• 한국세라믹학회지
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• 제43권10호
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• pp.666-672
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• 2006

GDC20($Ce_{0.8}Gd_{0.2}O_{1.9}$) powder was synthesized by oxalate co-precipitation and milling and its thermal decomposition, phase formation, and sinterability were investigated. As-prepared precipitates were non-crystalline due to the milling process and completely decomposed at 400$^{\circ}C$ The powder calcined at 800$^{\circ}C$ for 2 h contained fine p]sty particles with an average size of 0.69 $\mu$m. Attrition milling of the calcined powder for 2 h had a little milling effect, resulting in a slight decrease in the particle size to 0.45 $\mu$m. The milled powder consisted of small spherical primary particles and some large particles, which had been agglomerated during calcination. Due to the excellent sinterability of the powder, sintering of the powder compacts for 4 h showed relative densities of 78.7% at 1000$^{\circ}C$ and 97.8% at 1300$^{\circ}C$, respectively. Densification was found to almost complete at temperature above 1200$^{\circ}C$ and a dense and homogeneous microstructure was obtained. A rapid grain growth occurred between 1200$^{\circ}C$ and 1300$^{\circ}C$. Grains in 0.1$\sim$0.5 $\mu$m sizes at 1200$^{\circ}C$ grew to 0.2$\sim$2 $\mu$m and their size distribution became broader at 1300$^{\circ}C$.

• 한국세라믹학회지
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• 제52권3호
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• pp.215-220
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• 2015

Phase evolution, microstructure, and microwave dielectric properties of $Li_2O$ and $Al_2O_3$ doped $Zn_{1.9}Si_{1.05}O_4$, i.e., $Zn_{1.9-2x}Li_xAl_x-Si_{1.05}O_4$, ceramics (x = 0.02 ~ 0.10) were investigated. The ceramics were densified by $SiO_2$-rich liquid phase composed of the Li-Al-Si-O system, indicating that doped Li and Al contributed to the formation of the liquid. As the secondary phase, ${\beta}$-spodumene solid solution with the composition of $LiAlSi_3O_8$ was precipitated from the liquid during the cooling process. The dense ceramics were obtained for the specimens of $$x{\geq_-}0.06$$ showing the rapid densification above $1000^{\circ}C$, implying that a certain amount of liquid is necessary to densify. The specimen of x = 0.06 sintered at $1050^{\circ}C$ exhibited good microwave dielectric properties; the dielectric constant and the quality factor ($Q{\times}f_0$) were 6.4 and 11,213 GHz, respectively.

• 한국세라믹학회지
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• 제29권8호
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• pp.601-608
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• 1992

We investigated the influence of several processes, including the preparation of slurry and preform and the heat-treatment of the preform, on the properties of composites to fabricate the carbon-fiber reinforced glass composites having good mechanical properties. Cerander was determined to be the best binder among Cerander, Rhoplex and Elvacite 2045 by the dipping test and the binder within a preform could be completely eliminatd by burning out the specimen under 10-6 Torr at 400$^{\circ}C$ for more than 1h. The fracture behavior of a composite was largely dependent on the uniformity of carbon-fiber distribution within the composite and the heat-treatment condition of the composite. The higher the glass content, the more difficult to obtain uniform distribution of carbon-fiber. As the hot-pressing temperature increased, the densification process of the composite and the formation of pore due to oxidation of carbon fiber occurred competitively. But, above 1000$^{\circ}C$ the latter played a predominant role. We could fabricated the densest 15 vol.% carbon-fiber-content glass composite having the highest toughness and flexural strength of 250 MPa by hot-pressing under 15 MPa at 900$^{\circ}C$ for 30 min.

• 한국세라믹학회지
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• 제48권5호
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• pp.418-425
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• 2011

In this study, a high energy ball milling process was introduced in order to improve the densification of direct nitrided AlN powder. The sintering behavior and thermal conductivity of the AlN milled powder was investigated. The mixture of AlN powder and 5 wt% $Y_2O_3$ as a sintering additive was pulverized and dispersed by a bead mill with very small $ZrO_2$ bead media. The milled powders were sintered at $1700^{\circ}C-1800^{\circ}C$ for 4 h under $N_2$ atmosphere. The results showed that the sintered density was enhanced with increasing milling time due to the particle refinement as well as the increase in oxygen contents. Appropriate milling time was effective for the improvement of thermal conductivity, but the extensive millied powder formed more fractions of secondary phase during sintering, resulted in the decrease in thermal conductivity. The AlN powder milled for 10min after sintering at $1800^{\circ}C$ revealed the highest thermal conductivity, of 164W/$m{\cdot}K$ in tne densified AlN sintered at $1800^{\circ}C$.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2001년도 추계학술대회 논문집
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• pp.386-389
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• 2001

In this study, development of a new LTCC material using non-glassy system was attempted with respect to reducing the fabrication process steps and cost down. Lowering the sintering temperature can be achieved by liquid phase sintering. However, presence of liquid phases usually decrease dielectric properties, especially the quality factor. Therefore, the starting material must have quality factor as high as possible in microwave frequency range. And also, the material should have a low dielectric constant for enhancing the signal propagation speed. Regarding these factors, dielectric constants of various materials were estimated by the Clausius-Mosotti equation. Among them, ZnWO$_4$ was turned out the suitable LTCC material. ZnWO$_4$ can be sintered up to 98% of full density at 105$0^{\circ}C$ for 3 hours. It's measured dielectric constant, quality factor, and temperature coefficient of resonant frequency were 15.5, 74380GHz, and -70ppm/$^{\circ}C$, respectively In order to modify the dielectric properties and densification temperature, B$_2$O$_3$ and V$_2$O$_{5}$ were added to ZnWO$_4$. 40 mol% B$_2$O$_3$ addition reduced the dielectric constant from 15.5 to 12. And the temperature coefficient of resonant frequency was improved from -70 to -7.6ppm/$^{\circ}C$. However, sintering temperature did not change due to either lack of liquid phase or high viscosity of liquid phase. Incorporation of small amount of V$_2$O$_{5}$ in ZnWO$_4$-B$_2$O$_3$ system enhanced liquid phase sintering. 0.lwt% V$_2$O$_{5}$ addition to the 0.6ZnWO$_4$-0.4B$_2$O$_3$ system, reduced the sintering temperature down to 95$0^{\circ}C$ Dielectric constant, quality factor, and temperature coefficient of resonant frequency were 9.5, 16737GHz, and -21.6ppm/$^{\circ}C$ respectively.ively.

• 대한금속재료학회지
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• 제50권1호
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• pp.34-38
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• 2012

Titanium has good deformability, high hardness, high biocompatibility, excellent corrosion resistance and low density. Due to these attractive properties, it has been used in many industrial applications. Dense nanostructured Ti was sintered from mechanically activated Ti and $TiH_2$ powders by high frequency induction heating under pressure of 80 MPa. The advantage of this process is that it allows very quick densification to near theoretical density and inhibition of grain growth. $TiH_2$ powder was decomposed to Ti during sintering. The hardness of Ti increased and the average grain size of Ti decreased with increasing milling time. The average grain sizes of Ti samples sintered from Ti and $TiH_2$ powder milled for 5 hrs were about 26 nm, 44 nm, respectively. The hardness of Ti sintered from Ti and $TiH_2$ powder milled for 5 h was $504kg/mm^2$ and $567kg/mm^2$, respectively.

• 대한금속재료학회지
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• 제50권11호
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• pp.817-822
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• 2012

Nano powders of $Al_2O_3$ and $TiO_2$ compounds made by high energy ball milling were pulsed current activated sintered for studying their sintering behaviors and mechanical properties. The advantage of this process is that it allows very quick densification to near theoretical density and inhibition of grain growth. Nano-structured $Al_2TiO_5$ with small amount of $Al_2O_3$ and$TiO_2$ was formed by sintering at $1300^{\circ}C$ for 5 minute, in which average grain size was about 96 nm. Hardness and fracture toughness of the nano-structured $Al_2TiO_5$ compound with a small amount of $Al_2O_3$ and$TiO_2$ were $602kg/mm^2$ and $2.6MPa{\cdot}m^{1/2}$, respectively.

• 한국재료학회지
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• 제29권10호
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• pp.609-616
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• 2019

Oxide coatings are formed on die-cast AZ91D Mg alloy through an environmentally friendly plasma electrolytic oxidation(PEO) process using an electrolytic solution of $NaAlO_2$, KOH, and KF. The effects of PEO condition with different duty cycles (10 %, 20 %, and 40 %) and frequencies(500 Hz, 1,000 Hz, and 2,000 Hz) on the crystal phase, composition, microstructure, and micro-hardness properties of the oxide coatings are investigated. The oxide coatings on die-cast AZ91D Mg alloy mainly consist of MgO and $MgAl_2O_4$ phases. The proportion of each crystalline phase depends on various electrical parameters, such as duty cycle and frequency. The surfaces of oxide coatings exhibit as craters of pancake-shaped oxide melting and solidification particles. The pore size and surface roughness of the oxide coating increase considerably with increase in the number of duty cycles, while the densification and thickness of oxide coatings increase progressively. Differences in the growth mechanism may be attributed to differences in oxide growth during PEO treatment that occur because the applied operating voltage is insufficient to reach breakdown voltage at higher frequencies. PEO treatment also results in the oxide coating having strong adhesion properties on the Mg alloy. The micro-hardness at the cross-section of oxide coatings is much higher not only compared to that on the surface but also compared to that of the conventional anodizing oxide coatings. The oxide coatings are found to improve the micro-hardness with the increase in the number of duty cycles, which suggests that various electrical parameters, such as duty cycle and frequency, are among the key factors controlling the structural and physical properties of the oxide coating.

• Current Optics and Photonics
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• 제5권4호
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• pp.450-457
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• 2021

Upconversion (UC) properties of Y₂O₃:Ho³⁺/Yb³⁺ spherical particles synthesized by spray pyrolysis were investigated by changing the dopant concentration and calcination temperature. Citric acid (CA), ethylene glycol (EG) and N, N-dimethylformamide (DMF) were used to control the microstructure of Y₂O₃:Ho³⁺/Yb³⁺ particles. In terms of achieving the highest UC green emission intensity, the optimal concentrations of Ho³⁺ and Yb³⁺ were found to be 0.3% and 3.0%, respectively. In addition, the UC intensity of Y₂O₃:Ho³⁺/Yb³⁺ showed a linear relationship with the crystallite size. The use of organic additives allows Y₂O₃:Ho³⁺/Yb³⁺ particles to have a spherical and dense structure, resulting in significantly reducing the surface area while maintaining high crystallinity. As a result, the UC emission intensity of Y₂O₃:Ho³⁺/Yb³⁺ particles having a dense structure showed the UC emission intensity about 3.8 times higher than that of hollow particles prepared without organic additives. From those results, when Y₂O₃:Ho³⁺/Yb³⁺ particles are prepared by the spray pyrolysis process, the use of the CA/EG/DMF mixtures as organic additives has been suggested as an effective way to substantially increase the UC emission intensity.

• 한국분말재료학회지
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• 제28권3호
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• pp.216-220
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• 2021

The effect of tert-butyl alcohol (TBA) as a freezing solvent on the pore structure of a porous tungsten body prepared by freeze-drying is analyzed. TBA slurries with a WO3 content of 10 vol% are prepared by mixing with a small amount of dispersant and binder at 30℃. The slurries are frozen at -25℃, and pores are formed in the frozen specimens by the sublimation of TBA during drying in air. After hydrogen reduction at 800℃ and sintering at 1000℃, the green body of WO₃ is completely converted to porous W with various pore structures. Directional pores from the center of the specimen to the outside are observed in the sintered bodies because of the columnar growth of TBA. A decrease in pore directionality and porosity is observed in the specimens prepared by long-duration drying and sintering. The change in pore structure is explained by the growth of the freezing solvent and densification.