WO1991011391A1 - Superconducting metal oxyde compositions - Google Patents

Abstract

Compositions having the formula TlRxSr2-xCaCu2O7, wherein R is at least one element chosen from the group consisting of Nd, Pr and La and 0.2 < x < 0.8 are superconducting.

Description

SUPERCONDUCTING METAL OXIDE COMPOSITIONS BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to novel Tl-(La,Nd,Pr)- Sr-Ca-Cu-0 single phase compositions which are superconducting. Background Art Bednorz and uller, Z. Phys. B64, _.189 (1986), disclose a superconducting phase in the La-Ba-Cu-0 system with a superconducting transition temperature of about 35 K. The existence of this phase was subsequently confirmed by a number of investigators [see, for example, Rao and Ganguly, Current Science, 56, 47 (1987), Chu et al., Science 235, 567 (1987), Chu et al., Phys. Rev. Lett. 58, 405 (1987), Cava et al., Phys. Rev. Lett. 58, 408 (1987), Bednorz et al., Europhys. Lett. 3, 379 (1987)]. The superconducting phase has been identified as the composition

Laι-_X(Ba,Sr,Ca)_xCuθ4-y with the tetragonal K2NiF-j-type structure and with x typically about 0.15 and y indicating oxygen vacancies.

Wu et al., Phys. Rev. Lett. 58, 908 (1987), disclose a superconducting phase in the Y-Ba-Cu-0 system with a superconducting transition temperature of about 90 K. Cava et al., Phys. Rev. Lett. 58, 1676 (1987), have identified this superconducting Y-Ba-Cu-0 phase to be orthorhombic, distorted, oxygen-deficient perovskite YBa2Cu3θ9-d where d is about 2.1 and present the powder x-ray diffraction pattern and lattice parameters.

C. Michel et al., Z. Phys. B - Condensed Matter 68, 421 (1987) , disclose a novel family of superconducting oxides in the Bi-Sr-Cu-0 system with composition close to Bi2Sr2Cu2θ7+ . A pure phase was isolated for the composition Bi2Sr2C 2θ7+d. The X-ray diffraction pattern for this material exhibits some similarity with that of perovskite and the electron diffraction pattern shows the perovskite subcell with the orthorhombic cell parameters of a = 5.32 A (0.532 nm) , b = 26.6 A (2.66 nm) and c = 48.8 A (4.88 nm) . The material made from ultrapure oxides has a superconducting transition with a midpoint of 22 K, as determined from resistivity measurements and zero resistance below 14 K. The material made from commercial grade oxides has a superconducting transition with a midpoint of 7 K. H. Maeda et al., Jpn. J. Appl. Phys. 27, L209 (1988) , disclose a superconducting oxide in the Bi-Sr-Ca-Cu-0 system with the composition near

BiSrCaCu2θ_x and a superconducting transition temperature of about 105 K.

The commonly assigned application, "Superconducting Metal Oxide Compositions and Process For Making Them", S. N. 153,107, filed Feb. 8, 1988, a continuation-in- part of S. N. 152,186, filed Feb. 4, 1988, discloses superconducting compositions having the nominal formula Bi_aSrt,Ca_cCu3θχ wherein a is from about 1 to about 3, b is from about 3/8 to about 4, c is from about 3/16 to about 2 and x - (1.5 a + b + c + y) where y is from about 2 to about 5, with the proviso that b + c is from about 3/2 to about 5, said compositions having superconducting transition temperatures of about 70 K or higher. It also discloses the superconducting metal oxide phase having the formula Bi2Sr3-_zCa₂C 2θ8+ wherein z is from about 0.1 to about 0.9, preferably 0.4 to 0.8 and w is greater than zero but less than about 1. M. A. Subramanian et al., Science 239, 1015 (1988) also disclose the Bi2Sr3-₂Ca₂C 2θ8+w superconductor. Y. Yumada et al., Jpn. J. Appl. Phys. 27, L996 (1988) , disclose the substitution of Pb for Bi in the series Biι-_xPb_xS_rCaCu2θy where x = 0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1.0. The T_c increases from 75.5 K for x = 0, no Pb present, to a maximum of 85.5 K for x = 0.5. Tc decreases for higher Pb content to 76 K for x = 0.7. No superconductivity was observed for the samples with x = 0.9 and x = 1.

M. Takano et al., Jpn. J. Appl. Phys. 27, L1041 (1988), disclose that partial substitution of Pb for Bi in the Bi-Sr-Ca-Cu-O system results in an increase in the volume fraction of the high Tc phase. Coprecipitated oxalates containing the relevant ions in various ratios underwent thermal decomposition below 773 K. The samples in powder form were then heated in air to 1073 K for 12 hours and, after being formed into pellets, at 1118 K for various periods which extended to more than 240 hours in some cases. A starting composition of Bi:Pb:Sr:Ca:Cu = 0.7:0.3:1:1:1:8 was heated at 1118 K for 244 hours. The high-Tc phase shows an onset of superconductivity at around 115 K. This phase forms plate-like crystals and analysis of these crystals indicates that the cationic ratio is Bi:Pb:Sr:Ca:Cu = 67:5:100:85:180 so that there is considerably less Pb in the high-T_c than in the starting material.

M. Mizuno et al., Jpn. J. Appl. Phys. 27, L1225 (1988), also disclose that the addition of Pb to the Bi-Sr-Ca-Cu-0 system results in an increase in the volume fraction of the high-Tc phase and a lowering of the optimum temperature to obtain this phase to about 855'C.

E. V. Sampathkumaran et al., J. Phys. F: Met. Phys.

18, L163 (1988) disclose that the partial substitution of K or Pb for Bi in the Bi4Ca3Sr3Cu.}04 results in an enhancement of the' fraction of the phase superconducting at about 110 K.

Z. Z. Sheng et al.. Nature 332, 55 (1988) disclose superconductivity in the Tl-Ba-Cu-0 system in samples which have nominal compositions Tl2Ba2Cu3θ8+x and TlBaCu3θ5.5+_x. Both samples are reported to have onset temperatures above 90 K and zero resistance at 81 K.

Z. Z. Sheng et al.. Nature 332, 138 (1988) disclose superconductivity in the Tl-Ca-Ba-Cu-0 system in samples which have nominal compositions Tl2Ca2BaC 3θg+_x.

R. M. Hazen et al., Phys. Rev. Lett. 60, 1657 (1988) , disclose two superconducting phases in the Tl-Ba-Ca-Cu-0 system, Tl2Ba2Ca2C 3θιo and Tl2Ba2CaCu2θ8. The commonly assigned application, "Superconducting Metal Oxide Compositions and Process For Making Them", S. N. 153,107, filed Feb. 8, 1988, a continuation-in- part of S. N. 152,186, filed Feb. 4, 1988, discloses superconducting compositions having the nominal formula Tl_ePb_aCabSr_cCuoO_x wherein a is from about 1/10 to about 3/2, b is from about 1 to about 4, c is from about 1 to about 3, d is from about 1 to about 5, e is from about 3/10 to about 1 and x = (a + b + c + d + e + y) where y is from about 1/2 to about 3. Preferably, the sum of a+e is about 1, b is about 2, c is about 2, d is from about 3 to about 4 and y is from about 1/2 to about 2. The onset of superconductivity for these compositions is at least 70 K. The composition wherein e = a = 1/2, b = c = 2 and d = 3 is essentially single phase and has an onset of superconductivity at about 120 K.

P. Haldar et al.. Mater. Lett. 7, 1 (1988) disclose the superconductor (Tl,Bi) (Sr,Ca)2CUO .5+3 with an onset of superconductivity of 50 K. P. Haldar et al., J. Superconduct. 1, 211 (1988) disclose the superconductor

⁽Tl0.75Bi0.25)l.33Srι.33Caι.33Cu2θ6.67+a with the ideal stoichiometry (Tl,Bi)lSr2CaιCu2θ6.5+d and an onset of superconductivity at 92 K.

K. Chen et al., J. Appl.Phys. 65, 3574 (1989) report on intragrain critical current the measurements in (Tlo.75Biθ.25)1.33Srι.33Caι.33C 2O6.67+9-

Li et al., Physica C 157, 365 (1989) disclose the preparation of (Tl,Bi)Sr2CaC 2θy where the ratio of

Tl/Bi was about 1, y was about 7 and the T_c was about 95 K.

The commonly assigned application, "Superconducting Metal Oxide Compositions and Process For Manufacture", S. N. 390,944, filed Aug. 9,1988, discloses single phase superconducting compositions having the formula Tlι-_xBi_xSr2Ca2C 3θ9 wherein x is from about 3/20 to about 11/20. The onset of superconductivity for these compositions is above 110 K. 0. Inoue et al., Jpn. J. Appl. Phys. 28, L1375 (19_λ89), disclose the substitution of Y for Sr to give the nominal compositions Tlι.2Sr3- ₂Y_zCu2.35θ_x where z = 0, 0.15, 0.3, 0.5, 0.7, 1.0. Superconductive transitions above 50 K were observed for 0.15 ≤ z < 0.5. No superconductive transition was observed for Til.2Sr2.7ϊ0.3C 2.35θ_x.

Z. Z. Sheng et al., Solid State Co m. 71, 739 (1989) , disclose that the nominal compositions La2Tl2Sr2Cu3θn, Pr2Tl2Sr2C 3θn and NdTl2Sr2Cu3θ9.5 and a related Y-Tl-Sr-Cu-O composition have onsets of superconductivity at 41 K,

46 K, 65 K and 45 K respectively.

Itoh et al., Jpn. J. Appl. Phys. 28, L200 (1989), disclose compounds in the systems Tl-Ln-Sr-Cu-0 (Ln = Pr and Nd) and Tl-Pb-Ln-Sr-Cu-O (Ln = Pr, Nd and S ) that show superconductivty at about 40 K.

Vijayaragh van et al., Supercon. Sci. Technol. 2, 195 (1989) , note that it has not been possible to prepare the strontium analogues, Tl(Ca,Sr)_n+lCu_nθ2n+3. of the TlCan-lBa_nCunθ2n+3 superconductors. They state that it seems that the only way to stabilize the Sr analogues is either to partly substitute Tl by Pb or Ca by a rare earth as in TlCaι-_xLn_xSr2Cu_n02n+3 where Ln is any rare earth.

Liang et al.. Solid State Comm. 70, 661 (1989) disclose the crystal structure of TlSr2Cao.5Smo.5Cu2O6.75 which has a T_c of 37.6 K.

Liang et al., Z. Phys. B -Condensed Matter 76, 277 (1989) disclose the crystal structure of

TlSr2Cao.5Ero.5Cu2O6.75 which has a T_c of 34 K.

Ganguli et al., Appl. Phys. Lett. 55, 2664 (1989), disclose the thallium cuprate superconductors containing no calcium or barium. When x = 0.75-1.0, TlSr2- _xLn_xCuOy where Ln = La, Pr or Nd, has an onset of superconductivity at about 40 K and, when x = 0.4-0.5, TlSr3-_xLn_xCu2θy. where Ln = Pr or Nd, has an onset of superconductivity at about 80 K.

SUMMARY OF THE INVENTION This invention provides novel single phase superconducting compositions having the formula TlR_xSr2- _xCaC 2θ7 wherein R is at least one element chosen from the group consisting of Nd, Pr and La and 0.2 < x < 0.8. Preferably x is from about 0.4 to about 0.6, most preferably about 0.5. Preferably R is La.

These superconducting compositions are prepared by heating a mixture of the Tl, R, Sr, Ca and Cu oxides, the relative amounts chosen so that the atomic ratio Tl:R:Sr:Ca:Cu is l:x:2-x:l:2, to a temperature of about

880"C to about 910^*C and maintaining that temperature for about 6 to 12 hoursrin a controlled environment, e. g., in a sealed tube made of a non-reacting metal such as gold, said sealed tube serving to prevent the escape of the metals and the oxygen. The heating temperature is preferably about 890-900^*C.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows a plot of the flux excluded by a composition of the formula TlNdo. Srι. CaC 2θ7 as a function of temperature.

DETAILED DESCRIPTION OF THE INVENTION

The superconducting compositions of this invention are prepared by the following process. Quantities of the oxide reactants TI2O3, Srθ2, Caθ2, CuO and at least one of the R-oxides, Nd2θ3, PreOn and La2θ3, are chosen to provide an atomic ratio of Tl:R:Sr:Ca:Cu of l:x:2- x:l:2 where 0.2 < x < 0.8 and are mixed, for example, by grinding them together in a mortar. The mixed powder may then be heated directly or it can be first formed into a pellet or other shaped object and then heated. The superconducting composition of this invention is produced only when the environment in which the reactants are heated is carefully controlled. This controlled environment must prevent (1) the escape of the metals and the oxygen and (2) their reaction with other elements. One way to provide a controlled environment is to place the reactants in an air-filled container, e.g. a tube, made of a non-reacting metal such as gold and then seal the tube by welding. The sealed tube is then placed in a furnace and heated to about 880'C to about 910"C, preferably about 890-900^*C, and maintained at that temperature for about 6 to 12 hours. The sealed tube is then cooled to ambient temperature. For example, the power to the furnace is turned off and the tube is furnace-cooled to ambient temperature, about 20^*C, and then removed from the furnace. The tube is then opened and the black single phase product recovered. "The compositions of the invention prepared in this manner exhibit superconductivity and those with x near the middle of the indicated range, i. e., with x about 0.5, exhibit the highest onset of superconductivity with the onset occuring above 77 K. The lattice parameters of these superconducting compositions have been determined from X-ray diffraction powder pattern results indexed on a tetragonal unit cell. The lattice parameter a increases slightly and the lattice parameter c decreases slightly as x increases. The lattice parameter a « 0.38 nm and the lattice parameter c ~ 1.2 nm.

It is to be understood that the compositions of this invention include those compositions in which the amount of oxygen per formula unit deviates somewhat from the 7 indicated by the formula TlR_xSr2- CaCu2θ7 but which are single phase superconducting compositions with tetragonal unit cell parameters consistent with those given above. Typically, such deviations in the amount of oxygen would be less than about 5% of the amount of oxygen indicated by the given formula.

Superconductivity can be confirmed by observing magnetic flux exclusion, i. e., the Meissner effect. This effect can be measured by the method described in an article by E. Polturak and B. Fisher in Physical Review B, 36, 5586(1987).

The superconducting compositions of this invention can be used to conduct current extremely efficiently or to provide a magnetic field for magnetic imaging for medical purposes or for particle accelerators. Thus, by cooling the composition in the form of a wire or bar to a temperature below the superconducting transition temperature (T_c) , by exposing the material to liquid nitrogen or to liquid helium in a manner well known to those in this field, and initiating a flow of electrical current, one can obtain such flow without any electrical resistive losses. To provide exceptionally high magnetic fields with minimal power losses, the wire mentioned previously could be wound to form a coil or solenoid which would be exposed to liquid helium or liquid nitrogen before inducing any current into the coil. Such fields can also be used to levitate objects as large as railroad cars. These superconducting compositions are also useful in thin film-based Josephson devices such as SQUIDS (superconducting quantum interference devices) and in instruments that are based on the Josephson effect such as high speed sampling circuits and voltage standards.

All examples of the invention described below and the samples prepared in the comparison experiments were heated at 890"C. Examples and samples heated at 900"C were essentially the same as those prepared at 890"C.

EXAMPLES OF THE INVENTION EXAMPLES 1-14 Examples 1-14 were carried out using essentially the same procedure.

To make a composition having the formula TlR_xSr2- _xCaCu2θ7, quantities of TI2O3, Srθ2, Caθ2, CuO and one of the R-oxides, Nd2θ3, Pr6θn or La2θ3 corresponding to a Tl:R:Sr:Ca:Cu atomic ratio of l:x:l-x:l:2 were thoroughly ground together in an agate mortar for about 30 minutes. The specific quantities used in each Example are shown in Table I. In each instance, pellets, 10 mm in diameter and about 3 mm thick, were pressed from this mixed powder. Two pellets were loaded into a 1/2" (1.25 cm) diameter gold tube and the tube was sealed by welding both ends. The tube was placed in a furnace and heated at a rate of about 5^*C per minute to 890^*C and then maintained at 890^*C for 6 hours.

Power to the furnace was then shut off and the tube was allowed to cool to room temperature in the furnace. The tube was then removed from the furnace and cut open. The black product was recovered. Meissner effect measurements were carried out and the results showed an onset of superconductivity at about the temperature indicated in Table II for each Example. The results are shown for the product of Example 4, TlNdo.4Srχ. CaCu2θ7, -■-- Fig. 1 where the flux exclusion is plotted as a f nction of temperature. The plot shows the onset of superconductivity at about 80 K. An X-ray diffraction powder pattern of each of the products showed that the product was essentially single phase. The lattice parameters of the product were determined from the X-ray diffraction powder pattern results indexed on a tetragonal unit cell and are also shown in Table II along with the formula of the composition.

EXPERIMENTS Α-T Experiments A-I were carried out using essentially the same procedure used in Example 1.

Quantities of TI2O3, Srθ2, Caθ2, CuO and one of the R-oxides, d2θ3, Pr6θn qr La2θ3 corresponding to a Tl:R:Sr:Ca:Cu atomic ratio of l:x:l-x:l:2 were thoroughly ground together in an agate mortar for about 30 minutes. The specific quantities used in each Experiment are shown in Table I. In each instance, pellets, 10 mm in diameter and about 3 mm thick, were pressed from this mixed powder. Two pellets were loaded into a 1/2" (1.25 cm) diameter gold tube and the tube was sealed by welding both ends. The tube was placed in a furnace and heated at a rate of about 5^*C per minute to 890"C and then maintained at 890"C for 6 hours. Power to the furnace was then shut off and the tube was allowed to cool to room temperature in the furnace. The tube was then removed from the furnace and cut open. The black product was recovered. Meissner effect measurements were carried out and the results showed that the products of Experiments A, B, D, E, G and H were not superconductors at temperatures down to about 4 K. Meissner effect measurements were carried out and the results showed that the products of Experiments C, F and I showed an onset of superconductivity at about the temperature shown in Table II.

X-ray diffraction powder patterns of the products of Experiments C, F and I showed that these products were not single phase and the formula shown in Table II for each of these Experiments is a nominal one. X-ray diffraction powder patterns of the products of Experiments A, B, D, E, G and H showed that these products were essentially single phase and the lattice parameters are shown in Table II along with- the formula of the composition.

In summary, the products of Examples A, B, D, E, G and H are essentially single phase but are not superconductors. The products of Examples C, F and I exhibit superconductivity but are multiphase.

Claims

CLATMS Thf> Inven on Being Claiτngri ϊ <_:

1. A superconducting composition having the formula TlR_xSr2-_xCaCu2θ7 wherein R is at least one element chosen from the group consisting of Nd, Pr and La and 0.2 < x < 0.8.

2. The composition of Claim 1 wherein x is from about 0.4 to about 0.6.

3. The composition of Claim 2 wherein x is about

0.5,

4. The composition of Claim 1 wherein R is La.

5. The composition of Claim 2 wherein R is La.

6. The composition of Claim 3 wherein R is La.

7. A process for making a single phase composition having the formula TlR_xSr2-χCaCu2θ7 wherein R is at least one element chosen from the group consisting of Nd, Pr and La and 0.2 < x < 0.8, said process consisting essentially of a) forming an oxide mixture of the oxides of Tl, R, Sr, Ca and Cu, the relative amounts chosen so that the atomic ratio Tl:R:Ca:Sr:Cu is 1:x:2-x:1:2; b) heating said oxide mixture to a temperature of about 880"C to about 910^*C and maintaining said temperature for about 6 to 12 hours in a controlled environment; and c) cooling and recovering said composition.

8. The process of Claim 7 wherein the temperature to which the oxide mixture is heated is about 890^*C to about 900^*C.

9. The process of Claim 8 wherein the oxides are TI2O3, La2θ3, Srθ2, Caθ2 and CuO.

10. A method for conducting an electrical current within a conductor material without electrical resistive losses comp ising.the steps of: cooling a conductor material composed of a composition of Claim 1 to a temperature below the Tc of said composition; and initiating a flow of electrical current within said conductor material while maintaining said material below said temperature.