Coated Conductors Research Articles

Electromagnetic properties of curved HTS trapped field stacks under high-frequency cross fields for high-speed rotating machines

Superconducting electric propulsion systems, characterized by high power densities and efficiencies, provide a possibility to zero carbon emission for future aviation. Stacks of high temperature superconducting (HTS) coated conductors (CCs) have become an alternative for high field magnets applied to superconducting machines, given their excellent field trapping ability and thermal stability. High-frequency ripple fields always exist in high-speed electric machines. Most research work regarding HTS trapped field stacks (TFSs) was focused on their magnetization methods and amplitude of trapped flux density; however, their performance in the high-frequency environment remains unclear. Despite several numerical models established for flat HTS TFSs, a comprehensive analysis of curved ones is still lacking, which possess geometrical applicability for cylindrical rotating shafts. Aimed at exploring the electromagnetic properties of curved HTS TFSs applied to high-speed rotating machines, a 3D numerical model considering both the multilayer structure and the Jc (B) dependence of HTS CCs has been built. Current and magnetic flux density distributions, as well as loss properties of a curved HTS TFS have been studied in detail, under perpendicular and cross fields with varying frequencies ranging from 50 Hz to 20 kHz. Results have shown that, the widely adopted two-dimensional-axisymmetric models are inapplicable to study the electromagnetic distributions of TFSs because of the emergence of the electromagnetic criss-cross defined in this paper. High-frequency ripple fields can drive induced current towards the periphery of the HTS TFS due to the skin effect, leading to a fast rise of AC loss and even an irreversible demagnetization of the stack. This paper has qualified and quantified the high-frequency electromagnetic behaviours of curved HTS TFSs, providing a useful reference for their loss controlling and anti-demagnetization design in high-speed propulsion machines.

Evaluation of Electromechanical Properties in Multiple HTS Layered Conductors at 77 K

High engineering critical current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> ) is demanded in high-performance large-scale applications such as power cables, high-magnetic-field magnets, accelerators, and fusion reactors. In developing high current capacity and low vulnerability under a high magnetic field, high-temperature superconducting (HTS) rare-earth barium copper oxide coated conductors (CCs) have undergone various trials, and CC tape characteristics have ultimately been greatly improved. The Korea Electrotechnology Research Institute recently developed a “multiple HTS layers on one substrate” (MHOS) conductor that can carry high critical current, I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> , based on the number of superconducting layers deposited on one substrate. The superconducting layers in a MHOS conductor are much thicker than in a single-layered CC tape, possibly affecting the degradation behaviors of I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> under a mechanical load. To ensure the performance and reliability of MHOS conductors, it is necessary to evaluate the strain/stress response of I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> . In this study, the electromechanical properties of MHOS conductors were investigated using the uniaxial tension test method as well as the single-sided bending test at 77 K and self-field and at 0.5 T using a pair of neodymium permanent magnets.

Highly epitaxial YBa2Cu3O7−δ films grown on gradient La2−x Gd x Zr2O7-buffered NiW-RABiTS using all sol–gel process

The overall purpose of this work is to develop a reliable and low-cost technique for fabrication of coated conductors (CCs) on the flexible substrates, and to understand the effects of oriented growth and microstructure on the superconducting performance of CCs in-depth. The investigations of gradient La2−x Gd x Zr2O7 buffer architecture and YBa2Cu3O7−δ (YBCO) film on Ni-5at%W rolling-assisted biaxially textured substrate using all sol–gel process are reported. Combining x-ray diffraction and transmission electron microscopy (TEM) analysis, it is revealed that the gradient buffer architecture with increased lattice constant along the direction of film growth has an excellent c-axis orientation, while the inversed gradient architecture with decreased lattice constant shows the deteriorated orientation, indicating that the highly epitaxial growth mechanism most likely related to the interfacial lattice matching degree. The detailed analysis of YBCO films grown on gradient La2−x Gd x Zr2O7 under different deposition parameters, and the influence of microstructure on electrical properties are systematically discussed using x-ray diffraction, rocking curve test, scanning electron microscopy and TEM observations. Finally, the excellent biaxial texture YBCO film on gradient La2−x Gd x Zr2O7 buffer architecture is prepared at a proper heat-treatment temperature of 780 °C, and the full width at half maximum of ϕ-scan and ω-scan are 5.9° and 6.4°, respectively. The critical transition temperature T c,onset of the as-prepared YBCO film is 90 K. With elevating the temperatures, the appearance of a large number of weak-link and amorphous regions, being indicative of the microstructural collapse, is mainly responsible for the reduced superconductivity.

Improvement of the Delamination Strength of Yttrium Barium Copper Oxide (YBCO) Films by the Hamaker Method

Failure of yttrium barium copper oxide (YBCO) coated conductors (CC) tapes usually manifests as delamination occurring majorly in the YBCO films. Because the delamination strength of any material is greatly influenced by its microstructure, a good understanding of the intermolecular interactions in the YBCO films is essential to achieve the desired control of the delamination of YBCO CC tapes. Our previous works on the delamination behavior of multilayered YBCO CC tapes showed that the Hamaker based-models is viable for investigating the damage characteristics of YBCO films. Therefore, in this article, we investigated the intermolecular interactions of the constituent molecules of YBa2Cu3O7 films by Hamaker method. Then, used the deductions for the combining laws of the Hamaker coefficients for the interacting molecules to uncover and eliminate the interactions that cause the microstructural defects in the YBCO films. Our results show that increasing or decreasing the energy of the intervening medium between the interacting molecules to specific critical values eliminates the identified microstructural defects in the YBCO films. In particular, the elimination of the natural separation phenomenon among interacting molecules of the YBCO films improves its delamination strength significantly. Although based on our previous works, this technique had good agreement with the numerical, closed-form solution, and X-rays diffraction methods; further experimental validation is still under construction.

Current Distribution in Superconducting Tapes During Fast Current Ramping Based on Modified Brandt's Method

To reduce the time and risk of sample damage during critical current characterization of long Coated Conductors (CC), a pulse current method is proposed previously. The dynamic current distribution in the tape during pulse current ramping is studied in this work for better understanding of the behavior of the CCs under measurement, based on a current-driven version of Brandt's method. in addition, a coil with 2000 turn was used in the experiment to improve the signal-to-noise ratio. The ramping rate is compared with the voltage criterion in the standard four-point measurements. Especially, a scaling behavior is found for simple but precise description of the effect of the current ramping rate on the measured critical current, utilizing the concept of an effect electric field, which can be used to calculate the critical current with commonly used criterion of 1 μV/cm, with no free parameter. Effects of copper protection layers are also considered for fast pulses. The numerical results are also compared with those of experiments.

Hoop Stress Tests of an Epoxy-Impregnated REBCO Coil With Fluorine-Coated Polyimide Insulation

REBCO coated conductors (CCs) are suitable for high field magnets due to their high in-field critical current density ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) and high tensile strength. However, in order to apply the REBCO CCs to a high field magnet, it is important not only to evaluate the characteristics of short samples but also to clarify the behavior of the REBCO coil under strong electro-magnetic forces in high magnetic fields. In this study, we investigated the tolerance to the electro-magnetic force by applying hoop stresses to a REBCO double pancake coil in high magnetic fields. The REBCO coil was wound with BHO-EuBCO CCs and impregnated with epoxy resin. In order to prevent a degradation due to thermal delamination stress, fluorine-coated polyimide tapes were wound on the REBCO CCs. When a maximum hoop stress of 608 MPa was applied to the coil, normal voltage was generated and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> degradation was confirmed in one side of double pancake. As a result of the investigation, the degradation occurred only at lap joint part between CCs in the windings, and the cause of the degradation was thought to be stress concentration at the both edges of lap joint. Since there was no <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> degradation except for the lap joint part, it is considered that the REBCO coil can withstand larger hoop stress if we can reduce the stress concentration at the joint part. We therefore proposed a new joint structure and confirmed that it has higher tensile strength than a conventional lap joint structure.

Scanning Raman spectroscopy for inline characterization of 2G-HTS conductors

We report on a feasibility study of using Raman micro-spectroscopy as a characterization method for quality control of (REBCO, rare-earth barium copper oxide) coated conductors (CC). Fragments of long REBCO tape deposited using advanced metal organic chemical vapor deposition with dropouts in critical current (I c) were analyzed implementing Raman spectroscopy in area scan (map) and linear scan modes, in order to evaluate the level of information obtainable with the technique. The areal maps have been found to provide detailed information on the spatial distribution of the sample quality, providing maps of both intensity and wavelength shift characteristic of Raman-active peaks. The maps readily identify defective regions ranging from completely misoriented a-axis domains (REBCO a-axis || sample normal), as well as regions with well-textured c-axis orientation (REBCO c-axis || sample normal) but with significant variation in cation disorder. The results are complemented with linear scans carried out by two-dimensional x-ray diffraction, which do identify the a-axis grain regions but do not detect any apparent anomaly in the cation-disordered region detected by Raman micro-spectroscopy. The results indicate that scanning Raman spectroscopy has the potential to be utilized as a fast, insightful, and reliable characterization, quality, and process control technique for both CC optimization and increase of manufacturing yield.

Successful DC current limitation above 100 Vm−1 for 50 ms using HTS tapes with critical currents exceeding 750 A/cm-width

High-voltage direct current transmission systems are expected to allow the transmission of huge volumes of electricity over long distances. The use of superconducting fault current limiters (SFCLs) based on second-generation (2G) high-temperature superconductor (HTS) coated conductors (CCs) is a promising solution to mitigate fault currents in DC transmission systems. To fabricate a SFCL whose size remains acceptable, which means minimizing the length of the HTS tape used, the tape must sustain a high electric field during the whole fault duration. In this paper, high performance commercial 2G HTS CCs from THEVA (more than 750 A/cm-width at 77 K in self-field), on which a 500 µm thick Hastelloy shunt was soldered, were tested by submitting them to faults of different amplitudes and durations. Measurements revealed that these HTS tapes could sustain any type of fault up to 100 V m−1, lasting up to 50 ms. Three-dimensions finite element simulations were able to reproduce accurately the experiments by using the appropriate temperature dependence of the critical current density and power law index, and by accounting for the variations in the local critical current along the length of the HTS tapes.

Achievement of Low-Resistivity Diffusion Joint of REBCO Coated Conductors by Improving the Interface Connection of Ag Stabilizer

The achievement of low-resistance connection in REB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-z</sub> (REBCO, RE = Y, Gd) coated conductors (CCs) has become one of the strategic factors for magnet applications. In this letter, we fabricated low-resistivity (R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sj</sub> = 4.9 nΩ·cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) Ag-diffusion joints of REBCO CC by improving the interface connection of the Ag stabilizer. The effective contact area at the interface between two Ag stabilizers can be increased by polishing the surface of the Ag stabilizers, which can reduce joint resistance (R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">j</sub> ). Compared with a traditional diffusion temperature (400 °C), diffusion bonding of the Ag stabilizers at higher temperatures (800 or 850 °C, typical connection temperatures for superconducting joint) can promote interface connection between two Ag stabilizers. The oxygen content in REBCO will be reduced due to diffusion bonding at higher temperatures. Therefore, we etched some microchannels that serve as the fast oxygen diffusion paths on the surface of the Ag stabilizers to help restore the oxygen content, thereby recovering critical current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) and reducing R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">j</sub> . The connection mechanism of the interface between two Ag stabilizers was investigated through the cross-sectional analysis and delamination of the joint part. In addition, the REBCO layer can be completely peeled off and almost retain the original I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> by diffusion bonding of the Ag stabilizers at higher temperatures. This fabrication technique is a unique practicable solution to reduce the R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">j</sub> for the CC with thin Ag stabilizers and improve the peeling quality of the REBCO layer for efficiently fabricating the superconducting joint.

Magnetization Loss in HTS Coated Conductor Exposed to Harmonic External Magnetic Fields for Superconducting Rotating Machine Applications

Previous studies on magnetization AC loss of high temperature superconducting (HTS) coated conductors (CCs) taken into account the ideally sinusoidal external magnetic field, or non-sinusoidal by considering only the harmonics in phase with the fundamental component. However, realistically magnetic field often contains harmonics with different phase angles and amplitudes. In this paper, magnetization AC loss in HTS CC was studied, considering the phase angle φ of each harmonic; assuming superconductor is subjected to external magnetic field which is distorted by the 3 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rd</sup> and the 5 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> harmonics. Phase angle of each harmonic was considered in the range of 0 to π and amplitude of fundamental magnetic field, was assumed as 10, 20, 50, and 100 mT. Different distortion level of magnetic field was considered too. It is concluded that phase angle of field harmonic has a significant effect on magnetization loss of HTS CCs.

Characterization of the effect of ion irradiation on industrially produced GdBa2Cu3O7−δ superconducting tapes using a slow positron beam

To investigate the effect of irradiation-induced defects on the superconducting characteristics of industrially produced superconductor—GdBa2Cu3O7−δ (GdBCO)—coated conductors (CCs), we irradiated the GdBCO CCs with Au ions at 2 or 10 MeV and probed them using a slow positron beam. Vacancy clusters were detected in both unirradiated and irradiated GdBCO CCs. However, the effect of ion irradiation on the GdBCO CCs was characterized as a slight reduction in the positron annihilation rate with low-momentum electrons. We also found a correlation between the annihilation rate of low-momentum electrons and the superconducting transition temperature.

Large enhancement of the in-field critical current density of YBCO coated conductors due to composite pinning landscape

YBa2Cu3O7-based coated conductors (CCs) achieve the highest critical current densities (J c) of any known superconductor and are a key technology for applications such as rotatory machines, high-field magnets and power transmission. Incorporation of nano-sized non-superconducting second phases as additional vortex pinning centers has been considered the most amenable route to further enhance J c at an industrial scale, and has been successfully used in commercial CCs. The resulting pinning landscape is quite complex, with both synergistic and competing interactions among the various types of defects. Particle irradiation, on the other hand, allows for a controlled post-processing incorporation of a well-defined defect morphology. We have previously shown that irradiation with protons and other light ions can further enhance the in-field J c in commercial state-of-the-art CCs. Here we develop a combined irradiation process that increases J c above values previously achieved by irradiating with only one species. Our new approach involves sequentially irradiating with 250 MeV Au ions and 4 MeV protons. For example, at T∼ 27 K (liquid neon) and µ 0 H∼ 4 T, a region of interest for rotatory machines applications, we obtain J c ∼ 5 MA cm−2, which is about 40% higher than the values produced by the individual irradiations. Finally, we conclude that this is due to the synergistic pinning effects of the introduced splayed, non-uniform columnar defects and small clusters.

Joint characteristics of ultrasonic welded CC bridge joints for HTS coil applications

Long lengths of second-generation REBCO coated conductor (CC) tapes must be joined together when power cables, coils, and magnets requiring kilometer-scale lengths are fabricated. Various resistive joining techniques of CC tapes, such as soldering and diffusion reaction, have been studied. The joint method should provide good adhesion, low joint resistance (R j), no critical current (I c) degradation, cost effectiveness, and simplicity in processing. An ultrasonic welding (UW) process was developed and used to form various types of CC joint structures, such as lap- and butt-joints, using differently stabilized REBCO CC tapes. In this study, the UW process was used to form various bridge-joints of CC tapes for high-temperature superconducting coils. A mechanical-controlled soldering method which has provided a low contact resistance was also adopted for comparison. CC joints of various structures were fabricated using UW and soldering, and the retained I c and R j were measured at 77 K and self-field in each. The electromechanical properties of the CC bridge-joints were evaluated using a bending test. Results showed that R j varied in the CC joints depending on the structure type and joint area. Moreover, the additional Sn layer in Cu-stabilized CC tapes improved R j in each joint structure because it provided a solder-like bond at the joint interface. The bridge-joints formed using UW showed comparable bending diameter tolerances for irreversible I c degradation to those formed using mechanically controlled soldering. The multiple 4 mm wide CC bridges provided more flexibility at the joint part due to gaps between bridges and resulted to a better tolerance to tension bending than the single 12 mm wide ones.

Microstructure of YBa2Cu3Oy coated conductor using {100} ⟨001⟩ textured Cu tape with dual functions of metal substrate and electric stabilizing layer in order to develop low-cost high-TC superconducting wires

A new configuration of coated conductors (CCs), REBa2Cu3Oy (RE: rare earth elements such as Sm, Eu, Gd, and Y)/(Sr, La)TiO3 conductive buffer/Ni-electroplated {100}⟨001⟩ textured Cu and SUS316 lamination tape, has been proposed to reduce material costs. This conductive buffer layer is slightly modified from the previously reported Sr (Ti, Nb)O3 conductive buffer layers by changing the substitution element and site. However, a drastic change is found in the electric resistivity of the conductive buffer layers after oxygen annealing of YBa2Cu3Oy. The resistivity of Sr(Ti, Nb)O3 conductive buffer layers increases by more than three orders of magnitude, while the resistivity of (Sr, La)TiO3 is almost maintained and only minimally increases. Microscopic structural analysis of the CCs using the newly proposed conductive buffer layer is conducted using transmission electron microscopy (TEM). A very clean interface between the metal tape and the conductive buffer layer in the 600 nm thick (Sr, La)TiO3 buffer layers is confirmed by the TEM analysis, which has no reactive layers as well as no inter-diffusion of oxygen and metal elements. This result will soon lead to the development of low-cost and high-performance CCs.

Nanostructural evolution of intermediate grown superconducting joint layers between GdBa2Cu3Oy coated conductors

We have fabricated GdBa2Cu3Oy (GdBCO) jointed coated conductors (CCs) using a method in which intermediate grown superconductive (iGS) layers of (Y,Gd)Ba2Cu3Oy ((Y,Gd)BCO) are formed from an YBa2Cu3Oy precursor material to investigate their growth mechanism in the superconducting joint. We characterized the nanostructural evolution of the iGS layers by in-plane x-ray diffraction, transmission electron microscopy, and scanning transmission electron microscopy. The iGS layers contained Gd elements and were mainly composed of (Y,Gd)BCO grains. The c-axis oriented (Y,Gd)BCO grains nucleated on the GdBCO layers of both a joining strap and a CC. The thickness of the c-axis oriented (Y,Gd)BCO grains grew with time during the joining process at 800 °C, and finally both of the GdBCO layers were jointed together by the c-axis oriented (Y,Gd)BCO grains, resulting in a well-defined superconducting joint.

High homogeneity 10 cm long BaTiO3-doped YBa2Cu3O7-δ films by the trifluoroacetate metal-organic deposition process

The trifluoroacetate metal-organic deposition (TFA-MOD) is one of the most appealing routes for the growth of high-quality YBa2Cu3O7-δ (YBCO) coated conductors (CCs) due to its advantages of low cost and high efficiency. However, the puzzle of the inhomogeneities in microstructures and superconducting properties of YBCO CCs prepared based on the TFA-MOD route remain still a challenge. In this paper, 10 cm long YBCO films were prepared under different low-pressure conditions. These YBCO films were deposited on buffered Hastelloy C276 tapes using a dip coater. The effect of different ambient pressures on the microstructure and critical current density of YBCO films was investigated. High critical current density (JC) YBCO coated conductors with good homogeneity were obtained. The JC values (77 K, self-field) at different positions along the length direction of YBCO coated conductor were around 3.5–4 MA/cm2. The results showed that only under a suitable pressure the homogeneities of the microstructure and critical current density along the length direction of coated conductor can be greatly improved. The improvement was mainly ascribed to good texture and the absence of impurity phases BaF2 and Ba4Y2O7 in the films.

Dynamic loss and magnetization loss of HTS coated conductors, stacks, and coils for high-speed synchronous machines

Dynamic loss is an essential parameter to consider for the design of high temperature superconducting (HTS) synchronous machine windings. For aerospace electric propulsion systems, the fundamental frequency component and harmonics in electric machines can attain kHz level because of the high rotating speed. However, for HTS coated conductors (CC), the existing definition of dynamic loss only considers the HTS layer, the validity of which at high frequencies is questionable. Besides, the variation of dynamic loss and magnetization loss under skin effect due to high frequency is still unknown. Additionally, the influence of shielding effects among distinct turns on the dynamic loss of HTS stacks and coils remains unclear. In response to the above concerns, by use of the H-formulation based numerical multilayer modelling method which considers all layers of a CC, the frequency dependence of dynamic loss and magnetization loss of HTS CCs, stacks and coils over a wide range up to 20 kHz has been investigated. Results show that the existing definition of the dynamic region is no longer valid at kHz level, which shrinks rapidly with increasing frequency and magnetization loss plays a progressively important role due to skin effect. Meanwhile, the shielding effect in HTS stacks and coils can enhance the significance of dynamic loss. This paper clarifies the characteristics of dynamic loss and magnetization loss of HTS CCs, stacks, and coils over a wide frequency band, which can serve as a useful reference for accurate loss controlling of machine windings in future aerospace HTS propulsion systems.

Deposition of thick superconducting YBCO films using the surface laser heating

Cost reduction of REBa2Cu3O y (REBCO) coated conductors (CCs) is of interest in the recent applications. To improve the production speed of REBCO CCs is one solution to reduce costs. The larger I c with the thick REBCO layer is one solution to improve the production speed because the amount of REBCO CCs required for applications reduces. It is important that the deposition of the REBCO layer at a sufficiently high temperature for thick film. Therefore, various heating methods have been studied. In this study, we combined the conventional resistance heating with the laser heating to the surface of the CCs by using an infrared CW laser. As results, the a-axis oriented grains were suppressed with the laser heating. Furthermore, we have achieved I c values of up to 1500 A/cm-width at 77K and under a self-field in the YBCO films with a thickness of 8 µm.

Effect of post-annealing on the pinning properties of GdBa2Cu3O7-δ coated conductors prepared by RCE-DR

The pinning properties of GdBa2Cu3O7-δ (GdBCO) films fabricated by the reactive co-evaporation deposition and reaction (RCE-DR) process were very sensitive to the post-annealing temperature in a reduced oxygen atmosphere. For this study, as-grown GdBCO coated conductors (CCs) were post-annealed at the temperatures ranging from 450 to 750 °C in the oxygen pressure (PO2) of 300 mTorr, and subsequently oxygenated at 500 °C for 1 h in pure oxygen gas atmosphere. Compared with the pristine sample, samples post-annealed at the temperature region of 450 °C–600 °C for 1 h possessed higher stacking fault (SF) density and also included the GdBa2Cu4O8 (Gd124) phase while samples post-annealed at 650 °C for 1 h and 750 °C for 5 min showed lower SF density. The HAADF-STEM analyses on the sample post-annealed at 500 °C revealed that the Gd124 phase mainly existed near the CuO phase. At the temperature region of 50–77 K, while the minimum in-field critical current densities (Jc,min) of as-grown GdBCO CC were enhanced in relatively higher fields after post-annealing at 750 °C due to reduced SF density, both Jcab and Jc,min values were increased after post-annealing at 500 °C in relatively lower fields due to both increased SF density and the formation of Gd124.

Static Levitation Force and Its Thickness Dependence in HTS Tape Stack System

High-temperature superconductors (HTS) have great potential for magnetic levitation applications. Nowadays, coated conductor (CC) tapes are playing an increasingly important role in this field. Compared with HTS bulk materials, CC tapes have a higher critical current density and can be mass-produced, easy to shape, and have other important advantages for magnetic levitation applications. In order to gain an in-depth understanding of the relevant levitation characteristics of CC tapes, we studied the static magnetic levitation force between the permanent magnet and HTS tape stack, with the number of the CC tapes in the stack being 10 to 160. The experimental results show that the magnetic levitation force increases steadily as the thickness of the CC tape stack increases, but gradually reaches saturation.