NZ224205A - Metal oxide material comprising various mixtures of bi, tl, pb, sr, ca, cu, y and ag - Google Patents

Description

I 22 4 1 O Ci 4 J O i p»—-• $T^;- ' ^ '■ ,f-v< '• ■ Patents form No 5 PATENTS ACT 1953 COMPLETE SPECIFICATION METAL OXIDE MATERIALS v\ 3» 1 1 APR 19897 Number 224205/2281 32 Dated April 8, 1 98 8/ February 24, 1989 v-S. • I •fv V * >" > ?' 'A •}t/WE HER MAJESTY THE QUEEN in right of New Zealand, DSIR Physics and Engineering' Laboratory, Gracefield Road, Lower Hutt, New Zealand do hereby declare the invention for which #we pray that a Patent may be granted to M»/us, and the method by which it is to be performed, to be particularly described in and by the following statement: . 1 . - la - 224205 7 2 2 813 2 The invention comprises certain novel metal oxide materials which exhibit superconductivity at elevated temperatures.

It is known that certain classes of metal oxide will exhibit the phenomenon of superconductivity below a particular critical temperature referred to as T . These include as prototypes BaPb2_xBixC>2_ g* Ba2_xSrxCuO/j_YBa2Cu20^_ ^ as described in The Chemistry of High Temperature Superconductors, ed. by Nelson et al, American Chem. Soc. 1987, and Bi2Sr2CaCu20^_£ as described by Subramanian et al, Science 239, 1015 (1988). We have identified this last material as the n=2 member in a homologous series of approximate formula Bi2(Sr/Ca)n+^Cun02:n+^+.^r n=0,1,2,3, . .., obtained by inserting an additional layer of Ca and an additional square planar layer of CUO2 in order to obtain each higher member. These materials often exhibit intergrowth structures deriving from a number of these homologues as well as Bi substitution on the Sr and Ca sites. T is observed to rise as n increases from 1 to 2 to 3. c The material YBa2Cu40g+^ has a layered structure similar to the n=2 member of this series Bi2Sr2CaCu20g+,^ and we expect therefore that YBajCu^Og^ belongs to similar series. One such series could be obtained by insertion of extra Y-CUO2 layers resulting in the series of materials RnBa2Cun+302 5n_£> n=l,2,3,... and another by insertion of extra Ca-Cu02 layers resulting in the series RBa2CanCun+^Og+2n_^ / n=l,2,... By analogy it may be expected that Tc in these two series should rise with the value of n. 224k(J~> / 228132 - 2 - ' The invention provides certain novel metal oxide materials which exhibit superconductivity.

In broad terms in a first aspect of the invention comprises a metal oxide material having the formula: Bi~ (Sr, Ca) Cu 0,nLr. 2 v ' 'n+1 n 10+S wherein n=3, Bi may be partially replaced by Pb, Sr and Ca can be replaced in part by any alkali metal, alkali earth metal, or a combination thereof; and Cu can be replaced in part by Ag, Bi, Pb or Tl.

A particularly preferred n=3 material has the formula: Bi2.1Sr2Ca2Cu3°10+S- Preferred n=3 materials have the formula! Bi2-x Pbx (Sr' Ca>4 Cu3 O10tr /# £ ?o o wherein 0<x<0.4, or more specifically: *"» ^ CJ Bi2-x pbx Ca2+y-s Sr2-y Cu3 °10+4-wherein: 0<s<0.4; 0<z<0.4; -2<y<2; VO \o KJ ?24205 . 228132 2+y-s>0; and 0<x<0.4; and wherein A is an alkali metal, alkali earth metal, or a combination thereof.

Preferably in the n=3 materials of the invention £ is fixed in a range determined by annealing in air at between 300°C and 550°C, or by annealing in an atmosphere at an oxygen pressure or partial pressure and temperature equivalent to annealing in air at between 300°C and 550°C.

Especially preferred n=3 materials of the invention are ^ gPb0 # 35Ca2Sr2Cu3O10+s., Bi2 # ^Sr^O^ £l preferably wherein £ is fixed in a range determined by annealing in air at between 300°C and 550°C, or by annealing in an atmosphere at any oxygen pressure or partial pressure and temperature equivalent to annealing in air at between 300°C and 550°C.

In broad terms a further aspect of the invention comprises a metal oxide material having the formula ~ 5 FEB 1992; 0<x<0.15, B is Bi or Tl, or Bi or Tl partially replaced by Pb, B^Sr^Ca, R Cu 0o ^ 2 2 1-x x n 8-£ wherein n=2, 224205/228132 (""S _ 4 _ / Sr and Ca can be replaced in part by any alkali metal, alkali earth metal, or a combination thereof, and R is Y or any rare earth element.

Preferably R is Y.

Preferred n=2 materials have the formula: Bin <i Pb Sr„Ca, Y Cu~0o .. 2.1-y y 2 1-x x 2 8-J where 0<x<0.15 and 0<y<0.4.

In broad terms in a further aspect the invention comprises a metal oxide material having the formula B2-yPbySr2CaCun°8^-wherein n=2, o<y<o. 3 -5F C 8 19.9.2 •, B is Bi or Tl, and Sr and Ca can be replaced in part by any alkali metal, alkali earth metal, or a combination thereof. 224205 , 228132 Preferred examples are or Bi1.9Pb0.2Sr1.9CaCu2° 2 8-5 Preferably in the n=2 materials of the invention S is fixed in a range determined by annealing in air at between 700°C and 830°C or in 2% oxygen at between 600°C and 800°C, or by annealing in an atmosphere at an oxygen pressure or partial pressure and temperature equivalent to annealing in air at between 700°C and 800°C.

The materials of the invention may be formed as mixed phase or intergrowth structures incorporating structural sequences also including sequences from the material RBa2Cu30^_ ^ and its derivatives. For example, this includes RBajCu^ comprising, as it does, approximately alternating sequences of RBa2Cu20^_g and RBa2Cu^Og_^-. This also includes materials of the invention with n taking non-integral values allowing for the fact that, for example, a predominantly n=2 material may have n=l and n=3 intergrowths. This also includes materials of the invention with n taking non-integral values allowing for ordered mixed sequences of cells of different n values, for example, n=2.5 for alternating sequences of n=2 and n=3 slabs. .-prepared by solid state reaction of precursor materials such as metals, oxides, carbonates, nitrates, hydroxides, or any organic -5 FEB 1992.' Typically, the materials of the invention may be 2 2 4 2 C 5 22y132 salt or organo-metallic material, for example, such as 3120^, PbfNOg^/ Sr(NO^) Ca(N02^2 anc* Cu0 ^or Bj'-I)t,SrCaCu0 materials. The materials of the invention may also be prepared by liquid flux reaction or vapour phase deposition techniques for example, as will be known to those in the art. Following forming of the materials oxygen loading or unloading as appropriate to achieve the optimum oxygen stoichiometry, for example for superconductivity, is carried out. The above preparation techniques are described in "Chemistry of High Temperature Superconductors" - Eds. D L Nelson, M S Whittingham and T F George, American Chemical Society Symposium Series 351 (1987); Buckley et al, Physica C156, 629 (1988); and Torardi et al Science 240, 631 (1988), for example. The materials may be prepared in the form of any sintered ceramic, recrystallised glass, thick film, thin film, filaments or single crystals.

In order to achieve maximum strength and toughness for the materials, it is important that they are prepared to a density close to the theoretical density. As prepared by common solid-state reaction and sintering techniques, densities of about 80% theoretical density can readily be achieved. Higher densities may be achieved by, for example, spray drying or freeze drying powders as described for example in Johnson et al, Advanced Ceramic Materials 2, 337 (1987), spray pyrolysis as described for example in Kodas et al, Applied Physics Letters 52, 1622 (1988), precipitation or sol gel methods as described for example in Barboux et al J. Applied Physics 63., 2725 (1988), in order to achieve very fine particles of dimension 20 to 100 mm. After die-pressing these will sinter to high density.

I 2 2 4 2 C 5 2 2 y 1: 2 Alternatively, to achieve higher densities one may hot press, extrude, or rapidly solidify the ceramic material from the melt after solid state reaction or grow single crystals.

Preparation of the materials of the invention may be carried out more rapidly if in preparation of the materials by solid state reaction of precursor material any or all of the cations in the end material are introduced as precursors in the nitrate or hydroxide forms for rapid reaction of bulk material in the nitrate or hydroxide melt. Both the temperature and duration of the preparation reaction may be lowered by using nitrate or hydroxide precursors to introduce the cations.

Melting of the nitrate and/or hydroxide precursors allows intimate atomic mixing prior to decomposition and efflux of oxides of nitrogen.

After preparation the materials may be sintered or (re-)ground to small particles and pressed to shape and sintered as desired to form the end material for use, as is known in the art and/or annealed to relieve stresses and increase strength and toughness as is similarly known in the art for the unsubstituted materials. The materials after preparation may as necessary be loaded on unloaded with oxygen to achieve the optimum stoichiometry for superconductivity, optimised oxygen mobility, or other material properties. As stated, for n=2 ^ " 6 jv.

BCSCO materials for example this generally requires oxygen 1/0 *o 9j unloading into the materials and with known technique this is generally carried out by annealing at 700°C to 830°C in air over 1 to 4 hours followed by rapid quenching into liquid nitrogen, 2 2 -1 :j 0 ;] /' 22 813 2 - 8 - / for example. Most suitably, oxygen loading or unloading is carried out during cooling from the reaction temperatures immediately after the preparation reaction, where the materials are prepared by solid state reaction for example. Alternatively and/or additionally oxygen loading may be carried out during sintering or annealing in an oxygen containing atmosphere at an appropriate pressure or partial pressure of oxygen. Without loss of generality the materials may be annealed, cooled, quenched or subjected to any general heat treatment incorporating AgO or Ag20 as oxidants or in controlled gaseous atmospheres such as argon, air or oxygen followed by rapid quenching so as to control the oxygen stoichiometry of the novel materials, the said stoichiometry being described by the variables w or J. The materials may be used as prepared without necessarily requiring oxygen loading or unloading for forming electrodes, electrolytes, sensors, catalysts and the like O utilising high oxygen mobility property of the materials. r.-j Z-"J V3 The invention is further described with reference to' the following examples which further illustrate the preparation of materials in accordance with the invention. In the drawings which are referred to in the examples: Fig. 1 shows a plot of zero resistance Tc against the annealing temperature in air from which the sample was quenched into liquid nitrogen. (X) Bi2 ^Ca^r^u^O^Q; Bl2.lCaSr2Cu2°5' Bl2.1Ca0.5Sr2.5Cu2°5? (A) Bi2 iC3ii 5Sri 5Cu2°5' B:'-2 lCaSr2Cu2°5 cJuenched from 2% ? 2 4 20 5 228132 oxygen; (□) Bi-2 iCao 67Srl 33Cu06' ^ B^"2 iCaSrCu06• Inset: plot of maximum Tc versus n in the number of Cu layers.

Fig. 2 shows the XRD patterns for Pb-substituted compounds (a) n=2 x=0.2 and (b) n=3 x=0.35.

Fig. 3 shows the temperature dependence of the resistivity for Pb-substituted n=2 material x=0.2 (upper plot) and x=0.3 (lower plot).

Fiq. 4 shows the zero resistance T obtained as a 3 c function of anneal temperature for the n=2 and n=3 unsubstituted (open symbols, x=0) and Pb-substituted samples (filled symbols). 0:n=2, x=0.2, 21% oxygen; □ :n=2, x=0.2% oxygen; £, :n=2, x=0.2 0.2% oxygen; and O :n=3, x=0.35, 21% oxygen.

Fig. 5 shows the temperature dependence of resistivity for (a)n=2 and x=0.2 reacted at 800°C then annealed in air at 800, 7 00, 600 and 500°C before quenching into liquid nitrogen. A typical curve for the unsubstituted x=0 material is shown in the inset; (b) for n=3 and x=0.35 with typical behaviour for x=0 shown in the inset.

Fig. 6 shows the temperature dependence of the resistivity for Bi2 -^CaS^C^Og annealed in air at various temperatures shown in °C. (a) 5% Y substitution and (b) no substitution ^6 J99f : 224205 2281 Fig. 7 shows a plot of the zero resistance Tc as a function of air anneal temperature for 5% Y-substituted Bi2 ^CaS^C^Og (solid hexagons). For comparison, the Tc values for unsubstituted n=l, n= 2 and n=3 are also shown (open data points).

Fig. 8 shows a plot of the temperature dependence of the resistivity for the nominal composition Bi^ cjPkg 35Cao 9^0 lSr2Cu2°5 annealed anc* quenched at 500°C, 7 00°C and 820°C showing no change in T .

Fig. 9 shows a series of resistivity plots against temperature for n=3 and n=2 after annealing in air and then quenching into liquid nitrogen. The annealing temperatures are indicated in °C.

Fig. 10 shows the [551] zone axis electron diffraction o o o patterns for n=l, n=2 and n=3 indexed on a 5.4A x 5.4A x 2cA O cell where c=18.3+6.3n A.

Example 1 (n=2) "" i-Li 19 Samples of composition Bi2 2-xP^xCaSr2Cu2°8+ & with x=0, 0.1, 0.2, 0.3, 0.4 and 0.5 were prepared by solid state reaction of Bi203f CaCo^, SrCO^/ CuO and PbO for 12 hours at temperatures between 850 and 865°C. The samples were ground, pressed and sintered at the same temperature for a further 12 hours. Fig. 2 shows the XRD trace for x = 0.2 which confirms nearly single phase material. Minor impurity peaks are marked 224205 - ii - bi by dots. Pb substitution is confirmed by observing a systematic variation in lattice parameters with x, as follows a = 5.405 - 0.048 x £ and c = 30.830 - 0.094 x K Electron beam x-ray analysis of crystallites also confirmed the above compositions. Electron diffraction shows that the b-axis superstructure remains at about 4.75X for 0 < x < 0.2. For 0.2 < x < 0.35 this superstructure contracts to 4.5X and a second b-axis superstructure appears with length 7.3X. Substitution for x > 0.35 did not occur under the conditions of preparation. Samples were annealed at various 4 temperatures at oxygen partial pressures of 2.1 x 10 Pa (air), 3 2 2 x 10 Pa and 2 x 10 Pa then quenched into liquid nitrogen.

The DC resistivity of these samples was measured using a 4-terminal method and AC susceptibility was also measured. Fig. 3a and 3b show the resistivity curves for anneals in air for x=0.2 and x=0.3 respectively. Tc is seen to decrease with decreasing anneal temperature. Fig. 4 shows the zero resistance Tc versus annealing temperature for the three oxygen partial pressures for both x=0 and x=0.2. Tc is seen to pass through a maximum for an optimum oxygen content. The maximum Tc obtained is 93 K. This increase in not due to n=3 material which has a different behaviour also shown in Fig. 4. The optimised Tc is maximised at 93 K for x=0.2 and falls 4 K for x=0.3.

The sharpness of the resistive transitions should b'e . j-noted in Fig. 3 and compared with the typical best curve 224205 228132 obtained for x=0 shown in the inset in Fig. 5a which exhibits a typical resistive tail. The resistivity curves shown in the main part of Fig. 5a are for a x=0.2 sample prepared at the lower temperature of 800°C. The variation in Tc as a function of anneal temperature is similar to that shown in Fig. 3a but the normal state resistivity varies differently. Electron microprobe analysis indicated crystallites which were Pb-rich and deficient in Sr and Ca indicating substitution of Pb on the alkali-earth sites.

Example 3 (n=2) Samples of composition Bi2 xCal-xRxSr2Cu2^8+ & were prepared by solid state reaction of 6120^, CaCO^, SrCO^, CuO and **2^3 where R is Y or one of the rare earth elements.

Compositions with x=0, 0.05, 0.1, 0.2, 0.4, 0.9 and 1.0 were reacted at temperatures ranging from 860°C to 900°C as the rare earth content was increased. Samples were investigated by x-ray diffraction, electron diffraction, IR spectroscopy, thermal gravimetry, and the temperature dependence of resistivity and AC susceptibility. In the following example we deal exclusively ' with the results for Y-substitution.

FEB 1992.

The end member x=l was XRD phase pure and notably has the same structure as Bi2 ^CaS^C^Og^- except that the symmetry is reduced from tetragonal to orthorhombic as shown by the splitting of the (200) XRD peak. For Bi2 ^YS^C^C^£, annealed in air at 400°C the lattice parameters are a=5.430A, b=5.473A and c=30.180A. Electron diffraction also reveals the presence of an 8X incommensurate superstructure, of 43.5 A in the b-direction.

Attempts to study the metal to insulator transition at intermediate substitutions (0<x<l) were prevented by the absence of a substitutional solubility range under the conditions of preparation. In this intermediate range samples were a mixed phase of the x=0 and x=l end-members except at compositions close to x=0 and x=l where doping appears to be possible.

Interestingly, Y substitution for Ca at the 5% level in Bi2 ^CaSr2Cu20g+^ appears to raise Tc- Fig. 6a shows the temperature dependence of the resistivity for such a sample annealed and guenched at different temperatures in order to vary the oxygen stoichiometry, ^. This may be compared with the same data shown in Fig. 6b for the x=0 unsubstituted n=2 material.

The substitution has both sharpened the resistive transition and raised the zero resistance T . Otherwise the overall behaviour c is similar and quite distinct from the n=3 behaviour. ^ ?o O ....... rn The zero-resistance T is plotted in Fig. 7 as a if £ 'MD vO function of anneal temperature (solid data points) and evid&i,tly T is maximised at >101 K. Fig. 7 also includes T data for c ^ c unsubstituted n=l, n=2 and n=3 material for comparison. Like unsubstituted n=2 the Y-substituted material appears to exhibit a maximum T for anneals in air above 820°C. However, above c ' this temperature the effects of annealing and quenching are greatly modified by the proximity of a phase transition. In order to achieve maximum T anneals at an oxygen partial c pressure less than that of air is required. The highest zero t 2242C5 ?2a t 32 resistance Tc we have observed in this system is 102 K. The elevated T does not arise from the presence of n=3 material for c several reasons: i) We are able to prepare single-phase n=3 material by Pb-substitution for Bi. Attempts to substitute Y in this material at the 5% level drives the reacted material completely to the n=2 phase together with the binary Ca2Cu02. We would therefore hardly expect Y substitution of the n=2 material to promote n=3 material. ii) The annealing behaviour of Tc is similar to that for unsubstituted n=2 material with maxima occurring at 820°C or higher. The maximum Tc for n=3 occurs for anneals at 400°C and for anneals at 820°C the n=3 Tc is as low as 80 K.

Particle by particle analyses by SEM electron beam x-ray analysis indicates that Ca remains fixed at one per formula unit while Sr is slightly depleted. This suggests Y substitution on the Ca-site accompanied by Ca substitution on the Sr-site with the formula Ba2 i(CaQ 95Y0 05^Srl ■9SCa0. ,05)_ Cu20g. Starting compositions appropriately depleted in Sr indeed offered the best resistive transitions around 100 K with a minimal tail.

"J ^ r- -j It may be that the substitutional solubility tends to111 ^9/.',. occur only at grain boundaries as the sharp resistive transition. ^ - 15 - 224::f,;;/22fc to zero at 101 K is accompanied by only a small diamagnetic signal in the AC susceptibility commencing at 99 K. A sharp fall does not commence until 95 K at which point the diamagnetic signal is only about 5% of its fully developed value. The yttrium should therefore be dispersed more uniformly throughout a sample by reaction of nitrates or by melt processing.

This example and particularly the chemical formula deduced for the active phase responsible for raising T_, is presented by way of example without loss of generality. Because of the small diamagnetic signal appearing at ~100 K it may be that the active phase is of another composition and structure incorporating Bi/Ca/Y/Sr/Cu/O.

Example 4 (n=2 ) 'nn; 03 Co Predominantly single phase Bi^ 35<-ao 9^0 1^*2~ '• Cu208+£ was prepared by solid state reaction of a pressed disc of Bi203/ PbO, CaCO^/ SrCO^, Y2^3 anc* Cu<^ at ^or ^ hours.

By annealing in air at various temperatures then quenching into liquid nitrogen, the normal state resistivity is observed to change as shown in Fig. 8. However, the zero-resistance Tc does not change, remaining at just over 90 K for anneals at 500, 700 and 820. This is uncharacteristic of the parent n=2 superconductor as shown in Fig. 6 which required anneals at an oxygen partial pressure less than that of air in order to optimise Tc at just over 90 K. The combined Pb- and 224205 1 ' 16 ' 22a 132 Y-substitution therefore simplifies the processing requirements for n=2 material.

Example 5 (n=3^ A sample of nominal composition BiSrCaCu^O^. was prepared from the carbonates of Sr and Ca, CuO and bismuth oxycarbonate by reacting at 820°C for 9 hours, then for 10 hours at 850°C then for 10 hours at 860°C followed by air-quenching from the furnace. The sample was then annealed in air at temperatures ranging between 4 00°C and 800°C and quenched from the furnace into liquid nitrogen. Four terminal electrical DC resistivity and the AC susceptibility was measured for each anneal temperature. Fig. 9 shows the resistivity curves obtained for this sample after each anneal. The resistivity drop which occurs around 110 K is extrapolated to zero and the deduced zero resistance T is seen to be maximised at 105 K for c anneals at about 400°C. The annealing behaviour is seen to be quite different from that of the n=2 material.

This sample was pulverised, ground and investigated by XRD, SEM energy dispersive analysis of x-rays (EDX) and TEM electron diffraction. The EDX analyses indicated a high proportion (>70%) of particles with atomic ratios Bi:Sr:Ca:Cu of 2:2:2:3 though many of these particles showed Cu contents more like 2.8 to 2.9 indicating the occurrence of n=2 intergrowths in the n=3 material. Like the n=2 material, crystals of n=3 are platey and under TEM electron diffraction were found to exhibit a 5.4 & x 5.4 K subcell in the basal plane with the same 19/4-\. ' ' y y' 224205 228152 times incommensurate superlattice structure in the b-direction. The diffraction pattern for the [551] zone axis shown in Fig. 10 can be indexed on a 5.4 & x 5.4 k x 74 & cell suggesting a O sub-cell c-axis of 37 A with a superstructure which doubles the c-axis. XRD powder diffraction of this sample showed a broad O basal reflection corresponding to a c-repeat of about 18 A.

This leads to the natural conclusion that Bi2Sr2Ca2Cu20^g is structurally related to Bi2Sr2Ca^Cu20^ by the insertion of an extra pair of Ca~CuC>2 sheets per unit sub-cell.

Example 6 (n=3) Samples of composition Bi2 2-xP^>x<"a2<"U3^,10+^ were prepared by reaction of the oxides of Bi and Cu and the nitrates of Pb, Ca and Sr in stoichiometric proportions for 36 hours at 860 to 865°C in air. The XRD pattern shown in Fig. 2b indicates nearly single phase pseudo-tetragonal material with lattice parameters a=5.410 and c=37.125 Like the n=2 x=0.2 material, electron diffraction indicates a 4.5 times and a 7.3 times b-axis superlattice structure. The effect on resistivity curves of annealing in air at various temperatures is shown in Fig. 5b and the curve for the x=0 material is shown in the inset. The long resistive tail in the unsubstituted material is removed by Pb-substitution. The effect of annealing temperature in air on the zero resistance Tc is shown in Fig. 4 by the diamond shaped points for x=0.35 and x=0.

The foregoing describes the invention including , ^ preferred forms and examples thereof. The preparation of - 18 224205 derivative materials and forms other than sintered ceramic form, i.e. thin films, thick films, single crystals, filiaments and powders other than those specifically exemplified will be within the scope of those skilled in the art in view of the foregoing. The scope of the invention is defined in the following claims.

' ' -SEPjygj.

Claims (18)

224205 7 228152 - 19 - What We Claim Is:

1. A metal oxide material having the formula Bi2 (Sr, Ca|n+1 Cun O10+f wherein n=3, Bi may be partially replaced by Pb, Sr and Ca can be replaced in part by any alkali metal, alkali earth metal, or a combination thereof; and Cu can be replaced in part by Ag, Bi, Pb or Tl.

2. A metal oxide material having the formula Bi2.1Sr2Ca2Cu3°10+r

3. A metal oxide material according to claim 1 having the formula Bl2-x Pbx (Sr' Ca)4 Cu3 °10+£ wherein 0<x<0.4.

4. A metal oxide material according to claim 1 having the formula Bi0 Pb Ca„. Sr- A Cu0 0,n,r 2-x x 2+y-s 2-y z 3 10+£ wherein: 0<s<0.4; ;-SFE8!992.: - 20 - 0<2<0.4; -2<y<2; -1<CT<1; 2+y-s>0; and 0<x<0 .4 ; and. wherein A is an alkali metal, alkali earth metal, or a combination thereof.

5. A metal oxide material according to any one of claims 1 to 4 wherein ^ is fixed in a range determined by annealing in air at about 500°C.

6. A metal oxide material having the formula B~Sr0Ca, R Cu 0o <• 2 2 1-x x n 8-6 wherein n=2, 0<x<0.15, B is Bi or Tl, or Bi or Tl partially replaced by Pb, Sr and Ca can be replaced in part by any alkali metal, alkali earth metal, or a combination thereof, and R is Y or any rare earth element. - / -

7. A metal oxide material according to claim 6, wherein R is Y. ZZ£|J2_ 22-Y.'J>"> - 21 - /

8. A metal oxide material having the formula Bl2 . iSr2Cao . 95Y0 . 05Cu2°8-i' *

9. A metal oxide material having the formula Bl2.lSr2Ca0.9Y0.lCu2°8-^ *

10. A metal oxide material according to either one of claims 1 and 2, wherein B is Bi partially replaced by Pb.

11. A metal oxide material having the formula B12.l-yPbySr2Cal-xYxCu2°8-S - K> where .' O Tl rn co 0<x<0.15 and 0<y<0.4. ^

12. A metal oxide material having the formula Bll.9Pb0.35Sr2Y0.lCu2°8-£ *

13. A metal oxide material according to any one of the preceding claims, wherein ^ is fixed by annealing in an - 22 - 99 / 228 <- ' i ._ IJ < J oxygen partial pressure equal to or less than that of air at a temperature exceeding 75 0°C.

14. A metal oxide material according to any one of the preceding claims, wherein ^ is fixed by annealing in an oxygen partial pressure equal to or less than that of air at a temperature exceeding 750°C and after annealing is quenched rapidly to maintain the value of fa substantially unchanged.

15. A metal oxide material having the formula B2-yPVr2CaCun°8-i' wherein n=2, K) O -n rn o<y<o.3 ^ > sO r- B is Bi or Tl, and Sr and Ca can be replaced in part by any alkali metal, alkali earth metal, or a combination thereof.

16. A metal oxide material having the formula Bll. 9Pb0. 2Sr2Ca0. 9Cu2°8- <5 or Bll.9Pb0.2Sr1.9CaCu2°8-£• / P 9 -« - 23 - 0 .'j O O ; (> i

17. A metal oxide material according to claim 16, wherein & is fixed by annealing at a temperature exceeding 750°C in an oxygen partial pressure equal to or less than that of air.

18. A metal oxide material according to claim 17, wherein after annealing the oxide is quenched rapidly so as to maintain the value of ^ substantially unchanged. WEST-WALKER, NIcCABE per: ^ cl^ Lf^Ju' attorneys for the applicant ' >;> CO o -n rn CP Co KJ