U.S. patent application number 11/197050 was filed with the patent office on 2006-02-09 for method for fabricating ceramic articles and ceramic articles produced thereby.
Invention is credited to Douglas Munroe Beall, Irene Mona Peterson, David John Thompson.
Application Number | 20060030475 11/197050 |
Document ID | / |
Family ID | 35756633 |
Filed Date | 2006-02-09 |
United States Patent
Application |
20060030475 |
Kind Code |
A1 |
Beall; Douglas Munroe ; et
al. |
February 9, 2006 |
Method for fabricating ceramic articles and ceramic articles
produced thereby
Abstract
A method for fabricating a ceramic article which includes
providing a batch comprising components of (i) a mixture of
inorganic raw materials comprising talc, alumina, and silica; (ii)
a binder comprising a water-soluble organic binder and a fibrous
silicate mineral having a high aspect ratio in combination with a
large surface area; and (iii) a polar solvent; mixing the batch
components to form a homogenous and plasticized mass; shaping the
plasticized mass into a green body wherein the green body has
improved strength; and, sintering the green body by heating to a
temperature and for a time to initiate and sufficiently achieve
conversion of the green body into a fired ceramic article.
Inventors: |
Beall; Douglas Munroe;
(Painted Post, NY) ; Peterson; Irene Mona; (Elmira
Heights, NY) ; Thompson; David John; (Savona,
NY) |
Correspondence
Address: |
CORNING INCORPORATED
SP-TI-3-1
CORNING
NY
14831
US
|
Family ID: |
35756633 |
Appl. No.: |
11/197050 |
Filed: |
August 3, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10911083 |
Aug 3, 2004 |
|
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11197050 |
Aug 3, 2005 |
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Current U.S.
Class: |
501/119 ;
264/641; 501/128 |
Current CPC
Class: |
C04B 2235/3481 20130101;
C04B 2235/3218 20130101; C04B 2235/80 20130101; C04B 2235/3418
20130101; C04B 2235/5296 20130101; C04B 2235/3206 20130101; C04B
2235/3217 20130101; C04B 2235/9607 20130101; C04B 35/6263 20130101;
C04B 35/632 20130101; C04B 2235/3463 20130101; C04B 35/195
20130101; C04B 2235/349 20130101; C04B 2235/96 20130101; C04B
35/185 20130101; C04B 2235/5228 20130101; C04B 2235/3445 20130101;
C04B 2235/449 20130101; C04B 2235/6021 20130101; C04B 35/6365
20130101 |
Class at
Publication: |
501/119 ;
501/128; 264/641 |
International
Class: |
C04B 35/195 20060101
C04B035/195; C04B 33/36 20060101 C04B033/36 |
Claims
1. A method for fabricating a ceramic article, comprising:
providing a batch comprising components of (i) a mixture of
inorganic raw materials comprising talc, alumina, and silica; (ii)
a binder comprising a water-soluble organic binder and a fibrous
silicate mineral having a high aspect ratio in combination with a
large surface area; and (iii) a polar solvent; mixing the batch
components to form a homogenous and plasticized mass; shaping the
plasticized mass into a green body; and sintering the green body by
heating to a temperature and for a time to initiate and
sufficiently achieve conversion of the green body into a fired
ceramic article.
2. The method according to claim 1 wherein the green body has
improved strength in a temperature region between
300.degree.-900.degree. C. to resist cracking during the sintering
as compared to a like green body without the fibrous silicate
mineral.
3. The method according to claim 1 wherein the ceramic article has
improved final strength after the sintering as compared to a like
ceramic body without the fibrous silicate mineral.
4. The method according to claim 1 wherein the inorganic raw
materials are present in an effective which in combination with the
other batch components are capable of yielding a fired ceramic
article whose main phase is cordierite.
5. The method according to claim 1 wherein the organic binder is a
cellulose ether binder.
6. The method according to claim 5 wherein the cellulose ether
binder is a methylcellulose binder.
7. The method according to claim 6 wherein the methylcellulose
binder is added in an amount of 2.5-10% by weight
super-addition.
8. The method according to claim 7 wherein the methylcellulose
binder is added in an amount of 2.5-5% by weight
super-addition.
9. The method according to claim 1 wherein the fibrous silicate
mineral is attapulgite clay.
10. The method according to claim 1 wherein the fibrous silicate
mineral is added in an amount of 2-10% by weight.
11. The method according to claim 8 wherein the fibrous silicate
mineral is added in an amount of 5-10% by weight.
12. The method according to claim 1 wherein the fibrous silicate
mineral is added in an amount of 1-3% by weight.
13. The method according to claim 1 wherein the batch includes
other optional organic and inorganic components to be used as
processing aids.
14. The method according to claim 13 wherein the batch includes a
surfactant and a pore former.
15. The method according to claim 1 wherein the green body is a
honeycomb structure.
16. The method according to claim 1 wherein the batch comprises
100% by weight cordierite-forming inorganic raw materials, 2.0 to
10.0% by weight attapulgite clay, and based on 100% by weight
cordierite-forming raw materials 2.5 to 10% by weight
methylcellulose, up to and including 3% by weight sodium stearate,
up to and including 30% by weight graphite, and 25.0 to 40.0% by
weight water, as solvent.
17. The method according to claim 16 wherein the batch comprises
100% by weight cordierite-forming inorganic raw materials, 5.0 to
10.0% by weight attapulgite clay, and based on 100% by weight
inorganic raw materials 2.5 to 5.0% by weight methylcellulose, up
to and including 3% by weight sodium stearate, up to and including
30% by weight graphite, and 25.0 to 40.0% by weight water, as
solvent.
18. The method according to claim 1 wherein the batch comprises
100% by weight cordierite-forming inorganic raw materials, 1.0 to
3.0% by weight of the fibrous silicate mineral based on 100% by
weight inorganic raw materials, and 2.5 to 10.0% by weight of the
water-soluble organic binder.
19. The method according to claim 1 wherein the batch comprises
100% by weight cordierite-forming inorganic raw materials, 1.0 to
3.0% by weight attapulgite clay based on 100% by weight inorganic
raw materials, and 2.5 to 10.0% by weight methylcellulose.
20. The method according to claim 19 wherein the batch further
comprises up to and including 3% by weight sodium stearate, up to
and including 30% by weight graphite, and 25.0 to 40.0% by weight
water, as solvent.
21. The method according to claim 1 wherein the fibrous silicate
mineral has an aspect ratio greater than 500 and a surface area
greater than 100 m.sup.2/gm.
22. The method according to claim 1 further characterized by a
median particle size of 1-2 microns.
22. A ceramic article, comprising: a predominant phase of
cordierite having a composition, expressed on an oxide basis, of 33
to 41% of aluminum oxide, 46 to 53% of silica, and 11 to 17%
magnesium oxide wherein said article is manufactured from a batch
including a mixture of inorganic raw materials comprising talc,
alumina, and silica; a binder comprising a water-soluble organic
binder and a fibrous silicate mineral having an aspect ratio
greater than 500, a surface area greater than 100 m.sup.2/gm; and a
polar solvent.
Description
RELATED APPLICATIONS
[0001] This is a continuation-in-part application of U.S. patent
application Ser. No. 10/911,083 filed Aug. 3, 2004 entitled "Method
For Fabricating Ceramic Articles."
FIELD OF THE INVENTION
[0002] The present invention relates generally to a method for
fabricating ceramic articles from moldable powdered mixtures that
are formed by mixing inorganic particulate raw materials with a
binder system that includes a water-soluble organic binder and an
inorganic binder, and water as a solvent. More particularly, the
method relates to the manufacturing of cordierite articles having
improved strength to resist cracking and damage during
sintering.
BACKGROUND OF THE INVENTION
[0003] Popular cellular ceramic monoliths which are generally
formed by extrusion, such as cordierite honeycomb substrates which
find applications in catalytic converters, diesel particulate
filters, electrically heated catalyst, and chemical processing
catalyst, require binders and other similar aids for proper
processing. Typically, the binder which is an organic material must
meet a number of requirements.
[0004] For example, the binder must be compatible with the ceramic
material such that a flowable dispersion comprising a relatively
high loading of the ceramic material in the binder may be provided.
Also, the "green" perform produced by shaping the dispersion of
ceramic powder in the binder should have reasonable strength such
that it can be handled.
[0005] For desirable burnout, the binder should be removable from
the shaped ceramic part without incurring distortion or breakage of
the part. Also, the binder-free perform should be strong enough to
undergo defect-free consolidation. The formulations of binders
meeting these requirements is complex and a large number of
different binder formulations are known in the art.
[0006] Typically, water-soluble cellulose ether binders are used
for cordierite-forming batches. These binders result in
subsequently formed green bodies having good "wet" strength, as
well as good integrity in size and shape. "Wet" strength is used to
describe the strength of the body after extrusion but before
drying. "Green" strength refers to the strength of the body after
drying but before firing.
[0007] Cellulose ether binders which burnout in the temperature
region between 100.degree. C.-600.degree. C., and more specifically
around 300.degree. C., are difficult to remove without incurring
distortion or breakage of the ceramic part. Removal of organic
components during firing involves a sequence of simultaneous
reactions which are fairly complex, including, for example,
oxidation, volatilization, and thermal degradation.
[0008] Therefore, the major obstacle in working with plasticized
mixtures including organic binders is that the subsequently-formed
green ceramic article may crack when fired, particularly in thin
walled-honeycomb structures. The cracking is a result of internal
stresses developed during the removal of large amounts of organics
which causes excessive temperature or pressure gradients internal
to the bodies.
[0009] Accordingly, special considerations must be undertaken
during firing to avoid cracking of the ceramic body. For example,
long firing cycles, specially designed kilns, and similar means
have been used to control the burnout of organic binders and reduce
the thermal stresses, differential shrinkage and high cracking
frequency. These methods however, require expensive and
sophisticated equipment and increase the cost of firing and
manufacturing.
[0010] In light of the foregoing problems experience in the art,
there remains a need for a method of fabricating ceramic articles,
and in particular cordierite ceramic bodies having improved
strength to withstand the thermal stresses and shrinkage which form
in the body during sintering thereby enabling such articles to be
fired with less cracks and defects, in a cost-effective and
efficient manner.
SUMMARY OF INVENTION
[0011] The present invention is a method for fabricating a ceramic
article by providing a batch comprising powdered inorganic raw
materials, binder, and solvent. The binder includes a water-soluble
organic binder, such as cellulose ether binder, and a fibrous
silicate mineral. The fibrous silicate material preferably has a
high aspect ratio (preferably greater than 500) in combination with
a large surface area (preferably greater than 100 m.sup.2/gm). More
preferably, the fibrous silicate material exhibits a median
particle size of 1-2 microns. The batch preferably also includes a
polar solvent. A suitable polar solvent is, for example, water.
[0012] In one embodiment the inorganic powder materials are a
mixture of cordierite-forming raw materials and include silica,
talc, alumina, optionally clay and other cordierite-forming raw
materials, each of the raw materials present in an effective amount
which in combination with the other batch components, are capable
of yielding a fired ceramic article whose main phase is
cordierite.
[0013] The batch components are mixed together to form a
homogeneous and plasticized mass, which is then shaped into a green
body. The shaping can be performed according to any known method in
the art. In one embodiment the green body is a honeycomb monolith.
To form such a structure, the plasticized mass is preferably
extruded through a honeycomb die. Finally, the green body is
sintered to a temperature and for a time to initiate and
sufficiently achieve the conversion of the green body into a fired
ceramic article.
[0014] It has been found that by using organic and inorganic
components for the binder, ceramic articles of the type described
can be fired faster with less or no cracks. In particular the green
bodies have improved strength in a temperature region between
300.degree. C. to 900.degree. C., and are therefore more resistant
and less susceptible to cracking and being damaged during
subsequent sintering. The preferred amount of the inorganic
component, preferably a fibrous silicate mineral for providing
improved strength in a temperature region between 300.degree. C. to
900.degree. C. is between 2-10%. One preferable fibrous silicate
mineral is attapulgite clay. Furthermore, when using organic and
inorganic components for the binder in small amounts (for example
1-3%), the fired strength of the structure may be improved.
[0015] According to another aspect of the invention, a ceramic
article is provided, comprising a predominant phase of cordierite
having a composition, expressed on an oxide basis, of 33 to 41% of
aluminum oxide, 46 to 53% of silica, and 11 to 17% magnesium oxide
wherein said article is manufactured from a batch including a
mixture of inorganic raw materials comprising talc, alumina, and
silica; a binder comprising a water-soluble organic binder and a
fibrous silicate mineral having an aspect ratio greater than 500, a
surface area greater than 100 m.sup.2/gm; and a polar solvent.
BRIEF DESCRIPTION OF DRAWINGS
[0016] A complete understanding of the present invention may be
obtained with reference to the accompanying drawings, when
considered in conjunction with the subsequent detailed description,
in which:
[0017] FIG. 1 is shows shrinkage as a function of temperature up to
800.degree. C. for a green cordierite sample extruded without
attapulgite clay.
[0018] FIG. 2 shows shrinkage as a function of temperature up to
800.degree. C. for a green cordierite sample extruded with 5% by
weight attapulgite clay.
DESCRIPTION OF THE INVENTION
[0019] The invention is applicable to ceramic powder processing for
the fabrication of shaped articles from moldable batches including
inorganic raw materials, binder, and solvent. However, the
invention is particularly suitable to the formation of ceramic
articles which contain cordierite, and/or mullite. Examples of such
ceramic articles include mixtures of 2-60% mullite, and 30-97%
cordierite, with allowance for other phases, typically up to 10% by
weight.
[0020] Some ceramic batch material compositions for forming
cordierite that are especially suited to the practice of the
present invention are those disclosed in U.S. Pat. No. 6,541,407
which is herein incorporated by reference as filed. An embodiment
of a ceramic material which ultimately forms cordierite upon firing
is provided as follows (in percent by weight, assuming 100% by
weight): 33 to 41% of aluminum oxide, 46 to 53% of silica, and 11
to 17% magnesium oxide.
[0021] The inorganic raw materials used in the batch composition
can be synthetically produced materials such as oxides, hydroxides,
and the like, or they can be naturally occurring minerals, such as
clays, talcs, or any combination of these. The invention is not
limited to the types of powders or raw materials. These can be
chosen depending on the properties desired in the body.
[0022] Suitable cordierite-forming inorganic ceramic powder raw
materials for the purpose of forming cordierite-containing ceramic
articles may be selected from any source, and preferably include
high-purity talc, silica, alumina, clay, and magnesia-yielding raw
materials. Preferred raw materials are talc, alumina and
silica.
[0023] The talc has a median particle size (MPS) greater than 15
microns (mm) by less than 35 microns. It ha a platelet morphology
to promote low CTE in the sintered ceramic article. A suitable
morphology index for the talc (i.e., a measure of the degree of
platiness of the talc) is greater than 0.75, as further described
in U.S. Pat. No. 5,141,686. Alumina is used as a source for various
suitable kinds such as alpha-alumina, gamma-alumina, rho-alumina,
aluminum hydroxide, boehmite, and mixtures thereof. The alumina has
a median particle size of between 5 and 25 microns. Silica includes
but is not limited to quartz, cristobalite, non-crystalline silica
such as fused silica or a sol-gel silica, zeolite, diatomaceous
silica, and combinations thereof. The silica has a median particle
size of between 10 and 35 microns. Optionally, the inorganic raw
materials may include a clay, such as kaolin.
[0024] The binder in the present invention includes a water-soluble
organic binder, such a cellulose ether binder, and an inorganic
component. The inorganic component is preferably a fibrous silicate
mineral. Preferably, it has a high aspect ratio (preferably greater
than 500) in combination with a large surface area (preferably
greater than 100 m.sup.2/gm) that is highly charged and has a
strong interaction with a polar solvent (e.g., water). The fibrous
silicate material is preferably further characterized by a median
particle size of 1-2 microns.
[0025] For cordierite-forming batches, a suitable fibrous silicate
mineral is attapulgite clay, which is a hydrated magnesium
aluminosilicate clay. Attapulgite clay has fiber or needle-like
particles contained therein, which are very fine providing a high
aspect ratio and a large surface area. Typically, the aspect ratio
is at least 500. A source for this material is available under the
trade name of Acti-gel.TM. 208 from Active Materials Company (Hunt
Valley, Md.). To achieve the aforementioned green strength
improvement, the fibrous silicate mineral is added to the batch in
an amount of at least 2.0% by weight, but no more than 10.0% by
weight, preferably at least 5.0% by weight, but no more than 10.0%
by weight. However, as will be described below, addition of small
amounts (1-3%) achieve increases in final strength of the ceramic
article.
[0026] Suitable cellulose ether binders are methylcellulose,
ethylhydroxy ethylcellulose, hydroxybutyl methylcellulose,
hydroxymethylcellulose, hydroxypropyl methylcellulose, hydroxyethyl
methylcellulose, hydroxybutylcellulose, hydroxyethylcellulose,
hydroxypropylcellulose, sodium carboxy methylcellulose, and
mixtures thereof. Methylcellulose and/or methylcellulose
derivatives are especially suited with methylcellulose,
hydroxypropyl methylcellulose, or combinations of these being
preferred (available as Methocel.TM., Dow Chemical Co.).
[0027] The organic binder is preferably added in amount of between
2.5 to 10.0% by weight, with between 2.5 to 5.0% by weight being a
more preferred range. If the organic binder content is too low, the
plasticity of the batch may be compromised which may lead to
cracking during extrusion as the batch cannot stretch without
breaking. The organic binder is added as a super-addition to the
inorganic raw materials of the powdered mixture. By super-addition
is meant that to 100 grams of inorganic raw material mixture are
added for example between 2.5 to 10 grams of metal oxide.
[0028] The binder system and powder materials are mixed with a
solvent, such as water, which wets the powder materials and
provides a medium for the binder to dissolve in thus providing
plasticity to the batch. The batch may also include other organic
or inorganic components which are used as optional processing aids.
These include surfactants, lubricants, dispersants, oils and the
like. An oil component provides fluidity necessary for shaping of
the mixture, while maintaining the strength of the binder in the
solvent. Suitable oils include paraffinic oils, such as mineral
oils, hydrogenated polybutenes, alpha olefins, internal olefins,
polyphenyl ethers, polybutenes, and polyisobutylene.
[0029] A surfactant if present, promotes emulsification between the
solvent and the oil component. It disperses or wets the inorganic
powders. Typically, the surfactant by itself without other
substances, is insoluble in the solvent at room temperature.
Suitable surfactants are oleic acid, lauric acid, stearic acid and
combinations of these. A lubricant aids in the formation of a
plasticized batch as known in the art. A suitable example of such a
component is sodium stearate.
[0030] The batch may also include a pore former which is any
particulate substance (not a binder) that burns out of the green
body in the firing step. Suitable types of pore formers include
graphite, starch, polymers, cellulose, flour, and the like.
Graphite is one preferred pore former because it has the least
adverse effect on the processing. Combinations of pore formers may
also be employed.
[0031] In the preparation of ceramic bodies according to the
present invention a moldable batch is preferably prepared by mixing
the powdered raw materials with the binder system and the other
optional components to form a plasticized mixture. The batch
components are mixed in any desired amounts selected.
[0032] In one embodiment the batch includes 100% by weight
cordierite-forming inorganic raw materials, 2.0 to 10.0% by weight
attapulgite clay, and based on 100% by weight cordierite-forming
raw materials, 2.5 to 10% by weight methylcellulose, up to and
including 3% by weight sodium stearate, up to and including 30% by
weight graphite, and 25.0 to 40.0% by weight water, as solvent.
[0033] In another embodiment the moldable powder includes 100% by
weight cordierite-forming inorganic raw materials, 5.0 to 10.0% by
weight attapulgite, and based on 100% by weight cordierite-forming
raw materials 2.5 to 5.0% by weight methylcellulose, up to and
including 3% by weight sodium stearate, up to and including 30% by
weight graphite, and 25.0 to 40.0% by weight water, as solvent.
[0034] The individual components of the binder system are mixed
with the ceramic powder material, the other optional batch
components and an adequate amount of solvent (i.e., water) to form
a homogenous and formable mixture. Particularly, in the case of
batches for ceramic products, the batch formation takes place in
two stages prior to the shaping step. In the first stage or wetting
stage of batch formation, the ceramic materials along with the
binder components and other dry components are dry mixed followed
by addition of the water. The mixing can take place for example in
a Littleford mixer, as known in the art.
[0035] The second stage involves plasticization of the batch.
Typically the wet mix from the first stage is sheared in any
suitable mixer in which the batch will be plasticized, such as for
example in a twin-screw extruder/mixer, auger mixer, muller mixer,
or double arm, etc.
[0036] The resulting plasticized mixture is then shaped into a
green body by any known method for shaping plasticized mixtures,
but is best suited for extrusion through a die. The extrusion
operation, either vertical or horizontal, can be done using a
hydraulic ram extrusion press, or a two stage de-airing single
auger extruder, or a twin screw mixer with a die assembly attached
to the discharge end. In the latter, the proper screw elements are
chosen according to material and other process conditions in order
to build up sufficient pressure to force the batch material through
the die.
[0037] The extruded green body is preferably then dried according
to conditions well known in the art, and fired at a selected
temperature under suitable atmosphere and for a time dependent upon
the composition, size and geometry so as to result in a ceramic
article of the desired ceramic. For example, for a composition
which is primarily for forming cordierite honeycomb structures, the
subsequently formed parts are typically fired at a rate of between
15-100.degree. C. per hour to a maximum temperature of between
1405-1430.degree. C. with the holding times at these temperatures
ranging from about 6-25 hours. Firing times and temperatures depend
upon factors such as kinds and amounts of materials and the type of
equipment utilized.
[0038] In one embodiment ceramic article of the present invention
are honeycomb substrates composed of a cordierite-ceramic body.
Honeycomb structures are well known in the art. They are designed
to have an inlet end or face through which the exhaust gas enters
the body, and an outlet end or face opposite the inlet end, the
exhaust gas exiting the body at the outlet end. A multiplicity of
cells extend between the inlet and outlet ends, the cells having
porous walls. For purposes of a diesel particulate filter, part of
the total number of cells at the inlet end are plugged along a
portion of their lengths, and the remaining part of the cells that
are open at the inlet end are plugged at the outlet end, so that
exhaust passing through the cells of the honeycomb flows into the
open cells, through the cell walls, and out of the structure
through the open cells at the outlet end.
[0039] The benefits and advantages of the present invention having
the combination described above include: (1) a green body that has
good wet strength and shape retention after exiting the die; (2) a
green body that has improved and increased strength in a
temperature region between 300.degree. C. to 900.degree. C. to
withstand thermal stresses and differential shrinkage during
sintering, especially in the beginning of the firing cycle, where
the organics in the batch are removed or burned-off; (3) being able
to employ shorter firing cycles; (4) a more efficient and
cost-effective manufacturing process; and, (5) less wasted
ware.
[0040] The instant invention is thus suitably applied to the
fabrication of complicated formed ceramic bodies, especially
cordierite, that are usually formed by extrusion, and to the
manufacture of the corresponding fired bodies such as multicellular
honeycomb structures having a high cell density and exhibiting thin
cell wall dimensions.
[0041] To more fully illustrate the invention the following
non-limiting examples are provided.
EXAMPLES
[0042] Samples are prepared according to the compositions provided
in Table I below. The dry ingredients are weighed, and mixed with
water and other batch components, followed by kneading in a
stainless steel muller to form a plasticized batch which is then
extruded into cellular honeycomb bodies consisting of multiple
parallel channels of square cross section. The cellular bodies
contain approximately 200 cells per square inch (csi) and have a
wall thickness of 0.019 inches. TABLE-US-00001 TABLE I 1 2 3 4 5 6
7 Comp Inv Inv Inv Inv Inv Inv Raw Materials Wt % Wt % Wt % Wt % Wt
% Wt % Wt % Talc (MPS = mm) 39.96 39.16 37.99 36.02 39.16 37.99
36.02 .alpha.-Al.sub.2O.sub.3 21.54 21.14 20.79 20.32 21.14 20.79
20.32 (MPS = mm) Al(OH).sub.3 16.35 16.35 16.02 15.35 16.35 16.02
15.35 (MPS = mm) Silica (Quartz) 22.15 21.35 20.20 18.31 21.35
20.20 18.31 (MPS = mm) Acti-gel 208 .RTM. 0.00 2.00 5.00 10.00 2.00
5.00 10.00 (Attapulgite Clay) Methocel .TM. F240M 5.00 5.00 5.00
5.00 2.50 2.50 2.50 (Methylcellulose) Graphite 10.00 10.00 10.00
10.00 10.00 10.00 10.00 (MPS = mm) Sodium Stearate 1.00 1.00 1.00
1.00 1.00 1.00 1.00 Water 40 40 40 40 40 40 40
[0043] After drying the parts are fired according to a
predetermined firing cycle up to a maximum temperature of
1425.degree. C. and held there for 15 hours. The cellular bodies
are approximately 5.66 inches in diameter and were cut to be 10
inches in length. After firing, the samples are visually inspected
for cracks, and characterized for the coefficient of thermal
expansion (CTE) as measured by dilatometry between room temperature
(RT) and 975.degree. C., and modulus of rupture (MOR) strength as
measured in a four point bend test on cylindrical rods or cellular
bars. CTE is provided in units of 10.sup.-6.degree. C..sup.-1. The
MOR is measured at 400.degree. C. and 800.degree. C. to determine
the strength of the samples at high temperatures. MOR is provided
in units of pounds per square inch (psi). The measured properties
are provided in Table II below. TABLE-US-00002 TABLE II 1 2 3 4 5 6
7 Comp Inv Inv Inv Inv Inv Inv Properties MOR @ 400.degree. C.
(psi) 5* 179 301 579 165 387 522 MOR @ 800.degree. C. (psi) 5* 180
301 715 182 500 462 GTE (RT-975.degree. C.) 0.93 1.07 1.01 1.13
0.98 1.15 1.22 (10.sup.-6.degree. C..sup.-1) Cracks Many None None
None None None None *measurements taken on cellular bars
[0044] Rod samples of the comparative (Comp) sample were so weak at
both 400.degree. C. and 800.degree. C. that they crumbled before
any measurement could be taken. MOR measurements on cellular bars
indicated readings of 5 psi, the values which are reported in Table
II.
[0045] The inventive (Inv) samples show that the use of attapulgite
clay in combination with methylcellulose in the processing of
cordierite bodies, provides an improvement in the high temperature
green strength at both 400.degree. C. and 800.degree. C., such
during subsequent sintering the bodies can resist the thermal
stresses that are likely to lead to cracking during firing.
Consequently, the inventive samples all survived firing without
cracking. Whereas, the comparative sample cracked.
[0046] FIG. 1 shows shrinkage as measured up to 800.degree. C. for
comparative sample 1 which is processed without attapulgite clay.
Between about 275-590.degree. C. there occurs a large shrinkage
event which is associated with the burnout of the methylcellulose
binder. It is believed that the thermal gradients and stresses
resulting from this shrinkage event cause the sample to crack, as
it does not have the high temperature green strength to accommodate
this large dimensional change.
[0047] Referring now to FIG. 2 therein illustrated is the shrinkage
as measured up to 800.degree. C. for inventive sample 3 which is
processed with 5% by weight attapulgite clay binder. In comparison
with FIG. 1, the dimensional changes occurring between about
275-590.degree. C. are greatly reduced. Therefore, it is believed
that inventive sample 3 did not crack because of this reduction in
dimensional change in combination with the increased strength in
the aforementioned temperature region.
[0048] It has also been observed that the CTE of the inventive
sample increases slightly with increasing amount of the attapulgite
clay. However, by adding no more than 10% by weight of this
material as described above, the resulting CTEs are still within
useful limits for the intended use of honeycombs in catalytic
converters, diesel particulate filters, and the like.
[0049] Another embodiment of ceramic article of the present
invention is preferably a honeycomb substrate composed of a
cordierite-ceramic body, in particular, a diesel particulate filter
as described above. The batch used to form this article includes
only a very small amount of the fibrous silicate mineral, such as
attipulgite clay. In particular, adding 1-3% of the fibrous
silicate mineral in combination with the methocelulose binder was
discovered to increase the final fired strength of the article.
Accordingly, the advantages of this aspect of the present invention
are that the fired body has increased final strength as compared to
like (with same porosity, CTE) ceramic articles manufactured
without the use of the fibrous silicate mineral. In particular, the
strength increase may be as large as 25% (as based on MOR values as
described above) or more as compared to like ceramic articles
manufactured without the benefit of the small amounts of the
fibrous silicate mineral. Using smaller amounts of the fibrous
silicate mineral, such as attipulgite clay, has the additional
benefit that CTE of the ceramic article is less affected. In
particular, compositions having porosity of greater than 45% and
CTEs less than 0.5.times.10.sup.-6/.degree. C. (RT-800.degree. C.)
may be achieved utilizing the batch composition.
[0050] According to a preferred example of the invention which
exhibits enhanced final ceramic strength, the batch preferably
comprises 100% by weight cordierite-forming inorganic raw
materials, 1.0 to 3.0% by weight of the fibrous silicate mineral
based on 100% by weight inorganic raw materials, and 2.5 to 10.0%
by weight of the water-soluble organic binder. More preferably, the
high strength ceramic article is manufactured from an inventive
batch which comprises 100% by weight cordierite-forming inorganic
raw materials, 1.0 to 3.0% by weight attapulgite clay based on 100%
by weight inorganic raw materials, and 2.5 to 10.0% by weight
methylcellulose. In a most preferred embodiment, the invention
comprises a method for fabricating a ceramic article, comprising
providing a batch comprising a mixture of components. The
components include inorganic raw materials including talc, alumina,
and silica; a binder comprising the combination of 1) a
water-soluble organic binder and 2) a fibrous silicate mineral
having a high aspect ratio, a large surface area; and a polar
solvent; mixing the batch components to form a homogenous and
plasticized mass; shaping the plasticized mass into a green body;
and sintering the green body by to achieve conversion of the green
body into a fired ceramic article. Most preferably, the batch also
includes a pore former such as graphite. Preferably the article
includes predominant phase of cordierite.
[0051] While the invention has been described with respect to a
limited number of embodiments, those skilled in the art, having
benefit of this disclosure, will appreciate that other embodiments
can be devised which do not depart from the scope of the invention
as disclosed herein. Accordingly, the scope of the invention should
be limited only by the attached claims.
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