Superconductors

Abstract

Superconducting materials of the (T1,In)-Sr-Ca-Cu-O system-type are modified to raise their critical tempertures by substituting an element M for part of the Tl/In and optionally substituting Y or a rare earth element for some of the Ca.M is a transition metal with valency electrons in d orbitals in its normal state and having accessible tri-and/or tetra-valent states, that is V, Ti, Cr, Zr, Nb, Hf or Ta (which are preferred) Mn, Fe, Co, Ni, Mo, Rh, W, Os, Ir, Re or Ru.

Description

This invention relates to superconductors.

The discovery of superconductivity in the T1-Sr-Ca-Cu-O system (Nagashima, T. et. al., Jpn. J. Appl. Phys. 27, L1077-L1079 (1988); Sheng, Z. Z. et al. Phys.Rev. B38 7074-7076 (1988)) has led to the identification of two series of related superconducting cuprates. One has the formula (T1.sub.0.5 M.sub.0.5) Sr.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.2n+3, where M=Pb (Subramanian, M. A. et al., Science 242, 249-252 (1988); Ganguli, A. K., Nanjundaswamy D. S. & Rao, C. N. R. Physcia C 156, 788-790 (1988).; Liu, et al. Physica C 156, 791-794 (1988); Parise, J. B. et al., Physica C 159, 245-254 (1989), or M=Bi (Haldar, P. et al. J. Supercond. 1, 211-218 (1988); Li, S. & Greenlbatt, M. Physica C 157, 365-369 (1988); Subramanian, M. A., Gai, P.L. & Sleight, a. W. Mat. Res. Bull. 25, 101-106 (1990)); and n=2 or 3. The other has the formula T1Sr.sub.2 (Ca.sub.x-1 R.sub.x).sub.n-1 Cu.sub.n O.sub.2n+3, where (R=La or, Nd and Y and n=2 and O<x<1) (Sheng, Z. Z., Sheng, L., Fei, X. & Hermann, A. M. Phys.Rev. B39 2918-2920 (1989); Rao, C. N. R., Ganguli, A. K. & Vijayaraghavan, R. Phys.Rev. B40, 2565-2567 (1989). The n=2 and n=3 series in these materials have superconducting transition temperatures (T.sub.c) up to about 85K and 120K, respectively.

Recently, there have been reports (Liu et al. Physica C 159, 385-390 (1989); Wu, P. T. et al, Appl. Phys.Lett. 54, 2464-2466 (1989); Liang, J. M. et al. Physica C 165, 347-353 (1990); Huang, Y. T., Liu, R. S., Wang, W. N. & Wu, P. T., Jpn. J. Appl. Phys.28, L1514-L1517 (1989); Liu, R. S. et al, J. Solid State Chem. Vol 86 p 334-339 (1990)) of bulk superconductivity up to 110K in the (T1.sub.0.5 M.sub.0.5) Sr.sub.2 (Ca.sub.0.8 R.sub.0.2)Cu.sub.2 O.sub.7-delta system, where (M=Pb or Bi, R=Y or a rare earth element. Significantly, this latter system has a T.sub.c some 20-25K above those of the other cuprate systems having the so-called 1-2-1-2 structure, e.g. T1Ba.sub.2 CaCu.sub.2 O.sub.7 (Parkin, S. S. P. et al. Phys. Rev. B38 6531-6537 (1988)), (T1.sub.0.5 Pb.sub.0.5) Sr.sub.2 CaCu.sub.2 O.sub.y (Subramanian, M. A. et al. Science 242, 249-252 (1988)), and T1Sr.sub.2 (Ca,Y)Cu.sub.2 O.sub.y (Sheng, Z. Z., Sheng, L., Fei, X. & Hermann, A. M. Phys. Rev. B39 2918-2920 (1989); Rao, C. N. R., Ganguli, A. K. & Vijayaraghavan, R. Phys. Rev. B40, 2565-2567 (1989). The oxidation states of Pb ions substituted into the T1 sites in these materials have been established to be 4+ and Bi ions may be either 3+ or 4+ or a mixture of these.

The present invention involves the modification of T1-Sr-Ca-Cu-O systems by the partial substitution of T1 by an element M which is a transition metal with valency electrons in d orbitals in its normal state and having with accessible tri- and/or tetravalent states and in which In may be substituted for part or all of the remaining T1. Further, part or all of the Ca may be replaced by an element M' where M' is Y or a rare earth element.

The element M may be Ti, V, Cr, Mn, Fe Co, Ni, Zr, Nb, Mo, Ru, Rh, Hf, Ta, W, Re, Os and Ir; metals with not more than five d electrons in the valency shell, that is to say Ti, V, Cr, Zr, Nb, Mo, Hf, Re and W are preferred.

The following Example illustrates the invention, with reference to the accompanying drawings in which:

FIG. 1 is a graph showing temperature dependence of the normalised resistivity (expressed as a ratio to the resistivity at 300K) for selected samples of superconductors in accordance with the invention and some known superconductors for comparison;

FIG. 2 shows field cooled (FC) magnetisation versus temperature for selected superconductors in accordance with the invention in 100 gauss applied magnetic field;

FIG. 3 shows X-ray diffraction patterns of selected superconductor compositions;

FIG. 4 is an energy dispersion spectrum for samples of selected superconducting compositions;

FIG. 5 is a graph of modulated low-field microwave absorption measured at 77K for selected composition in accordance with the invention and;

FIG. 6 is a graph plotting resitivity as a function of temperature for a further group of superconducting compositions in accordance with the invention.

EXAMPLE 1

Samples with nominal compositions of T1Sr.sub.2 CaCu.sub.2 O.sub.y, (T1.sub.0.5 V.sub.0.5) Sr.sub.2 CaCu.sub.2 O.sub.y and (T1.sub.0.5 V.sub.0.5) Sr.sub.2 (Ca.sub.0.8 Y.sub.0.2)Cu.sub.2 O .sub.y were prepared by solid state reaction from stoichiometric mixtures of high purity T1.sub.2 O.sub.3, V.sub.2 O.sub.5, SrO.sub.2, CaO, Y.sub.2 O.sub.3 and/or CuO powders, as appropriate. The powders were weighed, mixed and pressed into pellets with dimensions of 10 mm diameter and 2 mm thick using a pressure of 5 tonne/cm.sup.2. The pellets were then wrapped with gold foil to alleviate loss of thallium during subsequent heat treatment. The sample was then sintered at 950.degree. C. for 3 hours in flowing oxygen and then cooled to room temperature at a rate of 5.degree. C./min. After heat treatment, all the samples were dense and black. Bar-shaped samples (1.5 mm.times.2 mm.times.8 mm) were cut from the sintered pellets and used subsequently for resistivity and magnetic measurements.

A standard four-point probe method was used for electrical resistivity measurements, and was fully automated for data acquisition. The electrical contacts to the sample were made by fine copper wires with a conductive silver paint, the temperature being recorded by a calibrated silicon diode sensor located close to the sample. Magnetisation data was obtained using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design), field-cooled (FC) dc magnetisation being measured in an applied field of 100 gauss. X-ray diffraction (XRD) studies were carried out with Copper-K-alpha radiation using a Spectrolab CPS-120 diffractometer. Variant chemical compositions were examined by energy dispersive X-ray spectrometry (EDS) in a Jeol JEM-200CX electron microscope. Molybdenum specimen grids were used and background spectra were studied to ensure that no copper signals were detected from the sample-free area. Freshly-prepared monophasic specimens, SrCuO.sub.2, Ca.sub.2 CuO.sub.3, BiVO.sub.4, Bi.sub.2 CuO.sub.4 as well as (T1.sub.0.5 Pb.sub.0.5)Sr.sub.2 (Ca.sub.0.8 Y0.2)Cu.sub.2 O (Liang, J. M. et al, Physica C 165, 347-353 (1990), have been used as standard materials in the EDS analysis. Selected area electron diffraction (SAED) patterns recorded from the same electron microscope have been used to confirm the unit cell parameters. Electron spin resonance (ESR) spectra were recorded between about 77K and room-temperature using a Bruker ER200D X-band spectrometer, with 100 KHZ field modulation and TE.sub.102 rectangular microwave resonator. This incorporated either a commercial variable temperature attachment or a quartz insert Dewar containing nitrogen.

FIG. 1 shows the temperture dependence of the (normalised) electrical resistivity for samples with nominal compositions of T1Sr.sub.2 CaCu.sub.2 O.sub.y (a), (T1.sub.0.5 V.sub.0.5) Sr.sub.2 CaCu.sub.2 O.sub.y (b) and (T1.sub.0.5 V.sub.0.5)Sr.sub.2 (Ca.sub.0.8 Y.sub.0.2)Cu.sub.2 O.sub.y (c). The T1Sr.sub.2 CaCu.sub.2 O.sub.y sample has a room temperature resitivity (300K) of 7.15 milliohm cm and clearly exhibits a metallic behaviour down to 4K except for a very small (but reproducible) resistivity drop around 84K, as shown in FIG. 1(a). This anomaly at 84K may arise from a small amount of superconducting phase present in this sample. The (T1.sub.0.5 V.sub.0.5)Sr.sub.2 CaCu.sub.2 O.sub.y sample [FIG. 1(b)] displayed a characteristic metallic behaviour in the normal state (resitivity at 300K about 3.2 milliohm cm) and had a two-step resistivity drop at 110K and 80K, and T.sub.c(zero) =57K. Interestingly, the (T1.sub.0.5 V.sub.0.5)Sr.sub.2 CaCu.sub.2 O.sub.y sample possessed the largest room temperature resistivity (17.4 milliohm cm) at 300K but also has the highest superconducting transition temperature, with T.sub.c(zero) =87K (FIG. 1 (c)).

FIG. 2 shows the results of field cooled measurements of the dc magnetisation in an applied field of 100 gauss on the samples (T1.sub.0.5 V.sub.0.5)Sr.sub.2 CaCu.sub.2 O.sub.y (a) and (T1.sub.0.5 V.sub.0.5)Sr.sub.2 (Ca.sub.0.8 Y.sub.0.2)Cu.sub.2 O.sub.y (b). The field-cooled curve of (T1.sub.0.5 V.sub.0.5)Sr.sub.2 CaCu.sub.2 O.sub.y [FIG. 2(a)] indicated a very small, but real, diamagnetic signal at 105K together with a large diamagnetic signal at 60K. Both features are consistent with the two-step electrical resistivity drop as shown in FIG. 1(b). The magnetisation measurement (field-cooled) of the sample (T1.sub.0.5 V.sub.0.5)Sr.sub.2 (Ca.sub.0.8 Y.sub.0.2)Cu.sub.2 O.sub.y indicated a diamagnetic response at 105K which is consistent with the electrical resistivity result [FIG. 1(c)].

The X-ray diffraction pattern of T1Sr.sub.2 CaCu.sub.2 O.sub.y sample revealed predominantly an unidentified phase with a small amount of the superconducting T1Sr.sub.2 CaCu.sub.2 O.sub.y (1-2-1-2) phase (Nagashima, T. et al Jpn J Appl Phys 27 L1077-L1079 (1988); Sheng, Z. Z. et al Phys Rev B38, 7074-7076 (1988)). However, when T1 and Ca were partially substituted by Pb and Y respectively single-phase (T1.sub.0.5 Pb.sub.0.5)Sr.sub.2 (Ca.sub.0.8 Y.sub.0.2)Cu.sub.2 O.sub.y was detected [FIG. 3(a)].

Importantly, in the present example, when V cations were introduced into T1 sites, the 1-2-1-2 phase was stabilized. FIG. 3 shows the powder X-ray diffraction patterns of the vanadium-containing samples with nominal compositios (T1.sub.0.5 V.sub.0.5)Sr.sub.2 CaCu.sub.2 O.sub.y (b) and (T1.sub.0.5 V.sub.0.5)Sr.sub.2 (Ca.sub.0.8 Y.sub.0.2)Cu.sub.2 O .sub.y (c) compared to the pattern of monophasic (T1.sub.0.5 Pb.sub.0.5)Sr.sub.2 (Ca.sub.0.8 Y.sub.0.2)Cu.sub.2 O.sub.y (a). Almost all of the strong peaks in the XRD pattern of the (T1.sub.0.5 V.sub.0.5)Sr.sub.2 CaCu.sub.2 O.sub.y sample [FIG. 3(b)] can be indexed on the basis of the T1Ba.sub.2 CaCu.sub.2 O.sub.y likely structure (Parkin, S. S. P. et al Phys. Rev B38 6531-6537 (1988)) structure having the tetragonal unit cell (P4/mmm) and lattice parameters a=3.791.ANG. and c=11.916.ANG.. These lattice parameters are slightly smaller than the corresponding (T1.sub.0.5 Pb.sub.0.5) Sr.sub.2 CaCu.sub.2 O.sub.y system [a=3.8023(3).ANG. and c=12.107(1).ANG.] (Parise, J. B. et al Physica C 156, 245-254 (1988)), probably as a result of the effects of the substitution of vanadium. We have attempted the substitution of 20% of Y.sup.3+ ions on Ca.sup.2+ sites in the (T1.sub.0.5 V.sub.0.5)Sr.sub.2 CaCu.sub.2 O.sub.y compound; the X-ray diffraction pattern of the resulting (T1.sub.0.5 V.sub.0.5)Sr.sub.2 (Ca.sub.0.8 Y.sub.0.2)Cu.sub.2 O.sub.y phase is given in FIG. 3(c). A contraction of lattice parameter c by 0.46% and expansion of the lattice parameter a by 0.10% was observed for (T1.sub.0.5 V.sub.0.5)Sr.sub.2 (Ca.sub.0.8 Y.sub.0.2)Cu.sub.2 O.sub.y as compared to the lattice parameters of (T1.sub.0.5 V.sub.0.5)Sr.sub.2 CaCu.sub.2 O.sub.y. This reduction in the lattice parameter c probably arises from the differences in ionic radii between the Y.sup.3+ ion (0.93.ANG.) and the Ca.sup.2+ ion (0.99.ANG.) and the expansion in the a lattice parameter a is attributed to a decrease in the average copper oxidation state, leading to larger Cu-O distances within the copper/oxygen sheets. A similar effect has been observed in earlier studies of Bi.sub.2 (Sr.sub.3-x Y.sub.x)Cu.sub.2 O.sub.8+delta (Subramanian, M. A. et al J. Solid State Chem. Vol177 pp196ff (1988)), BiSr.sub.2 (Ca.sub.1-x Y.sub.x)Cu.sub.2 O.sub.8+delta and T1Ba.sub.2 (Ca.sub.1-x Y.sub.x)Cu.sub.2 O.sub.7-de lta.

Compositions o individual single microcrystals in the prepared specimens, (T1.sub.0.5 V.sub.0.5)Sr.sub.2 CaCu.sub.2 O.sub.y and (T1.sub.0.5 V.sub.0.5)Sr.sub.2 (Ca.sub.0.8 Y.sub.0.2)Cu.sub.2 O.sub.y, have been examined using energy dispersive spectrometry. Thirty particles in each sample were randomly chosen. The majority phases in both samples were determined to be [T1.sub.0.72(4) V.sub.0.28(4) ]Sr.sub.2.1(1) Ca.sub.1.04(8) Cu.sub.2 O.sub.y and [1.sub.0.7(6) V.sub.0.27(4) ]Sr.sub.1.95(9) [Ca.sub.0.86(7) Y.sub.0.14(3) ]Cu.sub.2 O.sub.y, respectively as shown in FIG. 4. Both compositions fit the 1-2-1-2 phase very well. The composition of the main impurity phase in both the samples was determined by energy dispersive spectrometry to be close to Sr(V.sub.0.7 Cu.sub.0.2 Ca.sub.0.1)O.sub.y.

Selected area electron diffraction patterns for both the 1-2-1-2-like phase and the impurity phase have been recorded through tilting the specimen stage. We were able to confirm the unit cell Parameters of the 1-2-1-2 phase and determine the unit cell for the Sr, V-rich impurity phase (orthorhombic, with a=8.35.ANG., b=8.04.ANG. and c=17.36.ANG.). Using this data we have successfully indexed all extra peaks in FIG. 3(b) and 3(c). Although the detailed structure of this phase is not understood yet, we assume that the structure is probably related to pyrochlore type, say Sr.sub.2 [V.sub.1-x (Cu, Ca).sub.x ].sub.2 O.sub.7-8 and the unit cell shows a root 2 X root 2 X 3 superstructure based on the pyrochlore lattice.

Electron-spin resonance spectra of the two vanadium-containing superconducting phases with nominal compositions (T1.sub.0.5 V.sub.0.5)Sr.sub.2 CaCu.sub.2 O.sub.y and (T1.sub.0.5 V.sub.0.5)Sr.sub.2 Ca.sub.0.8 Y.sub.0.2)Cu.sub.2 O.sub.y were measured between room-temperature and 77K, with a view to probing the valence state of the vanadium ions. The synthetic conditions (see earlier), would almost certainly decompose V.sub.2 O.sub.5 to produce V.sub.2 O.sub.3 and/or VO.sub.2. The electron-spin resonance spectra of localised V.sup.4+ (3d.sup.1), V.sup.3+ (3d.sup.2) and V.sup.2+ (3d.sup.3) have all been extensively investigated. However, neither superconducting phase gave an electron-spin resonance signal. The V.sup.3+ ion has been previously detected as a dopant in A1.sub.2 O.sub.3 (Lambe, J, Kikuchi, C. Phys. Rev. vol 118 pp71-77 (1960)). It is characterised by a very large spin-lattice relaxation rate (a consequence of strong spin-orbit coupling and accessible, low-lying excited states) meaning the spectrum could only be observed at liquid helium temperatures. Clearly, low temperatures are required to probe the valence state of vanadium via electron-spin resonance, and in this regard, we were hampered by the appearance of a intense low-field microwave absorption that appeared on cooling the sample through T.sub.c. This response is characteristic of the superconducting state and has been widely studied. FIG. 5 shows the low-field microwave absorption from both samples at 77K, the signal from (T1.sub.0.5 V.sub.0.5)Sr.sub.2 (Ca.sub.0.8 Y.sub.0.2)Cu.sub.2 O.sub.y (b) being significantly more intense and broader than that from (T1.sub.0.5 V.sub.0.5)Sr.sub.2 CaCu.sub.2 O.sub.y (a), equivalent quantities of material being used in each case. The variation in the signal intensity of (T1.sub.0.5 V.sub.0.5)Sr.sub.2 CaCu.sub.2 O.sub.y as a function of microwave power is also shown as inset in FIG. 5; the signal exhibits an intial enhancement followed by saturation-type effect as the microwave power is raised, the latter being accompanied by a significant detuning of the microwave bridge.

EXAMPLE 2

The synthetic procedure set out in Example 1 was used to prepare compositions of the formula (T1.sub.0.5 M.sub.0.5) Sr.sub.2 (Ca.sub.0.8 Y.sub.0.2) CuO.sub.7-delta for M=Ti, Zr, Hf, Nb and Ta. Resistivity measurements were made in a similar manner and are plotted FIG. 6. Curve (b) is for M=Ti, curve (c) for Zr, curve (d) for Hf, curve (e) for Nb and curve (f) for Ta; curve (a) is a comparison curve for the prior-art composition (T1Sr.sub.2 (Ca.sub.0.8 Y.sub.0.2) Cu.sub.2 O.sub.7-delta, and it will be observed that all the substitutions result in increased T.sub.c compared with the prior art. There is no reason to believe that the compositions of this Example are optimum compositions for any of the metals M.

EXAMPLE 3

Various compositions of formula Tl-Cr-Sr-Ca-Cu-O were prepared from high purity Tl.sub.2 O.sub.3,Cr.sub.2 O.sub.3, SrCO.sub.3 and CaCO.sub.3, CaO and CuO which were mixed, ground, and heated at 900.degree. C.-950.degree. C. for about 24 hours with intermediate grinding to obtain uniform powders. These were pressed into pellets and heated for 5-15 minutes in an alumina crucible under a steam of oxygen. The furnace was cooled gradually to 850.degree. C. and that temperature was maintained for 8-24 hours before cooling to ambient temperatures.

The products were characterised as having a high proportion of 1-2-1-2 phase and T.sub.c 's in the range 100-110K.

(Examples 1 and 2 were conducted by the inventors at the Interdisciplinary Research Centre in Superconductivity, University of Cambridge and Example 3 has been reported by Sheng et al (Superconductor Week Feb. 4, 1991 page 2) working independently).

Claims

1. A superconducting material having a stoichiometric formula (Tl.sub.w, In.sub.1-w).sub.1-x M.sub.x Sr.sub.2 (Ca.sub.1-y M'.sub.y) Cu.sub.2 O.sub.z, in which W is in the range from 0 to 1 inclusive of both limits, x is greater than 0 but less than 1, y is in the range from 0 to 1 inclusive of both limits, and z is about 7, M is selected from the group consisting of V, Ti, Cr, Zr, Nb, Hf, Ta, Mn, Fe, Co, Ni, W, Os, or Ir, and M' is selected from the group consisting of Y and the rare earth elements.

2. A material according to claim 1 having the stoichimetric formula (T1.sub.1-x M.sub.x)Sr.sub.2 (Ca.sub.1-y M'.sub.y)Cu.sub.2 O.sub.z, in which O<x<1 and O<y<1 and z is about 7.

3. A material according to claim 2 in which 0.15<x<0.60.

4. A material according to claim 3 in which x is about 0.28.

5. A material according to claim 2 in which 1<y<0.5.

6. A material according to claim 1 in which M is V.

7. A material according to claim 1 in which M is selected from the group consisting of Ti, Cr, Zr, Nb, Hf and Ta.

8. A material according to claim 1 in which M is Cr.