Papers by Author: Young Rae Cho

Abstract: This study investigates the microstructure, tensile properties, and high-cycle fatigue resistance of twin-roll-cast Mg-3Al-1Zn (wt%) alloy strips with thicknesses of 1 mm, 1.5 mm, and 3 mm. The investigation results reveal that the 1-and 1.5-mm-thick strips show a fully dynamically recrystallized (DRXed) microstructure consisting of fine equiaxed DRXed grains, whereas the 3-mm-thick strip shows a partially DRXed microstructure containing very coarse elongated unDRXed grains because of the insufficient strain imposed during twin-roll casting. The inhomogeneous microstructure of the 3-mm-thick strip leads to a large deviation in its tensile elongation. The average grain size of the strips increases with increasing strip thickness, which results in reductions in both their tensile strength and their ductility because of the weakened grain-boundary hardening effect and the promoted formation of undesirable twins, respectively. The high-cycle fatigue resistance in the stress regime with finite fatigue life is similar for all three strips, but the fatigue strength with infinite fatigue life decreases from 175 MPa to 140 MPa as the strip thickness increases from 1 mm to 3 mm. The fatigue strength (FL) increases linearly with increasing yield strength (YS) according to the relationship FL = -199.5 + 2.03·YS.

Abstract: Anodic oxide films were formed on titanium using an electrolyte solution containing phosphoric acid for dental implants by an electrochemical method. The parameters of the oxidation process were current density and electrolyte concentration. The micro structural morphology, surface roughness, thickness of the oxide layer and the behavior of voltage-time curves were examined. The results show that the surface roughness, porosity size and the thickness of the oxide layer showed similar trends as the process parameters were changed. The surface morphology of the samples was observed for different current densities at a constant electrolyte concentration. The pore size, thickness and roughness of the anodic oxide films are directly proportional to the maximum voltage and the slope of the voltage to time transient (dV/dt) curves during the growth of the galvanostatic anode oxide film on plate type Ti.

Abstract: The mechanism of failure of patterned ITO electrodes on PET was investigated under static and cyclic bending in the mechanical stress mode. When mechanical stresses are increased, two types of micro-cracks are observed depending on the stress modes. The direction of the micro-crack in ITO electrodes is perpendicular to the load for the static mode. The electrical resistance of an ITO electrode is related to the density of the micro-cracks and failure mechanism of ITO electrode.

Abstract: CrN-based multi-component coatings were deposited by a hybrid coating system combining the arc ion plating (AIP) and sputtering technique. In this work, comparative studies on microstructure and mechanical properties of microhardness and wear behaviors among Cr-C-N, Cr-Si-N, and Cr-Si-C-N coatings were systematically conducted. Adding carbon and silicon atoms into CrN coatings had large effects on microstructural change and mechanical properties of CrN coatings. The hardness value of Cr-Si-C-N coatings showed about 44 GPa, while those of Cr-Si-N and Cr-C-N coatings were 34 and 23 GPa, respectively. The average friction coefficient of CrN-based coatings decreased from 0.65 to 0.4 with the incorporation of silicon and carbon content.

Abstract: The Platinum catalysts on the carbon nanotubes(CNTs) supports of various diameters were prepared by wet impregnation method using H2PtCl6 precursor. The samples using 100nm, 15~20nm, 10~15nm and 5~10nm diameters of CNTs and carbon nanofibers(CNFs) are named Pt/t- CNFs, Pt/MWNTs20, Pt/MWNTs10 and Pt/MWNTs5, respectively. The effects of CNTs diameter on the Pt particle size and distribution were investigated by the means of powder XRD and TEM observation. In addition, the electro-catalytic characteristics for methanol electro-oxidation were estimated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques. The average size of Pt particles increases as follows; Pt/MWNTs10 < Pt/MWNTs5 < Pt/MWNTs20 < Pt/t-CNFs. The electro-catalytic characteristics of Pt/MWNTs10 and Pt/MWNTs20 are found to be superior in comparison with the others. For preparation of the most effective supported Pt catalyst, the optimum diameter of CNTs support in the range of 10-20nm, is needed.

Abstract: The influence of annealing conditions on the crystallization and wear properties of bulk amorphous Zr41.2Ti13.8Cu12.5Ni10Be22.5 (BAZ) alloys was investigated. Nanocrystalline phases were precipitated from the bulk amorphous alloy matrix after an annealing treatment at both 670 K and at 780 K. The Vickers hardness (Hv) increased with the volume fraction (Vf) of the nanocrystalline phases with increasing annealing temperature. The friction coefficient and the wear loss showed a somewhat different trend with the volume fraction of the nanocrystalline phases. The friction coefficient had a minimum value of 0.46 at 670 K and a maximum value of 0.96 at 780 K. The wear loss of the BAZ showed a very similar trend of friction coefficient. The improved wear and hardness properties can be attributed to the formation of nanocrystalline precipitates in the Zr-based amorphous matrix.

Abstract: In this study, a 10"-sized panel with novel tetrode structure was tried to prevent broadening of electrons emitted from CNTs. The structure of the novel tetrode is composed of CNT emitters on a cathode electrode, a gate electrode, an extracting electrode coated on the top of a hopping electron spacer (HES), and an anode. HES contains funnel-shaped holes whose inner surfaces are coated with MgO. Electrons extracted through the gate are collected inside the funnel-shaped holes and hop along the hole surface to the top extracting electrode. The effects of HES on emission characteristics of field emission display (FED) were investigated. An active ozone treatment for the complete removal of residues of organic binders in the emitter devices was applied to the FED panel as a post-treatment

Abstract: Carbon nanotubes have attracted considerable attention because their high aspect ratio leads to a large electric field enhancement and a low operating voltage. The dependence of environmental gases such as Ar, O2 on the emission current from carbon nanotube emitters was examined in this study. Based on our experiments, the current density is decreased in single-wall carbon nanotubes (SWNTs), but is increased in multi-wall carbon nanotubes (MWNTs) as the vacuum level decreases from 10-7 Torr to 10-4 Torr by the inflow purging gases. The current density subsequently recovered as the vacuum level increased to 10-7 Torr when gas inflow stopped. From those results, we conclude that the MWNTs have completely different degradation characteristics in comparison to SWNTs. The increased current density of an MWNT in the high-pressure range of 10-4 Torr is a significant finding and would be widely applicable to low cost vacuum packages.

Abstract: The effects of magnetic annealing on a magnetiostriction in commercial grain-oriented 3.2 at.% Si steels were investigated. A combined (constant+strong pulsed) magnetic field during the magnetic annealing played a significant role in reducing the magnetostriction. The reduction ratio in magnetostriction was greatly dependent on surface conditions such as whether it was a bare metallic, tension coated and laser-scribed surface for grain-oriented electrical steels. For all three samples, the effect of the magnetic annealing on reduction in maganetostriction is more clearly observed in the compressive stress rather than in the tensile stress region.