Papers by Author: Hae Jin Hwang

Abstract: LBP has six steps, which are charging, exposure, development, transfer, fusing, cleaning and discharge, for the printing product. In these steps, we focused on the development step for the source of submicron inorganic pigments. The toner is a negatively charged combination of inorganic particles. A control blade holds the toner at a micro size distance from the drum. The inorganic pigment then moves from the control blade to the more positively charged latent image on the drum. The image in static electricity needs to be developed – made visible. The developer introduces small particles of toner onto the static-charged surface of the drum. In this study inorganic pigments were coated by functional silanes to provide particles with the electrostatic charge. The coating thickness and kinds of silanes were changed to evaluate the electrostatic charge of coated pigments. After coating process, the electrostatic charge of inorganic pigments coated with silanes were evaluated by the Faraday’s device. We will introduce the effect of 3-(N-phenylamino) propyltrimethoxy silane and phenyl tris (methylethylketoxime) silane coating on the electrostatic charge of inorganic pigments.

Abstract: This study reported the fabrication of tin oxide (SnO2) nanostructures on Co-coated Si substrates by the thermal heating of Sn powders. The microstructures and morphologies of the resultant nanostructures were studied by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and scanning electron microscopy (SEM). The product mainly comprised the tangled nanowires with average diameters in the range of 50-180 nm. The nanostructures were single-crystalline rutile structure of SnO2. The PL measurement with the Gaussian fitting exhibited visible light emission bands centered at 576 nm and 638 nm, respectively. We have discussed the possible growth mechanism of the nanostructures.

Abstract: Hydrophobic thin film silica aerogels were synthesized by ambient pressure drying method from silicic acid which was prepared from sodium silicate (water glass) solution. The pH value of the silica precursor sol was adjusted to make a spinable sol, and gel films were coated on a glass substrate by dip coating technique. Aerogel-like thin films with the thickness of about 1 μm could be successfully fabricated by repeating the dip coating process three times.

Abstract: In order to prevent the irreversible collapse of the silica aerogel backbone upon the drying of the liquid phase of silica wet gel derived from water glass based silicic acid, we tried to strengthen the aerogel back bone by aging silica wet gels in water and TEOS/ethanol solutions. Although aging of silica wet gels in water has been shown to grow the neck between silica particles of the aerogel backbone, it is hard to obtain a crack-free aerogel monolith. On the other hand, the mechanical stability of silica aerogels was improved significantly by aging the wet gel in TEOS/ethanol solutions with different TEOS content.

Abstract: Transition metal-doped TiO2 powders as a photocatalyst were prepared by sol-gel process and Sb, Bi and Nb were introduced into them as dopants. The photocatalytic behaviors of the doped TiO2 powder were studied as a function of dopant, doping concentration and preparation conditions. X-ray diffraction, FT-Raman, B.E.T. and scanning electron microscopy were applied for structural and microstructural studies. Optical properties of the doped TiO2 powders were studied by UV-Visible Spectrometer and photocatalytic activity of the doped TiO2 was characterized in terms of the degradation of 1,4-dichlorobenzene. X-ray difraction analysis showed that doping with a transition metal ion suppresses anatase-to-rutile phase transition compared with the pure TiO2. The Sb and Nb-doped TiO2 powders did not exhibit any other diffraction peaks except those belonging to TiO2. On the other hand, a diffraction peak of Bi4Ti3O12 appears for 5 at.% Bi-doped samples. All of the doped TiO2 powders had higher specific surface area than undoped TiO2. Surface area increased with increasing dopant concentration depending on the dopant, from 33.9 m2/g to 55.4m2/g. The UV-visible absorption spectra of doped samples were red-shifted by 20~50nm according to the doping level. Also transition metal doped TiO2 powders exhibited better photocatalytic activity than the undoped TiO2. The increase in photoactivity is probably due to the increase in the interfacial electron transfer, red shifts, and better crystallinity.

Abstract: Lanthanum strontium chromite (LSC) thin films were prepared from the solution of La, Sr, Cr nitrates, acetylacetone and 2-methoxyethanol. Dip-coating technique was used to decompose La0.8Sr0.2CrO3, LSC thin films. The obtained precursor solution was sufficiently spinable and dried gel films were crystallized after heat-treating above 500°C for 2 h. The oxidation at 800°C for 40 hr resulted in no Fe3O4 surface-insulating phase, which means that the LSC thin film would be an effective oxidation-protecting layer of SUS430 substrate. The carbon deposited on an uncoated SUS430 substrate was amorphous with spherical morphology. The obtained LSC thin film was found to be very effective on preventing carbon deposition under dry hydrocarbon atmosphere.

Abstract: Ceria-coated copper oxide powders were prepared by the precipitation method using cerium nitrate hexahydrate and ammonia as a precipitant. Heterogeneous nucleation and growth of ceria took place on the copper oxide surface by dispersing the copper oxide particles in cerium nitrate aqueous solution and subsequently adding ammonia into it. Process conditions such as the pH of the precursor solution and precipitation temperatures were carefully controlled for the homogeneous precipitation of ceria particles. It was found that copper oxide was homogeneously coated with nano-sized (~20 nm) ceria particles at room temperature. The morphology of ceria and ceria-coated copper oxide powder were examined by field-emission scanning electron microscopy (FE-SEM) as a function of precipitation temperature.

Abstract: Hydrophobic silica aerogels were synthesized by an ambient pressure drying method from silicic acid with a different pH value, which was prepared from sodium silicate solution (water glass). In this study we chose various hydrocarbon class solvents such as pentane, hexane, heptane, and toluene, and performed surface modification in TMCS (trimethylchlorosilane)/solvent solutions in order to improve reproducibility in aerogel production. Densities of the aerogels were about 0.1 ~ 0.3 g/cm3 , and apparent porosities were 88 ~ 96 %, depending on the processing conditions. Specific surface area was approximately 730 ~ 950 m2/g, and average pore size around 10 nm.

Abstract: Porous clay materials with columnar type pore channels were fabricated from water-based clay slurry and a specially designed mold by using a freeze casting technique. Ice was stimulated to grow vertical direction, and aligned macro pores were formed to the same direction. The phase separation between the ice and clay particles might be responsible for the formation of the columnar type pores. It appears that it is possible to control the size and morphology of pores by modifying a freezing mold, refrigerant temperature, and the weight fraction of clay in slurry.