Papers by Author: Do Kyung Kim

Abstract: Zirconium diboride based composites containing silicon carbide with relative densities in excess of 99 % were produced by hot-pressing. Oxidation test was conducted in air at 1500 °C. ZrB2-SiC composite showed relatively low oxidation resistance due to the non-uniform surface silica-rich layer. But in case of mixed boride-SiC composites further improvement of the oxidation performance were observed due to the phase separation in the surface silica-rich layer.

Abstract: The dielectric properties of -SiAlON and various cations doped -SiAlON bulk ceramics prepared by a hot-press method were investigated. The dielectric properties (dielectric constant and tangent loss) were characterized by a post-resonator method (Hakki-Coleman method) at room temperature in the microwave frequency range. The effect of z-values about -SiAlON was examined, and also the effects of various interstitial cations on dielectric properties of -SiAlON were studied. Dielectric properties of -SiAlON were compared with those of Si3N4 and -SiAlON and their relationship between the dielectric properties and the cationic species of SiAlON were discussed.

Abstract: -SiAlON with various z-values (z = 0.5~4.0) were produced by hot pressing. The dielectric properties (dielectric constant and tangent loss) of -SiAlON were characterized by the post-resonator method at room temperature and by the perturbation method from room temperature to 1200 oC at 2.45 GHz, respectively. Effect of z-values and temperatures with -SiAlON were investigated.

Abstract: The high thermal conductivity of Aluminum nitride, coupled with its high electrical resistivity and nontoxic nature, makes it a very promising material for electronic substrate. In this study, microstructural characterization on the thermal conductivity of AlN ceramics was investigated. An AlN ceramic was prepared with a dopant Y2O3 under a reducing nitrogen atmosphere with carbon. In order to obtain high thermal conductivity, cooling rate control and after-heat treatment was carried out. Morphology of the second phase was characterized using scanning electron microscopy (SEM). SEM studies showed that the microstructural change caused by after-heat treatment have a major influence on the thermal conductivity.

Abstract: The mechanical properties of brittle coating structures were characterized by various indentation techniques. The adhesion properties of the coatings were evaluated by in situ scratch and sphere indentation method. Physical vapor deposited TiN coatings on transparent substrates, sapphire, were scratched by diamond cone indenter and in situ observed through the transparent substrate. In situ scratch results reveal that the failure of coating is originated from the damage of the substrate and the plastic deformation of substrate is a primary factor for determining the adhesion breakage. The unique characterization technique for the strength measurement of brittle thin coating has been developed. The strength of the thin coating was evaluated by the sphere indentation on the trilayer structure. The CVD SiC coatings on graphite were characterized by the technique. It is concluded that the microstructure of SiC coatings influences the strength. In this paper, the various indentation technique were applied to evaluate the mechanical properties of TiN and SiC coatings and the effect of microstructure on the reliability of the brittle coating system was discussed.

Abstract: The dielectric properties of alumina ceramics have been measured using a free-space time-domain technique from room temperature to 1400 oC in the frequency range 8.2 – 12.4 GHz. The effects of thickness and lateral size of specimen were investigated with comparing the measured values to the calculated ones based on the half-wavelength and the Gaussian beam focusing. From these results the optimum specimen dimension for the high temperature dielectric measurement was suggested with experimental verification.

Abstract: Large areas of one-dimensional ZnO nanostructures (nanorods and nanowires) were prepared by simple solvothermal method. The synthesis involves a template-less and surfactant free aqueous methods. In this study, Zn metal sheet was used as both Zn2+ ion source for the growth of ZnO nano-structures and substrate for oxide film. The mixed solvent of 1-propanol with water was used as solvents for nanorods and ethylene glycol with water was for nanowires. The solvothermal treatment of a zinc metal plate with the solvents at the temperature of 125 oC for 10 hours resulted in the 1-D ZnO nanostructures, which were grown on the Zn metal sheet. The synthesized ZnO was well aligned on whole area of substrate and the nanorods were 150 nm in diameter and 3 μm in lengths, respectively.

Abstract: Two kinds of nano crystalline Gd-doped CeO2 particles were successfully synthesized via glycine-nitrate combustion and neutral precipitation with subsequent hydrothermal crystallization. It was revealed that the surface modification of nano particles significantly affected sintering behaviors and resulting physical properties. Glycine/nitrate ratio was a key parameter to control the surface state of nano particles in the glycine-nitrate process, but, in the neutral precipitation process, the solvent was important. To obtain the GDC nano particles with chemically and physically favorable properties, the effect of the surface state of nano particles on the sintering behavior was discussed with consideration of aggregation property and surface-adsorbed substances on nano particles.

Abstract: Contact damage resistances of silicon nitride ceramics with various grain boundary phases are investigated in this study. The grain boundary phases are controlled by the addition of different types of sintering additives, or the crystallization of intergranular phase in a silicon nitride. We control the microstructures of materials to have similar grain sizes and the same phases to each other. Contact testing with spherical indenters is used to characterize the damage response. The implication is that the grain boundary phase can be another controllable factor against contact damage and strength degradation even though it is not critical relative to the effect of grain morphology.

Abstract: Hertzian and explosive indentations were used to determine the damage behavior of SiC and Si3N4 ceramics. Specimens were selected with different microstructures. In order to observe the subsurface damaged zone, the bonded interface technique was adopted. It was found that the damage response depends strongly on the microstructure of ceramics. Examination of subsurface damage reveals a competition between brittle and quasiplastic damage mode: brittle fracture mode is dominant in fine grain microstructure; quasiplastic deformation occurs in coarse grain. Dynamic indentation induces subsurface yield zone which contains extensive micro-cracks. The role of microstructure on static and dynamic damage behavior are discussed in terms of the weakness of grain boundary and grain size.