Here reviewed are the recent progress and the present status of the organic superconductors. Aspects on synthesis, physical properties, electronic structure and mechanism of superconductivity are covered as for these several years. First we follow the development in the synthesis leading to the organic superconductor with Tc=13K. Basic molecules constituting organic superconductors have been extensively explored with the result that all successful ones are derivatives or an isolobal analogue of TTF. The investigation of the electronic structure by means of magneto-transport experiments has been fruitful. It has been established that the tight-binding band based on the highest occupied molecular orbital in the Hueckel approximation provides appropriate descriptions for the organic superconductors. The controversy over the nature of superconductivity in organics, i.e., s-type or d-type, is still hot. Concerning the mechanism of superconductivity the theory on the basis of the mediation by the intramolecular vibrations has recently received hopeful supports from isotope-effect experiments replacing the constituting atoms by their isotopes and from a point-contact tunneling experiment.
Control of cryogenic systems require flow-measurements and liquid level-measurements of cryogenic fluid. Further, stress-strain measurements are indispensable for mechanical strength testing of various materials of which cryogenic equipments compose. In this paper, measurement's principle and the way to use at low temperature are described in detail. Also, for new type measurement-systems, the prospects of cryogenic use are noted together.
With an additional circuit and a comparator of an OP Amplifier, two-level controller keeping the liquid nitrogen between an upper and a lower level was developed. A commercial Si diode was shown to be able to be used as a level sensor with good interchangeability, high speed response and 300°C heat resistance. A liquid nitrogen supply system of the controller and an auto pressurized vessel was constructed. The system could supply stably and reliably to a sorption pump even when consumption of liquid nitrogen was not constant at cooling down from room temperature.
When an a.c. transport current and/or an a.c. magnetic field are applied to multiply-twisted superconducting cables, magnetic field along strands composed of homogeneous and spatially periodic ones is induced in addition to the field perpendicular to the strands. The time variation of this longitudinal field along the strands also brings about the hysteresis and coupling-current losses. Theoretical expressions for the additional losses are derived both in the self-field configuration of the cable and in the external-transverse-field one. In the self-field configuration, the hysteresis loss due to the homogeneous component is dominant, which can be sufficiently decreased by the cabling procedure and the optimization of twist pitch in the strand. In the external transverse field, on the other hand, the development of high-current-density superconductors may be necessary for decreasing the losses in the a.c. cables in addition to optimization of the cable structure.
Superconducting properties and surface resistance, Rs, of YBa2Cu3O7-y strip lines prepared by the screen printing were investigated. The Tc of these strip lines with and without Ag2O and Ag additions was almost the same. However, Jc values increased with increasing the addition of Ag and Ag2O. The Rs values of these strip lines in superconducting state decreased with increasing the addition of Ag and Ag2O, and the optimum addition was about 10-15wt.%. The lowest values of Rs were about 15mΩ at 60K. The Rs values were proportional to frequency f1, in contradiction to f2 dependence expected from the two-fluid model.
Superconducting Y-Ba-Cu-O films have been prepared by on-axis DC magnetron sputtering from single sintered targets with various compositions, and relationships of transition temperature Tc to film composition and c-axis lattice parameter c0 have been examined. Films with zero-resistance transition temperature Tc, end exceeding 85K are easily obtained using Ba-poorer targets than stoichiometry under the conditions of lower oxygen pressures, 3-10mTorr, than those reported by other investigators. The Tc and c0 of these films strongly depend on X-ray intensity ratio Cu/Ba which has been obtained directly by EDX and is related to film composition ratio. As the Cu/Ba ratio is decreased from an optimum value, Tc values rapidly decrease and c0 values increases. Moreover, the c0 values are influenced by X-ray intensity ratio Ba/Y. Although some films with higher Ba/Y ratios have c0 values larger than 11.82Å (=tetragonal lattice parameter of bulk), they still show superconductivity. Their dependence of Tc on c0 is clearly different from that usually reported for bulk samples and post-annealed films. On the basis of these results, effects of oxygen pressure during sputter-deposition on Tc, film composition and c0 are discussed.
The superconducting electric propulsion system using superconducting motor seems to be an ideal propulsion system for the future. We have constructed a 480kW superconducting DC homopolar motor for the purpose of developing this new system. This motor is composed of superconducting field winding, segmented armature drum and copper-graphite current brushes. The superconducting field winding is cooled by the condensation heat exchangers and helium refrigerating system being built into the cryostat. We have obtained good test results that operating parameters of this DC homopolar motor accorded with the design parameters.
It is important to prepare bulk samples with high critical current density Jc to obtain the superconducting magnets of oxide superconductors. Many investigators have tried to improve the Jc values of Bi 2212 wire and tapes. We investigated the effect of intermediate cold press on Jc of Bi 2223 bulk specimens. The application of a uniaxial pressure on an already sintered sample (without intermediate crushing) leads to an improvement of Jc. Jc of the sample with intermediate coldpress is 2.5 times larger in zero field and 60 times larger in 0.1T than that of the sample without intermediate cold-press. After the application of the uniaxial pressure, density and texture of grains were increased. This means that the observed enhancement of Jc with intermediate cold-press is due to the densification (lowering the weak links) and texturing effects.