Microstructural Studies of Multiphase (Zr, Ti)(V, Cr, Mn, Co, Ni) 2 Alloys for NiMH Negative Electrodes
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Microstructural Studies of Multiphase (Zr,Ti)( V,Cr,Mn,Co,Ni)2 Alloys for NiMH Negative Electrodes L. A. Bendersky1, K. Wang1, W. J. Boettinger1, D. E. Newbury1, K. Young2 and B. Chao2 1
Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA 2 Energy Conversion Devices Inc., Rochester Hills, MI 48309, USA ABSTRACT The solidification microstructures of six Laves-based (Zr,Ti)(TM,Ni)2 alloys (TM= V,Cr, Mn,Co) intended for use as novel negative electrodes in Ni-metal hydride batteries were studied here; these alloys often have their best electrochemical properties when in the cast state. Solidification occurs by dendritic growth of a hexagonal C14 Laves phase followed by peritectic solidification of a cubic C15 Laves phase and formation of a cubic B2 phase in interdendritic regions. The observed sequence of Laves phase C14/C15 upon solidification agrees with predictions using effective compositions and thermodynamic assessments of the ternary systems, Ni-Cr-Zr and Cr-Ti-Zr. The paper also examines the complex internal structure of the interdendritic grains formed by solid-state transformation, which plays an important role in the electrochemical charge/discharge characteristics. By studying one alloy it is shown that the interdendritic grains solidify as a B2 (Ti,Zr)44(Ni,TM)56 phase, and then undergo transformation to Zr7Ni10-type, Zr9Ni11-type and martensitic phases. The transformations obey orientation relationships between the high-temperature B2 phase and the low-temperature Zr-Ni-type intermetallics, and consequently lead to a multivariant structure. Binary Ni-Zr and ternary Ti-NiZr phase diagrams were used to rationalize the formation of the observed domain structure. INTRODUCTION In recent two decades nickel metal hydride batteries (NiMH) have become very important in both portable electronic devices and vehicle propulsion applications due to the flex cell design, safe operation, high volumetric energy, high specific power, and environmentally friendly characteristics [1]. The NiMH battery’s electrochemical basis is in storing hydrogen in the solid hydride phase. The negative electrode of a conventional NiMH battery consists of a hydrogen storage material that can allow electrochemical storage and release of hydrogen during battery charge and discharge processes. The nickel hydroxide positive electrode is electrochemically reversible between Ni(OH)2 and Ni(OOH) nickel oxyhydroxide. At both electrodes, oxidationreduction reactions take place in an alkaline medium consisting of 30% by mass KOH in water. Most effort in improving capacity and performance of NiMH batteries is in developing and discovering better negative electrodes, which is equivalent to developing a hydrogen storage material that performs at ambient temperatures and is resistive to an alkaline medium. Most of today’s commercial NiMH products use AB5 (LaNi5 structural type) misch metal-based alloys as the negative electrode active material. Batteries made from AB2 Laves phase-based alloys with
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