Theoretical results of our own studies of the dp-model are briefly discussed, including the possibilities of a very narrow p-band in hole-doped cuprates and also of obtaining high-T_c superconductivity from the correlated hopping of doped carriers to be represented by the t-part of the tJ-model Hamiltonian.

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Nb-Ti and Nb₃Sn conductors with different cross-sectional configurations are being produced for matching the specifications of different applications. Magnetic fields exceeding 18T have been so far generated at 4.2K by a superconducting magnet. In this article, recent progresses achieved in Nb-Ti, Nb₃Sn and other metallic superconducting materials are reviewed. The developments of superconductors for AC use and those capable of generating fields over 20T are major topics in this area.

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Large Helical Device (LHD) is a superconducting heliotron type device for advanced fusion experiments. The SC coil system is composed of a pair of helical coils and 3 sets of poloidal coils with a total stored magnetic energy of 1.63 GJ, which will be the world maximum. The main machine parameters, major radius (R_C), coil minor radius (a_C), magnetic field, plasma minor radius (a_P), and plasma volume are 3.9m, 0.975m, 4T, 0.65m, and 30m³, respectively. The material of the super-conductor is NbTi, and the cooling systems are pool-boiling for helical coils and forced-flow for poloidal coils. Since the current density of the helical coils is as high as 53.3A/mm² with the maximum experienced magnetic field strength of 9.6T, the refrigeration with the super-fluid helium is required. LHD is expected to demonstrate the viability of the helical magnetic configuration extrapolatable to the fusion reactor. The major contributions to fusion technology area are seen in the superconductivity research and development. The necessary R & D programs, detailed design, and construction of parts are now in progress. The construction will be completed in 1997.

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Composites having a very fine dispersion of Ag in a Bi₂Sr₂Ca₂Cu₃O_10-δ matrix have been studied in order to provide a very robust conductor suitable for magnets. Conductors have a rather modest critical current density of 20, 000A/cm² at 4.2K, zero magnetic field and zero bending strain but they degrade much less under bending strain having 3000A/cm² at 20K, 20T for H perpendicular to the a-b plane and under 0.7% bending strain. The samples have typically had solder leads attached and removed a half dozen times, have been bent around different bending blocks, and have been quenched into liquid nitrogen, liquid helium, or liquid hydrogen many times before the measurements are made. These are very robust conductors.

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Directional steady-state solidification processes which are the Bridgman-type method and the zone melting method from the partial molten state, in which Y₂BaCuO₅ and liquid coexist in equilibrium, were performed under several different experimental conditions. In the Bridgman method, the temperature gradient (G) and the growth rate (R) were controlled independently at the same cooling rates. The degree of alignment of YBa₂Cu₃O_y crystals increased with increasing the G/R ratio even at the same cooling rate (6K/h). Higher G/R values were preferable for obtaining highly oriented crystal structures. The minimum limit for the continuous growth of YBa₂Cu₃O_y crystals exists between the G/R values of 301 and 482°Ch/cm². The high Jc value of 1.9×10⁴A/cm² (77K, 1T), estimated from the magnetic measurement, was obtained in the sample grown under the high G/R of 482°Ch/cm². The results of the samples grown by the zone melting method indicate that the continuous growth with large faceted solid/liquid interface can be achieved in the sample grown at 1mm/h and the volume fraction of 211 phase particles drasticaly decreases from in the liquid to the 123 phase solids at the interface. The proposed solidification model in which Y solute diffusion in the liquid limits the rate of the peritectic reaction agrees well with the experimental results.

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The contribution of the Y₂Ba₁Cu₁O₅ (211) phase and the effect of Pt addition on the critical current density (J_c) of the partial-melted Y-Ba-Cu-O filaments produced by suspension spinning and solution spinning are investigated. The J_c of the filament increases by adding the Pt element, whereas the J_c of decreases with increasing the Y-rich region from the stoichiometric composition of Y₁Ba₂Cu₃O_x.

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Partial melting and slow cooling process was applied for Bi₂Sr₂Ca₁Cu₂O_x/Ag tapes prepared by doctor blade and dip-coating processes. During slow cooling in molten state, plate-like 2212 crystals grow parallel to the tape surface, resulting in a highly oriented grain structure of c-axis alignment. Such a textured structure improves the intergrain coupling. We also found that the cooling condition after the solidification has a strong influence on the transport J_c and T_c. The transport J_c at 77K and 0T obtained so far is greater than 20, 000A/cm², although it is very sensitive to the applied magnetic field. At the temperatures below 30K, the tapes show excellent high field performance. Jc at 4.2K reaches to the practical level (>10⁵A/cm²) even in high magnetic fields. A 64 turn pancake coil was prepared from a 6m long tape produced by the continuous dip-coating process, which generated a central field of about 6, 000Gauss at 4.2K in a bias field of 12T.

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Bi-2201 system oxide superconducting crystals with or without doped La are grown by Verneuil method and travelling melt zone method by CO₂-laser heating. Single crystals though tiny reveal microstep line structures on their surfaces produced by lateral crystal growth mechanism. The fundamental step height is measured as 12Å which is equal to the half unit cell dimension. Bi, Cu and O, respectively, distribute nearly uniformly in the crystal, while Sr and La do not display uniform distribution. As an interesting characterics the concentration of Sr and La is complementary between the two. This suggests the a replacement of Sr in the crystal lattice by La is taking place. Occasionally magnetic susceptibility and electrical resistivity proved the occurrence of superconductivity but the reproducibility is rather inferior.

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We have studied intergranular flux depinning in Y₁Ba₂Cu₃O_7-δ ceramics. We measured do magnetization as a function of temperature at different applied magnetic fields for field-cooled ceramic and pressed-powder samples. Fast cooling of the ceramic sample down to low temperatures causes more intergrain flux trapping than slow cooling. During warming the flux depins which is associated with an increased Meissner expulsion. Two characteristic temperatures indicating the onset and completion of depinning allowed to develop a (H, T) magnetic phase diagram for the intergrain flux depinning, that occurs for magnetic fields and temperatures over a range 0-1500G and 92-30K, respectively.

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Microstructures of high-J_c YBaCuO material fabricated by Melt-Powder-Melt-Growth process were studied by transmission electron microscopy to clear the most effective pinning center in the material. It turns out that the interface between the YBa₂Cu₃O₇ matrix and Y₂BaCuO₅ inclusion is sharp. Such an interface may act as a strong pinning center, and the large J_c value of the MPMG processed material is thought to be brought by the 211 inclusions with sharp interfaces.

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Direct measurements of transport critical current and resistive state in YBCO single crystal microbridges and thin films have been performed, using pulse technique with microsecond current pulses. We have obtained critical currents values up to 10⁵A/cm² at 80K in zero field for single crystals and more than 10⁶A/cm² for thin films. Data on field dependence of ρ_f are presented and preliminary assumptions on the difference in flux flow mechanism in YBCO single crystals and thin films are made.

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Multilayered YBa₂Cu₃O_6.8/YBa₂(Cu_1-xNi_x)₃O_7-y and YBa₂Cu₃O_6.8/Bi₂Sr₂Ca_n-1Cu_nO_x (n=1, 2, 3) superconducting films have been grown by laser deposition technique. Critical currents were measured in the external magnetic field up to 5 tesla with different thicknesses of the interlayers. Results show that control of the number of layers as well as the thickness in multilayer structures could provide an effective means for improving the critical current in magnetic fields.

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Major bulk applications of the high T_c superconductors depend on their current-carrying capability. In order to achieve the desired critical current density of 10⁵A/cm² or higher, the enhancement of flux pinning is essential. In this paper, various techniques for flux pinning enhancement in bulk high T_c superconductors will be reviewed, and the chemical, microstructural, and processing parameters for introducing desirable flux pinning sites will be discussed.

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It is possible to disperse Y₂BaCuO₅(211) inclusions into YBa₂Cu₃O₇ (123) matrix by melt processes. Since nonsuperconducting particles are known to act as pinning centers, the 211 inclusions are expected to provide flux pinning. In this paper we give evidence that the 211/123 interfaces are effective in flux pinning in melt grown YBaCuO.

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Various issues involved in the development of phenomenological models of flux pinning and motion in high-T_c oxides are discussed. A simplified model is presented for the critical current density and is used to examine the question of whether flux flow results from an instability due to plasticity of the flux-line array or from pin breaking.

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The capacitance-voltage (C-V) characteristics were measured at various temperatures by using the Ag-MgO-high-T_c superconductor (Ba₂YCu₃O_x and Bi₂Sr₂CaCu₂O_x) MIS diode structure. With decreasing temperature, the capacitance of Ba₂YCu₃O_x films decreases. This temperature dependence of the capacitance will be discussed based on a marginal Fermi liquid theory.

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