• Journal of Advanced Marine Engineering and Technology
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• 제35권4호
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• pp.451-459
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• 2011

초저온 구조 재료로 극저온에서 기계적 특성이 우수한 300계열의 오스테나이트계 스테인리스강이 널리 사용되어진다. 이 중에서도 오스테나이트의 안전성 향상을 위해 몰리브덴을 첨가한 316강이나 용접 중에 입계 석출을 줄이기 위해 탄소의 함유율을 감소시킨 316L강, 그리고 질소를 첨가시켜 강도와 오스테나이트를 동시에 향상시킨 316LN강이 대표적으로 많이 사용된다. 하지만 초저온 재료들의 용접조건 및 온도변화에 대한 기계적 특성 연구가 미진한 상태이다. 본 논문에서는 초저온 구조물에서 많이 사용되어지는 304강의 용접조건 및 온도변화에 대한 기계적 특성을 연구하였다.

• 한국표면공학회지
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• 제37권1호
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• pp.64-70
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• 2004

Cryogenic characteristics of austenitic stainless steel based on 304 steel with nickel and nitrogen were investigated at room temperature and $-196^{\circ}C$. The alloys were fabricated by vacuum arc furnace and cold working after homogenization treatment. The addition of nickel and nitrogen decreased the stability of $\delta$-ferrite and induced the stability against the formation of martensite to result significantly in enhancing ductility at $-196^{\circ}C$. Nitrogen reduced Md temperature, which was beneficial to the tensile strength and elongation at $25^{\circ}C$ and -196$^{\circ}C$.

• 한국초전도ㆍ저온공학회논문지
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• 제21권3호
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• pp.1-5
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• 2019

The superconductor stability theories are consistently described by the integral formula. If the defined stability function is a simple decreasing function, it becomes a cryogenic stability condition. If the stability function has a maximum value and a minimum value, and the maximum value is less than 0, then it is a cold-end recovery condition. If the maximum value is more than 0, it can be shown that the unstable equilibrium temperature, that is, the MPZ (minimum propagation zone) temperature distribution can exist. The MPZ region is divided into two regions according to the current ratio. At the low current ratio, the maximum dimensionless temperature is greater than 1, and at the relatively high current ratio, the maximum dimensionless temperature is less than 1. In order to predict the minimum quench energy, the dimensionless energy was obtained for the MPZ temperature distribution. In particular, it was shown that the dimensionless energy can be obtained even when the MPZ maximum temperature is 1 or more.

• 한국정밀공학회:학술대회논문집
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• 한국정밀공학회 2008년도 추계학술대회 논문집
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• pp.569-570
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• 2008

• 한국초전도ㆍ저온공학회논문지
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• 제24권3호
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• pp.68-73
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• 2022

Thermoacoustic oscillations (TAOs) are spontaneous pressure oscillations frequently seen in hydrogen or helium cryogenic systems. Half-open tubes connected to cryogenic fluid with a closed room temperature end have a high potential for oscillation generation. Thermoacoustic oscillations will result in significant pressure fluctuations and additional heat load, endangering the security and stability of the cryogenic system. The goal of this paper is to investigate TAOs in superfluid helium using both theoretical and experimental methods. Five half-open tubes with varied typical inner diameters inserted into superfluid helium were installed in a test cryostat. The onset characteristics of thermoacoustic oscillations were presented and studied. The effect of temperature profile was discussed. Finally, a simple eliminating method was introduced.

• 한국초전도ㆍ저온공학회논문지
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• 제25권3호
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• pp.49-55
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• 2023

This paper presents the design of a re-liquefaction system as a BOG (boil-off gas) handling process in liquid hydrogen transport vessels. The total capacity of the re-liquefaction system was assumed to be 3 ton/day, with a BOR (boil-off rate) of 0.2 %/day inside the cargo. The re-liquefaction cycle was devised using the He-Brayton Cycle, incorporating considerations of BOG capacity and operational stability. The primary components of the system, such as compressors, expanders, and heat exchangers, were selected to meet domestically available specifications. Case studies were conducted based on the specifications of the components to determine the optimal design parameters for the re-liquefaction system. This encompassed variables such as helium mass flow rate, the number of compressors, compressor inlet pressure and compression ratio, as well as the quantity and composition of expanders. Additionally, an analysis of exergy destruction and exergy efficiency was carried out for the components within the system. Remarkably, while previous design studies of BOG re-liquefaction systems for liquid hydrogen vessels were confined to theoretical and analytical realms, this research distinguishes itself by accounting for practical implementation through equipment and system design.

• 한국재료학회지
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• 제29권3호
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• pp.189-195
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• 2019

The effect of C, Mn, and Al additions on the tensile and Charpy impact properties of austenitic high-manganese steels for cryogenic applications is investigated in terms of the deformation mechanism dependent on stacking fault energy and austenite stability. The addition of the alloying elements usually increases the stacking fault energy, which is calculated using a modified thermodynamic model. Although the yield strength of austenitic high-manganese steels is increased by the addition of the alloying elements, the tensile strength is significantly affected by the deformation mechanism associated with stacking fault energy because of grain size refinement caused by deformation twinning and mobile dislocations generated during deformation-induced martensite transformation. None of the austenitic high-manganese steels exhibit clear ductile-brittle transition behavior, but their absorbed energy gradually decreases with lowering test temperature, regardless of the alloying elements. However, the combined addition of Mn and Al to the austenitic high-manganese steels suppresses the decrease in absorbed energy with a decreasing temperature by enhancing austenite stability.

• 한국정밀공학회지
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• 제29권12호
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• pp.1331-1339
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• 2012

Safety valve used in LNG/LNG-FPSO ships is a high value valve, and it plays an important role in maintaining a fixed level of pressure by emitting LNG gas out of pipes in LNG piping system under the cryogenic and high-pressure condition when the pressure of the system connected with the LNG storage tank and pipes reaches over the set pressure. The structural stability is required for the inner pressure and thermal load because of the cryogenic and high-pressure condition, and a reliability of the safety valve is necessary for impact and deformation by opening the valve. But, the safety valve, which plays a key role for a safety of the transport and storage system, is depended on imports for over 90%, and in domestic production, the design of the valve is performed on the basis of experiences of the works without quantitative analysis for the inner operation characteristics and structural stability of the valve. In this study, impact velocity is calculated by theoretical analysis for obtaining the structural stability of the guide according to the impact load by opening the valve. The shape of the guide and the diaphragm for satisfying the structural stability are suggested and verified by using a thermal-structural analysis.

• 전자공학회논문지D
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• 제35D권3호
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• pp.1-10
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• 1998

A new oscillator configuration is presented and tested for Josephson voltage standard operated at the cryogenic(4.2K) temperature. Features of active devices are investigated in aspects of 1/f noise, output power, and current collapse at low temperature. The output power of oscillator is optimized by a nonlinear design approach called Harmonic Two Signal Method(HTSM). The embedding newworks of the generalized six oscillators with tow loads are derived. A HEMT oscilliator is designed in X-Band for the Josephson voltage standard and tested at room and cryogenic(4.2K) temperatures. Oscillation frequency, output power, C/N ratio, and fequency stability are compared at room and low temperatures.

• 한국초전도ㆍ저온공학회논문지
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• 제25권2호
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• pp.14-18
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• 2023

Superconducting layers deposited on the metal substrate using the pulsed laser deposition process (PLD) play a crucial role in exploring new applications of superconducting wires and enhancing the performance of superconducting devices. In order to improve the superconducting property and increase the throughput of superconducting wire fabricated by pulsed laser deposition, high temperature heating device is needed that provides high temperature stability and strong durability in high oxygen partial pressure environments while minimizing performance degradation caused by surface contamination. In this study, new heating device have been developed for PLD process that deposit and growth the superconducting material continuously on substrate using reel-to-reel transportation apparatus. New heating device is designed and fabricated using iron-chromium-aluminum wire and alumina tube as a heating element and sheath materials, respectively. Heating temperature of the heater was reached over 850 ℃ under 700 mTorr of oxygen partial pressure and is kept for 5 hours. The experimental results confirm the effectiveness of the developed heating device system in maintaining a stable and consistent temperature in PLD. These research findings make significant contributions to the exploration of new applications for superconducting materials and the enhancement of superconducting device performance.