Tae Young Yang

Recycling of Coal Fly Ash for Fabrication of Porous Mullite Composite

Unidirectional pore structured porous mullite composite has been fabricated using a TBA-based freeze-gelcasting of coal fly ash/alumina slurries. After sintering at 1300-1500o C, the shrinkage, apparent porosity and compressive strength of the resulting porous composites were investigated. After sintering at 1500o C, the composite showed the maximum compressive strength of ~51 MPa and the minimum apparent porosity of ~64%.

Preparation of Porous Mullite Composites through Recycling of Coal Fly Ash

Porous mullite/alumina composites have been fabricated by a freeze casting technique using TBA-based coal fly ash/alumina slurry. After sintering, unidirectional macropore channels aligned regularly along the TBA ice growth direction were developed; simultaneously, small sized micropores fromed in the outer walls of the pore channels. The physical and mechanical properties (e.g. porosity and compressive strength) of the sintered porous composites were roughly dependant of processing conditions, due to the complexity of the factors affecting them. However, with increasing solid loading and sintering temperature, the compressive strength generally increased and the porosity decreased. After sintering 1500℃ for 2 h, the porous specimen (porosity: 52.1%) showed a maximum compressive strength of 70.0㎫.

Freeze Casting of Aqueous Al2O3/Y-TZP Slurries

Freeze casting is a useful forming technique to produce complex-shaped ceramic parts. The colloidal processing stage is an essential step for preparation of freeze casting slurries. In order to optimize freeze casting slurries, several processing additives, such as dispersant, binder, surfactant, and cryo-protectant are usually employed. Dispersion and suspension rheology of aqueous alumina/ zirconia slurries with 55 and 60 vol.% solid loading have been characterized in terms of the effect of organic additives, and the resulting microstructures of the freeze cast green and sintered materials examined.

Fracture strength and damage resistance of slip cast Y-TZP/Ce-TZP layered composites

Asymmetric, three- and symmetric five-layer Y-TZP/Ce-TZP composites have been prepared by sequential slip casting and pressureless sintering at 1400–1600°C in air. The three-layer material sintered at 1500°C showed the maximum fracture strength (534 MPa), measured by a diametral compression test and failed by the triple-cleft mode of fracture. Contact damage resistance was superior in three-layer composite compared with five-layer, possibly due to the development of relatively large residual compressive stress.

Dispersion and Rheological Properties of Alumina Freeze Casting Slurries Containing Nanoparticle/Whisker SiC

The sedimentation density significantly decreased after addition of dispersant; the effect was more pronounced with pure alumina, as compared with SiC-containing slurry. With further addition of surfactant, the sedimentation density increased somewhat, but decreased with binderadditions. The suspension viscosity generally behaved in an opposite manner to the sedimentation density, i.e., low sedimentation density gave high low-shear viscosity, indicative of high structure formation in the suspended particles. Shear rate rheological measurements showed continuous shear thinning behavior.

Suspension characteristics and rheological properties of mullite/zirconia powder in methyl isobutyl ketone

Particle size and its distribution, sedimentation bulk density and rheology of mullite, zirconia, and mullite/zirconia mixed suspensions have been studied in terms of oxide loading (20, 30 vol%), and types of additives (dispersant, dispersant/plasticizer, dispersant/plasticizer/binder). Polyester/polyamine, dibutyl phthalate, poly(vinyl butyral), and methyl isobutyl ketone have been used as the dispersant, plasticizer, binder, and liquid medium, respectively. Sedimentation density significantly increased upon adding dispersant; the effect was more pronounced with zirconia suspension most likely due to the fine and hence high specific surface area of zirconia. With further addition of plasticizer and plasticizer/binder, the sedimentation density decreased. The suspension viscosity generally behaved in an opposite manner to the sedimentation density, i.e., low sedimentation density gave high low-shear viscosity, indicative of high order structure formation in the suspended particles. High shear rate rheology showed a shear thinning and its onset began at lower shear rate with higher solid loading. Mullite/zirconia mixed suspension gave intermediate sedimentation and rheological behavior, implying the two types of particles are non-interacting.

Dispersion stability of Y-TZP/Ce-TZP powder system and slip casting

The zeta potential and apparent viscosity measurements of 3Y-TZP, 12Ce-TZP and 3Y-TZP/12Ce-TZP suspensions, has allowed the slip casting conditions for the preparation of multilayer composites have been examined. The influence of heat treatment on the sintered density, microstructure and crystalline phase of multilayer composites was also studied. The isoelectric point of both 3Y-TZP and 12Ce-TZP suspensions was near pH 8 and that of 3Y-TZP/12Ce-TZP was at pH 8.6. The suspensions exhibited pseudoplastic flow, showing a decrease in viscosity with increasing shear stress. A small (0.3 wt%) addition of an organic deflocculant gave 3Y-TZP and 3Y-TZP/12Ce-TZP suspensions with 15 and 20 vol% solid contents an appropriate fluidity for slip casting, but an additional electrolyte was required to reduce viscosity in 12Ce-TZP. Dense (>98% of theoretical) multilayer composites with grain size of 0.3–2.2 μm were obtained after sintering at 1500°C.

The Effect of Processing Additives on Dispersion and Rheological Properties of Freeze Casting Alumina/Zirconia and Mullite/Zirconia Slurries

Freeze casting is a useful forming technique to produce complex-shaped ceramic parts. In order to optimize freeze casting slurries, several processing additives are usually employed. Therefore, the relationships between additives and suspension characteristic should be understood. The effects of incorporating processing additives on dispersion and viscosity functions of aqueous alumina/zirconia and mullite/zirconia suspensions have been investigated.

Preparation and Characteristics of Bioactive Silica-free Calcium Phosphate Glass-ceramics

Glass-ceramic materials, which consist of glass matrix phase containing crystalline β-Ca₃(PO₄)₂ and β-Ca₂P₂O?, have been prepared by heating at 750-900℃ of calcium phosphate invert glasses in the silica-free CaO-P₂O?-TiO₂-Na₂O system. With increasing heating temperature from 750 to 900℃, the crystallite size of precipitated β-Ca₃(PO₄)₂ in glass with 55CaOㆍ35P₂O?ㆍ3TiO₂ㆍ7Na₂O (㏖%) composition increased from 48 to 91 ㎚. With the extension of the immersion time in dilute acetic acid solution (pH = 5) to ≥200 min, the degree of dissolution of Ca²? and P?? ions in the glass-ceramics was linearly increased and the solution was constantly maintained at pH = ~7. Biomimetic nanostructured (62-88 nm in average dia.), sphere-shaped hydroxyapatite was homogeneously formed on the surface of the glass-ceramics when socked for 7-14 days in a Hanks’ solution, indicating bioactivity of the prepared glass-ceramics.

Freeze Casting of Aqueous Alumina/Silicon Carbide Slurries and Fabrication of Layered Composites: (I) Dispersion and Rheology of Slurries

Zeta potential, sedimentation bulk density and rheology in the dispersion system have been studied in terms of solid loading (40-55 vol%), and types of additives. Ammonium polymethacrylate, glycerol, ethoxylated acetylenic diol, and polyvinyl alcohol have been used as the dispersant, cryo-protectant, surfactant, and binder, respectively. Sedimentation density greatly increased upon adding dispersant; the effect was more pronounced with ionic alumina suspension compared with covalent silicon carbide. With further addition of cryo-protectant and surfactant to dispersant, the sedimentation density increased somewhat. The suspension viscosity generally behaviored in an opposite manner to the sedimentation density, i.e., high sedimentation gave low high-shear viscosity, indicative of low order structure formation in the suspended particles. Shear rate rheology in shear rate of 2-300 sec?¹ showed a shear thinning and its onset began at similar shear rate (~100 sec?¹), regardless of solid loading.