Preparation of Precursors for Yttrium-Containing Ceramic Superconductors

Copending application Ser. No. 121,013, filed 11-16-87, teaches the suspension of single-crystal grains of superconductive oxide in an alkoxide gel, where the alkoxide gel contains non-oxygen constituents of the superconductor, aligning the single crystal grains of oxide superconductor, and curing the gel in an oxidizing atmosphere to form an essentially single-crystal ceramic superconductor.

Copending application Ser. No. 121,012, filed 11-16-87, teaches a fabrication process to convert lanthanide ores into metal alkoxide precursors for advanced ceramics. It utilizes fluidized bed chlorination of a lanthanide ore followed by separation of at least one high value rare earth as a by-product, with the remaining rare earth mixture being processed into alkoxides and blended with zirconium alkoxide for fabrication of zirconium ceramic.

Copending application Ser. No. 121,010, filed 11-16-87, teaches making single-crystal superconductive oxide from an alkoxide gel, where the alkoxide gel contains non-oxygen constituents of the superconductor, contacting the gel with a single crystal substrate, and curing the gel starting at the gel substrate interface to form an essentially single-crystal ceramic superconductor.

The preceding applications are all assigned to the same assignee and are all hereby incorporated by reference.

1. Field of the Invention

This invention relates to superconducting ceramics, and in particular to a process utilizing fluidized bed chlorination to extract rare earth values from ore and the preparation of alkoxide precursors for the fabrication of superconducting ceramics.

2. Description of Related Art

The recent discovery by Muller and Bednorz of ceramic superconductors which are superconductive at relatively high temperatures (e.g. now 90 K. or above) has dramatically increased the interest in superconductivity and resulted in a large number of publications. The previous metallic superconductors such as niobium-tin or niobium-titanium required expensive liquid helium cooling. The new ceramic superconductors can be cooled by a relatively inexpensive means, such as liquid nitrogen. The ceramic superconductors have generally been oxides of at least one rare earth, at least one alkaline earth metal, and copper. Typically, the superconductors have been prepared by milling oxides of the rare earth and copper with an alkaline earth metal carbonate (e.g. yttrium oxide, copper oxide, and barium carbonate) and firing the mixture in an oxidizing atmosphere at 1000-1100 C., (typically regrinding, refiring, etc.) and then annealing the ceramic and oxygen at 400-900 C. for an extended period of time (e.g. 0.5-5 days).

Ku et al. in a Paper entitled "Superconductivity and Phase Stability in the Heavy Rare Earth Quaternary Compounds RBa.sub.2 Cu.sub.3 O.sub.7 (R=Ho, Er, Tm, Yb, Lu)" in the Symposium of the MRS spring meeting, 1987, discussed superconductivity in the lanthanum-barium-copper oxide system. Their powders were milled, pressed, sintered, ground, repressed and annealed to prepare samples.

Yoshizaki et al. in a paper entitled "Superconducting Properties of La.sub.1.85 Sr.sub.0.15 CuO.sub.4 Made by Hot Press and Sintered Methods" investigated superconducting transition properties by resistivity and magnetization in lanthanum-strontium-copper oxide for hot pressed and sintered samples. A single crystal was obtained in a portion of one sample.

Christen et al. in a paper entitled "Correlations Among Thermal Processing, Superconducting Properties and Microstructure in La.sub.1.85 Sr.sub.0.15 CuO.sub.4 " synthesized materials beginning with co-precipitation of lanthanum oxide, strontium oxide, and copper oxide from solution in molten urea followed by cold pressing into pellets, sintering/reacting at 1100 C. for four hours in air and annealing in flowing oxygen at 900 C. for 16 hours.

Willis et al. in a letter to the editor entitled "Superconductivity Above 90 K. in Magnetic Rare Earth-Barium-Copper Oxides" (Journal of Magnetism and Magnetic Materials 67, 1987, North Holland, Amsterdam) report measurements of superconducting and magnetic behavior on samples which were prepared by sintering the rare earth oxide, copper oxide, and barium carbonate in an oxygen atmosphere at 1000 C., regrinding and resintering at least twice more to promote reaction and obtain the desired phase.

Shamoto et al. in the Japanese Journal of Applied Physics, April, 1987, article entitled "Effect of Vacuum Annealing on the Superconducting Transition Temperature of La-Sr-Cu-O System" report the effect of vacuum annealing on the superconducting transition temperature of the superconductor lanthanum-strontium-copper oxide system. Their starting materials were apparently oxides pressed and sintered at about 1100 C.

Uwe et al. in the May, 1987 Japanese Journal of Applied Physics paper entitled "Affect of Hetero-Valiant Ion Doping in the High T.sub.C Y-Ba-Cu-O Superconductor" discussed the effect of cerium or lanthanum doping on the resistive transition of high T.sub.C superconductors (i.e. yttrium-barium-copper oxide). The samples were subjected to a procedure in which the material was pulverized, pressed and fired at 850-1000 C. for 2-10 hours in air or oxygen, with the procedure performed two or three times. Some of the samples were then annealed in oxygen at 700 C. for two or three hours. Their doping did not improve the properties and they saw some degradation.

Kasowski et al. in a paper received Mar. 25, 1987 entitled "Electronic Structure of Pure and Doped Orthorombic La.sub.2 CuO.sub.4 " investigate the electronic structure of orthorombic lanthanum-copper oxide and discuss the implications for superconductivity.

Cooke et al. in a paper entitled "Thermally Stimulated Luminescence from rare Earth Doped Barium Copper Oxides"discuss luminescence and emission spectrum measurements of rare-earth-doped barium-copper oxides. The loss of luminescence sensitivity with time, especially when the samples were maintained in vacuum, and the propensity of oxygen-defect perovskites to readily lose or gain oxygen were noted. It was suggested that these measurements might present a very sensitive way to investigate the problem of oxygen stability in these materials.

Braginski et al., in U.S. Pat. Nos. 4,411,959 and 4,575,927 issued Oct. 25, 1983 and Mar. 18, 1986, respectively, teach a submicron particle superconductor arrangement in which brittle superconductive particles remain unsintered in the fabricated wire, thus give a ductile wire, even though the superconducting material is brittle. The small particles provide spacing between particles of much less than the Ginzburg-Landau coherence lengths to avoid any significant degradation to T.sub.c. U.S. Pat. No. 4,419,125 to Charles et al. on Dec. 6, 1983 teaches using liquid alkali metal to co-reduce a mixture of solid halides to produce such submicron powders. These three patents are hereby incorporated by reference.

Naitou et al. in U.S. Pat. No. 4,650,652, issued Mar. 7, 1987, relates to a process for recovering high purity rare earth oxides from a waste rare earth phosphor. The process utilizes dissolving waste rare earth phosphor in an excess amount of acid, adding oxalic acid to obtain precipitates of rare earth oxylates, washing precipitates and baking precipitates.

Ozaki et al. in U.S. Pat. No. 4,507,254, issued Mar. 26, 1985, relates production of a rare earth metal alkoxide by reacting a rare earth metal carboxylate with an alkali metal alkoxide in an inert organic solvent or liquid under anhydrous conditions.

U.S. Pat. No. 4,244,935, issued to Dell on Jan. 13, 1981, relates a method of forming the chloride of a metal-oxygen containing substance based on a fluid coking technique. It should be noted that the commercial process for making zirconium metal utilizes a fluidized bed process in which the ore is subjected to a chlorination step which produces a relatively impure, hafnium-containing zirconium tetrachloride and by-product silicon tetrachloride (which by-product is relatively easily separated by differential condensation). U.S. Pat. No. 3,895,097, issued to Langenhoff et al. on July 15, 1975, also relates to a process for reacting metal oxides with chlorine.

U.S. Pat. No. 4,670,573, issued to Greco et al. on June 2, 1987, relates to the preparation of metal alkoxides from metals and alcohols. The description of the prior art lists references that relate to the reaction of metals with alcohol to form metal alkoxides. Kirk-Othmer also discusses metal oxides of higher, unsaturated, or branched alcohols made from lower metal alkoxides on page 1, lines 25-50.

U.S. Pat. No. 4,472,510, issued to January on Sept. 18, 1984, relates to a process of making glassy ceramics, including a method of preparing a carbon-containing monolithic glassy ceramic including a metal alkoxide which hydrolyzes and polymerizes in the presence of water.

This is a process for extracting a yttrium superconductor component from a rare earth ore and using the yttrium superconductor component to prepare a composite for use in the fabrication of a ceramic superconductor. This process utilizes feeding a rare earth (which may have been concentrated by known, leaching processes), to a fluidized bed chlorinator to produce yttrium chloride and chlorides of other rare earths contained in the ore, separating the yttrium chloride by differential condensation at 725-1200 C. from (preferably refluxed at 900-1100 C.) the other rare earth chlorides, anhydrously reacting the separated yttrium chloride with at least one alkoxide selected from the group consisting of the alkali metal alkoxides (preferably sodium alkoxide especially NaOCH.sub.3, NaOC.sub.2 H.sub.5, NaOC.sub.3 H.sub.7, NaOC.sub.4 H.sub.9 or NaOC.sub.5 H.sub.11) or alcohols to produce yttrium alkoxide, separating the yttrium alkoxide from the reaction by-products and excess reagents, and mixing the yttrium alkoxide with alkoxides of other non-oxygen constituents of the superconductor to produce an alkoxide composite for further processing into a superconductor.

Preferably, the separating of the yttrium alkoxide from reaction by-products and excess reagents is performed by evaporative crystallization followed by a distillative purification. Generally, the alkoxide reactant has 1-5 carbon atoms, and preferably one or two carbon atoms. Generally, the non-oxygen components are yttrium, at least one alkaline earth metal, and copper, however, some fluorine may also be included.

The utilization of alkoxides provides for extremely homogeneous superconductors. The process also allows introduction of stabilizers and dopants (to improve or modify electrical properties) which, heretofore had been ineffective due to inhomogeneous introduction.

The sole FIGURE is a graph of vapor pressure (in Torr) as a function of temperature for chlorides of yttrium, cerium and lanthanum.

This invention provides a process for fabrication of precursors of a ceramic superconductor. By mixing alkoxides of the non-oxygen constituents, an extremely homogeneous composite is prepared. This allows the final electrical or electronic device properties to be tailored to provide exacting control of chemical and physical properties. Compared to prior fabrication approaches which used oxide feedstocks, the present process had the advantages of achieving chemical homogeneity, and providing easy formation using liquid polymers. The physical homogeneity in the matrix translates directly to improved microstructure of the product. The excessive milling and blending steps required for prior art powders, with their inherent introduction of contaminants, are eliminated.

It should be noted that the term "rare earth", as used herein, includes yttrium. It should also be noted that, as used herein, "alkoxide" includes alcohol soluble compounds of non-oxygen components (especially copper) whose non-metallic (e.g. non-copper) decomposition products are gaseous at less than 600 C. (e.g. copper acetate or copper acetyl acetate). Generally, the actual alkoxides are used where solubility is not a problem and such other alcohol soluble compounds are utilized to supply the remaining material for a stoichiometric mixture.

For superconducting materials, superior product performance is obtained where metal alkoxide precursors are used as process feedstocks. This product performance stems from the precise control of chemical composition, chemical homogeneity, and improved microstructure available with the alkoxide feedstocks, but not from the oxide feedstocks normally utilized. The high cost of alkoxide materials, has in the past, been a major impediment to wide spread application. The present invention integrates chlorination technology to generate the required alkoxide feed stocks to provide cost effective metal alkoxide production. The process accepts the rare earth ores (e.g. xenotime or monazite). It generally utilizes fluidized bed chlorination of either the ore or of an ore concentrate produced by leaching of the ore mixed with carbon, at about 800-1250 C., with chlorine gas providing chlorination and, together with an inert gas providing fluidization (generally similar to the known process for chlorinating zirconium ore) to produce rare earth chlorides. The yttrium chloride is separated from the other rare earth chlorides by differential condensation at 725-1200 C. (and preferably reflux condensation at 900-1100 C.). It should be noted that the boiling point of yttrium chloride is 1507 C. and that the boiling points of the other rare earths are a little over 1700 C. and that while chlorination and condensing the other rare earths (with the yttrium remaining vapor and being condensed separately later) at 1550 C. or 1600 C. is theroetically possible, such extremely high temperatures are impractical for commercial production. This invention avoids such temperatures by operating both chlorination and differential condensation below the rare earth chloride boiling points at relatively low partial pressures (see the sole FIGURE, the vapor pressures of the remaining rare earth chloride are not shown, but are similar to those of the chlorides of cerium and lanthanium) and generally uses an inert gas to give a total system operating pressures of about one atmosphere and preferably uses a reflux condenser to improve separation condensing the other rare earth chlorides first and separately condensing the yttrium chloride downstream. The vapor from a 950 C. chlorinator can be introduced near the bottom of a reflux condenser, with the condenser operated, e.g. at about 900 C. with a reflux reboiler on the bottom operated at 1000 C., such that the reboiler receives condensate liquid from the condenser and vaporizes and refluxes most of the condensate, thus improving separation. Refluxing is required with monazite ore which contains only about 2.1% of the total lanthanon oxide as yttria, and preferred for ores such as xenotime (which contains about 60.8% of the lanthanon oxide as yttria). It should be noted that the other rare earths are generally not separated from one another by the process of this invention, and the by-product is generally mixed rare earth chlorides, depleted in yttrium.

The yttrium chloride is then reacted, generally with sodium alkoxide (e.g. NaOCH.sub.3), to produce an yttrium alkoxide (e.g. Y(OCH.sub.3).sub.3). By utilizing the chloride reaction with sodium alkoxide, yttrium alkoxide is directly produced, with the reaction going to completion, avoiding the use of high temperatures and pressures and/or expensive catalyst. Alkoxide is then preferably crystallized, by evaporation off (and recycling) excess reagent. The yttrium alkoxide may then be separated from by-product salt (e.g. sodium chloride) by heating to distill off the yttrium alkoxide. An alkaline earth metal alkoxide can be similarly formed by reacting the chloride with sodium alkoxide. The purified yttrium alkoxide can then be blended with other alcohol soluble compounds of the other non-oxygen constituents (e.g. BaOCH.sub.3 and copper acetyl acetate), to form a composite which may be gelled and eventually oxidized to produce the metal oxide ceramic superconductor.

This process provides for mixing on an atomic level and provides, at a reasonable cost, a homogeneous product which cannot be obtained by the prior art powder processes which utilize milling of oxides. The prior art processes, even with many cycles of grinding, firing, regrinding, refiring, etc., do not achieve mixing on an atomic level and inherently produce an inhomogeneous product. Applicants' invention, utilizing actual alkoxides or other alcohol soluble compounds produces a true solution and mixing of the (generally metallic), non-oxygen components on an atomic basis.

The process is broadly applicable to ceramic superconductors (e.g. yttrium-barium-strontium-copper oxides), however, the well-known yttrium-barium-copper oxide will be used for purposes of illustration. Thus the feed mixture for a gel may consist of stoichiometric amounts of yttrium and barium alkoxides with the copper generally being added as the aforementioned other alcohol soluble compound (e.g. copper acetyl acetate). The alcohol moieties on the feed stock alkoxides are selected to establish the desired microstructure and physical properties of the products, but in general are preferred to be C.sub.1 through C.sub.5 (and especially C.sub.1 and C.sub.2 as they require less oxygen during curing to the oxide) aliphatic chains. Table 1 below shows a procedure for forming an alkoxide gel.

Alternatively other alcohol/metal alkoxide combinations can be used. The reaction mix is polymerized in an exothermic reaction at the alcohol normal boiling point of 78.4 C. under reflux. The actual volume percent composition of the reaction mix can vary considerably.

The alkoxide gel can be fabricated into final devices, for example, utilizing suspension of single crystal grains of superconducting oxide in the gel, as taught in the aforementioned application Ser. No. 121,013, or by contacting the gel with a single crystal substrate and curing starting at the gel-substrate interface, as taught in the aforementioned copending application Ser. No. 121,010. Alternatively, the ungelled mixture can be spray oxidized to produce a powder. Such powder, being extremely homogeneous, can be used, for example, for sputtering. It should be noted that this extremely homogeneous material overcomes the homogeneity problems in sputtered films (generally sputtered films have been unsatisfactory due to compositional variations within the film, and the inventors believe this inhomogeneity is caused by inhomogeneity in the milled oxide powder sputtering targets used as a source of material for sputtered films in the prior art).

Thus, this invention provides a process for extracting yttrium from a rare earth ore and using the yttrium to prepare a composite (gelled or ungelled) for use in the fabrication of a ceramic superconductor. This process utilizes a fluidized bed chlorinator to produce yttrium chloride and by-product mixed chlorides of non-yttrium rare earths, separating the yttrium chloride from the other rare earth chlorides by differential condensation at 725-1200 C., anhydrously reacting the yttrium chloride with preferably sodium methyl alkoxide to produce ytrrium methyl alkoxide, and preferably mixing the yttrium alkoxide with at least one alkaline earth metal alkoxide and an alcohol soluble copper compound, with the copper compound preferably being copper acetate or copper acetyl acetate or combinations thereof. In addition, a source of fluorine (e.g. an alcohol soluable copper and fluorine compound), as well as stabilizer or dopant alkoxides may also be added.

The invention is not to be construed as limited to the particular examples described herein, as these are to be regarded as illustrative, rather than restrictive. The invention is intended to cover all processes which do not depart from the spirit and scope of the invention.