• 한국세라믹학회지
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• 제48권3호
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• pp.257-262
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• 2011

Dependencies of dielectric properties on $MgTa_2O_6$, $MgNb_2O_6$, and $MgWO_4$ (Mg-based ceramics) fillers of the polystyrene (PS) matrix composites were investigated as a function of frequency. With increasing frequency from 1 GHz to 7.3 GHz, the dielectric constant (K) of the composites was not changed significantly, while the dielectric loss (tan${\delta}$) of the composites was slightly decreased. The K, tan${\delta}$, and temperature coefficient of resonant frequency (TCF) of the composites were dependent on the type and amount of ceramics at 11 GHz. Also, several theoretical models have been employed to predict the effective dielectric constant of the composites and the results were compared with experimental data. Typically, a K value of 6.67, tan${\delta}$ of $0.56{\times}10^{-3}$, and TCF of -4.99 $ppm/^{\circ}C$ were obtained for the PS composites with 0.4 volume fraction of $MgNb_2O_6$ at 11 GHz.

• Journal of Ceramic Processing Research
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• 제19권6호
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• pp.504-508
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• 2018

Diopside is a ceramic material with excellent physical and chemical properties. However, when it is applied as an LED packaging material, heat dissipation of the LED element is not sufficient due to its relatively lower thermal conductivity, which may cause degradation of the LED function. In this study, glass-ceramics based on a $ZrO_2-CaO-MgO-SiO_2$ system, in which diopside is the main crystal phase, were prepared by heat-treating the glass, which was composed of zircon ($ZrO_2-SiO_2$) powders and diopside ($CaO-MgO-2SiO_2$) powders. The possibility of using the glass-ceramics as a packaging material for LEDs was then investigated by analyzing the density, shrinkage, thermal conductivity, and phases generated according to the amount of zircon powder added. The density and shrinkage of specimens decreased slightly and then increased again with the amount of $ZrO_2-SiO_2$ added within a range of 0~0.38 mol. Even though the crystal phase of zircon does not appear in the $ZrO_2-CaO-MgO-SiO_2$ system, the glass containing 0.38 mol zircon powder showed the highest thermal conductivity, 1.85 W/mK, among the specimens fabricated in this study: this value was about 23% higher than that of pure diopside. It was found that the thermal conductivity of the glass-ceramics based on a $ZrO_2-CaO-MgO-SiO_2$ system was closely related to the density, but not to the phase type. Zirconia ($ZrO_2$), a component oxide of zircon, plays an important role in increasing the density of the specimen. Furthermore the thermal conductivity of glass-ceramics based on a $ZrO_2-CaO-MgO-SiO_2$ system showed a nearly linear relationship with thermal diffusivity.

• 한국재료학회지
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• 제28권10호
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• pp.551-563
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• 2018

Visible and IR windows require a combination of high optical transparency and superior thermal and mechanical properties. Materials, fabrication and characterization of transparent ceramics for visible/IR windows are discussed in this review. The transparent polycrystalline $Y_2O_3$, $Y_2O_3-MgO$ nanocomposites and $MgAl_2O_4$ spinel ceramics are fabricated by advanced ceramic processing and the use of special sintering technologies. Ceramic processing conditions for achieveing fully densified transparent ceramics are strongly dependent on the initial powder characteristics. In addition, appropriate use of sintering technologies, including vacuum sintering, hot-pressing and spark plasama sintering methods, results in outstanding thermal and mechanical properties as well as high optical transparency of the final products. Specifically, the elimination of light scattering factors, including residual pores, second phases and grain boundaries, is a key technique for improving the characteristics of the transparent ceramics. This paper discusses the current research issues related to synthesis methods and sintering processes for yttria-based transparent ceramics and $MgAl_2O_4$ spinel.

• 한국재료학회지
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• 제22권3호
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• pp.111-117
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• 2012

The effect of Al addition on the precipitation behavior of a binary Mg-Zn alloy was investigated based on the changes in the morphology, distribution and element concentration of precipitates formed during aging treatment. The as-cast Mg-6.0 mass%Zn (Mg-6Zn) and Mg-6.0 mass%Zn-3.0 mass%Al (Al-added) were homogenized at 613 K for 48 h and at 673 K for 12 h; they were then solid solution treated at 673 K for 0.5 h and 1 h, respectively. The Mg-6Zn and Al-added alloys were aged at 403 K and 433 K. The peak hardness of the Al-added alloy was higher than that of the Mg-6Zn alloy at each aging temperature. Rod-like, plate-like, blocky, and lath-like precipitates were observed in the Al-added alloy aged at 433 K for 230.4 ks, although the rod-like and plate-like precipitates were observed in the TEM microstructure of the Mg-6Zn alloy aged at 433 K for 360 ks. Moreover, the precipitates in the Al-added alloy were refined and densely distributed compared with those in the Mg-6Zn alloy. The Cliff-Lorimer plots obtained by the EDS analysis of the rod-like ${\beta}_1^'$ and plate-like ${\beta}_2^'$ phases in the Al-added alloy peak aged at 433 K for 230.4 ks were examined. It was confirmed that the ${\beta}_2^'$ phases had higher concentration of solute Al atom than was present in the ${\beta}_1^'$ phases, indicating that the properties of precipitates can be changed by Al addition.

• 한국분말야금학회:학술대회논문집
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• 한국분말야금학회 2006년도 Extended Abstracts of 2006 POWDER METALLURGY World Congress Part 1
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• pp.368-369
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• 2006

By means of magnetic pulsed compaction and sintering of weakly aggregated alumina based nanopowders the jet forming nozzle samples for the hydroabrasive cutting were fabricated. The ceramics was obtained from pure alumina, as well as from alumina, doped by $TiO_2$, MgO and AlMg. It was shown that the samples sintered from AlMg doped $Al_2O_3$ powder have the best mechanical properties and structural characteristics: relative density ${\sim}0.97$, channel microhardness. - 18-20 GPa, channel surface roughness ${\sim}0.7\;{\mu}m$, average crystallite size ${\sim}1\;{\mu}m$.

• 한국결정학회지
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• 제11권3호
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• pp.176-181
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• 2000

Computer-based atomistic simulation methods are applied to address quantitatively the crystal defect chemistry of strontium hexaaluminate, SrAl/sub 12/O/sub 19/. Our calculations show that oxygen Frenkel disorder is the dominant intrinsic defect mode to be expected in the multi-component oxide, though Schottky disorder may also exist. When La and Mg enter into SrAl/sub 12/O/sub 19/. Mg prefers to occupy Al(3)4f tetrahedral sites in the magnetoplumbite structure. Our calculations also indicate that O/sub Sr/ defect is improbable in the structure.

• 한국산학기술학회논문지
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• 제10권8호
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• pp.2150-2156
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• 2009

지르코니아($3Y-ZrO_2$)에 frit을 첨가하고 MgO를 조제로 첨가하거나 혹은 하소된 성형체에 액상의 착색료에 침지하는 방법을 통하여 착색하고 얻어진 소결체의 물성과 색상 변화를 고찰하였다. Frit가 20 wt% 첨가된 지르코니아-frit조성에 MgO가 첨가됨에 따른 색상의 변화는 소결온도가 높아짐에 따라 구별가능한 정도로 효과가 있음을 확인하였다. 이 때 색상의 변화 정도는 MgO의 첨가량 변화에 의한 효과 보다는 소결 온도의 변화에 의한 효과가 더 크게 나타났음을 확인할 수 있었다. 액상의 착색료에 침지한 후 소결한 경우에서는 frit의 첨가량이 증가함에 따라 반사도가 증가하는 것으로 나타났으며 착색료에 침지하지 않은 경우에 비하여 색상의 변화가 감소함을 알 수 있었고 frit은 색상의 변화를 억제하는 것으로 나타났다.

• 한국세라믹학회지
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• 제45권1호
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• pp.1-29
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• 2008

In last two decades, an impressive progress has been recorded in terms of developing new materials or refining existing material composition/microstructure in order to obtain better performance in biomedical applications. The success of such efforts clearly demands better understanding of various concepts, e.g. biocompatibility, host response, cell-biomaterial interaction. In this article, we review the fundamental understanding that is required with respect to biomaterials development, as well as various materials and their properties, which are relevant in applications, such as hard tissue replacement. A major emphasize has been placed to present various design aspects, in terms of materials processing, of ceramics and polymer based biocomposites, Among the bioceramic composites, the research results obtained with Hydroxyapatite (HAp)-based biomaterials with metallic (Ti) or ceramic (Mullite) reinforcements as well as $SiO_2-MgO-Al_2O_3-K_2O-B_2O_3-F$ glass ceramics and stabilized $ZrO_2$ based bioinert ceramics are summarized. The physical as well as tribological properties of Polyethylene (PE) based hybrid biocomposites are discussed to illustrate the concept on how can the physical/wear properties be enhanced along with biocompatibility due to combined addition of bioinert and bioactive ceramic to a bioinert polymeric matrix. The tribological and corrosion properties of some important orthopedic metallic alloys based on Ti or Co-Cr-Mo are also illustrated. At the close, the future perspective on orthopedic biomaterials development and some unresolved issues are presented.

• 한국세라믹학회지
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• 제17권1호
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• pp.20-26
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• 1980

The crystallization behaviors of slag-based glass in $CaO-MgO-Al_2O_3-SiO_2(-Na_2O)$ system have been studied. The mother glass containing 16.50 CaO, 7.50MgO, 19.70Al2O3, 50.80SiO3 and 2.09wt% $Na_2O$ was prepared by using Korean domestic raw materials such as granulated slag, serpentine, sea sand and etc. The glass-ceramics composed of major crystalline phase diopside was produced by the heat treatment in a temperature range from 850$^{\circ}$ to 9$25^{\circ}C$ for 0-6hr. The composition and morphology of diopside phase formed in the system were examined by X-ray diffraction analysis and electron microscopy. The kinetic measurements such as J.M. A plot and Arrhenius plot indicated that the process of nucleation of the initially formed diopside phase could be described from the view point of instantaneous nucleation. It was also demonstrated that the linear crystal growth of diopside phase was proceeded by short range diffusion of $Mg^{2+}$ and $Ca^{2+}$ ion. The microstructures of the resulting glass-ceramics were consisted of leafroidal shaped crystalline aggregations.

• 한국재료학회지
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• 제22권7호
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• pp.329-334
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• 2012

Two types of nanoclusters, termed Cluster (1) and Cluster (2) here, both play an important role in the age-hardening behavior in Al-Mg-Si alloys. Small amounts of additions of Cu and Ag affect the formation of nanoclusters. Two exothermic peaks were clearly detected in differential scanning calorimetry(DSC) curves by means of peak separation by the Gaussian method in the base, Cu-added, Ag-added and Cu-Ag-added Al-Mg-Si alloys. The formation of nanoclusters in the initial stage of natural aging was suppressed in the Ag-added and Cu-Ag-added alloys, while the formation of nanoclusters was enhanced at an aging time longer than 259.2 ks(3 days) of natural aging with the addition Cu and Ag. The formation of nanoclusters while aging at $100^{\circ}C$ was accelerated in the Cu-added, Ag-added and Cu-Ag-added alloys due to the attractive interaction between the Cu and Ag atoms and the Mg atoms. The influence of additions of Cu and Ag on the clustering behavior during low-temperature aging was well characterized based on the interaction energies among solute atoms and on vacancies derived from the first-principle calculation of the full-potential Korrinaga-Kohn-Rostoker(FPKKR)-Green function method. The effects of low Cu and Ag additions on the formation of nanoclusters were also discussed based on the age-hardening phenomena.