Login| Sign Up| Help| Contact|

Patent Searching and Data


Title:
PROCESS FOR THE PRODUCTION OF MGB2 SUPERCONDUCTORS AND APPARATUS TO IMPLEMENT THE PROCESS
Document Type and Number:
WIPO Patent Application WO/2016/097986
Kind Code:
A1
Abstract:
The present invention relates to a process for the production of superconductors and to an apparatus adapted to implement such a process. More particularly, the process for the production of superconductors according to the present invention is characterized by a new step of treating the boron (B) and magnesium (Mg) powders used in the production of superconductors, as well as an apparatus adapted to implement such a process including such a treatment step. According to a variant of the process object of the present invention, it may also comprise a new step of treating the magnesium diboride (MgB2) powders.

Inventors:
TUMINO ANDREA (IT)
NARDELLI DAVIDE (IT)
GRASSO GIOVANNI (IT)
Application Number:
PCT/IB2015/059617
Publication Date:
June 23, 2016
Filing Date:
December 15, 2015
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
COLUMBUS SUPERCONDUCTORS S P A (IT)
International Classes:
H01L39/24; C04B35/58; B02C19/06
Domestic Patent References:
WO2002072501A22002-09-19
WO2013156465A12013-10-24
Foreign References:
US20080274902A12008-11-06
Other References:
None
Attorney, Agent or Firm:
LUALDI, Lorenzo et al. (Corso di Porta Vittoria 9, Milano, IT)
Download PDF:
Claims:
CLAIMS

1 . Process for the production of superconductors comprising at least the following steps:

preparing some boron (B) and some magnesium (Mg) powder; - dosing said boron (B) and said magnesium (Mg) powders with a stoichiometric ratio of 2:1 ;

mixing the pre-dosed amounts of boron (B) and magnesium (Mg) powders with the stoichiometric ratio 2:1 ;

subjecting the mixture of boron (B) and magnesium (Mg) powders to a thermal treatment until reaching a complete transformation of the mixture of boron and magnesium in magnesium diboride (MgB2); characterized in that said step of mixing boron (B) and magnesium (Mg) powders comprises a treatment step in which said powders are subjected to a jet-milling process.

2. Process for the production of superconductors according to the preceding Claim, characterized in that it further comprises a further step wherein the magnesium diboride powder (MgB2) obtained by means of a thermal treatment undergoes a further jet-milling process, which can be pure or can involve the addition of chemical additives to the jet-milling.

3. Process according to any one of the preceding Claims, wherein said powder treatment step by jet-milling is carried out using a gas.

4. Process according to any one the preceding Claims, wherein the jet-milling plant is inserted in a glove box, filled with a gas not containing oxygen, such as argon, nitrogen and helium, to avoid the powder exposure to atmospheric pollutants and oxidants.

5. Process for the production of superconductors according to any one the preceding Claims, wherein said powder treatment step by jet-milling comprises the addition of additives in the boron (B) and magnesium (Mg) mixture.

6. Process for the production of superconductors according to the preceding Claim, wherein said additives are composed at least by one or more of the following chemical elements: carbon, silicon, oxygen.

7. Process for the production of superconductors according to the preceding Claim, wherein said additives are added to said boron (B) and magnesium (Mg) mixture in replacement of a percentage comprised between 0 and 10% mol of boron powder.

8. Apparatus for the production of superconductors, characterized in that it comprises dosing means (10) for dosing boron (B) and magnesium (Mg) powders, at least a first jet-milling machine (30) for mixing said powders, supplying means (20) for conveying said pre-dosed boron (B) and magnesium (Mg) powders into said jet-milling machine (30), at least a reaction furnace for the thermal processing of said boron (B) and magnesium (Mg) powders in magnesium diboride (MgB2).

9. Apparatus for the production of superconductors according to the preceding Claim, characterized in that it further comprises a second jet-milling machine to carry out the jet-milling of magnesium diboride powders (MgB2).

Description:
PROCESS FOR THE PRODUCTION OF MGB2 SUPERCONDUCTORS AND

APPARATUS TO IMPLEMENT THE PROCESS

**********

Field of the invention

The present invention relates to a process for the production of superconductors and to an apparatus adapted to implement such a process.

More particularly, the production process according to the present invention is characterized by a new step of treating the boron B and magnesium Mg powders used in the production of superconductors, as well as an apparatus adapted to implement such a process including such a treatment step.

According to a variant of the process object of the present invention, it may also comprise a new step of treating the magnesium diboride MgB2 powders.

In a further variant, the magnesium and boron powders may be partially or fully replaced by powders of chemical compounds which contain magnesium and boron in the desired amount.

The possible jet-milling process is also included in order to add additives adapted to improve the superconducting properties of MgB2.

Prior art

The manufacturing of wires of superconductor material, in particular magnesium diboride (MgB2), is known.

Such wires are normally used in cryogenic conditions, at usage temperatures typically between 4 Kelvin and 40 Kelvin degrees, temperatures at which the magnesium diboride becomes superconductor.

In the process for the production of the superconductor wires of magnesium diboride, a critical stage is the step of deep mixing the boron and magnesium powders.

It is in fact known that boron and magnesium have very different physical and mechanical features, and therefore in the process for the production of the superconductor it is important to start from a mixture of boron and magnesium, and/or of chemical compounds containing both elements and/or chemical additives in the correct stoichiometric amounts, that is as homogeneous as possible in order to obtain the transformation of all the material into MgB2. Currently, conventional mechanical mixing techniques are used: the operator manually puts the powders in the correct stoichiometric amounts predetermined by the use of precision scales in a mechanical stirrer, such as ball mill systems with planetary motion, in which the rotation of suitably filled jars causes the collision of the Mg and B powders against the inner walls and against some metal balls contained therein.

The step of mixing the powders is very delicate not only, as said, due to the need of obtaining a homogeneous mixture free of undesired contaminants, but also due to the fact that all the steps in which the operator must handle the powders must be carried out in closed environments, where, in order to work, the operator must put his hands into special fixed gripping gloves crossing the safety glass separating the operator from the working environment.

In the processes of known type, the mixing step thus involves a manual intervention by the operator which implies a certain degree of difficulty and discomfort for the operator.

Therefore, the drawbacks afflicting the known processes for the production of the materials used in the processes for making superconductor wires, in particular of magnesium diboride, consist in a non-optimum mixing of the powders obtainable by means of the conventional mechanical mixing techniques, in the possible addition of undesired contaminants, as well as in the complexity and poor ergonomics of the operations which are carried out manually by the skilled expert who carries out the dosing and mixing of the powders as well as the difficult and onerous industrial scalability of such a process.

Summary

The task of the present invention is to provide a production process for the production of superconductors comprising a step of treating the powders which solves the drawbacks of the processes of known type.

Within this task, it is an object of the present invention to provide a process for the production of superconductors which optimizes the step of mixing the powders, allowing a more homogeneous powder mixture to be obtained compared to that obtainable through the mechanical mixing techniques used in the production processes of known type, and as far as possible free of undesired contaminants. It is a further object of the present invention to provide a process for the production of superconductors which allows the full automation of the step of mixing the powders, with consequent simplification and reduction of the field of intervention of the operator which leads the latter to work in conditions of greater comfort and greater safety from the health point of view.

It is another object to make the process cost-effective and easily scalable.

Last but not least, it is an object of the present invention to provide a process for the production of superconductors which allows the further grinding of the powders in the step of mixing them, with consequent optimization of the mixing of the powders and reduction of the anisotropy of the superconductor material, thereby increasing the electrical conduction properties thereof.

It is yet another object of the present invention to provide an apparatus adapted to implement the production process according to the present invention.

Brief description of the figures

Further features and advantages of the present invention will become more apparent from the following detailed description of a preferred but non-exclusive embodiment, shown by way of a non-limiting example with reference to the accompanying drawings, in which:

- figure 1 shows by way of example an overall view of a possible configuration of the apparatus adapted to implement the steps of the process for the production of superconductors object of the present invention.

Detailed description of the invention

The process for the production of superconductors to which the present invention relates includes an initial step of mixing the magnesium and boron powders in a stoichiometric ratio of about 1 to 2. The exact stoichiometric ratio may deviate slightly from 1 to 2 within a limited range MgiB2(i-x) with x between -0.1 and +0.1 to compensate for any stoichiometry changes which might occur during the subsequent thermo-mechanical process, as well as to compensate for or comprise any introduction of chemical additives adapted to improve the superconductor properties of the compound. At this point, the powder mixture undergoes a heat treatment which determines the transformation into MgB2: the mixture is placed in a reaction furnace in which the reaction of transformation into magnesium diboride MgB2 occurs.

Such a reaction occurs at a temperature between 600 °C and 950 °C.

At the end of the heat treatment, the magnesium diboride MgB2 is still in powder form.

In order to improve the features of the mixture, it is known to replace in a certain percentage, for example comprised between 0-10% mol, of boron with carbon, silicon or other additives.

According to the treatment methods of known type, at the end of the heat treatment the MgB2 may comprise agglomerates of powders formed during the heat treatment. Such non-uniformity of the MgB2 powder is undesirable, as it worsens the conductive behavior of the end product in addition to a non-optimal workability of the material itself.

In order to try to eliminate such agglomerates, the current production techniques include optionally subjecting the MgB2 to a mixing or mechanical crushing operation of the powders by means of mechanical systems of known type, for example by means of a ball mill with planetary motion, alternated to sieving steps. Once the magnesium diboride MgB2 powder has been obtained, this is used for the filling of the inner cavity of cylindrical billets of metal material.

The billet filled with MgB2 is then closed at the end used for filling and the production process continues according to the so-called ex-situ powder in tube (PIT) method, of known type. Very briefly, since it is outside the scope of protection of the present invention, this process provides for the billet filled with MgB2 to be subjected to mechanical processing by means of extrusion in combination with the lamination to obtain a superconductor wire with the desired geometric features. Such a step is both spaced out and followed by heat treatment processes having the function of reducing the mechanical stress caused by the deformation of the material.

The production process may provide, according to what is known, for a plurality of such conductor wires to be in turn inserted into a metal billet, which is extruded again to obtain a superconductor which internally comprises a plurality of wires. The method for the treatment of the powders used in the process for the production of superconductors according to the present invention, therefore, is inserted in the process for the production of superconductors described above, and in particular it includes a step of mixing the boron and magnesium powders through the jet-milling technique.

In the art, jet-milling is a milling process which uses a jet of pressurized gas. Hereinafter, jet-milling will be used to indicate this milling process, and the device which performs the jet-milling will be designated by the expression machine which carries out the jet-milling (jet-milling machine). The general term jet milling indicates all those industrial techniques including spiral jet-milling, loop jet-milling, opposed jet-milling, fluidized bed jet-milling, and high density jet-milling and which are essentially based on the same physical principle. The technique which will be described hereinafter is that of the spiral jet milling, which lends itself more directly to fulfil the required aim.

The spiral jet-milling technique involves the use, in a suitable device, of a flow of pressurized gas, preferably nitrogen and/or argon, but also helium, hydrogen or mixtures thereof, which entrains the powders through a box in a swirling motion. To this end, the box includes inner walls with a curved profile. Preferably, the box will be substantially cylindrical, so as to make the flow of pressurized gas, inserted in the box through a conduit arranged substantially tangentially with respect to the cylindrical box, take a swirling motion.

The solid particles entrained by the gas therefore take a swirling motion in which the particles of greater size, and therefore mass, will be pushed by the centrifugal force towards the perimeter of the box, bumping against the walls, while the particles of smaller size, and therefore mass, will come to be in the vicinity of the axis of the cylindrical box.

The swirling motion thus generates a series of impacts of the particles which collide with one another and against the inner walls of the boxes, decreasing in size due to the collisions, thus obtaining the milling of the powder inserted in the device. Once the particles reach the desired size for which the milling machine is specifically designed, the particles are removed from the center of the box, and thus of the vortex, through an axial outlet conduit.

The method of treating the powders used in the process for the production of superconductors according to the present invention includes inserting the boron and magnesium powders to be mixed in the machine which performs the jet- milling.

The jet-milling technique is therefore used in the method according to the present invention primarily to obtain the mixing of the boron and magnesium powders. The Applicant has found that the mixing of the boron and magnesium powders by means of the jet-milling technique allows obtaining a more homogeneous mixture compared to the mechanical mixing techniques traditionally employed in the field. As a secondary but highly appreciated effect in the production of superconductor wires, the mixing of the powders through the jet-milling technique allows obtaining a milling effect at the same time which further reduces the size of the granules due to the collisions between the particles themselves, which allows obtaining a mixture of particles of smaller size than the average sizes which characterize the powders commonly used in the production processes of superconductor wires. Finer powders allow a more uniform mixture and, ultimately, improved physical and mechanical properties of magnesium diboride MgB2, and therefore of the final superconductor.

A further advantage of the jet-milling technique is that the apparatus is compact in size and can therefore be accommodated in a glove box, containing a gas free of atmospheric contaminants or oxidants, such as argon, nitrogen and helium, thus preventing the powders from being contaminated throughout the mixing or milling process.

The method for the treatment of the powders used in the process for the production of superconductors described so far can further be used, according to a preferred aspect of the invention, in a step following the transformation of the powders in MgB2 as a result of heat treatment. After the heat treatment, in fact, the magnesium diboride powder may advantageously be subjected again to the jet-milling process in order to remove any agglomerates formed during the heat treatment of transformation into MgB2. As said, an object of the present invention is also an apparatus adapted to implement the treatment method of the powders as described herein.

Figure 1 shows by way of example a part of said apparatus. In particular, Figure 1 shows dosing means 10 of the powders, supply means 20 of the powders to a jet- milling machine 30, an inlet conduit 40 of pressurized gas, and an axial outlet conduit 50 of the mixed powders.

To this end, the apparatus is characterized in that it comprises at least a first machine 30 which carries out the jet-milling.

More particularly, the apparatus may further comprise dosing means 10 of the boron and magnesium powders, and supply means 20 of the powders to said jet- milling machine 30. Preferably, said dosing means 1 0 and said supply means 20 of the powders are automated.

Simply by way of example, the dosing means 10 may include automatic powder dispensers directly connected to tanks or hoppers containing the boron and magnesium, respectively, the dosing means 10 being of course programmed to dispense boron and magnesium in the desired stoichiometric ratio of about 2 to 1 , while the supply means 20 may include pneumatic dispensing systems or others of similar type. The introduction of pre-mixed powders or a manual step of mixing the powders may also be provided.

Advantageously, the apparatus according to the present invention may include a control panel by means of which the operator can impart a command to start the cycle which will cause the actuation of the dosing means 10. These will automatically dose the correct amounts of boron and magnesium powders or mixture which will be directed by the supply means 20 to the machine which carries out the jet-milling 30.

As said, the machine which carries out the jet-milling 30 has an inlet conduit of the powders, which already enter in the correct stoichiometric ratios, an inlet conduit 40 of the pressurized gas, and an axial outlet conduit 50 of the mixed powders. Means for extracting the mixed powders may advantageously be connected to said outlet conduit 50 of the powders, for example consisting of a suction system, which can directly carry the powder mixture to a filling station of a container suitable for being inserted in the reaction furnace to subject the powder mixture to the heat treatment of transformation into MgB2.

With reference to Figure 1 , the outlet conduit 50 of the powders is represented below the machine which carries out the jet-milling 30, while an outlet conduit 60 of the gas is suitably provided on top of the jet-milling machine 30, through which the exhaust gas used in the machine which carries out the jet-milling 30 and any waste material consisting of powder particles too thin in size are separated by filtration, the waste material being collected in a suitable container 70.

According to a preferred embodiment, the apparatus according to the present invention further comprises a second machine (or the same machine) for jet- milling in which the jet-milling treatment of the MgB2 powders is carried out.