A process for preparing a superconducting ceramic and particularly YBa{sub 2}Cu{sub 3}O{sub 7-{delta}}, where {delta} is in the order of about 0.1--0.4, is carried out using a polymeric binder which decomposes below its ignition point to reduce carbon residue between the grains of the sintered ceramic and a nonhydroxylic organic solvent to limit the problems with water or certain alcohols on the ceramic composition

Bloom, I. D.

Flandermeyer, B. K.

Poeppel, R. B.

UNT Digital Library

PAT_qTS-US--A7519981I_92 017178CDIZulIoI,,-,4G)IOO,-_Om_H_mPROCESSING METHOD FOR SUPERCONDUCTING CERAMICS_n Ira D. BloomURoger B. Poeppel_ Brian K. FlandermeyerO_ZX _ DISCLAIMERZ f::3 Thi;s report was prepared as an account of work sponsored by an agency of th,e United StatesI.-4 2: Government. Neither the United States Government nor any agency thereof, nor any of theirf./'j f...) employees, makes any warranty, express or implied, or assumesany legal liability or responsi-13_ bility for the accuracy, compl.eter_ess,or usefulness of any informati, on, apparatus, product, orU r,:..] prooess di.sclos_, or represents that its use wouM not infringe privately owned rights. Refer-O 13.,_:D en.ceherein to any specific commercial product, process,or servi.c¢by trade name, trademark,134tj} manufacturer, or otherwise does nol| r_eccssarily constitute or imply its endorsement, recom-mendatbn, or favoring by the United States Government or any agency thereof. The viewsariel, opinb,n=sor _utho,rs exprcs.u_ herein do n,o,t nec.euarily state er reflect thoc_eof thee-I United Stat¢_ Government o.r any agency t_ereof.CDd° MASTEBDtSTRtBUT;O_'.q C)F THIS DOCUMENT IS UNLI_'E_m'oCONTRACTUAL ORIGIN OF THE INVENTIONThe United States Government has rights in thisinvention pursuant to Contract No. W-31-109-ENG-38 betweenthe U.S. Department of Energy and The University of Chicagorepresenting Argonne National Laboratory.BACKGROUND OF THE INVENTIONThis invention relates to the preparation of metal oxidesuperconducting ceramics and more particularly to the use ofnew processing additives for preparing the sinteredi0 superconductors. Since the discovery of metal-oxidesuperconducting ceramics, considerable effort has beendirected to the development of compositions with high Tc andimproved performance with emphasis being placed onprocessing techniques. In these ceramic compositions, ithas been important that the grain contact areas have minimalcontamination with nonsuperconducting phases or othermaterials which would adversely affect the passage ofcurrent.PoOne of the important metal-oxide superconductors haswhere 6 may vary from aboutthe composition YBa2Cu307__0.1-0.4 with the composition being commonly identified as"123". Preparation of the composition involves the mixingof Y203 , CuO and BaCO 3, calcining of the mixture to formYBa2Cu307_6, grinding to reduce particle size, forming intoa "green" body and sintering to form the final product.Usually, a solvent and binder are included as processingadditives after calcining. The solvent is usually selectedI0 to have the property of being volatilized at temperaturesbelow about 100°C with the binder remaining until anignition temperature of about 500°C is reached with thesintering subsequently being carried out at about850-975oc. An illustration of a common solvent-bindercombination is methanol and polyvinyl alcohol.Since the binder is removed by burning, some residue isformed on the grains. While the extent of the problem hasnot been measured, the formation of residue between thegrains will increase the resistance between grains and limit20 the current density of the superconductor.Another problem which may occur in the processing isthe potential for an adverse effect of any moisture oralcohol on the composition or components. In thepreparation of the superconducting ceramic, it is ofconsiderable importance to maximize the percentage of theDdesired superconductor and avoid or limit the levels of....materials which are not superconducting.Accordingly, one object of this invention is thepreparation of a superconducting composition with limitedresidue formed between interconnecting grains of thesuperconductor. Another object of the invention is thepreparation of a superconducting composition in which thepresence of water or other hydroxy-containing compound isminimized. These and other objects of the invention willi0 become apparent from the following description.SUMMARY OF THE INVENTIONBriefly, this invention involves the preparation of ametal-oxide superconducting composition using a binder whichdecomposes below its igniti<_n temperature with the resultingdecomposition products being volatilized. The binders ofparticular interest are polymeric binders which depolymerizebelow about 500°C and preferably between 300-450°C with theresu_[ting products being volatilized. A preferable binderis polyisobutylene. Advantageously, the invention also20 includes the use of a solvent essentially free of water oralcohol. Preferably, the solvent is an ether such astetrahydrofuran, or a hydrocarbon such as cyclohexane or thelike. In the inventive method of preparation, thecomponents are mixed, heated to an elevated temperature forcalcining, processed in a grinding operation to reduceparticle size, and heated to a second elevated temperaturefor sintering, with a nonhydroxy containing solvent and apolymeric binder being added at a stage after the calcining.DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTThe invention is useful in the preparation of metal-oxide superconducting ceramics based on the presence of oneor more -Cu-O- planes and/or chains including those with thegeneral formula (Lal_xAx) CuO 4 where A may be Ca, Sr or Baand x is in the order of 0.1-0.2; RBa2Cu307_6 where R is ai0 rare earth; Bi2Ca2Sr2CU3Ol0, Bi2Ba2CaCu208, and those whereT1 may be substituted for Bi and Sr may be substituted forBa. The preferred superconductor is YBa2Cu307_ 6.In general, these are prepared from reactive mixturesof oxides, nitrates and/or carbonates .of the metals witheach reactant being added in an appropriate ratio for thedesired superconducting material. Suitable reactantsinclude Y203 , BaCO 3, CaCO 3, SrCO 3, Bi203 and TI203 and thecorresponding oxides and carbonates of the other metals.Advantageously, the reactants are milled in a vibratory mill20 with liquid hydrocarbon such as toluene and zirconia orother milling media at ambient temperatures for about 2-3 h.The mixture of reactants is then calcined at suitabletemperatures to form the superconducting composition orsimilar composition which may be subsequently converted to asuperconducting composition by treatment in an oxygenLatmosphere. Temperatures in the order of 700-750°C forabout 6-10 h are representative.Particularly when current density is of importance, thecomposition from calcining is processed further in agrinding operation to reduce particle size. Advantageously,the resulting particle size is in the order of about5-10 microns.Following the grinding operation, the composition isformed by pressing, extrusion or the like into a desiredI0 form and sintered. Advantageously, the sintering is attemperatures in the order of about 850-975°C to form anintegral mass of interconnected particles or grains.The inventive method further includes the use of asolvent _nd binder added to the composition following thecalcining and preferably before the grinding or pressingoperations. The solvent permits the extension of the binderthroughout the particle mixture.During the sintering step in the process, the solventis removed by evaporation at temperatures of about 60-80°C20 and preferably below about 100°C. Suitable solvents are thenonhydroxylic organics including ethers with less than abouteight carbon atoms such as tetrahydrofuran, hydrocarbonswith about 6-8 carbon atoms such as cyclohexane, and thelike. The binder remains in the composition after removalof the solvent until a decomposition temperature of the6Aorder of about 400°C is reached. The binder is an organicselected to form decomposition and preferably depolymerizedproducts which volatilize off at temperatures in the orderof about 300-450°C. Advantageously, the binder is polymericincluding polyisobutylene, polyisoprene and the like.Preferably, the binder is a polymer or copolymer ofbutene-l. These in general will have molecular weights inthe order of about 3,000-500,000 and preferably about50,000-500,000.i0 The following examples are provided for illustrativepurposes and are not intended to be restrictive as to thescope of the invention.EXAMPLE IA metal-oxide superconductor with the general formulaYBa2Cu307_ 6 was prepared. In the process, a mixture ofabout 5.0 gm Y203 , 17.37 gm BaCO 3 and 10.50 gm CuO wasmilled in about 50 ml toluene for about 2 hr. The tol_ enewas removed by drying overnight. The resulting mixture wasthen heated overnight under oxygen at a rate of about20 3.3°C/min to a temperature of about 686°C. After cooling,the product was composed of particles of a black solidcomposition. A mixture of polyisobutylene (mol. wt. ofabout 380,000) in tetrahydrofuran (about 1 gm or 0.5-1 wt.%)was added to the composition and the particles were pressedat about i0,000 psi into a pellet of about 1/4 inch indiameter. The pellet was heated to a temperature of about790°C and sintered overnight under flowing oxygen. AMeissner effect was demonstrated on the product.EXAMPLE IIFollowing the sintering of the composition ofExample I, the sintered product is examined by a scanningelectron microscope for evidence of organic residue fz:om thebinder. The product appears to be essentially fl1:ee ofcarbon-based residue.i0 The foregoing description of embodiments of theinvention has been presented for purposes of illustrationand description. It is not intended to be exhaustive or tolimit the invention to the precise form disclosed. Othermodifications and variations are possible in light of theabove teaching.i0ABSTRACTA process for preparing a superconducting ceramic andparticularly YBa2Cu307_6, where _ is in the order of about0.1-0.4, is carried out using a polymeric binder whichdecomposes below its ignition point to reduce carbon residuebetween the grains of the sintered ceramic and anonhydroxylic organic solvent to limit the problems withwater or certain alcohols on the ceramic composition.

Processing method for superconducting ceramics

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Processing method for superconducting ceramics

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