Papers by Author: In Sub Han

Abstract: Silica wet gels were prepared from water glass (29 wt.% SiO2) by using Amberlite as a ion exchange resin. After washing in distilled water, the wet gels were further aged in a solution of TEOS/EtOH to strengthen the 3-dimensional network structure. With the increasing TEOS content in aging solution, BET surface area and porosity of the ambient dried silica aerogel were decreased, and the average pore diameter was decreased from 30nm to 10nm. Also, higher density and compressive strength were obtained in case of higher TEOS content. This is due to the precipitation of SiO2 nano particle by TEOS. Hence, TEOS addition plays an important role in both strengthening and increasing stiffness of silica wet gel network. By adding 30 vol.% TEOS, a crack-free monolithic silica aerogel tiles were obtained and its density, compressive strength, and thermal conductivity were shown 0.232 g/cm3, 7.3 MPa, and 0.029 W/mK, respectively.

Abstract: A study is made of the damage resistance and strength degradation of nitrided pressureless sintered (NPS) silicon nitride ceramics. The silicon nitride is prepared by cost-effective NPS process combining by nitridation and consecutive pressureless sintering. Contact testing with spherical indenters is used to characterize the damage response. Examination of the indentation sites indicates a quasi-plastic damage modes are observed. Bend tests on specimens containing quasi-plastic contact damages reveal those materials to be not susceptible to strength degradation.

Abstract: As an alternative to degassing pipe and rotor blade using in molten aluminum industry, we investigate the mechanical properties of silicon nitride ceramic components prepared by nitrided pressureless sintered (NPS) process, which process is the continuous process of nitridation reaction process combined with pressureless sintering. Mechanical properties of silicon nitride prepared by NPS process with sintering additives of 5wt% Y2O3, 5wt% Al2O3 and 20wt% Si show high strength, >500 MPa, high hardness, 12.6 GPa, and superior damage tolerances with high fracture toughness, 9.8 MPam1/2.

Abstract: Hydrophobic thin film silica aerogels were synthesized by ambient pressure drying method from silicic acid which was prepared from sodium silicate (water glass) solution. The pH value of the silica precursor sol was adjusted to make a spinable sol, and gel films were coated on a glass substrate by dip coating technique. Aerogel-like thin films with the thickness of about 1 μm could be successfully fabricated by repeating the dip coating process three times.

Abstract: High temperature structural ceramics based on Al2TiO5-ZrTiO4 (ZAT) having excellent thermal-shock-resistance were synthesized by a reaction sintering. The ZAT ceramics sintered at 1600oC had a negative thermal expansions up to 1000oC and a much lower thermal expansion coefficient (0.3 ~ 1.3 x 10-6 /K) than that of polycrystalline Al2TiO5 (1.5 x 10-6 /K). These low thermal expansion are apparently due to a combination of microcracking caused by the large thermal expansion anisotropy of the crystal axes of the Al2TiO5 phase. The microstructural degradation of the composites after various thermal treatment for high temperature applications were analyzed by scanning electron microscopy, X-ray diffraction, ultrasonic and dilatometer.

Abstract: Yttria stabilized zirconia (YSZ) films with the thickness of up to 12 μm were prepared on alumina and NiO-YSZ substrates by electron beam physical vapor deposition (EB-PVD). The films showed nano-scaled columnar structures depending on the substrate temperature. Electrical conductivity of the YSZ films on alumina was also investigated at the temperature between 700 and 1000oC in oxidizing atmosphere. High activation energy of the conductivity (>1.03eV) indicated that the conduction via grain boundary controlled the ionic conduction in the films prepared by EB-PVD. La0.6Sr0.4CoO3-δ as a cathode was applied on the YSZ/NiO-YSZ in order to evaluate the performance of the YSZ electrolyte.

Abstract: This paper presents the work on the development of ceramic coating processing. Nano-structured zirconia coating has been developed with functions; substrate temperature and oxygen gas change in chamber by electron beam physical vapor deposition (EB-PVD). The microstructure of the coating layer has been characterized with FE-SEM, and SEM. The crystalline phase of the coating layer has been also characterized with XRD. The zirconia coating by EB-PVD had not monoclinic zirconia phase as shown in XRD pattern and Raman spectra and the thickness of coating were quite homogeneous. The fracture microstructure of the coating layer for a thickness of ~15 μm showed columnar or non-columnar structure and had nano-structure with nano scaled grain as shown in micrograph by FE-SEM.

Abstract: Silicon nitride ceramics were prepared by new nitrided pressureless sintering (NPS) process in this study. The microstructures, strengths and thermal properties of the NPS silicon nitride ceramics containing three types of Al2O3 and Y2O3 sintering additives were investigated. Additionally, we have investigated the effect of silicon metal contents changing with 0, 5, 10, 15 and 20 wt% in each composition. The silicon nitride was successfully densified using NPS process, particularly at the starting composition of 5 wt.% Al2O3, 5 wt.% Y2O3, and 5 wt.% Si addition. The maximum flexural strengths and relative densities of these specimens were 500 MPa and 98%, respectively. The flexural strength of sintered specimens after the thermal shock test between 30oC and 1300oC for 20,000 cycles was maintained with the original laboratory strength of 500MPa by low thermal expansion coefficient, 2.9 × 10-6/oC, and high thermal conductivity, 28 W/m⋅oC.

Abstract: Two different types of carbon fibre bundles were used for filament winding to obtain C/C preforms. C/C-SiC composites were produced from the C/C preforms by a silicon melt infiltration technique. The effect of the type of carbon fibre bundle on the mechanical and thermal properties of the resultant C/C-SiC composites was compared. The spun fiber preform yields C/C-SiC composites of better mechanical properties than the unidirectional continuous fiber preform. The strength of the composites from the SFP was 1.8 times higher than that from the CFP. The flexural strength and the O-ring strength of the composites from the SFP with a density of 2.35 g/cm3 were about 160 MPa and 170 MPa, respectively.