• Journal of Ocean Engineering and Technology
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• v.20 no.5 s.72
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• pp.77-81
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• 2006

In this study, monolithic liquid phase sintered SiC (LPS-SiC) was made by the hot pressing method with nano-SiC powder, whose particle size is 30 nm and less on the average. Alumina ($Al_{2}O_{3}$), yttria ($Y_{2}O_{3}$), and silica ($S_{i}O_{2}$) were used for sintering additives. To investigate the effects of $S_{i}O_{2}$, the $Al_{2}O_{3}/Y_{2}O_{3}$ composition was fixed and the ratio of $S_{i}O_{2}$ was changed, with seven different ratios tested. And to investigate the effects of the sintering temperature, the sintering temperature was changed, with $1760^{\circ}C,\;1780_{\circ}C$, and $1800_{\circ}C$ being used with a $S_{i}O_{2}$ ratio of 3 wt%. The materials were sintered for 1 hour at $1760^{\circ}C,\;1780^{\circ}C$ and $1800^{\circ}C$ under a pressure of 20 MPa. The effects on sintering from the sintering system used, as well as from the composition of the sintering additives, were investigated by density measurements. Mechanical properties, such as flexural strength, were investigated to ensure the optimum conditions for a matrix of SiCf/SiC composites. Sintered densityand the flexural strength of fabricated LPS-SiC increased with an increase in sintering temperature. Particularly, the relative density of a sintered body at $1800^{\circ}C$ with a non-content of $S_{i}O_{2}$, a specimen of AYSO-1800, was 95%. Also, flexural strength was about 750MPa.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.13 no.6
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• pp.485-492
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• 2000

The sintering characteristics and the effects of density on dielectric properties in 0.2Ba(Mg$_{1}$3(Nb$_{2}$3/)O$_3$-0.8Ba(Mg$_{1}$3//Ta$_{2}$3/)O$_3$ceramics were investigated. The samples were made by the powder mixing techniques with the two step calcining conditions. When the 1st and the 2nd calcining temperatures were 120$0^{\circ}C$ and the sintering temperature was 155$0^{\circ}C$the density of samples showed the highest value (7.45 g/cm$^3$, 98.5% of theoretical density) among them. The dielectric constant of samples was nearly independent of density but the tan $\delta$ and the temperature coefficient of dielectric constant decreased linearly with increasing of the density. The quality factor(Q$\times$f), the temperature coefficient of resonance frequency and the dielectric constant of Ba[Mg$_{1}$3(Nb$_{0.2}$/Ta$_{0.8}$)sub 2/3/]O$_3$ceramic were 79,548 GHz, +1.5 ppm/$^{\circ}C$ and 26 in the microwave range, respectively.ely.

• 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).).

• Transactions on Electrical and Electronic Materials
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• v.5 no.3
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• pp.89-92
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• 2004

For improving solar efficiencies, down conversion of high-energy photons to visible lights is discussed. The losses due to thermalization of charge carriers generated by the absorption of high-energy photons, can largely be reduced in a solar cell if more than one electron-hole pair can be generated per incident photon. The solar cell was constructed of dye-sensitized anatase-based TiO$_2$, approximately 30nm particle size, 6$\mu\textrm{m}$thickness, and 6${\times}$6$\textrm{mm}^2$ active area, Pt counter electrode and I$_3$$\^$-/I$_2$$\^$-/ electrolyte. After correction for losses due to light reflection and absorption by the conducting glass, the conversion of photons to electric current is practically quantitative in the plateau region of the curves. The incident photon to current conversion efficiency(IPCE) of N3 used as a dye in this work is about 80% at around 590nm and 610nm which is the emission spectrum of Eu doped LGF. The Eu doped LGF powder was prepared by conventional ceramic process, and used as a down converter for DSC after spin coated on the slide glass and fired.

• Proceedings of the Materials Research Society of Korea Conference
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• 2011.05a
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• pp.58.1-58.1
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• 2011

WC-Co and other similar cemented carbides have been widely used as hard materials in industrial cutting tools and as mould metals; and a number of techniques have been applied to improve its microstructural characteristics, hardness and ear resistance. Cobalt is used primarily to facilitate liquid phase sintering and acts as a matrix, i.e. a cementing phase between WC grains. A uniform distribution of metal phase in a ceramic is beneficial for improved mechanical properties of the composite. WC-Co, starting from initial powders, is vastly used for a variety of machining, cutting, drilling, and other applications because of its unique combination of high strength, high hardness, high toughness, and moderate modulus of elasticity, especially with fine grained WC and finely distributed cobalt. In this study, that started with two different compositions of initial powders, WC-7.5wt%Co and WC-12wt%Co with initial powder size being 1~3 ${\mu}m$, magnetic pulsed compaction followed by subsequent vacuum sintering were carried out to produce consolidated preforms. Magnetic Pulsed Compaction (MPC), a very short duration (~600 ${\mu}s$), high pressure (~4 Gpa), high-density preform molding method was used with varied pressure between 0.5 and 3.0 Gpa, in order to reach an initial high density that would help improve the sintering behavior. For both compositions and varied MPC pressure, before and after sintering, changes in microstructural behavior and mechanical properties were analyzed. With proper combination of MPC pressure and sintering, samples were obtained with better mechanical properties, densification and microstructural behavior, and considerably improved than other conventional processes.

• Korean Journal of Materials Research
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• v.20 no.9
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• pp.467-471
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• 2010

Composites of ceramic powders and an elastomer-based matrix were prepared by mixing $CaCO_3$ powders with polyethylene and polypropylene elastomers, and their mechanical and sound insulation properties were measured. $CaCO_3$ powders with 0.7 ${\mu}m$ and 35 ${\mu}m$ particle size were added to elastomers up to 80 wt%. Scanning electron microscopy photographs showed uniform distribution of the $CaCO_3$ powders in the matrix. While density and surface hardness increased, melt index, tensile strength and elongation of the composites decreased as the amount of added $CaCO_3$ powders increased. As more $CaCO_3$ powders were added sound transmission loss of the composites increased owing to the increase of density. Addition of 0.7 ${\mu}m$ sized $CaCO_3$ powders resulted in a slightly higher transmission loss than the addition of 35 ${\mu}m$ sized powders because of the increased interface area between the elastomer matrix and the $CaCO_3$ powders. Composites with a polyethylene matrix showed higher transmission loss than those with a polypropylene matrix because the tensile strength and hardness of the polyethylene-based composites were low and their elongation was high.

• Nuclear Engineering and Technology
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• v.33 no.3
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• pp.307-314
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• 2001

The changes of weight gain, structure, morphology and uranium oxidation states in l0wt% G $d_2$ $O_3$-doped U $O_2$ during the oxidation below 475$^{\circ}C$ and heat treatment at 130$0^{\circ}C$ in air were investigated using TGA, XRD, SEM, EPMA and XPS. The room temperature ( $U_{0.86}$G $d_{0.14}$) $O_2$Cubic Phase Converted to highly distorted ( $U_{0.86}$G $d_{0.14}$)$_3$ $O_{8}$ -type sing1e Phase by oxidation at 475 $^{\circ}C$ in air. This oxidized phase was reduced by annealing at 130$0^{\circ}C$ in air. The room temperature XRD pattern of the 130$0^{\circ}C$ annealed powder revealed that ( $U_{0.86}$G $d_{0.14}$)$_3$ $O_{8}$ -type single phase was separated into Gd-depleted $U_3$ $O_{8}$ and Gd-enriched ( $U_{0.7}$G $d_{0.3}$) $O_2$$_{+x}$ type cubic phase. The reduction and phase separation by the high temperature annealing of kinetically metastable and highly deformed ( $U_{0.86}$G $d_{0.14}$)$_3$ $O_{8}$ -type phase are interpreted in terms of cation size difference between G $d^3$$^{+}$ and U according to the oxidation state of U.U.U.U.U.te of U.U.U.U.U.

• Corrosion Science and Technology
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• v.17 no.3
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• pp.101-108
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• 2018

AA7010 is an Al-Zn-Mg-Cu alloy containing Zr, developed as an alternate to traditional AA7075 alloy owing to their high strength combined with better fracture toughness. It is necessary to improve the corrosion resistance and surface properties of the alloy by incorporating plasma electrolytic oxidation (PEO) method. AA7010-T7452 aluminum alloy has been processed through the forging route with multi-stage working operations, and was coated with $10{\mu}m$ thick $Al_2O_3$ ceramic aluminina coating using the plasma electrolytic oxidation (PEO) method. The corrosion, stress corrosion cracking (SCC) and nano-mechanical behaviours were examined by means of potentiodynamic polarization, slow strain rate test (SSRT) and nano-indentation tests. The results indicated that the additional thermomechanical treatment during the forging process caused a fully recrystallized microstructure, which lead to the poor environmental cracking resistance of the alloy in 3.5% NaCl solution, despite the overaging treatment. Although the fabricated PEO coating improved general corrosion resistance, the brittle nature of the coating did not provide any improvement in SCC resistance of the alloy. However, the hardness and elastic modulus of the coating were significantly higher than the base alloy.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.6 no.2
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• pp.213-219
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• 1996

Pellet type $Pt/MoO_{3}$ gas sensor which is operating at much lower temperature than conventional ceramic sensors such as $SnO_{2}$ or ZnO was fabricated. Morphology and crystal structure of $Pt/MoO_{3}$ according to calcination temperature have been characterized with Transmission Electron microscopy and X-Ray powder diffraction. The characterization indicates that as calcination temperature is increased, overlayers of $MoO_{3}$ on Pt are produced, but additionally, the Cl content associated with the Pt phase diminishes. The gas dasorption test showed that the change in surface morphology is closely related to hydrogen storage capacity of the sample. The gas sensitivities at $50^{\circ}C$ and $150^{\circ}C$ are very high.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2008.11a
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• pp.217-217
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• 2008

$BaTiO_3$는 perovskite 구조를 가지는 대표적인 강유전체 재료로서 MLCC (Multi Layer Ceramic Capacitor), PTC thermistor 등에 널리 사용되어지고 있으며, 그 특성을 향상시키기 위하여 많은 연구가 진행되고 있다. 현제 $BaTiO_3$ 분말 제조의 대표적인 합성법으로는 하소와 분쇄공정이 없는 수열합성법이 대표적이나, 나노 사이즈로 제작시 $BaTiO_3$는 마이크로 크기와 달리 입방정상으로 우세한 상태로 존재한다. 이는 제조과정 중의 hydroxyl defect의 영향과 나노 분말의 표면에너지 증가 때문이라고 보고된다. 따라서 본 연구는 이러한 문제점을 해결하기 위해 일반적인 세라믹 제조 방법인 고상반응법을 이용한 나노 사이즈의 $BaTiO_3$ 제조를 위한 최적의 공정 조건을 확립하기 위하여 본 연구를 진행하였다. 조성은 $BaTiO_3$와 반응온도를 낮추기 위한 anatase의 $TiO_2$를 사용하였고, $BaCO_3/TiO_2$ 의 조성비 (1. 1.01, 1.02, 1.03)를 제어하여 혼합한 후, 24h ball-mill 하여 하소 온도 ($860^{\circ}C{\sim}1000^{\circ}C$) 변화에 따른 입자 사이즈와 입도 분포를 측정하였다. 제조된 $BaTiO_3$분말의 결정 구조 분석을 위하여 XRD (X-ray diffraction) 분석을 수행 하였는데, 분석 결과로부터 제조된 분말들이 정방정 (tetragonal)의 perovskite구조를 갖고 있음을 확인하였다. 또한 분말의 미세구조 확인을 위하여 SEM (scanning electron microscope) 관찰을 수행하였는데, 나노 사이즈의 구형 분말을 얻을 수 있음을 확인할 수 있었다.