WO1996021252A1

WO1996021252A1 - HIGH Tc OXIDE SUPERCONDUCTORS

Info

Publication number: WO1996021252A1
Application number: PCT/US1995/017055
Authority: WO
Inventors: Dmitry M. Kocherginsky; Ramana G. Reddy
Original assignee: The Board Of Regents Of The University And Community College System Of Nevada
Priority date: 1994-12-30
Filing date: 1995-12-29
Publication date: 1996-07-11

Abstract

The present invention provides single-phase, homogenous superconducting materials with critical temperatures of superconducting transition higher than about 133 °K and having a composition represented by the general formula: (AgaAubPtcPbdBie) (Ba2-fSrf)Can-1CunO2n+1+x, where a + b + c + d is between about 0.75 and about 2.0 inclusive, a + b + c is a number greater than 0, f is a number between about 0 and about 2 inclusive; n is an integer between 1 and about 10 inclusive, and x is a number greater than 0. The following are preferred compositions of the present invention: Pt1Ba2Ca2Cu3O9, (Ag0.5Au0.5)Ba2Ca2Cu3O9, (Ag0.75Pb0.25)Ba2Ca2Cu3O9, (Ag0.75Bi0.25)Ba2Ca2Cu3O9, (Ag0.75Pb0.125Bi0.125)Ba2Ca2Cu3O9.

Description

HIGH T_c OXIDE SUPERCONDUCTORS

Field of the Invention

This invention provides superconducting materials, more specifically, superconducting materials comprising noble metals.

Background of the Invention

Superconductors are materials having essentially zero resistance to the flow of electrons below a certain critical temperature, T_c. Certain materials containing copper oxides exhibit superconductivity, e.g. YBa₂Cu₃O_7-y is superconductive below 93.4°K, Bi₂Sr₂Ca₂Cu₃O₁₀ below 110°K, Tl₂Ba₂Ca₂Cu₃O₁₀ below 125°K, and HgBa₂Ca₂CU₃O_8+y below 133ºK. (A. Bourdillon and N. X. Tan Bourdillon, "High Temperature Superconductors: Processing and Science," Academic Press, Inc., San Diego, Ca, 1994, 289p.) The compound Cu_1-xBa₂Ca_n-1Cu_nO_2n+4-8 has its superconductive transition at 116ºK. (H. Ihara, K.Tokiwa, H. Ozawa, M. Hirabayashi, A. Negishi, H. Matuhata, and Y.S. Song, Jpn. J. Appl. Phys., vol. 33 (1994), part 2, No 4A, pp. L503-L506.)

The temperatures at which known materials superconduct are too low for many practical applications. Attempts to produce superconductors having higher critical temperatures (the maximum temperature at which the material conducts electricity without losing energy to resistance), have been made. U.S. Patent No.

5,317,007 reports materials containing Bi, Sr, Ca, Cu, O, and Pb or Al. These materials have critical temperatures of approximately 83° to 95°K. U.S. Patent 5,264,413 describes superconducting materials containing Bi, Sr, Ca, Cu and O formed by molecular beam epitaxial deposition of atomic layers on a substrate. This patent reports that reduction of the amount of Bi in such compounds increases conductivity thereof. D.P.

Norton, B.C. Chakoumakos, J.D. Budai, D.H. Lowndes, B.C. Sales, J.R. Thompson, and D.K. Christen (1994), in "Superconductivity in SrCuO₂-BaCuO₂ Superlattices: Formation of Artificially Layered Superconducting Materials," Science 265:2074-2077, describe superconducting materials having critical temperatures as high as 70°K. This article also mentions superconductive materials having critical temperatures as high as 135°K comprising mercury- containing cuprates. U.S. Patent No. 5,217,945 discloses superconductive materials having critical temperatures up to about 86°K comprised of Tm, Yb or Lu, Ca, Ba, Sr, Cu and O. Non- reproducible, random sightings of superconductivity at temperatures near or above room temperature involving yttrium-barium based, mercury based, bismuth based, and calcium-strontium based copper oxides are reported in R.F. Service (1994), "Superconductivity Researchers Tease out Facts from Artifacts," Science 265: 2014-2015.

Previous attempts to increase the critical temperatures of superconductors by using oxides of noble metals have resulted in limited or no positive effects. Heating Ag₂O with oxides corresponding to the composition of the Bi₂Sr₂Ca,Cu₂O_x superconductor by melting in air followed by casting and quenching in silica glass tubes gave no positive effect on the critical temperature of the resultant material. Apparently Ag was not incorporated into the structure of the superconductor. (W.-H. Lee, Y. Abe, and E. Inukai, "Ag₂O-Doped Bi₂Sr₂Ca,Cu₂O_y Superconductors Prepared via Melt-Quenching, "J. Am. Ceram. Soc., V.76 (1993), 4, 849-56.)

Heating Ag₂O₂ and Au₂O₃ together with superconductor YBa₂Cu₃O_6+δ resulted in inclusions of Ag and Au. (W.D. MacDonald et. al. "Effect of Thermally Unstable Noble Metal Oxide Additions on Structure and Superconducting Properties of YBa₂Cu₃O₆σ," ed. by W.E. Mayo, p.227-233, TMS, Warrendale, 1988.) Again, no positive effect was observed on the critical temperature of the resultant material. U.S. Patents 4,826,808, 5,189,009, and 5,204,318 report a method of preparation of superconducting compounds by oxidation of metallic precursors containing noble metals (Ag, Au, Pt, Pd). Oxidation was done at temperatures at which oxides of these noble metals were unstable. The resultant oxidized materials were described as "composites" of superconductive oxide and a noble metal in which the noble metal was intimately mixed with the oxide phase. Inclusion of the noble metal was reported to improve the mechanical properties of the superconductors. However, the noble metal was not incorporated into the structure of the superconducting oxides and no effect on critical temperatures were observed.

Ihara et al. observed a very limited positive effect on critical temperature by incorporation of a noble metal into the structure of a superconductor. (H. Ihara, K.Tokiwa, H. Ozawa, M. Hirabayashi, H. Matuhata, A. Negishi, and Y.S. Song, Jpn. J. Appl. Phys., vol. 33 (1994), part 2, No 3A, pp. L300-L303.) The superconductor Ag_1-xCu_xBa₂Ca_n-1Cu_n0_2n+1-δ where x varies from 0.25 to 0.75 containing silver was produced under high pressure conditions from BaCaCu-containing oxides combined with Ago or Ag₂O. The authors report a structure in which the Ag atoms in the Ag planes were substituted by Cu. The temperature of superconducting transition of this compound was 117°κ, which is only one degree higher than for the Cu_l-xBa₂Ca_n-1Cu_nO_2n+4-δ superconductor. The ratio Cu:Ca was not n:n-1, where n is an integer as taught herein, and excess Cu substituted for Ag in the silver plane.

In a very recent report, Comert et al. describe incorporation of 0.01% of Ag into a Bi(Pb)SrCaCuO compound by diffusion doping. The Ag-doped material was reported to have a T_c of 118ºK, compared to a T_c of 105ºK for the undoped material and a T_c of 110°K for the high-temperature 2223 phase of the undoped material. (H. Comert, M. Altunbas, T.D. Dzhajarov, T. Kocohomeroglu, Y.G. Asadov and H. Karal, Supercond. Sci. Technol. (1994) 7:824-827.) All publications and patents referred to herein and all references cited in those publications and patents are hereby incorporated by reference in their entirety. Applicants make no admission that references discussed herein are prior art to this invention.

The present invention provides improved superconducting materials containing Ag, Au or Pt, preferably with critical temperature higher than about 133°K, more preferably higher than about 135°K, and still more preferably, higher than about 175°K. Summary of the Invention

Substantially single-phase, homogenous superconducting materials are provided having compositions represented by the general formula:

where a + b+ c + d is a number between about 0.75 and about 2.0 ± 0.2 (preferably about 2.0) inclusive, with a + b + c a number greater than 0, f is a number between about 0 and about 2 inclusive; n is preferably an integer between 1 and about 10 inclusive; and x is a number greater than 0. Preferably x is no more than about 5 and more preferably x is about 2. Compounds containing Ba are preferred over those containing Sr (i.e., f is preferably 2.0).

Compounds of formula I which contain Ag in the absence of other noble metals preferably contain 0.1 at. % or more of Ag. In a related embodiment, this invention provides improved superconducting materials in which a noble metal selected from Ag, Au, Pt or mixtures thereof is substituted for Pb or Bi or both in superconducting materials presently known in the art at a level equal to or greater than about 0.1 at. % in the material. More specifically, this invention provides improved superconducting materials with increased T_c in which Ag is substituted for Pb or Bi or both in superconducting materials presently known in the art at a level equal to or greater than about 0.1 at. %. Preferred improved superconductors with noble metals, particularly Ag, substituted for Pb or Bi are those containing about 1 at. % more of the noble metal. More preferred superconductor materials with a noble metal substituted for Pb or Bi or both are those with T_c equal to or greater than 133°K.

The superconducting materials of this invention are represented by the lattice structure set forth in Figure 3, in which the number of Cu-containing planes is preferably from 1 to about 10. Thus, n in formula I is preferably an integer between about 1 and about 10, plus or minus about 0.1. The top or bottom or both layers or planes in at least 0.1% of the unit cells of the material must comprise at least one element selected from the group Ag, Au or Pt. In a more preferred embodiment, the top or bottom layers or both contain at least one noble metal, Ag, Au or Pt.

The superconducting materials of this invention containing Ag, Au and Pt preferably have critical temperatures higher than about 133°K, more preferably higher than about 135°K, and still more preferably, higher than about 175°K.

These superconducting materials are useful for all purposes known to the art, and can be formed into wires, ribbon, sheets, rods or rings, or other shapes by known means, or can be formed into coatings or thin films on tubes, wires, rods or shaped articles all by means known to the art.

This invention also provides methods of making single-phase, homogenous compounds of a noble metal (Ag, Au or Pt) represented by the general formula given above. These compounds are preferably used as superconductive materials having critical temperatures above about 133°K. The methods of this invention for making the foregoing materials comprise heating stoichiometric amounts of Ag, Au, Pt or combinations thereof, and optionally stoichiometric amounts of Pb or Bi or combinations thereof, with stoichiometric amounts of said Ba or Sr or combinations thereof, and stoichiometric amounts of Cu and optionally stoichiometric amounts of Ca, in the presence of oxygen in the form of oxides and/or carbonates of any of the foregoing metals or free oxygen, for a sufficient time at a sufficient temperature, to form the single-phase, homogenous compounds of this invention. The stoichiometric amounts are those defined by the formula given above for the compositions of this invention.

Once the materials of this invention have been made, they may be tested for their superconductivity and critical temperatures by methods well-known to the art, such as the "four-point probe" method for measuring electrical conductivity.

Structures of the materials of this invention can be determined by standard X-ray techniques, for example those exemplified in Ihara et al. (1994) supra . Brief Description of the Drawings

Figure 1 graphically depicts the effect of temperature on equilibrium species distribution in known superconducting materials.

Figure 2 is a graph of critical temperatures of known superconductors versus the temperatures of completion of reaction to form barium and/or strontium peroxide from barium and/or strontium oxide (lowest temperature of reaction for each species is considered the temperature of completion of reaction).

Figure 3 illustrates the expected lattice structure for superconductors of this invention. Detailed Description of the Preferred Embodiments

The present invention provides new superconductor compositions with critical temperatures higher than about 133°K, more preferably higher than about 135°K, and still more preferably, higher than about 175°K.

These materials are represented by the general formula:

where a + b + c + d is a number between about 0.75 and about ± 2.0 (preferably about 2.0) inclusive, with a + b + c a number greater than 0, f is a number between about 0 and about 2 inclusive; n is an integer between 1 and about 10 inclusive, and x is a number greater than 0. Preferably x is no more than about 5 and more preferably x is 2.0.

The components inside the first set of parentheses in the formula are collectively referred to herein as the "first term" of the formula. The components inside the second set of parentheses in the formula are collectively referred to herein as the "second term" of the formula. The components not enclosed in parentheses are collectively referred to herein as the "third term" of the formula.

Ag, Au and Pt are referred to herein as "noble metals." Pb and Bi are referred to herein as "noble metal substitutes." There must be at least one noble metal present in the composition of the formula I; however, so long as at least one noble metal is present, a noble metal substitute can be used to make up the required stoichiometric amount of the first term of the formula. The first term has a minimum stoichiometric value of about 0.75 and a maximum stoichiometric value of about 2.0 plus or minus 0.2. The terms "homogenous" and "single phase" as used with respect to the superconducting materials of this invention are synonymous and refer to the fact that the materials comprise a composition of the given formula with bonding interactions between the elements thereof, rather than merely a mixture.

The term "substantially homogenous" or "substantially single-phase" as used herein refers to a superconducting material, a major portion of which is homogenous or single-phase as defined above. Materials of this invention may incorporate minor inhomogeneity or minor regions of separate phases so long as superconductivity properties are not significantly adversely affected. For example, as used herein a significant change in T_c is a change of about 10°K or more. It will be appreciated, however, by those in the art that any superconducting material with T_c equal to or greater than about 133°K represents a significant advance in the art. Superconductor materials of this invention also include those in which a noble metal selected from the group Ag, Au, Pt or mixtures thereof is substituted for Pb or Bi or both at a level of 0.1 at. % or more into superconductors presently known in the art. Preferred superconductors with noble metal substitution have 1 at. % or more of the noble metal. Preferred substitutions are those with Ag. Superconductors presently known in the art, including those with Pb and Bi, include without limitation those superconductors described in the patents and publications cited in this specification and those described in references cited in those patents and publications.

The lattice structure shown in Figure 3, represents the structure of the superconductive compositions of this invention. In this structure the top or bottom or both layers or planes at a minimum must comprise in at least 0.1% of the unit cells at least one noble metal (Ag, Au or Pt). The top and bottom layers may comprise a combination of these elements. These noble metals preferably do not appear in the intermediate planes between the top and bottom planes. The top and bottom planes may additionally comprise a noble metal substitute, i . e . Pb or Bi. Preferably, in compounds of formula I, the top or bottom or both layers comprise at least one element Ag, Au or Pt. The top and bottom planes may comprise three or four elements from the first term of the formula. The top and bottom planes preferably do not comprise Cu.

The processing conditions for making the superconducting compositions of this invention involve heating precursor materials together in the correct stoichiometric amounts as described above and in the Examples hereof. The processing conditions (such as pressure and temperature) are chosen such that the oxides of the noble metals or noble metal substitutes remain stable. Stability ranges for these oxides are given in Table 2. For the oxides which are unstable at the processing temperatures and pressures, the process should be carried out by heating a mixture of oxides of the desired elements in a sealed capsule to prevent oxygen loss. When oxides are stable at the processing conditions, the process can be carried out by oxidation of the metallic precursors.

In several of the methods exemplified herein, a precursor material having the formula BaCa₂Cu₃O₆ is heated with an oxide of Ag, Au, Pt or mixtures thereof to form the inventive compositions. In another method exemplified herein, stoichiometric amounts of Ag, Au, Pt or combinations thereof, Ba or Sr or combinations thereof, stoichiometric amounts of Ca and of Cu, all in elemental form, are heated in the absence of oxygen to form an alloy; said alloy is thereafter completely oxidized in oxygen, followed by sintering in a sealed environment, and annealing in the presence of oxygen for a sufficient period of time at a sufficient temperature to form the inventive compound. The processing times and temperatures sufficient to form the single-phase materials of this invention may vary due to the slow nature of the solid state diffusional processes necessary. Those skilled in the art will be able to readily optimize the process conditions for most efficient completion of the reactions. Without being bound by any theory governing the behavior of these materials, applicants postulate that the higher critical temperatures of the superconductive materials of this invention are correlated with the equilibrium species distribution therein.

To find the relationship between the chemical composition of known superconducting materials and their critical temperatures, calculations were performed to study the effect of temperature on the equilibrium species distribution in known superconducting materials. Thermodynamic equilibrium calculations were performed using chemical reaction and equilibrium computer software with an extensive thermochemical database, such as HSC for Windows, Outo-Kumpu Research Oy,

Finland (1993). The input data were the mixture of oxides corresponding to the composition of superconducting materials and the output data were the equilibrium compositions of the material at a given temperature. All calculations assume an ideal solution model and an Ar atmosphere.

Typically this approach is applicable only at high temperatures where the reactions are fast enough to achieve the equilibrium state. However, in a layered structure in which each layer can be represented by a simple binary oxide, the total composition can be considered as a combination of simple binary oxides for such calculations. High T_c superconductors are believed to have such a layered structure as indicated in Figure 3. For example, HgBa₂Ca₂Cu₃O_8+y where y = O (Bourdillon et al., supra) was assumed for purposes of calculation to be a mixture of CuO, CaO, BaO and HgO. The calculations predict that HgO, BaO, CuO and CaO predominate at temperatures higher than 40°K. In a transition interval between about 41°K to 31°K, HgO and BaO are transformed to Hg and Ba0₂ respectively. At temperatures above about 31°K, Hg reacts with Ba0₂ to form HgO and BaO. CuO and CaO remain constant over this transition interval as temperature is increased. Figure 1 is a graph representing the calculated effect of temperature on the equilibrium species distribution in the HgBa₂Ca₂Cu₃O_g system.

Similar calculations were performed for Bi₂Sr₂Ca₂Cu₃O₁₀, Tl₂Ba₂Ca₂Cu₃O₁₀, and Cu_1-xBa₂Ca_n-1Cu_nO_2n+4-δ where δ = 2. These compounds were chosen because they have highest critical temperatures in corresponding superconducting families. For these three superconductors it was predicted that oxygen transfer reactions can take place similar to that in the Hg-based superconductor. In each case, only the oxygen transfer reactions occurring at the highest temperature for each material were taken into account. The lowest temperature at which each reaction could occur was considered to be the temperature of reaction completion. Temperatures of completion for listed reactions for each superconductor together with the critical temperatures of each superconductor are summarized in Table 1. As is apparent, superconductive materials which include hard-to-oxidize materials such as noble metals Ag, Au and Pt as well as noble metal substitutes Pb and Bi will retain BaO₂ (or SrO₂) at higher temperatures and will have higher critical temperatures.

The critical temperature of each known superconductor was plotted versus the calculated temperature of completion of the corresponding highest temperature oxygen transfer reaction. See Figure 2. A good correlation between superconductor critical temperature and the temperature of completion of the highest temperature oxygen transfer reaction of a component metal oxide of the superconductor is observed. The critical temperature is approximately 100°K higher than the calculated reaction temperature.

To design new superconducting compositions with higher critical temperatures, calculations were performed to determine reaction completion temperatures for other oxides available in the thermodynamic databases.

The following reactions are irreversible at all temperatures up to the stability limits of oxides as shown in Table 2.

Based on these thermodynamic calculations and the observed correlation between critical temperature and temperature of oxygen transfer reactions, compounds of the general structure shown in Figure 3, in which the top and bottom layers or planes have Ag, Au, Pt, Pb or Bi atoms, are superconductors having critical temperatures higher than about 133°K. Hard-to-oxidize metals such as the noble metals and noble metal substitutes do not react with BaO₂ or SrO₂ until temperatures reach at least about 33°K and therefore superconductive materials made from these elements according to the teachings of this invention have critical temperatures in the range of about 133°K or higher. In preferred embodiments, the structures of the superconductors of this invention are similar to that of Ag_0.25Cu_0.75Ba₂Ca₂Cu₃O₉ superconductor of Ihara, et al. (1994), "New High-T_c Superconductor Ag_1-xCu_xBa₂Ca_n-1Cu_nO_2n+3-δ Family with T_c > 117°K," Jpn. J. Appl. Phys. 33:L300-L303. If the top and bottom planes as in the structure shown in Figure 3 are built on the base of Ag₂O, PtO, Au₂O₃, PbO₂, or any combination of these oxides, leaving all other parts of the structure the same as in known superconductors, such materials will have critical temperatures anywhere between 133ºK up to the stability limits of the listed oxides. As listed in Table 2, BaO₂ and SrO₂ are stable up to about 873°K and 373°K, respectively. In the mentioned oxides, the valence of Ag is +1, of Pt is +2, of Au is +3, and of Pb is +4. Bi is assumed to have a valence of +5 and form the oxide Bi₂O₅. To build top and bottom layers or planes it is necessary to have four metal atoms with the total valence number 7 or 8 in the corners of the quadrangle, or just three atoms assuming that one position can be vacant. This may be achieved by using four atoms of Pt, or 2 atoms of Au and 2 atoms of Ag, or 3 atoms of Ag and 1 atom of Pb, making the following preferred compositions:

Bi₂O₅ is unstable so that no thermodynamic data are available for this species but, because of its instability, it should react to form peroxides according to the reactions:

The Bi with valence +5 with three Ag atoms having a valence of +1 will give a total valence number of 8 making a structurally sound plane. Also it is known that Bi may substitute for Pb, and hence preferred superconductive compositions include this substitution.

Minimum and maximum values of stoichiometric coefficients of Ag, Au, Pt, Pb, Bi in formula I are determined on the basis of the following considerations. Assuming that one position out of 4 in a top or bottom plane may be vacant, this leads to a minimum of 3 atoms in the top and 3 in the bottom plane which gives a total of 6 atoms. Each atom belongs to eight unit cells. So the minimum stoichiometric coefficient for this structure is 6/8=0.75. The corresponding maximum stoichiometric coefficient is as in the composition of Bi₂Sr₂Ca₂Cu₃O₁₀, i.e. 2. Adding an amount, preferably 0.2, to provide the possibility for small deviations from stoichiometry gives 2.2. Because Pb and Bi have valences of +4 and +5 respectively they cannot alone form a stable plane and should be present with at least one element selected from the group consisting of Pt , Ag and Au. Hence the corresponding stoichiometric coefficients for Pt, Ag and Au are a + b + c > 0.

Leaving all other parts, i . e . , the Ba or Sr-containing layers and the Cu, Ca, O-containing layers, of the structure the same as in known superconductors and taking into account that Ba can be substituted by Sr we arrive at the following chemical composition for the materials of this invention having critical temperatures higher than about 133°K:

where a + b + c + d is a number between about 0.75 and about 2.0 plus or minus about 0.2 (preferably about 2.0) inclusive, a + b + c is a number greater than 0, f is a number between about 0 and about 2 inclusive; n is an integer between 1 and about 10 inclusive plus or minus about 0.1, and x is a number greater than 0. Preferably x is no more than about 5. The number n ± 0.1 provides the possibility for small deviations from stoichiometry which do not have significant adverse effect on the T_c of the material.

The structure for the (Ag_aAu_bPt_cPb_dBi_e) (Ba_2-fSr_f)Ca_n-1Cu_nO_2n+1+x compound is believed to be as shown in Figure 3. The metal components of the first term of formula I may exist in other valence states, for example Ag can have valence +2; Pb, valence +2 or +3; Pt, valence +3; and Bi, valence +2 or +3. Those of ordinary skill in the art will appreciate that use of listed metal species of different valence will result in variations of stoichiometric coefficients for the first term of formula I. This invention includes new superconductor compositions of formulae similar to formula I but in which the first term is adapted in view of different valences of Ag, Pb, Pt and/or Bi.

The stability of Ag, Au, Pt, Bi and Pb oxides was determined by thermodynamic calculation as described above. The valences used to determine the stoichiometric coefficients of formula I are those considered to be preferred to provide maximum T_c. The results are summarized in Table 2.

As seen from the table, oxides of Ag, Au, Pt, Pb and Bi are unstable at temperatures higher than 600°K. To prevent oxygen loss during processing to fabricate superconducting oxides in the bulk form, the mixture of oxides is preferably heated in a sealed capsule or metallic precursors are oxidized at low temperatures at which oxides are stable. Examples of methods of preparation of superconducting material of this invention are given below. Examples

Example 1.

PtO powder (0.1 mole) is mixed with BaO₂ powder (0.1 mole) and ground in a ball mill for 24 hours. The resultant mixture is pressed into a pellet and heated to 600°C in a furnace in flowing oxygen. After 8 hours the pellet is removed and crushed into a powder. The resultant material is a homogeneous compound PtBaO₃.

BaCO₃ (0.1 mole), CaCO₃ (0.2 moles) and CuO (0.3 moles) are mixed, ground in a ball mill for 1 hour and pressed into a pellet. The pellet is heated to 900° C in a furnace in flowing oxygen. After 24 hours the pellet is removed from the furnace and crushed into a powder. The resultant powder is precursor material BaCa₂Cu₃O₆. The powdered PtBaO₃ (0.1 mole) is mixed with the powdered BaCa₂Cu₃O₆ (0.1 mole) and this mixture is pressed into a pellet and the pellet is sealed in a gold capsule. The capsule is heated in a furnace at 900°C for 24 hours. The resultant sintered material is removed from the capsule and is then annealed in oxygen for 24 hours. The final material has the composition Pt₁Ba₂Ca₂Cu₃O₉ and exhibits a critical temperature higher than 133°K.

Example 2.

BaCa₂Cu₃O₆ (0.1 mole) powder, prepared in the same manner as in Example 1, is mixed with 0.025 moles of Au₂O₃, 0.025 moles of Ag₂O, and 0.1 mole of BaO₂ and ground in a ball mill for 24 hours. This mixture is pressed into a pellet and the pellet is sealed in a gold capsule. The capsule is heated in a furnace at 900°C for 24 hours, after which the capsule is opened. The resultant sintered material is annealed in oxygen at 100° C for 48 hours. The final product has the composition (Ag_0.5AU₀.₅)Ba₂Ca₂Cu₃O₉ and exhibits a critical temperature higher than 133ºK.

Example 3. Ag (0.75 moles), Pb (0.25 moles), Ba (0.2 moles), Ca (0.2 moles), and Cu (0.3 moles) are melted under an Ar atmosphere. The liquid alloy is melt spun to produce a ribbon of alloy. The ribbon is ground into a powder which is then completely oxidized in oxygen at 100° C. The oxidized powder is pressed into a pellet and the pellet is sealed into a gold capsule. The capsule is heated at 900°C for 10 hours, removed from the furnace and opened. The resultant sintered material is annealed in oxygen at 100° C for 48 hours. The final material has the composition (Ag_0.75Pb_0.25)Ba₂Ca₂Cu₃O₉ and exhibits a critical temperature higher than 133°K.

Example 4.

BaCa₂Cu₃O₆ (0.1 mole) powder, prepared in the same manner as in Example 1, is mixed with 0.0125 moles of Bi₂O₃, 0.05 moles of Ago, 0.0125 moles of Ag₂O and 0.1 mole of BaO₂ and ground in a ball mill for 24 hours. This mixture is pressed into a pellet and the pellet is sealed in a gold capsule. The capsule is treated in the same manner as in Example 1. The resultant sintered material is annealed in oxygen at 100° C for 48 hours. The final product has the composition (Ag_0.75Bi_0.25)Ba₂Ca₂Cu₃O₉ and exhibits a critical temperature higher than 133°K.

Example 5.

BaCa₂Cu₃O₆ (0.1 mole) powder, prepared in the same manner as in Example 1, is mixed with Bi₂O₃ (0.00625 mole), PbO₂ (0.125 mole), Ago (0.25 mole) and Ag₂O (0.25 mole) and BaO₂ (0.1 mole) and ground in a ball mill for 24 hours. This mixture is pressed into a pellet and the pellet is sealed in a gold capsule. The capsule is treated in the same manner as in Example 1. The resultant sintered material is annealed in oxygen at 100° C for 48 hours. The final product has the composition (Ag_0.75Pb_0.125Bi_0.125)Ba₂Ca₂Cu₃O₉ and exhibits a critical temperature higher than 133°K. The solid state diffusional processes which are taking place in the processing of these materials are slow and hence optimal reaction times and temperatures may vary significantly with the materials used. Adjustment of processing times and temperatures are within the routine ability of the ordinary skilled worker in the art based upon the teachings herein.

These examples are only given to illustrate the present invention and do not limit its scope. Equivalent methods and compositions will be apparent to the skilled worker and are intended to be encompassed within the scope of the claims hereof. Additional embodiments which will be appreciated by the skilled worker are within the scope of the following claims.

Claims

We Claim:

1. A substantially single-phase superconducting material having the composition represented by the general formula:

where a + b + c + d is a number between about 0.75 and about 2.0 inclusive, a + b + c is a number greater than 0, f is a number between about 0 and about 2 inclusive; n is an integer between 1 and about 10 inclusive, and x is a number greater than 0.

2. A superconducting material according to claim 1, in which x is less than or equal to about 5.

3. (Amended) A superconducting material according to claim 1, having a lattice structure as given in Figure 3 in which the top and bottom planes comprise an element selected from the group consisting of Ag, Au, Pt, and combinations thereof .

4. (Amended) A superconducting material according to claim 3, wherein said top and bottom planes additionally comprise an element selected from the group consisting of Bi and Pb.

5. (Amended) A superconducting material according to claim 3, wherein said top and bottom planes do not comprise Cu.

6. A superconducting material according to claim 1, in which a, b, d, and e = 0.

7. A superconducting material according to claim 1, in which c, d, and e = 0.

8. A superconducting material according to claim 1, in which b, c, and e = 0.

9. A superconducting material according to claim 1, in which b, c, and d - 0.

10. A superconducting material according to claim 1, in which b, and c = 0.

11. A superconducting material according to claim 1 in which f is 0.

12. A superconducting material according to claim 1 represented by the formula Pt₁Ba₂Ca_n-1Cu_nO_2n+1+x.

13. A superconducting material according to claim 1 represented by the formula Pt₁Sr₂Ca_n-1Cu_nO_2n+1+x.

14. A superconducting material according to claim 1 represented by the formula (Ag_0.5Au_0.5)Ba₂Ca_n-1Cu_nO_2n+1+x.

15. A superconducting material according to claim 1 represented by the formula (Ag_0.5Au_0.5) Sr₂Ca_n-1Cu_n0_2n+1+x.

16. A superconducting material according to claim 1 represented by the formula (Ag_0.75Pb_0.25)Ba₂Ca_n-1Cu_nO_2n+1+x.

17. A superconducting material according to claim 1 represented by the formula (Ag_0.75Pb_0.25)Sr₂Ca_n-1Cu_n0_2n+1+x.

18. A superconducting material according to claim 1 represented by the formula (Ag_0.75Bi_0.25)Ba₂Ca_n-1Cu_nO_2n+1+x.

19. A superconducting material according to claim 1 represented by the formula (Ag_0.75Bi_0.25)Sr₂Ca_n-1Cu_nO_2n+1+x.

20. A superconducting material according to claim 1 represented by the formula (Ag₀₇₅Pb_0.125Bi_0.125)Ba₂Ca_n-1Cu_nO_2n+1+x.

21. A superconducting material according to claim 1 represented by the formula (Ag_0.75Pb_0.125Bi₀,₂₅)Sr₂Ca_n-1Cu_nO_2n+1+x.

22. A superconducting material according to claim 1, represented by the formula Pt₁Ba₂Ca₂Cu₃O₉.

23. A superconducting material according to claim l, represented by the formula Pt₁Sr₂Ca₂Cu₃O₉.

24. A superconducting material according to claim l, represented by the formula (Ag_0.5Au_0.5)Ba₂Ca₂Cu₃O₉.

25. A superconducting material according to claim 1, represented by the formula (Ag_0.5Au_0.5) Sr₂Ca₂Cu₃O₉.

26. A superconducting material according to claim 1, represented by the formula (Ag_0.75Pb_0.25)Ba₂Ca₂Cu₃O₉.

27. A superconducting material according to claim l, represented by the formula (Ag_0.75Pb_0.25) Sr₂Ca₂Cu₃0₉.

28. A superconducting material according to claim 1, represented by the formula (Ag_0.75Bi_0.25)Ba₂Ca₂Cu₃O₉.

29. A superconducting material according to claim l, represented by the formula (Ag_0.75Bi_0.25) Sr₂Ca₂Cu₃O₉.

30. A superconducting material according to claim 1, represented by the formula (Ag_0.75Pb_0.125Bi_0.125)Ba₂Ca₂Cu₃O₉.

31. A superconducting material according to claim l, represented by the formula (Ag_0.75Pb_0.125Bi₀₁₂₅)Sr₂Ca₂Cu₃O₉.

32. A method of making a single-phase, composition comprising a noble metal represented by the general formula:

where a + b + c + d is between about 0.75 and about 2.0 inclusive, a + b + c is a number greater than 0, f is a number between about 0 and about 2 inclusive; n is an integer between 1 and about 10 inclusive, and x is a number greater than 0, comprising heating stoichiometric amounts of said Ag, Au, Pt or combinations thereof, and optionally stoichiometric amounts of Pb or Bi or combinations thereof, with stoichiometric amounts of said Ba or Sr or combinations thereof, and stoichiometric amounts of Cu and optionally stoichiometric amounts of Ca, in the presence of oxygen in the form of oxides of any of the foregoing or free oxygen, for a sufficient time at a sufficient temperature, to form said single-phase composition.

33. The method of claim 32 wherein a precursor material having the formula BaCa₂Cu₃O₆ is heated with an oxide of Ag, Au, Pt or mixtures thereof.

34. The method of claim 32 wherein stoichiometric amounts of said Ag, Au, Pt or combinations thereof, said Ba or Sr or combinations thereof, stoichiometric amounts of said Ca and of said Cu, all in elemental form, are heated in the absence of oxygen to form an alloy; said alloy is completely oxidized in oxygen, followed by sintering in a sealed environment, and annealing in the presence of oxygen for a sufficient period of time at a sufficient temperature to form said composition.

35. A method for making a superconducting material having a Tc greater than about 133°K which comprises making the single- phase composition of claim 32 and measuring the Tc of said composition.

36. A single-phase composition made by the method of claim 32.

37. A substantially single-phase superconducting material in which a noble metal is substituted for Pb or Bi or both in a superconducting material presently known in the art at a level of 0.1 at. % or more.

38. The superconducting material of claim 37 where the noble metal is Ag.

39. A method for producing an improved single-phase superconducting material which comprises the step of substituting a noble metal for Pb or Bi or both in a superconducting material presently known in the art at a level of 0.1 at. % or more.

40. The method of claim 39 wherein said noble metal is Ag.