Papers by Author: Sung Hoon Kim

Abstract: Different type carbon-based fabrics, namely woven or nonwoven fabric, were employed to investigate the electromagnetic wave shielding effectiveness of the fabrics in the wide operating frequency range, namely 0.4GHz to 40GHz. The surface and cross sectional morphologies of the fabrics, their electrical conductivities, and their electromagnetic wave shielding effectiveness were investigated. In the case of woven fabric, the value of the electrical conductivity was much different according to the measuring direction in the woven fabric. For the nonwoven fabric, however, this value was independent on the measuring direction. The shielding effectiveness of the woven fabric was above 20dB in the range of 0.04GHz to 4GHz and then it decreased to below 20dB in the range of 4GHz to 40GHz. In contrast, the shielding effectiveness of nonwoven fabric was above 40dB in the whole operating frequency range in this work. Based on these results, the dependence of the shielding effectiveness of the woven or nonwoven fabrics according to the operating frequency and the optimal shielding effectiveness material in the wide operating frequency range was suggested and discussed.

Abstract: The formation of aligned carbon microcoils could be achieved using C₂H₂ as a source gas and CS₂ as an incorporated additive gas under thermal chemical vapor deposition system. To elucidate the ratio of C₂H₂/CS₂ for the formation of the aligned carbon microcoils, the CS₂ flow rate was first manipulated under the identical C₂H₂ flow rate (500sccm) condition. The formation and the alignment of carbon microcoils could be only achieved under the ratio of C₂H₂/CS₂ = 33.3 condition, namely the flow rates of CS₂ = 15sccm and C₂H₂= 500sccm. The total flow rate of the used gases was varied under the identical C₂H₂/CS₂ flow rate ratio (33.3) condition. The C₂H₂ flow rate was manipulated under the identical CS₂ flow rate (15sccm) condition. It was found that the formation and the alignment of carbon microcoils could be only achieved under the condition of 15sccm of CS₂ flow rate in the range of 200 ~ 500sccm of C₂H₂ flow rate, regardless of the flow rate ratio of C₂H₂/CS₂ and the total flow rate. The crystal structure of the well-aligned CMCs reveals the increase in the (002) peak in XRD spectrum for the aligned carbon microcoils, indicating the existence of the more regular structure in the aligned carbon microcoils. Based on these results, the cause for the formation of the aligned carbon microcoils only in the case of the CS₂ flow rate = 15sccm with the imaginary pictures for the flow rate ratio of C₂H₂/CS₂ just above the substrate were proposed.

Abstract: Carbon coils were synthesized using C₂H₂ as source gas and CS₂ as an incorporated additive gas under the thermal chemical vapor deposition system. The flow rate of CS₂ varied according to the different reaction processes. Geometries of as-grown carbon nanomaterials were developed from vine-type into coil-type with increasing CS₂ flow rate from 5 to 20sccm. Above 20sccm of CS₂ flow rate, indeed, most of them were appeared as the tiny-sized wavelike nanocoil type geometries. To develop the double helix-type microcoils, namely the carbon microcoils, the injection of CS₂ flow was manipulated as the stepwise type on/off-cycle manner. Under the specific condition of the on/off-cycle number, the density of the carbon microcoils was enhanced. The cause for the enhanced formation density of the carbon microcoils by the stepwise type manipulation of CS₂ flow injection was suggested and discussed.

Abstract: The formation of the carbon microcoils could be achieved under the low temperature (550°C). Ni powders were used as the catalyst on the alumina substrate. C₂H₂ was used as a source gas and a few amount of SF₆ gas were used as an additive gas under the thermal chemical vapor deposition system. The surface morphologies of as-grown carbon materials at the low temperature (550°C) were also investigated according to the total pressure and the injection time of SF₆ flow. At 80Torr and 10min injection time of SF₆ flow, the carbon microcoils were highly developed on the entire surface of the sample. The lower or higher total pressure and the longer injection time of SF₆ flow deteriorated the formation of CMCs. The head area of the carbon microcoils was closely investigated and the growth mode for the initiation of the carbon microcoils at the low temperature was suggested.

Abstract: Carbon coils could be synthesized using C₂H₂/H₂ as source gases and SF₆ as an incorporated additive gas under thermal chemical vapor deposition system. Bunch-type large-sized (100 ~ 200 μm in diameter) Ni grains were used as the catalyst for the formation of the carbon coils. According to the different reaction processes, the injection time of SF₆ gas flow was varied from 1.0 min to 60 min. The characteristics (formation density, morphology, and geometry) of the deposited carbon coils on the substrates were investigated according to the different reaction processes. Finally, both the high production yield of carbon coils and their geometry control could be achieved merely by manipulating SF₆ gas flow injection time. Two cases growth patterns were proposed according to SF₆ gas flow injection time in association with the fluorine’s characteristics for etching the materials or enhancing the nucleation sites.

Abstract: The aligned carbon microcoils having the straight type overall geometry could be obtained using C₂H₂/H₂ as source gases and SF₆ as an incorporated additive gas under the thermal chemical vapor deposition system. Their morphologies and crystal structures were investigated and compared with the randomly grown carbon microcoils having the coil or twist type overall geometry. We could observe the enhancement of the (002) peak in XRD spectra of the aligned carbon microcoils indicating the existence of the more regular structure in the aligned carbon microcoils. The aligned carbon microcoils were formed as a bundle shape, while the randomly grown carbon microcoils were appeared as an individual shape. The systematic growth mode for the developing aspect of the aligned carbon microcoils was proposed.

Abstract: The carbon coils (d-CCs) having the diverse geometries were deposited on Al₂O₃ substrate by continuous injection of SF₆ in C₂H₂ source gas under the thermal chemical vapor deposition system. d-CCs with polyurethane (PU) composites (d-CCs@PU) were fabricated by dispersing d-CCs in PU solvent with dimethylformamide (DMF) additive. The electromagnetic wave shielding properties of d-CCs@PU composites were investigated in the frequency range of 0.25-1.5 GHz. The shielding effectiveness (SE) of d-CCs@PU composites were measured and discussed according to the weight percent of d-CCs in CCs@PU composites and the thickness of d-CCs@PU composites layers. Based on these results, we discussed the shielding properties of d-CCs@PU composites and the main mechanism of the SE in this work.

Abstract: As-grown Sr2SiO4:Eu thin films on Si (100) substrates have been reduced by hydrogen plasma reaction which was carried out by using radio frequency plasma enhanced chemical vapor deposition system. The photoluminescence spectra, the crystal structure and the surface morphologies of the plasma-reduced films were measured and compared with those of the as-grown films. At as-grown state, Eu3+ and Eu2+ ions co-exist so that the photoluminescence spectra of the films reveal several emission bands between 350 and 650 nm. They were due to f-f and f-d transitions within Eu3+ and Eu2+ ions. After the plasma-reduced process, the photoluminescence spectra of the films show a remarkable increase of blue-green emission from the transitions of Eu2+ ions and decrease of red emission from the transitions of Eu2+ ions.