• Journal of the Korean Ceramic Society
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• v.39 no.11
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• pp.1074-1082
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• 2002

Nanocomposite PMN-PT-BT/Ag/MgO was prepared by sintering at $950{\circ}C$ with addition of $AgNO_3$ and MgO sol to the PMN-PT-BT powder sinterable at $1200{\circ}C$. The low-temperature-sinterable PMN-PT-BT/Ag powder prepared by the modified mixed oxide method was calcined at $600{\circ}C$ for 1h and surface modified with the MgO sol of 0-10 wt% and then subjected to consolidation at $850-950{\circ}C$ for 4h under a flowing oxygen. The nanocomposite PMN-PT-BT/Ag/MgO(0.5wt%) sintered at $950{\circ}C$ showed the microstructure with grains of $1-3{\mu}m$, the second phase of MgO of $0.1-0.3{\mu}m$ by SEM and Ag of << $1{\mu}m$ qualitatively by SIMS. It showed the sintered relative density of 99%, the room temperature dielectric constant of 17200, the dielectric loss of 2.1% and the specific resistivity of $5.46{\times}10^{12}{\Omega}{\cdot}cm$. But the PMN-PT-BT/Ag/MgO(0 wt%) nanocomposite sintered at $950{\circ}C$ showed a little better properties : the sintered relative density of 99.5%, the room temperature dielectric constant of 19500, the dielectric loss of 2.1% and the specific resistivity of $7.30{\times}10^{12}{\Omega}{\cdot}cm$.

• Journal of the Korean Ceramic Society
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• v.36 no.1
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• pp.97-105
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• 1999

In the binary system of SiC and carbon, porosity and pore size distribution of green body was controlled by varying pH, by the addition of polyelectrolyte dispersants, and by using different particle size of starting powders. The preforms having different green microstructure were fabricated by slip casting from suspensions having different dispersion condition. The reaction bonding process was carried out for these preforms. The condition of reaction bonding was 1600$^{\circ}C$ and 20 min. under vacuum atmosphere. The analyses of optical and SEM were studied to investigate the effect of green microstructure on that of reaction bonded silicon carbide and subsequently the mechanical properties of sintered body was investigated. Different green microstructures were obtained from suspensions having different dispersion condition. It was found that the pore size could be remarkably reduced for a fine SiC(0.5$\mu\textrm{m}$). The bimodal microstructure was not found in the present study, which is frequently observed in the typical reaction bonded silicon carbide. It is considered that the ratio between SiC and C was responsible for the formation of bimodal microstructure. For the preform fabricated from the well dispersed suspension, the 3-point bending strength of reaction-bonded silicon carbide was 310${\pm}$40 MPa compared to the specimen fabricated from relatively agglomerated particles having lower value 260${\pm}$MPa.

• Journal of the Korean Ceramic Society
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• v.34 no.3
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• pp.289-295
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• 1997

Effect of ceramic processing was investigated on the microstructure and electronic properties of low loss Mn-Zn ferrite. Addition of CaO and SiO2 to calcined powder rather than to raw materials mixtured resulted in finer-grained microstructure. Higher oxygen pressure during sintering caused microstructural inhomogeneity and the increase in power loss and disaccommodation factor. Relatively low power loss was found for sintering up to 130$0^{\circ}C$ from powders calcined at high temperature and milled shortly. It was caused by slow densification rate and normal grain growth up to 130$0^{\circ}C$. Calcination at low temperature and prolonged milling enhanced den-sification, which gave a fine grained microstructure and low powder loss at sintering temperture below 120$0^{\circ}C$. Sintering temperature above 125$0^{\circ}C$, however, showed abnormal grain growth.

• Journal of the Korean Ceramic Society
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• v.39 no.10
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• pp.948-954
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• 2002

Porous reaction bonded SiC with high fracture strength was developed using Si melt infiltration method for use of the support layer in high temperature gas filter that is essential to develop the next generation power system such as integrated gasification combined cycle system. The porosity and pore size of porous RBSC developed in this study were in the range of 32∼36% and 37∼90 ${\mu}m$ respectively and the maximum fracture strength of porous RBSC fabricated was 120 MPa. The fracture strength and thermal shock resistance of porous RBSC fabricated by Si melt infiltration were much improved compared to those of commercially available porous clay bonded SiC due to the formation of the strong SiC/Si interface between SiC particles. The characteristics of pore structure of porous RBSC was varied depending on the amounts of residual Si as Well as the size of SiC particle used in green body.

• Journal of the Korean Ceramic Society
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• v.34 no.2
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• pp.195-201
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• 1997

The present paper describes the effect of dry mixed grinding on kaolinite-aluminum trihydroxide mixture with a planetary ball mill before sintering and its influence on mullite formation during sintering. The size reduction of the mixture is market in the early stage of grinding and the obtained fine particles agglomerate subsequently with an increase of grinding time. The crystal structure of the mixture is collapsed easily into an amorphous one by planetary ball milling, of which amount increases with an increase of grinding time. Only mullite phase except for anatase as an inherent impurity in kaolinite appeared in the sintered body of the mixtures with mixed grinding as relatively lower temperature 1523K, while corundum, cristobalite, and Al-Si spinel phases, besides mullite were formed in the sintered body of the mixture without mixed grinding. Therefore, the mixed grinding treatment is very effective to improve the homogeneous mixing and disp-ersion of the mixture of raw materials on a micro scale and to decrease the thermal decomposition tem-perature by crystal structure change of them so as to obatin direct preparation of mullite with high purity at relatively low temperature.

• Journal of the Korean Ceramic Society
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• v.41 no.3
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• pp.190-196
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• 2004

Yttrium Aluminum Garnet (YAG) powders were synthesized by precipitation of solutions of Al and Y nitrates using ammonium hydrogen carbonate as a precipitant. Y$_2$O$_3$ and YAG phases were formed in the precipitates, which had been attrition-milled. Well-crystallized, phase-pure YAG powders were obtained after calcination of the milled precipitates at 1100$^{\circ}C$ for 1 h. The powders were found to exhibit an excellent sinterability regardless of the addition of SiO$_2$(500 ppm Si) as a sintering aid. All samples already densified to relative densities greater than 70% at 1300$^{\circ}C$ and relative densities of ∼83％ at 1400$^{\circ}C$. The samples doped with SiO$_2$ showed a little improvement in densification as compared with those for the undoped samples and resulted in a relative density of 97% at 1600$^{\circ}C$.

• Journal of the Korean Ceramic Society
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• v.39 no.8
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• pp.769-774
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• 2002

The powder mixture in which Fe-Ni alloy particles of 20 nm were homogeneously dispersed on $Al_2O_3$ particle surfaces was prepared by hydrogen reduction of $Al_2O_3$ and metal oxide powders. $Al_2O_3$/Fe-Ni nanocomposites fabricated by pressureless sintering were only composed of $Al_2O_3$ and ${gamma}$-Fe-Ni phases and achieved over 98% of the theoretical density at the sintering temperature above $1350^{\circ}C$. The highest strength and toughness of the composites were 574 MPa and 3.9 MP$a{\cdot}m1/2$, respectively. These values were about 20% higher than these of monolithic $Al_2O_3$ sintered at the same conditions. Nanocomposites showed ferromagnetic properties and coercive force was increased with decrease of the average particle size of dispersions.

• Journal of the Korean Ceramic Society
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• v.36 no.1
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• pp.89-96
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• 1999

Manufacturing conditions of the porous alumina mold were established to overcome various limits of the gypsum mold. For the preparations of the porous alumina mold, an activated charcoal was added to the Al2O3 with the wt% variation and then mixed. The binary slurry was study dispersed based on the examination of the ESA and rheological behaviro. The cylinder type alumina mold was cast in the gyspum mold and characterized by the shrinkage rate at the variable sintering temperature and the resistance against wear. It was proper to make a sintering of the Al2O3 by the surface diffusion which was non-shrinkage sintering mechansim, and intergranular neck growed stronger while sintering was being made. We studied a sintering by three categories; 1) thermodynamic method below 1,000$^{\circ}C$, 2) kinetic method above 1,000$^{\circ}C$ and 3) combined method. In the results of the respective works, combined method was superiro to the others. The prepared Al2O3 mold had relatively high strength, low drying rate, the resistance against the acid or base and good casting behavior.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.33 no.6
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• pp.261-267
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• 2023

Crystal structure, microstructure, and dielectric properties of the (Ca, Sr)(Zr, Ti)O₃ (CSZT) system has been studied as a function of sintering temperature and MgO addition for microwave applications. A single-phase CSZT powder with the orthorhombic crystal structure was obtained by the solid-state reaction method. The powder compacts were sintered at 1200℃, 1300℃, and 1400℃ respectively. All the sintered samples had a single-phase orthorhombic crystal structure and grain size increased with sintering temperature. In the case of 1 mol% MgO addition, the orthorhombic crystal structure was the main phase; however, a secondary phase appeared during sintering at 1400℃, as determined by EDS analysis. At 1400℃, the undoped and MgO-doped CSZT had almost similar grain size distribution and densification but the grain size distribution became slightly narrow. The MgO-doped CSZT showed excellent low-loss dielectric properties: ε_r = 34.14, tanδ = 0.00047, τ_ε = -3.58 ppm/℃ at 1 MHz.

• Journal of the Korean Ceramic Society
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• v.39 no.10
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• pp.919-927
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• 2002

In order to fabricate double-layered porous materials powders of different particle sizes were pressed stepwise. Ford＇s equation which predicts the fired density with the change in pressed density was employed in order to adjust the difference in sintering shrinkage of the green body with double-layered porous structure. Double-layered porous materials were characterized by investigating microstructures and permeability. SEM micrographs showed the distinct difference in pore sizes of double-layered porous material. Permeability of single-layered porous material increased by increasing the starting particle sizes and porosity as well. Permeability of the double-layered porous material depends largely on the layer of small pore diameter.