Method of producing a superconducting wire using alloy preform

A method of producing a superconducting wire including A-B-C-D system oxide superconductor wherein A is an at least one element of group IIIa of the Periodic Table, B is an at least one alkali earth metal, C includes Cu and D includes O. In the method, a material containing at least one of both A and B is arranged inside or outside an elongated tubular coating layer, which includes a copper alloy and at least the other one, to form a composite element. Then, the composite element undergoes a heat treatment to produce the superconductor.

The present invention relates, though not exclusively, to a method of 
producing a superconducting wire which may be used in, for example, 
magnets for nuclear magnetic resonance or for particle acceleration and a 
superconducting wire produced according to the method. 
Recently, various oxide superconductors which exhibit very high critical 
temperatures (Tc) have been increasingly discovered. For producing such 
superconductors, for example, (La,Sr)CuO superconductor, a mixture of 
powders of a carbonate of Sr, an oxide of La and CuO is subjected to a 
heat treatment. 
However, the carbonate of Sr and the oxide of La are extremely hard to be 
worked and hence it is difficult to fabricate a long superconducting wire 
by extrusion or drawing without any trouble such as breaking. 
Accordingly, it is an object of the present invention to provide a method 
of producing a composite oxide superconductor which method reduces the 
above disadvantage of the prior art. 
It is another object of the present invention to provide a composite oxide 
superconductor produced according to the method. 
According to one aspect of the present invention, there is provided a 
method of producing a superconducting wire including A-B-C-D system oxide 
superconductor wherein A is an at least one element of group IIIa of the 
Periodic Table, B is an at least one alkali earth metal, C includes Cu and 
D includes O. In the method, a material containing at least one of both A 
and B is arranged inside or outside an elongated tubular coating layer, 
which includes a copper alloy and at least the other one, to form a 
composite element. Then, the composite element undergoes a heat treatment 
to produce the superconductor. 
Another aspect of the present invention is directed to a superconducting 
wire produced by the method. 
In the superconducting wire according to the present invention, a 
superconducting layer is formed in the interface between the tubular 
coating layer and the material, containing at least one of both the A and 
the B, and hence excellent adhesion of the superconducting layer to the 
coating layer is obtained. Further, the superconducting layer has a 
uniform thickness. 
The tubular coating layer may reinforce the superconducting layer. The 
thickness of the superconducting layer may be controlled by adjusting the 
content of each of the A and B. 
The IIa group element may include Be, Sr, Mg, Ca, Ba and Ra. 
The IIIa group element may include Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, 
Tb, Dy, Ho, Er, Tm, Yb and Lu. 
The component A may include more than two IIIa group elements above named 
while the component B may include more than two IIa group elements above 
named. When two elements are used for either A or B, for example, 
La-Yb-Ba-Cu-O or Y-Ba-Sr-Cu-O system superconductor is produced.

FIGS. 1 to 4 illustrate an embodiment of the present invention which is 
applied to production of a superconducting wire of (La,Sr)CuO system. 
As the first step of a process for producing this type of superconducting 
wire, a tube 1 as shown in FIG. 1 is prepared from a Cu-Sr alloy. The tube 
1 preferably contains about 5 to about 30 weight % of Sr. With the lower 
limit of about 5%, the tube 1 is provided with an appropriate hardness for 
diameter reduction. Further, the upper limit is preferable for ease in 
diameter reduction. 
Then, a powder mixture is formed by mixing a powder of a Cu-La alloy and a 
powder of CuO so that the following conditions are met: 
Cu: (La, Sr)=1:1 (mol ratio) 
La: Sr=(1-x): x 
0.1.ltoreq.x.ltoreq.0.9 (mol ratio) 
where, the Sr indicates that in the Cu-Sr alloy of the tube. 
The thus obtained powder mixture 2 is charged in the tube 1 as shown in 
FIG. 2 to form a preform composite, which is then subjected to extrusion 
and/or drawing to obtain a composite wire 3, shown in FIG. 3, of a desired 
diameter. The tube 1 containing the powder mixture has excellent 
workability, so that it is possible to draw it and form the continuous 
wire 3 of a considerable length without any breaking. 
The wire 3 is then subjected to a heat treatment which includes 1 to 
100-hour heating at a temperture between 800.degree. and 1100.degree. C., 
so that La and Sr in the composite wire 3 are diffused around the CuO 
particles and are made to react therewith, whereby a superconducting wire 
4 of (La ,Sr) CuO system is obtained. 
The described process may be modified so that the tube 1 is prepared from a 
Cu-La alloy, while the powder mixture charged in the tube 1 contains a 
Cu-Sr alloy powder and CuO powder. 
It is also possible to use a powder of La oxide in place of the Cu-La alloy 
powder. In such a case, the La oxide powder and the Cu-O powder are mixed 
to form a powder mixture 2 so as to meet the following conditions: 
Sr:La=2:3 
(Sr:La):Cu=1:1 (atomic ratio or mole ratio) 
The thus obtained powder mixture is charged in the tube 1, and the 
superconducting wire 4 is obtained through same process as that explained 
before. 
It is also possible to use a La powder in place of the La oxide powder in 
this process. 
This process also can be modified so that powder of an Sr compound or Sr 
powder is charged in the tube 1 made of a Cu-La alloy and the same 
procedures are followed as those of the above-described embodiment in 
FIGS. 3 and 4, so as to produce a superconducting wire. 
In fabricating an Y-Ba-Cu-O superconducting composite wire according to the 
present invention, an ingot of Cu-Y alloy may be made by arc melting or 
high-frequency induction heating. Then, a tube as shown in FIG. 1 is 
prepared from the ingot by boring or a suitable plastic work. A powder of 
BaO is then charged into the tube as shown in FIG. 2. Subsequently, the 
preform composite undergoes extrusion and/or drawing for diameter 
reduction to form a composite wire as shown in FIG. 3. During this 
processing, the tube exhibits good workability, so that a long continuous 
wire can be obtained without any trouble such as breaking. 
The wire is then subjected to a heat treatment which includes 1 to 300-hour 
heating at 700.degree. to 1300.degree. C. so that Y, Ba, Cu and O are made 
to react to form a superconducting substance of Y-Ba-Cu-O system, whereby 
a superconducting wire having a superconductor in it is obtained as shown 
in FIG. 4. 
The modified superconducting wire is a long continuous wire having a high 
critical temperature of 90 K or so and exhibits excellent 
superconductivity. 
In the described embodiment and modification, the portion of the finished 
wire where the superconducting substance is not formed, i.e., the portion 
outside the superconductor 4a, is constituted by a copper alloy which is 
workable, so that the finished wire may be wound into a coil. 
FIG. 5 shows another modified process of the present invention. As the 
first step of this process, a bar-shaped Cu-Y ingot alloy, with uniform 
orientation of the Cu-Y intermetallic compound phases 21 is prepared 
through conventional uni-axial solidification method or like. The Cu-Y 
ingot is then subjected to machining such as surface grinding and boring, 
whereby a Cu-Y alloy tube 22 is obtained. The Cu-Y alloy tube 22 
preferably contains about 5 to about 30% by weight of Y for the same 
reasons as the Cu-Sr alloy tube 1, i.e., for diameter reduction. 
Thereafter, a powder 23 of an oxide of a Cu-Ba alloy is charged into the 
Cu-Y alloy tube 22, thus obtaining a bar-shaped member as shown in FIG. 5. 
This bar-shaped member is then subjected to diameter reduction such as 
extrusion and drawing, followed by substantially the same heat treatment 
as in the preceding modification, whereby a superconducting wire is 
obtained. Though in the modified form in FIG. 5, the Cu-Y alloy tube 22 is 
charged with the powder 23 of Cu-Ba alloy oxide, the Cu-Ba alloy oxide 
powder may be substituted by other suitable material such as a powder of a 
Cu-Ba alloy fluoride, powder of Cu-Ba alloy chloride, a mixture of these 
powders and mixture of one of these powders and the Cu-Ba alloy oxide 
powder. When the Cu-Ba alloy oxide powder is not used, the heat treatment 
is preferably conducted in an atmosphere containing oxygen. 
FIG. 6 shows a known apparatus for producing the above-mentioned Cu-Y alloy 
ingot by uni-axial solidification. 
The apparatus has a high-frequency induction coil 24, a cylindrical 
graphite liner 25, cylindrical crucible 26 made of CaO disposed in the 
graphite liner, and a lifting means (not shown) for moving the crucible 26 
up and down within the graphite liner 25. When the Cu-Y alloy ingot is 
produced by this apparatus, a melt 27 of a Cu-Y alloy is charged in the 
crucible 26 and the melt 27 is heated by the high-frequency induction 
heating coil 24. Subsequently, the crucible in the heated state is lowered 
at a predetermined speed. As a result, the melt 27 in the crucible 26 is 
progressively cooled from the lower end towards the upper end thereof, 
whereby a bar-shaped Cu-Y alloy ingot is formed. During the 
solidification, a plurality of acicular Cu-Y intermetallic compound phases 
21 are formed in a uniform orientation in the axial direction of the Cu-Y 
alloy ingot. 
Thus, the described process includes the steps of preparing a tube 22 of a 
Cu-Y alloy having a structure with a multiplicity of Cu-Y intermetallic 
compound phases 21 oriented uniformly in the axial direction of the tube 
22, charging the tube 22 with powder 23 of the Cu-Ba alloy oxide, 
subjecting the tube with the powder 23 therein to a mechanical processing 
to reduce its diameter, and effecting a heat treatment on the tube of the 
reduced diameter. In this process, the Ba element of the Cu-Ba alloy oxide 
powder 23 is diffused so as to react with the Cu-Y intermetallic compound 
phases 21 in the Cu-Y alloy tube 22, whereby a superconductor wire is 
obtained in which a multiplicity of very long and thin oxide 
superconductors having a uniform crystalline structure are formed along 
the Cu-Y intermetallic compound phases 21 continuously over the entire 
length of the superconductor wire. It is therefore possible to obtain a 
fairly long superconducting wire having excellent superconductivity. In 
addition, oxide superconductors which are very brittle are formed in 
acicular form in the axial direction of the superconductor wire, so that 
bending strength of the superconducting wire is improved appreciably. It 
is also to be noted that the matrix of Cu-Y alloy surrounding the oxide 
superconductors effectively serves as a reinforcement for reinforcing the 
superconductors, whereby a high mechanical strength of the superconductor 
wire is ensured. 
Although a superconductor wire of Y-Ba-Cu-O system has been described 
specifically, it is to be understood that similar superconducting wires 
can be produced by using, in place of Y of the above-mentioned system, one 
of the elements belonging to group IIIa of the periodic table such as La, 
Sc, Yb, Dy, Ho and Er. It is also possible to produce similar 
superconducting wires by using, in place of Ba, one of elements of the 
group IIa of the periodic table, e.g., Sr. 
In the method in FIG. 5, the power of the oxide of the powder of a compound 
of an alloy, including copper and an element of group IIa of the periodic 
table, is charged in the tube which is made of an alloy of copper and an 
element of the group IIIa of the periodic table. This, however, is not 
exclusive and the method may be modified so that a tube of an alloy of 
copper and an element of the group IIa of the periodic table is charged 
with a powder of an oxide of an alloy of copper and an element of group 
IIIa of the periodic table. 
FIGS. 7 to 11 illustrate another embodiment applied to the production of a 
superconducting wire of (La, Sr) CuO system. The production process 
employs a copper tube 31 with finely dispersed CuO particles, the copper 
tube 31 being prepared by a suitable known method. 
Then, an La rod 32 of a circular cross-section is inserted into the copper 
tube 31 to form a preform, which is then subjected to extrusion and/or 
drawing into a composite wire shown in FIG. 9. The extrusion and drawing 
can be easily carried out by virtue of good workability exhibited by both 
the copper tube 31 and the rod 32. 
Then, a coating layer 34 of Sr is formed over the external surface of the 
wire 33, thus forming a coated wire 35. The formation of the coating layer 
of Sr over the wire 33 may be conducted by any known method such as 
plating, cladding and vapor deposition. 
Subsequently, the coated wire 35 is subjected to a heat treatment which 
includes 1 to 100-hour heating at 800 .degree. to 1100.degree. C., so that 
the fine CuO particles dispersed in the copper tube 31, La in the tube 32 
and Sr outside the tube 31 are made to react with one another, thus 
forming a superconducting wire 36 of (La, Sr) CuO system. 
The amounts of CuO dispersed in the copper tube 31, La in the rod 32 and Sr 
in the coating layer 34 may be the same as those in the embodiment 
described in connection with FIGS. 1 to 4. 
The embodiment shown in FIGS. 7 to 11 may be modified so that a rod of Sr 
is inserted into the copper tube 31 which is coated with a layer of La. 
The CuO particles dispersed in the copper tube 31 may be substituted by 
other copper oxide such as Cu.sub.2 O, Cu.sub.2 O.sub.3 and Cu.sub.4 O. It 
is also possible to use a copper alloy tube, such as Cu-La, Cu-La-Sr and 
Cu-Sr alloy tube, in place of the copper tube 31. 
It will be clear to those skilled in the art that a plurality of 
superconducting wires of the invention are bundled and cladded with a 
metal sheath to form a fine multi-core superconducting wire 37 as 
illustrated in FIG. 12 although the superconducting wires of the preceding 
embodiments are of single-core type. 
FIGS. 13 to 15 show still another embodiment of the invention applied to 
the production of a superconducting wire of La-Sr-Cu-O system. 
The production process includes preparation of a double tube 43 composed of 
an outer tube 41 and an inner tube 42. The outer tube 41 is made of a Cu 
alloy containing La as an example of the elements of group IIIa of the 
periodic table, while the outer tube 42 is constituted by Cu containing 
fine CuO particles dispersed therein. 
Then, a rod 44 made of Sr as an example of alkaline earth metal is inserted 
into the double tube 43, whereby a composite structure 45 as shown in FIG. 
14 is obtained. 
Then, the composite structure 45 is subjected to a line-forming process 
consisting of extrusion and drawing which are known per se and the thus 
obtained line is subjected to a suitable mechanical process for reducing 
its diameter. The line-forming operation effected on the composite 
structure 45 is performed successfully so that a long wire is obtained 
without any trouble such as cutting, because the double tube 43 and the 
rod 44 constituting the composite structure are highly workable. The wire 
thus formed can be coiled without difficulty. 
The wire is then heated at a temperature between 800 .degree. and 
1100.degree. C. for 1 to 100 hours, so that La, Sr, Cu and O in the wire 
are made to react with one another, whereby a superconducting wire 46 of 
La-Sr-Cu-O system is obtained as shown in FIG. 15. 
In the embodiment shown in FIGS. 13 to 15, the double tube 43 is composed 
of the outer tube 41 made of a copper alloy containing La and an inner 
tube 42 made of Cu containing CuO particles dispersed therein. This, 
however, is not exclusive and the double tube 43 may be constituted by an 
outer tube 41 made of Cu containing CuO particles dispersed therein and an 
inner tube 42 made of a copper alloy containing La. In the described 
embodiment, the Cu alloy used as the material of the tube 41 contains La, 
while the rod 44 inserted into the double tube 43 is made of Sr which is 
an alkaline earth metal. This material composition, however, may be 
reversed such that the Cu alloy as the material of the tube 41 contains 
Sr, while the rod 44 is made of La which is an element of the group IIIa 
of the periodic table. It is also to be noted that the CuO particle may 
besubstituted by other copper oxides, while the tube 42 made of Cu is a 
copper alloy tube. 
In a different embodiment shown in FIGS. 16 to 21, a cylindrical ingot made 
of a copper alloy containing an element of the group IIIa of the periodic 
table is prepared by melting, through arc melting method or high-frequency 
induction heating. The element of the group IIIa contained in the copper 
alloy may be one, two or more selected from a group consisting of Sc, Y, 
La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. The ratio 
of content between the element or elements of the group IIIa and the 
copper constituting the copper alloy may be determined suitably in 
accordance with factors such as the composition ratio of the material of 
the oxide superconductor which is to be obtained as the final product. 
Subsequently, a through hole 51a is formed through the center of the 
cylindrical ingot along the length thereof by a suitable boring such as 
drilling, whereby a tube 51 as shown in FIG. 16 is obtained. The inside 
diameter of the through hole 51a is determined such that the through hole 
51a can receive a later-mentioned rod. 
On the other hand, another cylindrical ingot made of a copper alloy 
containing an alkaline earth metal element is formed by melting, through 
arc melting, induction heating or other suitable method. One, two or more 
of elements selected from a group consisting of Be, Sr, Mg, Ba and Ra is 
used as the alkaline earth metal mentioned above. The composition ratio 
between the alkaline earth metal element and copper constituting the 
above-mentioned copper alloy may be suitably determined in accordance with 
factors such as the composition ratio between copper and the IIIa group 
element constituting the copper alloy used as the material of the tube 51 
and the composition ratio of the material of the final product of the 
oxide superconductor. The ingot is then subjected to a contracting 
mechanical processing such as swaging so that a cylindrical rod of a 
reduced diameter is obtained. The outside diameter of the rod is selected 
to be somewhat smaller than the diameter of the through hole 51a in the 
tube 51. 
Subsequently, the rod 52 is inserted into the through hole 51a in the tube 
51 as shown in FIG. 17, thus forming a composite body 53. The composite 
body 53 is then subjected to a series of diameter-reducing processings 
such as extrusion, groove roll processing, drawing and so forth, whereby a 
blank wire 54 of the desired diameter is obtained as shown in FIG. 18. As 
a result of the mechanical processings, the tube 51 and the rod 52 reduce 
their diameters so as to form an outer layer 54a and a core 54b of the 
blank wire 54. 
Then, an oxide layer 54c is formed on the blank wire 54 as shown in FIG. 
19. The oxide layer 54c may be formed by, for example, by anode oxidation 
or formation. FIG. 21 illustrates an example of an apparatus for carrying 
out anode oxidation. The apparatus has a treating tank 55 filled with a 
treating bath constituted by aqueous solution of hydroxide an alkaline 
metal or alkaline earth metal such as NaOH, HOH, Ba(OH).sub.2, 
Ca(OH).sub.2 and so forth, and a plurality of rollers 56 which form a path 
along which the wire passes through the treating bath in the treating tank 
55. In operation, the blank wire 54 mentioned above is made to pass 
through the treating bath in the treating tank 55 along the path formed by 
the rollers 56 so that an oxide layer 54c is formed on the outer periphery 
of the elementary wire 54 through anode oxidation. The oxide layer 54c has 
a thickness which can easily be controlled by varying the treating 
conditions such as current density and treating time in the anode 
oxidation. 
The blank wire 54 having an oxide film thus formed thereon is subjected to 
heat treatment under conditions which are suitably determined in 
accordance with factors such as the type of the oxide superconductor. In 
general, however, the treating temperature ranges between 700.degree. and 
1100.degree. C., preferably 800.degree. and 950.degree. C., while the 
treating time ranges between 1 to 300 hours, preferably several hours and 
100 hours. Preferably, the heat treatment is effected in a gas atmosphere 
of a suitable gas such as an inert gas, e.g., argon gas or nitrogen gas, 
oxygen gas, chlorin gas, fluorine gas, and so forth or a mixture of one or 
more of these gases. 
As a result of the heat treatment, oxygen and other elements in the oxide 
layer 54c on the elementary wire 54 are diffused into the blank wire 54 so 
as to react with the elements which constitute the materials of the outer 
layer 54a and the core 54b of the blank wire 54. In consequence, a 
superconductor layer 57 of oxide system is formed on the outer surface of 
the blank wire 54 as shown in FIG. 20, whereby an oxide superconducting 
wire 59 is obtained. The superconducting layer 57 formed as above has a 
thickness corresponding to the thickness of the oxide layer 54c on the 
blank wire 54. Thus, the thickness of the superconducting wire 57 can be 
controlled by varying the thickness of the oxide layer 54c which in turn 
is effected by varying the condition of, for example, anode oxidation. 
It is possible to form a stabilizing layer such as of aluminum on the oxide 
layer 54c of the blank wire 54. In such a case, the process may be such 
that the oxide layer 54c is covered by an oxidation-resistant metallic 
coating layer made of a noble metal such as Ag, Au, Pt, Ir, Os, Pd, Rh, Ru 
or the like or their alloy, and the thus covered blank wire 54 is 
subjected to a heat treatment whereby the stabilizing layer is formed on 
the metallic coating layer. It is also possible to produce a 
superconducting wire by bundling a plurality of blank wires 54, forming 
oxidation-resistant metallic coating layer on the bundle, subjecting the 
bundle to a heat treatment and then forming the stabilizing layer thereon. 
The oxidation-resistant metallic layer provided between the oxide layer 
54c and the stabilizing layer effectively prevents the oxygen in the oxide 
layer 54c from being diffused into the stabilizing layer without fail. 
The embodiment described in connection with FIGS. 16 to 21 features that 
the tube 51 is made of a copper alloy containing an element of the group 
IIIa of the periodic table, while the rod 52 is made of a copper alloy 
containing an alkaline earth metal element. This, however, may be modified 
such that the tube 51 is made of a copper alloy containing an alkaline 
earth metal element, while the rod 52 is made of a copper alloy containing 
an element of the group IIIa of the periodic table. The use of anode 
oxidation as a method of forming the oxide layer 54c on the blank wire 54 
is only illustrative and the oxide layer may be formed by a different 
method such as formation which employs, for example, a chromium oxide. 
FIGS. 21 to 28 shows a different embodiment of the invention. In this case, 
as shown in FIG. 22, a cylindrical tube 61, referred to as a copper alloy 
member, is prepared from a Cu-Y alloy, as shown in FIG. 22. A through hole 
61a having a circular cross-section is formed in the center of the tube 61 
so as to extend along the axis of the tube 61. The Y content of the alloy 
used as the material of the tube 61 is suitably determined taking into 
consideration various factors such as the drawing workability in a 
subsequent diameter-reducing mechanical processing, composition of the 
oxide superconductor in the superconducting wire which is to be obtained 
as the final product, and so forth. The Y content, however, usually ranges 
between 5 and 20 wt %. 
Meanwhile, a base member 62 made of Ni and having a cylindrical form as 
shown in FIG. 23 is prepared. This base member 62 constitutes the core of 
the superconducting wire as the final product. The base member 62 has an 
outside diameter which is smaller than the inside diameter of the through 
hole 61a formed in the tube 61. The use of Ni as the material of the base 
member 62 is only illustrative and other suitable mono-component metal or 
alloy can be used equally well, as the material of the base member 62. 
Examples of the material suitably used are mono-component metals such as 
noble metals e.g., Ag, At, Au, as well as Ti and Ta, and alloys such as 
Cu-Ni alloy, Cu-Al alloy, Ni-Al alloy, Ti-V alloy, monel metal, stainless 
steel and so forth. 
Subsequently, the base member 62 is inserted into the through hole 61a in 
the tube 61 so as to constitute a composite body which is then subjected 
to a diametrreducing mechanical processings including extrusion, drawing 
and so forth, whereby a sound wire having a circular cross-section is 
formed as shown in FIG. 24. The round wire 63 has a central core 63a of a 
circular cross-section formed from the base member 62 and a Y-containing 
copper alloy portion 63b surrounding the core 63a and formed from the tube 
61. 
The round wire 63 thus formed is then shaped to have a rectangular 
cross-section as shown in FIG. 25, whereby a flat wire 64 is formed. Thus, 
the flat wire 64 has a core 64a having a flat rectangular cross-section 
formed from the circular-cross-sectioned core 63a of the round wire and a 
copper alloy portion 64b having a flat rectangular cross-section formed 
from the circular-cross-sectioned copper apply portion 63b of the round 
wire 63. 
Subsequently, an oxidation treatment is effected on the surface of the 
copper alloy portion 64b of the flat wire 64, so that an oxide film 64c, 
composed of Cu.sub.2 O, CuO, Cu.sub.2 O.sub.3 or the like, are formed as 
shown in FIG. 26. The oxidation treatment maybe effected by dipping the 
flat wire 64 in an aqueous solution of hydrogen peroxide or nitric acid. 
It is also possible to effect the oxidation through an anode oxidation 
process which makes use of an electrolytic bath of an aqueous solution of 
hydroxide os an alkaline metal or alkaline earth metal, e.g., NaOH, KOH or 
the like, together with ethanol, methanol, formic acid or the like as 
necessitated. The film thickness of the oxide film 64c is suitably 
determined in accordance with factors such as the thickness of the 
superconducting layer of the superconducting wire which is to be obtained 
as the final product. In general, however, the thickness of the oxide film 
preferably fall within the range between 1 and 2 .mu.m. Provided that the 
core 64a of the flat wire 64a is made of the aforementioned anti-oxidation 
metallic material, there is no risk for the surface of the core 64a to be 
oxidized during the oxidation, whereby superior mechanical strength of the 
core 64a is ensured. 
Subsequently, a material layer 64d containing a barium compound is formed 
on the surface of the oxide film 64c, as shown in FIG. 27. Examples of 
such a barium compound are halogens such as an oxide, carbonate, sulfide, 
chloride and fluoride, as well as their mixtures. The thickness of the 
material layer 64d is determined in accordance with factors such as the 
thickness of the superconducting wire to be obtained, though it usually 
falls within the range between 10 and 50 .mu.m. The material layer 64d may 
be formed by any one of suitable known methods such as spray painting of a 
liquid suspending powder particles of the above-mentioned compound, flame 
spraying and various thin-film forming methods including CVD and 
sputtering. 
Thus, the flat wire 64 now has a flat rectangular core 64a, a copper alloy 
portion 64b on the core 64a, an oxide film 64c overlying the copper alloy 
portion 64b and the material layer 64d on the oxide film 64c. The flat 
wire 64 is then subjected to a heat treatment which consists in heating 
the wire for a period of one hour to several tens of hours at a 
temperature ranging between 600.degree. and 1000.degree. C. The heat 
treatment is preferably conducted within an atmosphere of pure oxygen, a 
gaseous mixture of oxygen and an inert gas such as argon gas and nitrogen 
gas, and a gas which is formed by mixing, with the above-mentioned gaseous 
mixture, a halogen gas such as fluorine gas, chlorin gas, bromine gas and 
so forth. 
As a result of the heat treatment, various elements in the flat wire 64 
such as copper and yttrium elements in the copper alloy portion 64b, 
copper, yttrium and oxygen elements in the oxide film 64c and barium and 
oxygen elements in the material layer 64d are diffused in one another, 
whereby superconducting layers 65 containing a Y-Ba-Cu-O system 
superconductor are formed in the boundary between the copper alloy portion 
64b and the oxide film 64c and the boundary between the oxide film 64c and 
the material layer 64d, so as to extend over the entire length of the flat 
wire 64. 
According to the described process, a uniform and continuous 
superconducting layer 65 including a superconductor of Y-Ba-Cu-O system is 
formed in the region from the boundary between the copper alloy portion 
64b and the oxide film 64c and the boundary between the oxide film 64c and 
the material layer 64d, whereby an elongated superconducting wire 66 
having superior superconductivity is produced. 
This embodiment may be modified such that the copper alloy portion 64b is 
constituted by a copper alloy containing at least one of elements 
belonging to the group IIIa of the periodic table or a copper alloy 
containing at least one of elements belonging to the group IIIa of the 
periodic table and at least one of elements belonging to the group IIa of 
the periodic table, while the material layer 64d is constituted by a 
compound of the above-mentioned at least one of elements of the group IIa 
or a compound containing the above-mentioned at least one of the elements 
of the group IIIa and the above-mentioned at least one of the elements of 
the group IIa of the periodic table. 
In a further embodiment of the present invention, the flat wire as shown in 
FIG. 25 is used, as in the case of the embodiment shown in FIGS. 22 to 25. 
Powder of BaO or a mixture powder of BaO and Y.sub.2 O.sub.3, having a mean 
particle size on the order of several micron meters (.mu.m) is sprayed 
onto the surface of the flat wire 64 so as to form a coating layer 75 
having a thickness of several tens of micron meters (.mu.m), whereby a 
coated composite body 76 is formed. 
The thus obtained coated composite body 76 is subjected to a heat treatment 
which is conducted by heating the body 76 at 400.degree. to 600.degree. C. 
within an inert gas atmosphere such as of argon gas for a period which may 
be varied between one hour and several hundreds of hours. Consequently, 
constituent elements of the coating layer 75 and the constituent elements 
of the copper alloy layer 64b containing Y are mutually diffused, whereby 
an intermediate layer 77 composed of Y, Ba and Cu is formed in the 
boundary between the coating layer 75 and the copper alloy layer 64b, as 
shown in FIG. 31. This intermediate layer 77 is formed by mutual diffusion 
of the constituent elements of the copper alloy layer 64b and the 
constituent elements of the coating layer 75, so that the copper alloy 
layer 64 and the coating layer 75 are bonded and united strongly. 
Subsequently, the coated composite body is subjected to a heat treatment 
consisting in 1 to 100-hourheating at 600.degree. to 1100.degree. C. in an 
oxidizing atmosphere containing oxygen gas, and cooling down to the room 
temperature at a rate of 100.degree.C./hr, whereby a superconducting wire 
78 having a sectional structure as shown in FIG. 32 is obtained. 
Although in the preceding embodiments, La and Y are used as IIIa group 
elements and Sr and Ba are used as IIa group elements, superconductors 
according to the present invention may be fabricated at similar conditions 
using other IIIa and IIa group elements. 
Although the multi-strand superconducting wire is illustrated only in FIG. 
12, it may be fabricated in a similar manner using single superconducting 
wires of the other embodiments. In the embodiment of FIG. 12, bundled 
composite wires 35 may be inserted in a barrier pipe made of Ni, Ti or a 
like conventional material, and the barrier covered composite wire bundle 
may be inserted into a stabilizing copper tube to form a composite 
assembly, which may be then subjected to diameter reduction. Thereafter, 
the composite assembly undergoes a heat treatment for producing the 
superconductor in conditions similar to those in the preceding 
embodiments. When the stabilizer is not used, the barrier tube is omitted. 
The present invention may be applied to a Bi-Sr-Ca-Cu-O system 
superconductor, in which case in FIGS. 1 to 4, a mixture including powders 
of Bi, Ca and CuO may be used in place of the mixture of powders of Cu-La 
and CuO. 
In view of thermal stresses during the heat treatment for producing a 
superconductor, it is, according to the present invention, preferable to 
raise the temperature of the wire composite at a rate of smaller than 
about 100.degree.C./hour from room temperature to the calcination 
temperature previously mentioned although it may be raised at a rate 
larger than about 300.degree. C. On the other hand, the rate of 
temperature decrease from the calcination temperature to room temperature 
is preferably about 100.degree. C. to 200.degree. C. although it may be 
about 300.degree. C. or higher. 
When a calcined material is cooled during the heat treatment for producing 
the Y-Ba-Cu-O system superconductor, it may be, according to the present 
invention, maintained for about 5 to 48 hours in a temperature range about 
400.degree. C. to about 500.degree. C., in which the crystal of the oxide 
superconductor transforms from a cubic system to a rhombic system and it 
may be then cooled to room temperature, so that the crystal structure is 
transformed to a rhombic system, which provides both a high critical 
temperature and a high critical current density. 
EXAMPLE 1 
A melt of a Cu-Y alloy containing 10 wt % of yttrium was poured into a 
cylindrical crucible made of CaO and the crucible was moved at a speed of 
1 mm/min through a high-frequency induction coil having a length of 100 
mm. In this case, the high-frequency induction heating coil was line at 
its inner side with a cylindrical graphite liner so that the interior of 
the crucible moved in the graphite liner was maintained at about 
1200.degree. C. As a result, an ingot of a Cu-Y alloy having a 
multiplicity of acicular Cu-Y intermetallic compound phase of about 40 to 
50 .mu.m.phi. was prepared. The ingot was then subjected to a surface 
grinding so that its outside diameter was reduced to 20 mm.phi. and the 
thus ground ingot was subjected to a boring whereby a Cu-Y alloy tube 
having the outside diameter of 20 mm.phi. and an inside diameter of 20 
mm.phi. was obtained. Meanwhile, a melt of a Cu-Ba alloy containing 50 wt 
% of barium was sprayed within an oxygen atmosphere whereby a powder of 
Cu-Ba alloy oxide having particle sizes ranging between 20 and 30 .mu.m 
was prepared. The powder of Cu-Ba alloy oxide thus formed was charged in 
the above-mentioned Cu-Ba alloy tube, and the tube was subjected to a 
diameter-reducing processing including extrusion and drawing, whereby a 
multiplicity of wires having an outside diameter of 1.0 mm.phi. were 
obtained. Then, 91 pieces of the thus formed wires were bundled and the 
bundle was placed in a copper tube having an outside diameter of 13 
mm.phi. and an inside diameter of 12 mm.phi.. The tube was then subjected 
to extrusion and drawing so that a multi-core wire having an outside 
diameter of 1.0 mm.phi. was obtained. Subsequently, the multi-core wire 
was subjected to anode oxidation which was conducted by dipping the wire 
in a 20 wt. % aqueous solution of NaOH so that a surface oxidation layer 
having a thickness of 1 .mu.m was formed. The wire was then heated at 
900.degree. C. for 24 hours so as to be changed into a superconducting 
wire. 
The long continuous superconducting wire thus formed exhibited superior 
superconductivity: namely, a critical temperature of 95 k and a critical 
current of 10.sup.3 A/cm.sup.2 at 77 k under magnetic flux density of 0 
(zero). In addition, a higher mechanical strength than known 
superconducting wire was confirmed. 
EXAMPLE 2 
A bar of 20 mm diameter and 200 mm long was prepared by arc melting method 
from a Cu-Y alloy containing 5 atomic % of Y. A through hole of 10 mm 
diameter was formed in the core of the bar along the longitudinal axis 
thereof. The tube was charged with BaO powder having particle sizes of 
about 1 .mu.m to form a composite tube so that Y, Ba, Cu and O are 
contained at a ratio 1:2:3:7 in it. The composite tube was subjected to a 
drawing, whereby a wire of about 1 mm in diameter was obtained. The thus 
formed wire was then heated at 800.degree. C. for about 50 hours, whereby 
a superconducting wire of Y-Ba-Cu-O system was obtained. A measurement of 
the critical current of this superconducting wire proved that the electric 
resistance is reduced to zero at a temperature around 90 k. 
EXAMPLE 3 
A Cu-Y alloy cylindrical ingot of the same size as that in Example 2 was 
prepared by the same method as Example 2, and the cylindrical ingot was 
subjected to boring so that a tube having a bore of 10.5 mm diameter was 
formed. 
On the other hand, a Cu.dbd.Ba alloy was prepared by induction heating 
melting such that the Ba content measures 10 atomic %, and a cylindrical 
ingot having an outside diameter of 30 mm and a length of 300 mm was 
produced from this alloy. 
The ingot was then swaged to reduce its diameter down to 10 mm so as to 
become a rod. The rod was inserted into the above-mentioned tube so that a 
composite body was obtained. The thus obtained composite body was then 
processed through groove rolling and drawing, so that a blank wire having 
an outside diameter of 1.0 mm was obtained. Subsequently, an oxide layer 
having a thickness of about 10 .mu.m was formed by anode oxidation on the 
outer periphery of the blank wire. The anode oxidation was executed by 
using a 10% aqueous solution of NaOH. The electric current density and the 
treating time were about 2A/cm.sup.2 and about 10 minutes, respectively. 
The blank wire having the thus formed oxide layer was heat-treated at 
800.degree. C. for 50 hours within nitrogen gas atmosphere, so that the 
oxygen in the oxide layer on the surface of the blank wire was diffused 
into the blank wire, whereby the objective oxide superconducting wire was 
produced. 
Measurement of characteristics showed that this superconducting wire has 
excellent critical temperature value of 90 k, while the critical current 
density was about 10.sup.3 A/cm.sup.2 at the temperature of liquid 
nitrogen. It was also confirmed that this superconducting wire has a 
uniform superconducting characteristics along the longitudinal axis 
thereof. 
EXAMPLE 4 
A pure Ni rod having an outside diameter of 14 mm was inserted into a Cu-Y 
alloy (Y content being 10 wt %) having an inside diameter of 15 mm and an 
outside diameter of 25 mm, thus forming a composite body. The composite 
body was then drawn to reduce its diameter down to 1.0 mm. The thus 
thinned wire was then flattened so that a flat wire having a thickness of 
0.5 mm and a width of 1.5 mm was obtained. Subsequently, the flat wire was 
anode-oxidized in a 20% aqueous solution of NaOH so that a copper oxide 
film of 2 .mu.m was formed on the surface of the copper alloy portion. 
Then, BaO powder was deposited onto the surface of the oxide film by 
spraying so that a material layer of 10 .mu.m thick was formed on the 
surface of the oxide film. 
The flat wire was then subjected to a heat treatment which was conducted by 
placing the flat wire in an oxygen stream at 950.degree. C. for 24 hours 
and was then cooled slowly down to the room temperature at a rate of 
100.degree. C./hr, whereby a superconducting wire of the structure 
substantially the same as that shown in FIG. 28 was obtained. 
The critical temperature Tc of this superconducting wire was measured and 
an excellent value of Tc=95 k was confirmed. A section of this 
superconducting wire was examined to confirm that a superconducting layer 
of about 10 .mu.m thick, composed of Y, Ba, Cu and O diffused in one 
another, was formed in the superconducting wire. In addition, diffraction 
fringe of YBa.sub.2 Cu.sub.3 O.sub.9-x (orthorhomic system) was observed 
through X-ray diffraction analysis. 
EXAMPLE 5 
A composite body was prepared by inserting an Ni rod having a diameter of 
14 mm into a base member of a Cu-10 wt %Y alloy having outside and inside 
diameters of 25 mm and 15 mm. The composite body was subjected to 
extrusion and groove rolling so that a tape of 0.5 mm thick and 1.5 mm 
wide was obtained. Then, BaO powder of particles sizes not greater than 2 
.mu.m was deposited by spraying onto the tape so that a coating layer of 
20 .mu.m thick was obtained. The tape was then heated at 800.degree. C. 
for 50 hours within an oxygen atmosphere, followed by a slow cooling down 
to the room temperature at a rate of 100.degree. C./hr. 
The thus formed oxide superconducting wire exhibited a critical temperature 
of 95 k which is excellent. An observation of a section of this 
superconducting wire proved that a reaction layer (superconducting layer) 
of about 10 .mu.m was formed. It was also confirmed, through an X-ray 
diffraction analysis, that this reaction layer has orthorhomic system of 
YBa.sub.2 Cu.sub.3 O.sub.9-x.