PROCESS FOR SYNTHESIZING A BORON CARBIDE POWDER WITHOUT EMISSION OF CARBON OXIDE

Abstract

A process for manufacturing a boron carbide powder by dry method from the reduction of an alkali boron oxide of chemical formula AyBxO(y+3x) / 2 or a precursor of this boron oxide, where A is an alkali chemical element and B is the chemical element boron, according to the overall reaction: 4 AyBxO(y+3x) / 2 +xC + 4zR → xB4C + 4yARzO(y+3x) / 2 (2) in which R is at least one reducing element selected from Al, Si and C is the carbon supplied by a carbon powder having a mass content of C of at least 95%. Powder obtained by said process.

Description

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

[0001] The invention relates to a new method for manufacturing or synthesizing a boron carbide powder.

[0002] 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 refractory applications where high thermal and electrical conductivity are an asset, particularly heat exchangers, structural ceramics, membranes in certain high-temperature applications or in very aggressive chemical environments, especially non-ferrous metals, as well as cutting tools, shielding, and abrasion-resistant coatings.

[0003] All these applications explain why the demand for this material is very high and growing at present.

[0004] Boron carbide does not exist in nature. Boron carbide, in particular, can be obtained, for example, by 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:

[0005] 2B2O3 (s) + 7C (s) B4C (s) + 6CO (g) (1)

[0006] 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.

[0007] 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).

[0008] Another known solution consists of a metallo-thermal reduction using, instead of carbon, an element in metallic form such as Mg, Ca. The exothermic reactions associated with the use of these reactants produce carbides by high-temperature self-propagating synthesis (SHS) but lead to incomplete conversion of the reactants and make the separation of the B4C material obtained very difficult with respect to the other reaction products such as magnesium and / or calcium oxides.

[0009] The object of the invention is thus to improve the synthesis processes described above in order to obtain a boron carbide powder:

[0010] -fine, that is to say typically a powder with a median diameter between 0.5 and 50 micrometers;

[0011] -of better purity, that is to say, whose elemental mass content in the sum of the following contaminants: oxygen (O), sulfur (S), carbon (C), nitrogen (N), iron (Fe), phosphorus (P), silicon (Si) and / or aluminum (Al), in particular 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 in contaminants is less than 3%, or even less than or equal to 2%;

[0012] while presenting:

[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 release of CO or CO2 and

[0016] -without resorting to a complex industrial synthesis process. Summary of the invention

[0017] 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.

[0018] 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 chosen from Na, K, Li, said process comprising the following steps:

[0019] - 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 AyBxO(y +3x) / 2 or a precursor of this oxide, in particular in the form of a hydrate of said oxide, the mass content of boron, expressed as B2O3 oxide, of at least 30%, preferably at least 40%, preferably at least 50%, where A is an alkali chemical 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 chosen from Al and / or Si, in particular an Al powder, a Si powder, a a mixture of Al powder and Si powder, or a powder of an ASi alloy; and c. a carbon powder D, the mass content of the carbon element C of which is at least 95%;

[0020] in the respective proportions leading, preferably corresponding, to the following overall reaction, expressed according to said AyBxO(y+3x) / 2 R and C:

[0021] 4AyBxO( y +3 xy 2 + xC + 4zR XB4C + 4yARzO( y +3Xy2 (2)

[0022] - heating said mixture in a chamber under a flow of rare gas, at a temperature above 600°C and below 1600°C, until the compound is obtained b4c,

[0023] said mixture of raw materials having the following characteristics:

[0024] - the median particle diameter of said powder comprising alkali boron oxide, or the precursor of this oxide, is between 1 and 100 micrometers; and

[0025] - the median particle diameter of the metal powder is between 5 and 200 micrometers; and

[0026] -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

[0027] -y is greater than or equal to 0.5, preferably less than 6, preferably less than or equal to 2; and

[0028] -z is greater than 0.5 and less than 2, preferably less than 1.5.

[0029] According to preferred but non-limiting embodiments of the present invention, which can be combined with each other where appropriate:

[0030] -The mass sum of said powder comprising an alkali boron oxide or an alkali boron oxide precursor, said metal 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.

[0031] - Alkaline boron oxide powder can be hydrated; it is preferably dehydrated;

[0032] -the mass content of said oxide AyBxO(y+3 x) / 2 of the powder comprising an alkali boron oxide or an alkali boron oxide precursor, is at least 95%;

[0033] - the metal powder can be hydrated, it is preferably dehydrated;

[0034] - the metal powder is a mixture of aluminum particles on the one hand and of silicon particles on the other hand. Preferably the metal powder is an aluminium powder.

[0035] - the alkali element A is preferably sodium (Na);

[0036] - the gas flow scavenging rate in said enclosure being between 0.5 and 10 L / min per m3 of enclosure;

[0037] - the median particle diameter of said powder comprising boron oxide alkaline is between 30 and 100 micrometers, preferably still between 30 and 50 micrometers;

[0038] - 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.

[0039] - 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.

[0040] 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 and allows an easy ability to extract said boron carbide powder without resorting to an industrially too complex powder synthesis process.

[0041] 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).

[0042] 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 an alkali salt in solution advantageously allows an optimal reaction, i.e. with a maximum material balance, while allowing easy separation of the boron carbide powder after synthesis in the chamber.

[0043] According to other preferred embodiments of the present invention, which may optionally be combined with each other:

[0044] - Alkaline boron oxide is chosen from sodium metaborate of formula NaBO2, anhydrous borax with the formula Na2B4O7, or even other borates containing other elements such as NaCaB5O9. Preferably, the alkali boron oxide is sodium metaborate with the chemical formula NaBO2 and / or anhydrous borax with the formula Na2B4O7.

[0045] - The alkali boron oxide precursor is generally chosen from the compounds in which boron is in a non-carburized, non-metallic form and in a form other than a salt, and in particular other than that of a halide.

[0046] - The precursor of alkali boron oxide can be a hydrated borate such as borax Natural borax with the formula Na2B4O7.10H2O, tincalconite with the formula Na2B4O7.5H2O, kemite with the formula Na2B4O7.4H2O, ulexite with the formula NaCaB5O9.8H2O, and proberite NaCaB5O9.5H2O. Preferably, the hydrated borate is chosen from natural borax with the formula Na2B4O7.10H2O, tincalconite with the formula Na2B4O7.5H2O, and kemite with the formula Na2B4O7.4H2O.

[0047] - According to one possible mode, when it is naturally in hydrated form, the The precursor of alkali boron oxide is pre-dehydrated in order to react more efficiently in reaction (2) and avoid the production of hydrogen.

[0048] - 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.

[0049] - The powder comprising alkali boron oxide is an alkali borate powder anhydrous, preferably anhydrous sodium borate powder. - 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. - 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. - 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. - 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. - 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. 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, of the powder comprising alkali boron oxide or its precursor, is less than 5%. Preferably, the mass content of SiO₂ in the powder comprising alkali boron oxide or its precursor is 2% or less. The mass content of Al₂O₃ and / or SiO₂ in said powder comprising alkali boron oxide or its precursor is preferably 2% or less. Preferably, the mass content of Fe₂O₃ + Na₂O + K₂O + CaO + MgO in said powder comprising alkali boron oxide or its precursor is 1% or less.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%. 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 SiO2 and / or of the element aluminium (Al), expressed as Al2O3, is preferably less than 2%, preferably less than 1%. The raw materials were previously dried at a temperature between ambient temperature and 150°C. The synthesis temperature, i.e. heating temperature in said chamber, 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. 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). The gas used to sweep the chamber is preferably a noble gas, for example Argon or Helium, preferably Argon. Preferably, the gas used to sweep the chamber, preferably 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 of chamber, preferably between 0.5 and 3 L / min per m³, preferably between 0.5 and 2 L / min per m³ of chamber. Excessive sweeping is not recommended. weak leads to an incomplete reaction, more specifically to undesirable residues present in the final boron carbide powder. - A non-oxidizing gas sweep flow rate ratio of 0.005 to 1 L / min per m3 of enclosure and per KW of enclosure heating power is particularly optimal, preferably between 0.01 and 0.5 / min per m3 of enclosure and per KW of enclosure heating power.

[0050] After reaction, the finely divided crude boron carbide powder can be easily extracted from the crude mixture from the chamber after the heating step.

[0051] 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 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, in order to allow the liquid containing the very fine residues of the other reaction products (2) to pass through.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.

[0052] According to one possible mode, 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 mode allows for the recovery of the product of reaction (2) with the formula ARO(y + 3x) / 2. Preferably, this possible mode is particularly advantageous when element R is Al and the alkali A is sodium.

[0053] The invention also relates to a boron carbide powder obtained according to the previous process.

[0054] 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:

[0055] - elemental oxygen (O): less than 1.3%, preferably less than 1.2%, of preference less than 1% or even less than 0.5%;

[0056] - elemental carbon (C): less than 0.5%, preferably less than 0.1%;

[0057] - elemental nitrogen (N): less than 0.5%; preferably less than 0.1%;

[0058] - 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;

[0059] - elemental iron (Fe): less than 0.45%, preferably less than 0.4%;

[0060] - elemental nickel (Ni): less than 0.4%, preferably 0.2%, or even less than 0.1%;

[0061] - elemental cobalt (Co): less than 0.4%, preferably 0.2%, or even less than 0.1%;

[0062] - elemental sum of alkali metals Li+Na+K+Rb+Cs: less than 1%, preferably less than 0.5%;

[0063] - elemental sum of alkaline earth metals (Be+Mg+Ca+Sr+Ba): less than 1%, of preference less than 0.5% or even less than 0.25%;

[0064] - content of element R in metallic form: less than 2%, preferably less than 1%, preferably even less than 0.5%, R preferably being different from M, R being at least one element chosen from Al, Si or their alloy,

[0065] the sum of the other elements being less than 2%, preferably less than 1%.

[0066] According to other preferred embodiments of the present invention, which may optionally be combined with each other:

[0067] - the boron carbide powder comprises the compound ARzO(y+3x) / 2, particularly under form of product of reaction (2), in particular in the form of an impurity.

[0068] - the mass content of the boron carbide powder in compound ARzO(y+3x) / 2 is greater than 0.01% and less than 1%, preferably less than 0.5%.

[0069] - the elemental sum oxygen (O) + nitrogen (N) + carbon (C) of the powder boron carbide is less than 1.5%, or even less than or equal to 1.2%.

[0070] - the mass content of silicon (Si) in metallic form of the carbide powder boron content is less than 0.1%.

[0071] - the mass content of aluminium (Al) in metallic form of the carbide powder boron content is less than 2%, preferably less than 1%, preferably less than 0.5%.

[0072] - the final boron carbide powder according to the invention does not comprise other crystallized phases such as B2O3, SiC, Al4C3, SiB4, SiB6, SiB3, as measured (detectable) by X-ray diffraction. Preferably said powder comprises only a crystallized phase of B4C, as measured (detectable) by X-ray diffraction.

[0073] - the ratio (DQo-Dio) / D5o of equivalent diameter of the particles of the raw powder, that is- that is, the powder after extraction from the raw mixture from the chamber after the step heating, in particular after separation of the product of reaction (2) of formula ARO( 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 Di0, 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.

[0074] Such a high-purity boron carbide powder with fine and regular particle size makes it possible to obtain by sintering a sintered ceramic body having 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 secondary metal borides which are not desired.

[0075] Such a powder makes it possible to obtain a sintered ceramic body in the form of a piece in which at least one dimension, preferably all of the overall dimensions, is greater than 5 cm, or even greater than 10 cm, and which has a total porosity also less than 10%, a very narrow pore size distribution, without deformation during sintering and without shrinkage cracking.

[0076] The (D9o-Dio) / D5o equivalent diameter ratio of boron carbide powder particles is advantageously less than 1.5, preferably even less than 1.2 or even less than or equal to 1.0.

[0077] 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 means 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, the fact that it contains more than 95% by mass of W2B5.

[0078] According to one possible mode, said mixture may also include an aluminium powder, a cobalt powder, a metallic iron powder partially or totally replacing said or said sintering powders.

[0079] The invention also relates to a method for manufacturing a sintered ceramic body comprising the following steps: a. preparation of a starting load comprising: - 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. - an aqueous solvent, in particular deionized water, - preferably, shaping additives, b. shaping the initial 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 into a furnace and baking the preform under an inert atmosphere, preferably under argon, or under vacuum, preferably at a temperature between 1600°C and 2200°C.

[0080] 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 in particular for the filtration of liquids or gases, of an anti-abrasion shielding or coating, of a refractory coating or block, of a heat exchanger, of a cooking support, of a metal melting crucible, in particular of non-ferrous metal, of a cutting tool.

[0081] Definitions:

[0082] The following indications and definitions are given in relation to the preceding description of the present invention:

[0083] - In this description, unless otherwise specified, all percentages are given in bulk, on a dried material basis.

[0084] - Alkaline boron oxide means any oxide comprising boron and oxygen, with at least one other element chosen from Na, K, Li. Preferably this alkali boron oxide contains no other elements than boron, oxygen and alkali.

[0085] - Herein, an alkali boron oxide precursor is understood to be a powder comprising the element boron (B) and an alkali, in hydrated form and / or possibly containing hydroxyl groups, which, by heating or oxidation, preferably in air, at a temperature below 600°C, produces the reagent AyBxO(y+3x) / 2 present in the chemical equation of reaction (2). For example, it may be an alkali boron hydrate or hydrated alkali borate, for example, a hydrated sodium borate.

[0086] - 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.

[0087] - The material yield is calculated by dividing the mass of raw carbide powder of boron obtained divided by that of the dry mixture of powder of the reactants (moisture less than 5%) of reaction (2) before heating.

[0088] -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.

[0089] -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. The particle size distribution characterization is conventionally performed with a laser particle size analyzer in accordance with ISO 13320-1. The laser particle size analyzer may 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% by mass of the 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, these first and second populations consisting only of particles larger or smaller than the median size, respectively. The D0 and D90 percentiles of a powder of grains or particles can also be determined; these are the diameters corresponding to the 10% and 90% percentages, respectively, on the cumulative distribution curve of grain diameter by volume, ranked in ascending order.

[0090] - The elemental chemical contents can be determined according to ISO 21068:2008. In particular, the following mass contents of:

[0091] - O, N, C, and S are measured by a LECO® brand analyzer,

[0092] - Si, Al, Co, Ni, alkaline (Li+Na+K+Rb+Cs), alkaline earth (Be+Mg+Ca+Sr+Ba), Fe, P can be determined by ICP (Induction Coupled Plasma),

[0093] - the contents of element M (in particular Ti, Zr, Hf) are preferably determined by ICP,

[0094] - aluminium and / or silicon in metallic form can be determined by X-ray diffraction,

[0095] - the B4C compound can be determined by X-ray diffraction,

[0096] - hydroxyls (OH) of the alkali boron oxide powder, can be measured by pH- metric,

[0097] - the actual powder density is measured by helium pycnometry, for example at using a Micromeritics AccuPycl330 device,

[0098] -The total porosity of a ceramic body is the ratio, expressed as a percentage, of the apparent density measured for example according to ISO 18754 to the absolute density measured for example according to ISO5018.

[0099] Unless otherwise stated, in this description all percentages are mass percentages. Detailed description

[0100] 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.

[0101] The starting raw material mixture comprises:

[0102] - a powder comprising alkali boron oxide AyBxO(y +3xy2) (for example, for a sodium metaborate powder NaBO2, A=Na, y=l; x=l). Preferably, the mass content of AyBxO(y +3xy2) in said powder is at least 95%, and

[0103] -a metallic powder of a 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, the mass content of elements other than Al and Si of which is preferably less than 0.5%, and

[0104] -a carbon powder D, selected from carbon, petroleum coke (residue of petroleum distillation), amorphous carbon, graphite, preferably black carbon.

[0105] The mixing is carried out under standard conditions for those skilled in the art. This dry mixing preparation step allows for intimate contact of the particles. In one possible configuration, it is carried out in a rubber ball mixer, a tumbler-type 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.

[0106] If necessary, certain hydrated raw materials, in particular the precursor of the alkali boron oxide powder, can 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 Na2B4O7.10H2O (sometimes also expressed as Na2B4O5(OH)4-8H2O), tincalconite of formula Na2B4O7.5H2O (sometimes also expressed as Na2B4O5(OH)4.3H2O), kernite of formula Na2B4O7.4H2O (sometimes also expressed as Na2B4O6(OH)2-3H2O), ulexite of formula NaCaB5O9.8H2O (sometimes also expressed as NaCaB5O6(OH)6-5H2O), 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 onto the surface of the powder or of hydroxyls OH. It allows to improve the rate of conversion into boron carbide which also results in a crude B4C powder having after synthesis a very low content of elemental reducing metal R.

[0107] Preferably, in order to maximize the conversion rate to boron carbide B4Cl, the hydroxyl (OH) content contributed by 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 AyBxO(y + 3x) / 2, of 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.

[0108] The median size or median diameter of the alkali boron oxide particles and that of the metallic particles of element reducer R is preferably between 10 and 100 micrometers, preferably between 30 and 80 micrometers.

[0109] Preferably, the median size ratio of the particles comprising the alkali boron oxide to that of the metallic particles of elemental reducer R is between 1 and 10.

[0110] 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).

[0111] 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 the reaction, preferably less than or equal to 10% by mass relative to the stoichiometric amount of carbon. This makes it possible to eliminate 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, preferably, the alkali boron oxide is sodium metaborate and the metal powder is aluminum and / or a silicon metal powder, preferably aluminum powder.

[0112] In said reaction (2), according to one possible mode, the quantity of said alkali boron oxide in the form AyBxO( y +3 x) / 2 in the mixture of raw materials is in excess but preferably less than 10%, or even less than 5% with respect to the stoichiometry, by mass.

[0113] The mixture is preferably dried 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, i.e. the residual mass content of H2O measured by a moisture meter well known to those skilled in the art, said mixture of raw materials is less than 5%, preferably less than 2%, or even more preferably less than 1%.

[0114] 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.

[0115] 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.

[0116] Preferably, a non-oxidizing gas sweep is carried out at a normal flow rate of 0.5 and 5 L / min per m3 of enclosure, preferably between 0.5 and 3 L / min / m3, preferably between 0.5 and 2 L / min / m3 of enclosure.

[0117] Preferably, the temperature ramp 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, 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.

[0118] Preferably, the plateau at the maximum temperature is at least one hour, preferably at least two hours.

[0119] 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.

[0120] Cooling can be free or forced, preferably according to a negative ramp of less than 20°C / min.

[0121] The resulting crude mixture typically has a particle size between 10 and 100 micrometers.

[0122] 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, makes it possible to eliminate agglomerations and separate the crude boron carbide powder.

[0123] 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. The 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 (3) to pass through. 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.

[0124] According to one possible mode, 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 mode potentially allows for the valorization of the product of reaction (2) with the formula ARzO(y+3xy2). Preferably, this possible mode is particularly advantageous when element R is Al and the alkali A is sodium.

[0125] After grinding the raw powder, it is possible to obtain a final finely divided boron carbide powder with a median diameter of between 0.5 and 50 micrometers of high purity, of micron size with very low size dispersion.

[0126] The final boron carbide powder allows obtaining by sintering a sintered ceramic body having a total porosity of less than 10%, or even less than 7% by volume without the use of additions of transition metals such as Ni, Fe or Co while exhibiting a very low electrical resistivity.

[0127] 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.

[0128] 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.

[0129] 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 load comprising: - 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 powders, magnesium diboride powders, zirconium diboride powders, titanium diboride powders, tungsten pentaboride powders, calcium hexaboride powders, 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. - an aqueous solvent, in particular deionized water, preferably representing: i. less than 20% of the total mass of the charge in the case of shaping by casting, ii. less than 15% of the total mass of the charge in the case of shaping by extrusion, iii. less than 10%, preferably less than 7% of the total mass of the charge in the case of shaping by pressing, - preferably, shaping additives such as binders like PVA (polyvinyl alcohol), plasticizers (like polyethylene glycol), lubricants, b. shaping the initial charge into a preform, preferably by pressing, extrusion or casting, 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 baking it under an inert atmosphere, preferably 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 mixture volume and the reactor power.

[0130] Any shaping technique known to a person skilled in the art may be applied according to the dimensions of the part to be produced, provided that all precautions are taken Precautions are taken to prevent contamination of the preform. For example, casting in plaster molds 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 those skilled in the art, are also applicable to other stages of the process. Thus, during sintering, the mold or matrix containing the preform should preferably be made of graphite.

[0131] Hot pressing, hot isostatic pressing or SPS (“Spark Plasma Sintering”) techniques are particularly suitable.

Claims

Demands

1. A process for synthesizing a boron carbide powder by reducing an alkali boron oxide, comprising the following steps: - preparing a mixture of raw materials comprising, and preferably consisting of: a. a powder comprising an alkali boron oxide of chemical formula AyBxO(y+3x) / 2 or a precursor of said alkali boron oxide, having a boron mass content, expressed as B2O3, of 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 D carbon powder whose mass content of the carbon element is at least 95%; in respective proportions leading to the following overall reaction, expressed according to the said AyBxO(y+3x) / 2, R and C: 4 AyBxO( y +3 x) / 2 +xC + 4zR xB4C + 4yARzO(y+3xy2 (2) - heating of said mixture in a chamber under a flow of rare gas, at a temperature greater than 600°C and less than 1600°C, 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 - x is between 0.5 and 10 and - y is greater than 0.5 and - 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 NaBO2 , anhydrous borax of formula Na2B4O7, NaCaB5O9 or the precursor of alkali boron is chosen from natural borax of formula Na2B4 O7.10H2O, tincalconite of formula Na2B4O7.5H2O kernite of formula Na2B4O7.4H2O, ulexite of formula NaCaB5O9.8H2O, proberite of formula NaCaB5O9.5H2O.

4. A method 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 method 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 method 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 pressure of the enclosure is maintained between 0.5 and 1.5 bars.

9. A process for synthesizing a boron carbide powder according to any one of the preceding claims, wherein the flow rate of rare gas sweeping the enclosure is preferably 0.5 to 5 L / min per m3 of enclosure.

10. Powder comprising more than 95% by weight of the boron carbide compound obtained according to any one of claims 1 to 9, 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%; - Elemental 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 being preferably different from M, R being at least one element chosen from Al, Si, the sum of the other elements being less than 2%.

11. Boron carbide powder according to claim 10, further comprising the compound ARzO(y+3x) / 2.

12. Boron carbide powder according to claim 11, having a content of compound ARzO(y+3x) / 2 greater than 0.01% and less than 1%.

13. Boron carbide powder according to claim 12, having the (DQo-Dio) / D5o ratio of equivalent particle diameter of the powder less than 1.5, preferably even less than 1.

2.

14. A mixture comprising between 90% and 99.9% by mass of a boron carbide powder according to any one of claims 10 to 13 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.

15. A method for manufacturing a sintered ceramic body comprising the following steps: a. preparation of a starting charge comprising: - boron carbide powder according to claim 10 to 13 or the powder mixture according to claim 14, - an aqueous solvent, in particular deionized water, - preferably, shaping additives, b. shaping the starting charge into a preform, cd demolding after hardening or drying, 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.

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

17. Use of the sintered ceramic body according to the preceding claim as all or part of a membrane, abrasion shielding or coating, refractory coating or block, heat exchanger, cooking support, metal melting crucible, in particular non-ferrous metal melting crucible, cutting tool.