Effect of Alloy Composition on the Structure of Zr Based Metal Alloys
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and B.C. CHAKOUMAKOS* Energy Conversion Devices, Inc., 1675 West Maple Road, Troy, MI 48084 *Neutron Scattering Section, Oak Ridge National Lab., Oak Ridge, TN 37831
Abstract The structures of Ovonic multi-element, multi-phase Zr-based transition metal alloys for hydrogen storage are studied. The alloys are designed to be multi-phase materials. The hexagonal and diamond cubic structures, known as C14 and C15 Laves structures respectively, are the two major hydrogen storage phases in the alloys. In both C14 and C 15 structures, Zr and Ti are the elements typically occupying the hydride former (A) sites and V, Cr, Mn, Fe, Co and Ni the catalytic (B) sites. Based on the application of Ovshinsky's design principles for disordered materials, both A and B sites are compositionally disordered by the corresponding elements in cubic (C15) structure. However, the hexagonal (C14) structure has two distinct B sites, B(I) and B(Il) with a 1:3 ratio. Preliminary neutron diffraction studies indicate that the B(I) sites are predominantly occupied by Ni and the B(ll) sites are randomly mixed with V, Cr, Mn, Fe, and Ni. It is then proposed that the formula of the hexagonal structure should become A2B(I)IB(II) 3 in the current multi-element Zr-based alloys. Minor phases close to the structures of Zr7 Nil 0 , Zr9 Ni1l, and ZrO2 are also present in some alloys. The average electron concentration factor (e/a) derived from the alloy composition dictates the alloy structures. The alloys with higher electron concentration factor (> 7.1) favor the diamond cubic structure. On the other hand, the hexagonal structure is associated with the alloys with lower electron concentration factor (< 6.5). The alloys having electron concentration factors in between are mixtures of the diamond cubic and hexagonal structures. Introduction Energy Conversion Devices, Inc. (ECD) and its subsidiary Ovonic Battery Company, Inc. (OBC), have pioneered the development and commercialization of nickel metal hydride secondary batteries. The introduction of the Ovonic alloys, based upon the multi-element, multiphase, compositional disorder design principles [I], led the way to the successful commercialization of the nickel metal hydride technology of both Zr-based and mischmetalbased systems. The Ovonic nickel metal hydride battery has become the battery of choice for electric vehicle application due to its superior properties [2]. Among the nickel metal hydride alloys, it has been known that the Zr based AB 2 Laves structure alloys have higher capacity [mAh/g] than the La based AB 5 alloys [3]. Recently, Young et al. have demonstrated that the advanced Ovonic multi-element Zr-Ti-V-Cr-Mn-Co-Ni alloys can deliver a capacity of 465 mAh/g [3]. Total capacity and the high rate discharge capability of the Ovonic alloys are superior to the mischmetal based AB 5 alloys currently used for commercial electronic products. ZrMn 2 and ZrV2 binary compounds have the C14 and C15 type Laves structures and can absorb hydrogen contents up to ZrMn2H 3.0 and ZrV2 H5 .2 , respectively [
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