• The Korean Journal of Ceramics
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• v.5 no.4
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• pp.390-394
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• 1999

Al2O3 particles were dispersed into carbon fiber reinforced epoxy composites to fabricate hybrid epoxy based composites. Interface behavior and mechanical properties of these hybrid composites were studied at room and liquid nitrogen temperature and liquid nitrogen temperature and the results were compared with the those of carbon fiber reinforced composites to investigate their applicability at room and cryogenic temperature. Young's modulus in-perpendicular to fiber direction and interlaminar shear strength at room temperature and the thermal contraction down to cryhogenic temperature were improved significantly by the addition of AL2O3 filler into the epoxy matrix. The effect of Al2O3 particle addition on mechanical properties were discussed.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2002.10a
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• pp.38-42
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• 2002

An apparatus was developed to repetitively apply a $-196^{\circ}C$ thermal load to coupon-sized mechanical test specimens. Using this device, IM7/5250-4 (carbon / bismaleimide) cross-ply and quasi-isotropic laminates were submerged in liquid nitrogen ($LN_2$) 400 times. Ply-by-ply micro-crack density, laminate modulus, and laminate strength were measured as a function of thermal cycles. Quasi-isotropic samples of IM7/977-3 (carbon / epoxy) composite were also manually cycled between liquid nitrogen and an oven set at $120^{\circ}C$ for 130 cycles to determine whether including elevated temperature in the thermal cycle significantly altered the degree or location of micro-cracking. In response to thermal cycling, both materials micro-cracked extensively in the surface plies fellowed by sparse cracking of the inner plies. The tensile modulus of the IM7/5250-4 specimens was unaffected by thermal cycling, but the tensile strength of two of the lay-ups decreased by as much as 8.5%.

• Journal of the Korean Society for Precision Engineering
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• v.15 no.4
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• pp.36-43
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• 1998

We experimented turning SCM440, called difficult-to-cut materials in general, using tungsten carbon tool(PIO) in order to elevate machinability by a new cutting method. The cutting tool designed and made to study was cooled to -17$0^{\circ}C$ in about 1 minute by liquid nitrogen. Then, we operated cryogenic cutting by cooling tool with liquid nitrogen and stuided the effect about cutting force, chip thickness, surface roughness, behavior of tool wear and cutting temperature. In addition, we investigated the possibility that sur face roughness of workpiece can be predicted analyzing cutting characteristics.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2003.05a
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• pp.151-155
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• 2003

An apparatus was developed to repetitively apply a -196 $^{\circ}C$ thermal load to coupon-sized mechanical test specimens. Using this device, IM7/5250-4 (carbon / bismaleimide) cross-ply and quasi-isotropic laminates were submerged in liquid nitrogen (L$N_2$) 400 times. Ply-by-Ply micro-crack density, laminate modulus, and laminate strength were measured as a function of thermal cycles. Quasi-isotropic samples of IM7/977-3 (carbon / epoxy) composite were also manually cycled between liquid nitrogen and an oven set at 120 $^{\circ}C$ for 130 cycles to determine whether including elevated temperature in the thermal cycle significantly altered the degree or location of micro-cracking. In response to thermal cycling, both materials micro-cracked extensively in the surface plies followed by sparse cracking of the inner plies. The tensile modulus of the IM7/5250-4 specimens was unaffected by thermal cycling, but the tensile strength of two of the lay-ups decreased by as much as 8.5 %.

• Progress in Superconductivity and Cryogenics
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• v.4 no.1
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• pp.119-123
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• 2002

In this study, we discussed the characteristics of breakdown and surface flashover of FRP in L$N_2$ for applying it to spacer of a superconducting power equipment. Fiberglass reinforced plastic(FRP) has high breakdown puncture strength and has high mechanical strength. So the research of its dielectric properties have been conducted in liquid nitrogen($LN_2$). We measured breakdown voltage of FRP and distinguished four types of surface flashover along solid insulator with arrangement of electrodes and measured the surface flashover voltages of each types of electrode arrangement. The type of a electrode set at the back of the insulator showed the worst dielectric property. The property of surface flashover depending on pressure was tested.

• The Transactions of the Korean Institute of Electrical Engineers
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• v.37 no.12
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• pp.872-878
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• 1988

During the past few years, a great deal of attention has been directed to the application of superconductivity to the electrical systems such as superconducting power transmission lines, superconducting magnet energy storage and so on. Yet in order to develop the practical model of these electrical equiqments utilizing suprconductivety and other phenomena at cryogenic temperautre, it is necessary to know the dielectric behaviour of insulating materials at cryogenic temperature in view of reliability, safety and economy of these machines. Investigation of dielectric properties of cryogenic liquids is very important due to the dual role of those as the dielectric and cooling medium. In this study, we investigated results measured over several kinds of dielectric characteristics of liquid nitrogen taking into consideration for application of high Tc superconductor. Dependence of breakdown voltage of gap width, polarity and pressure is reported in this paper and time delay characteristics of breakdown is also the subject of this discussion.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.16 no.10
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• pp.946-951
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• 2003

A high-Tc superconducting cable(HTS cable) is expected as an underground power line supplying the electrical power the densely populated city in future. The electrical insulation is very important for develop HTS cable system because it is operated a high voltage and in cryogenic temperature. We manufactured a mini-model cable and measured a tan$\delta$ of cable using schering bridge. The tan$\delta$ of PPLP was lower than that of Tyvek and Kraft at a given temperature, the tan$\delta$ of PPLP was 1.16${\times}$10-3. According to the increase of electric stress the tan$\delta$ increased because partial discharge occurred inside butt gap of mini-model cable. However, the tan$\delta$ decreased by increase of liquid nitrogen pressure. This reason is thought by decrease of part discharge between butt gap by increase of liquid nitrogen pressure.

• 유체기계공업학회:학술대회논문집
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• 2005.12a
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• pp.341-347
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• 2005

To perform a cryogenic development test for Tribo-elements in a turbopump, a cryogenic bearing and seal test facility (BSTF) is designed and currently under construction in KARI. The working fluid is liquid nitrogen operating at a temperature $-197^{\circ}C$. The maximum operating pressure and volume flow rate of BSTF are 100 bar and 10 liters per second, respectively. The development tests of floating ring seals, inter-propellant seals (IPS) and cryogenic ball bearings in a turbopump will be performed using the BSTF. This paper briefly described design requirements and procedures of BSTF.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1994.10a
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• pp.32-37
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• 1994

We experimented on cutting characteristics - cutting force, behavior of cutting temperature, surface roughness, behavior of chips-under low temperature,which generated by liquid nitrogen(77K). The workpieces were freezed to -195 .deg. C and liquid nitrogen was also sprinkled on cutting area in order to increase the efficiency of machining in low temperature. The workpiece was became to -195 .deg. C in 5 minutes. In cooled condition(CC) surface roughness of workpiece was better than normal condition(NC). In addition, we investigated the possibility that surface roughness of workpiece and shear angle can be predicted analyzing cutting condititions by the trained neural network.

• Proceedings of the Korean Institute of IIIuminating and Electrical Installation Engineers Conference
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• 1993.10a
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• pp.58-62
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• 1993

This paper shows the discharge characteristics of liquid nitrogen under plane to plane electrode, needle to plance electrode, inserting teflon insulator in plane to plane electrode at atmospheric pressure. The main experimental device of this paper is cryostat. The important results obtained from this study are sa follows. Breakdown voltage of LN2 under N-P electrode is higher than that of under P-P electrode at atmospheric pressure. Breakdown duration time is longer with increase of electode gap under the P-P, N-P electrodes. The formation of bubbles by evaporation is observed in spite of non-applying source at atmospheric pressure and the creation of corona confirmed. Breakdown voltage and breakdown duration time are in proportion to increase of diameter in case of inserting teflon insulator. Partial discharge characteristics of thickness 0.5[mm] is that 채굼 voltage of teflon insulator rose with increase of diameter in each [pps].