Papers by Author: Dae Ho Choi

Abstract: “Glass wool” and “rock wool” had been used as sound absorbing materials over several decades because they have excellent sound absorbing properties as well as being very productive and economical. Unfortunately, they were found to be toxic because of flying glass whiskers. To replace glass wool and rock wool as the sound absorbing panels, the recycled glass powders with zeolite and Na2SiO were foamed, adding foaming agents such as NaOH and CaCO3 at different foaming temperatures and time in an electrical furnace. Recycled glass powders from crushed passenger car window were heated up to 720°C in an electrical furnace for an optimal foaming condition. The compressive strength of the foamed glasses was measured using a dynamic materials tester. On the basis of microstructure observation of the foamed glasses, the excellent sound absorbing materials was tried to be fabricated by measuring the sound absorbing coefficient of the foamed glass materials by adding various amounts of NaOH and water.

Abstract: Mechanical and thermal properties of Si3N4 ceramics with various rare-earth oxides (La2O3, CeO2, Lu2O3, Dy2O3, Sm2O3, Nd2O3, Yb2O3, and RuO2) were investigated. Flexural strength of silicon nitride with addition of 5vol% Nd2O3, CeO2, Dy2O3, and Sm2O3 showed higher value than that of silicon nitride with Lu2O3 and La2O3 added because they form denser microstructure and smaller elongated grain. Thermal conductivity of silicon nitride with an addition of 5vol% RuO2 was more enhanced than that of silicon nitride added with Nd2O3, Sm2O3, and Dy2O3 because the addition of RuO2 depressed grain growth. It is also associated with lattice oxygen governing thermal conductivity of Si3N4 when added rare-earth oxides.

Abstract: Microstructure and mechanical behaviors of zirconia [ZR; ZrO2+3mol%Y2O3]–nano hydroxyapatite [HA; Ca10(PO4)6(OH)2] nanocomposites were studied. A pulse electric current sintering (PECS) method was applied to fabricate ZR-HA composites as biomaterials. The reaction between zirconia and hydroxyapatite was successfully avoided. It was a successful preparation of bioactive composites in the quasi-binary system Ca10(PO4)6(OH)2 + ZrO2. One of the most serious problems is that when a mixture of zirconia and nano-hydroxyapatite powders was sintered, extensive chemical reaction between zirconia and hydroxyapatite was unavoidable. By applying a novel super-fast consolidation technique, pulse electric current sintering (PECS), the deleterious reactions were inhibited kinetically. The specimens contain 0, 10, 20, and 30wt% of hydroxyapatite on zirconia as remainder. Samples were sintered using the PECS method at 1100°C , 1150°C, 1200°C, and 1250°C, with increasing 100°C /min, under a pressure of 50Mpa in vacuum atmosphere for 5min. Characterization of the samples was carried out using X-ray diffraction analysis at different sintering temperatures. Flexural strength was estimated with 3-point bending test. The relative density was acquired using an Archimedes method in toluene medium.