Papers by Author: Sang Yeup Park

Abstract: The nucleation and growth behavior of magentoplombite has been observed during the abnormal grain growth in the presence of liquid phase. Abnormal grain growth was explained in terms of grain orientation due to liquid phase. It has been observed that the nuclei formation for abnormal grain growth was driven by the capillary force due to the liquid phase. Because magnetoplumbite crystal structure has an anisotropic crystal nature, abnormal grain growth with faceted grain shape was observed depending on its chemical composition. The formation of nuclei and particle orientation was important for the occurrence of abnormal grain growth.

Abstract: In this study, we intend to synthesize the new bio-glass composition with fluoride additions, such as CaF2 and MgF2, instead of Na2O in bio-glass ceramic composition based on 45S5 (46.1SiO2-26.9CaO-24.4Na2O-2.6P2O5, mol%). Also, we intend to increase the SiO2 content up to 50 mol% to enhance the mechanical properties. When B2O3 (4 mol%) was added as a Na2O substitution, thermal expansion coefficient was decreased at the sintering temperature (650~950 oC). Compared to the low flexural strength (57±3 MPa) and vickers hardness (4.6 GPa) of sintered bio-glass ceramics without fluoride and B2O3, bio-glass ceramics substituted with 10mol% MgF2 for Na2O showed more higher mechanical properties (flexural strength: 141±5 MPa, vickers hardness: 5.6 GPa). Thermal expansion coefficient of bio-glass ceramics with the ion substitutions (Ca2+, Mg2+ and B3+) was decreased from 16×10-6/oC to 9.4~10×10-6/oC (~400 oC).

Abstract: The enhancement of physical properties of porous honeycomb filter for air purification was investigated using cordierite with the addition of pore former and zirconia additive. Because cordierite honeycomb has porous body, binder formulation was varied using graphite for a pore forming agent as well as lubricating agent during the extrusion process. Also, the effect of zirconia additives on the resultant physical properties of honeycomb filter such as porosity, thermal expansion coefficient and mechanical strength was investigated.

Abstract: In the study, we attempted to synthesize the cordierite by the reaction of fly-ash and alumina, silicon dioxide, magnesia powders. And also porous cordierite honeycomb filte r for air purification was fabricated from the combination of synthetic cordierite using fly as h and pore forming agent. As the extrusion processing parameters of honeycomb, binder formul ation and extrusion conditions were investigated. Graphite addition was effective for the reduce ment of extrusion pressures. Porous cordierite honeycomb was fabricated with high porosity ( 58%), and good compressive strength (69MPa) with 30wt% graphite at low sintering tempera ture(1150oC).

Abstract: Calcium-hexaluminate phase(CA6) phase is known to be effective for the crack shielding due to the spinel block crystal structure. In this study, we focused to control the CA6 morphology for the good damage tolerance behavior in alumina and zirconia/calcium-hexaluminate(CA6) composites. calcium-hexaluminate(CA6) composites were prepared from zirconia, alumina and calcium carbornate powders. Calcium-hexaluminate(CA6) phase was obtained by the solid reaction through the formation of intermediate phase(CA2). CA6 phase showed column type abnormal grain grown behavior composed of small blocks. Due to the typical microstructure of CA6, alumina and zirconia/calcium-hexaluminate composites provide well controlled crack propagation behavior.

Abstract: Cordierite is known as three different crystalline forms, such as metastable form (µ- cordierite), high temperature form (α–cordierite; indialite), and low temperature form (β-cordierite). In general, cordierite has a phase transition behavior from metastable form to high temperature form. In this study, we focused to synthesize the porous cordierite at low temperature using reaction method without metastable form. When we used a pure starting powders (Al2O3, MgO, and SiO2), metastable cordierite and Mg spinel phase was obtained during the heat treatment. However, fly ash based mixture used as a starting powder, we obtained a porous α–cordierite at low synthesis temperature through transition from sapphirine/spinel and mullite/spinel

Abstract: In this study, the effect of MgF2 additive on the mechanical properties of hydroxyapatite (HAp)/zirconia composites was investigated. As a fluoride additive, MgF2 was added to HAp/zirconia composites for the suppression of phase decomposition from HAp to tricalcium phosphate (TCP). Due to the dissolution of MgF2 at the phase decomposition temperature of HAp, the phase decomposition of non-stoichiometric HAp was suppressed. When MgF2 (5, 10 vol%) was added to HAp/zirconia composites, the decomposition of HAp was completely suppressed below 1400oC due to the ion substitution F- of for OH- in the HAp crystal structure. The flexural strength and Vickers hardness of composites was revealed more higher values compared with the MgF2-free composites.

Abstract: During the sintering process, the onset of abnormal grain growth is depend on various factors. The abnormal grain growth behavior, however, was focused only the growth stage compared to nucleation stage. Because most of abnormal grain growth occurred to a rapid mode, the observation of abnormal grain growth was impossible. In this study, we intend to in-situ observe the nucleation stage of abnormal grain using porous alumina. As a result, the liquid phase due to unintended impurities plays a main role for the formation of abnormal nuclei by the rearrangement of small grains due to the capillary force.

Abstract: Cordierite was synthesized by the reaction sintering using a high-energy ball milling process. After a high-energy ball milling for 32hr, the reaction temperature of cordierite was reduced to 1350oC from the conventional synthesis temperature of 1460oC. High-energy ball milling process was effective for the formation of amorphous silica layer to provide a low reaction temperature of cordierite.

Abstract: Mullite whiskers were synthesized by the reaction of fly-ash and alumina powder. During the reaction process, the effect of oxide additives on the formation of mullite whisker was investigated. Mullite whisker was synthesized above 1350oC with the addition of excess alumina, and the aspect ratio of mullite whisker increased with increasing sintering temperature and time. Also, the aspect ratio of mullite whisker is dependent on the liquid phase based on silicate composition. The effect of various oxide additives on the aspect ratio of mullite whisker was investigated.