High temperature superconducting materials based on either Y--Ba--Cu--O (YBCO) or Bi--Sr--Ca--Cu--O (BSCCO) are known to have high transition temperatures of about 90.degree. K and about 115.degree. K, respectively. Both of these systems have been successfully used to produce high quality thin films. In the YBCO system, large current densities exceeding 1 million amperes per square centimeter at 77.degree. K have been reported by many groups.
The formation of large bulk specimens (including wires) having very high current densities and high magnetizations has not been as successful as has the preparation of thin films, for reasons which have been associated with intergranular weak links and macro-defects, such as the presence of voids or other second phases, present in such larger bulk specimens. The advent of melt processing techniques in recent years, however, has allowed smaller bulk specimens to be produced, some of which are almost free from weak links, exhibit very high magnetization (about 5000 Gauss at 77.degree. K) and current densities up to about 5.times.0.sup.4 A/cm.sup.2. Unfortunately, these small articles have little practical application, and cannot be used, for example, in magnetic levitation, electric power transmission, as permanent magnets, or in magnetic energy storage, where much larger (or in the case of wires, longer) specimens are required.
Melt processed high temperature bulk superconductors have proven to be superior to sintered materials with respect to critical current density and magnetization. The melt processing techniques which have produced the best quality bulk specimens to date are those where the superconducting powder precursors (either calcined or non-calcined) are first melted and then cooled rapidly.
At this stage, however, the phases which are present are not superconducting. Thus, it is necessary to next go through several peritectic reactions in order to re-convert the majority of the material to a superconducting phase or phases. The conversion is accomplished by reheating the rapidly cooled (quenched) superconducting powder precursor to a temperature above the lowest peritectic temperature, typically above about 1080.degree. C., and then slowly cooling the mass to room temperature (about 25.degree. C.). During the phase decomposition associated with the peritectic reaction achieved by reheating the quenched material above the lowest peritectic temperature, the liquid phases which are formed are either drained from the specimen under gravitational force or are drawn to the exterior surfaces of the mass by interfacial surface energy. Under normal processing conditions both processes often take place simultaneously. A lack of sufficient liquid phase, and the rigidity of the material, which prevents changes in the volume of the solid phases, can lead to the presence of voids and insulating defects in the finished bulk specimens. See, for example, H. Hojaji, et al., "Melt-Processed YBCO Superconductors: Processing and Properties", High Temperature Superconductors, Ed. J. J. Pouch, et al., Mater. Res. Forum, Trans Tech Publications, Switz., 1992; H. Hojaji, et al., "Superconducting Cuprates Prepared by the Melt Quench Process and Containing Excess Y or Additives", Mat. Res. Bull., 25, 765-77, 1990; S. Hu, et al., "Bulk YBa.sub.2 Cu.sub.3 O.sub.x Superconductors Through Pressurized Partial Melt Growth Processing", J. Mater. Res., 7, 1-5, April, 1992.
In an attempt to produce better materials, zone melting and gradient melting have been used to produce texturing in a more controlled manner. See A. Goyal, et al., "Fabrication of Highly Aligned YBa.sub.2 Cu.sub.3 O.sub.7 --.delta.-A.G MELT-Textured Composites", Physica C, 182, 203-18 (1991). However, such techniques have not been totally successful in overcoming these defects in large superconducting specimens. Other innovative methods have been tried, such as mixing superconducting powders with a polymer matrix in order to form a composite material. See C. K. Chiang, et al., "Levitation of Superconducting Composites", Amsahts, 90, 181-85, NASA Publication 3100, Apr. 1, 1990. Although these methods are capable of producing large and complex shapes, they lack the ability to eliminate the inter-granular weak links discussed above.
Another method of fabricating improved bulk materials is a method in which the grains of a granular superconductor are first aligned in a magnetic field below the transition temperature (i.e., below 90.degree. K in the case of YBCO). See Y. Nakagawa, et al., J.J. of Appl. Phys.. 28, L547-50, April 1989. The magnetic field aligns the grains along the Cu-0 planes, as they normally carry more current than the perpendicular direction. The alignment is frozen in, and then the body is sintered to preserve the alignment Although this method is superior to regular solid state sintering, it does not eliminate the weak links in the structure.
It should be noted that in order to obtain high-quality superconducting specimens, insulating line defects, such as cracks and insulating grain boundaries, should be eliminated. However, localized phase impurities can be very useful by acting as flux pinning centers. Several methods have been proposed to obtain high quality melt based materials.
One such method is disclosed in Hojaji, et al. U.S. patent application Ser. No. 07/659,719, the contents of which are incorporated by reference herein. This method includes the introduction of an impurity phase to improve flux pinning and has proved to be beneficial in increasing the pinning forces, even at very high applied fields.
Another prior art melt processing method which uses secondary processing, i.e., a second thermal treatment of the melted and quenched material. This addresses a problem found in single stage processes: leakage of the liquid phase at elevated temperatures in such processes is detrimental to the formation of macro-defectfree large bulk superconductors.
A third prior art melt processing method involves mixing silver with ground melt processed material. This method, however, also suffers from certain drawbacks. For example, the presence of non-superconducting silver at the grain boundaries can hinder the supercurrent from circulating across the whole material. As a result, complete shielding of the magnetic flux cannot be obtained. This means, in turn, that partial penetration of the magnetic flux into non-superconducting regions of the bulk superconductor produces a lower levitation force and a correspondingly low current density compared to impurity free superconductors. See A. Goyal, et al., Physica C, 182. 1991.
Accordingly, it is an object of this invention to provide a method of processing bulk superconducting materials that produces highly dense bulk specimens with excellent magnetic and transport properties. It is believed, without limiting the scope of this invention, that the effectiveness of this inventive method is due to its success in eliminating the aforementioned defects and weak links which reduce current-carrying capability, while it permits the presence of flux pinning centers.
This method can be used to produce very large specimens with complex shapes that also have excellent mechanical integrity.