Papers by Author: Yong Ho Choa

Abstract: We fabricated a crack-free joining of Ni and Al2O3 using a functionally graded method. Because porosity reduction is important, particle size of the Ni and Al2O3 powders was varied to improve green body density and minimize shrinkage during sintering. As a result, crack-free joining of a Ni–Al2O3 sample with 10 layers was obtained. The ANSYS simulation tool was used to calculate residual stress. The hardness and modulus of each graded layer were measured using an Vickers’ indenter. The experimental values matched the simulation results, showing that this analysis is useful when residual stresses are very difficult to measure experimentally.

Abstract: The surface of magnetite nanoparticles which have been prepared by coprecipitation method was modified by carboxylic acid group of poly(3-thiophenacetic acid (3TA)). Then the egg white lysozyme was immobilized on the carboxylic acid group of the modification of the magnetite nanoparticles. Lysozyme immobilizing efficiency increased with increasing concentration of 3TA. And the functionalized magnetite particles had higher enzymatic capacity than non-functionalized magnetite nanoparticles.

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 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: Dense 8mol% yttria-stabilized zirconia (8YSZ) consisting of submicrometer-sized grains was prepared using spark plasma sintering (SPS) along with Al2O3 additives. The starting powder with average particle size of 50nm was densified to 98% of the relative density with short sintering time (5min) at 1200
while preserving a submicrometer grain size. The fracture toughness and bending strength showed maximum values of 2.54MPam1/2 and 380MPa at 2vol% alumina-added 8YSZ, due mainly to the higher relative density and small grain size. The electrical conductivity of 2vol% alumina-added 8YSZ was 0.0278 S/cm at 700
 in airThus, alumina additions in 8YSZ using the SPS method are an effective process to improve the mechanical strength and electrical conductivity.

Abstract: Fe/MgO nanocomposites, which are applicable to high frequency electronic devices, were fabricated by ultrasonic spray pyrolysis and selective reduction processes. Transmission electron micrographs showed that nano ferromagnetic Fe particles were isolated by MgO insulating matrix. With the increase of the reduction temperature, the particle size and saturation magnetization of the nanocomposites increased, which resulted in the decrease of the coercive force and the increase of the permeability. Furthermore, the ferromagnetic resonance peak of the nanocomposites was not observed up to 9 GHz.

Abstract: As the oxygen permeation flux of La0.7Sr0.3Ga0.6Fe0.4O3-δ (LSGF) membranes is lower than commercial membranes, we coated the nano porous La0.6Sr0.4CoO3-δ (LSC) particles to enhance the oxygen permeation flux. The nano porous LSC particles were synthesized in an advanced process to increase the volume fraction of nano pores by ultrasonic spray pyrolysis. The synthesized LSC particles consisted of nano sized primary particles and pores. They also had remarkably high surface area (22 m2/g). It was found that the LSGF membrane coated by the nano porous LSC resulted in significant improvement in the oxygen permeability.

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.