• 한국분말재료학회지
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• 제6권4호
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• pp.301-306
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• 1999

Mechanical properties of oxide based materials could be improved by nanocomposite processing. To investigate optimum route for fabrication of nanocomposite enabling mass production, high energy ball milling and Pulse Electric Current Sintering (PECS) were adopted. By high energy ball milling, the $Al_2O_3$-based composite powder with dispersed Cu grains below 20 nm in diameter was successfully synthesized. The PECS method as a new process for powder densification has merits of improved sinterability and short sintering time at lower temperature than conventional sintering process. The relative densities of the $Al_2O_3$-5vol%Cu composites sintered at $1250^{\circ}C$ and $1300^{\circ}C$ with holding temperature of $900^{\circ}C$ were 95.4% and 95.7% respectively. Microstructures revealed that the composite consisted of the homogeneous and very fine grains of $Al_2O_3$ and Cu with diameters less than 40 nm and 20 nm respectively The composite exhibited enhanced toughness compared with monolithic $Al_2O_3$. The influence of the Cu content upon fracture toughness was discussed in terms of microstructural characteristics.

• 한국분말재료학회지
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• 제5권4호
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• pp.286-292
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• 1998

Ni$_3$Al intermetallic compound has been tested as a binder phase, in order to improve the oxidation resistance and the mechanical properties of TiC-Ni cermet at a high temperature. The wettability of $Ni_3Al$ on TiC and the optimum sintering condition were investigated in TiC-(30, 40) vol% $Ni_3Al$ cermets with the sintering temperature (1380~$1430^{\circ}C$) and time (30~99 min). The results are summarized as follows: 1) Ni$_3$Al showed good wettability on TiC above 1400$^{\circ}C$ ; 2) The shrinkages of the specimens increased with the sintering temperature, the sintering time and the binder content, whereas the relative densities were decreased; 3) Any other phase did not appeared in the microstructures of all sintered cermets. The grain sizes of TiC became larger as the sintering temperature and the sintering time as well as the binder content increased; 4) The hardness of the cermets decreased with increase in the sintering temperature and the sintering time as well as the binder content; 5) The transverse-rupture strength of the cermets increased with the sintering temperature and the sintering time, whereas it decreased with the binder content.

• 대한화학회지
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• 제19권4호
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• pp.218-224
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• 1975

바닥상태에 대한 자기무당착장 상호작용 에너지 이론을 2-치환-아크릴로니트릴과 시클로펜타디엔의 Diels-Alder 반응에 적용하여 반응성과 입체 선택성을 조사하였다. 출발점으로는 $\sigma$전자와 $\pi$전자를 다 고려하는 CNDO/2 MO가 사용되었다. 1차 상호작용 에너지 계산으로 정성적인 반응성이 얻어 졌으며 활성화 에너지 값은 실측치 범위의 것이 계산되었다. 2차 상호작용 에너지 계산으로 얻은 입체 선택성은 치환기에 대하여 모두 endo 선택성을 보여주었다. 비대칭 폐환의 원인을 반응원자의 원잣가 비활성 전자밀도로서 설명하였다.

• 한국지반공학회논문집
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• 제15권5호
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• pp.205-215
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• 1999

파쇄성 사질토 지반은 세계각지의 해안을 중심으로 넓게 분포되어 있고, 칼슘성분을 다량 함유한 석회질 모래지반은 입자파쇄의 영향으로 높은 압축성을 나타내는 특징이 있다. 본 연구에서는 모래의 강도-변형특성에 관여하는 입자파쇄의 특성을 명백히 하기 위하여, 세가지 종류의 카보네이트계 모래와 실리카계의 모래를 이용하여 광범위한 응력영역에 대한 등방압축시험을 실시하였다. 다양한 상대밀도에 대한 등방압축 조건에서 압축 항복응력과 입자 파쇄응력과의 관계로부터 파쇄성지표 K를 제안하였다. 제안된 파쇄성지표 K는 흙입자 강도와도 밀접한 관계를 가지며, 흙의 파쇄성을 평가하기 위한 중요한 인자로 사료된다.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2010년도 하계학술대회 논문집
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• pp.22-22
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• 2010

0.1~5.0 at% CuO doped ZnO specimens were fabricated by a commercial ceramic process and sintered at 900~$1200^{\circ}C$ for 3h in air. The relative densities were over 97% for all samples and average grain size increased with CuO doping. The defect trap levels i.e. ionization energies of defects were increased linearly with CuO contents as 0.2 eV to 0.7 eV by using admittance spectroscopy and dielectric functions. The apparent activation energies of grain boundaries were varied but in the range of 0.96~1.1 eV. Dielectric constant were increased with CuO contents and sintering temperatures.

• 한국의학물리학회:학술대회논문집
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• 한국의학물리학회 2002년도 Proceedings
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• pp.272-274
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• 2002

The contribution of light in high-energy film dosimetry was examined using six commercially available solid water substitute phantoms. As six commercially available phantoms; RMI-451, Mix-DP, WE211, WE211-Black, PMMA and PMMA Black were evaluated in this study. It is difficult to evaluate the contribution of Cerenkov radiation and the optical permeability to the relative and/or absolute dosimetry using unpacked film in these phantoms. Therefore the contribution of Cerenkov radiation was estimated by the comparison between film densities in the shielded side (shutting off the light) and unshielded sides on a phantom. The effect of optical permeability was measured under ambient light by the time scale method. The results suggest that the use of black colored phantoms may improve the accuracy of dose measurement in film dosimetry.

• Geomechanics and Engineering
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• 제12권6호
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• pp.983-1001
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• 2017

The extent by which economy and safety concerns can be addressed in earth retaining structure design depends on the accuracy of the assumed failure surface. Accordingly, this study attempts to investigate and quantify mechanical backfill properties that control failure surface geometry of cohesionless backfills at the active state for translational mode of wall movements. For this purpose, a small scale 1 g physical model study was conducted. The experimental setup simulated the conditions of a backfill behind a laterally translating vertical retaining wall in plane strain conditions. To monitor the influence of dilative behavior on failure surface geometry, model tests were conducted on backfills with different densities corresponding to different dilation angles. Failure surface geometries were identified using particle image velocimetry (PIV) method. Friction and dilation angles of the backfill are calculated as functions of failure stress state and relative density of the backfill using a well-known empirical equation, making it possible to quantify the influence of dilation angle on failure surface geometry. As a result, an empirical equation is proposed to predict active failure surface geometry for cohesionless backfills based on peak dilatancy angle. It is shown that the failure surface geometries calculated using the proposed equation are in good agreement with the identified failure surfaces.

• 대한전자공학회:학술대회논문집
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• 대한전자공학회 2001년도 The 6th International Symposium of East Asian Resources Recycling Technology
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• pp.331-336
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• 2001

Aragonite type of precipitated calcium carbonate was synthesized by the successive reaction with hybridization of waste scallop shells with limestone. In the first step, carbonation was performed by using calcined limestone with low brightness, followed by the additional carbonation using calcined-hydrous scallop shells with high brightness. The temperature and the amount ratio of calcined limestone to calcined-hydrous scallop shells were examined as parameters in the experimental conditions. The products were characterized by XRD, SEM. in addition to measurements of brightness index. the specific surface area, and bulk density. The amount ratio of limestone to scallop shell affected not only the brightness but also morphologies of products. The increase in relative amounts of limestone leads to decrease in brightness and bumpy surface of particles. High temperature reaction produces aragonite particles with longer sizes and higher bulk densities. This study has made an attempt to establish the synthesis of aragonite with high brightness and high strength by utilization of waste scallop shells.

• 한국세라믹학회지
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• 제32권10호
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• pp.1194-1202
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• 1995

The influences of TiC additions to the α-SiC on microstructural, mechanical, and electrical properties were investigated. Electrical discharge machinability of SiC-TiC composites was also studied. Samples were prepared by adding 30, 45, 60 wt.% TiC particles as a second phase to a SiC matrix. Sintering of SiC-TiC composites was done by hot pressing under a vacuum atmospehre from 1000 to 2000℃ with a pressure of 32 MPa and held for 90 minutes at 2000℃. Samples obtained by hot pressing were fully dense with the relative densities over 99% except 60wt.% TiC samples. Flexural strength and fracture toughness of the samples were increased with the TiC content. In case of SiC samples containing 45 wt.% TiC, the fracture toughness showed 90% increase compared to that of monolithic SiC sample. The crack propagation and crack deflection were observed with a SEM for etched samples after Vicker's indentation. The electrical resistivities of SiC-TiC composites were measured utilizing the four-point probe. The electrical dischage machining of composites was also conducted to evaluate the machinability.

• 대한전기학회:학술대회논문집
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• 대한전기학회 1999년도 추계학술대회 논문집 학회본부 C
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• pp.879-881
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• 1999

Radical and ion densities in a CF4 plasma have been calculated as a function of input power density. 9as pressure and feed gas flow rate using simple 0 dimensional global model. Fluorine atom is found to be the most abundant neutral particle. Highly fragmented species such as CF and CF+ become dominant neutral and ionic radical at the high power condition. As the pressure increases. ion density increases but ionization rate decreases due to the decrease in electron temperature. The fractional dissociation of CF4 feed gas decreases with pressure after increasing at the low pressure range. Electron density and temperature are almost independent of flow rate within calculation conditions studied. The fractional dissociation of CF4 monotonically decreases with flow rate. which results in increase in CF3 and decrease in CF density. The calculation results show that the SiO2 etch selectivity improvement correlates to the increase in the relative density of fluorocarbon ion and neutral radicals which has high C/F ratio.