Abstract:
An oxide superconducting material having a composition represented by a formula 
     
       TlSr.sub.2 (Sr.sub.n-x Y.sub.x)Cu.sub.n+1 O.sub.5+2n 
     
     wherein n=1 or 2 and 0.1≦x≦n is provided. This material has a high critical current density Jc even by sintering at a relatively low temperature of 850°-880° C.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an oxide superconducting material for use in various superconductivity application apparatus and superconducting devices. 
     2. Description of the Prior Art 
     Up to the present, metal or alloy superconducting materials, intermetallic compound superconducting materials, etc. have already been put into practical use as superconducting materials. The superconducting materials are used for production of coils for superconducting magnets, electronics devices such as a Josephson device, etc. and are expected to realize applications particularly to a Superconducting Quantum Interference Device (=SQUID) and a precision measurement technique, utilizing the merits of high sensitivity, high precision and low noise properties of a Josephson junction, and also to an electronic computer, taking advantage of high speed responsibility and low electric power consumption properties of the Josephson junction. 
     It is desired that a superconductivity transition temperature Tc of a superconducting material is as high as possible. Since an oxide superconductor of La--Ba--Cu--O system having a Tc around 30 K. was found, there are successively found oxide superconductors of Ba--Y--Cu--O system having a Tc around 90 K., Bi--Sr--Ca--Cu--O system having a Tc around 110 K., Tl--Ba--Ca--Cu--O system having a Tc around 120 K., etc. These findings of the materials having Tc far exceeding a liquid nitrogen temperature are further enhancing expectations for use as practical materials. 
     For applying a superconducting material to an electronics device, it is preferred that the maximum temperature in the entire process for production is as low as possible. It is also important that the critical current density Jc is as large as possible, from the viewpoint of wiring materials. In a Tl--Ba--Ca--Cu system superconductor having not lower than 100 K. superconductivity transition temperature, it is necessary to carry out firing at a temperature of 890°-910° C. for obtaining a higher Jc. Any firing at a lower temperature does not complete sintering and so causes a lower Jc. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is therefore to present a superconductor which has an optimum firing temperature lower than that of the prior art superconductor and at the same time has a high Jc as obtained heretofore only at a higher firing temperature. 
     The present inventors have found through intensive and extensive researches that an oxide superconductor composition comprising a thallium (Tl). strontium (Sr), yttrium (Y) and copper (Cu) in a form of an oxide; a ratio of these elements being represented by a formula 
     
         TlSr.sub.2 (Sr.sub.n-x Y.sub.x)Cu.sub.n+1 O.sub.5+2n       (I) 
    
     wherein n=1 or 2 and x is in a range of 0.1≦x≦n, differently from the prior art composition particularly of a formula 
     
         Tl.sub.2 Ba.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.y              (II) 
    
     wherein n=1, 2 or 3, has a high Jc even by sintering at a relatively low temperature of 850°-880° C. and thus completed the present invention. The present inventors have found also that the properties of the sintered body can be further improved by wrapping the pressed molded body with a gold foil at sintering. 
     The composition of the present invention represented by the above formula (I) includes two cases, that is, a case wherein n=1 and a case wherein n=2, respectively corresponding to the following formulae: 
     
         TlSr.sub.3-x Y.sub.x Cu.sub.2 O.sub.7                      (IA) 
    
     wherein 0.1≦x≦1, and 
     
         TlSr.sub.4-x Y.sub.x Cu.sub.3 O.sub.9                      (IB) 
    
     wherein 0.1≦x≦2. Unit cells of the layered perovskite-like crystal structures of these compositions of the formulae (IA) and (IB) may be shown respectively as follows: 
     
         TlO/SrO/CuO.sub.2 /Sr or Y/CuO.sub.2 /SrO                  (IX) 
    
     
         TlO/SrO/CuO.sub.2 /Sr or Y/CuO.sub.2 /Sr or Y/CuO.sub.2 /SrO(IY) 
    
     from which it will be understood that the structures of these compositions are similar but different mainly in respect of the number of CuO 2  layers in the repeating cycle. 
     In effect, the present inventors have confirmed by experiments that the composition of the above formula (II) wherein n=2, that is, of the following formula: 
     
         Tl.sub.2 Ba.sub.2 CaCu.sub.2 O.sub.y                       (IIc) 
    
     has Jc of 100 A/cm 2  at 77 K. after sintering at 870° C. for one hour. 
     On the other hand, it has been found that the compositions according to the present invention, that is, of the formula (IA) wherein x=1, namely of a formula 
     
         TlSr.sub.2 YCu.sub.2 O.sub.7                               (Ia) 
    
     and of the formula (IB) wherein x=1, namely of a formula 
     
         TlSr.sub.3 YCu.sub.3 O.sub.9                               (Ib) 
    
     give structurally fine sintered body having Jc respectively of 800 A/cm 2  and 900 A/cm 2 , under the sintering condition same as the above mentioned condition for the composition (IIc). 
     It has also been found that the sintered body obtained from the composition of the formula (Ia) shows sharp transition into superconducting state at 85 K., and after such transition, the entire volume is in the superconducting state; and that obtained from the composition of the formula (Ib) shows sharp transition into superconducting state at 100 K., and after such transition, about 80% of the entire volume is in the superconducting state. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will be explained below further in detail with respect to its some embodiments shown as working examples. 
    
    
     EXAMPLE 1 
     As for starting materials, thallium oxide (Tl 2  O 3 ), strontium oxide (SrO), yttrium oxide (Y 2  O 3 ) and cupric oxide (CuO), respectively of 99% or more purity were used. These materials were taken up and weighed to show each of the composition ratios as shown in the following Table 1, well mixed in a mortar and then pressed into a pressed body in a size of 5 mm×10 mm×1 mm. This pressed body was sintered at 850°-870° C. for 1-10 hours in an oxygen atmosphere. 
     Then measurements were made on the sintered bodies of the compositions shown in the Table 1 as to their resistivities, critical current densities, and superconductivity volume fractions in order to evaluate their superconductivity properties. 
     The resistivity was measured by using a dc four probe method. The electrical contacts were made by using gold electrodes sputtered on the sample surface and the copper lead wires plated with tin. 
     The critical current density also was measured by the dc four probe method. The current value at which 0.1 μV or more voltage was generated between the voltage leads was employed as Jc. 
     The superconductivity volume fraction was obtained by an ac magnetic susceptibility measurement, which was carried out by determining any change of inductance L of a coil wherein a sample was placed, and calculating on the basis of ΔL at 4.2 K. of lead of the same volume and the same shape as equal to 100. 
     The resistivity measurement was made within a temperature range of from a room temperature to a temperature showing zero resistivity and the magnetic susceptibility measurement was made within a temperature range of from a room temperature to 4.2 K. 
     In the Table 1, the critical temperature at which the resistivity becomes zero and the critical current density at 77 K. as measured on the compositions are shown along with the composition ratios. 
     
                       TABLE 1______________________________________Sample                   Tc     JcNo.     Composition      (K)    (A/cm.sup.2)______________________________________1       TlSr.sub.2.9 Y.sub.0.1 Cu.sub.2 O.sub.7                    81     4702       TlSr.sub.2.8 Y.sub.0.2 Cu.sub.2 O.sub.7                    83     5303       TlSr.sub.2.5 Y.sub.0.5 Cu.sub.2 O.sub.7                    85     7004       TlSr.sub.2.2 Y.sub.0.8 Cu.sub.2 O.sub.7                    87     6905       TlSr.sub.2.0 Y.sub.1.0 Cu.sub.2 O.sub.7                    88     800______________________________________ 
    
     EXAMPLE 2 
     The procedures same as those described in the above Example 1 were repeated, excepting that the starting materials were mixed in the composition ratios as shown in the following Table 2, the sintering was made at 860°-880° C. and some of the samples were sintered after wrapped with gold foil. 
     The results are shown in the Table 2 along with the composition ratios. 
     
                       TABLE 2______________________________________Sample                   Tc     JcNo.     Composition      (K)    (A/cm.sup.2)______________________________________6       TlSr.sub.3.9 Y.sub.0.1 Cu.sub.3 O.sub.9                     90     5007       TlSr.sub.3.5 Y.sub.0.5 Cu.sub.3 O.sub.9                     98     8508       TlSr.sub.3.0 Y.sub.1.0 Cu.sub.3 O.sub.9                    100     9009       TlSr.sub.2.8 Y.sub.1.2 Cu.sub.3 O.sub.9                    107    100010      TlSr.sub.2.0 Y.sub.2.0 Cu.sub.3 O.sub.9                    110    1100______________________________________ 
    
     As shown in the above Examples 1 and 2, the compositions according to the present invention give large Jc even though they are sintered at a temperature lower than that of the prior art. 
     It was also found that in case the samples were wrapped with gold foil and sintered, the property values were improved for some percents. 
     It is to be noted however that if x was made less than 0.1, no superconductivity property was shown and so such x is not suitable for the purpose of the present invention, that if x was made larger than n, the volume fraction and therefore Jc was reduced, and that if the sintering was carried out at a temperature outside the range of 850°-880° C., no sufficient sintering could be reached or the volume fraction was reduced. 
     Thus the present invention provides a very useful superconducting material, in that the composition according to the present invention enables to reach high critical current density even by sintering at a lower temperature.