Physica C-superconductivity and Its Applications最新文献

Partially high-temperature superconducting generators (PHTSGs) feature large magnetic air gaps imposed by using cryostats for HTS field windings. This increased air gap significantly affects end winding inductance and can lead to elevated fault torque levels. This study investigates the influence of magnetic air gap variation on generator design and end winding inductance and its implications for short-circuit faults in PHTSGs. Through a combination of numerical simulations and analytical analysis, the study explores how changes in the magnetic air gap affect end winding inductance and subsequently influence short-circuit fault behavior. The results reveal a direct correlation between magnetic air gap length, end winding inductance, and key short-circuit parameters such as stator current, field currents, and electromagnetic torque. Notably, an increase in the magnetic air gap is observed to elevate stator and field currents and electromagnetic torque during short-circuit events. These insights underscore the importance of considering magnetic air gap variation and its impact on end winding inductance and its relationship with short circuit characteristics in the design and operation of PHTSGs, providing valuable insights for enhancing the resilience and performance of high-temperature superconductor-based generator systems.

High temperature superconducting (HTS) transformers have many advantages over conventional transformers in terms of volume, weight, and total efficiency. However, fault conditions limit the electro-thermal performance of an HTS transformer. Based on this, we investigate the thermal modeling of the HTS transformer in the fault-current state. For thermal modeling, heat transfer and parameters affecting it such as surface roughness and bubble behavior have been considered. At first, an accurate thermal model for a 10 kVA HTS transformer has been prepared. Experimental results show that the penalty of this model is about 2.5 K in the hot-spot point (HSP) temperature in fault-current conditions. In a second step, the impact of a roughness on the heat transfer and mass transfer rate in nucleated boiling mode is investigated. Results show that surface roughness decreases the HSP temperature by more than 10 K in fault-current conditions. These results have significant impacts on protecting HTS windings against quenching.

c-axis Josephson tunnelling in atomically thin twisted flakes of quasi-two-dimensional cuprate superconductors is one of the experimental techniques which can be used to reveal the pairing symmetry in HTS cuprates. Until very recent experiments reported by Zhao et al. (Science 382, 1422 (2023)), there was a widely accepted consensus that the c-axis Josephson currents, I_c(T), in twisted cuprate junctions manifest the s-wave gap symmetry for c-axis gap component. Here, I highlighted that this consensus was based on the analysis of the I_c(T) data by using the Ambegaokar-Baratoff equation, where the weak-coupling s-wave gap equation was utilized. In this study, I introduced a more accurate gap equation in the AB model and reanalyzed the I_c(T) data in atomically thin twisted Bi₂Sr_2-xLa_xCuO_6+y (Bi-2201) and Bi₂Sr₂CaCu₂O_8+x (Bi-2212) flakes. In the result, the analysis revealed evidences for d-wave pairing symmetry. Beside this, the analysis of the R_nI_c(T) data reported by Zhao et al. (Science 382, 1422 (2023)) by a piecewise AB model, where a linear low-T dependence of the I_c(T) is used, and where the threshold temperature T_M is a free-fitting parameter, confirmed the d-wave symmetry in these twisted Bi-2212 junctions.

Solar cells, electronics, and the semiconductor industry all need high-quality silicon crystals. The Czochralsky technique has been widely used to make a high-quality single silicon crystal from the silicon melt. However, this technique needs a strong magnetic field to stabilize the melt and control the temperature. In this paper, two coils with a curved shape have been optimized and simulated to produce a transverse magnetic field for the Czochralsky technique grower. The coils face each other and have an opening angle of 150°, and a NbTi superconducting wire was used to wind the coils with 5110 turns. The current was 106 A and flowing in the same direction. The finding results indicated that the magnetic field at the silicon melt center is 0.4 T, and the peak field in coils is 2.95 T. The optimized curve shape coils were compared with conventional coils (circle shape coils). The comparison indicated that the curve shape coils produced a highly uniform magnetic field, required less current, and significantly reduced the outer diameter of the superconducting magnet. In addition, heat generation by the current leads was calculated. In the case of the curve-shaped coils, less heat leakage has been achieved; as a result, the cooling capacity, magnet size, weight, and cost will be reduced. Furthermore, mechanical analysis was performed, and the results showed that the stress in the coils is within the permissible range for NbTi at 4.2 K.

The growth orientation of YBa₂Cu₃O_7−x (YBCO) is extremely sensitive to the growth atmosphere and the heat-treatment temperature. However, according to the existing literature, to obtain YBCO with preferred a-axis orientation, a buffer layer needs to be prepared on the substrate. In this work, YBCO films were grown via pulsed laser deposition, and the influence of N₂ and O₂ deposition atmospheres on the growth orientation of the YBCO films was studied. It was found that high-quality YBCO films with preferred c-axis orientation can be grown in O₂ atmosphere, but it is difficult to grow a-axis-oriented YBCO films in O₂ atmosphere without using a buffer layer; however, YBCO films with a high degree of preferred a-axis orientation can be grown in N₂ atmosphere without using a buffer layer. Furthermore, an a/c-oriented homogeneous YBCO bilayer with c-axis orientation in the lower layer and a-axis orientation in the upper layer was prepared. X-ray diffraction and scanning transmission electron microscopy results show that the bilayer is epitaxial with a good a/c orientation, and the resistance–temperature curve shows the occurrence of two transitions at temperatures of 91 and 71 K. We believe that these temperatures correspond to the superconducting onset transition temperatures of YBCO in the c- and a-axis growth orientations, respectively. The superconducting current usually flows within the Cu–O plane. With its sudden change in the direction of the current at the interface, the homogeneous junction prepared in this work is expected to provide new ideas for the research of high-temperature superconducting junction devices.

Y_0.5Gd_0.5Ba₂Cu₃O_7-σ (YGBCO) and Ho_0.5Gd_0.5Ba₂Cu₃O_7-σ (HGBCO) targets with similar densities were prepared by solid-phase reaction. Pulsed laser deposition (PLD) technology was used to fabricate superconducting films with these targets. During the experiments in this paper, we varied the laser energy by adjusting the optical lens. The structure and texture of RE_0.5Gd_0.5Ba₂Cu₃O_7-σ (REGBCO, where RE = Y, Ho) thin films were analyzed by X-ray diffraction (XRD). The surface morphology was observed by atomic force microscopy (AFM) and scanning electron microscopy (SEM), and the superconducting critical current was measured at 77 K using the standard four-probe method. The laser energy density and different doping elements can affect the properties of REGBCO films. This may be related to the ejection process of target particles and the size of rare earth atoms. Additionally, we prepared a 510-meter long second-generation high-temperature superconducting tape with the optimized parameters. The critical current of the tape is 350 A, and the current density is 3.9 × 10⁶ A/cm².

Two types of first-order gradiometers equipped with 50 mm diameter pickup coils were made out of an REBaCuO High-T_c superconducting (HTS) flexible tape. The HTS tapes were formed into the HTS gradiometers by ``cut-and-wind'' method without cable jointing.

The HTS gradiometers were examined at 77 K for magnetic sensitivity to a gradient field applied by an external coil. The axial-type HTS gradiometer eliminated the magnetic field output of the device induced by a uniform field, and enhanced the magnetic field output induced by a gradient field on the axis of the pickup coil. The planar-type HTS gradiometer made it possible to detect a 20 mm diameter steel cylinder placed beside the pickup coil in a uniform field of 100 µT by detecting the spatial field gradient in the vicinity of a ferromagnetic object.

Iron-based superconductors are regarded as prospective candidates for high magnetic field applications since they have very high upper critical field and low anisotropy. For practical applications, it is important to develop superconducting wires with strong current carrying capability. In this work, Cu/Ag composite sheathed (Ba, K)Fe₂As₂ (Ba-122) iron-based superconducting wires were fabricated by a two-axial rolling deformation process and a subsequent hot isostatic pressing (HIP) heat treatment. Compared with the previously reported Cu/Ag/Ba-122 wires prepared by groove rolling, the two-axial rolling allowed to use much thinner Cu/Ag composite sheath for wire fabrication, thus greatly increasing the filling factor of superconducting materials by about 5 times to 24 %, and lowering the volume fraction of silver down to 16 % in wires. By a comparative study on the microstructure and superconducting properties between the wires made by two-axial rolling and groove rolling processes, it is found that besides the significantly increased engineering critical current density, the former also exhibits improved homogeneity of mass distribution and uniformity of Ba-122/Ag interfaces, resulting in significantly enhanced n-values over 40 in magnetic fields up to 14 T. Our work suggests that two-axial rolling, combined with HIP processes, presents a promising approach to prepare iron-based superconducting wires with high filling factor, high uniformity and low cost for practical applications.