U.S. patent application number 09/870242 was filed with the patent office on 2001-09-20 for method and apparatus for producing free-standing silicon carbide articles.
This patent application is currently assigned to CVD, Inc.. Invention is credited to Goela, Jitendra Singh, Pickering, Michael A..
Application Number | 20010022408 09/870242 |
Document ID | / |
Family ID | 22110603 |
Filed Date | 2001-09-20 |
United States Patent
Application |
20010022408 |
Kind Code |
A1 |
Goela, Jitendra Singh ; et
al. |
September 20, 2001 |
Method and apparatus for producing free-standing silicon carbide
articles
Abstract
A process of producing relatively large, dense, free-standing
silicon carbide articles by chemical vapor deposition is enabled by
the provision of specially designed isolation devices. These
devices segregate silicon carbide deposits on the intended portions
of substrates, thereby alleviating the need to fracture heavy
silicon carbide deposits in order to remove, or otherwise move, the
substrate, with the heavy deposit thereon, from the deposition
furnace. The isolation devices enable the use of more efficient
vertically extended vacuum furnaces. The isolation devices also
enable the commercial production of relatively dense, large,
thin-walled, silicon carbide shells.
Inventors: |
Goela, Jitendra Singh;
(Andover, MA) ; Pickering, Michael A.; (Dracut,
MA) |
Correspondence
Address: |
EDWARDS & ANGELL, LLP
Dike, Bronstein, Roberts & Cushman, IP Group
P.O. Box 9169
Boston
MA
02209
US
|
Assignee: |
CVD, Inc.
Woburn
MA
|
Family ID: |
22110603 |
Appl. No.: |
09/870242 |
Filed: |
May 30, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09870242 |
May 30, 2001 |
|
|
|
09072927 |
May 5, 1998 |
|
|
|
6228297 |
|
|
|
|
Current U.S.
Class: |
264/81 ; 118/715;
118/720; 118/721; 118/728; 428/34.4 |
Current CPC
Class: |
Y10T 428/131 20150115;
C23C 16/325 20130101; C23C 16/01 20130101; C04B 35/565
20130101 |
Class at
Publication: |
264/81 ; 118/715;
118/720; 118/721; 118/728; 428/34.4 |
International
Class: |
B29C 031/00; C23C
016/00 |
Claims
We claim:
1. In a process for producing silicon carbide articles by chemical
vapor deposition comprising: providing a silicon carbide precursor
gas in proximity to a surface of a solid substrate in a deposition
chamber, reacting said silicon carbide precursor gas to provide a
silicon carbide deposit on a predetermined deposition zone on said
surface of said substrate, thereafter removing said substrate with
said silicon carbide deposit from said deposition chamber, and
recovering said deposit by separating it from said substrate, the
improvement comprising: providing at least one boundary zone on a
portion of said surface located between said predetermined
deposition zone and a proximate solid surface in said deposition
chamber, and producing a silicon carbide deposit on the
predetermined deposition zone which is substantially thicker than
the deposit produced in said boundary zone.
2. The process of claim 1, wherein the thickness of said deposit
produced in said boundary zone decreases as it extends away from
said deposition zone.
3. The process of claim 1, wherein essentially no deposit is formed
on the portion of said boundary zone which is closest to said
proximate solid surface.
4. The process of claim 1, wherein an isolation device is arranged
between said proximate solid surface and said substrate, and said
isolation device includes a side wall which extends over said
boundary zone.
5. The process of claim 4 wherein said proximate solid surface is a
surface of a second solid substrate.
6. The process of claim 4, wherein said proximate solid surface
supports said isolation device and said isolation device supports
said substrate.
7. A process of producing a silicon carbide article, comprising:
providing a silicon carbide precursor gas in proximity to a
predetermined deposition zone on a surface of a solid substrate in
a deposition chamber, reacting said silicon carbide precursor gas
to form a silicon carbide deposit on said predetermined deposition
zone, defining a boundary zone of diminished deposit thickness on
said surface adjacent said predetermined deposition zone, providing
a channel overlying said boundary zone, said channel being defined
by (a) said boundary zone, (b) an outer wall spaced from and
extending over said boundary zone, (c) a closed end extending
between said boundary zone and said outer wall, and (d) an open end
opposite said closed end and adjacent said deposition zone, and
recovering said silicon carbide deposit from said substrate
surface.
8. The process of claim 7, wherein the width of said channel at its
open end (w.sub.1) is one to two times the thickness of the
recovered deposit.
9. The process of claim 7, wherein the distance between the
channel's open end and its closed end (h) is 1,5 to 5 times the
width of said channel at its open end (w.sub.1).
10. The process of claim 7, wherein the width of said channel at
its open end (w.sub.1) is at least twice its width at its closed
end (w.sub.2).
11. The process of claim 7, wherein said substrate is separated
from another solid surface in said deposition chamber by an
isolation device.
12. The process of claim 11, wherein said outer wall and said
closed end are integral parts of said isolation device.
13. The process of claim 12, wherein said substrate is supported by
said isolation device.
14. The process of claim 13, wherein said isolation device is
supported by said another solid surface.
15. The process of claim 12, wherein said isolation device
separates two substrates.
16. The process of claim 7, wherein said substrate extends around a
hollow core.
17. The process of claim 16, wherein said substrate has a
cylindrical or frustroconical shape.
18. The process of claim 16, wherein said substrate comprises a
series of planar walls extending around said hollow core.
19. The process of claim 16, wherein said outer wall and said
closed end are integral parts of an isolation device which
separates the interior hollow core of said substrate from said
precursor silicon carbide gas in said deposition chamber.
20. An apparatus for forming solid deposits from gaseous
precursors, comprising: a solid substrate, a housing defining a
deposition chamber, said housing being capable of opening and
closing sufficiently to allow the insertion and removal of said
solid substrate, a source of a gaseous precursor material
operatively connected to said deposition chamber, an isolation
device located between said solid substrate and a proximate solid
surface in said deposition chamber, said isolation device
cooperating with said substrate to restrict the flow of said
gaseous precursor material over a boundary zone extending adjacent
the border of said substrate closest to said proximate solid
surface.
21. An apparatus according to claim 20, wherein: said isolation
device comprises an outer wall spaced from and extending over said
boundary zone from an open end to a closed end, said closed end
extending between said substrate and said outer wall.
22. An apparatus according to claim 21, wherein: said outer wall is
spaced a greater distance from said solid substrate at said open
end (w.sub.1) than it is spaced from said solid substrate at said
closed end (w.sub.2).
23. An apparatus according to claim 21, wherein: said open end of
said outer wall is spaced from said closed end a distance which is
2 to 5 times the distance, w.sub.1, the outer wall portion is
spaced from said substrate at said open end.
24. An apparatus according to claim 21, wherein: two solid
substrates are arranged one atop the other in said deposition
chamber, and said isolation device is located between the adjacent
solid surfaces of the two substrates.
25. An apparatus according to claim 24, wherein: said substrates
are generally cylindrical or frustroconical in shape.
26. An apparatus according to claim 20, wherein: said substrate is
supported in said deposition chamber by said isolation device.
27. A hollow silicon carbide shell having a ratio of external
perimeter to wall thickness greater than 50.
28. The hollow shell of claim 27 having a cylindrical shape.
29. The hollow shell of claim 27 having a frustroconical shape.
30. The hollow shell of claim 27, wherein the density of said
silicon carbide is at least 3.15 grams per cubic centimeter.
31. The hollow shell of claim 27, wherein said external perimeter
is in excess of 50 inches.
32. The hollow shell of claim 27, wherein said external perimeter
is in excess of 65 inches.
33. The hollow shell of claim 27, wherein said ratio is 200 or
greater.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] Silicon carbide's unique combination of properties make it a
particularly suitable material for a variety of applications in the
semiconductor, optical, electronic and chemical processing fields.
Moreover, chemical vapor deposition (CVD) techniques have been
widely used to provide thin films and coatings of a variety of
materials on various articles. Silicon carbide articles produced by
chemical vapor deposition (CVD) processing are recognized to
exhibit superior mechanical, thermal, physical and optical
properties. This invention is directed to improvements in a CVD
process of producing free standing, self-supporting silicon carbide
articles, and is particularly adapted to the production of hollow
shells of cylindrical, frustoconical or other shapes. Such shells
can be used in x-ray telescopes, semiconductor processing furnaces,
heat exchangers, laser tubes and chemical process equipment.
[0003] 2. Description of Related Art
[0004] The advantages of silicon carbide as a fabrication material
for astronomical X-ray telescopes and the experimental use of small
scale CVD processing to prepare conical silicon carbide shells was
recently described by Geril et al. in "Thin Shell Replication of
Grazing Incident (Wolter Type I) SiC Mirrors", SPIE Proc., 2478,
215 (1995).
[0005] The advantages of CVD produced free-standing silicon carbide
materials in applications requiring a high degree of surface
smoothness and polishability are described in U.S. Pat. No.
5,374,412. The patent describes apparatus and process conditions
which are used in the CVD production of free-standing silicon
carbide articles. This patent also refers to earlier U.S. Pat. Nos.
4,990,374; 4,997,678 and 5,071,596 as further describing CVD
processes of producing free-standing silicon carbide materials by
the pyrolytic deposit of SiC on a mandrel.
[0006] Several methods of controlling or isolating the deposit of
silicon carbide to the intended side of the substrate during
chemical vapor deposition are described in U.S. Pat. Nos. 4,963,393
and 4,990,374. In U.S. Pat. No. 4,963,393, a curtain of a flexible
graphite cloth is arranged to shield the backside of the substrate
from the flowing reacted precursor gases, whereby silicon carbide
deposits on the backside of the substrate are avoided. In U.S. Pat.
No. 4,990,374 a counterflow of a non-reactive gas is directed to
flow past the substrate's peripheral edge from behind the substrate
whereby the deposit is confined to the front face of the
substrate.
SUMMARY OF THE INVENTION
[0007] Chemical vapor deposition (CVD) has been used to produce
both free-standing articles and coatings of silicon carbide.
Typically, the process involves reacting vaporized or gaseous
chemical precursors in the vicinity of a substrate to result in
silicon carbide depositing on the substrate. The deposition
reaction is continued until the deposit reaches the desired
thickness. If a coated article is desired, the substrate is the
article to be coated and the coating is relatively thin. If a
free-standing article or silicon carbide bulk material is desired,
a thicker deposit is formed as a shell on the substrate and then
separated from the substrate to provide the silicon carbide
article.
[0008] In a typical silicon carbide bulk material production run,
silicon carbide precursor gases or vapors are fed to a deposition
chamber where they are heated to a temperature at which they react
to produce silicon carbide. The silicon carbide deposits as a shell
on a solid mandrel or other substrate provided in the deposition
chamber. The deposition is continued until the desired thickness of
silicon carbide is deposited on the substrate, or mandrel. The
mandrel is then removed from the deposition chamber and the shell
is separated from the mandrel. Monolithic silicon carbide plates
and cylinders have been produced by applying such chemical vapor
deposition (CVD) techniques with suitably shaped substrate or
mandrel forms.
[0009] Once the silicon carbide precursor gases or vapors are
brought to the appropriate conditions to cause them to react, they
produce silicon carbide which then deposits on any available
surface. The deposit generally is not limited to the intended
surface(s) of the mandrel(s) and generally extends past such
surfaces to adjoining surfaces as well as depositing on the walls
and housing of the deposition chamber. In the past, the silicon
carbide deposit has extended past the dimensional limits of the
mandrel over adjacent portions of the support structure holding or
supporting the mandrel in its position in the deposition chamber.
It is then necessary to fracture such deposits to remove the
mandrel from the deposition chamber. Fracturing of the deposit
often results in the formation of cracks which propagate through
the deposit from the point of fracture. Such cracks are not
acceptable in the intended applications of the silicon carbide
articles, and usually result in the article being rejected. The
prevalence of propagated cracks in relatively thick chemical vapor
deposits of silicon carbide have limited the size of articles that
can be produced commercially by this method. Moreover, recognition
of the potential capacity of CVD silicon carbide deposits to bridge
any joints between adjacent stacked mandrels and the subsequent
difficulty of separating and removing individual mandrels from such
a stack has prevented the use of stacked mandrels in the commercial
production of silicon carbide articles.
[0010] Optimal deposition conditions generally require less than
atmospheric pressures, which requires that the deposition be
conducted in a vacuum chamber. It is generally less expensive to
increase the production volume of vacuum chambers by increasing
their vertical dimensions rather than increasing their horizontal,
or floor space occupying, dimensions. Accordingly, it would be
economically advantageous to provide a commercial technique for
creating silicon carbide deposits on a plurality of mandrels,
wherein the mandrels are vertically stacked within a single
vertically extending deposition chamber. This, however, has not
been done in the past, at least in part because of the difficulty
in segregating, or isolating, the deposit on one mandrel from the
deposit produced on an adjoining mandrel.
[0011] The present invention is directed to a process, and
associated apparatus, which greatly restricts and, preferably,
completely avoids, the formation of deposits extending past the
dimensional limits of the mandrel. By limiting, or avoiding, the
formation of such deposits, removal of the mandrel from the
deposition chamber does not result in cracks which propagate
through the deposit. When practice of the invention avoids the
formation of a deposit at or adjacent the dimensional boundary of
the mandrel, the mandrel can be removed from the deposition chamber
without fracturing the deposit. When a greatly restricted deposit
forms at the dimensional boundary of the mandrel, it forms a thin
coating, substantially thinner than the main body of the deposit,
the fracture of which does not result in cracks extending into the
main body of the deposit.
[0012] The present invention also provides a process wherein
silicon carbide deposits are formed on a plurality of substrates,
or mandrels, as they are arranged in a vertical stack, one atop
another. The mandrels are then removed from the stack and the
deposits separated from the mandrels to result in free-standing
dense silicon carbide articles.
[0013] The invention further provides for the production of rigid,
thin-walled cylindrical or frustroconical shells of dense silicon
carbide having an aspect ratio, i.e., the ratio of the shell
diameter to its wall thickness, of 50 or greater. It also has
permitted the commercial production of large diameter, i.e. 18 inch
diameter and greater, cylindrical or frustoconical shells of dense
silicon carbide.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a schematic illustration, partially in section, of
a furnace utilizing the process and apparatus of the present
invention to produce an inventive relatively dense, thin-walled,
large silicon carbide shell.
[0015] FIG. 2 is a cross section of one type of isolation device
according to the present invention, deployed in the CVD furnace
with a deposit on a mandrel substrate.
[0016] FIG. 3 is a further cross section of the mandrel and
isolation device illustrated in FIG. 2.
[0017] FIG. 4 is a cross section of a further type of isolation
device according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0018] A chemical vapor deposition (CVD) furnace equipped for
producing silicon carbide shells according to the present invention
is illustrated in FIG. 1. The furnace 10 includes an outer housing
defining a vacuum chamber 12, which is connected to a vacuum source
through exhaust port 14. A deposition chamber 16, provided in the
vacuum chamber, is defined by side wall 18, top wall 20 and base
22. The base 22 is supported by columns 24 extending up from a
floor plate 26 provided near the bottom of the vacuum chamber 12. A
rotating platform 28 is supported by rails 30 which extend into
channels 32 on table 34. Shaft 36 transmits a motive force to
rotate the platform 28 from a motor/transmission system (not
shown). An arrangement of gas injectors 38 feed the deposit
precursor gases through the top wall 20 into the deposition chamber
16. Typically, the arrangement of injectors 38 involves multiple
injectors arranged around a central injector. A stack of two
mandrels 40 and 42 is arranged on the rotating platform. Each of
the mandrels comprise a side wall 44 formed from a hollow conical
graphite form which extends from a small end 46 to a large end 48.
The diameter of the mandrel at the small end 46 is less than the
diameter at its large end 48. As arranged in FIG. 1, the mandrels
are stacked with their large ends 48 adjacent each other. Suitable
heating means (not illustrated) provide the desired deposition
temperature at the mandrel surface. Graphite electrical resistance
heaters arranged along the side wall and beneath the bottom of the
mandrels have provided relatively uniform temperature distribution
over the mandrel surface.
[0019] In a typical silicon carbide production run, a single
mandrel 42 is located on the rotating platform 28, the deposition
chamber 16 assembled and the vacuum chamber 12 closed. The vacuum
chamber is purged of atmospheric gases by drawing a vacuum on the
chamber, injecting an inert gas through the gas injectors 38, and
redrawing a vacuum. These steps are repeated until the atmospheric
gases are adequately purged. The rotation of the platform 28 is
then initiated, the mandrels heated to the target deposition
temperature, the flow of reactive coating precursor gas initiated
and the target furnace pressure established. The flow of precursor
gas, the target mandrel temperature and the target furnace pressure
are continued until the deposit reaches the desired thickness, at
which time the flow of coating precursor gas is discontinued, and
the mandrel temperature and furnace pressure allowed to return to
normal, or ambient. The vacuum chamber is then opened (usually from
the bottom), the rotating platform lowered, and the mandrel removed
therefrom. The silicon carbide deposit is then separated and
recovered from the mandrel. In the past, removal of the mandrel
from the rotating platform was complicated by the silicon carbide
deposit which not only formed on the mandrel, but extended past the
end of the mandrel along the top surface of the rotating platform
28. Removal of the mandrel required fracturing this relatively
thick deposit which created cracks, which, in turn, could propagate
from the point of fracture throughout the deposit, in many cases
precluding the use of the silicon carbide article for its intended
use.
[0020] In the present process an isolation device 50 is provided
between the bottom mandrel 42 and the rotating platform 28. As best
seen in FIG. 2, the isolation device 50 includes a side or outer
wall portion 52 and a closed end portion 54. The outer wall portion
52 is spaced from the mandrel 42 and extends from the closed end
portion 54 to an open end 56. The closed end portion extends
between the mandrel 42 and the outer wall portion 52. Together, the
outer wall portion 52, the closed end portion 54 and the mandrel 42
define an open channel 58 which extends from the closed end to the
open end 56. As best seen in FIG. 3, at its open end 56, the width
of the open channel, w.sub.1, or the distance between the outer
wall portion 52 and the mandrel 42, is one to two times the
intended thickness of the final silicon carbide deposit 57.
Preferably, the width of the channel, w.sub.2, at the closed end 54
is less than 1/2 of the width w.sub.1. The provision of a smaller
channel width, w.sub.2, at the closed end assures that no deposit
will form on the mandrel where it adjoins the closed end 54. The
height, h, of the open channel 58 from the closed end 54 to the
open end 56 is greater than the width w.sub.1, and, preferably, is
approximately 1.5 to 5 times the width w.sub.1.
[0021] The open channel 58 defines a boundary zone 60 on that
portion of the surface of the mandrel, or substrate, which lies
adjacent the open channel, i.e. that portion of the mandrel
extending beneath the outer wall portion 52 of the isolation device
50. The boundary zone is distinguished from the remaining portion
of the mandrel surface, which portion is referred to as the
deposition zone 62, by its diminished availability to the reacting
coating precursor gases, resulting in the formation of a
substantially thinner silicon carbide deposit in the boundary zone.
The deposit thickness within the boundary zone decreases the
further it is from the channel's open end 56. Preferably, the
deposit thickness decreases sufficiently that essentially no
deposit is formed adjacent the closed end 54 of the channel.
[0022] Utilization of the isolation device 50 between the mandrel
42 and the platform 28 in the inventive process provides a boundary
zone 60 of diminished deposit thickness between the deposition zone
62 and that portion of the mandrel which is most proximate the
supporting solid surface of the rotating platform 28. Preferably,
the deposit thickness diminishes to essentially zero at the end of
the mandrel most proximate to the platform 28. Accordingly, removal
of the mandrel 42 with the silicon carbide deposit thereon from the
isolation device 50 does not require fracturing a thick extension
of the deposit and the accompanying possibility of creating
propagating cracks in the product article. The yield of acceptable
product is thereby substantially enhanced by minimizing or
eliminating the silicon carbide deposit where the removable mandrel
adjoins non-removable components of the furnace.
[0023] An isolation device 64, which is essentially identical to
the isolation device 50, is provided at the top of the upper
mandrel 40 where it serves to prevent the silicon carbide deposit
from extending over the upper rim and onto the interior surface of
the hollow upper mandrel 40. The closed end portion of isolation
device 64, like that of device 50, is a dish-like continuous solid
sheet spanning the space defined by the outer wall. Accordingly,
the isolation device 64 also functions to essentially close the
upper end of the stack thereby denying the reactive precursor gases
access to the interior of the stack of hollow mandrels 40 and 42
and avoiding unwanted deposits forming on the interior surfaces of
the hollow mandrels.
[0024] A further isolation device 66 is provided between the
stacked mandrels 40 and 42. The isolation devices 50 and 64 are
designed to cooperate with a single mandrel in defining a boundary
zone adjacent the periphery of such mandrel. Isolation device 66 is
designed to cooperate with two stacked mandrels in providing
isolation zones on each. As seen best in FIG. 4, isolation device
66 comprises a circular ring comprising a circumferentially
extending outer wall portion 70 with a radially disposed closed end
portion 72 extending inwardly from the center of the inner face of
the outer wall portion so as to result in the ring having a
generally T-shaped cross-section. As deployed, the radially
disposed closed end portion 72 is located on top of the upper edge
of the lower mandrel 42 and the lower edge of the upper mandrel 40
is located on top of the closed end portion 72. The
circumferentially extending outer wall portion 70 extends around
both lower and upper mandrels functioning with each of them and the
closed end portion 72 to define boundary zones 60 at each of the
mandrels' adjoining edges. During the deposition process the
isolation device restricts the flow of reacting precursor gases to
these boundary zones whereby the thickness of the deposit formed in
each of the boundary zones gradually decreases, preferably to zero,
as the deposit approaches the closed end. Since the deposit is
essentially completed before reaching the closed end and does not
extend across the junction of the mandrel with the isolation
device, the separation of the mandrel from the isolation device at
the completion of the run does not produce cracks which propagate
throughout the deposit. In the isolation device illustrated in FIG.
4, the radially disposed closed end portion 72 is sufficiently long
to be fully supported by the lower mandrel 42 and to provide full
support for the upper mandrel 40, but does not extend from one side
to the other, as do the illustrated isolation devices 50 and 64.
While the device 66 could be designed to completely isolate the
interiors of the adjacent mandrels, a savings in weight, material
and cost is achieved by providing a device 66 with the illustrated
annular closed end portion 72. The closed end portion of device 66
is thicker adjacent its outer end portion 70 than it is where it
contacts the mandrel. The change in thickness occurs at a step 74
located at a diameter equal to the external diameter of the
associated mandrel plus two times the intended w.sub.2 dimension,
i.e. the intended width at the closed end of the channel.
[0025] One of ordinary skill will recognize that the upper and
lower mandrels may have differing length, width and thickness
dimensions. The dimensions of the isolation devices can be readily
determined from the dimensions of the particular mandrels to be
isolated.
[0026] A vertical stack of two mandrels is illustrated in the FIG.
1 embodiment. The stack can include 4, 6, or any number of
mandrels, provided they are separated with isolation devices at
each junction. The capability of processing multiple mandrels in a
vertical stack enables the process to be conducted in vertically
oriented vacuum furnaces which generally require less floor space,
less capital and less maintenance expense than horizontally
oriented vacuum furnaces of the same capacity.
[0027] The substrates, or mandrels, are typically shaped around
hollow cores to minimize their cost and weight. The mandrels can
have generally circular or annular cross-sections, as in
cylindrical or frustroconical mandrels. Moreover, the mandrels may
incorporate several distinct shapes as they extend along their
axial lengths. Double frustroconical shaped mandrels in which two
cones having different side wall angles converge in the middle
portion of the mandrel have been used. Alternatively, when flat
sheets of silicon carbide are to be produced, the mandrel may
comprise a series of connected planar walls extending around a
hollow core, like the four side walls of a box.
[0028] The mandrels are fabricated from appropriate high
temperature materials such as alumina, graphite, molybdenum or
tungsten. Graphite is generally preferred because of its close
thermal expansion match with silicon carbide, its high temperature
properties and its availability in large sizes. The SiC-12 grade of
graphite produced by Toyo Tanzo Inc. is particularly preferred when
the deposit is formed on the exterior or peripheral surface of the
mandrel. The thermal expansion coefficient of this grade of
graphite is just slightly greater than silicon carbide's
coefficient, assuring that the mandrel will shrink slightly more
than the deposit during cool down. When graphite mandrels are used,
separation of the deposit from the mandrel is usually accomplished
by combustion of, or burning away, the graphite at a temperature
between 600.degree. and 800.degree. C. The isolation devices can be
fabricated from similar materials. Usually less expensive grades,
i.e. grades which might otherwise form small cracks at the
deposition conditions, of these materials can be used in the
isolation devices since they do not serve to shape the product
article.
[0029] The deposition process may produce the intended article as a
deposit on the exterior of the mandrel, as illustrated, or it may
produce the product deposit on the interior of a hollow substrate.
The deposit is usually machined to its final dimensions following
its removal from the mandrel. However, when it is not intended to
machine the removed deposit, the surface of the article with the
more critical surface dimensions is usually formed directly
adjacent the substrate.
[0030] A mold release coating may advantageously be applied to the
substrate surface prior to initiating the deposition, particularly
when large sized articles are deposited. Amorphous, glassy or
pyrolytic carbons are suitable release agents for use with graphite
mandrels.
[0031] The CVD production of bulk, or free-standing, silicon
carbide articles involves feeding a mixture of silicon carbide
precursor gases, such as a mixture of methyltrichlorosilane (MTS)
and H.sub.2, with an optional inert gas, such as argon, helium or
nitrogen, to the heated reactor/deposition chamber which is
maintained at a pressure between about 180 and 220 torr, and at a
temperature between about 1340.degree. and 1380.degree. C. The
mandrel(s) is rotated at a speed in the range of 1 to 5 rpm. The
relative partial pressure flow ratio of H.sub.2/MTS is maintained
in the range of about 4 to about 10. Silicon carbide is deposited
on the mandrel(s) at a deposition rate of about 1.0 to about 2.0
.mu.m/min. and is continued until the desired thickness of SiC is
deposited. Any desired thickness can be produced by merely
continuing the deposition for sufficient time, however, relatively
thin-walled shells are generally desirable based on weight, cost
and other considerations.
[0032] After the mandrel with the deposit is removed from the
rotating platform, the mandrel with the deposit thereon may be cut
to the required length and the outer surface of the deposit
machined. The mandrel is then removed by burning the graphite. If
necessary, the inside surface of the deposit can then be machined
to its required specification.
EXAMPLE
[0033] Two graphite shell mandrels, fabricated from SiC-12 grade of
graphite, were machined to final dimensions of 600-mm diameter and
240-mm length. They were then stacked in a CVD-furnace similar to
that shown in FIG. 1. Isolation devices wherein the dimensions
shown in FIG. 3 were a w.sub.1 of approximately {fraction (9/16)}th
of an inch, a height, h, of approximately one inch and a w.sub.2 of
approximately {fraction (3/16)}th of an inch. A reactive precursor
gas was injected through an array of seven injectors, six equally
spaced in an approx. 36 inch diameter circle and one located in the
circle's center. The precursor gas mixture was provided at a flow
rate through each injector of methyltrichlorosilane 4.4 standard
liters per minute (slpm), H.sub.2 22 slpm, and Ar 56.5 slpm. The
mandrels were rotated at a speed of 1.5 rpm and were maintained at
a target temperature of 1350.degree. C. for 90 hours. The deposits
provided on the deposition zones of the two mandrels varied between
0.149 and 0.348 inches in the axial direction. The thickness
variation in the radial direction was within 7%. The furnace was
opened and the mandrels and isolation devices removed without
introducing propagating cracks throughout the deposits. The
deposits were then cut to the specification length of 240-mm and
the outer surfaces machined to specification. The graphite mandrels
were then removed by burning the mandrels at temperatures in the
600-800.degree. C. range. The interior surface of a SiC shell was
then machined to provide a finished shell of 600-mm diameter by
240-mm length by 3-mm wall thickness.
[0034] The invention permits the commercial fabrication of
relatively dense, large-diameter, thin-walled silicon carbide
cylindrical and frustroconical shells or tubes. Chemical vapor
deposition techniques provide deposits of 3.15 g/cc and greater
densities, which correspond to at least 98% theoretical density.
Use of the isolation devices during CVD processing avoids, or at
least minimizes the formation of propagated cracks throughout the
deposit which previously had precluded the preparation of hollow
shells of 18 inch or larger diameters (i.e., shells having external
perimeters of 60 inches or greater). As illustrated in the
preceding example a shell of 24 inch diameter and 3-mm wall
thickness and having an aspect ratio (shell diameter/wall
thickness) of approximately 203 was prepared by this method. The
invention encompasses shells of dense silicon carbide having
external perimeters (i.e., circumferences) of 50 inches or greater,
and particularly, those having external perimeters of 65 inches or
greater; and having aspect ratios of 50 or greater; preferably,
shells having aspect ratios of 100 or greater; and most preferably,
those having aspect ratios of 200 or greater.
[0035] The foregoing is provided to enable workers in the art to
practice the invention, and to describe what is presently
considered the best mode of practicing the invention. The scope of
the invention is defined by the following claims.
* * * * *