The growth of barrier-type anodic films on aluminium and its alloys is reviewed, with particular consideration of ionic transport processes in the growing anodic films. The films formed are generally amorphous, and grow both at the film/electrolyte interface by outwards migration of cations and at the metal/film interface by inwards migration of ani-ons. Various types of foreign species are incorporated into the anodic films from electrolytes or alloy substrates. The in-corporated species are mobile inwards, or outwards or immobile under the high electric field of 106-107 V cm-2. This provides us with some important information on the complex ionic transport processes in amorphous anodic films. The anodic films formed on uniform Al-valve metal alloys are generally of two layers, due to the different migration rates of A13+ ions and valve metal ions. The properties and growth behavior of the films vary almost linearly with film composi-tion.
Anodization of aluminum was examined in nonaqueous solutions containing a small amount of water, and the resul-tant anodic oxide films were characterized by SEM, TEM, AES, XPS, XANES, and FT-IR. The film properties were dependent on not only the kind of nonaqueous solutions but also the amount of water in the solutions. When the water content in nonaqueous solutions was decreased below 1∼0.1 %, anions and/or organic solvents were incorporated into the films and very different anodic films were formed. A phthalate/γ-butyrolactone solution gave a fibrous porous-type film accompanying solution decomposition, whereas in a maleate/γ-butyrolactone solution fast and efficient film formation was observed and a barrier-type film containing maleate anions as an oxygen source was obtained. A vanadate/ethylene glycol solution provided a very thick film due to the incorporation of the solvent rather than vanadate anions. The optimum content of water to give the composite oxide films seems to depend on the viscosity of the solutions.
Recent research development and fundamental aspects on the structure and growth mechanism of porous anodic films formed on aluminum and magnesium are reviewed. Thick anodic film growth proceeds with the balance of oxide formation at the metal/film interface and the field-assisted dissolution of oxide at pore bases. Hexagonal cylindrical cell structure of the film, in which cell size is nearly proportional to the formation voltage, is attained as a result of inversely proportional uniform pore distribution to the applied voltage. More exactly, cell dimension is a function of voltage and electric field strength. Electrolyte anions are incorporated into the film under the effect of electric field. Defects in porous films are formed by the drilling effect of micro-breakdown between pore bases and triple points of cells on aluminum ridges. Long-range ordering of cell arrangement of porous anodic alumina for electric devise materials is signifi-cantly improved by appropriate electrolysis voltages and chemical texturing of aluminum surface. For the anodic film growth on magnesium, the cylindrical pore structure which is similar to the Keller's model of anodic alumina is confirmed. It proceeds mainly by the formation of MgF2 and Mgx+y/2Ox(OH)y at the metal/film interface and the dissolution of the film at pore bases.
Recent studies of the anodic oxide films on titanium are reviewed. In-situ electrochemical techniques including AC impedance, optical reflection like ellipsometry, photoelectrochemistry, and Raman spectroscopy have been used. The results are summarized as follows: (1) The TiO2 film is amorphous at potentials lower than 5 V but changes to anatase type at potentials higher than 5 V. (2) Thickness of the film is linearly proportional to the anodic potential in the poten-tials range lower than 7 V. However, the ratio of thickness to potential becomes larger if the potential is higher than 7 V probably due to formation of an ionic leakage path accompanied by crystallization of the film. (3) The film behaves as an n-type semiconductive electrode with a high concentration donor density and a bandgap of 3.2 eV.
Based on the literatures published over the past fifty years, a brief review is given of the studies on anodic oxide growth on tantalum. It has been demonstrated clearly that the oxide growth involves highly complex and unprecedented processes of ionic transport to which no satisfactory theory has yet been presented. Thus, in amorphous oxide and in the presence of a high electric field of 106-107 V cm-1 range, both Ta5+ and O2- ions migrate in a highly correlated manner, with new oxide formation proceeding simultaneously at the oxide/electrolyte and metal/oxide interfaces. Further small amounts of electrolyte anions are incorporated into the oxide which, depending on their nature, may be immobile or mobile inwardly or outwardly. To date, such complex processes of ionic transport may be best explained, though only qualitatively, by a “liquid droplet” model presented recently by Mott. However, further insights into the nature of liquid droplets, whose formation is thought to be an elemental process of ionic transport, are apparently required in the eventual development of an integrated theory of ionic transport. It is emphasized that the tracer experiments, in which migrations or non-migrations of various foreign cationic or anionic tracers are studied and interpreted in terms of the role of high electric fields, acting either generally or locally, on the dissociation or non-dissociation of various structural units in the oxides, provide an important and effective rote towards the required understanding.
An overview is provided on the properties and growth of passive films on Fe group metals and stainless steel. The growth kinetics of passive film might be explained by the Cabrera-Mott theory with some modifications. Passivation of stainless steel is characterized by Cr enrichment in the film with some critical Cr content above which the “stainless” passive film is obtained. The existence of the critical content is explained by using a percolation model and a competitive precipitation model. Modification of passive films is also described in terms of obtaining the much higher corrosion resistance and the surface functions.
Recent “in situ” observations of electrochemical oxidation of noble metals and group-VIII metals by means of scanning tunneling microscopy (STM) have been reviewed. Single crystal surfaces of gold and platinum are oxidized by anodic polarization in acidic media, resulting formation of amorphous oxides, which can be reduced by cathodic polarization. The metal atoms involved in oxidation processes are transported along the surface and thus the surface morphology changes. Single crystal surfaces of nickel and cobalt, after proper cleaning procedure, exhibit formation of wellordered epitaxial oxide layers in slightly acidic media. The lateral lattice structures of the oxide layers are close to lowindex planes embedded in the corresponding oxide crystals. Formation of stoichiometric oxides as well as hydrooxides is evidenced. Passivation takes place on Ni surfaces due to the oxides, whereas continuous anodic dissolution occurs on Co surfaces covered by oxide layers.
The depth profiles of natural mica were determined by XPS using Ar+ sputtering. The localities and the chemical states of the atoms were studied under proper analysis conditions. The Ar+ beam with an energy of 1 keV was used to make the sputter rate slow enough. A low take-off angle (20 degrees) was used to obtain a fine resolution of the depth profiles. As a result, the periodic variations in atomic concentration were observed at the peaks; O(1 s), Si(2 p), Mg (2 p), and K(2 p). The variations indicate periodic stacking of sheets; tetrahedral sheet, octahedral sheet, and interlayer cations. A periodic variation in the Al(2 p) binding energy in the range of 74.20 to 74.28eV was observed, and it suggests that the tetrahedral Al and the octahedral Al atoms existed alternately in the crystal structure along with an axis perpendicular to the surface.
The influence of surface structure on epitaxial growth, which has not been considered before in the crystal growth theory, is shown for the case of epitaxial growth of Si on Si(111)-7×7 substrate. In the lateral growth of islands on the 7×7 structure, the island size shows a discontinuous distribution according to the size of the unit cell of the surface structure, because the rate determinant process is the rearrangement of the surface lattice. This size distribution is understood by considering the activation energy of the rearrangement into the free energy change in the nucleation process. The surface structure also influences on the growth mode. In the initial growth stage, multilayer islands are formed because the lateral growth of the first layer is prevented by the stable 7×7 structure and some migrating atoms climb up the first layer and nucleate on it. However, the lateral growth of the second layer on the first one is not prevented and the layer-by-layer growth starts, because the structure of the first layer is composed of small domains with some metastable surface structures which is rather easier to rearrange than the 7×7 structure.
An ergonomic approach to the active tactile perception is described with an example of sensing surface conditions of a human skin. In a practical manipulation of softness and elasticity of human cheeks, a forefinger was used by applying a small amount of stroke, pushing releasing. A model is described for the input of a stroke to the object and the response detected as a stress change. The detection system has been developed using an actuator and a tactile sensor, which can be applied for measuring the dynamic properties of human skin. The system is controlled under the conditions derived from the analysis of forefinger movements, and a correlation is obtained between the parameters from the measurements and the softness and elasticity evaluated by experts. The importance of handling process for active sensing of pliant objects, such as the detection of conditions of human skin, is suggested.