METHOD FOR SYNTHESIZING DIBORIDE POWDER BY DRY ROUTE

Abstract

A method for manufacturing a diboride powder MB.sub.2 by dry route where M is a chemical element belonging to group 4 of the periodic table, from the reduction of an oxide MO.sub.2 of the element M according to the balance reaction MO.sub.2+B.sub.2O.sub.3+yR+xA.sub.2O.fwdarw.MB.sub.2+A.sub.2xR.sub.YO.sub.5+x, wherein R is a reducing element selected from Al, Si, Ti, Zr, Hf, Y, Sc, and the lanthanides and A.sub.2O is an oxide of alkali element A.

Claims

1. A method for synthesizing a diboride powder MB.sub.2, where M is a chemical element belonging to group 4 of the periodic table, by reducing an oxide of said M, comprising: preparing a mixture of raw materials comprising: a) a powder whose mass content of said oxide MO.sub.2 is at least 95% and b) a powder comprising a boron oxide or a precursor of a boron oxide, the boron content of which, expressed in B.sub.2O.sub.3, is at least 30%; and c) a metal powder of at least one reducing element R, R being selected from Al, Si, Ti, Zr, Hf, Y, Sc, the lanthanides; and d) an oxide powder of an alkali element A, having a A.sub.2O mass content of at least 70%, in respective proportions leading to the following balance reaction, expressed according to said MO.sub.2, B.sub.2O.sub.3, R and A.sub.2O:
MO.sub.2+B.sub.2O.sub.3+yR+xA.sub.2O.fwdarw.MB.sub.2+A.sub.2xR.sub.yO.sub.5+x(3) heating said mixture in an enclosure under a rare gas flow, at a temperature above 600 C. and below 1500 C., said mixture of raw materials having the following characteristics: a median particle diameter of said powder comprising the oxide MO.sub.2 is between 1 and 100 microns, and a median particle diameter of the powder comprising a boron oxide or a boron oxide precursor is between 5 and 200 microns; and x is greater than or equal to 1 y is greater than 0.5.

2. The method for synthesizing a diboride powder MB.sub.2, according to claim 1, wherein an hydroxyl mass content in the oxide powder of alkali metal element A, calculated in the form of the mass of OH to the alkali metal, is less than 40%.

3. The method for synthesizing a diboride powder MB.sub.2, according to claim 1, wherein the boron oxide or the boron oxide precursor is chosen from sodium metaborate of chemical formula NaBO.sub.2, anhydrous borax of formula Na.sub.2B.sub.4O.sub.7, or other borates.

4. The method for synthesizing a diboride powder MB.sub.2, according to claim 3, wherein the powder comprising boron oxide is an anhydrous alkali borate powder.

5. The method for synthesizing a powder of MB.sub.2, according to claim 1, wherein the median particle diameter of said powder comprising the oxide MO.sub.2, is greater than 7 micrometers and/or less than 50 micrometers.

6. The method for synthesizing a powder of MB.sub.2, according to claim 1, wherein the median particle diameter of said powder comprising the boron oxide, is greater than 30 micrometers and/or less than 100 micrometers.

7. The method for synthesizing a powder of MB.sub.2 according to claim 1, wherein a ratio of the median particle diameter of said powder comprising boron oxide to the median particle diameter of said powder comprising the oxide MO.sub.2, is less than 10 and/or greater than 1.

8. The method for synthesizing a powder of MB.sub.2 according to claim 1, wherein the heating temperature in said enclosure is greater than 700 C. and/or less than 1400 C.

9. The method for synthesizing a powder of MB.sub.2, according to claim 1, wherein the rare gas is a selected from argon or helium.

10. The method for synthesizing a powder of MB.sub.2, according to claim 9, wherein A is the element Na.

11. The method for synthesizing a powder of MB.sub.2, according to claim 10, wherein R is the element Al and/or Si.

12. The method for synthesizing a powder of MB.sub.2, according to claim 1, wherein M is the element Ti, A is the element Na, and R is the element Al and/or Si.

13. A powder comprising more than 95% by weight of the compound MB.sub.2 obtained according to claim 1, M being chosen among Ti, Zr, Hf, a median diameter of which is between 0.5 and 50 micrometers and a chemical composition of which comprises the following elemental mass percents: elemental oxygen (O): less than 1.3%; elemental carbon (C): less than 0.5%; elemental nitrogen (N): less than 0.5%; elemental sulfur (S): less than 400 ppm; elemental iron (Fe): less than 0.45%; elemental nickel (Ni): less than 0.4%; elemental cobalt (Co): less than 0.4%; elemental sum of the alkali metals (Li+Na+K+Rb+Cs): less than 1%; elemental sum of the alkaline earth metals (Be+Mg+Ca+Sr+Ba) less than 1%. content of element R in metal form: less than 2%, R being at least one element selected from Al, Si, Ti, Zr, Hf, Y, Sc, the lanthanides, a sum of the other elements being less than 2%.

14. A powder of TiB.sub.2 according to claim 13, the chemical composition of which comprises the following elemental mass percents: titanium (Ti): greater than 68% and/or less than 72%, boron (B): greater than 29% and/or less than 33%, optionally phosphorus (P) less than 0.3%, metallic aluminum: less than 2%, metallic silicon; less than 1%.

15. A powder of MB.sub.2 according to claim 13, wherein a ratio (D.sub.90D.sub.10)/D.sub.50 of equivalent diameter of the particles of the powder is less than 1.5.

16. A mixture comprising between 90% and 99.9% by mass of a MB.sub.2 powder according to claim 13 and between 0.1 and 10% by mass of one or more sintering powders chosen from powders of aluminum diboride, magnesium diboride, tungsten pentaboride, calcium hexaboride, silicon hexaboride, optionally zirconium diboride if M=Ti or Hf.

17. A method for manufacturing a sintered ceramic body, comprising: a) preparing a starting feedstock including: the powder of MB.sub.2 according to claim 13, an aqueous solvent, optionally, shaping additives, b) shaping the starting feedstock into the form of a preform; c) removal from the mold after setting or drying; d) optionally, drying the preform, e) loading in a furnace and firing the preform under an inert atmosphere.

18. A sintered ceramic body obtained by a method according to claim 17.

19. A method comprising providing a sintered ceramic body according to claim 18 as all or part of a membrane, a shielding or an anti-abrasion coating, a covering or a refractory block, an anode coating or block or a cathode coating or block, a heat exchanger, a metal melting crucible.

20. The method for synthesizing a powder of MB.sub.2, according to claim 1, wherein the mixture of raw materials consists of said a) powder whose mass content of said oxide MO.sub.2 is at least 95%, said b) powder comprising a boron oxide or a precursor of a boron oxide, said c) metal powder, and said d) oxide powder.

Description

FIGURES

[0141] FIG. 1 shows the raw powder according to example 2 according to the invention.

[0142] FIG. 2 shows the raw powder according to the comparative example 1.

[0143] FIG. 3 shows the raw powder of the comparative example 3.

DETAILED DESCRIPTION

[0144] The invention and its advantages will be better understood on reading the detailed description given below. Of course, the present invention is not limited to such a mode, in any of the aspects described below.

[0145] The mixture of starting raw materials comprises: [0146] a powder comprising oxide MO.sub.2 (for example a powder of titanium oxide, preferably rutile or anatase), the mass content of MO.sub.2 is at least 95%, and [0147] a powder comprising a boron oxide or a boron oxide precursor, the boron content of which, expressed in B.sub.2O.sub.3, is at least 30%; [0148] 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, whose 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 [0149] an oxide powder of an alkali element A, whose mass content of A.sub.2O is at least 70%, preferably at least 80%, preferably at least 90%, preferably at least 95%, preferably at least 98%, more preferably a powder of sodium oxide (Na.sub.2O) of greater purity of at least 99%, [0150] It is carried out under standard conditions for a person skilled in the art. This step of preparing the dry mixture allows intimate contact of the particles. According to one possible embodiment, it is carried out in a ball mixer or in a tumbler mixer or other devices known to the person skilled in the art. Prior co-grinding can be carried out to adjust the particle size of the starting raw materials if necessary.

[0151] If necessary, the certain raw materials such as borates or alkaline element oxide powder can be dried or calcined in order to reduce their H2O or hydroxyl content. In the case of starting materials such as the natural borax of formula Na.sub.2B.sub.4O.sub.7.Math.10H.sub.2O (also expressed in the form Na.sub.2B.sub.4O.sub.5(OH).sub.4.Math.8HO), the tinicalite of formula Na.sub.2B.sub.4O.sub.7.Math.5H.sub.2O (also expressed in the form Na.sub.2B.sub.4O.sub.5(OH).sub.4.Math.3H.sub.2O) the kernite of formula Na.sub.2B.sub.4O.sub.7.Math.4H.sub.2O (also expressed in the form Na.sub.2B.sub.4O.sub.6(OH).sub.2.Math.3H.sub.2O), the ulexite of formula NaCaB.sub.5O.sub.9.Math.8H.sub.2O (also expressed in the form of NaCaB.sub.5O.sub.6(OH).sub.6.Math.5H.sub.2O), the probertite NaCaB.sub.5O.sub.9.Math.5H.sub.2O (also expressed in the form of NaCaB.sub.5O.sub.7(OH).sub.4.Math.3H.sub.2O), this treatment reduces the presence of hydrogen present in the form of water H.sub.2O adsorbed on the surface of the OH powder or hydroxyls. It makes it possible to improve the conversion rate into the diboride of element M, which also results in a raw powder of MB.sub.2 having after synthesis a very low content of reducing metal at element R.

[0152] Preferably, in order to maximize the conversion rate into diboride MB.sub.2, the content of hydroxyl (OH) provided by the raw materials in the reaction (3) is minimized. In particular, the borates may be calcined in order to dehydroxylate them. Even more preferably, the oxide powder of alkali element A has a hydroxyl content, calculated by dividing its mass of OH by the mass of alkali metal A.sub.2O, is less than 40%, preferably less than 30%, more preferably less than 20%, or even less than 10%, or even less than 5% or even substantially zero.

[0153] The median size or median diameter of the oxide particles of element M is respectively between 1 and 100 microns, preferably between 7 and 80 microns. That of the particles comprising boron oxide, that of the metal particles of reducing element R and that of the oxide particles of alkali element A is preferably between 30 and 100 micrometers, preferably between 30 and 80 micrometers.

[0154] Preferably, the median size ratio of the particles comprising boron oxide to those of oxide of element M is between 1 and 10.

[0155] Preferably, in a mixture according to the invention comprises in mass proportion respectively 20 to 25% oxide of element M, 25 to 40% powder comprising a boron oxide or a boron oxide precursor, 20 to 30% metal powder of reducing element of element R and 15 to 25% an oxide powder of an alkali element A. In particular, in the case where the element M is Ti and A is Na, the mixture according to the invention respectively comprises, in mass proportion, 20 to 25% titanium oxide, 25 to 35% powder comprising a boron oxide or a boron oxide precursor, preferably sodium borate, 20 to 30% metallic powder of reducing element R, preferably Al and/or Si, and 15 to 25% sodium oxide powder.

[0156] The total content of alkali oxide calculated in form A.sub.2O in said mixture of raw materials is greater than or equal to the stoichiometric amount required for said reaction (3), preferably less than 10%, or even less than 5%;

[0157] 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 content, that is to say the residual mass content of H.sub.2O as measured by a moisture meter well known to a person skilled in the art, said mixture of raw materials is less than 5%, preferably less than 2%, or even more preferably less than 1%.

[0158] The mixture is placed in an inert crucible, preferably by diboride of element M or even of alumina, preferably of alumina coated with diboride of element M, for example in an induction furnace. The loose 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 MB.sub.2, or greater than 0.2 and/or preferably less than 0.5, less than 0.3 times the density of MB.sub.2.

[0159] A temperature rise is carried out to at least one temperature preferably higher than the melting point of the metal of element R selected 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 the 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 a flow of rare gas, in particular Argon in order to prevent oxidizing the powder of metal reducer R.

[0160] Preferably, the non-oxidizing gas sweeping is carried out at a normal flow rate of 0.5 and 5 L/min per m.sup.3 of enclosure, preferably between 0.5 and 3 L/min/m.sup.3, preferably between 0.5 and 2 L/min/m.sup.3 of enclosure.

[0161] Preferably, the temperature rise is less than 20 C./minute, preferably less than 10 C./minute, preferably less than 5 C./minute, or even less than 3 C./minute. This temperature rise ramp, like the duration of the plateau, can be adjusted as a function of the mixing volume and the power of the reactor. In particular, such a temperature rise range promotes better control of the exothermic effect due to the synthesis reaction of the powder according to the invention.

[0162] Preferably, the plateau at the maximum temperature is at least one hour, preferably at least two hours.

[0163] Preferably, an intermediate plateau is carried out between 60 and 1000 C., and/or a lower ramp, typically at least twice as low, is carried out after 600 C. in order to prevent the removal of the mixture and promote the reaction between the particles.

[0164] The cooling can be free or forced, preferably according to a negative ramp less than 20 C./min.

[0165] The raw mixture obtained has a particle size of typically between 10 and 100 micrometers.

[0166] 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 of light crushing or of vibration makes it possible to eliminate the agglomerations and to separate the raw powder of diboride of element M.

[0167] According to one possible embodiment, a sieving operation, or even of light crushing or of vibration, makes it possible to eliminate the agglomerates and to separate out the powder of diboride of element M. A suspension is carried out by adding to the previously ground crude mixture a solvent, preferably deionized water, in a mass ratio of 1 part crude mixture to at least 20, preferably 50 parts of solvent. Said suspension is filtered at an optimal size typically at 30 micrometers, preferably 20 micrometers, or 15 micrometers or less in order to allow through the liquid comprising the very fine residues of the other products of the reaction (3). The filtration retentate, consisting of the diboride powder of element M, is then calcined or dried, preferably in air, at a temperature above 80 C., preferably above 100 C. and/or preferably below 300 C., preferably below 200 C., preferably below 150 C.

[0168] According to one possible embodiment, said liquid resulting from the filtration of the suspension described above comprising products of the reaction (3) apart from the diboride powder of element M is heat-treated in the presence of water and a basic solution in order to form a hydrate of element R and an alkali hydroxide. This embodiment makes it possible to upgrade the reaction product (3) of formula A.sub.2xR.sub.yO.sub.5+x. Preferably, this possible embodiment is particularly advantageous in the case where the element R is Al and the alkali A is sodium.

[0169] After grinding the raw powder, it is possible to obtain a final diboride powder of finely divided element M whose median diameter is between 0.5 and 50 micrometers of large purity, of micron size, the size dispersion of which is very small.

[0170] The final powder of diboride of element M makes it possible to obtain by sintering a sintered ceramic body having a total porosity of less than 7% by volume without adding transition metals such as Ni, Fe or Co while exhibiting a very low electrical resistivity.

[0171] The final powder obtained according to the method of the invention also makes it possible to obtain a sintered ceramic body in the form of a part, all of the dimensions of which are at least one dimension greater than 5 cm without deformation upon sintering and without shrinkage cracking.

[0172] 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.Math.m.

[0173] The electrical resistivity can be measured according to the Van der Pauw method at 4 points on a sample with a diameter of 20-30 mm and a thickness of 2.5 mm. The sample being obtained by pressing a mixture consisting of said powder with 0.25% of a pressing additive (PVA) and 4.75% of deionized water by mass relative to the mass of M diboride powder in order to be cold-pressed under a pressure of 100 bar and 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 fired without pressure at a temperature of 1850 C. for 12 h in Argon.

[0174] A method for manufacturing a sintered ceramic body using the powder according to the invention in particular comprises the following steps: [0175] a) preparing a starting feedstock including: [0176] the diboride powder of element M, where M is a chemical element belonging to group 4 of the periodic table, in particular TiB.sub.2 powder according to the invention or a mixture of powders as described above, comprising said powder and one or more sintering powders, in particular chosen from powders of aluminum diboride, magnesium diboride, tungsten pentaboride, calcium hexaboride, silicon hexaboride, optionally zirconium diboride if M=Ti or Hf, the purity of said MB.sub.2 powder being greater than 95% by mass, preferably greater than 98% by mass, said MB.sub.2 powder preferably representing at least 90% of the total mass of the feedstock. [0177] an aqueous solvent, in particular deionized water, [0178] i. less than 20% of the total mass of the feedstock in the case of shaping by casting, [0179] ii. less than 15% of the total mass of the feedstock in the case of shaping by extrusion, [0180] iii. less than 10%, preferably less than 7% of the total mass of the feedstock in the case of a press-forming, [0181] preferably, forming additives such as binders such as PVA (polyvinyl alcohol), plasticizers (such as polyethylene glycol), lubricants, [0182] b) shaping the feedstock into the form of a preform, preferably by pressing, extrusion, or pouring, [0183] c) removal from the mold after setting or drying; [0184] d) optionally, drying the preform, preferably until the residual moisture content is comprised between 0 and 0.5% by weight, [0185] e) loading in 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., preferably according to a temperature rise ramp of less than 20 C./minute, preferably less than 10 C./minute. This temperature rise ramp, like the duration of the plateau, can be adjusted as a function of the mixing volume and the power of the reactor.

[0186] Any shaping technique known to the person skilled in the art can be applied as a function of the dimensions of the part to be made as soon as all the precautions are taken to avoid contamination of the preform. Thus, the casting in a plaster mold can be adapted by using graphite media between the mold and the preform or oils avoiding excessive contact and abrasion of the mold by mixing and finally contamination of the preform. These controlled precautions for use by a person skilled in the art are also applicable to other steps of the method. Thus, during sintering, the mold or the matrix used containing the preform will preferably be made of graphite.

[0187] 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.Math.m.

[0188] Hot pressing, hot isostatic pressing, or SPS (Spark Plasma Sintering) techniques are particularly suitable.

[0189] The following examples are for illustrative purposes only and do not limit the scope of the present invention in any of the aspects described.

EXAMPLES

Example 1 (Comparative)

[0190] The starting mixture was made with a powder of titanium oxide with a median diameter D.sub.50 of 10 m mainly in a crystallographic form of TiO.sub.2 in rutile form provided by Traxys France (95% purity), a powder of boron oxide B.sub.2O.sub.3 of median diameter D.sub.50 equal to 15 m and a carbon black powder of median diameter D.sub.50 of 0.2 m according to the following respective mass proportions 38.1% TiO.sub.2, 33.2% B.sub.2O.sub.3 and 28.7% C.

[0191] A mixture sample was placed in a graphite crucible with dimensions of 6 cm internal diameter, 8 cm external diameter and 8 cm tall. The open crucible is placed in an induction furnace in order to be subjected respectively to a heat treatment at 1600 C. for a plateau duration of 2 hours in an oven in an Argon flow of 1.25 L/min/m.sup.3.

[0192] 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)

[0193] The starting mixture was made with a powder of titanium oxide with a median diameter D.sub.50 of 10 m mainly in a rutile crystallographic form as in the previous example, a powder of sodium tetraborate (Na.sub.2B.sub.4O.sub.7) of median diameter D.sub.50 equal to 50 m from Sigma Aldrich with purity greater than 99% by mass and a metal aluminum powder with a median diameter D.sub.50 of 10 m from Alfa Aesar with purity greater than 99% by mass, a powder of sodium oxide (Na.sub.2Y) of median diameter D.sub.50 of 50 m from Sigma Aldrich with a purity of greater than 99% by mass, in the following respective mass proportions 23.3%, 29.3%, 26.2% and 21.2%. A mixture sample was placed in a graphite crucible of the same size as the previous example. The open crucible is placed in a tubular furnace in order to be subjected respectively to a heat treatment at 800 C. at a temperature rise of 2 C./minute in a tubular furnace followed by a stage of 2 hours in a furnace in an Argon flow of 1.25 L/min/m.sup.3 enclosure of the tubular furnace.

[0194] The corresponding balance reaction is:

3TiO.sub.2+1.5Na.sub.2B.sub.4O.sub.7+10Al+3.5Na.sub.2O.fwdarw.TiB.sub.2+10NaAlO.sub.2

or, expressed according to the reaction (3) as a function of the simple oxides TiO.sub.2, B.sub.2O.sub.3 and Na.sub.2O:

TiO.sub.2+B.sub.2O.sub.3+10/3Al+5/3Na.sub.2O+TiB.sub.2+10/3NaAlO.sub.2.

[0195] 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 1 g of powder for 50 ml of water. This mixture was filtered through passage through a VWR 185 mm 12-15 m paper to catch the titanium diboride particles. The retentate was dried at 110 C. to obtain the final dry titanium boride powder.

Example 3 (Comparative)

[0196] This example differs from example 2 in that the starting mixture comprises sodium hydroxide granules (NaH content greater than 99%) instead of a sodium oxide powder. The respective proportions by weight of the powders of titanium, sodium tetraborate, and aluminum metal, and the sodium hydroxide granules, were the following 21.9%, 27.6%, 24.7% and 25.8%.

TABLE-US-00001 TABLE 1 Example 1 Example 2 Example 3 (compara- (inven- (compara- Unit tive) tion) tive) recycled raw materials Titanium oxide D.sub.90 m 10 10 10 powder 30 30 30 B.sub.2O.sub.3 powder D.sub.50 m 10 NA NA D.sub.90 m 30 Carbon powder D.sub.50 m 0.2 NA NA D.sub.90 m 0.3 Aluminum D.sub.50 m NA 5 5 powder D.sub.90 m 10 10 Sodium D.sub.50 m NA 30 30 tetraborate D.sub.90 m 50 50 powder (Na.sub.2B.sub.4O.sub.7) Alkali oxide D.sub.50 m NA 50 NA powder Na2O D.sub.90 m 80 sodium D.sub.50 m NA NA about 100 hydroxide granules (NaOH) Heat treatment Max. C. 1600 800 800 temperature Ramp C./min 5 2 2 Plateau h 2 2 2 Atmosphere Argon Argon Argon Pressure mbar Atmos. Atmos. Atmos. sweep flow L/min/m.sup.3 1.25 1.25 1.25 rate of enclosure Material % 30 20.3 <5 efficiency N.M not measured; N.A not applicable = atmospheric pressure

[0197] The properties 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% of deionized water by mass relative to the mass of powder in order to be cold-pressed under a pressure of 100 bar and to form a cylinder with a diameter of 30 mm and a thickness of 10 mm. After demolding, each cylinder was dried at 11000 for 24 hours and then fired without pressure at a temperature of 1850 C. for 12 h in Argon. The electrical resistivity of each example was measured at room temperature according to the Van der Pauw method at 4 points on an obtained sintered body sample with a diameter of 20-30 mm and a thickness of 2.5 mm.

TABLE-US-00002 TABLE 2 Example 1 Example 2 Example 3 (compara- (inven- (compara- tive) tion) tive) Physical properties of the powder obtained after grinding, 1 min D.sub.50 m 5.2 2.6 6.2 D.sub.90 m 8.9 4.9 8.6 (D.sub.90 D.sub.10)/D.sub.50 m 1.3 0.9 1.4 Mass % final powder chemistry Ti (%) 61 >65 NM B(%) 26 32.0 NM O LECO (%) 0.7 <1 2.5 C LECO (%) >5% <0.1 <0.1 N LECO (%) 0.1 <0.1 <0.1 S LECO (ppm) 180 <50 <50 Fe (ppm) 785 <500 <500 Li + Na + K + Rb + Cs(ppm) 500 <500 >2000 Be + Mg + Ca + Sr + Ba <500 <500 <500 (ppm) P (ppm) <500 <500 <500 Ni-(ppm) <500 <500 <500 Co(ppm) <500 <500 <500 Metallic Si % <0.5 <0.5 <0.5 Metallic Al % <0.5 >5% Actual density 4.4 4.4 4.4 Characteristic of the ceramic body obtained Resistivity at 25 C. 0.20 0.16 >0.5 (micro-ohm .Math. m) NM = Not measured

[0198] These results of example 2 by difference with comparative example 1 show that it is possible to obtain, according to the method of the invention, a very pure, fine powder without releasing 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 that produced by the method of example 3 based on a soda solution (see D.sub.90D.sub.10)/D.sub.50).