Examination of Multiphase (Zr,Ti)(V,Cr,Mn,Ni) 2 Ni-MH Electrode Alloys: Part II. Solid-State Transformation of the Inter
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INTRODUCTION
FOR a number of Laves phase–based alloys used as negative electrodes of Ni-metal hydride (Ni-MH) batteries, improvement in the electrochemical properties has been attributed to the presence of secondary catalytic phases of the Zr-Ni system in addition to the major AB2 Laves phases.[1–7] These catalytic phases were identified as mainly Zr7Ni10 and Zr9Ni11, and in the case of ternary alloys of composition Zr(CrxNi1–x)2 (with x varying around the value x = 0.5), the optimal amount of the phases was found to be about 20 pct by mass; the electrochemical properties of Zr-Ni-Cr alloys have been studied by Joubert et al.,[3] and enhanced properties were attributed to the synergistic effect between Laves and Zr7Ni10 phases. In recent work on more complex alloys, the beneficial effect of the microsegregated secondary phases on the discharge capacity of electrodes was demonstrated by comparing the performance of as-cast and annealed (without secondary phase) materials.[7] The crystal structure for Zr7Ni10 was reported as orthorhombic with either Aba2 or Pbca space group.[8] L.A. BENDERSKY, Metallurgist, K. WANG, Research Associate, and W.J. BOETTINGER, NIST Fellow, Metallurgy Division, Materials Science & Engineering Laboratory, and D.E. NEWBURY, NIST Fellow, Surface and Microanalysis Science Division, Chemical Science and Standards Laboratory, are with the National Institute of Standards and Technology, Gaithersburg, MD 20899. Contact e-mail: [email protected] K. YOUNG and B. CHAO, Senior Research Scientists, are with Energy Conversion Devices Inc., Rochester Hills, MI 48309. Manuscript submitted January 27, 2010. Article published online May 29, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
Joubert et al. later refined the structure as centrosymmetic Cmca (a = 1.2381 nm, b = 0.9185 nm, c = 0.9221 nm).[9] The metastable tetragonal structure of Zr7Ni10 with space group I4/mmm was shown to form after dehydrogenation of its corresponding hydride or by rapid melt quenching.[10,11] The dominating crystal structure of TixZr7–xNi10 (x between 0 and 2.5) is also orthorhombic before and turns into tetragonal after hydrogenation.[12] With a small amount of vanadium included, the alloy Ti1.5Zr5.5V0.5Ni9.5 is still dominated by the orthorhombic Zr7Ni10 structure.[13] An I-centered tetragonal lattice was reported for Zr9Ni11 by Kirkpatrick and Larsen,[14] and the structure subsequently was shown to be isomorphous with Zr9Pt11,[15,16] with I4/m space group. Recent combined neutron and electron diffraction work showed that the crystal structure is tetragonal P4/m, with a = 0.9882 nm and c = 0.66089 nm at lower temperatures, and above 1273 K, the structure is body-centered I4/m.[17] The Zr9Ni11 is derived from the B2 (CsCl-type AB-ordered bcc) structure type by insertion of antisite Ni atoms, with structurally disordered one-dimensional chains of Zr and Ni atoms that are only weakly correlated with each other. It was found that additional Ni atoms are inserted in these chains to give a composition of Zr9Ni11+d (d 0.4).[17,18
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