Porosity of anodic alumina membranes from electrochemical measurements

A procedure based on the high-field mechanism of the growth of anodic oxides was developed in order to evaluate the morphological features of porous layers. Since the thickness of the barrier film, separating the porous layer from the metal, does not change during the steady-state growth of an anodic porous layer, the rate of displacement of the metal-oxide interface to the metal direction must be equal to the rate of displacement of the pore base to the oxide direction. As a consequence, porosity can be expressed in terms of the ratio i _diss/i _ion, where i _diss is the dissolution current density at the pore base, and i _ion is the ionic current density at the metal-oxide interface. Pore diameter can be determined from geometrical considerations, while average pore population can be obtained from the ratio of porosity to the average surface area of a single pore. This procedure was checked by comparison with experimental results relative to membranes prepared in various conditions. The satisfactory agreement between theoretical and experimental findings indicates that porosity can be evaluated by current density data and vice-versa. Therefore, anodic alumina membranes may be tailored for different applications by choosing operative conditions giving the desired value of i _diss/i _ion.