SUPERCONDUCTING MATERIALS, METHODS AND DERIVATED DEVICES

Abstract: The composition and valence state (and therefore superconductivity) of a superconducting material (10) is adjusted and maintained by the electrochemical addition or subtraction of an enhancement material from or to a donor or receptor control material (14) located in close proximity to the superconducting material (10). Where the superconducting material (10) is an oxide phase the valency is controlled by the electro-transport of oxygen to or from the material (14), by the application of electrochemical potential. Enhancement may occur through an ionic conductor (12) which may be a solid or liquid electrolyte. In one example oxygen is added through a solid electrolyte yttria zirconia, calciazirconia or of bismuth oxide containing strontium oxide, calcium oxide or lanthanum oxide. In other examples valency and therefore superconductivity is controlled by the transport of fluorine (F<->) or copper (Cu<+>).

Description: itle : SUPER CONDUCTING MATERIALS, METHODS AND DERIVATED DEVICES Field of Invention This invention concerns superconducting materials and their manufacture and is directed to improvements in the design and fabrication of high critical temperature superconducting films, components and wires, whereby highly reactive or chemically unstable superconducting materials can be adjusted in a controlled way to their optimum composition and state of oxidation or valence state and subsequently stabilised and maintained in this optimum condition. Background to the Invention Although superconducting components and wires have been available for some years they have, hitherto, depended on metallic superconducting materials for their properties. Examples of existing commercial materials are NbTi alloys and A15 intermetalliσ compounds such as b,Sn. A new class of very high critical temperature, high field, superconducting oxide phases, (known as ceramic superconductors), presents new opportunities for applications of superconductivity but raises new problems for the design of high performance components and wires. It is likely that other ceramic superconductors (such as sulphur nitride, molybdenum nitride) will present similar problems . <«, i^ £____> In the first place the materials are inherently brittle, and this raises acute problems for the support of a conductor under the extreme 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. From the recent extensive literature on oxide superconductors it is known that oxygen solubility in such materials varies considerably with the oxygen pressure. However, it is difficult to prepare these materials at an elevated temperature by exposure to oxygen and then retain the oxygen at low temperatures. Secondly, the range of oxygen partial pressures attainable using gases is limited. There are indications that some phases may superconduct near to room temperature but be prone to be transient in their behaviour in ambient atmospheres. Summary of the Invention The present invention provides a superconductor in which the composition and valence state (and therefore the s perconductivity) of the superconducting material is adjusted and maintained by the electrochemical addition or subtraction of material, on the application of electrochemical potential, from or to a donor or receptor control material, located in close proximity thereto. * The addition or removal of valence controlling material can be controlled very precisely using electrochemical techniques. The addition or removal can be referred to as enhancement and the material transferred can be thought of as the enhancement material. Using electrochemical techniques allows the chemical activities of chemical constituents of materials to be varied over a wide range. For example, it is possible to generate electrochemically, hydrogen activities equivalent to many tens of thousands of atmospheres of pressure in steel. A further advantage is that if a solid electrolyte is employed it is possible to create these pressures or activities completely in the solid state, without recourse to the gaseous or liquid environments. To avoid doubt the expression superconductor as employed herein is intended to mean a material which is capable of superconductivity. Preferably an ionic conductor such as an electrolyte is interposed between the superconductivity material and the donor or receptor material. Thus if the superconducting material is an oxide, the latter is placed in juxtaposition with an ionic conductor that can electro- transport oxygen to or, if necessary, from the material on the application of a suitable electrochemical potential. Typically the ionic conductor is an electrolyte and oxygen is transported through the electrolyte from a a solid or liquid or gaseous source of oxygen. The ionic conductor may cover certain external surfaces of the superconducting material or may fully encapsulate the superconducting material. In the case of a wire the ionic conductor may for example coat some or all of the outside of the wire. Alternately the ionic conductor may coat the sides of a passage through 'or hollow channels, cavities or ducts contained within a component or wire. In an alternative design the passages, channels, cavities or ducts may be filled with a solid enhancement material such as an oxide (where the enhancement material is to be oxygen) and if required, a conducting electrode. For some superconductors it is possible to control the oxygen partial pressure by exposing them to a mixture of metal and metal oxide. If, for example, it is found that the optimum superconductivity is at a partial pressure of 10 -13 atm, this could be achieved by heating the superconductor in a mixture of copper and cuprous oxide at 813K. By selection of the temperature and metal/metal oxide, a very wide range of possible oxygen pressures is possible. It is proposed that the level of enhancement be adjusted to an optimum value for the material, during fabrication and commissioning of the superconductor, and thereafter maintained at that level. The superconductor can be maintained in its optimum state by control of the electrochemical potential in service or during maintenance of the material. Thus active feed back control of the enhancement or optimum composition of the material may be achieved in superconductors with a sufficiently high superconducting transition temperature. Indeed the superconductivity of such materials may depend so critically on the composition or enhancement level that active control of this sort may be essential for practical applications, e.g. the control of oxygen concentration in the superconductor. As a further development of the invention the ionic conductor may be separated from the superconductor by, a layer of material which is readily permeable by the atoms or ions of the enhancement material which are to combine with or be removed from the superconductor to establish -the superconducting phase - e.g. oxygen, atoms or ions. The superconductor and the ionic conductor may be fabricated as one unit or joined after separate manufacture. Alternatively one component may be deposited or formed onto the other. Thus an oxide film may be deposited onto a substrate which comprises an ionic conductor. The invention also provides a superconductor in which the superconductor material may consist of a compacted powder pellet that has been appropriately pretreated, reacted, graded and possibly magnetically separated prior to compaction, and an encapsulating ionic conductor which may be similarly fabricated from compacted powders, and may be formed and heat treated in conjunction with the first material. The superconductor and the ionic conductor may be in intimate contact or separated by a thin deformable support or spacer, which at a later stage, is rendered oxygen permeable. The support or spacer .is preferably an electronic conductor. The invention also envisages an alternative method of £_£ J _£. ■ϋeg % construction in which the superconductor powder is contained in a tube or container (which may be formed from a metallic alloy) that is anodised or oxidised after fabrication to 1 - form a suitable ionic conductor. In a further alternative method the superconductor is formed first as * a metallic alloy and subsequently anodised or oxidised after fabrication to form an oxide superconductor. In a still further alternative method of construction the superconductor may surround the ionic conductor. Thus in the fabrication of a superconducting wire for example, a central core may be formed from a conductive metal, surrounded by a source of or sink for enhancement material which itself is separated from a cylindrical layer of superconductor by an ionic conductor also in the form of a cylindrical layer. If necessary the superconductor layer may be enclosed within a suitable covering material, to separate the superconductor from the environment. A conductive element may be required at the interface between the covering and the superconductor if the former is not electrically conductive, and the covering may itself be coated, or otherwise covered or incorporate an electrically insulating material. Just as it is possible to insert species from solid electrolytes, liquid electrolytes can also be used. For example, if lithium is required to be inserted this could be achieved by using an aluminium-lithium anode and a liquid lithium ion conductor, either organic or inorganic, to transport the lithium to the superconductor. The inorganic electrolyte could be molten lithium chloride at 900K. A further possible addition would be to include bismuth oxide containing strontium oxide, calcium oxide or lanthanum oxide as examples of solid electrolytes which conduct oxygen ions. The invention will now be described by way of example with reference to the accompanying drawings, in which:- Fig.l illustrates an oxygen ion conductor composite, Fig 2 illustrates the layers required in an example based on YBaCuO, Fig 3 is a cross section through a wire which is capable of functioning as a superconductor and which is stabilised in accordance with the invention, Fig 4a and 4b illustrate the layers required for controlling the oxygen and copper and fluorine levels in a superconducting composite, and Fig 5 illustrates a pellet-sandwich arrangement by which oxygen or fluorine penetration can be measured. An oxide 10, for example LaSrCuO or YBaCuO, is prepared in contact with a solid state ionic conductor 12 of oxygen ions such as calcia zirconia, or yttria zirconia or any other oxygen ion conductor. Both of these compounds are ionic conductors . By having a source of oxygen 14 on the other side of the ionic conductor such as oxygen gas or a - 3 - metallic oxide, and by applying a potential, oxygen ions are transported through the ionic conductor from or to the oxide superconductor depending on the polarity, as shown. In the case of an oxygen atmosphere, it may be preferable to coat the ionic conductor with a porous platinum layer to catalyse the " reaction. In this way it is possible to introduce substantial quantities of oxygen into the superconductor. For example, a difference in electrode potentials of 0.2V between the oxygen side and the oxide gives a pressure of oxygen in the superconductor of 10 atmospheres at 673 °K calculated using the expression -ZEF = RT ln(P0 2 "/P0 2 ' ) (1) where R is the gas constant,Z is the unit of charge carried, E is the potential, F is Faraday's constant, T is the temperature, and P0 " and P0„' are the pressures on either side of the electrolyte. By using this technique, oxygen activities equivalent to pressures far in excess of those which can normally be generated hydrostatically, can be achieved, and, therefore, the possibility of generating new superconducting phases exists. In order to prevent the oxygen escaping it may be necessary to coat the composite with an oxygen impermeable membrane such as a metal, e.g. Al or Au. If, however, it is necessary to maintain a very low oxygen partial pressure, the voltage and current can be applied in the reverse direction. For a voltage drop of 0.3V in the reverse direction the oxygen partial pressure would be i x -9 10 atmospheres, again a partial pressure which is difficult to attain using gas mixt es. Furthermore, from expression (1) above it is possible to measure the potential using a high impedance voltmeter after oxygen intercalation to confirm the value of the oxygen pressure or activity. For example for the oxygen defect perovskite a_Ba,CUgO, . , the following potentials would be measured at 673°K between (a) the perovskite and pure oxygen and (b) the perovskite and c u/Cu-0 . ( 5. (a) (b) 0 . 05 -7.68xlO~ 2 V +0.550V 0 . 19 -6.68xlO~ 2 V +0.560V 0 . 25 -5.67xlO~ 2 V +0.570V 0 . 3 1 ■ 4.34xlO~ 2 V +0.583V o . . 33 •3.34xlO~ 2 V +0.592V 0 , . 37 •2.33xlO~ 2 V +0.603V 0 , . 43 OV +0.626V Due to the fact that the ionic conductor is impermeable to oxygen molecules and oxygen atoms, the composition of the intercalated oxide will not change. Using this technique, it is possible to stabilise the oxide phase and, secondly, to extend the range of oxygen activities or pressures far in excess of those hitherto investigated. The above described technique also makes it possible to inject oxygen until the compound is saturated with intercalated oxygen, which can be detected by a constant potential measurement despite further oxygen intercalation. Alternatively, a potentiostat could be used. It may also be possible to control the oxygen pressure at the working temperature of the superconductor. The invention allows other compositions and states of -_- 2-f- oxidation to be controlled, for example, Cu /Cu , Cu 2+/Cu3+ or alkaline earth composition, by utilising an appropriate ion conductor, e.g. a copper ion conductor or an alkaline earth conductor instead of an oxygen ion conductor. Similarly fluorine may be introduced into materials using a fluorine ion conductor such as LaF- or any other fluorine ion conductor. The invention envisages that the composition of super conductors, may be altered, maintained, and monitored using ionic conductors and appropriate electrical potentials. In Figure 2 a layer of YBaCuO 16 which is to form a superconductor is sputtered onto an oxygen ion conductor, substrate 18 for example yttria zirconia and then coated with gold at 20. Oxygen either from the air or copper oxide is transferred electrochemically into the superconductor material. When the oxygen source is air, transfer occurs through a porous platinum electrode 22 on the underside of the substra-te, the platinum layer acting as an electronic conductor and a catalyst. When the oxygen source is copper oxide, the porous platinum layer is not required, the mixture acting as the conductor. The oxygen activity can be measured after the end of the transfer or intercalation and, due to the fact that the system is sealed, phases and superconduction compositions may be created which are unstable under ambient conditions. The oxygen transfer will most likely take place at temperatures above 673°K but with thin films of ionic conductor it may be possible to attain results at lower temperatures. It may be necessary to permanently maintain a potential across the electrolyte in order to prevent the phase from decomposing. In Figure 3 a copper wire (24)is surrounded by. copper oxide (26), within a tube of oxygen ion conductor (28), e.g. yttria zirconia. This is surrounded by an oxide superconductor (30) which is sheathed in copper (32) separated from the superconductor by a tantalum diffusion barrier (34). Passing a current from the outer copper sheath to the inner copper wire will ensure that oxygen atoms pass into the oxide superconductor. As described with reference to the previous example potentials may be measured ' etc. as previously mentioned. Figure 4a shows how the invention can enable both oxygen and copper concentrations to be controlled by use of two solid electrolytes, each one monitored and controlled independently, provided the central core is conductive. Thus in Figure 4a a ceramic superconductor in the form of a yttria barium copper oxide layer 36 is sandwiched between an oxygen ionic conductor of yttria zirconia 38 and a copper ionic conductor layer 40 of copper beta aluminium. A copper electrode 42 on the other surface, 40 provides copper ions/atoms and a copper/copper oxide oxygen ion/atom source 44 as provided below a porous platinum barrier film 46. Control of both oxygen and fluorine concentrations by use of two solid electrolytes is shown in Figure 4b. Here an Yttria Barium Copper Oxide superconductor layer 48 is sandwiched between a lanthanum fluoride layer 50 forming a fluorine ion conductor, and a layer 52 of Yttria Zirconia 52, forming an oxygen ion conductor. Porous platinum electrode layers 54 and 56 provide for the establishment of an electric potential through the junction and a source of fluorine ions/atoms and oxygen ions/atoms are provided beyond both electrodes. Overall, it may be possible to control the composition and structure of these materials rather like the doping of semiconductors and the invention applies to the fabrication and stabilisation of all ceramic superconductors, not only oxides. Figure 5 shows an arrangement whereby further tests were made to determine the transfer of oxygen and fluorine into a superconductor. Pellets of Cu,Cu 2 0, (58) yttria stabilised zirconia (60) and Y. Ba-Cu-O-, superconductor (62) were pressed together and heated to 750°C. On the application of 2.04V, a current of 15 mA cm flowed. This voltage includes the resistive and polarisation losses as well as the potential difference across the electrodes» After three hours titration it was found that the partial pressure of oxygen given by the superconductor was 1000 atm. The compressi e force was obtained from a spring (64) acting through a rod (66). Using exactly the same physical arrangement but substituting Ag/AgF for the Cu/Cu_0 as a source of fluorine and aF_ for Y„0^Zr0» as the F~conductor, fluorine was transferred to the superconductor at an _2 applied potential of 1.2V and 6 mA cm If, instead of a pressed pellet of aF. , a thin film of _2 LaF_ is vapour deposited on the superconductor, 100mA cm of current flowed at an applied potential of 0.777V. The higher current was attributed to the much better contact between the deposited film and the superconductor. By substituting pure Cu for Ag/AgF as the source of copper, and copper p- lumina for LaF_ as the Cu conductor, copper was transferred into the superconductor _2 at an applied voltage of 1.0V and a current of 8 mA cm The invention allows the exact enhancement activity (eg oxygen activity in an oxide superconductivity material) to be monitored by measuring the potential between the superconducting material and the reference across the ionic conductor (electrolyte), without the need for chemical analysis. Connection to the level of activity may be made by eg adjusting the electrochemical potential. Superconducting oxides are themselves very like ionic conductors and may be used in titration configurations without a separate ionic conductive layer, and the invention includes within its scope arrangements of such oxide superconductors in which the latter itself functions in part as an ionic conductor. Essentially the invention lies in the control of the conductivity of a mixture of elements so as to produce superconductivity by electrochemically altering the valence state of one of the elements and causing at least one other element to migrate into or out of the mixture to re-establish the charge neutrality of the mixture.

Claims: CLAIMS 1. A superconductor the composition and valence state (and therefore superconductivity) of which is adjusted and maintained by the electrochemical addition, or subtraction, on the application of electrochemical potential of material from, or to, a donor or receptor control material located in close proximity to the superconducting material. 2. A superconductor as claimed in claim 1 wherein an ionic conductor is interposed between the superconducting material and the donor or receptor material. 3. A superconductor as claimed in claim 2 in which the ionic conductor is an electrolyte through which material is transported from a solid or liquid or gaseous source of donor material. 4. A superconductor as claimed in claim 2 or 3 in which the ionic conductor is a solid material which covers at least some of the external surfaces of the superconducting material. 5. A superconductor in which the oxygen content of a superconducting oxide material is controlled by electrochemically controlling the valency of one of the elements in the superconducting material. 6. A superconductor as claimed in claim 5 in which the ^U Ϊ S- sii J ii. «ι-_.___«.-__- » valency controlled element is one of Cu, Bi, Tl, Mn or Fe. 7. A superconductor as claimed in any of claims 2 to 6 wherein the element which is transported through the ionic conductor is F~ or Cu . 8. A superconductor as claimed in any of claims 3 to 7 wherein the electrolyte is a solid. 9. A superconductor as claimed in any of claims 4 to 8 wherein the superconducting material is in the form of a wire and the ionic conductor is in the form of a coating around the outside of the wire. 10. A superconductor as claimed in any of claims 4 to 8 wherein the superconducting material is a hollow member and the ionic conductor coats some or all of the hollow interior. 11. A superconductor as claimed in claim 1 or 2 in which passages, channels, cavities or ducts in the superconducting material are filled with a solid donor or receptor material. 12. A superconductor as claimed in claim 9 in which the passages, channels, cavities or ducts also include a conducting electrode. 13. A superconductor as claimed in any of the preceding claims in which the control material provides for the addition of oxygen and the oxygen partial pressure is controlled by oxygen enhancement from a control material formed from a mixture of metal and metal oxide. , a - -πa ry 14. A superconductor as claimed in claim 11 in which the level of oxygen enhancement is adjusted to an optimum value for the material during fabrication and commissioning and thereafter is maintained at that level. 15. A superconductor as claimed in any of claims 1 to 12 in which a superconducting material consisting of compacted powder is encapsulated by an ionic conductor. 16. A superconductor as claimed in claim 13 wherein the ionic conductor is in intimate contact with the superconducting material. 17. A superconductor as claimed in any of claims 2 to 14 in which the ionic conductor is separated from the superconductor material by a layer of a material which is readily permeable to atoms or ions of the material which are to combine with, or be removed from, the superconducting material. 18. A superconductor as claimed in any one of claims 2 to 14 wherein the ionic and superconducting materials are separated by a thin deformable support or spacer, which, at a later stage, is rendered permeable to the atoms or ions which are to permeate to or from the superconducting material. 19. A superconductor as claimed in claim 16 in which the support or spacer is an electronic conductor. 20. A superconductor as claimed in any of claims 1 to 16 in which a powdered superconducting material is contained in a tubular container that is anodised or oxidised after fabrication to form a suitable ionic conductor. 21. A superconductor as claimed in claim 18 in which the container is formed from a metal alloy. 22. A superconductor as claimed in claim 1 in which superconductivity is achieved and maintained by the addition of oxygen through a solid electrolyte of Ca0Zr0„ or Y„0-,Zr02 or bismuth oxide containing strontium oxide, calcium oxide or lanthanum oxide. 23. A superconductor as claimed in claim 3 wherein the electrolyte is a Liquid. 24. A superconductor as claimed in claim 23 wherein the liquid electrolyte is a carbonate melt. 25. A method of constructing a superconductor in which the superconducting material is formed first as a metallic alloy and subsequently anodised or oxidised after fabrication to form an oxide having superconducting properties. 26. A method of constructing superconducting wire of which a central core is formed from a conductive metal, which is surrounded by a sleeve of control material for delivery or absorption of material to or from the superconducting material to control its composition and valence state and which itself is surrounded by a sleeve of the superconducting material but is separated therefrom by an ionic conductor also in the form of a cylindrical sleeve. 27. Method of increasing the conductivity of a mixture of elements by electrochemically altering the valence state of one of the elements and causing migration into or out of the mixture of at least one other element to re¬ establish charge neutrality of the mixture.

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: C01B13, C01G1, C01G3, C04B35, H01B12, H01L39