Kazuhisa Azumi

Mott‐Schottky Plot of the Passive Film Formed on Iron in Neutral Borate and Phosphate Solutions

The semiconductive properties of the passive film formed on iron were investigated by measuring the impedance of passivated iron electrodes in neutral borate and phosphate solutions. Mott‐Schottky type of plot was applied to the impedance data to obtain the donor density and the flatband potential as a function of film formation potential, oxidation time, and ion species present in the solution. The value of obtained for the passive film is in the range of and decreases with increasing formation potential and oxidation time, indicating that the structural and electronic defects in the passive film decrease with increasing film thickness. The value of of the passive film formed in phosphate solution is greater than that in borate solution. From the fact that when the electrolyte was changed from borate to phosphate solution increased from the value in borate solution to the value in phosphate solution in a time period of about 103s, it is evident that phosphate ions penetrate into the passive film producing a large number of defects.

Changes in electrochemical properties of the anodic oxide film formed on titanium during potential sweep

Abstract Changes in electronic properties of the anodic oxide films formed on titanium in weakly alkaline borate solution were investigated by using impedance measurements. Analysis of the cyclic voltammogram and capacitance during the cyclic potential sweep suggested that the outer part of the anodic oxide film was converted from an n-type semiconductor to a dielectric above 3 V, and converted to an n-type semiconductor again below 0.5 V. This phenomenon seems to be related to the dissociation of bound water in the film due to very high electric field applied to the oxide film. Formation of OH − in the film may compensate the positive charge of the donor states and form Ti peroxides such as TiO 3 . These compounds are reduced below 0.5 V to show a characteristic cathode current peak.

Heterogeneous Growth of Anodic Oxide Film on a Polycrystalline Titanium Electrode Observed with a Scanning Electrochemical Microscope

A scanning electrochemical microscope (SECM) was applied to study anodic oxide film grown on a polycrystalline titanium electrode in deaerated pH 8.4 borate solution. The probe current images of SECM could detect the heterogeneous growth of anodic oxide film, depending on the substrate crystal grains. This heterogeneity increased with increasing the film formation potential at the potential higher than 3 V (SHE). The study of the dependence of film thickness on the substrate grain has been also supported with Raman microprobe spectroscopy.

Use of a Liquid‐Phase Ion Gun for Local Breakdown of the Passive Film on Iron

A silver/silver chloride microelectrode was developed as a liquid-phase ion gun for investigation of precursor processes of pitting corrosion. This electrode was set as a probe electrode of a scanning electrochemical microscope and used to induce a local breakdown of passive film on iron. A small amount of chloride ions generated by cathodic polarization of the silver/silver chloride microelectrode has succeeded in inducing the local breakdown of the passive film formed on iron in deaerated pH 6.5 borate solution. Moreover, the microelectrode could detect ferric ions dissolved from the film at the initial stage of the film breakdown.

Hydrogen Evolution by Plasma Electrolysis in Aqueous Solution

Hydrogen has recently attracted attention as a possible solution to environmental and energy problems. If hydrogen should be considered an energy storage medium rather than a natural resource. However, free hydrogen does not exist on earth. Many techniques for obtaining hydrogen have been proposed. It can be reformulated from conventional hydrocarbon fuels, or obtained directly from water by electrolysis or high-temperature pyrolysis with a heat source such as a nuclear reactor. However, the efficiencies of these methods are low. The direct heating of water to sufficiently high temperatures for sustaining pyrolysis is very difficult. Pyrolysis occurs when the temperature exceeds 4000°C. Thus plasma electrolysis may be a better alternative, it is not only easier to achieve than direct heating, but also appears to produce more hydrogen than ordinary electrolysis, as predicted by Faradays laws, which is indirect evidence that it produces very high temperatures. We also observed large amounts of free oxygen generated at the cathode, which is further evidence of direct decomposition, rather than electrolytic decomposition. To achieve the continuous generation of hydrogen with efficiencies exceeding Faraday efficiency, it is necessary to control the surface conditions of the electrode, plasma electrolysis temperature, current density and input voltage. The minimum input voltage required induce the plasma state depends on the density and temperature of the solution, it was estimated as 120 V in this study. The lowest electrolyte temperature at which plasma forms is ~75°C. We have observed as much as 80 times more hydrogen generated by plasma electrolysis than by conventional electrolysis at 300 V.

A DFT study on the structures and electronic states of zinc cluster Znn (n = 2-32)

Ab-initio and density functional theory (DFT) calculations have been carried out for zinc clusters Znn (n = 2–32, n is the number of atoms to form a cluster) to elucidate the structure and electronic charge states of the clusters and the mechanism of clustering. The binding energies of Zn atoms were negligibly small at n = 2–3, whereas the energy increased significantly at n = 4 (the first transition). The second transition occurred at n = 8–16. In the larger clusters (n = 16–32), the binding energy increased slightly with increasing cluster size (n). The cluster size dependence of the binding energy and bond length between zinc atoms agreed well with that of the natural population of electrons in the 4p orbital of the zinc atom. In the larger clusters (n > 20), it was found that the zinc atoms in the surface region of the cluster have a positive charge, whereas those in the interior region have a negative charge with a large population in the 4p orbital. The formation mechanism of zinc clusters was discussed on the basis of the theoretical results.

Characterization of anodic oxide film formed on tin coating in neutral borate buffer solution

Properties of the oxide layer formed on a TiN coating by anodic oxidation in neutral borate buffer solution were investigated using cyclic voltammetry, AFM, FT-IR, impedance and photocurrent measurements. The voltammogram showed imperfections in the TiN coating and high susceptibility of anodic oxidation. The result of FT-IR showed the composition of an oxide layer, similar to TiO2, including OH− or water. Both impedance and photocurrent results confirmed that the oxide film has n-type semiconductor properties. The oxide would contain defects acting as intergap states, resulting in low efficiency of photo-induced free-carrier generation. It seems that these defects have their origin in degradation of the protective property of a TiN coating under anodic polarization.

Double Zincate Pretreatment of Sputter-Deposited Al Films

The characteristics of double zincate pretreatment of thin Al films deposited on glass plates using magnetron sputtering and ion-beam sputtering were investigated. Traces of Zn deposition and immersion potential as well as surface observations using scanning electron microscopy and atomic force microscopy showed that continuous dissolution of an Al film during the double zincate pretreatment occurred in the case of a magnetron sputter deposited film, resulting in Al film failure from the substrate. On the other hand, the substitution reaction of Al dissolution and Zn deposition occurring on the ion-beam sputter-deposited film ceased during the first and the second zincate treatment processes. The difference between the behaviors of the double zincate treatments for the two kinds of sputter-deposited films is related to the film structure. A magnetron sputter-deposited film has a columnar structure, resulting in higher susceptibility to the dissolution reaction in a concentrated alkaline zincate solution. On the other hand, an ion-beam sputter-deposited film has a fine microcrystalline structure with a low density of defects, resulting in lower susceptibility to the dissolution reaction.

Light Emission from Pt during High‐Voltage Cathodic Polarization

Light emission from cathodically polarized Pt electrodes was investigated at cell voltages up to 200 V in aqueous electrolyte solutions. The emission of light was observed when intense cathodic polarization caused the temperature of the Pt electrodes to exceed the boiling temperature of the electrolyte. A thin vapor layer was formed at the metal‐electrolyte interface in which a high electric field ionized vapor molecules to generate a plasma state. The emission of light was caused by the glow discharge at relatively low cell voltages and by the spark discharge at high cell voltages. The spectra of the emitted light were assigned to the constituents of the electrolyte solution, electrode material, and gaseous hydrogen evolved at the electrode.

Monitoring of Hydrogen Absorption into Titanium Using Resistometry

Hydrogen absorption into Ti electrodes during electrochemical cathodic polarization was monitored using resistometry. Electric resistance of Ti increased with H absorption due to growth of a hydride layer from the surface toward the inside. The growth rate of the hydride layer was estimated from resistance data and was found to depend on the polarization current density, existence of a preformed anodic oxide film, and shape of the specimen. For example, preformation of an anodic oxide film at a potential higher than the breakdown potential, rather, promotes hydrogen penetration. In the case of a thin wire electrode, the hydride layer grew in a nonuniform manner because the volume expansion induced cracking on the surface. Therefore, the average thickness of the hydride layer was estimated from the change in resistance.