Hydrogen Storage in Quasicrystals
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Hydrogen Storage in Ti-Zr-Ni Quasicrystals Certain titanium/zirconium-3d transition metals constitute the secondlargest class of icosahedral-quasicrystal (i-phase)-forming alloys. (Detailed discussions of their formation appear in References 1-4.) The Ti-Zr-Ni icosahedral quasicrystals are the best ordered of these and are thermodynamically stable.5'6 They also show promise for hydrogen-storage applications.57'8 Key qualities of a hydrogen-storage material are (a) an ability to load a significant amount of hydrogen, (b) an ability to get the hydrogen into and out of the metal at reasonable values of pressure and temperature and within a reasonable time, and (c) an ability to repeat this cycle many times without degradation of the intermetallic alloy. The favorable chemistry and presumed high density of tetrahedral sites make the Ti- and Zrbased quasicrystals candidates for storage applications. Our research group has identified the T-Zr-Ni i-phases as particularly promising. They demonstrate a large loading capacity, first from the gas phase at 230°C and 27 atm using a standard Sievert's apparatus,7 and later by gently ball-milling the samples in 1-5 atm of hydrogen at room temperature.8 The amount of hydrogen absorbed from the gas phase was determined directly by monitoring the hydrogen pressure during absorption and by measuring changes in the sample mass with hydrogenation using a Cahn electrobalance (±5-/ig accuracy). In all cases, an average hydrogen-to-metal atom ratio (H/M) of 1.7 was obtained (corresponding to approximately 2.5-wt% H). As will be discussed in comparisons with other metal hydrides in the following, these values for H/M are comparable or superior to those for metal hydrides currently used or under study. As demonstrated in Figure 1, the strain accompanying the absorbed hydrogen causes a 7-8% increase in the quasilattice constant aq (shifting the diffraction peaks to lower angle) and an increased peak
width. The x-ray-diffration pattern for an as-quenched ribbon (Figure la) is compared with the patterns for samples hydrogenated from the gas phase, either by heating 230°C in a moderate pressure of hydrogen (27 atm) (Figure lb) or by ballmilling the samples at room temperature in 5 atm of hydrogen (Figure lc). Figure Id shows the x-ray-diffraction pattern from a sample hydrogenated electrolytically to an H/M value of nearly 1.9 by cathodically biasing an electrode of the quasicrystal at 3.5 V with respect to a standard Pt electrode in a solution of 5 M KOH.9 Measured correlations between x-ray-peak shifts and the value of H/M in the sample showed that changes in aq scale linearly with the amount of hydrogen absorbed, allowing a determination of H/M from x-ray-peak positions, calibrated using the pressure drop and mass gain from gas-phase loading. Hydrogen loading from the gas phase without ball milling in i(TiZrNi) was preceded by a long induction time, varying from several hours to several days, depending on the exposure time to air. Once started, complete hydrogenation of the sample occurred ra
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