• 소성∙가공
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• 제31권6호
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• pp.376-383
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• 2022

The potential damage region of a WC-Co cemented carbide die is investigated for cold forging process of a wheel-nut by numerical simulation with its chemical composition considered. Numerical simulation is utilized to calculate internal stress, especially for the WC-Co die, during the forging process. Finite element model is established, in which the elasto-plastic properties are applied to the work-piece of bulk steel, and elastic properties are considered for the lower die insert of the WC-Co alloy. This stress analysis enables to distinguish the potential damage regions of the WC-Co die. The regions from calculation are comparatively analyzed along with the crack area observed in the die after repetitive manufacturing. Effect of chemical composition of the WC-Co is also evaluated on characteristics of potential damage region of the die with variance of mechanical properties considered. Derived from Mohr-Coulomb fracture model, furthermore, a new stress index is presented and used for die stress analysis. This index inherently considers hydrostatic pressure and is then capable of deducing wide range of its distribution for representing stress state by modification of its parameter implying pressure sensitivity.

• Advances in concrete construction
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• 제11권4호
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• pp.335-345
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• 2021

It is known from the literature that there are relatively few studies on the engineering properties of ultra-high performance concrete (UHPC) in early age. In fact, in order to ensure the safety of UHPC during construction and sufficient durability and long-term performance, it is necessary to explore the early behavior of UHPC. The test parameters (test control factors) investigated included the percentage of cement replaced by silica fume (SF), the percentage of cement replaced by ultra-fine silica powder (SFP), the amount of steel fiber (volume percent), and the amount of polypropylene fiber (volume percentage). The engineering properties of UHPC in the fresh mixing stage and at the age of 7 days were investigated. These properties include freshly mixed properties (slump, slump flow, and unit weight) and hardened mechanical properties (compressive strength, elastic modulus, flexural strength, and splitting tensile strength). Moreover, the effects of the experimental factors on the performance of the tested UHPC were evaluated by range analysis and variance analysis. The experiment results showed that the compressive strength of the C8 mix at the age of 7 days was highest of 111.5 MPa, and the compressive strength of the C1 mix at the age of 28 days was the highest of 128.1 MPa. In addition, the 28-day compressive strength in each experimental group increased by 13%-34% compared to the 7-day compressive strength. In terms of hardened mechanical properties, the performance of each experimental group was superior to that of the control group (without fiber and without additional binder materials), with considerable improvement, and the experimental group did not produce explosive or brittle damage after the test. Further, the flexural test process found that all test specimens exhibited deflection-hardening behavior, resulting in continued to increase carrying capacity after the first crack.

• 한국지진공학회논문집
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• 제25권5호
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• pp.213-221
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• 2021

Many Korean domestic masonry structures constructed since 1970 have been found to be vulnerable to earthquakes because they lack efficient lateral force resistance. Many studies have shown that the brick and mortar suddenly experience brittle fracture and out-of-plane collapse when they reach the inelastic range. This study evaluated the seismic retrofitting of non-reinforced masonry with Hybrid Super Coating (HSC) and Cast, manufactured using glass fiber. Four types of specimen original specimen (BR-OR), one layered HSC (BR-HS-O), two-layered HSC (BR-HS-B), one layered HSC, and Cast (BR-CT-HS-O) were constructed and analyzed using compression, flexural tensile, diagonal compression, and triplet tests. The specimen responses were presented and discussed in load-displacement curves, maximum strength, and crack propagation. The compressive strength of the retrofit specimens slightly increased, while the flexural tensile strength of the retrofit specimens increased significantly. In addition, the HSC and Cast also produced a considerable increase in the ductile response of specimens before failure. Diagonal compression test results showed that HSC delayed brittle cracks between the mortar and bricks and resulted in larger displacement before failure than the original brick. The triplet test results confirmed that the bonding strength of the retrofit specimens also increased. The application of HSC and Cast was found to restrain the occurrence of brittle failure effectively and delayed the collapse of masonry wall structures.

• 한국융합학회논문지
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• 제7권5호
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• pp.169-173
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• 2016

알루미늄의 양극산화는 황산법, 수산법과 황산과 수산을 활용한 혼산법 등이 있다. 산업체에서 만들어지는 양극산화는 황산법으로 전해액속의 황산농도가 15~20 wt %이다. 연질 양극산화피막을 생성할 경우는 전해액의 온도가 $20{\sim}30^{\circ}C$ 범위에서 생산되고 있으며, 생산 전압은 직류전압으로 13~15 V 이내가 가장 많이 사용된다. 경질 양극산화피막을 생성할 경우는 전해액의 온도가 $0{\sim}-5^{\circ}C$에서 생산되어지고 있다. 본 연구에서는 황산법과 수산법을 이용하여 $50{\mu}m$ 두께의 양극산화피막(알루미나)를 제작하였다. 황산법과 수산법에 의해 시편을 제조하여 열 피로시험을 수행하였다. 황산법과 수산법의 균열발생온도는 $500^{\circ}C$와 $600^{\circ}C$이었다. 황산법의 균열발생온도는 수산법의 균열발생온도보다 낮았다. 시험결과 수산법의 열 피로는 황산법의 열 피로 특성보다 좋았다. 그 이유를 알루미늄과 알루미나의 열팽창계수와 제조온도로 설명하였다. 고온에서 사용되는 양극산화제품의 제조방법을 여기서의 융합 연구를 통해 제안 가능하게 되었다.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2013년도 제44회 동계 정기학술대회 초록집
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• pp.183-184
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• 2013

Two main MBE growth techniques have been used: plasma-assisted MBE (PA-MBE), which utilizes a rf plasma to supply active nitrogen, and ammonia MBE, in which nitrogen is supplied by pyrolysis of NH3 on the sample surface during growth. PA-MBE is typically performed under metal-rich growth conditions, which results in the formation of gallium droplets on the sample surface and a narrow range of conditions for optimal growth. In contrast, high-quality GaN films can be grown by ammonia MBE under an excess nitrogen flux, which in principle should result in improved device uniformity due to the elimination of droplets and wider range of stable growth conditions. A drawback of ammonia MBE, on the other hand, is a serious memory effect of NH3 condensed on the cryo-panels and the vicinity of heaters, which ruins the control of critical growth stages, i.e. the native oxide desorption and the surface reconstruction, and the accurate control of V/III ratio, especially in the initial stage of seed layer growth. In this paper, we demonstrate that the reliable and reproducible growth of GaN on Si (110) substrates is successfully achieved by combining two MBE growth technologies using rf plasma and ammonia and setting a proper growth protocol. Samples were grown in a MBE system equipped with both a nitrogen rf plasma source (SVT) and an ammonia source. The ammonia gas purity was ＞99.9999% and further purified by using a getter filter. The custom-made injector designed to focus the ammonia flux onto the substrate was used for the gas delivery, while aluminum and gallium were provided via conventional effusion cells. The growth sequence to minimize the residual ammonia and subsequent memory effects is the following: (1) Native oxides are desorbed at $750^{\circ}C$ (Fig. (a) for [$1^-10$] and [001] azimuth) (2) 40 nm thick AlN is first grown using nitrogen rf plasma source at $900^{\circ}C$ nder the optimized condition to maintain the layer by layer growth of AlN buffer layer and slightly Al-rich condition. (Fig. (b)) (3) After switching to ammonia source, GaN growth is initiated with different V/III ratio and temperature conditions. A streaky RHEED pattern with an appearance of a weak ($2{\times}2$) reconstruction characteristic of Ga-polarity is observed all along the growth of subsequent GaN layer under optimized conditions. (Fig. (c)) The structural properties as well as dislocation densities as a function of growth conditions have been investigated using symmetrical and asymmetrical x-ray rocking curves. The electrical characteristics as a function of buffer and GaN layer growth conditions as well as the growth sequence will be also discussed. Figure: (a) RHEED pattern after oxide desorption (b) after 40 nm thick AlN growth using nitrogen rf plasma source and (c) after 600 nm thick GaN growth using ammonia source for (upper) [110] and (lower) [001] azimuth.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2000년도 제18회 학술발표회 논문개요집
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• pp.77-77
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• 2000

Titanium oxide (TiO2) thin films have valuable properties such as a high refractive index, excellent transmittance in the visible and near-IR frequency, and high chemical stability. Therefore it is extensively used in anti-reflection coating, sensor, and photocatalysis as electrical and optical applications. Specially, TiO2 have a high dielectric constant of 180 along the c axis and 90 along the a axis, so it is highlighted in fabricating dielectric capacitors in micro electronic devices. A variety of methods have been used to produce patterned self-assembled monolayers (SAMs), including microcontact printing ($\mu$CP), UV-photolithotgraphy, e-beam lithography, scanned-probe based micro-machining, and atom-lithography. Above all, thin film fabrication on $\mu$CP modified surface is a potentially low-cost, high-throughput method, because it does not require expensive photolithographic equipment, and it produce micrometer scale patterns in thin film materials. The patterned SAMs were used as thin resists, to transfer patterns onto thin films either by chemical etching or by selective deposition. In this study, we deposited TiO2 thin films on Si (1000 substrateds using titanium (IV) isopropoxide ([Ti(O(C3H7)4)] ; TIP as a single molecular precursor at deposition temperature in the range of 300-$700^{\circ}C$ without any carrier and bubbler gas. Crack-free, highly oriented TiO2 polycrystalline thin films with anatase phase and stoichimetric ratio of Ti and O were successfully deposited on Si(100) at temperature as low as 50$0^{\circ}C$. XRD and TED data showed that below 50$0^{\circ}C$, the TiO2 thin films were dominantly grown on Si(100) surfaces in the [211] direction, whereas with increasing the deposition temperature to $700^{\circ}C$, the main films growth direction was changed to be [200]. Two distinct growth behaviors were observed from the Arhenius plots. In addition to deposition of THe TiO2 thin films on Si(100) substrates, patterning of TiO2 thin films was also performed at grown temperature in the range of 300-50$0^{\circ}C$ by MOCVD onto the Si(100) substrates of which surface was modified by organic thin film template. The organic thin film of SAm is obtained by the $\mu$CP method. Alpha-step profile and optical microscope images showed that the boundaries between SAMs areas and selectively deposited TiO2 thin film areas are very definite and sharp. Capacitance - Voltage measurements made on TiO2 films gave a dielectric constant of 29, suggesting a possibility of electronic material applications.

• Nuclear Engineering and Technology
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• 제45권4호
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• pp.513-522
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• 2013

The ODSCC detected in the TSP position of Ulchin 3&4 SGs are typical ODSCC of Alloy 600MA tubes. The causative chemical environment is formed by concentration of impurities inside the occluded region formed by the tube surface, egg crate strips, and sludge deposit there. Most cracks are detected at or near the line contacts between the tube surface and the egg crate strips. The region of dense crack population, as defined as between $4^{th}$ and $9^{th}$ TSPs, and near the center of hot leg hemisphere plane, coincided well with the region of preferential sludge deposition as defined by thermal hydraulics calculation using SGAP computer code. The cracks developed homogeneously in a wide range of SGs, so that the number of cracks detected each outage increased very rapidly since the first detection in the $8^{th}$ refueling outage. The root cause assessment focused on investigation of the difference in microstructure and manufacturing residual stress in order to reveal the cause of different susceptibilities to ODSCC among identical six units. The manufacturing residual stress as measured by XRD on OD surface and by split tube method indicated that the high residual stress of Alloy 600MA tube played a critical role in developing ODSCC. The level of residual stress showed substantial variations among the six units depending on details of straightening and OD grinding processes. Youngwang 3&4 tubes are less susceptible to ODSCC than U3 and U4 tubes because semi-continuous coarse chromium carbides are formed along the grain boundary of Y3&4 tubes, while there are finer less continuous chromium carbides in U3 and U4. The different carbide morphology is caused by the difference in cooling rate after mill anneal. There is a possibility that high chromium content in the Y3&4 tubes, still within the allowable range of Alloy 600, has made some contribution to the improved resistance to ODSCC. It is anticipated that ODSCC in Y5&6 SGs will be retarded more considerably than U3 SGs since the manufacturing residual stress in Y5&6 tubes is substantially lower than in U3 tubes, while the microstructure is similar with each other.

• 콘크리트학회논문집
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• 제19권6호
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• pp.755-762
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• 2007

노후된 PSC I 거더교에 대한 내하력 평가 방법과 외부 강선으로 보강된 교량의 성능 평가 및 보강 효과 검증 방법은 재하 차량을 이용한 현장 실험이 많이 사용된다. 하지만 재하 차량에 의한 실험은 탄성 범위 내의 사용하중 상태에서 이루어지므로 활하중에 의한 교량의 거동 특성 분석만 유효하며, 균열하중 이후에 나타나는 비선형 거동과 극한 상태에서의 내하력을 평가하는 것은 불가능하다. 이 실험 연구에서는 27년 동안 공용된 PSC I 거더교를 대상으로 여러 가지 재하 시험을 실시하여 사용하중 및 극한하중에 대한 교량의 거동 특성과 내하 능력을 분석하였다 또한 외부 강선 보강 방법의 보강 효과를 검증하기 위하여 기존 교량의 내부 PS 강선을 절단하여 인위적으로 손실을 유도하고 평가하였으며, 절단된 양 만큼을 외부 강선으로 보강하여 보강 전후의 거동 변화를 비교하였다. 이 실험 결과를 이용하면 PSC I 거더교에 외부 강선 보강 방법을 적용할 경우에 요구되는 기존 교량의 내하력, 요구 보강량, 보강 공사시 품질관리 기준 등을 보다 명확하게 결정할 수 있을 것이다.

• 한국결정성장학회지
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• 제19권2호
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• pp.70-74
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• 2009

Sol-gel 법으로 제조한 $Pb(Sc_{1/2}Nb_{1/2})O_3$(이하 PSN) sol을 이용하여 스핀 코팅법으로 Pt(111)/Ti/$SiO_2$/Si 기판위에 제조한 박막의 제조 공정에 따른 영향을 연구하였다. Pt 기판위에 PSN sol을 증착, 건조한 후에 $370^{\circ}C$에서 5분간 열처리를 행한 후 $10^{\circ}C/sec$의 급속 가열로 $600{\sim}700^{\circ}C$에서 최종 열처리한 경우에 박막은 (111)면으로 우선 배향하는 것으로 나타났다. 그러나 중간 열처리를 거치지 않고, 급속가열에 의한 최종 열처리만을 행한 경우에는 (100)면으로 우선 배향하는 것으로 나타났다. 한편, 중간 열처리 후 $4^{\circ}C/min$의 승온속도로 관상로에서 최종 열처리를 행한 경우에는 (111)면과 (100)면이 동시에 나타나는 것으로 나타났다. 동일한 조건하에서 박막의 두께는 모두 300로 중간 열처리 공정이 어떠한 영향도 미치지 않는 것으로 나타났다.

• 한국결정성장학회지
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• 제24권3호
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• pp.89-93
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• 2014

V/III족 ratio의 변화에 따른 r-plane의 sapphire 위에 HVPE로 성장한 a-plane GaN 에피텍셜층의 특성변화를 연구하였다. V/III족 ratio가 증가함에 따라서, a-plane (11-20) GaN에 대한 Rocking Curve의 FWHM의 값이 감소하며, 성장된 GaN의 표면 거칠기도 감소하고, 성장성도는 증가하다가 V/III족 ratio 7까지는 증가하다가 다시 감소하는 경향을 보여준다. 즉 V/III족 ratio 10에서 a-plane (11-20) GaN에 대한 Rocking Curve의 FWHM의 가장 작은 829 arcsec값을 보이고, 표면거칠기도 가장 작은 1.58 nm 값을 보인다. 또한 광학현미경상에서 관찰되는 내부 Crack 또는 void가 가장 적게 발생하였다. 그리고 M모양의 Azimuth angle 의존도를 전 샘플에서 보이며, V/III족 ratio 10에서 FWHM 최대값과 최소값의 편차값이 439 arcscec로 가장 작은 차이를 보였다.