• Journal of the Korean Ceramic Society
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• v.31 no.9
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• pp.1021-1029
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• 1994

SiC ultrafine particles of 1, 10, 20 and 30 vol% were dispersed in $\alpha$-Si3N4 matrix and hot-pressed under the condition of 30 MPa at 1800 and 190$0^{\circ}C$ respectively. Physical, mechanical properties and microstructures of sintered Si3N4-SiC nanocomposites were investigated. Flexural strength and density of Si3N4-10 vol% SiC nanocomposites hot-pressed at 190$0^{\circ}C$ represented the 1002 MPa and 97.9%T.D respectively, and it was confirmed as a remarkable improvement of 67% compared to Si3N4 monolith. Fracture toughness was shown as 7.2 MPa.m1/2 when the same composition was hot pressed at 180$0^{\circ}C$. This effect was supposed to be due to the improvement of microstructure by the adequate suppression of the excessive growth of Si3N4 grain with SiC nano-particles.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.6 no.3
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• pp.392-398
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• 1996

$Si_{3}N_{4}/SiC$ nanocomposites reinforced with tow different mean particle size were fabricated by hot press. Grain growth of matrix gran was inhibited by adding of SiC particles, and then number of equiaxed and fine grains were increased. The effect of grain growth inhibition was higher in the nanocomposites dispersed small size SiC. herefore fracture strength and hardness were increased, but fracture toughness was decreased in small size SiC dispersed samples.

• Journal of the Korean Ceramic Society
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• v.37 no.2
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• pp.152-157
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• 2000

Si3N4/SiC nanocomposite ceramics containing 5 wt%dispersed SiC particles were prepared by gas-pressure-sintering at 200$0^{\circ}C$ under nitrogen atmosphere. SiC particles with average sizes of 0.2 and 0.5${\mu}{\textrm}{m}$ were used, and the effect of the SiC particle size on the microstructure was investigated. The addition of SiC particles effectively suppressed the growth of the Si3N4 matrix grains. The effect of grain growth inhibition was higher in the nanocomposites dispersed with fine SiC. SiC particles were dispersed uniformly inside Si3N4 matrix grains and on grain boundaries. When the fine SiC particles were added, large fraction of the SiC particles was trapped inside the grains. On the other hand, when the large SiC particles were added, most of the SiC particles were located on grain boundaries. Typically, the fraction of SiC particles located at grain boundaries was higher in the specimen prepared from $\beta$-Si3N4 than in the specimen prepared from $\alpha$-Si3N4.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.6 no.4
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• pp.552-557
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• 1996

Microstructural development of $Si_3N_4$/20 vol% SiC nanocomposites doped 2 wt% $Al_2O_3$ and 6 wt% $Y_2O_3$ as sintering additives were analyzed by sintering interruption. Density of samples was significantly increased between $1500^{\circ}C$ and $1700^{\circ}C$, and near full density was achieved at $1800^{\circ}C$. Transformation rate from $\alpha-Si_3N_4$ to $\betha-Si_3N_4$ was increased at $1700^{\circ}C$ and $1800^{\circ}C$, and then elongated matrix grains were appeared. Small size SiC particles had suppressive effect on densification rate and transformation of $Si_3N_4$ phase.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.6 no.3
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• pp.386-391
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• 1996

SiC particles (average size is 270 nm) of 0, 10, 20, 30, 40 vol% were dispersed in $Si_{3}N_{4}$, and $Si_{3}N_{4}/SiC$ nanocomposites were fabricated by hot press. After sintering, matrix phase, ${\alpha}-Si_{3}N_{4}$ was transformed to ${\beta}-Si_{3}N_{4}$, and second phase, ${\beta}-SiC$ was not changed. No grain boundary crystalline phase by adding of sintering additives was detected. Grain growth of $Si_{3}N_{4}$ was supressed with increasing of SiC contents, and then fine grain was occurred. The highest fracture strength was obtained at 10 vol% SiC, and fracture toughness was decreased, but hardness was linearly increased with SiC content.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.6 no.4
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• pp.558-563
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• 1996

Fracture strength of $Si_{3}N_{4}/20$ vol% SiC nanocomposites with fifferent sintering additives was measured. Strength of nanocomposites with 6 wt% $Y_{2}O_{3}$ and 2 wt% $Al_{2}O_{3}$ as sintering additives was higher at room temperature but significant strength degradation at elevated temperature was occured due to the softening of grain boundary phase. Fracture strength of 8 wt% $Y_{2}O_{3}$ doped sample was higher than that of $Al_{2}O_{3}$ added sample at $1400^{\circ}C$. The retention of high temperature strength in 8 wt% $Y_{2}O_{3}$ doped sample can be attributed to high softening temperature and crystallization of grain boundary glassy phase.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 1996.11a
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• pp.6-6
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• 1996

Ceramic based nanocomposite, in which nano-sized ceramics and metals were dispersed within matrix grains and/or at grain boundaries, were successfully fabricated in the ceramic/cerarnic and ceramic/metal composite systems such as $Al_2O_3$/SiC, $Al_2O_3$/$Si_3N_4$, MgO/SiC, mullite/SiC, $Si_3N_4/SiC, $Si_3N_4$/B, $Al_2O_3$/W, $Al_2O_3$/Mo, $Al_2O_3$/Ni and $ZrO_2$/Mo systems. In these systems, the ceramiclceramic composites were fabricated from homogeneously mixed powders, powders with thin coatings of the second phases and amorphous precursor composite powders by usual powder metallurgical methods. The ceramiclmetal nanocomposites were prepared by combination of H2 reduction of metal oxides in the early stage of sinterings and usual powder metallurgical processes. The transmission electron microscopic observation for the $Al_2O_3$/SiC nanocomposite indicated that the second phases less than 70nm were mainly located within matrix grains and the larger particles were dispersed at the grain boundaries. The similar observation was also identified for other cerarnic/ceramic and ceramiclmetal nanocornposites. The striking findings in these nanocomposites were that mechanical properties were significantly improved by the nano-sized dispersion from 5 to 10 vol% even at high temperatures. For example, the improvement in hcture strength by 2 to 5 times and in creep resistance by 2 to 4 orders was observed not only for the ceramidceramic nanocomposites but also for the ceramiclmetal nanocomposites with only 5~01%se cond phase. The newly developed silicon nitride/boron nitride nanocomposites, in which nano-sized hexagonal BN particulates with low Young's modulus and fracture strength were dispersed mainly within matrix grains, gave also the strong improvement in fracture strength and thermal shock fracture resistance. In presentation, the process-rnicro/nanostructure-properties relationship will be presented in detail. The special emphasis will be placed on the understanding of the roles of nano-sized dispersions on mechanical properties.

• Proceedings of the Materials Research Society of Korea Conference
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• 1997.05a
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• pp.72-72
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• 1997

• The Korean Journal of Ceramics
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• v.6 no.3
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• pp.245-249
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• 2000

Consolidation of amorphous powders is emerging as a route for synthesis of high strength composite materials. Diffusion processes necessary for consolidation are expected to be more rapid in amorphous state(SRO) than in the crystalline state(LRO). A new synthesis technique of exploiting polymeric ceramic precursors(polysilazane and polyborosilazane) is derived for Si$_3$N$_4$/SiC and boron-modified nanocomposites for extremely high temperature applications up to 200$0^{\circ}C$. The characterization methods include thermal analysis of DTA, and XRD, NMR, TEM, after pyrolysis, as a function of time and temperature.

• Proceedings of the Korea Association of Crystal Growth Conference
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• 1996.06a
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• pp.534-536
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• 1996