HK1069012B - Method for the preparation of higly densified superconductor massive bodies of mgb2, relevant solid end-products and their use - Google Patents

Description

MgB2Process for preparing high-density superconductor block, end product and use

The technical field is as follows:

the invention relates to the preparation of high-density superconductor massive MgB₂The related solid end products and their uses.

Background art:

recently, it has been found that borides of magnesium have superconducting properties up to 39K and can therefore be used in closed loop cryogenic systems (cryocoolers) which are less costly than systems based on the use of liquid helium (Nagamatsu et al, Nature, 410, 63; 2001).

As with all borides, up to about half a century of boride of the compound magnesium is known, which is characterized by an extremely high hardness when it is at high density.

However, magnesium boride hardens into a final product, which reaches its theoretical density (2.63 g/cm) upon powder compaction of its compound³) Near 100% values, it is generally necessary to use high pressures. Typically pressures of the order of several Gpa are used.

The compound MgB starting from a stoichiometric or non-stoichiometric mixture of boron and magnesium in powder form and in the form of a mass is also known in the literature₂Alternative synthesis methods of (1). In the latter case, however, the use of high pressure is indispensable for obtaining a high density of the final product.

One example is described by Canfieled et al, obtaining MgB from boron fibers that react with liquid or vapor phase Mg₂Fibers (phys. rev. lett.86, 2423(2001)) have an estimated density of about 80% of theoretical.

As a result, only magnesium boride end products having densities up to close to the theoretical value are obtained, and thus high-pressure high-temperature processes are used according to the known art, which are characterized by improved superconductivity and mechanical properties.

However, the use of high pressures at high temperatures limits the dimensions of the final product obtained and necessitates the use of equipment not suitable for mass production.

The invention content is as follows:

it is therefore an object of the present invention to overcome the existing drawbacks of the known art by providing MgB with bulk superconductor bodies having a density close to the theoretical value₂。

According to the invention, there is provided a process for the preparation of MgB having a density close to the theoretical value₂Method for superconductor massive bodies, said density being 2.25 to 2.63g/cm³The method is characterized by comprising the following steps:

a) with the formation of the activated powder, the crystalline boron is mechanically activated;

b) formation of an activated powdered porous ingot of crystalline boron;

c) combining an activated powdered porous ingot of crystalline boron and a metallic magnesium bulk precursor in a vessel and sealing them in an inert gas or low oxygen content atmosphere;

d) boron and magnesium in combination as above are heat treated at a temperature above 700 ℃ for a time greater than 30 minutes and then the liquid phase magnesium is infiltrated by the activated crystalline boron powder.

Another object of the invention relates to MgB having a density close to the theoretical value obtained by means of the process of the invention₂Superconductor massive bodies or solid end products.

Still another aspect of the invention relates to the method comprising In step c) using a mixture of one or more lower melting point metals, such as Ga, Sn, In, Zn, or Mg-based alloys with said metals.

The invention also relates to MgB obtainable using the method of the invention₂Block-shaped bodies as superconductors for current switching, variable inductance elements in current-limiting systems, in levitation systems, in elementary particle accelerators and detectors, in energy-accumulating systems, in linear or non-linear motors, permanent magnets used in generators.

The essential advantage of the process according to the invention lies in the fact that it allows solid MgB to be produced in a simple and economical manner₂The final superconductor product, having a density reaching a value close to the theoretical value, has improved characteristics with respect to the products obtainable by the state of the art processes known. From the application point of view, the values of density thus obtained reach MgB values close to the theoretical ones₂Allowing an increase in the current that can be delivered to the superconductor product and also improving the mechanical properties of the final product.

A further advantage resides in the fact that MgB₂Allows more successful use of deposition techniques, such as laser cutting or radio frequency sputtering, to obtain superconductor material deposited on source substrates in various thin film forms。

Description of the drawings:

FIG. 1 shows an X-ray diffraction pattern of boron powder;

FIG. 2 shows the container and protective liner used in example 2;

FIG. 3 shows an X-ray diffraction pattern of the product of example 2;

FIG. 4 shows a graph of the AC magnetization coefficient of the product of example 2;

figure 5 shows a graph of the AC magnetization coefficient for the product of example 4.

The specific implementation mode is as follows:

in particular for preparing densities close to the theoretical value, i.e. densities higher than or equal to 2.25g/cm³MgB of₂Superconductor massive bodies are produced by reacting boron and magnesium elements in a sealed container in an inert gas or low oxygen content (less than 20% atomic weight) atmosphere at elevated temperature, wherein at least the boron is present in the form of a powder of suitable particle size defined as an activator and having at least two crystalline phases resembling unit cells of rhombohedrons.

Mechanical activation step a) crystalline boron flakes with a size of a few millimeters and a purity of 99.4% or higher, are repeatedly crushed under "almost static" conditions, preferably with high load compression, which can be achieved, for example, in a hydraulic press. This activation not only minimizes the powder fragments to a fine particle size (e.g., below 20 microns), which is typical of a rotary ball milled product, but also allows for obtaining a powder that retains the crystal-type characteristics present in the starting flakes, thus making the powder more permeable to liquid magnesium.

Specifically, the activated crystalline boron powder is selected such that the mean volumetric particle diameter ranges from 30 to 70 microns and has a crystal type equal to the starting crystalline boron flake type and is virtually free of oxygen contamination. Step b) comprises forming a porous ingot of activated crystalline boron powder.

The shape of the porous ingot of activated crystalline boron powder is similar to the shape of the final product and must have a theoretical density higher than that of crystalline boron (2.35 g/cm)³) Apparent density of 50%.

The ingot of activated crystalline boron powder may additionally contain up to 20% atomic weight of magnesium. In this case, the ingot is generally composed of activated crystalline boron powder and magnesium powder virtually free of oxygen contamination and having a particle size lower than that of boron. The ingot can also consist of activated crystalline boron powder, the metallic Mg surfaces of which are covered and welded to each other by a heat treatment in an inert atmosphere, so as to maintain the porosity of the ingot while providing mechanical consistency to its treatment.

The ingot comprising magnesium must also meet the apparent density requirements defined above.

The subsequent step c) comprises combining the ingredients subjected to the heat treatment and converting in step d) into the final product. Containers for combining these ingredients are also important.

Step c) comprises placing the combination of the two components in a suitable container: the first component is an ingot produced from the above activated crystalline boron powder, having a purity of at least 99.4% or higher, a shape similar to the final product, and an apparent density higher than the theoretical density of rhombohedral crystalline boron (2.35 g/cm)³) Preferably, D is 50% and ranges from 51% to 56%. The second component consists of one or more metallic Mg blocks with a purity higher than 99%, which are saturated by the activated crystalline boron powder after melting in step d), with a purity higher than 99%.

The magnesium in the liquid phase is preferably derived from the melting of a bulk precursor of metallic Mg. There is virtually no contamination by oxygen.

The ratio between Mg and B depends mainly on the technique chosen for carrying out the reaction. In any case, they are remote from MgB₂Stoichiometric values of the compounds. Specifically, Mg is excessive so that the atomic weight ratio Mg/B is largeAt 0.5, the ratio is preferably greater than or equal to 0.55.

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

Atomic weight ratio values Mg/B, or (metal + Mg)/B, lower than the above specified limits, cause reactions that produce local densification of the product, which will reduce or completely eliminate the superconductivity with respect to the transmission of electric current.

The vessel in which step c) is carried out is made of a material that must not be attacked by boron and magnesium at temperatures up to 1000 c, such as Nb, Ta, MgO, Bn, etc. or any material resistant to high temperatures, internally lined by a jacket of one of the above materials. To prevent contamination of the boron ingot and the bulk of Mg by the elements forming the container. An example of such a container is provided in fig. 2.

The container must remain sealed and structurally unchanged during the entire processing time of step d). It is necessary to present inside the container an atmosphere of inert gas, or alternatively an atmosphere with a low oxygen content (less than 20% atomic weight), at a pressure such as to present magnesium in liquid phase during the whole treatment of stage d). The sealing and mechanical integrity of the vessel can be achieved by means of welding and/or fixing in suitable machines capable of balancing the internal pressure generated during the reaction and of preventing contamination by atmospheric oxygen.

Step d) of the process comprises a heat treatment at a temperature above 700 ℃ in the presence of an inert gas for at least 30 minutes to allow subsequent saturation of the magnesium, typically in the liquid phase, by activating the ingot of crystalline boron powder. Step d) is preferably carried out at a temperature in the range of 800 ℃ to 1000 ℃ for 1 to 3 hours.

The atmosphere within the container may also be a low oxygen content atmosphere (less than 20% atomic weight).

In particular, the impregnation may be achieved by infiltration by immersing a porous ingot of activated boron powder in molten magnesium maintained under inert gas pressure.

The impregnation can also be carried out in a sealed container, typically in the absence of oxygen or minimal oxygen content, at a sufficiently high temperature and pressure to allow the liquid magnesium to wet the activated boron powder.

According to the method of the invention, described in detail below, there is provided the activation of an ingot impregnated with boron powder, the quantity of metallic Mg necessary to be placed in a container-which, for the sake of simplicity, can be made of steel suitably protected by the above-mentioned lining which prevents its attack by magnesium and boron at high temperature-the remainder being collected in an inert gas or atmosphere with a low oxygen content at a pressure which ensures that the magnesium is present in the liquid phase at the reaction temperature. Magnesium metal Mg is present in amounts having an atomic weight ratio Mg/B greater than 0.5, and must be arranged so as to allow the penetration of liquid magnesium through the porous ingot once high temperatures in excess of 650 ℃ are reached.

The crystalline boron used in the present invention has a generally rhombohedral character characterized by the presence of at least two distinct phases for different unit cell parameters: it must be mechanically activated beforehand so as not to change the crystallinity itself and to obtain a more rapid and more effective saturation of the particle size. One method of activating boron is, for example, rolling of a few mm-sized impregnated sheet in a press by high-load pressing in an "almost static" state, which is different from the crushing achieved in a rotary ball mill. In fact this latter type of rolling not only produces much finer-grained powders (below 20 microns), but also causes undesirable changes in the crystallinity of the starting crystalline boron, detected by means of X-ray diffraction from the powder, when the diffraction line separation disappears, known to leave a known rhombohedral crystalline boron phase (described in database JCPDS, card # 11-618): this phenomenon is associated with the disappearance of the larger cells present in the starting crystalline B flakes, which are believed to favour the saturation of magnesium.

Ingots of activated crystalline boron powder can be prepared using conventional powder compaction techniques and must have a suitable apparent density. Alternatively, the ingot may be produced in the vessel itself by pouring activated crystalline boron powder directly inside and compacting it until the desired apparent density is reached.

As noted above, the ingot of activated crystalline boron powder may contain up to 20 atomic weight percent magnesium and may be composed of activated magnesium metal coated crystalline boron powder.

It has been surprisingly found that the use of an ingot suitably prepared as above, enclosed in a sealed container containing a suitable inert gas content or having a low oxygen content, and maintaining the reactants at a temperature above 700 ℃ for at least 30 minutes, allows the formation of MgB in the entire volume already occupied by the ingot₂B and Mg and small amounts of metallic Mg. The product was uniformly distributed and, in the final product, there were occasional empty regions with an average size of less than 20 microns. Whether the presence of metallic magnesium or the presence of empty regions has no significant effect on the abnormal superconducting properties of the final product.

Instead of pure liquid magnesium, the use of a mixture of this metal and one or more lower melting metals such as Ga, Sn, In, and Zn, or equivalent alloys, the latter being present In the required amount up to a percentage corresponding to the eutectic point of the equivalent alloy, enables equally high density MgB to be produced₂The final product, having superconductor properties similar to those obtained with pure metallic Mg.

At MgB₂The presence of a few phases, outside the crystal lattice and due to the metals used in the alloy, has proved not to hinder superconductivity. The use of these alloys, which have melting points lower than pure magnesium, allows the reaction to take place in a faster time and/or at lower temperatures by reducing the viscosity of the liquid metal at typical reaction temperatures, and is thus a useful method for reducing the cost of the process.

According to the previous observations, the main advantage of the process according to the invention is that it allows MgB to be produced in a simple and economical manner₂The superconducting solid end product, which has a density value which is at most close to the theoretical value, has improved properties with respect to the products obtained according to the methods known from the state of the art. From the applicable point of view, MgB is used, the density value of which is close to the theoretical value, thus obtained₂Allowing the reversible superconductor to be solid in the end productThe current delivered increases and also improves their mechanical properties.

The following examples are provided for a better understanding of the present invention.

Example 1

20g of activated crystalline boron powder (purity 99.4%, commercial source: grade K2 of H.C. STARK, Goslar (D)) are prepared starting from crystalline boron flakes of a size of a few millimetres, which are crushed by applying a high load, i.e. by placing them between two metal plates located between the pistons of a press, to which a load of up to 50 tonnes is repeatedly applied under "almost static" conditions. The powder thus crushed was sieved through a 100 micron mesh sieve. The X-ray diffraction spectrum of the powder thus screened, in the higher interplanar distance fraction, still has a separation of the diffraction peaks typical of the crystalline boron phase (rhombohedral cell described in the document JCPBS, corresponding to the cell edge a of a quasi-hexagonal crystal)₀＝1.095nm，c₀2.384nm card # 11-618). The appurtenant diffraction peaks present in the activated powder, having a density comparable to that of the rhombohedral phase, can be interpreted as belonging to the group having edges a similar to those of the unit cell corresponding to the quasi-hexagonal crystal₀＝1.102nm，c₀The unit cell phase for a 2.400nm diamond unit cell with a 1.8% sequential average volume expansion for the regular diamond crystalline boron phase. As an example, the separation of the first five reflections (bold lines) can be observed in the powder X-ray diffraction pattern shown in FIG. 1, which for simplicity also indicates the reflections corresponding to boron powder obtained from the same starting flake but with conventional repetition, i.e., using rotary ball milling.

Example 2

Figure 2 schematically shows a cylindrical steel vessel lined with Nb sheets of 100 micron thickness (figure 2, where 1 indicates the steel vessel and 2 indicates the protective lining). The leaf was wrapped twice around the inner wall and two Nb discs of the same thickness were placed under the bottom of the steel cylinder and the plug. Two magnesium cylinders, 15.2g in total, with a purity of 99% and a diameter allowing their precise insertion into an Nb liner, are inserted one after the other into a container thus lined; 10.7g of activated crystalline boron powder of example 1 was placed between the above two Mg cylinders and compacted by gravity with an apparent density equal to 52% of the theoretical density of rhombohedral crystals B.

The weight of the reactants is such as to obtain an atomic weight ratio Mg/B equal to 0.63.

The steel vessel was placed in an argon stream and then sealed by welding a plug to the electrode. It was then placed in a quartz tube where it was heated to 950 ℃ for 3 hours in a stream of argon. The gas trapped in the steel vessel creates a pressure of about 4 atmospheres at 950 ℃ sufficient to ensure a liquid Mg phase and MgB phase₂Stability of the equilibrium (see article: Preprint in Condensed-Matter Publ. Nr.0103335, March 2001, Zi-Kui et al).

After cooling, the metal container was opened and a uniformly densified cylinder having a density of 2.4g/cm, a diameter of about 17mm and a height of about 30mm was taken from the central part. By means of the analysis shown in FIG. 3 from powder X-ray diffraction, it was verified that the densified cylinders consist mainly of MgB₂Constituted, there are small amounts of metallic Mg phases and other small amounts of peaks that are not identifiable but in any case not attributable to MgO.

Then the MgB thus obtained is treated₂A portion of the cylinder was taken out in order to control its critical temperature by measuring its magnetic susceptibility in an alternating current as shown in fig. 4, verifying that the superconducting transition had an initial Tc of 39K and that the curve extended by Δ T at the turning point to 0.5K.

Then from the MgB₂The section of the cylinder is equal to 6.2mm²Rectangular bar with length equal to 28mm, resistance measurement of critical current is achieved at 4.2K temperature in the presence of high magnetic field.

With the measurement standards at the critical current corresponding to an electric field of 100 microvolts/m (European regulation EN 61788-1: 1998), the values of Table 1 were obtained:

TABLE 1

Magnetic field (Tesla)	Critical flow density (A/mm))
		9	29.0
10	12.0
		11	4.5
12	2.2

Example 3(comparison)

A similar container was prepared following the same procedure described in example 2, using the same amount of Mg and 11.58g of crystalline boron powder from the same source as in example 1, but not activated according to the procedure described in example 1. The atomic weight ratio between the Mg/B reactants is thus equal to 0.58. The crystalline boron powder was milled in a bushing ball mill and sieved through a 100 micron mesh screen. The much finer powder was compacted to an apparent density value equal to 57% of the theoretical crystalline boron density value.

After a heat treatment similar to example 2, the product obtained was taken out of the container, the product consisting of two densified MgBs₂A cylinder of diameter 17mm and height of about 8mm, and a cylindrical body located between the two densification cylindersPartially reacted boron powder.

Example 4

For the preparation of the container and the properties and method of using the activated crystalline boron powder, the procedure described in example 2 was followed. In addition to the two cylinders of metallic Mg, two disks of metallic Zn were used (purity 99%) in the following total amounts: mg 5.91g, Zn 4.64g, B5.10 g. The following atomic weight ratios were then used: (Zn + Mg)/B ═ 0.67; Mg/B is 0.52; Zn/Mg is 0.29.

The crystalline boron powder activated in the vessel was compacted to an apparent density of 54% of the theoretical value of diamond-shaped crystalline boron.

After a heat treatment at 850 ℃ for 2 hours, a uniform, densified cylinder having a diameter of 14mm, a height of 22mm and a density of 2.57g/cm was taken out of the vessel³Proved to be composed mainly of MgB upon X-ray diffraction analysis₂Composition, with a small amount of Zn-containing phase.

The MgB thus obtained is then taken out₂Fig. 5 verifies that the superconducting transition has an initial Tc of 38.4K and that the curve expands at the turning point by Δ T of 1.0K.

Claims (27)

1. Used for preparing MgB with density close to theoretical value₂Method for superconductor massive bodies, said density being 2.25 to 2.63g/cm³The method is characterized by comprising the following steps:

a) with the formation of the activated powder, the crystalline boron is mechanically activated;

b) formation of an activated powdered porous ingot of crystalline boron;

c) combining an activated powdered porous ingot of crystalline boron and a metallic magnesium bulk precursor in a vessel and sealing them in an inert gas or low oxygen content atmosphere;

d) boron and magnesium in combination as above are heat treated at a temperature above 700 ℃ for a time greater than 30 minutes and then the liquid phase magnesium is infiltrated by the activated crystalline boron powder.

2. The method according to claim 1, characterized in that said mechanical activation step a) of the crystalline boron consists in repeatedly crushing and rolling the crystalline boron flakes by high load compression.

3. The method of claim 1, wherein the activated crystalline boron powder has a mean volumetric particle diameter ranging from 30 to 70 microns and a crystal type equal to that of the starting crystalline boron flakes.

4. The method of claim 1, wherein the activated crystalline boron powder ingot is produced using conventional powder compaction techniques.

5. A process according to claim 1, wherein the ingot of activated crystalline boron powder is prepared in the vessel itself by pouring activated crystalline boron powder directly therein and compacting it.

6. The method of claim 1, wherein the activated crystalline boron powder ingot has an apparent density 2.35g/cm higher than the theoretical density value of crystalline boron³50% of the total.

7. The method of claim 1, wherein the activated crystalline boron powder ingot has a purity greater than or equal to 99.4%.

8. The method of claim 1, wherein the activated crystalline boron powder ingot comprises up to 20 atomic percent magnesium in the form of magnesium powder having a particle size below that of the boron powder.

9. The method of claim 1, wherein the ingot of activated crystalline boron powder is comprised of activated crystalline boron powder having a surface metallized with magnesium.

10. The method of claim 1, wherein step c) of combining the porous boron ingot and the bulk precursor of magnesium metal in the vessel is achieved with a bulk precursor of magnesium metal having a purity of greater than 99%.

11. The process according to claim 1, characterized in that there is an excess of Mg in step c) such that the atomic weight ratio Mg/B is greater than 0.5.

12. The process according to claim 1, characterized in that the atomic weight ratio Mg/B is higher than or equal to 0.55.

13. The method according to claim 1, wherein the container used in step c) is made of a material that is not attacked by boron and magnesium at temperatures up to 1000 ℃.

14. The method of claim 13, wherein the material is Nb, Ta, MgO, BN.

15. The method according to claim 1, characterized in that the container used in step c) is made of any material resistant to high temperatures, the inside being lined with a lining made of a material that is not attacked by boron and magnesium at temperatures up to 1000 ℃.

16. The method according to claim 1, wherein step d) comprises a heat treatment at a temperature ranging from 800 ℃ to 1000 ℃ for 1-3 hours.

17. The process according to claim 1, wherein the infiltration of step d) is achieved by infiltration by immersing the porous ingot of activated crystalline boron powder in molten magnesium maintained under inert gas pressure.

18. The process according to claim 1, characterized in that the bulk precursor of metallic Mg in step c) consists of bulk bodies of magnesium and one or more low melting metals or equivalent alloys.

19. The method according to claim 18, characterized in that the low-melting metal is present in such an amount as to achieve a percentage of eutectic point corresponding to the equivalent alloy.

20. The method of claim 18, wherein the atomic weight ratio of low melting point metal + magnesium/boron is greater than 0.55 and the atomic weight ratio of magnesium/boron is greater than 0.5.

21. The method according to claim 18, wherein the low melting point metal is selected from Ga, Sn, In and Zn.

22. MgB having a density close to the theoretical value obtained by the process according to claim 1₂The superconducting block of (1).

23. Use of the MgB of claim 22₂As a target for thin film vacuum deposition techniques such as laser cutting and radio frequency sputtering.

24. Use of the MgB of claim 22₂The superconducting bulk is used as a variable inductance element in a current-switching and current-limiting system, a permanent magnet in a suspension system, a medical magnetic resonance system, a basic particle accelerator and detector, an energy accumulation system, a linear or non-linear motor and a generator.

25. MgB having a density close to the theoretical value obtained by the process according to claim 1₂The superconducting solid end product of (1).

26. Use of the MgB of claim 25₂The superconducting solid final product is used as a target of a thin film vacuum deposition technology of laser cutting and radio frequency sputtering.

27. Use of the MgB of claim 25₂The superconducting solid end product is used as a variable inductance element in a current-switching and current-limiting system, a suspension system, a medical magnetic resonance system, a basic particle accelerator and detector, an energy accumulation system, a linear or non-linear motor and a permanent magnet in a generator.