Papers by Author: Dong Pyo 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: Non-oxide ceramic SiCN possesses unique physical and chemical properties, which make it a suitable material for various applications such as catalyst support, high temperature semiconducting devices, and metal or polymer matrix composites. In this work, SiCN foams with pore size of 500 and 1000 nm and high BET surface area of 184.5 and 71 m2/g were prepared using preceramic polymer, polyvinylsilazane, as a precursor and polystyrene sphere packed bed as a self-sacrificial template. The polystyrene sphere packed bed was prepared by the natural sedimentation of spheres. After the infiltration and curing of viscous preceramic polymer, the sample was pyrolyzed at 1000 °C under N2 atmosphere to produce the ceramic foams. It is a very simple synthesis because in the process of pyrolysing preceramic polymer, the polystyrene sphere template was removed simultaneously by pyrolysis with no additional etching step. SEM, N2 adsorption-desorption measurement and TGA were used to characterize the obtained samples. It was concluded from the TGA measurement that the obtained SiCN foam possessed very high thermal stability at 1000 °C under air.

Abstract: Interests on the fabrication of microfluidic devices have increased in the fields of micro total analysis system (μ-TAS) and MEMS (Microelectromechanical systems) due to their chemical inertness and high thermal stability. The thermal characterization of the SiCN preceramic polymer, polyvinylsilazane, showed that the cured polymer has ceramic properties at heat treatment temperature of 600 oC or above. In the characterization of the mechanical properties, the characteristic values of the elastic modulus and hardness notably increased for the heat-treated SiCN. The present study describes the preparation of nano-sized patterns and microfluidic channels using a soft lithographic technique. The study shows that the fabrication of microchannels using the cured inorganic polymers holds tremendous potential in the field of microfluidics, where materials with high optical transparency, thermal stability and chemical inertness are in demand as niche between conventional microfluidics using glass and polymeric materials.

Abstract: This paper reports fabrication of high temperature stable, chemically inert SiC and SiCN monolithic porous microchannels by micromolding in capillaries (MIMIC) method. These types of high surface area materials hold tremendous untapped potential in micro-total analysis systems, micro-reaction technology and clean energy systems. Owing to their excellent durability under harsh conditions, SiC and SiCN micro-components are imperative in fields like fuel cells, for hydrogen generation and as environmental sensors. The report also describes the use of template particles of different size and type with diameter of ranging from 0.5 to 1.5 µm to prepare porous, high temperature stable ceramic microchannels. In the present work, ceramic porous channels have been fabricated via polymer to ceramic conversion route by using commercially available polymer precursors.

Abstract: Microelectromechanical systems (MEMS) including microchemical devices have been widely concerned, in particular, when made of an extremely stable ceramic material for their use at harsh conditions. SiCBN preceramic polymer was derived from borazine (B3N3H6) modified allylhydridopolycarbosilane (AHPCS) via hydroboration of allyl groups with B-H groups at 0oC for 24 h by following the standard Schenk technique. The extent of polymer-to-ceramic conversion with an ultimate ceramic yield of 92 wt% was investigated by simultaneous TGA, 1H, 13C-NMR, IR, and XRD measurements. The polymer-derived SiCBN ceramic remained as an amorphous solid up to 1400oC. The ceramic product obtained after pyrolysis at 1000-1400oC exhibited excellent oxidation resistance in air. In addition, ceramic microstructures were fabricated by employing soft lithographic technique.

Abstract: Silver-containing porous frameworks have been extensively used in catalysis, electrochemistry, for heat dissipation and biofiltration. In this study, porous silver sponge was prepared by using a sacrificial macroporous carbon template. Aqueous AgNO3 was infiltrated into the carbon template and reduced to Ag by borohydride reduction reaction, followed by burning of the sacrificial template in air at 500~650°C. The products were characterized by SEM, XRD, and N2 adsorption-desorption isotherms. The Ag sponge exhibited a BET surface area of 12 m2/g.

Abstract: Nano-scale SiC-based ceramic patterns on Si substrates were fabricated via imprint lithography technique by using viscous polyvinylsilane as a ceramic precursor and economic nano-scale master such as CD, followed by pyrolysis at 800oC under nitrogen atmosphere. The thickness of residual layers was controlled by varying the spin-coating conditions (solution concentration, spinning speed) and the patterning conditions (molding pressure). In addition, for the effective removal of the remaining residual layer, the etching kinetics of both polymeric and ceramic patterns was also comparatively studied by Ar or reactive ion etching process.

Abstract: Formation of coatings with combined properties of being superhydrophilic / superhydrophobic is of immense applications in the field of optics. In our present work, we developed a hydrophilic, antifogging, transparent, and stable coating by employing an anionic electrolyte, Tiron and dip-coating of organic-inorganic mixture. One of the attempts was to adhere Tiron on metal oxide surface by chelating effect, and the other one was to apply an organicinorganic coating on glass or lens substrate. The latter was formulated by mixing appropriate ratios of organic silanol coupling agents and diacrylates component for strong adhesion to the substrate, and inorganic component of a hybrid mixture of silica, titania and Tiron for the antifogging behavior. The surface chemistry of the coated films was studied by taking ATR-IR measurements, and the wetting behavior was characterised by contact angle measurement

Abstract: Macroporous SiCBN and BCN ceramic were prepared by infiltrating these preceramic polymers into the colloidal silica crystalline array, followed by pyrolysis at 1400ı and subsequently etching off the silica spheres. It is generally observed by SEM and TEM that a highly ordered and interconnected 'honeycomb' pore structure was obtained by replicating the three dimensional close packed silica spheres which was fabricated by natural sedimentation. And BET surface areas in range 413 ~ 315 m2/g and pore sizes of 113 ~ 395 nm in the porous materials were tailored by controlling the sacrificial silica sphere sizes in range 145~500 nm even under volume shrinkage involved during pyrolytic step. The porous SiCBN ceramics and BCN ceramic showed a slight weight loss of 0.2~0.8% and 4%, respectively, indicating excellent resistance to oxidation.