Hydrogenation and dehydrogenation properties for DyNi5-H system

We have investigated the hydrogenation and dehydrogenation properties of DyNi5 binary intermetallic compound in the pressure range from 0.1 to 35 MPa. After initial activation, DyNi5 compound absorbs hydrogen to reach H/DyNi5 = 3.8 at 248 K. By evaluating the pressure-composition (P-C) isotherm part by part in detail, we found the P-C isotherm for DyNi5-H system comprises three steps of phase transitions during hydrogenation and dehydrogenation processes due to the presence of one solid solution (α) and three kinds of hydride (β, γ and δ). Below H/DyNi5 = 1.0, the first α-β phase transition appears with large hysteresis, whereas the second β-γ and third γ-δ phase transitions are superposed upon each other around H/DyNi5 = 2.3. Consequently, the P-C isotherm apparently shows two pressure plateaux during both hydrogenation and dehydrogenation. Between these plateaux during dehydrogenation, the hydrogen pressure decreases logarithmically with decreasing hydrogen content or log P ∝ H/DyNi5. This complex behavior of P-C isotherm is reversible.