• 마이크로전자및패키징학회지
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• 제31권3호
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• pp.91-98
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• 2024

This study reveals the feasibility and effectiveness of sinter bonding using an Ag nano-porous sheet at the lowest "theoretically" possible temperature of 145 ℃. By uniform pressure of 10 MPa for bonding times of 5 min and 10 min at 145 and 175 ℃, we achieved bonding strengths exceeding approximately 20 MPa with a only 5 min of bonding time at 145 ℃. In particular, it is interesting to note that in the pressure sintering bonding process at 145 ℃, bonding times of 5 and 10 min had no significant difference in strength. Even with a bonding temperature of 175 ℃, the difference in average bonding strength between bonding times of 5 min (i.e., 37.6 MPa) and 10 min (i.e., 43.0 MPa) was only 5 MPa. The bonding strength was fundamentally attributed to the thickness of the Ag sintered neck in the Ag sintered layer. Microstructural analysis revealed that as the bonding temperature increased to 175 ℃, the fraction of CSL Σ3 boundaries within the Ag sintered layer increased, indicating greater coalescence of Ag particles. This study systematically investigated the mechanism of bonding strength in extremely low-temperature pressure Ag sinter bonding, considering the relationship between microstructures and mechanical behaviors.

• Nuclear Engineering and Technology
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• 제55권2호
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• pp.780-791
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• 2023

To study oscillation characteristic of steam and non-condensable gas direct contact condensation through multi-hole sparger at low mass flux, a series of experiments of pure steam and mixture gas condensation have been carried out under the conditions of steam mass flux of 20-120kg/m²s, water temperature of 20-95 ℃ and mass fraction of non-condensable gas of 0-5%. The regime map of pure steam condensation through multi-hole sparger is divided into steam chugging, separated bubble, aggregated bubble and escaping aggregated bubble. The bubbles behavior of synchronization in the same hole columns and desynchronized excitation between different hole columns can be found. The coalescence effect of mixture bubbles increases with water temperature and non-condensable gas content increasing. Pressure oscillation intensity of pure steam condensation first increases and then decreases with water temperature increasing, and increases with steam mass flux increasing. Pressure oscillation intensity of mixture gas condensation decreases with water temperature and non-condensable gas content increasing, which is significantly weaker than that of pure steam condensation. The oscillation dominant frequency decreases with the rise of water temperature and non-condensable gas content. The correlations for oscillation intensity and dominant frequency respectively are developed in pure steam and mixture gas condensation at low mass flux.

• 대한기계학회논문집
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• 제16권1호
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• pp.53-62
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• 1992

본 연구에서는 약 10년 정도 사용하여 경년 열화가 되었다고 예상되는 실구조 물의 일부를 입수하였으며 열화재의 특성과 비교하기 위하여 열처리에 의해 충격치를 회복시킨 재료를 회복재로 하여 두가지 재료에 대해 시험편을 제작하였따.열확현상 을 파악하기 위하여 평활재로 피로과정, 즉 미소 균열의 발생, 진전 및 복수 균열이 간섭합체하여 파단에 달하는 과정에 대하여 파괴역학적 견지에서 열화재와 회복재를 해석하고 이결과로 부터 확율변수를 추정하여 통계학적인 수명예측방법의 하나를 제시 하여 실구조물에 적용하는 방법에 대해 시도해 보았다.

• 한국추진공학회지
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• 제22권6호
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• pp.84-93
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• 2018

냉각제 분사조건이 변화할 때 혼합가스 사출시스템 (또는 가스-스팀 발사체계) 냉각효율 및 발사체 사출성능의 변화를 확인하기 위해 3차원 수치모사를 수행하였다. 선행연구를 참조하여 구축한 1차원 모델을 통해 3차원 단상(single-phase) 계산모델을 검증한 후, Discrete Phase Model을 활용하여 고온의 기체에 냉각제 분사를 모사한 이상(two-phase) 유동 계산을 수행하였다. 냉각제 유량과 분사구 개수를 각각 변화시키면서 계산을 수행한 결과, 분사구 개수를 증가시켰을 경우 냉각효율이 보다 개선되었다. 또한 분사조건에 따라 냉각제 액적 서로간의 coalescence 발생 빈도 및 공간분포 변화가 발생하여 액적의 직경이 달라졌고, 이는 냉각제 증발률에 영향을 미쳤다. 냉각제의 증발은 breech 내부의 온도를 감소시키는 반면에 압력감소를 억제하여 사출시스템 최적화 설계에 있어 중요한 요소임을 확인하였다.

• Transactions on Electrical and Electronic Materials
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• 제7권1호
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• pp.36-41
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• 2006

In this paper, the time evolution of undoped GaN epilayers on a low-temperature GaN buffer layer grown on c-plane sapphire at a low pressure of 300 Torr was studied via a two-step growth condition in a horizontal MOCVD reactor. As a function of the growth time at a high-temperature, the surface morphology, structural quality, and optical and electrical properties were investigated using atomic force microscopy, high-resolution x-ray diffraction, photoluminescence, and Hall effect measurement, respectively. The root-mean-square roughness showed a drastic decrease after a certain period of surface roughening probably due to the initial island growth. The surface morphology also showed the island coalescence and the subsequent suppression of three-dimensional island nucleation. The structural quality of the GaN epilayer was improved with increasing growth time considering the symmetrical (002) and asymmetrical (102) rocking curves. The variations of room-temperature photoluminescence, background carrier concentration, and Hall mobility were measured and discussed.

• Composites Research
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• 제15권3호
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• pp.52-61
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• 2002

압전재가 부착된 원통형 패널의 공력열탄성학적인 해석을 수행하기 위하여, 다분야 층별 이론에 기초하여 기하학적 비선형 유한 요소를 개발하였다. Han Krumhaar의 초음속 피스톤 이론을 적용하여 압전재가 부착된 원통형 패널에 대하여 열하중과 열변형에 따른 초음속 플러터 해석을 수행하였다. 플러터 임계동압을 증가시키고, 압전재 층의 열탄성학적 변형을 줄이는 가능성에 대하여 압전 작동기를 사용하여 검토하였다. 이 논문의 해석 결과들은 압전 작동기가 플러터 모드의 병합을 늦추고, 열하중을 상쇄하여 임계 동압을 효과적으로 증가시킬 수 있음을 보였다.

• Journal of Mechanical Science and Technology
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• 제19권3호
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• pp.860-869
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• 2005

Local pool boiling on the outside and inside surfaces of a 51 mm diameter tube in horizontal direction has been studied experimentally in saturated water at atmospheric pressure. Much variation in local heat transfer coefficients was observed along the tube periphery. On the outside surface the maximum and the minimum are observed at ${\theta}=45^{\circ}$ and $180^{\circ}$, respectively. However, on the inside surface only the minimum was observed at ${\theta}=0^{\circ}$. Major mechanisms on the outside surface are liquid agitation and bubble coalescence while those on the inside surface are micro layer evaporation and liquid agitation. As the heat flux increases liquid agitation gets effective both on outside and inside surfaces. The local coefficients measured at ${\theta}=90^{\circ}$ can be recommended as the representative values of both outside and inside surfaces.

• 한국안전학회지
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• 제21권4호
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• pp.49-54
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• 2006

The evaluation of elastic-plastic fracture characteristic was investigated in ferrite steel SA 516- Gr70 used for reactor coolant piping elbow and support skirt of pressure vessels. This paper describes the effect of temperature on J-R curve characteristic of this material. The elastic-plastic fracture mechanics parameter J is obtained with unloading compliance method. The test method were analyzed according to ASTM E 813-89 and E 1152-89. Unloading compliance $J_{IC}$ tests were performed on 1 CT specimens at varied temperatures from $25^{\circ}C$ to about $400^{\circ}C$ using a high temperature extensometer. At all temperature, valid $J_{IC}$ measurements could be made and $J_{IC}$ decreased with increasing temperature. SEM fractography schematically illustrates microvoid initiation, growth and coalescence at the tip of a preexisting crack.

• 대한기계학회논문집B
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• 제29권2호
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• pp.239-246
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• 2005

Effects of subcooling on pool boiling heat transfer in vertical annuli with closed bottoms have been investigated experimentally. For the test, a tube of 19.1mm diameter and the water at atmospheric pressure have been used. Three annular gaps of 7.05, 18.15, and 28.20 have been tested in the subcooled water and results of the annuli are compared with the data of a single unrestricted tube. The increase in pool subcooling results in much change in heat transfer coefficients. At highly subcooled regions, heat transfer coefficients for the annuli are much larger than those of a single tube. As the heat flux increases and subcooling decrease, a deterioration of heat transfer coefficients is observed at the annulus of 7.05mm gap. Single-phase natural convection and liquid agitation are the governing mechanisms for the single tube while liquid agitation and bubble coalescence are the major factors at the bottom closed annuli.

• 한국자동차공학회논문집
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• 제12권4호
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• pp.65-73
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• 2004

Fatigue crack growth from surface defects is one of the most important subjects for the evaluation and the assurance of safety in pressure vessels, piping systems, LPG/LNG fuel tank and other various structures. This paper attempts to analysis some practical or general problems such as the estimation of crack growth life to penetrate the plate thickness, based on fatigue crack growth a single surface flaw and the interaction of multiple flaws. An experiment on the coalescence of multiple undercuts was carried out under cyclic tension condition as a attempt to the analysis of multiple crack problems. It is noted that the fracture strength is characterized by the analogy to that in a single crack growth.