METHOD FOR SYNTHESIZING TITANIUM DIBORIDE POWDER

Abstract

A method for synthesizing a TiB.sub.2 powder includes the reduction of titanium oxide by carbon in the presence of a source of boron, the method includes heating a mixture of a carbon source, a boron carbide powder whose median particle diameter is between 5 and 100 microns and a powder of titanium oxide whose median particle diameter is between 5 and 80 microns, the mixture being placed in an enclosure under an inert gas sweep flow rate between 0.5 and 10 L/min/m/m.sup.3 of enclosure at a temperature of between 1500 C. and 2000 C., as well as the TiB.sub.2 powder obtained by such a method.

Claims

1. A method for manufacturing a TiB.sub.2 powder, comprising reducing titanium oxide by carbon in the presence of a source of boron, said method comprising heating a mixture of raw materials consisting of: a) a titanium oxide (TiO.sub.2) powder, and b) a carbon source, and c) a boron carbide powder, at a temperature above 1500 C. and below 2000 C., in respective proportions leading to the reduction of the titanium oxide to titanium boride according to the balance reaction:
2TiO.sub.2+B.sub.4C+3C.fwdarw.2TiB.sub.2+4CO(2) wherein: a median particle diameter of the boron carbide powder is between 5 and 100 microns, and a median particle diameter of the titanium oxide powder is between 5 and 80 microns, and an excess boron carbide is less than 5% by mass relative to the stoichiometric amount necessary for the reaction (2) the synthesis is carried out in an enclosure under an inert gas flow, a flow rate of the inert gas flow in said enclosure is between 0.5 and 10 L/min per m.sup.3 of enclosure.

2. The method for the synthesis of a TiB.sub.2 powder, according to claim 1, wherein the median particle diameter of the boron carbide powder is greater than 7 micrometers and/or less than 80 micrometers.

3. The method for the synthesis of a TiB.sub.2 powder, according to claim 1, wherein the median particle diameter of the titanium oxide powder is greater than 7 micrometers and/or less than 50 micrometers.

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

5. The method for the synthesis of a TiB.sub.2 powder, according to claim 1, wherein the titanium oxide powder has a SiO.sub.2+Al.sub.2O.sub.3+ZrO.sub.2 mass percent less than 5%.

6. The method for the synthesis of a TiB.sub.2 powder, according to claim 1, wherein the carbon source is chosen from cokes.

7. The method for the synthesis of a TiB.sub.2 powder, according to claim 1, wherein the inert gas sweep flow rate is 0.005 to 1 L/min/m.sup.3 of enclosure/kW of heating power of the enclosure.

8. The method for the synthesis of a TiB.sub.2 powder, according to claim 1, wherein the inert gas is a noble gas.

9. The method for the synthesis of a TiB.sub.2 powder, according to claim 1, wherein an alkali metal salt is added to the mixture in a proportion of between 0.5 and 15% by mass of metal relative to the mass of the carbon source and of the particles of the boron carbide and titanium oxide powders.

10. The method for the synthesis of a TiB.sub.2 powder, according to claim 1, wherein said mixture comprises, in mass proportion, 62 to 65% of titanium oxide (TiO.sub.2), 21 to 23% of boron carbide (B.sub.4C) and 13 to 15% of carbon (C).

11. A TiB.sub.2 powder obtained according to the method of claim 1, the median diameter of which is between 0.5 and 50 micrometers and the chemical composition of which comprises the following elementary mass percents: 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%, a sum Li+Na+Rb+Cs of less than 1%, a sum of the other elements less than 2%.

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

13. The TiB.sub.2 powder according to claim 11, wherein the median diameter is between 0.5 and 50 micrometers and the chemical composition of which comprises the following elementary mass percents: 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%.

14. A TiB.sub.2 powder according to claim 11, comprising only a crystalline phase of TiB.sub.2, as measured by X-ray diffraction.

15. A mixture comprising between 90% and 99.9% by mass of a TiB.sub.2 powder according to claim 11 and between 0.1 and 10% by mass of one or more sintering powders chosen from aluminum diboride, magnesium diboride, zirconium diboride, tungsten pentaboride, calcium hexaboride.

16. A method for manufacturing a sintered ceramic body, comprising the following steps: a) preparing a starting feedstock comprising: the TiB.sub.2 powder according to claim 11, an aqueous solvent, 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.

17. A sintered ceramic body obtained by a method according to claim 16.

18. A method comprising providing the sintered ceramic body according to claim 17 as all or part of a membrane, a shielding or an anti-ballistic protection element, a covering or a refractory block, an anode coating or block or a cathode coating or block, a heat exchanger, a metal melting crucible.

19. The method for the synthesis of a TiB.sub.2 powder, according to claim 1, wherein a) the titanium oxide (TiO.sub.2) powder has a TiO.sub.2 mass percent of which is at least 95%, and b) the carbon source has a carbon mass percent that is at least 90%, and c) the boron carbide powder has a B.sub.4C mass percent of at least 90%.

20. The method for the synthesis of a TiB.sub.2 powder, according to claim 6, wherein the carbon source is chosen from petroleum coke, coal or from biomass, graphite or carbon black.

Description

FIGURES

[0103] FIG. 1 shows the crude powder after synthesis without adding NaCl according to example 2 according to the invention.

[0104] FIG. 2 shows the crude powder after synthesis including adding NaCl according to example 4 according to the invention.

[0105] FIG. 3 shows a reactor 1 allowing the implementation of the present method, comprising an enclosure 2 in order to sweep the mixture 3 with an inert gas 4 by heating it to obtain the crude powder according to the invention.

DETAILED DESCRIPTION

[0106] 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.

[0107] The starting mixture comprising a carbon source (for example carbon black, the C mass percent of which is greater than 90%, preferably greater than 95%), a powder of titanium oxide (for example a rutile or anatase powder, the TiO.sub.2 mass percent of which is greater than 95%) and a boron carbide powder (for example a powder whose B.sub.4C mass percent is greater than 90%), is carried out under standard conditions for the 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.

[0108] The median size of the boron carbide, titanium oxide, and carbon particles is respectively between 10 and 100 microns, between 5 and 80 microns and between 0.1 and 1 microns. Preferably, the median size of the boron carbide and titanium oxide particles is greater than 7 micrometers, greater than 8 micrometers, greater than 9 micrometers and/or less than 70 micrometers, less than 50 micrometers, less than 30 micrometers.

[0109] Preferably, the median size ratio of the boron carbide and titanium oxide particles is between 0.8 and 1.2.

[0110] Preferably, a mixture according to the invention comprises, in mass percent, respectively 62 to 65% titanium oxide, 21 to 23% boron carbide and 13 to 15% carbon, in particular in the form of carbon black.

[0111] The mixture according to the invention has an excess of B.sub.4C less than 5% relative to the stoichiometry of the reaction (2), calculated according to the invention on the basis of the amount of TiO.sub.2 introduced into said mixture.

[0112] Optionally, an alkali metal salt, preferably an alkali metal halide, in particular NaCl, is added in a proportion of between 0.5 and 15% by mass of metal relative to the preceding mass of mixture comprising boron carbide particles and titanium oxide and a carbon source.

[0113] The mixture is preferably air-dried, preferably at a temperature above 40 C., more preferably at a temperature above 100 C., in order to obtain a mixture whose moisture content is less than 2%, preferably less than 1%.

[0114] The mixture is placed in an enclosure in the form of an inert crucible 2, preferably of graphite, open in order to make the inert gas 4 sweep therein, the assembly being placed for example in an induction furnace 1 as shown in the attached FIG. 3. The induction furnace 1 is equipped with copper turns 5 placed around a quartz tube 6 inside which a fibrous thermal insulator 7 and a graphite susceptor 8 are placed. The inert gas is brought by a distributor 9. A discharge 10 allows the inert gas to flow and the reaction gases to be recovered, mainly CO. 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 represents less than 30% of the total volume of the enclosure in order to improve the circulation of the inert gas and the release of the gases produced by the reaction (2). A temperature rise is carried out up to at least 1500 C., preferably at least 1600 C. under an inert atmosphere, preferably under-sweeping of an inert gas, in particular Argon, the gas being brought into contact with the mixture. Preferably, the inert 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.

[0115] Preferably, the temperature rise is 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.

[0116] The maximum thermal treatment temperature is preferably between 160 and 2000 C., preferably between 160 and 1800 C. Preferably, the plateau at the maximum temperature is at least one hour, preferably at least two hours.

[0117] 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.

[0118] The cooling can be free or forced, preferably according to a negative ramp less than 20 C./min.

[0119] According to the method of the invention, the material yield is greater than 80%, or even greater than 90% or even greater than 95%, or even greater than or equal to 98%.

[0120] The crude powder obtained has a particle size of typically between 10 and 100 micrometers. An operation of sieving or of light crushing or of vibration makes it possible to eliminate the aggregates and to obtain a finely divided powder whose median diameter is between 0.5 and 50 micrometers, of large purity and very homogeneous. After milling, it is possible to obtain a micron-sized powder whose size dispersion is very reduced because of a narrow crystallite size.

[0121] A powder obtained with the preceding method, to which alkali metal salt was added during the synthesis of the powder, in the proportion as specified above, has a very high homogeneity which results in an even lower crystal size dispersion.

[0122] The final powder of TiB.sub.2 in particular has a high purity and a very reduced particle size dispersion making it possible to obtain by sintering a sintered ceramic body having a total porosity of less than 7% by volume without the use of the addition of transition metals such as Ni, Fe or Co while exhibiting a very low electrical resistivity.

[0123] The 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.

[0124] A method for manufacturing a sintered ceramic body using the powder according to the invention in particular comprises the following steps: [0125] a) preparing a starting feedstock comprising: [0126] the 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 aluminum diboride, magnesium diboride, zirconium diboride, tungsten pentaboride, calcium hexaboride, silicon hexaboride, the purity of said TiB.sub.2 powder being greater than 95% by mass, preferably greater than 98% by mass, said TiB.sub.2 powder preferably representing at least 90% of the total mass of the feedstock. [0127] an aqueous solvent, in particular deionized water, [0128] i. less than 20% of the total mass of the feedstock in the case of shaping by casting, [0129] ii. less than 15% of the total mass of the feedstock in the case of shaping by extrusion, [0130] iii. less than 10%, preferably less than 7% of the total mass of the feedstock in the case of a press-forming, [0131] preferably, forming additives such as binders such as PVA (polyvinyl alcohol), plasticizers (such as polyethylene glycol), lubricants, [0132] b) shaping the feedstock into the form of a preform, preferably by pressing, extrusion, or pouring, [0133] c) removal from the mold after setting or drying; [0134] d) optionally, drying the preform, preferably until the residual moisture content is comprised between 0 and 0.5% by weight, [0135] 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.

[0136] 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.

[0137] Hot pressing, hot isostatic pressing, or SPS (Spark Plasma Sintering) techniques are particularly suitable.

[0138] 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)

[0139] The starting mixture was made with a powder of titanium oxide with a mass percent of greater than 95% of TiO.sub.2 and of median diameter D.sub.50 of 0.8 m mainly in a rutile crystallographic form, a powder of B.sub.4C with a mass percent greater than 98% of B.sub.4C and a median diameter D.sub.50 equal to 7 m and petroleum coke, according to the following respective mass proportions 64.53% of TiO.sub.2, 22.59% of B.sub.4C and 12.89% of C. Such a mixture corresponds to an excess of boron carbide of 1.2%. An isopropanol solvent was added in order to subsequently obtain granules according to the teaching of the publication of the Journal International Journal of Refractory Metals and Hard Materials 25 (2007) page 345-350 by C. Subrmania et al.

[0140] Two mixture samples were subjected to a heat treatment without a particular sweep and in a vacuum of 4.Math.10.sup.5 mbar in a furnace according to a plateau duration of 2 h respectively at a temperature of 1600 C. and 1820 C.

Example 2 (According to the Invention)

[0141] A mixture was prepared under the same conditions as above, but without the granulation step after heat treatment, the milling being 3 minutes instead of 30 minutes. Furthermore, the starting powders consist of a powder of titanium oxide with a mass percent of greater than 95% of TiO.sub.2 (the remainder being essentially SiO.sub.2<2%, Al.sub.2O.sub.3<2%, ZrO.sub.2<1%, and traces of Fe) and of median diameter D.sub.50 of 10 m; powder of B.sub.4C with a mass percent greater than 98% B.sub.4C and a median diameter D.sub.50 of 15 m and a carbon black powder of median diameter D.sub.50 of 0.2 m according to the following respective mass proportions 63.6% TiO.sub.2, 22.1% B.sub.4C and 14.3% C. Such a mixture corresponds to an excess of boron carbide of 0.5% relative to the stoichiometry of the reaction. Two samples of mixtures were placed in a graphite crucible described above according to FIG. 3 serving as an enclosure respectively subjected to a heat treatment at 1600 C. and 1800 C. according to a 2 h plateau duration in a furnace under an argon sweep flow of 1.25 L/min/m.sup.3.

Example 3 (Comparative)

[0142] The starting mixture was carried out as in example 2, but the heat treatment was carried out without any particular sweep and in a vacuum of 4.Math.10.sup.5 mbar in a furnace according to a 2 h plateau duration at a temperature of 1600 C.

Example 4 (According to the Invention)

[0143] This example differs from example 2 in that the starting mixture comprises a further addition of NaCl representing 10% by weight of the dry mixture before heat treatment at 1600 C.

Example 5 (Comparative)

[0144] This example differs from example 2 in that the starting mixture comprises a titanium oxide powder of greater size before heat treatment at 2000 C.

[0145] For each of these examples, the raw powder mixture is then slightly crushed and sieved in order to separate the agglomerates to obtain a powder of particles, except for example 4 for which sieving alone was sufficient.

Example 6 (Comparative)

[0146] This example differs from example 2 according to the invention in that argon sweeping is adjusted to 0.25 L/min/m.sup.3.

Example 7 (Comparative)

[0147] This example differs from example 2 according to the invention in that a powder of B.sub.4C of purity >98% B.sub.4C by mass and median diameter D.sub.50 of the order of 150 m.

Example 8 (Comparative)

[0148] This example differs from example 2 according to the invention in that the respective mass proportions of titanium oxide powder, of B.sub.4C powder, and carbon black are the following 62.5% of TiO.sub.2, 23.2% of B.sub.4C and 14.3% of C. The excess of B.sub.4C relative to the stoichiometry of the reaction is about 7.7%.

[0149] The material yield was carried out according to the procedure described above in the application. The features of the method are compiled in table 1 which follows.

[0150] The properties of the final powders obtained are presented in table 2 below.

TABLE-US-00001 TABLE 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Diameter (comp.) (inv.) (comp.) inv. (comp.) (comp.) (comp.) (comp.) recycled raw materials Titanium D.sub.50 m 0.8 10 10 10 150 10 10 10 oxide D.sub.90 m N.M 30 30 30 250 30 30 30 powder Boron D.sub.50 m 6.7 15 15 15 15 15 150 15 carbide D.sub.90 m NM 30 30 30 30 30 220 30 powder Carbon D.sub.50 m 18 0.2 0.2 0.2 0.2 0.2 0.2 0.2 powder D.sub.90 m NM 0.3 0.3 0.3 0.3 0.3 0.3 0.3 NaCl D.sub.50 m N.A N.A N.A 10 N.A N.A N.A N.A powder D.sub.90 m 30 Excess % 1.2 0.5 0.5 0.5 0.5 0.5 0.5 7.7 B.sub.4C/Stoech. of reaction Heat treatment T max. C. 1600/1820 1600/1800 1600 1600 2000 1600 1600 1600 Ramp C./min 10 5 5 5 5 5 5 5 Platform h 3 2 2 2 2 2 2 2 Atmosphere Vacuum Argon Vacuum Argon Argon Argon Argon Argon Pressure mbar 4 .Math. 10.sup.5 Atmos. 4 .Math. 10.sup.5 Atmos. Atmos. Atmos. Atmos. Atmos. sweep flow L/min/m.sup.3 none 1.25 none 1.25 1.25 0.25 1.25 1.25 rate enclosure Yield % 60 98 63 98 66 78 <70 N.M N.M not measured; N.A not applicable = atmospheric pressure

TABLE-US-00002 TABLE 2 Ex. 1 Ex. 2 (comp.) (inv.) Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 1600/ 1600/ (comp.) (inv.) (comp.) (comp.) (comp.) (comp.) 1820 C. 1820 C. 1600 C. 1600 C. 2000 C. 1600 C. 1600 C. 1600 C. Physical properties of the powder obtained after grinding (3 min) D.sub.50 powder 0.8 2.2/2.9 1.8 1.8 3.8 N.M 3.8 N.M D.sub.90 powder 1.5 4.8/5.2 4.5 5.0 9.4 N.M 9.4 N.M Mass % final powder chemistry Ti (%) >67/>67 >67 >67 >67 >67 N.M N.M N.M B(%) NM 32/31.8 28 32.1 32 N.M N.M N.M O LECO 2 to 4/0.5 0.8/0.5 0.2 1.2 2.3 N.M N.M 2.1 (%) C Total 2 to 4/0.6 0.3/0.2 5.0 0.1 5.0 5.5 5.3 6.4 LECO (%) N LECO NM/0.5 0.1/0.1 0.2 0.1 0.1 0.1 0.1 0.1 (%) S LECO NM 300/150 150 30 900 300 300 300 (ppm) Fe (ppm) NM 785 5000 2200 3900 N.M N.M N.M Li + Na + NM 500 <500 <500 <500 N.M N.M N.M Rb + Cs(ppm) Be + Mg + NM <500 <500 <500 <500 N.M N.M N.M Ca + Sr + Ba (ppm) P (ppm) NM <500 <500 <500 <500 N.M N.M N.M Si (ppm) NM <500 <500 <500 <500 N.M N.M N.M XR Diffr. TiB.sub.2, TiB.sub.2 TiB.sub.2, TiB.sub.2 TiB.sub.2, N.M TiB.sub.2, TiB.sub.2, Phases Ti.sub.2O.sub.3, only Ti.sub.2O.sub.3, only B.sub.4C, C B.sub.4C B.sub.4C Ti.sub.3B.sub.4, C Ti.sub.3B.sub.4, C traces of SiC Actual NM 4.4/4.4 4.4 4.4 4.4 4.4 4.4 4.4 density N.M. = not measured

[0151] It can be seen in the data reported in Tables 1 and 2 that the TiB.sub.2 powders obtained by the method according to the invention are very pure and virtually free of contaminants (in particular oxygen, nitrogen, carbon, sulfur) further with a very good yield from the reaction.

[0152] Ceramic bodies were produced from the powders according to the preceding examples 2, 4, 6 and 8 (those obtained at 1600 C.) and two other ceramic bodies were produced according to the same method as described below, but the first from the commercial Hganas powder of grade SE and the second from the powder Japan New Metals of grade NF.

[0153] 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 titanium 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.

[0154] The porosity of the sintered bodies obtained was determined by dividing the ratio expressed as a percentage of the bulk density measured for example according to ISO 18754 to the absolute density measured according to ISO 5018. The electrical resistivity is measured at room temperature (20 C.) 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.

[0155] The properties of the final powders obtained are presented in table 3 below.

TABLE-US-00003 TABLE 3 Powder example 2 example 4 Japan example 6 example 8 employed (invention) (invention) Hognas New Metals (comparative) (comparative) (obtaining T) (1600 C.) (1600 C.) SE NF 1600 C. 1600 C. Mass % final powder chemistry O LECO (%) 0.8 1.2 1.5 1.4 N.M 2.1 C LECO (%) 0.3 0.1 0.5 0.4 5.5 6.4 N LECO (%) 0.1 0.1 0.7 0.6 0.1 0.1 S LECO (ppm) 300 30 NM NM 900 900 Physical properties of the sintered body obtained Total porosity 4.5 4.7 39.9 25.6 >20 >20 % vol. Resistivity at 0.12 0.20 0.55 0.23 0.31 0.35 25 C. (micro- ohm .Math. m) N.M. = not measured

[0156] On reading the results reported in the preceding tables, 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 bodies obtained with commercially available TiB.sub.2 powders. Furthermore, the grains of TiB.sub.2 used have levels of contaminants (elementary oxygen, carbon and nitrogen in particular) well below those obtained by a method as described in the prior art. These advantages were able to be obtained from a powder according to the invention after simple grinding following the heat treatment, without resorting to an additional granulation step.