• Journal of the Korean Ceramic Society
• /
• v.31 no.11
• /
• pp.1259-1264
• /
• 1994

$\beta$-Sialon has been regarded as one of promising materials showing high strength, fracture toughness, corrosion resistence and wear resistence. The improvement of the fracture toughness and tribological properties of $\beta$-Sialon (Z=1) has been attempeted by fabricating the $\beta$-Sialon/ SiC whisker composite. Each of green body composed of following ingredients, i.e., Si3N4, AlN, Y2O3 nd SiC, respectively, was first fired at 178$0^{\circ}C$ for 3hrs in N2 atmosphere and then post-HIPed at 173$0^{\circ}C$ for 1 hr under 170 MPa for N2 gas pressure. The fracture toughness, flexural strength and tribological properties increased with increasing SiC whisker content, despite the reduction of the relative density and hardness. $\beta$-Sialon/15 vol% SiC whisker showed a significant enhancement of wear resistance compared to the monolithic $\beta$-Sialon. The addition of SiC whisker caused the reduction of the density and hardness, but induced the increment of wear resistance.

• Journal of the Korean Ceramic Society
• /
• v.31 no.6
• /
• pp.623-628
• /
• 1994

TiO2-SiO2 homogeneous sol was prepared by means of complexing titanium iso-propoxide with acetylacetone. Crack-free, monolithic binary TiO2-SiO2 aerogels were obtained by evacuating the solvent under the pressure of 2,200 psi at 28$0^{\circ}C$, which was determined as a supercritical condition. The microstructure of the binary aerogels was observed to be finer and more homogeneous as the Ti content decreased. The microstructure was related to the optical transition of aerogels from opaque to transluscent.

• Journal of the Korean Ceramic Society
• /
• v.33 no.5
• /
• pp.485-494
• /
• 1996

Silica Aerogels with the density and porosity of 0.1g/cm3 and 96% were synthesized by two different supercri-tical drying processes (i.e additional solvent and intial pressure methods) Isoptopanol was chosen as sol-gel and supercritical drying solvents in order to synthesize aerogels at the lower temperature and pressure because the critical values of isopropanol are lower than those of methanol and ethanol commonly used. The P-V-T relationship of isopropanol was experimentally described for optimizing supercritical drying conditions such as the amount of extra solvent and supercritical drying temperature and pressure. In the addional solvent method monolithic and transparent aerogels were obtained by supercritical drying at 25$0^{\circ}C$ and 900 psing after 40% of the reactor volume was filled with isopropanol. Crack-free aerogels were synthesized at 25$0^{\circ}C$ and 1100~1200 psig by the initial pressure method with an intial nitrogen gas pressure of 400 psig and the isopropanol amount of 5% of the reactor volume.

• Proceedings of the IEEK Conference
• /
• 2000.11b
• /
• pp.21-24
• /
• 2000

This paper introduces the design and implementation of a Ku-band 5-bit monolithic phase shifter with a ceramic package. The 5-bit phase shifter MMIC was designed and fabricated by using GaAs MESFET switches. The packaged phase shifter demonstrates a phase error less than 11.3 $^{\circ}$ RMS and an insertion loss variation less than 1.0㏈ RMS for 13∼15㎓. For all 32 states, an insertion loss is measured to be 12.2${\pm}$2.2㏈, an input return loss more than 5.0㏈, and an output return loss more than 6.2㏈ from 13㎓ to 15㎓. The chip size of the 5-bit phase shifter MMIC is 2.35${\times}$1.65mm$\^$2/ including digital control circuits. The size of the ceramic packaged phase shifter is 7.2${\times}$6.2mm$\^$2/.

• The Korean Journal of Ceramics
• /
• v.4 no.4
• /
• pp.331-335
• /
• 1998

R-curve, of three kinds of silicon nitride-based ceramics were measured, using single edge notched beam (SENB) method at room and at elevated temperatures, up to $1200^{\circ}C$. Stable fraacture was seen on ceramic materials with SENB specimens if the machined notch is deep enough, even though the crack resistance did not increase with crack length. Hot pressed silicon nitride did not show the rising R-curve behavior at room temperature, but it showed some rising at $1000^{\circ}C$ and above. Si3N4 reinforced with SiC whiskers showed no rising behavior at room and elevated temperatures, as it has smaller grain size, compare to the monolithic specimen. Gas pressure sintered silicon nitride had very large and elongated grains, and it showed rising R-curve even at room temperature. However, it showed some creep behavior at $1200^{\circ}C$ and the calculated R-curve on this condition did not show a good result. We cannot apply this technique on this condition for obtaining the R-curve.

• Journal of the Korean Ceramic Society
• /
• v.37 no.10
• /
• pp.1001-1007
• /
• 2000

스크린 프린트법으로 MMIC(Monolithic Microwave Integrated Circuit)용 YIG 후막을 제조하였고, YIG의 조성과 프린트 조건의 변화가 다결정 $Y_{3-x}$C $a_{x}$ F $e_{5-x}$Z $r_{x}$ $O_{12}$ (x=0~0.3) 후막의 미세구조 및 자기적 특성에 미치는 영향을 고찰하였다. $Ca^{2+}$와 Z $r^{4+}$ 의 치환 첨가량이 0.2인 $Y_{2.8}$ C $a_{0.2}$F $e_{4.8}$ Z $r_{0.2}$ $O_{12}$ 조성의 paste로 제조된 YIG 후막의 경우, 포화 자화값이 최대를 나타내었으며, 강자성 공명 흡수선 폭은 최소를 나타내었다. 또한, YIG 후막의 두께 및 소결 유지시간 등의 제조조건을 제어함에 따라 치밀화 및 자기 특성이 향상됨을 확인할 수 있었다.다.다.

• Journal of the Korean Ceramic Society
• /
• v.26 no.4
• /
• pp.471-478
• /
• 1989

In this study we investigated the gelation time of sol which containing mixtures of TEOS with H2O, ethanol, HCl in the preparation of monolithic silica glass through Sol-Gel method. In this case apparent activation energies were observed by Arrhenius equation. We investigated the conversion from gel to glass in drying and heat treatment stages using DT-TG, FT-IR, XRD analysis and dried gel conversed to glass by heat treatment up to 85$0^{\circ}C$. The results of this experiment showed that the gelation time of solutions were promoted when increasing the amount of addition of H2O and HCl catalyst for TEOS and high reaction temperature of solution.

• Proceedings of the Korean Powder Metallurgy Institute Conference
• /
• 2006.09b
• /
• pp.1080-1081
• /
• 2006

In this investigation, $B_4C$ based ceramic composites were fabricated by in-situ reaction hot pressing using $B_4C$, TiC SiC powder as starting materials. The reaction synthesized composites by hot pressing at $1950^{\circ}C$ was found to posses very high relative density. The reaction synthesized $B_4C$ composites comprise $B_4C$, $TiB_2$, SiC and graphite by the reaction between TiC and $B_4C$. The newly formed $TiB_2$ and graphite was embedded both inside grain and at grain boundary $B_4C$. The mechanical properties of reaction synthesized $B_4C-TiB_2-SiC$-graphite composites were more enhanced compared to those of monolithic $B_4C$.

• Journal of the Korean Ceramic Society
• /
• v.28 no.8
• /
• pp.635-639
• /
• 1991

Monolithic gels in the ZrO2-SiO2 system containing up to 30 mol% ZrO2 were prepared from the mixed solutions of Zr(O.nC3H7)4 and partially prehydrolyzed Si(OC2H5)4 (Tetraethyl orthosilicate) by the sol-gel method. The effect of parameters such as the hydrolysis temperature, the amount of water and HCl on the hydrolysis condensation process was investigated and the obtained gels were studied by the IR spectra and TG-DTA. The results showed that the gelation time becomes shorter with increasing content of HCl, H2O and gelation temperature, and that the polymerization was more easily completed with the higher water volume causing the elimination of unreacted organic groups.

• The Journal of Korean Academy of Prosthodontics
• /
• v.57 no.2
• /
• pp.134-141
• /
• 2019

Purpose: The purpose of this study was to compare the clinical outcomes of lithium disilicate ceramic pressed zirconia prostheses and monolithic zirconia prostheses and to investigate the complications after two years of follow-up in posterior edentulous site. Materials and methods: A total 17 patients (male: 12, female: 5) were treated with 60 posterior fixed implant-supported prostheses (LP. lithium disilicate ceramic pressed zirconia prostheses: n = 30, MZ. monolithic zirconia prostheses: n = 30). After 24-month, clinical examination of Implant survival rate, marginal bone resorption, probing depth, plaque index, bleeding index, calculus and complications were evaluated. Results: There were no failed implants and all implants were normal in function without mobility. Marginal bone resorption was lower in LP group than MZ group at 12-month (P < .05), and 12-month probing depth and calculus deposit in LP group were significantly higher than MZ group (P < .05). Most common complications in MZ were marginal bone resorptions more than 1.5.mm and 2 chipping occurred in LP group. Conclusion: Within the limitations of the present study, lithium disilicate ceramic pressed zirconia is considered as a predictable treatment option as much as monolithic zirconia in posterior fixed implant-supported prostheses.