The present invention provides an intermetallic compound-based, composite superconductor suitable for use in superconducting coils for a high magnetic field, where the superconductive part is formed by thermal diffusion reaction, and which has such a structure that a metal layer of high melting point and high resistance is provided at the outermost periphery of a strand comprising a plurality of filaments containing a superconductive compound formed by diffusion reaction with the surrounding matrix, a stabilizer surrounding the matrix, and a diffusion barrier provided between the stabilizer and the matrix for preventing diffusion of superconductive compound-forming element in the matrix from diffusion into the stabilizer during the diffusion reaction, and the coupling current induced between the intermetallic compound-based superconductive wire members can be reduced by the provision of the metal layer of high melting point and high resistance. Furthermore, the superconductive parts can be formed by heat treatment, and the AC loss can be reduced thereby.

BACKGROUND OF THE INVENTION 
This invention relates to an intermetallic compound-based, composite 
superconductor, and more particularly to an intermetallic compound-based, 
composite superconductor suitable for use in superconducting coils, etc. 
for generating a high magnetic field, where the superconductive part is 
formed by a thermal diffusion reaction. 
Superconducting coils for use in a nuclear fusion reactor utilize an 
intermetallic compound-based superconductor having distinguished high 
magnetic field characteristics as a conductor owing to the necessity for 
generating a high magnetic field. According to the most popular structure 
of an intermetallic compound-based composite superconducting wire member, 
a large number of filaments are evenly distributed and contained in a 
matrix. The filaments contain a superconductive compound formed by 
diffusion reaction of the element in the filaments with the element in the 
matrix through a high temperature heat treatment, such as niobium-tin 
(Nb.sub.3 Sn), etc., and diffusion barriers are provided around the matrix 
to prevent a portion of the superconductive compound-forming element in 
the matrix from diffusion into a stabilizer during the thermal diffusion 
reaction. 
According to other examples of intermetallic compound-based, composite 
superconducting wire members having different structures from the 
above-mentioned one, a single diffusion barrier is provided in the 
stabilizer, or the individual filaments are surrounded by independent 
diffusion barriers, respectively. The individual intermetallic 
compound-base, composite superconducting wire members given above are used 
in coils for a relatively small electric current. 
On the other hand, the necessary superconductor for use in superconducting 
coils for a nuclear fusion reactor is a conductor for generating a high 
magnetic field and carrying a large electric current such as more than 10 
KA and more than 10 T, and thus it is composed from a plurality of the 
above-mentioned intermetallic compound-based, composite superconducting 
wire members. 
Generally, a conductor for a high magnetic field and a large electric 
current comprises a plurality of the above-mentioned intermetallic 
compound-based, composite superconducting wire members which are stranded 
and which are provided in a conduit made from a high strength structural 
material, where a coolant passes through interstices between the plurality 
of the stranded wire members in the conduit to cool the conductor. Since 
the conductor for a high magnetic field and a large electric current is in 
a structure that a plurality of intermetallic compound-based, composite 
superconducting wire members are stranded and encased in a conduit, a 
large AC loss is generated by the coupling currents induced between the 
intermetallic compound-based, composite superconducting wire members. 
The conventional alloy-based, composite superconducting wire member, for 
example, a NbTi-based composite superconducting wire member, requires no 
high temperature heat treatment, and thus the AC loss can be suppressed by 
applying an organic insulation to the surface of the wire member, whereas 
a conductor for a high magnetic field and a large electric current 
composed from a compound-based composite superconducting wire member 
requires a high temperature heat treatment, and thus it is hard to apply 
the organic insulation to the surface of the wire member. That is, there 
is such a disadvantage that the AC loss cannot be reduced. 
Intermetallic compound-based, composite superconducting wire members, where 
the superconductive part is formed by heat treatment, are disclosed in 
Japanese patent applications Kokai (Laid-open) Nos. 59-191208 and 
59-191213. 
SUMMARY OF THE INVENTION 
The present invention has been made considering the foregoing backgrounds, 
and an object of the present invention is to provide an intermetallic 
compound-based, composite superconductor with a small AC loss, using an 
intermetallic compound-based, composite superconducting wire member 
capable of forming a superconductive part by heat treatment. 
The object of the present invention can be attained by providing a metal 
layer of high melting point and high resistance at the outermost periphery 
of a superconducting compound formed by diffusion reaction with the 
surrounding matrix, a stabilizer surrounding the matrix, and a diffusion 
barrier provided between the stabilizer and the matrix for preventing 
diffusion of superconductive compound-forming element into the stabilizer 
during the diffusion reaction.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will be described in detail below, referring to the 
embodiments shown in the accompanying drawings. 
FIG. 1 shows one embodiment of the present invention, where superconductive 
elements each comprising a plurality of filaments 2 and a matrix 3, 
surrounded by a diffusion barrier 4 are distributed in a stabilizer 5, and 
a metal layer 7 of high melting point and high electrical resistance is 
provided at the outermost periphery of the stabilizer 5. Materials for the 
metal layer 7 of high melting point and high resistance include, for 
example, metals incapable of diffusing into the stabilizer 5 and 
contaminating the stabilizer during the heat treatment of the 
compound-based superconductor such as niobium (Nb), tantalum (Ta), 
vanadium (V), chromium (Cr), molybdenum (Mo), tungsten (W), etc. 
In this structure, high temperature heat treatment can be made to conduct 
diffusion reaction, and insulation can be made owing to the provision of 
the metal layer of high melting point and high electrical resistance, so 
that the coupling current induced between the intermetallic compound-based 
superconducting wire members can be reduced, and the AC loss can be thus 
suppressed. 
FIG. 2 shows another embodiment of the present invention, where another 
stabilizer 8 is further provided around the intermetallic compound-based, 
composite superconducting wire member according to the embodiment of FIG. 
1, and the outer surface of the stabilizer 8 is chemically treated to form 
an electric insulating layer 9. Materials for the stabilizer 8 for this 
purpose include, for example, copper, aluminum, etc. which are easy to 
process when drawing the superconducting wire member. The chemical 
treatment of the surface of the stabilizer 8 is, for example, oxidation or 
sulfurization to form oxides such as CuO, Al.sub.2 O.sub.3, etc., or 
sulfides such as CuS, etc. 
In this structure, the AC loss of an intermetallic compound-based composite 
superconductor can be much more reduced to provide a stabilized, 
intermetallic compound-based, composite superconductor for a high magnetic 
field and a large electric current. That is, a large, intermetallic 
compound-based superconducting coil with a high stability even against a 
large fluctuation in the magnetic field can be prepared. As a result, an 
amount of expensive liquid helium can be considerably reduced. This means 
a distinguished economical advantage. 
In the present intermetallic compound-based, composite superconductor, a 
metal layer of high melting point and high electrical resistance is 
provided at the outermost periphery of a strand comprising a plurality of 
filaments containing a superconductive compound formed by diffusion 
reaction with the surrounding matrix, a stabilizer surrounding the matrix, 
and a diffusion barrier provided between the stabilizer and the matrix for 
preventing diffusion of superconductive compound-forming element in the 
matrix into the stabilizer during the diffusion reaction, as described 
above, and thus the coupling current induced between the intermetallic 
compound-based superconducting wire members can be reduced by the 
provision of the metal layer of high melting point and high resistance. 
Furthermore, the superconductive parts can be formed by heat treatment, 
and the AC loss can be reduced thereby. These are the effects of the 
present invention.