Papers by Author: Kyung Ja Kim

Abstract: One method of improving the strength of porous ceramics is to add Al₂O₃ in ceramic raw materials and then they must be sintered at high temperature for densification. However, the non-plastic property of Al₂O₃ leads to a decline in plasticity of clay body, thus severely interfering with forming process. Besides porous ceramics have fine cracks and pores distributed within porous ceramics, these fine cracks on the surface also result in weakening of the sintered body. In this study, we attempted to improve the strength while compensating for the weakness of porous ceramics by the surface infiltration of strengthening materials after the first sintering step at 900^°C. MgCl₂, Al (NO₃)₃, and KNO₃ were used as surface strengthening materials. The effect of Infiltration factors, solution concentrations (1-3 moles) and infiltration time (1-5 seconds) on the mechanical properties of the sintered body are investigated. When the strength was assessed after the second sintering step at 1250^°C, an 30% increase in strength was obtained with Al (NO₃)₃ infiltrated in a 3 mole solution for 5 seconds. An analysis of the crystal phase of the strengthened body revealed a higher intensity of the mullite phase arising from the infusion of surface strengthening materials. Consequently, the mullite phase led to a higher strength value.

Abstract: The Magnesium hydroxide sulfate hydrate whiskers (5Mg(OH)₂.MgSO₄.3H₂O, abbreviated 513MHSH) have attracted much attention due to its practical applications as filler and reinforcement. However, it is difficult to produce high quality MHSH because plate-like Mg(OH)₂ impurities were formed in high concentrations of OH^- and interaction between Mg²⁺ and OH^-. In this work, to reduce formation Mg(OH)₂, molar ratio of^{Superscript text} magnesium oxide (MgO) and magnesium sulfate (MgSO₄.7H₂O) were controlled. As a result, when low concentration of SO₄^2-, MHSH whiskers co-existed with hexagonal plate Mg(OH)₂. The molar ratio of MgSO₄.7H₂O/MgO was high, uniform MHSH whiskers were formed without Mg(OH)₂. In addition, appropriate amount of NH₄OH has affected formation of high quality MHSH. Their morphologies and structures were determined by powder X-ray diffraction (XRD) scanning electron microscopy (SEM) and thermo-gravimetric analyzer (TGA)

Abstract: Promoting of mullite generation has been studied by replacing kaolinite with pyrophyllite because of mullite has excellent strength and thermal shock resistance. Effects of promoting of mullite generation and vitrification by replacing kaolinite with pyrophyllite on the mechanical and thermal properties were investigated. Addition of 45-55% pyrophyllite as a replacement of kaolinite (pyrophyllite (45-55%)-feldspar (30%)-clay (20%)) could vitrify samples (water absorption: 0.05%, bulk density: 2.66g/cm3) and improve the strength (122MPa) of samples fired at 1280°C. In ternary porcelain system, pyrophyllite-feldspar-clay, mullite generation of samples with 50% pyrophyllite reaches about 78.7% and thermal expansion coefficient is 5.4×10-6/K. Beyond 50% pyrophyllite addition, quartz and cristobalite phases increased. And thermal expansion coefficient of samples decreased with increasing of mullite amount.

Abstract: Thermal expansion behavior method in reheating process was used to evaluate firing temperature of white porcelains excavated in Yanggu area, Jangpyung-Ri No.1 (J1, 17~20 centuries) and Songhyun-Ri No.5 (S5, 14~15 centuries). For standard specimen used for evaluation of the firing temperature, we used starting materials (YC) that have similar chemical composition of J1 and S5 and then they were fired at various temperatures from 800 to 1300°C. Starting temperature of shrinkage and shrinkage rate were measured in thermal expansion behavior in reheating process of the of 1st fired specimens and the base line for original firing temperature was set based on the results from the measurement of the standard specimens. Effects of particle size of starting materials for standard specimens on the thermal expansion behavior in reheating process were also investigated and the base lines were also set. As a result, we found that difference in particle size for starting materials could cause different shrinkage rate and starting temperature of shrinkage in reheating process even with the same original firing temperature.

Abstract: Rod shaped nanocrystalline powders of hydroxyapatite (HAp) were synthesized by a low temperature chemical route involving calcium nitrate tetrahydrate and sodium phosphate. Three types of alkyl ammonium bromide surfactants i.e. tetrapropylammonium bromide, tetraethylammonium bromide and tetramethylammonium bromide were used to regulate the nucleation and crystal growth. The synthesized powders were characterized by XRD, SEM and FTIR. The effect of the various alkyl ammonium bromide surfactants on the phase formation, particle size and morphology of the hydroxyapatite powders has been investigated and reported.

Abstract: This work reported the development of the high throughput protein separation process with molecularly assembled silica-coated magnetic nanoparticles as a function of amino group numbers such as mono-, di-, and tri-aminofunctionality, in which the range of silica coating thicknesses were optimized to be interacted with protein. The protein separation efficiency was demonstrated as a function of each aminofunctional group and the particle sizes of the silica coated magnetic nanoparticles. The particles were prepared by the chemical precipitation of Fe2+ and Fe3+ salts with a molar ratio of 1:2 under basic solution. The silica coated magnetic nanoparticles were directly produced by the sol-gel reaction of a tetraethyl orthosilicate (TEOS) precursor, in which the coating layer serves as a biocompatible and versatile group for further biomolecular functionalization. To effectively capture the proteins, silica coated magnetic nanoparticles need to be functionalized reproducibly on the silica surface, and three kinds of amino functional groups were adapted as a function of number of amine using the mono-, di-, and tri-aminopropylalkoxysilanes.

Abstract: Porous hydroxyapatite coated with MCM-41/PCL has been utilized as the matrix for controlled drug delivery. SEM result shows porous hydroxyapatite block is formed of three dimensionally inter-connected pores of 250 –300 *m in size, similar to human cancellous bone. TEM observation confirms the pore size of MCM-41 scatters about 50 Å. Ibuprofen was loaded into the pores of MCM-41 and hybridized with polycaprolactone to coat on the surface of hydroxyapatite block and controlled release profiles were studied by soaking the samples in a simulated body fluid using a UV-VIS spectrophotometer.

Abstract: Selected MPEG-b-PDLLA block copolymers have been synthesized by ring-opening polymerization with systematic variation of the chain lengths of the resident hydrophilic and hydrophobic blocks. The size and shape of the micelles that spontaneously form in solution are then controlled by the characteristics of the block copolymer template. All the materials prepared in this study showed the tunable pore size of 20-80Å with the increase of hydrophobic chain lengths and up to 660m2/g of specific surface area. The formation mechanism of these nanoporous structures obtained by controlling the micelle size has been confirmed using both liquid and solid state 13C and 29Si NMR techniques. This work verifies the formation mechanism of nanoporous structures in which the pore size and wall thickness are closely dependent on the size of hydrophobic cores and hydrophilic shells of the block copolymer templates.

Abstract: This work describes an innovative approach to preparation of the highly controlled drug delivery materials that involves a self-assembly process at the molecular level based upon the silicified L3 phase silicates and thermoresponsive PNIPAm integrated L3 phase silicates. The materials designed by the integration of thermosensitive polymer have been prepared and demonstrated for the highly controlled drug releasing system over a longer period of time due to their high degree of continuity and contigunity in 3-D interconnected porous structure. This approach is suitable for long term drug delivery systems with constant release in hard tissue engineering due to nanodiffusion mechanism. The structural characterization was achieved by TEM, SEM, SAXD, solid-state 29Si NMR, and BET.

Abstract: A novel approach to load a hydrophilic bovine serum albumin (BSA) into the drug carriers was proposed in terms of thermosensitive dual-shell structures which were fabricated with poly(N-isopropylacrylamide), poly(lactide) and Au nanoparticles. Spherically well-defined dualshell structures were constructed by a modified-double-emulsion method (MDEM). The lower critical solubility temperature of the structures was shifted to 36.4 °C which was confirmed by UVVis spectroscopy.