH4, which was only half of the value of Mg2Ni alloy and the HT-phase Mg2NiH4, so it's a promising candidate for Ni-MH batteries.The XRD results showed that the grain size of nanocrystallme Mg2Ni was stable during the long time heating process when the temperature was below 200C and the hydrogen absorption rate and capacity were better than the conventional materials. If the temperature was above 300C,the hydriding and dehydriding characteristics of nanocrystallme Mg2Ni were depended on the heating time. The hydrogen absorption rate and capacity were decreased as the time increasing. The remarkable degradation was due to the decreasing of surface and grain boundaries region with the increasing of heating time. So the reaction temperature must be kept blow 200 C,the result was in good agreement with the result achieved through TG-DTA analysis in previous reports.The TG-DSC results indicated that the sample which grain size was unchanged started to desorb hydrogen at 172.1C,which was 75K lower than those synthesized by conventional methods. The total weight loss reached 2.42 wt.%. Noted that there were two obvious regions for hydrogen desorption,which corresponding to the two endotherms on the DSC curve, one was around 249.5C and the other at 354.6C .These were due to the decomposition of Mg2NiH4: Mg2NiH4 -Mg2NiHx +H2- Mg2Ni + H2 .With the increasing of hydriding-dehydriding cycles above 300C ,t... |
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