U.S. patent application number 11/120157 was filed with the patent office on 2006-11-02 for ceramic article, ceramic extrudate and related articles.
This patent application is currently assigned to SAINT-GOBAIN CERAMICS & PLASTICS, INC.. Invention is credited to Brad Cobbledick, Robin Crawford, John Shultis.
Application Number | 20060246389 11/120157 |
Document ID | / |
Family ID | 37234838 |
Filed Date | 2006-11-02 |
United States Patent
Application |
20060246389 |
Kind Code |
A1 |
Crawford; Robin ; et
al. |
November 2, 2006 |
Ceramic article, ceramic extrudate and related articles
Abstract
A ceramic article is provided. The ceramic article includes a
ceramic tube comprising a sintered helical tape. The sintered
helical tape has adjacent windings, which are joined together.
Another aspect provides an article comprising a ceramic extrudate
having an outer surface and a pattern of surface features along the
outer surface.
Inventors: |
Crawford; Robin; (Caledonia,
CA) ; Shultis; John; (Rockwood, CA) ;
Cobbledick; Brad; (Waterdown, CA) |
Correspondence
Address: |
LARSON NEWMAN ABEL;POLANSKY & WHITE, LLP
5914 WEST COURTYARD DRIVE
SUITE 200
AUSTIN
TX
78730
US
|
Assignee: |
SAINT-GOBAIN CERAMICS &
PLASTICS, INC.
Worcester
MA
|
Family ID: |
37234838 |
Appl. No.: |
11/120157 |
Filed: |
May 2, 2005 |
Current U.S.
Class: |
431/328 |
Current CPC
Class: |
F23D 14/46 20130101;
F23D 2213/00 20130101; F23D 2203/1012 20130101; F23D 2212/10
20130101 |
Class at
Publication: |
431/328 |
International
Class: |
F23D 14/12 20060101
F23D014/12 |
Claims
1. An article comprising: a ceramic tube comprising sintered
helical tape, the tape having adjacent windings that are joined
together.
2. The article of claim 1, wherein the sintered helical tape
comprises an extrudate.
3. The article of claim 1, wherein the ceramic tube is comprised of
a single layer of joined adjacent windings.
4. The article of claim 1, wherein the ceramic tube is cylindrical
or tapered.
5. The article of claim 4, wherein the ceramic tube is
cylindrical.
6. (canceled)
7. The article of claim 1, wherein the ceramic tube is comprised
mainly of ceramic.
8. (canceled)
9. (canceled)
10. The article of claim 7, wherein the ceramic is an oxide.
11. The article of claim 10, wherein the ceramic tube is comprised
of at least one oxide material selected from the group consisting
of SiO.sub.2, Al.sub.2O.sub.3, CaO, Na.sub.2O, MgO,
Fe.sub.2O.sub.3, TiO.sub.2, K.sub.2O, MnO, P.sub.2O.sub.5,
Cr.sub.2O.sub.3, and ZrO.sub.2 and combinations thereof.
12. (canceled)
13. (canceled)
14. The article of claim 1, wherein the ceramic tube is a burner
sheath.
15. (canceled)
16. The article of claim 1, the ceramic tube having of a pattern of
surface features.
17. The article of claim 16, wherein the pattern of surface
features is a pattern of perforations extending through the
thickness of the tube.
18. (canceled)
19. The article of claim 17, further comprising a transport tube
for delivering combustible gas to the burner sheath, the transport
tube being positioned inside the burner sheath.
20. The article of claim 19, wherein the transport tube has an open
first end and a closed second end, the transport tube having a
plurality of holes along its length through which the combustible
gas flows, to the burner sheath.
21. (canceled)
22. The article of claim 17, wherein the pattern of perforations
defines an open surface area not greater than about 50%.
23. The article of claim 22, wherein the open surface area is not
less than about 20%.
24. The article of clam 18, wherein the pattern of perforations is
comprised of perforations, each perforation having a diameter not
less than about 0.025 centimeters and not greater than 0.25
centimeters.
25. An article comprising: a ceramic extrudate having an outer
surface and a pattern of surface features along the outer surface,
the surface features of the pattern of surface features being
spread apart from each other along circumferential and longitudinal
directions.
26. The article of claim 25, wherein the ceramic extrudate
comprises a sintered ceramic extrudate.
27. The article of claim 25, wherein the ceramic extrudate is
hollow.
28. The article of claim 27, wherein the ceramic extrudate
comprises a helical tape having adjacent windings, the windings
being joined to form a tube.
29. (canceled)
30. The article of claim 28, wherein the tube has a tube wall and
the pattern of surface features comprises an array of perforations
extending through the tube wall.
31. (canceled)
32. (canceled)
33. The article of claim 28, wherein the ceramic extrudate is a
burner component, wherein the tube forms a sheath.
34. The article of claim 33, further comprising a transport tube
for delivering combustible gas to the sheath, the transport tube
being positioned inside the sheath.
35. he article of claim 25, wherein the pattern of surface features
is a pattern of perforations.
36. (canceled)
37. The article of claim 25, wherein the ceramic extrudate is
comprised mainly of ceramic.
38. (canceled)
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
BACKGROUND
[0001] 1. Field of the Disclosure
[0002] The following is directed generally toward ceramic articles.
Particularly, the following is directed towards extruded ceramic
articles which may find use in burner applications
[0003] 2. Description of the Related Art
[0004] Ceramics are a robust material capable of various
applications, for example, superconductors, semiconductors,
abrasives, cookware, and electrical and thermal insulators.
Superconducting and semiconducting ceramics are modern ceramic
materials characterized by their unique electrical properties and
involved processing requirements. Traditional ceramics, such as
insulating ceramics, are generally characterized by strong and
brittle fired bodies capable of thermal and electrical insulating
properties far superior to metals or polymers. Traditional ceramics
comprise a mixture of inorganic materials that, upon firing,
creates a chemically inert and stable body, making traditional
ceramics resilient to oxidation and other environmental effects
that plague polymers and metals.
[0005] Traditional ceramics are characterized by more traditional
forming methods such as slip casting, screening, pressing, molding
and extrusion. All of these traditional processes make use of a
ceramic slurry or moist ceramic powder body, which is created by
mixing ceramic powder in a solvent such as water. The density of
the slurry is controlled and altered depending upon the processing
demands of the desired processing method. However, all of these
processes have in common the fact that a green, or unfired, ceramic
body is created. Furthermore, traditional ceramics share the common
processing requirement of sintering, or a final firing, in which
the green or unfired ceramic is solidified in a high temperature
and long dwelling, firing process. The sintering process creates
the traditional ceramic characteristics of brittleness and heat
resistance.
[0006] Because of the heat resistant nature of traditional ceramic
bodies they are used in a variety of applications such as
electrical insulators, thermal insulators, cookware, and coatings
for metals. However, because of the brittle nature of the sintered
traditional ceramic material, post processing mechanical
alterations and manipulations of the ceramic body are very limited.
A sintered ceramic cannot be subject to high, post-firing strains,
otherwise the entire ceramic pieces will fail. For example, while a
sheet of metal after it has been formed may be punctured and
manipulated to fit a specific application, ceramics are generally
manipulated in the green state. This drawback has limited the use
of traditional ceramics in creating, for example, a more efficient
burner.
[0007] Burners are used in a variety of commercial and residential
applications. The applications range from uses in the kitchen, to
heating or boiling water, to boiling oils in deep fat fryers, to
melting metals and glass in commercial applications. Prior art
burners are characterized by a transport tube or diffuser tube for
transporting and delivering the gas to the combustion zone. The
blocks are characterized by a number of holes or pores along their
length allowing the gas to escape the block and combust on the
surface of the block emitting thermal radiation. Burners are
comprised of high-temperature metals or ceramics or a combination
of the two, to withstand the heat that is generated on the surface
of the burner.
[0008] Particularly, burners made of metal are desirable because
the pattern of holes or orifices in the transport tube are easily
made for uniform size and spacing. The pattern of the holes is
generally important because it determines the way in which the gas
is delivered to the combustion zone; the more uniform the pattern
of holes, the more uniform the thermal radiation that is created by
the burner. In some configurations, a burner will consist of
multiple metal tubes, inside one another, each tube having a unique
or different pattern of holes. Often, the inner most steel tube,
the diffuser, has a few large holes along its length in order to
effectively mix air and gas and diffuse the mixture into an outer
or secondary steel tube encompassing the diffuser. The outer, steel
tube encompassing the diffuser may have a greater number of holes,
those holes having a smaller cross section than the holes in the
diffuser, to further mix and diffuse the mixture of gas and air
effectively. The drawback with metal burners is that they are
typically incapable of withstanding high temperatures and are
susceptible to corrosion.
[0009] Alternatively, ceramic burners consist of at least one layer
of porous ceramic such as a reticulated ceramic. Ceramic burners
are typically coupled with the metal burners in a configuration
where the ceramic layer is the outer most layer, and acts as a high
temperature and corrosion resistant sheath for the burner. However,
a problem with such burners is that given the nature of producing
reticulated ceramics through a foam loss process or weaving of
ceramic fibers to create a high-porosity structure, the consistency
and uniformity of pore size are lost or at least difficult to
control. Reticulated ceramics are generally made through a foam
loss process whereby a high porosity foam is covered in a ceramic
slurry and upon firing of the ceramic and foam, the foam is burned
off, or lost. The remaining reticulated ceramic is a high porosity
structure, but the consistency and uniformity of the pores is poor.
The same problem exists for weaving of fibers of SiC or SiN. The
entanglement of the fibers creates a high porosity ceramic, but the
consistency and uniformity of the pores is poor. The inconsistent
pore size and spacing makes for inefficient and unequal
distribution of infrared radiation and makes the burner less
efficient. Furthermore, the attachment of the ceramic sheath to the
stainless tubes is difficult.
[0010] The industry continues to demand ceramic components having
novel structures and novel techniques for fabrication. The burner
industry in particular demands high temperature ceramic burners for
use in broad range of applications.
SUMMARY
[0011] According to the first aspect of the disclosed article, a
ceramic tube comprising a sintered helical tape is provided. The
sintered helical tape having adjacent windings, which are joined
together.
[0012] Another aspect provides an article comprising a ceramic
extrudate. The ceramic extrudate having an outer surface and a
pattern of surface features along the outer surface. The pattern of
surface features is spread apart from each other along
circumferential and longitudinal directions of the ceramic
extrudate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The present disclosure may be better understood, and its
numerous features and advantages made apparent to those skilled in
the art by referencing the accompanying drawings.
[0014] FIG. 1 is an illustration of a tube comprised of helical
windings.
[0015] FIG. 2A is an illustration of helical windings being
perforated and forming a tube.
[0016] FIG. 2B is an illustration of a pattern of perforations.
[0017] FIG. 2C is an illustration of an array of perforations.
[0018] FIG. 3 is an illustration of a diffuser tube inside a
perforated ceramic tube.
[0019] FIG. 4 is an illustration of a perforation having a
cylindrical contour.
[0020] FIG. 5 is an illustration of a perforation having a tapered
contour.
[0021] FIG. 6 is an illustration of a perforation having a
partially tapered contour.
[0022] FIG. 7 is an illustration of a tube having a tapered
contour.
[0023] FIG. 8 is an illustration of a tube having more than one
tapered contour.
[0024] FIG. 9A is an illustration of a rotating auger, die,
rotating extrudate and jig configuration.
[0025] FIG. 9B is an illustration of a die opening having an
annular contour.
[0026] FIG. 9C is an illustration of a die opening having a
rectangular contour.
[0027] The use of the same reference symbols in different drawings
indicates similar or identical items.
DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
[0028] FIG. 1 illustrates a ceramic tube 100 comprised of helical
windings 102. In one particular embodiment, the tube is formed by
winding a ceramic tape formed of the helical windings 102. The
ceramic tape may be formed by extrusion, however, as discussed
previously there are many other methods of forming ceramic tape
such as, casting, molding or deposition. In other embodiments,
discussed in more detail below, the ceramic tube 100 is formed by a
direct extrusion process in which the extrudate assumes the form of
a tube, in which helical windings are joined together prior to exit
from the die. Extrusion offers an advantage of a continuous
processing that provides an extrudate with a desirable green
strength and flexibility to undergo further manipulation, such as
surface texturing of the extrudate and/or winding of the tape into
a helix. The extruded ceramic body is a clay body or a ceramic
powder body having the consistency of a moist ceramic powder with
some plastic characteristics. In one embodiment, the moisture
content of the ceramic powder body is in a range of about 5% to 30%
by weight. In other embodiments, the moisture content of the
ceramic powder body is in a range of about 10% to 15% by weight and
as such, in a range of about 15% to 17% by weight.
[0029] In the case of tape extrusion, after forming the ceramic
tape, the tape is wound to form an extruded helical tape having
helical windings 102. The helical windings 102 are collected,
compressed and the adjacent windings are joined to form a tube. The
green ceramic tube is then fired to form a sintered helical tape,
which forms the sintered ceramic tube. As is understood in the art,
firing of ceramics involves sintering the ceramic body, which
typically involves holding the ceramic at high temperatures for a
long duration to solidify and strengthen the body.
[0030] Suitable ceramic materials are selected that will yield a
body capable of forming an extrudate as well as a final sintered
piece that has resilience to high temperatures. According to one
embodiment, it is preferable that the ceramic body withstands the
processing requirements of extrusion. As such, the final sintered
ceramic will generally contain not less than about 85% ceramic by
weight, while in other embodiments the final sintered ceramic will
contain not less than about 95% ceramics by weight. A wide range of
ceramics is available which generally includes materials such as
oxides, nitrides and carbides, or combinations of these materials.
For example, a body may contain oxides, such as SiO.sub.2,
Al.sub.2O.sub.3, CaO, Na.sub.2O, MgO, Fe.sub.2O.sub.3, TiO.sub.2,
K.sub.2O, MnO, P.sub.2O.sub.5, Cr.sub.2O.sub.3, and ZrO.sub.2, in
various proportions. As is understood in the art, the ceramic body
may make use of higher percentages of SiO.sub.2 and
Al.sub.2O.sub.3, in comparison to other oxides, because these
ceramic compounds are the basis for stable ceramic bodies. In
comparison, the other oxides may be used to a lesser degree as
fillers or fluxes. Other embodiments may make use of SiC or
SiN.
[0031] According to the embodiment shown in FIG. 2A, a tape 200 is
extruded and gathered on a shaft 204. The tape 200 is wound around
the shaft 204 to form a helix having adjacent windings 202. After
sufficient windings have been gathered on the shaft, the adjacent
windings 202 are joined to form a tube 206 or hollow cylinder from
a single layer of windings. The windings can be joined using a
variety of techniques. For example, one embodiment demonstrates
wetting the windings and compressing the windings to join the
single layer of adjacent windings to form the tube 206. Another
embodiment contemplates the use of lap joints, wherein the edges of
the adjacent windings overlap to form a tube. In this regard, the
lap joints may be formed during sintering processing, such that
natural shrinkage and densification of the extrudate forces the
windings together. The adjacent windings are solidified and
strengthened in the sintering process. Other embodiments show the
windings joined during the extrusion process. In the embodiment
shown in FIG. 2A, the windings are joined post-extrusion. In
contrast, as already noted above, the windings may be jointed in
situ, in the die prior to exit, such that the as-formed extrudate
is a tube rather than a tape.
[0032] The tube wall thickness is determined in part by the die
opening. The tube wall thickness is defined as the difference
between the inner radius of the tube wall and the outer radius of
the tube wall. In one embodiment, the tube wall thickness is not
less than about 5 millimeters, such as not less than 10
millimeters, and still, in other embodiments, the tube wall
thickness is not less than 20 millimeters. For example, in one
embodiment, the tube wall thickness is approximately 25
millimeters.
[0033] In one embodiment, post extrusion processing of the
extrudate includes providing surface features on the ceramic tape.
For example, in FIG. 2A the surface features are a pattern of
perforations 208 that are spread apart from each other along the
circumferential and longitudinal directions. The pattern of
perforations 208 extend through an entirety of a thickness of the
extrudate and provide 360 degrees of coverage. The perforations may
be provided in a variety of ways. For example, FIG. 2A demonstrates
providing the perforations using an array of reciprocating pins
210. In the embodiment, the array of reciprocating pins 210 is
connected to a motor 212, capable of translating the array of
reciprocating pins 210 in one direction. The pins translate in one
direction and perforate the extrudate through the thickness of the
extrudate. In one embodiment, the array of reciprocating pins 210
perforate the extrudate in the form of a tape as it exits the
extruder, but before helical windings or a tube is formed. Other
embodiments contemplate perforating the tape after the helical tape
202 forms the windings, as shown in FIG. 2A, while other
embodiments perforate an extrudate in the form of a rotating
ceramic tube emerging from the die. Still other embodiments utilize
multiple arrays of reciprocating pins, or an array of reciprocating
pins that can translated in various planes, whereby the pins are
rotated about the helical windings.
[0034] The embodiment illustrated in FIG. 2A utilizes an array of
pins to provide a pattern of perforations 208. As used herein, the
term `pattern` is defined as a repeatable unit of perforations. For
example, FIG. 2B illustrates a pattern, having a repeatable unit of
four perforations 260. However, the embodiment illustrated in FIG.
2A also contemplates an array of pins capable of providing an array
of perforations 208. As defined herein, an `array` is a species of
a pattern, and denotes a two-dimensional grid of perforations
having generally uniform spacing, and typically uniform shape and
size. An array of perforations is illustrated in FIG. 2C in which
each perforation is approximately an equal distance and orientation
from any other perforation.
[0035] As previously discussed, in some applications such as burner
components, there is a need for high temperature, high void area
ceramics. More particularly, there is a need for high temperature
ceramics manufactured with controlled uniformity of open porosity,
because the uniformity of the pores corresponds to uniform gas flow
from the burner component, and accordingly, uniform and efficient
burner operation. In one embodiment, illustrated in FIG. 3, a
burner component or burner sheath 302 is overlying a transport tube
306. The transport tube 306 is typically metal and delivers the
combustible gas to the burner sheath 302. The transport tube 306
typically has an open first end for receiving the gas, a closed
second end, and a plurality of holes along its length through which
the combustible gas flows. The transport tube 306 has larger but
fewer holes than the burner sheath 302. The gas flows through the
plurality of holes in the transport tube 306 and is delivered into
the burner sheath 302. The gas flows through the pattern of
perforations in the burner sheath 302 and is ignited by an igniter
on the surface of the burner sheath 302.
[0036] The open porosity in the form of through holes or
perforations is controlled not only in terms of the uniformity of
the spaces between the perforations but in the shape and size of
the perforations as well. FIGS. 4-6, illustrate perforation shapes.
To use the previous example, according to certain embodiments,
burner components and burner sheaths not only have uniformity of
open pore spacing, but uniformity of pore size and shape. The array
of reciprocating pins 210 may be modified to determine the size and
shape of the perforations. For example, in FIG. 4, the perforation
402 is shown as having a cylindrical contour through the thickness
of the extrudate 404. The cylindrical pin 406 perforates, extending
through the thickness of the extrudate 404 to form a perforation
402 having a cylindrical contour. The diameter of the perforation
is approximately the same as the diameter of the cylindrical pin
406.
[0037] Another embodiment, as illustrated in FIG. 5, illustrates a
tapered perforation 502 having a tapered contour. Tapered
perforations can vary, in the degree to which the hole is tapered,
as well as the direction in which the tapered contour extends. In
burner components, a tapered perforation produces a pressure drop
across the thickness of the tube, forcing the gas out of the tube
and through the hole while distributing the gas across a wider
surface at the outer surface of the extrudate, where the gas is
ignited. As shown in FIG. 5, typically the tapered perforation 502
has a greater cross sectional area at the outer surface of the
extrudate 504 than the cross sectional area of the perforation at
the inner surface 506. The tapered perforation 502 is made using a
pin 508 having a tapered head that perforates the thickness of the
extrudate 510.
[0038] Yet another embodiment, illustrated in FIG. 6, shows a
partially tapered perforation 602, wherein the tapered contour of
the perforation 604 extends only partially through the thickness of
the extrudate, and the remaining portion of the perforation extends
having a cylindrical contour 606. The partially tapered pin 608
perforates the thickness of the extrudate 610 to make the
perforation 602 having the combination of the tapered contour 604
and the cylindrical contour 606. Other perforations having unique
contours are contemplated but not illustrated.
[0039] Further, according to embodiments herein, the open surface
area as a result of the perforations on the surface of the ceramic
tube, is variable. The term `open surface area` is herein defined
as the percentage of surface area of the ceramic tube that is
consumed by the perforations. For example, the open surface area
for a tube as described previously is calculated as the total
surface area of the perforations divided by the total surface area
of the outer surface of the tube without the perforations. In
applications such as burner sheaths, the open surface area is
related to the rate at which the gas burns and the amount of heat
that is emitted on the surface area of the ceramic tube. According
to one embodiment, the open surface area on the surface of the
ceramic tube is not less than about 20%, such as not less than
about 35%. In various embodiments, the open surface area is not
greater than about 50%.
[0040] The open surface area may be varied by manipulating the
number of perforations per unit of area or the size of the
perforations. The number of perforations per unit of area is
determined by the number of reciprocating pins in the array of
reciprocating pins and/or the speed of extrusion. The size of the
perforations is determined by the diameter of each pin in the array
of reciprocating pins. In the instance of burner components,
generally the larger the diameter of the ceramic tube required, the
larger the diameter of the perforations necessary to accommodate
the increased gas pressure. However, it is recognized that the size
of the perforations depends in part upon the application and the
size of the perforations may be determined based upon the gas
velocity and flame velocity dynamic. Moreover, the larger the
diameter of the burner component required, the thicker the
extrudate and the thicker the tube wall. A thicker tube wall may
require a larger diameter pin to effectively perforate through the
proportionally thicker extrudate. According to embodiments herein,
the diameter of the perforations created by the array of
reciprocating pins is not less than about 0.025 centimeters, such
as not less than about 0.050 centimeters, or even not less than
about 0.10 centimeters. The diameter of the perforations is
typically not greater than about 0.25 centimeters.
[0041] The shape of the tube is variable in order to accommodate a
variety of applications. The tube as shown in FIGS. 1 and 2 is a
hollow cylinder, but other embodiments are contemplated. A
cylindrical tube provides a common shape and sufficient surface
area to be used in a variety of applications. For example, in the
case of burners, cylindrical tubes or housings are common because
the gas is easily and evenly distributed through the symmetry of
the tube, making the radiation emitted by the burner uniform,
making the burner efficient. Moreover, a cylindrical tube is
compatible with a variety of metal transport tubes for burner
applications, as illustrated in FIG. 3.
[0042] In other embodiments, as shown in FIGS. 7 and 8, the tube
has a tapered shape. FIG. 7 illustrates that the tube may be
tapered in one direction to form a conical contour, wherein the
diameter of the tube decreases at a generally constant rate along
the length of the tube. In another embodiment, as illustrated in
FIG. 8 the contour of the tube is more complex, having a bi-conical
tapered shape such that the diameter of the tube expands from an
interior, smallest diameter portion. The tapered tube in this
embodiment has flanges 802 at opposite ends of the tube and is
constricted between the flared ends. Other embodiments are
contemplated, for example, closing one end or both ends of the
cylinder in a variety of ways, depending upon the anticipated final
application of the tube.
[0043] FIG. 9A illustrates a generalized setup of an extrusion
apparatus for extruding tapes and tubes. A die punch 902 is
positioned at the open end of the extrusion barrel 904 to force the
ceramic powder body 906 into helical channels die 912.
Alternatively, the die punch 902 can be an auger or any mechanism
used to advance the ceramic powder body 906 into the helical
channels 912. The helical channels 912 are defined by an inner
barrel 910 housed within the extrusion barrel 904, and a spiraling
divider 914 spaced circumferentially between the extrusion barrel
910 and inner barrel 910 and spiraling around the inner barrel 910.
The ceramic powder body 906 is forced into the helical channels 912
and is formed into a helical-shaped ceramic powder body 916. The
helical-shaped ceramic powder body 916 moves through the helical
channels 912 to a die exit region 918. The die exit region 918 is
positioned at the opposite end of the extrusion barrel 904 from the
die punch 902 and is defined by a circumferentially tapered contour
for constricting the helical-shaped ceramic powder body 916 exiting
the helical channels 912. In one embodiment, the circumferentially
tapered contour of the die exit region 918, illustrated in FIG. 9,
compresses the windings of the helical-shaped ceramic powder body
916 so that the adjacent windings are joined and a hollow tube 920
is formed prior to exit from the die exit region 918.
[0044] In one embodiment, the hollow tube 920 is extruded through a
die opening 922 positioned at the exit end of the extrusion barrel
904. The hollow tube 920 rotates during extrusion and exits from
the die exit region 918, through the die opening 922 and is
gathered on a shaft 924. Rotation of the hollow tube 910 along the
shaft 924 enables formation of a pattern of surface features, such
as perforations, using a fixed position perforation mechanism, such
as an array of pins 926 as discussed above. More specifically, the
array of pins 926 as shown in the embodiment, perforate the hollow
tube 920 as it rotates and exits through the die opening 922. In
this embodiment the ceramic tube extrudate 910 is gathered on the
shaft 912 and is not extruded as a tape that requires winding. In
addition, extrusion of an as-formed tube from internally joined
helical windings may ease mechanical strains in the extrudate, as
compared to embodiments relying on tape extrusion. Further, the
circumferentially tapered contour of the die exit region 918 allows
for continuous processing without need for winding a tape. The
embodiment shows a continuous process that improves throughput.
[0045] The shape of the extruded ceramic powder body is determined
in part, by the contour of the die opening 922. FIGS. 9B and 9C
illustrate two embodiments of die opening contours. FIG. 9B is a
die opening having an annular contour. The annular contour of the
die opening defines an extrudate with an annular cross section.
According to one embodiment, as previously discussed, the annular
contour of the die opening 922 allows the helical-shaped ceramic
powder body 916 to be extruded as a hollow tube 920 upon exit from
the extruder.
[0046] FIG. 9C is a die opening having a rectangular contour.
According to one embodiment, the die opening may have a polygonal
contour, while other embodiments contemplate a die opening having
the contour of a parallelogram. The die opening having a
rectangular contour, as illustrated in FIG. 9C, allows the
extrudate to form a tape, which can form a helical tape having
windings. In one embodiment the tape is gathered on a jig, while
other embodiments make use of a shaft and the tape is wound around
the shaft to form helical windings. Some embodiments make use of
both the shaft and a jig. As discussed previously, the windings may
be joined in the green state or during firing. Still, in another
embodiment the windings may not be joined and remain spaced apart
from each other forming gaps between the windings.
[0047] The die opening 908 defines in part, the cross sectional
contour of the extrudate. For example, the die opening having a
rectangular contour, illustrated in FIG. 9C, can be described by a
dimension ratio of the die opening measurements. The dimension
ratio of the die opening is a ratio between the measurement of the
width and the thickness. In one embodiment, the die opening has a
dimension ratio of not less than about five to one, where the width
is five times the measurement of the thickness. Still, in another
embodiment the dimension ration is not less than about ten to one,
where the width is ten times the measurement of the thickness.
[0048] While particular aspects of the present invention have been
described herein with particularity it is well understood that
those of ordinary skill in the art may make modifications hereto
yet still be within the scope of the present claims. The previously
mentioned embodiments and examples, in no way limit the scope of
the following claims.
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