Systematic Study on Morphology of Anodic Alumina Produced by Hard Anodization in the Electrolytes Modified with Ethylene Glycol

Abstract

The morphology of anodic aluminum oxide (AAO) produced by hard anodization (HA) in oxalic acid electrolyte modified with various amount of ethylene glycol (EG) was investigated. The EG induces a considerable changes in the AAO morphology. The AAO transforms from continuous nanoporous film to separated AAO nanotubes upon addition of increasing amount of EG. In the sample II (4:1 v/v water to EG mixture) well separated nanotubes with variable wall thickness are produced. In the sample III (1:1 v/v water to EG solution) the nanotubes “imprisoned” in a partially dissolved cell skeleton with regularly spaced apertures along the cell are formed. In the electrolyte with the highest amount of EG (1:4 v/v water to EG mixture) an irregular AAO consisted of formless oxide and the oxide in a form of separated tubes of thick walls and small pores is fabricated. Based on the data obtained in this work it is concluded that the C containing ionic species originating from the EG dissociation along with the high electric field (E) operating during the HA were responsible for the separation phenomena. These ions, driven by the high E, were transported from the electrolyte to the pore base, where they were being embedded into the AAO framework generating strong mechanical stresses at cell boundaries and initiating the cell cleavage process. Moreover, some of these charged particles were ionized under the high E providing additional electrons to the overall current flow and giving rise to a sudden current density boost in the samples II and III.