Papers by Author: Byong Taek Lee

Abstract: Novel continuously porous alumina (25%monoclinic-Zirconia) (Al2O3-(m-ZrO2)) and tetragonal Zirconia (t-ZrO2) composites were fabricated with concentric multilayer shells of Al2O3-(m-ZrO2) and t-ZrO2 by the multi-pass extrusion process. Nanocrystaline Al2O3-(m-ZrO2)/ t-ZrO2 powders and graphite powder as a pore forming agent were mixed with polymer in extrudable form. The microstructure was tailored in the green bodies by the extrusion process with high flexibility of control. The green body was burnt-out and sintered to obtain the continuously porous ceramics. The pore diameter and pore frame thickness were around 198±10.5 μm and 158.5±10.2 μm, respectively. The pore frame region was furnished with 9 alternating shells of Al2O3-(m-ZrO2) and t-ZrO2 with uniform thickness of about 17.6±1.1 μm. The interfaces of the alternate layers were designed corrugated shape. The detailed microstructure of the porous bodies was characterized by SEM techniques.

Abstract: Using 6wt%Y2O3-2wt%Al2O3 as sintering additives and Si as a raw powder, the continuously porous in-situ Si2N2O-Si3N4 bodies were fabricated by multi-pass extrusion process and their microstructures were investigated depending on the addition of carbon (0-9wt %) in the mixture powder. The introduction of Si2N2O fibers observed in the unidirectional continuous pores as well as in the pore-frame regions of the nitrided bodies can be an effective method in increasing the filtration efficiency. In the case of no carbon addition, the network type Si2N2O fibers with high aspect ratio appeared in the continuous pores with diameters of 150-200nm. However, in the case of 9wt% C addition, the fibers were found without any network type and had diameters of 200-250nm.

Abstract: Carbon nanotube (CNT) reinforced hydroxyapatite (HAp) composites were fabricated by using the spark plasma sintering process with surfactant modified CNT and HAp nano powder. Without the dependency on sintering temperature, the main crystal phase existed with the HAp phase although a few contents of β-TCP (Tri calcium phosphate) phase were detected. The maximum fracture toughness, (1.27 MPa.m1/2) was obtained in the sample sintered at 1100 oC and on the fracture surface a typical intergranular fracture mode, as well as the pull-out pmhenomenon of CNT, was observed.

Abstract: Micro powder metal injection molding has received attention as a manufacturing technology for microparts. Small powder size is very useful in achieving detailed structures. STS nanopowders with an average diameter of 100 nm and STS micropowders with an average diameter of 5 micron were utilized to produce feedstock. The mixing behavior of the feedstock was indicated that the nanoparticle feedstock produce highest mixing torque at various powder_loading than the micropowder feedstock. Ares rheometer was utilized to examine visco-elatic flow behavior. The nanoparticles feedstocks showed that elastic properties are dominant in flow behavior and high viscosity. Whereas the micropowders feedstocks, viscous properties are dominant in flow behavior and less viscosity.

Abstract: The continuously porous t-ZrO2 bodies were fabricated by the extrusion process. The average pore size of the 2nd passed samples was about 260μm in diameter. The maximum bending strength value was about 177MPa. For the evaluation of their biocompatibility, human osteoblast like MG-63 cells and osteoclast like Raw 264.7 cells were cultured on the top surface of the porous t-ZrO2 bodies. The osteoblast cells were grown with spindle shape, condensed circular growth and three-dimensional network type. In contrast, the osteoclast cells appeared with pebble stone structure.

Abstract: Using microwave synthesized HAp nano powder and polymethyl methacrylate (PMMA) as a pore-forming agent, the porous biphasic calcium phosphate (BCP) ceramics were fabricated depending on the sintering temperature. The synthesized HAp powders was about 70-90 nm in diameter. In the porous sintered bodies, the pores having 150-180 μm were homogeneously dispersed in the BCP matrix. Some amounts of pores interconnected due the necking of PMMA powders which will increase the osteoconductivity and ingrowth of bone-tissues while using as a bone substrate. As the sintering temperature increased, the relative density increased and showed the maximum value of 79.6%. From the SBF experiment, the maximum resorption of Ca2+ ion was observed in the sample sintered at 1000°C.

Abstract: Ag-doped hydroxyapatite films were deposited on a ZrO2 substrate using r.f. magnetron sputtering to improve the bioaffinity and mechanical properties of the hydroxyapatite. The resulting hydroxyapatite films exhibited a variation in their microstructure and mechanical properties relative to the Ag content. The variation in the (Ca, Ag)/P ratios suggested that some of the Ca2+ ions in the hydroxyapatite were replaced with Ag+ ions. After annealing at 800oC, the hydroxyapatite films showed a microstructure with crystalline nano-grains, whereas the Ag-doped hydroxyapatite films revealed the formation of crystallites embedded in the amorphous matrix. The hydroxyapatite films showed an average roughness of about 3~4nm, very smooth surface, and dense microstructure. The hardness and modulus of the films decreased with an increasing Ag content.