Tuning the thermal conductivity of hydrogenated porous magnesium hydride composites with the aid of carbonaceous additives

We synthesized solid porous pellets of the Mg-2 wt.% multiwall carbon nanotubes composite and determined their thermal conductivity in partially hydrogenated state (80-90% of their maximum hydrogen storage capacity). The thermal conductivities of the composite pellets with carbon nanotubes were up to 20% higher than thermal conductivities of the reference pellets of pure Mg prepared in a similar way. We demonstrated that the high energy ball milling employed in the synthesis of the composites has destroyed the carbon nanotubes and transformed them into chains of carbon nanoparticles of 20-30 nm in diameter. The hydride phase grew preferentially along these chains, resulting in anisotropic microstructure, with residual Mg phase forming an interpenetrating, percolating network contributing to the long-range thermal transport. On the contrary, the residual Mg formed isolated pockets in the reference pellets of pure Mg. Our work demonstrates that anisotropic additives to the hydride forming metals can improve their thermal conductivity and hydrogen storage properties by changing the topology of the two-phase metal-hydride microstructure.