Earticle

This paper reviews the development status of magnetic refrigeration system for hydrogen liquefaction. There is no doubt that hydrogen is one of most important energy sources in the near future. In particular, liquid hydrogen can be utilized for infrastructure construction consisting of storage and transportation. Liquid hydrogen is in cryogenic temperatures and therefore high efficient liquefaction method must be studied. Magnetic refrigeration which uses the magneto-caloric effect has potential to realize not only the higher liquefaction efficiency > 50 %, but also to be environmentally friendly and cost effective. Our hydrogen magnetic refrigeration system consists of Carnot cycle for liquefaction stage and AMR (active magnetic regenerator) cycle for precooling stages. For the Carnot cycle, we develop the high efficient system > 80 % liquefaction efficiency by using the heat pipe. For the AMR cycle, we studied two kinds of displacer systems, which transferred the working fluid. We confirmed the AMR effect with the cooling temperature span of 12 K for 1.8 T of the magnetic field and 6 second of the cycle. By using the simulation, we estimate the total efficiency of the hydrogen liquefaction plant for 10 kg/day. A FOM of 0.47 is obtained in the magnetic refrigeration system operation temperature between 20 K and 77 K including LN2 work input.

We have investigated the effect of thickness of the MgB2 film on the grain growth direction as well as on their superconducting properties. MgB2 films of various thicknesses were fabricated on c-cut Al2O3 substrates at a temperature of 540 °C by using hybrid physical-chemical vapor deposition (HPCVD) technique. The superconducting transition temperature (Tc) was found to increase with increase in the thickness of the MgB2 film. X-ray diffraction analysis revealed that the orientation of grains changed from c-axis to a-axis upon increasing the thickness of the MgB2 film from 0.6 to 2.0 μm. MgB2 grains of various orientations were observed in the microstructures of the films examined by scanning electron microscopy. It is observed that at high magnetic fields the 2.0-μm-thick film exhibit considerably larger critical current density (Jc) as compared to 0.6-μ m-thick film. The results are discussed in terms of an intrinsic-pinning in MgB2 similarly as intrinsic-pinning occurring in high-Tc cuprate superconductors with layered structure.

The pure and carbon (C)-doped MgB2 thin films were fabricated on Al2O3 (0001) substrates at a temperature of 650 °C by using hot-filament-assisted hybrid physical–chemical vapor deposition technique. The Tc value for pure MgB2 film is 38.5 K, while it is between 30 and 35 K for carbon-doped MgB2 films. Expansion in c-axis lattice parameter was observed with increase in carbon doping concentration which is in contrast to carbon-doped MgB2 single crystals. Significant enhancement in the critical current density was obtained for C-doped MgB2 films as compared to the undoped MgB2 film. This enhancement is most probably due to the incorporation of C into MgB2 and the high density of grain boundaries, both help in the pinning of vortices and result in improved superconducting performance.

The magneto-resistivity and electric field-current density (E-J) curves were investigated up to a magnetic field 9 T in the optimally doped BaFe1.8Co0.2As2 single crystal with a superconducting temperature (Tc) of 24.6 K. The E-J Scaling behaviors below and above vortex glass transition temperature (Tg) were found, confirming the existence of the vortex glass phase transition. The critical exponents for the diverging spatial and time correlations at Tg, were obtained as ν = 1.1 ± 0.1 and z = 4.5 ± 0.3, respectively. The obtained critical exponents are in good agreement with the predicted values of ν ~ 1 – 2 and z > 4 within the 3D vortex glass theory.

For sensitive measurements of micro-Tesla nuclear magnetic resonance (μT-NMR) signal, a low-noise superconducting quantum interference device (SQUID) system is needed. We have fabricated a liquid He dewar for an SQUID having a large diameter for the pickup coil. The initial test of the SQUID system showed much higher low-frequency magnetic noise caused by the thermal magnetic noise of the aluminum plates used for the vapor-cooled thermal shield material. The frequency dependence of the noise spectrum showed that the noise increases with the decrease of frequency. This behavior could be explained from a two-layer model; one generating the thermal noise and the other one shielding the thermal noise by eddy-current shielding. And the eddy-current shielding effect is strongly dependent on the frequency through the skin-depth. To minimize the loop size for the fluctuating thermal noise current, we changed the thermal shield material into insulated thin Cu mesh. The magnetic noise of the SQUID system became flat down to 0.1 Hz with a white noise of 0.3 fT/√Hz, including the other noise contributions such as SQUID electronics and magnetically shielded room, etc, which is acceptable for low-noise μT-NMR experiments.

In this study, the homogeneity of critical current, Ic, along the lengthwise direction in the coated conductor (CC) tape under uniaxial tension was investigated using a multiple voltage tap configuration. Initially, a gradual and homogeneous Ic degradation occurred in all subsections of the tape up to a certain strain value. This was followed by an abrupt Ic degradation in some subsections, which caused scattering in Ic values along the length with increasing tension strain. The Ic degradation behaviour was also explained through n-value as well as microstructure analyses. Subsections showed Ic scattering corresponding to damaged areas of the CC tape revealed that transverse cracks were distributed throughout the gauge length. This homogeneous Ic degradation behaviour under tension is similar with the case under torsion strain but different with the case under hard bending which were previously reported. This behaviour is also different with the case using Bi-2223 HTS tapes under tension strain.

The understanding of the strain dependence of critical current, Ic, in the reversible region is important for the evaluation of the performance of coated conductor (CC) tapes in practical applications. In this study, the stress/strain tolerance of Ic in GdBCO CC tapes with stainless steel substrate stabilized by additional Cu and brass laminate was analyzed quantitatively through Ic–strain measurement at 77 K under self-field. The variation in irreversible strain limits of CC tapes by the addition of stabilizing layers was analyzed through the consideration of the pre-strain induced on the GdBCO coating film. The results were then compared with the ones previously reported for GdBCO CC tapes with Hastelloy substrate. As a result, GdBCO CC tapes with stainless steel substrate showed much higher strain tolerance of Ic as compared with those adopting Hastelloy substrate.

AC Losses for face to face stacks of four identical coated conductors (CCs) were numerically calculated using the H-formulation combined with the E-J power law and the Kim model. The motive sample was the face to face stack of four 2 mm-wide CC tapes with 2 μm thick superconducting layer of which the critical current density, Jc, was 2.16×106 A/cm2 on IBAD-MgO template, which was suggested for the mitigation of ac loss as a round shaped wire by Korea Electrotechnology Research Institute. For the calculation the cross section of the stack was simply modeled as vertically aligned 4 rectangles of superconducting (SC) layers with E = Eo(J(x,y,t)/Jc(B))n in x-y plane where Eo was 10-6 V/cm, Jc(B) was the field dependence of current density and n was 21. The field dependence of the critical current of the sample measured in four-probe method was employed for Jc(B) in the equation. The model was implemented in the finite element method program by commercial software. The ac loss properties for the stacks were compared with those of single 4 cm-wide SC layers with the same critical current density or the same critical current. The constraint for the simulation was imposed in two different ways that the total current of the stack obtained by integrating J(x,y,t) over the cross sections was the same as that of the applied transport current: one is that one fourth of the external current was enforced to flow through each SC. In this case, the ac loss values for the stacks were lower than those of single wide SC layer. This mitigation of the loss is attributed to the reduction of the normal component of the magnetic field near the SC layers due to the strong expulsion of the magnetic field by the enforced transport current. On the contrary, for the other case of no such enforcement, the ac loss values were greater than those of single 4cm-wide SC layer and . In this case, the phase difference of the current flowing through the inner and the outer SC layers of the stack was observed as the transport current was increased, which was a cause of the abrupt increase of ac loss for higher transport current.

MgB2 bulk samples are synthesized through solid state reaction route using Mg precursors with different particle size by keeping the boron precursor unchanged. Scanning electron microscopy study of the fractured surface for all the samples depicts quite distinct structure depending on the Mg precursor. Big size of Mg precursor resulted in to largely elongated and deep pores while smaller one gave roughly ellipsoidal and shallow pore structure. Influence of the Mg particle size on the grain to grain connectivity reflected in the critical current density value which was greater for samples with smaller Mg precursor. All the synthesized samples undergo a superconducting transition at around 36.5 K irrespective of different Mg precursor particle size.

Large AC loss from the second generation (2G) high temperature superconducting (HTS) wires has been one of the major bottlenecks in power applications with HTS materials. Moreover, the large power applications also require the large current capacity from the HTS wires, which makes them produce larger AC losses. In order to reduce the AC loss from the HTS conductors with large current capacity, an HTS compact cable with some striations on the superconducting layers has been proposed. In this paper, we prepared some sample HTS compact conductors with striations, and measured their magnetization loss from the external magnetic field. We also made some slits on the superconducting layer of the HTS wire by laser cutting to reduce the aspect ratio of the superconducting layers. It would make the low eddy current loss and magnetic decoupling. Finally, the magnetization losses of the sample HTS compact conductors were measured and analyzed.

The thermal contact resistance (TCR) is one of the important components in the cryogenic systems. Especially, cryogenic measurement devices using a cryocooler can be affected by TCR because the systems have to consist of several metal components in contact with each other for heat transferring to the specimen without cryogen. Therefore, accurate measurement and understanding of TCR is necessary for the design of cryogenic measurement device using a cryocooler. The TCR occurs at the interface between metals and it can be affected by variable factors, such as roughness of metal surface, contact area and contact pressure. In this study, we designed TCR measurement system at various temperatures using a cryocooler as a heat sink and used steady state method to measure the TCR between metals. The copper is selected as a specimen in the experiment because it is widely used as a heat transfer medium in the cryogenic measurement devices. The TCR between Cu and Cu is measured for various temperatures and contact pressures. The effect of the interfacial materials on the TCR is also investigated.