Effect of Joule Heating on Formation of Porous Structure of Thin Oxalic Acid Anodic Alumina Films

Abstract

The effect of electrolyte temperature and anodizing voltage on the surface morphology of thin oxalic acid anodic alumina films formed on the SiO2/Si and glass-ceramic substrate was studied. It was shown when anodizing voltage increased from 10 to 20 V pore diameter increased from 12.2 to 15.5 nm and from 14.5 to 17.7 nm for the films formed on SiO2/Si and glass ceramic substrates, respectively. For the films formed at anodizing voltage (Ua) 30 and 40 V, pore diameter was 20.5 and 25.5 nm, respectively, and it was independent of the substrate type. It was demonstrated that when thin aluminum films were oxidized in oxalic acid, anodizing voltage of 30 V was a turning point at which the mechanism of the formation of anodic porous structure changed. By performing the experiments on the substrates with different thermal conductivity, we showed that at Ua < 30 V generated in the barrier layer Joule heating was absorbed by the substrate, but at Ua > 30 V the heat warmed up the electrolyte at the pore bottom that resulted in the changes of morphology of the films. We also suggested the empiric equation that connected pore diameter, anodizing current and generated Joule heating.