Hydrogen Release and Microstructure of MgH_2 Based Composite Powders Containing a Relevant Amount of LaNi_5

摘要

Micro-composite materials based on MgH_2 with the addition of a relevant amount of LaNis have been synthesized by reactive ball milling. The powder microstructure has been studied by a combination of X-ray diffraction and scanning electron microscopy, while the decomposition behaviour and the hydrogen release properties have been obtained by differential thermal analysis and thermo-gravimetric measurements. Both temperature scans and constant temperature isotherms have been used to this purpose. Experimental results allow identifying optimum processing condition for synthesis of material that shows the onset of hydrogen release at temperatures as low as 450 K. The decomposition kinetics has been studied by isothermal measurements which show that the whole process cannot be described by just one mechanism limiting the reaction velocity. In fact in the first decomposition step the reaction is kinetically limited by the second phase nucleation, while, for partially decomposed samples the bulk diffusion appears to limit the process. On the basis of the experimental results we propose a mechanism of phase transformation where a percolating network of the two phases is formed.