Hydriding-dehydriding and structural properties of composite materials produced by disproportionation reaction of Y₅Mg₂₄ and Y₅Mg_22.5Ni_1.5 with hydrogen

Composite materials were prepared by disproportionation reaction of Y₅Mg₂₄ and Y₅Mg_22.5Ni_1.5 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 Y₅Mg_22.5Ni_1.5 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 Y₅Mg_22.5Ni_1.5, respectively. (2) Even at low temperatures below 100°C, Y₅Mg_22.5Ni_1.5 was hydrogenated and decomposed into YH₂, MgH₂, Mg₂NiH_0.3 and a small amount of nonreactive Y₅Mg_22.5Ni_1.5 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 Y₅Mg_22.5Ni_1.5. (4) The dehydriding reaction from the hydrogenated Y₅Mg₂₄ occurred at 350°C, whereas the dehydriding reaction for all the hydrogenated Y₅Mg_22.5Ni_1.5 in the temperature range from 24 to 400°C started to occur arround 250°C, which is close to the dehydriding temperature of Mg₂NiH₄.