Papers by Author: Sung Hun Cho

Abstract: With respect to metal doping into TiO₂, the doping limits for V, Cr, Zr, Nb, Mo, and W are predicted to be higher than other transition metals according to the binding energy calculations in a unit cell model of anatase TiO₂, which suggests that Cr and W can be doped into anatase structure more easily than Ag or Pt, for example. Our investigation has twofold research objectives. One is to prepare metal-doped TiO₂ nano-powders from these transition metals, and the other is to test photocatalytic activity of each resulting powder. For the former, sonochemical process has been used to produce Cr-doped, W-doped, Ag-doped, and Pt-doped TiO₂ nanoparticles. For the latter, we have performed photocatalytic oxidation of methylene blue. The combined results of the morphology and photocatalytic activites have enabled characterization of the physicochemical properties of these transition metal-doped nanoparticles.

Abstract: Sol-gel with sonochemical process and sol-gel method were used for the fabrication of photocatalytic TiO₂ powder and the properties of the TiO₂ powder thus prepared were examined and compared. For the synthesis of TiO₂ powder, the starting material was Ti-butoxide and H₂O and the reaction was carried out at 80 °C in a sol-gel method. On the other hand, Ti-butoxide was reacted with H₂O in an ultrasonic frequency of 23 kHz with ultrasonic powers of 800 W for two hours. After evaporation, 24 hours aging time was kept at 400 °C in both cases, the final product appeared was the anatase phase. Ultrasonic chemical method was found to be excellent for flocculation and crushing effects where as in sol-gel method, the product obtained was of high purity and high homogeneity. Sol-gel with sonochemical process has found to be excellent for flocculation and crushing effects. From Sol-gel method, the size of TiO₂ particles produced were found to be about 20 nm, spherical, homogeneous while severe agglomeration. In contrast, by sonochemistry method, the size of TiO₂ particles produced were about 10 ~ 22 nm size and it has shown more photocatalytic activity than sol-gel sample which is about 28.7 % greater efficiency

Abstract: Alumina (Al₂O₃)-zirconia (ZrO₂)-hydroxyapatite (HA) composite is used as biomaterials. Biomaterials must be bio-compatible to the body and should have excellent mechanical and tribological properties. 1 wt.% of rare earth oxide (Dy₂O₃) was added to the basic composition of composite Al₂O₃ -ZrO₂ (15wt.%)-HA(1wt.%). The powder mixture was hot-pressed at 1300°C in Ar gas environment under 30 MPa pressure for densification sintering. The microstructural analysis, phase transformation, surface morphology and mechanical properties like hardness, fracture toughness, mechanical strength of the composite were observed and evaluated by SEM , XRD, Vickers hardness, etc. The results demonstrated that the addition of 1 wt.% dysprosia has increased the mechanical properties of composite.

Abstract: High temperature low friction materials are sought for use in engines in order to reduce energy consumption of the machines. Due to the high service temperatures solid lubricating materials are necessary. This study is designed to find the optimal processing conditions for preparing these materials by pulsed electric current sintering. In this study, the Al₂O₃ - 15wt% ZrO₂ (AZ) nanocomposite was modified with 3 wt% of self-lubricating component (CaF₂, BaF₂, MoS₂, WS₂, h-BN, or graphite). After the preparation of the alumina-zirconia powder mixture solid lubricant powder was added. Powders were then mixed in ethanol for 24 h, dried in a rotary evaporator, and in oven at 80°C for 24 h. The particle size distribution of the powders was established with the laser method. Powders were compacted by using pulsed electric current sintering technique at 1300 °C with 50 MPa for 5 min in vacuum. The structure of the materials was studied with XRD and SEM. Density of the compacts was measured with the Archimedes method and their hardness was evaluated by applying HV1 hardness with the instrumented indentation techniques. Their mechanical behavior was further studied with the instrumented scratch testing.

Abstract: Ni-SiC nano composite coatings were fabricated using electrodeposition technique with the aid of ultrasound. The properties of the nano composite were investigated using SEM and Vicker’s microhardness tester. The results demonstrated that the microhardness of composite coatings under ultrasonic vibrations was improved significantly as compared to conventional electrodeposition techniques without ultrasound. The nano particles were found to be distributed homogeneously with reduced agglomeration due to the ultrasonic vibration.

Abstract: The ceramic channel is very important in SRL hot dipping system. High strength and fracture toughness of ceramic channel materials can improve the quality, productivity and economic feasibility of zinc plated steel. The purpose of this research was to find out the most suitable conditions of the ceramic channel that have best fracture strength and fracture toughness. The hot pressed composite materials was carried out by hot pressing Al2O3 with different content of ZrO2. The composite contained from 0-20 wt.% ZrO2. Hot pressed composite materials were observed for mechanical properties (density, hardness, fracture toughness and flexural strength) and microstructure.

Abstract: Solid lubricants were suited for the applications of high temperature parts. Recently, researches for self-lubricating ceramic matrix composites using solid lubricants, such as MoS2, WS2, h-BN, and graphite etc., became active. This work is to use the spark plasma sintering (SPS) technique in order to make self-lubricating Al2O3/15wt% ZrO2 nanocomposites with higher mechanical properties. To optimize the self-lubricating Al2O3/15wt% ZrO2 nanocomposite systems were added with different solid lubricants (CaF2, BaF2, MoS2, WS2, h-BN, and graphite). The coefficient of friction of CaF2, h-BN and graphite added composites in steady-state at room temperature are below 0.3.

Abstract: Nano-technology is a super microscopic technology to deal with the structures of 100 nm or smaller. This technology also involves the developing of TiO2 materials or TiO2 devices within that size. The aim of the present paper is to synthesize the silver doped nano-TiO2 by Sonochemistry method and to evaluate the effect of different percentages (0.2-1.5%) of silver load on TiO2 in the algae inactivation, The sample were characterized by using different techniques as XPS, TEM, SEM. Photo-catalytic activity of Silver doped TiO2 were evaluated through the inactivation of red tide (Cochlodinium) using UV-irradiation (315-400nm). The best result was found with silver doped TiO2.

Abstract: The commercial nano-sized TiO2 powder (P25) coated with PtO2 powder by a sonochemistry method was immobilized on the foamed waste-glass substrates by using plasma spray coating technology. The immobilized TiO2-Pt coatings were applied to treat both green tide and red tide under illumination of an UV light. Their photocatalytic characterizations were evaluated as functions of the number of residual algae, transmission as well as pH value with illumination time. It’s found that the plasma sprayed Pt-doped TiO2 nanostructured coatings on foamed waste-glass showed higher photocatalytic activity, which was effective to treat the green tide and red tide.