Sachiko Ono

Self‐Ordering of Cell Arrangement of Anodic Porous Alumina Formed in Sulfuric Acid Solution

Self-ordering of the cell arrangement of the porous structure of anodic alumina has been studied in a sulfuric acid solution. Ordering of the cell arrangement was dependent on the applied potential, and a highly ordered structure was obtained under anodization at a constant potential of 25 to 27 V. Self-ordering of the porous structure proceeded with the growth of the oxide layer under anodization at an appropriate potential, and a porous film with an almost ideal hexagonal honeycomb structure was formed over an area of several micrometers after a long period of anodization.

Controlling Factor of Self-Ordering of Anodic Porous Alumina

The controlling factor of self-ordering of anodic porous alumina was investigated by focusing on the current density during film growth. The homogeneity of cell size was improved with increasing formation voltage accompanied by the exponential increase in current density. The maximum anodizing voltage for proceeding uniform oxide growth while avoiding extremely high current accompanied by gas evolution was identical with the previously established self-ordering voltage. With the increase in formation voltage up to the self-ordering voltage, the ratio of pore diameter to cell diameter d pore /d cell lowered and converged to ∼0.3 regardless of the electrolyte type. Moreover, domains of highly self-ordered pore arrays were found in the film formed during burning, where extremely high current was locally concentrated. This suggests that the condition inducing film growth under high current density, i.e., high electric field strength is the key controlling factor of self-ordering. Based on the above knowledge, a new self-ordered porous alumina with a 600 nm pore interval was fabricated in citric acid just under the critical voltage of burning.

A TEM investigation of naturally formed oxide films on pure magnesium

The morphology and structure of oxide films formed naturally on pure magnesium by exposure to humid air and water have been investigated by use of ultramicrotomed cross sections and TEM. The initial film formed immediately after exposing fresh surface by scratching in air is thin and dense, and it has an amorphous structure. In humid air, a hydrated layer forms between the metal and the initial layer as a result of water ingress through the initial layer and metal oxidation. The film formed in water contains an additional top layer with platelet-like morphology, formed by redeposition of sparingly soluble magnesium, which migrates outward also through the initial layer. The properties of these layers and their significance in determining the corrosion behavior of magnesium are discussed.

Morphology and Structure of Oxide Films Formed on Magnesium by Exposure to Air and Water

Oxide films formed naturally on pure Mg are investigated by the use of ultramicrotomed cross sections and transmission electron microscopy. The film formed in air immediately after scratching the metal surface is initially thin, dense, amorphous, and relatively dehydrated. Continuing exposure to humid air or exposure to water leads to the formation of a thicker hydrated film adjacent to the metal. The film formed in water contains an additional top layer characterized by a plateletlike morphology. The structure of these layers and their significance in corrosion protection are discussed. The changes occurring in these structures as a result of exposure to the electron beam are reported.

Morphology and Structure of Water‐Formed Oxides on Ternary MgAl Alloys

Oxides on ternary magnesium alloys MgAlZn and MgAlRE were investigated by transmission electron microscopy using ultramicrotomed film sections. These films have a three-layered structure, similar to pure Mg and binary MgAl alloys, characterized by a hydrated inner layer, a thin and dense intermediate region, and a platelet-like outer layer. Zinc and rare-earth elements present in the two types of ternary alloys become incorporated in the oxide film so as to increase its stability in an aqueous environment, in particular by reducing hydration and increasing resistance to magnesium egress of the inner layer, which is responsible for the passivity of the surface. The apparent presence of trace amounts of rare-earth oxides in the film is particularly effective in improving passivity of the surface and, thereby, the corrosion resistance of MgAlRE alloys. The presence of aluminum together with rare-earth elements (RE) in the alloy is an essential factor in obtaining these results.

Evaluation of pore diameter of anodic porous films formed on aluminum

The porosity of anodic films formed in four major electrolytes, which was measured by pore-filling technique, decreases with increasing voltage. Comparing the porosity at an identical voltage, it increases in the order of the films formed in sulfuric acid, oxalic acid, chromic acid, and phosphoric acid solutions. When the voltage decreases down to lower than 10 V, the porosity increases remarkably, especially in the films formed in phosphoric acid. The pore diameter estimated from the porosity and cell diameter measured by pore-filling technique decreases with decreasing voltage down to 5 V. However, at the voltages lower than 5 V, the pore diameter increases while the cell diameter decreases. Pore and cell diameters of the films formed in the four electrolytes at an identical voltage increase in the order: sulfuric acid, oxalic acid, chromic acid, and phosphoric acid. These results are basically consistent with those reported in our previous TEM study, although the pore sizes are slightly larger than the latters. The reason of the discrepancy between the methods is discussed.

Self-Ordering of Anodic Porous Alumina Induced by Local Current Concentration: Burning

The self-ordering behavior of porous anodic alumina induced by the burning phenomenon on the specimen where extremely high and local current concentrations occurred was studied. The domains of highly self-ordered cell arrangement were found at the locally thickened film formed by burning. At the center of the burnt area, the regularity of cell arrangement was higher and the barrier layer thickness as well as the cell size was remarkably lower. These results clearly suggest that the condition inducing film growth under high current density, i.e., high electric field is the key factor determining the self-ordering.

Structure of anodic films formed on magnesium

The cylindrical pore structure and barrier layer of anodic films formed on magnesium which are similar to the Keller`s model of anodic alumina are determined by direct cross-sectional observation. The ratios of cell diameter and barrier layer thickness to the forming voltage are notably smaller than those associated with anodic alumina. It is assumed that anodic film growth proceeds mainly by the formation of MgF{sub 2} and Mg{sub x+y/2}O{sub x}(OH){sub y} (magnesium oxyhydroxide) at the metal/film interface and the dissolution of the film at pore bases.

Growth of anodic porous alumina with square cells

We have studied pore nucleation and growth of anodic porous alumina with square cells in the initial stage of anodizing by measuring current density transient at constant voltage and by observation of film structure at different stages. The initiation sites of pores were laid out in square lattice patterns using imprinting process. The obtained oxide film was formed by a close-packed array of square cells following the imprinting pattern. The shape of the pore was circular at the surface, but square at the bottom of the film corresponding to the shape of square cell at the steady-state growth stage. At the same time, crystalline particles located at fourfold point of cells were detected. The surface of substrate after removal of oxide film revealed that the crystals were regularly ordered aluminum pillars placed at every fourfold point of cell junction. From these characteristic structures, it is concluded that the unusual arrangement of initiation sites, which is different from ordinary hexagonal cell arrangement, could control the growth of anodic porous alumina significantly.

Transfer of nanoporous pattern of anodic porous alumina into Si substrate

Nanohole arrays in a Si substrate with a self-ordered configuration having a 100 nm hole periodicity were fabricated by the pattern transfer of the hole configuration of anodic porous alumina. The self-ordered anodic porous alumina used as a mask was directly prepared by anodizing an aluminum film sputtered on a Si substrate. The transfer of the nanoporous pattern of anodic alumina into the Si substrate could be achieved by removing silicon oxide, which was produced by the anodic oxidation of the local part of the Si substrate underneath the barrier layer corresponding to the pore base. In addition, we confirmed that the transformation of the nanostructure of porous alumina grown on a Si substrate is comparable to the current transient during alumina film formation.