Papers by Author: Jae Pyong Ahn

Abstract: Nanophase tin powder having sizes ranging from 6 to 40 nm was synthesized by the inert gas condensation method using helium as the convection gas. As-synthesized particles smaller than 8 nm were the amorphous tin oxide. As-synthesized particles larger than 10 nm can be characterized by the core-shell structure comprising inner crystalline tin core and outer amorphous tin oxide shell having a thickness of about 4 nm. Upon annealing in air, the oxidation of nanophase tin particles strongly depended on particle size. With increasing particle size, the transformation into the crystalline phases took places at a higher temperature. Calculation of the size dependent melting temperature of tin particles indicates that melting of the tin encapsulated with the amorphous tin oxide took place prior to the oxidation.

Abstract: TiO2 nanoparticle was synthesized by the flame method using a metal organic precursor of titanium tetraisopropoxide (TTIP, Ti(OC3H7)4), which was controlled by varying the ratio and flow rate of gas mixtures consisting of oxygen (oxidizer), methane (fuel) and nitrogen (carrier gas). The morphology and the size distribution of nanoparticles were observed with TEM and FESEM, and the phase evolution was analyzed by XRD measurement using a monochromator. The crystalline phases of TiO2 nanoparticle depended strongly on the temperature distribution in the flame, whereas the morphology was not sensitive. During the flame synthesis of TiO2 nanoparticle, anatase TiO2 nanoparticle was predominantly synthesized at the high flame temperature and rapid flame cooling condition. The low flame temperature and long flame length enabled to form almost rutile TiO2 nanoparticle (>95%). The anatase nanoparticle was formed by a homogeneous nucleation and has finally kept the anatase phase without the phase transformation any more in the flame. However, the rutile TiO2 nanoparticle was not formed directly and homogeneously in flame, and was manufactured by the phase transformation such as amorphous
anatase
rutile. It was proved that the rutile phase was nucleated heterogeneously from the amorphous or anatase particles.

Abstract: The spherical anatase TiO2 nanoparticle of 50 nm in diameter was manufactured by flame method and was subsequently heat-treated to investigate the transformation behavior from anatase to rutile using TEM observation. The anatase particle was facetted at the free surface and a neck formed between the anatase particles prior to the phase transformation. This resulted in the severe lattice distortion at the region of the interface. Unfortunately, we could not find the rutile grain nucleated in the anatase particle due to very fast grain growth. All the phase boundaries observed in HRTEM images existed in the contact between anatase and rutile particles. The nucleation of rutile phase in anatase particle was suppressed at the low heat-treated temperature but the grain growth of rutile particles after the phase transformation grew very fast by the sweeping phenomena of grain boundary. It leaded to the microstructure without the rutile phase traped in anatase particle.

Abstract: This paper describes the morphology and microstructure of graphitic nanofibers(GNFs) synthesized by the catalytic decomposition of ethylene in the presence of Ni:Cu(70wt%:30wt%) as catalyst at 550°C to 700°C. The catalyst was prepared by the coprecipitation method using aqueous solution of metal nitrates. The catalysts and the graphitic nanofibers(GNFs) were characterized by FE-SEM, HR-TEM and EDXS. The shape, composition and microstructure of the catalyst according to the synthesis temperature of graphitic nanofibers(GNFs)were changed so that it brought about the change of morphology and microstructure of graphitic nanofibers(GNFs). In particular, it should be noted that the composition change of catalysts with different synthesis temperature is one of the key factors in determining the morphology of graphitic nanofibers(GNFs)