• Progress in Superconductivity
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• 제10권2호
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• pp.128-132
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• 2009

Polycrystalline samples of $(Ru_{0.8}Nb_{0.2})Sr_2(Gd_{1.5-x}Nd_xCe_{0.5})Cu_2O_z$($ 0{\leq}x{\leq}1.25$) have been synthesized by a solid-state reaction route. The X-ray diffraction data revealed that the Nd solubility limit can be placed between x=0.5 and x=0.75. The superconducting transition temperature decreased with increasing Nd content, confirming that Nd entered the lattice. Room-temperature thermoelectric power measurements showed that all the samples are in the underdoped state and the partial substitution of Nd for Gd resulted in a decrease in the carrier density on the superconducting layers.

• 한국초전도ㆍ저온공학회논문지
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• 제25권4호
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• pp.1-4
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• 2023

Recently, a claim of material, named LK-99 (a lead apatite-based compound), exhibiting a superconducting transition temperature of over 400 K under standard atmospheric pressure, was reported [1, 2]. This claim has generated considerable attention from scientists worldwide. Here, we synthesized five LK-99 samples following the method detailed in the original papers [1, 2], and measured structural and resistivity data for each of these samples. The structure of the synthesized samples (P6₃/m, a=9.82 Å , c=7.34 Å ) was very close to the reported one. Contrary to the report, however, no hint of room-temperature superconductivity was noted from any of the samples. The results of Energy-dispersive X-ray spectroscopy (EDXS) measurements demonstrate that the atomic distribution in the sample was inhomogenous, and unreacted precursors were included in the samples. To investigate the intrinsic superconducting properties of LK-99, we propose to synthesize samples having high structural purity and chemical uniformity.

• 한국초전도ㆍ저온공학회논문지
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• 제14권3호
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• pp.60-64
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• 2012

In this paper, an investigation of a room temperature active magnetic regenerative refrigerator is carried out. Experimental apparatus includes two active magnetic regenerators containing 186 g of Gd spheres. Four E-type thermocouples are installed inside the Active magnetic regenerator(AMR) to observe the instantaneous temperature variation of AMR. Both warm and cold heat exchangers are designed for large temperature span. The cold heat exchanger, which separates the two AMRs, employs a copper tube with length of 80 mm and diameter of 6.35 mm. In order to minimize dead volume between the warm heat exchanger and AMRs, the warm heat exchangers are located close to the AMRs. The deionized water is used as a heat transfer fluid, and maximum 1.4 T magnetic field is supplied by Halbach array of permanent magnets. The AMR plate, which contains the warm and the cold heat exchangers and the AMRs, has reciprocating motion using a linear actuator and each AMR is alternatively magnetized and demagnetized by a Halbach array of permanent magnet. Since the gap of the Halbach array of permanent magnets is 25 mm and two warm heat exchangers have the motion through it, a compact printed circuit heat exchanger (PCHE) is used as a warm heat exchanger. A maximum no-load temperature span of 26.8 K and a maximum cooling power of 33 W are obtained from the fabricated Active Magnetic Regenerative Refrigerator (AMRR).

• 한국초전도ㆍ저온공학회논문지
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• 제15권4호
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• pp.59-62
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• 2013

The conductive link is used as a cooling medium between a cryocooler and magnet in a cryogen-free superconducting magnet system. The low temperature superconducting (LTS) magnet has one solenoidal configuration with a metal former which has a 52 mm room temperature bore. The superconducting coil is installed in the cryostat maintaining high vacuum and cooled by a two-stage cryocooler. In order to maintain the operating temperature of magnet at the designed level, the cold head temperature of the cryocooler must be lower so that heat can be removed from the superconducting coil. Also, temperature difference is occurred between the magnet and cryocooler and its magnitude is dependent upon the contact resistance at the interfacial surface between metals in the conductive link. In the paper, the performance of the LTS magnet is investigated with respect to the conductive link between the magnet former and the cold head of the cryocooler. The effects of the contact pressure and interfacial materials on the temperature distribution along the conductive link are also presented.

• 한국초전도ㆍ저온공학회논문지
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• 제13권2호
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• pp.29-32
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• 2011

The thermal properties of insulation material are essential to develop a high-temperature superconducting (HTS) power cable to be operated at around liquid nitrogen temperature. Unlike metallic materials, nonmetallic materials have a high thermal resistance; therefore special attention needs to be paid to estimate heat flow correctly. Thus, we have developed a precise instrument for measuring the thermal conductivity of insulating materials over a temperature range from 40 K to near room temperature using a cryocooler. Firstly, the measurement of thermal conductivity for Teflon is carried out for accuracy confirmation. For a supplied heat flux, the temperature difference between warm and cold side is measured in steady state, from which the thermal conductivity of Teflon is calculated and compared with published result of NIST. In addition, the apparent thermal conductivity of Polypropylene laminated paper (PPLP) is presented and its temperature dependency is discussed.

• 한국초전도ㆍ저온공학회논문지
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• 제12권3호
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• pp.1-6
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• 2010

Since the fiber reinforced polymeric (FRP) composites are considered in next generation of space transportation systems, reliable thermal expansion properties should be well provided for structural design of composite materials. To obtain accurate mechanical behaviors at a cryogenic temperature, precise strain measurement and calibration must be provided. In this work, apparent strains (or thermal output) of temperature self-compensated strain gages were deliberately investigated for epoxy, CTBN modified epoxy and carbon fabric composite system from room temperature to liquid nitrogen temperature. Also, fourth-order thermal output curves were presented for the further calibration. The results showed that the thermal output is heavily dependent on test materials and a large amount of apparent strains were observed for the polymer resins.

• 한국초전도ㆍ저온공학회논문지
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• 제11권1호
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• pp.5-8
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• 2009

MgO nanodots have been deposited and formed on top of the substrate surface. Mg was sputtered to form the MgO nanodots on the single crystal substrates by rf-sputtering method and followed by heat treatment in the oxygen ambient. The deposition and formation of MgO nanodots have been controlled systematically using the process variables such as substrate temperature, sputtering time, and rf-power. As the substrate temperature increased from the room temperature the density of MgO nanodots decreased. The optimal conditions of MgO nanodots formation using the rf-sputtering was investigated and the maximum density of more than $230/{\mu}m^2$ on single crystal substrates was obtained when the rf-power of 100 watts was applied for 30 seconds at room temperature. The typical size of MgO nanodots was identified to be <160 nm(diameter) and 4-30nm (height) by atomic force microscopy. The modulated surface morphology was examined through surface images and cross-section analysis and discussed for the artificial pinning sites in the superconducting films.

• 한국초전도ㆍ저온공학회논문지
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• 제14권4호
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• pp.32-35
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• 2012

A high-$T_c$ superconducting (HTS) DC application has advantages such as the ultimately lower loss, more compact dimensions, and large capacity compared to AC application. In order to optimize the insulation design of a HTS DC machines, it is important to understand the high voltage insulation and materials at cryogenic temperature. Polypropylene laminated paper (PPLP) has been widely used as an insulating material for HTS AC machines. However, the fundamental data under DC voltage have not been revealed satisfactorily until now. In this paper, we will discuss mainly on the breakdown and dielectric characteristics of PPLP in liquid nitrogen ($LN_2$). The polarity effects of DC and impulse voltage were studied by using the semi-rod to cylindrical electrode. The volume resistivity of PPLP in $LN_2$ was studied. Also, the space charge distribution at room temperature was studied. However, it is necessary to study this topic at cryogenic temperature in the near future.

• 한국초전도ㆍ저온공학회논문지
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• 제18권1호
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• pp.33-36
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• 2016

Suppression of the superconducting transition temperature ($T_c$) of NbN thin films in superconductor/ferromagnet multilayers has been investigated. Both superconducting NbN and ferromagnetic FeN layers were deposited on thermally oxidized Si substrate at room temperature by using reactive magnetron sputtering in an $Ar-N_2$ gas mixture. The thickness of FeN films was fixed at 20 nm, while the thickness of NbN films was varied from 3 nm to 90 nm. $T_c$ suppression was clearly observed in NbN layers up to 70 nm thickness when NbN layer was in proximity with FeN layer. For a given thickness of NbN layer, the magnitude of $T_c$ suppression was increased in the order of Si/FeN/NbN, Si/NbN/FeN, and Si/FeN/NbN/FeN structure. This result can be used to design a spin switch whose operation is based on the proximity effect between superconducting and ferromagnetic layers.

• 한국초전도ㆍ저온공학회논문지
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• 제14권2호
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• pp.32-35
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• 2012

The objective of the present work is to develop a precise instrument for measuring the thermal property of insulating material over a temperature range from 30 K to near room temperature by utilizing a cryocooler. The instrument consists of two thermal links, a test sample, heat sink, heat source and vacuum vessel. The cold head of the cryocooler as a heat sink is thermally anchored to the thermal link and used to bring the apparatus to a desired temperature in a vacuum chamber. An electric heater as a heat source is placed in the middle of test sample for generating uniform heat flux. The entire apparatus is covered by thermal shields and wrapped in multi-layer insulation to minimize thermal radiation in a vacuum chamber. For a supplied heat flux the temperature distribution in the insulating material is measured in steady and transient state. The thermal conductivity of insulating material is measured from temperature difference for a given heat flux. In addition, the specific heat of insulating material is obtained by solving one-dimensional heat diffusion equation.