Hydrogenation and microstructural properties of hydriding combustion synthesized Mg-Ni-C composite ball-milled with NbF5 catalyst

The present investigation evaluates the effect of milling the hydriding combustion synthesized (HCSed) MgNiC composite with NbF5 catalyst. X-ray analysis revealed that h-Mg-(1 0 1) peak positions of the ball-milled samples shifted by an amount that varied with the milling time. Morphologically, among the SPEX ball-milled samples the samples with the NbF5 catalyst showed slightly delayed refinement/agglomeration with milling time which was attributed to lubricating-behavior of NbF5 during milling. The final products from both (planetary and SPEX) ball mills showed insignificant agglomeration. A general trend of decreasing surface area with increasing milling time was observed. The HCSed sample manually mixed with catalyst showed the largest surface area of 23.78 m2/g. The presence of the catalyst and different interfaces, such as Mg-Nb, Mg-Ni and Mg-CNTs, in the samples played a crucial role in decreasing the thermal constraints for dehydrogenation. Analysis of hydrogenation kinetics revealed that the catalyst added samples requires activation and can be easily activated. By relating the hydrogenation kinetic data with the JMAK model, the value of the reaction order, n, was found to be close to 1 for all the samples, suggesting a possible diffusional phase transition. The lowest activation energy observed in synthesized sample for hydrogenation was 64.9 kJ/mol.