METHOD FOR PRODUCING BORON SUBPHOSPHIDE B12 P2 in SHS

Abstract

Method for producing boron subphosphide B 12 P 2 by self-propagating synthesis, the method comprising a step of mixing reactants, in the form of powders, followed by a step of initiating an exothermic reaction, the exothermic reaction being carried out in a closed chamber.

Description

[0001] Process for the production of boron subphosphide Bi2P2 in SHS

[0002] FIELD OF THE INVENTION

[0003] The invention relates to the production of boron subphosphide B12P2, which has interesting mechanical, thermal and electronic properties.

[0004] Boron subphosphide B12P2 exhibits high hardness values, low densities, high compressive strength, and also has a very high melting temperature of around 2400 K.

[0005] STATE OF THE ART

[0006] Boron subphosphide B12P2 is often seen as an impurity in processes that primarily aim to produce boron phosphide BP.

[0007] Nevertheless, various methods have been proposed in the prior art for the synthesis of boron subphosphide B12P2.

[0008] According to a first type of synthesis, the boron subphosphide is deposited on a substrate, for example silicon or silicon carbide. Document US2004005768 (Hersee, 2005) describes epitaxial growth of boron subphosphide on a silicon carbide substrate, by chemical vapor deposition.

[0009] According to a second type of synthesis, boron subphosphide B12P2 is obtained by thermal decomposition of boron phosphide BP, at a temperature of more than 1500 K, in a reducing atmosphere. The document Slack et al. (Melt growth and properties of B6P crystals, Journal of the Physics and Chemistry of Solids 1983, vol.44, no. 10, pp. 1009-13, 10. 1016 / 0022-3697(83)90151 -8) describes the production of boron subphosphide, of composition very close to B12P2, by heating boron phosphide BP at a temperature of 1250°C for 11 hours, under a flow of hydrogen.

[0010] In a third type of synthesis, boron subphosphide is obtained by direct synthesis from boron powder and phosphorus powder. Yang et al. (Synthesis and cell refinement for icosahedral boron phosphide B12P2, 10. 1017 / s0885715600014949) describes such a process, with the mixture of the two powders being heated by induction at 1400°C for one hour under 50 bar (750 psi) argon pressure.

[0011] It has also been proposed to produce boron subphosphide B12P2 by a SHS (Self-propagating High-temperature Synthesis) type process, sometimes called combustion synthesis or self-propagating synthesis.

[0012] SHS processes have been proposed for the production of various ceramic materials (borides, silicides, oxides, nitrides, carbides, in particular titanium carbide) or intermetallic materials.

[0013] An SHS type reaction consists of an intimate mixture of reactants, generally in the form of powders, followed by high-pressure compaction, the reaction then being initiated by a sudden and local supply of energy, in the open air. The reaction, highly exothermic, propagates until the reactants are completely consumed.

[0014] The principles of SHS processes are presented in the book by Tavadze et al. (Production of advanced materials by methods of selfpropagating high-temperature synthesis, Springer, 2013). Subcategories of SHS processes can be identified, see for example Morsi (The diversity of combustion synthesis processing: a review, J Mater Sci 2012, 10. 1007 / s 10853.01 1 -5926-5).

[0015] Mukhanov et al. (Self-propagating high-temperature synthesis of boron subphosphide B12P2, Journal of Superhard Materials 36, 2014, 10.3103 / S 1063457614010031) propose two syntheses of boron subphosphide B12P2 by an SHS process under argon atmosphere from a compacted element of reactants.

[0016] A first synthesis is carried out from a mixture of a glass B 1 2 P2O 23 and magnesium, under argon, according to the following reaction: the reaction temperature being above 1300 K.

[0017] A second synthesis is a reduction of boron phosphate, according to the following reaction:

[0018] US 201 7203965 describes a process for producing boron phosphide BP comprising a step of mixing boron phosphate BPO4 and metallic magnesium, a step of compacting the mixture at a pressure of the order of 20,000 psi (137 MPa), and an ignition step by applying an energy input, using a heated fusible wire, the compaction making it possible to reduce the formation of impurities such as subphosphide B12P2.

[0019] State-of-the-art synthesis processes for the production of boron subphosphide B12P2 have many disadvantages. The synthesis processes of boron subphosphides are slow, expensive, complicated, dangerous (use of toxic elements), with low yield, and the resulting boron subphosphide B12P2 has low purity.

[0020] The use of long, high-temperature heat treatments is very energy-intensive, polluting and dangerous, making such processes economically uncompetitive.

[0021] The durations and difficulties of implementing the methods of synthesis of boron subphosphide Bi2P2 are such that, despite its interesting mechanical, thermal and electronic properties, boron subphosphide B12P2 is still little used in industry.

[0022] STATEMENT OF THE INVENTION

[0023] The invention aims to overcome the drawbacks of the prior art processes, by proposing the synthesis of boron subphosphide B12P2 in large volume, at lower cost, with low energy consumption.

[0024] The invention provides a simple, safe and rapid solution for the production of B12P2 subphosphide with inexpensive reagents that are readily available in large quantities.

[0025] For these purposes, there is proposed, according to a first aspect, a method for producing boron subphosphide B12P2 by self-propagating synthesis, the method comprising a step of mixing reagents, in the form of powders, followed by a step of initiating an exothermic reaction, the exothermic reaction being carried out in a closed enclosure. Advantageously, the method for producing boron subphosphide B12P2 by self-propagating synthesis at high temperature, advantageously greater than 1800K, comprises: a step of homogeneous mixing of reagents, in the form of powders, comprising at least one alkaline earth metal such as magnesium or calcium, and a compound comprising boron B and phosphorus P, such as boron phosphate BPO4 or B12 2O23 a step of filling a reactor with the homogeneous mixture of reagents,the reactor comprising at least one heating element and a layer of thermal insulation on an inner wall of the bottom of the reactor and a layer of thermal insulation on one or more inner side walls, the thermal insulation being chemically inert and being intended to surround the homogeneous mixture of reactants during the reaction, and to separate it from the walls of the reactor, a step of covering the upper part of the homogeneous mixture of reactants with a layer of chemically inert thermal insulation, a step of hermetically sealing the reactor, a step of initiating an exothermic reaction in the homogeneous mixture of reactants, by triggering the heating element, a step of high temperature exothermic reaction which self-propagates throughout the mixture of reactants of the reactor to obtain boron subphosphide B12 2.,

[0026] The hermetic closure of the reactor prevents gaseous magnesium from escaping from the reactor during the reaction, and allows thermal confinement by avoiding contact with air, to enable obtaining a high purity of B12P2 at least greater than 80%, or even advantageously greater than 90%.

[0027] During the exothermic reaction, temperatures of the order of 2000K can be reached locally. The enclosure in which the exothermic reaction is carried out forms a closed reactor, avoiding oxidation reactions.

[0028] In some implementations, the reactants include phosphoric acid, boric acid, and magnesium, with the exothermic reaction being according to reaction equation (2)

[0029] In other implementations, the reactants include boron phosphate, magnesium, and magnesium diboride, with the exothermic reaction being according to reaction equation (1)

[0030] 2 BPO4 + 3 Mg + 5 MgB2 — * B12P2 + 8 MgO U )- The enclosure in which the exothermic reaction is carried out forms a closed reactor, preventing the escape of vapors, in particular magnesium vapors in the air, from the enclosure. The boiling point of magnesium, of the order of 1363 K, is much lower than the temperatures of the exothermic reaction, of the order of 2000 K.

[0031] Advantageously, the magnesium is in excess in the mixture of reactants, relative to the stoichiometry of equation (1) or equation (2). In certain implementations, the magnesium is in excess of 22% in the mixture of reactants, relative to the stoichiometry of equation (1)-

[0032] Advantageously, magnesium diboride is in excess in the mixture of reactants, compared to the stoichiometry of equation (1). The excess magnesium in the mixture of reactants ensures complete reduction of the reactants, and also allows the final temperature of the reaction to be controlled, by the introduction of an additional heat capacity and high latent heats of fusion and especially boiling of magnesium.

[0033] Advantageously, the step of homogeneous mixing of the reactants is carried out by means of a two-dimensional powder mixer or a three-dimensional dynamic powder mixer, such as a mixer capable of providing rotational movements, translational movements and inversion movements to a mixing container, for example for a period greater than 1 hour, advantageously greater than 10 hours. Such a mixer is for example of the type marketed under the brand name Turbula®.

[0034] In some implementations, the ignition step involves heating a resistive element. The resistive element is, for example, a tungsten filament. In particular implementations, the tungsten filament has a diameter of about a hundred microns and a length of a few centimeters.

[0035] Advantageously, the method comprises a step of placing a thermally insulating material between the mixture of powdered reagents and the wall of the closed enclosure. The thermal insulating material is advantageously chemically inert, for example a sodium chloride powder, or another salt such as KCl or MgO, or mineral sand. Advantageously, the insulating material is in powder form. Advantageously, the method comprises a step of cooling the enclosure forming the reactor, the enclosure being kept closed until the end of cooling. Cooling is for example ensured by a circulation of heat transfer fluid.

[0036] The method comprises a step of extracting the product of the exothermic reaction, the product extracted from the reactor being subjected to grinding and washing, the washing advantageously comprising leaching.

[0037] In some implementations, the powder resulting from the grinding of the product of the exothermic reaction is placed in a hydrochloric acid bath, the solution obtained then being subjected to vacuum filtration, for example with nanometric filters, chosen in particular for their resistance to acids, such as Millipore-M F polyethersulfone filters.

[0038] In particular implementations, washing includes aqua regia leaching.

[0039] Advantageously, the mixture of powdered reagents is not compacted before the priming step. In other words, the filling of the mixture of powdered reagents into the reactor is advantageously carried out without compaction.

[0040] The absence of compaction limits powder handling, saves considerable time, and avoids the purchase of expensive equipment.

[0041] Advantageously, the filling rate in the reactor, corresponding to the ratio between the volume of the sample and the volume of the reactor, is greater than 25%.

[0042] In certain implementations, the method is such that: the filling rate is greater than 25%, and the excess magnesium is between 10% and 50%, advantageously between 20% and 25%, advantageously with an excess of magnesium diboride up to 20%, making it possible to obtain temperatures greater than 1,500°C throughout the mixture of reactants during the self-propagating exothermic reaction step, in order to have a purity of the mixture greater than 80% of B12P2, advantageously greater than 95% of B12P2.

[0043] Advantageously, the exothermic reaction is carried out without solvent. Advantageously, the reactor has a volume allowing the production of masses of boron subphosphide greater than 50g, advantageously greater than 100g, and even more advantageously greater than 500g.

[0044] Advantageously, the filling step comprises the insertion of an intermediate wall held in the internal space of the reactor, the time for filling with the insulation of the space located between the lateral interior walls of the reactor and the intermediate wall, then the time for filling with the reagent mixture of the central volume of the reactor defined by the internal space of the reactor surrounded by the intermediate wall up to a given height in the reactor.

[0045] The invention relates, according to a second aspect, to a boron subphosphide B12P2 obtained according to the method presented above, the boron subphosphide B12P2 obtained comprising: B12P2 particles having sizes which vary from 50 to 500 nm with rounded icosahedral shapes, B12P2 particles of micron sizes with a “faceted rod” type morphology, for example with a length of between 1 and 10 microns.

[0046] Advantageously, a mixture is proposed comprising only: boron subphosphide B12P2 as presented below, and boron phosphide BP, with a purity of the mixture in boron subphosphide B12P2 greater than 90%, advantageously greater than 95%, advantageously greater than 98%.

[0047] According to a third aspect, the use of boron subphosphide obtained by a process as presented above is proposed as a hard material in machining or cutting tools, wire drawing dies, wear protection coatings, structural tools for construction, mining or quarrying, drilling tools, armor-piercing munitions, bullet-proof vests.

[0048] According to a fourth aspect, the use of boron subphosphide obtained by a process as presented above is proposed as a heat-conducting material or thermoelectric material.

[0049] According to a fifth aspect, there is provided the use of boron subphosphide obtained by a process as presented above, as a filler in a thermosetting resin. According to another aspect, there is provided the use of boron subphosphide obtained by a process as presented above, as a neutron absorbing material.

[0050] According to another aspect, there is proposed the use of boron subphosphide obtained by a process as presented above, as a powder material for the manufacture by sintering of a ceramic for shielding.

[0051] DESCRIPTION OF FIGURES

[0052] Other objects and advantages of the invention will appear in the light of the description of embodiments, given below with reference to the appended drawings in which: Figure 1 is an external view of three closed reactors having respective capacities of 50g, 100g and 500g, for the synthesis of boron subphosphide B12P2; Figure 2 is a diffractogram of a boron subphosphide powder B12P2 obtained in a closed reactor with a capacity of 100g as shown in Figure 1; Figure 3 are two scanning electron microscopy images of a boron subphosphide powder B12P2 obtained in a closed reactor with a capacity of 100g as shown in Figure 1; Figures 4 to 12 are views of the steps of filling a reactor, according to one implementation.

[0053] DETAILED DESCRIPTION OF THE INVENTION

[0054] The process advantageously uses a closed reactor, non-compacted reagents and leaching, to produce boron subphosphide B12P2 on a large scale and at low cost.

[0055] The leaching processes have been well optimized to eliminate reagents not converted into B12P2.

[0056] Figure 1 shows three closed reactors, capable of containing sufficient quantities of reagents to synthesize 50g, 100g and 500g of powders, respectively. The process for synthesizing boron subphosphorus B12P2 is based on highly exothermic reactions between solid or gaseous reagents, in the absence of solvent, these oxidation-reduction reactions being as follows:

[0057] 2 BPO4 + 3 Mg + 5 Mg Bs — * B12P2 + 8 MgO or

[0058] 1 2 H3BO3 + 2 H3PO4 — * B12P2O23 + 21 H2O then B12P2O23 + 23 Mg — > B12P2 + 23 MgO

[0059] In other implementations, calcium is used instead of magnesium.

[0060] For simplicity, we present here only the results concerning boron subphosphide B12P2 synthesized using the reaction

[0061] 2 BPO4 + 3 Mg + 5 Mg B2 — * B12P2 + 8 MgO.

[0062] The reactive mixture uses boron phosphate BPO4 and magnesium diboride MgB2 as sources of boron and phosphorus, and magnesium Mg as the main reducing agent.

[0063] It is noteworthy that magnesium diboride Mg B2 also contributes to this role.

[0064] The reaction is extremely exothermic, with a final temperature that can exceed 2000 K, which is well beyond the boiling point of magnesium.

[0065] To ensure complete reduction of the reactants, magnesium is advantageously in excess in the mixture.

[0066] This excess magnesium also makes it possible to control the final temperature of the reaction, by introducing an additional thermal capacity and high latent heats of fusion and especially of boiling of magnesium.

[0067] Furthermore, magnesium diboride Mg B2 is advantageously in excess in the mixture, so as to be able to limit the quantity of boron phosphide BP present as a by-product, in the final powders of boron subphosphide B12P2.

[0068] These experimental conditions made it possible to completely eliminate boron phosphate BPO4, and the final yield of the reaction is 80%. This yield corresponds to the mass of boron subphosphorus B12P2 obtained, compared to that which could have been expected based on the stoichiometry of the reaction.

[0069] Advantageously, a calculation of the adiabatic temperature is carried out, using the N IST (National Institute of Standards and Technology) databases of the different reaction products.

[0070] Figure 2 shows the X-ray diffractograms of the boron subphosphide powder B12P2 obtained for batches of 50g and 100g in the intermediate reactor with a capacity of 100g shown in Figure 1. A Rietveld refinement of the diffractograms was carried out, using the MAU D (Material Analysis Using Diffraction) software on these two batches of powders.

[0071] Rietveld refinement is presented for example by Stephens ( Uniting electron crystallography and powder diffraction, 2012, 10. 1007 / 978-94- 007-5580-2 2). Rietveld refinement is a method for global simulation of X-ray diffraction patterns on polycrystalline samples, this method being implemented in different software such as FullProf, Jana2006 and MAU D. The MAU D software was developed by Luca Lutterotti (L. Lutterotti et al. MAUD: a friendly Java program for material analysis using diffraction. lUCr: Newsletter of the CPD, 21: 14- 15, 1999).

[0072] The powders consist of 97.441% boron subphosphide B12P2 and 2.559% boron phosphide BP as an impurity.

[0073] The rhombohedral structure is observed for the different diffractograms, with a space group R-3m. The unit cell parameters of boron subphosphide B12P2 are a = 6.01025 Å and c = 11.90010 Å in the hexagonal unit cell.

[0074] Figure 3 shows the morphological analysis of the powders obtained, by scanning electron microscopy.

[0075] Two distinct populations of particles are observed.

[0076] A very small number of particles have micron sizes, with a faceted rod-like morphology.

[0077] Otherwise, the vast majority of boron subphosphide B12P2 particles have sizes ranging from 50 to 500 nm, with slightly rounded icosahedral shapes.

[0078] The steps of a process will now be described in more detail. The powders are first weighed, then homogenized in a mixer for 24 hours.

[0079] The mixer can be a rotating tank mixer (cube, rotating drum, double cone), or a convective mixer comprising a fixed tank and a rotating element inside the tank, a fluidized bed mixer or a static mixer.

[0080] Advantageously, the mixer is of the complex movement type, providing rotational, translational and inverting movement to a mixing container, the movements being pulsating.

[0081] The mixer is, for example, of the type marketed under the Turbula® brand by Willy Bachofen AG. A presentation of Turbula mixers is provided by Mayer-Laigle (Dynamic study and effect of scale change for several particulate systems in Turbula® mixer, thesis 2012).

[0082] This mixer allows you to obtain a very high quality final mixture.

[0083] The powders are then placed in the reactor, surrounded in all directions by insulating walls, ensuring thermal and chemical insulation with the outer walls of the reactor.

[0084] The insulating wall is, for example, formed from a powdered material, such as sodium chloride or mineral sand.

[0085] The thickness of this thermal insulating wall must be chosen to limit the cooling rate of the sample, which advantageously increases the purity of the product by reducing the quantity of BP, and to allow a more gradual evacuation of heat and thus avoid excessive heating of the reactor walls which could harm their integrity.

[0086] For example, it is between 2 and 3 cm, for a characteristic transverse length of the homogeneous mixture of reagents between 4 and 6 cm and a characteristic height between 10 and 16 cm.

[0087] This cooling must be optimized, so as to be slow enough, in order to optimize the time that the sample spends at high temperature, allowing to maximize the quantity of B12P2, to the detriment of BP, without harming productivity. This time is for example from one to several hours. This cooling is linked to all the characteristics of the thermal insulator. For example, for a powdered thermal insulator, allowing a lower thermal conduction, the characteristics of the insulator include the size of the particles, the thickness of the insulator, the nature of the insulator.

[0088] The magnesium was evaporated during the reaction, and is found mixed with the insulating salts, and therefore the rapid opening would cause an oxidation of the products and of the magnesium, which would not allow the high reaction yields obtained by the implementation of the invention to be achieved.

[0089] Figures 4 to 12 illustrate an implementation of the filling of a reactor.

[0090] In a first step, shown in Figure 4, a heating element 10 such as a resistive filament is placed in the lower part of the reactor. The resistive filament 10 is for example a tungsten filament. For information, the tungsten filament has a diameter of around a hundred microns and a length of a few centimeters. The two ends of the filament are each connected to a pad 11 mounted on the base 12 of the reactor, with an inner wall 12a in contact with the thermal insulation.

[0091] In a second step, shown in Figure 5, the inner wall 13 of the reactor is placed on the base 12. In the embodiment shown, the inner wall 13 is a cylinder of revolution, the axis of slenderness of which is perpendicular to the base 12 of the reactor. In other implementations, the inner wall is in the form of a profiled part of square or polygonal cross-section. In order to facilitate understanding, the wall 13 is shown in partial section, in Figures 5 to 11. The inner wall 13 is for example fitted or fitted onto the base 12 of the reactor. When the inner wall 13 is in place, an internal volume of the reactor is defined laterally by this inner wall 13 and by the base 12 of the reactor. This volume is open in the upper part, allowing the reactor to be filled. The resistive filament is placed in this internal volume.

[0092] In a third step, shown in Figure 6, an insulating wall or base 14a is placed in the bottom of the internal volume of the reactor.

[0093] Advantageously, this insulating wall 14a or base is formed from a powder of a thermally and chemically inert insulating material, for example a salt powder such as sodium chloride or another salt, KCI or MgO, making it possible to confine the thermal energy during the reaction and to avoid, during the highly exothermic reaction, the migration of chemical elements from the walls of the reactor to the homogeneous mixture of reactants, making it possible to obtain a high purity of the product obtained B12P2. The insulating wall 14a is thus easily placed in the internal space of the reactor, below the resistive filament 10.

[0094] In a fourth step, shown in Figure 7, an intermediate wall or filling mold 15 is inserted and held in the internal space of the reactor, this internal wall separating a peripheral annular volume 16 from a central volume 17. In the implementation shown, the intermediate wall is in the form of a cylindrical part of revolution, for example a steel cylinder, such as a strip. It must not damage the heating element 10 during its insertion and must have openings or the like to allow the filament to pass through, or be located in abutment on the filament located at the end of the height of the base 14a.

[0095] In a fifth step, shown in Figure 8, an insulating wall or side layer 14b is formed in the peripheral annular space 16, advantageously by filling this annular space with a powder of insulating and chemically inert material. This powder is advantageously the same as that having formed the insulating wall 14a in the lower part of the reactor R.

[0096] In a sixth step, shown in Figure 9, the reactive powder mixture is placed in the central volume 17 of the reactor. This reactive powder mixture is in contact with the resistive filament 10.

[0097] In a seventh step, shown in Figure 10, the intermediate wall or filling mold 15 is removed.

[0098] In an eighth step, shown in Figure 11, an upper insulating wall or layer is formed in the upper part of the reactor. Advantageously, this upper insulating wall or layer 14c is formed by an insulating and chemically inert powder, in particular that used to form the insulating wall 14a in the lower part of the reactor (Figure 6), or that used to form the insulating wall or side layer 14b of the reactor (Figure 8), for example a salt or sand powder. The insulating powder is placed in the reactor until the reactor is completely filled.

[0099] In a ninth step, shown in Figure 12, the reactor is hermetically sealed, allowing confinement of the homogeneous mixture of reactants, thermally isolated from the ambient atmosphere in the reactor. Advantageously, the reactor comprises a lower flange forming the base 12 of the reactor and an upper flange forming the cover 18 of the reactor, reinforcing rods 19 connecting the two flanges.

[0100] The reaction is initiated, once the reactor is hermetically sealed, advantageously by a heating element, such as a fusible heating filament, for example a tungsten filament positioned within the reactive mixture. The tungsten filament has, for example, a diameter of around a hundred microns, and a length of a few centimeters, and is heated by the Joule effect.

[0101] Once initiated, the reaction is complete after a few seconds.

[0102] The reactor is then kept closed until the sample has cooled, then it is extracted, crushed and leached to dissolve the sodium chloride NaCl and the magnesium oxide MgO.

[0103] In an advantageous implementation, the powders obtained are placed in a 2M hydrochloric acid bath, at 110°C, with magnetic stirring for 1 h, in order to dissolve the magnesium oxide MgO and the sodium chloride NaCl.

[0104] The solution is then filtered under vacuum, for example using Millipore-M F® 220 nm polyethersulfone filters chosen in particular for their resistance to acids.

[0105] This operation is advantageously repeated twice in succession.

[0106] Advantageously, a final leaching is carried out using aqua regia, with an HNO3 / HCl ratio of 1 / 3 to eliminate the last impurities.

[0107] The final product is then washed several times with distilled water, in order to eliminate residues present on the surface of the powders.

[0108] Finally, the powders are dried in an oven.

[0109] Several parameters influence the reaction yield, and a parametric study was carried out, with the objectives of obtaining the most complete reaction possible, by minimizing the oxide phase (BPO4) and favoring the formation of boron subphosphide B12P2 compared to that of boron phosphide BP.

[0110] The excess of magnesium Mg advantageously allows a better reduction of the oxide phase.

[0111] The excess of magnesium diboride Mg B2 advantageously allows to reduce the quantity of boron phosphide BP produced, while providing a slight improvement in the reducing power of the reaction mixture.

[0112] The overall volume of the reactor and the ratio of the sample volume to the mass of the reaction mixture determine the ratio of the sample volume to the reactor volume, i.e. the filling rate of the reactor with reaction mixture.

[0113] Advantageously: the filling rate is greater than 25%, and the excess magnesium is between 10% and 50%, advantageously between 20% and 25%, allowing a complete reduction of the BPO4 oxide. In addition to this excess magnesium, advantageously there can be an excess of magnesium diboride up to 20%, allowing the stoichiometric ratio B:P of 6: 1 to be adjusted as closely as possible.

[0114] It was found that a 22% excess of magnesium Mg, without excess of magnesium diboride Mg B2, achieved a yield approaching 80%.

[0115] The space occupied by the sample in the reactor has a significant impact on the final purity of the product.

[0116] The purity of boron subphosphide BI2P2 in a 50g capacity reactor is thus 86%, and this purity increases to 98% in the intermediate reactor with a capacity of 100g. In both cases, the main and only impurity in boron subphosphide B I2P2 is boron phosphide BP.

[0117] Advantageously, the reactor has an internal diameter of 10 cm and a height of 20 cm, the total internal volume of the reactor being 1,570 cm 3 .

[0118] Advantageously, the powder occupies a volume in the reactor of between 1 and 80 cm 3 and 350 cm 3 These volumes correspond to cylinders with a diameter of 5 cm and a height of between 9 and 18 cm.

[0119] Advantageously, the reagent powder is not compressed to facilitate the initiation of the reaction. Advantageously, the reagent powder has a low possibility of expansion, so that the different precursors remain in contact during the propagation of the reaction, which allows a more complete synthesis.

[0120] Advantageously, the entire reactor is filled, the part not occupied by the reaction mixture containing an insulating material, for example sodium chloride or sand.

[0121] In this way, the residual heat is retained for a longer time, which promotes self-purification phenomena and greatly favors the synthesis of boron subphosphide B12P2 compared to that of boron phosphide BP.

[0122] BP, at high temperature, decomposes into B12P2, BP being stable in air only up to 1400 K (Mukhanov et al. Dalton trans., 2016, 45, 10122, DOI: 10. 1039 / c6dt00435k).

[0123] The method according to the invention has numerous advantages.

[0124] The process is safe, practical, fast and inexpensive, with simple implementation and ease of scale-up, for the production of boron subphosphide B12P2.

[0125] The process allows the synthesis of a high added-value material, using inexpensive reagents and an easy-to-implement device.

[0126] The process also requires little labor, reducing costs.

[0127] The process allows, by using SHS reactions in confined spaces (closed reactors), to synthesize quantities of B12P2 powders of up to 100g, from non-compacted reagents.

[0128] The yield is remarkably very high, around 80%.

[0129] The purity is also extremely high, around 97%.

[0130] The process is rapid and has very low energy consumption. The invention thus allows the synthesis of boron subphosphide B12P2, in large volumes, at low cost, with low energy consumption.

[0131] The use of this material with exceptional mechanical properties was until now little developed, precisely because of the lack of possibility of large-scale synthesis with good economic potential. The process allows a production of this boron subphosphide B12P2 in large volume, the subphosphide having a very high hardness, a remarkable chemical and thermal stability, the subphosphide being refractory, a wide band gap semiconductor, and having a thermoelectric power with a high figure of merit at high temperature.

[0132] The boron subphosphide obtained by the process according to the invention is advantageously used as a hard material, as an alternative to tungsten carbide.

[0133] The synthesis of CW tungsten carbide is very expensive, and occurs from the elements at very high temperatures, between 1700 K and 2300 K.

[0134] In addition, the sintering of CW tungsten carbide is done by adding a binder, cobalt. Chronic exposure to cobalt dust associated with tungsten carbide leads to occupational, skin, respiratory and tumor diseases (see for example Robert (Temporal analysis of French occupational exposures and pathologies linked to cobalt and hard metal dust, from the end of the 1990s to 2020, pharmacy thesis, 2021).

[0135] Boron subphosphide B12P2 has properties far superior to tungsten carbide (harder, more resistant to chemical or thermal attacks) and the process according to the invention is much less expensive than that allowing the synthesis of tungsten carbide CW.

[0136] Furthermore, binderless sintering of boron subphosphides is possible, suggesting possibilities for low-pressure binderless sintering by conventional methods.

[0137] The obtained boron subphosphide B12P2 (hardness ~35 GPa, comparable to commercial c-BN) finds advantageously many uses, today reserved for tungsten carbide (or c-BN), such as machining and cutting tools (saws, drills, reamers, guide bushes, milling cutters, lathe tools, punches), wire drawing or compacting dies, wear protection coatings (for turbine blades of aircraft engines, gas power plants, metallurgical furnaces), structural tools (public works, land leveling, drilling tool heads, mining and quarrying, tunnel boring machines), but also pen balls, armor-piercing ammunition, bullet-proof vests. The boron subphosphide obtained by the process according to the invention is advantageously used as a heat-conducting material, for example in integrated circuits.

[0138] Indeed, with the constant increase in the power density of microelectronic components, the thermal stress on these components increases exponentially.

[0139] With high heat densities, heat dissipation requires materials with remarkable thermophysical characteristics, such as hard materials such as boron subphosphide B12P2.

[0140] The boron subphosphide obtained by the process according to the invention is advantageously used as a high-temperature thermoelectric material.

[0141] In the field of microelectronics, it is possible to dope boron subphosphide B12P2 of p or n type, and therefore to produce diodes operating in aggressive environments (chemical, thermal or mechanical) or even for power electronics.

[0142] Also, boron subphosphide B12P2 may have important applications as a high-temperature thermoelectric material.

[0143] The boron subphosphide obtained by the process according to the invention is advantageously used as a filler in a thermosetting resin.

[0144] The addition of a hard, lightweight, thermally conductive material to thermosetting resins makes it possible to strengthen their thermomechanical and chemical resistance.

[0145] These thermosetting materials are of growing interest for their remarkable properties: very good mechanical strength, as well as good resistance to chemicals (non-reactive materials) and heat.

[0146] The boron subphosphide obtained by the process according to the invention is advantageously used in the nuclear field, due to its high neutron absorption cross section, as an alternative to boron or boron carbide.

[0147] Pure boron is difficult to produce in specific forms (e.g. for control rods in nuclear power plants), so boron carbide is usually substituted in these applications, providing a high concentration of boron atoms in a solid, refractory form that is resistant to chemical attack.

[0148] However, the production of boron carbide B4C is carried out at very high temperatures and is relatively complex.

[0149] Also, the simplicity of low-cost synthesis of boron subphosphide B12P2 could allow this material to be promoted in nuclear applications, and thus replace boron carbide B4C in certain specific cases.

Claims

CLAIMS 1. Process for producing boron subphosphide B12 2 by self-propagating synthesis at high temperature, above 1500°C, by closed, thermally insulated reactor, the process comprising: a step of homogeneous mixing of reactants, in the form of powders, comprising at least: an alkaline earth metal such as magnesium or calcium, and a compound comprising boron B and phosphorus P, or a compound comprising boron B and a compound comprising phosphorus P, such as boron phosphate BPO4 or Bi 2P2O23 a step of filling a reactor (R) with the homogeneous mixture (M) of reactants, the reactor comprising at least: a heating element (10), a layer or base of thermal insulation (14a) on an inner wall of the bottom of the reactor (12a) and a layer of lateral insulation (14b) on one or more inner lateral walls (13),the thermal insulation being chemically inert and being intended to surround the homogeneous mixture (M) of reactants during the reaction and to separate it from the walls of the closed reactor (12a, 13), a step of covering the upper part of the homogeneous mixture (M) of reactants as well as the lateral insulation layer (14b), with an upper layer of chemically inert thermal insulation (14c), making it possible to confine the homogeneous mixture (M) of reactants in the reactor, this mixture (M) being completely surrounded by the base insulation layer (14a), the lateral thermal insulation layer (14b), and the upper layer of thermal insulation (14c), a step of hermetically closing the reactor, using a cover (18), a step of initiating in the closed reactor, a high-temperature exothermic reaction in the homogeneous mixture (M) of reactants, by triggering the heating element (10), in contact with the mixture (M), - a high-temperature exothermic reaction step, above 1500°C, which self-propagates throughout the mixture (M) of reactants in the reactor R to obtain boron subphosphide B 1 2 P 2 , the insulating material allowing the residual heat to be retained for longer in the homogeneous mixture (M) of reactants, to promote self-purification phenomena and promote the synthesis of boron subphosphide B12P2, compared to that of boron phosphide BP, the mixture of reactants being said to be confined in the reactor, the powder of reactants thus having a low possibility of expansion, so that the different precursors remain in contact with each other, during the propagation of the reaction, magnesium being in excess in the mixture of reactants to increase the reduction of the oxides, - a step of extracting the product of the exothermic reaction, the product extracted from the reactor (R) being subjected to washing which includes one or more leaching operations, then rinsing, to obtain boron subphosphide B 1 2 P 2.

2. Method according to claim 1, characterized in that the homogeneous mixture (M) of reactants is produced by a two-dimensional powder mixer or a three-dimensional powder mixer, such as a mixer capable of providing rotational movements, translational movements and inversion movements to a mixing container, for example for a period greater than 1 hour, advantageously greater than 10 hours.

3. Method according to one of claims 1 to 2, characterized in that the filling and confinement of the mixture of powdered reagents in the reactor (R) is carried out without compaction.

4. Method according to any one of claims 1 to 3, characterized in that the filling rate in the reactor (R) corresponding to the ratio between the volume of the sample of reagents and the volume of the reactor (R) is greater than 25%.

5. Method according to any one of claims 1 to 4, characterized in that the reagents comprise phosphoric acid, acid boric and magnesium, the exothermic reaction being according to the reaction equation then B12P2O23 + 23 Mg — > B12P2 + 23 MgO (2) 6. A method according to any one of claims 1 to 4, characterized in that the reactants comprise boron phosphate, magnesium and magnesium diboride, the exothermic reaction being according to the reaction equation 2 B U )- 7. Method according to any one of claims 5 or 6, characterized in that the magnesium is in excess in the mixture of reactants, compared to the stoichiometry of equation (1) or equation (2).

8. Method according to any one of claims 6 or 7, characterized in that the magnesium diboride is in excess in the mixture of reactants, compared to the stoichiometry of equation (1).

9. Method according to any one of claims 6 to 8, characterized in that: the filling rate greater than 25%, and the excess magnesium between 10% and 50%, advantageously between 20% and 25%, advantageously with an excess of magnesium diboride up to 20%, make it possible to obtain temperatures greater than 1,500°C in the entire mixture of reactants during the self-propagating exothermic reaction step, in order to have a conversion rate of the reactants into B12P2, advantageously greater than 80%, and after leaching for only B12P2 and BP products, with a proportion of B12P2 advantageously greater than 95%.

10. Method according to any one of claims 1 to 9, characterized in that the heating element (10) is a resistive element, such as a tungsten filament. 1 1. Method according to any one of claims 1 to 1 0, characterized in that the thermally insulating and chemically inert material is sodium chloride or another salt such as KCl, or MgO.

12. Method according to any one of claims 1 to 11, characterized in that the method comprises a step of cooling the reactor (R), kept closed until the end of cooling, for example for a duration greater than 1 hour and less than 10 hours, advantageously less than 5 hours, advantageously less than 2 hours.

13. Method according to any one of claims 1 to 12, characterized in that the step of extracting the product of the exothermic reaction comprises a grinding step before leaching.

14. Method according to any one of claims 1 to 13, characterized in that the washing of the extraction step comprises one or more leaches, for example one or more leaches with hydrochloric acid, followed by a leach with aqua regia.

15. Method according to claim 14, characterized in that the powder resulting from the grinding of the product of the exothermic reaction is placed in a hydrochloric acid bath, the solution obtained then being subjected to vacuum filtration, for example with nanometric filters chosen in particular for their resistance to acids, such as Millipore-M F® polyethersulfone filters.

16. Process according to any one of claims 1 to 15, characterized in that the exothermic reaction is carried out without solvent.

17. Method according to any one of claims 1 to 16, characterized in that the reactor (R) has a volume making it possible to produce masses of B12P2 greater than 50g, advantageously greater than 100g, advantageously greater than 500g.

18. Method according to any one of claims 1 to 17, characterized in that the step of filling the reactor comprises - a step of filling the thermal insulation at the bottom of the reactor, to constitute a base (14a) of thermal insulation, for example up to the height of the heating element (10); - a step of inserting an intermediate wall (15) or filling mold, held in the internal space of the reactor (R), for the time of filling with the thermal insulation, in the space located between the lateral interior walls of the reactor (R) and the intermediate wall (15), to produce a layer of thermal insulation (14a) on the lateral wall(s) of the reactor, by the mixture of reactants, in the central volume (17) of the reactor (R) defined by the internal space of the reactor (R) surrounded and delimited by the intermediate wall (15) up to a given height in the reactor (R) - a step of removing the intermediate wall (1 5), - a step of depositing a layer of chemically inert thermal insulation (14c), to cover the upper part of the homogeneous mixture (M) of reagents as well as the upper part of the lateral thermal insulation layer (14b).

19. Final product resulting from the process defined by one of claims 1 to 18, characterized in that the final product has: B12P2 particles which have sizes which vary from 50 to 500 nm with rounded icosahedral shapes, B12P2 particles of micron sizes with a “faceted rod” type morphology.

20. Final product according to claim 19, comprising only: boron subphosphide B12P2, and boron phosphide BP, with a quantity of boron subphosphide B12P2 in the mixture greater than 90%, advantageously greater than 95%, advantageously greater than 98%, the remainder being boron phosphide BP.

21. Use of the final product according to any one of claims 19 or 20, as a hard material in machining or cutting tools, wire drawing dies, wear protection coatings, tools for structural work in construction, mining or quarrying, drilling tools, armor-piercing ammunition, bullet-proof vests.

22. Use of the final product according to any one of claims 19 or 20, as a heat-conducting material or thermoelectric material.

23. Use of the final product according to any one of claims 19 or 20, as a filler in a thermosetting resin.

24. Use of the final product according to any one of claims 19 or 20, as a neutron absorbing material.

25. Use of the final product according to any one of claims 19 or 20, as a powder material for the manufacture by sintering of a ceramic for shielding.

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