HYDRIDING-DEHYDRIDING AND STRUCTURAL-PROP ERTIES OF COMPOSITE-MATERIALS PRODUCED BY DISPROPORTIONATION REACTION OF Y5MG24 AND Y5MG22.5NI1.5WITH HYDROGEN

Composite materials were prepared by disproportionation reaction of Y5 Mg24 and Y5Mg22.5Ni1.5 with hydrogen in the temperature range from 24 to 400 degrees 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 an d a thermo-gravimetry analysis. The results obtained are summarized as follows: (1) At the first hydriding process under 400 degrees C and 3 MPa H-2, Y5Mg24 and Y5Mg22.5Ni1.5 were respectively decomposed into t wo and three main phases of YH3 and MgH2, and YH3, MgH2 and Mg2NiH4. T he external region of products aas covered by YH3 and lamellar-like YH 3/ MgH2, and the internal regions were composed of MgH2 and YH3 with t he size of several micrometers for Y5Mg24, and MgH2, YH3 and entectic mixture MgH2/Mg2NiH4 with the size of several hundred nanometers for Y 5Mg22.5Ni1.5, respectively. (2) Even at low temperatures below 100 deg rees C, Y5Mg22.5Ni1.5 was hydrogenated and decomposed into YH2, MgK2, Mg2NiH0.3 and a small amount of nonreactive Y5Mg22.5Ni1.5 With nanomet er size. (3) After several cycles of hydriding-dehydriding process, th e products became to show stable hydriding properties and maximum hydr ogen content reached 3.8 mass% at 302 degrees C Within 10 minutes for Y5Mg22.5Ni1.5. (4) The dehydriding reaction from the hydrogenated Y5Mg 24 occurred at 350 degrees C, whereas the dehydriding reaction for all the hydrogenated Y5Mg22.5Ni1.5 in the temperature range from 24 to 40 0 degrees C started to occur arround 250 degrees C, which is close to the dehydriding temperature of Mg2NiH4.