A duplicated fluorination technique for hydrogen storage alloys

The fluorination technique developed in 1991 has been experimentally confir med to be effective for modifying alloy surfaces to extremely high activati on characteristics and a highly protective nature against poisoning materia ls such as air, water vapour, and carbon oxides in gas-solid reaction. Howe ver, the dissolution of a considerable amount of Ni during the fluorination process has been left as an unsolved problem for further development. The aim of this work is to prevent the decrease of the Ni concentration on the treated surface. A fluorination technique named as the duplicated fluorinat ion technique, presented here aims at enriching the metallic Ni onto the in creased surface area of the alloy by varying the pH-value down and up repea tedly in a definite range. Experimental studies have been performed extensi vely on the duplicated fluorination of LaNi4.7Al0.3, and LmNi(5) based allo y particles. The surface reaction characteristics were found to be greatly improved as the results of (1) the removal of oxides, (2) its high affinity to both ionic and molecular hydrogen, and (3) its high electron conductivi ty of the functionally-graded surface composed of the enriched Ni and fluor ide layer. The microstructure, element distribution, and composition analys is were examined by using scanning electron microscopy (SEM), electron prob e microanalysis (EPMA) and inductively coupled plasma spectrometry (ICPS). (C) 1999 Elsevier Science S.A. All rights reserved.