The Effect of Rapid Solidification on Ti-Zr Alloys for Application in Ni-MH Battery Anodes
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Metal hydrides find applications in the storage of hydrogen and also as anode materials forthe nickel-metal hydride batteries(Ni-MH). The properties of these materials can be tunedby selecting appropriate elemental composition and by controlling the microstructure. Theoverall goal in the development of advanced metal hydrides is to obtain materials with highhydrogen storage capacity, fast and completely reversible hydrogen absorption and desorption,and tunable phase stability (with an adsorption/desorption pressure around 1 bar atambient conditions for the Ni-MH batteries), as well as good stability in the working environment(6-9 M KOH electrolyte in Ni-MH batteries). The goal of the present work was to study the effect of rapid solidification(RS) as ananostructuring technology applied to obtain novel and improved anode materials for theNi-MH battery. The focus of the work was on the TiZr-based AB2 Laves phase alloys, whichcontain significantly less rare earth metals (RE) compared to the commercialized alloys usedtoday, which has the composition (RE(Ni,Co)5). The alloys were melt spun to obtain agrain refinement, and consequently improve the discharge capacity and high rate dischargeperformance of the alloys. Based on earlier experiments performed at IFE, two basic alloys with compositions(TixZr1-xLay)(Ni1.2Mn0.7V0.12Fe0.12) were chosen. Particular compositions were selected asx=0.15, and y=0.03 (Alloy 1), and x=0.2 and y=0.01 (Alloy 2). Both alloys contain minoramounts of La to help achieving fast activation of the material. The alloys were melt spun, with wheel surface speeds varying from 3.1-62.8 m/s. As themelt spinning process proved to modify the alloy composition, resulting in depletion of Mnwhich is easily vaporized, the effect of excess Mn addition to mitigate this problem was alsostudied for alloy 1. The effect of RS on the microstructure and phase compositions was characterizedby using SEM, EDS, AES and XRD. Furthermore, the electrochemical propertieswere studied through electrochemical cycling performed at different current densities as wellas by electrochemical impedance spectroscopy and PCT. The SEM studies showed a successful refinement of the microstructure by RS. The grainsize gradually decreased from the original 2 μm in the as cast alloys to around 250 nm forthe alloys melt spun with the highest cooling rate. The morphology of alloy 1 showed anisotropic reduction in grain size with RS, with a related formation of small particles of thesecondary phases at the grain boundaries. Alloy 2, on the other hand, showed a formation of a lamellar structure after the melt spinning. The XRD data show that both alloys contain a mixture of two Laves phase compounds;cubic C15 and hexagonal C14 structure. The preferred C15 crystal structure is dominatingfor the initial alloys. RS however increases the amount of the C14 phase. The EDS andXRD results confirm that the main phase observed in the SEM micrographs corresponds tothe C15 phase, while the particles or secondary phase consists of the C14 crystal structure.Electrochemical results showed that RS casted samples have a more sluggish activation performance.Furthermore, alloy 2 proved to have more sluggish activation than alloy 1, as wellas a sloped working potential. Lastly, for alloy 1, the high rate of discharge performance was improved by RS, as thereis an increase in capacity for all tested current densities. This is a result of the refinedmicrostructure leading to an increased diffusion coefficient of H through the material.