U.S. patent application number 10/852612 was filed with the patent office on 2004-10-28 for thermostructural composite material bowl.
Invention is credited to Coupe, Dominique, Georges, Jean-Michel, Guirman, Jean-Michel.
Application Number | 20040211354 10/852612 |
Document ID | / |
Family ID | 26212578 |
Filed Date | 2004-10-28 |
United States Patent
Application |
20040211354 |
Kind Code |
A1 |
Guirman, Jean-Michel ; et
al. |
October 28, 2004 |
Thermostructural composite material bowl
Abstract
A one-piece bowl of thermostructural composite material
comprising fiber reinforcement densified by a matrix. The bowl is
made by supplying deformable fiber in plies that are whole, being
free from slots or cutouts, superposing said plies on a former of
shape corresponding to the bowl to be made, deforming the plies,
and bonding the superposed plies to one another by means of fibers
extending transversely relative to the plies, e.g. by needling so
as to obtain a bowl preform which is then densified. The bowl can
be used as a support for a crucible in an installation for
producing monocrystalline silicon.
Inventors: |
Guirman, Jean-Michel;
(Begles, FR) ; Coupe, Dominique; (Le Haillan,
FR) ; Georges, Jean-Michel; (Blanquefort,
FR) |
Correspondence
Address: |
WEINGARTEN, SCHURGIN, GAGNEBIN & LEBOVICI LLP
TEN POST OFFICE SQUARE
BOSTON
MA
02109
US
|
Family ID: |
26212578 |
Appl. No.: |
10/852612 |
Filed: |
May 24, 2004 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10852612 |
May 24, 2004 |
|
|
|
09889862 |
Jul 23, 2001 |
|
|
|
09889862 |
Jul 23, 2001 |
|
|
|
PCT/FR00/03276 |
Nov 24, 2000 |
|
|
|
Current U.S.
Class: |
117/13 ; 117/900;
428/113; 428/293.4; 428/34.1; 428/34.6; 428/408 |
Current CPC
Class: |
D04H 3/002 20130101;
C23C 16/045 20130101; D04H 18/02 20130101; D04H 3/07 20130101; C04B
2235/5256 20130101; Y10T 428/24124 20150115; Y10T 428/249928
20150401; C04B 2235/5268 20130101; Y10T 428/30 20150115; C30B 15/10
20130101; D04H 3/04 20130101; C04B 35/83 20130101; C04B 2235/483
20130101; Y10T 428/13 20150115; C04B 2235/612 20130101; C04B
2235/616 20130101; C04B 2235/94 20130101; C30B 35/002 20130101;
D04H 1/498 20130101; Y10T 156/1062 20150115; D04H 3/105 20130101;
C04B 2235/614 20130101; D04H 3/115 20130101; D04H 3/12 20130101;
Y10T 428/1317 20150115; D04H 13/00 20130101 |
Class at
Publication: |
117/013 ;
117/900; 428/034.1; 428/034.6; 428/113; 428/293.4; 428/408 |
International
Class: |
C30B 015/10; C30B
035/00; B32B 001/00; B32B 005/12 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 24, 1999 |
FR |
99 14766 |
Aug 11, 2000 |
FR |
00 10564 |
Claims
1. A bowl of thermostructural composite material formed by fiber
reinforcement densified by a matrix, in which the fiber
reinforcement comprises superposed two-dimensional fiber plies, the
bowl being characterized in that the fiber plies are bonded
together by fibers extending transversely relative to the
plies.
2. A bowl according to claim 1, characterized in that it is a
one-piece bowl and has two-dimensional reinforcing plies that are
whole, without cutouts or slots.
3. A bowl according to claim 1, characterized in that the fiber
plies are formed of unidirectional sheets superposed in different
directions.
4. A bowl according to claim 3, characterized in that the fiber
plies are made of carbon fibers.
5. A bowl according to claim 4, characterized in that the matrix is
formed at least in part out of pyrolytic carbon.
6. A bowl according to claim 4, characterized in that the matrix is
made at least in part out of ceramic.
7. A bowl according to claim 6, characterized in that the matrix is
made at least in part out of silicon carbide.
8. A bowl according to claim 1, characterized in that at least its
inside face is coated in a layer of pyrolytic carbon.
9. A bowl according to claim 1, characterized in that at least its
inside face is coated in a layer of silicon carbide.
10. The use of a bowl according to claim 1 for supporting a
crucible in an installation for producing monocyrstalline silicon
ingots, the use being characterized in that a protective layer is
interposed between the bowl and the crucible.
11. A bowl according to claim 10, characterized in that a
protective layer of thermostructural composite material is
used.
12. A bowl according to claim 2, characterized in that the fiber
plies are formed of unidirectional sheets superposed in different
directions.
13. A bowl according to claim 5, characterized in that the matrix
is made at least in part out of ceramic.
14. A bowl according to claim 7, characterized in that at least its
inside face is coated in a layer of pyrolytic carbon or silicon
carbide.
15. The use of a bowl according to claim 14 for supporting a
crucible in an installation for producing monocyrstalline silicon
ingots, the use being characterized in that a protective layer is
interposed between the bowl and the crucible, and characterized in
that a protective layer of thermostructural composite material is
used.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application under
.sctn.1.53(b) of prior application Ser. No. 09/889,862 filed Jul.
23, 2001; which was a .sctn.371 filing of PCT/FR00/03276 filed Nov.
24, 2000, entitled: METHOD FOR MAKING A BOWL IN THERMOSTRUCTURAL
COMPOSITE MATERIAL, RESULTING BOWL AND USE OF SAME AS CRUCIBLE
SUPPORT; which claimed priority under 35 USC .sctn.119(a)-(d) to
French Application No. 99 14766 filed Nov. 24, 1999 and to French
Application No. 00 10564 filed Aug. 11, 2000.
FIELD OF THE INVENTION
[0002] The invention relates to manufacturing hollow parts out of
thermostructural composite material, more particularly parts having
a deep stamped shape that cannot be developed, that is not
necessarily axially symmetrical, with an end wall portion and a
side wall portion interconnected by portions in which the radius of
curvature can be relatively small. For convenience, such parts are
referred to throughout the remainder of the description and in the
claims under the generic term of "bowls". A field of application of
the invention is, for example, manufacturing bowls for receiving
crucibles containing molten metal, such as silicon, in particular
for drawing ingots of silicon, or of other metals in other
metallurgical fields.
[0003] The term "thermostructural composite material" is used to
mean a material comprising fiber reinforcement made of refractory
fibers, e.g. carbon fibers or ceramic fibers, and densified by a
refractory matrix, e.g. of carbon or of ceramics. Carbon/carbon
(C/C) composite materials and ceramic matrix composite (CMC)
materials are examples of thermostructural composite materials.
BACKGROUND OF THE INVENTION
[0004] A well-known method of producing silicon single crystals in
particular for manufacturing semiconductor products consists in
melting silicon in a receptacle, in putting a crystal germ having a
desired crystal arrangement into contact with the bath of liquid
silicon, so as to initiate solidification of the silicon contained
in the crucible with the desired crystal arrangement, and in
mechanically withdrawing an ingot of single crystal silicon
obtained in this way from the crucible. This method is known as the
Czochralski method or as the "CZ" method.
[0005] The receptacle containing the molten silicon is frequently a
crucible made of silica or quartz (SiO.sub.2) placed in a bowl,
sometimes called a susceptor, which is generally made of graphite.
Heating can be provided by radiation from an electrically
conductive cylindrical body made of graphite, e.g. heated by the
Joule effect, which surrounds the bowl. The bottom of the bowl
stands on a support. For this purpose, the bottom of the bowl is
generally machined, in particular so as to form a bearing surface
for centering purposes and also a support zone. In addition, in the
application in question, very high purity requirements make it
necessary to use raw materials that are pure, with methods that do
not pollute them, and/or with methods of purification in the
finished state or in an intermediate state of bowl manufacture. For
carbon-containing materials (such as graphite or C/C composites),
methods of purification by high temperature treatment (at more than
2000.degree. C.) under an atmosphere that is inert or reactive
(e.g. a halogen) are known and are commonly used.
[0006] The pieces of graphite used as bowls are fragile. They are
often made up of as a plurality of portions (so-called "petal"
architecture) and they cannot retain molten silicon in the event of
the crucible made of silica leaking or rupturing. This safety
problem becomes more critical with the increasing size of the
silicon ingots that are drawn, and thus with the increasing mass of
the liquid silicon. Furthermore, graphite bowls are generally of
short lifetime while being very thick and thus bulky and heavy.
[0007] To avoid these drawbacks, proposals have already been made
to make bowls out of C/C composite material. Such a material has
much better mechanical strength than graphite. Making bowls of
large diameter, e.g. as great as or even more than 850 millimeters
(mm) can then be envisaged in order to deal with the requirement
for monocrystalline silicon ingots of larger section. In addition,
the thickness of such bowls can be decreased compared with the
thickness of graphite bowls, thus improving the transmission of
heat flux to the crucible and reducing bulk. Furthermore, C/C
composite materials are less exposed than graphite to becoming
brittle following corrosion from SiO coming from the crucible.
[0008] The manufacture of a C/C composite material piece, or more
generally a piece of thermostructural composite material, usually
comprises making a fiber preform having the same shape as the piece
that is to be made, and that constitutes the fiber reinforcement of
the composite material, and then densifying the preform with the
matrix.
[0009] Techniques presently in use for making preforms include
winding yarns by coiling yarns on a mandrel having a shape that
corresponds to the shape of the preform that is to be made, draping
which consists in superposing layers or plies of two-dimensional
fiber fabric on a former matching the shape of the preform to be
made, the superposed plies optionally being bonded together by
needling or by stitching, or indeed by three-dimensional weaving or
knitting.
[0010] The preform can be densified in well-known manner using a
liquid process, a gas process, or a dual process combining both of
them. Liquid process densification consists in impregnating the
preform--or in pre-impregnating the yarns or plies making it
up--with a matrix precursor, e.g. a carbon or ceramic precursor
resin, and in transforming the precursor by heat treatment. Gas
densification, known as chemical vapor infiltration, consists in
placing the preform in an enclosure and in admitting a
matrix-precursor gas into the enclosure. Conditions, in particular
temperature and pressure conditions, are adjusted so as to enable
the gas to diffuse into the core of the pores of the preform, so
that on coming into contact with the fibers it forms a deposit of
matrix-constituting material thereon by one of the components of
the gas decomposing or by a reaction taking place between a
plurality of components of the gas.
[0011] For pieces that are of hollow shape that cannot be
developed, for example pieces that are bowl shaped, a particular
difficulty lies in making a fiber preform having the right
shape.
[0012] The filament-winding technique is very difficult to
implement in order to obtain a bowl shape as a single piece. A
solution that can be recommended is to make the side wall of the
bowl preform by winding a filament and to make the portion of the
preform that corresponds to the bottom of the bowl separately.
[0013] The technique of draping plies is also difficult to
implement for shapes that are this complex when it is desired to
avoid forming extra thickness due to folds in the plies. A known
solution consists in cutting the plies, in particular to form
slots, as a function of the shape that is to be made so that the
plies can fit closely on this shape with the lips of the cutouts or
slots coming together once draped and shaped. Such plies must be
precut with very great precision. Cut plies also present the
drawback of leaving discontinuities in the yarns of the
preform.
OBJECT AND SUMMARY OF THE INVENTION
[0014] In one of its aspects, an object of the invention is to
propose a method of manufacturing a bowl out of thermostructural
composite material making it possible to avoid the drawbacks of the
prior art, while remaining simple and low in cost.
[0015] According to the invention, the method comprises the steps
which consist in:
[0016] providing deformable two-dimensional fiber plies;
[0017] superposing the plies while deforming them on a former
having a shape that corresponds to the shape of the bowl to be
made, the plies fitting closely on said former by deforming and
without forming folds; and
[0018] bonding together the deformed plies by means of fibers that
extend transversely relative to the plies, so as to obtain a bowl
preform which is subsequently densified.
[0019] The invention is remarkable in that the bowl preform can be
made of unitary plies that do not include any slots for enabling
them to fit closely to the desired shape. This contributes to
providing better mechanical and cohesive behavior to the bowl
obtained by densifying the preform, and to providing a high level
of safety in the event of the crucible breaking, in the context of
the application to drawing silicon ingots.
[0020] The plies are made of deformable fiber fabric.
Advantageously, a fabric is used made up of a plurality of
unidirectional sheets superposed in different directions, for
example two unidirectional sheets superposed with directions at an
angle preferably lying in the range 45.degree. to 60.degree.
relative to each other, the sheets being bonded together so as to
form deformable individual mesh loops. The sheets can be bonded
together by needling or by knitted thread or by stitching. Entire
plies are cut out to the desired dimensions in the deformable
fabric. Plies are thus obtained having a capacity for deformation
which is sufficient to enable them to take up the desired shape
merely by deforming, without forming folds or increases in
thickness.
[0021] Advantageously, the deformed plies are bonded to one another
by needling, so as to transfer fibers taken from the plies by the
needles in a direction that extends transversely relative to the
plies. Each new draped ply can be needled onto the underlying
structure, advantageously while controlling the density of
transferred fibers throughout the thickness of the preform.
[0022] In a variant, the deformed sheets can be bonded together by
stitching or by implanting threads.
[0023] In another implementation of the method of the invention,
the deformable fiber fabric constituting the plies is a knit.
[0024] The fibers constituting the plies are preferably made of
carbon or of a carbon precursor. If they are made of precursor,
heat treatment is performed after the preform has been made so as
to transform the precursor into carbon.
[0025] After the preform has been made, it can be subjected to a
step of consolidation by a liquid method, and to heat treatment for
stabilizing the fibers and for purification which can be performed
after or prior to consolidation.
[0026] The optionally consolidated preform is preferably densified
by chemical gas infiltration.
[0027] In a preferred implementation, deformable two-dimensional
fiber plies are used that are whole, having no cutouts or slots, so
as to obtain a complete bowl preform as a single piece, and
densification is performed on the complete bowl preform. A
one-piece bowl of thermostructural composite material can thus be
obtained directly.
[0028] In another implementation, one-piece deformable
two-dimensional fiber plies are likewise used that are free from
any cutouts or slots so as to obtain a complete bowl preform, but a
hole is made through the bottom of the preform prior to
densification by chemical vapor infiltration. The presence of this
hole enhances flow of the gas, thereby making it possible to
increase densification efficiency, particularly with bowls of large
dimensions. After the preform has been densified, at least in part,
the hole is closed with a plug. It is possible to use a plug made
of thermostructural composite material. After the hole has been
closed by the plug, a final step of densification by chemical vapor
infiltration can be performed.
[0029] In yet another implementation, one-piece two-dimensional
fiber plies are used that present a substantially central opening,
and the plies are superposed on the former with their openings in
alignment so as to obtain a bowl preform presenting a hole passing
through the bottom of the preform and constituted by the aligned
openings in the plies. After the preform has been densified at
least in part by chemical vapor infiltration, the hole is closed by
a plug. As mentioned above, the bowl can be made of a
thermostructural composite material and a final step of chemical
vapor infiltration can be performed.
[0030] In any event, once the preform has been densified, a bowl
blank is obtained.
[0031] Final purification heat treatment can then optionally be
performed.
[0032] In addition, a final deposition of pyrolytic carbon and/or
of silicon carbide (SiC) can be performed, at least on the inside
face of the bowl.
[0033] In another aspect, the invention also provides a bowl of
thermostructural composite material of the kind that can be
obtained by the above-defined method.
[0034] In the invention, such a bowl is characterized in that it
comprises fiber reinforcement comprising two-dimensional fiber
plies which are superposed and bonded together by fibers extending
transversely relative to the plies.
[0035] Advantageously, the fiber reinforcement constitutes a single
piece made up of one-piece two-dimensional plies free from slots
and cutouts.
[0036] A coating of pyrolytic carbon can be present on the surface
of the bowl, at least on the inside of the bowl.
[0037] The invention also provides the use of such a bowl as a
crucible support, in particular for producing monocrystalline
silicon. A protective layer, e.g. of thermostructural composite
material, such as a C--C composite, can be interposed between the
bowl and the crucible.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] 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:
[0039] FIG. 1 is a highly diagrammatic half-section view showing a
bowl of composite material used as a crucible support in an
installation for producing silicon ingots;
[0040] FIG. 2 is a flow chart showing the successive steps in a
first implementation of a method in accordance with the
invention;
[0041] FIGS. 3A, 3B, and 3C are views showing a two-dimensional ply
with deformable mesh loops suitable for implementing the method of
FIG. 2;
[0042] FIG. 4 is a highly diagrammatic view of needling apparatus
in a form suitable for implementing the method of FIG. 2;
[0043] FIG. 5 is a fragmentary diagrammatic view showing additional
draping of plies over a portion of the bottom of the bowl;
[0044] FIG. 6 is a flow chart showing successive steps in a second
implementation of a method in accordance with the invention;
[0045] FIG. 7 is a diagrammatic section view showing a plug closing
a hole formed in the bottom of a preform;
[0046] FIG. 8 is a flow chart showing the successive steps in a
third implementation of a method in accordance with the invention;
and
[0047] FIG. 9 is a highly diagrammatic view showing plies being
draped in yet another implementation of a method in accordance with
the invention.
DETAILED DESCRIPTION OF IMPLEMENTATIONS OF THE INVENTION
[0048] As already mentioned, a non-limiting example to which the
invention applies is that of making bowls of thermostructural
composite material for supporting crucibles in installations that
produce single crystal silicon ingots.
[0049] Highly diagrammatic FIG. 1 shows such a bowl of composite
material, e.g. C/C composite material supporting a crucible 5 which
is usually made of silica. The bowl 1 stands on an annular support
formed with a ring 2 mounted at the end of a shaft 3 having a
setback 4 therein. The bowl has a bottom portion 1a and a surround
portion 1b, part of which is substantially cylindrical and is
connected to the bottom portion via a region of rounded profile.
The bottom portion of the bowl 1 is machined so as to form a
centering bearing surface corresponding to the setback 4 and a
support surface on the ring 2.
[0050] After the crucible has been filled with silicon, the
assembly is placed in a furnace and the temperature in the furnace
is raised to a value which is high enough to cause the silicon to
liquefy. At this temperature, greater than 1420.degree. C., the
silica crucible softens and it matches the shape of the bowl. A
germ having a crystal arrangement is then brought into contact with
the bath of silicon and an ingot is extracted slowly therefrom,
thereby forming a column between the germ and the bath. An ingot
can thus be drawn, and its length can lie in the range 1 meter (m)
to 2 m.
[0051] That method of manufacturing silicon ingots is well known
and does not form part of the invention, such that a more detailed
description is not required.
[0052] Because thermostructural composite materials have the
ability to conserve good mechanical properties and good dimensional
stability at high temperatures, they are particularly suitable for
making bowls for use in the above application.
[0053] The description below relates more particularly to making
bowls out of C/C composite materials with carbon fiber
reinforcement and a carbon matrix, or at least a matrix that is
essentially made of carbon. The invention also covers making bowls
out of CMC type composite materials, i.e. having ceramic fiber
reinforcement (e.g. made of SiC fibers) and a matrix that is also
ceramic (e.g. likewise of SiC), where technologies for making CMCs
are well known.
[0054] The fiber reinforcement can be made from carbon yarns of the
kind commercially available, but preferably free from any of the
surface treatment normally performed to provide surface functions
that encourage bonding with an organic matrix when such yarns are
used to form fiber/resin type composite materials that are not
intended for high temperature applications. The absence of surface
functions makes it possible to avoid internal stresses during the
process of manufacturing the composite material using the method of
the invention.
[0055] In a variant, before or immediately after making the fiber
reinforcement, it is possible to subject ordinary commercial carbon
yarns to heat treatment seeking to eliminate surface functions, and
the fibers constituting the yarns can be provided with a thin
interphase coating of pyrolytic carbon, typically having a
thickness less than or equal to 0.1 micrometers (.mu.m). The
interphase coating of pyrolytic carbon can be obtained by chemical
vapor deposition, as described in the Applicants' U.S. Pat. No.
4,748,079.
[0056] A first implementation of the method for manufacturing a
composite material bowl is described below with reference to FIG.
2.
[0057] A first step 10 of the method consists in providing
deformable two-dimensional plies of carbon fiber.
[0058] The plies are made of a deformable fiber fabric,
advantageously made up of unidirectional sheets of carbon yarns
having no surface functions, which are superposed with different
directions and bonded together so as to form deformable individual
mesh loops.
[0059] The sheets can be bonded together by light needling which
provides the fabric with cohesion while leaving it with sufficient
ability to deform. It is also possible to bond the sheets to one
another by stitching using a thread passing from one face to the
other of the fabric.
[0060] The sheets are preferably bonded together by knitting a
thread passing from one face to the other of the fabric, as shown
in FIGS. 3A to 3C. Such deformable fabrics are known and they are
described in the Applicants' document WO 98/44182. They are
constituted by two unidirectional sheets superposed with the
directions between each other making an angle of less than
90.degree., preferably an angle lying in the range 45.degree. to
60.degree..
[0061] FIGS. 3A and 3B show the front and back faces of the fabric
102 while FIG. 3C shows in greater detail the knitting stitch 108
used. The stitch is in the form of interlaced loops 108a that are
elongate in a longitudinal direction of the fabric 102 and that
form a plurality of parallel rows, together with V-shaped or zigzag
paths 108b which interconnect loops in adjacent rows. The fabric
102 is situated between the paths 108b situated on the front face
(FIG. 3A) and the loops 108a situated on the back face (FIG. 3B),
giving the knit the appearance of a zigzag stitch on one face and
the appearance of a chain stitch on the other face. The knitting
stitch overlies a plurality of yarns in each unidirectional sheet
depending on the gauge selected for the knitting machine.
[0062] The bridges between the zigzag paths 108b and the loops
108a, such as points A, B, C, and D in FIG. 3C define the vertices
of the deformable individual mesh loops. Under such circumstances,
both the loops defined by the knitting stitch and the loops defined
by the cross-points between the threads of the sheets are
deformable, with the cross-points serving to constitute deformable
parallelograms.
[0063] The knitting thread used 106 can be a carbon thread or a
thread made of a carbon precursor, or indeed a thread made of a
sacrificial material, i.e. a material that is to be removed by
being dissolved or by heat so as to leave no residue at a
subsequent stage in the manufacture of the composite bowl. An
example of a sacrificial thread is a thread made of polyvinyl
alcohol (PVA) that is soluble in water.
[0064] The plies are cut out from the deformable fabric to the
outside dimensions that are required by the shape and dimensions of
the bowl to be made. The plies are whole, constituting single
pieces that are free from any internal cutouts or slots.
[0065] In the following step 20 of the method, the plies are draped
onto tooling having a shape corresponding to the shape of the bowl
that is to be made. Draping can be performed manually.
[0066] Because the mesh loops of the plies are deformable and
because of the way in which the plies are draped, it is possible to
give the superposed plies the desired shape without forming any
folds, while nevertheless using plies that are whole without any
slots or cutouts.
[0067] Compared with the technique consisting in forming cutouts in
two-dimensional plies that are not sufficiently deformable, e.g. so
as to form petals that can fit closely to the desired shape without
folds or extra thicknesses, the use of plies having deformable mesh
loops presents the advantages of being easier to drape and of
preserving the integrity of the structure of the plies. This point
is particularly important for the mechanical properties of the
final bowl.
[0068] The plies are superposed by being offset angularly about the
axis of the preform passing through the top thereof, so as to avoid
exact superposition of their patterns, since that can lead to
non-uniformity in the structure.
[0069] The plies are stacked until the desired thickness has been
obtained for the bowl preform, and they are bonded to one another
by needling (step 30).
[0070] The needling can be performed after the plies have been
draped, or preferably as the draping is taking place, e.g. by
needling each newly-draped ply.
[0071] By way of example, a needling installation is used of the
kind described in the Applicants' U.S. Pat. No. 5,266,217. As shown
very diagrammatically in FIG. 4, such an installation comprises a
table 300 supporting a former 302, a robot 304 having a control
unit 306 connected to an operator console 308 and a needling head
310 fixed to the end to the arm 312 of the robot 304. The other end
of the arm 312 is hinged about a vertical axis to a vertically
movable support 314. In the vicinity of the needling head, the arm
312 has a multi-axis articulation 316.
[0072] The needling head 310 thus possesses the degrees of freedom
required for being brought into the desired position with the
desired orientation for needling the plies draped on the former
302, and to perform needling along preestablished trajectories with
a predetermined direction of incidence, generally normal to the
plies.
[0073] The former 302 is provided with a backing coating, e.g. a
felt into which the needles of the head 310 can penetrate without
being damaged.
[0074] The needling head 310 is provided with a presser plate 310a
having perforations through which the needles pass. The presser
plate is urged resiliently to exert controlled pressure on the
plies being needled.
[0075] Advantageously, needling is performed while controlling the
density of fibers transferred by the needles transversely relative
to the plies. This can be achieved by controlling the penetration
depth of the needles so as to obtain needling density that is
substantially constant throughout the thickness of the preform.
[0076] The preform 320 constituted by the draped and needled plies
102 is advantageously associated with additional plies being draped
(step 40) having dimensions that are restricted to the dimensions
of the bottom portion of the bowl that is to be made.
[0077] As shown in FIG. 5, the additional plies 104 (which can be
of the same kind as the plies 102) are draped on the bottom of the
preform 320 until sufficient thickness has been obtained to enable
the bottom portion of the bowl to be machined so as to form a
support face with a centering bearing surface.
[0078] The plies 104 are bonded together and to the plies 102 by
needling. This is done using a needling installation of the kind
described above.
[0079] The resulting fiber preform is then subjected to a
consolidation process using a liquid.
[0080] For this purpose, the fiber preform 54 is placed in a mold
56 and impregnated with a liquid precursor of carbon (step 50).
Impregnation can be performed using a phenolic resin, for
example.
[0081] After the resin has been polymerized in the mold, the
preform is removed from the mold and is subjected to heat treatment
to carbonize the resin.
[0082] In a variant, impregnation can be performed on the preform
while it is maintained on the former, after needling. For this
purpose, the resin is inserted into the preform while it is covered
in a flexible cover, e.g. made of elastomer, possibly associated
with suction. The cover can then be withdrawn and the preform
removed, after the resin has polymerized and before it is
carbonized.
[0083] The following step 60 of the method consists in performing
heat treatment to stabilize the carbon fibers dimensionally and to
purify the consolidated preform. The heat treatment is performed at
a temperature that preferably lies in the range 1600.degree. C. to
2800.degree. C. It serves to prevent subsequent dimensional
variation of the fibers during continued manufacture of the bowl
when the fibers have not previously been exposed to a temperature
not less that that to which they are exposed subsequently, in
particular during densification. This encourages removal of the
impurities contained in the fibers and in the coke of the
consolidation resin.
[0084] Thereafter, the preform is densified by a matrix of
pyrolytic carbon using chemical vapor infiltration (step 70). To
this end, in well-known manner, the preform can be placed in an
enclosure into which a gas is introduced that contains a precursor
of carbon, e.g. methane. The pressure and temperature conditions
are selected in such a manner as to enable the gas to diffuse
within the pores of the consolidated preform, and to enable the
methane to decompose so as to give a deposit of pyrolytic
carbon.
[0085] Chemical vapor infiltration can be implemented under
constant-temperature, constant-pressure conditions, or with a
temperature gradient, both of which processes are well known.
[0086] Infiltration can also be performed by immersing the
consolidated preform in a liquid precursor and by heating the
preform so as to develop a film of gaseous precursor at its
surface. Such a method is described, for example in the Applicants'
document FR 2 784 695.
[0087] In another variant, the preform can be densified using a
liquid in the form of a precursor for the matrix, e.g. a resin.
[0088] After densification, the resulting bowl blank is machined
(step 80) in particular for the purpose of forming the centering
bearing surface and the support surface at the bottom of the
bowl.
[0089] A final heat treatment step is performed (step 90) e.g. at a
temperature in the range 2200.degree. C. to 2700.degree. C. in
order to purify the resulting C/C composite bowl. In conventional
manner, the purification treatment can be performed in the presence
of a halogen.
[0090] A final deposit of pyrolytic carbon (step 100) can be
performed by chemical vapor deposition. This deposit is formed on
the surface of the bowl, at least on the inside. In variant, this
final deposit can be of silicon carbide (SiC), likewise obtained by
chemical vapor deposition.
[0091] The final deposition of pyrolytic carbon or of SiC could
alternatively be performed prior to the final purification heat
treatment.
[0092] Another implementation of the method of the invention is
described below with reference to FIGS. 6 and 7.
[0093] The method whose successive steps are shown in FIG. 6 has
the same initial steps 10 to 60 as the method of FIG. 2, i.e.
supplying deformable two-dimensional fiber plies (step 10), draping
the plies on a former (step 20), bonding the draped plies together
by needling (step 30), draping additional plies (step 40),
impregnating with a resin for consolidation purposes (step 50), and
stabilization and purification heat treatment (step 60).
[0094] Thereafter, the method of FIG. 6 differs from that of FIG. 2
in that prior to densifying the preform, a hole 52 is machined
through the bottom of the consolidated preform 58 (step 65). It
should be observed that the hole 52 can be formed in the
non-consolidated preform prior to impregnation with the resin, or
immediately after the resin has been polymerized but prior to the
resin being carbonized.
[0095] The presence of the hole 52 can be beneficial during
densification of the preform by chemical vapor infiltration. The
hole 52 encourages flow of the gas through the enclosure in which
the preform is placed.
[0096] Step 70 of densifying the preform is thus preferably
performed by chemical vapor infiltration.
[0097] The preform densified in this way is machined (step 80) in
particular the bottom of the preform is machined.
[0098] A plug is then made (step 82) for inserting in the hole 52
(step 86).
[0099] The plug can be made of various different materials, e.g. of
graphite, or preferably of a thermostructural composite material
such as a C/C composite. The plug can be made as one or more parts
that are obtained by densifying one or more corresponding preforms.
The or each preform is formed by superposing two-dimensional plies,
e.g. of woven fabric, which are bonded together by needling or by
stitching. Densification by means of a carbon matrix is then
performed using a liquid or by chemical vapor infiltration. In the
example shown in FIG. 7, the plug 84 comprises two parts 84a and
84b. The part 84a is cup-shaped with a lip-shaped outline that
bears against a setback 52a formed in the wall of the hole 52 on
the inside of the bowl preform. The outside face of the part 84a is
of a shape that fits continuously with the inside face of the bowl.
The part 84b is also cup-shaped with an outline in the form of a
lip which bears against the outside face of the bottom of the
preform of the bowl around the hole 52. The parts 84a and 84b can
be bonded together by screw fastening, with the part 84a having a
projecting central portion which is screwed into a housing in the
part 84b. The parts 84a and 84b are thus clamped to the rim of the
hole 52.
[0100] After the plug has been put into place, a new step 88 of
chemical vapor infiltration can be performed so as to complete
assembly of the plug 84 with the bottom portion of the bowl preform
and so as to finish off densification. The densification performed
in step 80 could then have been performed in partial manner.
[0101] Steps 90 and 100 of final heat treatment for purification
and for depositing pyrolytic carbon can then be performed as in the
method of FIG. 2.
[0102] Yet another implementation of the method of the invention is
briefly described with reference to FIG. 8.
[0103] This method comprises steps 10 to 100 that are the same as
the method of FIG. 2 with the exception of steps 30 and 40 of
bonding draped plies and of draping additional plies.
[0104] In the method of FIG. 8, plies 102 are bonded to one another
(step 30') by stitching using a thread 202 which passes through the
set of draped plies 102. A similar thread 204 is used in the
following step 40' to bond additional plies 104 to one another and
to the plies 102, with the thread 204 passing through all of the
plies 102 and 104.
[0105] The threads 202 and 204 can be carbon threads similar to
those used for forming the plies 102 and 104. In a variant, it is
possible to use threads of a sacrificial material, i.e. a material
suitable for being eliminated by being dissolved or by heat at a
subsequent stage in preparation of the bowl.
[0106] It is also possible to bond the plies 102 together by
stitching and to bond the plies 104 together and to the plies 102
by needling, as in step 40 of FIG. 6.
[0107] It should also be observed that the method of bonding the
plies 102 and 104 together by stitching could be used instead of
the needling technique used in the method of FIG. 6.
[0108] In yet another implementation of the method of the
invention, deformable two-dimensional plies are used that present a
substantially central opening.
[0109] As shown in FIG. 9, the plies 202 provided with a central
opening 203 are draped on a former, e.g. the same former 302 as is
shown in FIG. 4. The plies 202 are draped so that the openings 203
are in alignment in the central portion of the bottom of the
preform that is being built up.
[0110] The plies 202 are bonded together by needling, as in the
methods of FIGS. 2 and 6, or by stitching as in the method of FIG.
6.
[0111] Additional plies 204 are draped on the bottom of the
preform, the plies 204 having substantially central openings 205
that are in alignment. The plies 204 are bonded to one another and
to the plies 202 by needling or by stitching.
[0112] The aligned openings 203, 205 define a hole 152 passing
through the bottom of the preform.
[0113] After the plies 202 and 204 have been draped and bonded
together, manufacture of the bowl can continue with steps of
consolidation by resin impregnation, stabilization and purification
heat treatment, densification by chemical vapor infiltration,
machining, preparing and installing a plug closing the hole 152,
final densification by chemical vapor infiltration, purification
heat treatment, and deposition of pyrolytic carbon, as in steps 50,
60, 70, 80, 82, 86, 88, 90, and 100 of the method of FIG. 6.
[0114] Other variants can be applied to the method described above
without going beyond the ambit of the protection defined by the
accompanying claims. Thus, the preform can be made out of yarns
made of carbon precursor fibers instead of carbon fibers. Suitable
carbon precursors include, in conventional manner, preoxidized
polyacrylonitril (PAN), phenolic compounds, and pitch. The
precursor is transformed into carbon by heat treatment after the
preform has been built up.
[0115] The stage of consolidating the preform could be omitted. The
preform with the superposed and bonded-together plies could then be
placed in tooling of a shape corresponds to the shape of the bowl
to be made so as to be inserted in an enclosure for densification
by chemical vapor infiltration. The tooling can be withdrawn after
a first densification stage that provides sufficient cohesion to
enable densification to be continued without tooling.
[0116] The stage of heat treating the preform prior to
densification could be omitted, in particular when the fibers do
not need to be stabilized dimensionally. This can be the case when
the carbon fibers of the preform have already been raised to a
temperature not less than the temperature encountered subsequently.
Purification can then be performed in a single operation after
densification.
[0117] Alternatively, the final purification stage could be
omitted, providing the degree of purity obtained for the preform by
the heat treatment prior to densification, and when densification
is performed with a carbon precursor under conditions that do not
introduce significant quantities of impurities. When a high level
of purity needs to be complied with for the material contained in
the crucible supported by the manufactured bowl, as is the case for
silicon that is for use in manufacturing semiconductors, the level
of residual impurities in the bowl must preferably below 5 parts
per million (ppm).
[0118] In addition, the preform could be densified using a matrix
that is made at least in part out of a ceramic material, e.g.
silicon carbide obtained by chemical vapor infiltration and using a
gas precursor such as methyltrichlorosilane.
[0119] Finally, although the description above relates to forming a
coating of pyrolytic carbon or of SiC particularly on the inside
face of the bowl, other forms of protection could be adopted,
instead of or as well as a coating of pyrolytic carbon or of
SiC.
[0120] In particular, a protective layer could be interposed
between the bowl and the crucible, to avoid attacking the composite
material of the bowl, as can happen with a crucible made of silica
and a bowl made of C/C composite material.
[0121] By way of example, the protective layer can itself be made
of a thermostructural composite material, such as a C/C composite
and it can behave as a "consumable" layer that needs to be replaced
periodically. The C/C composite material used can be made up of
two-dimensional plies of carbon fibers bonded together by a matrix
of carbon obtained by using a liquid or chemical vapor
infiltration.
[0122] Such a protective layer 6 fitting closely to the shape of
the inside surface of the bowl 1 is shown in FIG. 1.
* * * * *