Pressure Dependence of Hydrogen-Induced Amorphization in C15 Laves Phases TbM<SUB>2</SUB> (M=Fe, Co and Ni)

Abstract

The hydrogen pressure dependence of structural changes in C15 Laves phases TbM 2 (M = Fe, Co and Ni) were investigated between 0.1 and 5.0 MPa H 2 in order to elucidate conditions and the mechanism of hydrogen induced amorphization (HIA). General thermal reactions occur on heating as follows; hydrogen absorption in the crystalline state, hydrogen-induced amorphization (HIA), precipitation of TbH 2 (or TbNi 5 H x ) and decomposition of the remaining amorphous alloys. Exceptionally, a single-phase amorphous a-TbFe 2 H x was not formed below 0.2 MPa H 2 in TbFe 2 , because HIA and precipitation of TbH 2 simultaneously occurred. On the other hand, a single-phase a-TbNi 2 H x was not formed below 1.0 MPa H 2 because of the low hydrogen contents in c-TbNi 2 H x . The present work indicates that hydrogen absorption more than 1.0 (H/M) is essential for the occurrence of HIA in the C15 Laves phases TbM 2 .

Process and mechanism of hydrogen-induced amorphization in C15 Laves phases RFe2

“…Recently, the present authors have shown that C14 Laves phase NdMn 2 with the atomic size ratio R Nd /R Mn = 1.45 amorphizes also by hydrogenation [17]. Furthermore, it has been reported that structural changes in C15 Laves phases RFe 2 depend on the kind of R [12,13]. That is, hydrogen absorption, HIA, the precipitation of RH 3 and the decomposition of the remaining amorphous phase occur in RFe 2 (R = Sm, Gd, Tb, Dy and Ho).…”

“…HIA is closely related to the disproportionation reaction that dominates a lifetime of hydrogen storage alloys, so that it is important to make clear the controlling factors of HIA. HIA has been observed in the intermetallic compounds A x B 1−x consisting of hydride forming (A) and non-hydride forming metals (B) with the crystal structures such as C15, B8 2 , C23, D0 19 and L1 2 [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Among these, structural changes in the C15 Laves phases RFe 2 (R = rare earth metals) are particularly interesting, because both a crystalline and an amorphous hydride form depending on the hydrogenation temperature [12].…”

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

“…Among the amorphizing compounds, C15 Laves phases RM 2 (R = rare earth metals, M = Fe, Co, Ni) have attracted much attention, because a crystalline and an amorphous hydride are formed depending on the temperature. 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.…”

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

“…Furthermore, T p of HIA for C15 Laves RFe 2 (R ¼ Tb, Er) shows also a same strong negative pressure dependence. [15][16][17][18] It has been proposed that HIA of C15 Laves RM 2 occurs by the short-range diffusion of R and M atoms. Therefore, it is interesting to compare the pressure dependence of HIA for DyCo 2 with that for DyFe 2 .…”