The present invention relates to a method for the preparation of highly densified superconductor massive bodies of MgB2, the relevant solid end-products and their use.

It has recently been discovered that magnesium boride has superconductor properties up to 39 K and can therefore be applied in closed circuit cryogenic systems (cryo-refrigerators), which are less costly than those based on the use of liquid helium (Nagamatsu et al., Na- ture, 410,63; 2001).

Like all borides, magnesium boride, a compound which has been known for about half a century, is characterized by extreme hardness when it is highly densified.

The densification of magnesium boride however into end-products, reaching values close to 100% of its theo- retical density (2.63 g/cm3), effected by the compacting of the.. powders of the compound itself, normally requires the use of high pressures. Pressures in the order of sev- eral GPa are generally used.

Alternative synthesis methods of the compound MgB2 starting from stoichiometric, or non-stoichiometric, mix- tures of boron and magnesium, both in powder form and in the form of massive bodies, are also known in literature.

In the latter case, however, the use of high pressures is indispensable for obtaining highly densified end- products.

An example is described by Canfield et al., whereby, MgB2 fibres are obtained, starting from boron fibres re- acted with liquid Mg or in vapour phase, (Phys. Rev.

Lett. 86,2423 (2001)), having an estimated density of about 80% of the theoretical value.

It is consequently only possible to obtain an end- product of magnesium boride densified up to values close to the theoretical value, and therefore characterized by improved superconductivity and mechanical properties, with the methods of the known art, by the use of high pressures at a high temperature.

The use of high pressures at a high temperature how- ever limits the dimensions of the end-products obtained and necessitates the use of equipment which is not suit- able for a mass production.

An objective of the present invention is therefore to obtain superconductor massive bodies of MgB2 with a density close to the theoretical value with a method

which overcomes the drawbacks present in the known art.

An object of the present invention relates to a method for the preparation of superconductor massive bod- ies of MgB2, having a density close to the theoretical value, which comprises the following passages: a) mechanical activation of crystalline boron with the formation of activated powders ; b) formation of a porous preform of activated powders of crystalline boron; c) assembly of the porous boron preform and massive precursors of metallic magnesium in a container and sealing thereof in an atmosphere of inert gas or with a low oxygen content; d) thermal treatment of the boron and magnesium as as- sembled above, at a temperature higher than 700°C for a time greater than 30 minutes, with the conse- quent percolation of the magnesium, in liquid phase, through the activated crystalline boron powders.

A further object of the present invention relates to a superconductor massive body or solid end-product of MgB2, having a density close to the theoretical value, obtained by means of the method of the present invention.

Another object of the present invention also relates to a method which comprises in step c) the use of magne- sium mixed with one or more lower-melting metals, such as

Ga, Sn, In, Zn, or an Mg-based alloy with said metals.

The present invention also relates to the use of the massive bodies of MgB2 obtainable with the method accord- ing to the present invention for superconductors to be used as electric current cut-ins, variable induction ele- ments in current limitation systems, permanent magnets to be used in levitation systems, in medical magnetic reso- nance systems, in elementary particle accelerators and detectors, in energy accumulation systems, in linear or non-linear motors, in power generators.

The fundamental advantage of the method according to the present invention lies in the fact that it allows the production, in a simple and economic way, of solid super- conductor end-products of MgB2, densified up to values close to the theoretical value, with improved character- istics with respect to the products obtainable with the known methods in the state of the art. From an applica- tive point of view, the use of MgB2, densified up to val- ues close to the theoretical value, thus obtained, allows the current which can be conveyed to the superconductor products to be increased and also improves the mechanical properties of said end-products.

A further advantage also consists in the fact that highly densified targets of MgB2 allow deposition tech- nologies such as laser ablation or radio-frequency sput-

tering, to be applied with greater success, to obtain su- perconductor material deposited on substrates of various origins, in the form of thin films.

In particular, the method for the production of su- perconductor massive bodies of MgB2, having a density close to the theoretical value, i. e. a density higher than or equal to 2.25 g/cm3 consists in reacting the bo- ron and magnesium elements in a sealed container in an atmosphere of inert gas or with a low oxygen content (lower than 20% atomic), at a high temperature, wherein at least the boron is present in the form of powders, de- fined as active, with a suitable particle-size and having at least two crystalline phases similar to unit cells of the rhombohedral type.

The mechanical activation step a) crystalline boron flakes having dimensions of a few millimeters and a pu- rity higher than or equal to 99.4%, preferably consists in a repeated crushing by high load compression, under "almost static"conditions, as for example can be ef- fected in a hydraulic press. This activation not only minimizes the powder fraction with a finer particle-size (for example lower than 20 micrometres) which is the typical grinding product of a rotating ball mill, but also allows a powder to be obtained, which maintains the characteristics of the crystallinity type present in the

starting flakes, thus making the powders more permeable to liquid magnesium.

In particular, the activated crystalline boron pow- ders are selected so as to have an average volumetric particle diameter ranging from 30 to 70 microns and are practically without oxygen contamination. Step b) com- prises the formation of a porous preform of activated crystalline boron powders.

The porous preform of activated crystalline boron powders has a shape similar to that of the end-product and must have an apparent density higher than 50% of the theoretical density of the crystalline boron (2.35 g/cm 3).

The preform of activated crystalline boron powders may alternatively contain up to 20% atomic of magnesium.

In this case, the preform prevalently consists of acti- vated crystalline boron powder and magnesium powder prac- tically without oxygen contamination and a particle-size lower than that of the boron. The preform can also con- sist of activated crystalline boron powders, surface- covered by metallic Mg and welded to each other by ther- mal treatment in an inert atmosphere, so as to maintain the porosity of the preform and at the same time provide mechanical consistency for its handling.

Preforms containing magnesium must also satisfy the

requisite of apparent density defined above.

The following step c) comprises the assembly of the components which will undergo thermal treatment and transformation to the end-product in step d). The con- tainer in which these components are assembled, is also important.

Step c) comprises the insertion in a suitable con- tainer of a combination of two components: the first com- ponent is the preform produced with the above-mentioned activated crystalline boron powder, having a purity at least higher than or equal to 99.4%, which has a shape similar to that of the end-product and an apparent den- sity higher than 50% of the theoretical density of rhom- bohedral crystalline boron (2.35 g/cm3), preferably rang- ing from 51% to 56%. The second component consists of one or more massive bodies of metallic Mg having a purity higher than 99% which in step d), after melting, perco- lates through the activated crystalline boron powder.

The magnesium in liquid phase preferably derives from the melting of massive precursors of metallic Mg. It is also practically free from oxygen contamination.

The proportions between Mg and B largely depend on the technology selected for carrying out the reaction. In any case they are far from the stoichiometric values of the MgBz compound. In particular, there is an excess of

Mg which is such that the atomic ratio Mg/B is greater than 0.5, preferably said ratio is greater than or equal to 0.55.

When mixtures of Mg with other metals are used, the atomic ratio (metals + Mg)/B should be greater than 0.55, with Mg/B contemporaneously greater than 0.5.

Atomic ratio values Mg/B, or (metals + Mg)/B, lower than the limits defined above, cause a reaction which produces a partial densification of the product, reducing or completely cancelling the superconductivity character- istics relating to the conveying of the electric current.

The container in which step c) is effected, consists of a material which cannot be attacked by boron and mag- nesium at temperatures up to 1000°C, such as Nb, Ta, MgO, BN, etc. or any material resistant to high temperatures, internally lined by a sheath of one of the above materi- als in order to prevent contamination of the boron pre- form and massive bodies of Mg due to the elements forming the container. An example of said container is provided in figure 2.

The container must be kept sealed and structurally unaltered during the whole treatment time of step d). An atmosphere of inert gas or, alternatively, an atmosphere with a low oxygen content (less than 20% atomic) must be present inside the container, at a pressure which is such

as to ensure the presence of magnesium in liquid phase during the whole treatment of passage d). The sealing and mechanical integrity of the container can be effected by means of welding and/or by fixing in a suitable machine capable of counter-balancing the internal pressure which is generated during the reaction and capable of prevent- ing contamination with atmospheric oxygen.

Step d) of the method comprises thermal treatment at a temperature higher than 700°C for a time of at least 30 minutes, in the presence of an atmosphere of inert gas, to allow the consequent percolation of the magnesium, prevalently in liquid phase, through the preform of acti- vated crystalline boron powder. Step d) is preferably carried out at temperatures ranging from 800°C to 1000°C for 1-3 hours.

The atmosphere inside the container can also be an atmosphere with a low oxygen content (less than 20% atomic).

In particular, the percolation can be effected by infiltration of the porous preform of activated boron powder, immersed in molten magnesium, maintained under a pressure of inert gas.

The percolation can also be effected in a sealed container, at a temperature which is sufficiently high and a gas pressure which is such as to allow the liquid

magnesium to wet the activated boron powder, constantly in the absence of oxygen or with a minimum oxygen con- tent.

The following detailed description of the method ac- cording to the present invention provides that the pre- form of activated crystalline boron powder, the necessary quantity of metallic Mg, be inserted inside the container - a container which, for the sake of simplicity, can be made of steel suitably protected with the sheath de- scribed above, preventing it from being attacked by the magnesium and boron at high temperatures-remaining trapped in an atmosphere of inert gas or with a low oxy- gen content, at such a pressure as to guarantee the pres- ence of magnesium in liquid phase at the reaction tem- peratures. The metallic Mg, present in such a quantity as to have an atomic ratio Mg/B greater than 0.5, must be arranged so as to allow, one the high temperatures, i. e. over 650°C, have been reached, the percolation of the liquid magnesium through the boron preform.

The crystalline boron used in the present invention has a prevalent crystallinity of the rhombohedral type characterized by the presence of at least two distinct phases for different unit cell parameters: it must be previously mechanically activated, so as not to modify the crystallinity itself and obtain a particle-size which

is such as to be more rapidly and more effectively perme- ated by the liquid magnesium. One way of activating the boron is by grinding, for example in a press, the crys- talline flakes having dimensions of a few millimetres by high load compression crushing under"almost static"con- ditions, said grinding being different from that effected in a rotating ball mill. This latter type of grinding, in fact, not only produces powders with a much finer parti- cle-size (lower than 20 micrometres), but also induces undesired variations in the crystallinity of the starting crystalline boron, said variations being detected by means of X-ray diffraction from powders, as the disap- pearance of the splitting of the diffraction lines, leav- ing the known rhombohedral crystalline boron phase alone (described in database JCPDS, card #11-618) : this phe- nomenon is associated with the disappearance of a larger unit cell phase, present in the starting crystalline B flakes, whose presence can be considered as being favour- able for the permeation of the magnesium.

The preform of activated crystalline boron powders can be prepared with the usual powder compacting tech- niques and must have an appropriate apparent density. The preform can alternatively be produced in the container itself by pouring the activated crystalline boron powder directly inside and compacting it until the desired ap-

parent density is reached.

As specified above, the preform of activated crys- talline boron powder can contain up to 20% atomic of mag- nesium and can consist of activated crystalline boron powders, surface-coated by metallic Mg.

It has been surprisingly found that the use of pre- forms suitably prepared as described above, closed inside a sealed container containing appropriate contents of in- ert gas or with a low oxygen content and maintaining the reagents at temperatures higher than 700°C for at least 30 minutes, allows the reactive transformation of B and Mg forming MgB2 and minority metallic Mg in the whole volume already occupied by the preform. The products are homogeneously distributed, also inside the end-products, with the occasional presence of empty zones having aver- age dimensions of less than 20 micrometres. Neither the presence of metallic magnesium nor the presence of empty zones has a significant influence on the extraordinary superconductor characteristics of the end-products.

By using as reagent, instead of pure liquid Mg, a mixture of this with one or more lower-melting metals, such as for example Ga, Sn, In and Zn, or an equivalent alloy, the latter present in the desired quantity up to the percentage corresponding to the eutectic point of the alloy, it is equally possible to produce highly densified

end-products of MgB2 having analogous superconductor properties to those obtained using pure metallic Mg.

The presence of minority phases, foreign to the crystalline lattice of MgB2 and due to the metals used in the alloy, has proved not to be obstacle for the super- conductivity. The use of these alloys, having melting points lower than that of pure magnesium, by reducing the viscosity of the liquid metal at the typical reaction temperatures, allows the reaction to take place in more rapid times and/or at lower temperatures and is therefore a useful method for reducing the process costs.

The main advantage of the method according to the present invention, as previously observed, consists in that it allows the production, in a simple and economic way, of superconductor solid end-products of MgB2, densi- fied up to values close to the theoretical value, with improved characteristics with respect to the products ob- tained with the known methods in the state of the art.

From an applicative point of view, the use of MgB2, den- sified up to values close to the theoretical value, thus obtained, allows the current which can be conveyed into the superconductor solid end-products, to be increased and also improves their mechanical characteristics.

The following examples are provided for a better un- derstanding of the present invention.

EXAMPLE 1 20 g of activated crystalline boron powder are pre- pared starting from flakes of crystalline boron having dimensions of a few millimetres (purity 99.4%, of commer- cial origin: grade K2 of H. C. STARK, Goslar (D)), grind- ing the flakes by applied high load crushing, i. e. by placing them between two metallic plates situated between the pistons of a press, to which loads of up to 50 tons are repeatedly applied, under"almost static"conditions.

The powders thus ground are sieved with a 100 micrometre mesh sieve. The X-ray diffraction spectrum of the powders thus sieved, still has splitting, on the part of the higher interplanar distances, of the diffraction peaks typical of the crystalline boron phase (rhombohedral cell described in the file JCPDS, card#11-618 corresponding to pseudohexagonal cell sides ao = 1.095 nm, co = 2.384 nm).

The supplementary diffraction peaks, present in the acti- vated powder, have an intensity comparable with those of the rhombohedral phase and can be interpreted as belong- ing to a phase having a cell similar to a rhombohedral cell, corresponding to pseudohexagonal cell sides ao = 1.102 nm, co = 2.400 nm, with a consequent average volume expansion of 1.8%, with respect to the regular rhombohe- dral crystalline boron phase. As an example, the split- ting of the first five reflexes can be observed in the X-

ray diffraction diagram of powders represented in figure 1 (thick line) which also indicates for comparative pur- poses (thin line) the corresponding reflexes of a boron powder obtained from the same starting flakes, but ground with a conventional method, i. e. with a rotating ball mill.

EXAMPLE 2 A cylindrical steel container, schematically illus- trated in figure 2, is lined with a sheet of Nb having a thickness of 100 micrometres (figure 2 wherein 1 indi- cates the steel container and 2 the protective sheaths).

The sheet is wrapped twice around the internal wall and two disks of Nb having the same thickness are placed on the bottom and below the plug of the steel cylinder. Two magnesium cylinders, having a total weight of 15.2 g, a purity of 99% and a diameter which is such as to allow them to be accurately inserted inside the Nb sheath, are subsequently inserted into the container thus lined; 10.7 g of the activated crystalline boron powder of Example 1 are placed between the above two Mg cylinders and com- pacted by gravity, with an apparent density equal to 52% of the theoretical density of rhombohedral crystalline B.

The weights of the reagents are such as to obtain an atomic ratio Mg/B equal to 0.63.

The steel container is placed in a stream of Argon

and then sealed by welding the plug to the electrode. It is then placed in a quartz tube where it is heated, in a stream of Argon, to a temperature of 950°C, for 3 hours.

The gas entrapped in the steel container generates a pressure of about 4 atmospheres at 950°C, sufficient for ensuring the stability of the liquid Mg phase in equilib- rium with MgB2 (see the article of Zi-Kui Liu et al.: Preprint in Condensed-Matter Publ. Nr. 0103335, March 2001).

After cooling, the metallic container is opened and a homogeneously densified cylinder, having a density of 2.4 g/cm3, a diameter of about 17 mm and a height of about 30 mm, is removed from the central part. Analysis by means of X-ray diffraction from powders, represented in figure 3, verifies that said densified cylinder mainly consists of MgB2, with the presence of a minority phase of metallic Mg and other minority peaks, non-identifiable but in any case not attributable to MgO.

A part of the MgB2 cylinder thus obtained is then removed to control its critical temperature by measuring the magnetic susceptibility in alternating current, rep- resented in figure 4, verifying that the superconductive transition has an incipient Tc of 39 K and the broadening of the curve, in the inflection point, is AT = 0.5 K.

A rectangular bar with a section equal to 6.2 mm2

and a length equal to 28 mm, is then cut from the MgB2 cylinder, and resistive measurements of the critical cur- rent are effected in the presence of high magnetic fields at a temperature of 4.2 K.

With the criterion of the critical current measure- ment at the electric field corresponding to 100 micro- volts/m (European regulation EN61788-1 : 1998, the values of Table 1 were obtained: Table 1 Magnetic field (Tesla) Critical stream density (A/mm2) 929.0 1012.0 114.5 122.2 EXAMPLE 3 (Comparative) Following the same procedure described in Example 2, an analogous container is prepared, using the same quan- tity of Mg and 11.58 g of crystalline boron powder, of the same origin as that of Example 1, but not activated according to the procedure described in Example 1. The atomic ratio between the Mg/B reagents is therefore equal to 0.58. The crystalline boron powder was ground conven-

tionally in a rotating ball mill and sieved with a sieve having a mesh of 100 micrometres. The powder, which is much finer, is compacted to an apparent density value equal to 57% of the theoretical density of rhombohedral crystalline boron.

After thermal treatment analogous to that of Example 2, the resulting product is removed from the container, consisting of two densified cylinders of MgB2, having a diameter of 17 mm and a height of about 8 mm, and par- tially reacted boron powder, situated between the two densified cylinders.

EXAMPLE 4 The procedure described in Example 2 is followed, both for the preparation of the container and for the na- ture and method of use of the activated crystalline boron powder. In addition to two cylinders of metallic Mg, two disks of metallic Zn (purity 99%) are also used, in ac- cordance with the following overall quantities: Mg = 5.91 g, Zn = 4.64 g, B = 5.10 g. The following atomic ratios are therefore used: (Zn+Mg)/B = 0.67; Mg/B = 0.52; Zn/Mg = 0. 29.

The activated crystalline boron powder was compacted in the container to an apparent density of 54% of the theoretical value of rhombohedral crystalline boron.

After thermal treatment carried out at 850°C for 2

hours, a homogeneously densified cylinder is removed from the container, having a diameter of 14 mm and a height of 22 mm and a density = 2.57 g/cm3 which, upon X-ray dif- fraction analysis, proves to mainly consist of MgB2, with minority phases containing Zn.

A part of the cylinder of MgB2 thus obtained is then removed to control its critical temperature by measuring the magnetic susceptibility in alternating current, fig- ure 5, verifying that the superconductive transition has an incipient Tc of 38.4 K and the broadening of the curve, in the inflection point, is AT = 1.0 K.