Material characteristics and capacitive properties of aluminum anodic oxides formed in various electrolytes

Abstract

The microstructure and composition of aluminum oxide films, formed by a two-step anodization process in various electrolytes at 100 V then subject to an intermediate heat treatment (500 °C, 2 min), were investigated. The anodization electrolytes used included ammonium adipate electrolyte, phosphoric acid electrolyte, ammonium dihydrogen phosphate electrolyte, and their mixtures. The cross-section morphologies, crystal structure, and chemical composition of aluminum anodic oxides were examined by transmission electron microscopy. X-ray photoelectron spectroscopy was carried out to study the surface chemical state of the anodic films. The corresponding capacitances and retention voltages of these oxide films were also explored. The results indicated that amorphous-to-crystalline transformation of the oxide, primarily in films formed in ammonium adipate electrolyte, was induced by the heat treatment. Electron diffraction analyses further revealed the oxide films consisted of two distinct zones, which included an inner amorphous layer and an outer layer containing crystalline γ′-Al2O3. This study found that the phosphorous species in either the primary or the re-anodization electrolytes had a potential to cause changes of Al and O distributions within the oxides. The oxide film primarily anodized in ammonium adipate and re-anodized in phosphoric acid had the highest capacitance due to its high degree of crystallinity and thinness compared to oxides formed in other electrolytes. The presence of phosphorus, from the primary anodization electrolytes, in the oxides could inhibit the formation of crystalline γ′-Al2O3 and, consequently, decrease the capacitances of the anodic films. At the same time, the retention voltage and hydration resistance of these oxide films were improved.