This invention relates to a method of forming films, thin films or other like deposits of superconducting ceramics and the superconducting ceramic films made by the method. More particularly, the method is directed to the electrodeposition of a mixture of metals of the type and in a proportion sufficient to be oxidized into a superconducting ceramic, with the subsequent step of, after electrodeposition of the mixture of metals, oxidizing the electrodeposited mixture of metals to form the superconducting ceramic film.
Superconducting materials, as discussed in copending application Ser. Nos. 052,830, filed in May, 1987, and 097,994, filed Sept. 17, 1987, both of which are commonly assigned, have been known since 1911. However, the synthesis of superconductors having relatively high transition temperatures above 30.degree. K. is a quite recent development. By superconductors we herein mean such high transition temperature superconductors.
One class of these materials has been found to be superconducting near 90.degree. K. and has been identified as an oxygen deficient perovskite corresponding to the general composition MBa.sub.2 Cu.sub.3 O.sub.y (referred to hereinafter as the 1-2-3 material), where M is La, Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu or Th or combinations of these elements. Two sub-classes of the 1-2-3 materials are: (a) an oxygen-reduced form, with an oxygen content of about 6.7 atoms per unit cell, which has a transition temperature (Tc) of about 60 K., and (b) a doped form referred to sometimes as the 3-3-6 structure of general formula M(Ba.sub.2-x M.sub.x)Cu.sub.3 O.sub.7+.delta. in which M=Y, La, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu or Th, where Tc ranges from 0 to about 60 K. depending on x and annealing conditions. A second independent class with a Tc of between 20 and 40 K. consists of perovskite materials of composition corresponding to La.sub.2-x M.sub.x CuO.sub.4, where M is Sr, Ba or Ca. These materials have been characterized by a variety of techniques (Extended Abstracts of the Materials Research Society Spring Meeting, Anaheim, Calif., 1987 and "High Temperature Superconductors", Materials Research Society Symposium Proceedings, Vol. 99 (1988)). More recently Bi and Tl containing compositions and phases such as Bi.sub.2 Sr.sub.2 Ca.sub.1 Cu.sub.2 O.sub.8 and Tl.sub.2 Ba.sub.2 Ca.sub. 1 Cu.sub.2 O.sub.8, superconducting near 110 K. and a Tl.sub.2 Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10 phase superconducting near 127 K., (Proceedings of Conference on Materials and Mechanisms of High Tc Superconductivity, Interlaken, Switzerland, 1988, to be published in Physica B.) have been identified.
Thus, as can be seen, a lot of work has been done in superconductors, but up to now, no effective way of putting such high temperature superconducting compositions to use in, for example, circuit or superconducting wire applications, have been developed.
More particularly, prior art methods of manufacturing superconducting compositions involved mixing together amounts of compounds having the desired metals in ratios as they are found in superconducting compounds, and treating the materials in a complex series of steps which ultimately involve firing in an oven to oxidize the metals into a ceramic composition which is superconducting (Extended Abstracts of the Materials Research Society Symposium, Anaheim, Calif., 1987). The resultant materials are typically powder in form and, thus, are not easily used.
Other methods of making the superconducting ceramics involve, for example, (i) the firing under oxygen of a metal mixture formed by molten metal processing and (ii) the solution deposition of an organometallic precursor followed by a firing step under oxygen ("High Temperature Supeconductors", Materials Research Society, Symposium Proceedings, Vol. 99 (1988)).
One prior art alternative approach to developing materials, such as the 1-2-3 phase in a useable form has involved chemical vapor deposition of the metals. For example, in the case of the 1-2-3 composition, the metals are deposited by chemical vapor deposition, and thereafter oxidized into a ceramic film. This technique however, is complicated, and precision deposition of the film on desired areas or on desired paths has not yet been achieved. Moreover, the technique itself is complicated, requiring high vacuum, high deposition temperatures, as well as requiring very high temperatures to fire the metals in an oxygen atmosphere to oxidize and then form the ceramic film ("Thin Film Processing and Characterization of High Temperature Superconductors", No. 165, American Vacuum Society Series, editors J. M. E. Harper, R. J. Colton and L. C. Feldman, 1988). The former complications are avoided by the method of the invention.
The application of electrochemical techniques to the formation of high temperature superconductors has been restricted to a method of electrochemically varying the oxygen content of certain high temperature superconductors, ("High Temperature Superconductors", Materials Research Society, Symposium Proceedings, Vol. 99 (1988)). No known prior art exists for electrochemically forming combinations of metals that are precursors to high temperature superconductors. In addition, no known precedent exists for the electrochemical formation of combinations of metals similar to those found in high temperature superconductors.
In particular, there is no known precedent for the codeposition of metals whose deposition potentials differ by about 3 V and, therefore, whose deposition rates and characteristics can be expected to differ dramatically.
Those of ordinary skill in this art would not codeposit such combinations of metals by conventional electrodeposition methods because such combinations comprise one or more metals whose deposition from an electrolyte requires application of a highly cathodic potential (i.e., highly reductive potential). Aqueous electrolytes, used in conventional electrodeposition, are, themselves, reactive with material having such highly cathodic reduction potentials at these potentials. Thus, those of ordinary skill in this art would expect that such metals having highly negative reduction potentials would not be effectively deposited on the substrate. By cathodic potential is meant a potential which allows electrons to be liberated, e.g., from an electrode to reduce the charge of a species in an electrolyte. By highly reducing potential is meant that which is substantially negative of the potential at which H.sup.+ is reduced to 1/2 H.sub.2 as at a normal hydrogen electrode (NHE). For example, each known precursor combination includes one or more metals that can be deposited only at potentials more than 2 V cathodic (negative) of NHE (e.g., Ca.sup.+2 at potentials &lt;-2.76 V vs normal hydrogen electrode, Sr.sup.+2 at &lt;-2.89 V, Ba.sup.+2 at &lt;-2.90 V, Y.sup.+3 at &lt;-2.37 V). For comparison, copper, which is typically also required for formation of the high transition temperature superconductors, has a much more positive reduction potential for Cu.sup.+2 of +0.34 eV.