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.