Electrochemical and Thermal Properties of Hydrogen-absorbed Mg 67 Ni 28 Pd 5 Amorphous Alloy
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We have examined electrochemical properties and thermal stability of the amorphous Mg67Ni28Pd5 alloy produced by melt-spinning subjected to electrochemical hydrogen charge. In the cyclic life test, the discharge capacity of the alloy increases significantly with increasing cycle number and reaches 411 mA h/g at the 15th cycle. The hydrogen-absorbed amorphous alloy crystallizes through two stages of primary crystallization of Mg2Ni, followed by precipitation of Mg2NiH4 at the second stage. The completed crystallization temperature of the hydrogen-absorbed alloy increases by 65 K as compared with the as-solidified one. It is thus concluded that stability of the Mg-based amorphous alloy against crystallization increases by hydrogen absorption.
I. INTRODUCTION
It is well known that Mg–Ni alloys are highly promising hydrogen-storage materials, though their absorption/desorption temperatures are slightly higher than room temperature.1 In recent years, it has been reported that Mg–Ni amorphous and nanocrystalline alloys produced by mechanical alloying (MA)/mechanical grinding (MG) possess lower absorption/desorption temperatures than conventionally crystallized ones and indicate the possibility of electrochemical absorption/desorption of hydrogen at room temperature.2–11 Nowadays, Mg–Ni amorphous and nanocrystalline alloys have attracted considerable attention owing to their hopeful hydriding properties. Consequently, great efforts have been devoted to improve hydriding properties by tailoring their structure. The previous studies have demonstrated that the MA process has the following disadvantages: (1) the necessity of long time to obtain an amorphous phase; (2) difficulty of mass-production; (3) contamination caused by dissolution of the material of chamber and balls used in ball milling. The melt-spinning technique is useful for production of amorphous or nanocrystalline phase and more suitable for industrial mass production. It is expected that amorphous hydrogen-storage alloys produced by melt-spinning can exhibit better hydriding characteristics at lower temperatures than that for the MA alloys. Consequently, we used the melt-spinning technique to produce Mg-based amorphous hydrogenstorage alloys in this study. Since the Mg2Ni phase has high hydrogen-storage capacity, it is preferable to use
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Graduate Student, Tohoku University. Present address: Tokin Corp. 60
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J. Mater. Res., Vol. 17, No. 1, Jan 2002 Downloaded: 15 Mar 2015
amorphous alloys with the compositions around Mg67Ni33. However, the composition in which an amorphous phase is formed in MgxNi100−x binary system has been limited to the range of x ⳱ 75 to 92 Mg.12,13 Consequently, we added the third element to produce the amorphous alloys of x ⳱ 67. Palladium with similar electronegativity and significantly different atomic size mismatch against nickel was added to the Mg–Ni binary alloy to synthesize a Mg67Ni28Pd5 amorphous alloy by the melt-spinning technique. The objective of this study is to examine electrochemical pr
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