Abstract:
A superconducting element comprising a high temperature superconducting film deposited on a stainless steel substrate that includes a composition of nickel; chromium; silicon; manganese; and iron. Optionally, carbon; phosphorous; sulfur; nitrogen; and unavoidable impurities will be present.

Description:
BACKGROUND OF THE INVENTION 
     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 (T c ) superconducting layers with high critical current densities (j c ) of &gt;4 MA/cm 2  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 “technical” 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® (Quiton et al. in High Temperature Superconductivity, Research review 1998, ed. W. Y. Liang, 135-141), Inconel®, 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 YBa 2 Cu 3 O 7-δ.  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-T c  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 (1×1 cm 2 ) substrates. For longer (&gt;10 cm) substrates, the high-T c  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® and, partly, for Hastelloy® 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 Fe—Ni—Cr 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. 
     SUMMARY OF THE INVENTION 
     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&#39;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 17×10 −6 /K between 20° and 400° C.; of 16 to 19, preferably 18×10 −6 /K between 20° and 800° C.; of 17 to 20, preferably 19×10 −6 /K between 20° and 1000° C.; and of 18 to 20.5, preferably 19.5×10 −6 /K between 20° and 1200° 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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Reference is now made more particularly to the drawing, which illustrates the best presently known mode of carrying out the invention and wherein similar reference characters indicate the same parts throughout the views. 
     FIG. 1 is a view schematically illustrating an apparatus for preparation of HTSC films on ribbon-type substrates in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     In the apparatus, illustrated in FIG. 1, a foil substrate  1  consisting of stainless steel is fixed in a helix form on a cylindrical tubular holder  2  with an outer diameter of 68 millimeters and mounted together with the holder in a motion guide comprising rotating supports  3  and linear pulling devices  4  built in a vacuum chamber  5 . The rotating supports  3  and the pulling devices  4  are connected to motor drives  6 ,  7 , respectively. In a film deposition area, the substrate  1  with the holder  2  can be heated by a tubular quasi-equilibrium heater, which consists of a heating element  8  and a rotating chopper  9 . The heating element  8  is provided with one deposition window  10  and the chopper  9  is equipped with three similar windows. All of the parts mentioned above are installed co-axially, relative to the rotation axis  11  of the tubular holder  2 . Opposite to the deposition window  10  a ceramic target  12  is placed in the vacuum chamber  5 . The target  12  is a YBa 2 Cu 3 O 7-δ  composition. The target  12  is capable of linear oscillation in a direction perpendicular to the plane of the drawing of FIG.  1  and is capable also of periodic turns around an axis  13 , thus providing variations of the azimuth of incoming laser beams, relative to the target. This also provides for stabilizing the laser plume during a long-term deposition process. The motions of the target  12  are provided by a two-dimensional drive  14 . To provide large area pulsed laser deposition, the target  12  can be ablated by a pulsed laser beam  15  which, after a reflection at an oscillating mirror  16 , is capable of scanning the target surface by a deflected beam  17  entering the vacuum chamber  5  through a window  18 . An excimer laser with a wave-length of 308 nanometers, an energy of pulses of 0.5 J, and a pulse repetition rate of 300 Hz was employed as a source of the laser beam. 
     In operation, the chamber is pumped down to a pressure of &lt;10 −3  mbar and the substrate  1  with the holder  2  are heated up to the temperature of 760° C. During the heating step and subsequent film deposition, the chopper  9  rotates with the frequency of 23 Hz, and holder  2  rotates with the frequency of 4 Hz. Then the oxygen pressure of 0.4 mbar is introduced into the chamber. The oscillating motions of the target  12 , the mirror  16  and the axial pulling of the holder  2  are started. The laser is turned on and the laser pulses are synchronized with phases of motions of the chopper  9  and the mirror  16  so that each ablation pulse appears only at the moment when one of the deposition windows in the rotating chopper  9  coincides with the deposition window  10  in the heating element  8 , i.e., when the substrate  1  is exposed to a plasma plume  19 . Due to the rotation of the holder  2  and scanning of the target surface with the laser beam, a homogeneous film deposition over the substrate surface is provided. After the film deposition, the oxygen pressure in the chamber is increased up to 300 mbar within 10 minutes. Simultaneously, the substrate temperature is reduced down to 500° C. during the following 10 minutes and afterwards down to 150° C. during the next 60 minutes. 
     One illustrative example in accordance with the present invention is a superconducting element comprising a high temperature superconducting film deposited on a technical substrate consisting of a stainless steel which is composed of 
     C 0 to 0.20% 
     Si 1.5 to 2.5% 
     Mn 0 to 2.00% 
     P 0 to 0.045% 
     S 0 to 0.030% 
     Cr 18.0 to 28.0% 
     Ni 16.0 to 25.0% 
     N 0 to 0.11% 
     with the remainder being iron and unavoidable impurities. 
     The Si content is in a range between 1.6 and 2.5%, or more preferably in a range between 1.7 and 2.5%. 
     EXAMPLE 1 
     0.1 mm-thick stainless steel with a composition of Ni 19%, Cr 24%, Si 2%, Mn 1%, and with the rest of iron was used as the substrate  1 . Substrate surface was galvanically polished in water solution of H 3 PO 4  (30%), CrO 3  (15%) and H 2 SO 4  (7%) using a current density of 0.5 A/cm 2 . Polishing time was of 10 s. After the polishing the substrate  1  was thoroughly washed in de-ionized water in supersonic bath. Then the substrate was covered with an Yttrium Stabilised Zirconia 1 micrometer thick buffer layer by ion beam assisted deposition (IBAD). 
     By using the apparatus illustrated in FIG.  1  and described above the YBa 2 Cu 3 O 7-δ  film was deposited which showed a density of the critical current in said superconducting film measured at the temperature of 77 K of 2 MA/cm 2 . Transition temperature was 90 K. No substrate oxidation was observed after 120 min. lasting film deposition process. 
     The price of the substrate is 20 Duetche Marks per square meter (DM/m 2 ), i.e. about $10 per square meter. 
     EXAMPLE 2 
     A 0.1 mm-thick stainless steel with the composition of Ni 25%, Cr 24%, Si 1.5%, Mn 2%, and with the rest of iron was used as the substrate  1 . Buffer layer and superconducting film was applied as described in Example 1. 
     The density of the critical current in the superconducting film was of 1.8 MA/cm 2 . Transition temperature was 90 K. No substrate oxidation was observed after 120 min. 
     The price of the substrate is 25 Duetche Marks per square meter (DM/m 2 ), i.e. about $12.50 per square meter. 
     EXAMPLE 3 
     A 0.1 mm-thick stainless steel with the composition of Ni 16%, Cr 24%, Si 2%, Mn 1%, and with the rest of iron was used as the substrate  1 . Buffer layer and superconducting film was applied as described in Example 1. 
     The density of the critical current in the superconducting film was 1.5 MA/cm 2 . Transition temperature was 89 K. No substrate oxidation was observed after 120 min. 
     The price of the substrate is 20 Duetche Marks per square meter (DM/m 2 ), i.e. about $10 per square meter. 
     EXAMPLE 4 
     A 0.1 mm-thick stainless steel with the composition of Ni 19%, Cr 18%, Si 2%, Mn 2%, and with the rest of iron was used as the substrate  1 . Buffer layer and superconducting film was applied as described in Example 1. 
     The density of the critical current in the superconducting film was of 1.7 MA/cm 2 . Transition temperature was 89 K. No substrate oxidation was observed after 120 min. 
     The price of the substrate is 18 Duetche Marks per square meter (DM/m 2 ), i.e. about $9 per square meter. 
     EXAMPLE 5 
     A 0.1 mm-thick stainless steel with the composition of Ni 19%, Cr 28%, Si 1.5%, Mn 1%, and with the rest of iron was used as the substrate  1 . Buffer layer and superconducting film was applied as described in Example 1. 
     The density of the critical current in the superconducting film was 1.5 MA/cm 2 . Transition temperature was 88 K. No substrate oxidation was observed after 120 min. 
     The price of the substrate is 25 Duetche Marks per square meter (DM/m 2 ), i.e. about $12.50 per square meter. 
     EXAMPLE 6 
     The stainless steel of Example 1 was used, by instead of galvanic polishing the substrate was annealed at 850° C. in vacuum during 2 hours. 
     The density of the critical current in the superconducting film was 1.7 MA/cm 2 . Transition temperature was 90 K. No substrate oxidation was observed after 120 min. 
     The price of the substrate is 20 Duetche Marks per square meter (DM/m 2 ), i.e. about $10 per square meter. 
     EXAMPLE 7 
     A 0.1 mm-thick stainless steel with the composition of Ni 8%, Cr 18%, Mn 2%, and with the rest of iron was used as the substrate  1 . Buffer layer and superconducting film was applied as described in Example 1. 
     The density of the critical current in the superconducting film was 0.1 MA/cm 2 . Transition temperature was 88 K. A pronounced substrate oxidation was observed after 120 min. 
     The price of the substrate is 15 Duetche Marks per square meter (DM/m 2 ), i.e. about $7.50 per square meter. 
     EXAMPLE 8 
     A 0.1 mm-thick stainless steel with the composition of Ni 9%, Cr 18%, Ti 0.5%, Mn 2%, and with the rest of iron was used as the substrate  1 . Buffer layer and superconducting film was applied as described in Example 1. 
     The density of the critical current in the superconducting film was 0.1 MA/cm 2 . Transition temperature was 88 K. A pronounced substrate oxidation was observed after 120 min. lasting film deposition process. 
     The price of the substrate is 22 Duetche Marks per square meter (DM/m 2 ), i.e. about $11 per square meter. 
     EXAMPLE 9 
     A 0.1 mm-thick stainless steel with the composition of Ni 57%, Cr 16%, Fe 6%, Mo 17%, W 5% (Hastelloy® C) was used as the substrate  1 . Buffer-layer and superconducting film was applied as described in Example 1. 
     The density of the critical current in the superconducting film was 1.5 MA/cm 2 . Transition temperature was 88 K. A local substrate oxidation was observed after 120 min. 
     The price of the substrate is 2,700 Duetche Marks per square meter (DM/m 2 ), i.e. about $1,350 per square meter. 
     EXAMPLE 10 
     A 0.1 mm-thick stainless steel with the composition of Ni 100% (Nickel) was used as the substrate  1 . Buffer layer and superconducting film was applied as described in Example 1. 
     The density of the critical current in the superconducting film was 1.4 MA/cm 2 . Transition temperature was 88 K. A pronounced substrate oxidation was observed after 120 min. 
     The price of the substrate is 1,300 Duetche Marks per square meter (DM/m 2 ), i.e. about $650 per square meter. 
     EXAMPLE 11 
     A 0.1 mm-thick stainless steel with the composition of Ni 53%, Cr 19%, Fe 19%, Mo 3%, Nb+Tb 5%, Ti 1% (Inconel®718) was used as the substrate  1 . Buffer layer and superconducting film was applied as described in Example 1. 
     The density of the critical current in the superconducting film was 0.15 MA/cm 2 . Transition temperature was 87 K. A local substrate oxidation was observed after 120 min. 
     The price of the substrate is 6,300 Duetche Marks per square meter (DM/m 2 ), i.e. about $3,150 per square meter. 
     EXAMPLE 12 
     A 0.1 mm-thick stainless steel with the composition of Ni 72%, Cr 16%, Fe 8%, Mn 1%, Si 0.5%, Co (Inconel®600) was used as the substrate. Buffer layer and superconducting film was applied as described in Example 1. 
     The density of the critical current in the superconducting film was 0.1 MA/cm 2 . Transition temperature was 86 K. A large substrate oxidation was observed after 120 min. 
     The price of the substrate is 900 Duetche Marks per square meter (DM/m 2 ), i.e. about $450 per square meter. 
     EXAMPLE 13 
     At the same conditions as Example 1 but with a substrate composition of Ni 19%, Cr 25%, Fe 53%, Mn 2%, Si 0.75% (steel SEW No. 1.4845 or ASTM No. 310 S). 
     The density of the critical current in the superconducting film was 0.9 MA/cm 2 . Transition temperature corresponds to 88 K. Some local substrate oxidation was observed after 120 mm. The price of the substrate is 614 Duetche Marks per square meter (DM/m 2 ), i.e. about $307 per square meter. 
     The results of the experiments according to the above examples are summarized in Table 1. 
     It should be noted that all indications in % are based on weight (wt %). 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Comparison of quality of high-Tc superconducting films and substrate prices for different metallic substrates 
               
             
          
           
               
                   
                 Quality of high-Tc film 
                   
                 Film deterioration 
               
             
          
           
               
                   
                 Substrate 
                 Substrate composition 
                 Substrate 
                 Transition 
                 Critical current 
                 Price of 
                 caused by substrate 
               
             
          
           
               
                 No. 
                 material 
                 Ni 
                 Cr 
                 Fe 
                 other elements 
                 treatment 
                 temperature 
                 density 
                 substrate 
                 oxidation 
               
               
                   
               
               
                 1 
                 Stainless steel 
                 19% 
                 24% 
                 ˜53% 
                 Si 2%, Mn 1% 
                 galvanic 
                 90 K. 
                   2 MA/cm 2   
                 20 DM/m 2   
                 no 
               
               
                 2 
                 Stainless steel 
                 25% 
                 24% 
                 ˜47% 
                 Si 1.5%, Mn 2% 
                 galvanic 
                 90 K. 
                 1.8 MA/cm 2   
                 25 DM/m 2   
                 no 
               
               
                 3 
                 Stainless steel 
                 16% 
                 24% 
                 ˜56% 
                 Si 2%, Mn 1% 
                 galvanic 
                 89 K. 
                 1.5 MA/cm 2   
                 20 DM/m 2   
                 no 
               
               
                 4 
                 Stainless steel 
                 19% 
                 18% 
                 ˜58% 
                 Si 2%, Mn 2% 
                 galvanic 
                 89 K. 
                 1.7 MA/cm 2   
                 18 DM/m 2   
                 no 
               
               
                 5 
                 Stainless steel 
                 19% 
                 28% 
                 ˜50% 
                 Si 1.5%, Mn 1% 
                 galvanic 
                 88 K. 
                 1.5 MA/cm 2   
                 25 DM/m 2   
                 no 
               
               
                 6 
                 Stainless steel 
                 19% 
                 24% 
                 ˜53% 
                 Si 2%, Mn 1% 
                 thermal 
                 90 K. 
                 1.7 MA/cm 2   
                 20 DM/m 2   
                 no 
               
               
                 7 
                 Stainless steel 
                  8% 
                 18% 
                 ˜71% 
                 Mn 2% 
                 galvanic 
                 87 K. 
                 0.1 MA/cm 2   
                 15 DM/m 2   
                 large 
               
               
                 8 
                 Stainless steel 
                  9% 
                 18% 
                 ˜70% 
                 Ti 0.5%, Mn 2% 
                 galvanic 
                 86 K. 
                 &lt;0.1 MA/cm 2    
                 22 DM/m 2   
                 large 
               
               
                 9 
                 Hastelloy ® C 
                 57% 
                 16% 
                  6% 
                 Mo 17%, W 5% 
                 galvanic 
                 88 K. 
                 1.5 MA/cm 2   
                 2700 DM/m 2   
                 local 
               
               
                 10  
                 Nickel 
                 100%  
                  0% 
                  0% 
                   
                 galvanic 
                 88 K. 
                 1.4 MA/cm 2   
                 1300 DM/m 2   
                 large 
               
               
                 11  
                 Inconel ® 718 
                 53% 
                 19% 
                  19% 
                 Mo 3%, Nb + Ta 
                 galvanic 
                 87 K. 
                 0.15 MA/cm 2   
                 6300 DM/m 2   
                 local 
               
               
                   
                   
                   
                   
                   
                 5%, Ti 1% 
               
               
                 12  
                 Inconel ® 600 
                 72% 
                 16% 
                  8% 
                 Mn 1%, 
                 galvanic 
                 86 K. 
                 0.1 MA/cm 2   
                 900 DM/m 2   
                 large 
               
               
                   
                   
                   
                   
                   
                 Co, Si 0.5% 
               
               
                 13  
                 Stainless steel 
                 19% 
                 25% 
                 ˜53% 
                 Si 0.75%, Mn 2% 
                 galvanic 
                 88 K. 
                 0.9 MA/cm 2   
                 614 DM/m 2   
                 small, local 
               
               
                   
               
             
          
         
       
     
     Table 1 shows that the stainless steel according to the present invention (according to SEW steel No. 1.4841 (corresponding to ASTM 314 or SUS-314) shows a double as high critical current density compared with the stainless steel of Example 9 having the SEW steel No. 1.4845 or ASTM 310 or SUS-310). Surprisingly the higher Si content of the steel according to the present invention has the effect of a much improved critical current density of the superconducting element.