The effect of Sr addition on the flexural strength of bulk SiOC ceramics was investigated in poly... more The effect of Sr addition on the flexural strength of bulk SiOC ceramics was investigated in polymer-derived SiOC ceramics prepared by conventional hot pressing. Crack-free, dense SiOC discs with a 30 mm diameter were successfully fabricated from commercially available polysiloxane with 1 mol% strontium isopropoxide derived Sr as an additive. Agglomerates formed after the pyrolysis of polysiloxane led to the formation of domain-like structures. The flexural strength of bulk SiOC was strongly dependent on the domain size formed and Sr addition. Both the minimization of the agglomerate size in the starting powders by milling after pyrolysis and the addition of Sr, which reinforces the SiOC structure, are efficient ways to improve the flexural strength of bulk SiOC ceramics. The typical flexural strength of bulk Sr-doped SiOC ceramics fabricated from submicron-sized SiOC powders was ~209 MPa.
International Journal of Applied Ceramic Technology
Polycrystalline SiC ceramics with 10 vol% Y 2 O 3-AlN additives were sintered without any applied... more Polycrystalline SiC ceramics with 10 vol% Y 2 O 3-AlN additives were sintered without any applied pressure at temperatures of 1900-2050°C in nitrogen. The electrical resistivity of the resulting SiC ceramics decreased from 6.5 × 10 1 to 1.9 × 10 −2 Ω•cm as the sintering temperature increased from 1900 to 2050°C. The average grain size increased from 0.68 to 2.34 μm with increase in sintering temperature. A decrease in the electrical resistivity with increasing sintering temperature was attributed to the grain-growth-induced N-doping in the SiC grains, which is supported by the enhanced carrier density. The electrical conductivity of the SiC ceramic sintered at 2050°C was~53 Ω −1 •cm −1 at room temperature. This ceramic achieved the highest electrical conductivity among pressureless liquidphase sintered SiC ceramics.
For cost-effective production of dense silicon carbide (SiC) ceramic parts, pressureless sinterin... more For cost-effective production of dense silicon carbide (SiC) ceramic parts, pressureless sintering of SiC ceramics at lower temperatures is preferred. This study suggests a new additive composition based on Al 2 O 3 Y 2 O 3 MgO CaO that leads to successful densification of SiC without applied pressure at a temperature as low as 1800°C. The effect of sintering temperature on the mechanical and thermal properties of SiC ceramics sintered with the new quaternary additive was investigated. Fracture toughness of the SiC ceramics continuously increased with increasing sintering temperature because of the growth of platelet SiC grains at temperatures ²1750°C. In contrast, the flexural strength and hardness showed maxima at 1800°C due to the decreased density and increased grain size at higher sintering temperatures. The thermal conductivity of SiC ceramics increased with increasing sintering temperature from 1700 to 1900°C as a result of the decrease in lattice oxygen content in the SiC lattice. Typical sintered density, fracture toughness, hardness, flexural strength, and thermal conductivity of the 1800°C-sintered SiC ceramics were 98.8%, 5.2 MPa•m 1/2 , 29.3 GPa, 347 MPa, and 83 Wm ¹1 K ¹1 at room temperature, respectively.
Highly porous silicon carbide (SiC) ceramics were fabricated from polysiloxane, SiC and carbon bl... more Highly porous silicon carbide (SiC) ceramics were fabricated from polysiloxane, SiC and carbon black fillers, AlN-Y 2 O 3 additives, and poly (ether-co-octene) (PEOc) and expandable microsphere templates. Powder mixtures with a fixed PEOc content (30 wt%) and varying SiC filler contents from 0-21 wt% were compression-molded. During the pyrolysis process, the polysiloxane was converted to SiOC, the PEOc generated a considerable degree of interconnected porosity, and the expandable microspheres generated fine cells. The polysiloxane-derived SiOC and carbon black reacted and synthesized nano-sized SiC with a carbothermal reduction during a heattreatment. Subsequent sintering of the compacts in a nitrogen atmosphere produced highly porous SiC ceramics with porosities ranging from 78 % to 82 % and a flexura lstrength of up to ~7 MPa.
Low-cost ceramic membrane supports with pore sizes in the range of 0.52-0.62 µm were successfully... more Low-cost ceramic membrane supports with pore sizes in the range of 0.52-0.62 µm were successfully prepared by uniaxial dry compaction method using inexpensive raw materials including kaolin, bentonite, talc, sodium borate, and alkaline-earth oxides in carbonate forms (e.g., MgCO 3 , CaCO 3 , and SrCO 3). The prepared green supports were sintered at 1000 o C for 8 hr in air. The effect of alkaline-earth oxide additives on the flexural strength of clay-based membrane supports was investigated. The porosity of the clay-based membrane supports was found to be in the range of 33-34%. The flexural strength of the clay-based membrane supports with 1% alkaline-earth carbonates was found to be in the range of 42.8-52.7 MPa. The addition of alkaline-earth carbonates to clay-based membrane supports resulted in large increases (47-80%) in the flexural strength of the membrane supports, compared to that of membrane supports without alkaline-earth carbonates. The typical flexural strength of the clay-based membrane support with 1% SrCO 3 was 52.7 MPa at 33.8% porosity.
Cullet-loess tile bodies are successfully fabricated using cullet, loess, hollow microspheres, an... more Cullet-loess tile bodies are successfully fabricated using cullet, loess, hollow microspheres, and sintering additives (borosilicate glass frit, boric acid, or fumed silica) as starting materials. The effects of the additive composition and sintering temperature on the sintered density and flexural strength of the cullet-loess tile bodies are investigated. The sintered density of the cullet-loess tile bodies increases with an increase in the sintering temperature as a result of the enhanced densification of pore walls through the viscous flow of a liquid phase formed from the glass frit and sintering additives. The flexural strength of the cullet-loess tile bodies increases with increases in the sintering temperature and the cullet content in the starting composition. A maximal flexural strength of 40 MPa is obtained in cullet-loess tile bodies sintered with glass frit at 800 o C in air.
β-SiC powders were synthesized by a carbothermal reduction process using SiO 2-C precursors fabri... more β-SiC powders were synthesized by a carbothermal reduction process using SiO 2-C precursors fabricated by a sol-gel process using phenol resin and TEOS as starting materials for carbon and Si sources, respectively. The C/Si molar ratio was selected as an important parameter for synthesizing SiC powders using a sol-gel process, and the effects of the C/Si molar ratio (1.4-3.0) on the particle size, particle size distribution, and yield of the synthesized β-SiC powders were investigated. It was found that (1) the particle size of the synthesized β-SiC powders decreased with an increase in the C/Si molar ratio in the SiO 2-C hybrid precursors, (2) the particle size distribution widened with an increase in the C/Si molar ratio, and (3) the yield of the β-SiC powder production increased with an increase in the C/Si molar ratio.
Kaolin-based membranes with a pore size of 0.30-0.40 µm were successfully prepared by a simple pr... more Kaolin-based membranes with a pore size of 0.30-0.40 µm were successfully prepared by a simple pressing route using low-cost starting materials, kaolin and sodium borate. The prepared green bodies were sintered at different temperatures ranging between 900 and 1200 o C. The sintered membranes were characterized by X-ray diffraction, mercury porosimetry, scanning electron microscopy, and capillary flowmetry. It was observed that the porosity decreased with an increase in both the sintering temperature and the sodium borate content, whereas the flexural strength increased with an increase in both the sintering temperature and the sodium borate content. The air flow rate decreased with an increase in the sodium borate content. The typical porosity, flexural strength, and specific flow rate of the kaolin-based membrane sintered with 5 wt% sodium borate at 1100 o C were 37%, 19 MPa, and 1 × 10 −3 L/min/cm 2 , respectively, at a p of 30 psi.
Macroporous silicon carbide (SiC) ceramics were fabricated by powder processing and polymer proce... more Macroporous silicon carbide (SiC) ceramics were fabricated by powder processing and polymer processing using carbon-filled polysiloxane as a precursor. The effects of the starting SiC polytype, template type, and template content on porosity and flexural strength of macroporous SiC ceramics were investigated. The β-SiC powder as a starting material or a filler led to higher porosity than α-SiC powder, owing to the impingement of growing α-SiC grains, which were transformed from β-SiC during sintering. Typical flexural strength of powder-processed macroporous SiC ceramics fabricated from α-SiC starting powder and polymer microbeads was 127 MPa at 29% porosity. In contrast, that of polymer-processed macroporous SiC ceramics fabricated from carbon-filled polysiloxane, β-SiC fillers, and hollow microspheres was 116 MPa at 29% porosity. The combination of α-SiC starting powder and a fairly large amount (10 wt%) of Al 2 O 3-Y 2 O 3 additives led to macroporous SiC ceramics with excellent flexural strength.
The effect of SrCO 3 content on the microstructure, porosity, flexural strength, and pore size di... more The effect of SrCO 3 content on the microstructure, porosity, flexural strength, and pore size distribution of clay-based membrane supports was investigated. Green compacts prepared from low cost materials such as kaolin, bentonite, talc, sodium borate, and strontium carbonate were sintered at 1000 o C for 8 h in air. It was possible to control the porosity of the clay-based membrane supports within the range of 33% to 37% by adjusting the SrCO 3 content. The flexural strength of the clay-based membrane supports was found to strongly depend on their porosity. In turn, the porosity was affected by the SrCO 3 content. The average pore size and flexural strength of the clay-based membrane supports containing 4 wt% SrCO 3 were 0.62 µm and 33 MPa at 34% porosity.
The effect of Sr addition on the flexural strength of bulk SiOC ceramics was investigated in poly... more The effect of Sr addition on the flexural strength of bulk SiOC ceramics was investigated in polymer-derived SiOC ceramics prepared by conventional hot pressing. Crack-free, dense SiOC discs with a 30 mm diameter were successfully fabricated from commercially available polysiloxane with 1 mol% strontium isopropoxide derived Sr as an additive. Agglomerates formed after the pyrolysis of polysiloxane led to the formation of domain-like structures. The flexural strength of bulk SiOC was strongly dependent on the domain size formed and Sr addition. Both the minimization of the agglomerate size in the starting powders by milling after pyrolysis and the addition of Sr, which reinforces the SiOC structure, are efficient ways to improve the flexural strength of bulk SiOC ceramics. The typical flexural strength of bulk Sr-doped SiOC ceramics fabricated from submicron-sized SiOC powders was ~209 MPa.
International Journal of Applied Ceramic Technology
Polycrystalline SiC ceramics with 10 vol% Y 2 O 3-AlN additives were sintered without any applied... more Polycrystalline SiC ceramics with 10 vol% Y 2 O 3-AlN additives were sintered without any applied pressure at temperatures of 1900-2050°C in nitrogen. The electrical resistivity of the resulting SiC ceramics decreased from 6.5 × 10 1 to 1.9 × 10 −2 Ω•cm as the sintering temperature increased from 1900 to 2050°C. The average grain size increased from 0.68 to 2.34 μm with increase in sintering temperature. A decrease in the electrical resistivity with increasing sintering temperature was attributed to the grain-growth-induced N-doping in the SiC grains, which is supported by the enhanced carrier density. The electrical conductivity of the SiC ceramic sintered at 2050°C was~53 Ω −1 •cm −1 at room temperature. This ceramic achieved the highest electrical conductivity among pressureless liquidphase sintered SiC ceramics.
For cost-effective production of dense silicon carbide (SiC) ceramic parts, pressureless sinterin... more For cost-effective production of dense silicon carbide (SiC) ceramic parts, pressureless sintering of SiC ceramics at lower temperatures is preferred. This study suggests a new additive composition based on Al 2 O 3 Y 2 O 3 MgO CaO that leads to successful densification of SiC without applied pressure at a temperature as low as 1800°C. The effect of sintering temperature on the mechanical and thermal properties of SiC ceramics sintered with the new quaternary additive was investigated. Fracture toughness of the SiC ceramics continuously increased with increasing sintering temperature because of the growth of platelet SiC grains at temperatures ²1750°C. In contrast, the flexural strength and hardness showed maxima at 1800°C due to the decreased density and increased grain size at higher sintering temperatures. The thermal conductivity of SiC ceramics increased with increasing sintering temperature from 1700 to 1900°C as a result of the decrease in lattice oxygen content in the SiC lattice. Typical sintered density, fracture toughness, hardness, flexural strength, and thermal conductivity of the 1800°C-sintered SiC ceramics were 98.8%, 5.2 MPa•m 1/2 , 29.3 GPa, 347 MPa, and 83 Wm ¹1 K ¹1 at room temperature, respectively.
Highly porous silicon carbide (SiC) ceramics were fabricated from polysiloxane, SiC and carbon bl... more Highly porous silicon carbide (SiC) ceramics were fabricated from polysiloxane, SiC and carbon black fillers, AlN-Y 2 O 3 additives, and poly (ether-co-octene) (PEOc) and expandable microsphere templates. Powder mixtures with a fixed PEOc content (30 wt%) and varying SiC filler contents from 0-21 wt% were compression-molded. During the pyrolysis process, the polysiloxane was converted to SiOC, the PEOc generated a considerable degree of interconnected porosity, and the expandable microspheres generated fine cells. The polysiloxane-derived SiOC and carbon black reacted and synthesized nano-sized SiC with a carbothermal reduction during a heattreatment. Subsequent sintering of the compacts in a nitrogen atmosphere produced highly porous SiC ceramics with porosities ranging from 78 % to 82 % and a flexura lstrength of up to ~7 MPa.
Low-cost ceramic membrane supports with pore sizes in the range of 0.52-0.62 µm were successfully... more Low-cost ceramic membrane supports with pore sizes in the range of 0.52-0.62 µm were successfully prepared by uniaxial dry compaction method using inexpensive raw materials including kaolin, bentonite, talc, sodium borate, and alkaline-earth oxides in carbonate forms (e.g., MgCO 3 , CaCO 3 , and SrCO 3). The prepared green supports were sintered at 1000 o C for 8 hr in air. The effect of alkaline-earth oxide additives on the flexural strength of clay-based membrane supports was investigated. The porosity of the clay-based membrane supports was found to be in the range of 33-34%. The flexural strength of the clay-based membrane supports with 1% alkaline-earth carbonates was found to be in the range of 42.8-52.7 MPa. The addition of alkaline-earth carbonates to clay-based membrane supports resulted in large increases (47-80%) in the flexural strength of the membrane supports, compared to that of membrane supports without alkaline-earth carbonates. The typical flexural strength of the clay-based membrane support with 1% SrCO 3 was 52.7 MPa at 33.8% porosity.
Cullet-loess tile bodies are successfully fabricated using cullet, loess, hollow microspheres, an... more Cullet-loess tile bodies are successfully fabricated using cullet, loess, hollow microspheres, and sintering additives (borosilicate glass frit, boric acid, or fumed silica) as starting materials. The effects of the additive composition and sintering temperature on the sintered density and flexural strength of the cullet-loess tile bodies are investigated. The sintered density of the cullet-loess tile bodies increases with an increase in the sintering temperature as a result of the enhanced densification of pore walls through the viscous flow of a liquid phase formed from the glass frit and sintering additives. The flexural strength of the cullet-loess tile bodies increases with increases in the sintering temperature and the cullet content in the starting composition. A maximal flexural strength of 40 MPa is obtained in cullet-loess tile bodies sintered with glass frit at 800 o C in air.
β-SiC powders were synthesized by a carbothermal reduction process using SiO 2-C precursors fabri... more β-SiC powders were synthesized by a carbothermal reduction process using SiO 2-C precursors fabricated by a sol-gel process using phenol resin and TEOS as starting materials for carbon and Si sources, respectively. The C/Si molar ratio was selected as an important parameter for synthesizing SiC powders using a sol-gel process, and the effects of the C/Si molar ratio (1.4-3.0) on the particle size, particle size distribution, and yield of the synthesized β-SiC powders were investigated. It was found that (1) the particle size of the synthesized β-SiC powders decreased with an increase in the C/Si molar ratio in the SiO 2-C hybrid precursors, (2) the particle size distribution widened with an increase in the C/Si molar ratio, and (3) the yield of the β-SiC powder production increased with an increase in the C/Si molar ratio.
Kaolin-based membranes with a pore size of 0.30-0.40 µm were successfully prepared by a simple pr... more Kaolin-based membranes with a pore size of 0.30-0.40 µm were successfully prepared by a simple pressing route using low-cost starting materials, kaolin and sodium borate. The prepared green bodies were sintered at different temperatures ranging between 900 and 1200 o C. The sintered membranes were characterized by X-ray diffraction, mercury porosimetry, scanning electron microscopy, and capillary flowmetry. It was observed that the porosity decreased with an increase in both the sintering temperature and the sodium borate content, whereas the flexural strength increased with an increase in both the sintering temperature and the sodium borate content. The air flow rate decreased with an increase in the sodium borate content. The typical porosity, flexural strength, and specific flow rate of the kaolin-based membrane sintered with 5 wt% sodium borate at 1100 o C were 37%, 19 MPa, and 1 × 10 −3 L/min/cm 2 , respectively, at a p of 30 psi.
Macroporous silicon carbide (SiC) ceramics were fabricated by powder processing and polymer proce... more Macroporous silicon carbide (SiC) ceramics were fabricated by powder processing and polymer processing using carbon-filled polysiloxane as a precursor. The effects of the starting SiC polytype, template type, and template content on porosity and flexural strength of macroporous SiC ceramics were investigated. The β-SiC powder as a starting material or a filler led to higher porosity than α-SiC powder, owing to the impingement of growing α-SiC grains, which were transformed from β-SiC during sintering. Typical flexural strength of powder-processed macroporous SiC ceramics fabricated from α-SiC starting powder and polymer microbeads was 127 MPa at 29% porosity. In contrast, that of polymer-processed macroporous SiC ceramics fabricated from carbon-filled polysiloxane, β-SiC fillers, and hollow microspheres was 116 MPa at 29% porosity. The combination of α-SiC starting powder and a fairly large amount (10 wt%) of Al 2 O 3-Y 2 O 3 additives led to macroporous SiC ceramics with excellent flexural strength.
The effect of SrCO 3 content on the microstructure, porosity, flexural strength, and pore size di... more The effect of SrCO 3 content on the microstructure, porosity, flexural strength, and pore size distribution of clay-based membrane supports was investigated. Green compacts prepared from low cost materials such as kaolin, bentonite, talc, sodium borate, and strontium carbonate were sintered at 1000 o C for 8 h in air. It was possible to control the porosity of the clay-based membrane supports within the range of 33% to 37% by adjusting the SrCO 3 content. The flexural strength of the clay-based membrane supports was found to strongly depend on their porosity. In turn, the porosity was affected by the SrCO 3 content. The average pore size and flexural strength of the clay-based membrane supports containing 4 wt% SrCO 3 were 0.62 µm and 33 MPa at 34% porosity.
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Papers by Young-Wook Kim