The present invention relates generally to superconducting elements and more particularly to a superconducting element having a high temperature superconducting film deposited on a technical substrate which consists of stainless steel containing Ni, Cr, and Fe and other alloying elements, especially Mn and Si.
The recent progress in the technology of high temperature superconducting (HTSC) films demonstrates the first real possibility of obtaining high critical temperature (Tc) superconducting layers with high critical current densities (jc) of  greater than 4 MA/cm2 on various single crystalline substrates. Several methods based on pulsed laser deposition, molecular beam deposition, sputtering, etc. were developed and successfully applied for this purpose.
A crucial point in these developments is choosing an appropriate substrate material which can be manufactured in larger pieces (from 1 to 100 m) and which is not as expensive as single crystals. Thus, several attempts have been made to employ a number of different xe2x80x9ctechnicalxe2x80x9d materials, such as Ni (Garcia-Moreno Usoskin, Freyhardt et al., IOP Conf. Ser. 158 (1997) 909-912), Yttrium Stabilised Zirconia (YSZ) ceramics (Kinder et al. IEEE Trans. on Appl. Superconductivity, Vol. 5 No. 2 (1995) 1575-1580), Hastelloy(copyright) (Quiton et al. in High Temperature Superconductivity, Research review 1998, ed. W. Y. Liang, 135-141), Inconel(copyright), etc. A buffer layer is deposited between the HTSC films and the substrate for influencing the texture of the deposited HTSC film and for acting as a diffusion barrier inhibiting an ion diffusion between substrate and HTSC film. The most well known HTSC film which is generally used for the present invention is YBa2Cu3O7-xcex4. In the case of polycrystalline substrates of the above-mentioned kind, an artificially induced alignment by ion-beam assisted deposition (IBAD)(Iijima et al., J. Appl. Phys. 7 (1993), 1905), inclined substrate deposition (ISD) (Hasegawa et al. The 1998 Internationale Workshop on Superconductivity, 73-76) of the buffer layers or rolling assisted biaxially texturing of metallic substrate (RABiTs) (Goyal et al. Appl. Phys., Lett. 69, 1795) is required to provide a well textured growth of high-Tc films.
Nevertheless, no substrate material with sufficient properties has been found for long HTSC coated tapes. For example, Ni sheets exhibit a high performance when used for smaller (1xc3x971 cm2) substrates. For longer ( greater than 10 cm) substrates, the high-Tc film is generally destroyed due to local oxidation of the substrate caused by oxygen diffusion through weak parts in the buffer layer. This is also true for Inconel(copyright) and, partly, for Hastelloy(copyright) substrates. Ceramic substrates are shown to be sufficient to provide high critical currents in superconducting films, but they are not flexible enough to perform winding in coils. All of the aforementioned substrates are expensive. One square meter of each of them costs at present more than 1,000 DM (about $500).
The above-mentioned draw backs preclude the use of HTSC films in numerous fields of their industrial applications in electronics, HF-technique and power engineering, where HTSC films on a single crystalline substrate are useless because of their strictly limited dimensions and insufficient mechanical flexibility.
EP 0 312 015 A2 discloses a substrate for an oxide superconductor shaped body which is formed of an Fexe2x80x94Nixe2x80x94Cr steel alloy, e.g. SUS-310 or SUS-410. For these substrates a barrier layer of a noble metal is positioned between the substrate and the superconductor. Therefore, the costs of a superconducting element of this kind are high.
Accordingly, it is an aspect of the present invention to provide a superconducting element of the above-mentioned kind, which provides a high critical current density and a high transition temperature and allows a reduction of production costs.
The superconducting element according to the present invention allows both an unexpected increase of the critical current densities and transition temperatures of high temperature superconducting films and a drastically reduced substrate price, at least, by a factor of 100. A reason for the improved film performance may be the optimal combination of chosen the substrate material""s properties, namely its austenitic microstructure, high thermal expansion, peculiar surface structure, temperature resistance against oxidation, etc.
In a preferred embodiment of the present invention, the steel shows heat elongation of 15 to 18, preferably 17xc3x9710xe2x88x926/K between 20xc2x0 and 400xc2x0 C.; of 16 to 19, preferably 18xc3x9710xe2x88x926/K between 20xc2x0 and 800xc2x0 C.; of 17 to 20, preferably 19xc3x9710xe2x88x926/K between 20xc2x0 and 1000xc2x0 C.; and of 18 to 20.5, preferably 19.5xc3x9710xe2x88x926/K between 20xc2x0 and 1200xc2x0 C.
A preferred steel according to the present invention is known under the EURO norm designation as X15CrNiSi25-21 or as X15CrNiSi25-20 according to DIN E EN 10096 (12/95) and has Cr content in a range of 24.0 to 26.0%, a Ni content in a range of 19.0 to 22.0% and a Si content of 1.5 to 2.5%. Said steels are normally used as parts of furnaces, which are heat resistant and have a high mechanical strength. Another embodiment of the invention is a superconductor-coated tape of the stainless steel as described above. The above aspect should not be deemed as all-inclusive, but is merely an illustrative example of the myriad of aspects associated with this present invention.