• The Korean Journal of Ceramics
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• 제6권2호
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• pp.155-158
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• 2000

NMR study has been used for measuring precise quantity of the amorphous phase in the $Si_3N_4$powder. Care must be taken to allow the $^{29}$Si nuclear spin system to fully relax between pulses in order to make the signals proportional to the number of nuclei in each phase. $^{29}$Si MAS spectrum was decomposed into the three spectra of $\alpha$-, $\beta$-, and amorphous $Si_3N_4$assuming pseudo-Voigt function. Moreover, the Rietveld analysis of the powder X-ray diffraction data was performed to measure quantity of crystalline phases as $\alpha/\beta$ ratio.

• 대한금속재료학회지
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• 제50권11호
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• pp.839-845
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• 2012

The effect of an AlN additive on the thermoelectric properties of SiC ceramics was studied. Porous SiC ceramics with 48-54% relative density were fabricated by sintering the pressed ${\alpha}-SiC$ powder compacts with AlN at $2100-2200^{\circ}C$ for 3 h in an Ar atmosphere. In the undoped specimens, the Seebeck coefficients were positive (p-type semiconducting) possibly due to a dominant effect of the acceptor impurities (Al, Fe) contained in the starting powder. With AlN addition, the reverse phase transformation of 6H-SiC to 4H-SiC was observed during the sintering process. The electrical conductivity of the AlN doped specimen was larger than that of the undoped specimen under the same conditions, which might be due to a reverse phase trans-formation. The Seebeck coefficient of the AlN doped specimen was also larger than that of the undoped specimen. The density of specimen and the amount of addition had significant effects on the thermoelectric properties.

• 한국분말야금학회:학술대회논문집
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• 한국분말야금학회 2006년도 Extended Abstracts of 2006 POWDER METALLURGY World Congress Part2
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• pp.1091-1092
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• 2006

Silicon nitride - silicon carbide composite was developed by using an abrasive SiC powders as a raw material. The composites were prepared by mixing abrasive SiC powder with silicon, pressing and sintering at $1400^{\circ}C$ under nitrogen atmosphere in atmosphere controlled vacuum furnace. The proportion of silicon in the initial mixtures varied from 20 to 50 wt%. After sintering, crystalline phases and microstructure were characterized. All composites consisted of ${\alpha}-Si_3N_4$ and ${\beta}-Si_3N_4$ as the bonding phases in SiC matrix. Their physical and mechanical properties were also determined. It was found that the density of the obtained composites increased with an increase in the $Si_3N_4$ content formed in the reaction.

• 한국분말재료학회지
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• 제5권2호
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• pp.133-138
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• 1998

The effect of $\beta$-$Si_3N_4$ seeding on microstructural development of silicon nitride based materials has been investigated. In particular, to observe more distinctly the abnormal grain growth in pressureless sintering, fine $\alpha$-$Si_3N_4$(mean particle size: 0.26 ${\mu}m$) powder classified by sedimentation method was used. It was possible to prepare silicon nitride with abnormally grown grains under low nitrogen pressure of 1 atm thanks to the heterogeneous nucleation on $Si_3N_4$ seed particles. The size and morphology of silicon nitride grains were strongly influenced by the presence of $\beta$-$Si_3N_4$ seed and overall chemical composition. For specimens with initially low $\beta$-content, the large grains grew without a significant impingement by other large grains. On the contrary, for specimens with initially high $\beta$-content, steric hindrance was effective. The resulting microstructure was less inhomogeneous and characterized by unimodal grain size distribution.

• 한국세라믹학회지
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• 제22권4호
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• pp.33-39
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• 1985

A composition containing a-$Si_3N_4$ with 5w/0 $Y_2O_3$ and 4w/0 $Al_2O_3$ was hot-pressed at 1, $650^{\circ}C$ and 350kg/$cm^2$ for 1.5hrs and specimens of the same composition were pressureless-sintered at 1, 75$0^{\circ}C$ for 1.5 and 5hrs. By X-ray diffraction it was found that hot-pressed specimens were consisted of $\alpha$-and $\beta$-$Si_3N_4$ and sintered specimens were consisted of $\beta$-$Si_3N_4$ and $Si_3N-4Y_2O_3$ which was crystallized out from the grainboundary phase. The 5-hr sintered specimens had higher degree of crystallization than the 1.5 hr sintered specimens. Among these three different specimens the 5-hr sintered specimens showed the highest strength by hot MOR test at 1, 00$0^{\circ}C$. The SPT diagram for the 5-hr sintered $Si_3N_4$ was constructed by measurements of the stress rate dependence of fracture strength.

• 한국세라믹학회지
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• 제21권1호
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• pp.11-18
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• 1984

$eta$-Sialon synthesis was investigated via the simultaneous reduction and nitridation of Hadong Pink Kaolin using the graphite as a reducing agent at 135$0^{\circ}C$ under 80% $N_2-20%H_2$ atmosphere. When Hadong Pink Kaolin-graphite-silicon nitride seed(molar ratio ; $SiO_2:C:Si_3N_4$=1;3.5:0.05) mixture was heated at 135$0^{\circ}C$ for as long as 20h in 80%$N_2-20%H_2$ atmosphere a homogeneous $eta$-Sialon$(Si_{3.5}Al_{2.5}O_{2.5}N_{2.5})$ was mainly formed together with a small amount of $\alpha$-$Si_3N_4$.

• 한국분말재료학회지
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• 제10권2호
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• pp.118-122
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• 2003

Alpha-SiAlON ceramics having various compositions and modifying cations were investigated with respect to their phase stability, transformation kinetics. and resulting microstructures. Each composition was heat treated at 150$0^{\circ}C$ for 1h and measured the $\alpha$-SiAlON transformation. The phase-boundary composition in the single-phase $\alpha$-SiAlON region showed sluggish transformation from $\alpha$-$Si_3N_4$ to $\alpha$-SiAlON compared to the phase-center composition in the diagram. Using the different rare earth modifying cations, dependence of transformation kinetics on the phase stability in a fixed composition was also explained. By changing size of the stable u-phase region with exchanging cations, systematic change in transformation was observed. Transformation rate of $\alpha$-SiAlON at low temperature has an important role on controlling the final microstructure. Less transformation gives more chances to develop elongated grain in the microstructure.

• 한국세라믹학회지
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• 제34권4호
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• pp.406-412
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• 1997

Al2O3/SiC hybrid-composite has been fabricated by the conventional powder process. The addition of $\alpha$-Al2O3 as seed particles in the transformation of ${\gamma}$-Al2O3 to $\alpha$-Al2O3 provided a homogeneity of the microstructure. The grain growth of Al2O3 are significantly surpressed by the addition of nano-size SiC particles. Dislocation were produced due to the difference of thermal expansion coefficient between Al2O3 and SiC and piled up on SiC particles in Al2O3 matrix, resulting in transgranular fracture. The high fracture strength of the composite was contributed to the grain refinement and the transgranular fracture mode. The addition of SiC platelets to Al2O3/SiC nano-composite decreased the fracture strength, but increased the fracture toughness. Coated SiC platelets with nitrides such as BN and Si3N4 enhanced fracture toughness much more than non-coated SiC platelets by enhancing crack deflection.

• 한국결정성장학회지
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• 제7권2호
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• pp.207-215
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• 1997

Ionized Cluster Beam Deposition(ICBD)방법을 이용하여 Si(100)기판과 TiN(60 nm)/Si(100)기판위에 Al 박막을 증착하였다. 증착된 Al 박막의 증착특성은 $\alpha$-step, four-point-probe, XRD, SEM, AES 측정장치를 가지고 조사해 보았다. 도가니 온도가 증가함에 따라 Al 박막의 증착속도는 증가하였고 비저항 값은 감소하였다. 도가니 온도가 $1800^{\circ}C$인 경우 가속전압이 증가함에 따라 연속적이며 평평한 박막이 형성되고 비저항이 감소되었다. 최소의 비저항 값은 Si 기판에서는 가속전압이 4 kV일 때 3.4 $\mu \Omega \textrm {cm}$, TiN 기판에서는 가속전압이 2kV일 때 3.6 $\mu \Omega \textrm {cm}$. AES 분석결과 형성된 박막내에서는 불순물이 존재하지 않는 것을 알 수 있었다. 따라서 Al 박막의 비저항은 박막충의 미세구조에 의해 영향을 받는다.

• 한국세라믹학회지
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• 제27권3호
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• pp.412-422
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• 1990

Porous SiC ceramics were proposed to be promising materials for high-temperature thermoelectric energy conversion. Throughthe thermoelectric property measurements and microstructure observations on the porous alpha SiC and the mixture of $\alpha$-and $\beta$-SiC, it was experimentally clarified that elimination of stacking faults and twin boundaries by grain growth is effective to increase the seebeck coefficient and increasing content of $\alpha$-SiC gives rise to lower electrical conductivity. Furthermore, the effects of additives on the thermoelectric properties of 6H-SiC ceramics were also studied. The electrical conductivity and the seebeck coefficient were measured at 35$0^{\circ}C$ to 105$0^{\circ}C$ in argon atmospehre. The thermoelectric conversion efficiency of $\alpha$-SiC ceramics was lower than that of $\beta$-SiC ceramics. The phase homogeneity would be needed to improve the seebeck coefficient and electrical conductivity decreased with increasing the content of $\alpha$-phase. In the case of B addition, XRD analysis showed that the phase transformation did not occur during sintering. On the other hand, AlN addiiton enhanced the reverse phase transformation from 6H-SiC to 4H-SiC, and this phenomenon had a great effect upon the electrical conductivity.