Superconductor technology: apparatus – material – process – Processes of producing or treating high temperature... – Process of making wire – tape – cable – coil – or fiber
Reexamination Certificate
1989-03-17
2003-05-06
Derrington, James (Department: 1731)
Superconductor technology: apparatus, material, process
Processes of producing or treating high temperature...
Process of making wire, tape, cable, coil, or fiber
C264S621000, C264S622000, C264S623000, C501S012000, C505S440000, C505S450000, C505S510000, C505S735000, C505S737000, C505S739000, C505S740000, C505S741000, C505S779000, C505S780000, C516S088000, C516S089000
Reexamination Certificate
active
06559103
ABSTRACT:
The present invention is directed to a method for producing superconducting oxide compounds which are formable into fibers and other desired shapes. The present invention is also directed to an improved method for making superconducting oxide compounds with advantageous homogeneity and current-carrying capacity.
BACKGROUND OF THE INVENTION
Until recently, known superconducting materials have been metallic materials which exhibit superconducting properties only at temperatures close to absolute zero. However, recently a novel class of superconducting materials which take the form of ceramics, or mixed metal oxides, have been discovered, some of which exhibit superconductivity above the temperature of liquid nitrogen, 77° K. (−321° F.), at ambient pressure, which signifies the ability to prepare and maintain superconductive materials now in virtually any laboratory. These mixed metal oxides are made typically by sputtering the appropriate metals and metal oxides onto a substrate and sintering to form the requisite ceramic structure. Another method involves coprecipitating the appropriate metals from aqueous solutions of their nitrate salts, then heating the precipitate at 900° C. to 950° C. to form the appropriate ceramic structure.
One of the disadvantages of the sintering method is that it produces islands of the correct compound composition for superconductivity, interconnected by areas where the stoichiometry is not optimized for superconductivity. This problem of nonhomogeneity may partially account for the low current-carrying characteristics of the mixed metal oxide superconductors. If these superconducting materials are to have a broad base industrial application, methods need to be devised to increase their current carrying capacity.
Another problem is that the ceramic material is brittle, hard and difficult to handle without damaging the ceramic, and is particularly difficult to form into a wire or fiber, which is useful for high-current applications.
For small-scale applications, such as for use as components in electronic devices, the low current-carrying capacity problem may be solved by fabricating the ceramic materials in the form of single crystals. However, the technology for making large single crystals suitable for high-current industrial uses is not practically available.
It is therefore an object of the present invention to provide a method for producing mixed metal oxide superconducting materials which can be made into virtually any desired shape or form and which, in particular, may be cast or extruded.
It is another object of the present invention to provide a method for making mixed metal oxide superconductors characterized by advantageous homogeneity.
It is a further object of the present invention to provide a method for making mixed metal oxide superconductors whereby precise dopant concentration can be tailored to create optimum oxide charge states in the superconductor.
It is yet another object of the present invention to provide novel mixed metal oxide superconductors according to the processes disclosed hereinafter.
These and other objects of the present invention will be apparent from the following description of the preferred embodiments of the invention, the appended claims, and may from the practice of the invention.
SUMMARY OF THE INVENTION
The present invention provides a method for preparing solid superconducting mixed-metal oxides in a predetermined shape and form, comprising the steps of providing a solution of salts of the metals contained in the desired superconducting mixed-metal oxide of predetermined composition, wherein each of the salts is present in an amount necessary to provide the predetermined stoichiometric amount of each respective metal required in the desired superconductive mixed-metal oxide; and wherein the counterions, or hydrolysis products thereof, of the metal ions for each of the salts in the solution are removable from the solution by evaporative methods; subjecting the solution to hydrolyzing conditions and removing the counterions and/or hydrolysis products thereof, and a substantial portion of the solvent, from the solution by evaporative methods; converting the metal ions to a mixed metal oxide precursor of the superconducting mixed metal oxide; peptizing the mixed metal oxide precursor to form a viscous polymeric sol; forming the viscous polymeric sol into a predetermined shape or form and heat-setting the sol to a flexible, ductile gel; firing the heat-set gel in the presence of oxygen at a temperature and for a period of time sufficient to oxidize and volatilize any remaining vapors and organic materials from the gel and to form the solid superconducting mixed metal oxide. Novel superconducting mixed metal oxides according to the present invention are also provided, as well as novel viscous, castable, extrudable mixed metal oxide precursors.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention provides a method for producing mixed metal oxide superconducting materials of a predetermined shape, such as tape, fibers, and coatings. In the context of the description of the invention the term sol will have its accepted technical meaning: a colloidal solution. The term gel will have its accepted technical meaning: a colloidal solution of a liquid in a solid. The starting materials for the method are soluble salts, in particular the soluble organic salts, of the metals which comprise the final mixed metal oxide superconductor. The metal salts may be soluble in water, or water miscible alcohol, mixtures thereof, or any other water-miscible solvent which can be removed by evaporation without a reaction which is deleterious to the formation of the desired mixed metal oxide structure. If an appropriate soluble salt of a desired metal is not readily available, but is available as an insoluble metal halide, the metal may be incorporated, alternatively, as a colloidal gel by reacting the metal halide (such as a metal chloride) with water to make a colloidal metal hydroxide. Such a colloidal metal hydroxide may be separated from an ammonium chloride solution, then reacted with the sol containing hydroxides and/or oxides of the other metals to be incorporated into the mixed oxide superconductor.
While not intending to be limited by any particular theory, it is believed that the method of preparation of the mixed metal oxides of the present invention permit the formation of the appropriate metal oxide structure to occur on a colloidal level among particles the size of about 1-10 nm in diameter in the proper stoichiometry and lattice conformation, valence ratio and phase relationships, thereby producing compounds which are believed to be more homogeneous than a mixed metal oxide superconductor of similar composition made by sputtering and sintering of mixed-metal oxide or carbonate powders. Sputtering of metal oxide or carbonate powders allows mixing of particles on the order of 1-10 &mgr;m in size, which are sintered more slowly than the colloidal particles according to the method of the present invention.
A solution is first prepared containing soluble salts of the metals ultimately required in the mixed metal oxide superconductor. These salts are preferably soluble in water or in a water-miscible alcohol, such as methanol, ethanol, isopropanol, ethylene glycol and the like. The appropriate salts include those which provide, as a counterion to the metal ion, an ion which is removable by evaporative methods, or at least the hydrolysis product of which is removable by evaporative methods. This thus includes the organic counterions such as the acetates and formates, as well as counterions which evolve as gases at an appropriate pH, such as the carbonates. To assist in solubilizing the metal salts, polyhydroxy compounds, such as, ethylene glycol, and organic acids, such as citric acid, malonic acid, acetic acid, and the like, may be added to form the metal salt solution. These polyhydroxy compounds and organic acids retain metal salts in solution, since some salts would precipitate under subsequent distillation
Kirkwood Brad Lee
Luhman Thomas S.
Stephenson Ronald Roy
Strasik Michael
Derrington James
Hammar John C.
The Boeing Company
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