• Journal of Korean Ophthalmic Optics Society
• /
• v.6 no.1
• /
• pp.145-148
• /
• 2001

SiC composites were developed by incorporating glass phase into SiC, in the light of improving mechanical properties of material for spectacle lens cutting. Specimens for spectacle lens cutting with glass phase as sintering additives have been fabricated by hot-pressing at $1810^{\circ}C$ for 2 hr under a pressure of 25 MPa. The fracture toughness and hardness of hot-pressed specimens were characterized and compared with previous works. Typical hardness and fracture toughness of materials for spectacle lens cutting were 12 GPa and $5.1MPa{\cdot}m^{1/2}$ respectively.

• Journal of the Korean Crystal Growth and Crystal Technology
• /
• v.18 no.6
• /
• pp.237-241
• /
• 2008

Hot-press sintering of AlN ceramics were carried out with $Y_2O_3$ as sintering additive at a sintering temperature $1,750{\sim}1,850^{\circ}C$. The effect of $Y_2O_3$ addition and sintering time on sintering behavior and thermal conductivity of AlN ceramics was investigated. $Y_2O_3$ added AlN showed noticeably higher denazification rate than pure AlN. The thermal conductivity of AlN specimens was promoted by the addition of $Y_2O_3$ in spite of the formation of YAG secondary phase in AlN grain boundaries and grain boundary triple junction because $Y_2O_3$ addition could reduced the oxygen contents in AlN lattice which is primary factor of thermal conductivity. Typically, the thermal conductivity of 5 wt% $Y_2O_3$ added specimen was dramatically improved by the increase of sintering time because the elimination of YAG secondary phases from the grain boundary due to the evaporation, as well as the grain-growth of AlN grains.

• Applied Chemistry for Engineering
• /
• v.16 no.5
• /
• pp.648-652
• /
• 2005

A super ionic conductor, $K^+$-beta-aluminas, which is known to be difficult to obtain in the form of dense sintered density under atmospheric pressure, was pulverized to 350 nm mean particle size using attrition mill. The sample were pressed into tablet form by uniaxial pressing. The specimen was sintered under atmospheric pressure in powder form. Sintering temperature range was $1400^{\circ}C$ to $1650^{\circ}C$ at $50^{\circ}C$ intervals. Additionally, zone sintering was carried out to control the growth grain at high temperature ($1600^{\circ}C$). The density of specimens that were sintered at $1600^{\circ}C$ and $1650^{\circ}C$, and sintered at $1600^{\circ}C$ by zone sintering were about 93% and 95%, respectively. In the case of the lengthened sintering time to 2 h, the density of specimen was reduced to lower than 90%, since the particles were grown to the duplex microstructure.

• The Journal of Engineering Geology
• /
• v.16 no.1 s.47
• /
• pp.1-14
• /
• 2006

Various measurements were carried out to estimate the modulus of deformation in two dominant rock types in Korea: granite and gneiss. Four most commonly used methods were utilized: Goodman jack tests, PS well logging, laboratory ultrasonic tests and laboratory uniaxial loading tests. Laboratory static and dynamic Young's moduli depend on the magnitude of the applied axial stress, range of Sequency used for measurement and the loading/unloading condition. As the laboratory measurement condition approaches to that in situ, the resultant moduli also appear to be comparable to that in situ. This suggests that the simulation of in situ stress condition is important when the modulus of rock is determined in the laboratory Dynamic Young's modulus is generally higher than static Young's modulus because of (micro)crack behavior in response to the stress, different range of frequency used for measurements, and the effect of the amplitude of deformation. Understanding of the relations in moduli from different measurement methods will help estimate appropriate in situ values.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
• /
• 2008.11a
• /
• pp.39-39
• /
• 2008

이동통신 시스템의 소형화, 다기능화 추세에 따라 이동통신 부품들의 모듈화, 고집적화 추세로 급진전되고 있어, 고집적 세라믹 기판 모듈 제작을 위한 핵심공정 기술인 그린시트의 층간 정밀도 및 소성후 수축율 제어의 중요성이 증대되고 있다. 본 연구에서는 일축가압 이용한 PAS(Pressure Assisted Sintering) 법과 Al2O3를 희생층으로 이용한 Constrained Sintering법을 혼합하여 저온 동시소성 세라믹 기판의 x-y 축 수축율을 zero로 제어하고자하였다. $Al_2O_3$/LTCC/$Al_2O_3$인 샌드위치 구조로 세라믹 시트를 적층하여 Load 값과, LTCC 두께에 따른 x-y축, z축 소성 수축율 및 Edge Curvature의 Radius와 warpage 현상을 관찰하고, 이때 미세구조 및 밀도를 측정하였다. 그 결과 symmetic한 구조일 때 소성온도 $900^{\circ}C$에서 $Al_2O_3$ 두께가 $30{\mu}m$ 이상일 때 LTCC의 글라스가 $Al_2O_3$에 Infiltration 되는 두께는 $30{\mu}m$를 나타내었다. 또한 $Al_2O_3$ 두께 $500{\mu}m$, LTCC 두께 $2,000{\mu}m$, Load값이 800g/$cm^2$ 일 때 x-y 축 수축율<1%, z축 수축율 40%, 소결밀도는 2.99g/$cm^3$로 우수한 무수축 기판 특성을 나타내었다.

• Journal of Ocean Engineering and Technology
• /
• v.21 no.6
• /
• pp.95-100
• /
• 2007

In the present work, silicon nitride was fabricated with $Y_3Al_5O_{12}$ as a sintering additive and its mechanical properties were investigated. Silicon nitride with 3, 5, and 7wt% of $Y_3Al_5O_{12}$ was prepared and sintered by a Hot Pressing (HP) technique at 1750 and $1800^{\circ}C$ for 2 h. The process was performed under different process pressures of 30 and 45 MPa. Mechanical properties (density, strength, hardness, and fracture toughness) were investigated as a function of the $Y_3Al_5O_{12}$ content in $Si_3N_4$. $Si_3N_4\;-Y_3Al_5O_{12}$ ceramics showing similar mechanical properties compared with $Si_3N_4-Y_2O_3-Al_2O_3$ ceramics. But its high temperature strength was considerably higher than that of $Si_3N_4-Y_2O_3-Al_2O_3$ ceramics.

• Journal of the Microelectronics and Packaging Society
• /
• v.26 no.2
• /
• pp.45-49
• /
• 2019

Herein we developed the hot extrusion technology to prepare n-type Bi-Te-Se-based thermoelectric materials with high reliability. Starting ingot was fabricated via melt-solidification process, then pulverized it into powders (${\sim}30{\mu}m$) by using high energy ball milling. By optimization of mold design and temperature-pressure conditions for hot extrusion, dense extrudate of 1.8 mm in diameter with high 00l orientation could be obtained from disc-shape compacted powders (20 mm in diameter). High power factor ${\sim}4.1mW/mK^2$ and enhanced mechanical strength ~50 MPa were simultaneously observed at 300 K.

• Journal of Korean Tunnelling and Underground Space Association
• /
• v.22 no.6
• /
• pp.623-641
• /
• 2020

About 5 km length of tunnels were constructed by mechanized tunnelling method using closed type shield TBM. In order to avoid construction delay problems for ensuring timely electricity transmission, it is necessary to increase the prediction accuracy of the excavation process involving machines according to rock mass types. This is important to corroborate the project duration and optimum operation for various considerations involved in the machine. So, full-scale tunnelling tests were performed for developing the advance rate model to be appropriately used for 3.6 m diameter shield TBM. About 100 test cases were established and performed using various operational parameters such as thrust force and rotational speed of cuttterhead in representative uniaxial compressive strengths. Accordingly, relationships between normal force and penetration depth and, between UCS and torque were suggested which consider UCS and thrust force conditions according to weathered, soft, hard rocks. Capacity analysis of cutterhead was performed and optimum operational conditions were also suggested based on the developed model. Based on this study, it can be expected that the project construction duration can be reduced and users can benefit from the provision of earlier service.

• Journal of the Korean Ceramic Society
• /
• v.46 no.1
• /
• pp.81-85
• /
• 2009

$B_4C$ ceramics were fabricated by a hot-press sintering method and their sintering behavior, microstructure and mechanical properties were evaluated. Relative density of $B_4C$ ceramics were obtained by a hot-press sintering method reached as high as 99% without any sintering additives. The mechanical properties of $B_4C$ ceramics was improved by a methanol washing process which can remove $B_2O_3$ phase from a $B_4C$ powder surface. This improvement results from the formation of homogeneous microstructure because the grain coarsening was suppressed by the elimination of $B_2O_3$ phase. Particularly, mechanical properties of the sintered specimen using a methanol washed powder improved compared with the specimen using an as-received commercial powder.

• Journal of Korean Ophthalmic Optics Society
• /
• v.6 no.2
• /
• pp.111-114
• /
• 2001

This paper presents the influence of the additive composition on flexural strength and hardness of SiC-TiC composites materials for spectacle lens cutting materials. The materials were hot-pressed at $1800^{\circ}C$ and subsequently annealed at $1910^{\circ}C$ for 3h. The heating rate was $15^{\circ}C/min$ and the cooling rate about $25^{\circ}C/min$ in from the sintering temperature to $1300^{\circ}C$. The growth of particles of spectacle lens cutting materials was analysed by SEM and crystalline phases were discussed by x-ray diffractometry. Typical fracture toughness and hardness of materials for spectacle lens cutting were $6.1MPa{\cdot}m^{1/2}$ and 14.9 GPa, respectively.