Method for producing boron hypophosphide B12P2 in SHS

Abstract

Process for producing boron hypophosphide B12P2 by self-propagating synthesis, comprising a step of mixing reactants in powder form followed by a step of initiating an exothermic reaction, the exothermic reaction being carried out in a closed chamber.

Description

Technical Field

[0001] The present invention relates to boron phosphide (B) having interesting mechanical, thermal and electronic properties 12 Production of P2.

[0002] Boron phosphide B 12 P2 has particularly high hardness values, low density, high compressive strength and a very high melting temperature of approximately 2400K. Background Art

[0003] Boron phosphide B 12 P2 is generally considered an impurity in the processes primarily used to produce boron phosphide (BP).

[0004] However, the prior art has proposed a method for synthesizing boron phosphide B. 12 Various methods of P2.

[0005] In a first type of synthesis, boron subphosphide is deposited on a substrate, for example made of silicon or silicon carbide. Document US2004005768 (Hersee, 2005) describes the epitaxial growth of boron subphosphide on a silicon carbide substrate by chemical vapor deposition.

[0006] According to the second type of synthesis, boron phosphide B 12 P2 is obtained by thermally decomposing boron phosphide BP in a reducing atmosphere at a temperature exceeding 1500 K. The document by Slack et al. (Melt growth and properties of B6Pcrystals, 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 a compositionally similar boron phosphide BP by heating boron phosphide BP to a temperature of 1250° C. for 11 hours under a hydrogen flow. 12 P2 is very similar to boron phosphide.

[0007] According to the third type of synthesis, icosahedral boron phosphide B is obtained by direct synthesis from boron powder and phosphorus powder. 12 P2, 10.1017 / s0885715600014949) describes a method in which a mixture of two powders is induction heated to 1400°C for one hour under an argon pressure of 50 bar (750 psi).

[0008] The use of the SHS (self-propagating high temperature synthesis) method (sometimes called combustion synthesis or self-propagating synthesis) to produce boron phosphide B has also been proposed. 12 P2.

[0009] The SHS method has been proposed for the production of various ceramic materials (borides, silicides, oxides, nitrides, carbides, in particular titanium carbide) or intermetallic materials.

[0010] SHS-type reactions involve intimate mixing of reactants, usually in powder form, followed by high-pressure compaction. The reaction is then initiated in open air by a sudden, localized input of energy. The highly exothermic reaction propagates until all reactants are consumed.

[0011] The principles of the SHS process are presented in Tavadze et al. (Production of advanced materials by methods of self-propagating high-temperature synthesis, Springer, 2013). Subcategories of the SHS process can be identified, see for example Morsi (The diversity of combustion synthesis processing: a review, J Mater Sci 2012, 10.1007 / s10853.011-5926-5).

[0012] Mukhanov et al. (Self-propagating high-temperature synthesis of boronsubphosphide B 12 P2, Journal of Superhard Materials 36, 2014, 10.3103 / S1063457614010031) proposed two methods for preparing boron phosphide (BPS) by SHS method under argon atmosphere from compacted components of reactants. 12 Synthesis of P2.

[0013] Under argon, B was reacted according to the following reaction 12 P2O 23 A mixture of glass and magnesium undergoes the first synthesis:

[0014] B 12 P2O 23 +23Mg→B 12 P2+23MgO

[0015] The reaction temperature is higher than 1300K.

[0016] The second synthesis is based on the reduction of boron phosphate by the following reaction:

[0017] 2BPO4+5MgB2+3Mg→B 12 P2+8MgO.

[0018] US2017203965 describes a method for producing boron phosphide (BP), which comprises a step of mixing boron phosphate (BPO4) and magnesium metal, a step of compacting the mixture at a pressure of the order of 20,000 psi (137 MPa), and a step of igniting the mixture by applying an energy input using a heated fuse. The compacting makes it possible to reduce impurities such as secondary phosphide (BPO4). 12 The formation of P2).

[0019] Used to produce boron phosphide B 12 The prior art synthesis methods of P2 have many disadvantages. The methods for synthesizing boron phosphide are slow, expensive, complicated, dangerous (using toxic elements) and low yield, and the obtained boron phosphide B 12 The purity of P2 is lower.

[0020] The use of prolonged high temperature thermal treatments is energy intensive, polluting and hazardous, making such processes economically uncompetitive.

[0021] Used for the synthesis of boron phosphide B 12 The P2 method is very time-consuming and difficult to implement, although it has interesting mechanical, thermal and electronic properties. 12 P2 is still rarely used in industry. Summary of the Invention

[0022] The present invention aims to propose a method for synthesizing boron phosphide B in large quantities at low cost and low energy consumption. 12 P2 is used to overcome the shortcomings of the existing technology methods.

[0023] The present invention provides a simple, safe and rapid solution for producing boron phosphide (B) using readily available and inexpensive reactants. 12 P2.

[0024] For these purposes, according to a first aspect, a method for producing boron phosphide (B) by self-propagating synthesis is proposed. 12 The method of P2 comprises a step of mixing the reactants in powder form, followed by a step of initiating an exothermic reaction, the exothermic reaction being carried out in a closed chamber.

[0025] Advantageously, for producing boron phosphide B by self-propagating high-temperature synthesis, advantageously at a temperature above 1800K 12 P2's methods include:

[0026] - a step of uniformly mixing the reactants in powder form, the reactants comprising at least

[0027] Alkaline earth metals, such as magnesium or calcium, and

[0028] Compounds containing boron B and phosphorus P, such as boron phosphate BPO4 or B 12 P2O 23

[0029] - a step of filling a reactor with a homogeneous mixture of reactants, the reactor comprising at least one heating element and a layer of thermal insulation on the inner wall of the bottom of the reactor and on one or more inner side walls, the thermal insulation being chemically inert and intended to surround the homogeneous mixture of reactants and to insulate it from the reactor walls during the reaction,

[0030] - a step of covering the top of the homogeneous mixture of reactants with a layer of chemically inert thermal insulation,

[0031] - a step of hermetically sealing the reactor,

[0032] - a step of initiating an exothermic reaction in the homogeneous mixture of reactants by triggering a heating element,

[0033] - a high temperature exothermic reaction step, which self-propagates in the reactant mixture in the reactor to obtain boron phosphide B 12 P2.

[0034] The airtight seal of the reactor prevents magnesium gas from escaping from the reactor during the reaction and achieves thermal sealing by avoiding contact with air, making it possible to obtain high-purity B 12 P2 is at least greater than 80%, and advantageously even greater than 90%.

[0035] During the exothermic reaction, temperatures of approximately 2000 K can be reached locally. The chamber in which the exothermic reaction takes place forms a closed reactor, thereby preventing oxidation reactions.

[0036] In some embodiments, the reactants include phosphoric acid, boric acid, and magnesium, and the exothermic reaction proceeds according to equation (2):

[0037] 12H3BO3+2H3PO4→B 12 P2O 23 +21H2O, then

[0038] B 12 P2O 23 + 23 Mg → B 12 P2 + 23 MgO (2)

[0039] In other embodiments, the reactants include boron phosphate, magnesium, and magnesium diboride, and the exothermic reaction proceeds according to equation (1):

[0040] 2 BPO4 + 3 Mg + 5 MgB2 → B 12 P2 + 8 MgO (1).

[0041] The chamber in which the exothermic reaction takes place forms a closed reactor, thereby preventing vapors, particularly magnesium vapors, from being released from the chamber into the air. The boiling point of magnesium at approximately 1363K is much lower than the temperature of the exothermic reaction of approximately 2000K.

[0042] Advantageously, magnesium is present in excess in the reactant mixture relative to the stoichiometry of equation (1) or equation (2). In some embodiments, magnesium is present in excess of 22% relative to the stoichiometry of equation (1).

[0043] Advantageously, magnesium diboride is present in excess in the reactant mixture relative to the stoichiometry of reaction formula (1).

[0044] The excess magnesium in the reactant mixture ensures complete reduction of the reactants and also helps control the final reaction temperature by introducing additional heat capacity and high latent heat for melting the magnesium, and especially for boiling the magnesium.

[0045] 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 imparting to the mixing container a rotational motion, a translational motion and an overturning motion for a period of, for example, more than 1 hour, advantageously more than 10 hours. Such a mixer is, for example, Type of brand name sales.

[0046] In some embodiments, the initiating step comprises heating a resistive element. The resistive element is, for example, a tungsten wire. In a particular embodiment, the tungsten wire has a diameter of approximately one hundred microns and a length of several centimeters.

[0047] Advantageously, the method comprises the step of placing an insulating material between the mixture of powdered reactants and the wall of the closed chamber. The insulating material is advantageously chemically inert, for example sodium chloride powder, or another salt (such as KCl or MgO), or mineral sand. Advantageously, the insulating material is in powder form.

[0048] Advantageously, the method comprises the step of cooling the chamber forming the reactor, the chamber remaining closed until cooling is complete. Cooling is provided, for example, by circulating a heat transfer fluid.

[0049] The method comprises a step of extracting the product of the exothermic reaction, the product extracted from the reactor being ground and washed, the washing advantageously comprising leaching.

[0050] In some embodiments, the powder obtained by grinding the product of the exothermic reaction is placed in a bath of hydrochloric acid and the resulting solution is then vacuum filtered, for example using a nanofilter specifically selected for its acid resistance, such as a Millipore-MF polyethersulfone filter.

[0051] In specific embodiments, washing comprises leaching with aqua regia.

[0052] Advantageously, the mixture of powdered reactants is not compacted before the initiation step. In other words, advantageously the mixture of powdered reactants is filled into the reactor without compacting.

[0053] No compaction means less powder handling, saving significant time and eliminating the need to purchase expensive equipment.

[0054] Advantageously, the filling rate in the reactor, corresponding to the ratio between the sample volume and the reactor capacity, is greater than 25%.

[0055] In some embodiments, the method is such that:

[0056] - Fill ratio is greater than 25%, and

[0057] an excess of magnesium between 10% and 50%, advantageously between 20% and 25%, advantageously an excess of magnesium diboride of at most 20%,

[0058] enables temperatures exceeding 1500°C to be obtained throughout the reactant mixture during the self-propagating exothermic reaction step in order to achieve greater than 80% B in the mixture. 12 P2 purity, advantageously greater than 95% B 12 P2.

[0059] Advantageously, the exothermic reaction is carried out in the absence of a solvent.

[0060] Advantageously, the reactor has a volume allowing the production of a weight of boron subphosphide greater than 50 g, advantageously greater than 100 g, even more advantageously greater than 500 g.

[0061] Advantageously, the filling step comprises inserting the intermediate wall which remains in the interior space of the reactor during filling of the space between the inner side wall and the intermediate wall of the reactor with insulation and then filling the central volume of the reactor defined by the interior space of the reactor surrounded by the intermediate wall with the reactant mixture up to a given height in the reactor.

[0062] According to a second aspect, the present invention relates to boron phosphide B obtained according to the method presented above 12 P2, obtained boron phosphide B 12 P2 includes:

[0063] - B with a rounded icosahedral shape ranging in size from 50 nm to 500 nm 12 P2 particles,

[0064] -Micron-sized B with "polyhedral rod" morphology 12 The length of the P2 particles is, for example, between 1 μm and 10 μm.

[0065] Advantageously, a mixture is proposed comprising only:

[0066] - Boron phosphide B as shown below 12 P2, and

[0067] - Boron phosphide BP,

[0068] Boron phosphide B in the mixture 12 The P2 purity is greater than 90%, advantageously greater than 95%, advantageously greater than 98%.

[0069] According to a third aspect, it is proposed to use the boron subphosphide obtained by the process as presented above as a hard material in machining or cutting tools, wire drawing dies, wear protection coatings, construction, mining or quarrying tools, drilling tools, armor-piercing ammunition and bulletproof vests.

[0070] According to a fourth aspect, it is proposed to use the boron subphosphide obtained by the method as presented above as a thermally conductive or thermoelectric material.

[0071] According to a fifth aspect, it is proposed to use the boron subphosphide obtained by the method as presented above as a filler in a thermosetting resin.

[0072] According to another aspect, it is proposed to use the boron subphosphide obtained by the method as presented above as neutron absorbing material.

[0073] According to another aspect, it is proposed to use the boron subphosphide obtained by the method as presented above as powder material for producing a shielding ceramic by sintering. BRIEF DESCRIPTION OF THE DRAWINGS

[0074] Further objects and advantages of the present invention will become apparent from the following description of some embodiments provided with reference to the accompanying drawings, in which:

[0075] - Figure 1 It is used to synthesize boron phosphide B 12 External view of three closed reactors of P2 with capacities of 50 g, 100 g, and 500 g, respectively;

[0076] - Figure 2 yes Figure 1 Boron phosphide B was obtained in a closed reactor with a capacity of 100 g as depicted in FIG. 12Diffraction pattern of P2 powder;

[0077] - Figure 3 Shown Figure 1 Boron phosphide B was obtained in a closed reactor with a capacity of 100 g as depicted in FIG. 12 Two SEM images of P2 powder;

[0078] - Figures 4 to 12 is a diagram of a reactor filling step according to one embodiment. DETAILED DESCRIPTION

[0079] The method advantageously uses a closed reactor, non-compacted reactants, and leaching to produce boron phosphide (B) on a large scale and at low cost. 12 P2.

[0080] The leaching method has been optimized to eliminate the unconverted 12 reactants of P2.

[0081] Figure 1 Three closed reactors are depicted that can accommodate sufficient amounts of reactants to synthesize 50 g, 100 g, and 500 g of powder, respectively.

[0082] Used for the synthesis of boron phosphide B 12 The P2 method is based on highly exothermic reactions between solid or gaseous reactants in the absence of solvent. These redox reactions are as follows:

[0083] 2BPO4+3Mg+5MgB2→B 12 P2+8MgO

[0084] or

[0085] 12H3BO3+2H3PO4→B 12 P2O 23 +21H2O, then

[0086] B 12 P2O 23 +23Mg→B 12 P2+23MgO

[0087] In other embodiments, calcium is used instead of magnesium.

[0088] For the sake of simplicity, we only present in this paper the synthesis of boron phosphide B using the following reaction 12 Results of P2

[0089] 2BPO4+3Mg+5MgB2→B 12 P2+8MgO.

[0090] The reactant mixture uses boron phosphate BPO4 and magnesium diboride MgB2 as boron and phosphorus sources, and magnesium Mg as the main reducing agent.

[0091] It should be noted that magnesium diboride MgB2 also contributes to this effect.

[0092] The reaction is extremely exothermic, with final temperatures exceeding 2000 K, well above the boiling point of magnesium.

[0093] In order to ensure complete reduction of the reactants, an excess of magnesium is advantageously added to the mixture.

[0094] This excess of magnesium makes it possible to control the final reaction temperature by introducing additional heat capacity and high latent heat for melting the magnesium, and especially for boiling the magnesium.

[0095] Furthermore, magnesium diboride MgB2 is advantageously present in excess in the mixture in order to limit the amount of boron phosphide B 12 The amount of boron phosphide BP present as a by-product in the final powder of P2.

[0096] These experimental conditions allowed for the complete elimination of boron phosphate BPO4 and a final yield of 80% for the reaction.

[0097] This yield corresponds to the amount of boron phosphide B obtained relative to the weight that might be expected based on the stoichiometry of the reaction. 12 The weight of P2.

[0098] Advantageously, adiabatic temperature calculations are performed using the NIST (National Institute of Standards and Technology) database of various reaction products.

[0099] Figure 2 Shown in Figure 1 Boron phosphide B obtained for 50 g and 100 g batches in an intermediate reactor with a 100 g capacity depicted in 12 X-ray diffraction patterns of P2 powder. Rietveld refinement of the diffraction patterns was performed on both powder batches using MAUD (Materials Analysis using Diffraction) software.

[0100] Rietveld refinement is disclosed, 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 performing global simulations on X-ray diffraction patterns from polycrystalline samples, and is implemented in various software packages, such as FullProf, Jana 2006, and MAUD. The MAUD software was developed by Luca Lutterotti (L. Lutterotti et al., MAUD: a friendly Java program for material analysis using diffraction. IUCr: Newsletter of the CPD, 21:14-15, 1999).

[0101] The powder consists of 97.441% boron phosphide B 12 It is composed of P2 and 2.559% of boron phosphide BP as an impurity.

[0102] For various diffraction patterns, a rhombohedral structure with an R-3m space group was observed. 12 The grid parameters of P2 in the hexagonal grid are and

[0103] Figure 3 Shown are morphological analyses of the powders obtained by scanning electron microscopy.

[0104] Two discrete populations of particles were observed.

[0105] A very small number of particles are micrometer-sized with a multifaceted rod-like morphology.

[0106] Otherwise, most of the secondary boron phosphide B 12 The size of P2 particles ranges from 50 nm to 500 nm, with a slightly rounded icosahedral shape.

[0107] The steps of the method will now be described in more detail.

[0108] The powder was first weighed and then homogenized in a mixer for 24 hours.

[0109] The mixer can be a rotating container mixer (cube, drum, double cone), or a countercurrent mixer comprising a stationary container and a movable member rotating inside the container, a fluidized bed mixer, or a static mixer.

[0110] Advantageously, the mixer is of the complex motion type, providing the mixing container with rotational, translational and tumbling motions, these motions being pulsating.

[0111] Mixers are manufactured, for example, by Willy Bachofen AG under the Type of mixer sold under a brand name. The Turbula mixer was introduced by Mayer-Laigle (Etude dynamique et effet du changement d'échellepour plusieurs systèmes particulaires en mélangeur thesis 2012).

[0112] This mixer makes it possible to obtain a high-quality final mixture.

[0113] The powder is then placed in a reactor surrounded by insulating walls on every side, thereby ensuring thermal and chemical insulation from the outer walls of the reactor.

[0114] The insulating wall is formed, for example, from a powdery material such as sodium chloride or mineral sand.

[0115] The thickness of this insulating wall must be chosen to limit the cooling rate of the sample, which advantageously improves the purity of the product by reducing the amount of BP and allows for a more gradual heat discharge, avoiding overheating of the reactor walls which could compromise their integrity.

[0116] For example, for a characteristic lateral length of the homogeneous mixture of reactants between 4 cm and 6 cm and a characteristic height between 10 cm and 16 cm, the thickness is between 2 cm and 3 cm.

[0117] This cooling must be optimized to be slow enough to optimize the time the sample spends at the high temperature so that B can be used without compromising productivity. 12 The amount of P2 reaches its maximum to impair BP, and this time is, for example, one to several hours.

[0118] This cooling is related to all the properties of the thermal insulator. For example, for powdered thermal insulation that allows for lower thermal conductivity, the properties of the insulation include particle size, thickness of the insulation, and type of insulation.

[0119] The magnesium evaporates during the reaction and mixes with the insulating salt, so a rapid opening would result in oxidation of the product and the magnesium, which would not allow the high reaction yields obtained by practicing the present invention to be achieved.

[0120] Figures 4 to 12 One embodiment of the filling of the reactor is illustrated.

[0121] exist Figure 4 In the first step depicted in FIG, a heating element 10 (such as a resistance wire) is placed at the bottom of the reactor. Resistance wire 10 is, for example, tungsten. By way of illustration, the tungsten wire has a diameter of approximately 100 microns and a length of several centimeters. Each end of the wire is connected to a stud 11 mounted on the base 12 of the reactor, with the inner wall 12a in contact with the thermal insulator.

[0122] exist Figure 5 In the second step depicted in FIG, the inner wall 13 of the reactor is positioned on the base 12. In the depicted embodiment, the inner wall 13 is a cylindrical surface of revolution, the longitudinal axis of the inner wall being perpendicular to the base 12 of the reactor. In other embodiments, the inner wall is in the form of a profiled part having a square or polygonal cross section. For ease of understanding, in FIG. Figures 5 to 11 In FIG, wall 13 is depicted in partial cross-section. Inner wall 13 is, for example, mounted or inserted onto base 12 of the reactor. When in place, the volume inside the reactor is laterally defined by inner wall 13 and base 12. This volume is open at the top, allowing the reactor to be filled. A resistance wire is placed within this internal volume.

[0123] exist Figure 6 In the third step depicted in , an insulating wall or base 14a is placed at the bottom of the interior volume of the reactor.

[0124] Advantageously, this insulating wall 14a or base is formed of a powder of a thermally insulating and chemically inert material, for example a salt powder (such as sodium chloride) or another salt (KCl or MgO), which enables thermal confinement during the reaction and avoids the migration of chemical elements from the walls of the reactor to the homogeneous mixture of the reactants during the highly exothermic reaction, making it possible to obtain a high purity of the obtained product B 12 P2. Therefore, the insulating wall 14a is easily positioned in the inner space of the reactor, below the resistance wire 10.

[0125] exist Figure 7 In the fourth step depicted in FIG, an intermediate wall or filling mold 15 is inserted and held in the interior space of the reactor, the inner wall separating the peripheral annular volume 16 from the central volume 17. In the embodiment depicted, the intermediate wall is in the form of a cylindrical rotating part, for example a steel cylinder, such as a belt. When the intermediate wall is inserted, it must not damage the heating element 10 and must have a hole or other opening to allow the wire to pass through, or be placed on the wire at the end located at the level of the base 14a.

[0126] exist Figure 8In a fifth step depicted in , an insulating wall or lateral layer 14b is formed in the peripheral annular space 16, advantageously by filling this annular space with a powder of chemically inert insulating material. This powder is advantageously the same as that used to form the insulating wall 14a at the bottom of the reactor R.

[0127] exist Figure 9 In the sixth step depicted in FIG, a reactive powder mixture is placed in the central volume 17 of the reactor. The reactive powder mixture is in contact with the resistance wire 10.

[0128] exist Figure 10 In a seventh step depicted in , the intermediate wall or filling mould 15 is removed.

[0129] exist Figure 11 In the eighth step depicted in FIG, an upper insulating wall or layer is formed at the top of the reactor. Advantageously, this upper insulating wall or layer 14c is formed from an insulating and chemically inert powder, in particular the powder used to form the insulating wall 14a at the bottom of the reactor ( Figure 6 ), or powder for forming the insulating wall or side layer 14b of the reactor ( Figure 8 ), such as salt or sand powder. The insulating powder is placed in the reactor until it is completely filled.

[0130] exist Figure 12 In the ninth step depicted in FIG, the reactor is hermetically sealed so that the homogeneous mixture of the reactants to be mixed can be thermally isolated from the atmosphere surrounding 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, with a reinforcement rod 19 connecting the two flanges.

[0131] Once the reactor is hermetically sealed, the reaction is initiated advantageously by a heating element located within the reactant mixture, such as a fusible heating wire, for example a tungsten wire, which has a diameter of about one hundred micrometers and a length of several centimeters and is heated by the Joule effect.

[0132] Once initiated, the reaction is complete within seconds.

[0133] The reactor was then kept closed until the sample cooled, after which it was extracted, ground and leached to dissolve the sodium chloride, NaCl, and magnesium oxide, MgO.

[0134] In an advantageous embodiment, the powder obtained is placed in a 2M hydrochloric acid bath at 110° C. for 1 h under magnetic stirring in order to dissolve the magnesium oxide MgO and the sodium chloride NaCl.

[0135] The solution is then cooled under vacuum, for example using a 220 nm ionizer selected particularly for its acid resistance. Polyethersulfone filter for filtration.

[0136] This operation is advantageously repeated twice in succession.

[0137] Advantageously, a final leaching is carried out using aqua regia with a HNO3 / HCl ratio of 1 / 3 to remove the last impurities.

[0138] The final product was then washed with distilled water several times to remove any residue from the powder surface.

[0139] Finally, the powder was dried in an oven.

[0140] Several parameters affect the yield of this reaction and a parameter study was conducted with the aim of minimizing the oxide phase (BPO4) and favoring the boron phosphide B 12 The most complete reaction possible is achieved by the formation of P2 rather than the formation of boron phosphide BP.

[0141] An excess of magnesium Mg advantageously enables a better reduction of the oxide phase.

[0142] An excess of magnesium diboride MgB2 advantageously reduces the amount of boron phosphide BP produced while slightly increasing the reducing power of the reaction mixture.

[0143] The total volume of the reactor and the ratio between the volume of the sample and the weight of the reaction mixture determine the ratio between the volume of the sample and the volume of the reactor, ie the filling rate of the reactor with the reaction mixture.

[0144] Advantageously:

[0145] - a fill rate greater than 25%, and

[0146] The excess of magnesium is between 10% and 50%, advantageously between 20% and 25%, so as to allow complete reduction of the oxide BPO4.

[0147] In addition to this excess of magnesium, up to a 20% excess of magnesium diboride may be added to match the 6:1 B:P stoichiometric ratio as closely as possible.

[0148] It has been found that a 22% excess of magnesium Mg, without an excess of magnesium diboride MgB2, makes it possible to achieve a yield close to 80%.

[0149] The position of the sample in the reactor has a significant impact on the final purity of the product.

[0150] Therefore, in a reactor with a capacity of 50 g, boron phosphide B 12 The purity of P2 was 86%, and in the intermediate reactor with a capacity of 100 g, this purity rose to 98%. In both cases, the boron phosphide B 12 The main and only impurity in P2 is boron phosphide BP.

[0151] Advantageously, the reactor has an internal diameter of 10 cm and a height of 20 cm, for a total internal volume of 1,570 cm 3 .

[0152] Advantageously, the powder occupies a space of 180 cm in the reactor. 3 Up to 350cm 3 These volumes correspond to cylinders with a diameter of 5 cm and heights between 9 cm and 18 cm.

[0153] Advantageously, the reactant powders are not compressed to facilitate initiation of the reaction.

[0154] Advantageously, the reactant powders have low swelling capacity, allowing the different precursors to remain in contact during the propagation of the reaction, thereby enabling a more complete synthesis.

[0155] Advantageously, the entire reactor is filled, the portion not occupied by the reaction mixture containing insulating material, such as sodium chloride or sand.

[0156] In this way, the waste heat is retained for a longer time, thus promoting the self-purification phenomenon and being very beneficial to the BPS 12 The synthesis of P2 instead of boron phosphide BP.

[0157] At high temperatures, BP decomposes into B 12 P2, BP is only stable in air up to 1400 K (Mukhanov et al., Dalton trans., 2016, 45, 10122, DOI: 10.1039 / c6dt00435k).

[0158] The method according to the invention has numerous advantages.

[0159] This method is targeted at boron phosphide B 12 The production of P2 is safe, practical, fast and cheap, simple to implement and easy to scale.

[0160] This method can synthesize high-value-added materials using low-cost reactants and easy-to-implement equipment.

[0161] Furthermore, the method requires very less manpower, thereby reducing costs.

[0162] By using the SHS reaction in a confined space (closed reactor), this method can synthesize up to 100 g of B using non-compacted reactants. 12 P2 powder.

[0163] The yield is very high, about 80%.

[0164] The purity is also extremely high, about 97%.

[0165] The method is fast and requires very little energy. Therefore, the present invention can synthesize a large amount of boron phosphide B at low cost and low energy consumption. 12 P2.

[0166] Until now, the use of this material with excellent mechanical properties has remained unexplored precisely due to the lack of large-scale synthesis possibilities with good economic potential.

[0167] This method can produce a large amount of secondary boron phosphide B 12 P2, subphosphide, is extremely hard and has remarkable chemical and thermal stability. Subphosphide is a refractory wide-bandgap semiconductor and exhibits thermoelectric power with a high quality factor at high temperatures.

[0168] The boron phosphide obtained by the process according to the invention is advantageously used as a hard material as a replacement for tungsten carbide.

[0169] Tungsten carbide CW is very expensive to synthesize and is made from elements at very high temperatures between 1700K and 2300K.

[0170] Furthermore, tungsten carbide CW is sintered by adding a binder: cobalt. Long-term exposure to cobalt powder associated with tungsten carbide leads to occupational, skin, respiratory and tumor diseases (see, for example, Robert (Analyse temporelledesexpositions et des pathologies professionnelles Liées au cobalt et aux poussières de métaux durs, de la fin des années 1990 à 2020, pharmacy thesis, 2021).

[0171] Boron phosphide B 12 P2 has far superior properties to tungsten carbide (harder, more resistant to chemical or thermal attack), and the method according to the invention is much cheaper than the method for synthesizing tungsten carbide CW.

[0172] Furthermore, boron subphosphide can be sintered without a binder, which indicates the possibility of sintering without a binder at low pressure by conventional methods.

[0173] Boron phosphide B 12P2 (hardness of about 35 GPa, comparable to commercial c-BN) is used in a wide range of applications currently reserved for tungsten carbide (or c-BN), such as machining and cutting tools (saws, drill bits, reamers, guide bushings, milling cutters, turning tools, punches), wire drawing or compacting dies, wear protection coatings (for turbine blades in aircraft engines, gas power stations, metallurgical furnaces), heavy tools (civil engineering, land leveling, drilling tool heads, mining and quarrying, tunnel boring machines), as well as pen balls, armor-piercing ammunition, and bulletproof vests.

[0174] The boron subphosphide obtained by the method according to the invention is advantageously used as a thermally conductive material, for example in integrated circuits.

[0175] As the power density of microelectronic components continues to increase, the thermal load on these components increases exponentially.

[0176] Due to the high heat density, heat dissipation requires materials with significant thermophysical properties, such as hard materials such as boron phosphide B 12 P2.

[0177] The boron phosphide obtained by the method according to the invention is advantageously used as a high-temperature thermoelectric material.

[0178] In the field of microelectronics, boron phosphide B 12 P2 can be p-type or n-type doped, enabling the production of diodes for use in harsh environments (chemical, thermal or mechanical) or for use in power electronics.

[0179] Boron phosphide B 12 P2 may also find major applications as a high-temperature thermoelectric material.

[0180] The boron phosphide obtained by the process according to the invention is advantageously used as filler in thermosetting resins.

[0181] Adding hard, lightweight, thermally conductive materials to thermoset resins enhances their thermomechanical and chemical resistance.

[0182] These thermoset materials are gaining increasing attention due to their remarkable properties: very good mechanical strength, as well as good resistance to chemicals (non-reactive materials) and heat.

[0183] Boron subphosphide obtained by the process according to the invention can be advantageously used in the nuclear field as a replacement for boron or boron carbide due to its high neutron absorption cross section.

[0184] Pure boron is difficult to produce in certain shapes (for example, for control rods in nuclear power plants), which is why boron carbide is often used instead in these applications, providing a high concentration of boron atoms in a solid, refractory form that is resistant to chemical attack.

[0185] However, the production of boron carbide B4C requires very high temperatures and is relatively complex.

[0186] Boron phosphide B 12 The simplicity of the low-cost synthesis of P2 could also allow the material to be promoted in nuclear applications, thereby replacing boron carbide B4C in some special cases.

Claims

1. A method for producing boron phosphide (B) by self-propagating high-temperature synthesis at a temperature above 1500°C in a closed adiabatic reactor 12 The method of P2, comprising: - a step of uniformly mixing reactants in powder form, said reactants comprising at least: √ alkaline earth metals, such as magnesium or calcium, and √ Compounds containing boron B and phosphorus P, or compounds containing boron B and compounds containing phosphorus P, such as boron phosphate BPO4 or B 12 P2O 23 - a step of filling the reactor (R) with a homogeneous mixture (M) of reactants, The reactor comprises at least: √ Heating element (10), a layer or base (14a) of thermal insulation on the inner wall (12a) of the bottom of the reactor, and a side insulating layer (14b) on one or more inner side walls (13), said thermal insulation being chemically inert and serving to surround said homogeneous mixture of reactants (M) during the reaction and to separate it from the walls (12a, 13) of the closed reactor, - a step of covering the top of said homogeneous mixture (M) of reactants and said side insulation layers (14b) with an upper layer (14c) of chemically inert thermal insulation, so that said homogeneous mixture (M) of reactants can be confined in said reactor, said mixture (M) being completely surrounded by said base insulation layer (14a), said side insulation layers (14b) and said upper insulation layer (14c), - a step of hermetically sealing the reactor with a cover (18), - a step of initiating a high-temperature exothermic reaction in said homogeneous mixture (M) of reactants in said closed reactor by triggering said heating element (10) in contact with said mixture (M), - a step of high temperature exothermic reaction at a temperature higher than 1500° C., said high temperature exothermic reaction self-propagating throughout the reactant mixture (M) of said reactor R to obtain said boron phosphide B 12 P2, The insulating material allows the residual heat to be retained longer in the homogeneous mixture (M) of the reactants, promoting the self-purification phenomenon and favoring the boron phosphide B 12 For the synthesis of P2, rather than the synthesis of boron phosphide BP, the mixture of reactants is defined as being confined in the reactor, The reactant powders therefore have a low expansion capacity, allowing the various precursors to remain in contact with each other as the reaction propagates. The magnesium is in excess in the reactant mixture to enhance oxide reduction, - a step of extracting the product of the exothermic reaction, washing said product extracted from said reactor (R), said washing comprising one or more leachings followed by rinsing, in order to obtain said sub-boron phosphide B 12 P2.

2. The method according to claim 1, characterized in that Said homogeneous mixing (M) of said reactants is performed by means of a two-dimensional powder mixer or a three-dimensional powder mixer, such as a mixer capable of imparting rotational, translational and tumbling movements to the mixing container, for example for a period of more than 1 hour, advantageously more than 10 hours.

3. The method according to claim 1 , wherein the method comprises: The filling and confinement of the mixture of pulverulent reactants in the reactor (R) is carried out without compaction.

4. The 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 reactant sample and the capacity of the reactor (R), is greater than 25%.

5. The method according to any one of claims 1 to 4, characterized in that The reactants include phosphoric acid, boric acid and magnesium, and the exothermic reaction is according to reaction formula (2): 12H3BO3+2H3PO4→B 12 P2O 23 +21H2O, then B 12 P2O 23 + 23 Mg → B 12 P2 + 23 MgO (2)。 6. The method according to any one of claims 1 to 4, characterized in that The reactants include boron phosphate, magnesium and magnesium diboride, and the exothermic reaction is according to reaction formula (1): 2 BPO4 + 3 Mg + 5 MgB2 → B 12 P2 + 8 MgO (1)。 7. The method according to any one of claims 5 or 6, characterized in that Magnesium is in excess in the reactant mixture relative to the stoichiometry of reaction formula (1) or reaction formula (2).

8. The method according to any one of claims 6 or 7, characterized in that Magnesium diboride is in excess in the reactant mixture relative to the stoichiometry of reaction formula (1).

9. The method according to any one of claims 6 to 8, characterized in that: - a fill rate greater than 25%, and an excess of magnesium between 10% and 50%, advantageously between 20% and 25%, advantageously an excess of magnesium diboride of at most 20%, enabling temperatures exceeding 1500° C. to be obtained throughout said reactant mixture during the self-propagating exothermic reaction step, To convert the reactant into B 12 The conversion of P2 is advantageously greater than 80%, and only B is leached. 12 After P2 and BP products, B 12 The proportion of P2 is advantageously greater than 95%.

10. The 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 wire.

11. The method according to any one of claims 1 to 10, characterized in that The thermally insulating and chemically inert material is sodium chloride or another salt such as KCl or MgO.

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

13. The method according to any one of claims 1 to 12, characterized in that Said step of extracting said product of said exothermic reaction comprises a grinding step prior to leaching.

14. The method according to any one of claims 1 to 13, characterized in that The washing of the extraction step may comprise one or more leachings, for example one or more leachings with hydrochloric acid followed by leaching with aqua regia.

15. The method according to claim 14, characterized in that The powder obtained by grinding the product of the exothermic reaction is placed in a bath of hydrochloric acid and the resulting solution is then vacuum filtered, for example using a nanofilter selected especially for its acid resistance, such as Polyethersulfone filter.

16. The method according to any one of claims 1 to 15, characterized in that The exothermic reaction is carried out in the absence of solvent.

17. The method according to any one of claims 1 to 16, characterized in that The volume of the reactor (R) is such as to be able to produce more than 50 g, advantageously more than 100 g, advantageously more than 500 g of B 12 P2 weight.

18. The 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 form a base (14a) of the thermal insulation, for example up to the height of the heating element (10); - a step of inserting an intermediate wall (15) or a filling mold which remains in the internal space of the reactor (R) during the filling operation carrying out the filling operation in the space between the inner side wall and the intermediate wall (15) of the reactor (R) by means of the thermal insulator so as to produce a layer (14a) of thermal insulation on the side wall or walls of the reactor, The filling operation is carried out by the reactant mixture in the central volume (17) of the reactor (R) defined by the inner 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 said intermediate wall (15), - A step of depositing a layer (14c) of chemically inert thermal insulation to cover said top of said homogeneous mixture (M) of reactants and on top of said side thermal insulation layer (14b).

19. A final product obtained using the method defined in one of claims 1 to 18, characterized in that The final product has: - B with a rounded icosahedral shape ranging in size from 50 nm to 500 nm 12 P2 particles, -Micron-sized B with "polyhedral rod" morphology 12 P2 particles.

20. The final product according to claim 19, comprising only: - Boron phosphide B 12 P2, and - Boron phosphide BP, Boron phosphide B in the mixture 12 The amount of P2 is greater than 90%, advantageously greater than 95%, advantageously greater than 98%, with the remainder being boron phosphide BP.

21. Use of the end 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, construction, mining or quarrying tools, drilling tools, armor-piercing ammunition, bulletproof vests.

22. Use of the final product according to any one of claims 19 or 20 as a thermally conductive material or a thermoelectric material.

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

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

25. Use of the end product according to any one of claims 19 or 20 as powder material for producing shielding ceramics by sintering.

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