Remarkable progresses in the development of high temperature superconductors (HTS) such as BSCCO-2223 tapes and YBCO coated conductors have been achieved in recent years, where very high engineering critical current densities (Je) were reached in long conductor length. It is however necessary to realize simultaneously high strain tolerance of Je, low AC losses and high mechanical strength in order to apply them for practical uses. In the first part of the present review, some critical techniques to improve microstructures for achieving total performance of BSSCO tapes as well as YBCO coated conductors are suggested. In the major part, the recent progress of evaluation techniques of mecahno-electromagnetic properties is introduced. The HTS’s are typical composite material consisting of essentially five components. Here the analytical technique is proposed to make clear the mechanical properties based on the rule of mixture, while the quantitative experimental method to measure tensile properties is introduced. The critical current is very sensitive on strain. The strain dependency could be divided into two regions. In the reversible region, the critical current decreases monotonously for BSCCO tapes. On the other hand, YBCO coated conductors give a so-called Ekin’s intrinsic behavior for the change of critical current, where a maximum of critical current appears during the process of increasing tensile strain. In order to understand fully the strain dependences of critical current, it is absolutely necessary to elucidate the strain state exerted on the superconducting component in the composite. Recently the direct measurements of local strain have been succeeded by means of diffraction techniques using neutron and synchrotron radiation. Their interesting results including a new science are reported in the present review.
The separation method using electromagnetic force is receiving attention as an elimination method for the removal of impurities from molten metal. In this study, the relation between electromagnetic force and the motion of particles in molten metal was discussed through experiment and calculation using metals with low melting temperatures. Electromagnetic separation was examined under conditions which were decided by the results of calculations (average flow velocity: 0.1 m/s, electric current density: 2.1×104 A/m2, magnetic flux density: 10 T).The relation between particle size and separation efficiency can be obtained through calculation. It was successfully demonstrated that the insulating lead balls in metals melted at low temperature could be separated by electromagnetic force.
We developed a new multi-turning system reel-to-reel (MT-RTR) system furnace with a vertical gas flow mode to increase the production rate of the crystallization step and attained a uniform reaction for the tapes in the multi-turn system. We fabricated YBCO films with different conditions including partial pressures of water vapor, the total pressure and the gas flow volume using the MT-RTR furnace with the vertical gas flow system, in order to investigate these influence of the processing parameters on the growth rate and the superconducting properties of the YBCO films. It was found that the growth rate of the YBCO layer increased under the conditions of the high gas flow rate, the low total pressure and the high partial water vapor pressure. As a result, 21 times higher Jc value was attained with the 17 times faster growth rate than that before optimizing the processing parameters in the crystallization step. This is thought to be due to the suppression of the coarsening of Y2Cu2O5 and CuO particles in the precursor film. Consequently, a high Ic,end-to-end value of 250 A/cm-w was achieved in a 5 m-long tape fabricated at a production rate of 3 m/h using only 2 lanes of the entire MT-RTR crystallization furnace originally designed for 10 lanes.