Abstract:
Superconductors having a composition Bi 2-a-b-c  Sn a  Pb b  Sb c  Sr 2  Ca n-1  Cu n  O 2n+4  can be made by firing the appropriate mixtures of designated oxides or carbonates. In the composition a=about 0.001-0.4, b about 0-0.4, c about 0-0.4, and n 2 or 3. The addition of Sn improves the superconductive properties of comparable compositions that lack Sn. For example, Bi 1 .6 Sn 0 .4 Sr 2  Ca 2  Cu 3  O x  (x=ca 10), calcined at 830° C. for 15 hours shows superconductivity by magnetic susceptibility tests at about 77° K. whereas Bi 2  Sr 2  Ca 2  Cu 3  O 10  prepared under the same conditions but lacking Sn, shows nothing at this temperature.

Description:
RELATED APPLICATION 
     U.S. Ser. No. 324,482 filed Mar. 16, 1989, N. D. Spencer, and owned by the same assignee hereof, discloses a ceramic mixed oxide of Bi-Pb-Sb-Sr-Ca-Cu. 
     FIELD OF THE INVENTION 
     This invention relates to the preparation of ceramic multi-metal oxides. 
     BACKGROUND OF THE INVENTION 
     It has long been known that the effective resistivity of certain metals was sometime substantially eliminated when the metal was exposed to low temperature conditions. Of particular interest were the metals and metal oxides which can conduct electricity under certain low temperature conditions with virtually no resistance. These have become known as superconductors. Certain metals, for example, are known to be superconductive when cooled to about 4° on the Kelvin scale (° K.), and certain niobium alloys are known to be superconductive at about 15° K., some as high as about 23° K. 
     Discovery of superconductivity in the system La-Ba-Cu-O (J. G. Bednorz and K. A. Muller, Zeit. Phys. B 64, 189-193 [1986]) has stimulated the search for other systems, particularly with a view to substituting other elements for the rare earths (RE) used in the earlier materials. For example, replacement of RE by Bi and Tl has been reported. (See M. A. Subramanian et al.,) Science, 239, p. 1015 (1988); L. Gao et al., Nature, 332, pp. 623-624 (1988). 
     Superconductors of the formula Bi 2  Sr 2  Ca n-l  Cu n  O 2n+4  where n=1, 2, or 3 are known. See Science, 242, 1519 (Dec. 16, 1988). Researchers have replaced some of the Bi with Sb and/or Pb. Thus Bi-Pb-Sb-Sr-Ca-Cu-O superconductors have been made by heating together Bi 2  O 3 , CuO, SrCO 3 , CaCO 3 , PbO and Sb 2  O 5  in air, at, e.g. 830° C. for 15 hours, followed by grinding, pelletizing, sintering at 880° C. for 12 hours, followed by furnace cooling in air. Processes of this type are disclosed in the following preprints: 
     Mao, X., et al, The Influence of Pb composition on the Upper Critical Magnetic Field of Bi 1 .9-x Pb x  Sb 0 .1 Sr 2  Ca 2  Cu 3  O y  system. 
     Liu, H. et al, Zero Resistance at 132° K. in the Multiphase System of Bi 1 .9-x Pb x  Sb 0 .1 Sr 2  Ca 2  Cu 3  O y  with x=0.3, 0.4. 
     Liu, H., et al, Superconducting Transition above 160° K. in Bi-Pb-Sb-Sr-Ca-Cu-O System. 
     Liu, H. et al, Superconducting Properties in (Bi 2-x-y  Pb x  Sb y )Sr 2  Ca 2  Cu 3  O z  System (x=0, 0.1, 0.3, 0.5; y=0, 0.1). 
     A published article is similar but omits PbO: 
     Liu et al, The Superconducting Properties in Bi 2-x  Sb x  Sr 2  Ca 2  Cu 3  O 4  Compounds (x=0.05, 0.1, 0.15, 0.2) Physica C 156, pp. 804-806 (1988). 
     High Tc Update, (Nota Bene), Jan. 15, 1989, describes Bi-Pb-Sb-Sr-Ca-Cu-O y  superconductors. 
     So far as I have been able to determine, Sn has never before been used in an oxide superconductor. 
     SUMMARY OF THE INVENTION 
     The invention is directed to the preparation of novel ceramic oxides of the formula 
     
         Bi.sub.2-a-b-c Sn.sub.a Pb.sub.b Sb.sub.c Sr.sub.2 Ca.sub.n-l Cu.sub.n O.sub.2n+4 
    
     where a=about 0.001-0.4, b=about 0-0.4, c=about 0-0.4, the sum a+b+c is about 0.001-0.4, and n=2 or 3. Suitably, a=about 0.1-0.4, b=about 0.1-0.4, c=about 0.1-0.2, and (a+b+c)=about 0.4-0.5. Typically the new compounds are superconductors, though they may have other uses in the general field of ceramics, e.g., as insulators, glazes, circuit boards, etc. 
     The new compounds may be regarded as Bi 2  Sr 2  Ca 2  Cu 3  O x  (x=10), a known superconductor, in which some of the Bi is replaced with Sn, with or without addition of Pb and/or Sb. The Sn addition improves the superconductivity of the Bi-Sr-Ca-Cu-oxide. 
     The new compounds can be made by mixing the appropriate oxides or carbonates of the designated metals, followed by compression pelletization, firing, comminution, and repeating the pelletization and firing. 
     THE FIGURE 
     The FIGURE provides a comparison of superconducting properties of three Bi-Sr-Ca-Cu containing oxides. The comparison standard is Bi 2  Sr 2  Ca 2  Cu 3  O x . In another compound, some of the Bi is replaced with Pb, and in the third (Example 2 below), some of the Bi is replaced with Sn. All products were processed similarly, viz., pellets were tested after firing at 830° C. in air for 15 hours. Magnetic susceptibility of the tin-substituted sample (this invention) clearly has a sharper superconducting transition (sharp downward turn) than the other two samples. The lead-substituted sample has some superconducting phase (note the &#34;blip&#34;), but the non-substituted sample (i.e., Bi only) is not superconducting. On further calcining (865° C., 60 hours) the Bi 2  Sr 2  Ca 2  Cu 3  O x  sample showed a small &#34;blip&#34;. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Sn-containing Bi-Sr-Ca-Cu-oxide can be made using processes available for making Bi 2  Sr 2  Ca 2  Cu 3  O x  itself. For example, the appropriate individual oxides or carbonates (as the case may be) are intimately mixed, then pelletized, fired, ground, pelletized, and fired again. As a variant, a portion of the oxide/carbonate mix (cf Example 1) may be initially prepared, then ground with the remaining oxide/carbonate components. Various other processes are available. For example, a solution of nitrates of the metals may be treated with a quaternary ammonium carbonate to precipitate the metals as mixed carbonates, followed by drying the precipitate, and firing. Or the solution of the metal salts (as nitrates, formates, acetates, etc.) may be spray dried, and the product calcined. 
     The following examples illustrate without limiting the invention. 
     The type of these starting compounds is not critical provided the compound is calcinable to the oxide. Suitable compounds include oxides, carbonates, bicarbonates, nitrates, mono- and di-carboxylates (formates, acetates, oxalates, etc.), and the like, and mixtures thereof. 
     EXAMPLE 1 
     Sr 2  Ca 2  Cu 3  -oxygen Base Mixture 
     For ease of preparing Sn-containing Bi-Sr-Ca-Cu oxides with various amounts of Sn, I have found it convenient first to prepare a mixture of carbonates of Sr and Ca, with Cu oxide, as follows: 
     After vacuum-drying at 100° C. for 48 hours, a 200 g batch of SrCO 3 , CaCO 3 , and CuO in an Sr:Ca:Cu atomic ratio of 2:2:3 was wet-ball milled in a polyurethane-lined jar using zirconia balls for about 5 hours. The resulting mixture was dried in an air oven, sieved (100-mesh sieve) in air, and then the material was stored in a vacuum oven. 
     The base mixture can be compressed, fired, and ground prior to addition of the other components, but this is not necessary. 
     When making a given Sn-containing composition of the invention, it is not necessary to start with the base mixture as above described. It is also convenient simply to start with a mixture of Bi 2  O 3 , SrCO 3 , CaCO 3 , CuO, and SnO 2  (or SnO), with or without one or more of the other additives herein described, i.e., PbO (or PbO 2  or Pb 3  O 4 ) and Sb 2  O 3  (or Sb 2  O 4  or Sb 2  O 5 ). These materials are intimately mixed in amounts to provide atomic ratios of Bi 2-a-b-c  Sn a  Pb b  Sb c  Sr 2  Ca n-l  Cu n  O 2n+4 , as explained in the Summary above. Obviously, if b is zero, no Pb is present, and if c is zero, no Sb is present. 
     The amount of oxygen in the products of this invention will depend on the precise final mixture. In the typical case it is close to theory. Thus, for Bi 2  Sr 2  Ca 2  Cu 3  O x , x would calculate theoretically as (3+2+2+3)=10, and indeed this particular compound is generally reported with x=10. When Bi is partially replaced by SnO 2 , additional oxygen is put in the composition. Thus for Bi 1 .6 Sn 0 .4 Sr 2  Ca 2  Cu 3  O x , instead of 3 oxygens for Bi, theory requires [(1.6/2)×3]+(0.4×2)=3.2, so that theoretically x=10.2. Similar calculations can be made for the other Sn-modifications of Bi 2  Sr 2  Ca n-l  Cu n  O 2n+4  herein described. 
     EXAMPLE 2 
     
         ______________________________________Bi.sub.1.6 Sn.sub.0.4 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x ; x = ca______________________________________10Bi.sub.2 O.sub.3     6.3882 g.SnO.sub.2            1.0327 g.Sr.sub.2 Ca.sub.2 Cu.sub.3 -oxygen Base Mixture                12.5792 g.______________________________________ 
    
     The above materials were ground together by mortar and pestle, pressed into pellets (about 3/4&#34; diameter, about 8 g) at 20,000 psi, and the pellets were fired at about 830° C. for 15 hours in air. The pellets of this first firing showed superconductivity at about 77° K. by magnetic susceptibility tests. (See FIGURE.) The fired pellet was cooled, reground with mortar and pestle, repressed, then refired at about 865° C. in air for 60 hours. The pellet of this second firing showed superconductivity at 115° K. by magnetic susceptibility. The pellet when submerged in liquid nitrogen (77° K.) floated a chip of rare earth magnet, demonstrating superconductivity by the Meissner effect. 
     EXAMPLE 3 
     
         ______________________________________Bi.sub.1.5 Pb.sub.0.4 Sn.sub.0.1 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x ; x =ca 10______________________________________Bi.sub.2 O.sub.3     5.8840 g.PbO                  1.5030 g.SnO.sub.2            0.2535 g.Sr.sub.2 Ca.sub.2 Cu.sub.3 -oxygen Base Mixture                12.3591 g.______________________________________ 
    
     The above materials were mixed and processed as in Example 2. The pellet of the first firing showed a T c  of 74° K. by magnetic susceptibility; the pellet of the second firing when immersed in liquid nitrogen floated a small rare earth magnet, indicating superconductivity by the Meissner effect (at 77° K.). 
     EXAMPLE 4 
     
         ______________________________________Bi.sub.1.5 Pb.sub.0.3 Sn.sub.0.1 Sb.sub.0.1 Sr.sub.2 Ca.sub.2 Cu.sub.3O.sub.x ; x = ca 10______________________________________Bi.sub.2 O.sub.3     5.9224 g.PbO                  1.1348 g.SnO.sub.2            0.2552 g.Sb.sub.2 O.sub.3     0.2467 g.Sr.sub.2 Ca.sub.2 Cu.sub.3 -oxygen Base Mixture                12.4407 g.______________________________________ 
    
     The above materials were mixed and processed as in Example 2. the pellet of the first firing showed &#34;blips&#34; in magnetic susceptibility at 85° K. and at 77° K. indicating superconductivity. The pellet of the second firing when immersed in liquid nitrogen floated a small rare earth magnet. 
     EXAMPLE 5 
     
         ______________________________________Bi.sub.1.6 Sb.sub.0.2 Sn.sub.0.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x ; x =ca 10______________________________________Bi.sub.2 O.sub.3     6.3921 g.Sb.sub.2 O.sub.3     0.4998 g.SnO.sub.2            0.5168 g.Sr.sub.2 Ca.sub.2 Cu.sub.3 -oxygen Base Mixture                12.5897 g.______________________________________ 
    
     The above materials were mixed and processed as in Example 2. The pellet of the first firing showed &#34;blips&#34; in magnetic susceptibility indicating superconductivity at 86° K. and at 78° K. The pellet of the second firing when immersed in liquid nitrogen floated a small rare earth magnet. 
     FIRING CYCLE 
     The two-step firing cycle described here provides good uniformity without melting intermediate compounds. The first firing is suitably carried out at about 750°-850° C. for about 5°-20 hours. Following this, the product is cooled, ground, re-pelletized, and fired again. The second firing is suitably at about 850°-880° C. for about 20-70 hours. The second firing may not be required in all instances, but in the general case it improves superconductivity of the product. Grinding and compression are not essential. However, grinding aids uniformity, and compression helps the components react as well as keeps the powder together for testing or other shaping. 
     In summary, all of the Sn-containing invention compounds (Examples 2-5), as a result of the first firing, showed superconductivity by magnetic susceptibility measurements, but none floated a magnet. All the invention pellets of the second firing floated a magnet (Meissner effect). 
     In tests carried out so far, use of Sn in accordance with the invention gave results in some respect superior to results when Sn is omitted, other things being equal. (See FIGURE, and discussion, above.) For example, all of the Bi-Sr-Ca-Cu-oxides that lacked Sn showed little or no superconductivity by magnetic susceptibility after firing at 830° C. for 15 hours, whereas all comparable Sn-containing Bi-Sr-Ca-Cu-oxides showed superconductivity (where there was none in the Sn-lacking materials), or improved superconductivity (if there was any in the Sn-lacking materials). Where the Sn-lacking material(s) did show some superconductivity (always faint), addition of Sn to the composition enhanced the indications, as shown by magnetic susceptibility tests. 
     The Sn-lacking Bi-Sr-Ca-Cu-oxides tested included Bi 2  Sr 2  Ca 2  Cu 3  O 10  ; Bi 1 .6 Pb 0 .4 Sr 2  Ca 2  Cu 3  O x  ; Bi 1 .6 Sb 0 .4 Sr 2  Ca 2  Cu 3  O x  ; and Bi 1 .5 Pb 0 .4 Sb 0 .1 Sr 2  Ca 2  Cu 3  O x . 
     The Table summarizes relevant test data. 
     
                       TABLE______________________________________Replacement of Bi in Bi.sub.2 [A][A] = Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x, x = ca 10            AC Susceptibility.sup.(1)            After First Heat TreatmentCompound         (830° C., 15 hours)______________________________________Bi.sub.2 [A]     no superconductivitySn[A]            no superconductivityBi.sub.1.6 Pb.sub.0.4 [A].sup.(2)            Blip at 86° K.Bi.sub.1.6 Sb.sub.0.4 [A].sup.(2)            Blip at 77° K.Bi.sub.1.6 Sn.sub.0.4 [A].sup.(2)            Big drop at 77° K.Bi.sub.1.5 Pb.sub.0.4 Sb.sub.0.1 [A].sup.(3)            Big blip at 77° K.Bi.sub.1.5 Pb.sub.0.4 Sn.sub.0.1 [A].sup.(3)            Big drop at 74° K.______________________________________ .sup.(1) As a sample is cooled and becomes superconducting, the AC magnetic susceptibility drops. Larger drops indicate more of the sample i superconducting. Small drops (&#34;blips&#34;) indicate less of the sample is superconducting. .sup.(2) Lead and antimony help, but adding tin is better. .sup.(3) Antimony helps, but tin is better.