EP4463418A1 - Method for dry-method synthesis of a diboride powder - Google Patents

Abstract

The invention relates to a method for dry-method manufacture of a diboride powder MB2, wherein M is a chemical element belonging to group 4 of the periodic table, from the reduction of an oxide MO2 of the element M according to the reaction MO2 + B2O3 + yR + xA2O → MB2 + A2xRyO5+x, wherein R is a reducing element selected from Al, Si, Ti, Zr, Hf, Y, Sc and the lanthanides and A2O is an oxide of an alkali element A. The invention also relates to a powder obtained by means of said method.

Description

Description

Title of the invention: PROCESS FOR THE SYNTHESIS OF A DIBORIDE POWDER BY A DRY WAY

The invention relates to a new process for the manufacture or synthesis of element diboride of group 4 of the periodic table, in particular titanium diboride.

Element diborides of 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.

Titanium diboride in particular is a ceramic material having a low density (approximately 4.5 g/cm ³ ), high hardness, high thermal conductivity and low electrical resistivity. This makes it a potentially interesting material for several applications such as refractory applications where thermal conduction and high electrical conduction are an asset, in particular heat exchangers, coating or even the composition of anodes or cathodes of electrolysis reactors , or even membranes in certain temperature applications or in very aggressive chemical environments, but also metal melting crucibles, in particular non-ferrous metals, or even cutting tools or shielding or even an anti-abrasion coating.

All these applications explain why the demand for this material is very important and growing at the present time.

Diborides do not occur naturally. 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 in reacting a mixture of powders according to the following simplified reaction at a temperature above 1500°C:

However, this process has a theoretical material yield of approximately 30%. Another less known process consists in particular in replacing the boron oxide powder with boron carbide, as illustrated by the following balance reaction:

The advantage of such a reaction is its higher theoretical material yield (55.4%) and therefore less release of carbon monoxide, 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.

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 less than 5 micrometers).

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-synthesis. -propagate at high temperature (SHS) but lead to the incomplete conversion of the reactants, which conventionally requires a second reaction typically with boric acid H3BO3 in order to obtain a better conversion rate to diboride.

Another solution also consists of a synthesis in a medium of molten salts or in solution. The patent application published under W02020073767A1 from the Wuhan University of Science and Technology thus proposes a method for preparing a less expensive and more environmentally friendly TiB2 or (Zr,Hf)B2 powder 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 alkaline salt. The alkaline salt can be chosen from a hydrate, a silicate or a sodium, lithium or potassium carbonate in order to form a liquid phase at low temperature. The addition of the silicate, however, leads to reaction products which 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. The object of the invention is thus to improve the synthesis methods described above in order to obtain a powder of element diboride from group 4 of the periodic table, in particular TiBz:

-fine, i.e. typically a powder with a median diameter of between 0.5 and 50 micrometers;

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

-a satisfactory material yield, for example of at least 20%;

-an easy ability to extract said diboride powder from the reaction products;

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

-without resorting to an industrial process that is too complex for the synthesis of powder. Summary of the invention:

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

More specifically, the present invention relates to a process for manufacturing a powder of diboride 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 process comprising the steps following:

- preparation of a mixture of raw materials comprising, and preferably 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 precursor of boron oxide, whose mass content of boron, expressed in BzCh, 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, lanthanides; and d) an oxide powder of alkaline element A, the A2O mass content of which is at least 70%; in the respective proportions leading, preferably corresponding, to the following balance reaction, expressed according to said MCh, B2O3, R and A2O:

- heating of said mixture in an enclosure under a flow of rare gas, at a temperature above 600°C and below 1500°C, to obtain compound MB2, said mixture of raw materials having the following characteristics: -the diameter median of particles 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.

According to preferred but non-limiting embodiments of the present invention, which can be combined with each other if necessary:

- The mass sum of the powder whose mass content of said oxide MO2 is at least 95%, of the powder comprising a boron oxide or a precursor of boron oxide, of the metal powder of at least one element reducer R and alkaline element oxide powder A represent more than 80% by weight of said mixture, or even for more than 90% by weight, or even more than 95% by weight of said mixture.

- The powder whose mass content of MO2 oxide is at least 95% comprises several MO2 oxides, in particular is a mixture of TiCL, ZrC and/or HfC. -The powder whose mass content of said oxide MO2 is at least 95% comprises a mixed oxide with at least two different M elements, M being chosen from Ti, Zr and Hf,

- The powder consists of the oxide MO2, preferably consists of titanium oxide.

-The powder comprises a boron oxide or a boron oxide precursor, the mass content of boron of which, expressed as B2O3, is at least 40%.

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

- The alkaline element A oxide powder, preferably sodium oxide (NazO), has a mass content of A2O of at least 80%

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

- The rate of sweeping of the gas flow in said enclosure being between 0.5 and 10 L/min per m ³ of enclosure.

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

- The median particle diameter of the powder comprising boron oxide is between 10 and 100 micrometers.

- the total content of said mixture of raw materials in alkaline 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.

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.

-y is greater than 1 .

According to one possible mode, the mixture of raw materials comprises an MO2 oxide powder of a first chemical element M belonging to group 4b of the periodic table and a second MO2 oxide powder of a second chemical element M belonging to group 4b of the periodic table different from the first element, M being chosen from Ti, Zr, or HfC .

According to another mode, which can be combined with the previous one, the mixture of raw materials comprises at least one MO2 oxide powder in which M is a mixture of at least two, preferably two, chemical elements belonging to group 4b of the table periodic.

As will be described in more detail below, such a combination of parameters advantageously makes it possible to obtain a fine powder of diboride MB2 of high purity with a satisfactory material yield, using a process that does not release or release little of CO or CO2 and allowing an easy ability to extract said diboride powder without resorting to an industrially too complex powder synthesis process.

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

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

Thus, the balance reaction (3) is written, in the case of the use of boron oxide B2O3:

The balance reaction (3) is written, in the case of the use of a precursor of boron oxide Na2B4Û7: with M=Ti A=Na R=Al x=5/3 and y=10/3

In particular, unlike the synthetic processes using molten salts or dissolution in a solvent, in particular water, the process according to the invention by the dry route and in particular by the use of an oxide powder of weakly or non-hydroxylated alkaline element A rather than the use of alkaline 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 a easy separation of the element M diboride powder after synthesis in the chamber. According to other preferred embodiments of the present invention, which can be combined with each other if necessary:

- The boron oxide precursor is chosen from compounds in which the 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.

- The boron oxide is chosen from B2O3, sodium metaborate with the chemical formula NaBC, anhydrous borax with the formula N 326407, or other borates such as natural borax with the formula Na2B4O?.10H2O, tincalconite with the formula Na2B4O ?.5H2O kernite with formula Na2B4O?.4H2O, ulexite with formula NaCaBsO9.8H2O, proberite NaCaBsOç.SlO, sassolite with formula H3BO3. Preferably, the boron oxide is chosen from sodium metaborate of chemical formula NaBÛ2, anhydrous borax of formula Na2B4O7, or other borates such as natural borax of formula Na2B4O7.10H2O, tincalconite of formula Na2B4O7.5H2O la kernite with the formula Na2B4O7.4H2O, ulexite with the formula NaCaBsO9.8H2O, proberite NaCaBsOg.SlO;

- The powder comprising boron oxide is an anhydrous alkaline borate powder, preferably anhydrous sodium borate powder.

- The median diameter (D50) of particles of said powder comprising the oxide MO2 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 diameter D90 of particles of said powder comprising the oxide MO2 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 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 diameter (D50) of particles of said metal powder of reducing agent 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. - The alkaline element oxide powder A 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 is less than or equal to 10 and/or greater to 5, preferably greater than 2. This ratio making it possible to optimize the rate of conversion to element M diboride.

- 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.

- A is the element Na.

- R is the element Al and/or Si

- M is the Ti element, A is the Na element and R is the Al and/or Si element.

- The powder comprising the oxide MO2 is a titanium oxide powder, preferably a rutile or anatase powder, more preferably rutile.

- The SiO2+Al2O3+Fe2O3+Na2O+K2O+CaO+MgO mass content of the powder comprising the MO2 oxide, preferably titanium oxide, is less than 5%. Preferably, the S1O2 mass content of the powder comprising the MO2 oxide, preferably titanium oxide, is preferably less than or equal to 2%. The mass content of Al2O3 of said powder comprising MO2 oxide, 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 MO2 oxide, 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 or equal to 0.5%.

- Apart from the metal powder of reducing agent R which may contain this element when it is in particular silicon powder, the mass content of the mixture of raw materials before reaction in the element silicon (Si), expressed in the form of Sit , is preferably less than 2%, preferably less than 1%. - The raw materials have been previously dried at a temperature between room temperature and 150°C.

- The synthesis temperature, that is to say heating in said enclosure, is greater than 700°C, preferably greater than 800°C and/or less than 1400°C, preferably less than 1300°C, so more preferably below 1200°C.

- The pressure of the enclosure is kept almost constant, for example between 0.5 and 1.5 bars and more 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. Preferably, the gas sweeping the enclosure, preferably a noble gas, is brought into the enclosure, in contact with the mixture of raw materials. The flow rate is preferably from 0.5 to 5 L/min per m ³ of enclosure, preferably between 0.5 and 3 L/min per m ³ , preferably between 0.5 and 2 L/min per m ³ d 'pregnant. Scanning too low leads to an incomplete reaction, more particularly to undesirable residues present in the final MB2 diboride powder.

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

After reaction, the finely divided raw powder of element diboride from group 4 of the periodic table can be easily extracted from the raw mixture coming from the enclosure after the heating step.

According to one possible mode, a sieving operation, typically to a diameter of 100 micrometers, preferably 50 micrometers, or even light crushing or vibrating makes it possible to eliminate the agglomerations and to separate the raw powder of element diboride M. 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 optimum 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 reaction products (3) to pass. The filtration residue, consisting of the powder of element M diboride, 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.

According to one possible mode, said liquid resulting from the filtration of the suspension described previously comprising reaction products (3) apart from the powder of element M diboride is heat-treated in the presence of water and a basic solution in order to form a hydrate of the R element and an alkali hydroxide. This mode makes it possible to upgrade the product of reaction (3) of formula AzxRyOs+x. Preferably, this possible mode is particularly advantageous in the case where the element R is Al and the alkali A is sodium.

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 TiBz, obtained according to the preceding process.

Said powder comprises more than 95% by weight of 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 also comprises the following mass contents:

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

- elementary carbon (C): less than 0.5%, preferably less than 0.1%;

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

- 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;

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

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

- elemental cobalt (Co): less than 0.4%, preferably 0.2%, or even less than 0.1%; - elementary sum of the alkalis Li+Na+K+Rb+Cs: less than 1%, preferably less than 0.5%;

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

- 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, lanthanides, the sum of the other elements being less than 2%, preferably less than 1%.

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

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

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

Preferably, the final powder of MB2 according to the invention does not comprise crystallized phases such as M2O3, M3B4 phases as measured (detectable) by X-ray diffraction. Said powder preferably comprises only a crystallized phase of MB2, such than measured (detectable) by X-ray diffraction.

Preferably the ratio (D ₉ O-DIO)/D ₅ O of equivalent diameter of the particles of the raw powder, that is to say of the powder after extraction of the raw mixture from the enclosure after the step of heating, in particular after separation of the product of reaction (3) of formula A2 _X RyOs _+x , is less than 2, preferably less than 1.5, more preferably less than 1.2 or even less than 1. The percentiles D10, D50 and D90 being the diameters corresponding respectively to the percentages of 10%, 50% and 90% on the cumulative grain diameter distribution curve by volume classified in ascending order of said powder. If M=Ti, the mass elementary content of Ti is preferably greater than 68% and/or less than 72% and the mass elementary 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%.

Such a powder of element M diboride from group 4 of the periodic table, in particular a TiBz 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% in volume without recourse to additions of transition metals such as Ni, Fe or Co which are liable to lead to the formation of secondary metal borides from these metals which are not desired.

Such a powder makes it possible to obtain a sintered ceramic body in the form of a part with at least one dimension. preferably all of the overall dimensions are greater than 5 cm, or even greater than 10 cm, and exhibiting a total porosity also less than 7%, a very narrow pore size distribution, without sintering deformation and without cracking of withdrawal.

Preferably, M is Ti.

Said powder of element M diboride from group 4 of the periodic table is then a powder of the compound TiBz which also comprises one or more of the following mass contents:

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

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

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

- preferably phosphorus (P): less than 0.3%, preferably less than 0.2%, preferably less than 0.1%,

Said powder of element M diboride from group 4 of the periodic table is then a TiBz powder, the chemical composition of which comprises the following elemental contents by mass: - 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 aluminium: less than 2%,

- metallic silicon: less than 1%.

The (D ₉ O-DIO)/D ₅ O ratio of equivalent particle diameter of the MBz powder is advantageously less than 1.5, more preferably less than 1.2 or even less than or equal to 1.0.

The invention also relates to a mixture comprising between 90 and 99.9% by mass 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 diboride titanium (TiBz), according to the invention and between 0.1 and 10% by weight of one or more sintering powders chosen from powders of aluminum diboride, magnesium diboride, zirconium diboride, pentaboride of tungsten, calcium hexaboride, preferably silicon hexaboride, optionally zirconium diboride if M=Ti or Hf, the purity of which is greater than 95% by weight, preferably greater than 98% by weight. Purity greater than 95% by mass means that of said phase or of the most stable main compound: for example in the case of a powder of aluminum diboride more than 95% by mass of AlBz or for a powder of tungsten pentaboride the fact that it contains more than 95% by mass of W ₂ B ₅ .

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

- the powder of diboride of element M from group 4 of the periodic table, preferably TiBz and/or (Zr,Hf)B2, preferably TiB2, as obtained by a process according to the invention or from a mixture 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 the form of a preform, preferably by pressing, c) demoulding after hardening or drying, d) optionally, drying the preform, preferably so that the residual humidity 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.

The invention also relates to the sintered ceramic body thus obtained and the use of the sintered ceramic body obtained by the preceding process as all or part of a membrane, in particular for the filtration of liquids or gases, of shielding or of anti-abrasion 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, a heat exchanger, a melting crucible for metal, in particular non-ferrous metal, a cutting tool.

Definitions:

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

- In the present description, unless otherwise specified, all percentages are given by weight, on the basis of dried material.

- By boron oxide is meant any oxide comprising boron and oxygen, optionally with at least one other element chosen in particular from Na, Ca, the oxide optionally being hydrated.

-Here, boron oxide precursor means a powder comprising the element boron (B) which by oxidation, for example by heating under an oxidizing gas, preferably in 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 of element M diboride obtained divided by that of the dry powder mixture of the reagents (humidity less than 5%) before heat treatment. - By crude mixture is meant the mixture obtained directly at the outlet of the enclosure after heating and reaction of the mixture of raw materials and before additional treatment for extraction of the raw powder of element diboride M, for example by screening or a slight grinding.

-The median diameter (or the median "size") of the particles constituting a powder, is given within the meaning of the present invention by a characterization of the particle size distribution, in particular by means of a laser particle sizer. The characterization of the particle size distribution is conventionally carried out with a laser particle sizer in accordance with the ISO 13320-1 standard. The laser particle sizer can be, for example, a Partica LA-950 from the company HORIBA. Within the meaning of the present description and unless otherwise stated, the median diameter of the particles designates respectively the diameter of the particles below which there is 50% by mass of the population. We call "median diameter" or "median size" of a set of particles, in particular of a powder, the percentile D50, that is to say 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 than, or less than, respectively, the median size. It is also possible to determine the D10 and D90 percentiles of a powder of grains or particles which are the diameters corresponding respectively to the percentages of 10% and 90% on the cumulative grain diameter distribution curve in volume classified in ascending order.

- the elementary chemical contents can be determined according to the ISO 21068 standard of 2008.

In particular the following mass contents of:

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

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

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

- the hydroxyl content (OH) of the alkali element oxide powder A, can be measured by PH-metry. - the actual powder density is measured by helium pycnometry, for example using AccuPyc1330 equipment from Micromeritics.

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

Unless otherwise indicated, in the present description, all the percentages are mass percentages.

Figures:

Figure 1 shows the raw powder according to Example 2 according to the invention.

Figure 2 shows the raw powder according to Comparative Example 1.

Figure 3 shows the raw powder of Comparative Example 3.

detailed description

The invention and its advantages will be better understood on 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.

The starting raw material mix includes:

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

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

-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 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

-an alkaline element oxide powder A, the mass content of A2O of which is at least 70%, preferably at least 80%, preferably at least 90%, preferably at least 95%, of preferably at least 98%, more preferably a sodium oxide powder (Na2O) of higher purity of at least 99%, It is carried out under standard conditions for those skilled in the art. This step of preparing the dry mix allows intimate contact of the particles. According to one possible mode, it is carried out in a mixer with rubber balls or in a mixer of the tumbler type or other devices known to those skilled in the art. A preliminary co-grinding can be carried out to adjust the particle size of the starting raw materials if necessary.

If necessary, certain raw materials such as borates or alkaline 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 the formula NazB ^ . HzO (also expressed as Na2B4O5(OH)4-8H2O), tincalconite with the formula Na2B4O?.5H2O (also expressed as Na2B4O5(OH)4.3H2O) kernite with the formula Na2B4O?.4H2O (also expressed as form Na2B4O6(OH)2-3H2O), ulexite with the formula NaCaBsO9.8H2O (also expressed in the form NaCaBsO6(OH)6-5 H2O), probertite NaCaBsOg.S^O (also expressed in the form 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 the powder or of OH hydroxyls. It improves the rate of conversion to element M diboride, which also results in a crude MB2 powder having, after synthesis, a very low content of element R reducing metal.

Preferably, in order to maximize the degree of conversion into diboride MB2, the content of hydroxyls (OH) provided by the starting materials in reaction (3) is reduced to a minimum. In particular borates can be calcined in order to dehydroxylate them. More preferably, the alkaline element oxide powder A has a hydroxyl content calculated by dividing its mass of OH over the mass of alkaline oxide A2O is less than 40%, preferably less than 30%, of more preferably less than 20%, or even less than 10%, even less than 5% or even substantially zero.

The median size or median diameter of the oxide particles of element M 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 reducer R and that of the particles of alkaline element oxide A is preferably between 30 and 100 micrometers preferably between 30 and 80 micrometers. Preferably, the median size ratio of the particles comprising boron oxide to that of element M oxide is between 1 and 10.

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

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

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, of said mixture of raw materials is less than 5%, preferably less than 2%, or even more preferably less than 1%.

The mixture is placed in an inert crucible, preferably made of element M diboride or even alumina, preferably alumina coated with element M diboride, for example in an induction furnace. The unpacked density of the mixture before heat treatment measured according to the ASTM D7481 - 18 standard 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.

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

Preferably, non-oxidizing gas is flushed at a normal flow rate of 0.5 and 5 L/min per m ³ of enclosure, preferably between 0.5 and 3 L/min/m ³ , preferably between 0 .5 and 2 L/min/m ³ of enclosure.

Preferably, the rise in temperature 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 plateau, can be adjusted according to the volume of mixture and the power of the reactor. In particular, such a temperature rise range promotes better control of the exothermic effect due to the powder synthesis reaction according to the invention.

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

Preferably, an intermediate plateau is made between 600 and 1000°C and/or a lower ramp, typically at least twice as low, is performed after 600°C in order to avoid decohesion of the mixture and promote the reaction between the particles.

The cooling can be free or forced, preferably according to a negative ramp of less than 20° C./min.

The crude mixture obtained has a particle size typically comprised between 10 and 100 micrometers.

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 vibrating makes it possible to eliminate the agglomerations and to separate the raw powder of element M diboride.

According to one possible mode, a sieving or even light crushing or vibration operation makes it possible to eliminate the agglomerations and to separate the powder of element M diboride. 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 crude mixture for at least 20, preferably 50 parts of solvent. Said suspension is filtered to an optimal size typically at 30 micrometers, preferably 20 micrometers, even 15 micrometers or less in order to allow the liquid comprising the residues of the other reaction products (3). The filter 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.

According to one possible mode, said liquid resulting from the filtration of the suspension described previously comprising reaction products (3) apart from the powder of element M diboride is heat-treated in the presence of water and a basic solution in order to form a hydrate of the R element and an alkali hydroxide. This mode makes it possible to upgrade the product of reaction (3) of formula AzxRyOs+x. Preferably, this possible mode is particularly advantageous in the case where the element R is Al and the alkali A is sodium.

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.

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

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, all the dimensions of which are at least one dimension greater than 5 cm without deformation on sintering and without shrinkage cracks.

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

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 sodium diboride powder. M in order to be cold pressed under a pressure of 100 bars and to form a cylinder with a diameter of 30 mm and a thickness of 10 mm. After demoulding, each cylinder was dried at 110°C for 24 hours then cooked without pressure at a temperature of 1850°C for 12 hours under Argon.

A process for 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 element M diboride powder where M is a chemical element belonging to group 4 of the periodic table, in particular TiBz, according to the invention or a mixture of powders as described above, comprising said powder and one or more powders sintering process, chosen in particular 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 filler.

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

- preferably, shaping additives such as binders such as PVA (polyvinylalcohol), plasticizers (such as polyethylene glycol), lubricants, b) shaping of the starting charge in the form of a preform , preferably by pressing, extrusion or casting, c) demoulding after hardening or drying, d) optionally, drying the preform, preferably so that 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 lower than 20°C/minute, preferably less than 10°C/minute. This temperature rise ramp, like the duration of the plateau, can be adjusted according to the volume of mixture and the power of the reactor.

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 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 avoiding too intimate contact and abrasion of the mold by the mixture and ultimately contamination of the preform. These usage precautions 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.

A sintered ceramic body obtained from the powder according to the invention advantageously has an electrical resistivity, measured at 25° C. and at atmospheric pressure, of less than 0.2 microOhm. Mr.

Hot pressing (or “Hot Pressing”), hot isostatic pressing (or “Hot Isostatic Pressing”) or SPS (“Spark Plasma Sintering”) techniques are particularly suitable.

The examples which follow are given purely by way of illustration and do not limit the scope of the present invention under any of the aspects described.

Examples:

Example 1 (comparative):

The starting mixture was made with a titanium oxide powder with a median diameter D50 of 10 μm mainly in a crystallographic form of TiCL in rutile form supplied by Traxys France (purity 95%), a boron oxide powder B2O3 with a median diameter D50 equal to 15 μm and a black carbon powder with a median diameter D50 of 0.2 μm according to the following respective mass proportions 38.1% TiCh, 33.2% B2O3 and 28.7% C. A sample of the mixture was placed in a graphite crucible with dimensions 6 cm in internal diameter, 8 cm in external diameter and 8 cm in height. The open crucible is placed in an induction furnace in order to be subjected respectively to a heat treatment at 1600° C. according to a stage duration of 2 hours in a furnace under a sweep under Argon of 1.25 L/min/m ³ .

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):

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 powder of sodium tetraborate (Na ₂ B ₄ O7) of 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 (Na ₂ O) with a median diameter D50 of 50 μm of 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%. A sample of the mixture was placed in a graphite crucible of the same size as the previous example. The open crucible is placed in a tube furnace in order to be subjected respectively to a heat treatment at 800° C. according to a temperature rise of 2° C./minute in a tube furnace followed by a 2 hour plateau in a furnace under a scanning under Argon at 1.25 L/min/m ³ of the enclosure of the tubular furnace.

The corresponding balance reaction is:

3TiO ₂ +1.5Na ₂ B ₄ O ₇ +10Al +3.5Na ₂ O ->TiB ₂ +10 NaAlO ₂ or again, expressed according to reaction (3) as a function of the simple oxides TiO ₂ , B ₂ Û3 and Na ₂ O:

The synthesis mixture obtained was ground for 1 minute in order to obtain a powder with a median size of less than 30 microns. The powder obtained was mixed with deionized water according to the following proportion of 1g of powder for 50ml of water. This mixture was filtered by passing through VWR 185mm 12-15µm paper in order to retain the titanium diboride particles. The retentate was dried at 110°C. in order to obtain the dry final titanium boride powder. Example 3 (comparative): This example differs from Example 2 in that the starting mixture comprises soda granules (with an NaOH content greater than 99%) instead of a sodium oxide powder. The respective mass proportions of the titanium powders, of sodium tetraborate, of aluminum metal, of the soda granules were as follows: 21.9%, 27.6%, 24.7% and 25.8%. [Table 1]

N.M not measured; N.A not applicable; atmosphere. = atmospheric pressure

The characteristics of the final powders obtained are presented in Table 2 below. Each powder was mixed with 0.25% of a pressing additive (PVA) and 4.75% deionized water by mass relative to the mass of powder in order to be cold pressed under a pressure of 100 bars and to form a cylinder with a diameter of 30 mm and a thickness of 10 mm. After demoulding, each cylinder was dried at 110° C. for 24 h then baked without pressure at a temperature of 1850° C. for 12 h under Argon. The electrical resistivity of each example was measured at room temperature according to the Van der Pauw method at 4 points on a sample of sintered body obtained with a diameter of 20-30 mm and a thickness of 2.5 mm. [Table 2]

NM = not measured

These results of example 2 by difference with comparative example 1 show that it is possible to obtain according to the process of the invention a very pure, fine powder without release of CO and at a lower synthesis temperature. The 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 method of Example 3 based on a sodium hydroxide solution (cf. ( D ₉₀ - DIO)/D ₅ O).

Claims

- 26 - Claims 1 . Process for the synthesis of a MB2 diboride 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 comprising, and preferably 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 precursor of boron oxide, the boron mass content of which, 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, lanthanides; and d) an oxide powder of alkaline element A, the A2O mass content of which is at least 70%; in respective proportions leading to the following balance reaction, expressed according to said MCh, B2O3, R and A2O: - heating of 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 the MO2 oxide 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. Process for the synthesis of a powder of diboride MB2, according to claim 1, in which the mass content of the powder of alkali element A in hydroxyls, calculated as the mass of OH on the mass of alkaline is less than 40%. 3. Process for the synthesis of a MB2 diboride powder, according to claim 1 or 2, in which the boron oxide or the boron oxide precursor is chosen from sodium metaborate with the chemical formula NaBC>2, anhydrous borax with the formula Na2B4O?, or other borates such as natural borax with the formula Na2B4O?.10H2O, tincalconite with the formula Na2B4O?.5H2O, kernite of formula Na2B4O?.4H2O, ulexite of formula NaCaBsOg.SFhO, proberite NaCaBsO9.5H2O, sassolite of formula H3BO3, boron nitride. 4. Process for the synthesis of a MB2 diboride powder, according to the immediately preceding claim, in which the powder comprising boron oxide is an anhydrous alkali borate powder, preferably anhydrous sodium borate powder. 5. Process for synthesizing an MB2 powder, according to one of the preceding claims, in which the median particle diameter of said powder comprising the oxide MO2 is greater than 7 micrometers and/or less than 50 micrometers. 6. Process for synthesizing an MB2 powder, according to one of the preceding claims, in which the median particle diameter of said powder comprising boron oxide is greater than 30 micrometers and/or less than 100 micrometers. 7. Process for synthesizing an MB2 powder according to one of the preceding claims, in which 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. Process for synthesizing an MB2 powder according to one of the preceding claims, in which the heating temperature in said enclosure is greater than 700°C and/or less than 1400°C. 9. Process for synthesizing an MB2 powder according to one of the preceding claims, in which the rare gas is chosen from argon or helium. 10. Process for the synthesis of an MB2 powder, according to the immediately preceding claim, in which A is the element Na. 11. Process for the synthesis of an MB2 powder, according to the immediately preceding claim, in which R is the element Al and/or Si. 12. Process for the synthesis of an MB2 powder, according to one of the preceding claims, in which 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 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 (O): less than 1.3%; - elementary carbon (C): less than 0.5%; - elemental nitrogen (N): less than 0.5%; - elemental sulfur (S): less than 400 ppm; - elemental iron (Fe): less than 0.45%; - elemental nickel (Ni): less than 0.4%; - elemental cobalt (Co): less than 0.4%; - elementary sum of alkalis (Li+Na+K+Rb+Cs): less than 1%; - elementary sum of alkaline earth metals (Be+Mg+Ca+Sr+Ba): less than 1%, - content of element R in metallic form: less than 2%, R preferably being different from M, R being at least one element chosen from Al, Si, Ti, Zr, Hf, Y, Sc, lanthanides, the sum of other elements being less than 2%. 14. TiB2 powder according to claim 13, the chemical composition of which comprises the following elementary contents by mass: - 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 aluminium: less than 2%, - metallic silicon: less than 1%. - 29 - 15. MB2 powder according to one of claims 13 or 14, in which the ratio (D ₉ O-DIO)/D ₅ O of equivalent diameter of the particles of the powder is less than 1.5, preferably even less than 1,2. 16. Mixture comprising between 90% and 99.9% by weight of an MB2 powder according to one of claims 13 to 15 and between 0.1 and 10% by weight 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 whose purity is greater than 95% by mass. 17. Process for manufacturing a sintered ceramic body comprising the following steps: a) preparation of a starting charge comprising: - the MB2 powder according to claim 13 to 15 or the mixture of powders according to claim 16, - an aqueous solvent, in particular deionized water, - preferably, shaping additives, b) shaping of the starting charge in the form of a preform, c) demolding after hardening or drying, d) optionally, drying of the preform, preferably in such a way until the residual humidity 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. 18. Sintered ceramic body obtained by a method according to the preceding claim. 19. Use of the sintered ceramic body according to the preceding claim as all or part of a membrane, a shielding or an anti-abrasion coating, a coating or a refractory block, a coating or an anode block or a coating or a cathode block, a heat exchanger, a melting crucible for metal, in particular non-ferrous metal, a cutting tool.