EP2424818A1

EP2424818A1 - Method for producing a powder including carbon, silicon and boron

Info

Publication number: EP2424818A1
Application number: EP10719750A
Authority: EP
Inventors: Hicham Maskrot; Benoît GUIZARD; François TENEGAL
Original assignee: Commissariat a lEnergie Atomique CEA; Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2009-04-29
Filing date: 2010-04-29
Publication date: 2012-03-07
Also published as: FR2945035B1; CN102414126A; KR20120024545A; JP2012525313A; US9139478B2; US20120152724A1; JP5645925B2; CN102414126B; FR2945035A1; WO2010125149A1; KR101841558B1

Abstract

The invention relates to a method for producing a powder including carbon, silicon and boron, the silicon being in the form of silicon carbide and the boron being in the form of boron carbide and/or free boron, including the following steps: a step of placing in contact a carbon-based precursor, a silicon-based precursor and a boron-based precursor BX3, where X is a halogen atom, such as to obtain a mix of said three precursors; a step of subjecting the resulting mixture to laser pyrolysis, the boron-based precursor BX3 being heated, prior to the step of placing in contact and/or simultaneously with the step of placing in contact, to a temperature higher than the condensation temperature of said precursor.

Description

PROCESS FOR PRODUCING A POWDER COMPRISING CARBON, SILICON AND BORON

DESCRIPTION

TECHNICAL AREA

The present invention relates to a process for producing a powder comprising carbon, silicon and boron, the silicon being in the form of silicon carbide and the boron being in the form of boron carbide and / or boron alone.

Such powders, especially because they comprise boron, such as powders comprising less than 5% by weight of boron, are powders which lend themselves easily to sintering and thus to the design of consolidated parts from said powders. .

Thus, the powders obtained by the process of the invention can find their application in the design of parts obtained by sintering.

The powders obtained by the process of the invention can also be used for the design of self-healing matrices, especially when they comprise boron at a content greater than 5% by weight of boron. Indeed, the self-healing phase of said matrices must have a contact surface with the highest possible oxygen. Due to the surface / volume ratio, the reactivity towards oxygen is increased and boron carbide oxidizes as B2O3 at lower temperatures and kinetically faster.

STATE OF THE PRIOR ART Powders comprising carbon, silicon and boron have so far been produced by three types of techniques:

- the technique of mechanosynthesis;

- the solution technique; - the thermal technique.

The technique of mechanosynthesis consists of a mechanical grinding, in a device of the grinder or attritor type, of submicron-sized silicon carbide (SiC) and boron carbide (B ₄ C) powders for a sufficient time.

(often several hours) to try to obtain a powder intimately mixed having a silicon carbide phase and a boron carbide phase.

However, it has not been reported by this method to obtain nanoscale powders. Moreover, the mechanical grinding of powders can cause pollution of the resulting powders by elements from the grinding device. Therefore, the powders resulting from this process can not be used in applications requiring high purity powders.

According to the solution technique, powders comprising carbon, silicon and boron have been synthesized by sol-gel using silicon-based precursors, carbon-based precursors and boron-based precursors. Such is the case, in particular, of the method described in Zhi-min et al. (Trans.Nonferrous Met Soc., China, 16 (2006), 470-473), which comprises respectively:

a step of mixing tetraethoxysilane, ethanol, sucrose and water;

a step of adding to the resulting mixture of tributyl borate, the pH being maintained until homogenization to a value of between 3 and 4;

a drying step in an oven at 60 ^° C. so as to obtain a gel;

a carbothermic reduction step of the silica formed.

According to the thermal technique, the powders comprising carbon, silicon and boron can be prepared from gaseous precursors using different heat sources such as a laser (in which case we will speak of laser pyrolysis) or a plasma.

Thus, Vassen et al. (Journal of Materials Science 31 (1996) 3623-3637) have synthesized, by laser pyrolysis, powders comprising both carbon, silicon and boron, from a mixture of precursors: SiH ₄ -C ₂ H ₄ -B ₂ H6 with a boron content not exceeding 4% by mass. This method of synthesis has, among others, the disadvantage of using diborane B ₂ H ₆ , which is an unstable gas with a high cost, and therefore difficult to use for the production of powders having higher levels of boron than the above-mentioned content. Guo et al. (Journal of Materials Science,

32 (1997), 5257-5269) synthesized powders comprising carbon, silicon and boron by the use of a thermal plasma from a mixture comprising solid silicon, BCI3 boron trichloride and CH ₄ methane. The powders obtained by this method of synthesis comprise less than 4% by weight of boron and finally have no boron carbide (B ₄ C) phase but boron nitride. The formation of boron nitride is generated by the reaction of the nitrogen N ₂ from the plasma on the boron carbide formed transiently according to the following reaction:

(1/2) B ₄ C + N ₂ → 2 BN + (1/2) C

Moreover, the powders obtained by this process have a submicron size.

Other authors have tried the synthesis of powders comprising carbon, silicon and boron by thermal plasma, in particular by using a radiofrequency plasma torch, such as Saiki et al. in US 4,847,060 starting from a mixture of reactants such as a mixture of SiCl ₄ , CH ₄ and BCI 3, resulting in submicron powders having a bad distribution of boron in the boron carbide grains and a boron content not exceeding 5% by mass.

On the basis of the disadvantages of the processes of the prior art, the authors have set themselves the objective of proposing a process for the preparation of powders comprising at the same time carbon, silicon and boron in the form of silicon carbide and boron carbide and or boron alone, which method is used simple and inexpensive, and which, thanks to a motivated combination of steps and reagents, makes it possible to obtain powders that can meet the following characteristics: a nanometric size of grains constituting the powder;

- a narrow size dispersion;

a boron content which may be high (for example up to 30% by weight relative to the total mass of the other elements present in the powder);

good dispersion of the silicon carbide and boron carbide phases;

a control of the composition of the powder, especially with regard to the Si / B ratios,

Si / C and B / C.

STATEMENT OF THE INVENTION

Thus, the invention relates to a process for producing a powder comprising carbon, silicon and boron, the silicon being in the form of silicon carbide and the boron being in the form of boron carbide and or boron alone comprising the following steps:

a step of bringing into contact a precursor based on carbon, a precursor based on silicon and a precursor based on boron BX ₃ , X being a halogen atom, so as to obtain a mixture of of these three precursors;

a step of subjecting the resulting mixture to laser pyrolysis, the boron precursor BX ₃ being heated, prior to the contacting step and / or simultaneously with the contacting step, at a temperature above its condensation temperature.

This newly implemented process has the following advantages:

- Incorporation of boron in the synthesized powders up to the amount of boron precursor implemented in the process, which allows to obtain, if desired, high boron levels in the resulting powders;

obtaining a powder having an average size of nanometric grains and a narrow size distribution.

Thus, as mentioned above, the invention comprises a step of heating the boron precursor BX ₃ , X being a halogen atom, at a temperature above its condensation temperature before the contacting step. and / or during said contacting step, for example, at a temperature ranging from 40 to 60 ^° C. By proceeding in this way, a condensation of the precursor is prevented, before being subjected to pyrolysis, and thus to have a quantity of precursor that can not be incorporated in the powder due to this condensation. Thanks to this step, all the boron resulting from the precursor used in the context of this process will enter into the constitution of the powders. By adjusting the amount of boron precursor introduced, it will thus be possible to directly predict the amount of boron that will be incorporated into the powder and thereby control the boron content therein. This heating step, in that it excludes the condensation of the boron precursor, also prevents the apparatus, in which the process is implemented, from being damaged, for example by clogging of the injection nozzles by the product resulting from the condensation of the boron precursor.

The heating step can take place before the contacting step, for example, before contacting the boron precursor with the other precursors (ie, the silicon-based precursor and the precursor-based of carbon), this heating step can be carried out in an enclosure comprising the boron precursor (this chamber may be a bottle having, for example, an outlet pressure of at least 0.4 bar) and / or in the injection pipe of said precursor for conveying said precursor into the chamber where it will be brought into contact with the other precursors.

The heating step may make it possible, in addition to avoiding the condensation of BX ₃ , to increase the flow rate of BX ₃ , and in particular to have access to a flow rate sufficient to obtain large quantities of powder (for example, a rate of at least 100 g / h).

The heating step can also take place during the contacting step in which case the heating step will concern all the precursors contacted during this step. It is not excluded within the meaning of the invention that the heating step can take place at a time before the contacting step and during the contacting step, so that there is no condensation of the boron precursor before the pyrolysis step. The boron precursor may be BCI3 boron trichloride.

When the boron precursor is boron trichloride BCI3, the latter may be heated prior to the contacting step and / or simultaneously with the contacting step at a temperature ranging from 40 ^° C. to 50 ^° C. ⁰ C.

As mentioned above, the method of the invention comprises a step of contacting, prior to laser pyrolysis, a boron precursor BX ₃ , X being a halogen atom, a precursor based on carbon and a precursor based on silicon.

Preferably, the boron precursor is boron trichloride.

The carbon-based precursor may be a compound selected from alkanes, such as methane, alkenes, such as ethylene and alkynes, such as acetylene.

Preferably, the carbon-based precursor may be a gaseous alkyne, such as acetylene C2H2, which has the particularity of being very reactive during the pyrolysis step, because it decomposes more rapidly than the methane CH ₄ and ethylene C2H4 and at lower temperature.

The silicon-based precursor is preferably a silane compound, such as SiH ₄ . Preferably, the boron-based precursor is BCI3, the carbon-based precursor is C2H2 acetylene, and the silicon-based precursor is SiH ₄ , this precursor mixture being advantageous from a cost point of view and having also thermokinetic properties particularly suited to the process of the invention, in particular to obtain homogeneous powders in terms of size and composition. The carbon-based precursor, the silicon precursor and the boron precursor are conventionally contacted in a mixing chamber, which can be heated to a temperature above the condensation temperature of the precursor based on silicon. boron. The introduction of the carbon-based precursor, the silicon-based precursor and the boron-based precursor is advantageously carried out separately in the enclosure, so that there is no contact precursors before contacting in the enclosure. This also makes it possible to avoid chemical reactions between the precursors before they are introduced into the mixing chamber. To do this, the mixing chamber can be provided with three separate injection ports. Preferably, the injection of the precursors into the chamber takes place vertically, which means, in other words, that the precursors are injected into a vertical enclosure in the upper part thereof and are concentrated by the effect of gravity in the lower part of the enclosure after introduction. The precursors are introduced at predetermined flow rates according to the desired powder characteristics (in terms in particular of boron content, carbon content and silicon content). The resulting mixture is then subjected to a laser pyrolysis step.

Laser pyrolysis is based on the interaction between gaseous precursors (in this case, the carbon-based precursor, the boron precursor, and the silicon-based precursor) and a laser , generally a CO2 laser, which interaction results in a resonance between the emission spectrum of the laser and the absorption spectrum of the precursors. Absorption is the excitation of the vibrational levels of the precursor molecules, which absorb the energy of the laser radiation. The energy of the excited precursor molecules propagates from molecules to molecules, causing the dissociation thereof to form a supersaturated vapor, in which the nucleation and growth of the constituent particles of the powder occur. A so-called "incandescent" flame can then be observed. The particles formed undergo a quenching effect at the flame outlet, which has the effect of stopping the growth of the particles.

From a practical point of view, the mixture obtained during the aforementioned mixing step is conventionally injected via an injection nozzle into a laser pyrolysis chamber, where a laser beam is emitted. The pressure in the pyrolysis chamber can range from 100 mbar to 900 mbar. The laser used may be a gas laser, in particular a carbon dioxide laser capable of emitting in the infrared (their main wavelength band being centered between 9.4 and 10.6 μm). The power of such a laser can be up to 20000 W, for example ranging from 200 to 700 W.

Each fraction of the mixture (i.e., the mixing fraction passing through the laser beam) is subjected to a short residence time, for example from 1 to 10 ms and at a temperature ranging from 1000 ^° C. to more than 2500 ^° C. .

It results from this process nanoscale powders comprising carbon, silicon and boron, the silicon being in the form of silicon carbide, the boron being in the form of boron carbide and / or free boron, the carbon in addition to its presence in the form of carbide (s) may also be in the form of free carbon. The powders obtained have a narrow size distribution and may have a boron content ranging from 1 to 30% by weight relative to the total mass of the other elements present in the powder.

The powder obtained advantageously consists of nanometric grains, themselves advantageously constituted by B ₄ C boron carbide and silicon carbide SiC phases.

The powders obtained by this process can then be collected in a collection device.

These powders can be used to produce parts by sintering having advantageous mechanical characteristics due to the fact that nanometric nature of the powders or to make self-healing matrices.

The method of the invention can be implemented in a device comprising respectively: an injection chamber, in which the precursors are injected, which chamber can consist of a heated mixing chamber, a rod of injection connected to the upper part of the chamber, two injection rods connected to the lateral part of the chamber, which allow the separate injection of the various precursors, an injection nozzle for injection into a pyrolysis chamber;

a pyrolysis chamber, in which a laser beam is emitted which will interact with the precursor mixture to form the aforementioned powder.

The invention will now be described with reference to the following examples given for illustrative and non-limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 relates to a schematic representation of a reactor capable of enabling the implementation of the method of the invention. FIG. 2 is an X-ray diffractogram (the abscissa representing the angle 2Θ and the ordinate the intensity I (in arbitrary units ua)) of the powder obtained according to the example given below. DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

EXAMPLE

The present example illustrates the preparation of a powder comprising both a silicon carbide phase and a boron carbide phase by laser pyrolysis implemented in a reactor 1 shown in FIG. 1 comprising the following elements:

an injection chamber, into which the precursors are injected and then mixed before being subjected to laser pyrolysis, which chamber consists of a heated mixing chamber 5, an injection rod 7 connected to the part upper chamber, two injection rods 9, 11 connected to the side portion of the chamber, an injection nozzle 13 for injecting the mixture in a pyrolysis chamber; a containment chimney 15 disposed around the injection nozzle;

a pyrolysis chamber 17 in which a laser beam 19 is emitted from a laser emission device 21 which will interact with the precursor mixture to form the aforementioned powder.

The confinement chimney makes it possible on the one hand to keep the powders produced in a laminar flow and, on the other hand, it prevents any contact with the metal walls of the reaction chamber and thus to avoid any pollution.

The operating protocol is as follows. In the mixing chamber, the following precursors are introduced: a silicon-based precursor: SiH ₄ ; a precursor based on carbon: acetylene C2H2;

a boron precursor BCI3 at the following flow rates: 3.6, 1.8 and 0.14 L / min, the flow rates being controlled by mass flow controllers, which precursors are mixed in said heated enclosure at a temperature of 45 ° C.

In the context of this example, BCI3 boron trichloride is injected into the chamber by the injection rod 7, while the acetylene and SiH ₄ are injected into the chamber by the injection rods 9 and 11.

BCI3 boron precursor is preheated before being injected into the reactor at 45 ° C both before and during its passage in the injection rod.

BCI3 boron precursor is derived from a bottle containing it, this bottle having an outlet pressure of at least 0.4 bar, this bottle being heated to 45 ° C and stirred to accelerate the diffusion of heat inside and thus BCI3 boron precursor transfer.

The mixture of precursors obtained in the mixing chamber is then injected through the injection nozzle into the pyrolysis chamber at a flow rate of 7.2 L.min ^-1 where it is subjected to an infrared laser beam ( IR), more precisely a CO2 laser used at a working power of 5000 W for a residence time of 2.8 ms. The rate of production of powder at the outlet of the pyrolysis chamber is of the order of 391 g / h.

The powder obtained was analyzed by the following techniques: X-ray diffraction, the diffractogram of which is represented in FIG. 2;

observation of powder samples obtained by transmission microscopy.

It appears on the X-ray diffractogram of a sample obtained according to the protocol defined above the presence of the following phases:

a phase of β-SiC silicon carbide, the presence of which is indicated by an arrow in FIG. 2;

a crystalline boron carbide phase and an amorphous boron carbide phase, the presence of which is indicated by two distinct arrows, the amorphous boron carbide phase being a minority. The width of the diffraction peaks on the diffractogram testifies to the nanometric nature of the crystallites constituting these powders.

The powder obtained was also the subject of elementary chemical analysis, so as to determine the percentage by mass of each of the chemical elements present in it.

The values obtained by the elemental chemical analysis are as follows:

Boron: 27% by weight

Silicon: 36% by weight; Carbon: 28% by weight; Oxygen: 8% by mass measurement errors can be of the order of 2 to 3% by mass depending on the elements. The average size of the constituent grains of the powder (ie the average diameter of the latter) has also been measured by two methods:

- by powder transmission electron microscopy, which made it possible to determine an average grain size of 25 nm, with a narrow size distribution centered around this value;

by measuring the specific surface area by the BET method and by measuring the density by pycnometry with helium, which made it possible to determine an average grain size of 22 nm.

By BET method, a BET surface area of 102 m ² / g was also determined.

Claims

A process for producing a powder comprising carbon, silicon and boron, the silicon being in the form of silicon carbide and the boron in the form of boron carbide and / or free boron comprising the steps following:

a step of subjecting the resulting mixture to laser pyrolysis, the boron precursor BX ₃ being heated, prior to the contacting step and / or simultaneously with the contacting step, at a higher temperature at the condensation temperature of said precursor.

2. Production process according to claim 1, wherein the boron precursor is boron trichloride BCl ₃ .

3. Production process according to claim 1 or 2, wherein the carbon-based precursor is selected from alkanes, alkenes, and alkynes.

The process of any of the preceding claims, wherein the carbon precursor is acetylene.

The method of any of the preceding claims, wherein the silicon precursor is a silane compound.

The method of any of the preceding claims, wherein the silicon-based precursor is SiH ₄ .

7. A process according to any one of the preceding claims, wherein the boron precursor is boron trichloride, the carbon precursor is acetylene and the silicon precursor is SiH ₄ .

8. Process according to any one of the preceding claims, in which, when the boron precursor is boron trichloride BCI3, the latter is heated before the contacting step and / or simultaneously with the step contacting at a temperature ranging from 40 to 50 ^° C.

A process according to any one of the preceding claims, wherein the contacting step is carried out in an enclosure, wherein the carbon precursor, the silicon precursor and the boron precursor are injected separately.

The method of any of the preceding claims, wherein the laser used in the laser pyrolysis step is a gas laser.

The method of claim 10, wherein the gas laser is a carbon dioxide laser.

12. Method according to any one of the preceding claims, wherein the powder has a boron content ranging from 1 to 30% by weight relative to the total weight of the elements present in the powder.

13. The method of any one of the preceding claims, wherein the powder is nanoscale grains.

14. Method according to any one of the preceding claims, wherein the powder consists of nanoscale grains consisting of phases B ₄ C and SiC.