Han-Jun Oh

Electrochemical characteristics of alumina dielectric layers

Abstract The electrochemical characteristics of alumina dielectric layers were studied using a surface roughness factor and an impedance spectroscopy. From the limiting diffusion current method, the surface area factor of the dielectric anodic layer with low electrical conductivity was estimated to be 1.03. As alumina dielectric films on Al have a variable stoichiometry, the electrochemical behavior of Al 2 O 3 layer can be monitored by evaluating an equivalent circuit with Young impedance of dielectric constant with a vertical decay of conductivity.

Stress corrosion cracking properties of environmentally friendly unleaded brasses containing bismuth in Mattsson's solution

The purpose of the present investigation was to study the stress corrosion cracking (SCC) properties of environmentally friendly Bi-brasses in Mattssons solution (0.05 M copper sulfate and 0.5 M ammonium sulfate, pH 7.4) at room temperature. Three unleaded brasses with Bi content ranging from 1 to 3 wt.% and a conventional leaded brass for comparison were investigated. The effect of Bi on SCC of unleaded brasses was studied through polarization test, electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and slow strain rate tests (SSRT). Potentiodynamic polarization studies indicated that the corrosion resistance of Bi-containing brasses was improved as the content of Bi, which shown passivating characteristic in Mattssons solution, increased. The Pb- and Bi-containing brasses presented an inductive behavior on impedance data in low frequency region at the open-circuit potential (OCP). Such behavior could be attributed to the adsorption process on the alloy surface. The observed dezincified layers after SSRT should be particularly good initiators of SCC as they could induce tensile stress. As the content of Bi increased, the thickness of dezincified layer and the volume fraction of preferentially attacked phase decreased, therefore, SCC resistance of the Bi-containing brasses was improved.

Surface roughness factor of anodic oxide layer for electrolytic capacitors

Abstract The roughness factor of barrier-type anodic oxide layer with low electrical conductivity was evaluated using electrochemical measurement in electrolyte with redox couple. To evaluate the surface roughness factor, the surface of barrier-type oxide layer was covered with thin layer of platinum by sputtering. From the limiting diffusion current method the surface roughness factor of barrier-type Al 2 O 3 estimated to be 1.03. This factor is in good agreement with the results of the cross-sectional characteristics by using transmission electron microscope.

Photocatalytic performance of graphene/Ag/TiO 2 hybrid nanocomposites

To improve photocatalytic efficiency, graphene/Ag/TiO₂ nanotube catalyst was synthesized, and its surface characteristics and photocatalytic activity investigated. For deposition of Ag nanoparticles on the TiO₂ nanotubes, a polymer compound containing CH₃COOAg/poly(Llactide) was utilized, and the silver particles were precipitated by reducing the silver ions during the annealing process. Graphene deposition on the Ag/TiO₂ nanotubes was achieved using an electrophoretic deposition process. Based on the dye degradation results, it was determined that the photocatalytic efficiency was significantly affected by deposition of silver particles and graphene on the TiO₂ catalyst. Highly efficient destruction of the dye was obtained with the new graphene/Ag/TiO₂ nanotube photocatalyst. This may be attributed to a synergistic effect of the graphene and Ag nanoparticles on the TiO₂ nanotubes.

Effect of surface modification of carbon felts on capacitive deionization for desalination

Surface modified carbon felts were utilized as an electrode for the removal of inorganic ions from seawater. The surfaces of the carbon felts were chemically modified by alkaline and acidic solutions, respectively. The potassium hydroxide (KOH) modified carbon felt exhibited high Brunauer-Emmett-Teller (BET) surface areas and large pore volume, and oxygencontaining functional groups were increased during KOH chemical modification. However, the BET surface area significantly decreased by nitric acid (HNO3) chemical modification due to severe chemical dissolution of the pore structure. The capability of electrosorption by an electrical double-layer and the efficiency of capacitive deionization (CDI) thus showed the greatest enhancement by chemical KOH modification due to the appropriate increase of carboxyl and hydroxyl functional groups and the enlargement of the specific surface area.

Impedance of Barrier-Type Oxide Layer on Aluminum

The impedance characteristics of barrier-type oxide layers on aluminum was studied using impedance spectroscopy. Since anodic films on Al have a variable stoichiometry with a gradual reduction of oxygen deficiency towards the oxide-electrolyte interface, the interpretation of impedance spectra for oxide layers is complex and the impedance of surface layers differs from those of ideal capacitors. This frequency response of the layer with conductance gradients cannot be described by a single resistance–capacitance (RC) element. The oxide layers of Al are properly described by the Young model of dielectric constant with a vertical decay of conductivity.

Fabrication Parameters for Highly Ordered Nano-Pore Arrays in Alumina Template

The fabrication parameters for highly ordered nanopore arrays formed by self-organized anodization on aluminum have been investigated. To fabricate ordered nanopore arrays in alumina layer a two-step anodization process was used in oxalic acid solution. The regular nanopore arrangement, interpore distance and pore diameter in alumina surface were strongly dependent on the applied potential at anodization. The interpore distance and pore diameter increased with anodic applied potential, and hexagonally ordered pore arrangements in alumina were obtained by anodization at 40V. For the effects of the heat treatment of aluminum substrate on size of domains with same pore orientation, the domain size increased with increasing annealing time.

Effects of Hydration Treatments on the Phase Transition of Anodic Aluminum Oxide Layers

Hydration treatments were performed on the pure aluminum substrate at followed by anodizing and heat treatments on the layers. The transformation behaviors of the oxide layers according to the hydration treatment were studied using TEM, XRD, RBS etc. Above the hydrous oxide film could be formed, which were turned out to be hydrous oxides(AlOOH O). The anodization on the hydrous oxide film was more effective for the transition of amorphous anodic oxides to the crystalline comparing with the case for anodizing on the aluminum substrate without hydration treatment And additional heat treatments were also helpful for the acceleration of the transformation of the hydrous oxide to . During the heat treatment the interface between and the hydrous oxide layers migrated to the outer side of hydrous layer.

Effects of Anodic Voltages of Photcatalytic TiO 2 and Doping in H 2 SO 4 Solutions on the Photocatalytic Activity

To compare the photocatalytic performances of titania for purification of waste water according to applied voltages and doping, films were prepared in a 1.0 M solution containing at different anodic voltages. Chemical bonding states of F-N-codoped were analyzed using surface X-ray photoelectron spectroscopy (XPS). The photocatalytic activity of the co-doped films was analyzed by the degradation of aniline blue solution. Nanotubes were formed with thicknesses of 200-300 nm for the films anodized at 30 V, but porous morphology was generated with pores of 1-2 for the anodized at 180 V. The phenomenon of spark discharge was initiated at about 98 V due to the breakdown of the oxide films in both solutions. XPS analysis revealed the spectra of F1s at 684.3 eV and N1s at 399.8 eV for the anodized in the solution at 180 V, suggesting the incorporation of F and N species during anodization. Dye removal rates for the pure anodized at 30 V and 180 V were found to be 14.0% and 38.9%, respectively, in the photocatalytic degradation test of the aniline blue solution for 200 min irradiation; the rates for the F-N-codoped anodized at 30 V and 180 V were found to be 21.2% and 65.6%, respectively. From the results of diffuse reflectance absorption spectroscopy (DRS), it was found that the absorption edge of the F-N-codoped films shifted toward the visible light region up to 412 nm, indicating that the photocatalytic activity of is improved by appropriate doping of F and N by the addition of .

Solid state reactive sintering of cold pressed thermoelectric Mg 3 Sb 2

We intended to prepare Mg Sb compound bodies through solid state reactive sintering after cold-pressing mixtures of elementary Mg and Sb powders and investigated the crystal phases of the sintered bodies according to Mg/Sb mole ratios and reaction temperatures. The Mg Sb bodies sintered at the temperatures of 773~843 K showed typical crystalline phases of Mg Sb compounds, but their diffraction angles in XRD patterns were slightly different along with the vertical axis of the bodies obtained. All the bottom parts of the sintered Mg Sb bodies were composed of the typical crystalline phases of Mg Sb compounds and their diffraction angles were completely in accord with those of the α-Mg Sb phase, when Mg : Sb = 3.15 : 1.85 at 823 K, or when the Mg moles were greater than or equal to 3.10 at 843 K. It was considered that the slightly remaining Mg phases were formed by precipitation from α-Mg Sb phases during the solidification process of liquid phase.