FR3016305A1 - COMBINING MACROPORES OF A FIBROUS SUBSTRATE BY HOMOGENEOUS NUCLEATION - Google Patents

Abstract

The present invention relates to a process for filling macropores of a porous fibrous substrate by forming carbon grains, characterized in that a fluid composition containing tetramethylsilane and a dilution fluid is diffused within the substrate under conditions of temperature comprised between between 650 and 1100 ° C and under pressure with an initial pressure lower than the critical pressure of tetramethylsilane and the dilution fluid, preferably less than or equal to 2 MPa, and for maintaining the tetramethylsilane and the dilution fluid in the subcritical state and spontaneously initiating uniform phase nucleation within the macropores of the substrate from tetramethylsilane to directly form the carbon grains.

Description

The present invention relates to the filling of macropores of a porous fibrous substrate by formation of carbon grains. This formation therefore takes place at the heart of the porosity and not at the surface of the substrate. It uses a fluid composition containing tetramethylsilane and a dilution fluid, the whole being maintained under subcritical pressure conditions. More particularly, the invention relates to the production of thermostructural composite material parts comprising reinforcements refractory fibers (carbon or ceramic) densified by a refractory matrix (carbon or ceramic) whose macropores are filled with carbon grains. Typical thermostructural materials are carbon / carbon composite materials, denoted C / C (carbon fibers and matrix), and ceramic matrix composite materials, or CMCs. They possess the mechanical properties required to form structural elements and have the ability to retain these properties at high temperatures. Chemical vapor infiltration (CVI) processes are well known and commonly used to densify porous substrates with a solid material. The substrates to be densified are placed in an oven into which a gaseous reaction phase is injected which, under conditions of determined temperature and pressure, makes it possible to directly form the desired solid deposit on the heated fibrous substrate by decomposition of the reaction gas phase or by reaction between several of its constituents.

The CVI processes, in particular for the formation of carbon or ceramic matrix, are generally carried out at a temperature of about 900 ° C. and at most 1100 ° C. and under a reduced pressure of less than 100 kPa, typically between 1 kPa. and 50 kPa. Indeed, a low pressure is recommended to promote the diffusion of the gaseous phase in the core of the fibrous preforms and to avoid too fast sealing of the surface porosity preventing further densification in the core of the porous substrates and resulting in a strong gradient densification within the substrates. This results in a significant manufacturing time for CMC parts and makes the process expensive. Generally machining is necessary to reopen the periphery porosity and allow access to the gases at heart. In any case, the matrix densification is stopped when the porosity reaches a value close to 10% to 15% (due in particular to the presence of macropores). Thus, this long and expensive process does not allow the total filling of the macropores. It is also known to use densification methods slip or ceramic or sol-gel. This process consists in impregnating the fibrous preforms with a slip or a sol (mixture of particles of (sub-) micrometric ceramic size, sintering additions and liquid solvents) and then drying and sintering the assembly at 1600. -1800 ° C generally under pressure. This method is reserved for unidirectional composites, in particular because of the removal of the matrix during sintering. Some of the additions may be detrimental to the ultimate properties of the material. In addition, the development of carbon xerogel uses products classified CMR (carcinogenic, mutagenic, toxic for the reproduction), which makes difficult the industrial production. A hybrid slip / CVI process combining the slip route (without the addition of sintering) and the gaseous route may appear to be an interesting solution. After impregnation of the fibrous preform with the slip, the conventional CVI infiltration of the green composite would then allow the matrix to be densified. However, the high compactness of an (sub-) micrometric agglomerated powder is a barrier to good infiltration. The heart of the vintage is densified badly because of the premature closure of the porosity on the periphery of the vintage. The reactive species hardly reach the small pores and their concentration decreases very rapidly from the periphery to the heart which slows down strongly and then prevents the growth of the consolidation layer. Thus, this method does not allow the filling of macropores. By replacing the classical CVI with the pulsed CVI, it is possible to consolidate micrometric powders (4-5 μm) forming a raw compact of millimeter thickness. The purge and the filling of the raw compact allow to periodically lower the natural concentration gradient of the gaseous species between core and periphery. But the feasibility has not been reported for sub-micron powders and the process seems difficult to industrialize. The use of a fluid in the supercritical state in a process of densification of a porous substrate is described in the prior art. US 4,552,786 discloses a method of densifying a porous ceramic substrate comprising dissolving a ceramic precursor in a fluid (propane) which is brought to the supercritical state to penetrate with the precursor in the porosity of the substrate, and the release of the liquid precursor within this porosity back to non-supercritical conditions which is accompanied by a decrease in the propane dilution power. The transformation of the liquid precursor into ceramic is then carried out by heat treatment. It is therefore in fact a process of densification by liquid way, i.e the fluid in the supercritical state being used only to make penetrate the liquid precursor in the porous substrate. CN 1377855A discloses a similar method in which solid charges (carbon or silicon carbide) are added to the ceramic liquid precursor to reduce the densification time of the substrate. US 5,789,027 and US 6,689,700 disclose methods of forming solid deposits on a surface by dissolving a precursor of the material to be deposited in a solvent in the supercritical state. Exposing the substrate to this solution under conditions where the precursor is in a stable state, and introducing into the solution a reaction agent to trigger a chemical reaction involving the precursor and causing the deposition of the material on the surface of the precursor. substrate. The intended application is the production of a deposit on substrates of metal, metal oxide or semiconductor films. A process of this type is contemplated in US 5,789,027 to also deposit a material in a microporous or nanoporous substrate, but without a precise description of an implementation. Note that the substrate is maintained at a relatively low temperature, about 200 ° C, which may be sufficient to provide polymerization but too low to allow the formation of ceramic or carbon. The filling of the macropores of a porous fibrous-type substrate, thus macroporous (namely a pore size of a few tens to a few hundred microns up to a few mm), is not envisaged in these two last documents. The deposition of a material in a microporous substrate (pore size of the order of one micron) or nanoporous (pore size of a few nanometers to a few hundred nanometers) is only performed on a limited depth, close to the surface . It is noted in this regard that US 6,689,700 describes the use of photolysis which can occur only on the surface. In addition, this known process imposes conditions in which the precursor remains stable until the initiation of the deposit resulting from the introduction of a reaction agent, that is to say that it imposes a limit of substrate temperature at at most 250 ° C or 300 ° C. In addition, separate introduction of a reaction agent should be provided.

The techniques mentioned above, therefore hardly fill the macropores of a fibrous preform. It is possible to penetrate powders in the pores of the preform, by suction of submicron powders (APS), or electrophoresis. These powders are not bonded together, it is necessary to continue densification by CVI or RMI (Reactive Melt Infiltration), to infiltrate a preceramic polymer or to sinter these powders at high temperature, ... However, the rate of porosity after densification of these materials remains significant.

It is also known from application WO2007 / 0038013 to rapidly densify a porous substrate by forming a solid deposit within the porosity of the substrate. This method uses a fluid composition diffused within the substrate and containing a precursor reactive fluid of the material constituting the solid matrix deposition to be formed, possibly in admixture with a dilution fluid, the precursor fluid and / or the dilution fluid being the supercritical state during the implementation of the method and this method for spontaneously and directly forming the solid deposit of refractory matrix within the substrate from the precursor reactive fluid. However, in addition to the fact that the dilution fluid and / or the precursor fluid is in supercritical conditions, the method described in this document favors a continuous deposit around each fiber and seeks to avoid the formation of powder by nucleation in homogeneous phase so avoid disturbing the access to the internal porosity of the substrate (page 6 lines 30 - 32 and page 16 line 34 - page 17 line 2). In the only examples in which tetramethylsilane (TMS) is used (Examples 5 and 6), the initial pressure of the TMS + dilution fluid (nitrogen) mixture is set at 4 MPa or 4.1 MPa and thus the dilution fluid is in the supercritical state. These conditions favor the formation of silicon carbide in the form of continuous deposit on the fibers and do not allow the filling of macropores by formation of carbon grains. The person skilled in the art is therefore not encouraged to read this document to modify the initial pressure conditions during the implementation of the process, since on the contrary it is not recommended to avoid clogging the external porosity and therefore to prevent the TMS from reaching the core of the substrate. There is therefore a need to find a new process, fast, inexpensive, industrializable that allows the filling of macropores without prematurely plugging the external porosity of the substrate. However, the inventors have surprisingly discovered that it is possible to fill almost completely the macropores of the substrate without prematurely plugging the external porosity of the substrate using conditions of initial pressure and very specific temperature in which the TMS and the fluid are in the subcritical state. In addition, this filling is done using carbon grains, in particular nodules (grains having a spherical shape), thanks to nucleation in homogeneous phase. This process is fast: in less than an hour, it is possible to densify a fibrous preform several millimeters deep and fill in its macropores. This process is therefore due to its various advantages less expensive, faster and industrializable. In addition, it does not require the use of products classified CMR, nor dispersant nor to suspend suspension of powders in a liquid.

Moreover, this process has the advantage of requiring virtually no effluent treatment since the TMS is almost completely transformed during the process, either methane or hydrogen, easier to reprocess. This process is therefore industrially more ecological and more economical.

The present invention therefore relates to a process for filling macropores of a porous fibrous substrate by formation of carbon grains, characterized in that a fluid composition containing tetramethylsilane and a dilution fluid (advantageously the fluid composition is constituted by TMS and the dilution fluid) is diffused within the substrate under conditions of temperature between 650 and 1100 ° C, and under pressure with an initial pressure lower than the critical pressure of the tetramethylsilane and the dilution fluid and to maintain the tetramethylsilane and the dilution fluid in the subcritical state and to spontaneously cause nucleation in homogeneous phase within the macropores of the substrate from tetramethylsilane to directly form the carbon grains.

For the purposes of the present invention, the term "initial pressure", the pressure that is used at the beginning of the process. Indeed, during the process the pressure can increase spontaneously due to (i) the increase in temperature and (ii) the production for example of hydrogen or other gases such as methane, when setting up implementation of the method. For the purposes of the present invention, the term "subcritical state of the dilution fluid and TMS" any state in which neither the dilution fluid nor the TMS are in the supercritical state. Thus, neither the dilution fluid nor the TMS are compressed at a pressure beyond their critical pressures (Pc). The pressure is therefore chosen as a function of the critical pressure (Pc) of the dilution fluid and the TMS. The critical pressure of TMS is 2.8 MPa and that of nitrogen is 3.4 MPa. So the pressure must always be less than 2.8 MPa. However, in the context of the process according to the present invention, the pressure must still be greater than atmospheric pressure, the process being carried out under pressure. Advantageously it is less than or equal to 2 MPa, more advantageously between 0.8 and 2 MPa, in particular between 0.8 and 1.5 MPa, more particularly about 1 MPa.

Moreover, in the context of the process according to the present invention, the temperature must be chosen at a value sufficient to allow the spontaneous chemical reaction of deposition of carbon grains from the TMS. Thus, the temperature is between 650 and 1100 ° C., advantageously between 700 and 1100 ° C., in particular between 750 ° C. and 1000 ° C. This temperature is that measured at the heart of the substrate. In particular, it is necessary to maintain a temperature gradient between the core of the substrate and its periphery, which makes it possible to avoid filling the pores at the periphery before those present in the core of the substrate. These temperature and pressure conditions make it possible to maintain the TMS and the dilution fluid under pressure but in the subcritical state and to spontaneously cause nucleation in homogeneous phase within the macropores of the substrate and the growth of carbon grains. These parameters are therefore controlled to obtain such a nucleation and favor the formation of carbon grains. Indeed the grains obtained by the process consist essentially of carbon, but they can sometimes contain a little silicon carbide, but in proportions much lower than carbon. Thus, depending on the pressure and temperature conditions used, the process according to the present invention makes it possible to fill the macropores with carbon grains alone, or with grains containing a mixture of carbon and silicon carbide, the proportion of carbon of the grain being always much greater than that of silicon carbide. In all cases, we will speak of carbon grains within the meaning of the present invention, even if the composition of these grains contains a little silicon (probably in the form of carbides). The carbon grains generally have an average diameter of a few micrometers, for example between 1 and 2 amps, measured by scanning electron microscopy (SEM). The presence of silicon carbide in the grain composition does not change their dimensions.

In all cases, advantageously, the grains (carbon or carbon + silicon carbide) do not form a foam. It is therefore a discontinuous phase. For the purposes of the present invention, the term "dilution fluid", any fluid that does not participate in the chemical reaction of formation of carbon grains. Thus, advantageously the dilution fluid is chemically neutral vis-à-vis the formation of carbon grains, and in particular is selected from the inert gases of the atmosphere, such as for example nitrogen and argon, more particularly nitrogen. The initial molar level of the dilution fluid in the composition may be relatively high, for example up to 90% or more, advantageously up to 98% or 99%. For the purposes of the present invention, the term "macropores" means pores whose average diameter is greater than 50 nanometers. The diameter of these pores may even be greater than 200 μm, or even be of the order of a few mm. These macropores are generally present in the composites at the cross between the fibers. In a particular embodiment, the porous fibrous substrate is formed from carbon or ceramic fibers, preferably carbon fibers. The method according to the present invention may also be implemented on a shaped substrate, for example which constitutes a fibrous preform, in particular whose shape is close to that of a piece of composite material to be produced. More particularly, the porous fibrous substrate is a carbon fiber preform. This porous fibrous substrate may also be refractory. The substrate can be consolidated before carrying out the process according to the present invention. The consolidation consists of a limited partial densification of the substrate, sufficient to bind the fibers together so that the substrate can be manipulated while retaining its shape without the aid of holding tools. Consolidation by partial densification can be carried out by liquid, as known per se. In another particular embodiment, the porous fibrous substrate is not electrically conductive. This is particularly advantageous, unlike the substrate described in the patent application WO2007 / 003813. The porous fibrous substrate according to the present invention comprises macropores which must be filled. In an advantageous embodiment of the process according to the present invention, this substrate whose macropores are to be filled has an open porosity, measured by helium pycnometry or by a mercury porosimeter, of at most 80%. It is therefore possible to fill the macropores of very porous fibrous substrates, for example having an open porosity of between 70 and 80%. For the purposes of the present invention, the term "open porosity" of the substrate is understood to mean pores which communicate with each other and with the outside via channels, which allows the circulation of fluids (provided that the size of the molecules that make up the fluid less than the pore size).

The measurement of the open porosity of a substrate by helium pycnometry or by a mercury porosimeter is well known to those skilled in the art.

The process according to the present invention can be carried out in a closed enclosure (Batch process) or with continuous circulation of the fluid composition (semi-continuous or continuous process). In the first case, the method according to the present invention comprises the steps of (a) introducing a quantity of fluid composition into an enclosure containing the substrate and (b) establishing in the chamber temperature and humidity conditions. pressure to cause nucleation in homogeneous phase of carbon grains within the macropores of the substrate while maintaining in the subcritical state tetramethylsilane and the dilution fluid. Those skilled in the art understand that before carrying out step (a) of the process according to the present invention, it is necessary to evacuate the reactor. The process can be repeated several times on the same substrate with a higher temperature at each repetition, preferably at least twice (first time with a lower temperature than the second time), especially at least 3 times (with 3 temperatures). different). There is therefore in this case a gradation in the temperatures used. Preferably, the process is not repeated.

In addition, in this case, the method according to the present invention may comprise a preliminary step (A) or an additional step (B) of diffusion within the substrate of a fluid composition containing tetramethylsilane and a dilution fluid, in temperature and pressure conditions for maintaining the dilution fluid in the supercritical state and for spontaneously initiating uniform phase nucleation within the macropores of the substrate from tetramethylsilane to directly form silicon carbide grains.

For the purposes of the present invention, the term "dilution fluid in the supercritical state" any dilution fluid which is heated beyond its critical temperature (Tc) and compressed beyond its critical pressure (Pc). The fluid then has the following properties: a viscosity close to that of the gases, a high diffusivity, a density close to that of the liquids, and a solvent power. The temperature and the pressure are thus chosen as a function of the pressure and the critical temperature (Pc and Tc) of the dilution fluid, the temperature also to be chosen at a value sufficient to allow the spontaneous chemical reaction to deposit from the TMS. . The dilution fluid may be the same as that used in the preceding or following step (that is to say under subcritical conditions according to the present invention) and in particular it is nitrogen. The molar level of TMS may also be the same or different, in particular different. Advantageously, the temperature of this preliminary or additional step is between 600 ° C and 1100 ° C, preferably between 750 and 1000 ° C, in particular between 900 and 1000 ° C. In a particularly advantageous manner, the temperature is the same as that of the other steps of the process (in particular of the step under subcritical conditions). In particular, the pressure during this preliminary or additional step is between 4 MPa and 6 MPa, more particularly between 4 and 5 MPa. These temperature and pressure conditions make it possible to maintain the dilution fluid in the supercritical state and to spontaneously cause nucleation in homogeneous phase within the macropores of the silicon carbide grain substrate. These parameters are therefore controlled to obtain such nucleation. In this step, unlike the additional or prior step (b) under subcritical conditions, it is the formation of silicon carbide grains that is preferred, to the detriment of carbon grains. This formation of silicon carbide grains also depends on the temperature used. Low temperatures, for example between 650 and 900 ° C, also favor the formation of silicon carbide. The combination of these two steps (supercritical dilution stage + TMS, followed by a dilution fluid step and TMS in the subcritical state, or vice versa) thus makes it possible to obtain a mixture of grains of silicon carbide and carbon grains within the macropores of the substrate in the desired proportions.

To implement this batch process, the device described in WO2007 / 003813 can be used, provided that the walls of the reactor are cooled so that there is a temperature gradient between the core of the porous substrate and the outside, this which prevents the outer pores from filling up before the pores present in the heart.

As illustrated in FIG. 5, this device (1) can also be modified by inserting a carbon tube (2) held by jaws (6) between the two electrodes (3) and (4) and disposed within the porous substrate (5). The carbon tube serves to heat the porous substrate and allows the process to be used on porous substrates that are not electrically conductive.

This modified device (1) therefore also comprises a reactor (7), an inlet and outlet of the fluid composition (8). The temperature is measured by a thermocouple (9). In the second case, the method according to the present invention comprises the steps of continuous admission of a flow of fluid composition into an enclosure containing the substrate, continuous extraction of a flow of effluent fluid out of the enclosure and of maintaining conditions of substrate temperature and pressure in the chamber to cause homogenous nucleation of carbon grains within the macropores of the substrate while maintaining in the subcritical state tetramethylsilane and the dilution fluid. The substrate obtained by the process according to the present invention can be used for the manufacture of thermostructural composite material parts. In particular the thermostructural material is a carbon / carbon composite material, or C / C (carbon fibers and matrix), or a ceramic matrix composite material, or CMC. It has the mechanical properties required to form structural elements that have the ability to maintain these properties at elevated temperatures. The material obtained by the process according to the present invention can then be used as a substrate for densification by CVI. It can also be the medium of a RMI reaction. Thus, advantageously, the process according to the present invention comprises an additional step (c) after step (b) or after the optional step (B) of RMI treatment of the substrate so as to transform the carbon grains into carbide. The present invention will be better understood on reading the description of the figures and the examples which follow. FIG. 1 shows Scanning Electron Microscopy (SEM) observation at 10pm scale of carbon grains within macroporosities on a fiber coil positioned around the graphite tube obtained by the method according to the present invention with Nitrogen as a diluting fluid according to the conditions of Example 1. FIG. 2 represents scanning electron microscope (SEM) observation at the 20 μm scale of silicon carbide grains within the macroporosities on a fiber coil positioned around the graphite tube obtained by a supercritical process with nitrogen as a diluting fluid according to the conditions of Comparative Example 1 at a temperature of 900 ° C.

FIG. 3 represents scanning electron microscope (SEM) observation at a 5 μm scale of silicon carbide grains within the macroporosities on a second fiber coil positioned around the graphite tube obtained by a process under supercritical conditions with nitrogen as the diluting fluid according to the conditions of Comparative Example 1 at a temperature of 1000 ° C. FIG. 4 represents the scanning electron microscope (SEM) observation at different magnifications (A: 300 μm scale, B: 50 μm scale and C scale: 10 μm scale) of carbon grains and silicon carbide within the macroporosities. on a fiber coil positioned around the graphite tube obtained by the process according to the present invention with nitrogen as a diluting fluid according to the conditions of Example 2 (after the two consecutive batches). FIG. 5 represents a device that can be used for carrying out the method according to the present invention.

Example 1 A laboratory reactor of 0.5 L capacity was used for matrix densification of a porous substrate consisting of a coil of carbon fibers 2 cm high by 1.5 cm in diameter. The device used is that shown in Figure 5. The coil is located around the carbon tube of the reactor, and the system is heated by joule effect. After emptying the reactor of the air it contains, 4 ml of TMS under 1 MPa of N 2 (dilution fluid in the subcritical state) was introduced, the fluid composition consisting solely of TMS + N 2. The initial pressure in the reactor was therefore set at 1 MPa. It rose gradually to about 2.1 MPa (due to an increase in temperature and the production of dihydrogen and / or methane). The heating of the substrate is carried out with a rise in temperature from room temperature to the desired temperature (750 ° C.) at the heart of the substrate. As soon as the desired temperature at the center of the substrate was obtained, the infiltration time by the TMS was set at 10 minutes. A temperature gradient is present within the porous substrate.

A single 10 minute infiltration cycle was performed at the temperature of 750 ° C and almost all of the TMS was transformed. Grains consisting of carbon and some SiC a few micrometers in diameter are observed filling the macroporosity of the preforms (Figure 1).

Comparative Example 1: Two coils of carbon fibers were used under the same experimental conditions and the same device as in Example 1 with the exception of the following differences: the dinitrogen pressure was 4 MPa. It is therefore in the supercritical state; the quantity of TMS is 1 ml; 1 cycle of 10 minutes was carried out on each of the coils, at a temperature of 900 ° C (1st coil) or at a temperature of 1000 ° C (2nd coil). According to the SEM images taken at the heart of the substrate and supplemented by X-ray microprobe analyzes, the chemical composition of the grains elaborated from TMS in supercritical fluid evolves according to the temperature. The formation of SIC is favored at low temperature (900 ° C - Figure 2), while the formation of carbon (in the form of grains) is favored at high temperature accompanied by the presence of SIC grains (1000 ° C - Figure 3). ). EXAMPLE 2 A coil of SiC fibers around the carbon tube was produced and treated under the same experimental conditions and the same device as in Example 1 with the exception of the following differences: the fibers of the coil are fibers Hi Nicalon S (electrical non-conductive fibers) 2 consecutive batches are used on the same preform: o Batch 1: 4m1 of TMS, 5 minutes, under 5 MPa of N2 at 1000 ° C, (supercritical conditions) o Batch 2: 4m1 of TMS, 5 minutes, under 1 MPa of N2 at 1000 ° C (sucritical conditions). Between the two batches, it was expected that the reactor would cool down to room temperature, and then the reactor was evacuated before the implementation of the 2nd batch.

The chemical composition of the grains elaborated from TMS in super and subcritical fluid evolves according to the pressure. Batch 1 (high pressure supercritical conditions) makes it possible to promote the nucleation of SIC grains while batch 2 (lower pressure, subcritical conditions) favors that of carbon.

The grains obtained in the macroporosities of the preform following these two batches are a mixture of silicon carbide grains and micron-sized carbon grains (FIG. 4).

Claims (15)

REVENDICATIONS1. Process for filling macropores with a porous fibrous substrate by formation of carbon grains, characterized in that a fluid composition containing tetramethylsilane and a dilution fluid is diffused within the substrate under temperature conditions of between 650 and 1100 C. and under pressure with an initial pressure lower than the critical pressure of the tetramethylsilane and the dilution fluid, advantageously less than or equal to 2 MPa, while maintaining the tetramethylsilane and the dilution fluid in the subcritical state and provoking spontaneously nucleation in homogeneous phase within the macropores of the substrate from tetramethylsilane to directly form the carbon grains. 2. Process according to claim 1, characterized in that the temperature is between 700 and 1100 ° C, advantageously between 750 ° C and 1000 ° C. 3. Method according to any one of claims 1 or 2, characterized in that the initial pressure is between 0.8 and 2 MPa, preferably it is 1MPa. 4. Process according to any one of claims 1 to 3, characterized in that the porous fibrous substrate is formed from carbon or ceramic fibers, preferably carbon fibers. 5. Method according to claim 4, characterized in that the porous fibrous substrate is a carbon fiber preform. 6. Process according to any one of claims 1 to 5, characterized in that the porous fibrous substrate is not electrically conductive. 7. Process according to any one of Claims 1 to 6, characterized in that the substrate on which the process is carried out has an open porosity, measured by helium pycnometry or by a mercury porosimeter, from plus 80%. 8. Method according to any one of claims 1 to 7, characterized in that the dilution fluid is chemically neutral vis-à-vis the formation of carbon grains, and advantageously is selected from the inert gases of the atmosphere . 9. The method of claim 8, characterized in that the dilution fluid is selected from nitrogen and argon, advantageously it is nitrogen. 10. Method according to any one of claims 1 to 9, characterized in that it comprises the steps (a) of introducing a quantity of fluid composition into an enclosure containing the substrate and (b) establishment in the enclosure of the temperature and pressure conditions for causing nucleation in homogeneous phase of carbon grains within the macropores of the substrate while maintaining in the subcritical state tetramethylsilane and the dilution fluid. 11. Method according to any one of claims 1 to 10, characterized in that it comprises a preliminary step (A) or an additional step (B) of diffusion within the substrate of a fluid composition containing tetramethylsilane and a dilution fluid, under conditions of temperature and pressure to maintain the dilution fluid in the supercritical state and spontaneously cause nucleation in homogeneous phase within the macropores of the substrate from tetramethylsilane to directly form carbide grains of silicon. 12. The process as claimed in claim 11, characterized in that the temperature of this preliminary or additional stage is between 600 ° C and 1100 ° C, advantageously between 750 and 1000 ° C. 13. Method according to any one of claims 11 or 12, characterized in that the pressure of this preliminary step or additional is between 4MPa and 6MPa. 14. Method according to any one of claims 11 to 13, characterized in that it comprises an additional step (c) after step (b) or after the possible step (B) RMI treatment of the substrate of way to transform the carbon of grains into carbide. 15. Method according to any one of claims 1 to 9, characterized in that it comprises the steps of continuous admission of a stream of fluid composition in a chamber containing the substrate, continuous extraction of a flow of fluid effluent out of the chamber and maintaining substrate temperature conditions and pressure in the chamber to cause a homogenous phase nucleation of carbon grains within the macropores of the substrate while maintaining in the subcritical state the tetramethylsilane and the dilution fluid.