Papers by Author: Sung Tag Oh

Abstract: Thermal behavior and microstructural characterization of the CNTs/Al2O3 nanocomposites with different relative densities were studied by TGA and SEM. The onset temperature for weight loss corresponded to a decomposition of CNTs in TGA increased with an increase of relative density. The activation energy for CNTs decomposition obtained by the Kissinger method increased with increasing relative density. The difference in thermal behavior was explained by the porosity effect on the oxidation and decomposition of CNTs.

Abstract: An optimum route to synthesize Al2O3-based composite powders with a homogeneous dispersion of carbon nanotubes (CNTs) was investigated. CNT/Metal/Al2O3 nanocomposite powders were fabricated by thermal chemical vapor deposition (CVD) over a metal catalyst homogeneously dispersed into an Al2O3 matrix by the means of chemical and selective reduction processes. The nanocomposite powders were densified by Pulse Electric Current Sintering (PECS). The experimental results show that the CNT/Metal/Al2O3 nanocomposites have unique electrical properties.

Abstract: The influence of relative density on the thermal stability of CNTs-dispersed Al2O3 nanocomposites has been investigated by using thermogravimetric analysis (TGA). TGA revealed that the onset temperature for weight loss increased with an increasing sintering temperature and CNT content. By taking account of the XRD and SEM results, it is suggested that the weight loss is caused by a decomposition of CNTs phase by oxidative atmosphere. The difference in thermal stability was explained by the porosity effect on the oxidation and decomposition behavior of CNTs, because the open pores promote the chemical reaction and gas flow of reactants.

Abstract: The effect of Cu on the hydrogen reduction of molybdenum oxide powders was investigated by measuring the humidity change during a non-isothermal process of hydrogen reduction. The presence of Cu induced a shift in the reduction temperature and strongly affected the reduction processes of MoO3→Mo4O11→MoO2, which comprised the contained chemical vapor transport of MoOx(OH)2. This study suggests that the surface of the Cu grains acts as a nucleation site for the reduction of MoOx(OH)2 to MoO2 particles from MoO3 or Mo4O11 phases. Such an activated reduction process results in the deposition of metallic Mo and MoO2 particles on the surface of the Cu grains.

Abstract: The effect of working temperature on microstructure and mechanical properties of ultrafine grained monolithic Al and Al-5vol.%SiCp composite processed by accumulative roll bonding (ARB) was studied. The ARB was performed up to eight cycles (an equivalent strain of ~6.4) without lubricant. The working temperature was varied from ambient temperature to 200
C. The samples processed at temperatures below 100
C exhibited an ultrafine grained structure over almost all regions. However, the samples processed at 200
C showed an inhomogeneous structure in which a few coarse grains due to an occurrence of conventional recrystallization is partially seen. The tensile strength of both the monolithic Al and the composite decreased with increasing the ARB working temperature. The variation of microstructure and mechanical properties of the composite with the working temperature was compared to that of the monolithic aluminum.

Abstract: The microstructure and electrical conductivity of CNTs dispersed Al2O3 nancomposites depending on the powder processing and CNTs content were demonstrated. The composite powders with homogeneous dispersion of CNTs could be synthesized by a catalytic route for direct formation of CNTs on nano-sized Fe dispersed Al2O3 powders. The sintered nanocomposite using the composite powder with directly synthesized CNTs showed homogeneous microstructure and enhanced elelctrical conductivity. The influence of powder processing on the properties of sintered nanocomposites was discussed by the observed microstructural features.

Abstract: The microstructure and mechanical property of hot-pressed Al2O3/Cu nanocomposites with a different temperature for atmosphere changing from H2 to Ar have been studied. When the atmosphere changed from H2 to Ar gas at 1450°C, the hot-pressed composite was characterized by inhomogeneous microstructure and low fracture strength. On the contrary, when the atmosphere changed at a lower temperature of 1100°C, a more homogeneous microstructure and higher fracture strength was observed.

Abstract: CNTs/Fe/Al2O3 nanocomposites were prepared by thermal CVD and SPS methods. The dispersion of CNTs in the Fe/Al2O3 matrix were controlled by an attrition milling process. FESEM analysis revealed that the CNTs of 5 vol.% were homogeneously dispersed in the Fe/Al2O3. The effects of CNTs dispersion on the mechanical and electrical properties of the specimens were investigated. Fracture strength and electrical conductivity of 5 vol.% CNTs/Fe/Al2O3 specimen were measured at 641 MPa and 2.93
10-11 mS/m, whereas that of a 20 vol.% CNTs/Fe/Al2O3 specimen were 208 MPa and 8.46
10-7 mS/m, respectively. In comparison with an Al2O3 monolith, the specimen with 5 vol.% CNTs showed enhanced fracture strength and increased electrical conductivity.

Abstract: The microstructure and mechanical properties including wear resistance of Al2O3-based nanocomposites with 5 vol% of Cu and Ni-Co dispersions were investigated. Al2O3/Cu and Al2O3/ Ni-Co nanocomposites were fabricated by hydrogen reduction and sintering process using metal oxide and metal nitrates. The composites showed homogeneous microstructures with nano-sized metal dispersions and enhanced fracture strength and toughness compared with monolithic Al2O3. In particular, high toughness and hardness were measured for the Al2O3/Ni-Co nanocomposite consolidated by PECS. A minimum wear coefficient of 2.33 x 10-5 mm3/Nm was obtained for the Al2O3/Ni-Co nanocomposite, while the monolithic Al2O3 showed a value of 2.0 x 10-5 mm3/Nm. Wear behavior is discussed in terms of microstructure and mechanical properties of the nanocomposites.

Abstract: The reduction behavior of Al2O3/CuO powder mixtures, prepared from Al2O3/CuO or Al2O3/Cu-nitrate, was investigated by using thermogravimetry and hygrometry. Also, the influence of powder characteristics on the microstructure and properties of hot-pressed composites is analyzed. It was found that the hydrogen reduction process of nano-sized oxide particles and properties of sintered composites strongly depends on their distribution and size in composite powder. The formation mechanisms of nano-sized Cu dispersions are discussed based on the powder characteristics and reduction kinetics. In addition, the dependence of the microstructure and mechanical properties of hot-pressed composites on powder characteristics is discussed in terms of the initial size and distribution of Cu particles.