DRY METHOD FOR SYNTHESIZING A DIBORIDE POWDER

Abstract

A process for manufacturing a diboride powder MB2 by dry method where M is a chemical element belonging to group 4 of the periodic table, from the reduction of an oxide MO2 of said element M according to the overall reaction MO2 + B2O3 + yR + xA2O → MB2 + A2xRyO5+x, in which R is a reducing element chosen from Al, Si, Ti, Zr, Hf, Y, Sc, and the lanthanides and A2O is an oxide of alkali element A. Powder obtained by said process.

Description

Title of the invention: SYNTHESIS METHOD OF A DIBORIDE POWDER BY DRY METHOD

[0001] The invention relates to a new process for manufacturing or synthesizing diboride of an element from group 4 of the periodic table, in particular titanium diboride.

[0002] Diborides of elements from group 4 of the periodic table, in particular titanium diboride, zirconium diboride or hafnium diboride, have many advantages including high refractoriness and toughness and excellent chemical inertness.

[0003] Titanium diboride in particular is a ceramic material with a low density (about 4.5 g / cm3), high hardness, high thermal conductivity and low electrical resistivity. This makes it a potentially interesting material for several applications such as refractory applications where high thermal conduction and electrical conduction are an asset, in particular heat exchangers, the coating or even the composition of anodes or cathodes of electrolysis reactors, or even membranes in certain applications at high temperature or in very aggressive chemical environments, but also crucibles for melting metals, in particular non-ferrous metals, or even cutting tools or shielding or even an anti-abrasion coating.

[0004] All these applications explain why the demand for this material is very high and growing at present.

[0005] Diborides do not exist in their natural state. Titanium diboride in particular can be obtained, for example, by direct reaction of titanium (or its oxides or hydrides) with elemental boron at 1000°C or by carbothermic reduction of titanium oxide and boron oxide. In the latter case, the reaction consists of reacting a mixture of powders according to the following simplified reaction at a temperature above 1500°C:

[0006] TiO2 (s) + B2O3 (s) + 5C (s) TiB2 (s) + 5CO (g) (1)

[0007] This process, however, has a material yield of approximately 30%.

[0008] Another less well-known process consists in particular of replacing the boron oxide powder with boron carbide, as illustrated by the following balanced reaction:

[0009] 2 TiO2 + B4C + 3C TiB2 + 4CO (2)

[0010] The advantage of such a reaction is its higher material yield but it has the disadvantage of requiring a reaction temperature above 1600°C and generates a significant quantity of CO in gaseous form, which poses health, safety and environmental problems.

[0011] The manufacturing processes for this material are also all the more expensive and energy-consuming as the final titanium diboride powder sought is fine (typically with a median diameter of between 5 and 50 micrometers) or even ultrafine (median diameter of less than 5 micrometers).

[0012] Another known solution consists of a metallo-thermal reduction using instead of Carbon an element in metallic form such as Al, Si, Mg, Ca. The exothermic reactions linked to the use of these reagents produce diborides by self-propagating synthesis at high temperature (SHS) but lead to the incomplete conversion of the reagents, which classically requires a second reaction typically with boric acid H3BO3 in order to obtain a better rate of conversion into diboride.

[0013] Another solution also consists of a synthesis in a molten salt medium or in solution. The patent application published under WO2020073767A1 from the Wuhan University of Science and Technology thus proposes a method for preparing a TiB2 or (Zr,Hf)B2 powder that is less expensive and more environmentally friendly from a mixture comprising a source of titanium (or zirconium or hafnium), a source of boron, a reducing agent (Si or Al in metallic form) and an alkali salt. The alkali salt can be chosen from a hydrate, a silicate or a carbonate of sodium, lithium or potassium in order to form a liquid phase at low temperature. The addition of the silicate, however, leads to reaction products that are difficult to separate. The addition of carbonate(s) poses the problem of the release of CO2 due to the decomposition of the carbonate during the synthesis reaction.

[0014] The aim of the invention is thus to improve the synthesis processes previously described in order to obtain a powder of diboride of an element from group 4 of the periodic table, in particular TiB2:

[0015] -fine, that is to say typically a powder with a median diameter of between 0.5 and 50 micrometers;

[0016] -of better purity, that is to say whose elementary 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), alkalis (Li+Na+K+Rb+Cs), alkaline earths (Be+Mg+Ca+Sr+Ba) is less than 5%, and preferably that is to say whose total elementary mass content in contaminants is less than 3%, or even less than or equal to 2%;

[0017] while presenting:

[0018] -a satisfactory material yield, for example at least 20%;

[0019] - an easy capacity for extracting said diboride powder from the reaction products;

[0020] -a very low, preferably no, release of CO or CO2 and

[0021] -without resorting to an industrial process for powder synthesis. Summary of the invention

[0022] In particular, according to a first aspect, the present invention relates to an alternative method for manufacturing a powder of diboride of an element from group 4 of the periodic table, in particular titanium diboride TiB2 at a temperature below 1500°C, preferably below 1400°C, preferably below 1300°C, or even 1200°C, meeting this aim thanks to an appropriate choice of starting powders, without the use of solvent or surfactant.

[0023] More specifically, the present invention relates to a method for manufacturing a diboride powder MB2 where M is a chemical element belonging to group 4 of the periodic table, by reduction of an oxide MO2 of said element M, said method comprising the following steps:

[0024] - preparation of a mixture of raw materials consisting of: a. a powder whose mass content of said MO2 oxide is at least 95% and b. a powder comprising a boron oxide or a boron oxide precursor, the mass content of which in boron, expressed as B2O3, is at least 30%; and c. a metal powder of at least one reducing element R, R being chosen from Al, Si, Ti, Zr, Hf, Y, Sc, the lanthanides; and d. an alkali element A oxide powder, the mass content of which is at least 70% in A2O;

[0025] in the respective proportions leading, preferably corresponding, to the following balanced reaction, expressed according to said MO2jB2O3, R and A2O:

[0026] MO2 + B2O3 + yR + xA20 MB2 + A2xRyO3+x (3)

[0027] - heating said mixture in an enclosure under a flow of rare gas, at a temperature above 600°C and below 1500°C, to obtain the compound MB2,

[0028] said mixture of raw materials having the following characteristics:

[0029] -the median particle diameter of said powder comprising MO2 oxide is between 1 and 100 micrometers; and

[0030] - the median particle diameter of the powder comprising a boron oxide or a boron oxide precursor is between 5 and 200 micrometers; and

[0031] - x is greater than or equal to 1

[0032] - y is greater than 0.5.

[0033] According to preferred but non-limiting embodiments of the present invention, which may be combined with each other where appropriate:

[0034] - The powder is made of MO2 oxide, preferably made of oxide of titanium.

[0035] -The powder comprises a boron oxide or a boron oxide precursor, the mass content of boron expressed in the form B2O3, of which is at least 40%.

[0036] - The metal powder comprises at least one reducing element R chosen from the element aluminum (Al) and / or silicon (Si).

[0037] - Alkaline element A oxide powder, preferably sodium oxide (Na2 O), has a mass content of A2O of at least 80%

[0038] - The hydroxyl (OH) content of the alkali element A oxide powder, calculated in the form of the mass of OH on the mass A2O being preferably less than 30%, more preferably less than 20%, or even less than 10%, or even less than 5% or even substantially zero.

[0039] - The flow rate of the gas flow in said enclosure being between 0.5 and 10 L / min per m3 of enclosure.

[0040] - The median particle diameter of said powder comprising MO2 oxide is between 30 and 100 micrometers, more preferably between 30 and 50 micrometers.

[0041] - The median particle diameter of the powder comprising boron oxide being between 10 and 100 micrometers.

[0042] - the total content of said mixture of raw materials in alkali oxide, calculated in A2O form, is equal to or greater than the quantity necessary for said reaction (3), such that x is greater than 1, preferably x is between 1.5 and 5, more preferably x is between 1.5 and 2.

[0043] -the residual mass content of H2O in said mixture of raw materials is less than 5%, as measured at a temperature of 400°C at atmospheric pressure

[0044] -y is greater than 1.

[0045] As will be described in more detail below, such a combination of parameters advantageously makes it possible to obtain a fine powder of high-purity MB2 diboride with a satisfactory material yield, using a process which releases little or no CO or CO2 and which allows easy extraction of said diboride powder without resorting to an industrially overly complex powder synthesis process.

[0046] The respective proportions leading to the reduction of the oxide of element M to diboride of element M are the substantially stoichiometric quantities of the different reagents mentioned in points a) to d) above leading to the balance reaction (3).

[0047] For example, in the case of the use of a boron oxide precursor of the Na2 B4O7 type, one mole of B2O3 in reaction (3) corresponds to a contribution of half a mole of the Na2B4O7 reagent.

[0048] Thus, the balanced reaction (3) is written, in the case of the use of boron oxide B2O3:

[0049] MO2 + B2O3 + R + xA20 MB2 + A2xRO5+x

[0050] The balanced reaction (3) is written, in the case of the use of a boron oxide Na2B4O7:

[0051] MO2 + B2O3 + y R + xA20 MB2 + A2xRyO3+x

[0052] with M=Ti A=Na R=A1 x=5 / 3 and y=10 / 3

[0053] In particular, unlike synthesis processes using molten salts or dissolving in a solvent, in particular water, the process according to the invention by the dry route and in particular by the use of a weakly or non-hydroxylated alkali element A oxide powder rather than the use of alkali salt in solution as described for example by WO2020073767A1 advantageously allows an optimal reaction, that is to say with a maximum material balance, while allowing easy separation of the element M diboride powder after synthesis in the enclosure.

[0054] According to other preferred embodiments of the present invention, which may be combined with each other where appropriate:

[0055] - The boron oxide precursor is chosen from 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, for example boron nitride.

[0056] - The boron oxide is chosen from B2O3, sodium metaborate of formula chemical NaBO2, anhydrous borax of formula Na2B4O7, or other borates such as natural borax of formula Na2B4O7.10H2O, tincalconite of formula Na2B4O7.5H2O, kemite of formula Na2B4O7.4H2O, ulexite of formula NaCaB5O9.8H2O, proberite NaCaB5O9.5H2O, sassolite with formula H3BO3. Preferably, the boron oxide is chosen from sodium metaborate of chemical formula NaBO2, anhydrous borax of formula Na2B4O7, or other borates such as natural borax of formula Na2B4O7.10H2O, tincalconite of formula Na2B4O7.5H2O, kernite of formula Na2B4O7.4H2O, ulexite of formula NaCaB5O9.8H2O, proberite NaCaB5O9.5H2O;

[0057] - The powder comprising boron oxide is an alkali borate powder anhydrous, preferably anhydrous sodium borate powder. - The median particle diameter of said powder comprising MO2 oxide is greater than 7 micrometers, preferably greater than or equal to 10 micrometers and / or less than 50 micrometers, or even less than 30 micrometers. - The particle diameter D90 of said powder comprising MO2 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 particle diameter of said powder comprising boron oxide is greater than 10 micrometers, preferably greater than or equal to 30 micrometers and / or less than 100 micrometers, or even less than 50 micrometers. The median particle diameter of said reducing metal powder 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, or even less than 30 micrometers. Alkali element A oxide powder is a powder in the form of granules whose size is greater than 30 micrometers and / or less than 100 micrometers. The ratio of the median particle diameter of said powder comprising boron oxide to the median particle diameter of said metal oxide powder M is less than 30, preferably less than or equal to 10 and / or greater than 5, preferably greater than 2. This ratio makes it possible to optimize the conversion rate into diboride of element M. The ratio of the median particle diameter of said powder comprising boron oxide to the median particle diameter of said powder comprising MO2 oxide is less than 10 and / or greater than 1. M is Ti, and A is the element Na. M is Ti and R is the element Al and / or Si. The powder comprising the MO2 oxide is a titanium oxide powder, preferably a rutile or anatase powder, more preferably rutile. The mass content SiO2+Al2O3+Fe2O3+Na2O+K2O+CaO+MgO of the powder comprising the oxide MO2, preferably titanium oxide, is less than 5%. Preferably, the mass content of SiO2 of the powder comprising the oxide MO2, preferably titanium oxide, is preferably less than or equal to 2%. The mass content of Al2O3 of said powder comprising the oxide MO2, preferably titanium oxide, is preferably less than or equal to 2%. Preferably, the mass content of Fe2O3+Na2O +K2O+CaO+MgO of said powder comprising the oxide MO2, preferably titanium oxide, 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 the oxide MO2, preferably titanium oxide, is preferably less than or equal to 1%, preferably less than or equal to 0.5%. - Apart from the metal powder of reducer R which may contain, in particular when it is silicon powder, the mass content of the mixture of raw materials before reaction in the element silicon (Si), expressed in the form of SiO2, is preferably less than 2%, preferably less than 1%. - The raw materials were previously dried at a temperature between room temperature and 150°C. - 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 1400°C, preferably less than 1300°C, more preferably less than 1200°C. - The pressure of 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 sweeping the enclosure is preferably a noble gas, for example Argon or Helium, more preferably argon. The flow rate is preferably 0.5 to 5 L / min per m3 of enclosure, preferably between 0.5 and 3 L / min per m3, preferably between 0.5 and 2 L / min per m3 of enclosure. Too little sweeping leads to an incomplete reaction, more particularly to undesirable residues present in the final MB2 diboride 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.

[0058] After reaction, the final finely divided powder of group 4 element diboride of the periodic table can be easily extracted from the crude mixture from the enclosure after the heating step.

[0059] According to one possible embodiment, a screening operation, typically to a diameter of 100 micrometers, preferably 50 micrometers, or even light crushing or vibration makes it possible to eliminate agglomerations and separate the element M diboride powder. A suspension is made by adding to the previously ground raw mixture a solvent, preferably deionized water, according to a mass ratio of 1 part of raw mixture for at least 20, preferably 50 parts of solvent. Said suspension is filtered to an optimal size typically less than 30 micrometers, preferably less than 20 micrometers in order to allow the liquid comprising the very fine residues of the other products of the reaction (3) to pass through. The filtration retentate consisting of the element M diboride 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.

[0060] According to a possible embodiment, said liquid resulting from the filtration of the suspension described above comprising products of reaction (3) except the diboride powder of element M is heat-treated in the presence of water and a basic solution in order to form a hydrate of element R and an alkali metal hydroxide. This embodiment makes it possible to use the product of reaction (3) of formula A2xRyO5+x. Preferably, this possible embodiment is particularly advantageous in the case where element R is Al and alkali metal A is sodium.

[0061] The invention also relates to a powder of diboride of element M from group 4 of the periodic table, in particular a powder of titanium diboride TiB2, obtained according to the preceding process.

[0062] Said powder comprises more than 95% by mass of the compound MB2, M being chosen from Ti, Zr and Hf. The median particle diameter of this powder is between 0.5 and 50 micrometers, and it further comprises the following mass contents:

[0063] - elemental oxygen (O): less than 1.3%, preferably less than 1.2%, of preferably less than 1% or even less than 0.5%;

[0064] - elemental carbon (C): less than 0.5%, preferably less than 0.1%;

[0065] - elemental nitrogen (N): less than 0.5%;

[0066] - 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;

[0067] - elemental iron (Fe): less than 0.45%, preferably less than 0.4%;

[0068] - elemental nickel (Ni): less than 0.4%, preferably 0.2%, or even less than 0.1%;

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

[0070] - elementary sum of alkalis Li+Na+K+Rb+Cs: less than 1%, preferably less than 0.5%;

[0071] - elementary sum of alkaline earths (Be+Mg+Ca+Sr+Ba): less than 1%, of preferably less than 0.5% or even less than 0.25%;

[0072] - content of element R in metallic form: less than 2%, preferably less than 1%, more preferably less than 0.5%, R preferably being different from M, R being at least one element chosen from Al, Si, Ti, Zr, Hf, Y, Sc, the lanthanides,

[0073] the sum of the other elements being less than 2%, preferably less than 1%.

[0074] Preferably, the elemental sum oxygen (O) + nitrogen (N) + carbon (C) of the diboride powder of element M is less than 1.5%, or even less than or equal to 1.2%.

[0075] Preferably, the mass content of silicon (Si) in metallic form of the element M diboride powder is less than 0.1%.

[0076] Preferably, the mass content of aluminum (Al) in metallic form of the element M diboride powder is less than 2%, preferably less than 1%, preferably less than 0.5%.

[0077] Preferably the ratio (DQo-Dio) / D5o of equivalent diameter of the particles of the raw powder raw powder, that is to say of the powder after extraction of the raw mixture from the enclosure after the heating step, in particular after separation of the product of the reaction (3) of formula A2xRyO5+x, is less than 2, preferably less than 1.5, preferably 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 curve of distribution of grain diameter by volume classified in ascending order of said powder.

[0078] If M = Ti, the mass elemental content of Ti is preferably greater than 68% and / or less than 72% and the mass elemental content of boron is preferably greater than 29% and / or less than 33%. Preferably, the mass elemental content of phosphorus is less than 0.3%, preferably less than 0.2% or even less than 0.1%.

[0079] Such a powder of diboride of element M from group 4 of the periodic table, in particular a TiB2 powder, of high purity and of fine and regular particle size makes it possible to obtain by sintering a sintered ceramic body having a total porosity of less than 7% by volume without resorting to additions of transition metals such as Ni, Fe or Co which are likely to lead to the formation of secondary metal borides from these metals which are not desired.

[0080] Such a powder makes it possible to obtain a sintered ceramic body in the form of a part of which at least one dimension, preferably all of the overall dimensions, is greater than 5 cm, or even greater than 10 cm, and having a total porosity also less than 7%, a very narrow pore size distribution, without deformation during sintering and without shrinkage cracks.

[0081] Preferably, M is Ti.

[0082] Said powder of diboride of element M from group 4 of the periodic table is then a powder of the compound TiB2 which further comprises one or more of the following mass contents:

[0083] - titanium (Ti): greater than 68% and / or less than 72%,

[0084] - boron (B): greater than 29% and / or less than 33%.

[0085] - oxygen (O): less than 1%, preferably less than 0.5%, or sulfur (S) less than 400 ppm, less than 300 ppm, less than 100 ppm, preferably less than 50 ppm,

[0086] - preferably phosphorus (P): less than 0.3%, preferably less than 0.2%, of preferably less than 0.1%,

[0087] Said diboride powder of element M from group 4 of the periodic table is then a TiB2 powder whose chemical composition comprises the following mass elemental contents:

[0088] -titanium (Ti): greater than 68% and / or less than 72%,

[0089] - boron (B): greater than 29% and / or less than 33%,

[0090] - preferably phosphorus (P) less than 0.3%,

[0091] - metallic aluminum: less than 2%,

[0092] - metallic silicon: less than 1%.

[0093] The invention also relates to a mixture comprising between 90 and 99.9% by mass of diboride of element M from group 4 of the periodic table or even consisting of a powder of diboride of element M from group 4 of the periodic table, preferably titanium diboride (TiB2), according to the invention and between 0.1 and 10% by mass of one or more sintering powders chosen from powders of aluminum diboride, magnesium diboride, zirconium diboride, tungsten pentaboride, calcium hexaboride, silicon hexaboride preferably, optionally zirconium diboride if M=Ti or Hf, 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 the said phase or of the most stable main compound: for example in the case of an aluminium diboride 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.

[0094] The invention also relates to a method of manufacturing a sintered ceramic body comprising the following steps: a. preparation of a starting charge comprising: - the diboride powder of element M from group 4 of the periodic table, preferably TiB2 and / or (Zr,Hf)B2, preferably TiB2 as obtained by a process according to the invention or a mixture of powders as described above comprising said powder and one or more of said sintering powders. - an aqueous solvent, in particular deionized water, - 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 is between 0 and 0.5% by weight, e. loading into an oven and baking the preform under an inert atmosphere, preferably under argon, or under vacuum, preferably at a temperature between 1600°C and 2200°C.

[0095] The invention also relates to the sintered ceramic body thus obtained and the use of the sintered ceramic body obtained by the preceding method as all or part of a membrane, in particular for the filtration of liquids or gases, of an abrasion-resistant shielding or coating, of a refractory coating or block, of an anode coating or block or of a cathode coating or block, in particular of an electrolysis reactor, of a heat exchanger, of a crucible for melting metal, in particular non-ferrous metal, or of a cutting tool. Definitions:

[0096] The following indications and definitions are given, in relation to the preceding description of the present invention:

[0097] - In this description, unless otherwise specified, all percentages are given by weight, based on dried material.

[0098] - Boron oxide means any oxide comprising boron and oxygen, with possibly at least one other element notably chosen from Na, Ca, the oxide being optionally hydrated.

[0099] -Here, the term boron oxide precursor is understood to mean a powder comprising the element boron (B) which, by oxidation, for example by heating under an oxidizing gas, preferably under air, at a temperature below 600°C, or by contact with an oxide present in the mixture of raw materials, oxidizes in order to produce the reactant B2O3 present in the chemical equation of reaction (3). - The material yield is calculated by dividing the mass of raw powder obtained divided by that of the dry mixture of powder of the reactants (humidity less than 5%) before heat treatment.

[0100] -By raw 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 additional treatment of extraction of the raw powder of diboride of element M for example by screening or light grinding.

[0101] -The median diameter (or median "size") of the particles constituting a powder is given in the sense of the present invention by a characterization of particle size distribution, in particular by means of a laser particle size analyzer. The characterization particle size distribution is conventionally carried out with a laser granulometer in accordance with ISO 13320-1. The laser granulometer may be, for example, a Partica LA-950 from HORIBA. For the purposes of this description and unless otherwise stated, the median particle diameter designates respectively the particle diameter below which 50% by mass of the population is found. The term "median diameter" or "median size" of a set of particles, in particular of a powder, is the D50 percentile, i.e. the size dividing the particles into first and second populations equal in volume, these first and second populations comprising only particles having a size greater, or less respectively, than the median size.We can also determine the percentiles Di0 and D90 of a powder of grains or particles which are the diameters corresponding respectively to the percentages of 10% and 90% on the cumulative curve of distribution of grain diameter in volume classified in ascending order.

[0102] -elementary chemical contents can be determined according to ISO 21068 of 2008.

[0103] In particular the following mass contents of:

[0104] - O, N, C, and S are measured by LECO® brand analyzer,

[0105] - 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" in English or inductively coupled plasma spectrometry in French),

[0106] -the contents of element M (in particular Ti, Zr, Hf) are preferably determined by ICP.

[0107] - Aluminum in metallic form or Silicon in metallic form or compound MB2 can be determined by X-ray diffraction.

[0108] - the hydroxyl (OH) content of the alkali element A oxide powder, can be measured by pH-metry.

[0109] - the actual density on powder is measured by helium pycnometry, for example at using Micromeritics AccuPycl330 equipment.

[0110] -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.

[0111] Unless otherwise indicated, in this description, all percentages are mass percentages. Brief description of the drawings

[0112] [Fig. 1] shows the raw powder according to Example 2 according to the invention.

[0113] [Fig.2] shows the raw powder according to comparative example 1.

[0114] [Fig.3] shows the raw powder of comparative example 3. Detailed description

[0115] The invention and its advantages will be better understood upon reading the detailed description which follows. Of course, the present invention is not limited to such a mode, in any of the aspects described below.

[0116] The starting raw material mixture comprises:

[0117] -a powder comprising MO2 oxide (for example a titanium oxide powder, preferably rutile or anatase) whose mass content of MO2 is at least 95%, and

[0118] - a powder comprising a boron oxide or a boron oxide precursor whose mass content of element Boron (B), expressed as B2O3, is at least 30%;

[0119] -a metal powder of a reducing element R chosen from Al, Si, Ti, Zr, Hf, Y, Sc, and the lanthanides, their mixture or their alloy, the mass content of elements other than Al, Si, Ti, Zr, Hf, Y, Sc, and the lanthanides of which is less than 1%, preferably less than 0.5%, for example a powder of the element aluminum (Al) and / or of the element silicon (Si), and

[0120] -an alkali element A oxide powder, the mass content of A2O of which is at least 70%, preferably at least 80%, preferably at least 90%, preferably at least 95%, preferably at least 98%, more preferably a sodium oxide (Na2O) powder of purity greater than at least 99%,

[0121] It is carried out under standard conditions for those skilled in the art. This step of preparing the dry mixture allows intimate contact of the particles. According to one possible mode, it is carried out in a rubber ball mixer or in a tumbler type mixer or other devices known to those skilled in the art. Preliminary co-grinding can be carried out to adjust the particle size of the starting raw materials if necessary.

[0122] If necessary, certain raw materials such as borates or alkali element oxide powder can be dried or even calcined in order to reduce their H2O or OH hydroxyl content. In the case of starting materials such as natural borax of formula Na2B4O7.10H2O (also expressed as Na2 B4O5(OH)4-8H2O), tincalconite of formula Na2B4O7.5H2O (also expressed as Na2B4O5(OH)4.3H2O), kernite of formula Na2B4O7.4H2O (also expressed as Na2B4O6(OH)2-3H2O), ulexite of formula NaCaB5O9.8H2 O (also expressed as NaCaB5O6(OH)6-5 H2O), probertite NaCaB5 O9.5H2O (also expressed as NaCaB5O7(OH)4.3H2O), this treatment makes it possible to reduce the presence of hydrogen present in the form of water H2O adsorbed on the surface of powder or hydroxyls OH. It helps improve the conversion rate in diboride of element M which also results in a raw MB2 powder having after synthesis a very low content of reducing metal of element R.

[0123] Preferably, in order to maximize the conversion rate to diboride MB2j, the hydroxyl (OH) content provided by the raw materials in reaction (3) is reduced to a minimum. In particular, the borates can be calcined in order to dehydroxylate them. More preferably, the alkali element oxide powder A has a hydroxyl content calculated by dividing its mass of OH by the mass of alkali oxide A2O 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.

[0124] The median size or median diameter of the particles of element M oxide is respectively between 1 and 100 micrometers, preferably between 7 and 80 micrometers. That of the particles comprising boron oxide, that of the metal particles of element R reducer and that of the particles of alkali element A oxide is preferably between 30 and 100 micrometers, preferably between 30 and 80 micrometers.

[0125] Preferably, the median size ratio of the particles comprising boron oxide to that of element M oxide is between 1 and 10.

[0126] Preferably, in a mixture according to the invention comprises in mass proportion respectively 20 to 25% of oxide of element M, 25 to 40% of powder comprising a boron oxide or a precursor of boron oxide, 20 to 30% metal powder of reducer of element R and 15 to 25% of a powder of oxide of alkali element A. In particular in the case where the element M is Ti and A is Na, the mixture according to the invention comprises in mass proportion respectively 20 to 25% of titanium oxide, 25 to 35% of powder comprising a boron oxide or a precursor of boron oxide, preferably a sodium borate, 20 to 30% metal powder of reducer of element R, preferably Al and / or Si, and 15 to 25% of a sodium oxide powder.

[0127] The total content of said mixture of raw materials in alkali oxide calculated in the form A2O is equal to or greater than the stoichiometric quantity necessary for said reaction (3), preferably less than 10%, or even less than 5%;

[0128] 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 humidity, i.e. the residual mass content of H2O measured by a humidity meter well known to those skilled in the art, of said mixture of raw materials is less than 5%, preferably less than 2%, or even more preferably less than 1%.

[0129] The mixture is placed in an inert crucible, preferably made of diboride of element M or even alumina, preferably alumina coated with diboride of element M, for example example in an induction furnace. The unpacked density of the mixture before heat treatment measured according to ASTM D7481 - 18 is preferably greater than 0.1 times the density of MB2, or even greater than 0.2 and / or preferably less than 0.5, less than 0.3 times the density of MB2.

[0130] A temperature rise is carried out up to at least one temperature preferably higher than the melting point of the metal of element R chosen from Al, Si, Ti, Zr, Hf, Y, Sc, and the lanthanides, their mixture or their alloy, the content of elements other than Al, Si, Ti, Zr, Hf, Y, Sc, and the lanthanides, preferably higher than 600°C, preferably higher than 700°C, preferably higher than 800°C, and lower than 1500°C, preferably lower than 1300°C, in a non-oxidizing atmosphere, preferably under rare gas scavenging, in particular Argon so as to avoid oxidation of the metal reducing agent powder R.

[0131] 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.

[0132] Preferably the temperature rise 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 rise ramp, like the duration of the stage, can be adjusted according to the volume of mixture and the power of the reactor.

[0133] Preferably, the plateau at the maximum temperature is at least one hour, preferably at least two hours.

[0134] Preferably, an intermediate level is carried out between 600 and 1000°C and / or a lower ramp, typically at least twice as low, is carried out after 600°C in order to avoid decohesion of the mixture and promote the reaction between the particles.

[0135] Cooling can be free or forced, preferably according to a negative ramp of less than 20°C / min.

[0136] The raw mixture obtained has a particle size typically between 10 and 100 micrometers.

[0137] A sieving operation, typically to a diameter of 100 micrometers, preferably to a diameter of 80 micrometers, preferably to a diameter of 50 micrometers, or even light crushing or vibration makes it possible to eliminate agglomerations and separate the raw powder of element M diboride.

[0138] According to one possible mode, a sieving operation or even light crushing or vibration makes it possible to eliminate the agglomerations and to separate the powder of diboride of element M. A suspension is produced by adding to the previously ground raw mixture a solvent, preferably deionized water, according to a mass ratio of 1 part of raw mixture for at least 20, preferably 50 parts of solvent. Said suspension is filtered to an optimal size typically 30 micrometers, preferably 20 micrometers, or even 15 micrometers or less in order to allow the liquid comprising the residues of the other products of the reaction (3) to pass through. The filtration retentate consisting of the diboride powder of element M 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.

[0139] According to a possible embodiment, said liquid resulting from the filtration of the suspension described above comprising products of reaction (3) except the diboride powder of element M is heat-treated in the presence of water and a basic solution in order to form a hydrate of element R and an alkali metal hydroxide. This embodiment makes it possible to use the product of reaction (3) of formula A2xRyO5+x. Preferably, this possible embodiment is particularly advantageous in the case where element R is Al and alkali metal A is sodium.

[0140] After grinding the raw powder, it is possible to obtain a final powder of finely divided element M diboride whose median diameter is between 0.5 and 50 micrometers of high purity, of micron size whose size dispersion is very reduced.

[0141] The final powder of element M diboride makes it possible to obtain by sintering a sintered ceramic body having a total porosity of less than 7% by volume without resorting to additions of transition metals such as Ni, Fe or Co while having a very low electrical resistivity.

[0142] 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 part of which all the dimensions are at least one dimension greater than 5 cm without deformation during sintering and without shrinkage cracks.

[0143] The powder material according to the invention has an electrical resistivity, measured at 25°C and at atmospheric pressure, of less than 0.2 microOhm.m.

[0144] The electrical resistivity can be measured according to the Van der Pauw method at 4 points on a sample with a diameter of 20-30 mm and a thickness of 2.5 mm. The sample being obtained by pressing a mixture consisting of said powder with 0.25% of a pressing additive (PVA) and 4.75% of deionized water by mass relative to the mass of M diboride powder in order to be cold pressed under a pressure of 100 bars and to constitute 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.

[0145] A method of manufacturing a sintered ceramic body using the powder according to the invention comprises in particular the following steps: a. preparation of a starting charge comprising: - the diboride powder of element M where M is a chemical element belonging to group 4 of the periodic table, in particular TiB2 according to the invention or a mixture of powders as previously described, comprising said powder and one or more sintering powders, in particular chosen from powders of aluminum diboride, magnesium diboride, tungsten pentaboride, calcium hexaboride, silicon hexaboride, optionally zirconium diboride if M = Ti or Hf, the purity of said MB2 powder being greater than 95% by mass, preferably greater than 98% by mass, said MB2 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 starting charge in the form of a preform, preferably by pressing, extrusion or casting, c. demolding after curing or drying, d. optionally, drying the preform, preferably until the residual moisture is between 0 and 0.5% by weight, e. loading into an oven and baking the preform under an inert atmosphere, preferably under argon, or under vacuum, preferably at a temperature between 1600°C and 2200°C, preferably according to a temperature rise ramp of less than 20°C / minute, preferably less than 10°C / minute. This temperature rise ramp, like the duration of the stage, can be adjusted according to the volume of mixture and the power of the reactor.

[0146] Any shaping technique known to those skilled in the art can be applied depending on the size of the part to be produced, provided that all precautions are taken are taken to avoid contamination of the preform. Thus, casting in a plaster mold can be adapted by using graphite media between the mold and the preform or oils to avoid too intimate contact and abrasion of the mold by the mixture and ultimately contamination of the preform. These precautions for use mastered by those skilled in the art are also applicable to other stages of the process. Thus, during sintering, the mold or the matrix used containing the preform will preferably be made of graphite.

[0147] Hot pressing, hot isostatic pressing or SPS (Spark Plasma Sintering) techniques are particularly suitable.

[0148] The following examples are given purely for illustrative purposes and do not limit the scope of the present invention in any of the aspects described. Example 1 (comparative)#:

[0149] The starting mixture was made with a titanium oxide powder with a median diameter D50 of 10 pm mainly in a crystallographic form of TiO2 in rutile form supplied by Traxys France (purity 95%), a boron oxide powder B2O3 with a median diameter D50 equal to 15 pm and a carbon black powder (carbon black) with a median diameter D50 of 0.2 pm according to the following respective mass proportions: 38.1%, 33.2% and 28.7%.

[0150] A sample of mixture was placed in a graphite crucible subjected respectively to a heat treatment at 1600°C according to a holding time of 2 hours in a furnace under an Argon sweep of 1.25 L / min / m3.

[0151] The synthesis mixture obtained was ground for 3 minutes in order to obtain a powder with a median size of less than 10 microns. Example 2 (according to the invention)#:

[0152] The starting mixture was made with a titanium oxide powder with a median diameter D50 of 10 μm mainly in a crystallographic form of rutile as in the previous example, a sodium tetraborate powder (Na2B4O7) with a median diameter D50 equal to 50 μm from Sigma Aldrich with a purity greater than 99% by mass and an aluminum metal powder with a median diameter D50 of 10 μm from Alfa Aesar with a purity greater than 99% by mass, a sodium oxide powder (Na2O) with a median diameter D50 of 50 μm from Sigma Aldrich with a purity greater than 99% by mass, according to the following respective mass proportions: 23.3%, 29.3%, 26.2% and 21.2%.

[0153] A sample of mixture was placed in an alumina crucible subjected respectively to a heat treatment at 800°C according to a holding time of 2 hours in a furnace under an Argon sweep of 1.25 L / min / m3.

[0154] The corresponding balance reaction is:

[0155] 3TiO2 + 1.5Na2B4O7 +10A1 +3.5Na2O TiB2 +10 NaA102

[0156] or, expressed according to reaction (3) as a function of the simple oxides TiO2, B2O3 and Na2O:

[0157] TiO2 +B2O3 + 10 / 3 Al +5 / 3Na2O TiB2 +10 / 3NaA102.

[0158] The synthesis mixture obtained was ground for 1 minute in order to obtain a powder with a median size of less than 30 microns.

[0159] The obtained powder was mixed with deionized water in the following proportion of 1g of powder to 50ml of water. This mixture was filtered by passing through a 185mm 12-15pm VWR paper in order to retain the titanium diboride particles. The retentate was dried at 110°C. in order to obtain the final dry titanium boride powder. Example 3 (comparative)#:

[0160] This example differs from Example 2 in that the starting mixture comprises sodium hydroxide granules (NaOH) instead of sodium oxide powder. The respective mass proportions of the titanium, sodium tetraborate, aluminum metal powders, and sodium hydroxide granules were 21.9%, 27.6%, 24.7%, and 25.8%.

[0161] [Tables 1] unit Example 1 (comparative) Example 2 (invention) Example 3 (comparative) raw materials of the mixture Titanium oxide powder D50pm 10 10 10 D90pm 30 30 30 B2O3 powder D50pm 10 NA NA D90pm 30 Carbon powder D50 pm 0.2 NA NA D90pm 0.3 Aluminum powder D50 pm NA 5 5 D90pm 10 10 Sodium tetraborate powder D50 pm NA 30 30 dium (Na2B4O7) D90pm 50 50 Alkali oxide powder Na D50 pm NA 50 50 20 D90pm 80 80 Heat treatment Max. temperature °C 1600 800 800 Ramp °C / min 5 2 2 Stage h 2 2 2 Atmosphere Argon Argon Argon Pressure mbar Atmos. Atmos. Atmos. Sweep flow rate L / min / m3 of enclosure 1.25 1.25 1.25 Material yield % 30 20.3 <5

[0162] NM not measured; NA not applicable; Atmos. = atmospheric pressure

[0163] The characteristics of the final powders obtained are presented in Table 2 below:

[0164] [Tables2] Example 1 (comparative) 1600 Example 2 (invention) 800°C Example 3 (comparative) 800°C Physical characteristics of the powder obtained after grinding, 1 min D50 pm 5.2 2.6 6.2 D90pm 8.9 4.9 8.6 (D9o-D1o) / D5o pm 1.3 0.9 1.4 Final powder chemistry % by mass Ti (%) 61 >65 NM B (%) 26 32.0 NM O LECO (%) 0.7 <1 2.5 C LECO (%) >5% <0.1 <0.1 N LECO (%) 0.1 <0.1 <0.1 S LECO (ppm) 180 <50 <50 Fe (ppm) 785 <500 <500 Li+N a+K+Rb+C s(ppm) 500 <500 >2000 Be+Mg+Ca+Sr+Ba (pp m) <500 <500 <500 P (ppm) <500 <500 <500 Ni-(ppm) <500 <500 <500 Co(ppm) <500 <500 <500 Metallic Si % <0.5 <0.5 <0.5 Metallic Al % - <0.5 >5% Actual density 4.4 4.4 4.4

[0165] NM = not measured

[0166] These results of Example 2, in contrast to Comparative Example 1, show that it is possible to obtain, according to the process of the invention, a very pure, fine powder without CO release and at a lower synthesis temperature. Comparison of Example 2 with Examples 1 and 3 shows that the raw powder according to the invention is significantly less dispersed, in particular than that produced by the process of Example 3 based on a sodium hydroxide solution (cf. (D9o-Dio) / D5o).

Claims

Claims

1. A process for synthesizing a diboride MB2 powder, where M is a chemical element belonging to group 4 of the periodic table, by reduction of an oxide of said M, comprising the following steps: - preparation of a mixture of raw materials consisting of: a. a powder whose mass content of said oxide MO2 is at least 95% and b. a powder comprising a boron oxide or a boron oxide precursor, whose mass content of boron, expressed as B2O3, is at least 30%; and c. a metal powder of at least one reducing element R, R being chosen from Al, Si, Ti, Zr, Hf, Y, Sc, the lanthanides; and d.an alkali element A oxide powder, the mass content of A2O of which is at least 70%; in respective proportions leading to the following balanced reaction, expressed according to said MO2 B2O3, R and A2O: MO2 + B2O3 + yR + xA20 MB2 + A2xRyO5+x (3) - heating said mixture in an enclosure under a flow of rare gas, at a temperature above 600°C and below 1500°C, said mixture of raw materials having the following characteristics: - the median particle diameter of said powder comprising the oxide MO2 is between 1 and 100 micrometers; and - the median particle diameter of the powder comprising a boron oxide or a boron oxide precursor is between 5 and 200 micrometers; and - x is greater than or equal to 1 - y is greater than 0.

5.

2. A method for synthesizing a MB2 diboride powder, according to claim 1, wherein the mass content of the alkali element A oxide powder in hydroxyls, calculated as the mass of OH over the mass of alkali is less than 40%.

3. A method of synthesizing a MB2 diboride powder, according to claim 1 or 2, wherein the boron oxide or boron oxide precursor is selected from, sodium metaborate of chemical formula NaBO2, anhydrous borax of formula Na2B4O7, or other borates such as natural borax of formula Na2B4O7.10H2O, tincalconite of formula Na2B4O7.5H2O kernite of formula Na2B4 O7.4H2O, ulexite of formula NaCaB5O9.8H2O, proberite NaCaB5 O9.5H2O, sassolite of formula H3BO3, boron nitride.

4. A method of synthesizing a MB2 diboride powder, according to the immediately preceding claim, wherein the powder comprising boron oxide is an anhydrous alkali borate powder, preferably an anhydrous sodium borate powder.

5. A method of synthesizing an MB2 powder, according to one of the preceding claims, wherein the median particle diameter of said powder comprising MO2 oxide is greater than 7 micrometers and / or less than 50 micrometers.

6. A method of synthesizing an MB2 powder, according to one of the preceding claims, wherein the median particle diameter of said powder comprising boron oxide is greater than 30 micrometers and / or less than 100 micrometers.

7. A method of synthesizing an MB2 powder according to one of the preceding claims, wherein the ratio of the median particle diameter of said powder comprising boron oxide to the median particle diameter of said powder comprising MO2 oxide is less than 10 and / or greater than 1.

8. A method of synthesizing an MB2 powder according to one of the preceding claims, wherein the heating temperature in said enclosure is greater than 700°C and / or less than 1400°C.

9. Method for synthesizing an MB2 powder according to one of the preceding claims, in which the rare gas is chosen from argon or helium.

10. A method of synthesizing a TiB2 powder, according to the immediately preceding claim, wherein A is the element Na.

11. A method of synthesizing a TiB2 powder, according to the immediately preceding claim, wherein R is the element Al and / or Si.

12. A method of synthesizing an MB2 powder, according to one of the preceding claims, wherein M is the element Ti, A is the element Na and R is the element Al and / or Si.

13. Powder comprising more than 95% by weight of the compound MB2 obtained according to one of claims 1 to 12, M being chosen from Ti, Zr, Hf, whose median diameter is between 0.5 and 50 micrometers, and whose chemical composition includes the following mass elemental contents: - elemental oxygen (0): 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 alkalis (Li+Na+K+Rb+Cs): less than 1%; - elemental sum of alkaline earths (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, Ti, Zr, Hf, Y, Sc, the lanthanides, the sum of the other elements being less than 2%.

14. TiB2 powder obtained according to claim 13, the chemical composition of which comprises the following mass elemental contents: - titanium (Ti): greater than 68% and / or less than 72%, - boron (B): greater than 29% and / or less than 33%, - preferably phosphorus (P) less than 0.3%, - metallic aluminum: less than 2%, - metallic silicon: less than 1%.

15. Mixture comprising between 90% and 99.9% by mass of an MB2 powder according to one of claims 13 to 14 and between 0.1 and 10% by mass of one or more sintering powders chosen from powders of aluminum diboride, magnesium diboride, tungsten pentaboride, calcium hexaboride, silicon hexaboride, optionally zirconium diboride if M = Ti or Hf, preferably the purity of which is greater than 95% by mass.

16. A method of manufacturing a sintered ceramic body comprising the following steps: a. preparing a starting charge comprising: - the MB2 powder according to claim 13 to 14 or the mixture of powders according to claim 15, - an aqueous solvent, in particular deionized water, - preferably, shaping additives, b. shaping the starting charge in the form of a preform, c. demolding after hardening or drying, d. optionally, drying the preform, preferably until the residual moisture is between 0 and 0.5% by weight, e. loading into an oven and baking the preform under an inert atmosphere, preferably under argon, or under vacuum, preferably at a temperature between 1600°C and 2200°C.

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