Process for the preparation of a superconductor material of the mixed oxide type

The invention relates to a process for the preparation of a superconductor material of the mixed oxide type, such as oxides of the YBaCuO and Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8 type. This process consists of the deposition by electrolysis on a conductive substrate of successive layers of metallic elements entering in the constitution of the superconductor material, using a single element in each layer and carrying out, following the deposition of at least one of the layers, an intermediate oxidation-reaction heat treatment for fixing the element of said layer before depositing the following layer, optionally repeating one or more times at least part of the aforementioned operations, and subjecting all the layers to a final, oxidation heat treatment to form the mixed superconductor oxide. In this process, as a result of the order of the deposition of the layers and the performance of one or more intermediate, oxidation-reaction, heat treatments, it is possible to obtain a narrow superconductor transition and high critical current densities.

This is a national stage application of PCT/FR95/00262 filed Mar. 8, 1995. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a process for the preparation of a superconductor 
material of the mixed oxide type at a high critical temperature. 
2. Description of the Related Art 
Superconductivity is a phenomenon developing at temperatures below the 
critical temperature of a material, i.e. the temperature as from which the 
electrical resistance of said material becomes equal to 0 and a perfect 
diamagnetism is observed. This phenomenon is very interesting, because it 
permits power transmission at temperatures below the critical temperature 
without any energy loss. 
The first superconductor materials discovered had very low critical 
temperatures requiring a cooling by means of liquid helium. In 1986, 
Bednorz and Muller discovered a new superconductor of the oxide type 
having a critical temperature above 30K and no longer requiring a cooling 
by liquid helium. Other mixed superconductor oxides were then found, such 
as e.g. the oxide YBa.sub.2 Cu.sup.3 O.sub.7-x, whose critical temperature 
is approximately 90K, so that there is no need for liquid helium for 
cooling it. 
However, for most superconductor applications, it is necessary to prepare 
said materials in the form of either layers deposited on substrates in 
order to obtain thin films intended for microelectronics with a surface 
area of a few square centimeters and a thickness below 1 micrometer, or 
wires, ribbons or other conductors for high power electrical engineering 
and whose length will be several hundred or even several thousand meters 
and having a thickness of a few micrometers to a few millimeters. 
For the preparation of thin films, it is possible to use conventional film 
vapour deposition methods such as laser ablation, cathode, radiofrequency 
and other sputtering processes, vapour deposition of organometallic 
compounds, etc. With these methods, products are obtained having an 
excellent electrical and magnetic quality, but the use of these methods 
for producing ribbons or products of great length is difficult to envisage 
due to the constraints involved in obtaining said quality. Thus, the 
excellence of the performance characteristics is only achieved on 
monocrystalline substrates, which are difficult to produce in great 
lengths. Moreover, in all these methods, the deposition rate is generally 
too slow to be able to produce at an acceptable cost layers with a 
thickness of a few micrometers over hundreds of meters. Finally, research 
has shown that for thicknesses exceeding 1 micrometer, the layers prepared 
lose their texture and consequently transmit lower current densities. 
Moreover, for the preparation of very long products, consideration has been 
given to the use of powder metallurgy processes starting from the 
superconductor oxide in powder form or pulverulent precursor oxides which 
can be transformed into a superconductor by a heat treatment, filling a 
metal tube with these powders and then bringing it into the form of wires 
or ribbons by mechanical operations such as spinning, accompanied or not 
by intermediate annealing operations. The composites obtained are finally 
annealed in order to join together the superconductor grains and increase 
the power transmission capacity of the product obtained. However, this 
process suffers from the following disadvantages: 
1. requires a large number of operations, 
2. requires an optimization in order that the section of the superconductor 
oxide remains constant over the entire wire length, because a single 
necking is sufficient to reduce to zero the overall superconductivity and 
3. retain in the metal sheath gases prejudicial to the quality of the 
product. 
A process for the preparation of long products is known, which makes use of 
the deposition of metal coatings by electrolytic processes, such as is 
described in U.S. Pat. No. 5,162,295. 
According to this document, deposition takes place on a conductive 
substrate of layers of metals entering into the composition of the mixed 
superconductor oxide, the layers deposited then undergoing an oxidation 
treatment under conditions such that the mixed superconductor oxide is 
formed. However, this procedure suffers from the disadvantage of leading 
to superconductor films with a much too wide resistive transition and an 
extremely low critical current density. 
SUMMARY OF THE INVENTION 
The present invention relates to a process for the preparation of a 
superconductor material of the mixed oxide type which, although using said 
electrolytic deposition method, leads to superconductor materials having a 
narrow resistive transition and a high critical current density. 
According to the invention, the process for the preparation of a 
superconductor material of the mixed oxide type having metallic elements 
comprises the following stages: 
a) depositing by electrolysis on a conductive substrate successive layers 
of p metallic elements, each layer containing a single element and having 
a thickness such that it is not detached from the substrate or the 
previously deposited layer, and the order of the layers is such that the 
elements liable to react with the substrate are separated therefrom by at 
least one layer of another element, and performing, following the 
deposition of at least one of the layers, an intermediate, 
oxidation-reaction, heat treatment for fixing the element of said layer 
before depositing the following layer, 
b) optionally repeating one or more times at least certain of the 
electrolytic deposition and intermediate heat treatment operations of 
stage a) to obtain the desired superconductor material thickness and 
c) then subjecting all the layers to a final, oxidation, heat treatment at 
an adequate temperature for forming the mixed superconductor oxide. 
In this process, the performance of at least one intermediate, 
oxidation-reaction, heat treatment, as well as the order in which the 
metal layers are deposited, make it possible to avoid that, during the 
final heat treatment, parasitic reactions do not occur with the substrate. 
Thus, during the final oxidation of the metal layers, various liquids with 
a low melting point can appear and react with the substrate and this is 
avoided in the present invention due to the intermediate heat treatment or 
treatments performed on the elements liable to give rise to the formation 
of such liquids. Moreover, the order of the deposits makes it possible to 
move away from the substrate the most reactive elements with respect 
thereto, particularly at the temperature of the final heat treatment. 
Moreover, by limiting the thickness of the layers, no exfoliation 
phenomena occur which are prejudicial to the quality of the product. This 
phenomenon is generally observed with layers of alkaline earth metals 
beyond a thickness of 2 micrometers, when they have a tendency to separate 
from the substrate. The thickness of the alkaline earth metal layers is 
generally limited to less than 2 .mu.m. 
Preferably, the process also comprises a prior treatment stage for the 
substrate with a view to facilitating the attachment of the first layer 
deposited thereon and/or improve the homogeneity of said first layer. 
This treatment can consist of one or more cleaning, scouring, pickling and 
annealing operations using conventional methods improving the homogeneity 
and/or adhesion of the first deposit. This treatment is chosen as a 
function of the nature of the substrate used. Thus, for metal substrates, 
a simple and effective treatment consists of scouring followed by high 
temperature annealing, e.g. for 2 to 30 min, under air at a temperature of 
700.degree. to 800.degree. C. in the case of silver. 
The substrates which can be used can be constituted by various electricity 
conducting or non-conducting materials. In the case where they are made 
from non-conducting material, on the same is deposited a conductive layer, 
e.g. a silver layer using various processes, such as e.g. cathode 
sputtering, vapour deposition, dip coating, etc. In addition, the 
substrate material is chosen in such a way that, under the deposition and 
heat treatment conditions used, it does not give rise to parasitic 
reactions, even of a partial nature, with the elements of the layers 
deposited for forming the superconductor material. Examples of materials 
usable as the substrate are refractory, stainless steels. 
The use as the substrate of oxidizable materials such as nickel or certain 
nickel alloys, which may or may not be coated with silver, can be of 
interest when it is wished to electrically insulate the superconductor 
layer. 
Thus, with such materials, during the final heat treatment, the substrate 
oxidizes over a varying thickness, which insulates it electrically from 
the superconductor layer, which is beneficial for applications where a 
relatively high resistivity in the normal state is sought. 
The order in which are deposited the layers of the different elements is 
chosen with a view to minimizing the interactions with the substrate, but 
also in order to avoid incompatibilities between the successive 
electrolysis baths and obtain after the first intermediate, 
oxidation-reaction treatment a sufficiently conductive layer to permit the 
following deposits. Thus, the intermediate, oxidation-reaction, heat 
treatment is performed on layers of metallic elements liable to form on 
the substrate with the elements of the previously deposited layers oxides 
which are sufficiently conductive to permit the deposition of the 
following layer. 
In the production of each layer, the deposited quantities are not of an 
arbitrary nature, but instead a function of the final desired thickness 
and composition. 
The thickness of each layer is chosen as a function of the critical 
thickness of the layer which is most easily exfoliated. When said 
thickness is chosen, a calculation takes place of the thicknesses of the 
following layers in such a way that they correspond to stoichiometric 
proportions with respect to the quantity of atoms of the layer which is 
exfoliated most easily. If necessary, the deposition of the layers takes 
place by the number of occasions necessary for obtaining the desired 
superconductor material thickness. Normally the total thickness of the 
deposited layers is 2 to 20 .mu.m and the thickness of each layer 0.5 to 2 
.mu.m. 
It is also possible to forecast that certain elements will be deposited in 
the form of several layers, whereas other elements will only be deposited 
in the form of a single layer. In this case, the thickness of the single 
layer must correspond to stoichiometry with respect to the total quantity 
of the element deposited in the form of several layers. 
Thus, the process according to the invention can be very flexibly 
performed, because it is not necessary for the layer thickness of each 
metallic element to correspond to stoichiometry with respect to the other 
layers. 
According to the invention, an intermediate, oxidation-reaction, heat 
treatment is performed on the layers of elements least stable from the 
mechanical and/or chemical standpoint or the least electricity conducting. 
Thus, when the mixed oxide incorporates one or more alkaline earth metals, 
generally an intermediate, oxidation-reaction, heat treatment is performed 
after the deposition of each alkaline earth metal layer in order to fix 
the latter whilst obtaining a sufficiently conductive oxide mixture to 
permit the performance of other deposits. Therefore the process is of 
great interest, because by stabilizing in this way certain elements before 
the final treatment, it is possible to obviate parasitic reactions and the 
performance characteristics of the superconductor material are improved. 
Moreover, this process can be performed for the preparation of very long 
elements such as wires or ribbons, by carrying out in a continuous manner 
the deposition and heat treatment operations. 
Moreover, with this process, the final machining operations of the product 
obtained with a view to bringing it into its definitive form can be 
performed before or after the final heat treatment. 
This process can also be performed for producing superconductor elements 
incorporating braided or twisted fibres using several products obtained by 
this process in order to form braids, including them in an appropriate 
sheath and then transforming the assembly by thermomechanical operations 
such as hammering and rolling-drawing, into superconductor elements having 
bundles of superconductor fibres. 
This process can also be used for producing superconductor deposits 
directly on substrates having a complex shape, such as braids, twists, 
felts or foams. 
The process according to the invention can also be performed in such a way 
as to form on the substrate superconductor circuits having a desired 
profile, either by directly depositing the metals in accordance with the 
desired line, or by subsequently machining the substrate covered with the 
deposited layers using mechanical, ionic, photochemical or lithographic 
processes in order to eliminate the layers from certain areas of the 
substrates, prior to or following the final heat treatment stage c). 
With this process it is also possible to deposit at the end of the 
operation one or more substance layers for the protection, insulation, 
reinforcement, improvement of electrical and magnetic properties or 
thermalization of the superconductor materials. It is also possible to 
deposit a final metal layer serving as a substrate in order to recommence 
the sequence of operations leading to the superconductor material with a 
view to producing multistrand superconductors. 
Another interesting possibility consists of firstly depositing a silver 
layer in which are dispersed atoms of another more oxidizable metal, so as 
to form during the final heat treatment a composite sheath reinforced by a 
fine dispersion of very hard oxide. 
The process of the invention is easy to implement, because it essentially 
comprises deposition stages by electrolysis and heat treatments. 
The electrolytic deposits can be produced by conventional methods using as 
the electrolytic deposition bath a solution of a salt of the element to be 
deposited and in particular a chloride or a nitrate, in an appropriate 
solvent such as water, dimethyl sulphoxide, acetonitrile or mixtures of 
the two latter substances. Generally, for the deposition operation, the 
substrate serves as the cathode and is associated with two 
counterelectrodes, which can be soluble anodes of the same nature as the 
deposited metal or insoluble anodes of platinum or platinum alloy in 
exemplified manner. 
The intermediate heat treatments and the final heat treatment can be 
performed in air. The temperatures and durations of these treatments are 
chosen as a function of the elements and the superconductor material to be 
formed. The final treatment is generally performed in two stages with 
optionally an intermediate quenching or cooling stage. 
The process of the invention is applicable to numerous mixed superconductor 
oxides. As examples of such oxides reference can be made to Y--Ba--Cu--O, 
La--Ba--Cu--O and La--Sr--Cu--O, or other mixed oxides derived therefrom 
by the partial or total substitution of yttrium by an element from the 
group of rare earths, and barium by strontium, whereas the copper can only 
be replaced in a very small proportion by other atoms. 
Other examples of mixed superconductor oxides are Bi--Sr--Ca--Cu--O and 
Tl--Ba--Ca--Cu--O, Hg--Ba--Ca--Cu--O, Ag--Ba--Ca--Cu--O and their 
substituted derivatives (at present about 100 high temperature 
superconductors are known). 
Other features and advantages of the invention will become more apparent 
from reading the following description given in an illustrative and 
non-limitative nature with reference to the attached drawings, wherein 
show:

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Example 1 
First Method of Preparing a Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8 Ribbon 
The starting item is a substrate constituted by a rolled silver ribbon with 
a thickness of 50 .mu.m, a width of 20 mm and a length of 35 mm. The 
substrate firstly undergoes scouring with trichloroethane and is then 
annealed in air at 850.degree. C. for 15 min. Following this prior 
substrate treatment stage, the deposition of bismuth, strontium, calcium 
and copper layers takes place, working in the following way. 
Having observed by X-ray diffraction and thermogravimetric analysis that 
bismuth melted before completely oxidizing during a temperature rise in 
air and finding that liquid bismuth reacted strongly with the silver 
substrate, it was concluded that this metal must not be deposited directly 
on the substrate, but instead as far away as possible therefrom. It was 
also observed that strontium and calcium did not form stable layers and it 
was therefore necessary on the one hand not to exceed a thickness of 1.5 
to 2 .mu.m corresponding to 3.10.sup.8 atoms/cm.sup.2 in order to have an 
adhesive deposit and on the other hand subject them to an intermediate, 
oxidation-reaction, heat treatment in order to fix these elements. 
Therefore the following stages were successively performed: 
1) deposition of a copper layer, 
2) deposition of a strontium layer, 
3) intermediate, oxidation-reaction, heat treatment of the strontium and 
copper layers, 
4) deposition of a bismuth layer, 
5) deposition of a calcium layer, 
6) intermediate, oxidation-reaction, heat treatment of the calcium layer, 
7) repetition of stages 2, 3, 4, 5 and 6 and 
8) final heat treatment. 
In stage 1), the thickness of the copper layer corresponds to the total 
copper quantity required, i.e. n.epsilon. copper atoms. 
Conversely, in stages 2) and 4), the thickness of the layer only 
corresponds to .epsilon. atoms and in stage 3), the thickness of the layer 
corresponds to .epsilon./2 calcium atoms. It is therefore necessary to 
repeat (n-1) times the stages 2), 3), 4) and 5) and (n-2) times stage 6). 
A description will now be given of these different stages in the case where 
.epsilon.=2.5.multidot.10.sup.18 atoms/cm.sup.2. 
1) Deposition of the copper layer 
Uniform deposition takes place of approximately 5.multidot.10.sup.18 atoms 
of copper per cm.sup.2 on each face of the silver ribbon using as the 
electrolysis bath the mixture H.sub.2 O+H.sub.2 SO.sub.4 
(0.5M)+CuSO.sub.4, 5H.sub.2 O(0.25M to 0.5M), placing the silver substrate 
between two copper counterelectrodes and passing through a total current 
of 180 mA. Following the deposition operation, elimination takes place of 
the final solution traces and the coated ribbon is then dried in air. 
2) Deposition of the strontium layer 
Uniform deposition takes place on the preceding layer of 
2.5.multidot.10.sup.18 strontium atoms per cm.sup.2 using as the 
electrolysis bath a 0.1 mole/l strontium chloride solution in a mixture of 
dimethyl sulphoxide (DMSO) and acetonitrile (AN) (1/6 vol.), placing the 
copper-coated silver ribbon between two platinum counterelectrodes and 
passing through a total current of 180 mA. After deposition, the final 
solution traces are eliminated. 
3) Intermediate heat treatment 
This treatment consists of annealing the ribbon coated with the copper and 
strontium layers in air at 850.degree. C. and for 5 min, which makes it 
possible to stabilize the strontium deposit. 
4) Deposition of a bismuth layer 
Uniform deposition takes place of approximately 2.5.multidot.10.sup.18 
bismuth atoms per cm.sup.2 on the ribbon subjected to the preceding heat 
treatment, using an electrolysis bath constituted by a 1 mole/l bismuth 
nitrate solution in DMSO, placing the ribbon between two platinum 
counterelectrodes and passing through a total current of 180 mA. After 
deposition, the final traces of solution are eliminated. 
5) Deposition of the calcium layer 
Uniform deposition takes place of approximately 1.25.multidot.10.sup.18 
calcium atoms/cm.sup.2 on the preceding deposit, using an electrolysis 
bath constituted by a 1 mole/l calcium chloride solution in DMSO, placing 
the coated ribbon between two platinum counterelectrodes and passing 
through a total current of 180 mA. Following deposition, the final traces 
of solution are eliminated. 
6) Intermediate heat treatment 
In order to perform this treatment, the ribbon coated in the preceding 
stages undergoes annealing in air at 850.degree. C. and for 2 minutes. 
7) Repetition of stages 2 to 5 
8) Final heat treatment 
To carry out this heat treatment in air, the product constituted by the 
silver substrate coated with the different layers is introduced into a 
furnace heated to 800.degree. C. and the temperature is rapidly increased 
at a rate of 1000.degree. C./h to 850.degree. to 860.degree. C., which is 
maintained for 2 minutes, followed by quenching cooling at a rate of 
approximately 1000.degree. C./min to about 20.degree. C., followed by 
reheating at a rate of 60.degree. C./h to a temperature of 800.degree. C., 
maintaining the latter for 60 hours and finally carrying out rapid 
quenching in air to ambient temperature. 
The resistivity of the thus obtained superconductor material is then 
determined as a function of the temperature in K. 
The results obtained are given in FIG. 1, where Curve 1 represents the 
variations of the resistivity .rho. expressed in .rho.(T)/.rho.(100K), as 
a function of the temperature in K under a current density of 500 
A/cm.sup.2. 
In Curve 1, it can be seen that the superconductor transition is narrow and 
is at approximately 82K. Moreover, a measurement of the density of the 
critical current at 77K indicates that it is 10,000 to 15,000 A/cm.sup.2. 
The layer is also studied by X-ray diffraction and the diagram 
corresponding to Curve 2 of FIG. 1 is shown in FIG. 2. It can be seen that 
the various grains of the layers have virtually all their planes (a, b) 
parallel to the plane of the ribbon, which explains the very high critical 
current densities measured. 
Example 2 
Second Method for Preparing a Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8 Ribbon 
The operating procedure of Example 1 is adopted for preparing a silver 
ribbon coated with said superconductor material, but the final heat 
treatment is performed using a temperature of 840.degree. C. for 12 hours 
for the final range in place of 800.degree. C. for 60 hours. 
Curve 2 of FIG. 1 represents the resistivity of the material obtained as a 
function of the temperature under a current density of 500 A/cm.sup.2. In 
this case, the superconductor transition is observed as a slightly lower 
temperature of 80K. The critical current density at 77K is also 10,000 to 
15,000 A/cm.sup.2. 
It should be noted that similar results are obtained on thicker layers 
prepared according to Example 1, repeating a first time the deposition and 
intermediate heat treatment stages 2 to 6 and a second time stages 2 to 5 
using a larger copper deposit in the first stage. 
Example 3 
Preparation of the superconductor material YBa.sub.2 Cu.sub.3 O.sub.7 
In order to produce this material, the starting product is constituted by a 
rolled silver plate with a thickness of 50 .mu.m, a width of 20 mm and a 
length of 35 mm, which, as in Example 1, undergoes a prior trichloroethane 
scouring treatment, followed by annealing in air at 850.degree. C. and for 
15 minutes. This is followed by the following stages: 
1) deposition of a copper layer having approximately 7.5.multidot.10.sup.18 
copper atoms per cm.sup.2, working as in Example 1, 
2) deposition of an yttrium layer having approximately 
1.25.multidot.10.sup.18 yttrium atoms per cm.sup.2 and using as the 
electrolysis bath a 0.1 mole/l yttrium nitrate solution in DMSO and 
placing the silver plate between two platinum counterelectrodes with a 
total current of 180 mA, followed by the elimination of the residual 
electrolyte, 
3) annealing under air at 850.degree. C. and for 2 minutes, 
4) deposition of a barium layer having 2.5.multidot.10.sup.18 barium atoms 
per cm.sup.2 based on a 0.1 mole/l barium chloride solution in a mixture 
of DMSO and AN (1/6 vol.), using a total current of 180 mA and two 
platinum counterelectrodes, followed by an elimination of the final traces 
of solution, 
5) annealing in air at 850.degree. C. for 2 minutes, 
6) repetition of stages 2 to 4, and 
7) final heat treatment under purified oxygen involving successively a 
rapid temperature rise of 300.degree. C./h to 920.degree. C., a period of 
15 h at this temperature, cooling at a rate of 1.degree. C./min to 
450.degree. C., a period of 2 h at the latter temperature and finally 
cooling at 1.degree. C./h to ambient temperature. 
Determination then takes place, as in Example 1, of the resistivity 
variations of the YBa.sub.2 Cu.sub.3 O.sub.7 layer as a function of the 
temperature and under a current density of 500 A/cm.sup.2. The results 
obtained are given in FIG. 3, which illustrates the resistivity variation 
.rho.(T)/.rho.(110) as a function of the temperature in K. FIG. 3 shows 
that the superconductor transition is narrow and corresponds to a critical 
temperature under weak current of 86 to 90K. The critical current density 
at 77K is between 2,000 and 5,000 A/cm.sup.2. 
FIG. 4 shows the X-ray diffraction diagram of the layer obtained. It can be 
seen that the various grains of the layer have virtually all their planes 
(a, b) parallel to the ribbon. 
Conversely, the microstructure of this layer differs significantly from 
that of Example 1. Thus, the Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8 layers 
are constituted by several thicknesses of extremely flat grains, whereas 
the YBa.sub.2 Cu.sub.3 O.sub.7 layer has less of these. Moreover, in the 
Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8 layer, the stack of grains forms 
terraces with relatively sharp angles covering the entire surface of the 
substrate, whereas in YBaCuO, the grains are more rounded and are so 
strongly engraved at the joints that the silver of the substrate appears 
at the triple junctions. 
However, the use of substrates with a higher melting point than that of 
silver would make it possible to perform the final heat treatment at a 
higher temperature in order to melt the YBa.sub.2 Cu.sub.3 O.sub.7 and 
obtain by slow solidification much higher critical currents. In the same 
way, the use of a substrate having a linear expansion coefficient closer 
to that of YBa.sub.2 Cu.sub.3 O.sub.7 than silver would eliminate the 
cracking and increase the critical current density. 
On comparing the results obtained in these examples with those obtained in 
U.S. Pat. No. 5,162,295, it can be seen that the resistive transitions are 
narrower with the process according to the invention (17 to 90K in the 
case of the US patent and 86 to 90K in the invention) and that the 
critical current densities are much higher in the invention. The YBaCuO 
prepared according to the US patent is not superconductive at 77K, whereas 
that prepared according to the invention has, at the same temperature, a 
critical current density of approximately 5000 A/cm.sup.2. 
Thus, the process according to the invention is very interesting, because 
it significantly improves the performance characteristics of the 
superconductor material compared with the process of U.S. Pat. No. 
5,162,295. Moreover, it can be performed for obtaining superconductor 
elements of various forms and in particular composite elements, electronic 
circuits, ribbons, wires, filaments, etc. The process can be performed so 
as to have several complimentary deposition stages of one or more layers 
intended for the protection, insulation, reinforcement or thermalization 
of the superconductor material.