• Journal of Power System Engineering
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• v.6 no.1
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• pp.88-95
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• 2002

본 연구에서는 균열 치유 거동을 가지는 소결된 Mullite/SiC의 모재, 열처리재, 균열재, 치유 균열재의 기계적 특성이 논의되었다. 반타원형 균열의 치수는 $100{\mu}m$ 과 $200{\mu}m$이다. 얻어진 결과는 다음과 같다. (a) Mullite/SiC 복합 세라믹스는 균열 치유 능력이 있었다. (b) 최적의 균열 치유 열처리 조건은 $1300^{\circ}C$, 1시간이었다. (c) 치유 가능한 최대 균열 길이는 직경 $100{\mu}m$의 반타원 균열이다. (d) 균열 치유부는 $1200^{\circ}C$이상에서 충분한 강도를 가졌고, 대부분의 시험편은 균열 치유부 이외의 영역에서 파단 하였다. (e) 공기중에서 예열처리는 본 재료의 피로강도 향상에 유용하였다.

• Journal of the Korean Ceramic Society
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• v.32 no.5
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• pp.626-634
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• 1995

The SiC-porcelain powder mixtures containing 51.9wt% SiC are produced as by-products from the surface abrasion process of porcelain cores. This raw powders were used as starting materials for the synthesis of SiC containing ceramics. The specimen, which was fired at 135$0^{\circ}C$ from raw powders, had SiC, $Al_{2}O_{3}$, , cristobalite, mullite as crystalline phases, and the fractured microstructure showed dispersed SiC crystalline particles almost wetted with glassy matrix and spherical pores. Although the oxidation of SiC containing powder compacts wetted with glassy matrix and spherical pores. Although the oxidation of SiC containing powder compacts started at the range of 600~80$0^{\circ}C$ form the analysis of weight gain, the presence of $SiO_{2}$ crystallien phase and cristobalite was confirmed at 100$0^{\circ}C$ by XRD analysis. Mullitization of specimens was accelerated by preheating before the final firing. The specimen sintered at 135$0^{\circ}C$ after 100$0^{\circ}C$ preheating consisted of SiC, cristobalite, mullite as crystalline phases, and revealed 2.24g/$cm^{3}$ bulk density, 11.73% water adsorption, porous microstructure with small amount of glassy phase. SiC contents of specimens, which was 51.9 wt% in the raw powders, reduced to 37~22 wt% after firing at 135$0^{\circ}C$ depending on the preheating condition.

• Journal of the Korean Ceramic Society
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• v.47 no.6
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• pp.509-514
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• 2010

Porous mullite-bonded SiC (MBSC) ceramics were fabricated at temperatures ranging from 1400 to $1500^{\circ}C$ for 2 h using silicon carbide (SiC), alumina ($Al_2O_3$), strontium oxide (SrO), and poly (methyl methacrylate-coethylene glycol dimethacrylate) (PMMA) microbeads. The effect of template content on porosity, pore morphology, and flexural strength were investigated. The porosity increased with increasing the template content at the same sintering temperature. The flexural strength showed maximum after sintering at $1450^{\circ}C$/2 h for all specimens due to small pores and dense strut. By controlling the template content and sintering temperature, it was possible to produce porous MBSC ceramics with porosities ranging from 30% to 54%. A maximum flexural strength of ~51MPa was obtained at 30% porosity when no template were used and specimens sintered at $1450^{\circ}C$/2 h.

• Journal of the Korean Ceramic Society
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• v.36 no.7
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• pp.773-781
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• 1999

A new technique to fabricate complex-shaped mullite has been developed through gelcasting without organic binder. The suspension of a commercial mullite powder and boehmite-SiO2 sol as binder was elecrtrosterically stabilized. Ammonium polymetharylyate was employed as a dispersant for the electrosteric stabilization. Complex-shaped bodies were made through casting and gelation at room temperature. The green bodies were dried at a temperature of 30$^{\circ}C$ and relative humidity of 98% without crack development. The dried bodies were then pressureless-sintered at 1650$^{\circ}C$ for 2 h to get a relative density of 98% The Vickers hardness and 4-point flexural strength of sintered parts were 11.0 GPa and 150MPa respectively which are similar to those of commercially available sintered mullite.

• Journal of the Korean Ceramic Society
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• v.48 no.6
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• pp.604-609
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• 2011

Additives for low temperature sintering of mullite ceramics were investigated for matching Mo-Cu conducting paste with that ceramics at 1,400$^{\circ}C$. $SiO_2$, MgO and $Y_2O_3$ were chosen as the additives for low temperature sintering, and the amounts of those additives were varied with sintering temperature of 1,400$^{\circ}C$ to 1,500$^{\circ}C$. With additives of 1.0 wt% of $SiO_2$, 1.0 wt% of MgO, and 1.5 wt% of $Y_2O_3$, the densest sintered body of 3.12 g/$cm^3$ was obtained at 1,400$^{\circ}C$ in reducing atmosphere. The flexural strength of that was 325 MPa and the CTE (Coefficient of thermal expansion) was 4.33 ppm/$^{\circ}C$.

• Journal of the Korean Ceramic Society
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• v.51 no.5
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• pp.481-486
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• 2014

Environmental barrier coatings (EBCs) are used to protect SiC-based ceramics or composites from oxidation and corrosion due to reaction with oxygen and water vapour at high temperatures above $1000^{\circ}C$. Mullite ceramics have been studied for environmental barrier coatings for Si-based ceramics. More recently, rare earth silicate ceramics have been identified as more water vapour-resistant materials than mullite for environmental barrier coatings. In this study, we fabricate mullite and yttrium silicate ceramics by an atmospheric plasma spray coating method using spherical granules fabricated by spray drying. As a result, EBCs with thicknesses in the range of $200-300{\mu}m$ are successfully fabricated without any macroscopic cracks or interfacial delamination. Phase and microstructure analysis are conducted, and the basic mechanical properties, such as hardness and indentation load-displacement curves are evaluated.

• Korean Journal of Materials Research
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• v.28 no.12
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• pp.719-724
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• 2018

Cordierite composed of an alumina-silica-magnesia compound has a low coefficient of thermal expansion(CTE) and excellent thermal shock resistance. It also has a low dielectric constant and high electrical insulation. However, due to low mechanical strength, it is limited for use in a ceramic heater. In this study, $ZrO_2$ is added to an 80 wt% cordierite-20 wt% mullite composition, and the effect of $ZrO_2$ addition on the mechanical strength and thermal shock resistance is investigated. With an increasing addition of $ZrO_2$, cordierite-mullite formed $ZrO_2$, $ZrSiO_4$ and spinel phases. With sintering conducted at $1400^{\circ}C$ with the addition of 5 wt% $ZrO_2$ to 80 wt% cordierite-20 wt% mullite, the most dense microstructure forms along with an excellent mechanical strength with a 3-point flexural strength of 238MPa. When this composition is quenched in water at ${\Delta}T=400^{\circ}C$, the 3-point flexural strength is maintained. Moreover, when this composition is cooled from $800^{\circ}C$ to air, the 3-point flexural strength is maintained even after 100 cycles. In addition, the CTE is measured as $3.00{\times}10^{-6}{\cdot}K^{-1}$ at $1000^{\circ}C$. Therefore, 80 wt% cordierite-20 wt% mullite with 5 wt% $ZrO_2$ is considered to be appropriate as material for a ceramic heater.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.30 no.12
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• pp.762-767
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• 2017

Mullite ($3Al_2O_3{\cdot}2SiO_2$) has emerged as a promising candidate for high-temperature structural materials due to its erosion resistance, chemical and thermal stabilities, relatively low thermal expansion coefficient, excellent thermal shock and creep resistances, and low dielectric constant. However, since the pure mullite sintering temperature is as high as $1,600{\sim}1,700^{\circ}C$, there is an increasing need for a sintering additive capable of improving the strength characteristics while lowering the sintering temperature. Herein we have tried to obtain the optimal sintering additive composition by adding MgO, $Cr_2O_3$, and $Y_2O_3$ to mullite, followed by sintering at $1,325{\sim}1,550^{\circ}C$ for 2 h. With additives of 2 wt% of MgO, 2 wt% of $Cr_2O_3$, 4 wt% of $Y_2O_3$, A density of $3.23g/cm^3$ was obtained for the sintered body at $1,350^{\circ}C$ upon using 2 wt% MgO, 2 wt% $Cr_2O_3$, and 4 wt% $Y_2O_3$ as additives. The three-point flexural strength of that was 275 MPa and the coefficient of thermal expansion (CTE) was $4.15ppm/^{\circ}C$.

• Korean Journal of Materials Research
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• v.10 no.3
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• pp.233-240
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• 2000

In order to fabricate dense cordierite ceramics without sintering aid, thermal behavior of Mg-Al-Si compounds during sintering was investigated. The dispersibility of cordierite suspension in aqueous media was measured by ESA(electrokinetic sonic amplitude). To prevent aggregation and insufficient dispersion of the cordierite sol, the pH of the suspension was controlled to 1.03 and 8.30 by adding $2N\;HNO_3$ and $2N\;NH_4OH$, respectively. Magnesium-aluminum-silicate complex gel coexisted in the specimen which has been gelled at $150^{\circ}C$ fir 12 hours, however several metastable phase such as ${\mu}-cordierite(Mg_2Al_4Si_5O_{18}),\;spine(MgAl_2O_4)\;and\;mullite(Al_6Si_2O_{13})$ existed below $1300^{\circ}C$ Nucleation rates of the two suspension were similar, but densification of the gel was sensitive to the pH of the sol. Densification of the sol with the pH of 8.3 was more pronounced than that of the sol with pH of 1.63.

• Journal of the Korean Ceramic Society
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• v.35 no.4
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• pp.311-318
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• 1998

The reaction bonded alumina ceramics with reinforced particles which have low shrinkage were pro-duced by blending of SiC or TiC or ZrO2 powders to the mixture of Al metal and Al2O3 powder. The powd-ers were attrition milled isostantically pressed and preheated tio 110$0^{\circ}C$ with a heating rate of $1.5^{\circ}C$/min The specimens were then sintered at the temperature range 1500 to 1$600^{\circ}C$ for 5 hours with a heating rate of 5$^{\circ}C$/min. The specimens showed 5-9% weight gain and 2-9% dimensional expansion through the complete oxidation of Al after preheating up to 11--$^{\circ}C$ the overall dimensional change of the specimens after the reaction sintering at 1500-1$600^{\circ}C$ was 6-12% The maximum densities were 92% theoretical. The fine grain-ed(average grain size :0.4 ${\mu}{\textrm}{m}$) microstructure were observed in the specimen with ZrO2 and SiC. But the microstructure of specimen with TiC was relatively coarse.(average grain size : 2.1 ${\mu}{\textrm}{m}$) The mullite phase was formed by the reaction of Al2O3 and SiO2 in a specimen with SiC. In the TiC contained specimen TiC was oxidized into TiO2 and finally reacted with Al2O3 to form Al2TiO5 during sintering.