Authors: A.C. Geiculescu, J.L. Stevens and T.F. Strange
Affilation: St. Jude Medical, United States
Pages: 170 - 173
Keywords: FESEM, TEM, aluminum oxide, anodic alumina
Anodizing of aluminum in solutions such as sulfuric acid has been widely used for many years to produce nano-porous coatings on aluminum for architectural, decorative or abrasion–resistance purposes. It is well characterized in the literature as being an amorphous aluminum oxide that can be grown electrolytically to thicknesses of many microns with a relatively well organized array of nano-pores extending nearly down to the metal surface. Furthermore, these films grow under compressive stress, which imparts an inherent ability to resist cracking. Somewhat less well known is the electrolytic conversion of anodized films on aluminum to a totally amorphous, dielectric barrier layer material for use in pulse discharge aluminum electrolytic capacitors. Oxide for normal aluminum electrolytic capacitor applications is produced by different formation processes without the use of prior anodization. However, this nano-crystalline aluminum oxide forms in the tensile stress state which can lead to localized ruptures within the dielectric film. This paper shows that by adding a hydrothermal treatment between the anodization step and the barrier layer formation step, we have produced a nano-composite barrier layer that is up to 80% nano-crystalline while maintaining a compressive stress, amorphous oxide layer adjacent to the metal.It is expected that this nano-composite structure will offer the low stress, high stability features of amorphous oxide while maintaining high capacitance typical of totally nano-crystalline oxide.