Papers by Author: Yun Soo Lim

Abstract: To achieve lower price and high performance of pitch carbon fiber, coal tar pitch has to restructure through high-purity process, thermal and acid treatment process, and separation process. Thermal treatment and different oxidation treatments were used to increase the softening point and yield of the precursor pitch for carbon fiber. These treatments have the effect on the properties of carbon fiber because they cause considerable change in the chemical composition of pitches. Low molecular weight materials could be removed by the thermal treatment. The oxygen containing structure formed by the acid treatments would promote the poly-condensation and polymerization reactions. The oxidation treatment with sulfuric acid, nitric acid, and air blowing would lead to the remarkable change in the chemical composition of pitches, and resulted in considerable increase in the softening point and yield of the pitches.

Abstract: Polyurethanes crystallized at the aluminum surface, but the crystalline interphase varied with polyol OH number. Early stage of spherulite formation was characterized using AFM after removing amorphous polyurethane. The crystallite shapes of polyurethanes were correlated with the bond strength measured from indentation debonding. Interestingly, the samples involving non-specific shape of crystallites displayed high bond strength, while the polyurethanes with rod-like crystallite poorly adhered to aluminum substrate. Although crystallite shape did not unequivocally relate to bond strength, the results propose that there is a probable correlation.

Abstract: Electromagnetic interference (EMI) shielding refers to the reflection and/or adsorption of electromagnetic radiation by materials, which acts as a shield against the penetration of the radiation through the shield. In this study, two directional carbon fibers reinforced composites with carbon nanotubes as fillers were studied for the high frequency electromagnetic properties and mechanical properties. Carbon nanotubes were prepared by a catalytic chemical vapor deposition method with ferrocene/xylene mixture. The electromagnetic insulating properties were measured by micro strip line method in this study. Electromagnetic interference shielding properties of composites were investigated in the frequency region of 0.3-6 GHz. The experimental results indicated that the EMI shielding effectiveness of carbon composite is sensitive to the existence of CNT.

Abstract: In order to apply to the high-power anode materials of lithium ion battery, various cokes samples were prepared by milling, pitch coating, and following heat treatment. The samples were milled and the larger particles were removed by sieving. Two types of raw cokes and four pitch coated cokes treated at different temperatures were tested as the anode materials for lithium ion battery, and their electrical performance was compared with the cokes without pitch coating. Although the anode materials prepared with cokes showed lower charge-discharge capacity than the graphite anode materials, their power capability was superior to that of graphite. The electrochemical performance of various anodes with the pitch coated cokes was demonstrated as a function of preparation conditions.

Abstract: We produced highly uniform nanoporous thin films of the dense array of titania (TiO2) pores of 70~80 nm in diameter with nanoimprinting method. Titania in HCl and 2-propanol solution was coated on an indium tin oxide (ITO) surface and embossed with an array of PMMA nanopoles which was produced using a nanoporous alumina (Al2O3) template. Two-step anodization was introduced to produce highly uniform and dense nanopores on the aluminum surface. The polymethyl methacrylate (PMMA) was poured onto and infiltrated into the nanoporous alumina surface which was heated at 150 oC. The alumina nanopores and aluminum plate were removed by wet-etching leaving an array of PMMA nanopoles. These highly uniform nanostructured titania films will be very useful for photovoltaic and photocatalytic applications where nanostructuring of surface with controlled dimensions are essential.

Abstract: Artificial graphite generally manufactured by carbonization sintering of shape-body of Kneaded mixture using granular cokes as filler and pitch as binder, going through pitch impregnation process if necessary and finally applying graphitization heat treatment. The effect of thermal oxidation with air or CO2 atmosphere on IG-11 graphite samples was investigated. The results show a localized oxidation process which progressively reveals the large coke particles with increasing overall weight loss in air. The surface of graphite was peel off which was not changed the specific gravity after air oxidation. However, the specific gravity of graphite continuously decreased by CO2 oxidation. Decreasing of specific gravity in the CO2 oxidation was because that CO2 gas ingress the surface to interior. The pore shape after the CO2 oxidation showed the longneck beakers.

Abstract: The growth of carbon nanotubes (CNTs) from surface of carbon fibers rods (CFRs) as used supports is under investigation using the Fe-catalytic chemical vapor deposition method. TEM studies indicate that under the experimental conditions Fe catalysts on CFRs produces mostly bundles of multiwall carbon nanotubes (MWCNTs). SEM results show the best presence of bundles of MWCNTs for Fe catalysts of 6.5 mol % at 800°C.

Abstract: Carbon nanofibers and nano-size metal particles were incorporated into the surface of graphite particles. The carbon nanofibers prepared from the decomposition of ethylene over nickelcopper catalyst and the micro-size silicon particles were directly introduced into the graphite particles by mixing. Nano-size tin particles were also incorporated into the graphite particles by impregnation. The three different graphite composites were tested as anode base materials for lithium ion secondary battery. The incorporation of a certain amount of carbon nanofibers into the graphite electrode improved the cyclic performance as well as the initial charge/discharge capacity. With the introduction of silicon, the initial charge/discharge capacity increased but exhibited the bad cyclic characteristics. With the modification with tin, an improved electrochemical performance was observed in both initial charge/discharge capacity and cyclic characteristics.

Abstract: Activated carbon fibers were prepared from stabilized PAN-based fibers by chemical activation using hydroxides at different concentrations. The experimental data showed variations in specific surface area, microstructure, pore size distribution, and amounts of iodine adsorbed by the activated carbon fibers. Specific surface area of about 2244m2/g and iodine adsorption of 1202mg/g were obtained in the KOH 1.5M. However, the use of NaOH in the activation process rather than KOH and using the same time/ temperature profiles resulted in a carbon with a much lower surface area. KOH is a more developed pore structure than NaOH, which means that KOH is a better activation agent in producing ACF than NaOH.