Cycle life and hydrogen storage properties of mechanical alloyed Ca1-xZrxNi5-yCry; (x=0, 0.05 and y=0, 0.1)
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CaNi5–based alloys have been synthesized by mechanical alloying followed by isothermal annealing. The formation of the CaNi5 structure occurred when the milled powders were heated at 800 °C under vacuum for 3 h. The abundance of CaNi5 phase in the alloys ranges from 60 to 70 wt.%. Replacement of Zr into the Ca site reduces the unit cell volume of CaNi5 whilst replacement of Cr into the Ni site slightly increases the unit cell volume. The hydrogen storage capacity of all substituted alloys is decreased and the hydrogen sorption plateau regions are narrowed compared to those of pure CaNi5. Substitution of Zr into the Ca site extinguishes the flat plateau region unlike replacement of Cr into the Ni site where a flat plateau is maintained. The reaction enthalpy ΔH for both absorption and desorption are directly proportional to the unit cell volume of the alloys. The hydrogen storage capacity of all alloys rapidly decays for the first 50 cycles at 85 °C followed by a more gradual decline after 50 further cycles. The hydrogen storage capacity of the alloys after 200 cycles is in the range of 65–75% of the initial capacity.
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