Papers by Author: Seog Young Yoon

Abstract: The sedimentation bulk density and rheology of tertiary-butyl alcohol (TBA)-based hydroxyapatite/calcium phosphate glass ceramic suspension system have been examined in terms of oxide solid loading (10, 15 vol.%), and types of additives (dispersant, dispersant/gelator, dispersant/gelator/surfactant), and calcium phosphate glass ceramic content. Sedimentation density of hydroxyapatite decreased by adding calcium phosphate glass ceramic mainly due to the coarse and hence low surface area of calcium phosphate glass ceramic powder; the decrease effect was nearly offset when the dispersant was added. The suspension viscosity generally behaved in an opposite manner to the sedimentation density. A shear thinning rheology was shown in applied shear rate, regardless of processing variables.

Abstract: Sedimentation bulk density and rheology of TBA-based hydroxyapatite suspensions have been investigated in terms of solid loading and types of additives (gelators, gelators/dispersant, gelators/dispersant/surfactant).

Abstract: Particulate composites prepared by sintering compacts of Al2O3 and TZP powders, in air at 1500 and 1600 oC have been characterized in terms of microstructure and mechanical properties. The mechanical properties of the composites appear to be controlled by three main factors: the grain size of major phase, type of intergranular TZP, and presence of residual stress.

Abstract: Mullite whiskers have been manufactured by firing compacts of kaolin, Al2(SO4)3 and (NH4)2SO4 powders, with a small addition of Na2SO4, in air at 1400o C for 15 h. From the batch composition of Al2O3/SiO2 = 0.7, molar ratio, alumina-deficient (Al2O3/SiO2 = 1.02, molar ratio), orthorhombic mullite whiskers with an aspect ratio of >30 (0.2-0.4 μm in diameter) were obtained. With increasing Al2O3/SiO2 molar ratio, the size and aspect ratio of the mullite whiskers decreased.

Abstract: Fiber-reinforced plastics consist of fibers of high strength and modulus embedded in, or bonded to a matrix with distinct interfaces between them. Because fiber configuration plays a key role in determining mechanical strength of fiber-reinforced plastic rods, especially bending strength of fiber-reinforced plastic rods was measured and simulated numerically in variation with winding angles. Also, stress distribution in fiber-reinforced plastic rods was simulated numerically under the condition of constant bending load to fiber-reinforced plastic rods. The measured bending strength of fiber-reinforced plastic rods in variation with winding angles was different from that of simulated. The difference between measured and simulated results was due to the effect of shear stresses on the strength of fiber-reinforced plastic rods.

Abstract: Particle separations occurred frequently in sintered hydroxyapatite when immersed in distilled water or simulated body fluid. This dissolution initiated at grain boundary creating nano-size defects such as small pores and grew up to micro scale by increasing immersion time. The dissolution, probably due to the appearance of secondary phases in grain boundary, resulted in grain separation at the surfaces and finally in degradation and fracture. And the dissolution concentrated on those grains adjacent to pores rather than those in the dense region. Hydroxyapatite ceramics incorporated with calcium silicate glass were prepared by slip casting to enhance the sinterability as well as to reduce dissolution. Glass phase was incorporated into hydroxyapatite to act as sintering aids followed by crystallization in order to improve the mechanical properties without reducing biocompatibility. From dissolution test, significant damage was reduced even more than 7 days and the dissolution pattern somewhat changed than pure hydroxyapatite. X-ray diffraction and SEM showed no decomposition of secondary phases in grain boundary and fracture toughness somewhat increased.

Abstract: Porous HAp/chitosan-alginate composite scaffolds were successfully synthesized by insitu co-precipitation method. During the preparation of HAp/chitosan-alginate composite scaffolds, the interaction between chitosan-alginate molecules would be reduced with increasing HAp content, with the resulting that the chitosan-alginate molecules were homogeneously dispersed in the composite scaffolds. The chitosan-alginate content was found to be almost consistent as initially added during the preparation. These results imply that chitosan-alginate was almost perfectly incorporated into the composites. It was found that the pore structure of the composite scaffolds with low HAp content was similar to chitosan-alginate scaffolds, and the morphology of uniform microstructure was unaffected by the presence of HAp. However, the pore diameter decreased with increasing the HAp content up to HAp content of 30 wt%, eventually the pore structure was collapsed and the composites scaffolds appeared to be agglomerated at higher HAp content.

Abstract: The β-tricalcium phosphate (β-TCP)/ poly(lactide-co-glycolide) (PLGA) composites for biodegradable scaffolds in bone tissue engineering were synthesized by in situ polymerization with microwave energy. The degradation behavior of β-TCP/PLGA composite was investigated by soaking in simulated body fluid (SBF) for 4 weeks. The molecular weight of the β-TCP/PLGA composites decreased with soaking time until week 2, whereas the loss rate of molecular weight reduced after week 2. The incubation time was needed for the degradation of the β-TCP, indicating that the β-TCP should be detached from the PLGA matrix and then degraded into SBF solution. The studies of mass loss of the composites with the soaking time revealed that the degradation behavior of PLGA would be processed with the transformation from the polymer to the oligomer followed by the degradation. Morphological changes, whisker-like, due to transformation and degradation of polymer in the composites were observed after week 2. On the basis of the results, it found that the degradation behavior of β-TCP/PLGA composites was influenced by the β-TCP content in the composite and the degradation rate of the composite could be controlled by the initial molecular weight of PLGA in the composite.

Abstract: The organic-inorganic hybrid nanocomposites were successfully obtained by the sol-gel process with tetraethylorthosilicate (TEOS) as an inorganic networking precursor and 2-hydroxyethylmethacrylate (HEMA) as the nonsurfactant template in the presence of benzil initiator. The characteristics of the obtained hybrid were examined by means of TGA, FTIR, SEM, and TEM. The synthesized HEMA/SiO2 hybrid was nearly transparent, monolithic, and monodispersed with the average size of 25 nm. It was found that the hybrid structure could be defined by intertwining organic and inorganic polymeric networks.

Abstract: Freeze casting is a useful forming technique to produce complex-shaped ceramic parts. In order to optimize freeze casting slurries, several processing additives are usually employed. Therefore, the relationships between additives and suspension characteristic should be understood. The effects of incorporating processing additives on dispersion and viscosity functions of aqueous alumina/zirconia and mullite/zirconia suspensions have been investigated.