Papers by Author: Hwan Kim

Abstract: The surface dissolution of hydroxyapatite ceramics in distilled water was investigated by using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The disks made of hydroxyapatite powder were sintered at 1200oC for 2 h in air with under moisture protection. After sintering, the disks were polished to smoothness using 1 μm diamond and they were soaked in 40 ml of pH 7.4 distilled water at 37oC for certain period of time. The morphological change of hydroxyapatite surface, specifically at the initial stage of immersion, and related surface roughness were observed with immersion time. The present study suggested that an artificial fact like surface scratch in this case initiated to be eliminated prior to the surface dissolution along grain boundary. That is, the surface roughness decreased at the initial stage of immersion, and then increased due to the surface dissolution.

Abstract: To prevent the shrinkages by the densification during the application of unfired Al2O3-C refractories or Al2O3 castables in steel making conditions, MgO was added as aggregate or matrix powder and the expansion caused by spinel formation was studied. Because the spinel was formed at the contacting areas between Al2O3 and MgO particles and the volume of in-situ formed spinel increased more abnormally at the side of Al2O3 particles than MgO, the addition of MgO aggregates was not recommendable due to the formation of large voids around the MgO aggregates. Nevertheless, corrosion resistance was increased with the amount of fine MgO added, and the finer MgO powder added, the better residual expansion and minute structure formation was observed. In the contrary, in case of castables volume expansion due to spinel formation was not obvious because the degree of densification was less than high-pressure formed refractories. But CA6 phase would not form around alumina aggregates during corrosion so the corrosion resistance was much more enhanced.