Papers by Author: Hai Doo Kim

Abstract: In this study, two-dimensional (2D) ordered macroporous SiCN ceramics were prepared by infiltrating sacrificial colloidal silica (SiO2) templates with the low molecular weight preceramic polymer, polysilazane. This was followed by a thermal curing step, pyrolysis at 1250°C in a N2 atmosphere, and finally the removal of the templates by etching with diluted HF. In particular, 100large-scale monolayer silica crystals were prepared on the cleaned Si substrates by spin coating. Two-dimensional SiCN ordered pore arrays were fabricated by a solution-dipping template strategy.

Abstract: The interaction between gas atmosphere and silicon during reaction bonded silicon nitride(RBSN)process leads to a non-uniform band formation of alpha silicon nitride. The reaction layer, α -Si3N4, was formed near the surface of the sample in the early stage of RBSN. Reactive nitrogen gas was supplied as static state using computer controlled gas delivery system. The formation of α -Si3N4 band near the surface of the sample can be explained thermodynamically, based on the nitrogen partial pressure in the gas mixtures.

Abstract: Using 6wt%Y2O3-2wt%Al2O3 as sintering additives and Si as a raw powder, the continuously porous in-situ Si2N2O-Si3N4 bodies were fabricated by multi-pass extrusion process and their microstructures were investigated depending on the addition of carbon (0-9wt %) in the mixture powder. The introduction of Si2N2O fibers observed in the unidirectional continuous pores as well as in the pore-frame regions of the nitrided bodies can be an effective method in increasing the filtration efficiency. In the case of no carbon addition, the network type Si2N2O fibers with high aspect ratio appeared in the continuous pores with diameters of 150-200nm. However, in the case of 9wt% C addition, the fibers were found without any network type and had diameters of 200-250nm.

Abstract: Prediction of porosity level for the Si3N4 filter substrate manufactured by reaction bonding of Si compact and subsequent post-sintering was attempted and its validity was verified by comparison between calculation and measurement. Both are well matched in the specimens of asnitrided and low temperature post-sintering, while few percents higher measurement in high temperature post-sintered specimens resulted from weight loss during the process.

Abstract: Processing techniques for producing microcellular silicon carbide, mullite, and cordierite ceramics with cell densities greater than 108 cells/cm3 and cells smaller than 30 μm have been developed by a reaction method that incorporates a polysiloxane and reactive fillers. The techniques developed in this study offer substantial flexibility for producing microcellular ceramics whereby cell size, cell density, degree of interconnectivity, composition, and porosity can all be effectively controlled. It is demonstrated that the adjustment of filler composition enables the possibility of tailoring the composition and properties of the microcellular ceramics. The present results suggest that the proposed novel processing techniques are suitable for the manufacture of microcellular ceramics with high morphological uniformity.

Abstract: Recently, a novel processing route for fabricating microcellular ceramics has been developed. The strategy for making the microcellular ceramics involves: (i) forming some shapes containing a mixture of preceramic polymer, expandable microspheres and optional fillers by a conventional ceramic forming method, (ii) foaming the compact by heating, (iii) cross-linking the foamed body, and (iv) transforming the foamed body into microcellular ceramics by pyrolysis. The flexural strength and compressive strengths of the microcellular ceramics were investigated; values up to 30 MPa and 100 MPa, respectively, were obtained at room temperature. The superior mechanical properties were attributed to homogeneous distribution of cells in microcellular ceramics.

Abstract: The b–Si3N4 whisker randomly and unidirectionally reinforced Si3N4 composites were fabricated respectively. The high whisker alignment was obtained by extrusion processing. The R-curve behaviors were studied for the Si3N4 ceramics with addition of 3wt%, 5wt% and 10wt% b–Si3N4 whisker seeds respectively, and the effects of whisker orientation and content on the R-curve behaviors were analyzed and discussed. The ceramics with different whisker content and orientation showed well-defined R-curve behaviors. However, for the whisker random distribution, as its whisker content increased from 3wt% to 10wt%, the corresponding fracture toughness changed at an irregular trend; for the whisker unidirectional reinforcement, the fracture toughness in the direction perpendicular to whisker alignment was considerably improved, and most of indentation impressions tend to be very abnormal. The effects of whisker orientation and contents on the R-curve behaviors of the Si3N4 ceramics were further discussed.

Abstract: Porous silicon nitride was prepared by extrusion of silicon followed by nitridation at 1723 K. PMMA spheres with 20 µm in diameter were employed as the pore-forming precursors. b-silicon nitride whiskers were added to the dough for extrusion and their effect on the properties of porous RBSN were examined. The nitridation rate that was obtained from the weight change of the sample due to the nitridation process was between 75% and 80%. However, XRD patterns of the samples after nitridation had no Si peak. That means the actual nitridation rate of the sample was higher than that obtained from the weight change. Porosity of the sample was between 45% and 55%. The XRD patterns from the surfaces of the samples with the silicon nitride whiskers parallel and perpendicular to the extrusion direction showed a slight anisotropy. The pore size distributions of the samples showed a highly populated pores smaller than 3 micrometer, especially for the samples with the whiskers. The room temperature flexural strengths of the samples were between 25 MPa and 35 MPa, the sample with 5 wt% whiskers showing the highest value. The microstructures of the samples contained pores with about 100 micrometer in diameter as well as fine pores with a few micrometer in diameter. Closer observation of the fracture surface of the samples revealed that fine whiskers were inside the pores. A small honeycomb was fabricated by reaction bonding of silicon.

Abstract: In this study, SHS process has been employed to fabricate porous MoSi2 material with electric-resistive heating capability through the control of pore size. The preform for SHS reaction was consisted of molybdenum powder with different sizes and silicon powder with different contained quantity. The size of the MoSi2 particles thus formed was determined by the generated heat of combustion, not by the size of molybdenum powder. However, the pore size of MoSi2 composite was proportional to the particle size of molybdenum powder. That is, the coarser the molybdenum powder used, the larger the formed pore size. Based on these results, the porous MoSi2 composite could be fabricated with a desired pore size. By orienting the porous molybdenum disilicide-based material in the form of pore size gradient, porous materials used for filters with improved dirt-holding capacity can be manufactured.

Abstract: In this study, SiC whiskers were grown in porous alumina substrate in order to enhance the filtering efficiency, performance, and durability by controlling pore morphology. This experiment was performed by chemical vapor infiltration (CVI) in order to obtain the whiskers on the inside of pores as well as on the surface of porous the Al2O3 substrate. The deposition morphology was changed remarkably with the deposition position and temperature. First, the mean diameter of whisker was decreased as the position of observation moved into the inside of substrate due to ‘the depletion effect’ and ‘the pressure effect’. Second, the deposition temperature caused the changes of the deposition type such as debris, whiskers and films and these changes of morphology affect the various properties. When SiC films were deposited, the gas permeability and the specific surface area decreased. However, the whisker showed the opposite result; a large specific surface area provides the absorption site and the whiskers in gas traveling path hinder the particles from easily flowing. Comparing with the normal pores (inter-grain open pores), the pores formed by the whiskers have relatively large volume fraction under the same pore size. Porous ceramic filters with whisker will be expected to increase the filtering efficient and gas permeability simultaneously. It is the main advantage of our whiskered filter. Therefore the porous alumina body which deposited the SiC whisker will be the promising material in order to apply to the particulate filter.