Pressure Dependence of Hydrogen-Induced Transformations in C15 Laves Phase DyCo<SUB>2</SUB>

Abstract

Structural changes in C15 Laves phase DyCo 2 heated in a hydrogen atmosphere between 0.1 and 5.0 MPa were investigated using a pressure differential scanning calorimeter (PDSC) and were compared with those of DyFe 2 . Four exothermic reactions resulting from 1) hydrogen absorption forming crystalline and amorphous hydrides, 2) hydrogen-induced amorphization (HIA), 3) the precipitation of BiF 3 -type DyH 3 in the amorphous hydride and 4) the decomposition of the remaining amorphous hydride into DyH 3 + -Co were observed with increasing temperature. HIA in DyCo 2 occurred for every hydrogen pressure in contrast to DyFe 2 , because the pressure dependence of it is small. The mechanism of the phase transformations for DyCo 2 in a hydrogen atmosphere was discussed on the basis of the experimental results such as the pressure dependence of T p and T p =T m (the peak temperature/the melting point of DyCo 2 ), the enthalpy change ÁH, the activation energy E A and so on.

A study on hydrogen-induced amorphization in C15 Laves phase DyNi2 under different hydrogen pressures

“…The broken lines and the arrows indicate the base lines and the temperatures to which the samples were heated and then cooled, respectively. The shape of these PDSC curves shows a similarity with those of DyCo 2 [17].…”

“…12 shows the relation between the hydrogen pressure and the peak temperature T p for the thermal reactions of DyNi 2 heated at 0.17 K/s. This pressure dependence of T p for DyNi 2 is complex in comparison with that of DyFe 2 [16] and DyCo 2 [17]. A single-phase amorphous hydride a-DyNi 2 H x is formed at 2.0 MPa H 2 or higher hydrogen pressure, but is not formed below 2.0 MPa H 2 .…”

“…hydrogen absorption forming crystalline (c) and amorphous a-DyNi 2 H x in contrast to DyFe 2 [16] and in the same way as DyCo 2 [17]. Since both hydrogen absorption and HIA occur exothermically, the enthalpy change of the first exothermic peak is very large in comparison with that of HIA.…”

“…DyNi 2 absorbs hydrogen forming c-and a-DyNi 2 H x at the first exothermic in the same way as DyCo 2 [17], but in contrast to DyFe 2 [16]. Furthermore, it has been reported that C15 Laves phases RFe 2 (R = Y, Sm, Gd, Tb, Dy, Ho, Er) absorb hydrogen forming c-RFe 2 H x at the first exothermic peak [9,10,[12][13][14][15], although a-CeFe 2 H x is exceptionally formed [15,18].…”

“…For instance, TbFe 2 [12,13] and ErFe 2 [14] absorb hydrogen forming the crystalline hydride at the first exothermic peak and transform to the amorphous hydride at the second exothermic peak, although the amorphous hydride is formed at the first exothermic peak in CeFe 2 and no crystalline hydride is formed [15]. On the other hand, recent investigations on structural changes in DyFe 2 [16] and DyCo 2 [17] have indicated that the hydrogen pressure controls whether HIA occur or not at low hydrogen pressure. That is, T p of HIA and of the precipitation of DyH 3 in DyFe 2 show a large and negative, and a positive pressure dependence, respectively.…”