Magnesium diboride-based composite and composition and method for its production

Abstract

MgB2-based composite material comprising the following additives: carbon nanotubes, graphene, silicon carbide, magnesium oxide, and optionally a compatibilizing agent. There is also described a production method that leads to obtaining sintered spherical particles of the composite, which exhibits synergistically enhanced superconducting performance, mechanical robustness, thermal stability and resistance to oxidation over a wide temperature range.

Description

[0001] MAGNESIUM DIBORIDE-BASED COMPOSITE AND COMPOSITION AND METHOD FOR ITS PRODUCTION DESCRIPTION

[0002] The present invention relates to a magnesium diboride-based composite, a composition for its production and to a method for producing said composite by using said composition.

[0003] BACKGROUND OF THE INVENTION

[0004] The superconductivity of magnesium diboride (MgB2), which was discovered in 2001, falls within the scope of research into the development of superconducting materials capable of operating at critical high temperature, called High Temperature Superconductors (HTSs), i.e., of operating at temperatures that do not require the use of liquid helium as coolant. The possibility of operating with a system that is simpler and more manageable, as well as much less expensive, can guarantee a greater diffusion and applicability of superconducting materials, significantly increasing the performance of electrotechnical devices.

[0005] Although other HTSs have been known for some time, MgB2 exhibits numerous advantages, such as:

[0006] i. the absence of barriers during the passage of currents between the grain boundaries, making it unnecessary to synthesize a perfect single crystal by means of complex and costly preparation methods, as occurs for Yttrium barium copper oxide (YBCO)-based superconductors;

[0007] ii. the relative abundance and low cost of the raw materials;

[0008] iii. easy production of the material, combined with very low density and excellent mechanical properties.

[0009] In particular, MgB2 is a type-II superconductor: below the transition temperature between normal state and superconducting state (Tc=39 K), as well as the disappearance of electrical resistance, a particular magnetic behaviour is observed. In fact, by applying an external magnetic field, MgB2 exhibits perfect diamagnetism below a critical field value (Hci) and, for magnetic fields (H) in the range HCI<H<HC2, the magnetic field is penetrated through quantized flux lines, while, for fields higher than Hc2, a total loss of superconductivity occurs. In the intermediate field regime, HCI<H<HC2, the flux lines are subject to Lorentz forces, which cause lateral displacements, dissipating energy and hindering complete formation of the superconducting state. In this context, doping of the HTS materials becomes necessary, by introducing pinning centres capable of immobilizing the flux lines, in this way preventing energy dissipation during the passage of supercurrents.

[0010] BACKGROUND ARTThe scientific and patent literature describes examples of doping MgB2 with other composites or elements.

[0011] WO 03 / 005460 A2 mentions doping MgB2 with alkaline or alkaline earth metals.

[0012] CN 113354417 A describes a method for the production of MgB2 doped with graphene.

[0013] Xun Xu, Wenxian Li, Xiaolin Wang and Shi-Xue Dou, “Superconducting Properties of Graphene Doped Magnesium Diboride, Applications of High-Tc Superconductivity”, Edited by Dr. Adir Luiz; ISBN 978-953-307-308-8, 27 June 2011, www.intechopen.com, describe a form of polycrystalline MgB2 doped with graphene.

[0014] US 2002 / 173428 Al discloses a method of making a doped MgB2 superconductor by mixing Mg and B powders with a doping agent by high energy milling. It does not disclose using graphene.

[0015] De Silva K.S.B et al; “A significant improvement in the superconducting properties of MgB2 by co-doping with graphene and nano SiC”; Scripta Materialia; vol. 7, No. 10; 16 July 2012, pp. 802-805, disclose the improvement in the superconducting properties of MgB2 by co-doping with graphene and nano-SiC. It does not disclose using MgO doping.

[0016] Muralidhar M. et al; “Review on high-performance bulk MgB2 superconductors”; Journal of Physics D: Applied Physics; November 2023, disclose bulk MgB2 superconductors.

[0017] Kovac P et al; “Transport current improvements of in situ MgB2 tapes by the addition of carbon nanotubes, silicon carbide or graphite”; Superconductor Science and Technology, vol. 20, No.1, January 2007, pp. 105-111, disclose MgB2 tapes. The do not mention MgO nor graphene. US 2008 / 020137 Al discloses a method to make a superconducting doped MgB2. It does not disclose MgO nor CNT.

[0018] CN 113368517 A discloses the production of a silicon boride powder. It does not disclose MgB2.

[0019] It would be desirable to improve the characteristics and performance of these superconducting materials with regard to current density, stability and mechanical properties, and thermal properties, to make them more suitable for applications in medical devices, in long-range power transmission devices, in superconducting magnets and in advanced electronic devices.

[0020] It would also be desirable to have a composite material that, regardless of the superconducting properties at cryogenic temperatures, exhibits good electrical and thermal conductivity also at non cryogenic temperatures, for example at room temperature or above, and which is in any case characterized by high mechanical strength and resistance to corrosion.

[0021] Finally, it would be desirable to have a method that allows an MgB2-based composite material to be produced in an effective and efficient manner.SUMMARY OF THE INVENTION

[0022] An aspect of the invention consists of a composition for the production of an MgfU-based composite material comprising:

[0023] a) 62 to 71% by weight of magnesium diboride (Mgfh);

[0024] b) 2 to 3% by weight of carbon nanotubes;

[0025] c) 3 to 4% by weight of graphene

[0026] d) 6 to 8% by weight of silicon carbide;

[0027] e) 2 to 3% by weight of magnesium oxide (MgO);

[0028] f) 9 to 12% by weight of a binding agent selected from polyvinylpyrrolidone and isopropanol;

[0029] g) 7 to 8% by weight of a compatibilizing agent selected from the group consisting of zinc oxide, titanium carbide, mixture of zinc oxide and titanium carbide, epoxy resins, functionalized silanes, mixtures of epoxy resins and functionalized silanes.

[0030] Such composition is also defined as the precursor composition of the desired MgfU-based composite.

[0031] Another aspect of the invention consists of a method for the preparation of an MgfU-based composite material using the composition reported above, comprising the steps of:

[0032] A) mixing magnesium and boron powders in the atomic ratio 1 :2 at room temperature and heating the mixture to a temperature between 650°C and 750°C in an inert gas atmosphere and obtaining the formation of MgEE;

[0033] B) cooling the magnesium diboride obtained in step A) in an inert gas atmosphere to obtain MgB2 in the form of a crystalline powder (component a));

[0034] C) mixing the components a) - g) of the composition in the quantities defined in the composition reported above and obtaining a composite consisting of doped MgB2, in the form of a powder;

[0035] D) melting the composite by heating it to a temperature sufficient to allow atomization, preferably up to 1000°C, and atomizing it in an inert gas environment to obtain a powder with a spherical morphology;

[0036] E) cooling the spherical powder obtained in step D) in an inert gas atmosphere to room temperature.

[0037] The material obtained with the aforesaid method is a MgB2-based composite in the form of sintered spherical powder, which can be used to produce the desired articles by means of further processing steps.When the compatibilizing agent of the precursor composition is entirely made of an organic substance which is released during the method of production of the composite, the MgB2-based composite comprises:

[0038] a) 77.5 to 84.5% by weight of magnesium diboride (MgB2);

[0039] b) 2.4 to 3.75% by weight of carbon nanotubes;

[0040] c) 3.6 to 5.0% by weight of graphene

[0041] d) 7.1 to 10.0% by weight of silicon carbide;

[0042] e) 2.4 to 3.75% by weight of magnesium oxide (MgO).

[0043] The amount of components a) - e) in the MgB2-based composite is increased compared to the amount of the same components in the precursor composition due to the absence of the organic binding agent, which is released or destroyed during the method of production of the composite. The MgB2-based composite can comprise an amount of zinc oxide and / or titanium carbide or a mixture of zinc oxide and titanium carbide, if the precursor composition comprises such inorganic compounds as compatibilizing agent, as this type of compatibilizing agent is not released or destroyed during the production method. Since the amount of compatibilizing agent in the precursor composition is from 7 to 8% of the total precursor composition and the binding agent is released during the production method, the amount of the inorganic compatibilizing agent in the MgB2-based composite is increased due to the absence of the binding agent. The increased amount of the inorganic compatibilizing agent in the MgB2-based composite is of no more than 9% of the total composition and the amount of components a) - e) is proportionally decreased of no more than 2% each.

[0044] DETAILED DESCRIPTION OF THE INVENTION

[0045] In the present description, the terms “comprising” and “containing”, whose meaning does not exclude the presence of other elements, in addition to those defined after these terms, are used interchangeably. Therefore, the scope of this meaning also comprises the term “consisting of’. The terms “comprising” and “containing” have a broader meaning than “consisting of’, but do not exclude it.

[0046] In the present description, the terms “magnesium diboride” and the formula “MgB2” are used interchangeably as they relate to the same compound.

[0047] In the present description, the term “composite” and “composite material “are used interchangeably.

[0048] Magnesium diboride is a superconducting material that finds application in various technological fields, mainly due to its electrical conductivity properties at relatively high temperatures with respect to other superconductors.The main applications of MgB2 are the following:

[0049] - Medical sector: MgB2 is used in magnetic resonance (MRI) scanners thanks to its ability to generate stable and powerful magnetic fields at less extreme cryogenic temperatures with respect to conventional superconductors. This reduces operating costs related to cooling. - Energy sector: in long-range power transmission projects without losses, as MgB2 allows current to be transported with practically zero resistance when it is superconductive, improving the efficiency of energy systems.

[0050] Superconducting magnets: employed in scientific experiments and industrial applications that require strong magnetic fields (e.g., particle accelerators).

[0051] Advanced electronic devices: the superconductivity of MgB2 can be exploited to create high speed circuits in environments that require high precision and reliability.

[0052] Being a type-II superconductor, magnesium diboride remains superconductor up to temperatures of around 39 K (-234 °C), which make it more advantageous than other superconducting materials, which require even lower temperatures. However, some characteristics of magnesium diboride, such as current density at 20 K between 105- 106A / cm2, reduced thermal stability above sopra 40 K, brittleness and oxidation sensitivity, limit its use, and not even the addition of graphene improves those characteristics in a completely satisfactory way.

[0053] It has now surprisingly been found that many of the properties of magnesium diboride, such as critical current density, stability and thermal conductivity, mechanical strength and resistance to corrosion, can be improved by doping with a specific group of additives that make it possible to obtain a composite that is particularly effective, both as superconductor and as material for other applications.

[0054] Therefore, an aspect of the invention consists of a composition for the production of an MgB2-based composite material comprising:

[0055] a) 62 to 71% by weight of magnesium diboride (MgB2);

[0056] b) 2 to 3% by weight of carbon nanotubes;

[0057] c) 3 to 4% by weight of graphene;

[0058] d) 6 to 8% by weight of silicon carbide (SiC);

[0059] e) 2 to 3% by weight of magnesium oxide (MgO);

[0060] f) 9 to 12% by weight of a binding agent selected from polyvinylpyrrolidone and isopropanol;g) 7 to 8% by weight of a compatibilizing agent selected from the group consisting of zinc oxide, titanium carbide, mixture of zinc oxide and titanium carbide, epoxy resins, functionalized silanes, mixtures of epoxy resins and functionalized silanes.

[0061] Preferably, magnesium diboride is present in the composition in the amount from 64 to 70% by weight, more preferably from 66 to 69% by weight.

[0062] Preferably, carbon nanotubes (“CNTs”) are present in the composition in the amount from 2.2 to 2.8% by weight, more preferably from 2.4 to 2.6% by weight.

[0063] Preferably, graphene is present in the composition in the amount from 3.2 to 3.8% by weight, more preferably from 3.4 to 3.6% by weight.

[0064] Preferably, silicon carbide is present in the composition in the amount from 6.5 to 7.5% by weight, more preferably from 6.8 to 7.2% by weight.

[0065] Preferably, magnesium oxide is present in the composition in the amount from 2.2 to 2.8% by weight, more preferably from 2.4 to 2.6% by weight.

[0066] Preferably, the binding agent is present in the composition in the amount from 9.5 to 11.5% by weight, more preferably from 10 to 11% by weight.

[0067] Preferably, the compatibilizing agent is present in the composition in the amount from 7.2 to 7.8% by weight, more preferably from 7.4 to 7.6% by weight.

[0068] Binding agent

[0069] A binding agent is selected from polyvinylpyrrolidone and isopropanol and has adhesive and cohesive properties which, mixed with the other components, contributes to form a composite with high mechanical strength. Being an organic compound, the binding agent is released or destroyed during the method to produce the composite material, due to the high temperatures reached in the method, particularly in step A) and D).

[0070] Compatibilizing agent

[0071] A compatibilizing agent is a substance that makes different and unrelated materials "compatible", i.e., it makes them miscible with one another, contributing to increase adhesion between the surfaces of the various components and thereby allowing the formation of a stable composite.

[0072] The compatibilizing agent is selected in the group consisting of zinc oxide, titanium carbide, mixture of zinc oxide and titanium carbide, epoxy resins, functionalized silanes, mixtures of epoxy resins and functionalized silanes. When the compatibilizing agent is and epoxy resins or a functionalized silane, it is released during the method to produce the composite material, due to the high temperatures reached in the method, particularly in step A) and D). According tothe invention, the compatibilizing agent is either of organic nature or of inorganic nature, i.e., the organic and inorganic compatibilizing agents are not mixed.

[0073] The above composition is a precursor of the desired composite material and is converted into the composite material by means of the method according to the present invention.

[0074] The MgB2-based composite obtained by treating the precursor composition by means of the inventive method comprises:

[0075] a) 77.5 to 84.5% by weight of magnesium diboride (MgB2);

[0076] b) 2.4 to 3.75% by weight of carbon nanotubes;

[0077] c) 3.6 to 5.0% by weight of graphene

[0078] d) 7.1 to 10.0% by weight of silicon carbide;

[0079] e) 2.4 to 3.75% by weight of magnesium oxide (MgO).

[0080] The composite optionally comprises also the compatibilizing agent when it consists of zinc oxide, titanium carbide or mixture thereof, as they are not released or destroyed at the temperature of the production method.

[0081] As mentioned above, the MgB2-based composite can optionally comprise an amount of zinc oxide and / or titanium carbide if the precursor composition comprises such inorganic compounds as compatibilizing agent. Since the amount of compatibilizing agent in the precursor composition is from 7 to 8% of the total precursor composition and the binding agent is released during the production method, the amount of the inorganic compatibilizing agent in the MgB2-based composite is proportionally increased due to the absence of the binding agent. The increased amount of the inorganic compatibilizing agent in the MgB2-based composite is of no more than 9% of the total composition and the amount of components a) - e) is proportionally decreased of no more than 2% each.

[0082] The MgB2-based composite preferably comprises said magnesium diboride in the amount from 78.5 to 83.5% by weight, more preferably from 79 to 83% by weight.

[0083] The MgB2-based composite preferably comprises said carbon nanotubes in the amount from 2.6 to 3.6% by weight, more preferably from 2.8 to 3.4% by weight.

[0084] The MgB2-based composite preferably comprises said graphene in the amount from 3.8 to 4.8% by weight, more preferably from 4.0 to 4.6% by weight.

[0085] The MgB2-based composite preferably comprises said silicon carbide in the amount from 7.4 to 9.6% by weight, more preferably from 7.6 to 9.4% by weight.

[0086] The MgB2-based composite preferably comprises said magnesium oxide in the amount from 2.6 to 3.6% by weight, more preferably from 2.8 to 3.4% by weight.In the final composite material, the MgB2-based composite has a hexagonal crystalline structure in which the magnesium atoms (Mg) are arranged in alternating layers with boron atoms (B), creating a lattice.

[0087] The additives CNT, graphene, SiC, MgO are generally dispersed at nanometric level or placed on the layers of MgB2. The additives do not bond chemically with MgB2 but integrate into the structure to improve its properties. The resulting structure of composite material is in fact able to carry higher currents, withstand unfavourable mechanical and thermal conditions and maintain superconducting properties at higher temperatures. Moreover, pinning centres of the lines of the magnetic field are created in the structure, thus raising the critical current density (Jc).

[0088] The main characteristics of each component of the composite according to the invention are described below.

[0089] a) Magnesium diboride

[0090]

[0091] High purity MgB2, preferably > 99%, with a particle size from 1 to 5 pm, with a hexagonal crystalline structure, necessary for superconductivity, is used.

[0092] The hexagonal structure of MgB?, typically of the group AIB2, allows a superconducting transition. The critical temperature Tc of MgB? (around 39 K) is a property derived from the bond structure B-B and Mg-B. The purity > 99% minimizes the impurities that would cause a degradation of superconductivity, increasing the critical current density Jc.

[0093] b) Carbon nanotubes (CNTs)

[0094] High purity CNTs, preferably > 99%, with a diameter from 10 to 30 mm and a length from 1 to 5 pm, slightly oxygenated, i.e. with an oxygen content from 0.2 to 0.5% by weight, are used. Multi -walled carbon nanotubes (MWCNTs) offer a higher mechanical strength and stability than single-walled nanotubes. The chemical bond between the oxygen and the carbon improves dispersion, reducing the tendency to cluster. The tensile strength of MWCNTs is of around 60 GPa, contributing to a total strength of the composite of over 800 MPa. Oxygen content is determined by elemental analysis CHNS(O).

[0095] c) Graphene

[0096] High purity graphene, preferably > 99%, with a particle size from 0.5 to 2 pm, slightly oxygenated, i.e., with an oxygen content from 0.2 to 0.5% by weight, determined by elemental analysis CHNS(O), is used.

[0097] Graphene consisting of a few layers, preferably from 2 to 10, offers a combination of optimal mechanical stability and conductivity. Oxygen functionalization improves interaction with the MgB? matrix without compromising the conductive properties. The thermal conductivity ofpure graphene (> 5000 W / m K) is reduced to around 2000 W / m K with oxygenation, while still remaining higher than most conductors.

[0098] d) Silicon carbide (SiC)

[0099] High purity silicon carbide, preferably > 99%, with a particle size from 1 to 2 pm, spherical in shape, homogeneously dispersed, is used. The particle size is measured by laser diffraction method.

[0100] SiC, which has a modulus of elasticity of around 424 GPa, improves the fracture toughness and thermal stability of the composite material. Particles of 1-2 pm ensure high dispersion without negatively influencing the superconducting properties. In a superconducting environment, SiC acts as pinning centre, stabilizing magnetic vortices and improving the critical current density Jc in high fields. The particle size is measured by laser diffraction method.

[0101] e) Magnesium oxide

[0102] High purity MgO, preferably > 99%, with a particle size from 1 to 5 pm, homogeneously dispersed, is used.

[0103] MgO offers a barrier against the unwanted diffusion of oxygen and other elements, maintaining the purity of the MgB?. The chemical stability of MgO contributes to the resistance of the material to corrosion and to the high temperature reactive environments. Moreover, thanks to a density of 3.58 g / cm3, the particles of MgO improve the thermal inertia of the composite. The density can be measured with a pycnometer.

[0104] Magnesium oxide does not act only as a filler but plays a multifunctional role within the composite. In particular, MgO contributes simultaneously to: (i) limiting oxygen diffusion within the MgB? matrix, (ii) increasing the thermal inertia of the composite, and (iii) enhancing long-term corrosion resistance. Such a combination of effects cannot be achieved by carbonbased additives or silicon carbide alone and results in a synergistic stabilization of both the superconducting and non-superconducting properties of the composite.

[0105] If the superconducting composite according to the invention is not cooled to below its critical temperature, it does not reach the superconducting state and behaves similarly to a normal conductive material, maintaining the characteristics of its base components. The main characteristics of the composite at room temperature or in non-cryogenic conditions are listed below.

[0106] Electrical Conduction (Non-Superconducting)

[0107] At temperatures exceeding 50 K, the composite does not have superconductivity but has a finite electrical resistance just as a normal conductor.The resistance of the material increases as the temperature increases, as occurs in the majority of conducting materials. The ability to conduct electricity is much lower with respect to superconducting state and is subject to resistive losses.

[0108] Magnetic Effect (Absence of Meissner Effect)

[0109] At normal temperatures, the composite does not expel magnetic fields (phenomenon known as Meissner effect). The material is therefore subject to external magnetic fields and does not exhibit the characteristics of levitation typical of superconductors. Just as a normal conducting material, the composite is subject to the conventional laws of electromagnetism and does not have the ability to shield external magnetic fields

[0110] Mechanical Strength and Structural Stability

[0111] Even at room temperature, the composite maintains its improved mechanical properties. The addition of additives such as graphene, carbon nanotubes (CNTs), MgO and SiC provides a higher fracture toughness and mechanical robustness with respect to pure MgEh. This makes it resistant to mechanical stress and to wear even at normal temperatures.

[0112] Even without superconductivity, the composite can be used as an advanced structural material thanks to its high mechanical strength and stability, especially in environments that require light but robust materials.

[0113] Resistance to Corrosion and Oxidation

[0114] It was found that the presence of magnesium oxide (MgO) and graphene makes the composite highly resistant to corrosion and to oxidation even at room temperature. This makes it a material suitable for hostile operating environments, such as industrial or marine environments. Therefore, even in the absence of superconductivity, the composite can be used in applications that require corrosion resistance and durability overtime.

[0115] Thermal Conductivity

[0116] It was found that the presence of graphene and carbon nanotubes gives the composite an improved thermal conductivity with respect to pure MgEE. Even at room temperature, this additive makes MgE a good heat sink, useful in applications that require heat management, such as advanced electronics or industrial components.

[0117] The composite can be used as base material for heat sinks or in applications that require efficient heat management, preventing overheating in electronic devices.

[0118] Absence of Superconductivity

[0119] At normal temperatures the composite does not exhibit the phenomenon of conduction with no resistance. The electrical current that passes through the material is subject to the normal laws of electrical conduction, with consequent energy losses in the form of heat. Therefore, withoutcryogenic cooling, the composite is not suitable for applications that require superconductivity, such as lossless transmission lines or superconducting magnets.

[0120] Potential Applications at Room Temperature

[0121] Even without superconductivity, the composite can be used in various applications, thanks to its mechanical, corrosion resistance and thermal conductivity properties:

[0122] Advanced Structural Materials: the composite can be used in environments that require light, strong and durable materials, such as aerospace, marine constructions or infrastructures.

[0123] High Performance Electronic Devices: thanks to its thermal conductivity and mechanical strength, the composite can be used as heat sink material in electronic devices, printed circuits and industrial components.

[0124] Resistance to Extreme Environmental Conditions: in environments exposed to corrosion or oxidation, such as offshore platforms or industrial plants, the composite can be used as coating or protective structural material.

[0125] The composition of the composite can be varied as a function of the anticipated use.

[0126] In conclusion, at cryogenic temperatures, the composite according to the invention exhibits superconductivity with conduction with no resistance, magnetic levitation, and other unique properties of superconductors. Instead, at room temperature the behaviour of the composite becomes closer to that of a normal conducting material, without any of the superconducting properties, but with advantages in terms of mechanical strength and chemical and thermal resistance.

[0127] The composite according to the invention can comprise further optional components, besides those indicated above as components a) - g). In this case, the components a) - g) are present in an amount such as to allow the presence of the optional components while falling within the previously defined quantitative ranges.

[0128] According to an embodiment, carbon fibres are an optional component that can be associated with CNTs or partly replace CNTs in the formulation of the composite.

[0129] Carbon fibre (CF) is a material with a high tensile strength and a good thermal conductivity. It is easier to process with respect to carbon nanotubes and can be used to strengthen composite materials. Carbon fibres have an excellent mechanical strength, can be woven or layered to create a more ductile material, facilitating processing of the material in the form of sheets or strips.

[0130] However, the thermal conductivity of carbon fibres is lower with respect to that of CNTs, although still adequate for many mechanical and heat management applications.Expanded graphite is also an optional component that can be associated with CNTs, or partly replace CNTs, in the formulation of the composite to improve thermal conductivity and provide mechanical strength to composite materials.

[0131] Expanded graphite has a high thermal conductivity, which makes it ideal for applications that require heat dissipation, such as aerospace structures or electronic devices. It is also resistant to corrosion and improves the durability of the material in hostile environments.

[0132] According to an embodiment, the composite comprises an optional component consisting of alumina nanofibres associated with the components a) - g) in the formulation of the composite. Alumina nanofibres are ceramic materials that offer high mechanical strength and thermal stability. Alumina is characterized by a high degree of hardness and corrosion resistance. Alumina nanofibres offer an excellent mechanical strength, are resistant to heat and can withstand very high temperatures without degrading, making them ideal for aerospace or industrial applications.

[0133] According to an embodiment, the composite comprises an optional component consisting of nanoparticles of hexagonal boron nitride (h-BN) associated with the components a) - g) in the formulation of the composite.

[0134] Hexagonal boron nitride (h-BN) is a material that has properties similar to those of graphene in terms of heat conduction, but is also an excellent electrical insulator, has excellent thermal conduction, which makes it ideal for dissipating heat.

[0135] According to an embodiment, the composite comprises an optional component consisting of basalt fibres associated with the components a) - g) in the formulation of the composite. Basalt fibres are an interesting alternative for strengthened composites, thanks to their high mechanical strength and resistance to corrosion. Basalt fibres offer excellent tensile and compressive strength, making them ideal for strengthening composite materials. Basalt is resistant to high temperatures and can thus be used in applications that require stability in extreme environments. The composite material according to the invention can be produced in various physical forms but preferably in the form of spherical particles. These particles preferably have a diameter from 0.50 to 80 pm, more preferably from 1.0 to 60 pm. the particles diameter is measured by laser diffraction method.

[0136] Laser diffraction measures particle size distributions by measuring the angular variation in intensity of light scattered as a laser beam passes through a dispersed particulate sample. Large particles scatter light at small angles relative to the laser beam and small particles scatter light at large anglesJust as for the percentages of the various components of the composite, which can be varied according to use, also the particle size can be defined during the production phase, according to use and to the market of reference, in particular and purely by way of example:

[0137] <

[0138] <

[0139]

[0140] The spherical particles can then be further treated to obtain the desired final article, for example cables, sheets, strips, plates or other articles of different forms. In an intermediate treatment step, the spherical particles obtained with atomization can be thickened to improve their mechanical and conducting properties, as will be described below.

[0141] Production Method

[0142] An aspect of the invention relates to the method for the preparation of an MgB2-based composite material having the composition indicated above, comprising the steps of:

[0143] A) mixing magnesium and boron powders in the atomic ratio 1 :2 at room temperature and heating the mixture to a temperature between 650°C and 750°C in an inert gas atmosphere and obtaining the formation of MgB2;

[0144] B) cooling the magnesium diboride obtained in step A) in an inert gas atmosphere to obtain MgB2 in the form of a crystalline powder;

[0145] C) mixing the components a) - g) of the superconductor in the quantities defined in the composition reported above and obtaining a composite consisting of doped MgB2 in the form of a powder;

[0146] D) melting the composite by heating it up to 1000°C and atomizing it in an inert gas environment to obtain a powder with a spherical morphology;

[0147] E) cooling the spherical powder obtained in step D) in an inert gas atmosphere to room temperature.

[0148] In the following description, any reference to specific commercial equipment is provided solely by way of non-limiting example. Equivalent devices and systems capable of performing thesame technical function under comparable operating conditions may be used without departing from the scope of the present invention.

[0149] Production of the composite takes place through various synthesis and heat treatment steps. The mains steps include:

[0150] Step A)

[0151] High temperature synthesis of MgB2

[0152] In this step a pure and stable MgB2 matrix is formed as base of the composite. Pure magnesium (fine powder with purity >99.9%) and pure amorphous boron (fine powder with purity >99.9%) in stoichiometric proportions (molar ratio 1:2) are used to form the MgB2. For example, 2.58 kg of Mg and 7.42 kg of B are used to produce 10 kg of MgB2.

[0153] Synthesis takes place according to the reaction Mg+2B^MgB2.

[0154] The powders are mixed in a rotary drum at low speed, for example between 30 and 100 rpm, for 15-20 minutes.

[0155] The mixture is placed in a controlled atmosphere reactor, for example using argon or nitrogen, to avoid oxidative contamination of the magnesium and of the boron. The furnace Carbolite Gero HTF 1800, with a chamber volume of 30 L, in which the crucible containing the mixture of Mg and B is placed, can advantageously be used.

[0156] The mixture is heated gradually to 650-750°C to start the reaction for the formation of MgB2. The reaction is exothermic and maintains the material in molten state.

[0157] Synthesis is completed in 2-3 hours.

[0158] Step B)

[0159] Cooling of MgB2

[0160] At the end of step A) the Mg diboride is cooled slowly, again in an inert gas atmosphere. A crystalline powder is obtained.

[0161] Slow cooling takes place directly in the furnace to maintain a controlled and uniform temperature.

[0162] The inert gas atmosphere (e.g. argon) is maintained to avoid unwanted reactions with oxygen or humidity, which could oxidize the MgB?. This step does not require stirring, as MgB? forms naturally as crystalline powder during solidification and cooling, provided that temperatures are uniformly distributed. The cooling parameters are as follows:

[0163] Initial temperatures: end of the synthesis temperature of the MgB?.

[0164] Cooling speed:

[0165] • Slow and controlled: 10-20°C / min up to room temperature (~25°C).

[0166] • Avoid rapid cooling to minimize internal stresses and uneven fragmentation.Protective atmosphere:

[0167] • Inert gas (argon) with a purity >99.999%.

[0168] • Slightly positive pressure to prevent the entry of oxygen.

[0169] Step C)

[0170] Mixing MgB2 with the other components of the composite.

[0171] The components b) - g) of the superconductor, in the amounts defined in the composition reported above, are mixed with the Mgfh powder obtained in step B).

[0172] According to an embodiment, the carbon nanotubes (component b) and the graphene (component c) are dispersed in a liquid medium, for example ethanol, at a concentration from 0.05 to 2% weight / volume.

[0173] Dispersion takes place in an ultrasonic homogenizer at a frequency between 20 and 40 kHz for 60 min. The sonicator Hielscher UTP1000 hdt can advantageously be used.

[0174] The dispersion of CNTs and graphene is then added gradually to the MgB2 powder and finally the additives silicon carbide (component d) and magnesium oxide (component e) are incorporated.

[0175] Some compatibilizing and binding agents can be added in subsequent steps of the method, for example in the atomization step D) or in treatments downstream of the cooling step E), i.e., in the processing stages that lead to obtaining the desired final product.

[0176] However, compatibilizing agents consisting of functionalized silanes and zinc oxide are preferably added in the mixing Step C).

[0177] The mixture consisting of the MgB2 powder and of the additives b) - e) and of the aforesaid compatibilizing agents is then placed under high energy mechanical stirring. The mechanical stirrer IK A RW 20 Digital, with a chamber volume of 10 L, set at a speed of 2000 rpm, for 30 minutes, can advantageously be used.

[0178] The final mixture obtained is heated to remove the dispersing liquid. The Memmert UF75 Plus drying oven, at 80°C for 12 hours, can advantageously be used.

[0179] Step D)

[0180] Conversion of the dried powder into uniform spherical particles

[0181] To facilitate the subsequent compacting and sintering steps, the powder obtained in step C) is converted into uniform spherical particles.

[0182] The MgB2-based composite material is melted by heating the composite to a temperature sufficient to allow atomization and morphological spheroidization of the material under inert atmosphere conditions. Typically, the composite is heated up to 1000°C and the molten material is atomized in an inert gas environment, such as argon, obtaining a powder with a sphericalmorphology. The atomizer furnace Vacuum Technologies - Atomizer HDG can advantageously be used. The dried mixture is placed in a ceramic crucible, the argon flow is set to 10 L / min and the temperature to 800°C. The molten composite passes through an atomization nozzle and after around 60 min of atomization the spherical particles of composite are collected. The particle size produced during atomization is defined by the following formula (I):

[0183]

[0184] where:

[0185] G is the surface tension of the molten material, which for MgB2 is of around 0.48 N / m; p is the density of the molten material, which for MgB2 is of around 2.6 g / cm3;

[0186] v is the output speed of the material from the nozzle.

[0187] For example, with an output speed (v) of the molten material from the atomization nozzle of 10 m / s, the formula (I) calculates that spherical particles with diameter d = 1.85 pm are obtained, hence perfectly within the desired size range.

[0188] The above relationship is provided by way of example to illustrate the influence of process parameters on particle size.

[0189] Step E)

[0190] Cooling

[0191] The spherical particles produced in step D) are cooled to room temperature in an inert gas atmosphere, to prevent oxidation. The cooling chamber COOLVAC by Leybold, equipped with cryogenic pump, can advantageously be used.

[0192] The spherical particles obtained with Step E) are a semi-finished product that is further treated to obtain the desired final article, for example cables, sheets, strips, plates or other articles of different forms.

[0193] For example, the spherical particles can be sintered at low pressure to consolidate them and obtain a dense and stable composite, with improved mechanical and superconducting properties.

[0194] Subsequently, the sintered particles can be subject to rolling, which makes it possible to obtain sheets or cables with precise thicknesses and forms, adaptable to the intended applications. For example, the sintered spherical particles can be hot rolled, obtaining sheets with a thickness from 0.5 to 10 mm.

[0195] The aim of sintering is to reach a final density of the composite close to the theoretical density without generating porosity, using low pressure sintering that preserves the microstructure.The theoretical density (ptheoreticai) of the composite according to the invention is calculated as weighted average of the density of the components a) - e) of the superconductor, with the following formula (II):

[0196] Ptheoreticai = X\[gB2 * PMgB2 + XcNT * PCNT + Xgraphene * Pgraphene + XsiC * PSiC + X\[g() * pMgO (II) where:

[0197] X is the percentage of each component a) - e)

[0198] p is the density of each component.

[0199] It was found that in an embodiment of the method according to the invention the density of the particles before sintering was 1.57 g / cm3, i.e., around 60% of the theoretical density.

[0200] This initial density is increased during low pressure sintering, suitable to preserve the desired microstructure of the composite without generating porosity, obtaining a density after sintering between 2 and 3 g / cm3, preferably between 2.38 and 2.45 g / cm3.

[0201] The density can be measured, for example, with a pycnometer.

[0202] To perform sintering, the atomized particles are arranged uniformly inside the sintering container, consisting of a cylindrical or rectangular mould, so as to avoid significant voids. An isostatic press, such as the AIP - Hot Isostatic Press (HIP) QIH 15, can advantageously be used. Sintering takes place at a temperature between 400°C and 600°C, and at a pressure between 1 and 5 MPa, to facilitate atomic diffusion without excessive deformation of the structure and without causing excessive grain growth. The sintering time is between 2 and 4 hours, to ensure gradual densification. In this way, a homogeneous distribution of CNTs and of graphene is obtained, maintaining the pinning centres required to improve the critical current density and maximize the mechanical strength and thermal stability of the composite.

Claims

CLAIMS1. MgB2-based composite comprising:a) 77.5 to 84.5% by weight of magnesium diboride (MgfL);b) 2.4 to 3.75% by weight of carbon nanotubes;c) 3.6 to 5.0% by weight of graphened) 7.1 to 10.0% by weight of silicon carbide;e) 2.4 to 3.75% by weight of magnesium oxide (MgO).

2. Composite according to claim 1, characterized in that said magnesium diboride is present in the amount from 78.5 to 83.5% by weight, more preferably from 79 to 83% by weight.

3. Composite according to claim 1 or 2, characterized in that said carbon nanotubes are present in the amount from 2.6 to 3.6% by weight, more preferably from 2.8 to 3.4% by weight.

4. Composite according to one or more of the preceding claims, characterized in that said graphene is present in the composite in the amount from 3.8 to 4.8% by weight, more preferably from 4.0 to 4.6% by weight.

5. Composite according to one or more of the preceding claims, characterized in that said silicon carbide is present in the amount from 7.4 to 9.6% by weight, more preferably from 7.6 to 9.4% by weight.

6. Composite according to one or more of the preceding claims, characterized in that said magnesium oxide is present in the amount from 2.6 to 3.6% by weight, more preferably from 2.8 to 3.4% by weight.

7. Composition for the preparation of a Mgfh-based composite comprising:a) 62 to 71% by weight of magnesium diboride (MgfL);b) 2 to 3% by weight of carbon nanotubes;c) 3 to 4% by weight of graphened) 6 to 8% by weight of silicon carbide;e) 2 to 3% by weight of magnesium oxide (MgO).f) 9 to 12% by weight of a binding agent selected from polyvinylpyrrolidone and isopropanol.g) 7 to 8% by weight of a compatibilizing agent selected from the group consisting of zinc oxide, titanium carbide, mixture of zinc oxide and titanium carbide, epoxy resins, functionalized silanes, mixtures of epoxy resins and functionalized silanes.

8. Composition according to claim 7, characterized in that said binding agent is present in the amount from 9.5 to 11.5% by weight, preferably from 10 to 11% by weight.

9. Composition according to claim 7 or 8, characterized in that said compatibilizing agent is present in the amount from 7.2 to 7.8% by weight, preferably from 7.4 to 7.6% by weight.

10. Composite according to one or more of claims 1 - 6, characterized in that it is in the form of spherical particles with a diameter from 0.50 to 80 pm, more preferably from 1.0 to 60 pm, wherein the diameter of the particles is measured by laser diffraction method.

11. Composite according to one or more of claims 1-6 and 10, characterized in that said carbon nanotubes have an oxygen content from 0.2 to 0.5% by moles, wherein the oxygen content is measured by elemental analysis method.

12. Composite according to one or more of claims 1-6 and 10-11, characterized in that said graphene has an oxygen content from 0.2 to 0.5% by moles, wherein the oxygen content is measured by elemental analysis method.

13. Composite according to one or more of claims 1-6 and 10-12, characterized in that it comprises one or more additives selected from the group consisting of: carbon fibres, expanded graphite, alumina nanofibres, hexagonal boron nitride, basalt fibres.

14. Composite according to one or more of claims 1-6 and 10-13, characterized in that it comprises a compatibilizing agent selected from zinc oxide, titanium carbide and mixture thereof15. Method for the preparation of an MgB2-based composite according to one or more of claims 1-6 and 10-14 by using the composition of one or more claims 7-9, comprising the steps of:A) mixing magnesium and boron powders in the atomic ratio 1:2 at room temperature and heating the mixture to a temperature between 650°C and 750°C in an inert gas atmosphere and obtaining the formation of MgB2;B) cooling the magnesium diboride obtained in step A) in an inert gas atmosphere to obtain MgB2 in the form of a crystalline powder.C) mixing the components b) - g) of the composite in the quantities defined in the composition reported above and obtaining a composite in the form of a powder; D) melting the composite by heating it to a temperature sufficient to allow atomization, preferably up to 1000°C, in an inert gas environment to obtain a powder with a spherical morphology;E) cooling the spherical powder obtained in step D) in an inert gas atmosphere to room temperature.

16. Method according to claim 15, characterized in that the spherical powder obtained in step E) is sintered.

17. Method according to claim 16 characterized in that the spherical powder obtained in step E) is sintered at a temperature between 400°C and 600°C and at a pressure between 1 and 5 MPa to obtain a sintered composite.

18. Method according to claim 16 or 17 characterized in that the sintered particles have a density between 2.0 and 3.0 g / cm3, preferably between 2.38 and 2.45 g / cm3, wherein the density is measured by using a pycnometer.

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