Process for preparing YBa2 Cu3 O7-x superconductors
This application is a continuation of application Ser. No. 08/066,418, filed May 25, 1993. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an improvement in a process for preparing YBa 2  Cu 3  O 7-x  superconductors having a high critical current density. More particularly, the present invention is concerned with an isothermal peritectic reaction technique for preparing YBa 2  Cu 3  O 7-x  superconducting wires or tapes. 
     2. Description of the Prior Art 
     The discovery of high temperature superconductivity in copper-oxide compounds led to intense investigations and researches thereon owing to their immense industrial applicability. In contrast to the initial expectations, the progress toward major applications of the oxide-based superconducting wires and tapes has been hampered by their low critical current density values. 
     Specifically, polycrystalline YBa 2  Cu 3  O 7-x  (hereinafter, referred to as &#34;123&#34;) superconductors have a lower critical current density (about 10 2  A/cm 2 ) compared with single crystal or epitaxially grown thin films. It has been reported that such a low critical current density is mainly due to the anisotropy of electric conductivity, the second phases or impurities present in the grain boundaries, low sintering temperature, the presence of micro-crack and stress zone, and so forth. See, J. W. Ekin, Adv. Ceram. Mat., 2[3B], p.586 (1987); and S. Jin, T. H. Tiefel, R. C. Sherwood, M. E. Davis, R. B. van Dover, G. W. Kammlott, and R. A. Fastnacht, Phys. Rev. B., 37, p.7850 (1988). 
     It was discovered by Dimos et al. that the ratio of the critical current density at the grain boundaries to the critical current density within the grain decreases rapidly as the anisotropic angle of 123 bicrystal increases. See, P. Chaudhari, J. Mannhart, D. Dimos, C. C. Tsuei, J. Chi, M. M. Oprysko, and M. Scheuermann, Phys. Rev. Lett., 60, p.1653 (1988); D. Dimos, P. Chaudhari, J. Mannhart, and F. K. LeGoues, Phys. Rev. Lett., 61, p.219 (1988); J. Mannhart, P. Chaudhari, D. Dimos, C. C. Tsuei, and T. R. McGuire, Phys. Rev. Lett., 61, p.2476 (1988); and D. R. Clarke, T. M. Shaw, and D. Dimos, J. Am. Ceram. Soc., 72[7], p.1103 (1989). The above discovery indicates that such a low critical current density of oxide-based polycrystalline superconductors depends on the properties intrinsic to the grain boundaries and is affected concomitantly by the extrinsic factors such the second phases, micro-crack, low sintered density, and so on. 
     In order to improve the critical current density of 123 superconductors, it is necessary to minimize the problems involved in the grain boundaries by aligning the 123 grains in a direction. It has been reported that superconductors having a high critical current density of above 10 4  A/cm 2  can be obtained by aligning the grains in a direction so that only the low angle grain boundaries are arranged in the direction of the current flow. See, S. Jin, T. H. Tiefel, R. C. Sherwood, M. E. Davis, R. B. van Dover, G. W. Kammlott, and R. A. Fastnacht, Phys. Ref. B, 37, p.7850 (1988); M. Murakami, M. Morita, K. Doi, K. Miyamoto, and H. Hamada, Jpn. J. Appl. Phys., 28[3], L399 (1989); M. Murakami, M. Morita, K. Doi, K. Miyamoto, Jpn. J. Appl. Phys, 28[7], p.1189 (1989); M. Murakami, H. Fujimoto, T. Oyama, S. Gotoh, Y. Shiohara, N. Koshizuka, and S. Tanaka, High Tc Information Service, Vol 3, No. 5 (1990); K. Salama, V. Selvamanickam, L. Gao and K. Sun, Appl. Phys. Lett., Vol. 54[23], p.2352 (1989). 
     The above prior art processes comprises heating a pre-sintered 123 specimen to a temperature higher than the peritectic temperature of 123 to obtain a melt and Y 2  BaCuO 5  (hereinafter, referred to as &#34;211&#34;) phase, and very slowly cooling the resultant. The time usually needed to prepare a wire of 1-2 cm long is a few tens of hours. Therefore, the process requires an extremely long period of time and a high temperature of above about 1,000° C. 
     K. Bose et al. proposes a process for preparing super-conducting wires or tapes, which comprises mixing YBa 2  Cu 3  O 7-x  powder with certain polymers, and compressing the resulting mixture into a wire or tape shape followed by sintering below the peritectic temperature of 123. See, K. Bose, &#34;A Simple Method for the Preparation of YBa 2  Cu 3  O x  Superconducting Wires and Tapes Using Common Polymers,&#34; Jap. J. Appl. Phys. Vol. 30[5A], p. L823 (1991). According to this process, 123 wires or tapes can be prepared in a continuous manner at a relative low temperature for a short period of time. However, the resulting 123 super-conductors are hardly used in the practical application owing to their critical current density less than about 10 2  A/cm 2 . 
     We, the inventors of the present application, have intensively conducted a wide range of experiments in order to solve the problems encountered in the prior art processes. As a result, the inventors have now developed an isothermal peritectic reaction (IPR) process by which superconductors having a high critical current density can be obtained by heat-treatment at a lower temperature and for a shorter period of time as compared with the conventional melting techniques. 
     SUMMARY OF THE INVENTION 
     It is, therefore, object of the invention to provide an improved process for preparing superconductors having a high critical current density by means of isothermal peritectic reaction (IPR) process. 
     It is another object of the invention to provide a process for preparing superconducting wires or tapes in a continuous manner at a relatively low temperature for a short period of time. 
     It is still another object of the invention to provide superconductors having a high critical current density. 
     Any additional objects of the invention will become apparent through reading the remainder of the specification. 
     These and other objects of the invention have been accomplished by an improved process of the invention which comprises surrounding a pre-sintered cold-molded 211 material with liquid-forming powder to give a compact and subjecting the compact to isothermal heat-treatment at a temperature below the peritectic reaction temperature of 123 superconductors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 illustrates micrograph showing well aligned 123 grains in a disc-type specimen prepared by IPR process developed in the present invention. 
     FIG. 2 illustrates micrograph showing poorly aligned 123 grains in a disc-type specimen. 
     FIG. 3 illustrates micrograph showing well aligned 123 grains in a wire-type specimen prepared by IPR process developed in the present application. 
    
    
     DISCLOSURE OF THE INVENTION 
     According to the present invention, the sintered cold-molded material is first prepared. The preparation method thereof is known in the art. For example, Y 2  O 3 , BaCO 3  and CuO powders are mixed in a Y:Ba:Cu molar ratio of 2:1:1, and the mixture is then calcined at a temperature of about 900° to 950° C. for about 20 to 25 hours to synthesize 211 powder. The resulting powder is cold-molded, for example, in the form of disc or wire under pressure and sintered in air at a temperature of 1,000° to 1,100° C. for about 10 to 20 hours. 
     Independently, BaCO 3  and CuO powders are mixed in a Ba:Cu molar ratio of 3:5, and then calcined at a temperature of about 850° to 900° C. for about 25 to 30 hours in order to make the liquid-forming powder. 
     The 211 sintered cold-molded material is surrounded with the liquid-forming powder. The resulting compact is then subject to heat-treatment under oxygen atmosphere at a temperature of about 800° to 1,000° C. for 15 to 50 hours, preferably 20 hours to obtain 123 phase. During the heat-treatment, the following reaction occurs. 
     
         Y.sub.2 BaCuO.sub.5 +L(3BaCuO.sub.2 +2CuO)→2YBa.sub.2 Cu.sub.3 O.sub.7-x +(2x-1)/2 O.sub.2 ↑ 
    
     wherein, L represents a liquid phase at the temperature of the heat-treatment, namely, below the peritectic temperature, and x is less than 1. 
     The 123 superconductor with an aligned grain structure in the form of a disc is prepared by conducting heat-treatment at 945° C. for 20 hours under oxygen atmosphere. Alternatively, the heat-treatment may be carried out in two steps of heating at 945° C. for 6 hours and then at 965° C. for 14 hours. In order to prepare a 123 superconducting wire, it is preferred to use an elevated temperature of about 10° C. higher than that in the case of producing the same in the form of a disc. 
     The process for preparing YBa 2  Cu 3  O 7-x  by using the IPR process according to the invention, in which the liquid-forming powder is infiltrated into the 211 pre-sintered cold-molded material at a temperature below the peritectic temperature of 123 phase has advantages that: 
     First, the process is completed within a relatively short period of time and can be carried out continuously; 
     Second, a dense 123 phase can be obtained, attributing to the liquid phase participated in the reaction; 
     Third, an aligned grain structure can be obtained, attributing to the direction of the liquid infiltration from the surface of the sintered cold-molded 211 material; and 
     Fourth, the pinning effect by the 211 grains distributed within the formed 123 grains can be expected in the final product. 
     PREFERRED EMBODIMENT OF THE INVENTION 
     The present invention will be described in greater detail by way of the following examples. The examples are presented for illustrative purpose only and should not be construed as limiting the invention which is properly delineated in the claims. 
     EXAMPLE 1 
     Five sets of YBa 2  Cu 3  O 7-x  superconductors in the form of disc were prepared according to the following procedures: 
     BaCO 3  and CuO powders were mixed in a Ba:Cu molar ratio of 3:5. The resulting mixture was calcined at 870° C. for 26 hours to synthesize liquid-forming powder (3BaCuO 2  +2CuO). 
     Separately, Y 2  O 3 , BaCO 3  and CuO powders were mixed in a Y:Ba:Cu molar ratio of 2:1:1. The resulting mixture was calcined at 925° C. for 23 hours to synthesize Y 2  BaCuO 5  powder. The Y 2  BaCuO 5  powder was then cold-molded in a disc form under pressure and sintered in air at 1,050° C. for 15 hours to give a 211 compact. 
     Subsequently, the Y 2  BaCuO 5  material thus sintered and cold-molded was surrounded with the liquid-forming powder. The resulting compact was subject to a cycle of heating under oxygen atmosphere to give YBa 2  Cu 3  O 7-x  specimens. The heating temperature and times were shown in Table I below. 
     
                       TABLE I                                                     
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Specimen No.                                                              
          945° C.                                                  
                     965° C.                                       
                                Atmosphere                                
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1         20 hours   --         O.sub.2                                   
2         --         20 hours   O.sub.2                                   
3         40 min.    19 hours plus                                        
                                O.sub.2                                   
                     20 min.                                              
4          2 hours   18 hours   O.sub.2                                   
5          6 hours   14 hours   O.sub.2                                   
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     Among the above, Specimens 1 and 5 showed good alignment of 123 grains as shown in FIG. 1. Although the grains at the surface region are randomly oriented, the grains grown in the interior of the specimen are well aligned toward the center of the specimen. In contrast, the Specimen 2 showed randomly oriented grain structure as shown in FIG. 2. This is believed to be due to the high temperature employed in the heat-treatment. Specimens 3 and 4 also showed randomly oriented grains. This was believed to be due to the shortage of heat-treatment time at 945° C. 
     EXAMPLE 2 
     YBa 2  Cu 3  O 7-x  superconductors in the form of wire were prepared by grinding the 211 sintered material prepared in Example 1 on SiC paper to give specimens in the form of short wire. Each specimen thus obtained was surrounded with the same liquid-forming powder as in Example 1 and then subject to heat-treatment. The wire specimen having a well aligned grain structure, as shown in FIG. 3, was obtained by conducting the heat-treatment at a temperature about 10° C. higher than that in the production of the specimen in the form of disc. The critical current density of the 123 superconducting wire was measured to be 1,200 A/cm 2 . This value is about 10 times higher than that (about 100 A/cm 2 ) of the sintered material prepared by the conventional sintering methods.