Papers by Author: Soo Wohn Lee

Abstract: Two types of CuFe matrix composites with different reinforcements: silica and alumina particulates were developed using powder metallurgy. The mechanical properties were determined from Brinell hardness and flexural strength. The dry sliding friction and wear performance of the composites were investigated by the friction test. The results indicated that mechanical properties of alumina were superior to those of silica. The friction tests of the composites showed that the alumina reinforcement particle provides better braking performance. A wear track examination of composites showed that same abrasive wear. Our results indicated that composites with alumina reinforcement particles of high compatibility are to be preferred for braking performance.

Abstract: Gelcasting/pressure less sintered Al₂O₃/SiC nanocomposites has a low sinterability. Also, mechanical and wear properties of these nanocomposites was degraded. Wear mechanism of low sinterability gelcasting nanocomposites was dominated by fracture mode of surface during wear tests. In this study, gelcasting processing and followed plressureless (PL) and hot isostatic pressing (HIP) sintering was attempted to fabricate dense Al₂O₃/SiC nanocomposites. Wear behaviors of high densities gelcasting nanocomposites were investigated under the identical wear test condition. The comparative specimen was used to hot pressed nanocomposites. Wear rates of dense gelcasting nanocomposites were related to closely initial friction coefficient.

Abstract: Cross-linked polyethylene (XLPE) is a main kind of insulation materials, used in medium and high voltage power cable. High voltage XLPE cable production mainly adopts peroxide crosslinking method, and the performance of XLPE cable will be affected by decomposing by-products of crosslinking agent (DCP, dicumyl peroxide).In this paper, the XLPE cable was degassed with different times. The XLPE samples were analyzed by infrared spectrum analysis, DSC analysis and electron microscopy, and the influences of degassing on XLPE crystalline morphology were analyzed. The results showed that the crystal structure of XLPE in the cable was affected by degassing treatment. With the degassing treatment time went on, the by-products kept releasing. The content of by-products in 110kV XLPE cable decreased obviously in 2-7 days’ degassing treatment, and changed slowly after 7 days’. The growth and perfection of the original crystal in XLPE material were obtained, and the new small grains were formed in the material.

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: Oxide ceramics such as alumina and zirconia are industrially utilized as cutting tools, a variety of bearings, biomaterials, and thermal and corrosion-resistant coatings due to their high hardness, chemical inertness, high melting point, and ability to retain mechanical strength at elevated temperatures. In this research, the effect of other ceramic additives (TiO₂) and h-BN within alumina(α-Al₂O₃) and yttria-stabilized tetragonal (Y-TZP) composite was studied with respect to the mechanical and tribological properties. The lowest coefficient of frction of 0.45 was observed for the ZTA ceramic composite with hBN-TiO₂. The highest hardness, fracture toughness and flexural strength were obtained as 15.7GPa, 5.2MPam^-1/2, 712MPa, respectively.

Abstract: The coefficient of friction was very important factor for the applications of high temperature parts. In vehicles, the coefficient of friction was decreased due to lubricants as like engine oil etc. Lubricant such as oils is difficult to apply at high temperature. To apply high temperature parts, lubricants were demanded for high temperature stability. This work is to use the pulsed electric current sintering (PECS) technique and the atmospheric plasma spraying (APS) method in order to make self-lubricating Al₂O₃-15wt% ZrO₂-solid lubricant composites. We focused on the coefficient of friction for the fabrication method of self-lubricating Al₂O₃-15wt% ZrO₂-solid lubricant composites. We compared with the coefficient of friction of PECSed and APSed composites. The surface roughness of PECSed Al₂O₃-15wt% ZrO₂-solid lubricant composites were 0.06 ~ 0.31 μm of Ra and 10.16 ~ 33.12 μm of Ry. In the case of APSed Al₂O₃-15wt% ZrO₂-solid lubricant composites, as-coated samples were 6.56 ~ 11.42 μm of Ra and 59.68 ~ 81.79 μm of Ry, and polished samples were 1.12 ~ 3.70 μm of Ra and 11.66 ~ 32.22 μm of Ry. The coefficient of friction of PECSed and APSed Al₂O₃-15wt% ZrO₂-solid lubricant composites were 0.19 ~ 0.49 and 0.41 ~ 0.61, respectively.

Abstract: In this work it was carried out the synthesis of Bi2InTaO7 pyrochlore-type structure compound by the sol-gel method. By X-Ray powder diffraction the sol-gel compound calcined at 600°C showed the pyrochlore-type structure phase and by scanning electron microscopy its morphology revealed the presence of particles with size lower than 1 micron. Its chemical elemental composition was determined by EDS analysis. Results of SBET area and Eg values of this compound suggested higher properties than those obtained by the compound prepared by solid state reaction. Bi2InTaO7 sol-gel was used as active photocatalyst on the degradation reaction of organic compounds such as Alizarin Red S, and Red and Green Tide, under UV-light. Half time life results showed that Bi2InTaO7 sol-gel compound could be a good candidate for the degradation of organic compounds because after few minutes showed high efficiency on the degradation reaction of Alizarin Red S (t1/2 = 30 min), and Green Tide inactivation (t1/2 = 17 min).

Abstract: The addition of h- BN to a polycrystalline Si3N4 was to increase the fracture toughness and other mechanical properties such as flexural strength and hardness of the material. The hot pressed samples were prepared from the mixture of α-Si3N4, AlN, MgO and h-BN. The composite contained from 0 to 2 wt.% BN powder with sintering aids (9% AlN + 3% MgO). The transparency, mechanical properties and microstructure of hot pressed polycrystalline Si3N4-BN composite materials were investigated by UV/VIS spectrophotometer, scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. The transparency decreased with increasing the content of h-BN into Si3N4.