Method of manufacturing a superconductive cable

A superconductive conductor or cable comprising a core, which comprises at least one string of a ceramic, superconductive material, and where the core is encapsulated by a metal cap. In order to manufacture a conductor or a cable with an encapsulation, and in which it is possible as well during the manufacturing process as under the operation to maintain a controlled atmosphere around the superconductive core, at least one layer of not sintered, ceramic powderous material is provided between the cap and the core, which material has a higher sintering temperature than the superconductive material in the core. The superconductive core may be sintered for formation of the superconductive ceramics by placing in the tubular metal cap a starting material in powderous form, and shaped as a core in the other ceramic powder material and subsequently forgeing the the metal cap with its content, preferably by swaging at an ambient temperature, which is below the sintering temperature of the core.

BACKGROUND OF THE INVENTION 
The present invention relates to a superconductive conductor or cable 
comprising a core, which comprises at least one string of a ceramic 
superconductive material, and where the core is encapsulated by a metallic 
cap. Furthermore the invention relates to a method of manufacturing such a 
conductor or cable. 
Ceramic compounds of the type, which are often designated by, the formula 
Y-Ba-Cu-O have metals in oxidized form and exhibit electric 
superconductive characteristics at a substantially higher temperature than 
that of conventional superconducting materials. Because of the relatively 
high marginal temperature of the superconductive characteristics, this 
type of compounds has become more interesting for use in a greater variety 
of industrial application. The ceramic compounds form a whole group of 
materials, in which copper and oxygen seem to be the only required 
materials, as they, in addition to or instead of the metals yttrium and 
barium, may also comprise e.g. scandium, various lanthanides, calcium or 
strontium. Superconductors comprising other materials, e.g. bismuth and/or 
thallium are also known, as such materials may completely or partly 
replace the abovementioned metals. Furthermore, it is known that the 
superconductive characteristics may be improved in some cases, if fluorine 
replaces part of the oxygen. 
The use of the superconductive materials is restricted by the fact that the 
manufacture of these conductors in sufficient lengths and with a 
sufficiently homogeneous structure for the industrial application of 
fabricating cables is difficult and costly. An essential problem in this 
connection is that in the manufacturing process a solid-reaction between 
suitable compounds, e.g. oxides, carbonates or oxalates must take place; 
that the reaction must take place under a controlled set of temperature, 
and that the composition of the surrounding atmosphere may be of 
considerable importance. In particular, a high partial pressure of oxygen 
may be necessary. Furthermore there is a considerable risk that the 
reaction product thus formed may be unstable and liable to split off e.g. 
oxygen. It may, therefore, be necessary to surround the superconductive 
core, also under later operation, by a controlled atmosphere. 
These problems are discussed in Fujikura Technical Review, No. 17, Feb. 
(Tokyo JP), H. Osani et al, pages 1-4, in which different materials for 
the metal cap have been tested, and in Advanced Ceramic Materials--Ceramic 
Superconductors, Volume 2, No. 38, Special Issue, Jul. 1988, (Westerville, 
Oh., US), R. W. McCallum et al pages 388-400, in which silver is 
identified as a preferred material for encapsulation because of its 
permeability to O.sub.2, and in which addition of silver oxides inside the 
cap has been suggested as a means for controlling the atmosphere inside 
the cap during the sintering process of the superconductive material. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a conductor or a cable 
with an encapsulation, and in which it is possible both during the 
manufacturing process and during the later application to maintain a 
controlled atmosphere around the superconductive core. 
This object is achieved by means of a conductor or a cable of the 
abovementioned kind, the cable being characterized by the presence of a 
ceramic powderous material between the cap and the ceramic core of the 
cable. 
Due to the fact that the ceramic, superconductive core is surrounded by an 
only partially sintered ceramic powderous material, a flow of gas and 
diffusion in the pore volume, which exists in such material, can take 
place. On account of the higher sintering temperature of the surrounding 
material, it will remain porous also during various manufacturing 
processes, to which the cable will be subjected in the production process, 
and it is therefore ensured that the surrounding material will retain its 
permeability to gas. 
According to the invention it is preferable that the surrounding layer has 
a high temperature stability, and preferred embodiments of the invention 
hot and cold resistant oxides can be mentioned, especially oxide ceramics 
preferably MgO, Al.sub.2 O.sub.3 or ZrO.sub.2. 
In order to be able to maintain a controlled atmosphere around the 
conductive core, it is preferable according to the invention that the 
cable comprises elements for introducing a maintaining or regenerating 
gas, e.g. O.sub.2 in the non-sintered material. 
The invention further relates to a method of manufacturing an electric 
superconductive conductor or cable, in which method one or more materials, 
which under specified circumstances can be made superconductive, are 
placed in the form of at least one continuous string in a metallic, 
tubular, closed cap, which thereafter is subjected to a forging process 
producing a deformation of the closed cap. The deformation can be caused 
by swaging, hammering or causing an explosion in the cap producing a 
tightly confined materials reduction of the cross section surrounding the. 
From Japanese Patent No. 6113663-A, dated Jun. 24, 1986, it is known in a 
continuous process to convey a superconductive bundle of thread, 
comprising an armoring thread, which is to be formed into cylindrical 
shape together with a metal band, which will be rolled around the bundle 
of thread in such a way that cavities are eliminated. Subsequent to seam 
welding of the metal band for the purpose of forming a tube, the cross 
section of the tube is reduced by at least 80% by swaging and finally it 
is rolled into square cross section. The superconductive threads are 
manufactured by use of heat treatment of a suitable compound. The metal 
band is made of Cu or Al and the reinforcing threads are stainless steel, 
Mo or W. 
The present invention also is directed to a method of manufacturing a 
superconductive cable involving the steps of placing a continuous string 
of superconductive ceramic material inside a tubular, closed metal cap. 
Thereafter, the closed metal cap is subjected to a forging process by 
means of swaging, hammering or an explosive process, all of which result 
in the reduction of the cross-section of the cap to thereby tightly 
surround and compress the enclosed materials. The superconductive ceramic 
material being of such a type that it achieves superconductive properties 
under specified heating conditions when it is surrounded by a powderous 
ceramic material the sintering temperature which is higher than the 
reaction temperature of the composition of the core itself which is 
comprised of superconductive ceramic material. 
Due to the fact that the sintering temperature of the surrounding material 
is higher than the reaction temperature of the solid substance of the 
superconductive material it is ensured that around the superconductive 
core there will be a permeable layer, through which it is possible to 
introduce the maintaining atmosphere around the core. 
Preferably, the composition forming the superconductive ceramic material 
core is introduced into the cap as a powder. During the forging process, 
the cross-section of the cap will be reduced and the powderous composition 
is sintered under pressure to form the superconductive ceramic material 
core. 
Surprisingly it has proved that the necessary solid substance reaction, 
which normally takes a protracted heating under high pressure and with a 
controlled composition of the surrounding atmosphere, may be attained 
through a forging of the cap, when the forging takes place with such an 
intensity that the powder is compressed simultaneously with the reduction 
of the diameter of the tube. In practice the forging may preferably be 
carried out through swaging, i.e. in a reduction machine, and it has 
proved that the desired reaction sets in quickly and that a sintering of 
the superconductor takes place in such a way that the desired 
characteristics are achieved even though several meters of tubular cap are 
advanced through the machine per minute of time. 
According to the invention it is preferable that the tube is made from a 
hard and ductile metal, e.g. a corrosion resistant steel, which provides a 
relatively high resistance to the forging. It is particularly important 
that the tube material does not show any tendency to absorb oxygen, which 
may be liberated from the superconductive ceramic material. 
Thus a method is provided, in which the previously so problem-filled 
sintering process may be carried out in a fast and efficient way, the 
hitherto existing limitations on the length of the sintered strings being 
eliminated. 
In this connection it should be mentioned that from U.S. Pat. No. 4,717,627 
it is known to manufacture a fine-grained, superconductive or magnetic 
material by means of the following procedure: On a first layer of solid 
material a second layer of powder and a third layer of solid material are 
placed, and the assembly is placed in a sturdy container. Then a 
supersonic wave is transmitted through the first, the second, and the 
third layer in the sequence mentioned, so that the second layer is heated 
to a temperature exceeding the melting point of the material by means of a 
shockwave pressure which is higher than 50 kBar. The melted, compressed 
layer will rapidly cool by heat transmission to the other two layers.

DETAILED DESCRIPTION 
As shown in the drawing, the superconductive conductor or cable according 
to the invention comprises an outer metallic cap 1, in which a core 2 of a 
superconductive ceramic material is placed. In the space between the core 
2 and the cap 1, a diffusive ceramic material 3 is placed, which has a 
higher sintering temperature than the superconductive core 2. The 
diffusive material makes it possible before, under and/or after the 
sintering of the superconductive material, to optimize the surrounding 
atmosphere around the core, providing it with properties which act to 
preserve or regenerate the core. In most cases the problem is to control 
the contents of O.sub.2, the superconductive core preferably consisting of 
a ceramic material in Y-Ba-Cu-O category, but which comprises a great 
number of superconductive ceramic materials, which become superconductive 
at temperatures about and somewhat over the boiling point of liquid N. 
These materials may contain elements other than the above mentioned: 
Yttrium, barium, copper and oxygen. 
The cable may be provided with means for maintaining a predetermined 
atmosphere in the material 3, e.g. in the form of welded stubs. The 
material 3 is a ceramic powder, which has a higher sintering temperature 
than the superconductive core 2. Preferably a temperature-resistant oxide 
of one of the ceramic oxides will be used, e.g. MgO, Al.sub.2 O.sub.3 or 
ZrO.sub.2. 
In manufacturing the cable care must be taken that the cap closely 
surrounds the core and the powder-like layer. This may be achieved 
following the method according to the invention. In a special aspect of 
the method the necessary solid matter reaction, which transforms a 
superconductive powder into a sintered superconductive ceramic string of 
material is achieved. 
In addition, the core of the cable may comprise elements for mechanical 
reinforcement of the core, for cooling thereof or for dividing it into a 
number of parallel conductors. 
A superconductive cable according to the invention and of a limited length 
can be manufactured in the following way: A tube of Inconel, which is a 
temperature-resistant and corrosion-resistant compound with a high content 
of Ni and corrosion-resistant compound with a high content of Ni and 
having an outer diameter of 14 mm and an inner diameter of 11 mm is filled 
with an intermediate layer of an isolating, ceramic powder Al.sub.2 
O.sub.3 adjacent to the metal wall and a core of a powder which 
constitutes the superconductive core in the following way: Within the 
Inconel tube a tube, made for example of glass is placed, said tube having 
an outer and an inner diameter of 8 mm and 6 mm respectively, and within 
this tube is placed a tube with an outer and an inner diameter of 4 mm and 
3 mm respectively. In the inner tube is placed a metal bar of a thickness 
of 2 mm. By means of funnels two different kinds of powders are dosed. By 
means of the first funnel Al.sub.2 O.sub.3 is added to the interspace 
between the Inconel tube and the glass tube, and by help of the second the 
powder having the composition YBa.sub.2 Cu.sub.3 O.sub.x and a powder 
grain dimension of 0-60 is fed, said powder forming the superconductive 
string, in the space between the inner tube and the metal bar. Firstly, 
approximately 5 cm Al.sub.2 O.sub.3 powder is filled into the outer 
opening while the outer glass tube is being used for stamping. Then the 
inner tube is drawn up 1-2 cm, and superconductive powder is added which 
powder then is stamped with the metal bar. Subsequently, the glass tube is 
again lifted and there is filled up and stamped until a height of 4-4.5 cm 
is achieved Again Al.sub.2 O.sub.3 powder is added, said powder being 
stamped etc., until the tube is filled with powder. The tube is then 
sealed with on-welded terminals and subsequently forged to the specified 
diameter. In this case the tube was processed in a swaging machine thereby 
achieving a reduction in diameter from 14 mm to 9 mm. At the same time a 
corresponding pressing of the ceramic powder was achieved. The reduction 
of the diameter in the swaging machine took place with an advancing speed 
of 1-2 m/minute. 
The superconductive cable manufactured in this way was tested and the 
following measurements carried out on the superconductive string according 
to the formula YBa.sub.2 Cu.sub.3 O.sub.x : 
______________________________________ 
Dimension of crystallite 
approx. 1300 .ANG. 
(measured by X-ray diffraction) 
Dimension of crystallite 
approx. 1300 .ANG. 
(measured by X-ray diffraction) 
Porosity approx. 1.5 Vol % 
(measured by Hg-porosimetry) 
Skeleton density approx. 5.55 g/ml 
(measured by Hg-porosimetry) 
Positive indication of Meisner-effect at 77K. 
______________________________________ 
After the heating treatment of the cable in 2 h at 930.degree. C. the 
superconductive string has been investigated for shrinkage, and it was 
found that the string lies solidly embedded in the surrounding materials. 
Before the heating treatment it was found that a regular sintering of the 
Al.sub.2 O.sub.3 layer had not taken place, but it is found as a compact 
powder. 
The powder, being used for making the superconductive string, had a normal 
composition for the forming of a superconductor of the type Y-Ba-Cu-0. 
However the same method will also be applicable in connection with other 
compositions of ceramic superconductors. Furthermore, it is possible to 
support the sintering of the superconductor in the forging process by 
means of heating to a temperature, which is below 900.degree. C. said 
temperature being a common temperature for achieving sintering by heating 
alone. At such temperatures below 900.degree. C. the surrounding ceramic 
insulation will maintain its permeability, whereby it will be possible 
also during the forging to regulate the atmosphere surrounding the core. 
In addition to swaging, which is used in the example, the forging may take 
place by hammering or by an explosion-deformation. Furthermore, the 
forging may take place in such a way, that the tube achieves another form 
than the normally used circular form. Moreover, it is possible in the core 
to embed reinforcing- or cooling elements and elements which divide the 
core into a number of parallel strings. 
The manufacture of larger lengths of superconductive cables may be 
performed by application of modifications of the technology which is known 
from manufacturing of continuously filled, tubular packings. When the 
various strings of powder have been placed in the tube forming the cap and 
stamped appropriately, the sintering of the core may be carried out by 
swaging in particular, which allows treatment of even very large lengths 
of material.