The present invention relates to a method for the production of superconductors. More in particular, the method according to present invention relates to the production of simple or composite wires made of magnesium diboride (MgB2). Such a method is applied to the production of electromagnets used in the most varied sectors and of wires for transporting electrical currents. By way of example only, sectors include the medical field (in magnetic resonance imaging machines) and the field of experimental physics (in particle accelerators).

Prior art

It is known to produce wires made of superconductor material, in particular of magnesium diboride (MgB2).

Such wires are normally used in cryogenic conditions, at temperatures of use typically comprised between 4 Kelvin and 40 Kelvin, temperatures at which magnesium diboride becomes superconducting.

In the known method of production, an operator manually introduces the correct stoichiometric quantities of powders into an agitator typically of mechanical type, such as a planetary ball mill system, in which the rotation of appropriately filled jars causes the Mg and B powders to knock into each other, against the inner walls and against some metal balls contained therein.

The step of mixing of the powders is very delicate not only, as mentioned, for the need to obtain a homogenous mixture free from undesired contaminants, but also because all the steps in which the operator must handle the powders must be carried out in closed environments, to operate inside which the operators must insert their hands in specific fixed gripping gloves through the security glass which separates the operator from the working environment.

Consequently, the step of mixing in the known type processes requires a manual intervention by the operator which implies a given degree of difficulty and discomfort for the operator.

After having completed the mixing, the operator fills a tube made of metal or appropriate alloy with the mixed powders (billet). Such a billet is thus mechanically deformed, typically by drawing and/or rolling processes as needed, to produce a superconductor wire of smaller diameter and greater length than the initial one. Finally, the wire is subjected to a thermal treatment so as to trigger the reaction between the boron and the magnesium and thus make the magnesium-diboride- based compound inside the metal or alloy sheath which constituted the initial containing tube.

Disadvantageously, the production of MgB2 superconductors starting from mixing powders implies a substantial risk of lack of uniformity inside the material. In practice, in case of imperfect mixing, a prevalence of boron or of magnesium powder may be created in some zones. Consequently, such a material in excess does not react and remains as impurity in the superconductor. Furthermore, this creates lack of uniformity in the distribution of the MgB2, which reflects on a lack of uniformity and lowers the mechanical properties of the wire itself. Additionally, since the mass density of the MgB2 compound is significantly higher than the mass densities of the boron and magnesium elements in mixed state, the aforesaid thermal reaction causes the formation of a spongiform MgB2 structure, in which the superconductor particles are surrounded by holes, which may result as interruptions of continuity of the superconductor structure, thus significantly limiting the electrical properties.

Summary

In this context, the technical task underlying the present invention is to suggest a method for the production of superconductors which overcomes the drawbacks of the prior art mentioned above.

In particular, it is the object of the present invention to make available a method for the production of superconductors capable of guaranteeing better mechanical and electrical properties to the product.

It is a further object of the present invention to make available a method for the production of superconductors capable of reducing the product anisotropies to the minimum.

The technical task and the specified objects are substantially achieved by a method for the production of superconductors comprising the technical features illustrated in one or more of the appended claims. In particular, the method for the production of superconductors according to the present invention comprises a step of preparing a sheet of a first material. Such a first material is preferably magnesium, either pure or possibly with the addition of impurities chosen and controlled to increase the transporting properties or the mechanical properties. The sheet has a longitudinal development axis.

A predetermined amount of powder of a second material is thus dosed. Such a second material is preferably boron, either pure or possibly with the addition of impurities chosen and controlled to increase the transporting properties or the mechanical properties. Such a powder is spread on the sheet according to a predetermined dosing so as to define a layer of second material. In particular, the dosing is performed according to a stoichiometric ratio Mg:B of 1 :2, except for variations which take dopants or additives into account.

The sheet is thus wrapped along the longitudinal axis to obtain a blank. Optionally, such a step of wrapping may comprise the sub-step of preparing a support having a prevalent development axis parallel to the longitudinal development axis.

The sheet is thus wrapped to obtain a blank. Such a blank is then processed by drawing and rolling to turn it into a wire.

The wire is then thermally treated to obtain the formation of the MgB2 superconductor by means of the reaction between the boron powders and the magnesium sheet.

The superconductor resulting from the method according to the present invention will have an internal spiral structure. Such a structures is basically more uniform than that known until now in the prior art, and also has considerably improved mechanical properties, because the voids present are uniformly distributed along the length and the spacing between the planes of the spiral allows movements of the MgB2 planes without causing the creation of fractures caused by the bending of the wire.

Brief description of the figures

Further features and advantages of the present invention will be more apparent in the following indicative and consequently non-limitative description of a preferred, but not exclusive embodiment of a method for the production of superconductors, as shown on the accompanying drawings, in which: - figure 1 diagrammatically shows a first step of the method for the production of superconductors according to the present invention;

- figure 2 diagrammatically shows a second step of the method for the production of superconductors according to the present invention; and

- figure 3 diagrammatically shows a third step of the method for the production of superconductors according to the present invention.

Detailed description of the invention

A method for the production of superconductors according to the present invention will now be described with reference to the accompanying figures.

In particular, figure 1 shows a step of preparing a sheet 1 of a first material. Such a first material is preferably magnesium. The sheet has a longitudinal development axis "A". More in particular, the sheet 1 has a rectangular shape. In different embodiments of the invention, the development axis "A" may be defined in parallel to the either longer side or shorter side of the sheet 1 .

A predetermined amount of powder of a second material is thus dosed. Such a second material is preferably boron. Such a powder is spread on the sheet 1 according to a predetermined dosing to define a layer 2 of second material. Dosing occurs in particular according to a stoichiometric ratio between the first and the second material 1 :2.

In further detail, the distribution of the second material on the sheet 1 may be carried out homogeneously, e.g. by roughly applying the powder and then distributing it with a predetermined spacer. In order to make such a composition more stable, a pressure can be applied (e.g. by means of a metal surface) on the layer 2 so as to compact the powder and make it adhere to the surface of the sheet 1 in a preliminary manner so as to wrap it. In an alternative embodiment (not shown), a further sheet of material is added and pressed on the layer 2 to define a containment for the powders.

Alternatively, the powders of second material may be mixed with an appropriate substance which is easily evaporable but not water-based, such as, for example ultra-pure isopropyl alcohol, which may help to spread the powders of second material uniformly on the sheet 1 . The added liquid may be subsequently removed by evaporation either at ambient temperature or at higher temperature, possibly in vacuum conditions to accelerate the process. Advantageously, in this manner, the uniform spreading of the layer 2 is facilitated and the required position and size of the layer 2 is maintained.

With reference to figure 2, the sheet 1 covered with the layer of powder 2 is wrapped along the longitudinal development axis "A" to obtain a blank 3. Such a step of wrapping may comprise the sub-step of preparing a support 4 having a prevalent development axis "B" parallel to the longitudinal development axis "A". The sheet 1 may be wound on such a support 4. In particular, the support is a wire or a tube. In particular, such a support 4 may also have a function of improving the mechanical and/or electrical properties of the wire once completed.

The method comprises the further step of inserting the blank 3 in a casing 7. In particular, such a casing 7 comprises a tube, preferably cylindrical, having a cavity 7a adapted to house the blank. The casing 7 is preferably made of steel, or of various alloys, in particular nickel-based. It is worth noting that, according to with the present invention, the step of inserting the blank 3 inside the casing 7 is carried out before the step of thermal treating.

Subsequently, the blank 3 is subjected to drawing to obtain a wire 5. Such a wire 5 may be cut, in turn, into a plurality of pieces of predetermined length. The wires 5 may thus be stacked again, according to a predetermined mode, and re- introduced into a further casing (not shown). The assembly may be processed again in manner similar to the wire 5, so as to obtain a wire comprising a plurality of MgB2 cores, named "multifilamentary" wire in the technical jargon of the sector. The blank 3 is thus thermally treated to obtain a wire 5 of superconductor 6. In particular, such a superconductor is magnesium diboride. In further detail, the thermal treatment cycle comprises at least one step in which the wire 5 is taken to a temperature higher than the melting temperature of magnesium. In this manner, the sheet 1 passes either partially or totally to the liquid state and, by reacting with the boron powder, produces the superconductor 6 uniformly along the entire wire 5.

The method further comprises a step of pressurizing of the blank 3, during the thermal treatment or subsequently thereafter. In particular, a mechanical or pneumatic pressure is exerted externally to the wire 5 on the surface thereof so as to compress the external diameter. Advantageously, this allows to either reduce or eliminate possible areas devoid of superconductor inside the wire 5.

Optionally, a plurality of pieces of a plurality of pieces of blank 3 can be inserted inside a further casing (not shown). Such a casing is thus pressurized and thermally treated. In particular, the further casing filled with the pieces of blank 3 may be processed in manner similar to the blank 3 itself, so as to obtain a wire comprising a plurality of MgB2 cores.