Papers by Author: J.H. Lee

Abstract: The anodic titanium dioxide (TiO2) films for photocatalyst were prepared by anodization in acid solution. The characteristics of surface layer and the photocatalytic efficiency of the anodic film have been investigated. The chemical states of anodic film were analyzed using XPS, and the photocatalytic efficiency was evaluated by the degradation rate of aniline blue. From XPS results the component elements of the electrolyte, P and S, were observed in the anodic film, which were incorporated from the electrolyte into the oxide layer during anodization. The porous TiO2 film exhibited high photocatalytic efficiency for degradation of aniline blue.

Abstract: This study placed a focus on fabrication factors and surface morphology characteristics to obtain anodic alumina layers with ordered nanopore arrays using two-step anodization process. The regularity of nanopore arrangement in anodic alumina was prepared in oxalic acid at 40 V. The regular array was shown to be more strongly dependent on the time of first anodization than that of second anodization. The widening process of oxide surface in H3PO4 solution clearly exhibits that the nanopore diameter of the alumina layer subjected to widening time increased linearly at a rate of 0.6 nm/min at 30°C, and 0.8 nm/min at 40°C. The pore arrays in anodic alumina were analyzed by SEM.

Abstract: The increase of capacitance on dielectric layer is associated with enlargement of etched surface area of electrodes for aluminum electrolytic capacitors. Etched tunnels extending along [100] directions are formed by crystallographic dissolution of aluminum in a chloride solution. This study has been focused primarily on the effects of addition of inhibitors to hydrochloric acid solution on the increase of the surface area by Electrochemical Impedance Spectroscopy (EIS). The addition of 3 % ethylene glycol was more effective for the enlargement of the surface area comparing with those of 1M sulfuric acid and no additive. And it was observed by TEM that the tunnels were grown straight and parallel to [100] directions.

Abstract: The microstructure and photocatalytic characteristics of anodic TiO2 film have been investigated for applications in waste water treatment. The major structure of the fabricated anodic TiO2 film consisted of porous cell structure, of which the main crystal structure bas been found to be anatase type. The photocatalytic efficiencies of anodized TiO2 were evaluated by the degradation rate of aniline blue and the chemical states of anodic film were estimated by P2P spectrum. The reaction order and the rate constant for degradation of dye have been determined to be first dimension and 1.608410-4s. Due to the high bonding strength between TiO2 film, and the substrate and the high specific surface area, the anodized TiO2 can be used as efficient photocatalyst with high reactivity for waste water treatment and water purification.

Abstract: The influence of thermal fatigue test temperature on crack propagation behavior of the surface treated tool steel for die-casting was investigated. For this purpose, thermal fatigue system consisted of induction heating and water spray quenching unit was constructed to simulate the service condition and Lm is proposed as the index representing the susceptibility to crack initiation and propagation. The thermal fatigue tests were conducted at the maximum temperature of 600°C, 700°C and 720°C using as-heat treated or nitrided specimens. The ion nitrided specimen showed lower Lm value than as-heat treated at all test temperature. But in the case of maximum and average crack length, the ion nitrided specimen exhibited higher value than those of as-heat treated specimen.

Abstract: The dimension of cold forged part is larger than that of the die cavity. The difference could be originated from the various features, such as the elastic characteristics of die and workpiece, thermal influences, and machine-elasticity. Among these features, elastic behavior of die and workpiece has been considered as the most important element for the precision cold forging. In the present study, the effect of material properties of both workpiece and die on the dimensional precision of forged part was investigated using FE and experimental analyses. Three materials, such as SCM420H, Cu-OFHC and Al6061 alloy, were used to divide the effect of the elastic modulus and flow stress on the dimension specifically. From the results of FEA, the elastic deformation of die was found to be more dominant than that of workpiece. The die expansion depends on the flow stress of workpiece during a loading stage. On the other hand, the die contraction during unloading was affected by both the flow stress and elastic modulus of workpiece. The elastic modulus of workpiece affects the elastic recovery of forged part after ejecting stage.

Abstract: Thermomechanical behavior of Al-Mg-Si alloys has been studied to investigate the effect of microstructural features such as pre-existing substructure and distribution of particles on the deformation characteristics. The controlled compression tests have been carried out to get the basic information on how the alloy responds to temperature, strain amount and strain rate. Microstructural features after forging have been discussed in terms of the thermomechanical response of Al-Mg-Si alloys. As already well mentioned, we have found that the high temperature deformation of Al-Mg-Si brought the recovered structure. However, the abnormally large grains seems to occur in a certain condition as a result of deformation assisted grain growth, which means that hot forging of Al-Mg-Si alloys could lead to the undesirable microstructures and the consequent mechanical properties such as fatigue strength.