Oxide superconducting material and process for preparing the same
This is a Continuation of application Ser. No. 07/914,520, filed Jul. 17, 1992 now abandoned, which is a continuation of 07/852,331 filed Mar. 17, 1992 now abandoned, which is a continuation of 07/436,011 filed Nov. 13, 1989 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to superconducting oxide materials and processes for preparing the same. More particularly, it relates to thallium-containing superconducting oxide materials having high superconducting critical temperature (Tc) and higher superconducting critical current density (Jc) and processes for preparing the same. 
     2. Description of the Related Art 
     Discovery of new oxide type superconductor by Bednorz and Muller revealed the possibility of high temperature superconductors (Z. Phys. B64, 1986 p 189). 
     The new type oxide superconductor discovered by Bednorz and Muller is represented by [La, Sr] 2  CuO 4  which is called the K 2  NiF 4  -type oxide having a crystal structure similar to known perovskite type oxides. The K 2  NiF 4  -type compound oxides show such higher Tc as 30 K which are extremely higher than known superconducting materials. 
     C. W. Chu et al. reported another superconducting material of so-called YBCO type represented by YBa 2  Cu 3  O 7-x  having the critical temperature of about 90 K (Physical Review letters, Vol. 58, No. 9, p 908). However, the critical temperature of this oxide superconductor is not so different from a boiling point 70 K of liquid nitrogen and hence the other oxide materials which have much higher critical temperature, in other words, which have larger temperature margin have been demanded. 
     Maeda et al reported the other type new superconducting compound oxide of Bi--Sr--Ca--Cu--O system which show the critical temperature of mor than 100 k (Japanese Journal of Applied Physics. Vol. 27, No. 2, p 1209 to 1210). 
     Thallium type compound oxides are also high Tc superconductors of more than 100 K. The present inventors disclosed several kinds of thallium type compound oxides superconductors in U.S. patent application No. 223,634 filed on Jul. 25, 1988 and Hermann et al. reported Tl--Ba--Ca--Cu--O system in Appl. Phys. Lett. 52 (20) p 1738. U.S. Pat. No. 4,870,052 discloses a kind of thallium-containing oxide superconductor. Thallium type compound oxides have such a very important merit that superconductors which show such a high Tc as more than 100 K can be obtained without using rear earth elements as a material so that the production cost can be reduced. 
     Although these known Tl--Ba--Ca--Cu--O system and Bi--Sr--Ca--Cu--O system oxide superconductors show very high Tco&#39;s at which the superconducting property is observed, their Tci&#39;s at which the apparent electrical resistance become zero or undetectable are such low as about 80 to 90 K which is not so different from that of the Y 1  Ba 2  Cu 3  O 7-x  system because these compound oxide superconductors contain several different phases. Still more, their critical current density values (Jc) are inferior to that of the Y 1  Ba 2  Cu 3  O 7-x  system. 
     Still more, in the case of production of thallium type oxide superconductors, there is a special problem cause by such a fact that thallium is a very volatile element and toxic for human. In fact, it is difficult to obtain a thallium-containing oxide having a desired composition because the vapour pressure of thallium is relatively higher than the other elements. Further, the special attention must be paid to handle the material of the thallium-containing superconductor because thallium element is toxic for human. 
     An object of the present invention is to overcome the problems of the prior arts and to provide thallium-containing superconductors which is relatively easy to be obtained as a single phase and also which have improved superconducting properties and a method for producing the same. 
     SUMMARY OF THE INVENTION 
     The present invention provides a superconducting oxide material, characterized in that the superconducting oxide material contain compound oxide having a composition represented by the formula: 
     
         (Tl.sub.(l-p-q) Bi.sub.p Pb.sub.q).sub.y γ.sub.z (α.sub.(l-r) β.sub.r).sub.s Cu.sub.v O.sub.w 
    
     in which each of &#34;α&#34; and &#34;γ&#34; is an element selected in IIa group of the periodic table, &#34;β&#34; is an element selected from a group comprising Na, K, Rb and Cs, &#34;y&#34;, &#34;z&#34;, &#34;v&#34;, &#34;w&#34;, &#34;p&#34;, &#34;q&#34;, &#34;r&#34; and &#34;s&#34; are numbers each satisfying respective range of 0.5≦y≦3.0, 0.5≦z≦6.0, 1.0≦v, 5.0≦w, 0≦p≦1.0, 0≦q≦1.0, 0≦r≦1.0 and 0.5≦s≦3.0. 
     The expression &#34;contain&#34; means that the superconducting oxide material according to the present invention can contain additionally or inevitably the other compound oxides. In fact, a bulk oxide superconductor usually may consist of more than one phase. Therefore, all compound oxides containing the compound oxide defined by the present invention are included in the scope of the present invention. 
     The present invention provides also a process for preparing the superconducting oxide material, characterized by mixing oxide powders each containing Tl, Bi, Pb, an element α, an element β, an element γ and Cu in proportions of 
     
         Tl:Bi:Pb:γ:α:β:Cu=h:i:j:k:l:m:n, 
    
     in which each of &#34;α&#34; and &#34;γ&#34; is an element selected in IIa group of the periodic table, &#34;β&#34; is an element selected from a group comprising Na, K, Rb and Cs, &#34;h&#34;, &#34;j&#34;, &#34;k&#34;, &#34;l&#34;, &#34;m&#34; and &#34;n&#34; are numbers each satisfying respective range of 0&lt;h≦3.0, 0≦i&lt;3.0, 0≦j&lt;3.0, 0.5≦k≦6.0, 0≦l≦3.0, 0≦m≦3.0 and 1.0≦n, and then sintering the resulting powder mixture at a temperature between 820° and 950° C. for 6 to 100 hours in oxygen gas atmosphere. 
     An essence of the present invention reside in that the oxide superconductor according to the present invention contain a compound oxide having a layered crystal structure of a tetragonal system which has one layer of Tl--O, (Tl, Pb)--O, (Tl, Bi)--O or (Tl, Pb, Bi)--O in the crystal. 
     Preferable compound oxides are represented by one of the following general formulas: 
     
         (Tl.sub.(l-q) Pb.sub.q).sub.y γ.sub.z α.sub.s Cu.sub.v O.sub.w( 1) 
    
     in which each of &#34;α&#34; and &#34;γ&#34; is an element selected in IIa group of the periodic table, &#34;y&#34;, &#34;z&#34;, &#34;v&#34;, &#34;w&#34;, &#34;q&#34; and &#34;s&#34; are numbers each satisfying respective range of 0.5≦y≦3.0, 0.5≦z≦6.0, 2.0≦v, 5.0≦w, 0≦q≦1.0 and 0.5≦s≦3.0, preferably 0&lt;q≦0.5. This compound oxide has preferably such a crystal structure as having two to six layer of CuO corresponding to v=2 to 6. 
     
         (Tl.sub.(l-p) Bi.sub.p).sub.y α.sub.s Ca.sub.z Cu.sub.v O.sub.w( 2) 
    
     in which &#34;α&#34; is Ba or Sr, &#34;y&#34;, &#34;z&#34;, &#34;v&#34;, &#34;w&#34;, &#34;p&#34; and &#34;s&#34; are numbers each satisfying respective range of 0.5≦y≦3.0, 0.5≦z≦4.0, 1.0≦v≦5.0, 5.0≦w15.0, 0≦p≦1.0 and 0.5≦s≦3.0, preferably 0.2≦p≦0.8. 
     
         (Tl.sub.(l-p-q) Bi.sub.p Pb.sub.q).sub.y α.sub.s Ca.sub.z Cu.sub.v O.sub.w                                                   ( 3) 
    
     in which &#34;α&#34; is Ba or Sr, &#34;y&#34;, &#34;z&#34;, &#34;v&#34;, &#34;w&#34;, &#34;p&#34;, &#34;q&#34; and &#34;s&#34; are numbers each satisfying respective range of 0.5≦y≦3.0, 0.5≦z≦4.5, 1.0≦v≦5.0, 5.0≦w≦15.0, 0≦p≦1.0, 0≦q≦0.6 and 0.5≦s≦3.0, preferably 0.2≦p≦0.8. 
     
         Tl.sub.y Ca.sub.z (α.sub.(l-r) β.sub.r).sub.s Cu.sub.v O.sub.w( 4) 
    
     in which &#34;α&#34; is Ba or Sr, &#34;β&#34; is an element selected from a group comprising Na, K, Rb and Cs, &#34;y&#34;, &#34;z&#34;, &#34;v&#34;, &#34;w&#34;, &#34;r&#34; and &#34;s&#34; are numbers each satisfying respective range of 0.5≦y≦3.0, 0.5≦z≦4.0, 2.0≦v≦5.0, 5.0≦w≦15.0, 0≦r≦0.8 and 0.5≦s≦3.0. 
     
         (Tl.sub.(l-p-q) Bi.sub.p Pb.sub.q).sub.y Ca.sub.z (α.sub.(l-r) β.sub.r).sub.s Cu.sub.v O.sub.w                      ( 5) 
    
     in which &#34;α&#34; is Ba or Sr, &#34;β&#34; is an element selected from a group comprising Na, K, Rb and Cs, &#34;y&#34;, &#34;z&#34;, &#34;v&#34;, &#34;w&#34;, &#34;p&#34;, &#34;q&#34;, &#34;r&#34; and &#34;s&#34; are numbers each satisfying respective range of 0.5≦y≦3.0, 0.5≦z≦4.0, 2.0≦v≦5.0, 5.0≦w≦15.0, 0≦p≦0.5, 0≦q≦0.5, 0≦r≦0.8 and 0.5≦s≦3.0. 
     The superconducting oxide material containing the above-mentioned compound oxide according to the present invention is preferably prepared by mixing one of oxide powders consisting of the following elements listed below with their proportions and then by sintering the resulting powder mixture at a temperature between 820° and 950° C. for 6 to 100 hours in oxygen gas atmosphere: 
     (1) Tl:Pb:Ca:Ba:Cu=h:j:k:l:n 
     in which 0&lt;h&lt;3.0, 0&lt;j&lt;3.0, 0.5≦k≦6.0, 0.5≦l≦3.0 and 2.0≦n, 
     (2) Tl:Bi:Ca:Ba:Cu=h:i:k:l:n 
     in which 0&lt;h&lt;3.0, 0&lt;i&lt;3.0, 0.5≦k≦6.0, 0.5≦l≦3.0 and 2.0≦n, 
     (3) Tl:Bi:Ca:Sr:Cu=h:i:k:l:n 
     in which 0&lt;h&lt;3.0, 0&lt;i&lt;3.0, 0.5≦k≦6.0, 0.5≦l≦3.0 and 2.0≦n, 
     (4) Tl:Bi:Pb:Ca:Sr:Cu=h:i:j:k:l:n 
     in which 0&lt;h&lt;3.0, 0≦i&lt;3.0, 0≦j&lt;3.0, 0.5≦k≦4.0, 0.5≦l≦3.0 and 1.0≦n≦5.0, 
     (5) Tl:Ca:α:β:Cu=h:k:l:m:n 
     in which &#34;α&#34; is Ba or Sr, &#34;β&#34; is an element selected from a group comprising Na, K, Rb and Cs and 0.5≦h≦3.0 0.5≦k≦4.0, 0.1≦l≦3.0, 0≦m≦2.4 and 2.0≦n≦5.0, and 
     (6) Tl:Bi:Pb:Ca:α:β:Cu=h:i:j:k:l:m:n 
     in which &#34;α&#34; is Ba or Sr, &#34;β&#34; is an element selected from a group comprising Na, K, Rb and Cs and 0.5≦h≦3.0, 0.5≦i≦3.0, 0.5≦j&lt;3.0, 0.5≦k≦4.0, 0.1≦l≦3.0, 0≦m≦2.4 and 2.0 ≦n≦5.0. 
     In operation, the material powder mixture is preferably wrapped with a metallic foil made of one of precious metals such as gold or their alloys. The sintering operation can be effected in an ordinary furnace. The interior of the furnace is preferably in the condition of oxygen-rich atmosphere. The oxygen gas is preferably fed continuously into the furnace during the sintering operation preferably at a rate of more than 0.1 liter per minute at 1 atm. Usual oxygen gas pressure is about 1 atm but the sintering can be effected also at a higher oxygen pressure than 1 atm. According to the present invention, the sintering is effected at a temperature between 820° and 950° C. When the sintering temperature is not higher than 820° C., the resulting sintered mass becomes a mixture of different phases each having different critical temperature, so that the apparent Tc of the sintered mass become lower. To the contrary, if the sintering temperature is not lower than 950° C., the evaporation of thallium increase excessively so that it is difficult to adjust the composition of the sintered mass to desired atomic ratios and also it increases precipitates which doesn&#39;t contribute the superconductivity. The sintering can be effected for a time duration between 6 to 100 hours. When the sintering time is not longer than 6 hours, the material powder mixture is not sintered satisfactorily so that a desired superconductor can not be obtained. Longer sintering time than 100 hours may not effective to improve the superconducting property. 
     As is described hereinabove, the oxide superconductors according to the present invention exhibit very higher critical current density than the known oxide superconductors such as Bi--Sr--Ca--Cu--O system and Tl--Sr--Ca--Cu--O system while they exhibit the same or even higher critical temperature with respect to such known oxide superconductors. It is thought that such advantage might be obtained from such facts that the oxide superconducting materials according to the present invention have a novel layered crystal structure of tetragonal system having one layer of Tl--O, (Tl, Pb)--O, (Tl, Bi)--O, or (Tl, Pb, Bi)--O and that the oxide superconducting materials according to the present invention are relatively easily obtained as a single phase. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 to FIG. 5 are X-ray diffraction charts of five oxide superconductors (Sample No. 1 to 5) obtained in Example 1 according to the present invention. 
     FIG. 6 is a X-ray diffraction chart of an oxide superconductor (Sample No. 6) obtained in Example 3 according to the present invention. 
     FIGS. 7 to 9 show respective temperature dependency of magnetic susceptibility of oxide superconductors (Sample No. 3 to 5) obtained in Example 1 according to the present invention. 
     FIG. 10 shows a temperature dependency of electric resistance of an oxide superconductor (Sample No. 1) obtained in Example 3 according to the present invention. 
     FIG. 11 and FIG. 12 show respective temperature dependency of electric resistance and of magnetic susceptibility of an oxide superconductor (Sample No. 6) obtained in Example 3 according to the present invention. 
     Now, the present invention will be described in more details by examples, but the scope of the present invention should not be limited to the following special examples. 
    
    
     EXAMPLE 1 
     Oxide superconducting materials according to the present invention were prepared by the process according to the present invention. Powders of Tl 2  O 3 , PbO, CaO, BaO 2  and CuO used as materials were mixed in such proportions that the atomic ratios of Tl:Pb:Ca:Ba:Cu in each sample became as followings: 
     (1) 0.8:0.2:3:l:3 
     (2) 0.75:0.25:3:1:3 
     (3) 0.8:0.2:3:1:3 
     (4) 0.95:0.05:3:1:3 
     (5) 0.95:0.05:3:1:3. 
     Then, each of the resulting powder mixture was compacted under a pressure of about 100 kg/cm 2  into a pellet. Each pellet was wrapped with a gold foil of 50 micron thick and was sintered at a fixed temperature selected from a range between 850° C. and 900° C. for about 10 hours in a furnace into which O 2  gas is flown at a rate of 200 ml/min to obtain an oxide superconductor according to the present invention. 
     The powder mixture of Sample No. 4 and 5 had the same atomic ratios but they were sintered at different sintering temperature of 870° C. (sample No. 4) and 850° C. (sampel No. 5) respectively. 
     All of the resulting oxide superconductors were found that they contained compound oxide represented by the general formula: 
     
         (Tl.sub.(l-q) Pb.sub.q).sub.y Ca.sub.z Ba.sub.s Cu.sub.v O.sub.w 
    
     in which &#34;y&#34;, &#34;z&#34;, &#34;v&#34;, &#34;w&#34;, &#34;q&#34; and &#34;s&#34; are numbers each satisfying respective range of 0.5≦y≦3.0, 0.5≦z≦6.0, 2.0≦v, 5.0≦w, 0≦q≦1.0 and 0.5≦s≦3.0. 
     Table 1 shows the properties of each oxide superconducting material obtained in FIGS. 1 and 5 show X-ray diffraction data thereof. FIGS. 7 to 9 show the result of magnetic susceptibility measured on the samples No. 3, 4 and 5 respectively. In the composition shown in Table 1, there are such cases that the ratio of Tl to Pb and the ratio of Ca to Cu don&#39;t coincide with the theoretical values which are expected by the analysis of crystal structure because the values shown in Table 1 are measuered values and conain a tolerance. 
     For a comparison, the coresponding properties of known superconducting materials represented by Tl 2  Ca 2  Ba 2  Cu 3  O 10  and (Bi, Pb) 2  Ca 2  Sr 2  Cu 3  O 10  are also shown in Table 1. 
     From the Table 1, it is apparent that the oxide superconducting material according to the present invention show higher critical current density Jc than the known oxide superconducting materials while same or even higher critical temperature Tc is observed. 
     
                       TABLE 1                                                     
______________________________________                                    
sam-                                    Jc                                
ple                     lattice    Tci  (A/                               
No   composition        constant   (K)  cm.sup.2)                         
______________________________________                                    
1    (TI, Pb).sub.1 Ca.sub.1 Ba.sub.2 Cu.sub.2 O.sub.7                    
                        a =  3.86 Å                                   
                                    80  400                               
                        c = 12.95 Å                                   
2    (TI, Pb).sub.1 Ca.sub.2 Ba.sub.2 Cu.sub.3 O.sub.9                    
                        a =  3.86 Å                                   
                                   102  410                               
                        c = 16.11 Å                                   
3    TI.sub.0.80 Pb.sub.0.20 Ca.sub.3.0 Ba.sub.2.0 Cu.sub.3.9 O.sub.x     
                        a =  3.85 Å                                   
                                   121  520                               
                        c = 19.36 Å                                   
4    TI.sub.0.82 Pb.sub.0.15 Ca.sub.4.0 Ba.sub.2.0 Cu.sub.4.9 O.sub.y     
                        a =  3.85 Å                                   
                                   113  470                               
                        c = 22.22 Å                                   
5    TI.sub.0.90 Pb.sub.0.16 Ca.sub.5.0 Ba.sub.2.0 Cu.sub.5.8 O.sub.z     
                        a =  3.85 Å                                   
                                   102  --                                
                        c = 25.97 Å                                   
Comparative                                                               
TI.sub.2 Ca.sub.2 Ba.sub.2 Cu.sub.3 O.sub.10                              
                           118    270                                     
(Bi, Pb).sub.2 Ca.sub.2 Sr.sub.2 Cu.sub.3 O.sub.10                        
                           108    255                                     
______________________________________                                    
 
    
     EXAMPLE 2 
     Oxide superconducting materials according to the present invention were prepared by the process according to the present invention. Powders of Tl 2  O 3 , Bi 2  O 3 , BaO 2 , CaO and CuO (purity is higher than 99.9%) used as materials were weighted and mixed in the proportions shown in Table 2 to prepare two samples of powder mixture. Then, the resulting powder mixtures were compacted into pellets. the resulting each pellet was wrapped with a gold foil and was sintered at a temperature of 870° C. for 12 hours. 
     The resulting two samples of oxide superconductors were found that they contained compound oxide represented by the general formula: 
     
         (Tl.sub.(l-p) Bi.sub.p).sub.y Ba.sub.s Ca.sub.z Cu.sub.v O.sub.w 
    
     in which &#34;y&#34;, &#34;z&#34;, &#34;v&#34;, &#34;w&#34;, &#34;p&#34; and &#34;s&#34; are numbers each satisfying respective range of 0.5≦y≦3.0, 0.5≦z≦4.0, 1.0≦v≦5.0, 5≦w≦15.0, 0≦p≦1.0 and 0.5≦s≦3.0. 
     On the resulting oxide superconductors, the critical temperature and the critical current density at liquid nitrogen temperature were measured and X-ray diffraction charts were obtained in order to check phases produced. 
     For a comparison, a comparative superconducting material was prepared by sintering a powder mixture containing solely of Tl 2  O 3 , BaO 2 , CaO and CuO and the superconducting properties were determined by the same method as above. 
     Table 2 shows the ratios of elements in the material powder mixture, the superconducting properties and the crystal structure (phase) determined by X-ray diffraction analysis including the value &#34;y&#34; in the general formula. 
     From the Table 2, it is apparent that the oxide superconducting material according to the present invention show much higher critical current density Jc than the known oxide superconducting material while same or even higher critical temperature Tc is observed. 
     
                       TABLE 2                                                     
______________________________________                                    
Composition of                                                            
the powder mixture                                                        
                Phase        Tci    Jc                                    
TI      Bi    Ba    Ca  Cu  obtained y   (K)  (A/cm.sup.2)                
______________________________________                                    
Exam- 1.4   0.6   2   2   3   a =  3.85 Å                             
                                       2   110  400                       
ple                           c =15.8 Å                               
                              tetragonal                                  
                              system                                      
      1.6   0.4   2   2   3   a =  3.85 Å                             
                                       2   115  450                       
                              c = 15.8 Å                              
                              tetragonal                                  
                              system                                      
Com-  2     --    2   2   3   a =  3.85 Å                             
                                       --  112  270                       
para-                         c = 35.8 Å                              
tive                                                                      
______________________________________                                    
 Note: Tci is a temperature where perfect zero resistance was observed    
 
    
     EXAMPLE 3 
     A plurality of oxide superconducting materials according to the present invention were prepared by the process according to the present invention. Powders of Tl 2  O 3 , Bi 2  O 3 , PbO, CaO, SrO and CuO (purity is higher than 99.9%) used as materials were weighted and mixed in the proportions shown in Table 3, Table 4 and Table 5 to prepare powder mixture samples. Then, the resulting powder mixture samples were compacted into pellets. The resulting each pellet was wrapped with a gold foil and was sintered at a temperature of 870° C. for 12 hours. 
     All of the resulting oxide superconductors were found that they contained compound oxide represented by the general formula: 
     
         (Tl.sub.(l-p) Bi.sub.p).sub.y Sr.sub.s Ca.sub.z Cu.sub.v O.sub.w 
    
     in which &#34;y&#34;, &#34;z&#34;, &#34;v&#34;, &#34;w&#34;, &#34;p&#34; and &#34;s&#34; are numbers each satisfying respective range of 0.5≦y≦3.0, 0.5≦z≦4.0, 1.0≦v≦5.0, 5≦w≦15.0, 0≦p≦1.0 and 0.5≦s≦3.0, 
     or compound oxide represented by the general formula: 
     
         (Tl.sub.(l-p-q) Bi.sub.p Pb.sub.q).sub.y Sr.sub.s Ca.sub.z Cu.sub.v O.sub.w 
    
     in which &#34;y&#34;, &#34;z&#34;, &#34;v&#34; &#34;w&#34; &#34;p&#34; &#34;q&#34; and &#34;s&#34; are numbers each satisfying respective range of 0.5≦y≦3.0, 0.5≦z≦4.5, 1.0≦v≦5.0, 5.0≦w≦15.0, 0≦p≦1.0, 0≦q≦0.6 and 0.5≦s≦3.0. 
     On the resulting oxide superconductors, the critical temperature and the critical current density at liquid nitrogen temperature were measured and X-ray diffraction charts were obtained in order to check phases produced. The value &#34;y&#34; in the general formula was also determined in several samples. 
     From these experimental data, it was confirmed that a novel layered crystal structure of a tetragonal system having a mixed layer of Tl and Bi or a mixed layer of Tl and Pb was produced in the resulting oxide superconductor. From the analysis of the proportion of the phase obtained by X-ray diffraction chart, it was concluded that this novel phase contributes to a superconducting phase. 
     For a comparison, the same test as above was repeated for comparative superconducting materials which were prepared by sintering a powder mixture containing solely of Bi 2  O 3 , SrO, CaO and CuO and a powder mixture containing solely of Bi 2  O 3 , SrO, CaO, CuO and PbO. 
     The ratios in the material powder mixture and the results of tests are also summarized in Table 3, 4 and 5. FIG. 10 shows a temperature dependency of electric resistance measured on the sample No. 1. FIG. 6 shows a X-ray diffraction chart obtained from the sample No. 6. FIG. 11 and 12 show a temperature dependency of magnetic susceptibility and a temperature dependency of electric resistance obtained from the sample No. 6 respectively. 
     All of the oxide superconducting materials according to the present invention show much higher critical current density Jc than the known oxide superconducting material while same or even higher critical temperature Tc is observed. 
     
                       TABLE 3 (1)                                                 
______________________________________                                    
        Composition of                                                    
Sample  the powder mixture   Tci    Jc                                    
No      TI     Bi     Pb  Sr    Ca   Cu  (K)  (A/cm.sup.2)                
______________________________________                                    
Example I                                                                 
 (1)    0.8    0.2    --  2     2    3   115  900                         
 (2)    0.7    0.3    --  2     2    3   100  450                         
 (3)    0.6    0.4    --  2     2    3   105  550                         
 (4)    0.4    0.6    --  2     2    3   100  500                         
 (5)    0.2    0.8    --  2     2    3    98  400                         
 (6)    1      1      --  2     2    3   105  780                         
 (7)    0.6    0.3    0.1 2     2    3   108  650                         
 (8)    0.56   0.24   0.2 2     2    3   110  700                         
 (9)    0.42   0.18   0.4 2     2    3   113  750                         
(10)    0.35   0.15   0.5 2     2    3   110  680                         
(11)    0.28   0.12   0.6 2     2    3   108  650                         
(12)    0.21   0.09   0.7 2     2    3    80   90                         
______________________________________                                    
 Note: Tci is a temperature where perfect zero resistance was observed. Th
 phase obtained was a tetragonal system having the lattice constant of a =
 3.8 Å and c = 15.3 Å in all samples prepared.                    
 
    
     
                       TABLE 3 (2)                                                 
______________________________________                                    
         Composition of                                                   
Sample   the powder mixture  Tci    Jc                                    
No       TI    Bi    Pb    Sr  Ca    Cu  (K)  (A/cm.sup.2)                
______________________________________                                    
Comparative                                                               
Example I                                                                 
(13)     0.9   0.1   --    2   2     3    78   10                         
(14)     0.1   0.9   --    2   2     3    80  100                         
(15)     0     2     --    2   2     3    80   90                         
(16)     1     0     --    2   2     3    75  --                          
(17)     0     1.6   0.4   2   2     3   103  120                         
______________________________________                                    
 Note: Tci is a temperature where perfect zero resistance was observed    
 
    
     
                       TABLE 4                                                     
______________________________________                                    
         Composition of                                                   
Sample   the powder mixture  Tci    Jc                                    
No       TI    Bi    Pb    Sr  Ca    Cu  (K)  (A/cm.sup.2)                
______________________________________                                    
Example II                                                                
(18)     0.7   0.3   --    2   1     2   86   120                         
(19)     0.6   0.4   --    2   1     2   90   150                         
(20)     0.4   0.6   --    2   1     2   85   120                         
(21)     0.2   0.8   --    2   1     2   85   120                         
(22)     0.6   0.3   0.1   2   1     2   95   300                         
Comparative                                                               
Example II                                                                
(23)     0.9   0.1   --    2   1     2   78    10                         
(24)     0.1   0.9   --    2   1     2   78    20                         
(25)     0     2     --    2   1     2   78    20                         
(26)     1     0     --    2   1     2   75   --                          
(27)     0     1.6   0.4   2   1     2   90   180                         
______________________________________                                    
 Note: Tci is a temperature where perfect zero resistance was observed    
 
    
     
                                           TABLE 5                                 
__________________________________________________________________________
       Composition of                                                     
Sample the powder mixture                                                 
                   Lattice  Tci                                           
                               Jc                                         
No     Bi                                                                 
         TI                                                               
           Pb                                                             
             Sr                                                           
               Ca                                                         
                 Cu                                                       
                   constant (Å)                                       
                          y (K)                                           
                               (A/cm.sup.2)                               
__________________________________________________________________________
Example III                                                               
(28)   1.2                                                                
         0.8                                                              
           --                                                             
             2 2 3 a =  3.8                                               
                          2 100                                           
                               450                                        
                   c = 15.3                                               
                   tetragonal                                             
(29)   0.8                                                                
         1.2                                                              
           --                                                             
             2 2 3 a =  3.8                                               
                          2 105                                           
                               550                                        
                   c = 15.3                                               
                   tetragonal                                             
(30)   1.0                                                                
         0.6                                                              
           0.4                                                            
             2 2 3 a =  3.8                                               
                          2 110                                           
                               500                                        
                   c = 15.8                                               
Comparative                                                               
Example III                                                               
(31)   2 --                                                               
           --                                                             
             2 2 3 a =  5.41                                              
                          --                                              
                             78                                           
                                10                                        
                   c = 36.8                                               
                   a =  5.41                                              
                   c = 30.6                                               
(32)   1.6                                                                
         0.4                                                              
           --                                                             
             2 2 3 a =  5.41                                              
                          --                                              
                            103                                           
                               120                                        
                   c = 36.8                                               
__________________________________________________________________________
 Note: Tci is a temperature where perfect zero resistance was observed    
 
    
     EXAMPLE 4 
     Oxide superconducting materials according to the present invention were prepared by the process according to the present invention. Powders of Tl 2  O 3 , CaO, BaO 2 , Cs 2  CO 3  and CuO (purity is higher than 99.9%) used as materials were weighted and mixed in the proportions shown in Table 6 to prepare five samples of powder mixture. Then, the resulting powder mixtures were compacted into pellets. The resulting each pellet was wrapped with a gold foil and was sintered at a temperature of 850° C. for 12 hours in oxygen gas stream. For a comparison, a comparative superconducting material was prepared by sintering a powder mixture containing the same materials except Cs 2  CO 3 . 
     The resulting all samples of oxide superconductors were found that they contained compound oxide represented by the general formula: 
     
         Tl.sub.y Ca.sub.z (Ba.sub.(l-r) Cs.sub.r).sub.s Cu.sub.v O.sub.w 
    
     in which &#34;y&#34;, &#34;z&#34;, &#34;v&#34;, &#34;w&#34;, &#34;r&#34; and &#34;s&#34; are numbers each satisfying respective range of 0.5≦y≦3.0, 0.5≦z≦4.0, 2.0≦v≦5.0, 5.0≦w≦15.0, 0≦r≦0.8 and 0.5≦s≦3.0. 
     On the resulting oxide superconductors, the critical temperature Tc and the critical current density Jc at liquid nitrogen temperature were measured. The results are summarized in Table 6. 
     All of the oxide superconducting materials according to the present invention show much higher critical current density Jc than known oxide superconducting material while same or even higher critical temperature Tc is observed. 
     
                       TABLE 6                                                     
______________________________________                                    
         Composition of                                                   
Sample   the powder mixture  Tci    Jc                                    
No       TI    Ca    Ba     Cs   Cu    (K)  (A/cm.sup.2)                  
______________________________________                                    
Comparative                                                               
         2     2     2.0    --   3     110  300                           
(1)      2     2     1.95   0.05 3     113  280                           
(2)      2     2     1.8    0.2  3     116  350                           
(3)      2     2     1.0    1.0  3     116  400                           
(4)      2     2     0.4    1.6  3     117  450                           
(5)      2     2     0.2    1.8  3     --*  --*                           
______________________________________                                    
 Note: *: no superconductor above 4.2 K.                                  
 
    
     EXAMPLE 5 
     Oxide superconducting materials according to the present invention were prepared by the same process as Example 4 except that Bi 2  O 3  and PbO were added additionally. The resulting oxide superconductor was evaluated by the same method as Example 4. Table 7 shows the composition of the material powder and the properties of the resulting oxide superconductor. 
     The resulting oxide superconducting materials was found that it contained compound oxide represented by the general formula: 
     
         (Tl.sub.(l-p-q) Bi.sub.p Pb.sub.q).sub.y Ca.sub.z (Ba.sub.(l-r) Cs.sub.r).sub.s Cu.sub.v O.sub.w 
    
     in which &#34;y&#34;, &#34;z&#34;, &#34;v&#34;, &#34;w&#34;, &#34;p&#34;, &#34;q&#34;, &#34;r&#34; and &#34;s&#34; are numbers each satisfying respective range of 0.5≦y≦3.0, 0.5≦z≦4.0, 2.0≦v≦5.0, 5.0≦w≦15.0, 0≦p≦0.5, 0≦q≦0.5, 0≦r≦0.8 and 0.5≦s≦3.0. 
     This oxide superconducting material according to the present invention shows much higher critical current density Jc than known oxide superconducting material while same or even higher critical temperature Tc is observed. 
     
                       TABLE 7                                                     
______________________________________                                    
Composition of                                                            
the powder mixture       Tci    Jc                                        
Sample No                                                                 
        TI    Bi    Pb  Ca    Ba   Cs  Cu  (K)  (A/cm.sup.2)              
______________________________________                                    
(6)     1.6   0.2   0.2 2     1    1   3   116  350                       
______________________________________                                    
 
    
     EXAMPLE 6 
     Oxide superconducting materials according to the present invention were prepared by the same process as Example 4 except that BaO 2  is replaced by SrO 2 . The resulting oxide superconductor was evaluated by the same method as Example 4. Table 8 shows the composition of the material powder and the properties of the resulting oxide superconductor. 
     All of the resulting oxide superconducting materials were found that they contained compound oxide represented by the general formula: 
     
         Tl.sub.y Ca.sub.z (Sr.sub.(l-r) Cs.sub.r).sub.s Cu.sub.v O.sub.w 
    
     in which &#34;y&#34;, &#34;z&#34;, &#34;v&#34;, &#34;w&#34;, &#34;r&#34; and &#34;s&#34; are numbers each satisfying respective range of 0.5≦y≦3.0, 0.5≦z≦4.0, 2.0≦v≦5.0, 5.0≦w≦15.0, 0≦r≦0.8 and 0.5≦s≦3.0. 
     These oxide superconducting materials according to the present invention show much higher critical current density Jc than known oxide superconducting material while same or even higher critical temperature Tc is observed. 
     
                       TABLE 8                                                     
______________________________________                                    
         Composition of                                                   
         the powder mixture  Tci    Jc                                    
Sample No.                                                                
         TI    Ca    Sr     Cs   Cu    (K)  (A/cm.sup.2)                  
______________________________________                                    
Comparative                                                               
         2     2     2.0    --   3      70  --                            
 (7)     2     2     1.95   0.05 3      72  --                            
 (8)     2     2     1.8    0.2  3     100  280                           
 (9)     2     2     1.0    1.0  3     105  400                           
(10)     2     2     0.4    1.6  3     103  350                           
(11)     2     2     0.2    1.8  3      --*  --*                          
______________________________________                                    
 NB: *: no superconductor above 4.2 K.                                    
 
    
     EXAMPLE 7 
     Oxide superconducting material according to the present invention was prepared by the same process as Example 6 except that Bi 2  O 3  and PbO were added additionally. The resulting oxide superconductor was evaluated by the same method as Example 6. Table 9 shows the composition of the material powder and the properties of the resulting oxide superconductor. 
     The resulting oxide superconducting material was found that it contained compound oxide represented by the general formula: 
     
         (Tl.sub.(l-p-q) Bi.sub.p Pb.sub.q).sub.y Ca.sub.z (Sr.sub.(l-r) Cs.sub.r).sub.s Cu.sub.v O.sub.w 
    
     in which &#34;y&#34;, &#34;z&#34;, &#34;v&#34;, &#34;w&#34;, &#34;p&#34;, &#34;q&#34;, &#34;r&#34; and &#34;s&#34; are numbers each satisfying respective range of 0.5≦y≦3.0, 0.5≦z≦4.0, 2.0≦v≦5.0, 5.0≦w≦15.0, 0≦p≦0.5, 0≦q≦0.5, 0≦r≦0.8 and 0.5≦s≦3.0. 
     The oxide superconducting material according to the present invention show much higher critical current density Jc than known oxide superconducting material while same or even higher critical temperature Tc is observed. 
     
                       TABLE 9                                                     
______________________________________                                    
        Composition of                                                    
        the powder mixture   Tci    Jc                                    
Sample No                                                                 
        TI    Bi    Pb  Ca    Sr   Cs  Cu  (K)  (A/cm.sup.2)              
______________________________________                                    
(12)   1.6    0.2   0.2 2     1    1   3   108  380                       
______________________________________