Papers by Author: B.S. Jun

Abstract: The reaction-sintered zirconia-alumina and zirconia-spinel ceramics having low firing shrinkage were prepared from ZrO2(Ca-PSZ)/Al and ZrO2(Baddellyite)/MgAl powder mixtures via the attrition milling and the effect of the characteristics of used raw powders was investigated. Flaky Al powders mixed with coarse Ca-PSZ powders was not effectively comminuted due to lower hardness of zirconia powders. So by using the alumina ball media or coarse Al2O3 powders rather than Al, the milling efficiency could be much more increased. When fused Ca-PSZ powder was reaction-sintered with Al at 1550
for 3 hours, the reaction-sintering and densification were somewhat difficult because the Ca-PSZ/Al powder mixture was not effectively comminuted. And the Ca ion in Ca-PSZ grains diffused into alumina grains during sintering so that the unstabilization of Ca-PSZ body was occurred which gave the cracks in the specimens. But when MgAl alloy powder was added to monoclinic zirconia, Mg and Al became to oxide at first and subsequently converted to spinel(MgAl2O4) during heating and finally unreacted MgO seemed to stabilize the zirconia. The oxides which formed at the oxidation process would have very fine grain size so that the reaction sintering was more effective to densification and homogeneous microstructure. The mechanical properties of reaction-sintered stabilized zirconia/spinel composite were better than only MgO stabilized zirconia. Sintering behavior in reaction and mechanical properties of sintered body were examined, with emphasis on the relations between spinel formation and mechanical properties.

Abstract: The optimum values of the porosity, pore size and transmittance of the coating layer were investigated to give the lower thermal conductivity of the smart glazing. The porosity of the aerogel film is 90%, and the pores with 50micron size are uniformly distributed through the film. The transmittance and thermal conductivity of smart glazing with 500 μm thickness is at about 80 % in the visible range(380-780nm) and 0.15 W/(m.K) respectively.

Abstract: The suspension stability was investigated as the function of particle number concentration, applied force, and the distance between cathode and anode. The suspension was prepared by using BaTiO3 nanoparticles in a mixed solution with ethanol and acetylacetone. The highest zeta potential value was 30 mV, when the mixing ratio of EtOH and Acac was 75 : 25. EPD conditions of BaTiO3 nanoparticles suspension with 0.05 wt% were settled at the conditions of an applied dc field of 20 V/
and an electrode distance of 1.5
at 300 seconds of deposition time.

Abstract: Porous concretes with continuous voids have been gaining more interest as an ecological material because of their useful functions such as water permeability and adsorption ability. So pore size distribution and the shape of especially open pores are the key point to permeability. In this study, the size and shape of pores of water permeable concrete were primary controlled by the size and shape of aggregates and secondly by the expanding agents to the cement pastes. 2 types of raw coase aggregate, type I and type C which mean fabricated by impact crusher and concrusher, were taken into uses and 3 sizes of aggregate were used, namely 3 to 5mm, 5 to 13 mm, and 13 to 20 mm. The compressive strength was found to be higher when using impact crushed aggregate. And the smaller size of aggregate was used, the higher compressive strength was achieved.

Abstract: Porous HAp scaffolds have been prepared by using the slurry including HAp and magnesia based on the replication of polymer sponge substrate. The influence of HAp and MgO content in slurry on the pore morphology and size, and density, porosity, and mechanical strength of porous HAp scaffolds was investigated. The obtained scaffolds with average pore sizes ranging 150 to 300 μm had open, relatively uniform, and interconnected porous structure regardless of HAp and MgO content. As the MgO content increased, the pore network frame of scaffolds became to be relatively stronger, even though the pore size was not much changed. The compressive strength of the scaffolds increased rapidly with the increase of MgO content at a fixed HAp content because of increasing the pore wall thickness and density of the scaffolds. As a result, the porosity, density, and compressive strength of the porous HAp scaffolds scaffolds prepared by the sponge method were significantly affected by the addition of MgO.

Abstract: To prevent the shrinkages by the densification during the application of unfired Al2O3-C refractories or Al2O3 castables in steel making conditions, MgO was added as aggregate or matrix powder and the expansion caused by spinel formation was studied. Because the spinel was formed at the contacting areas between Al2O3 and MgO particles and the volume of in-situ formed spinel increased more abnormally at the side of Al2O3 particles than MgO, the addition of MgO aggregates was not recommendable due to the formation of large voids around the MgO aggregates. Nevertheless, corrosion resistance was increased with the amount of fine MgO added, and the finer MgO powder added, the better residual expansion and minute structure formation was observed. In the contrary, in case of castables volume expansion due to spinel formation was not obvious because the degree of densification was less than high-pressure formed refractories. But CA6 phase would not form around alumina aggregates during corrosion so the corrosion resistance was much more enhanced.

Abstract: Polymer/layered silicate nanocomposities were prepared by in situ polymerization with microwave process. The influence of the amount of clay on the structure and thermal properties for the synthesized nanocomposites were characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and thermal gravimetric analysis (TGA). It was found that the structure of nanocomposites, an intercalated/exfoliated structure, depended on the clay content.

Abstract: Unagglomerated α-Al2O3 powders of 100 to 200nm were synthesized by combustion spray pyrolysis with droplet filtration. The ignition conditions of the oxidizer and fuel were well elucidated by the calculation of partial equilibrium species and thermal analysis. A metal screen filter with 500 mesh was employed to dilute the number concentration of the sprayed droplets. The Reynold’s number of the aerosol fluid was kept at the value of 1,200 to keep a short residence time and a laminar flow.

Abstract: Nanocrystalline α-Al2O3 powders have been prepared by the pyrolysis of a resin compound of aluminum with polyester by a two-step calcination process. A polymeric precursor was prepared using a complexing agent to keep the metal ions in homogeneous solution, which gives sufficient flexibility for the system to exist homogeneously throughout the reaction without undergoing precipitation. The metal-ion-polyester resin forms the precursor material on complete polymerization reaction of aluminum nitrate, citric acid and ethylene glycol. A single-phase α-Al2O3 powder resulted after calcinations above 1150°C, but during heat treatment α-Al2O3 particles grow very fast by coalescence. So 2-step calcination was used, where the first step was done in a reducing atmosphere at above 1150°C and second calcinations were done in oxidizing atmosphere at the relatively low temperature of 1000°C. The precursors and the heat-treated final powders have been characterized by X-ray diffractometry, thermogravimetry , transmission electron microscopy(TEM), and BET surface area analysis. The nanocrystalline α-Al2O3 particles obtained by this 2-step calcinations method had an average specific surface area of >170m2/g, with an average particle size between 40 and 60nm.