Method for synthesizing titanium diboride powder

Abstract

The present invention relates to a method for the synthesis of TiB2 powder, comprising the reduction of titanium oxide by carbon in the presence of a boron source, said method comprising heating a mixture of a carbon source, a boron carbide powder having a median particle size of 5-100 microns, and a titanium oxide powder having a median particle size of 5-80 microns, said mixture and the TiB2 powder obtained by such method are heated in a container for 1 m 3 The furnace is heated at 1500℃~2000℃ under the inert gas sweep rate of 0.5~10L / min.

Description

[Technical field]

[0001] The present invention relates to a novel method for producing or synthesizing titanium diboride. [Background technology]

[0002] Titanium diboride has a low density (approximately 4.5 g / cm 3 ), high hardness, high thermal conductivity and low electrical resistivity, which may make it a potentially interesting material for various applications, in particular for shielding and ballistic protection, for refractory applications where thermal and high electrical conductivity are advantageous, in particular for the construction of anodes or cathodes of heat exchangers, coatings or even electrolytic reactors, or even membranes in certain temperature applications or in highly aggressive chemical media, as well as crucibles for melting metals, in particular non-ferrous metals, or cutting tools.

[0003] All these applications explain why the demand for this material is very high and currently increasing.

[0004] TiB2 does not exist in the natural state. Titanium diboride can be obtained, for example, by direct reaction of titanium (or its oxide or hydride) with elemental boron at 1000°C, or by carbothermal reduction of titanium oxide with boron oxide. In the latter case, the reaction consists of reacting a mixture of powders at temperatures above 1500°C according to the following simplified reaction: TiO2(s)+B2O3(s)+5C(s) → TiB2(s)+5CO(g) ··· (1)

[0005] However, this method has a theoretical material efficiency of only about 30%.

[0006] Another less well known method is to specifically replace boron oxide powder with boron carbide, as shown in the following reaction balance: 2TiO2+B4C+3C → 2TiB2+4CO ··· (2)

[0007] The advantage of such a reaction is its relatively good theoretical material yield (55.4%) and therefore relatively low emissions of carbon monoxide, but it has the disadvantage of requiring relatively high reaction temperatures.

[0008] The source of titanium oxide is generally a mineral source containing more than 95% TiO2. The carbon source is generally and preferentially petroleum coke (oil distillation residue) or carbon black. Boron carbide is also a synthetic material available on the carbide powder market, especially on the abrasives market.

[0009] However, the methods of producing this material become more expensive and energy consuming as the final titanium diboride powder becomes fine (typically having a median diameter of 5-50 micrometers) or ultrafine (median diameter less than 5 micrometers).

[0010] The article "Synthesis and consolidation of titanium diboride" published in the international review "Journal of Refractory Metals and Hard Materials 25 (2007) page 345-350 by C. Subrmania et al." suggests, for example, a method that makes it possible to obtain a very pure final powder (content of oxygen, nitrogen and carbon of about 0.5%). The method involves reacting a mixture of high purity powders (content of majority compounds is more than 95%), titanium oxide with a median diameter of 0.8 micrometers, boron carbide with a median diameter of 6.7 micrometers and petroleum coke in a dry organic solvent and then heating it at a temperature of at least 1800 °C for 4.10 to 1800 °C. -5It consists in heating in a vacuum corresponding to a residual pressure of less than mbar. Below this temperature, the purity of the resulting powder is too low. This article teaches the use of reactant powders, in particular titanium oxide, of fine (submicron (micrometer)) particle size and therefore relatively high reactivity, thereby improving the reaction yield and reaction rate.

[0011] Another solution to obtain a better control of the reaction consists in including an excess of BC, at least 10 percent by weight, or even 20 percent by weight, relative to the stoichiometric amount required for reaction (1). This additional addition makes it possible to make up for the loss of gaseous boron at high temperatures and reduces the presence of TiC and residual carbon, but at the expense of the actual material yield of the process. Summary of the Invention [Problem to be solved by the invention]

[0012] It is therefore an object of the present invention to improve the synthesis method described above and exemplified by formula (2), whereby pure fine TiB2 powders, i.e. having a mass percentage of more than 95%, or even very pure, i.e. having a purity of 98% or more, having a low elemental oxygen content and advantageously also a low elemental carbon content, are obtained while retaining a high material yield and without recourse to industrially complex powder synthesis processes. [Means for solving the problem]

[0013] In particular, according to a first aspect, the present invention relates to an alternative method for producing TiB2 at temperatures below 2000 °C, which meets this objective by benefiting from specific atmospheric conditions and by benefiting from a suitable selection of starting powders without the use of catalysts or surfactants.

[0014] More specifically, the present invention relates to a method for producing TiB2 powder, which comprises reducing titanium oxide with carbon in the presence of a boron source, said method comprising: (a) a titanium oxide powder, preferably in powder form, the weight percent of which is at least 95 weight percent TiO; and (b) a carbon source, preferably a carbon source having a carbon mass percentage of at least 90%, and (c) boron carbide powder, preferably having a B4C mass percentage of at least 90%, preferably at least 95%; A mixture of raw materials comprising, and preferably consisting of, at temperatures above 1500 ° C, preferably above 1600 ° C, and below 2000 ° C, preferably below 1900 ° C, - The reaction balance of titanium dioxide to titanium boride: 2TiO2+B4C+3C → 2TiB2+4CO ··· (2) The solution is heated in the respective proportions, which results in reduction by

[0015] The method is characterized in that: - the median particle size of the boron carbide powder is between 5 and 100 microns (micrometers); and - the median particle size of the titanium dioxide powder is 5 to 80 microns (micrometers); and the excess of boron carbide is less than 5 percent by weight, preferably less than 2 percent by weight, relative to the stoichiometric amount required for reaction (2) above; and - the synthesis is carried out in a vessel under a flow of inert gas; - The gas flow velocity in the above vessel is 1m 3 The flow rate is 0.5 to 10 L / min.

[0016] According to the invention, an inert gas is introduced into the vessel in contact with the mixture of raw materials. The invention resides not only in the selection of the particle size of the starting powders described above, but also in the selection of the specific synthesis conditions previously mentioned, such a combination advantageously making it possible to obtain a fine TiB2 powder of high purity with maximum material yield, as will be explained in more detail below.

[0017] The method of the invention may in particular include one or more of the following preferred features: The boron carbide powder has a median particle size greater than 7 micrometers, preferably greater than or equal to 10 micrometers. The median particle size of the boron carbide powder is less than 80 microns (micrometers), preferably less than 50 microns (micrometers), or even more preferably less than 30 microns (micrometers). The titanium dioxide powder has a median particle size of more than 7 micrometers, preferably 10 micrometers or more. - according to a preferred embodiment, the boron carbide powder has a median particle size greater than 7 micrometers and the titanium oxide powder has a median particle size greater than 7 micrometers; - Median particle size of titanium dioxide powder (D 50 ) is less than 50 micrometers, preferably less than 30 micrometers. - diameter D of the particles of the boron carbide powder 90 is less than 100 micrometers, preferably less than 80 micrometers, preferably 50 micrometers or less, more preferably 40 micrometers or less; - diameter D of the titanium dioxide powder particles 90 is less than 100 micrometers, preferably less than 80 micrometers, preferably 50 micrometers or less, more preferably 40 micrometers or less; the ratio of the median particle size of the boron carbide powder to the median particle size of the titanium dioxide powder is greater than 0.8, preferably greater than or equal to 1; the ratio of the median particle size of the boron carbide powder to the median particle size of the titanium oxide powder is less than 5, preferably less than 2; The titanium oxide powder is a rutile or anatase powder, preferably rutile. - The mass percentage of SiO2+Al2O3+ZrO2 in the titanium dioxide powder is less than 5%.

[0018] In particular, the mass content of SiO2 in the titanium oxide powder is preferably 2% or less. The mass percentage of Al2O3 in the titanium oxide powder is preferably 2% or less, more preferably less than 1%. The mass percentage of ZrO2 in the titanium oxide powder is preferably 1% or less. The elemental oxygen content in the boron carbide powder is less than or equal to 5%, preferably less than 3%, more preferably less than 2%. the carbon source is chosen from coke, in particular petroleum coke, from coal or biomass, graphite or carbon black; The mass percentage of elemental carbon in the carbon source is greater than 95%, preferably greater than 97%. - the carbon source, when it is in the form of coke, is subjected to a dehydrogenation treatment, so that its mass percentage of elemental hydrogen is less than 1%, very preferably less than 0.5% or even more preferably less than 0.1% according to the ISO / TS 12902 standard. Preferably, the content is less than 10 ng / mg for each of the following HAP compounds: naphthalene, acenaphthene, fluorene, phenanthracene, chrysene, anthracene, pyrene, benz[a]anthracene, benzo[a]pyrene, dibenzo(a,h)anthracene, benzo[ghi]perylene, benzo[k]fluoranthene, fluoranthene, benzo[b]fluoranthene and In(1,2,3,c,d,)P). - The raw materials were pre-dried at temperatures between room temperature and 150°C. the synthesis temperature is greater than or equal to 1600°C, preferably less than 1800°C; - The pressure in the vessel is kept substantially constant, for example between 0.5 and 1.5 bar, more preferably the vessel is under atmospheric pressure (1 bar). The gas sweeping the vessel in which the mixture is placed is preferably a noble gas, for example argon or helium, preferably argon. The flow rate measured under normal pressure and temperature conditions is preferably 1 m 2 of the vessel. 3 0.5 to 5 L / min per m, preferably 1 m 3 0.5 to 3 L / min per 1 m of container, preferably 3The flow rate is 0.5-2 L / min per 1 m of the vessel. Too low a flow rate leads to incomplete reactions and, more particularly, to the presence of undesirable carbon residues in the final titanium diboride powder. Too high a flow rate impairs the yield of reaction (2) since it requires a relatively high energy supply associated with the chemical reaction kinetics. 3 Gas sweep flow rates of 0.5 to 10 L / min per minute are relatively particularly suitable for reactors having an energy output typically comprised between 20 and 80 KW. Such reactors are used to heat mixtures that may range up to 500 g for a vessel volume equal to 2.5 liters in volume. - 1m container 3 An inert sweep gas flow rate of 0.005 to 1 L / min per 1 m of vessel and per 1 KW of vessel heating power is particularly optimal, preferably 3 The flow rate is 0.01 to 0.5 L / min per 1 kW of container heating power.

[0019] According to a first possible embodiment, the addition of the alkali metal salt can be carried out, for example, according to a proportion of metal of 0.5 to 15%, preferably 5 to 15 mass percent, relative to the total mass of the carbon source, the boron carbide powder particles and the titanium oxide particles, which reduces the presence of agglomerates in the synthesis powder, which may interfere with the firing process of the sintered ceramic body obtained from this TiB2 synthesis powder.

[0020] Addition of less than 0.5% of the alkali metal salt is insufficient for temperatures above 1500° C., especially for temperatures between 1600 and 2000° C. Supplying more than 15% leads to excessive evaporation of boron during synthesis of the TiB2 powder.

[0021] According to an advantageous embodiment of the invention, the alkali metal is selected from Li, Na, K. Preferably, the alkali metal salt is an alkali metal halide, preferably a chloride. More preferably, it is sodium chloride.

[0022] The median size of the alkali metal salt particles is preferably 0.5 to 100 micrometers, and more preferably 5 to 50 micrometers.

[0023] The present invention also relates to a TiB2 powder obtained according to the aforementioned method, the median particle size of which is 0.5 to 50 microns (micrometers) and which contains the following elemental mass percentages: Titanium (Ti): more than 67%; Boron (B): more than 28%; - oxygen (O): less than 1.3%, preferably less than 1.2%; - Carbon (C): less than 0.5%; - Nitrogen (N): less than 0.5%; - sulfur (S): less than 400 ppm, preferably less than 300 ppm, or even less than 150 ppm; - Iron (Fe): less than 0.45%; - preferably the sum of Li+Na+Rb+Cs is less than 1%; Preferably, the sum of other elements is less than 2%.

[0024] Preferably, the sum of oxygen (O) + nitrogen (N) + carbon (C) is less than 1.5%, or even less than or equal to 1.2%.

[0025] Such high purity and defined particle size TiB2 powders allow for sintered ceramic bodies having a total porosity of less than 7 volume percent to be obtained by sintering without the use of additions of transition metals such as Ni, Fe, or Co, which can result in the formation of secondary metal borides from these metals, which are undesirable.

[0026] The powder obtained by the above-mentioned method, in which the alkali metal salt is added in the above-mentioned proportions during the synthesis of the powder, has a very high homogeneity, which results in a very small dispersion of the crystal size. Such powder makes it possible to obtain sintered ceramic bodies in the form of parts, with at least one dimension, preferably all overall dimensions, greater than 5 cm, or even greater than 10 cm, a total porosity of less than 7%, a very limited pore size distribution, which do not deform during sintering and are free of shrinkage cracks.

[0027] Preferably, the TiB2 powder according to the invention further comprises one or more of the following elemental mass percentages: Titanium (Ti): greater than 68% and / or less than 72%; Boron (B): greater than 29% and / or less than 33%; - Carbon (C): less than 0.5%; oxygen (O): less than 1%, preferably less than 0.5%, or sulfur (S): less than 300 ppm, less than 100 ppm, preferably less than 50 ppm; - Nitrogen (N): less than 0.5%; - Iron (Fe): less than 0.4%; - preferably phosphorus (P): less than 0.3%, preferably less than 0.2%, preferably less than 0.1%; - preferably less than 0.1%, preferably less than 500 ppm of silicon (Si); - Alkaline earth elements (Be+Mg+Ca+Sr+Ba): less than 0.25%.

[0028] The TiB2 powder further comprises a SiC content of less than 1%, preferably less than 0.5%, and a TiC content of less than 1%, preferably less than 0.5%.

[0029] The TiB2 powder according to the invention does not contain any crystalline phases, such as B4C or TiC phases, or Ti2O3, Ti3B4, SiC, etc., as determined (detectable) by X-ray diffraction. Preferably, the powder contains only the crystalline phase of TiB2, as determined (detectable) by X-ray diffraction.

[0030] The present invention also relates to a mixture comprising or even consisting of 90 to 99.9 percent by weight of the TiB2 powder according to the invention and 0.1 to 10 percent by weight of one or more sintering powders selected from aluminum diboride, magnesium diboride, zirconium diboride, tungsten pentaboride, calcium hexaboride, silicon hexaboride, preferably having a purity of more than 95 percent by weight, preferably more than 98 percent by weight.

[0031] The purity of greater than 95 percent by weight is that of the above phase or most stable predominant compound: for example, in the case of aluminum diboride powder, greater than 95 percent by weight of AlB2, or in the case of tungsten pentaboride powder, greater than 95 percent by weight of W2B5.

[0032] The invention also relates to a method for producing a sintered ceramic body, which comprises the following steps: (a) generating a starting feedstock comprising: - a TiB2 powder obtainable by the method according to the invention, or a mixture of the abovementioned powders, comprising said powder and one or more of the abovementioned sintering powders, - aqueous solvents, in particular deionized water, - preferably a molding additive, (b) forming the feedstock into the shape of a preform, preferably by pressing; (c) Removal from the mold after solidification or drying; (d) optionally drying the preform, preferably until the residual moisture content is between 0 and 0.5 weight percent; (e) Loading into a furnace and firing the preform under an inert atmosphere, preferably under an argon atmosphere or under vacuum, preferably at a temperature between 1600° C. and 2200° C.

[0033] The invention also relates to the sintered ceramic bodies thus obtained and to the use of the sintered ceramic bodies obtained by the process described above as membranes, in particular for the filtration of liquids or gases, shielding or ballistic elements, coatings or refractory blocks, anode coatings or blocks or cathode coatings or blocks, in particular as all or part of electrolytic reactors, heat exchangers, metal melting crucibles, in particular for non-ferrous metals, cutting tools.

[0034] The following indications and definitions are provided in connection with the foregoing description of the invention: - In this specification, unless otherwise stated, all percentages are given by weight, based on dry ingredients. - the material yield is calculated by dividing the mass of the TiB2 raw powder obtained by the theoretical mass of TiB2 raw powder that would have been obtained from the theoretical mass of the dry mixed powder of the reactants (moisture content less than 2%) before the heat treatment. For example, under the stoichiometric conditions according to reaction (2), a 100% yield corresponds to a mass of 55.4 g of crude powder starting from 100 g of a mixture of titanium oxide powder, carbon powder and boron carbide powder. - coarse powder means the powder obtained directly at the outlet of the vessel after heating and reaction of the mixture, before any further treatment, such as screening or grinding. - the median diameter (or median "size") of the particles constituting a powder is given in the sense of the present invention by characterizing the particle size distribution, in particular by means of a laser granulometer. The characterization of the particle size distribution is conventionally carried out by means of a laser granulometer according to the standard ISO 13320-1. The laser granulometer can for example be a Partica LA-950 from HORIBA. For the purposes of this specification, unless otherwise stated, the median particle size denotes, respectively, the diameter of the particles below which 50 mass percent of the population are found. The "median diameter" or "median size" of a collection of particles, in particular of a powder, is given by D 50It is called a percentile, i.e. a size that separates the particles into a first and a second population of equal volume, which contain only particles having a size larger than the median size or only particles having a size smaller than the median size, respectively. - The content of elemental chemicals may be determined according to the ISO 21068:2008 standard.

[0035] In particular, - The O, N, C and S content is measured by a LECO® brand analyzer; - the content of Si, alkali, alkaline earth Fe, P may be measured by ICP (Induction Coupled Plasma); the B and Ti contents may preferably be determined by ICP, - the presence and relative content of TiB2 as well as other crystallized compounds may be conventionally determined by X-ray diffraction analysis; the total porosity of a ceramic body, expressed as a percentage, of the bulk density, measured, for example, according to ISO 18754, to the absolute density, measured, for example, according to ISO 5018.

[0036] Unless otherwise noted, all percentages herein are percent by weight. [Brief description of the drawings]

[0037] [Figure 1] FIG. 1 shows the crude powder after synthesis without the addition of NaCl according to Example 2 according to the invention. [Diagram 2] FIG. 2 shows the crude powder after synthesis including the addition of NaCl according to Example 4 according to the invention. [Diagram 3] FIG. 3 shows a reactor 1 making it possible to carry out the method, which comprises a vessel 2, by means of which the mixture 3 is swept with an inert gas 4 whilst it is heated, in order to obtain a coarse powder according to the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] Detailed Description

[0039] The invention and its advantages will be better understood upon reading the detailed description set forth below. Of course, the invention is not limited to any of the embodiments described below.

[0040] A starting mixture is produced under standard conditions for those skilled in the art, comprising a carbon source (e.g. carbon black, the C mass percentage of which is greater than 90%, preferably greater than 95%), a titanium oxide powder (e.g. rutile or anatase powder, the TiO mass percentage of which is greater than 95%), and a boron carbide powder (e.g. powder with a B4C mass percentage of greater than 90%). This step of producing a dry mixture allows for an intimate contact of the particles. According to one possible embodiment, it is carried out in a ball mixer or tumbler mixer, or in other equipment known to those skilled in the art. A prior co-grinding may be carried out, if necessary, to adjust the particle size of the starting raw materials.

[0041] The median sizes of the boron carbide, titanium oxide, and carbon particles are 10 to 100 microns (micrometers), 5 to 80 microns (micrometers), and 0.1 to 1 microns (micrometers), respectively. Preferably, the median sizes of the boron carbide and titanium oxide particles are greater than 7 micrometers, greater than 8 micrometers, greater than 9 micrometers, and / or less than 70 micrometers, less than 50 micrometers, or less than 30 micrometers.

[0042] Preferably, the median size ratio of the boron carbide particles to the titanium oxide particles is 0.8 to 1.2.

[0043] Preferably, the mixture according to the invention contains, in percentages by weight, 62-65% titanium oxide, 21-23% boron carbide and 13-15% carbon, in particular in the form of carbon black.

[0044] The mixture according to the invention has an excess of B4C of less than 5% relative to the stoichiometry of reaction (2), which is calculated according to the invention based on the amount of TiO2 introduced into said mixture.

[0045] Optionally, an alkali metal salt, preferably an alkali metal halide, especially NaCl, is added in a proportion of 0.5 to 15 weight percent of the metal, based on the aforementioned weight of the mixture containing the boron carbide particles, titanium oxide, and carbon source.

[0046] The mixture is preferably air dried, preferably at a temperature above 40° C., more preferably above 100° C., thereby obtaining a mixture having a moisture content of less than 2%, preferably less than 1%.

[0047] The mixture is placed in a container in the form of an inert crucible 2, preferably made of graphite, which is open, thereby allowing the inert gas 4 to flow through it, and the assembly is placed in an induction furnace 1, for example as shown in the attached FIG. 3. The induction furnace 1 comprises a copper turn 5, which is arranged around a quartz tube 6, inside of which a fibrous heat breaker 7 and a graphite susceptor 8 are arranged. The inert gas is introduced by a distributor 9. An outlet 10 allows the inert gas to flow and allows the reaction gases, mainly CO, to be recovered. The loose density of the mixture before heat treatment, measured according to the ASTM D7481-18 standard, is preferably greater than 0.5, greater than 0.6, greater than 0.7 and / or preferably less than 2.0, less than 1.8. Preferably, the mixing volume is less than 30% of the total volume of the container, thereby improving the circulation of the inert gas and the release of the gases produced by the reaction (2). The heating is carried out to at least 1500° C., preferably at least 1600° C., under an inert atmosphere, preferably under an inert gas sweep, in particular under an argon sweep, so that the gas is brought into contact with the mixture. Preferably, the inert gas sweep is carried out in a volume of 1 m of the vessel. 3 0.5 to 5 L / min per m, preferably 1 m 30.5 to 3 L / min per 1 m of container, more preferably 3 The usual flow rate is 0.5 to 2 L / min per injection.

[0048] Preferably the temperature ramp is less than 20° C. / min, preferably less than 10° C. / min. This temperature ramp, as well as the duration of the plateau, can be adjusted as a function of the mixing volume and the reactor power output.

[0049] The maximum heat treatment temperature is preferably 1600 to 2000° C., more preferably 1600 to 1800° C. Preferably, the plateau at the maximum temperature is at least 1 hour, preferably at least 2 hours.

[0050] Preferably, an intermediate plateau is performed between 600°C and 1000°C, and / or a relatively low ramp, typically at least two times lower, is performed after 600°C, thereby preventing removal of the mixture and promoting reaction between the particles.

[0051] Cooling may be free or forced and is preferably performed according to a negative ramp of less than 20° C. / min.

[0052] According to the methods of the present invention, the material yield is greater than 80%, or even greater than 90%, or even greater than 95%, or even 98% or greater.

[0053] The resulting coarse powder typically has a particle size of 10 to 100 micrometers. Sieving, light grinding or vibration operations make it possible to remove agglomerates and obtain a highly homogeneous, finely divided powder with a median size of 0.5 to 50 micrometers, of high purity. After grinding, it is possible to obtain a micron (micrometer) sized powder with a very reduced size distribution due to the limited crystallite size.

[0054] The powders obtained by the above-mentioned method, in which the alkali metal salt is added in the above-mentioned proportions during the synthesis of the powder, have a very high homogeneity, which further results in a relatively small dispersion of the crystal size.

[0055] In particular, the final powder of TiB2 has a high purity and a very reduced particle size distribution, which makes it possible to obtain sintered ceramic bodies having a total porosity of less than 7 volume percent by sintering, without the use of additions of transition metals such as Ni, Fe or Co, while exhibiting very low electrical resistivity.

[0056] The powders obtained according to the method of the invention also make it possible to obtain sintered ceramic bodies in the form of parts, which do not deform during sintering and do not suffer from shrinkage cracks, and which have all dimensions greater than at least 5 cm.

[0057] The method for producing a sintered ceramic body using the powder according to the invention comprises in particular the following steps: (a) generating a starting feedstock comprising: - a TiB2 powder according to the invention or a mixture of powders as described above, comprising said powder and one or more sintering powders, in particular one or more sintering powders selected from aluminium diboride, magnesium diboride, zirconium diboride, tungsten pentaboride, calcium hexaboride, silicon hexaboride, said TiB2 powder having a purity of more than 95 mass percent, preferably more than 98 mass percent, said TiB2 powder preferably representing at least 90% of the total mass of the feedstock, - aqueous solvents, in particular deionized water, i. In the case of casting, it is less than 20% of the total mass of the feedstock; ii. in the case of extrusion, less than 15% of the total mass of the feedstock; iii. in the case of press molding, it is less than 10% of the total mass of the feedstock, preferably less than 7%; - preferably molding additives, such as binders, for example PVA (polyvinyl alcohol), plasticizers (for example polyethylene glycol), lubricants, etc. (b) forming the feedstock into the shape of a preform, preferably by pressing, extrusion, or pouring; (c) Removal from the mold after solidification or drying; (d) optionally drying the preform, preferably until the residual moisture content is between 0 and 0.5 weight percent; (e) Loading into a furnace and firing the preform under an inert atmosphere, preferably under argon or under vacuum, at a temperature preferably between 1600° C. and 2200° C., preferably following a temperature ramp of less than 20° C. / min, preferably less than 10° C. / min. This temperature ramp as well as the duration of the plateau can be adjusted as a function of the mixing volume and the reactor power.

[0058] Any molding technique known to those skilled in the art can be applied depending on the dimensions of the part to be made, provided that all precautions are taken to avoid contamination of the preform. Thus, casting in a gypsum mold can be adapted by using a graphite medium between the mold and the preform, or by using oil to avoid excessive contact and wear of the mold due to mixing, and ultimately contamination of the preform. These controlled precautions for those skilled in the art to use can also be applied to the other steps of the method. Thus, the mold or matrix used to contain the preform during sintering will preferably be made of graphite.

[0059] Hot pressing, hot isostatic pressing or SPS (spark plasma sintering) techniques are particularly suitable.

[0060] The following examples are for illustrative purposes and are not intended to limit the scope of the invention in any aspect described. EXAMPLES

[0061] Example 1 (Comparative Example)

[0062] The starting mixture has a mass percentage of TiO2 of more than 95% and a median diameter D 50 0.8 μm, predominantly in rutile crystalline form, and the mass percentage of B4C is greater than 98%, with a median diameter D 50A mixture was prepared using B4C powder with a diameter equal to 7 μm, and petroleum coke according to the following respective mass ratios: 64.53% TiO2, 22.59% B4C, and 12.89% C. Such a mixture corresponds to a boron carbide surplus of 1.2%. An isopropanol solvent was added, whereby granules were then obtained according to the teachings of C. Subrmania et al., International Journal of Refractory Metals and Hard Materials 25 (2007) page 345-350.

[0063] The two mixed samples were mixed at 4.10°C without any specific flushing. -5 They were subjected to a heat treatment in a furnace at temperatures of 1600° C. and 1820° C., respectively, with a plateau duration of 2 hours in a vacuum of 1000 mbar.

[0064] Example 2 (embodiment according to the invention)

[0065] The mixture was produced under the same conditions as above, but without the granulation step after heat treatment and milling for 3 minutes instead of 30 minutes. In addition, the starting powder had a mass percentage of TiO2 of more than 95% (the remainder being essentially SiO2<2%, Al2O3<2%, ZrO2<1%, and traces of Fe) and a median diameter D 50 A titanium oxide powder having a diameter of 10 μm, a mass percentage of B4C of more than 98%, and a median diameter D 50 B4C powder with a median diameter of 15 μm and D 50 The mixture is made up of carbon black powder with a diameter of 0.2 μm, according to the following respective mass ratios: 63.6% TiO2, 22.1% B4C, and 14.3% C. Such a mixture corresponds to a surplus of 0.5% boron carbide with respect to the stoichiometry of the reaction. Two samples of the mixture were placed in the above-mentioned graphite crucible according to FIG. 3, which served as a container, and were then poured into a 1 m 3 The samples were subjected to heat treatment at 1600° C. and 1800° C. with a plateau duration of 2 hours in a furnace under an argon sweep of 1.25 L / min per well.

[0066] Example 3 (Comparative Example)

[0067] The starting mixture was prepared as in Example 2, but the heat treatment was carried out without specific flushing, at 4.10 -5 The reaction was carried out in a furnace at a vacuum of 1000 mbar at a temperature of 1600° C. with a plateau duration of 2 hours.

[0068] Example 4 (Example according to the invention)

[0069] This example differs from Example 2 in that the starting mixture includes an additional addition of NaCl, 10 weight percent of the dry mixture, prior to heat treatment at 1600°C.

[0070] Example 5 (Comparative Example)

[0071] This example differs from Example 2 in that the starting mixture contains relatively large sized titanium oxide powder prior to the heat treatment at 2000°C.

[0072] And for each of these examples, except for Example 4 where sieving alone was sufficient, the raw powder mixture was slightly ground and sieved, thereby separating agglomerates to obtain a particulate powder.

[0073] Example 6 (Comparative Example)

[0074] In this example, the argon flush is 1m 3 This example differs from Example 2 according to the present invention in that the flow rate is adjusted to 0.25 L / min per minute.

[0075] Example 7 (Comparative Example)

[0076] This example is a powder of B4C having a purity of more than 98 mass percent B4C and a median diameter D 50 This example differs from Example 2 according to the present invention in that the thickness of the first and second electrodes is about 150 μm.

[0077] Example 8 (Comparative Example)

[0078] This example differs from Example 2 according to the invention in that the respective mass ratios of titanium dioxide powder, B4C powder and carbon black are: 62.5% titanium dioxide, 23.2% B4C and 14.3% C. The surplus of B4C with respect to the stoichiometry of the reaction is about 7.7%.

[0079] The material yield was determined according to the procedure described above in this application, and the characteristics of the method are summarized in Table 1 below.

[0080] The properties of the final powder obtained are shown in Table 2 below.

[0081] [Table 1]

[0082] NM is not measured; NA is not applicable; Atmos. is atmospheric pressure.

[0083] [Table 2]

[0084] NM is not measured.

[0085] From the data reported in Tables 1 and 2 it can be seen that the TiB2 powder obtained by the method according to the invention is very pure and practically free of contaminants (in particular oxygen, nitrogen, carbon, sulfur) and furthermore the yield from the reaction is also very good.

[0086] Ceramic bodies were made from powders according to Examples 2, 4, 6 and 8 above (obtained at 1600°C), and two other ceramic bodies were made according to the same method described below, but the first was made from commercial Hoganas powder of grade SE and the second was made from Japan New Metals powder of grade NF.

[0087] Each powder was mixed with 0.25 weight percent of a pressing additive (PVA) and 4.75 weight percent of deionized water, relative to the weight of the titanium diboride powder, and cold pressed under a pressure of 100 bar to form a cylinder with a diameter of 30 mm and a thickness of 10 mm. After demolding, each cylinder was dried at 110° C. for 24 hours and then pressureless fired at a temperature of 1850° C. in argon for 12 hours.

[0088] The porosity of the obtained sintered body was determined by dividing the ratio expressed as a percentage of the bulk density measured according to ISO 18754 by the absolute density measured according to ISO 5018. The electrical resistivity was measured at room temperature (20°C) according to the Van der Pauw method at four points on a sample with a diameter of 20 to 30 mm and a thickness of 2.5 mm.

[0089] The properties of the final powder obtained are shown in Table 3 below.

[0090] [Table 3]

[0091] NM is not measured.

[0092] Reading the results reported in the preceding table, it is observed that the sintered bodies according to the invention have a very low resistivity and a porosity that is much lower than that of the sintered bodies obtained with commercial TiB2 powder. Moreover, the TiB2 grains used have levels of contaminants (in particular the elements oxygen, carbon and nitrogen) that are much lower than those obtained by the methods as described in the prior art. These advantages could be obtained from the powder according to the invention after simple grinding followed by heat treatment, without resorting to an additional granulation step.

Claims

1. TiB ２ 1. A method for producing a powder comprising reducing titanium oxide with carbon in the presence of a boron source, said method comprising: (a) titanium oxide powder, preferably in powder form, the TiO ２ Titanium oxide powder having a mass percentage of at least 95%; and (b) a carbon source, preferably a carbon source whose carbon mass percent is at least 90%, and (c) boron carbide powder, preferably at least 90% B ４ boron carbide powder having a weight percent of C; A mixture of raw materials consisting of at temperatures above 1500°C and below 2000°C, - Reaction balance (2) of the titanium oxide to titanium boride: 2TiO ２ +B ４ C+3C → 2TiB ２ +4CO ・・・ (2) and heating the mixture at respective rates to effect reduction by The method comprising: the boron carbide powder has a median particle size of 5 to 100 micrometers; and the titanium oxide powder has a median particle size of 5 to 80 micrometers; and the excess of boron carbide is less than 5 percent by weight relative to the stoichiometric amount required for reaction (2); the synthesis is carried out in a vessel under a stream of inert gas; - The flow rate of the gas flow in the container is ３ 0.5 to 10 L / min per minute, A method characterized by:

2. 2. The TiB powder of claim 1, wherein the boron carbide powder has a median particle size of more than 7 μm and / or less than 80 μm. ２ Powder synthesis method.

3. 3. The TiB powder according to claim 1, wherein the titanium oxide powder has a median particle size of more than 7 μm and / or less than 50 μm. ２ Powder synthesis method.

4. 3. The TiB powder according to claim 1, wherein the ratio of the median particle size of the boron carbide powder to the median particle size of the titanium oxide powder is greater than 0.8 and / or less than 5. ２ Powder synthesis method.

5. The titanium oxide powder contains less than 5% of SiO ２ +Al ２ O ３ + ZrO ２ 3. The TiB composition according to claim 1 or 2, having a mass percentage of ２ Powder synthesis method.

6. 3. The TiB composition according to claim 1 or 2, wherein the carbon source is selected from coke, in particular petroleum coke, derived from coal or biomass, graphite, or carbon black. ２ Powder synthesis method.

7. The inert gas sweep velocity is ３ 3. The TiB composition according to claim 1, wherein the temperature is 0.005 to 1 L / min per 1 kW of heating power of the vessel. ２ Powder synthesis method.

8. 3. The TiB according to claim 1 or 2, wherein the inert gas is a noble gas, preferably a noble gas selected from argon or helium. ２ Powder synthesis method.

9. 3. The TiB2O3 composition according to claim 1, wherein the alkali metal salt is added to the mixture in a proportion of 0.5 to 15 mass percent of the metal based on the mass of the carbon source and the particles of boron carbide powder and titanium oxide powder. ２ Powder synthesis method.

10. The mixture contains, by mass ratio, 62 to 65% titanium oxide (TiO ２ ), 21-23% boron carbide (B ４ 3. The TiB composition according to claim 1 or 2, comprising 13 to 15% carbon (C), in particular in the form of carbon black. ２ Powder synthesis method.

11. TiB obtained by the method according to claim 1 or 2 ２ A powder having a median diameter of 0.5 to 50 μm and a chemical composition of the following elemental mass percentages: Titanium (Ti): greater than 67%; Boron (B): greater than 28%; oxygen (O): less than 1.3%; Carbon (C): less than 0.5%; Nitrogen (N): less than 0.5%; sulfur (S): less than 400 ppm; Iron (Fe): less than 0.45%; the sum of Li+Na+Rb+Cs is less than 1%; - The sum of other elements is less than 2%; Including TiB ２ powder.

12. 12. The TiB of claim 11, wherein the sum of oxygen (O) + nitrogen (N) + carbon (C) is less than 1.5%. ２ powder.

13. A median diameter of 0.5 to 50 μm and a chemical composition of the following elemental mass percentages: Titanium (Ti): greater than 68% and less than 72%; Boron (B): greater than 29% and less than 33%; Carbon (C): less than 0.5%; oxygen (O): less than 1% or sulfur (S): less than 300 ppm; Nitrogen (N): less than 0.5%; Iron (Fe): less than 0.4%; preferably less than 0.1% silicon (Si); preferably less than 0.3% phosphorus (P); preferably the sum of alkaline earth metals (Be+Mg+Ca+Sr+Ba) is less than 0.25%; The TiB of claim 11, ２ powder.

14. TiB as measured by X-ray diffraction ２ 12. The TiB of claim 11, comprising only the crystalline phase ２ powder.

15. The mixture of 90 to 99.9 mass percent of TiB according to claim 11 ２ and 0.1 to 10 weight percent of one or more sintering powders selected from aluminum diboride, magnesium diboride, zirconium diboride, tungsten pentaboride, calcium hexaboride, silicon hexaboride, preferably having a purity greater than 95 weight percent.

16. 1. A method for producing a sintered ceramic body, comprising the steps of: (a) generating a starting feedstock comprising: TiB according to claim 11 ２ a powder or mixture comprising 90 to 99.9 percent by weight of the TiB 2 powder according to claim 11 and 0.1 to 10 percent by weight of one or more sintering powders selected from aluminum diboride, magnesium diboride, zirconium diboride, tungsten pentaboride, calcium hexaboride, silicon hexaboride, preferably having a purity of more than 95% by weight; aqueous solvents, in particular deionized water, - preferably molding additives, (b) forming the starting feedstock into the shape of a preform; (c) removing from the mold after solidification or drying; (d) optionally drying the preform, preferably to a residual moisture content of 0 to 0.5 weight percent; (e) loading into a furnace and firing the preform under an inert atmosphere, preferably under argon, or under vacuum, preferably at a temperature between 1600°C and 2200°C; A method comprising:

17. 17. A sintered ceramic body obtainable by the method of claim 16.

18. 18. Use of the sintered ceramic body according to claim 17 as all or part of a membrane, a shielding or ballistic element, a coating or refractory block, an anode coating or block or a cathode coating or block, a heat exchanger, a metal melting crucible, in particular a metal melting crucible for non-ferrous metals, a cutting tool.

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