• Progress in Superconductivity and Cryogenics
• /
• v.25 no.4
• /
• pp.70-74
• /
• 2023

The Small Punch Test (SPT) was developed to evaluate the softening and embrittlement of materials such as power plants and nuclear fusion reactors by taking samples in the field. Specimens used in the SPT are very thin and small disk-shaped compared to specimens for general tensile test, and thus have economic advantages in terms of miniaturization and repeatability of the test. The cryogenic SPT can also be miniaturized and has a significantly lower heat capacity than conventional universal test machines. This leads to reduced cooling and warm-up times. In this study, the cryogenic SPT was developed by modifying the existing room temperature SPT to be cooled by liquid nitrogen using a super bellows and a thermal insulation structure. Since the cryogenic SPT was first developed, basic experiments were conducted to verify the effectiveness of it. For the validation, aluminum alloy 6061- T6 specimens were tested for mechanical properties at room and cryogenic temperature. The results of the corrected tensile properties from the SPT experiment results were compared with known room temperature and cryogenic properties. Based on the correction results, the effectiveness of the cryogenic SPT test was confirmed, and the surface fracture characteristics of the material were analyzed using a 3d image scanner. In the future, we plan to conduct property evaluation according to the development of various alloy materials.

• Progress in Superconductivity and Cryogenics
• /
• v.15 no.1
• /
• pp.45-50
• /
• 2013

Closed-loop J-T (Joule-Thomson) refrigeration cycle is advantageous compared to common open loop $N_2$ decompression system in terms of nitrogen consumption. In this study, two closed-loop pure $N_2$ J-T refrigeration systems with sub-atmospheric device for cooling High Temperature Superconductor (HTS) power cable are investigated. J-T cooling systems include 2-stage compressor, 2-stage precooling cycle, J-T valve and a cold compressor or an auxiliary vacuum pump at the room temperature. The cold compressor and the vacuum pump are installed after the J-T valve to create sub-atmospheric condition. The temperature of 67 K is possible by lowering the pressure up to 24 kPa at the cold part. The optimized hydrocarbon mixed refrigerant (MR) J-T system is applied for precooling stage. The cold head of precooling MR J-T have the temperature from 120 K to 150 K. The various characteristics of cold compressor are invstigated and applied to design parameter of the cold compressor. The Carnot efficiency of cold compressor system is calculated as 16.7% and that of vacuum pump system as 16.4%. The efficiency difference between the cold compressor system and the vacuum pump system is due to difference of enthalpy change at cryogenic temperature, enthalpy change at room temperature and different work load at the pre-cooling cycle. The efficiency of neon-nitrogen MR J-T system is also presented for comparison with the sub-atmospheric devices. These systems have several pros and cons in comparison to typical MR J-T systems such as vacuum line maintainability, system's COP and etc. In this paper, the detailed design of the subcooled $N_2$ J-T systems are examined and some practical issues of the sub-atmospheric devices are discussed.

• Proceedings of the Korea Institute of Applied Superconductivity and Cryogenics Conference
• /
• 1999.02a
• /
• pp.173-176
• /
• 1999

Vacuum insulated pipes (VIP) are one of the important equipments for cryogenic fluids' transfer. Flange type of VIP, which can easily be installed at the site, uses a set of male and female bayonet with very small gap between them. In order to prevent leakage of liquid from inner pipe to bayonet. Teflon or Kel-F is located outside both the inner pipe of male and the guide of female. Even though liquid may leak at room temperature, it cannot leak at cryogenic temperature since Teflon shrinks much more than pipes and adheres closely to the inner pipe and guide. Teflon seal method has the advantage of easy fabrication, low cost and effective sealing compared to the conventional method.

• Progress in Superconductivity and Cryogenics
• /
• v.1 no.1
• /
• pp.48-55
• /
• 1999

thermodynamic cycle analysis has been performed for the power generation systems to utilize the cold energy of liquefied natural gas (LNG). The power cycle used the air or water at room temperature as a heat source and the LNG at cryogenic temperature as a heat sink. Among manypossible configurations of the cycle. the open Rankine cycle. and the closed Brayton cycle, and the closed Rankine cycle are selected for the basic analysis because of their practical importance. The power output per unit mass of LNG has been analytically calculated for various design parameters such as the pressure ratio. the mass flow rate. the adiabatic efficiency. the heat exchanger effectiveness. or the working fluid. The optimal conditions for the parameters are presented to maximize the power output and the design considerations are discussed. It is concluded that the open Rankine cycle is the most recormmendable both in thermodynamic efficency and in practice.

• Progress in Superconductivity and Cryogenics
• /
• v.21 no.4
• /
• pp.34-38
• /
• 2019

Multilayered high-temperature superconductor coated conductor (CC) tapes are used in an extensive range of applications and are exposed to many stresses such as hoop stress, radial/transverse tensile stress under large Lorentz forces, and thermal stress while cooling if thermal expansion properties differ. Loads induced transversely at the tape surface inevitably create delamination phenomena in the multilayered CC tapes. Thus, delamination behaviors of CC tapes along the c-axis under transverse loading conditions, which can vary based on manufacturing process and constituent layers, must be characterized for applications. The anvil test method was used to mechanically investigate the delamination characteristics of various commercially available Ag-stabilized CC tapes at room temperature and 77 K, finding superior strength at the latter. The wide variations found depended on tape structure and fabrication technique. Fractographic morphologies of delaminated tapes supported the findings under transverse loading conditions.

• Progress in Superconductivity and Cryogenics
• /
• v.24 no.3
• /
• pp.68-73
• /
• 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.

• Progress in Superconductivity and Cryogenics
• /
• v.19 no.2
• /
• pp.53-57
• /
• 2017

In superconducting power devices including power cables in which high temperature superconducting (HTS) tapes are utilized, a reliable electrical insulation should be achieved for its maximum performance. For an efficient design of HTS superconducting devices, a comparative evaluation of the mechanical and thermal propperties for various insulation materials at cryogenic temperatures is required. Especially, in the process of the property evaluation of the sheet-shaped insulation materials, anisotropy according to the machining direction should be considered because the mechanical and thermal properties are significantly influenced by the sample orientation. In this study, the cryogenic thermal and mechanical properties of various insulation material sheets such as PPLP, Cryoflex, Teflon, and Kapton were determined considering sample orientation. All samples tested at cryogenic temperature showed significantly higher tensile strength as compared with that of room temperature. The ultimate tensile strength at both temperature conditions significantly depended upon the sample orientation. The thermal properties of the insulation materials exhibited a slight difference among samples depending on the orientation: for the PPLP and Cryoflex, the CD orientation showed larger thermal contraction up to 77 K as compared to the MD one. MD samples in PPLP and Cryoflex showed a lower CTE and thermal contraction which made it more promising as an insulation material due to its comparable CTE with HTS CC tapes.

• Progress in Superconductivity and Cryogenics
• /
• v.24 no.4
• /
• pp.71-77
• /
• 2022

Sub-Kelvin cooling has been generally demanded for the fields of low temperature physics, such as physical property measurements, astronomical detection, and quantum computing. The refrigeration system with a small size can be appropriately introduced when the measurement system does not require a high cooling capacity at sub-Kelvin temperature. The dilution refrigerator which is a common method to reach sub-Kelvin, however, must possess a large ³He circulation equipment at room temperature. As alternatives, a sorption refrigerator and a magnetic refrigerator can be adopted for sub-Kelvin cooling. This paper describes those coolers which have been developed by various research groups. Furthermore, a cold-cycle dilution refrigerator of which the size of the ³He circulation system is minimized, is also introduced. Subsequently, a new concept of dilution refrigerator is proposed by our group. The suggested cooler can achieve sub-Kelvin temperature with a small size since it does not require any recuperator and turbo-molecular vacuum pump. Its architecture allows the compact configuration to reach sub-Kelvin temperature by integrating the sorption pump and the magnetic refrigerators. Therefore, it may be suitably utilized in the low temperature experiments requiring low cooling capacity.

• Progress in Superconductivity and Cryogenics
• /
• v.15 no.3
• /
• pp.1-4
• /
• 2013

We investigated the effect of Nb substitution for Ru on the structural and magnetic properties of $(Ru_{1-x}Nb_x)\;Sr_2(Sm_{1.4}Ce_{0.6})Cu_2O_z$ Samples. X-ray diffraction measurements indicated that nearly single-phase samples are formed in the range from x = 0 to 1.0. The superconducting transition temperature determined from the inflection in the field-cooled magnetic susceptibility decreased only slightly from $T_c$ = 25 K for x = 0 to $T_c$ = 22 K for x = 1.0, in consistent with the change in room temperature thermopower of the samples. However, the Nb substitution for Ru above x = 0.25 significantly suppressed the weak ferromagnetic component of the field-cooled magnetic susceptibility. It was also found that the Nb substitution for Ru results in an enhanced diamagnetic susceptibility with Nb content above x = 0.5 in both zero field-cooled and field-cooled magnetization measurements, in contrast to the behavior of the samples with $x{\leq}0.5$ in which the diamagnetic susceptibility decreases as the Nb content increases.

• Progress in Superconductivity and Cryogenics
• /
• v.7 no.2
• /
• pp.39-43
• /
• 2005

Toward the practical applications, on operation of conduction-cooled HTS SMES at temperatures well below 77 K should be investigated, in order to take advantage of a greater critical current density of HTS and considerably reduce the size and weight of the system. Recently, research and development concerning application of the conduction-cooled HTS SMES that is easily movement are actively progressing in Korea. Electrical insulation under cryogenic temperature is a key and an important element in the application of this apparatus. However, the behaviors of insulators for cryogenic conditions in air or vacuum are virtually unknown. Therefore, this work focuses on the breakdown and flashover phenomenology of dielectrics exposed in vacuum for temperatures ranging from room temperature to cryogenic temperature. Firstly, we summary the insulation factors of the magnet for HTS SMES. And a surface flashover as well as volume breakdown in air and vacuum has been investigated with two kind insulators. Finally, we will discuss applications for the HTS SMES including aging studies on model coils exposed in vacuum at cryogenic temperature.