Patent Publication Number: US-5840659-A

Title: Method of preparing oxide superconductive material

Description:
This is a continuation of application Ser. No. 08/487,126, filed Jun. 7, 1995, now abandoned, which is a divisional application of application Ser. No. 08/186,831, filed Jan. 25, 1994, now abandoned, which is in turn a continuation of application Ser. No. 07/891,356, filed May 29, 1992, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an oxide superconductive material and a method of preparing the same, and more particularly, it relates to a Tl--Ba--Ca--Cu--O superconductive material and a method of preparing the same. 
     2. Description of the Background Art 
     In 1988, Bi--Sr--Ca--Cu--O and Tl--Ba--Ca--Cu--O superconductors were discovered with critical temperatures (Tc) exceeding 100 K. Thereafter many researchers have been made to develop superconductive materials having critical temperatures of at least 120 K, such as Tl 2  Ba 2  Ca 2  Cu 3  O 10  (Tc=125 K) and TlBa 2  Ca 3  Cu 4  O 11  (Tc=122 K). 
     However, although Tl 2  Ba 2  Ca 2  Cu 3  O 10  (2223 phase) and TlBa 2  Ca 3  Cu 4  O 11  (1234 phase) having critical temperatures of at least 120 K completely lose electrical resistance at 125 K and 122 K respectively, methods of preparing such materials are so delicate that it has been difficult to prepare a material which exhibits superconductivity under a temperature exceeding 125 K with excellent reproducibility. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a 2223-phase oxide superconductive material which exhibits superconductivity under a temperature exceeding 125 K, and a method of preparing the same. 
     The inventive oxide superconductive material is expressed in the following composition formula: 
     
         Tl.sub.x Ba.sub.2 Ca.sub.y Cu.sub.3 O.sub.z 
    
     where x, y and z are in relations satisfying 1.5≦x≦2.0, 2.0≦y≦2.5, x+y=4.0 and 9.0≦z≦11.0, and comprises tetragonal system superconducting phases having lattice constants of a=0.385 to 0.386 nm and c longer than 3.575 nm, that is, 0.385&lt;a&lt;0.386, and c&gt;3.575. 
     The oxide superconductive material according to the present invention preferably exhibits superconductivity under a temperature of at least 125 K. 
     The inventive method is employable for preparation of the aforementioned oxide superconductive material. In more concrete terms, the inventive method is adapted to prepare an oxide superconductive material which is expressed in the following composition formula: 
     
         Tl.sub.x Ba.sub.2 Ca.sub.y Cu.sub.3 O.sub.z 
    
     where x, y and z are in relations satisfying 1.5≦x≦2.0, 2.0 ≦y≦2.5, x+y=4.0 and 9.0 and ≦z≦11.0, and comprises a step of mixing powder raw materials in blending ratios for satisfying the composition formula, a step of sintering the as-formed mixed powder in flowing oxygen gas or in air to obtain a sintered body, and a step of annealing the sintered body in a closed atmosphere at 700° to 800° C. for at least 10 hours. 
     According to the present invention, the sintered body is annealed in a closed atmosphere for at least 10 hours, preferably 10 to 500 hours. 
     The oxide superconductive material according to the present invention has lattice constants of a=0.385 to 0.386 nm and c longer than 3.575 nm. Thus, it is possible to optimize distances between copper atoms and oxygen atoms as well as the number of carriers to attain a high critical temperature. The oxide superconductive material completely loses electrical resistance under a temperature of at least 125 K when the c-axis length is at least 3.575 nm, and preferably c=3.575 to 3.580 nm. If the c-axis length is less than 3.575 nm, distances between copper atoms and oxygen atoms in the crystal structure are so extremely reduced that excess carriers are introduced into the structure. When the c-axis length exceeds 3.580 nm, on the other hand, the distances between the copper atoms and the oxygen atoms are so excessively increased that carriers cannot be sufficiently introduced into the structure. 
     According to the present invention, the sintered body is annealed at 700° to 800° C. Due to this annealing, the atoms may conceivably be diffused in the sample. In the present invention, the annealing temperature must be at least 700° C. since the atoms are so insufficiently diffused that the critical temperature is not much increased if the annealing temperature is less than 700° C. If the annealing temperature exceeds 800° C., on the other hand, Tl atoms contained in the sample are extremely evaporated to decompose the 2223 phases, which contribute to superconductivity. Also when the annealing time is too long, the Tl atoms are evaporated to decompose the 2223 phases. 
     The oxide superconductive material according to the present invention exhibits superconductivity under a temperature of at least 125 K, to implement a stable superconducting state. 
     According to the method of the present invention, it is possible to prepare an oxide superconductive material which exhibits superconductivity under a temperature of at least 125 K, with excellent reproducibility. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates resistance/temperature characteristics in relation to Example 1 according to the present invention; 
     FIG. 2 illustrates powder X-ray diffraction patterns in relation to Example 1 according to the present invention; 
     FIG. 3 illustrates resistance/temperature characteristics in relation to Example 2 according to the present invention; 
     FIG. 4 illustrates susceptibility /temperature characteristics in relation to Example 2 according to the present invention; 
     FIG. 5 illustrates a lattice constant c of Example 2 according to the present invention; and 
     FIG. 6 illustrates a lattice constant a of Example 2 according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     EXAMPLE 1 
     (1) Starting raw materials were prepared by commercially available Tl 2  O 3 , BaO 2 , CaO, and CuO powder materials. These powder raw materials were mixed with each other in blending ratios for attaining a blending composition of Tl 1 .7 Ba 2  Ca 2 .3 Cu 3  O 9 .5. 
     The as-formed mixed powder was press-molded into a rectangular parallelopiped of 2 mm by 2 mm by 20 mm, which in turn was wrapped up in gold foil, sintered in flowing oxygen gas at 850° to 900° C. for 5 hours, and cooled in a furnace. 
     (2) The as-obtained sintered body was again wrapped up in gold foil and introduced into a silica tube, which in turn was decompressed with a vacuum pump until its internal pressure was 10 -4  Torr and sealed. This silica tube was introduced into a furnace and annealed at 750° C. for 80 hours. 
     The as-formed sample of Example 1 was taken out from the furnace, and temperature dependency of resistance was measured by a four-probe method. On the other hand, comparative example 1 was prepared by carrying out a step similar to the aforementioned step (1) and then directly annealing the as-obtained sintered body in air at 750° C. for 80 hours, without introducing the same in a silica tube. Then temperature dependency of resistance was measured by a four-probe method similarly to Example 1. FIG. 1 shows the result. 
     As clearly understood from FIG. 1, Example 1 exhibited a critical temperature (zero-resistance temperature) of 125 K, while the comparative example 1 attained absolutely no superconductivity, with no metallic temperature change of resistance. 
     Another comparative example 2 was prepared through a step similar to the aforementioned step (1), followed by no annealing step (2). Example 1 and the comparative examples 1 and 2 were subjected to powder X-ray diffraction analysis. FIG. 2 shows the diffraction patterns. 
     As to the pattern of the comparative example 2 shown in FIG. 2, most peaks are in superconductor structures called 2223 phases, having lattice constants a=0.38 nm and c=3.57 nm. Thus, it is understood that the 2223 phases contributing to superconductivity were already synthesized in the step (1). 
     As to the pattern of Example 1 shown in FIG. 2, most of the peaks are also in 2223 phases, with a relatively large number of peaks which may be impurity phases as compared with the comparative example 2. This is conceivably because vaporizable TQ atoms were evaporated from the sample during the step (2), to break the 2223 phases. 
     On the other hand, the pattern of the comparative Example 1 shown in FIG. 2 hardly has peaks which may conceivably be in 2223 phases, since most of the 2223 phases contributing to superconductivity were decomposed by annealing. Thus, it may conceivably be possible to greatly suppress evaporation of Tl atoms during the annealing step by enclosing the sample with a silica tube. 
     EXAMPLE 2 
     In order to investigate optimum conditions for the annealing step (2) in the aforementioned Example 1, samples of oxide superconductive materials were prepared in a similar manner to Example 1, except for that sintered bodies were heat treated under conditions of annealing temperatures and times shown in Table 1. 
     Table 1 shows zero resistance temperatures of the respective samples and diamagnetic signal starting temperatures obtained by measuring dc susceptibility values. 
     
                       TABLE 1                                                     
______________________________________                                    
                        Diamagnetic                                       
              Zero      Signal    Lattice                                 
Annealing Condition                                                       
              Resistance                                                  
                        Starting  Constant                                
Sam- Temperature                                                          
               Time   Temperature                                         
                              Temperature                                 
                                      a     c                             
ple  (°C.)                                                         
               (hr)   (K.)    (K.)    (nm)  (nm)                          
______________________________________                                    
1    --        --     118     120     0.3855                              
                                            3.571                         
2    600       80     118     120                                         
3    650       80     119     121                                         
4    700       80     123     125                                         
5    750       80     125     126     0.3856                              
                                            3.575                         
6    800       80     124     125                                         
7    850       80     120     122                                         
8    900       80     110     115                                         
9    750       2      120     122                                         
10   750       10     123     126                                         
11   750       150    126     128     0.3854                              
                                            3.576                         
12   750       250    127     130     0.3855                              
                                            3.577                         
13   750       500    126     129                                         
14   750       700    125     127                                         
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     As clearly understood from Table 1, the samples prepared under the annealing conditions of 700° to 800° C. and 10 to 500 hours exhibited high zero resistance temperatures and diamagnetic signal starting temperatures. Particularly the sample No. 12, which was annealed at 750° C. for 250 hours, exhibited the highest zero resistance temperature of 127 K. 
     The samples having c-axis lengths of at least 3.575 nm exhibited zero resistance temperatures of at least 125 K. On the other hand, the a-axis lengths, which had no relation to the zero resistance temperatures, were substantially constant in a range of 0.385 to 0.386 nm Thus, it is understood that a zero resistance temperature of at least 125 K can be obtained when the a-axis length is 0.385 to 0.386 nm and the c-axis length is at least 3.575 nm. Particularly the sample No. 12 having the lattice constants of a=0.3855 nm and c=3.577 nm exhibited the highest zero resistance temperature of 127 K. 
     FIG. 3 shows resistance/temperature characteristics of the sample No. 12 according to Example 2 and those of the comparative example 2, which was not annealed. 
     FIG. 4 illustrates susceptibility/temperature characteristics of Example 2 (sample No. 12) and the comparative example 2. 
     As clearly understood from FIGS. 3 and 4, the sample No. 12 according to Example 2 exhibited the zero resistance temperature and the diamagnetic signal starting temperature at 127 K. 
     FIGS. 5 and 6 illustrate lattice constants of the inventive samples Nos. 5, 11 and 12 according to Example 2 of the present invention and the sample No. 1, which was out of the annealing conditions for the inventive method, respectively. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.