DIRECT OBSERVATION OF ANODIC FILMS ON TANTALUM.

Direct observation of anodic films on tantalum is possible by preparation of electron transparent section via ultramicrotomy. Retention of the film on the substrate is favored by using relatively porous (low density) tantalum substrates rather than bulk tantalum, where mechanical deformation during sectioning enhances film spalling. The resultant sections of the anodic films, showing remarkable similarity to barrier-type anodic alumina films, appear relatively uniform and featureless at the resolution and magnifications employed. Prolonged exposure of the films to the electron beam, under the conditions employed, does not induce crystallization which may relate to the inhibiting effect of incorporated phosphate species and other film properties. Energy dispersive analysis of X-rays confirms the presence of phosphorous, although further work is required to identify clearly the extent of such incorporation across the section of the film material.