EP1625598B1 - Method for the production of an electrically insulating and mechanically structuring sleeve on an electric conductor - Google Patents

Abstract

The fabrication of an electrically insulating sleeve mechanically structured on an electric conductor (2) comprises: (a) the formation of a ceramic precursor (10) in the form of a fluid solution; (b) the formation of a coating on the conductor with this ceramic precursor; and (c) the heat treatment of the coating to form the ceramic from this ceramic precursor.

Description

TECHNICAL AREA

The present invention relates to a method of manufacturing an electrically insulating and mechanically structuring sheath on an electrical conductor.

The invention makes it possible to obtain an electrically isolated conductor that can be used over a wide range of temperatures and more particularly at very low temperatures, less than or equal to 4.2K, corresponding to the operating range of the superconducting magnets used for generate strong magnetic fields.

The invention thus applies in particular to the manufacture of such superconducting magnets.

It also applies to the manufacture of polar parts of electric motors.

STATE OF THE PRIOR ART

Superconductive electromagnets are already known, made from Nb ₃ Sn type alloys. Such alloys are capable of producing intense magnetic fields of up to 24 Tesla, which gives them a definite advantage over the NbTi-type alloys usually employed in such electromagnets.

However, the characteristics of Nb ₃ Sn make it difficult to implement because, unlike NbTi which is a very ductile and easily extrudable alloy, it is difficult to manufacture multi-filament Nb ₃ Sn compounds.

Indeed, Nb ₃ Sn is a polycrystalline intermetallic material which, to be formed, must undergo a long heat treatment of up to 3 weeks at temperatures of 600 ° C to 720 ° C under an inert atmosphere. Once treated, it becomes brittle and its superconducting properties are very sensitive to any mechanical deformation.

Thus, when it is desired to manufacture an electromagnet from the alloy Nb ₃ Sn, it is necessary to shape the winding of the electromagnet with a cable formed using a " precursor "of this alloy and to subsequently undergo a treatment, namely a thermal cycle, allowing the formation of Nb ₃ Sn.

This treatment is also called "reaction" in the following description and the cable formed using a precursor Nb ₃ Sn is called "unreacted cable" (in English "non-reacted cable").

The implementation of the electrical insulation of the cable is particularly difficult because, for this insulation, it is difficult to use a conventional material of organic type. Indeed, such a material is not resistant to a heat treatment during which the temperature exceeds 600 ° C.

• WO 03/010781A invention de Jean-Michel Rey, Sandrine Marchant, Arnaud Devred et Eric Prouzet.

We will refer to the following document:

• WO 03 / 010781A invention of Jean-Michel Rey, Sandrine Marchant, Arnaud Devred and Eric Prouzet.

This document discloses a method of manufacturing an electrically insulating and mechanically structuring sheath on an electrical conductor and proposes the use of a gelled solution, containing an organic binder, for the deposition of a ceramic precursor or directly on the conductor to isolate either on a ribbon serving to surround this conductor.

However, the use of a gel requires the use of an acid to generate this gel. Moreover, the presence of an organic binder is not desirable because it can lead to the creation of carbon residues which are detrimental to the insulating properties of the ceramic. This undesirable effect therefore requires a phase of elimination of the organic binder.

• US 6 387 852 B, E. Celik, Y. Hascicek et I. Mutlu.

We will also refer to the following document:

• US 6,387,852 B, E. Celik, Y. Hascicek and I. Mutlu.

This document describes a method for coating superconductors with an electrical insulator. However, this process also uses a sol-gel solution requiring oxides and organic solvents, i.e. isopropanol and acetyl-acetone, to form the ceramic precursor.

STATEMENT OF THE INVENTION

The present invention aims to overcome the above disadvantages.

In the invention, no organic binder is used and the suspension for forming the ceramic precursor is not a gel but a fluid solution without any organic element.

The method which is the subject of the invention leads to a simplification of the compositions used for its implementation and to a clear separation of the phases of elaboration of the isolated conductor, as will be seen later.

• formation d'un précurseur de céramique sous forme d'une solution fluide, ce précurseur de céramique étant un liquide constitué par une solution comprenant de l'eau, de la fritte de verre et une argile en suspension dans l'eau, sans aucun élément organique,
• formation d'un revêtement du conducteur avec ce précurseur de céramique, et
• traitement thermique de ce revêtement, ce traitement thermique étant apte à former la céramique à partir du précurseur de céramique.

Specifically, the subject of the present invention is a method for manufacturing an electrically insulating and mechanically structuring sheath on an electrical conductor, in particular a non-superconducting metal conductor, a superconductive metal conductor or a superconducting precursor conductor, this method being characterized in that it comprises the steps of:

• forming a ceramic precursor in the form of a fluid solution, said ceramic precursor being a liquid consisting of a solution comprising water, glass frit and a clay suspended in water, without any element organic,
• forming a coating of the conductor with this ceramic precursor, and
• heat treatment of this coating, this heat treatment being able to form the ceramic from the ceramic precursor.

Preferably, the clay is selected from the group of smectites and in this group, montmorillonite is preferably selected.

According to a preferred embodiment of the invention, the solution comprises, in weight percent, 35% to 50% water, 8% to 15% clay and 35% to 55% glass frit.

According to a first particular embodiment of the method which is the subject of the invention, the conductor is a precursor of a superconductor, in particular Nb ₃ Sn, and an overall heat treatment of this conductor provided with the coating is carried out. global heat treatment being able to form the superconductor and the ceramic.

According to a second particular embodiment, the conductor is either a non-superconducting metal or a superconducting metal and a heat treatment of this conductor provided with the coating is carried out, this heat treatment being able to form the ceramic.

According to a particular embodiment of the invention, the step of forming the coating comprises a step of depositing the ceramic precursor on a fiber ribbon and then a step of arranging the ribbon provided with the ceramic precursor around the conductor.

In this case, there is a coating of the ribbon by the ceramic precursor and the fibers may be made of a material selected from type E glass, type C glass, type R glass, type S2 glass pure silica, an alumina and an aluminosilicate.

Preferably, the fiber ribbon is previously desensitized, for example thermally or chemically.

According to a particular embodiment of the method which is the subject of the invention, the conductor provided with the coating is shaped before the heat treatment step capable of forming the ceramic.

To shape the conductor, it can for example wind this conductor (provided with the coating), before the heat treatment step capable of forming the ceramic.

BRIEF DESCRIPTION OF THE DRAWINGS

• la figure 1 illustre schématiquement des étapes d'un mode de mise en oeuvre particulier du procédé objet de l'invention,
• la figure 2 illustre schématiquement une application particulière de l'invention, et
• les figures 3 et 4 montrent des courbes d'écoulement de suspensions céramiques ayant des compositions différentes.

The present invention will be better understood on reading the description of exemplary embodiments given below, purely by way of indication and in no way limiting, with reference to the appended drawings in which:

• FIG. 1 schematically illustrates steps of a particular mode of implementation of the method which is the subject of the invention,
• FIG. 2 schematically illustrates a particular application of the invention, and
• Figures 3 and 4 show flow curves of ceramic suspensions having different compositions.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

The electrical insulation technique proposed in the present invention makes it possible to deposit a ceramic cladding on an unreacted conductive cable (made of a Nb ₃ Sn precursor), before forming a superconducting magnet coil.

The ceramic cladding will react simultaneously during the thermal cycle necessary for the formation of Nb ₃ Sn superconductor and will thus contribute to the electrical insulation and mechanical cohesion of the coil (structuring function).

In order to facilitate the industrial exploitation of the insulation process, the phases of preparation of the ceramic precursor, preparation of the ceramic sheath (for example by coating a ribbon of glass fibers) and sheathing of the conductive cable (covering) are distinct.

• assurer l'isolation électrique du câble conducteur,
• garantir une cohésion mécanique à la bobine résultant de la mise en forme du conducteur isolé,
• maintenir une bonne résistance mécanique dans une plage de températures allant de la température ambiante (environ 300K) jusqu'à 1,6K, et
• présenter si possible une certaine porosité afin de permettre la diffusion d'hélium jusqu'à la surface du conducteur pour les applications relatives aux aimants supraconducteurs.

The ceramic cladding of the conductor must have certain properties to ensure the proper functioning of the superconducting cable that will eventually be formed. This sheathing must:

• ensure the electrical insulation of the conductor cable,
• guarantee a mechanical cohesion to the coil resulting from the shaping of the insulated conductor,
• maintain good mechanical strength in a temperature range from room temperature (about 300K) up to 1.6K, and
• if possible, provide a certain porosity in order to allow the diffusion of helium up to the surface of the conductor for applications relating to superconducting magnets.

• préparation d'une suspension formant un précurseur céramique,
• fabrication d'un gainage céramique par enduction d'un ruban de fibres de verre au moyen de cette suspension,
• guipage, au moyen de ce ruban, d'un câble conducteur constitué d'un précurseur de Nb₃Sn non-réagi,
• formation d'une bobine à partir du câble conducteur ainsi guipé, et
• accomplissement d'un cycle thermique nécessaire à la réaction du précurseur de Nb₃Sn. Ce cycle thermique réalise simultanément la transformation du précurseur Nb₃Sn en supraconducteur et du revêtement en précurseur céramique en céramique.

In one example of the invention, the elaboration of an electrically insulated Nb ₃ Sn superconducting cable is carried out in several distinct phases, namely:

• preparation of a suspension forming a ceramic precursor,
• manufacturing a ceramic cladding by coating a fiberglass ribbon with this suspension,
• covering, by means of this ribbon, a conductive cable consisting of an unreacted Nb ₃ Sn precursor,
• forming a coil from the conductive cable thus covered, and
• completion of a thermal cycle necessary for the reaction of the Nb ₃ Sn precursor. This thermal cycle simultaneously performs the transformation of the Nb ₃ Sn precursor into a superconductor and the ceramic ceramic precursor coating.

Thus, a superconducting coil of Nb ₃ Sn electrically insulated and having a mechanical cohesion is obtained.

In the following, the preparation of a ceramic precursor is explained.

The solution used in the invention for the formation of this precursor has no organic component, especially binder type, to prevent the formation of carbon residues that are known to be harmful to good electrical insulation.

This solution is preferably a ternary mixture of a montmorillonite-type clay, glass frit and water which form a ceramic suspension.

In one example, the montmorillonite used is produced by Arvel SA under the trade name Expans.

This clay makes it possible to give the necessary plasticity to the impregnated tape that will be used during the wrapping of the conductive cable (made of a precursor of the Nb ₃ Sn alloy). In addition, it allows bending radii of about 2mm for the cladding tape.

In comparison with other clays, its high plasticizing power makes it possible to reduce the quantity used and to proportionally increase the glass frit.

The glass frit used is manufactured by Johnson & Mattey, under the reference 2495F. Its melting point is 538 ° C.

The glass frit is a fuse element that contributes to the cohesion of the ceramic insulation after the heat treatment.

The water makes it possible to adjust the viscosity of the suspension.

At the end of the present description, the rheological behavior of two particular compositions of the ceramic suspension has been considered. As indicated, the experimental conditions are such that we are in the regime described by the start of the behavior curves.

The clay and the glass frit are heated at 100 ° C for 12 hours in an oven to remove any traces of moisture. Then two powders of clay and glass frit are ground separately until a particle size of less than 20 .mu.m is obtained. The glass frit is then mixed with water with a magnetic stirrer.

The solution resulting from this mixture is then subjected to the effects of a Bioblock Scientific brand ultrasound gun, Vibracell model 72412, used at a power of 300 watts. This treatment is intended to break the possible aggregates of particles.

Then the solution is left stirring for 4 hours to allow the stabilization of the pH value. This expectation of stabilization makes it possible to ensure the reproducibility of the experimental conditions in the preparation of the ceramic precursor.

The clay is then incorporated by successive additions, which facilitates the mixing of the whole, then the suspension obtained is again treated with the aid of the ultrasonic gun to obtain a homogeneous mixture.

It is found experimentally that a gelation of the suspension is obtained.

This suspension is then stirred. To do this, in the example described, it is placed on a roller stirrer for 12 hours, in a polyethylene bottle containing twenty or so porcelain balls of diameter 20 mm. Thanks to this stirring technique, a good homogenization of the solution is obtained and the suspension is given a fluid appearance.

In practice, the agitation breaks the gelling process previously observed.

The reduced viscosity of the mixture is necessary for a good impregnation of the fiberglass ribbon which will be used for the cladding of the conductor.

A volume of about 600 milliliters of mixture is constituted for each preparation.

The field of composition of the suspension is now indicated.

• 35% à 50% pour l'eau,
• 35% à 55% pour la fritte de verre, et
• 8% à 15% pour l'argile de type montmorillonite.

In the ceramic precursor, the mass percentages may vary in the intervals given below (the sum of the percentages must of course be equal to 100% for a given ceramic precursor):

• 35% to 50% for water,
• 35% to 55% for the glass frit, and
• 8% to 15% for montmorillonite clay.

In what follows, the manufacture of the ceramic sheath is explained.

In the example described, the ceramic sheath is made of a glass fiber ribbon which is impregnated with the ceramic suspension described above. The fibers of this ribbon may be type E, C, R or S2 glass. These fibers may equally well be pure silica, alumina or aluminosilicate.

Before being impregnated, the ribbon undergoes heat treatment - it is maintained at 350 ° C for 12 hours - to eliminate the organic size of the fibers of which it is made.

This size is indeed detrimental to good fiber coating by the ceramic suspension and is a source of carbon elements, which can reduce the insulating properties of the ceramic.

The coating of the glass fiber ribbon with the ceramic solution is carried out by means of an impregnation bench which is diagrammatically shown in FIG.

The desensitized ribbon, in the form of a roller 2, is attached to a brake system 4 which allows unrolling the ribbon while keeping a constant tension. Pulleys 6 guide the ribbon through the various components of the impregnation bench. The direction of movement is indicated by the arrow F.

In a first step, the ribbon passes into an impregnating tank 8 containing the ceramic suspension 10. This is kept under stirring, thanks to a magnetic stirrer 12, during the impregnation phase of the ribbon, in order to preserve the homogeneity of the latter and avoid sedimentation problems.

At the outlet of the tray 8, the ribbon 2 passes through a scraper system 14 which limits the thickness of the ceramic deposit 16 formed on the ribbon (due to its passage through the ceramic suspension).

A drying column 18, heated to 150 ° C, allows complete evaporation of water from the ceramic solution deposited on the ribbon.

At the outlet of the column, the sheath, ceramic precursor, is completely dry. She is packaged in the form of a roller 20, thanks to a motor 22 which maintains a constant running speed of 20cm per minute.

The following describes the manufacture of a quadrupole electromagnet using the present invention.

The construction of such an electromagnet requires the manufacture of four identical windings, each winding consisting of 75m of superconducting cable Rutherford type.

The Rutherford cables are approximately trapezoidal in cross-section and consist of 36 conductive strands which are twisted together and finally made of Nb ₃ Sn in the example.

These strands are distributed so as to form a flat two-layer conductor, the cross section of which has the following approximate dimensions: 1.3 mm for the short side, 1.6 mm for the long side and 15.1 mm for the width .

The ceramic cladding, consisting of the glass fiber ribbon impregnated with the ceramic precursor, is wrapped around the Rutherford conductor cable (formed of the Nb ₃ Sn precursor), in two layers offset by half a width, as it is see figure 2.

In this figure, the references 24, 26, 28 and 30 respectively represent the cable (before the treatment intended to form Nb ₃ Sn), the strands of the cable, the first layer of the ribbon and the second layer of the ribbon.

For each of these layers, the edge of one turn of ribbon is against the edge of the adjacent turn. In addition, the first layer 28 is placed first on the cable and the second layer 30 ensures the continuity of the electrical insulation, as seen in Figure 2.

After covering the conductor cable by means of the two ceramic cladding layers 28 and 30, this cable is formed into coils according to means known in the state of the art. Then the coils thus obtained from the conductive cable, consisting of the precursor covered with the ceramic sheath, are subjected to a heat treatment under a neutral gas such as argon.

This treatment comprises a slow rise in temperature, at a speed close to 6 ° C. per hour, up to the temperature of 660 ° C., then a plateau at 660 ° C. for 240 hours, then a slow cooling down to the temperature ambient temperature (20 ° C to 23 ° C) in the furnace chamber.

This treatment allows the reaction of the precursor cable and obtaining a Nb ₃ Sn superconducting material having the desired properties.

During this heat treatment, a continuous flow of neutral gas takes place inside the oven. The use of such an inert atmosphere during the heat treatment makes it possible to avoid the harmful reactions between the Nb ₃ Sn precursor and the oxygen of the air, which can form various metal oxides, capable of reducing the properties of the superconductor formed. .

The use of a temperature of 660 ° C under neutral gas is an important constraint for the development of a suitable ceramic insulation.

Indeed, the glass frit used in the example of the invention has a melting point of 540 ° C. It therefore melts during the heat treatment necessary for the formation of the Nb ₃ Sn superconductor (during which the temperature is maintained at 660 ° C.) and thus provides, after cooling to ambient temperature, the electrical insulation and the mechanical cohesion necessary for the applications of the invention, such as the formation of superconducting coils.

During the operation of the superconducting electromagnets, each coil is cooled to the temperature of liquid helium (4.2K at atmospheric pressure) or that of superfluid helium (temperature below 2.1K under reduced pressure) to superconducting the alloy Nb ₃ Sn component conductor which is formed the winding cable.

When the electromagnet is traversed by an excitation current, considerable Lorentz forces appear in each winding. The mechanical cohesion provided by the ceramic insulation facilitates the handling of the coils after the heat treatment and makes it possible to withstand the forces generated by the operation of the electromagnet to intense magnetic fields.

In the invention, in place of montmorillonite, any other clay from the group of smectites can be used.

• un précurseur de Nb₃Al ou
• un précurseur d'un supraconducteur à base d'oxyde de cuivre, tel que YBa₂Cu₃O₇, Bi₂Sr₂CaCu₂O₂ ou Bi₂Sr₂Ca₂CU₃O₁₀, ou
• un métal qui n'est pas supraconducteur, par exemple le cuivre, ou
• tout conducteur y compris un supraconducteur supportant le traitement thermique que l'on fait subir au précurseur céramique.

In addition, the invention can be implemented with other conductors than a precursor of Nb ₃ Sn, for example:

• a precursor of Nb ₃ Al or
• a precursor of a superconductor based on copper oxide, such as YBa ₂ Cu ₃ O ₇ , Bi ₂ Sr ₂ CaCu ₂ O ₂ or Bi ₂ Sr ₂ Ca ₂ CU ₃ O ₁₀ , or
• a metal that is not superconducting, for example copper, or
• any conductor including a superconductor supporting the heat treatment that is subjected to the ceramic precursor.

• à la fabrication de petits solénoïdes supraconducteurs compacts, dépourvus d'éléments métalliques structurants, utilisés principalement à basse température,
• à la fabrication de bobinages de machines électriques tournantes supraconductrices,
• à la fabrication de bobinages de machines électriques tournantes non supraconductrices, destinées à fonctionner à des températures supérieures à 300°C, par utilisation de conducteurs classiques, et
• à l'isolation électrique de câbles conducteurs devant résister pendant un certain temps à de fortes températures, sans dégagement de vapeurs nocives en cas d'incendie.

The invention applies in particular:

• the manufacture of small compact superconducting solenoids, devoid of structuring metal elements, used mainly at low temperatures,
• the manufacture of windings of superconducting rotating electrical machines,
• the manufacture of windings of non-superconducting rotating electrical machines, intended to operate at temperatures above 300 ° C, using conventional conductors, and
• the electrical insulation of conductor cables that must withstand high temperatures for a certain period without the release of harmful vapors in the event of fire.

Let us now consider the rheological behavior of two particular ceramic suspensions, usable in the invention.

Referring to Figures 3 and 4 which show the flow curves for two ceramic suspensions having different compositions: Figure 3 corresponds to a first composition and Figure 4 to a second composition, different from the first.

Each of these flow curves represents the variations of the stress τ (expressed in Pa) as a function of the shear rate γ (expressed in s ^-1 ).

The behavior is not Newtonian type, the average viscosities of the two compositions are close, around 45mPa.s, but only the first composition (Figure 3) gives a sufficient deposit on the glass ribbon.

This difference is explained by the variation of the thixotropic behavior of the two suspensions. Indeed, the two descent curves D1 and D2 are equivalent but, for the rise curves M1 and M2, the first composition has a more shear-thinning behavior which leads to a greater thixotropy.

However, the low speed movement of the glass ribbon in the ceramic suspension creates low shear rates. Thus, during the impregnation phase, the experimental conditions are such that the rheological behavior corresponds to the beginning of the flow curves.

The composition of the two suspensions is given in Table I below. The clay used for the two suspensions is montmorillonite marketed by Arvel SA under the name Expans. Table 1 Clay (% by mass) Glass frit (% by mass) Water (% by mass) Suspension 1 11.5 46 42.5 Suspension 2 10 50 40

Claims (12)

1. Method for manufacturing an electrically insulating and mechanically structuring sheath on an electrical conductor (2), in particular a non-superconducting metal conductor, a superconducting metal conductor or a superconductor precursor conductor, said method comprising the steps of:

   - forming a ceramic precursor (10) in the form of a fluid solution, said ceramic precursor (10) being a liquid consisting of a solution containing water, glass frit and a suspension of clay in water,

   - forming a coating for the conductor with said ceramic precursor, and

   - heat-treating said coating, said heat treatment being capable of forming the ceramic from the ceramic precursor,

   said method being characterized in that in the step of forming a ceramic precursor said liquid contains no organic element.
2. Method of claim 1, in which the clay is selected from the smectite group.
3. Method of claim 2, in which the clay is montmorillonite.
4. Method as claimed in any of claims 1 to 3, in which the solution comprises, in per cent by weight, 35% to 50% water, 8% to 15% clay and 35% to 55% glass frit.
5. Method as claimed in any of claims 1 to 4, in which the conductor (2) is a superconductor precursor, in particular Nb₃Sn, and a global heat treatment of said conductor provided with the coating is carried out, said global heat treatment being capable of forming the superconductor and the ceramic.
6. Method as claimed in any of claims 1 to 4, in which the conductor (2) is made of either a non-superconducting metal or a superconducting metal, and a heat treatment of said conductor provided with the coating is carried out, said heat treatment being capable of forming the ceramic.
7. Method as claimed in any of claims 1 to 6, in which the step of forming the coating comprises a step of depositing the ceramic precursor on a fiber tape, then a step of arranging the tape provided with the ceramic precursor around the conductor.
8. Method of claim 7, in which the fibers are made of a material selected from among type E glass, type C glass, type R glass, type S2 glass, pure silica, an alumina and an aluminosilicate.
9. Method as claimed in any of claims 7 and 8, in which the fiber tape is first desized.
10. Method of claim 9, in which the fiber tape is first desized thermally or chemically.
11. Method as claimed in any of claims 1 to 10, in which the conductor (2) provided with the coating is formed prior to the heat treatment step capable of forming the ceramic.
12. Method of claim 11, in which the conductor (2) provided with the coating is wound prior to the heat treatment step capable of forming the ceramic.

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