J. T. Dusek

Transport critical current density and microstructure in extruded YBa2Cu3O7−x wires processed by zone melting

YBa2Cu3O7−x compounds were extruded into long wires with the diameter of 1 mm after sintering. The sintered wires were subsequently zone melted to develop a highly textured microstructure. Magnetization experiments at 77 K indicated a Jc value of 1×105 A/cm2 at 1 T. Transport measurements at 77 K showed a greatly enhanced field dependence of the critical current density. Transmission electron microscopy revealed an important grain‐boundary feature which eliminated the weak‐link behavior. Large amounts of dislocations have also been found in the zone‐melted sample which may contribute to flux pinning in the system.

Role of p O2 in microstructural development and properties of YBa 2 Cu 3 O x superconductors

An evaluation of the effects of oxygen partial pressure (pO2) on sintering behavior and the resulting microstructure of YBa2Cu3Ox (YBCO) indicates that sintering kinetics are enhanced at reduced pO2. The density of specimens sintered at 910 °C increased from 79 to 94% theoretical when pO2 was decreased from 0.1 to 0.0001 MPa. It is believed that increase in density with decrease in pO2 is the result of enhanced sintering kinetics, due probably to increased defect concentration, decreased activation energy of the rate-controlling species, and possibly the presence of a small amount of liquid phase. Sintering at 910 °C resulted in a fine-grain microstructure, with an average grain size of ≍4 μm. Such a microstructure results in reduced microcracking. Consequently, strength as high as 191 MPa is achieved. Reduced microcracking may have important implications for developing microstructures with improved critical current density.

Calcination of YBa2Cu3O7−x powder☆

YBa2Cu3O7−x was synthesized by mixed-oxide reaction of Y2O3, BaCO3, and CuO. The starting constituents were mixed, followed by multiple steps of calcination with interposed grinding. Heating rapidly to 900°C was shown to minimize formation of liquid during calcination while still producing a phase-pure powder. Slow heating to 900°C or fast heating to 950°C resulted in formation of a liquid phase deleterious to YBa2Cu3O7−x synthesis.

Shape forming high-Tc superconductors

Before the potential of high-temperature superconductors can evolve from the realm of science fantasy to the marketplace of practical reality, materials scientists and engineers must first develop viable methods for fabricating these revolutionary materials into a wide assortment of functional shapes and sizes.

High-Tc superconductors: fabricating technologies and future perspectives

Processing methods for production of bulk high-{Tc} superconductors are reviewed. Conductors made by conventional forming and sintering methods can generally carry only small current densities. Melting and powder-in-tube methods yield superior conductors, but further improvements are necessary before these superconductors can be employed in most large-scale applications. 36 refs., 6 figs.

FABRICATION OF HIGH-Tc SUPERCONDUCTORS

ABSTRACT An overview is presented of techniques for fabricating high-Tc superconductors based on Y-Ba-Cu-O, Bi-Sr-Ba-Cu-0 and TI-Ca-Ba-Cu-O. Processing of both thin films and bulk forms such as wires and tapes is discussed. Thin films with high current densities can now be made reproducibly. In addition, several techniques for etching precise patterns into the films have been developed. Bulk forms have yet to be produced with current densities comparable to those in thin films. Improvement in superconducting current density remains the principal focus of much research on bulk forms of high-Tc superconductors.

Effect of Magnetic Field on Critical Current Density in Bulk Superconducting Wires

Bulk YBa{sub 2}Cu{sub 3}O7{minus}x (YBCO) wires and tubes were fabricated by a conventional extrusion technique and sintered above 900{degree}C. The critical current density (Jc) was measured as a function of applied magnetic field at 77 K, and was found to decrease significantly at fields below 100 G. A manifestation of the magnetic field dependence of Jc is the correlation between critical current density and specimen geometry. In addition, when a concentric magnetic field was generated by passing a current though a copper wire, the external field from the wire could constructively or destructively interfere with the magnetic field produced by current in the YBCO tube. Jc was found to increase by 10% when the current in the YBCO tube was opposite to the current in the copper wire. The large change in electrical properties with magnetic field has been attributed to weak link behavior at the grain boundaries. Batch to batch differences in the field dependence of Jc imply the possibility of reducing the dependence by processing modifications. 10 refs., 3 figs.

Atmosphere Control During Preparation of YBa2Cu3O7−x Magnet Windings

Large coils of YBa2Cu3O7−x can be fired successfully if the furnace atmosphere is carefully controlled. Organics added during processing produce CO2 during the initial portions of the firing schedule. Transmission electron microscopy of material fired in atmospheres containing various levels of CO2 clearly shows the extent of grain boundary degradation caused by CO2. Coils with acceptable critical current density can be produced if the rate of CO2 removal is adequate.

Fabricating polycrystalline high-Tc superconductors

Many applications of high-temperature superconductors require the ability to carry large currents in high magnetic fields. Electrical properties of single crystals and thin films offer hope that suitable electrical properties will be obtained in polycrystalline materials. Novel microstructures can be produced by melt-recrystallization processes that can be characterized by oriented crystallites. The large critical current densities required for many applications have been demonstrated by powder-in-tube and melt-texturing processes. There are several competing processes to form bulk polycrystalline materials. Plastic forming techniques, such as extrusion and tape casting, offer the advantage of fabricating long and continuous coils. Nearterm products that operate in low magnetic fields include current leads, shielding devices and high-quality resonant cavities.

Ceramic Processing Of High-T c Superconductors

The means by which superconducting YBa2Cu307-x powders can be synthesized are summarized. Methods of forming powders into useful shapes are then described. Lastly, heat treatment schedules used for consolidation of the powders into dense, sound bodies with good superconducting properties are discussed.