Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Title of the invention
An active cooling panel of thermostructural composite
material and method for its manufacture
Background of the invention
The present invention relates to an active cooling
panel of thermostructural composite material.
The term "active cooling panel" is used herein to
mean a panel having a cooling fluid passing therethrough
for the purpose of taking away the heat received by the
panel being exposed to high temperature or high heat
flux.
The term "thermostructural composite material" is
used herein to mean a composite material having
mechanical properties which make it suitable for
constituting structural elements and having the ability
to conserve these mechanical properties at high
temperature. Thermostructural composite materials are
typically carbon-carbon (C/C) type composite material
comprising a reinforcing structure made of carbon fibers
densified by a matrix of carbon, and ceramic matrix
composite (CMC) materials comprising a reinforcing
structure of refractory fibers (in particular carbon
fibers or ceramic fibers) densified by a ceramic matrix.
Applications of the invention lie in particular in
constituting the walls of a combustion chamber in an
aircraft engine, which walls convey a cooling fluid which
may be constituted by the fuel that is injected into the
chamber, or walls for the diverging portions of rocket
engines which are likewise cooled by fluid, which fluid
may be a propellant component injected into the
combustion chamber of the rocket engine, or indeed the
walls of a plasma confinement chamber in a nuclear fusion
reactor. In such applications, the panel acts as a heat
exchanger between its face that is exposed to high
temperature or high heat flux and the fluid it conveys.
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In such heat exchanger walls, the use of active
cooling panels made of thermostructural composite
material enables the operation of systems including such
heat exchangers to be extended to higher temperatures
and/or enables the lifetime of such systems to be
extended. Increasing operating temperature can enable
performance to be increased, in particular the efficiency
of combustion chambers or nozzles in aviation or space
engines, and can also reduce the amount of pollution
l0 emitted by aircraft engines.
Making a part out of thermostructural composite
material generally requires a porous fiber preform to be
prepared of a shape that is close to the shape of the
part that is to be made, with the preform then being
densified.
Densification can be performed by a liquid technique
or by a gas technique. Liquid densification consists in
impregnating the preform with a liquid that is a
precursor of the matrix material, which precursor is
generally a resin, and in transforming the precursor,
usually by heat treatment. The gas technique or chemical
vapor infiltration (CVI) consists in placing the preform
in an enclosure and in admitting a reaction gas into the
enclosure, which gas diffuses under determined conditions
of pressure and temperature into the pores of the preform
and forms a solid deposit therein by one or more of the
components of the gas decomposing or reacting together.
Both techniques, using a liquid or CVI are well known and
they can be combined, for example by performing
predensification or consolidation of the preform using a
liquid followed by CVI.
Whatever the densification method used,
thermostructural composite materials present residual
porosity so they are unsuitable for use on their own in
forming cooling panels having internal fluid-conveying
passages, since the walls of such passages are not
leakproof.
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Several solutions have been proposed to overcome
this difficulty and to enable active cooling by means of
a flowing fluid to be combined with the use of porous
refractory materials.
A first solution consists in making a panel having a
front plate made of graphite on its side that is exposed
to high temperatures, and a rear plate made of metal, in
particular steel, with the channels for conveying the
cooling fluid being made therein. The two plates are
assembled together by brazing, with layers of metal being
interposed to match the different coefficients of thermal
expansion of steel and of graphite. The presence of
solid metal is penalizing in terms of the mass of the
cooling panel. In addition, the length of the path along
which heat travels through the graphite plate and the
metal plate puts a limit on capacity to cool the exposed
surf ace .
Another solution consists in forming passages in a
block of thermostructural composite material and in
making the walls of the passages leakproof by brazing a
metal lining, e.g. made of copper.
Yet another solution consists in making two plates
out of thermostructural composite material, one of which
plates presents channels machined in its face that is to
be assembled with a facing face of the other plate, with
assembly being performed by brazing.
The second and third solutions are satisfactory in
terms of mass and of shortening heat flow path length,
but leakage problems can arise due to the metal lining or
the brazing cracking following repeated exposure to very
high temperatures and excess stresses induced by the
shape of the channels.
Object and summary of the invention
In one of its aspects, the invention seeks to
provide an active cooling panel of thermostructural
composite material that presents leaktightness which is
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effective and durable relative to a fluid flowing in
internal passages of the panel.
This object is achieved by a panel of the type
comprising first and second parts of thermostructural
composite material each having an inside face and an
opposite outside face, the parts being assembled together
by bonding their inside faces together, and channels
being formed by indentations formed in the inside face of
at least one of the first and second parts, which panel,
according to the invention, further comprises a sealing
layer bonded to at least one of the first and second
parts and situated at a distance from the assembled-
together inside faces thereof.
Such a panel is remarkable in that sealing is
achieved not at the interface between the parts, i.e. at
the walls of the flow channels, but at a different level
of the panel, remote from said interface.
Thus, the integrity of the sealing layer and its
bond with the thermostructural composite material are not
affected by excess stresses of the kind that are
encountered if the sealing layer is to follow or be
subjected to the indentations of the channels at the
interface between the parts. In addition, it is then
possible to displace the sealing layer further away from
the face of the panel that is exposed in operation to
high temperatures, thereby reducing the thermomechanical
stresses to which the sealing layer is exposed.
In an embodiment of the panel, a sealing layer is
situated within at least one of the first and second
parts, separating the part into two portions between its
inside face and its outside face, the two portions being
bonded together by the sealing layer.
In another embodiment, a sealing layer covers at
least one of the outside faces of the first and second
parts.
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Advantageously, the sealing layer is a thin metal
layer, for example a metal selected from niobium, nickel,
tantalum, molybdenum, tungsten, and rhenium.
When the sealing layer is formed within a part, it
5 is possible to provide for the sealing layer and the
portion situated on the outside of the part provided with
the sealing layer to project around the periphery of the
panel, in particular to facilitate installing a sealing
gasket around the periphery of the panel.
Preferably, the channels are formed in the inside
face of the part whose outside face constitutes the face
of the panel that is to be exposed to high temperatures
while the panel is in use.
The panel may be provided with stiffening ribs which
project from the outside face of the part situated on the
side opposite from the side that is to be exposed to high
temperatures while the panel is in use.
The inside faces of the first and second parts may
be bonded together by brazing.
In a variant, the inside faces may be provided with
metal coatings that are bonded directly to each other.
In another aspect, the invention seeks to provide a
method of manufacturing an active cooling panel as
defined above.
This object is achieved by a method of the type
comprising the steps consisting in providing first and
second parts of thermostructural composite material, each
having an inside face and an outside face opposite to the
inside face, the inside face of at least one of the parts
presenting indentations forming channels, and in
assembling the first and second parts together by bonding
their inside faces together in such a manner as to obtain
a cooling panel made of thermostructural composite
material having circulation channels integrated therein,
in which method, according to the invention, at least one
of the first and second parts is provided with a sealing
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layer situated at a distance from the inside face of the
part.
In a particular implementation of the method, a
sealing layer is integrated within at least one of the
first and second parts between its inside face and its
outside face.
For this purpose, advantageously, at least one of
the first and second parts is made up of two distinct
portions, and the portions are assembled together with
the sealing layer interposed between them.
In another implementation of the method, the outside
face of at least one of the first and second parts is
provided with a sealing layer.
In either case, the sealing layer may be implemented
as a metal foil, e.g. made of a metal selected from
niobium, nickel, tantalum, molybdenum, tungsten, and
rhenium.
The metal foil may be assembled to the composite
material of the first or second part by hot compression,
in particular by hot isostatic pressing.
The inside faces of the first and second parts may
be assembled together by brazing.
In a variant, it is possible to form at least one
metal coating layer on the inside faces of the first and
second parts and to assemble said inside faces together
by hot compression, in particular by hot isostatic
pressing.
Advantageously, prior to assembling together the
inside faces of the first and second parts, treatment is
performed to reduce the surface porosity of the
thermostructural composite material in at least one of
said inside faces of the parts.
The treatment for reducing porosity may comprise
applying a suspension to the surface of at least one of
said inside faces of the part, the suspension comprising
a ceramic powder and a ceramic material precursor in
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solution, the treatment further comprising transforming
the precursor into a ceramic material.
The precursor is typically a polymer which is cross-
linked and transformed into ceramic by heat treatment.
Optionally, after the precursor has been transformed
into ceramic material, chemical vapor deposition or
chemical vapor infiltration is performed.
Brief description of the drawings
The invention will be better understood on reading
the following description given by way of non-limiting
indication and made with reference to the accompanying
drawings, in which:
- Figure 1 is a cross-section view of an embodiment
of an active cooling panel in accordance with the
invention;
- Figure 2 is a fragmentary section view on plane
II-II of Figure 1;
- Figure 3 is a section view on plane III-III of
Figure 2;
- Figures 4 to 8 show successive steps in
implementing a method in accordance with the invention
for manufacturing a panel of the type shown in Figure 1;
and
- Figures 9 to 13 are cross-section views of other
embodiments of an active cooling panel in accordance with
the invention.
Detailed description of embodiments
A first embodiment of an active cooling panel 10 is
shown in Figures 1 to 3.
The panel 10 comprises two parts 20 and 30 that are
generally in the form of rectangular parallelepipeds and
that are assembled to each other via their inside faces
21 and 31. In this example, assembly is performed by
brazing 12. The part 20 whose outside face 22 opposite
from its face 21 defines the front face of the panel that
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is to be exposed to high temperatures or to intense heat
flow is made of a thermostructural composite material.
Channels 24 for circulating a cooling fluid are formed by
indentations formed in the inside face 21. A plurality
of channels 24 parallel to two opposite sides of the
panel 10 extend between two manifolds 40, 42 that are
internal to the panel 10 and that are situated close to
two other opposite sides thereof.
The part 30 comprises two portions 34 and 36 in the
form of plates made of thermostructural composite
material. The portions 34 and 36 are assembled via
facing faces 35, 37 with a sealing layer 38 being
interposed between them. The faces of the portions 34
and 36 that are opposite from their faces 35 and 37
define respectively the inside face 31 and the opposite
outside face 32 of the part 30. The face 32 constitutes
the rear face of the panel 10.
The manifolds 40, 42 are formed by elongate openings
or slots formed in the portion 34. The manifolds 40, 42
communicate with the outside of the panel via holes 41,
43 formed through the sealing layer 38 and the portion
36, and provided with metal inserts 44, 46 enabling the
panel to be connected with a circuit for circulating
fluid and/or with an adjacent panel by means of a
connecting coupling.
In a variant, the channels 24 may each have at least
one end opening out into a side end of the part 20.
After the cooling panel has been made, the open ends of
the channels can then be connected by means of couplings
either to a manifold external to the panel, or else to
similar channels in an adjacent panel.
The part 20 and the part 30 (portions 34 and 36) are
made of a C/C or a CMC thermostructural composite
material. For applications at very high temperature, in
particular in an oxidizing medium, it is preferred to use
CMC, typically comprising composite materials reinforced
by silicon carbide (SiC) fibers or carbon fibers with a
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matrix of SiC or a matrix that has at least an outer
phase of SiC. The channels and the manifolds may be made
by machining.
Whatever the thermostructural composite material
used, it presents residual porosity. The sealing layer
38 makes it possible to prevent any fluid flowing along
the channels 24 from leaking to the rear face 32 of the
panel 10.
In the example shown in Figures 1 to 3, the part 20
is not provided with a sealing layer. This is acceptable
when there is no requirement for a high degree of
leaktightness between the channels 24 and the front face
22 of the panel 10. This can apply for an active cooling
panel for a combustion chamber wall when the cooling
fluid used is a fuel and when a certain amount of leakage
into the combustion chamber can be tolerated.
The sealing layer 38 is a metal layer bonded to the
faces 35, 37 of the portions 34, 36 of the part 30, e.g.
a layer of niobium, nickel, tantalum, molybdenum,
tungsten, or rhenium.
A method of manufacturing a cooling panel of the
kind shown in Figures 1 to 3 is described below with
reference to Figures 4 to 8.
The part 20 and the portions or plates 34, 36 of the
part 30 are made separately out of thermostructural
composite material, in particular C/C or CMC material.
The recesses needed for forming the channels 24 and the
manifolds are formed by machining the inside face 21 of
the part 20 and the portion 34 of the part 30. It should
be observed that the part 20 and the portions 34, 36 may
be cut out from a single block of thermostructural
material prior to machining the locations for the
channels and the manifolds.
The detailed views of Figure 4 show in highly
diagrammatic manner the surface porosity of the
thermostructural composite material.
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Advantageously, treatment is applied to reduce the
porosity of the inside face 21 of the part 20 in which
the channels 24 are formed, and the face 31 of the
portion 34, i.e. those faces that are to be assembled
5 together.
Porosity can be reduced by applying a suspension
onto the faces 21 and 31, the suspension containing a
solid filler in the form of a ceramic powder and a
ceramic precursor in solution, and then transforming the
10 precursor into ceramic material. The precursor may be a
polymer which is cross-linked and then transformed into
ceramic by heat treatment. By way of example, for the
precursor it is possible to use a polycarbosilane (PCS)
or a polytitanocarbosilane (PTCS) as a precursor for SiC,
which precursor is put into solution in a solvent, e.g.
xylene. The ceramic powder contributes to filling in
surface pores effectively. It is possible to use an SiC
powder, fox example.
The liquid composition may be applied using a brush
or a spray gun, with the quantity of solvent being
selected to make application easy and to encourage
penetration of the liquid composition into the surface
pores.
After the Liquid composition has been applied and
has been dried by eliminating the solvent, the precursor
polymer is cross-linked and then transformed into
ceramic. When using PCS, for example, cross-linking can
be performed by raising the temperature to about 350°C,
and ceramization by raising the temperature to about
900°C.
After ceramization, it is optionally possible to
shave the surface of the part in order to restore it to
its initial shape.
Two detail views in Figure 5 show in highly
diagrammatic manner how pores are filled in by the
material 51 comprising the ceramization residue and the
ceramic powder.
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It is also advantageous for pores to be filled in
further by forming a deposit of ceramic, e.g. SiC, by
chemical vapor infiltration or deposition, thus making it
possible to obtain a uniform and continuous coating 52
anchored to the thermostructural composite material.
The ceramic coating 52 obtained by chemical vapor
infiltration or deposition (shown in the detailed views
of Figure 5) may be formed not only on the inside faces
21 and 31, but also on the other faces on the outside of
the part 20, and in particular its outside face 22 and on
other surfaces on the outside of the portion 34.
It should be observed that the method of filling in
pores by depositing a suspension containing a ceramic
powder and a ceramic precursor polymer, and then
transforming the precursor into ceramic, followed by
shaving and then forming a ceramic coating by chemical
vapor infiltration is described in the patent application
in the name of the present Applicant and entitled "A
method of surface-treating a thermostructural composite
material part and its application to brazing
thermostructural composite material parts".
The following step of the method consists in
interposing a sealing layer between the portions 34 and
36, possibly after machining the faces 35 and 37 of the
portions 34 and 36 in order to lay bare the composite
material. The sealing layer is advantageously formed by
a metal foil 38 (Figure 6), e.g. made of a metal selected
from niobium, nickel, tantalum, molybdenum, tungsten, and
rhenium. The thickness of the foil 38 typically lies in
the range 0.05 millimeters (mm) to 0.3 mm.
The portions 34 and 36 are bonded together and to
the foil 38 by hot compression.
This can be done using known methods such as the hot
isostatic pressing (HIP) assembly method or the method of
hot pressing in a press.
Bonding by hot isostatic pressing is performed by
placing the elements for assembly against each other in
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an enclosure while encapsulating the part in a leakproof
cover 45 (Figure 7). Temperature and pressure are then
raised in substantially uniform manner in the enclosure.
Bonding is achieved by metal from the foil 38 diffusing
into the surface pores of the faces 35, 37. The
leakproof cover 45 encapsulating the parts is
constituted, for example, by a metal film such as a film
of niobium, or indeed of nickel, of iron, or of an alloy
thereof. Tooling elements such as plates of graphite 46,
47 may be interposed between the metal film and the
outside surfaces of the portions 34, 36 in order to
prevent the metal of the film 45 becoming embedded in
said surfaces due to the hot isostatic pressing when the
presence of said metal on said surfaces is undesirable.
This may apply in particular to the face 31, depending in
particular on the method used subsequently for bonding it
to the face 21 of the part 20.
Bonding by pressing in a press consists in raising
the temperature of the elements to be assembled together
and in pressing them against one another by exerting
pressure on the faces 31 and 32 in a press.
The pressure used for hot compression bonding lies,
for example, in the range 80 megapascals (MPa) to
120 MPa. The temperature is a function of the nature of
the metal sealing layer used for bonding the parts
together. It is substantially lower than the melting
temperature of the metal of said metal layer, generally
lying in the range 60% to 80% of said melting
temperature.
When the metal sealing layer is made of niobium, the
temperature is selected more particularly to lie in the
range 900°C to 1200°C both for bonding by hot isostatic
pressing and for bonding by pressing in a press.
Once the part 30 has been made, it is assembled to
the part 20, e.g. by brazing. For this purpose, a layer
of brazing 48 is interposed between the reduced-porosity
faces 21 and 31 (Figure 8).
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Brazing together parts made of thermostructural
composite material is, in itself, known. For example, it
is possible to use a brazing material based on silicon of
the type described in the French patent applications
published under the Nos. 2 748 471 and 2 749 787. Other
brazing compositions can be used, in particular
compositions based on silicon or on titanium such as
those sold under the name TiCuSil~ by Wesgo Metals, a
division of the US supplier Morgan Advanced Ceramics.
In a variant, the parts 20 and 30 can be bonded
together by hot compression.
For this purpose, the surfaces 21 and 31 are
initially provided with metal coatings which, in addition
to providing bonding by hot compression, can also perform
a sealing function.
By way of example, each face 21, 31 is provided with
a first layer of a metal that advantageously performs a
barrier function against chemical reaction with the
underlying material and/or a matching function, and a
second metal layer having the ability to bond by hot
compression.
The second layer may be a metal selected from
nickel, copper, iron, or an alloy of at least one of
them. Nickel (Ni) or a nickel alloy present the
advantages of good thermal conductivity, good ability to
bond by hot compression, and a high melting temperature
avoiding passage into the liquid state during bonding by
hot compression.
The first layer may be made of a metal selected from
rhenium, molybdenum, tungsten, and tantalum. When the
thermostructural composite material has an SiC matrix and
fiber reinforcement of carbon or of SiC, and/or when a
coating of SiC has previously been formed thereon,
rhenium presents the advantage of not reacting with SiC.
It also presents good conductivity and it has a high
melting temperature ensuring that it does not pass to the
liquid state during subsequent bonding under hot
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compression. Furthermore, rhenium has a coefficient of
expansion that is intermediate between those of SiC and
Ni and therefore also constitutes a mechanical matching
layer when the second metal layer is constituted at least
in part by Ni.
The first and second metal layers are deposited in
succession. It is possible to use conventional
deposition methods of the physical vapor deposition type
or the plasma sputtering type.
Prior to bonding the parts together by hot
compression, a metal foil may be interposed between the
facing inside faces of the parts, which metal foil is
preferably made of the same material as the second metal
layer of the metal coating formed on the inside surfaces
21, 31.
The parts 20 and 30 are bonded together by hot
compression, possibly after inserting a metal foil.
It is possible to use the hot isostatic pressing
assembly method or the method of pressing in a press as
described above.
When the parts 20 and 30 are bonded together by hot
compression, it is possible to make said bond
simultaneously with the bond between the portions 34, 36
and the sealing layer 38, after forming the metal
coatings on the inside faces 21 and 31.
Figures 9 to 13 illustrate various other embodiments
of an active cooling panel in accordance with the
invention.
Thus, the panel of Figure 9 differs from that of
Figures 1 to 3 in that the portion 36 of the part 30 and
the sealing layer 38 project around the periphery of the
panel.
The panel can then be housed in a frame 54
comprising a base 55 from which there projects a rim 56.
A sealing gasket 58 is disposed in the space defined by
the base 55, the periphery of the panel 30 in the
vicinity of the part 20 and the portion 34, the
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projecting portion 36 and layer 38, and the rim 56. The
gasket 58 serves to contain cooling fluid leaks around
the periphery of the panel.
The panel of Figure ZO differs from that of
5 Figures 1 to 3 in that the part 30 is provided with
stiffeners 60. These are in the form of stiffening ribs
projecting from the outside face 32 of the portion 36 of
the part 30.
The ribs 60 may be made integrally with the portion
10 36.
The ribs 60 give the panel greater ability to
withstand the forces to which it is subjected, preventing
deformation which might damage the bonds between the
portions 34, 36 of the part 30 and between the parts 20
15 and 30.
The panel of Figure 11 differs from that of
Figures 1 to 3 in that not only the part 30, but also the
part 20 is provided with a sealing layer 62 integrated
within the part 20 and at a distance from the interface
between the parts 20 and 30.
The layer 62 may be of the same kind and may be put
into place in the same manner as the sealing layer 38, in
which case the part 30 is likewise made by assembling
together two distinct portions with the layer 62 being
interposed between them.
The panel of Figure 12 differs from that of
Figures 1 to 3 in that the part 30 is a single piece of
thermostructural composite material and the sealing layer
64 is disposed on the outside face 32 of the part 30
instead of being disposed within it.
The layer 64 may be of the same kind as the sealing
layer 38 and it may likewise be assembled to the part 30
by hot compression.
As shown in Figure 13, the part 20 of a panel of the
kind shown in Figure 12 may also be provided with a
sealing layer 66 assembled to its outside face 22.
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The panels of Figures 12 and 13 are easier to make
than those of the other panels. However integrating the
sealing layer within a part, between two portions of
thermostructural composite material, enables said sealing
layer to be protected against oxidation by the presence
of the composite material. In addition, placing the
sealing layer on the outside face of a part can make it
necessary for the sealing layer to be shaped so as to
take account of the possible presence of stiffeners or
interfaces with the outside of the panel.
Naturally, a single panel may be provided with a
sealing layer based on an outside face of one of the two
parts of the panel, and with a sealing layer disposed
within the other part.
It is also possible to place the panels of the
embodiments shown in Figures 10 and 11 in a frame, as
shown for the embodiment of Figure 9.
Example
A part 20 and portions 34, 36 of the kind shown for
the embodiment of Figures 1 to 3 have been made out of
C/SiC thermostructural composite material, with the
channels and the manifolds being formed by machining.
The porosity of the inside surfaces 21, 31 was
reduced by brushing thereon a composition containing an
SiC powder of mean grain size equal to about 9 microns
(um) in a solution of PCS in xylene. After drying in
air, the PCS was cross-linked at about 350°C and then
transformed into SiC by raising the temperature to about
900°C. A thin coating of SiC having thickness equal to
about 100 um was then deposited by chemical vapor
infiltration, said coating then being formed over the
entire outside surface of the part 20 and the portion 34,
and not only on the inside faces 21 and 31. In
combination with the residue of ceramizing the PCS in
association with the SiC powder, the SiC coating
contributes to effective reduction of porosity.
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The faces 35, 37 of the portions 34, 36 were then
machined in order to lay bare the composite material so
as to present open pores, encouraging mechanical bonding
with the foil subsequently put into place between these
faces. A 0.1 mm thick niobium foil was interposed
between the faces 35 and 37, and assembly was then
performed by hot isostatic pressing. For this purpose,
the elements 34, 38, and 37 were encapsulated in a 0.5 mm
thick niobium foil with plates of graphite being
interposed between the outside surfaces of the elements
to be assembled together and the niobium foil.
Hot isostatic pressing was performed at a pressure
of about 90 MPa and at a temperature of about 1000°C.
The part 30 as obtained in this way was assembled to
the part 20 by brazing using a silicon-based brazing
composition.
