Electron Beam Induced Texture Change of the Anodic Films Formed in the Molten Melt

The material comprising the film was sensitive to electron beam irradiation in the vacuum of the microscope; therefore, it was necessary to characterize further the appearance of the film material substructure observed in the transmission electron microscope. Figure 1 shows the ultramicrotomed film sections, formed originally to 60 V and 10 Am2 in the molten melt at 418 K, and subsequently exposed to the electron beam for different time periods. The continued exposure of the same area in the ultramicrotomed film section to the electron beam produced remarkable changes in the film substructure (Figure 1). After short exposure times (30 s), there was a coarsening of the texture, with a relatively light region (about 15 nm in thickness) adjacent to the film/substrate interface (Figure 1(b)). After 1 min, there was a further coarsening of the texture to produce the relatively larger, light zones (Figure 1 (c)). With further exposure, this trend was continued, with discrete zones apparent but with more complicated zone shapes observed in other regions, particularly adjacent to the film/substrate interface (Figure 1 (d)). The complicated zone shapes possibly arose from coalescence or merging of growing zones; a further important consideration is that the image observed in the transmission electron microscope is influenced by zones above or below one another, giving rise to the apparent overlapping of the zones and apparently complicated shapes (Figure 1 (e)). After longer exposure to the electron beam, relatively large zones were evident, with significantly less overlapping of zones, which suggest that the zones extended throughout the thickness of the ultramicrotomed section (Figure 1 (f)). During the exposure of the film, a decrease in the thickness was observed with some zones evidently obliterated by dark material. This may indicate a volume contraction during the creation of the zone structure and/or a compaction of the film material. Any further changes with exposure to the electron beam were slow to proceed and the examination was discontinued at this stage. In order to characterize the crystallization behavior by electron beam induced heating, the ultramicrotomed film sections, after 30 s and 30 minutes exposure to the electron beam, were investigated by selected area electron diffraction. The resultant diffraction patterns of the now heavily textured films exhibited two broad diffuse rings, typifying the amorphous film structure. For the barriertype films formed in aqueous environments, electron beam induced crystallization shows that the inner relatively pure alumina region crystallizes at a faster rate than the outer contaminated material. This infers that the changes result from the different nature, i.e. relative compactness, composition and extent of hydration, etc., of the contaminated film material. For the contaminated film materials developed in the major acid electrolytes, the rate of change of the film material texture also appears to depend on the type of incorporated acid anion and increases in the order, sulphuric acid > oxalic acid > phosphoric acid > chromic acid. In order to investigate the possible composition variations between the outer textured region and the inner region of the film found in the molten melt, an EDX analysis of the film was performed.