Deug Joong Kim

Effects of ammonia on the alignment of carbon nanotubes in metal-assisted thermal chemical vapor deposition

The effects of ammonia on alignment of carbon nanotubes in an atmospheric pressure thermal chemical vapor deposition assisted by Ni were investigated. It was confirmed that ammonia is critical to the alignment of nanotubes at temperatures of 800–950°C. Both synthesis by C2H2 after ammonia pretreatment and synthesis by NH3/C2H2 mixture flow could establish alignment. Alignment of carbon nanotubes due to ammonia was realized not by its influence on the morphology of the metal particles, but by its role of inhibiting a formation of amorphous carbon during the synthesis, particularly in its initial stage.

Growth of elongated grains in α-SiAlON ceramics

Abstract The microstructural evolution of pressureless sintered α-SiAlON, which was made from different particle sizes of α-Si 3 N 4 as starting powders have been investigated. The microstructure of the specimens prepared from as-received powder of 0.55 μm in average size, which was sintered at 1850°C, consisted of equiaxed α-SiAlON grains. However, by decreasing the initial α-powder size to 0.27 μm, the growth of elongated grains was enhanced, resulting in a microstructure with elongated grains of a high aspect ratio. In addition, when the specimen prepared from as-received α-Si 3 N 4 powder of 0.55 μm in average size was heat treated at 1600°C for 10 h, and then sintered at 1950°C for 1 h, elongated grains were also formed, probably due to the nuclei size reduction by the dissolution of remaining α-Si 3 N 4 during intermediate heat treatment. The specimen prepared from finer powders showed increased values of hardness and fracture toughness as a result of the microstructure containing elongated grains.

The Oxidation of TiB2 Ceramics Containing Cr and Fe

The oxidation behavior of TiB2, TiB2–0.5 wt.% Cr–0.5 wt.% Fe and TiB2–1 wt.% Cr–1 wt.% Fe was studied at 800, 900, and 1000°C in static air. These ceramics oxidized rather rapidly and formed thick oxide scales. The oxidation rates of TiB2-base ceramics were comparable to TiO2 formation on pure titanium. The scale formed on TiB2 consisted of TiO2 and B2O3. For TiB–Cr–Fe ceramics, a small amount of Cr- and Fe-oxides was additionally formed. B2O3 formed during oxidation tended to evaporate because of its high vapor pressure, making oxide scales porous and fragile. The oxidation of the TiB2-base ceramics appeared to be governed by the inward transport of oxygen via the highly porous oxide scale. The oxidation resistance of TiB2–Cr–Fe ceramics was similar to or better than that of TiB2.

Polymer-filler derived Mo2C ceramics

Abstract Manufacturing, microstructure and properties of novel reaction bonded Mo 2 C materials derived from polymer/reactive filler mixtures were investigated. Mo powder was used as a filler to react with carbon bearing decomposition products of poly(methyl- and phenysiloxanes) during pyrolysis in nitrogen atmosphere. Microcrystalline composites with the filler reaction products Mo 3 C, Mo 3 Si, Mo 5 Si 3 embedded in a silicon oxycarbide glass matrix could be formed with complex geometry owing to near net shape polymer/ceramic conversion. Depending on the precursor composition and pyrolysis conditions, ceramic hard materials with a density up to 97% theoretical density, a hardness of 10 GPa, a Young’s Modulus of 250 GPa, a fracture toughness of 5 MPam 1/2 and a flexural strength of 330 MPa were obtained.

A Mesoporous Silica Thin Film as Uptake Host for Guest Molecules with Retarded Release Kinetics

Uptake and release processes of various fluorescent rhodamine dyes and antitumor drugs to/from an ordered mesoporous silica film are investigated by means of UV/Vis absorption and fluorescence spectroscopies. The pores in the 160 nm-thick silica film strongly withdraw the dyes from water, thus allowing the storage of several micrograms of guest molecules per square centimeter of film. The binding equilibrium of the dyes follows a Langmuir-type adsorption. The dissociation constant, K(d), and the maximum binding amount to the film, N(ads)(infinity), are determined by fitting the binding curves. The release kinetics of the guests from the film to a simulated body fluid (SBF) solution follows a bimodal first-order exponential behavior. The release kinetics from the mesoporous thin film is remarkably retarded relative to that from mesoporous powders. Among all the studied dyes, rhodamine 101 is released most slowly, which implies that the release rate depends not only on the interactions between the guests and the silica surface, but also on intermolecular interactions between the guest molecules. Comparison of the release kinetics of different antitumor drugs, such as actinomycin D and mitoxantrone, into an SBF solution shows that mitoxantrone is released much slowly. This slower release is attributed to the positive molecular charge and the formation of dimers in the pores.

The Reversible Phase Transition and Dielectric Properties of BaNb2O6 Polymorphs

Barium metaniobate, BaNb2O6 with polymorphic orthorhombic and hexagonal forms was prepared by solid-state reaction. Hexagonal BaNb2O6 is the low temperature phase, and can be transformed to the orthorhombic phase over a broad temperature range above 1150°C. It was observed that the hexagonal-to-orthorhombic phase transition is reversible due to the structural similarity of these two polymorphs. The difference in transition kinetics and associated mechanisms could be confirmed from the analysis of time dependence of transformed phase. We also found the dielectric properties of each phase to be significantly different. In particular, orthorhombic BaNb2O6 has good microwave dielectric properties: Q×f=43,000 GHz, er=30, τf=-45 ppm/°C.

Effect of β-Si3N4 starting powder size on elongated grain growth in β-Si3N4 ceramics

Abstract The microstructural evolution of pressureless sintered silicon nitride ceramics prepared from different particle sizes of β-Si 3 N 4 as starting powders, has been investigated. When the specimen prepared from as-received β-powder of 0.66 μm in average size, was sintered at 1850°C, equiaxed β-Si 3 N 4 grains were observed. As the size of the initial β-powder went down to 0.26 μm, however, the growth of elongated grains was enhanced, which resulted in a whisker-like microstructure similar to that made from α-starting powder. When the sintering temperature was increased to 2000°C, the elongated grains were also developed even in the specimen made from 0.66 μm β-powder. The observed results were discussed with relation to the two dimensional nucleation and growth theory for faceted crystals. In addition, fracture toughness of the specimen consisting of elongated grains, which was prepared from finer powders, increased.

In situ measurements and analysis of imidization extent, thickness, and stress during the curing of polyimide films

The effects of scanning rate and pre-baking time on the imidization extent, thickness, and stress of polyimide films during the curing process were simultaneously analyzed using FTIR and the strip end deflection detector complemented with interferometer systems. Film thickness and stress increased, but imidization extent decreased with increasing scanning rates. Longer pre-baking times significantly reduced the initial film thickness and stress. Imidization extent, thickness, and stress behavior of polyimide films during the curing process were closely related one another.

The oxidation of Ni3Al containing decomposed SiC-particles

Abstract Alloys of Ni 3 Al–(0, 1, 3, 5 vol.%)SiC were prepared via powder metallurgical routes and their oxidation kinetics were studied between 1273 and 1473 K in air. The initially added SiC particles were unstable and decomposed within the matrix. It was found that initially added SiC increased the isothermal oxidation resistance to a certain extent, but adversely decreased the cyclic oxidation resistance because silica made the Al 2 O 3 -rich oxide layer more susceptible to spallation. For SiC-added alloys, the oxide scales consisted primarily of α-Al 2 O 3 and SiO 2 with and without NiAl 2 O 4 . During oxidation, Al, Si and a bit of Ni diffused outwardly to form the oxide scale, while oxygen and most of Ni transported inwardly.

Controlled drug release using nanoporous anodic aluminum oxide

Hardening of blood vessels has steeply increased every year, and implantation stent was used to solve that the conditions of illness. However, in-stent restenosis continues to make problem. To overcome the problem of in-stent restenosis, the concept of local delivery of antiproliferative or immunosuppressive drugs has been introduced into interventional cardiology. Local drug delivery system was demonstrated by using drug-eluting stents coated with nanoporous anodic aluminum oxide (AAO) used for controlled drug release. Effects of diameter and depth of AAO on the diffusion characteristics of drug were investigated. 2-deoxyadenosine was tested. AAO pore diameter and depth showed image by SEM and characterization of diffusion of drug from the AAO nanoporous pore diameter and depth was carried out using by HPLC (high performance liquid chromatography).