Yun Soo Lim

Electrical Properties of PVdF/PVP Composite Filled with Carbon Nanotubes Prepared by Floating Catalyst Method

The multi-wall carbon nanotubes (MWNTs)|with graphite crystal structure were synthesized by the catalytic decomposition of a ferrocene-xylene mixture in a quartz tube reactor to use as the conductive filler in the binary polymer matrix composed of poly(vinylidene fluoride)|(PVdF)|and poly(vinyl pyrrolidone)|(PVP)|for the EMI (electromagnetic interference)|shielding applications. The yield of MWNTs was significantly dependent on the reaction temperature and the mole ratio of ferrocene to xylene, approaching to the maximum at 800°C and 0.065 mole ratio. The electrical conductivity of the MWNTs-filled PVdF/PVP composite proportionally depended on the mass ratio of MWNTs to the binary polymer matrix, enhancing significantly from 0.56 to 26.7 S/cm with the raise of the mass ratio of MWNTs from 0.1 to 0.4. Based on the higher electrical conductivity and better EMI shielding effectiveness than the carbon nanofibers (CNFs)-filled coating materials, the MWNTs-filled binary polymer matrix showed a prospective possibility to apply to the EMI shielding materials. Moreover, the good adhesive strength confirmed that the binary polymer matrix could be used for improving the plastic properties of the EMI shielding materials.

Microstructure and Microwave Dielectric Properties of Ba(Co1/3Nb2/3)O3 Ceramics

Ba(Co1/3Nb2/3)O3 (BCN) ceramics has a 1:2 ordered hexagonal structure and the degree of the 1:2 ordering slightly decreased when the sintering temperature exceeded 1400°C. A large amount of the liquid phase was found in the BCN ceramics sintered above 1400°C the formation of which is related to the evaporation of CoO. The liquid phase contains high concentrations of Ba and Nb ions. The grain size increased for the specimens sintered above 1400°C due to the presence of the liquid phase during the sintering. The Q-value of BCN increased with increasing sintering temperature and the specimen sintered at 1400°C had the maximum Q-value. When the sintering temperature exceeded 1400°C, however, the Q-value significantly decreased. The presence of a large amount of liquid phase could be responsible for the decrease of the Q-value. BCN ceramics were also sintered for various times at 1400°C and 1450°C and the variations of the microwave dielectric properties were explained in terms of the grain size and the relative density.

Preparation of Activated Carbon Fibers by Chemical Activation Method with Hydroxides

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.

Chemical and micro-structural changes in glass-like carbon during high temperature heat treatment

A glass-like carbon was fabricated using furan resin. The influence of heat treatment temperature during fabrication process on the chemical and micro-structural changes was studied by various analytical and spectroscopic methods including TGA, FT-IR, CHN, TEM and XRD. The chemical resistance properties of the fabricated glasslike carbon were also investigated. It has been found that the heat-treated samples at higher temperature up to 2600 °C in N2 atmosphere had little weight loss, small amounts of functional groups, and high carbon content. The fabricated glass-like carbons upon heat treatment at 2600 °C showed an amorphous stage without any grain growth and/or reconstruction of structure. The glass-like carbon had much better chemical resistance than the artificial graphite, and exhibited a high chemical resistance due to its low surface areas, minimum impurities, and low graphite crystallites.

Structural and Property Changes in Glass-like Carbons Formed by Heat Treatment and Addition of Filler

Glass-like carbon precursors shrink significantly during curing and carbonization, which leads to crack formation and bending. Cured furan resin powder and ethanol were added to furan resin to diminish the weight loss, to suppress the shrinkage and bending, and to readily release the gases evolved during polymerization and curing. Curing and carbonization were controlled by pressure and slow heating to avoid damage to the samples. The effect of the filler and ethanol on the fabrication process was examined by measuring the properties of the glass-like carbon, such as the specific gravity, bending strength, electrical resistivity, and microstructural change. The specific gravities of the filler-added glass-like carbons were higher than those of the ethanol-added samples because of the formation of macropores from the vaporization of ethanol during the curing and polymerization processes. Although the ethanol-added glass-like carbons exhibited lower bending strengths after carbonization than did the filler-added samples, the opposite result was observed after aging at 2,600 oC. We found that the macropores created from ethanol were contracted and removed upon heat treatment. The electrical resistivity of the glass-like carbon aged at 2,600°C was lower than those of the samples carbonized at 1,000 oC. We attribute this phenomenon to the fact that aging at high temperature led to well-developed microstructures, the removal of macropores, and the reduction of the surface area.

Preparation and characterization of high-power anode materials using soft carbon precursors for lithium ion battery

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.

Growth of Carbon Nanotubes on Surface of Carbon Fibers Rod

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.

Properties of Coal Tar Pitch Modified with Acid and Oxidation Treatment

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.

Phase Transition of Titania Nano-Materials Controlled by Hydrothermal Method

TiO 2 nanofibers of various sizes and layered structure from inorganic titanium compounds by hydrothermal reactions were prepared. Using Titanium Etoxide as a raw material can make anatase- and rutile-structure particles, and these two type particles were mixed with different composition rate use to prepare TiO 2 nanowire with NaOH solution. The microstructure of TiO 2 nanowire made from mixtures of 75% of anatase structured particles and 25% of rutile structured particles showed rutile structure by XRD. The shapes of nanowires were change from fiber to needle in according to composition change from anatase of 100% to rutile of 100%.

Preparation and Characterization of Chemical Activated Fibers on Various Carbon Fibers

OXI-PAN fibers, Kynol fibers, and rayon fibers were used as precursors for the preparation of activated carbon fibers (ACFs) by chemical activation with KOH at 800°C. The effects of different precursor fibers and fiber/KOH ratios on the final ACFs are discussed. The precursor fibers used were appropriate for the ACFs in a single stage pyrolysis process. The OXI-PAN fibers, which were activated with KOH of 2.0M, showed a specific surface area of 2328m2/g, however, lost the fiber shape because of low yields. The Kynol fibers and Rayon fibers showed the high yields, but lower specific surface areas of 900m2/g and 774m2/g, respectively, at KOH of 1.5M. The OXI-PAN fibers, which were activated with KOH of 1.5M, have a specific surface area of 1028m2/g and higher micro-pore volumes and lower yields rather than Kynol-1.5 and Rayon-1.5 samples. This phenomenon is attributed to higher chemical resistance of the Kynol and Rayon fibers rather than OXI-PAN fibers. However, the Kynol fibers were the best precursors on KOH activation at 800°C when carbon yields, surface areas, and micropore volumes are condisered.