Examination of Multiphase (Zr,Ti)(V,Cr,Mn,Ni) 2 Ni-MH Electrode Alloys: Part I. Dendritic Solidification Structure
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THE history of the interstitial hydrogen absorption of Laves phases is long.[1–3] The relatively low enthalpy of hydride formation and fast kinetics provided by the interstitial nature of the hydrogen absorption/desorption process are advantageous. No significant rearrangement of the host metal matrix elements is required as hydrogen is taken up and released. Interstitial hydrogen-absorbing alloys have been commercially used as a negative electrode of Ni-metal hydride (NiMH) batteries since the 1990s.[4,5] The nickel metal hydride (NiMH) battery is an emerging technology used both in electronic devices and vehicle propulsion applications. AB2-type Zr,Ti-based Laves phase metal hydrides have been attracting much attention recently because of their larger hydrogen storage capacity and relatively longer electrochemical charge/discharge cycle life than the commercialized AB5-type alloys.[4–8] The use of complex multielement, multiphase metal hydride alloys have enabled the better capacity and kinetics of NiMH batteries, while overcoming the crucial barrier of unstable oxidation/corrosion behavior to obtain long cycle life.[9,10] Accordingly, multicomponent AB2-type alloys with A = (Ti,Zr) and W.J. BOETTINGER, NIST Fellow, K. WANG and C. CHIU, Research Associates, L.A. BENDERSKY, Metallurgist, and U.R. KATTNER, Physical Scientist, Metallurgy Division, Materials Science and Engineering Laboratory, and D.E. NEWBURY, NIST Fellow, Surface and Microanalysis Science Division, Chemical Science and Standards Laboratory, are with NIST, 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 25, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
B = (Ni,Mn,V,Cr,Fe,Co,Al,Sn) have been studied by a number of research groups for their hydrogenation/ discharge characteristics and their phase constitution.[11–14] An extensive list of the alloys studied is given in the introduction by Young et al.[11] To optimize the hydrogen capacity and improve the electrochemical charge/discharge properties, compositional modifications of the Laves phases to adjust metalhydrogen bond strength were studied by different research groups by varying the ratio of Ti and Zr on the A site, and the variation of the other components on the B site, or stabilization of the different crystal structures of the Laves phase C14, C15, and C36.[11–14] For example, partially substituting zirconium with titanium in Zr(VxNi1x)2 alloys has been demonstrated to enhance the hydrogen storage characteristics of an alloy. When used as the metal hydride (MH) electrode in NiMH batteries, the best properties are obtained for multicomponent alloys that contain a mixture of Laves and other phases.[9,15,16] The interaction between the main working Laves phases and secondary minor phases appears to play an important role in improving the electrochemical properties of the alloys.[17–20] In addition,
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