Behaviour of a fast migrating cation species in porous anodic alumina

The behaviour of Nd³⁺ ions is examined in porous anodic alumina films formed at 5 mA cm⁻² in 0.4 M phosphoric acid at 293 K on aluminium substrates that contain a buried 5 nm-thick tracer band of Al-Nd alloy. The Nd³⁺ ions migrate outward in the barrier region about twice as fast as Al³⁺ ions. The neodymium was located in the anodic film by transmission electron microscopy and scanning electron microscopy, and quantified by Rutherford backscattering spectroscopy. The Nd³⁺ ions migrated to the cell walls and to pore base, depending upon their location in the substrate relative to the alumina cells and pores. Nd³⁺ ions that reached the pore base were lost to the electrolyte. The outward transport of the Nd³⁺ ions was greatest beneath the pores and least at the cell boundaries, resulting in transformation of the planar tracer layer of the substrate to a roughly hemispherical shape in the film. The behaviour contrasts with that of a tracer band of slowly migrating W⁶⁺ ions, which reveals an approximately inverse distribution, while W⁶⁺ ions are retained within the film.