| We report on ball-milled MgH2 composite powder containing 5 mol.% FeF3, which were characterized by Small Angle Neutron Scattering (SANS) measurements carried out during in-situ H2 absorption/desorption cycles. In the room temperature SANS curves for the as-milled powder, a power-law regime can be recognized for 0.02 Å-1 < q < 0.15 Å-1. Within this range, a linear fit yields a power-law scattering exponent a of about 2.4, indicating that the scattering entities can be associated with mass fractals with fractal dimension D — 2.4. Scanning Electron Microscopy confirms the presence of powder particle aggregates and self-similarity features, which are characteristic of fractal structures. Upon cycling, a power-law regime is again observed for 0.02 Å-1 < q < 0.15 Å-1, α exhibiting values of about 3.5 and 4 after desorption and absorption, respectively. The former value can be related to surface fractals with fractal dimension D of about 2.5, whereas for α = 4 Porods law is satisfied. This suggests the development, upon cycling, of a pore structure when Mg forms and its disappearance when MgH2 forms. |
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