Method of making electrical contacts on bi-based oxide superconductors

The invention relates to solid bodies made of high-temperature superconducting material to which contact is made by solid, compact metallic conductors. To produce said solid bodies, high-temperature superconducting material is either melted completely or a melt is obtained from the oxides of bismuth, strontium, calcium and copper, and, optionally, of antimony and lead. The solid, compact metallic conductors made of silver, gold, a platinum metal or an alloy containing said metals are then partially encased in the melt and the melt is allowed to solidify. Finally, the solid body obtained is annealed together with the conductors in a first stage at temperatures of from 710.degree. to 810.degree. C. and in a second stage in an oxygen-containing atmosphere at temperatures of from 750.degree. to 880.degree. C.

FIELD OF THE INVENTION 
The present invention relates to solid bodies made of high-temperature 
superconducting material to which contact is made by metallic conductors, 
and to processes for producing them. 
BACKGROUND AND PRIOR ART 
From U.S. Pat. No. 5,047,391, it is known to produce moldings of 
high-temperature superconducting material by producing a homogeneous melt 
from the oxides of bismuth, strontium, calcium and copper in a molar ratio 
of the metals of 2:2:1:2 in the temperature range from 870.degree. to 
1100.degree. C., casting it in molds and then allowing it to solidify. The 
castings removed from the molds are first heat-treated at temperatures 
from 780.degree. to 850.degree. C. and finally they are treated in an 
oxygen atmosphere at temperatures from 600.degree. to 830.degree. C. 
In the process for making contact to high-temperature superconductors based 
on yttrium, bismuth and thallium according to U.S. Pat. No. 4,963,523, the 
surface of the high-temperature superconducting material is thoroughly 
cleaned, in a first step, by etching with air excluded and a metallic 
contact made of noble metals or their alloys is applied directly, in a 
second step, to the high-temperature superconducting material. 
It is also common to apply suspensions of noble metals in the form of a 
lacquer or paste to high-temperature superconducting material, in which 
case a fine metal film, to which solder contacts can be applied, remains 
behind on the surface of the high-temperature superconducting material 
after the solvent has evaporated. 
A disadvantage of the contacts obtained by the known processes is their low 
mechanical ruggedness. In addition, a heat treatment is usually necessary 
after applying the noble metal to obtain a good electrical contact, as a 
result of which the superconducting properties of the material may be 
adversely affected. 
SUMMARY OF THE INVENTION 
It is therefore the object of the present invention to provide solid bodies 
made of high-temperature superconducting material to which contact is made 
by metallic conductors, and also to specify processes for producing them 
in which the contact point produced has a very low electrical resistance, 
for example in the region of microohms, accompanied by good mechanical 
ruggedness. The solid bodies, according to the invention, made of 
high-temperature superconducting material which can be obtained from the 
oxides of bismuth, strontium, calcium and copper, and optionally, of 
antimony and lead are made contact to by solid, compact metallic 
conductors. 
The solid bodies according to the invention may optionally also be further 
developed in that 
a) they contain the oxides of bismuth, strontium, calcium and copper in a 
molar ratio of the metals of 2:2:1:2; 
b) they are made contact to by silver, gold, a platinum metal or an alloy 
containing said metals as solid, compact metallic conductors. 
A process for producing the solid bodies according to the invention is one 
which comprises completely melting the high-temperature superconducting 
material, partially encasing the solid, compact metallic conductors with 
the melt, allowing the melt to solidify and annealing the solid body 
obtained together with the conductors in a first stage at temperatures 
from 710.degree. to 810.degree. C. and in a second stage in an 
oxygen-containing atmosphere at temperatures from 750.degree. to 
880.degree. C. 
A further process for producing the solid bodies according to the invention 
is one which comprises completely melting a mixture of the oxides of 
bismuth, strontium, calcium and copper, and, optionally, of antimony and 
of lead, partially encasing the solid, compact metallic conductors with 
the melt, allowing the melt to solidify and annealing the solid body 
obtained together with the conductors in a first stage at temperatures 
from 710.degree. to 810.degree. C. and in a second stage in an 
oxygen-containing atmosphere at temperatures from 750.degree. to 
880.degree. C. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
The processes according to the invention may furthermore optionally also be 
refined in that 
c) the complete melting of the high-temperature superconducting material is 
achieved by heating to temperatures of up to 1100.degree. C.; 
d) the high-temperature superconducting material is heated to temperatures 
of up to 1000.degree. C., preferably up to 950.degree. C.; 
e) the complete melting of the oxides of bismuth, strontium, calcium and 
copper, and, optionally, of antimony and lead, is achieved by heating to 
temperatures of up to 1100.degree. C.; 
f) the oxides are heated to temperatures of up to 1000.degree. C., 
preferably up to 950.degree. C.; 
g) the annealing in the first stage is carried out in an inert gas 
atmosphere; 
h) the annealing in the first stage is carried out in air; 
i) the first stage of the annealing is carried out for 0.5 to 60 hours; 
j) the annealing in the second stage is carried out in a pure oxygen 
atmosphere; 
k) the annealing in the second stage is carried out in air; 
l) the annealing in the second stage is carried out in an atmosphere 
containing oxygen and inert gas; 
m) the second stage of the annealing is carried out for 6 to 100 hours. 
In the solid body, according to the invention, made of high-temperature 
superconducting material to which contact is made by solid, compact 
metallic conductors, the contact resistances of the contact points are 
particularly low-resistance if the metallic conductors are not oxidized 
either by atmospheric oxygen or by the molten high-temperature 
superconducting material. That is the case, for example, with silver, gold 
and the platinum metals. 
Furthermore, in the solid body according to the invention, it is 
advantageous for the purpose of achieving a low contact resistance if the 
metallic conductors are sunk into the high-temperature superconducting 
material only over a short length, while they have a large cross section 
owing to an appreciable contact area in the high-temperature 
superconducting material. 
In the process according to the invention, the annealing in the first stage 
can also be carried out in a mixture of inert gas and air. 
In the process according to the invention, the two annealings are carried 
out at temperatures which are markedly below the melting point of the 
noble metals used in each case. 
The partial melting occurring in the solid body in the process according to 
the invention during the annealing brings about the intimate contact 
between the high-temperature superconducting material and the metallic 
conductor. 
In the process according to the invention, the conversion of the solidified 
melt to the superconducting state and the formation of the good electrical 
contact is achieved by the two annealing stages, which may be carried out 
in one heat-treatment cycle.

EXAMPLE 1 
(Comparison Example) 
An approximately 50 mm long silver tube (outside diameter: 1.6 mm) was 
pushed in each case into a rectangular copper mold (dimensions 
15.times.15.times.60 mm.sup.3), open at the top, through small drilled 
holes in its end faces in such a way that about 30 mm of silver tube 
projected on each side. A homogeneous melt of binary oxides of bismuth, 
strontium, calcium and copper was then cast into the copper mold, the 
molar ratio of the metals to one another being 2:2:1:2. 
The silver tubes were mechanically firmly joined to the solidified melt. 
Furthermore, a polished cross section of the solidified melt with the 
silver tube contained in it revealed that the tube was completely 
smooth-edged and completely surrounded by the solidified melt, so that it 
was apparently not possible for interactions to have taken place between 
the melt and the silver tube. 
Since the solidified melt was not yet superconducting, it was removed with 
the silver tubes contained in it from the copper mold and heated up in a 
tubular furnace to 850.degree. C. at 300.degree./h and left in air at this 
temperature for 36 hours. After this heat treatment, the mechanical joint 
between the silver tubes and the solidified melt had been released and it 
was possible to pull the silver tubes out of the solidified melt. The 
solidified melt was, however, superconducting (critical temperature: 90K). 
EXAMPLE 2 
(Comparison Example) 
Example 1 was repeated, with the modification that the solidified melt with 
the silver tubes contained in it was heated to 820.degree. C. and left in 
air at this temperature for 40 hours. 
A polished cross section of the solidified melt with the silver tube 
contained in it revealed a slight contact between solidified melt and 
silver tube. The resistance between silver tube and solidified melt, which 
was now superconducting, was measured as 1.2 .OMEGA. at 77K. 
EXAMPLE 3 
(in accordance with the invention) 
Example 1 was repeated, with the modification that the solidified melt with 
the silver tubes contained in it was heat-treated in air first for 20 
hours at 750.degree. C. and then for 60 hours at 850.degree. C. After 
this, the silver tubes were mechanically firmly joined to the solidified 
melt, which was now superconducting. 
A polished cross section revealed that the outer surface of the silver 
tubes was heavily fissured and crystallites of the solidified melt had 
grown into the wall of the silver tubes. It was also possible to detect 
silver in the boundary zone of the solidified melt with the aid of an 
electron-beam microprobe. 
The resistance across the entire specimen, i.e. measured in each case at 
the points of emergence of the silver tubes from the solid body made of 
high-temperature superconducting material, was 60 .mu..OMEGA. at 77K. The 
surface area of the part, contained in the solid body, of each silver tube 
was 1 cm.sup.2 in each case. 
EXAMPLE 4 
(in accordance with the invention) 
Example 3 was repeated, with the modification that the two silver tubes 
were replaced in each case by five silver wires (diameter: 0.5 mm, length 
in solid body: 17 mm) (cf. figure). 
The resistance across the entire specimen, i.e. measured in each case at 
the points of emergence of the silver wires from the solid body, was 45 
.mu..OMEGA. at 77K. 
EXAMPLE 5 
(in accordance with the invention) 
Example 4 was repeated, with the modification that, instead of the silver 
wires, gold wires of the same dimensions were used. 
The resistance was 18 .mu..OMEGA. at 77K. 
EXAMPLE 6 
(in accordance with the invention) 
Example 4 was repeated, with the modification that, instead of the silver 
wires, platinum wires of the same dimensions were used. 
The resistance was 29 .mu..OMEGA. at 77K. 
EXAMPLE 7 
(in accordance with the invention) 
Example 3 was repeated, with the modification that the silver tubes were 
replaced by a silver sheet (dimensions: 17.times.7.3.times.1 mm.sup.3). 
The resistance was 45 .mu..OMEGA. at 77K.