Abstract:
A mixture suitable for the production of melt-processed high-temperature superconductors capable of producing a high levitation force. The mixture contains YBa 2  Cu 3  O 7-x  powder with a very low content of copper oxide, i.e. copper not bound in with the YBa 2  Cu 3  O 7-x , and a very low carbon content. Also included are stabilizing (so-called &#34;flux-pinning&#34;) additives. Also disclosed is a method or producing the mixture, as well as YBa 2  Cu 3  O 7-x  powder with suitable low free copper oxide and carbon contents, used to prepare the mixture.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a superconductor premixture prepared for fusion processing, a method for the production thereof, and a YBa 2  Cu 3  O 7-x  powder which can be used in preparing the premixture. 
     YBa 2  Cu 3  O 7-x  powder (x=0 to 0.5) is used for the preparation of technical superconductors. The powder is advantageously mixed with stabilizing additives which form sticking or pinning centers. The resulting premixture is pressed into compacts and heat treated, preferably by fusion processing. In fusion processing the surfaces of the powder particles of the premixture are heat-softened, so that in the cooling that follows the particles are bonded tightly to one another. The flow of the electric current through the grain boundaries is thus improved. The stabilizing additives usually have the effect of reducing the melting temperature of the premixture, so that the softening of the bodies that are to formed and the escape of molten material otherwise observed at higher fusion texturizing temperatures, can be prevented. The additives, which form sticking or pinning centers, permit increasing the critical current density in the superconductors. The pinning centers provide for an anchoring of the magnetic lines of flux, so that high critical currents can also be achieved in the magnetic field. Flux pinning additives for superconductors composes of copper oxide are disclosed, for example, in European Patent Applications EP-A-0 292 126 and 296 380. 
     The bodies obtained by fusion processing from high-temperature superconductor material can be used, for example, in the manufacture of electric motors and in the production of the so-called &#34;flywheel.&#34; The flywheel rotates under superconducting conditions in the magnetic field and serves to store kinetic energy. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to provide an improved premixture composed of YBa 2  Cu 3  O 7-x  powders with stabilizing additives or those producing flux pinning. This object is achieved by the premixture described below. 
     The invention is based on the knowledge that premixtures which comprise the YBa 2  Cu 3  O 7-x  powders and stabilizing or flux pinning additives are especially well suited for the preparation of fusion-processed bodies if they have a small content of carbon combined with a small content of free copper oxide not bound in the YBa 2  Cu 3  O 7-x  phase. 
     The superconductor premixture according to the invention, which is designed for fusion processing, comprises additives which produce flux pinning as well as YBa 2  Cu 3  O 7-x  powder, this YBa 2  Cu 3  O 7-x  powder containing less than 0.6 wt.-% of free copper oxide not bound in the YBa 2  Cu 3  O 7-x  phase and less than 0.1 wt.-% carbon, relative to the YBa 2  Cu 3  O 7-x  powder. 
     The stabilizing or flux pinning additives can basically be added in any amount in which they result in the desired effect. The stabilizing action lies, for example, in the fact that the melting temperature of the particles in the superconductor premixture is lowered. The additives that produce the flux pinning cause the magnetic flux lines to be anchored in the superconducting material, and consequently high critical currents become possible. Any customary stabilizing or flux pinning additives can be used. There are additives, such as Y 2  BaCuO 5 , of which it is assumed that they have both a stabilizing and a flux pinning action. Especially well suited as additives are Y 2  O 3 , Y 2  BaCuO 5 , PtO 2 , Ag 2  O, CeO 2 , SnO 2 , ZrO 2 , BaCeO 3  and BaTiO 3 . 
     Preferred superconductor premixtures according to the invention contain 0.1 to 50 wt.-% of stabilizing additives or additives that produce flux pinning, relative to the YBa 2  Cu 3  O 7-x  powder taken as 100%. 
     A content of Y 2  O 3 , Y 2  BaCuO 5 , PtO 2 , Ag 2  O, CeO 2 , SnO 2 , ZrO 2 , BaCeO 3  and/or BaTiO 3  is preferred as stabilizing or flux pinning additives in the superconductor premixture according to the invention. 
     If Y 2  O 3  is contained, an addition of 0.1 to 50 wt.-% of this compound, with respect to the YBa 2  Cu 3  O 7-x  powder taken as 100 wt.-%, is very advantageous; if Y 2  BaCuO 5  is contained, 0.1 to 50 wt.-% is advantageous. If PtO 2  is contained, 0.5 to 5 wt.-% of PtO 2  additive is advantageous. If Ag 2  O is contained, and an addition of 1 to 20 wt.-% is advantageous. If CeO 2 , SnO 2 , ZrO 2 , BaCeO 3  and/or BaTiO 3  is contained, a content of 0.1 to 5 wt.-% is preferred. 
     A superconductor premixture of the invention with especially advantageous properties in fusion texturizing contains less than 0.05 wt.-% of carbon, relative to the YBa 2  Cu 3  O 7-x  phase. Free copper oxide not bound in the YBa 2  Cu 3  O 7-x  phase is preferably contained in an amount of less than 0.2 wt.-%. 
     It has proven to be advantageous if the superconductor premixture comprises particles in the grain size range from above 30 up to 500 μm. It is especially advantageous if 40 to 70 wt.-% of the amount is in the stated grain size range. 
     If desired, the superconductor premixture according to the invention can be in compressed form, for example in the form of compacts pressed axially or isostatically. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The preparation of the superconductor premixture of the invention will be described below. For this purpose a YBa 2  Cu 3  O 7-x  powder containing less than 0.6 wt.-% of free copper oxide not bound in the YBa 2  Cu 3  O 7-x  phase, relative to the YBa 2  Cu 3  O 7-x  powder, and less than 0.1 wt.-% of carbon relative to the YBa 2  Cu 3  O 7-x  powder, is mixed according to the invention with stabilizing and/or flux pinning additives. The mixture, i.e., the premixture, can then be pressed if desired, axially or isostatically for example, to form compacts. Powder with preferred properties, the especially good stabilizing and/or flux pinning additives, and their preferred amounts, have already been mentioned above. 
     The preparation of a YBa 2  Cu 3  O 7-x  powder is explained hereinafter, which has the specification required for use in the superconductor premixture of the invention with regard to the free copper oxide content and the carbon content. This powder, which is especially suitable for use in the superconducting premixture of the invention is likewise subject matter of the invention. 
     This YBa 2  Cu 3  O 7-x  powder according to the invention, usable to special advantage in the superconductor premixture, is prepared by mixing together powdered yttrium, barium and copper compounds which include anions which have oxygen as well as, in some cases, hydrogen and/or carbon, and which are used in such amounts that the atomic ratio of yttrium, barium and copper is substantially 1:2:3, 
     a) calcining the finely divided starting material by heating it to a temperature of 850 to 920° C. in an oxygen-containing atmosphere, holding it at this temperature, then cooling it to room temperature, and then finely dividing the partially sintered powder body, 
     b) repeating step a) one or more times, and after the last repetition, upon cooling in the temperature range of 380 to 420° C., inserting a holding phase of up to 50 hours, until the mixture obtained is fully oxidized, and in the obtained mixture the detectable residue of copper oxide not bound in the YBa 2  Cu 3  O 7-x  phase is less than 0.6 wt.-% and carbon less than 0.1 wt.-% with respect to the YBa 2  Cu 3  O 7-x  powder, and 
     c) comminuting the material obtained in step b). 
     It is advantageous to use a powder in which at least 40% of the material is in the grain size range of 30 to 500 μm. It is especially preferred to prepare the superconductor premixture of the invention by admixing a YBa 2  Cu 3  O 7-x  powder in which 40 to 70 wt.-% of the material is in the grain size range of 30 to 500 μm. If the powder is produced by the method described above, it is preferred to grind, and optionally sort, the material after the holding phase. 
     The YBa 2  Cu 3  O 7-x  powder of the invention is advantageously prepared by using Y 2  O 3 , CuO and barium hydroxide, hydrated barium hydroxide or barium carbonate. 
     The calcination in the above-described step a) can advantageously be performed by calcining the starting material in loose or precompressed form by initially heating it in air at a rate of 100 to 400° C./hour from ambient temperature to an end temperature of 850 to 920° C., letting it stand at this temperature for 100 to 400 hours and then cooling it to ambient temperature at 100 to 200° C. per hour. The resulting material can then be ground as mentioned above, if desired, and sorted, if desired. At the end of the holding phase, the YBa 2  Cu 3  O 7-x  powder is advantageously mixed with the additive or additives and, if desired, axially or isostatically pressed to form compacts. 
     The superconductor premixture according to the invention is outstandingly suitable for the preparation of fusion-processed high-temperature superconductors with high levitation power. The compacts which are made with the premixture according to the invention are very stable; in tests of large numbers of samples they exhibit reproducible high levitation forces. 
     The following example is intended to further explain the invention without limiting its scope. 
    
    
     EXAMPLE 
     a) Preparation of the YBa 2  Cu 3  O 7-x  powder. 
     The yttrium compound used was commercial Y 2  O 3  with a purity of 99.99%, the copper compound was commercial CuO with a purity of 99.999% and the barium compound commercial BaCO 3  with a purity of 99.4%. The starting materials were measured out according to the desired composition for producing an amount of 500 g, and were homogeneously mixed. The mixture was loosely filled into in Kawenite crucibles (diameter 50 mm, height 35 mm). The crucibles were placed in a laboratory muffle kiln with low temperature gradients (ΔT&lt;1 K/cm) and heated at 300 K/hour to an end temperature of 905° C. The powder was held at this temperature for 96 hours and thereafter cooled at 200 K/hour to room temperature. The powder cake was crumbled and sieved, and the crucibles were again filled with this granulate. The granulate was subjected to a second heating cycle over 72 hours, to a third heating cycle over 62 hours, and to a fourth heating cycle over 70 hours, at the same temperature and at equal heating and cooling rates, and crumbled and sieved after each heating cycle. After the last cycle the granulate was held at 400° C. for one hour, cooled at 200 K/hour to room temperature, and comminuted. All of the heating cycles were performed in air. The detectable free copper oxide in the fully reacted powder was measured by differential thermal analysis and thermogravimetry, and is under 0.2 wt.-%. The carbon content according to chemical analysis is less than 0.05 wt.-%. The extensive conversion of the premixture to the orthorhombic phase was detectable by means of the doublet cleavage in the X-ray diffraction spectrum. 
     b) Verification of the Premixture. 
     The powder was premixed with 9 wt.-% of yttrium oxide and 1 wt.-% of platinum oxide. The premixture was pressed axially at 3 MPa to compacts 35 mm in diameter and 20 mm in height. The compacts were heated at 300 K/hour to 1190° C. and held for 0.5 hours, then cooled at 400 K/hour to 980° C., and slowly fusion-processed from this temperature at 1 K/hour to 950° C. in the temperature gradient of 5 K/cm. Cooling to room temperature was performed at 400 K/h. In a separate heat process between 400 to 600° C. over 144 hours the material was converted to the orthorhombic phase. In the superconductive state at 77 Kelvin, when the compact approaches a samarium-cobalt permanent magnet 25 mm in diameter and 10 mm in thickness, at a distance of 0.5 mm levitation forces of 40 to 60 N are produced, corresponding to 30 to 35% of the repulsive forces, for perfect shielding.