A novel approach in mechanical nanostructuring synthesis of metal hydride: Hydrogen sorption enhancement by High Pressure Torsion Extrusion

In this study, we evaluated the influence of a new mechanical nanostructuring technique called High Pressure Torsion Extrusion (HPTE) on the microstructural evolution of niobium and the subsequent effects on the mechanical properties and hydrogen storage behaviour. Two different regimes with the extrusion speeds of ν = 7 mm/min and ν = 10 mm/min were implemented in the experiments. A remarkable microstructural refinement and increase in hardness were achieved after one pass of HPTE. The initial grain size of 16.5 μm decreased to 600 nm and the initial hardness of 80 Hv increased to 284 Hv. Using a Sievert apparatus, it was found that the HPTE processed sample could absorb hydrogen to its full capacity within about 6 h while the as-received sample did not absorb even after one day of exposure to hydrogen gas. Rate limiting step modelling of the hydrogen absorption revealed that the absorption is a 3-dimensional diffusion-controlled reaction with a constant or decreasing interface velocity, depending on the HPTE regime.