The present invention relates to superconducting magnets, particularly to magnets fabricated from high T.sub.c superconducting materials, and more particularly to high-field magnets fabricated from high T.sub.c superconducting ceramic thin films.
Since the discovery of superconducting material development efforts have been underway to utilize this material for various applications including coils, solenoids, magnets, etc. The early metal type superconductor, such as a Ti-Nb alloy and Nb.sub.3 Ge had a critical temperature (T.sub.c) which could not exceed 23.2 K and hence the use of liquidized helium (boiling point of 4.2 K) as the cryogen for superconductivity, and thus limited the application of superconducting materials.
The discovery of a new type of superconducting material, generally referred to as an oxide type superconductor, having a much higher T.sub.c was revealed by Bednorz and Miller in 1986, and had a T.sub.c of 30 K. Also, in 1987 the discovery of another type of superconducting material was reported by C. W. Chu et al. having a critical temperature of about 90 K and referred to as YBCO, being a compound oxide of the Ba-Y system represented by YBa.sub.2 Cu.sub.3 0.sub.7-x.
Since the discovery of high T.sub.c oxide superconductor in 1986, research has furiously been underway worldwide to understand and optimize critical parameters of these materials in order to build useful devices based on their extraordinary characteristics. Various classes of these materials can be routinely fabricated by a number of techniques with T.sub.c well above liquid nitrogen (LN.sub.2) temperature (77.3 K at 1 atm pressure) and upper critical fields (B.sub.c2) that are extremely high (measured to be in excess of 100 Tesla (T) for YBCO at 6 K). However, only recently have large area (&gt;1 cm.sup.2) high T.sub.c thin films been grown that have critical current densities (J.sub.c) which surpass the best low T.sub.c superconductors in the presence of magnetic fields. The extentiveness and volume of these prior efforts are exemplified by the following U.S. patents issued during the July-October 1990 time period: U.S. Pat. No. 4,942,142 issue Jul. 17, 1990 to H. Itozaki et al.; U.S. Pat. No. 4,948,779 issued Aug. 14, 1990 to W. C. Keur et al.; U.S. Pat. No. 4,959,346 issued Sep. 25, 1990 to A. Mogro-Campero et al.; U.S. Pat. No. 4,959,348 issued Sep. 25, 1990 to K. Higashibata et al.; U.S. Pat. No. 4,962,086 issued Oct. 9, 1990 to W. J. Gallagher et al.; and U.S. Pat. No. 4,965,247 issued Oct. 23, 1990 to M. Nichiguchi.
The best results are on substrates with: 1) good lattice match to the film, 2) which do not react with the film at the high temperatures required by deposition (about 750.degree. C.), and 3) which have a reasonably well matched thermal coefficient of expansion. The above-referenced U.S. Pat. Nos. 4,948,779 and 4,959,346 have attempted to satisfy these requirements by the addition of a buffer layer between the substrate and the YBCO. These three requirements are well met by substrates fabricated from strontium titanate (SrTiO.sub.3) and lanthanum illuminate (LaAlO.sub.3). Yttria-stabilized zirconia (YSZ) is useful as a substrate at all but the highest deposition temperatures, where it starts to react with the YBCO. Large area films of YBCO expitaxially grown on LaAlO.sub.3 were measured to have J.sub.c (4 K, 1 T)=1.times.10.sup.7 A/cm.sup.2 and J.sub.c (77 K, 0 T)=5.times.10.sup.6 A/cm.sup.2. More recent data show J.sub.c about a factor of 2 better than those cited above for LaAlO.sub.3 and SrTiO.sub.3 at the similar conditions, wherein J.sub.c (77 K, 2 T) has been measured to be .about.5.times.10.sup.6 A/cm.sup.2.
An additional requirement for very large area films (&gt;10 cm.sup.2) is an inexpensive substrate with high thermal conductivity for rapid removal of heat (if necessary) during operation of superconductive devices. High thermal conductivity during deposition is beneficial, but not necessary. Such a condition would be met by a substrate of sapphire, but high T.sub.c films tend to react with this substrate at high temperatures. It has recently been reported that high J.sub.c films on sapphire (.about.2-3 times lower than the best results reported previous) using a buffer layer of SrTiO.sub.3. This J.sub.c data is 2-3 orders of magnitude better than the best available data in YBCO wires and tapes. This is the main reason why there are not many new high-field T.sub.c superconducting magnets. In addition, these oxides are very brittle making them extremely difficult to wind.
Further, properly fabricated superconducting material will remain superconducting only if operated: 1) below a certain critical temperature T.sub.c,2 ) below the J.sub.c, and 3) below a critical magnetic field (or magnetic induction) B.sub.c. These three parameters are interdependent and must be known to optimize the design of a useful magnet.
It is thus seen that while researchers have attempted to use high-critical-temperature, superconducting ceramic (HTSC) materials to fabricate useful superconducting magnets that can operate at temperatures well in excess of liquid helium (He) temperature, most of the work has been focused on fabricating wires and tapes from bulk HTSC materials with sufficiently high critical current density (J.sub.c), and although progress has been made, the J.sub.c for wires and tapes remains about two orders of magnitude less than that for large-area HTSC thin films. Thus, there is a need for a magnet fabricated from high-critical-temperature superconducting ceramic thin film. The present invention satisfies that need by providing a high-field magnet using HTSC films formed by stackable disk-shaped substrates coated with HTSC thin films and which can generate fields greater than 10 T.