Amorphous hydrides of the Ti₄₅Zr₃₈Ni_17-xCo_x nano-powders

The Ti-Zr-Ni alloys can be synthesized in crystalline, amorphous and quasicrystalline phases. They are produced by mechanical alloying or melt-spinning and they are good candidate materials for hydrogen storage applications. In this communication we modified the Ti₄₅Zr₃₈Ni₁₇ compounds by substituting 3d metals (cobalt) for Ni to obtain amorphous phase. The cobalt atoms are located at the same positions as nickel. We describe a series of phase transitions associated with a hydrogen uptake in the amorphous Ti₄₅Zr₃₈Ni_17-xCo_x alloys that were prepared by mechanical alloying. The investigated alloys exhibited a high capacity for gaseous H₂ reaching 2.2 wt.% at elevated temperatures. It has been found, that the amorphous powders were decomposed into simple hydrides being exposed to H₂ at temperatures above 200 °C. In order to overcome that problem, a processing route to obtain the amorphous hydrides was established. The heated amorphous hydrides underwent an unusual and fascinating transformation into the glassy quasicrystal phase with no hydrogen loss. A further temperature increase triggered another structural transformation into the cubic phase, which was associated with hydrogen release from the structure. The crystal structure evolution was characterized by the variety of diffraction techniques. Thermodynamic properties were studied by differential scanning calorimetry and thermal desorption spectroscopy.