Metal oxide materials

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_c. These include as prototypes BaPb_2-xBi_xO_3-δ, Ba_2-xSr_xCuO_4-δ, YBa₂Cu₃O_7-δ as described in The Chemistry of High Temperature Superconductors, ed. by Nelson et al, American Chem. Soc. 1987, and Bi₂Sr₂CaCu₂O_8+δ as described by Subramanian et al, Science <u>239,</u> 1015 (1988). We have identified this last material as the n=2 member in a homologous series of approximate formula Bi₂(Sr,Ca)_n+1Cu_nO_2n+4+δ , 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_c is observed to rise as n increases from 1 to 2 to 3. The material YBa₂Cu₄O_8+δ has a layered structure similar to the n=2 member of this series Bi₂Sr₂CaCu₂O_8+δ and we expect therefore that YBa₂Cu₄O_8+δ belongs to similar series. One such series could be obtained by insertion of extra Y-CuO₂ layers resulting in the series of materials R_nBa₂Cu_n+3O_3-5+2-5n-δ, n=1,2,3,... and another by insertion of extra Ca-CuO₂ layers resulting in the series RBa₂Ca_nCu_n+4O_8+2n-δ , n=1,2,... By analogy it may be expected that T_c in these two series should rise with the value of n.

The invention provides certain novel metal oxide materials which exhibit superconductivity.

The invention comprises a metal oxide material having the formula:         Bi_2·1Ca₂Sr₂Cu₃O_10+δ wherein n=3, and Bi can be replaced in a minor part by Pb; <ul id="ul0001" list-style="none"><li>Sr and Ca can be replaced in a minor part by</li><li>Na, K, Rb, Cs or Ba or a combination thereof; and</li><li>Cu can be replaced in part by Bi, Pb or T1.</li></ul>

Preferably in the n=3 materials of the invention 6 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.

An especially preferred n=3 material of the invention is Bi_2.1Ca₂Sr₂Cu₃O_10+δ, preferably wherein δ 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.

The materials of the invention may be formed as mixed phase or intergrowth structures incorporating structural sequences 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.

Typically, the materials of the invention may be prepared by solid state reaction of precursor materials such as metals, oxides, carbonates, nitrates, hydroxides, or any organic salt or organo-metallic material, for example, such as Bi₂O₃, Pb(NO₃)₂, Sr(NO₃)₂, Ca(NO₃)₂ and CuO for BiPbSrCaCuO 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 <u>C156,</u> 629 (1988); and Torardi et al Science <u>240,</u> 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 <u>2</u>, 337 (1987), spray pyrolysis as described for example in Kodas et al, Applied Physics Letters <u>52</u>, 1622 (1988), precipitation or sol gel methods as described for example in Barboux et al J. Applied Physics <u>63</u>, 2725 (1988) in order to achieve very fine particles. After die-pressing these will sinter to high density. 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 or unloaded with oxygen to achieve the optimum stoichiometry for superconductivity, optimised oxygen mobility, or other material properties. 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 Ag₂O 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 variable δ.

The following examples illustrate the preparation of superconducting materials in accordance with the invention. In the drawings which are referred to in the examples:

Fig. 1 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. 2 shows the [551] zone axis electron diffraction patterns for n=1, n=2 and n=3 indexed on a 5.4Å x 5.4Å x 2cÅ cell where c=18.3+6.3n Å.

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

Fig. 4 shows the temperature dependence of resistivity for (a)n=2 and x-0.2 reacted at 800°C then annealed in air at 800, 700, 600 and 500°C before quenching into liquid nitrogen. A typical curve for the unsubstituted x=O material is shown in the inset; (b) for n=3 and x=0.35 with typical behaviour for x=O shown in the inset.

Fig. 5 shows the zero resistance T_c obtained as a function of anneal temperature for the n=2 and n=3 unsubstituted (open symbols, x=O) and Pb-substituted samples (filled symbols). o:n=2, x=0.2, 21% oxygen; □:n=2, x=0.2m 2% oxygen; Δ:n=2, x=0.2 0.2% oxygen; and ◇:n=3, x=0.35, 21% oxygen.

Example 1 (n=3)

A sample of nominal composition BiSrCaCu₃O_X 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 400°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. 1 shows the resistivity curves obtained for this sample after each anneal. The resistivity drop which occurs around 110K is extrapolated to zero and the deduced zero resistance T_c is seen to be maximised at 105K for anneals at about 400°C. The annealing behaviour is seen to be quite different from that of the n=2 material for which we have shown similar resistivity curves after anneals at different temperatures in Fig. 1 also.

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 (&gt;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 Å x 5.4 Å subcell in the basal plane with the same 19/4 times incommensurate superlattice structure in the b-direction. The diffraction pattern for the [5<maths id="math0001"><math display="inline"><mrow><mover accent="true"><mrow><mtext>5</mtext></mrow><mo>¯</mo></mover></mrow></math><img file="EP0336450B1_D0001.tif" /></maths>1] zone axis shown in Fig. 2 can be indexed on a 5.4 Å x 5.4 Å x 74 Å cell suggesting a sub-cell c-axis of 37 A with a superstructure which doubles the c-axis. XRD powder diffraction of this sample showed a broad basal reflection corresponding to a c-repeat of about 18 Å. This leads to the natural conclusion that Bi₂Sr₂Ca₂Cu₃O₁₀ is structurally related to Bi₂Sr₂Ca₁Cu₂O₈ by the insertion of an extra pair of Ca-CuO₂ sheets per unit sub-cell. We have shown similar diffraction patterns for n=1 and n=2 samples in Fig. 2 also.

Example 2 (n=3)

Samples of composition Bi_2.2-xPb_xCa₂Sr₂Cu₃O_10+δ, 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. 3b indicates nearly single phase pseudo-tetragonal material with lattice parameters a=5.410 Å and c=37.125 Å. We have shown an XRD pattern for a Pb substituted n=2 sample in Fig. 3a. 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. 4b 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. We have shown similar curves for the n=2 sample in Fig. 4a. The effect of annealing temperature in air on the zero resistance T_c is shown in Fig. 5 by the diamond shaped points for x=0.35 and x=0, which also shows the effect of annealing temperature in air on the zero resistance Tc for various n=2 samples.

The foregoing describes the invention including preferred forms and examples thereof. The preparation of 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.