The influence of the annealing temperature on the electrochemical properties and the structure of the melt-spun ML(NiMnTiCo)5 hydrogen-storage alloy

The influence of the annealing temperature on the electrochemical properties and the structure of the melt-spun ML(NiMnTiCo)(5) hydrogen-storage alloys has been investigated in detail by electrochemical measurement, X-ray diffraction (XRD), differential thermal analysis (DTA), transmission electron microscope (TEM) and positron annihilation techniques. The electrochemical properties of the melt-spun alloy were very sensitive to the annealing temperature. For a given annealing time we found that the higher the annealing temperature, the higher the activation rate and the discharge voltage, the larger the discharge capacity and the flatter the discharge plateau. The optimum annealing temperature was about 700 K. Striking differences due to annealing were found in the phase structure and microstructure. Annealing at a temperature higher than 873 K could introduce new phases which were also found in the as-cast alloy and led to grain growth. The structure change due to high-temperature annealing led to a negative influence on the cycle durability and should be avoided by choosing a proper annealing temperature. With the positron annihilation technique, it was found that annealing could decrease the size of the defects in the melt-spun alloy, but this quantity increased after annealing. (C) 1998 Elsevier Science S.A.