Patent Application
20240409420
The invention relates to a novel method for manufacturing or synthesizing titanium diboride.
Titanium diboride is a ceramic material having a low density (about 4.5 g/cm3), a high hardness, a high thermal conductivity and a low electrical resistivity. This makes it a potentially interesting material for several applications, in particular shielding and antiballistic protection, refractory applications where the thermal conduction and the high electrical conduction are an asset, in particular heat exchangers, the coating or even the composition of anodes or of cathodes of electrolysis reactors, or even membranes in certain temperature applications or in very aggressive chemical media, as well as the cutting tools of metals, in particular for non-ferrous metals, or cutting tools.
All these applications explain that the demand for this material is very high and currently increasing.
TiB2 does not exist in the natural state. The titanium diboride can be obtained for example by direct reaction of titanium (or its oxides or hydrides) with elemental boron at 1,000° C. or by carbothermic reduction of titanium oxide and boron oxide. In the latter case, the reaction consists of reacting a mixture of powders according to the following simplified reaction at a temperature above 1500° C.:
TiO2(s)+B2O3(s)+5C(s)→TiB2(s)+5CO(g) (1)
However, this method has a theoretical material efficiency of only about 30%.
Another less known method consists in particular in replacing the boron oxide powder with boron carbide, as shown by the following balance reaction:
2TiO2+B4C+3C→2TiB2+4CO (2)
The advantage of such a reaction is its better theoretical material yield (55.4%) and therefore release of less carbon monoxide, but it has the disadvantage of requiring a higher reaction temperature.
The source of titanium oxide is generally a mineral source with a content greater than 95% TiO2. The carbon source is generally and preferentially petroleum coke (petroleum distillation residue) or carbon black. Boron carbide is also a synthetic material available on the market of carbide powders, in particular abrasives.
However, methods for manufacturing this material are all the more expensive and energy-consuming as the final powder of titanium diboride obtained becomes finer (typically with a median diameter between 5 and 50 micrometers) or ultra-fine (median diameter less than 5 micrometers).
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 making it possible to obtain a very pure final powder (content of oxygen, of nitrogen and of carbon of the order of 0.5%). The method consists of reacting a mixture of high-purity powders (majority compound content greater 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 an organic solvent dried and then heated to a temperature of at least 1,800° C. and in a vacuum corresponding to a residual pressure of less than or equal to 4·10−5 mbar. Below this temperature, the powder obtained is too pure. This article teaches the use of reagent powder of fine (submicron) particle size and therefore more reactive, in particular titanium oxide, to improve the reaction yield and kinetics.
Another solution for better controlling the reaction consists of a method with an excess of B4C of at least 10%, or even 20%, by mass relative to the stoichiometric theoretical amount required for the reaction (1). This additional addition makes it possible to fill the loss of boron in gaseous form at high temperature and reduces the presence of TiC and residual carbon but penalizes the actual material yield of the method.
The purpose of the invention is thus to improve the synthesis method described above and illustrated by equation (2), in order to obtain a pure fine TiB2 powder, that is, one with a mass percent greater than 95%, or even very pure, that is, with a purity of greater than or equal to 98%, said powder having a low elemental oxygen content and advantageously also a low elemental carbon content, while retaining a high material yield, without resorting to an industrially complex powder synthesis process.
In particular, according to a first aspect, the present invention relates to an alternative method for manufacturing TiB2 at a temperature below 2000° C., satisfying this purpose by virtue of particular atmosphere conditions and an appropriate choice of starting powders without any catalysis or surfactant additive.
More specifically, the present invention relates to a method for manufacturing a TiB2 powder, comprising reducing titanium oxide by carbon in the presence of a source of boron, said method of heating a mixture of raw materials comprising, and preferably consisting of:
2TiO2+B4C+3C→2TiB2+4CO (2)
According to the invention, the inert gas is brought into the enclosure in contact with the mixture of raw materials. The present invention lies in the choice not only of the particle size of the starting powders described above but also in the selection of the preceding particular synthesis conditions, such a combination advantageously making it possible to obtain a fine TiB2 powder of high purity with a maximum material yield, as will be described in more detail below.
The method of the invention particularly comprises one or more of the following preferred features:
In particular, the content of SiO2 in the titanium oxide powder by mass is preferably less than or equal to 2%. The mass percent of Al2O3 in the titanium oxide powder is preferably less than or equal to 2%, preferably less than 1%. The mass percent of ZrO2 in the titanium oxide powder is preferably less than or equal to 1%.
According to a first possible mode, an addition of alkali metal salt can be carried out, for example according to proportions of between 0.5 and 15%, preferably between 5 and 15%, by mass of metal relative to the total mass of the carbon source, of the particles of the boron carbide powders and of the titanium oxide particles. This supply reduces the presence of agglomerates in the synthesis powder which are likely to disrupt the step of firing the sintered ceramic body obtained from this TiB2 synthesis powder.
An addition of alkali metal salt of less than 0.5% is insufficient to a temperature greater than 1500° C., in particular between 160° and 2000° C. A supply of greater than 15% leads to excessive evaporation of the boron during the synthesis of the TiB2 powder.
According to an advantageous embodiment of the present invention, the alkali metal is chosen from Li, Na, K. Preferably, the alkali metal salt is an alkali metal halide, preferably a chloride. More preferably, it is sodium chloride.
The median size of the alkali metal salt particles is preferably between 0.5 and 100 micrometers, more preferably between 5 and 50 micrometers.
The invention also relates to a TiB2 powder obtained according to the preceding method. The median particle diameter of this powder is between 0.5 and 50 microns, and it comprises the following elementary mass percents:
Such a TiB2 powder of high purity and of defined particle size makes 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 which are capable of resulting in the formation of secondary metal borides from these metals which are not desired.
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 a very low crystal size dispersion. Such a powder makes it possible to obtain a sintered ceramic body in the form of a part, at least one dimension of which, preferably all of the overall dimensions, is greater than 5 cm, or even greater than 10 cm, and having a total porosity also less than 7%, a very narrow pore size distribution, without deformation on sintering and without shrinkage cracking.
Preferably, the TiB2 powder according to the invention further comprises one or more of the following elementary mass percents:
Said 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%,
The TiB2 powder according to the invention does not comprise a crystallized phase such as B4C or TiC phases, or Ti2O3, Ti3B4, SiC as measured (detectable) by X-ray diffraction. Preferably, said powder comprises only a crystalline phase of TiB2, as measured (detectable) by X-ray diffraction.
The invention also relates to a mixture comprising between 90 and 99.9% by mass or even consisting of the TiB2 powder according to the invention 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, silicon hexaboride, preferably whose purity is greater than 95% by mass, preferably greater than 98% by mass.
A purity greater than 95% by mass is that of said phase or of the most stable main compound: for example, in the case of a powder of aluminum diboride, more than 95% by mass of AlB2 or for a tungsten pentaboride powder, containing more than 95% by mass of W2B5.
The invention likewise relates to a method for manufacturing a sintered ceramic body, comprising the following steps:
The invention also relates to the sintered ceramic body thus obtained and the use of the sintered ceramic body obtained by the preceding method as all or part of a membrane, in particular for the filtration of liquids or gases, 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, in particular an electrolysis reactor, a heat exchanger, a metal melting crucible, in particular for non-ferrous metal, a cutting tool.
The following indications and definitions are given in connection with the preceding description of the present invention:
In particular:
Unless otherwise specified, all percentages in this description are mass percentages.
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.
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 TiO2 mass percent of which is greater than 95%) and a boron carbide powder (for example a powder whose B4C 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.
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.
Preferably, the median size ratio of the boron carbide and titanium oxide particles is between 0.8 and 1.2.
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.
The mixture according to the invention has an excess of B4C less than 5% relative to the stoichiometry of the reaction (2), calculated according to the invention on the basis of the amount of TiO2 introduced into said mixture.
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.
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%.
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
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.
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.
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.
The cooling can be free or forced, preferably according to a negative ramp less than 20° C./min.
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%.
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.
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.
The final powder of TiB2 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.
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.
A method for manufacturing a sintered ceramic body using the powder according to the invention in particular comprises the following steps:
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.
Hot pressing, hot isostatic pressing, or SPS (Spark Plasma Sintering) techniques are particularly suitable.
The following examples are for illustrative purposes only and do not limit the scope of the present invention in any of the aspects described.
The starting mixture was made with a powder of titanium oxide with a mass percent of greater than 95% of TiO2 and of median diameter D50 of 0.8 μm mainly in a rutile crystallographic form, a powder of B4C with a mass percent greater than 98% of B4C and a median diameter D50 equal to 7 μm and petroleum coke, according to the following respective mass proportions 64.53% of TiO2, 22.59% of B4C 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.
Two mixture samples were subjected to a heat treatment without a particular sweep and in a vacuum of 4·10−5 mbar in a furnace according to a plateau duration of 2 h respectively at a temperature of 1600° C. and 1820° C.
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 TiO2 (the remainder being essentially SiO2<2%, Al2O3<2%, ZrO2<1%, and traces of Fe) and of median diameter D50 of 10 μm; powder of B4C with a mass percent greater than 98% B4C and a median diameter D50 of 15 μm and a carbon black powder of median diameter D50 of 0.2 μm according to the following respective mass proportions 63.6% TiO2, 22.1% B4C 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
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 4·10−5 mbar in a furnace according to a 2 h plateau duration at a temperature of 1600° C.
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.
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.
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.
This example differs from example 2 according to the invention in that argon sweeping is adjusted to 0.25 L/min/m3.
This example differs from example 2 according to the invention in that a powder of B4C of purity >98% B4C by mass and median diameter D50 of the order of 150 μm.
This example differs from example 2 according to the invention in that the respective mass proportions of titanium oxide powder, of B4C powder, and carbon black are the following 62.5% of TiO2, 23.2% of B4C and 14.3% of C. The excess of B4C relative to the stoichiometry of the reaction is about 7.7%.
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.
The properties of the final powders obtained are presented in table 2 below.
It can be seen in the data reported in Tables 1 and 2 that the TiB2 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.
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 Höganas powder of grade SE and the second from the powder Japan New Metals of grade NF.
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.
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.
The properties of the final powders obtained are presented in table 3 below.
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 TiB2 powders. Furthermore, the grains of TiB2 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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2110464 | Oct 2021 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR2022/051873 | 10/4/2022 | WO |
