As the first lecture of the series on high-temperature superconducting materials, this article provides a basic introduction so that non-specialists can catch up with current topics and understand some details about the copper oxides which show high-Tc superconductivity. The crystal structure and the electronic properties of the cuprates are described as a function of carrier doping. Many investigators have recognized that the anisotropic Cooper pairs with d x2-y2-symmetry are formed in the two-dimensional CuO2 planes. Some proposals, which may lead to the basic pairing mechanism for the superconductivity in the cuprates, are discussed. The most likely scenarios have been proposed based on both antiferromagnetic interaction and strong correlation between 3d electrons of Cu. Some trials to improve Tc are reviewed with an optimistic objective achieving room-temperature superconductors.
Through monitoring the temperature inside cryogenic equipment such as superconducting magnets, it is possible to detect indicators of failures and prevent equipment from becoming damaged. An optical fiber sensor is considered a suitable means of measuring the temperature inside cryogenic equipment because of its low heat invasion and high voltage insulating properties. In this study, we used a fiber Bragg grating (FBG) to investigate the reproducibility and multipoint measurement characteristics over a temperature gradient inside the cryogenic equipment. We also investigated the improvement of the wavelength shift change ratio at cryogenic temperature using a coated optical fiber sensor to enhance the thermal expansion coefficient.
We prepared MgB2 thin films on non-textured aluminum (Al) tapes using an electron-beam evaporation technique. Single phases of MgB2 films with a c-axis and without biaxial crystal orientation were obtained with the critical temperatures (Tc) of 19-29.8 K, corresponding to the substrate temperature of 200-280°C. The magnetic field dependences of the critical current density (Jc) were measured at 4.2, 10 and 20 K in the field of 0-12 T applied parallel and perpendicular to the film surface. The Jc at 10 and 20 K in the self field were 3.1×106 A/cm2 and 1.2×106 A/cm2, respectively. Jc at 4.2 K exceeded 106 A/cm2 in the parallel and perpendicular fields of between 0 and 10 T. These results suggest that a MgB2 coated conductor may be one of the most promising candidates for practical MgB2 superconducting wires.
This study considers a method to evaluate AC transport current losses in Bi-2223 superconducting coils through integrating local external field losses of stacked, taped wires over the winding part of the coils. We measured the transport current losses of two kinds of pancake superconducting coils of Bi-2223 Ag-sheathed tapes, which were composed of 8 or 16 doublepancake (DP) coils, at 0.008 and 0.04 Hz. We also measured the AC external field losses of short specimens of six-layer Bi-2223 wires in both perpendicular and parallel field configurations using a method based on saddle-shaped pickup coils. The approximate local external field loss was calculated as the sum of perpendicular and parallel field losses of the short specimen. In the calculation, a demagnetization effect was considered in order to introduce the geometrical situation of winding in the coil, in addition to the distributions of the perpendicular and parallel magnetic-field components. The comparison between the experimental results of the transport current loss and the numerical results revealed that there is good agreement in the lower amplitude region of the transport current, and deviation in the higher amplitude region. The former finding indicates that this method is applicable as a general means to estimate the transport current losses in superconducting coils. The latter finding suggests that the effects of the transport current may cause significant changes to the AC loss properties of the superconducting coils.