FR3114329A1 - Process for treating a residual gaseous phase resulting from a CVI technique - Google Patents

Abstract

Process for treating a residual gaseous phase resulting from a CVI technique The invention relates to the treatment of a residual gas phase comprising radical chlorosilanes resulting from a process of chemical vapor infiltration of silicon carbide, in which the residual gas phase is diluted in a pumping line so as to avoid the deposit of pyrophoric species. Figure for abstract: Fig. 1.

Description

Process for treating a residual gaseous phase resulting from a CVI technique

The invention relates to the treatment of a residual gaseous phase resulting from a process of chemical infiltration in the vapor phase of silicon carbide, for which the quantity of pyrophoric deposits downstream of the reaction chamber is reduced, or even eliminated, as well as an associated facility.

Ceramic Matrix Composites (CMC) are of interest as materials operating at high temperatures (1300°C) under high stresses (rotating parts of aircraft engines for example). Their ability to damage themselves, necessary for their use in this type of environment, is given to them by their composite architecture. Their manufacture includes in particular the infiltration of a fibrous preform in the vapor phase (“Chemical Vapor Infiltration”; “CVI”).

The material mainly deposited in this way is silicon carbide SiC. It can be deposited from the gaseous precursor methyltrichlorosilane CH ₃ SiCl ₃ . It is a chlorinated precursor, which decomposes in the hot zone of densification furnaces before forming the SiC ceramic via recombinations of effective precursors. However, the reaction never displays a 100% yield, in particular when it is carried out at low pressure (<20 mbar) to favor the infiltration of the parts. Transformed reactive species can therefore leave the hot zone to recondense in the cold parts of the pumping pipes to form a pyrophoric polymer which ignites spontaneously in air or under the action of very slight friction. This pyrophore can pose a problem in terms of maintenance of the pumping network.

It is therefore desirable to reduce, or even avoid, the formation of pyrophoric species at the outlet of the reaction chamber.

The present invention relates to a process for treating a residual gas phase comprising radical chlorosilanes resulting from a process of chemical infiltration in the vapor phase of silicon carbide having taken place in a reaction chamber, the residual gas phase being entrained at the outside the reaction chamber by a pumping device,
characterized in that the residual gaseous phase passes through, at the outlet of the reaction enclosure, a pumping line into which a diluent gas is introduced so as to mix with the residual gaseous phase and thus form a gaseous mixture, and in which the mixture gas is maintained at a temperature greater than or equal to 100°C.

Chemical vapor infiltration of silicon carbide uses low pressure to achieve densification of porous structures. This low pressure leads to a certain quantity of intermediate species in the residual gas phase, in this case radical chlorosilanes, which have not reacted. These radical chlorosilanes can form a pyrophoric polymer (Cl-Si-H) _n when cold. In order to reduce or avoid the formation of this pyrophoric polymer, the radical chlorosilanes are diluted by the diluent gas to keep them away from each other and avoid their polymerization, and to maintain them at a sufficient temperature as soon as they leave the reaction chamber. avoids the uncontrolled condensation of radical chlorosilanes on the wall of the pumping pipe.

In an exemplary embodiment, the gas mixture passes through a removable condenser positioned upstream of the pumping device on which chlorinated species are deposited from the radical chlorosilanes present in the gas mixture.

In the text, the expressions "upstream" and "downstream" refer to the direction of flow of the residual gaseous phase or of the gaseous mixture.

The condenser is at a temperature low enough to allow the condensation of chlorinated species from the radical chlorosilanes of the gas mixture. The chlorinated species are thus grouped together at the level of the condenser and do not reach the pumping device. The condenser constitutes a removable part of the rest of the system, thus allowing an easy and complete washing of these species. The presence of the condenser is however not mandatory. Indeed, in the case where the pump operates in a liquid medium (eg: water ring pump), the chlorosilanes are dissolved, without forming pyrophoric polymers.

In an exemplary embodiment, the volume fraction of diluent gas in the gas mixture is greater than or equal to 50%.

Such a feature makes it possible to further reduce the risk of formation of pyrophoric species downstream of the reaction chamber.

In an exemplary embodiment, the diluent gas comprises dinitrogen.

The invention also relates to an installation for the treatment of a residual gaseous phase comprising radical chlorosilanes, comprising at least:
- a silicon carbide chemical vapor infiltration furnace comprising a reaction chamber in which a silicon carbide chemical vapor infiltration process is intended to take place,
- a pumping line in communication with one of the reaction enclosure provided with a channel for introducing a diluent gas and a heating element configured to maintain the inside of the pumping line at a higher temperature or equal to 100°C, and
- A pumping device in communication with the reaction chamber and the pumping pipe.

The installation is suitable for implementing the process described above.

In an exemplary embodiment, the installation further comprises a removable condenser positioned upstream of the pumping device and in communication with the pumping pipe, said removable condenser being able to allow the deposition of chlorinated species from radical chlorosilanes.

FIG. 1 is a schematic and partial representation of the treatment of the residual gas phase within the framework of an example of a process according to the invention.

FIG. 2 illustrates a succession of possible reactions in the reaction chamber.

Figure 3 illustrates the dilution of radical chlorosilanes.

Figure 4 illustrates the deactivation of the condensation of chlorinated species in the pump line.

Figure 5 illustrates the deposition of chlorine species in the condenser.

FIG. 6 illustrates the washing of deposits of chlorinated species.

There is shown in Figure 1 a furnace 1 for the chemical vapor infiltration of silicon carbide. The furnace 1 comprises a reaction chamber 3 delimited by a wall which defines a useful zone intended to receive porous structures (not shown) in the porosity of which silicon carbide is intended to be deposited from a gaseous phase by technique of chemical vapor infiltration. The porous structures can be fibrous preforms comprising ceramic and/or carbon yarns. The fibrous preforms can be woven, for example be obtained by three-dimensional weaving, in particular with an interlock weave. The fibrous preforms are intended to form the fibrous reinforcement of a part made of composite material with a ceramic matrix to be obtained. The yarns of the fiber preforms treated with the gas phase can be coated with an interphase, for example of boron nitride or pyrolytic carbon. The furnace 1 comprises a heating member (not shown) capable of heating the reaction chamber 3. The heating member may comprise a susceptor coupled with an inductor, the heating of the reaction chamber 3 being ensured by radiation from the heated susceptor by inductive coupling with the inductor. The gaseous phase comprises a silicon carbide precursor in gaseous form, in particular methyltrichlorosilane (MTS, CH ₃ SiCl ₃ ). A pumping device P ensures circulation of the gaseous phase in the furnace and maintains the desired reduced pressure inside the latter. The gaseous phase is sucked through the reaction chamber 3 by the pumping device P. The system for injecting the gaseous phase into the furnace, the useful zone and the pumping device correspond to elements known per se, need not be detailed further here.

By way of illustration, the temperature imposed in the useful zone can be between 900° C. and 1200° C., and the pressure imposed in the useful zone can be between 1 mbar and 100 mbar.

In the useful zone, the silicon carbide precursor forms reaction intermediates resulting in the formation of silicon carbide inside the pores of the treated porous structures. In particular, the precursor forms radical chlorosilanes, for example SiCl ₃ ^● radicals. FIG. 2 provides, by way of illustration, the succession of reactions producing the silicon carbide from an MTS precursor in the useful zone.

The residual gaseous phase GR having passed through the reaction chamber 3 is evacuated from the latter through an outlet 5 and then enters directly into a pumping pipe 8 extending from the outlet 5. The pumping pipe 8 which is located outside the reaction chamber 3 is maintained at a temperature, at a temperature sufficient to prevent the condensation of radical chlorosilanes on the wall. This temperature may be greater than or equal to 100°C, for example between 100°C and 350°C. In particular, the temperature of the pumping line 8 is lower than that imposed in the reaction chamber 3. The pumping line 8 is maintained at temperature by a heating element 10, for example a resistive heating element. Heating element 10 may be in the form of a heating pad, as shown. As illustrated, the heating element 10 can extend over substantially the entire length of the pumping line 8 and all around it. In particular, the heating element 10 can extend from the outlet 5 of the reaction chamber 3, as illustrated.

The pumping line 8 is provided with an introduction channel 12 through which a diluent gas GD is introduced into it. The introduction channel 12 is in communication with a source of diluent gas GD (not shown), for example with a tank containing the diluent gas GD. The introduction channel 12 can be spaced from the outlet 5 of the reaction enclosure 3 by a distance d less than or equal to 5 meters, for example between 0.5 meter and 1 meter. The diluent gas GD mixes with the residual gaseous phase GR, this mixing taking place at a temperature between 100°C and 350°C. The diluent gas GD mixes with the residual gaseous phase GR outside the reaction enclosure 3. The diluent gas GD does not penetrate into the reaction enclosure 3 and in particular does not come into contact with the porous structures densified in the reaction enclosure 3. The diluent gas GD can comprise at least one of dinitrogen or argon. The diluent gas GD can be inert with respect to the residual gaseous phase GR. As indicated above, the diluent gas GD makes it possible to space out the radical chlorosilanes in order to avoid their polymerization and the formation of a pyrophoric polymer. The gaseous mixture M obtained after mixing with the diluent gas GD may comprise a volume fraction of diluent gas GD greater than or equal to 50%, the remainder possibly being formed by the residual gaseous phase GR leaving the reaction chamber 3. The case of A dilution of radical chlorosilanes with dinitrogen is shown in Figure 3.

The gas mixture M is maintained at a temperature greater than or equal to 100° C. in line 8, for example between 100° C. and 350° C. The temperature of the mixture M in the pump line 8 can be substantially constant. Maintaining the temperature of the gas mixture M makes it possible to avoid the condensation of chlorinated species, of formula SiClx as illustrated in figure 4.

A condenser 20 is downstream of the reaction chamber 3 and of the pumping pipe 8. The condenser 20 is upstream of the pumping device P. In the example illustrated, before being sucked in by the pumping device P, the gaseous mixture M flows to a cooled condenser 20, providing surface area to promote the condensation of the chlorinated species SiClx. The condenser 20 can be at a temperature lower than or equal to 30°C. The forced condensation in a condenser is illustrated in figure 5 where the radical chlorosilanes form SiClx deposits.

The condenser 20 is designed for easy disassembly and cleaning. The condenser 20 is separable from the pipe 8 and from the pumping device P. The cleaning of the condensed chlorinated species can be carried out with water, or with solvents which are easier to dry such as ethanol, or even anhydrous such as acetone. . The fact that the condenser is removable ensures effective and total cleaning of the condensed species, the condenser can for example be immersed in the cleaning liquid to carry out a complete hydrolysis of the deposits before a new use. The washing of deposits is illustrated in Figure 6.

We have just described an example where a condenser 20 is present between the pipe 8 and the pumping device P but it does not depart from the scope of the invention if the condenser is omitted, the pumping pipe extending to the device pumping device P. The pumping device P can in this case operate in a liquid medium, for example be a liquid ring pump.

The expression “between … and …” must be understood as including the limits.

Claims (6)

Process for treating a residual gaseous phase (GR) comprising radical chlorosilanes resulting from a process of chemical infiltration in the vapor phase of silicon carbide having taken place in a reaction chamber (3), the residual gaseous phase being entrained in the outside of the reaction enclosure by a pumping device (P),
characterized in that the residual gaseous phase passes through, at the outlet of the reaction chamber, a pumping line (8) into which a diluent gas (GD) is introduced so as to mix with the residual gaseous phase and thus form a gaseous mixture (M), and in which the gaseous mixture is maintained at a temperature greater than or equal to 100°C. Process according to Claim 1, in which the gaseous mixture (M) passes through a removable condenser (20) positioned upstream of the pumping device (P) on which chlorinated species are deposited from the radical chlorosilanes present in the gaseous mixture. Process according to Claim 1 or 2, in which the volume fraction of diluent gas (GD) in the gas mixture (M) is greater than or equal to 50%. A method according to any of claims 1 to 3, wherein the diluent gas (GD) comprises dinitrogen. Installation for the treatment of a residual gas phase (GR) comprising radical chlorosilanes, comprising at least:
- a silicon carbide chemical vapor infiltration furnace (1) comprising a reaction chamber (3) in which a silicon carbide chemical vapor infiltration process is intended to take place,
- a pumping line (8) in communication with an outlet of the reaction chamber provided with a channel (12) for introducing a diluent gas (GD) and a heating element (10) configured to maintain inside the pumping line at a temperature greater than or equal to 100°C, and
- a pumping device (P) in communication with the reaction chamber and the pumping pipe. Installation according to claim 5, further comprising a removable condenser (20) positioned upstream of the pumping device (P) and in communication with the pumping pipe (8), said removable condenser being able to allow the deposition of chlorinated species from radical chlorosilanes.