• Journal of the Korean Ceramic Society
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• v.28 no.5
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• pp.399-405
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• 1991

Mullite-PSZ powders were synthesized by the sol-gel process using Al(sec-OC4H9)3, Si(OC2H5)4, ZrOCl2$.$8H2O and YCl3 solution and the characteristics of synthesized powders were studied. The sinterability and mechanical properties of powder compacts sintered at 1670$^{\circ}C$ for 4hr were also studied for various PSZ contents. ${\gamma}$-Al2O3(Al-Si spinel) formed at 980$^{\circ}C$ from amorphous dried gel, and mullite as well as ZrO2 formed above 1200$^{\circ}C$. At the room temperature, ZrO2 was a mixture of tetragonal and monoclinic phases. The specimens were densified to 97∼98% except the specimen containing 25 vol% PSZ which showed the relative density of 94%. The K1c value increased with the PSZ content and showed a maximum value of 4.1 MN/m3/2 at 25 vol% PSZ; this value was about 50% higher than that of the mullite without PSZ. Flexural strength had a maximum value of 280 Mn/㎡ at 20 vol% PSZ. In contrast, at 25 vol%, the flexural strength was even lower than that of the mullite possibly due to higher porosity of 6%.

• Korean Journal of Materials Research
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• v.9 no.6
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• pp.630-634
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• 1999

Porous mullites were fabricated using $Al(OH)_3$ and amorphous $SiO_2$ as starting materials by reaction sintering method. The molar ratios of alumina and silica varied from stoichiometric mullite composition to silica-rich and alumina-rich compositions. $AlF_3$ of 0, 1, 5, 10 wt% wad added to each composition, and the effects of composition and the additive for the formation of mullite were examined. The temperature of mullite formation decreased as the amount of $AlF_3$ increased, and themullite phase was formed in the stoichiometric composition in addition of 5 wt% $AlF_3$ at $1250^{\circ}C$ and porous whiskered mullite were synthesized in the sample sintered at $1300^{\circ}C$ and higher temperature. The effect of temperature on the mullite formation was not observed for the sample with 5 wt% or higher content of $AlF_3$, and the body showed little contraction after sintering.

• Journal of the Korean Ceramic Society
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• v.17 no.2
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• pp.61-68
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• 1980

The effect of the addition of various fluoreides on the mullitization of Korean crude kaolin was studied by X-ray powder diffraction and scanning electron microscopic methods. Kaolin without any addition of fluoreides began to be transformed into the mullite at 1, 10$0^{\circ}C$. Mullite peaks were discernible in the X-ray diffraction patterns of the specimens which contained fluorides equivalent to about 2 wt % fluorine, and which were sintered at 1, 05$0^{\circ}C$. The higher the concentration of fluorine in kaolin, the lower was the initiatinig temperature of mullitization. Experiments , for example, showed that mullite could be formed at 95$0^{\circ}C$ from kaolin mixed with 3.4% fluorine. Of the fluoride, addtives, sodiumsiliconfluoride $(Na_2SiF_6)$ was must effective in mullite formation of kaolin. In order of accelerating mullitization, the fluorides except $Na_2SiF_6$ could be placed in following sequence ; (1) sodium (NaF) (2) aluminium$(AlF_3)$ (3)potassium(KF) (4) ammonium$(NH_4F)$ (5) magnesium$(MgF_2)$ (6) calcium$(CaF_2)$. It was considered that the intrinsic characteristics of fluorides, such as size of ionic radiu, charge , bond strength between cation and anion, and electronegativity of cation affected millitization of halloysite, a main constituent mineral of kaolin.

• Journal of the Korean Ceramic Society
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• v.30 no.5
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• pp.381-388
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• 1993

Due to the anisotropy of thermal expansion, LAS system which has low thermal expansion property is hard to obtain a dense sintered body. Therefore, the thermal expansion coefficient and the mechanical strength were decreased. In this study, mullite, which has good mechanical properties in high temperature and comparatively low thmeral expansion coefficient, was taken as a additive in LAS system. And then, sintering, thermal, and mechanical properties were investigated. The results are follows; When mullite is added in eucryptite composition (Li2O.Al2O3.2SiO2) of LAS system, the creation of liquid phase results in the densification of sintered body and the specimen sintered at 136$0^{\circ}C$ for 2 hours shows optimum sintering condition. With the addition of mullite in eucryptite composition, mechanical strength is increased by the control of grain growth. Especially, flexual strength of EM0 specimen was about double value than the basic composition. Thermal expansion coefficients of EM0 and EM15 specimens sintered at 136$0^{\circ}C$ were -8.23$\times$10-6/$^{\circ}C$ and -4.90$\times$10-6/$^{\circ}C$ in the temperature range of RT.~80$0^{\circ}C$. As the mullite content are increased, negative thermal expansion ratios are decreased.

• 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.41 no.1
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• pp.51-56
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• 2004

Mullite whiskers were synthesized using $Al(OH)_3$, amorphous $SiO_2$ and $AIF_3$ as starting materials by a vapor-solid reaction at relatively low temperature of $1100^{\circ}C$. Leaflet Fluorotopaz was observed for the specimen heat-treated at $800^{\circ}C$. Fluorotopaz was decomposed at $1000^{\circ}C$ and only mullite phase was observed for the specimen sintered at $1100^{\circ}C$ and above. It was found that the synthesized mullite at $1200^{\circ}C$ had grown to [001] direction and side planes were the {110}. The distance of (110} plane was $5.34^{\AA}C$. The amounts of $Al_2O_3$ in mullite was 73.56(1.39)wt%.

• Journal of Korean Society for Atmospheric Environment
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• v.20 no.3
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• pp.291-302
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• 2004

The CuO/$3AL_2O_3{\cdot}2SiO_2$ catalyst impregnated ceramic candle filters for nitrogen oxides removal were prepared by porous mullite($3AL_2O_3{\cdot}2SiO_2$) support and CuO catalyst deposited on this support to achieve uniformly dispersed CuO deposition, which are impregnated into the pores of available alumino-silicate ceramic candle filter. The CuO/3$AL_2O_3{\cdot}2SiO_2$ catalyst impregnated ceramic candle filters were characterized by XRD, BET, air permeability, pore size, SEM and catalytic tests in the reduction of NOx by NH$_3$. The observed effects of CuO/3$AL_2O_3{\cdot}2SiO_2$ impregnated ceramic candle filters in SCR reaction are as follows : (1) when the content of CuO catalyst increased further, activity of NO increased. (2) NO conversion at first increased with temperature and then decreased at high temperatures (above 40$0^{\circ}C$), possibly due to the occurrence of the ammonia oxidation reaction. (3) In pilot plant test for 3 months, NO conversion was greater than 90%.

• Journal of the Korean Ceramic Society
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• v.51 no.5
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• pp.392-398
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• 2014

$Al_2O_3$-SiC composites reinforced with SiC whisker ($SiC_w$) were fabricated using three different methods. In the first, $Al_2O_3-SiC_w$ starting materials were used. In the second, $Al_2O_3-SiC_w$-SiC particles ($SiC_p$) were used, which was intended to enhance the mechanical properties by $SiC_p$ reinforcement. In the third method, reaction-sintering was used with mullite-Al-C-$SiC_w$ starting materials. After hot-pressing at $1750^{\circ}C$ and 30 MPa for 1 h, the composites fabricated using $Al_2O_3-SiC_w$ and $Al_2O_3-SiC_w-SiC_p$ showed strong mechanical properties, by which the effects of reinforcement by $SiC_w$ and $SiC_p$ were confirmed. On the other hand, the mechanical properties of the composite fabricated by reaction-sintering were found to be inferior to those of the other $Al_2O_3$-SiC composites owing to its relatively lower density and the presence of ${\gamma}-Al_2O_3$ and ${\gamma}-Al_{2.67}O_4$. The greatest hardness and $K_{1C}$ were 20.37 GPa for the composite fabricated using $Al_2O_3-SiC_w$, and $4.9MPa{\cdot}m^{1/2}$ using $Al_2O_3-SiC_w-SiC_p$, respectively, which were much improved over those from the monolithic $Al_2O_3$.

• Journal of the Korean Ceramic Society
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• v.26 no.5
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• pp.593-600
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• 1989

Fine spinel powder was prepared from the Mg-Al double alkoxide synthesized using magnesium powder, aluminum foil and sec-butyl alcohol. This powder was compared with powder prepared by mixing two commercial alkoxides. The spinelization was started at 50$0^{\circ}C$ and was almost completed at 100$0^{\circ}C$ with a good crystallinity in the double alkoxide system. In mixed alkoxide system, homogeneous spinel powder was not obtained and MgO existed as a second phase because of solubility and hydrolysis rate differences of two alkoxides. The relative density of specimen prepared by double alkoxide was 99% and specimen prepared by mixed alkoxide was 95%. The modulus of rupture of specimens prepared by double alkoxide and mixed alkoxide was 49.9kg/$\textrm{mm}^2$ and 41.6kg/$\textrm{mm}^2$, respectively.

• Journal of the Korean Ceramic Society
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• v.16 no.4
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• pp.206-212
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• 1979

The phase relations and microstructure appeared at 1700℃ in a system of Si3N4-Al2O3-SiO2 were studied. The samples were pressurelessly sintered at 1700℃ for 1hr and reheated at 1600℃ for 1hr under nitrogen atmosphere. The compounds formed were identified by X-ray diffraction method and the microstrues were observed by SEM. The stable phases appeared in this system were X-phase, Si2ON2, β'-Si3N4 and Mullite. From the results of those experiments, it was concluded that the X-phase has very close composition to that proposed by G, K. Layden, Si3Al6O12N2. SEM photographs showed that Si2ON2 was a plate phase and X-phase was a rectagular plate phase.