Heating – Heating or heat retaining work chamber structure
Reexamination Certificate
2002-05-06
2003-06-03
Wilson, Gregory (Department: 3749)
Heating
Heating or heat retaining work chamber structure
C432S241000, C118S725000, C118S724000
Reexamination Certificate
active
06572371
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to densification of porous annular substrates by chemical vapor infiltration (CVI).
A particular field of application of the invention is the making of annular parts in a thermostructural composite material, such as carbon/carbon (C/C) composite brake discs for airplanes or land vehicles.
Thermostructural composite materials are remarkable because they possess mechanical properties that enable them to be used for making structural parts and have the ability to conserve these properties at high temperatures. Typical examples of thermostructural composite materials are C/C composite materials having a reinforcing fibrous texture of carbon fibers densified by a pyrolytic carbon matrix, and ceramic matrix composites (CMCs) having a reinforcing texture of refractory fibers (carbon or ceramic) densified by a ceramic matrix.
In a CVI process, substrates to be densified are placed in a reaction chamber of a furnace in which they are heated. A reactive gas containing one or more gaseous precursors of the material that is to constitute the matrix is introduced into the reaction chamber. The temperature and pressure inside the reaction chamber are adjusted to enable the reactive gas to diffuse within the pores of the substrate and deposit the matrix-constituting material therein by one or more components of the reactive gas decomposing or reacting together. The process is performed under low pressure in order to enhance diffusion of the reactive gas into the substrates. The temperature at which the precursor(s) is transformed to form the matrix material, such as pyrolytic carbon or ceramic, is usually greater than 900° C., and is typically close to 1000° C.
In order to enable substrates throughout the reaction chamber to be densified as uniformly as possible, whether in terms of increasing density or in terms of microstructure of the matrix material deposited, it would ideally be necessary to have a substantially uniform temperature within the reaction chamber and to allow the reactive gas to reach all substrates relatively uniformly.
CVI furnaces usually include a gas preheater situated inside the furnace between the reactive gas inlet into the furnace and the reaction chamber. Typically, a gas preheater zone comprises a heat exchange assembly in the form of a plurality of perforated plates through which the reactive gas passes before entering the reaction chamber.
The substrates, like the heat-exchange assembly, are heated because they are located in the furnace. The latter is generally heated by means of a susceptor, e.g. made of graphite. The susceptor defines the side of the wall of the reaction chamber and is heated by inductive coupling with an inductor surrounding the reaction chamber or by resistors surrounding the furnace.
The applicants have found that the efficiency of the gas preheater is not always as good as desired. A significant example is that of densifying porous substrates constituted by annular preforms of carbon fibers or pre-densified annular blanks for use in making C/C composite brake disks.
The annular substrates are loaded in vertical stacks in the reaction chamber above the gas preheater which is situated at the bottom of the furnace. In spite of the reactive gas being preheated, a temperature gradient is often observed in the reaction chamber, with the temperature close to substrates situated at the bottom of the stacks possible being several tens of ° C. lower than the temperature that applies in the remainder of the stacks. This may give rise to a large densification gradient between the substrates in a same stack, depending on the position of a substrate within the stack.
In order to solve that problem, it would be possible to increase the efficiency with which the reactive gas is preheated by increasing the size of the gas preheater. However, for a given volume of the furnace, that would reduce the loading capacity for the substrates. Since CVI processes require large amounts of industrial investment and long processing time, it is highly desirable for furnaces to have the highest possible productivity, and thus as high as possible a ratio of volume dedicated to the load of substrates over the volume dedicated to preheating the reactive gas.
Another problem resides in the fact that a temperature gradient is observed not only in the vertical direction, along the stacks of substrates, but also in the horizontal direction, between different stacks. In particular, it has been noted that stacks located in a central part of the reaction chamber may not benefit from the heat radiated by the susceptor in the same way as stacks located closer to the internal side wall of the susceptor.
This also results in a gradient of densification between substrates belonging to different stacks.
OBJECTS AND SUMMARY OF THE INVENTION
An object of the invention is generally to provide means for achieving an efficient and cost effective substantially uniform densification of porous annular substrates in a CVI furnace.
A particular object of the invention is to provide a gas preheater which allows such a substantially uniform densification to be achieved without significantly affecting the productivity of the CVI substrate.
According to one aspect of the invention, in a CVI furnace for the densification of annular porous substrates arranged in a plurality of vertical annular stacks of substrates, comprising a susceptor having an internal side wall delimiting a gas preheating zone and a reaction chamber within the furnace and a bottom wall, and at least one gas inlet opening through the bottom wall of the susceptor, a gas preheater is provided which comprises:
a sleeve made of heat conductive material resting upon the susceptor bottom wall and delimiting a gas preheating chamber, with the at least one gas inlet opening in the gas preheating chamber,
a heat exchange assembly located in the gas preheating chamber,
a gas distribution plate resting upon the sleeve, covering the gas preheating chamber and provided with a plurality of passages for pre-heated gas,
a load supporting plate for supporting stacks of annular substrates to be loaded in the reaction chamber for densification and provided with a plurality of passages in communication with respective passages of the gas distribution plate and in registration with internal volumes of respective stacks of annular substrates, and
nozzles inserted in passages communicating the gas preheating zone with the internal volumes of respective stacks of annular substrates for adjusting the flows of preheated gas respectively admitted in said internal volumes.
The sleeve, which is preferably formed of a massive body made in one piece of heat conductive material, achieves different functions:
resting upon the susceptor bottom wall and being thus surrounded by the susceptor side wall, it enables an efficient heating of the preheating zone to be reached,
it encloses the preheating zone and contributes to the sealing thereof, avoiding a large fraction of the reactive gas admitted to reach the reaction chamber without having fully passed through the gas preheater, and
it supports the load of substrates through the gas distribution plate and load supporting plate and transfers the weight to the susceptor bottom wall without the need for a separate supporting structure for the load supporting plate.
The above contributes to the efficiency of the gas preheating and compactness of the structure located at the bottom of the furnace.
The provision of flow adjusting nozzles which may be inserted in the passages of the gas distribution plate, makes it possible to feed stacks of substrates with a larger flow of reactive gas compared to other stacks of substrates. It is thus possible to compensate for a gradient of temperature between different stacks of substrates in order to achieve a substantially uniform densification. Indeed, the deposition rate of the matrix material varies as a function of the temperature and of the flow of reactive gas.
According to a particular aspect of the invention, the heat excha
Baudry Yvan
Sion Eric
Messier-Bugatti
Weingarten Schurgin, Gagnebin & Lebovici LLP
Wilson Gregory
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