Metallic Hydrides III: Body-Centered- Cubic Solid- Solution Alloys
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Metallic Hydrides III: Body-CenteredCubic SolidSolution Alloys E. Akiba and M. Okada Abstract Hydrogen-absorbing alloys with bcc (body-centered-cubic) structures, such as Ti-V-Mn, Ti-V-Cr, Ti-V-Cr-Mn, and Ti-Cr-(Mo, Ru), have been developed since 1993. These alloys have a higher hydrogen capacity (about 3.0 mass%) than conventional intermetallic hydrogen-absorbing alloys. Generally, bcc metals and alloys exhibit two plateaus in pressure–composition isotherms, but the lower plateau is far below atmospheric pressure at room temperature. Many efforts have been made to increase hydrogen capacity and raise the equilibrium pressure of this lower plateau. The crystal structure and morphology of Laves-phase-related bcc solid-solution alloys are reviewed. Keywords: bcc structure, hydrogen storage, metallic hydrides, solid-solution alloys.
Introduction Body-centered-cubic (bcc) metals and alloys intrinsically have a large hydrogen capacity, but they have not been used for any hydrogen-storage applications so far. Fewer studies have been done on hydride formation in bcc alloys than in intermetallics such as AB5 and AB2 alloys.1–7 In this article, we will review new bcc hydrogenabsorbing alloys based on a new concept, the Laves-phase-related bcc solid solution, as proposed by one of the authors. Recent progress on the materials development of these alloys and crystal structure analysis will be introduced.
Hydrogenation Properties of bcc Metals and Alloys Figure 1 shows the pressure–composition (PC) isotherms of vanadium,7 one of the most studied hydrogen-absorbing bcc metals. Vanadium has two clear plateaus with remarkably different equilibrium pressures. Figure 1 shows only the plateau between monohydride (H/V 1 ) and dihydride (H/V 2). Papathanassopoulos and Wenzl8 measured both equilibrium pressures. The lower desorption equilibrium pressure was 101 Pa at 353 K, while the higher one was about 105 Pa at 263 K.
MRS BULLETIN/SEPTEMBER 2002
Body-centered-cubic metals and alloys generally show two plateaus in their PC isotherms. Their plateau region commonly exhibits a slope. The slope in the plateau region and the pressure difference between the two plateaus are undesirable for any hydrogen-storage applications. The reported disadvantages of bcc metals and alloys as hydrogen-absorbers include slow kinetics, difficulties in activation, and the slope in the PC plateau. Therefore, even though bcc alloys have an intrinsically large hydrogen capacity, studies on their hydrogen-absorption properties have been limited. Libowitz and Maeland1 found bcc solidsolution alloys that showed extremely rapid reaction kinetics with hydrogen. They are Ti-V-based alloys such as Ti-V-Fe, Ti-V-Mn, Ti-V-Co, Ti-V-Cr, and Ti-V-Ni.3,9,10 The alloys Ti70V30-10at.%M (M Fe, Mn, Co, Cr, Ni) reacted rapidly with hydrogen in a glass apparatus at room temperature just after evacuation.9 Lynch et al.3 reported hydride formation in the Ti-V-Fe system with a bcc structure. They found that dihydride formation, as well as the lattice parameter, depends o
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