• Journal of the Korean Ceramic Society
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• v.38 no.5
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• pp.479-484
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• 2001

고순도 알루미나 분말에 소결조제로서 MgO를 0ppm 및 500ppm 첨가하여 열분무건조기를 사용하여 준비한 과립을 80$0^{\circ}C$에서 하소를 함으로써 얻어진 과립으로 제작한 소결체의 MgO 첨가량에 따른 미세구조와 꺾임강도에 미치는 영향을 고찰하였다. 또한 시편두께를 50$mu extrm{m}$의 박편으로 가공하여 투광법으로 내부구조도 관찰하였다. MgO를 500ppm 첨가하여 제작한 과립을 180 MPa의 냉간정수압성형성 후 1$600^{\circ}C$에서 소결한 소결체는 상대밀도가 100%에 도달하였고 균일한 결정립성장의 치밀한 미세구조를 나타내었다. MgO를 500ppm 첨가한 시편의 4점 꺾임강도 또한 각 소결온도에서 MgO를 9ppm 첨가한 시편보다 23%-32%정도 높은 강도를 나타내었으며, 특히 소결온도 1$600^{\circ}C$에서는 꺾임강도가 501MPa 그리고 weibull 계수가 20의 고강도이면서 신뢰성이 높은 알루미나 소결체가 얻어졌다. 본 연구에서는 weibull 계수의 값이 높은 시편일수록 내부구조의 결함크기가 작고 이 작은 결함이 시편내부에 균일하게 분포되어 있음을 확인할 수 있었다.

• Journal of the Korean Ceramic Society
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• v.40 no.12
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• pp.1213-1219
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• 2003

Porous ceramics for microoganism carriers were prepared with amorphous alumina and pore formers by hydrothermal reaction, burn-out and wash-out method. Activated carbon with average size of 67,222, and 405 $\mu\textrm{m}$, organic polymer and inorganic salt were used as pore formers. Specimens were hydrothermally treated at 200$^{\circ}C$ for 24 h, heat-treated at 650$^{\circ}C$ for 5 h, and washed out at 80$^{\circ}C$ for 48 h. The formation of crystalline phase, porosity, pore size distribution and compressive strength were measured. The specimen with activated carbon was transformed to boehmite phase, but organic polymer and inorganic salt inhibited the aquohydroxoy complex gel and crystalline formation. The porous ceramics for microoganism carriers using activated carbon as a pore formers was successfully prepared, which is composed of ${\gamma}$-alumina phase with porosity of above 70 vol% and the compressive strength of 40 kgf/$\textrm{cm}^2$.

• Korean Journal of Materials Research
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• v.11 no.12
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• pp.1086-1090
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• 2001

Composition effects on microstructure and metallizing properties of the alumina sintered body were evaluated to develop the metallized alumina tubes having superior properties for electronic devices. SEM observation revealed that resultant micrographs and fractographs were varied with composition chance of additives and $SiO_2$-rich specimens showed better microstructural characteristics with uniform distribution of fine and round particles than other CaO-rich or MgO-rich ones. The resultant interfacial microstructure of the $SiO_2$-rich metallized alumina tubes also showed good metallizing properties with no defects between layers and uniform thickness of metallizing layer.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.10 no.1
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• pp.73-79
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• 2011

In this study, coring with diamond core drill on the sintered $Al_2O_3$ ceramic plate were carried out with different coring conditions such as various cutting speed and feed rate to evaluate their effectiveness on the wearing behavior of diamond tool and coring quality. The wearing rate of diamond core drill were getting better with increasing cutting speed and feed rate but the quality of cored hole were getting worse as increasing cutting speed and feed rate.

• Journal of the Korean Ceramic Society
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• v.30 no.5
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• pp.347-356
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• 1993

In joining alumina ceramics to metal by using Mo-Mn metallizing process the effects of metallizing thickness, temperature, and the composition of paste on the bond strength and the microstructure of joining interface were investigated. The bond strength variation in the range of metallizing temperature, 1350~155$0^{\circ}C$ was more than 150MPa above 145$0^{\circ}C$ and the optimum metallizing thickness was 30${\mu}{\textrm}{m}$. The optimum contents of Mn in Mo-Mn paste was 5% due to the bond strength decrease with the increase of addition. The effect of SiO2 addition in paste on bond strength was saturated around 200MPa. It was also observed that as the particle size of Mo decreased, the joinning with higher bond strength was shown in spite of low metallizing temperature.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 1997.04a
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• pp.33-40
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• 1997

This study was undertaken to investigate tribology characteristics of the alumina ceramics($Al_2O_3$) for the vauiation of ambient condition such as air and distilled water. The results obtained were as follows. As the sliding speed increases, the friction coefficient in the air decreased due to the reduction of sheafing stress caused by the heat accumulation of contact interface. And the friction coefficient in the distilled water decreased due to an activation of the tribochemical reaction. As the contact load increases, the friction coefficient is small in the air due to temperature rise of the contact interface. However, at the low speed side in the distilled water, the friction coefficient holds a large value due to decrease of the tribochemical reaction. The friction surface of ceramics can be protected in the air by the influence of the oxides tansfered from STB2 and also in the distilled water by the influence of the corrosive productive hydroxides.

• Journal of the Korean Ceramic Society
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• v.31 no.10
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• pp.1099-1106
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• 1994

It is suggested that the microstructural toughening process in the initial rising portion of R-curves observed in polycrystalline alumina should be different from the grain bridging mechanism identified in the long crack regime. Microcracking in the advancing crack front seems to be a prerequisite for the development of unbroken bridging ligaments behind the crack tip. In order to test such a proposition, attempts were made to identify experimentally the presence of microcracks in the frontal zone of propagating cracks. In-situ observation is made of crack growth in a miniature double cantilever beam specimen of a average grain size of 10 ${\mu}{\textrm}{m}$ alumina. Presence of a few microcracks was identified in front of crack tip on the propagating crack plane. The R-curves were re-evaluated based on the observation.

• Journal of the Korean Ceramic Society
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• v.36 no.6
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• pp.662-670
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• 1999

Porous alumina was fabricated from pressureless powder packing forming method using powders granulated by spray drying. It was investigated the pore size distribution of fabricated porous alumina. The results of microstructural observation showed that intraganular pore size and intragranular pore size. At 1700$^{\circ}C$ there were no intragranular pores but it showed homogeneous distribution of intergranular pore size. The bending strength and shrinkage increased as porosity decreased. In case of thermal shock resistance sudden decrease of bending strength to $\Delta$T was not shown because intergranular large pore prevented sudden crack propagation.

• Journal of the Korean Ceramic Society
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• v.32 no.1
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• pp.113-119
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• 1995

The green body was fabricated by a new forming method, pressureless powder packaing forming method, and the characteristics of sintered specimen were investigated. It was found that alumina ceramics prepared by the present method showed porous structure with narrow pore size distribution, and in case of abrasive powder sintered body, compared with dry-pressed specimen, had the nearly same density. Especially, the specimen prepared with spray-dried granules showed the characteristic that granules were not either deformed or fractured during forming and sintering process. Therefore, it was found that this new forming method was effective method in fabrication of porous ceramics on account of easy control of porosity and pore size and its high thermal stability.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 1997.10a
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• pp.66-75
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• 1997

This study was undertaken to investigate tribology characteristics of the alumina ceramics($Al_2O_3$) of the various purity for the variation of ambient condition such as air and vacuum. The wear test was carried out under different experimental condition using the wear test device, which was designed for this study, and in which the annular surface of wear testing specimens as well as mate specimen made of STB2 steel, were subjected to sliding speed, applied load and the sliding distance. The results obtained were as follows. As the ambient pressure decreases, the friction coefficient increases because the protective layer made of absorption due to decrease of the amount of ambient gas can not be formed. As the friction coefficient paticularly for 85% alumina lower than 95% and 99.7% in the alumina purity increases by an influence of heat accumulation caused by small elastic modulus and thermal conductivity. The friction surface of ceramics can be protected in the air by the influence of the oxides transfered from STB2. However, in the vacuum, the protective layer made of the absorption substance can not be formed due to the decrease of ambient gas.