METHOD OF MAKING HIGH CRITICAL TEMPERATURE SUPERCONDUCTORS, AND STARTING MATERIAL THEREFOR

Abstract: A method of fabricating a superconducting ceramic based material in which a constituent of the ceramic microstructure which gives the final product its superconducting properties is replaced by a halide prior to thermomechanical processing of the material so as to improve the ductility of the combined material for processing, and the halide is then replaced by the desired constituent to provide the superconducting properties after processing. The halide is preferably fluorine. The introduction or replacement of constituents may be carried out electrochemically using fast ionic compounds.

Description: Title: Method of making high critical temperature superconductors, and starting material therefor Field of the Invention This invention concerns superconducting ceramic materials and their manufacture and is directed to improvements in the fabrication of high critical temperature superconducting materials to form components and wires. Background to the Invention Although superconducting comporients and wires have been available for some years they have in general been based on metallic compounds. Examples of existing commercial materials are NbTi alloys and A15 interϊietallic compounds such as Nb Sri. A new class of very high critical temperature, high field, superconduc ing oxide phases, (known as ceramic superconductors), presents new opportunities for applications of superconductivity but raises new problems associated with the design of components and wires, because of the brittle nature of the material involved. It is likely that other ceramic- superconducting materials (such as sulphur nitride, and molybdenum nitride) will present similar problems. In the first place, these ceramic materials are inherently brittle, and this raises acute problems for the support of a conductor under the conditions of mechanical stress encountered in many high field applications. Secondly, in their optimum superconducting state these materials are chemically extremely active, in particular exhibiting extreme sensitivity to the presence of oxygen and moisture, and in view of their extreme chemical activity, the components are also susceptible to corrosion and environmental degradation. Thirdly, the brittle nature of ceramics limits the solid state processing of these materials into wires and components and furthermore makes it difficult to achieve in a controlled way the special microstructures that are essential if these materials are to support the large superconducting critical current density necessary for most commercial applications. Summary o£ the Invention According to one aspect of the present invention a starting material from which a ceramic superconductor is to be formed comprises a ceramic base and a replaceable ceramic microstructure constituent which gives the combination significant ductility to enable thermomechanical working to be performed to produce a desired shape or configuration of the combined material, and which constituent is replaceable by another constituent material which gives the desired superconducting properties to the combined material. According to another aspect of the invention there is provided a method of making a superconducting ceramic based material, in which a constituent " of the ceramic microstructure which is required to produce the superconducting properties (the superconductivity constituent), is substituted or replaced during at least an initial stage of the manufacturing process by a constituent which provides the ceramic microstructure with significant ductility, so that the material can be thermomechanically processed in the solid state, and thereafter the substituted constituent is replaced by the superconductivity constituent. Thus in a method of making a component from a superconducting ceramic based material, the ceramic microstructure is controlled during solid state thermo- mechanical processing so that the ceramic has a particular chemical composition which gives the material ductility and after the solid state thermomechanical processing has been employed to•produce the final desired shape of the component, the chemical composition of the material is adjusted to give the material the desired superconducting properties. After processing the chemical composition of the material is altered because high temperature ductility and good superconducting properties may not be found together in ceramic compounds which exhibit superconducting properties. To avoid doubt the expressions superconductor and superconducting material as employed herein are intended to mean materials which are capable of superconducti ity. The ceramic microstructure with the substituted constituent has significant ductility (typically having a Vickers hardness value similar to or below that of steel) at least at elevated temperatures at which conventional thermomechanical processing procedures are carried out (typically about 700°C), although in some cases the material may have significant dutility even at room temperature. The composition of the material is conveniently temporarily altered to improve ductility by addition of a halide, particularly a fluoride. Thus, where the superconducting ceramic-based material is to be an oxide phase or oxy-halide phase ceramic, the material may be prepared, either initially or at some intermediate stage, as a halide so as to possess improved ductility, and after thermomechanical working, may be oxidised to remove and replace some or all of the halide constituent to produce the superconducting oxide or oxy- halide phase. Oxidation to remove halide may be carried out, eg, by . heating the material in pure oxygen gas, controlling the partial pressure of oxygen if necessary to obtain the desired degree of oxidation. Alternatively, oxidation may be carried out electrochemically by depositing an oxygen electrolyte (such as Bi D •Y 2°3' Y 2°3* Zr °2 ° r C C0 - ZrO 2 ^ and a halide electrolyte (such as LaF_ for fluorides) in contact with the material and applying a suitable electric potential across the electrolytes. The halide electrolyte may not be essential in all cases, and i-t may be possible to achieve the desired result by applying a suitable potential to the oxygen electrolyte. If a wire is required the ductile material may be formed into a wire by conventional wire processing techniques, typically by wire drawing, extrusion or swaging. The material may also comprise a composite with suitable cladding and internal structure, as described in our copending International Application No. PCT/GB88/00381, and if desired only selected parts of the composite may be rendered ductile while other parts may remain brittle or in the form of a powder, during the fabrication processing. A- particularly beneficial feature of the method of ductile wire processing described is the formation of an elongated textured microstructure. Such a microstructure is known to be very beneficial for supporting a high critical current density in conventional commercial superconductors (e.g. ductile NbTi alloys). After thermomechanical processing the wire may be subjected to further annealing prior to conversion " to the required superconducting phase.. The material can be adjusted to the required composition for optimum superconducting properties by more than merely one step, and a plurality of conversion steps may be employed to achieve full superconductivity. A fully processed conductor may be electrochemically adjusted into its optimum composition and state of oxidation or valence state so as to maximise its superconducting properties, as described in our copending International Application No. PCT/GB88/00381. In particular according to another feature of the invention the orientation of the ceramic material structure has been found to be significantly improved to allow high critical current densities to be obtained by titrating oxygen out of the pure oxide superconducting ceramic and then allowing the oxygen to return to restore the equilibrium over a period of time. Titration may be achieved using a solid electrolyte fast ionic conductor in contact with the superconductor and electrochemically titrating oxygen from the latter into the electrolyte. The oxygen level in the superconducting ceramic may be restored by allowing stoichiometry to reach its equilibrium value by slow cooling or by reversing the electrochemical process. The material, in addition or alternatively, may be further processed to maximise its superconducting properties by successive annealing treatments in a vacuum or low pressure gas, on the one hand, and in gas with a high partial pressure of oxygen on the other hand. In this way the composition may be changed in a series of discrete annealing cycles. The superconductor may be required to be in the form of a tape or component of a particular shape. In this case the ductile ceramic may be hot-formed by -roiling or forging. The material may be clad and may have a composite structure as in the case of wire. After forming to achieve the desired shape and microstructure the composition and state of oxidation or valence state may be adjusted as described above in the case of wire. The invention also envisages an alternative- method of processing- whereby the ceramic is initially compacted and partially processed as a brittle powder (or distribution of different brittle and ductile powders). Suitable cladding and appropriate separating barriers may be incorporated. After partial processing the brittle powder that is eventually intended to be the superconducting material is converted electrochemically or otherwise into a ceramic phase with ductility, and thermomechanical processing of the material in the solid state is performed in a manner such as described above, whereafter the ductile phase is altered, eg into an oxide or oxy-halide phase, to provide the desired superconducting properties. It may be convenient for certain applications to remove those parts of the composite connected with adjusting the composition, subsequent to optimising the superconductor. Thus where for example copper/copper oxide is employed as a source or sink material, and/or fast ionic conductors are used to assist the substitution (or control) step, such source or sink material and any fast ionic conductors may need to be removed prior to the processing of the ceramic to enhance its superconducting properties. The invention will now be described by way of example with reference to Figure 1 of the accompanying drawing which illustrates how a circular section composite in the form of a wire can be constructed in accordance with the invention, and by reference to various examples and tests performed on superconducting oxide phase ceramic materials. Figure 1 is a cross section through a wire which is capable of functioning as a superconductor and which is constructed in accordance with the invention. In the drawing a copper wire 24 is surrounded by copper oxide 26, within a tube of yttria bismuth oxide 52 forming an oxygen ion conductor which is itself surrounded by a layer 48 of yttria barium copper oxide superconductor. This in turn is surrounded by a lanthanum fluoride layer 50 forming a fluorine ion conductor. A layer of porous platinum 56 provides for the establishment of an electric potential through the junction, to enable electrochemical / titration of oxygen and fluorine to be effected. In a series of tests to establish the change in hardness and ductility a number of samples of oxide phase- and oxyhalide phase- superconducting ceramic materials were prepared and tested. Example 1 Using the following as starting materials:- Y O "+ BaCO + CuO + CuF in the molar ratio:- 0.5 Ϊ 2 : 3 . 0 calcination was performed for 24 hours in air at 920°C. The material was then sintered in air for 24 hours at 950°C and furnace cooled to 650°C and thereafter cooled in air. A material having the composition YBa Cu 0 was 2 3 -o,• produced. A constant load compression test was performed on a pellet of the prepared material in which the temperature was ramped to 820°C. No detectable deviation from a linear relationship between force and reduction in thickness was noted. The material was then subjected to a constant load at 820°C for 2 hours and no change in dimension was noted to have occured. The results from this test were used as a comparison with the results to be obtained on an oxyhalide-phase of the same material. Example 2 Using the same starting materials but in the molar ratio 0.5 : 2 : 1 : 2 calcination was performed for 24 hours in air at 900°C. Thereafter the material was sintered for 24 hours in air at 900°C and subsequently annealed for 120 hours in air at 500°C and furnace cooled. Material having the composition YBa Cu_ " 0 - F. was produced. A constant load compression test was performed on a pellet of the prepared material in which the temperature was ramped to 760°C at 16 β C per minute. The applied load was 0.24kN. Up to this temperature no detectable deviation from a linear relationship between force and reduction in thickness was noted. Beyond 760°C deviations from the linear relationship was observed and at 780"C considerable deformation under constant load was noted to be occuring and the test was - 10 - terminated . The sample was found to have suffered a 22% reduction in thickness . Thus whils t the initial dimens ions of the pallet were 5 .0mm x 4 .0mm x 1 .52mm, the / final dimensions were 5 .6mm x 4.4mm x 1 .18mm. Thus the yield point in compression at 780 °C for the -2 sample was 12 MN which is comparable to that for ultra pure ductile metals with a cubic close packed structure. As " a comparison the yield strength for aluminia is 5000 MNm -2 that for zirconniia is 4000 MNm-2 and that for magnesia is 3000 MNm-2 Example 3 Using the same starting materials as in Example 1, it is possible to produce a slightly less oxygen-rich material having an expected composition of YBa_Cu ' 0_ CL „F_ by using J o.O+X 2 a different molar ratio of 0.5 s 2 s 2 : 1 and subjecting the starting materials to the same heat treatment as in Example 2. Performing Vickers hardness tests on pellets of the final materials from Examples 2 and 3 at room temperature produced results in the range 70-170. These values are to be compared with the following Vickers hardness values for the following materials:- YBa Cu 0 ? 1000 Al O 1800 i0 2 1100 Si C 3500 NaCl 250 The introduction of fluorine into the ceramic material thus rendered the material very soft and ductile, even at room temperatures. As a comparison, the value of 70 is /the value usually associated with pure lead. The hardness tests were made using a diamond indentor which is pressed nto the surface of the sample under a known load and the size of the indentation produced is a measure of the hardness.

Claims: CLAIMS 1. A method of making a superconducting ceramic based material, in which a constituent of the ceramic microstructure which is required to produce the superconducting properties (the superconductivity constituent), is substituted or replaced during at least an initial stage of the manufacturing process by a constituent which provides the ceramic microstructure with significant ductility, so that the material can be thermomechanically processed in the solid state, and thereafter the substituted constituent is replaced by the superconductivity constituent. 2. The method as claimed in claim 1 in which the thermomechanical processing is employecT to produce the final desired shape of a component. 3. The method as claimed in claim 1 or 2 in which the superconducting ceramic based material is to be an oxide phase ceramic wherein the material is prepared as a halide so as to possess ductility at elevated temperatures and after thermomechanical working, is oxidised to remove and replace all of the halide constituent to produce a superconducting oxide phase. 4. The method as claimed in claim 1 or 2 which the superconducting ceramic based material is to be an oxy- halide ceramic wherein the material is prepared as a halide so as to possess ductility at elevated temperatures and after thermomechanical working is oxidised so as to remove some of the halide constituent to produce a superconducting oxy-halide phase. 5. The method as claimed in claim 3 or 4 wherein the halide is fluorine. 6. The method as claimed in any of the preceding claims in which only part of the ceramic microstructure is rendered ductile for the thermomechanical processing and the substitution of the superconductivity constituent during processing is limited to that part which is to be thermomechanically processed. 7. The method as claimed in any of the preceding claims wherein the thermomechanical processing step comprises drawing, extrusion, swaging, rolling or forging. 8. The method as claimed in any of the preceding claims wherein the material is to be a wire- " and the material is subjected to at least one annealing step before the ductile substituted constituent is replaced by the less ductile superconducting constituent. 9. The method as claimed in claim 8 in which the annealing is performed in part in a vacuum (or in a low pressure gaseous environment) and subsequently in one or more gaseous environments having a high partial pressure of oxygen, thereby to cause oxidation to occur during the annealing steps. 10. The method as claimed in any of the preceding claims wherein the replacement, substitution or adjustment of the superconductivity constituent, to enhance the ductility of the material (or control its chemical composi ion), is achieved electrochemically. 11. The method as claimed in any of the preceding claims wherein the replacement (or control of the chemical composition) step, is performed after partial processing of the starting material. 12. A starting material for a method as claimed in any of the preceding claims comprising a ceramic base and a replaceable ceramic microstructure constituent which gives the combination sufficient ductility to enable thermomechanical working to be performed to produce a desired shape or configuration of the combined materials and which constituent is replaceable by another constituent material which gives the desired superconducting properties to the combined material. 13. A superconducting ceramic based conductor constructed in accordance with the method as claimed in any one of claims 1 to 14.

Inventors: EVETTS JAN EDGAR, FRAY DEREK JOHN, EVETTS, JAN, EDGAR, FRAY, DEREK, JOHN
Applicants: EVETTS JAN E, FRAY DEREK JOHN, EVETTS, JAN, EDGAR, FRAY, DEREK, JOHN
Application year: 1988
Main classification: H01L39
IPC codes: C01G1, C04B35, C04B41, H01B12, H01B13, H01L39