FR2680604A1 - Method of producing a composite structure in the form of a metal sheet/superconductor with high critical temperature - Google Patents

Abstract

Method of producing a composite structure in the form of a sheet consisting of two juxtaposed layers, a metal-based layer and a layer based on a superconducting material with a high critical temperature, characterised in that: - in a first step, with the aid of a heating calender and with organic binders, powder of the said superconducting material or of its precursors is agglomerated, in such a way as to obtain a paste which forms a flexible plasticised strip, - in a second step, the said flexible plasticised strip is solidified by compression or co-rolling onto a strip based on a metal compatible with the said superconducting material, - in a third step, heat treatment is carried out.

Description

Process for producing a composite structure in the form of a metal sheet / superconductor at high critical temperature
The present invention relates to a method for producing a composite structure in the form of a sheet metal / superconductor at high critical temperature.

The metal is selected from metals or alloys stable in the presence of oxygen. The superconductor at high critical temperature is chosen from the phases of the type
Ln2 Ba4 CU6 + n 014 + n
(BP) 2 Sr2 Can CU (l + n) 06 + 2n with: Ln = lanthanide BP = Bi, Bi x Pbx, T1
n = O; i; 2.

 The metal and the superconductor are in the form of two juxtaposed layers defining a sheet.

 There are currently three methods under development to realize such structures.

The first method, which is the most widespread and the most advanced in terms of performance, is described in particular in the article by KEN-ICHI SAIO and the Proceedings of 2nd International Symposium on
Supraconductivity 14-17 Nov. 1989 TSUKUBA p.335 to 340 ".

 This method consists in vacuum introduction of the superconducting powder into a generally silver or cupro-aluminum tube, then after closure, to reduce the diameter by wire drawing and possibly convert the wire obtained into ribbon by rolling. These mechanical operations are interspersed with heat treatments intended to relax the metal and to restructure the superconducting ceramic. This method leads to good results but has the disadvantage of confining the superconducting material, whose performance is known to be very affected by the treatment atmosphere; it is thus difficult to control the oxygen content inside the tube and even more difficult to eliminate the very harmful carbonates of barium, calcium or strontium trapped in the tube.

 The second method is described in the articles by K. KOSHINO "Jap.Journal of Applied Physics Vol 28 No. 7, July 1989 pp. 1214-1216" and TH TIEFEL and S. SIN "Applied Physic Letters 58 (17) 29.04 .91 pp. 1917-19.

 It consists in depositing, by screen printing or spraying, an ink loaded with superconducting powder on a metal substrate, and then, after heat treatment, rolling or pressing this structure. This time, the process is of the "open" type, but the silkscreened deposits are limited to a few tenths of a millimeter thick and are very poor in ceramic charge. The removal of cooking is very high, resulting in cracks difficult to eliminate by rolling and pressing.

The third method is described in K. TOGANO's article "Proceedings of ICMC" 90 May 1990 Garmisch - Partenkirchen.
HC Freyhardt Verlag ".

 It consists in first making a strip by casting: a slip consisting of bismuth superconducting ceramic powder suspended in a solvent is spread with a knife on a support, then dried and peeled off. In a second time, this band is placed on a silver strip, heat treated until the beginning of fusion to cause adhesion, then the whole is rolled and pressed. This process has the advantage over serigraphy of increasing the thickness; on the other hand, the need to melt, even partially, impairs the final purity of the phase or bismuth at 11OK whose melting irreversibly leads to a mixture of phases at low critical temperatures (20K and 85K) and of inert CaCO 3. In addition, obtaining by casting a dense, uncracked and reproducibly thick strip is quite difficult, especially in the case of reactive superconducting phases with which many additions and solvents (water in particular ) are incompatible.

 The present invention aims to implement a method for producing a composite sheet economically, by using a common material, offering a great freedom of choice of parameters, including that of the thickness of the superconducting layer.

 The present invention relates to a method for producing a composite sheet-shaped structure consisting of two juxtaposed layers, a metal layer and a layer based on superconducting material at high critical temperature, characterized in that - in a first step is agglomerated, using a heating calender and with organic binders, the powder of said superconductive material or its precursors, so as to obtain a paste that forms a flexible plasticized strip, - in a second step on solidarise said flexible plasticized strip by compression or bonding on a strip based on a metal compatible with said superconductive material, - in a third step a heat treatment is carried out to remove said binders, possibly synthesize the superconducting material if one left said precursors and sintering the resulting two-layer sheet.

 The different binders will be specified later.

 Said metal-based strip can be dense and obtained by rolling solid metal, but preferably it is obtained as said flexible strip by agglomeration of powder of said metal by organic binders using a heating calender good then a porous metal band. The respective binder levels in the plasticized strip and the metal strip are then chosen so as to equalize the shrinkage of these two strips during said heat treatment and to avoid distortions due to the bimetallic effect.

To produce said plasticized flexible strip, a powder is first prepared by dry grinding - a powder of a superconductive material chosen from
Ln2 Ba4 Cu6 + n 014 + n with: Ln = lanthanide or: (BP) 2 Sr2 Can Cu1 + n 6 + 2n with: BP = Bi, Bi x Pbx, T1 and n = O; 1; 2.

or a stoichiometric mixture of precursor powders corresponding to the desired superconducting material and chosen from
Ln 2 O 3, BaCO 3, SrCO 3, CaCO 3, Bi 2 O 3, Bi 2 (CO 3) 3, PbO, Pb 3 O 4, PbCO 3.

 In order to facilitate the grinding operation of such a powder and to increase its affinity with respect to said organic binders, a surfactant agent is added chosen from fatty acid esters, such as glycerol trioleate, glycol or triethanolamine stearate, glycerol triplamitate.

 Furthermore, between the rolls heated between 800C and 1500C of said calender, said binders are kneaded and first a thermoplastic polymer, such as for example polyethylene, polypropylene, ethylene vinyl acetate, polystyrene.

 Preferably, a microcrystalline wax such as ozokerite or a fatty acid ester, such as those mentioned above, is added to this polymer.

 After obtaining the pasty fusion of the thermosplastic polymer and the wax, the previously ground powder is incorporated in small portions; if this incorporation is difficult, it can be helped by adding small amounts of plasticizer, for example a heavy ester, such as dibutyl or dioctyl phthalate.

The following table summarizes the compositions of the dough (in mass)
- powder itself 80 to 80%
surfactant agent 0.2% to 1%

<tb><SEP> polymer <SEP> thermosplastic <SEP> 5% <SEP> to <SEP> 15%
<tb> binders1 <SEP> - <SEP> wax <SEP> microcrystalline <SEP> 1% <SEP> to <SEP> 5%
<tb><SEP> | <SEP> - <SEP> agent <SEP> plasticizer <SEP> 0 <SEP> to <SEP> 5%.
<Tb>

<SEP> L
<Tb>
After complete incorporation, the dough is worked to perfect homogeneity, the rolls are brought closer so as to calibrate the band to the desired thickness; the heating of a cylinder is then cut off, which has the effect of grouping the dough on the hot cylinder, the rolls are spread apart and allowed to cool after having made a longitudinal groove in the dough. The latter is detached from itself at cooling; a simple heating in an oven at 1000C brings it back to the flat shape.

Such a plasticized strip is solid and flexible, very easy to calibrate and handle. Its realization requires only the use of a current and cheap equipment.

 During the second step of the process according to the invention, said flexible plasticized strip is secured to said metal strip by a compression operation or a roll-forming operation.

 If one operates by compression, using a frame whose thickness is chosen so that the volume it defines internally is 1% to 10% less than the volume of material placed therein. The flexible strip and the metal strip are placed in this frame and placed between two compression plates with the interposition of two sheets of aluminum foil.

The assembly is brought to a temperature ranging from 1500C and 2500C between the trays of a heating press, and then compressed between 50 and 200 bar when using a dense metal strip and between 10 and 50 bar when using a porous metal strip. Cool and unmold. Peelable aluminum foil prevents sticking on compression plates
If the joining of the two strips by bonding is carried out, the procedure is as follows.

Between the rolls of a calender machine heated between 100 ° C and 1500 ° C, the spacing of which is adjusted so as to obtain a creep of the material of between 5% and 10% and the speed of rotation of which is set for a feed rate of 10%. and 100 cm / minute, we introduce the two strips to be secured previously placed between two aluminum foils. The colaminated sheet is recovered at the outlet of the rolls after peeling the aluminum foil.

 In the process which has just been defined, a flexible plasticized superconductive strip has always been secured to a metal strip. Other variants may be envisaged with a flexible plasticized strip between two metal strips or a metal strip between two flexible plasticized strips. Any other structure comprising several juxtaposed previous structures may also be made according to the invention.

 Whatever the "raw" structure thus obtained, a heat treatment is carried out in said third stage of the process according to the invention.

 This step involves, in the first place, elimination of the binders by slow rise (300C / hour) of the temperature under neutral gas (N2 - Ar ...) up to 5000C or 6000C. The rest of the treatment is conventional and depends on the superconducting material to be treated.

 In the case of compounds with yttrium (Y2Ba2Cu3O7), it is a step of 5 to 10 hours if the material has already been synthesized; if it is preferably in the precursor state, the level is 12 to 30 hours. In both cases, the operation is conducted under oxygen (between 10 and 100%) and at temperatures of 8800C to 9500C respectively. This stage is followed by a descent at 600 hours to 5000C under the same atmosphere, then a descent at 200C / hour up to 2500C preferentially under pure oxygen, and finally a descent to 1200C / hour.

 In the case of compounds (BP) 2 Sr 2 Can CUl + n 06 + 2 n 'the step is from 10 to 20 hours for the material already synthesized; in the preferred case where precursors are employed, the plateau is 20 to 40 hours (when n = 1) or 50 to 200 hours (when n = 2). The operation is preferably carried out under oxygen (between 5 and 20%) at temperatures of 7800.degree. C. to 8700.degree. This level is followed by a descent between 60 and 2400C / hour up to 250C, preferably with oxygen.

 The heat treatment is preferably supplemented by a phase of longitudinal orientation of the crystallographic planes of the superconducting material. It consists in known manner, and as indicated in the references cited above, to apply mechanical stresses on the two-layer sheet. It is crushed by rolling or uniaxial pressing at pressures greater than 10,000 bar, followed by annealing for a few hours. This operation is repeated until optimum performance is achieved.

 Other features and advantages of the present invention will appear in the following description of embodiments given for illustrative but not limiting.

EXAMPLE 1
11.29 g of Y 2 O 3 were ground for 1 hour in a planetary mill, 39.47 g of BaCO 3 and 23.86 g of CuO in the presence of 0.7 g of a surfactant such as "Lubromin" from PROTEX.

 Between the rolls of a calender machine heated to 1000 ° C., a mixture of 8 g of low density polyethylene and 1 g of ozokerite is introduced into the pasty fusion, and then the ground powder is incorporated in small portions. To facilitate the incorporation of the last fractions, 0.5 g of dibutyl phthalate are added dropwise. The dough is kneaded for 30 minutes, the gap between the rolls is adjusted to 0.45 mm and a cylinder cooled, which has the effect of grouping the dough around the hot roll which is cooled in turn. The dough is longitudinally slit which, upon solidification, peels off in the form of a curved plastic strip which is flattened by heating it in an oven at 1000 ° C.

 Stacking - a compression plate, - aluminum foil, - a frame 100 x 100 mm, thickness 0.5 mm, containing a silver strip 0.1 mm thick acid-cleaned 50% nitric and said cut plasticized strip, a second aluminum foil and a second compression plate.

 The whole is brought to 2000C between the plates of a hydraulic press, then compressed to 200 Kilonewtons for one minute. The press is cooled to 800C under pressure and then opened. The silver / superconducting composite sheet according to the invention is demolded and has a tendency to bend by bimetallic effect.

Cut a sample that is placed in a furnace programmed as follows
a rise at 300C / hour up to 5000C under N2,
- a rise to 600 hours up to 9200C under 2 / N2 10%
- a bearing at 9200C of 15 hours under 2 / N2 10%
- a descent at 600 hours up to 5000C under 02 / N2 10%
- descent at 200C / hour up to 2500C under 02
- a descent at 1200C / hour up to 250C under 02.

 We recover the sample which tends to wind on itself; its thickness is increased from 0.4 mm to 0.1 mm by rolling, and annealed in a treatment identical to the previous one except the rise which is a ramp at 1200C / hour up to 8900C and the duration of the bearing which is brought back at 5 o'clock at 8900C.

 Such a sample reveals a critical current density at 77K of 40 A / mm 2.

EXAMPLE 2
It is identical to the preceding except for the silver band which, instead of being cut from the foil is a plasticized strip obtained by mixing in the heating calandreuse a mixture of silver powder and 10% of polyethylene ; after cooling and peeling, a strip constituted respectively by volume of 56.4% of polyethylene and 43.3% of silver is obtained.

 The compression is performed in the same equipment as that described in Example 1, but the applied pressure is reduced to 100 kilonewtons all conditions being equal. The composite sheet obtained has that of Example 1 the advantage of not being bent; moreover, the adhesion between the two layers is very much improved.

 This structure, treated under the same conditions as that of Example 1 does not roll on itself, which facilitates rolling operations for orienting the crystallites, resulting in an improvement of the transport capacity of supracurrant.

EXAMPLE 3
18.64 g of Bi 2 O 3 are ground for one hour in a planetary mill. 5.34 g of PbCO 3 are added; 14.76 g of SrCO3: 10.0 g of CaCO3 and 11.93 g of CuO in the presence of 0.6 g of "Lubromin".

 This powder is shaped plasticized strip by the method described in Example 1, then a composite sheet is made by justaposition to a silver band powder plasticized with 10% polyethylene according to the method described in Example 2 .

Cut a sample that is placed in a furnace programmed as follows
- a rise to 300C / hour up to 5000C under N2
- a rise to 600 hours up to 8100C under 02 / N2 10%
- a level of 150 hours at 81000 under 02 / N2 10%
- a descent at 1200C / hour at 250C under pure 02.

 The sample is recovered and its thickness is increased from 0.4 mm to 0.1 mm by rolling and is annealed for 24 hours at 8000 ° C. under 10% N 2 / N 2.

 This tape has a critical temperature of 107K and its resistance becomes zero at 92K.

EXAMPLE 4
It is identical to the previous one except for the compressed structure: the juxtaposition is carried out so as to obtain a band of superconducting precursors sandwiched between two plasticized silver powdered strips. The synthesis heat treatment is identical to that of Example 3, but the rolling operations for orienting the crystallites are greatly facilitated; the creep is more regular and the tendency or tearing very attenuated.

EXAMPLE 5
Two 0.5 mm thick plastic strips are prepared respectively in Y1Ba2Cu3O7 precursor powder and in silver powder; both are plasticized with 10% polyethylene and 1% azokerite. The temperature of the rolls is set at 1500 ° C, the feed rate is 10 cm / minute and the gauge is 0.95 mm; the two strips slid between two aluminum sheets 0.05 mm thick are introduced between the rolls; the glued assembly is recovered by the effect of heat and pressure, and the aluminum foils are peeled off. These operations can be carried out continuously which constitutes a clear economic advantage. The rest of the process is in accordance with Example 2.

 Naturally, the invention is not limited to the embodiments that have just been described. Without departing from the scope of the invention, any means may be replaced by equivalent means.

Claims (14)

1 / A process for producing a composite sheet-shaped structure consisting of two juxtaposed layers, a metal-based layer and a layer based on superconducting material with a high critical temperature, characterized in that - in a first step, agglomerates, using a heating calender and with organic binders, the powder of said superconductive material or its precursors, so as to obtain a paste which forms a flexible plasticized strip, - in a second step, said strip is secured flexible plasticized by compression or bonding on a strip based on a metal compatible with said superconductive material, - in a third step a heat treatment is carried out to remove said binders, optionally synthesize the superconducting material if one has left said precursors, and sinter the two-layer sheet obtained. 2 / A method of manufacture according to claim 1, characterized in that said metal-based strip compatible with said superconducting material is obtained as said flexible strip, by agglomeration of powder of said metal by organic binders with the aid of a heating radiator. 3 / A method according to claim 1, characterized in that said powder of said superconductive material has the formula Ln2 Ba4 CU6 + n 014 + n with: Ln = lanthanide or: (BP) 2 Sr2 Can Cu1 + n 6 + 2n with: BP = Bi, Bi1 x Pbx, T1 and n = O; 1; 2. 4 / A method according to claim 1, characterized in that said powder is a stoichiometric mixture of precursors chosen from Ln 2 O 3, BaCO 3, SrCO 3, CaCO 3, Bi 2 O 3, Bi 2 (CO 3) 3, PbO, Pub304, Pic03. 5 / A method according to one of claims 3 and 4, characterized in that, during the agglomerating operation using the heating calender, said paste has the following composition (by weight)  - powder itself 80 to 80%  surfactant agent 0.2% to 1%

<tb> <SEP> polymer <SEP> thermosplastic <SEP> 5% <SEP> to <SEP> 15% <tb> binders1 <SEP> - <SEP> wax <SEP> microcrystalline <SEP> 1% <SEP> to <SEP> 5% <tb> <SEP> | <SEP> - <SEP> agent <SEP> plasticizer <SEP> O <SEP> to <SEP> 5%. <Tb>  <SEP> L 6 / A method according to claim 5, characterized in that said surfactant agent is selected from fatty acid esters such as glycerol trioleate, glycol or triethanolamine stearate, glycerol triplamitate. 7 / A method according to claim 5, characterized in that said thermoplastic polymer is selected from polyethylene, polypropylene, ethylene vinyl acetate, polystyrene. 8 / A method according to claim 5, characterized in that said plasticizer is a heavy ester such as dibutyl phthalate or dioctyl. 9 / A method according to claim 1, characterized in that, in said second step, is compressed between the trays of a heating press heated to a temperature between lO0C and 250 C a frame containing said flexible strip and said strip-based of metal, so as to secure them. 10 / A method according to claim 9, characterized in that one compresses at a value between 50 and 200 bars if the metal band is dense. 11 / A method according to claim 9, characterized in that one compresses to a value between 10 and 50 bars if the metal strip is porous. 12 / A method according to claim 1, characterized in that, in said second step, are introduced the two strips to be bonded by roll bonding between the rolls of a calender machine whose spacing is adjusted so as to obtain a creep between 5 % and 10% and whose rotation speed allows a scrolling bands between 10 and 100 centimeters per minute. 13 / A method according to one of the preceding claims, characterized in that it implements a second metal-based strip compatible with said superconductive material so that said plasticized flexible strip is secured on both sides to a strip metallic. 14 / A method according to one of claims 1 to 12, characterized in that it implements a second superconducting flexible plasticized strip so that the metal strip is secured on both sides to a superconductive flexible plasticized strip. 15 / A method according to claim 1, characterized in that said heat treatment is completed by an orientation operation in which is carried out several times of rolling and annealing operations, so as to extend said sheet at the expense of its thickness and to orient the crystalline planes of said superconducting material.