Method for synthesising a boron carbide powder without emitting carbon monoxide

Abstract

The invention relates to a dry route method for producing a boron carbide powder from the reduction of an alkaline boron oxide of chemical formula AyBxO(y+3x) / 2 or a precursor of this boron oxide, where A is an alkaline chemical element and B is the chemical element boron, according to the balanced reaction equation: 4 AyBxO(y+3x) / 2 +xC + 4zR → xB4C + 4yARzO(y+3x) / 2 (2), where R is at least one reducing element selected from Al and Si, and C is the carbon provided by a carbon powder having a content by weight of the element C of at least 95%. The invention also relates to the powder resulting from said method.

Description

[0001] Description

[0002] Title of the invention: METHOD FOR SYNTHESIZING A BORON CARBIDE POWDER WITHOUT EMISSION OF CARBON OXIDE

[0003] The invention relates to a new method for manufacturing or synthesizing boron carbide powder.

[0004] Boron carbide offers numerous advantages, including high refractoriness and hardness, very high electronic and thermal conductivity, and excellent chemical inertness. This makes it a potentially interesting material for several applications, such as refractories where high thermal and electrical conductivity are beneficial, including heat exchangers, structural ceramics, membranes in certain high-temperature or highly aggressive chemical environments, particularly non-ferrous metals, cutting tools, shielding, and abrasion-resistant coatings.

[0005] All these applications explain why the demand for this material is very high and growing at present.

[0006] Boron carbide does not exist in nature. Boron carbide, in particular, can be obtained, for example, by the carbothermic reduction of boron oxide. In this latter case, the reaction consists of reacting a mixture of powders according to the following simplified reaction at a temperature above 1500°C:

[0007] However, this process has a theoretical material yield of 24.7%. It generates a significant amount of CO in gaseous form, which poses hygiene, safety and environmental problems.

[0008] The manufacturing processes for this material are also all the more costly and energy-intensive as the final boron carbide powder sought is fine (typically with a median diameter between 5 and 50 micrometers) or even ultrafine (median diameter less than 5 micrometers).

[0009] Another known solution involves metallo-thermal reduction using a metallic element such as Mg or Ca instead of carbon. The exothermic reactions associated with the use of these reagents produce carbides by high-temperature self-propagating synthesis (SHS), but lead to incomplete conversion of the reagents and make the separation of the resulting B4C material very difficult from other reaction products such as magnesium and / or calcium oxides.

[0010] The aim of the invention is therefore to improve the synthesis processes described above in order to obtain a boron carbide powder:

[0011] -fine, that is to say typically a powder with a median diameter between 0.5 and 50 micrometers;

[0012] -of better purity, that is to say, whose elemental mass content of the sum of the following contaminants: oxygen (O), sulfur (S), carbon (C), nitrogen (N), iron (Fe), phosphorus (P), silicon (Si) and / or aluminum (Al), particularly in metallic form, cobalt (Co), nickel (Ni), alkali metals (Li+Na+K+Rb+Cs), alkaline earth metals (Be+Mg+Ca+Sr+Ba) is less than 5% and preferably, that is to say, whose total elemental mass content of contaminants is less than 3%, or even less than or equal to 2%; while exhibiting:

[0013] -a satisfactory material yield, for example of at least 10%;

[0014] -an easy ability to extract said carbide powder from the reaction products;

[0015] -no CO or CO2 emissions and

[0016] -without resorting to a complex industrial synthesis process.

[0017] Summary of the invention:

[0018] In particular, according to a first aspect, the present invention relates to an alternative process for manufacturing boron carbide powder at a temperature below 1600°C, preferably below 1500°C, preferably below 1400°C, preferably below 1300°C, or even 1200°C, achieving this goal in particular through an appropriate choice of starting powders, without the use of solvent or surfactant.

[0019] More specifically, the present invention relates to a process for manufacturing boron carbide powder by reducing an alkali boron oxide, the alkali element preferably being selected from Na, K, Li, said process comprising the following steps: - preparation of a mixture of raw materials comprising, and preferably consisting of: a) a powder comprising, and preferably consisting of, an alkali boron oxide of chemical formula A y B x O( y+3x) / 2 or a precursor of this oxide, in particular in the form of a hydrate of said oxide, the mass content of which in boron, expressed as oxide B2O3, is at least 30%, preferably at least 40%, preferably at least 50%, where A is an alkali metal element preferably selected from Na, K, Li and B is the element Boron; and b) a metallic powder of at least one reducing element R, R being selected from Al and / or Si, in particular an Al powder, a Si powder, a mixture of an Al powder and a Si powder, or a powder of an ASi alloy; and c) a carbon powder, the mass content of the element carbon C being at least 95%; in the respective proportions leading, preferably corresponding, to the following balanced reaction, expressed in terms of said A y B x O( y+ 3x) / 2, R and C:

[0020] 4A y B x O(y + 3x) / 2 + xC + 4zR - xB4C + 4yAR z O( y+ 3x) / 2 (2)

[0021] - heating said mixture in a chamber under a flow of rare gas, at a temperature above 600°C and below 1600°C, until the B4C compound is obtained, said mixture of raw materials having the following characteristics: - the median particle diameter of said powder comprising alkali boron oxide, or the precursor of this oxide, is between 1 and 100 micrometers; and

[0022] - the median particle diameter of the metal powder is between 5 and 200 micrometers; and

[0023] -x is between 0.5 and 10, preferably x is less than 5, preferably less than or equal to 4, or even less than or equal to 2; and

[0024] -y is greater than or equal to 0.5, preferably less than 6, preferably less than or equal to 2; and

[0025] -z is greater than 0.5 and less than 2, preferably less than 1.5.

[0026] These parameter ranges are particularly well suited to obtain a fine, high-purity boron carbide powder with a satisfactory yield, without CO release and with easier final powder extraction.

[0027] According to preferred but non-limiting embodiments of the present invention, which may optionally be combined with each other:

[0028] -The mass sum of said powder comprising an alkali boron oxide or an alkali boron oxide precursor, said metallic powder of at least one reducing element R, and said carbon powder C represents more than 80% by weight of said raw material mixture, or even more than 90% by weight, or even more than 95% by weight of said mixture. Preferably, said mixture consists solely of said three powders.

[0029] - Alkaline boron oxide powder can be hydrated, but it is preferably dehydrated;

[0030] -the mass content of said oxide A y B x O( y+ 3x) / 2 of the powder comprising an alkali boron oxide or an alkali boron oxide precursor, is at least 95%;

[0031] - metallic powder can be hydrated, but it is preferably dehydrated;

[0032] - The metal powder is a mixture of aluminum particles on the one hand and silicon particles on the other. Preferably, the metal powder is an aluminum powder.

[0033] - the alkali element A is preferably sodium (Na);

[0034] - the gas flow rate sweep rate in said enclosure being between 0.5 and 10 L / min per m 3 enclosure;

[0035] - the median particle diameter of said powder comprising alkali boron oxide is between 30 and 100 micrometers, preferably between 30 and 50 micrometers;

[0036] - The median diameter of the carbon powder is greater than 1 nm and less than 10 micrometers, preferably less than 1 micrometer. Carbon black is preferably used.

[0037] - the residual mass content of H2O of said mixture of raw materials is less than 5%, as measured at a temperature of 400 °C at atmospheric pressure.

[0038] As will be described in more detail later, such a combination of parameters advantageously makes it possible to obtain a fine powder of high purity boron carbide with a satisfactory material yield, using a process that does not release CO or CO2 and allows for easy extraction of said boron carbide powder without resorting to an industrially complex powder synthesis process.

[0039] The respective proportions leading to the reduction of alkali boron oxide to boron carbide are the substantially stoichiometric quantities of the different reactants mentioned in points a) to c) above leading to the overall reaction (2).

[0040] In particular, unlike synthesis processes using molten salts or dissolving in a solvent, especially water, the process according to the invention by dry means and in particular by using a weakly or non-hydroxylated alkali boron oxide powder rather than the use of alkali salt in solution advantageously allows an optimal reaction, i.e. with a maximum mass balance, while allowing easy separation of the boron carbide powder after synthesis in the chamber.

[0041] According to other preferred embodiments of the present invention, which may optionally be combined with each other:

[0042] - The alkali boron oxide is chosen from sodium metaborate with the chemical formula NaBCh, anhydrous borax with the formula Na2B4U7, or even other borates containing other elements such as NaCaBsOc. Preferably, the alkali boron oxide is sodium metaborate with the chemical formula NaBCh and / or anhydrous borax with the formula Na2B4U7.

[0043] - The precursor of alkali boron oxide is generally chosen from compounds in which boron is in a non-carbide, non-metallic form and in a form other than a salt, and in particular other than that of a halide.

[0044] - The precursor of alkali boron oxide can be a hydrated borate such as natural borax with the formula Na2B4O7.10H2O, tincalconite with the formula Na2B4O7.5H2O, kernite with the formula Na2B4O7.4H2O, ulexite with the formula NaCaBsOg.BhbO, proberite NaCaBsOç.ShbO. Preferably, the hydrated borate is chosen from natural borax with the formula Na2B4O7.10H2O, tincalconite with the formula Na2B4O7.5H2O, and kernite with the formula Na2B4O7.4H2O.

[0045] - According to one possible mode, when naturally in hydrated form, the precursor of alkali boron oxide is pre-dehydrated in order to react more efficiently in reaction (2) and avoid the production of hydrogen.

[0046] - According to another possible mode, in the case of the presence of alkali boron oxide used is in hydrated form, the hydrogen produced by the precursor of alkali boron oxide on the side of reaction (2) is recovered to be stored separately in order to be valorized.

[0047] - The powder comprising the alkali boron oxide is an anhydrous alkali borate powder, preferably an anhydrous sodium borate powder.

[0048] - The median diameter (D50) of particles of said powder comprising alkali boron oxide is greater than 10 micrometers, preferably greater than or equal to 30 micrometers and / or less than 100 micrometers, preferably less than 80 micrometers, or even less than 50 micrometers.

[0049] - The diameter D90 of particles of said powder comprising alkali boron oxide is less than 100 micrometers, preferably less than 80 micrometers, preferably less than or equal to 50 micrometers, more preferably less than or equal to 40 micrometers.

[0050] - The median diameter (D50) of particles of said metallic powder of reducer R is greater than 10 micrometers, preferably greater than or equal to 30 micrometers and / or preferably less than 100 micrometers, preferably less than 50 micrometers.

[0051] - The ratio of the median particle diameter of said powder comprising alkali boron oxide to the median particle diameter of said metallic powder of at least one reducing element R is less than 30, preferably less than or equal to 10 and / or greater than 1, preferably greater than 2, or even greater than 5. This ratio allows the conversion rate to boron carbide to be optimized.

[0052] - The ratio of the median particle diameter of said powder comprising alkali boron oxide to the median particle diameter of said carbon powder is less than 100 and / or greater than 10, preferably greater than 20, or even greater than 50.

[0053] The total mass content of the elements Si, Al, Fe, Na, K, Ca, and Mg, expressed as the sum of their oxides SiO2+Al2O3+Fe2O3+Na2O+K2O+CaO+MgO, in the powder containing alkali boron oxide or its precursor, is less than 5%. Preferably, the mass content of S1O2 in the powder containing alkali boron oxide or its precursor is less than or equal to 2%. The mass content of Al2O3 and / or SiO2 in said powder containing alkali boron oxide or its precursor is preferably less than or equal to 2%. Preferably, the mass content of Fe2O3+Na2O+K2O+CaO+MgO in said powder containing alkali boron oxide or its precursor is preferably less than or equal to 1%.Preferably, the mass content of the sum of the elements carbon (C) + nitrogen (N) of said powder comprising alkali boron oxide or the precursor of this boron oxide is preferably less than or equal to 1%, preferably less than or equal to 0.5%.

[0054] - Apart from the metallic reducing powder R which may contain this element when it is in particular silicon powder, the mass content of the mixture of raw materials before reaction of the element silicon (Si), expressed as SiCh, and / or of the element aluminium (Al), expressed as Al2O3, is preferably less than 2%, preferably less than 1%.

[0055] - The raw materials were previously dried at a temperature between ambient temperature and 150 °C.

[0056] - The synthesis temperature, i.e. the heating temperature in said enclosure, is greater than 700°C, preferably greater than 800°C and / or less than 1500°C, preferably less than 1300°C, more preferably less than 1200°C.

[0057] - The pressure in the enclosure is kept almost constant, for example between 0.5 and 1.5 bars and preferably the enclosure is at atmospheric pressure (1 bar).

[0058] The gas used to sweep the chamber is preferably a noble gas, for example Argon or Helium, preferably Argon. Preferably, this gas, a noble gas, is introduced into the chamber and brought into contact with the raw material mixture. The flow rate is preferably 0.5 to 5 L / min per m³ 3 enclosure, preferably between 0.5 and 3 L / min per m 3 , preferably between 0.5 and 2 L / min per m 3chamber. Too weak a sweep leads to an incomplete reaction, more specifically to undesirable residues present in the final boron carbide powder.

[0059] - A non-oxidizing gas scavenging flow rate ratio of 0.005 to 1 L / min per m 3 The enclosure heating power per kW is particularly optimal, preferably between 0.01 and 0.5 / min per m² 3 enclosure and per KW of enclosure heating power.

[0060] After reaction, the finely divided crude boron carbide powder can be easily extracted from the crude mixture from the chamber after the heating step.

[0061] According to one possible method, a sieving operation, typically at a diameter of 100 micrometers, preferably 50 micrometers, or even light crushing or vibration, allows for the removal of agglomerates and the separation of the crude boron carbide powder. A suspension is made by adding a solvent, preferably deionized water, to the previously ground crude mixture, in a mass ratio of 1 part crude mixture to at least 20, preferably 50 parts solvent. This suspension is filtered to an optimal size, typically less than 30 micrometers, preferably less than 20 micrometers, to allow the liquid containing the very fine residues of the other reaction products to pass through (2).The filtration retentate, consisting of boron carbide powder, is then calcined or dried, preferably under air, at a temperature above 80°C, preferably above 100°C and / or preferably below 300°C, preferably below 200°C, preferably below 150°C.

[0062] According to one possible method, the liquid resulting from the filtration of the previously described suspension, comprising products of reaction (2) other than boron carbide powder, is heat-treated in the presence of water and a basic solution to form a hydrate of element R and an alkali hydroxide. This method allows for the valorization of the product of reaction (2) with the formula AR z O( y+ 3x) / 2. Preferably, this possible mode is particularly advantageous in the case where the element R is Al and the alkali A is sodium.

[0063] The invention also relates to a boron carbide powder that can be obtained, preferably obtained according to the previous process.

[0064] Preferably, said powder comprises by mass more than 95%, preferably more than 97%, or even more than 98% of compound B4C. The median particle diameter of this powder is between 0.5 and 50 micrometers, and it further comprises the following mass contents: - elemental oxygen (O): less than 1.3%, preferably less than 1.2%, preferably less than 1% or even less than 0.5%;

[0065] - elemental carbon (C): less than 0.5%, preferably less than 0.1%;

[0066] - elemental nitrogen (N): less than 0.5%; preferably less than 0.1%;

[0067] - elemental sulfur (S): less than 400 ppm, preferably less than 300 ppm or even less than 150 ppm, or even less than 100 ppm or less than 50 ppm;

[0068] - elemental iron (Fe): less than 0.45%, preferably less than 0.4%;

[0069] - elemental nickel (Ni): less than 0.4%, preferably 0.2%, or even less than 0.1%;

[0070] - elemental cobalt (Co): less than 0.4%, preferably 0.2%, or even less than 0.1%;

[0071] - elemental sum of alkali metals Li+Na+K+Rb+Cs: less than 1%, preferably less than 0.5%;

[0072] - elemental sum of alkaline earth metals (Be+Mg+Ca+Sr+Ba): less than 1%, preferably less than 0.5% or even less than 0.25%;

[0073] - content of element R in metallic form: less than 2%, preferably less than 1%, preferably even less than 0.5%, R being preferably different from M, R being at least one element chosen from Al, Si or their alloy, the sum of the other elements being less than 2%, preferably less than 1%.

[0074] According to other preferred embodiments of the present invention, which may optionally be combined with each other:

[0075] - boron carbide powder comprises the compound AR z O( y+ 3x) / 2, in particular as a product of reaction (2), especially as an impurity.

[0076] - the mass content of the boron carbide powder in compound AR z O( y+ 3x) / 2 is greater than 0.01% and less than 1%, preferably less than 0.5%. - the elemental sum oxygen (O) + nitrogen (N) + carbon (C) of the boron carbide powder is less than 1.5%, or even less than or equal to 1.2%.

[0077] - the mass content of silicon (Si) in metallic form of boron carbide powder is less than 0.1%.

[0078] - the mass content of aluminium (Al) in metallic form of boron carbide powder is less than 2%, preferably less than 1%, preferably less than 0.5%.

[0079] - the final boron carbide powder according to the invention does not comprise other crystalline phases such as B2O3, SiC, Al4C3, SiB4, SiBô, SiBs, as measured (detectable) by X-ray diffraction. Preferably said powder comprises only a crystalline phase of B4C, as measured (detectable) by X-ray diffraction.

[0080] - the (D9O-DIO) / D5O ratio of equivalent particle diameter of the crude powder, i.e., the powder after extraction from the crude mixture from the chamber after the heating step, in particular after separation of the product of reaction (2) of formula AR zO(y+3x) / 2, is less than 2, preferably less than 1.5, preferably even less than 1.2 or even less than 1. The percentiles D10, D50 and D90 being the diameters corresponding respectively to the percentages of 10%, 50% and 90% on the cumulative distribution curve of grain diameter by volume classified in ascending order of said powder.

[0081] Such a high-purity boron carbide powder with fine and regular particle size makes it possible to obtain by sintering a sintered ceramic body with a low total porosity of less than 10% by volume without the need for additions of transition metals such as Ni, Fe or Co which are likely to lead to the formation of undesirable secondary metal borides.

[0082] Such a powder makes it possible to obtain a sintered ceramic body in the form of a single piece in which at least one dimension, preferably all overall dimensions, is greater than 5 cm, or even greater than 10 cm, and which exhibits a total porosity also less than 10%, a very narrow pore size distribution, without sintering deformation and without shrinkage cracking. The (D9O-DIO) / D5O ratio of equivalent particle diameter of the boron carbide powder is advantageously less than 1.5, preferably even less than 1.2, or even less than or equal to 1.0.

[0083] The invention also relates to a mixture comprising between 90 and 99.9% by mass of a boron carbide powder according to the invention and between 0, 1, and 10% by mass of one or more sintering powders selected from aluminum diboride, magnesium diboride, zirconium diboride, titanium diboride, tungsten pentaboride, calcium hexaboride, and silicon hexaboride powders, the purity of which is greater than 95% by mass, preferably greater than 98% by mass. Purity greater than 95% by mass is understood to mean that of said phase or of the most stable principal compound: for example, in the case of an aluminum boron carbide powder, more than 95% by mass of AlB2, or for a tungsten pentaboride powder, that it contains more than 95% by mass of W2B5.

[0084] According to one possible method, said mixture may also include an aluminium powder, a cobalt powder, a metallic iron powder partially or totally replacing said sintering powder(s).

[0085] The invention also relates to a method for manufacturing a sintered ceramic body comprising the following steps: a) preparation of a starting charge comprising:

[0086] - boron carbide powder according to the invention, or a mixture of powders as described above comprising said boron carbide powder and one or more of said sintering powders.

[0087] - an aqueous solvent, in particular deionized water,

[0088] - preferably, shaping additives, b) shaping the starting charge into a preform, preferably by pressing, c) demolding after hardening or drying, d) optionally, drying the preform, preferably until the residual moisture content is between 0 and 0.5% by weight, e) loading the preform into a furnace and firing it under an inert atmosphere, preferably under argon, or under vacuum, preferably at a temperature between 1600°C and 2200°C. The invention also relates to the sintered ceramic body obtained by the preceding process and the use of said sintered ceramic body as all or part of a membrane, particularly for filtering liquids or gases, as abrasion-resistant shielding or coating, as a refractory lining or block, as a heat exchanger, as a firing support, as a metal melting crucible, particularly for non-ferrous metals, of a cutting tool.

[0089] Definitions:

[0090] The following indications and definitions are given, in relation to the preceding description of the present invention:

[0091] - In this description, unless otherwise specified, all percentages are given by mass, on a dry matter basis.

[0092] - Alkaline boron oxide is defined as any oxide comprising boron and oxygen, with at least one other element, in particular chosen from Na, K, Li. Preferably, this alkali boron oxide does not contain any other elements than boron, oxygen and alkali.

[0093] -Here, an alkali boron oxide precursor is defined as a powder comprising the element boron (B) and an alkali metal, in hydrated form and / or possibly containing hydroxyl groups, which, by heating or oxidation, preferably in air, at a temperature below 600°C, produces reagent A y B x O( y+3x) / 2 present in the chemical equation of reaction (2). For example, it could be an alkali boron hydrate or hydrated alkali borate, for example a hydrated sodium borate.

[0094] - An alloy of two metallic elements is understood to be the product of the fusion between these two elements within the same amorphous or crystalline structure.

[0095] - The material yield is calculated by dividing the mass of crude boron carbide powder obtained by that of the dry powder mixture of reactants (moisture less than 5%) of reaction (2) before heating.

[0096] -By crude mixture is meant the mixture directly obtained at the outlet of the enclosure after heating and reaction of the mixture of raw materials and before further treatment of extraction of the crude boron carbide powder for example by screening or light grinding.

[0097] The median diameter (or median "size") of the particles constituting a powder is obtained, according to the present invention, by characterizing the particle size distribution, in particular using a laser particle size analyzer. This characterization is conventionally performed with a laser particle size analyzer in accordance with ISO 13320-1. The laser particle size analyzer could be, for example, a Partica LA-950 from HORIBA. For the purposes of this description, and unless otherwise stated, the median particle diameter refers to the particle diameter below which 50% of the mass population lies.The median diameter or median size of a set of particles, particularly a powder, is defined as the D50 percentile. This is the size that divides the particles into first and second populations equal in volume, each containing only particles larger or smaller than the median size. The D10 and D90 percentiles of a powder of grains or particles can also be determined. These are the diameters corresponding to the 10th and 90th percentiles, respectively, on the cumulative distribution curve of grain diameters by volume, ranked in ascending order.

[0098] -The elemental chemical concentrations can be determined according to ISO 21068:2008. In particular, the following mass concentrations of:

[0099] - O, N, C, and S are measured using a LECO® brand analyzer.

[0100] - Si, Al, Co, Ni, alkali metals (Li+Na+K+Rb+Cs), alkaline earth metals (Be+Mg+Ca+Sr+Ba), Ti, Zr, Hf, B, Fe, P can be determined by ICP (“Induction Coupled Plasma”).

[0101] - Aluminium and / or silicon in metallic form can be determined by X-ray diffraction,

[0102] - Compound B4C can be determined by X-ray diffraction,

[0103] - Hydroxyl (OH) from alkali boron oxide powder can be measured by pH measurement.

[0104] - The actual powder density is measured by helium pycnometry, for example using Micromeritics AccuPyc1330 equipment,

[0105] -The total porosity of a ceramic body is the ratio, expressed as a percentage, of the apparent density measured for example according to ISO18754 to the absolute density measured for example according to ISO5018.

[0106] Unless otherwise stated, all percentages in this description are mass percentages. Detailed description

[0107] The invention and its advantages will be better understood upon reading the detailed description that follows. Of course, the present invention is not limited to such a method in any of the aspects described thereafter.

[0108] The initial raw material mixture includes:

[0109] -a powder comprising alkali boron oxide A y B x O( y+ 3x) / 2 (for example, for a sodium metaborate powder NaBCh, A=Na, y=1; x=1). Preferably, the mass content of A y B x O( y+ 3x) / 2 of said powder is at least 95%, and

[0110] -a metallic powder of a reducing element R chosen from Al, Si, in particular an Al powder, a Si powder, a mixture of an Al powder and a Si powder, or a powder of an ASi alloy, the mass content of elements other than Al and Si of which is preferably less than 0.5%, and

[0111] -a D carbon powder, chosen from carbon, petroleum coke (residue from petroleum distillation), amorphous carbon, graphite, preferably black carbon.

[0112] The mixing is carried out under standard conditions for those skilled in the art. This dry mixing preparation stage ensures intimate contact between the particles. Depending on the method, it is performed in a rubber ball mixer, a tumbler mixer, or other devices known to those skilled in the art. Pre-grinding may be carried out to adjust the particle size of the starting raw materials if necessary.

[0113] If necessary, certain hydrated raw materials, in particular the precursor of alkali boron oxide powder, may be dried or even calcined in order to reduce their H2O or OH hydroxyl content and obtain said alkali boron oxide. This is particularly preferred in the case where the precursor of the alkali boron oxide powder is chosen from natural borax of formula Na2B4O?.10H2O (sometimes also expressed as Na2B4O5(OH)4-8H2O), tincalconite of formula Na2B4O?.5H2O (sometimes also expressed as Na2B4O5(OH)4.3H2O), kernite of formula Na2B4O?.4H2O (sometimes also expressed as Na2B4Oô(OH)2-3H2O), ulexite of formula NaCaBsOç.S^O (sometimes also expressed as NaCaB5Oô(OH)6-5 H2O), probertite NaCaB5O9.5H2O (sometimes also expressed as NaCaB5O7(OH)4.3H2O). This treatment reduces the presence of hydrogen in the form of water H2O adsorbed on the surface of the powder or hydroxyl OH.It improves the conversion rate to boron carbide, which also results in a crude B4C powder with, after synthesis, a very low content of elemental reducing metal R.

[0114] Preferably, to maximize the conversion rate to boron carbide B4C, the hydroxyl (OH) content of the raw materials in reaction (2) is minimized. In particular, borates can be calcined to dehydroxylate them. Even more preferably, the alkali boron oxide powder has a hydroxyl content calculated by dividing its mass of OH by the mass of alkali boron oxide A y B x O( y+ 3x) / 2 is less than 40%, preferably less than 30%, more preferably less than 20%, or even less than 10%, or even less than 5% or even substantially zero.

[0115] The median size or median diameter of the alkali boron oxide particles and that of the R-element reducing metal particles is preferably between 10 and 100 micrometers, preferably between 30 and 80 micrometers. Preferably, the ratio of the median size of the particles comprising the alkali boron oxide to that of the R-element reducing metal particles is between 1 and 10.

[0116] Preferably, in a mixture according to the invention comprises alkali boron oxide, carbon powder and a metallic powder of elemental reducer R in the stoichiometric proportions of reaction (2).

[0117] Without departing from the scope of the invention, the addition of carbon powder can be such that there is a slight excess of carbon in reaction (2), preferably less than or equal to 10% by mass relative to the stoichiometric amount of carbon. This allows for the elimination of the residual oxygen content of the boron carbide powder. A higher carbon content in the reaction has the disadvantage of favoring the formation of carbides other than B4C. In particular, and preferably, the alkali boron oxide is sodium metaborate and the metal powder is aluminum and / or a silicon metal powder, preferably aluminum powder.

[0118] In said reaction (2), according to one possible mode, the quantity of said alkali boron oxide in the form A y B x O( y+The amount of 3x) / 2 in the raw material mixture is in excess, but preferably less than 10%, or even less than 5%, relative to the stoichiometry, by mass. The mixture is dried preferably in air, preferably at a temperature above 40°C, more preferably at a temperature above 100°C, in order to obtain a mixture whose residual moisture content, that is to say the residual mass content of H2O measured by a moisture meter well known to those skilled in the art, of said raw material mixture is less than 5%, preferably less than 2%, or more preferably less than 1%.

[0119] The mixture is placed in an inert crucible, preferably made of boron carbide or even alumina, preferably alumina coated with boron carbide, for example in an induction furnace. The loose density of the mixture before heat treatment, measured according to ASTM D7481-18, is preferably greater than 0.1 times the density of B4C, or even greater than 0.2, and / or preferably less than 0.5, less than 0.3 times the density of B4C.

[0120] A temperature increase is carried out up to at least a temperature preferably above the melting point of the element metal R chosen from Al, Si, preferably above 600°C, preferably above 700°C, preferably above 800°C, and below 1600°C, preferably below 1500°C, preferably below 1300°C, under a non-oxidizing atmosphere, preferably under rare gas sweeping, in particular Argon so as to avoid oxidation of the metallic reducing powder R.

[0121] Preferably, a non-oxidizing gas purge is carried out at a normal flow rate of 0.5 and 5 L / min per m 3 enclosure, preferably between 0.5 and 3 L / min / m 3 , preferably between 0.5 and 2 L / min / m 3 enclosure.

[0122] Preferably, the temperature ramp rate is less than 20°C / minute, preferably less than 10°C / minute, preferably less than 5°C / minute, or even less than 3°C / minute. This temperature ramp rate, as well as the dwell time, can be adjusted according to the mixing volume and the reactor power. In particular, such a temperature ramp range promotes better control of the exothermic effect due to the powder synthesis reaction according to the invention.

[0123] Preferably, the plateau at the maximum temperature is at least one hour, preferably at least two hours.

[0124] Preferably, an intermediate step is carried out between 600 and 1000°C and / or a weaker ramp typically at least twice as weak is carried out after 600°C in order to avoid decohesion of the mixture and to promote the reaction between the particles.

[0125] Cooling can be free or forced, preferably according to a negative ramp of less than 20°C / min.

[0126] The resulting raw mixture typically has a particle size between 10 and 100 micrometers.

[0127] A sieving operation, typically at a diameter of 100 micrometers, preferably at a diameter of 80 micrometers, preferably at a diameter of 50 micrometers, or even light crushing or vibration, allows the agglomerations to be eliminated and the raw boron carbide powder to be separated.

[0128] A suspension is made by adding a solvent, preferably deionized water, to the previously ground crude mixture in a mass ratio of 1 part crude mixture to at least 20, preferably 50 parts solvent. This suspension is filtered to an optimal size, typically 30 micrometers, preferably 20 micrometers, or even 15 micrometers or less, to allow the liquid containing the residues of the other reaction products to pass through (3). The filter retentate, consisting of boron carbide powder, is then calcined or dried, preferably in air, at a temperature above 80°C, preferably above 100°C and / or preferably below 300°C, preferably below 200°C, preferably below 150°C.

[0129] According to one possible method, the liquid resulting from the filtration of the previously described suspension, comprising products of reaction (2) other than boron carbide powder, is thermally treated in the presence of water and a basic solution to form a hydrate of element R and an alkali hydroxide. This method potentially allows for the valorization of the product of reaction (2) with the formula AR z O( y+ 3x) / 2. Preferably this possible mode is particularly advantageous in the case where the element R is Al and the alkali A is sodium.

[0130] After grinding the raw powder, it is possible to obtain a final finely divided boron carbide powder with a median diameter between 0.5 and 50 micrometers of high purity, micron size with very low size dispersion.

[0131] The final boron carbide powder allows for the production by sintering of a sintered ceramic body with a total porosity of less than 10%, or even less than 7% by volume, without the need for additions of transition metals such as Ni, Fe or Co, while exhibiting very low electrical resistivity.

[0132] The final powder obtained according to the process of the invention also makes it possible to obtain a sintered ceramic body in the form of a piece in which all dimensions are at least one dimension greater than 5 cm without deformation during sintering and without shrinkage cracking.

[0133] The powder material according to the invention advantageously exhibits an electrical resistivity, measured at 25 °C and atmospheric pressure, of less than 0.2 microohms·m. The electrical resistivity can be measured using the four-point Van der Pauw method on a sample with a diameter of 20–30 mm and a thickness of 2.5 mm. The sample was obtained by pressing a mixture of said powder with 0.25% of a pressing additive (PVA) and 4.75% deionized water by mass relative to the mass of boron carbide powder M, and then cold-pressed under a pressure of 100 bar to form a cylinder with a diameter of 30 mm and a thickness of 10 mm. After demolding, each cylinder was dried at 110 °C for 24 hours and then baked without pressure at a temperature of 1850 °C for 12 hours under argon.

[0134] A method for manufacturing a sintered ceramic body using the powder according to the invention includes in particular the following steps: a) preparation of a starting charge comprising:

[0135] - boron carbide powder according to the invention or a mixture of powders as previously described, comprising said powder and one or more sintering powders, in particular selected from aluminum diboride, magnesium diboride, zirconium diboride, titanium diboride, tungsten pentaboride, calcium hexaboride, silicon hexaboride powders, the purity of said B4C powder being greater than 95% by mass, preferably greater than 98% by mass, said B4C powder preferably representing at least 90% of the total mass of the charge.

[0136] - an aqueous solvent, in particular deionized water, preferably representing: i. less than 20% of the total mass of the filler in the case of shaping by casting, ii. less than 15% of the total mass of the filler in the case of shaping by extrusion, iii. less than 10%, preferably less than 7% of the total mass of the filler in the case of shaping by pressing,

[0137] - preferably, shaping additives such as binders like PVA (polyvinyl alcohol), plasticizers (like polyethylene glycol), lubricants, b) shaping of the starting material into a preform, preferably by pressing, extrusion, or casting, c) demolding after hardening or drying, d) optionally, drying of the preform, preferably until the residual moisture content is between 0 and 0.5% by weight, e) loading into a furnace and baking of the preform under an inert atmosphere, preferably under argon, or under vacuum, preferably at a temperature between 1600°C and 2200°C, preferably with a temperature ramp rate of less than 20°C / minute, preferably less than 10°C / minute. This temperature ramp rate, as well as the dwell time, can be adjusted according to the mixing volume and the reactor power.

[0138] Any shaping technique known to a person skilled in the art can be applied, depending on the size of the part to be produced, provided that all precautions are taken to avoid contamination of the preform. For example, casting in a plaster mold can be adapted by using graphite media between the mold and the preform, or oils to prevent excessive contact and abrasion of the mold by the mixture, ultimately leading to contamination of the preform. These precautions, mastered by a person skilled in the art, are also applicable to other stages of the process. Thus, during sintering, the mold or die containing the preform should preferably be made of graphite.

[0139] Hot pressing, hot isostatic pressing, and spark plasma sintering (SPS) techniques are particularly suitable.

Claims

[CLAIMS] 1. Process for the synthesis of a boron carbide powder, by reduction of an alkali boron oxide, comprising the following steps: - preparation of a mixture of raw materials comprising, and preferably consisting of: a) a powder comprising an alkali boron oxide of chemical formula A y BxO(y + 3x) / 2 or a precursor of said alkali boron oxide, the mass content of which, expressed as B2O3, is at least 30%, A being an alkali chemical element and B being the element Boron; and b) a metallic powder of at least one reducing element R selected from Al, Si, in particular an Al powder, a Si powder, a mixture of an Al powder and a Si powder, or a powder of an ASi alloy; and c) a carbon powder the mass content of the element carbon C being at least 95%; in respective proportions leading to the following balanced reaction, expressed in terms of said A y B x O(y+ 3x) / 2, R and C: - heating said mixture in a chamber under a flow of rare gas, brought into contact with the raw materials at a temperature above 600°C and below 1600°C, until the B4C compound is obtained; said mixture of raw materials having the following characteristics: - the median particle diameter of said powder comprising alkali boron oxide or the precursor of alkali boron oxide is between 1 and 100 micrometers; and - the median particle diameter of said metallic powder is between 5 and 200 micrometers; and - the median diameter of said carbon powder is greater than 1 nm and less than 10 micrometers; and - x is between 0.5 and 10; and - y is greater than 0.5 and less than 6; - z is greater than 0.5 and less than 2.

2. A method for synthesizing a boron carbide powder, according to any one of the preceding claims, wherein A is the element Na.

3. A process for synthesizing a boron carbide powder, according to any one of the preceding claims, wherein the alkali boron oxide is selected from sodium metaborate of chemical formula NaBCh, anhydrous borax of formula Na2B4O₂, NaCaBsO₂ or the alkali boron precursor is selected from natural borax of formula Na2B4O₂.10H₂O, tincalconite of formula Na2B4O₂.5H₂O, kernite of formula Na2B4O₂.4H₂O, ulexite of formula NaCaBsO₂.11W, proberite of formula NaCaBsO₂.11W.

4. A process for synthesizing a boron carbide powder, according to the immediately preceding claim, wherein the powder comprising boron oxide is an anhydrous alkali boron oxide powder, preferably an anhydrous sodium borate powder.

5. A process for synthesizing a boron carbide powder, according to any one of the preceding claims, wherein the median particle diameter of said powder comprising alkali boron oxide is greater than 10 micrometers and / or less than 100 micrometers.

6. A process for synthesizing a boron carbide powder, according to any one of the preceding claims, wherein the diameter D90 of particles of said powder comprising alkali boron oxide is less than 100 micrometers.

7. A process for synthesizing a boron carbide powder according to any one of the preceding claims, wherein the ratio of the median particle diameter of said powder comprising alkali boron oxide to the median particle diameter of said metallic powder of at least one reducing element R, is less than 30 and / or greater than 1.

8. A method for synthesizing a boron carbide powder according to any one of the preceding claims, wherein the median particle diameter of said metal powder is less than 100 micrometers.

9. A method for synthesizing a boron carbide powder according to any one of the preceding claims, wherein the pressure of the enclosure is maintained between 0.5 and 1.5 bars.

10. A process for synthesizing boron carbide powder according to any one of the preceding claims, wherein the flow rate of noble gas sweeping the chamber is preferably 0.5 to 5 L / min per m 3 enclosure.

11. A process for synthesizing a boron carbide powder according to any one of the preceding claims, wherein the crude mixture from the chamber after the heating step is sieved to a diameter of 100 micrometers, or even slightly crushed or vibrated, in order to remove agglomerations and separate the crude boron carbide powder, a suspension being made by adding to said mixture a solvent, preferably deionized water, in a mass ratio of one part of said mixture to at least twenty parts of solvent, said suspension being filtered to a size less than 30 micrometers in order to allow the liquid containing the very fine residues of the other products of the reaction (2) to pass through, the retentate being constituted by the boron carbide powder which is then dried or calcined at a temperature greater than 80°C and less than 300°C.

12. Powder comprising more than 95% by weight of the boron carbide compound obtainable by a process according to any one of claims 1 to 11, having a median diameter between 0.5 and 50 micrometers, and having a chemical composition comprising the following elemental mass contents: - elemental oxygen (O): less than 1.3%; - elemental carbon (C): less than 0.5%; - elemental nitrogen (N): less than 0.5%; - elemental sulfur (S): less than 400 ppm; - elemental iron (Fe): less than 0.45%; - elemental nickel (Ni): less than 0.4%; - elemental cobalt (Co): less than 0.4%; - elementary sum of alkali metals (Li+Na+K+Rb+Cs): less than 1%; - Elemental sum of alkaline earth metals (Be+Mg+Ca+Sr+Ba): less than 1%, - content of element R in metallic form: less than 2%, R preferably being different from M, R being at least one element chosen from Al, Si, - the sum of the other elements being less than 2%; said powder further comprising compound AR z O( y+ 3x) / 2.

13. Boron carbide powder according to claim 12, having a content of compound AR z O(y+3x) / 2 is greater than 0.01% and less than 1%.

14. Boron carbide powder according to claim 13, wherein the ratio (D 90 - DIO) / D 5O the equivalent diameter of the powder particles is less than 1.5, preferably even less than 1.

2.

15. Mixture comprising between 90% and 99.9% by mass of a boron carbide powder according to any one of claims 12 to 14 and between 0.1% and 10% by mass of one or more sintering powders selected from aluminum diboride, magnesium diboride, zirconium diboride, titanium diboride, tungsten pentaboride, calcium hexaboride, silicon hexaboride powders, the purity of which is greater than 95% by mass.

16. A process for manufacturing a sintered ceramic body comprising the following steps: a) preparation of a starting charge comprising: - boron carbide powder according to claim 12 to 14 or the powder mixture according to claim 15, - an aqueous solvent, in particular deionized water, - preferably, shaping additives, b) shaping of the starting charge into a preform, c) demolding after hardening or drying, d) optionally, drying of the preform, preferably until the residual moisture is between 0 and 0.5% by weight, e) loading into a furnace and baking of the preform under an inert atmosphere, preferably under argon, or under vacuum, preferably at a temperature between 1600°C and 2200°C.

17. Sintered ceramic body obtained by a process according to the preceding claim.

18. Use of the sintered ceramic body according to the preceding claim as all or part of a membrane, abrasion shield or coating, refractory lining or block, of a heat exchanger, cooking support, metal melting crucible, especially for non-ferrous metals, cutting tool.

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