Kouichi Higuchi1, Hironobu Fujii2,Shin-ichi Orimo2 and Kazuhiko Terata3
1Hiroshima Industrial Technology Organization., Higashi-Hiroshima 739
Composite materials were prepared by disproportionation reaction of Y5Mg24 and Y5Mg22.5Ni1.5 with hydrogen in the temperature range from 24 to 400°C. The structural features were examined by using X-ray diffraction technique, SEM- and TEM- observations, while the hydriding and dehydriding properties were studied by using a Sieverts' method and a thermo-gravimetry analysis.
The results obtained are summarized as follows: (1) At the first hydriding process under 400°C and 3 MPa H2, Y5Mg24 and Y5Mg22.5Ni1.5 were respectively decomposed into two and three main phases of YH3 and MgH2, and YH3, MgH2 and Mg2NiH4. The external region of products was covered by YH3 and lamellar-like YH3/MgH2, and the internal regions were composed of MgH2 and YH3 with the size of several micrometers for Y5Mg24, and MgH2, YH3 and eutectic mixture MgH2/Mg2NiH4 with the size of several hundred nanometers for Y5Mg22.5Ni1.5, respectively. (2) Even at low temperatures below 100°C, Y5Mg22.5Ni1.5 was hydrogenated and decomposed into YH2, MgH2, Mg2NiH0.3 and a small amount of nonreactive Y5Mg22.5Ni1.5 with nanometer size. (3)After several cycles of hydriding - dehydriding process, the products became to show stable hydriding properties and maximum hydrogen content reached 3.8 mass% at 302°C within 10 minutes for Y5Mg22.5Ni1.5. (4)The dehydriding reaction from the hydrogenated Y5Mg24 occurred at 350°C, whereas the dehydriding reaction for all the hydrogenated Y5Mg22.5Ni1.5 in the temperature range from 24 to 400°C started to occur arround 250°C, which is close to the dehydriding temperature of Mg2NiH4.
(Received April 11, 1997)
hydrogen, magnesium, nickel, yttrium, disproportionation reaction, composite materials, hydriding property, dehydriding property, structural features
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