DUCTILE SUPERCONDUCTING COMPOSITES AND METHOD OF PRODUCTION THEREOF FIELD OF THE INVENTION The invention herein relates to superconducting materials.
BACKGROUND OF THE INVENTION In recent months there have been dramatic breakthroughs in the technology of superconducting materials. Reports from a number of laboratories describe numerous oxide materials which have transition temperatures as high as 120'K. Of particular current interest are the superconducting oxides formed from yttrium, lanthanum or certain of the lanthanide elements (all except Ce, Pr, Pm and Tb) in combination with barium or strontium and copper. These superconducting oxides are well described by Geballe et al. in Science, 239, 367 (Jan. 22, 1988). Even more recent are the superconducting oxides formed of bismuth, strontium, calcium, copper and, in some cases, aluminum, as described by aldrof in Science. 239. 730 (Feb. 12, 1988). These materials are normally formed as the oxides and as such are quite brittle. This effectively prevents them from being readily formed into wires, thin films, sheets or other useful shapes. In addition, when such shapes are formed by special techniques such as casting, they are unable to resist normal stresses to which such shapes are subject and readily fracture because of their brittleness.
Work recently reported by Yurek et al. in j.
Electrochem. Socy.. 134. 10, 2653 (Oct., 1987) shows the combination into an alloy of gold with an equal amount of the metallic elements (europium, barium and copper) from an
Eu-Ba-Cu-0 superconducting oxide. This alloy was then subjected to oxidation at low temperature, then at high temperature and then again at low temperature to form a composite of the gold metal dispersed as discrete particles throughout an oxide matrix. The resulting composite remained brittle. It is speculated in the article that silver and platinum group metals can be substituted for the gold.
It is evident that in order for the superconducting oxides to be useful in many electrical applications, they will need to be effectively combined in some manner with ductile conductive metals. The presence of these metals will be necessary to enable the superconducting oxides to be electrically joined to components of a network or circuit or to enable them to be coiled, bent or otherwise shaped to fit available spaces in electrical devices and so forth. It would therefore be of considerable value to have available composites of such oxides and metals that would retain the ductile properties of the metals while exhibiting the superconductivity of the oxides, as well as having convenient and relatively simple methods for production of such materials.
BRIEF SUMMARY OF THE INVENTION In one aspect the invention herein is a composite material comprising a superconducting oxide in association with a ductile metal; the superconducting oxide formed of at least one metallic element and oxygen; the ductile metal being having a lower oxidation potential than those metallic elements and having the ability to dissolve oxygen
in the liquid phase or diffuse oxygen in the solid phase; and the ductile metal being present in a continuous phase as at least 50% of the composition.
In a specific embodiment of this aspect, the ductile metal is also miscible in the liquid phase with all metallic elements of said superconducting oxide and the ductile metal and metallic elements can be formed into a homogeneous alloy.
In another aspect the invention is a method of forming a composite material comprising a ductile metal and a superconducting oxide composed of at least one metallic element and oxygen which comprises: a. selecting as the ductile metal a metal that has. a lower oxidation potential than those metallic elements and has the ability to dissolve oxygen in the liquid phase or diffuse oxygen in the solid phase; b. forming a mixture of the ductile metal and the metallic elements of the superconducting oxide, with the ductile metal being present in the mixture in an amount such that following oxidation the metal will be present as at least 50% of the composite material; c. forming the mixture into a desired shape; and d. subjecting the mixture to an oxidizing atmosphere for a sufficient period of time and at a sufficient temperature to cause the metallic elements to oxidize to form the superconducting oxide but not to oxidize the ductile metal, with the ductile metal being a continuous phase in the composite.
In a specific embodiment of this aspect, the ductile metal is miscible in the liquid phase with all metallic elements of the superconducting oxide and the mixture produced is an alloy of the ductile metal and the metallic elements of the oxide.
In yet another aspect the invention is a method of forming a composite material comprising a ductile metal and a superconducting oxide composed of at least one metallic element and oxygen which comprises: a. selecting as the ductile metal a metal which is miscible in the liquid phase with all metallic elements of the superconducting oxide, has a lower oxidation potential than those metallic elements, and has the ability to dissolve oxygen in the liquid phase or diffuse oxygen in the solid phase; b. forming an alloy of the ductile metal and the metallic elements of the superconducting oxide, with the ductile metal being present in the alloy in an amount such that following oxidation the metal will be present as at least 50% of the composite material; c. forming the alloy into a desired shape; and d. first subjecting the alloy to an oxidizing atmosphere at a temperature on the order of 850°C-950 β C and subsequently subjecting the alloy to the oxidizing atmosphere at a temperature on the order of 500"C-700°C to cause the metallic elements to oxidize to form the superconducting oxide but not to oxidize the ductile metal, with the ductile metal being a continuous phase in the composite.
Yet another aspect of the invention is a composite material formed by a method as described in the preceding paragraph and comprising a superconducting oxide in association with a ductile metal; the superconducting oxide formed of at least one metallic element and oxygen; the ductile metal being miscible in the liquid phase with all metallic elements of the superconducting oxide, having a lower oxidation potential than those metallic elements, and having the ability to dissolve oxygen in the liquid phase or diffuse oxygen in the solid phase; and the ductile metal being present as at least 50% of the composite material.
Preferably the ductile metal will comprise at least 75%, more preferably 85%, and most preferably 85%-95%, or the composite. All precentages herein are percentages by weight unless otherwise noted.
DETAILED DESCRIPTION OF THE INVENTION
The subject mentioned involves two interrelated aspects: the ductile superconductive materials and their method of manufacture. The materials will first be described, followed by descriptions of the various procedures one can use to obtain these desirable products.
The superconducting oxides may be any of the known superconducting materials which have been described in the literature, such as those described in the Science articles cited above. It is recognized that the superconducting oxide field is rapidly evolving and that new superconducting oxides can be expected. The nature of the present invention is such that it is anticipated that these new materials will be equally applicable in the present invention if they are brittle as are the currently known materials.
The metallic elements in a typical superconducting alloy are commonly yttrium, lanthanum or the lanthanide elements (with the exception of Ce, Pr, Pm and Tb) ; europium has been specifically described in the literature as a suitable component. Other metallic elements in the superconductors are commonly the Group II elements calcium, strontium or barium; copper, bismuth and aluminum. Some other elements which have been used in the older generation of superconductors include niobium, germanium and titanium. Of most interest at the present time are the superconducting oxides in the R-A-Cu-0 system (in which R represents yttrium, lanthanum or the aforementioned lanthanides in various proportions and A is an alkaline earth element such as Ca, Ba or Sr or an alkali metal such as Na) and the newly described oxides in the Bi-Sr-Ca-Cu-[Al]-0 system. The particular compositions are not critical to the present invention, except to the extent that the metallic elements must be those which can be mixed or alloyed with the ductile metal, as will be described below. It will be noted that in most of these compositions, copper has the lowest oxidation potential. It is not critical that copper be the element of the lowest oxidation potential, but this will serve as a convenient point of reference for the discussion which follows. The ductile metal used in the present invention may be silver, gold, the platinum group metals (platinum, palladium, osmium, iridium, rhodium and ruthenium) or any other metal which can be mixed, or preferably alloyed, with the metallic elements of the superconducting oxide and has the following properties: it has a lower oxidation potential
than any of the metallic elements of the superconducting oxide (with copper usually being the limiting metallic element) , it has the ability to dissolve oxygen in the liquid phase and to diffuse oxygen in the solid phase, and, preferably, it is miscible with all of the metallic elements in the liquid phase, he last property is preferred because this enables the ductile metal and the metallic elements to be alloyed prior to oxidation. Preferred among the ductile metals is silver, since it is commonly available at a relatively low price and has excellent ductility. It is also easily melted and does not oxidize above 200 β C. It should be noted, however, that because of its melting point of 962"C, care must be taken at the upper end of the first oxidation range (described below) where the oxidation temperature limit is approximately 950 β C. When silver is used as the ductile metal, it is preferred to keep the first oxidation temperature at about 900"C or below to avoid having the composite deform because of softening of the silver. It is also possible to use various combinations (mixtures or preferably alloys) of two or more ductile metals. The mixture or alloy formed with the metallic elements of the oxide will then include also all of the elements which comprise the ductile metal component. The ductile metal will be present in the final composite as at least 50% of the composite, preferably at least 75% and more preferably at least 85%-90% of the composite. It has been found that, in comparison to the prior art materials in which the oxide content predominated, the requisite ductility cannot be obtained unless these
minimum amounts of ductile metal are present in the composite. The exact amount of ductile metal present will depend upon the metal itself as well as on the method by which the original mixture or alloy is worked into the desired shape. Cold rolling normally produces a less ductile composite and therefore ductile metal concentrations of 85-90% or greater are required. Hot rolling and other hot working techniques, however, do not reduce ductility as much and lower metal contents are satisfactory. Between 50% and 75% metal content, however, the brittle properties of the oxide become more evident, and below 50% those properties will will dominate.
The composite materials of the present invention can be formed in a variety of ways, all of which involve carefully controlled oxidation. Depending on the type of oxidation step used, one can obtain a ductile metal substrate having the oxide incorporated on the surface thereof, a layered composite in which the oxide layers alternate with the ductile metal layers or a composite in which the ductile metal forms a continuous matrix through which the oxide is dispersed in small particles such as filaments or granules.
All of the production procedures of the present invention begin with alloying of the metallic elements of the oxide and then incorporation with the ductile metal or metals. If the desired composite is to be a metal matrix in which the oxide is dispersed, the incorporation will be by mixing of the metallic element alloy with the ductile metal, usually by such techniques as plating the ductile metal onto the metallic element alloy or by simply physically inserting particles of alloy into a body of the ductile metal. For
the other forms of the composite, however, it is preferred to combine the ductile metal directly into the alloy with the metallic elements. Alloying, whether of the metallic elements alone or in combination with the ductile metal, is accomplished by normal alloying techniques which are known to those skilled in the art. The temperatures for forming the alloy melt will be determined by the particular metals which are present. Normally the alloying will be done in an inert atmosphere so as to prevent premature oxidation of any of the various metals. The inert atmosphere may be one of helium, argon or other gas which will neither oxidize nor otherwise react with any of the metallic elements. In some instances nitrogen will also be usable, but care must be taken that the nitrogen does not react with any of the metallic elements present such as yttrium.
The mixture or alloy is then formed into any desired shape for oxidation. In most cases it will be at this point where the general final shape of the composite is determined. The material may be in the form of an ingot or similar shape to be oxidized. Commonly, however, the wire, thin film or other desired shape will be formed at this stage and the material then oxidized while in that shape. The particular choice of shaping techniques and timing will be determined to a large extent by the particular form of the composite. Those composites in which the superconducting oxide is formed within the matrix of the metal may be shaped more extensively following oxidation. On the other hand those in which the oxide forms as a surface layer or as internal layers will usually be shaped prior to oxidation so that the layers remain intact after
oxidation. Those schooled in the art can readily determine the appropriate time for formation of the desired shape or whether the shaping should be done in two stages, a partial shaping following mixture or alloy formation and a final shaping following oxidation.
Once the desired shape for oxidation has been produced with the mixture or alloy, it is subjected to the particular type of oxidation best suited to produce the composite structure desired. Most commonly the material will be subjected to a high temperature first stage oxidation with temperatures of 850°C-950 β C, preferably on the order of 900'C In this temperature range oxidation is quite rapid, particularly with high surface area shapes. This is desirable for promoting surface oxidation where the oxide is to be in the form of a surface coating on an underlying ductile substrate. The particular length of time for which the oxidation is conducted at this temperature will be a function of the temperature and the particular metals to be oxidized. Such functions are well known to those skilled in the art and the optimum time and temperature for any particular combination of elements in the material will be easily determined by standard techniques. Measurements of oxidation rate have been discussed in several publications, such as by Turner et al. in Surface Science. 147. 648 (1984) . Normally the first stage oxidation at the higher temperatures for the yttrium or lanthanum superconductor metallic elements will be on the order of 12 to 72 hours, depending on the thickness of the sample. Following the high temperature oxidation, the temperature is lowered to approximately 500 β C-700°C and a controlled low temperature
oxidation is conducted, to provide the optimum oxygen content in the superconductor oxide. Again time and temperature will be dependent on the oxides to be formed and the oxygen content and rate of oxidation can be determined by the published techniques.
The oxidizing atmosphere may be any of a variety of different oxygen containing atmospheres. Commonly one will use an atmosphere of pure oxygen gas. Alternatively, and equally suitable, are those atmospheres which are essentially inert atmospheres but into which oxygen is etered at a ^predetermined rate. Thus one can have a mixture of helium and oxygen, argon and oxygen or the like. Control of the partial pressure of the oxygen in such mixtures will determine the rate of oxidation. Other oxidation atmospheres, but ones which are less preferred because they are more difficult to control, include air and mixtures of hydrogen and water vapor. Those skilled in the art will be aware of other oxidizing atmospheres which will be suitable. The particular atmosphere used is not critical to the present invention. Rather what will determine the , preferred atmosphere will normally be the degree to which one can control the oxygen presence and therefore the rate and degree of oxidation.
In those instances where it is desired to have the superconducting oxide present as discrete particles or filaments within the ductile metal matrix, one forms an alloy of the metallic elements of the oxide, forms the alloy into filaments or particles and then disperses those throughout the ductile metal matrix or coats the alloy filaments or particles with the ductile metal. This
heterogeneous mixture of the ductile metal and the metallic alloy will then be subjected to shaping and oxidation as described above.
In an alternative procedure, a homogenous alloy of the ductile metal and the metallic elements from the oxide is formed. The alloy is then shaped and subjected to oxidation as described above to produce the forms with surface or layered structures. Also, to form the internally layered configurations, one follows the above-described oxidation procedures but with the exception that the temperature of the first oxidation stage (the high temperature oxidation) is repeatedly cycled over a small temperature range (generally +5 β C-10 β C on either side of the normal oxidation temperature) . The cycles are usually on the order of about 10-15 minutes in length. These cycles, and the cyclic diffusion of oxygen that they produce, will cause the metallic elements to be oxidized in a planar manner such that the final composite will be in the general form of alternating layers of superconductor oxide and ductile metal.
The resulting composites of this invention with their high ductility have been found to possess properties not present in the materials of the prior art. For instance, it is known that in conventional superconductor materials prepared by sintering of oxides, there are a wide variety of grains present with different orientations. The presence of the grain boundaries causes severe problems and prevents the oxides from being used for many practical applications. The present materials, because of their method of manufacture, have a substantially reduced number of grain boundaries and
many of the grains are oriented in generally parallel directions, thus substantially reducing the grain boundary effects.
The materials of this invention are " much more homogeneous and have more uniform densities than many of the prior art superconductor materials. Densities close to theoretical can often be obtained by the methods herein.
It will also be evident that the presence of the ductile metal makes the entire superconductor much more mechanically deformable and flexible, thus eliminating virtually all of the prior art problems with brittle superconductors.
In addition, the compositions of this invention will have much greater current density capacity than those of the prior art.
It will be evident from the above that there are many aspects of this invention which, while not specifically set forth above, are clearly within the scope and spirit of the present invention. The above description is therefore intended to be exemplary only and the scope of the invention is to be limited solely by the appended claims. We claim: