Method of Manufacturing a Superconducting Wire

The invention will be described more fully with reference to the the following example and the drawing, in which:

FIG. 1 shows a cross-section of a preform used in the method according to the invention,

FIG. 2 shows in cross-section a preform obtained by compacting the preform shown in FIG. 1,

FIG. 3 shows in cross-section a superconducting wire obtained in accordance with the invention, and

FIG. 4 shows in cross-section another embodiment of a superconducting wire obtained in accordance with the invention.

FIG. 5 shows the Y.sub.2 O.sub.3 --BaO--CuO phase diagram.

A mixture comprising 60% by weight of a pulverulent superconducting material corresponding to the formula YBa.sub.2 Cu.sub.3 O.sub.7-.delta., where .delta.=0.1-0.5 and 2% by weight of lecithine and for the remaining part a binder, such as especially nitrocellulose, is ground for a few hours in the presence of a solvent for the binder in a ball-mill. The mixture is processed by means of an extruder to a number (such as three) of cylindrical rods, which are denoted in FIG. 1 by reference numeral 1. In the same manner, rods 2 are manufactured, which comprise instead of the said superconducting material a pulverulent non-superconducting oxidic material corresponding to the formula Y.sub.2 BaCuO.sub.5. This material is non-reactive with respect to the aforementioned YBa.sub.2 Cu.sub.3 O.sub.7-.delta.. The rods 1 and 2 are arranged in the manner shown in FIG. 1. The rods have a diameter of a few millimeters, for example 5 mm. Subsequently, a pressure is exerted in radial direction on the bundle of rods, during which process the spaces between the rods disappear. The preform obtained is shown in FIG. 2, in which the same reference numerals are used as in FIG. 1.

The preform is then deformed by means of an extrusion process to a wire having a diameter of 15 .mu.m. The solvent evaporates in air. The wire is shaped into the desired configuration and is wound, for example, helically. The wire is then subjected to a first temperature treatment, in which the wire is heated to a temperature of 400.degree. C. As a result, the nitrocellulose is entirely decomposed to gaseous products so that no residue remains in or on the wire. Subsequently, a second temperature treatment is carried out, in which the wire is kept for a few hours at a temperature of 1300.degree. C. The wire is sintered due to this temperature treatment. The superconducting wire obtained is shown in FIG. 3, in which reference numeral 3 denotes the filaments obtained of the superconducting material having the aforementioned composition and reference numeral 4 denotes the matrix of non-superconducting material of the aforementioned formula. The filaments are entirely embedded in the matrix.

In quite the same manner as described above, a preform is manufactured, in which the rods 1 comprise the non-superconducting oxidic material of the formula Y.sub.2 Cu.sub.2 O.sub.5 and the rods 2 comprise the non-superconducting oxidic material of the formula BaCuO.sub.2. After extrusion of the preform, baking-out of the binder and sintering, a superconducting wire is obtained which is shown in FIG. 4.

In FIG. 4, reference numeral 5 denotes the sintered material of the formula Y.sub.2 Cu.sub.2 O.sub.5 and reference numeral 6 denotes the sintered material of the formula BaCuO.sub.2. During the sintering process, the superconducting compound of the formula YBa.sub.2 Cu.sub.3 O.sub.7-.delta. is formed at the interface of the two materials so that tubular filaments 7 of superconducting material are obtained, which are embedded in the non-superconducting materials 5 and 6.

In FIG. 5 the phase diagram of CuO--BaO--YO.sub.1,5 is represented. As shown by means of a partly dotted line the non-superconducting compound Y.sub.2 Cu.sub.2 O.sub.5 will combine with the non-superconducting compound BaCuO.sub.2 to form the superconducting compound YBa.sub.2 Cu.sub.3 O.sub.7-.delta.. Also the non-superconducting superconducting CuO combines with the non-superconducting Y.sub.2 Ba.sub.4 O.sub.7 to form the above given superconducting compound. Furthermore the diagram shows three segregation (decomposition) zones. The triangle with the corners BaCuO.sub.2 --Y.sub.2 BaCuO.sub.5 --YBa.sub.2 Cu.sub.3 O.sub.7-.delta. represents one decomposition zone in which any compound present in said zone will decompose into the products represented by said corners. A second decomposition zone is defined by the corners of the triangle CuO--Y.sub.2 BaCuO.sub.5 --YBa.sub.2 Cu.sub.3 O.sub.7--.delta.. Every compound with a composition falling inside said zone will decompose to said corner compositions. A third decomposition zone is the triangle with the corners YBa.sub.2 Cu.sub.3 O.sub.7-.delta. --CuO--BacuO.sub.2. Every compound within this zone will decompose into the products represented by said corner compositions.

Thus, any compound having a composition which lies inside the zone confined by the larger triangle CuO--BaCuO.sub.2 --2 Y.sub.2 BaCuO.sub.5 will at decomposition at least produce the superconductive YBa.sub.2 Cu.sub.3 O.sub.7- .delta.. It may be emphasized that the non-superconductive compound Y.sub.2 BaCuO.sub.5 in a stable compound which will not decompose and not produce the aforesaid superconductive material.