Papers by Author: Sang Heum Youn

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: 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.

Abstract: Several sintering additives for hydroxyapatite have been tested in order to enhance its sinterability without decomposing the hydroxyapatite(Ca10(PO4)6(OH)2) and without decreasing the bioactivity and/or biocompatibility. In case of sintered pure hydroxyapatites significant dissolution was occurred after immersion in distilled water or in simulated body fluid. At first the dissolution was iniciated at grain boundary creating the nano-size defects such as small pores and grew up to micro scale by increasing the immersion time. This dissolution resulted in grain separation at the surfaces and finally in fracture. And the dissolution was concentrated on those grains adjacent to pores rather than those in the dense region. So glass-reinforced hydroxyapatite(GR-HA) ceramics were prepared to strengthening the grain boundary to prevent dissolution. Several glasses were added at 0 to 10wt.% and sintered at 1200 °C for 2h in air with moisture protection. Glass phase was incorporated into hydroxyapatite to act as the sintering aids followed by crystallization in order to improve the mechanical properties without reducing the biocompatibility. From dissolution test, significant damage was reduced even more than 7days. X-ray diffraction and SEM showed no decomposition of hydroxyapatite to secondary phases and fracture toughness was increased more than pure hydroxyapatite.

Abstract: The reaction-sintered alumina ceramics with low firing shrinkage were prepared from Al/Al2O3 powder mixture by attrition milling and the effect of milling characteristics of raw powders on reaction sintering was investigated. Powder mixtures of flaky shape Al with coarse alumina was much more effectively comminuted by the attrition milling than the mixtures of globular shape Al with coarse alumina powders. Furthermore the coarse alumina was much more useful in pulverizing and grinding the ductile Al particles than fine alumina. After attrition milling and isopressing at 400MPa, the Al/Al2O3 specimen was oxidized at 1200°C for 8 hours followed by sintering at 1550°C for 3 hours. Because mixed powder of coarse alumina with flaky Al was much more effectively comminuted than the globular Al, sintered body of more than 97% theoretical density was achieved, but low contents of Al leads to relatively higher shrinkage of about 8%. As the coarse alumina particles are much more useful in cutting and reducing the ductile Al particles, the use of the coarse alumina powder was much more effective in reaction-sintering.

Abstract: Nanocrystalline α-Al2O3 powders have been prepared by the pyrolysis of a resin compound of aluminum with polyester by a two-step calcination process. A polymeric precursor was prepared using a complexing agent to keep the metal ions in homogeneous solution, which gives sufficient flexibility for the system to exist homogeneously throughout the reaction without undergoing precipitation. The metal-ion-polyester resin forms the precursor material on complete polymerization reaction of aluminum nitrate, citric acid and ethylene glycol. A single-phase α-Al2O3 powder resulted after calcinations above 1150°C, but during heat treatment α-Al2O3 particles grow very fast by coalescence. So 2-step calcination was used, where the first step was done in a reducing atmosphere at above 1150°C and second calcinations were done in oxidizing atmosphere at the relatively low temperature of 1000°C. The precursors and the heat-treated final powders have been characterized by X-ray diffractometry, thermogravimetry , transmission electron microscopy(TEM), and BET surface area analysis. The nanocrystalline α-Al2O3 particles obtained by this 2-step calcinations method had an average specific surface area of >170m2/g, with an average particle size between 40 and 60nm.

Abstract: To improve the mechanical properties of Al2O3/ZrO2 composites, the homogeneous dispersion of ultra low size ZrO2 particles in Al2O3 ceramics have been controlled by partial dispersion of ZrO2 by chemical processes such as coprecipitation or polymeric precursor method(Pechini process). So nanosized Zr/Y hydroxide were coprecipitated or polymerized directly to the surfaces of commercial sub-micron size α-alumina powder(Sumitomo: AES-11
(0.4 μm)) using ZrOCl2 /Y(NO3)3 solution. By the partial coprecipitation method, dispersion of relatively small sized ZrO2 in Al2O3/ZrO2 composites could be achieved at 1500~1600° C of sintering temperature. In case of the polyesterization of Zr/Y(NO3)3-citric acid solution in ethylene glycol directly to the commercial sub-micron size α-alumina powder, more homogeneous dispersion of relatively low sized ZrO2 in Al2O3/ZrO2 composites could be obtained at 1450~1600°C of sintering temperature range and their mechanical strength was more enhanced.

Abstract: Waste sludges produced from the recycling of concretes contain a large amount of aggregate powders and pre-reacted cements so that they have no more cementing properties. However, since they contain unreacted Ca(OH)2, pozzolanics such as blast slag or meta kaoline can activate the waste concrete, and they can be reused as low-grade cement or pozzolanics. When meta kaoline was added with some other pozzolanic materials such as gypsum or blast slags, the density was increased so that the compressive strength, especially early stage of curing time, was enhanced. When meta kaolin was added, more densified C-S-H gel networks were formed at aggregates, and ettringites were produced between the C-S-H gel networks.

Abstract: To improve the mechanical properties of concretes containing recycled aggregates, pozzolanic materials such as Silica Fume and Meta Kaolin were used to decrease the porosity of the recycled aggregates. These pozzolanic aterials were adhered on the surface of recycled aggregates and closed the open pores so that the water absorption was decreased 1~2% as the amount of adsorption was increased. Compressive strength of cement mortars and concretes using surface treated recycled aggregates reached above 95% of the strength of its natural counterparts. Investigation of the microstructures using the scanning lectron micrographs showed the formation of dense interface after the adsorption treatment of pozzolanics to recycled aggregates.

Abstract: To improve the mechanical properties of concretes containing recycled aggregates, pozzolanic materials were used to decrease the porosity of the recycled aggregates. These pozzolanic materials were adhered on the surface of recycled aggregates and closed the open pores so that the water absorption was decreased 1~2% as the amount of adsorption was increased. Compressive strength of cement mortars and concretes using surface treated recycled aggregates reaches above 95% of the strength of its natural counterparts. Investigation of the microstructures using the scanning electron micrographs showed the formation of dense interface after the adsorption treatment of pozzolanics to recycled aggregates.