In the power applications of superconductors, the most important key element is the high-temperature superconducting tapes. The essential conditions for superconducting tapes are a high transport critical current density (Jc), shape uniformity of long-length tape and mechanical performance. From these points of view, Bi-2223 superconducting tapes have been considered promising. Short-length tapes with Jc of 40kA/cm2 and tapes over 1km in length with Jc of 20kA/cm2 have been obtained at 77K. In order to attain higher Jc values, we must consider the content of the Bi-2223 phase, grain alignment, grain connectivity and pinning as important factors of limitation. With the objective to identify the most important factor of limitation, we investigated tapes in detail by XRD, SEM, SQUID and so on.
Bi-2212 superconductor is known as one of the candidates for practical superconducting wires. The “partial melting and slow cooling process” realizes a well-oriented microstructure, good connectivity between superconducting grains and excellent critical current density below 30K. Many investigations of phase changes during heat treatment have been carried out and the results have been quite helpful for controlling the microstructure, thus improving superconducting properties. There are three factors which affect superconducting properties: oxygen concentration in the atmosphere, composition of the superconducting powders and heat-treatment schedule. The highest Jc value, approximately 5×105A/cm2 at 4.2K and 0T, was achieved using the powder-in-tube technique and has been applied to produce an insert magnet of a high-field magnet. The issues which remain to be solved are the introduction of effective pinning centers above 30K and improvement of the mechanical strength and homogeneity of long wires.
YBa2Cu3O7-x oxide superconductor has intrinsically strong pinning properties at 77K, which suggests its potential for applications at liquid nitrogen temperatures. Formerly, YBa2Cu3O7-x was considered difficult for use in forming conductors because its severe intergranular weak links could not be easily removed due to difficulty in crystalline alignment control. In recent years several possibilities of using flexible coated conductor have been indicated with the effective removal of intergranular weak links to produce “biaxially aligned structures.” Four kinds of methods have been proposed to produce “biaxially aligned structures, ” such as ion-beam-assisted deposition (IBAD), modified bias sputtering, inclined pulsed laser deposition, and rolling assisted biaxially textured substrates (RABiTS). This article reviews the current status of the development of YBa2Cu3O7-x oxide superconducting tapes produced by vapor phase deposition. Biaxially aligned YBa2Cu3O7-x coated conductors have the following advantages: (a) high Jc values: stable Jc values over 105A/cm2 (77K, 0T) were achieved, and Jc values over 106A/cm2 (77K, 0T) were obtained in short samples; (b) the possibility of large-current conductors: Ic (77K, 0T) values over 100A were obtained in 2-μm-thick YBa2Cu3O7-x films; (c) strong pinning properties at 77K; (d) excellent flexibility and mechanical durability; and (e) the low a.c. losses caused by thin-strip geometry and the low conductivity of substrates. YBa2Cu3O7-x tape conductors are unexplored technologies which require long-length, flexible tapes with uniform biaxial texturing. It has been a great challenge to remove the intergranular weak links and derive the intrinsic performance of oxide superconductors, which deserves further persistent research & development.