• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2004.11a
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• pp.214-217
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• 2004

숭화법을 이용한 탄화규소(Silicon carbide) 단결정 성장시 사용되는 원료의 상(phase)이 단결정 성장에 미치는 영향을 알아보기 위해 알파형 탄화규소 분말(${\alpha}-SiC$ powder)과 베타형 탄화규소 분말(${\beta}-SiC$ powder)을 각각 사용하였다. 알파형 탄화규소 분말을 사용한 경우에 단결정(single-crystal)을 성장할 수 있었으나, 베타형 탄화규소 분말을 사용하였을 때에는 다결정(poly-crystal)이 성장되었다. 다결정 형성요인에 관한 EPMA 분석결과, 베타형 탄화규소 분말의 탄소에 대한 실리콘의 원소조성비$(N_{Si}/N_C\;=\;1.57)$가 알파형 탄화규소 분말의 경우보다$(N_{Si}/N_C\;=\;0.81)$ 높음을 확인하였다. 따라서 흑연도가니(crucible) 내부의 실리콘 원자가 알파형 탄화규소 분말을 사용하는 경우보다 높은 과포화상태가 되어 종자정 표면에 미세한 실리콘 액적(droplet)이 중착되고 이것으로부터 일정하지 않은 방향성(random orientation)을 갖는 탄화규소 다결정(다양한 방향성을 갖는 다형 포함)이 성장한 것으로 실리콘과 탄소 원소에 대한 EPMA 지도(map) 결과를 통해 확인할 수 있었다.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.8 no.4
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• pp.645-650
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• 1998

Alumina- and yttria-coated SiC powder was prepared by the surface-induced precipitation method, and sintered properties of silicon carbide prepared from this powder were investigated. After a well dispersion of SiC powders in the aqueous solution of $Al_2(SO_4)_3$ and $Y_2(SO_4)_3$, the mixed precursors of aluminum hydroxide, aluminum carbonate, yttrium hydroxide, and yttrium carbonate were precipitated on the surfaces of SiC particles through the hydrolysis reaction of urea. SiC specimens with alumina and yttria exhibit, 97.8% of theoretical density after the sintering at $1900^{\circ}C$ for 2 hrs. During annealing at $2000^{\circ}C$, $\beta$longrightarrow$\alpha$ phase transformation of SiC had taken place and resulted with a rodlike microstructure. Toughness of sintered SiC was enhanced by crack deflection around the rodlike grains. In case of annealing less than that of 3 hr, the fracture toughness of SiC was slightly improved with increasing the amount of sintering aid. However, annealed specimens for a long time showed constant fracture toughness even though the amount of sintering aid increased. It is resulted that the main factor for toughening in annealed SiC for a long time is the pullout effect of rodlike grains during the propagation of cracks, and the amount of sintering aids is less effective on the fracture toughness of SiC.

• Journal of the Korean Ceramic Society
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• v.37 no.11
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• pp.1091-1096
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• 2000

본 연구에서는 기상 반응법을 이용하여 TMS(Tetramethylsilane:Si($CH_3$)$_4$)와 NH$_3$그리고 H$_2$의 혼합기체로부터 반응 온도 1000~120$0^{\circ}C$ 및 입력비(NH$_3$/Si($CH_3$)$_4$) 1~3의 조건에서 초미분의 SiC-Si$_3$N$_4$복합 분말을 합성하였다. 합성되어진 복합 분말들의 결정상의 변화와 평균 입경을 알아보기 위해 XRD와 TEM 분석을 행한 결과, 구형의 비정질 분말이 형성되었으며, 입자의 크기는 약 70~130nm이었다. 입자의 크기는 입력비에 관계없이 거의 일정하였으나 반응 온도가 증가함에 따라서 감소하였다. FT-IR과 EA 분석 결과, 합성되어진 분말은 Si, N, C, 그리고 H로 이루어진 화합물임을 확인할 수 있었다. 또한 입력비가 다른 조건에서 합성되어진 분말을 $N_2$분위기 하에서 155$0^{\circ}C$로 2시간 열처리를 행한 결과, 낮은 입력비인 경우 $\beta$-SiC, $\alpha$-Si$_3$N$_4$와 $\beta$-Si$_3$N$_4$의 결정상들이 혼재하였으나, 높은 입력비인 경우는 결정화 후 $\alpha$-Si$_3$N$_4$상만이 존재하였다.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2001.07a
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• pp.381-384
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• 2001

The $\beta$-SiC+ZrB$_2$ and $\beta$-SiC+TiB$_2$ceramic electroconductive composites were pressureless-sintered and annealed by adding l2wt% A1$_2$ $O_3$+Y$_2$ $O_3$(6 : 4wt%) powder as a function of sintering temperature. The relative density showed highest value of 84.92% of the theoretical density for SiC-TiB$_2$ at 190$0^{\circ}C$ sintering temperature. The phase analysis of the composites by XRD revealed of $\alpha$-SiC(6H), TiB$_2$, $Al_{5}$Y$_2$ $O_{12}$ and $\beta$-SiC(15R). Flexural strength showed the highest of 230 MPa for SiC-ZrB$_2$ composites sintered at 190$0^{\circ}C$. The vicker's hardness increased with increasing sintering temperature and showed the highest for SiC-ZrB$_2$ composites sintered at 190$0^{\circ}C$. Owing to YAG, the fracture toughness showed the highest of 6.50 MPa . m$^{1}$2/ for SiC-ZrB$_2$ composites at 190$0^{\circ}C$. The electrical resistivity was measured by the Rauw method from $25^{\circ}C$ to $700^{\circ}C$. The electrical resistivity of the composites showed the PTCR(Positive Temperature Coefficient Resistivity).).

• Proceedings of the KIEE Conference
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• 2011.07a
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• pp.1449-1450
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• 2011

The SiC-$ZrB_2$ composites were produced by subjecting a 40:60 (vol.%) mixture of zirconium diboride($ZrB_2$) powder and ${\beta}$-silicon carbide (SiC) matrix to spark plasma sintering(SPS) under argon atmosphere at 50MPa(P50) and 60MPa(P60) pressure. The relative density, 94.13% of P60 sample was lower than that, 94.75% of P50 sample. Reactions between ${\beta}$-SiC and $ZrB_2$ were not observed via x-ray diffraction (hereafter, XRD) analysis. The trend of flexural strength of SiC-$ZrB_2$ composites were in accordance with the relative density. The properties of a SiC-$ZrB_2$ composites through SPS under argon atmosphere were positive temperature coefficient resistance in the temperature range from $25^{\circ}C$ to $500^{\circ}C$, and electrical resistivity of P50 and P60 sample were $6.75{\times}10^{-4}$ and $7.22{\times}10^{-4}{\Omega}{\cdot}cm$ at room temperature, respectively.

• Journal of the Korean Ceramic Society
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• v.17 no.4
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• pp.197-202
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• 1980

The oxidation mechanism of the not sintered pellets and sintered bodies of $Si_3N_4$ was investigated. in air over the temperature range of 800~130$0^{\circ}C$. The $\beta$-cristobalite was instantaneously formed and covered the particles of powder packed in the not sintered and weakly sintered porous $Si_3N_4$ bodies by molecular diffusion of oxygen through the porous Si3N4 bodies and an immediate oxidation. The diffusion of oxygen ion through the formed $\beta$-cristobalite surface layer is assumed to control the further oxidation of the $Si_3N_4$ particles of the porous $Si_3N_4$ bodies. The diffusion coefficients and activation energies of oxygen ion through the $\beta$-cristobalite layer were obtained by the use of a derived equation.

• Journal of the Korean Ceramic Society
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• v.29 no.11
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• pp.877-887
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• 1992

Tetragonal zirconia powder with 3 mol% Y2O3 mas mixed with up to 30 vol% of ${\beta}$-SiC powders, and the mixtures were hot-pressed at 1500$^{\circ}C$ for 60 min under a pressure of 30 MPa in Ar atmosphere. Flexural strength and fracture toughness were measured at room-and high-temperature (1000$^{\circ}C$). Evolution of microstructure was also conducted to investigate the effects of SiC addition on the properties of 3Y-TZP ceramics. Average grain size of the composites was about 0.5 $\mu\textrm{m}$, and decreased with SiC addition. Both room- and high-temperature mechanical properties of the composites were improved with SiC content. Particularly, high-temperature strength and fracture toughness of 3Y-TZP/30v/o SiC composite were twice as high as those of 3Y-TZP. The hardness of the composites also increased with SiC content and reached maximum value at 3Y-TZP/30v/o SiC composite.

• Proceedings of the KIEE Conference
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• 1999.07d
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• pp.1913-1915
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• 1999

The ${\beta}-SiC+ZrB_2$ ceramic composites were hot-press sintered and annealed by adding 1, 2, 3wt% $Al_2O_3+Y_2O_3$(6 : 4wt%) powder as a liquid forming additives at $1950^{\circ}C$ for 4h. In this microstructures, no reactions were observed between $\beta$-SiC and $ZrB_2$, and the relative density is over 90.79% of the theoretical density and the porosity decreased with increasing $Al_2O_3+Y_2O_3$ contents. Phase analysis of the composites by XRD revealed of $\alpha$-SiC(6H, 4H), $ZrB_2$, $Al_2O_3$ and $\beta$-SiC(15R). Flexural strength showed the highest of 315.46MPa for composites added with 3wt% $Al_2O_3+Y_2O_3$ additives at room temperature. Owing to crack deflection and crack bridging of fracture toughness mechanism, the fracture toughness showed the highest of $5.5328MPa{\cdot}m^{1/2}$ for composites added with 2wt% $Al_2O_3+Y_2O_3$ additives at room temperature.

• Journal of Korean Ophthalmic Optics Society
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• v.5 no.1
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• pp.61-65
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• 2000

${\beta}$-SiC powder and ${\alpha}$-SiC powders of different particle sizes, containing 5.7wt% $Al_2O_3$ and 4.3wt% $Y_2O_3$ as sintering aids, were hot-pressed at $1780^{\circ}C$ and subsequently annealed at $1950^{\circ}C$ to initiate grain growth. All the hot-pressed and annealed materials consisted of large SiC grains and elongated SiC grains. Typical hardness and fracture toughness of materials for spectacle lens cutting were 15.6 GPa and $5.7MPa{\cdot}m^{1/2}$, respectively.

• Proceedings of the KSME Conference
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• 2004.04a
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• pp.204-209
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• 2004

SiC materials have been extensively studied for high temperature components in advanced energy system and advanced gas turbine. However, the brittle characteristics of SiC such as low fracture toughness and low strain-to fracture still impose a severe limitation on practical applications of SiC materials. For these reasons, $SiC_f/SiC$ composites can be considered as a promising for various structural materials, because of their good fracture toughness compared with monolithic SiC ceramics. But, high temperature and pressure lead to the degradation of the reinforcing fiber during the hot pressing. Therefore, reduction of sintering temperature and pressure is key requirements for the fabrication of $SiC_f/SiC$ composites by hot pressing method. In the present work, Monolithic LPS-SiC was fabricated by hot pressing method in Ar atmosphere at 1760 $^{\circ}C$, 1780 $^{\circ}C$, 1800 $^{\circ}C$ and 1820 $^{\circ}C$ under 20 MPa using $Al_2O_3-Y_2O_3$ system as sintering additives in order to low sintering temperature. The starting powder was high purity ${\beta}-SiC$ nano-powder with an average particle size of 30 nm. Monolithic LPS-SiC was evaluated in terms of sintering density, micro-structure, flexural strength, elastic modulus and so on. Sintered density, flexural strength and elastic modulus of fabricated LPS-SiC increased with increasing the sintering temperature. In the micro-structure of this specimen, it was found that grain of sintered body was grown from 30 nm to 200 nm.