U.S. patent application number 09/872414 was filed with the patent office on 2002-01-24 for fabrication of ultra low thermal expansion cordierite structures.
Invention is credited to Beall, Douglas M., Merkel, Gregory A..
Application Number | 20020010073 09/872414 |
Document ID | / |
Family ID | 22337072 |
Filed Date | 2002-01-24 |
United States Patent
Application |
20020010073 |
Kind Code |
A1 |
Beall, Douglas M. ; et
al. |
January 24, 2002 |
Fabrication of ultra low thermal expansion cordierite
structures
Abstract
Disclosed is sintered ceramic article that exhibits a primary
crystalline phase of cordierite and analytical oxide composition,
in weight percent, of 49-53% SiO.sub.2, 33-38% Al.sub.2O.sub.3,
12-16% MgO and exhibits a coefficient of thermal expansion no
greater than about 4.0.times.10.sup.-7/.degree. C. over the
temperature range of about 25.degree. C. to about 800.degree. C.
and a transverse-I ratio of not less than about 0.92. Also
disclosed is a method for producing a sintered cordierite ceramic
article involving preparing a plasticizable raw material,
comprising a magnesium source, a SiO.sub.2-forming source and an
additional component of either: (a) a clay-free,
Al.sub.2O.sub.3-forming source having a surface area of greater
than about 5 m.sup.2/g; or, (b) a clay and Al.sub.2O.sub.3-forming
source combination wherein the clay comprises no greater than about
30%, by weight, of the total inorganic mixture, and the
Al.sub.2O.sub.3-forming source exhibits a surface area of greater
than about 40 m.sup.2/g. The mixture is formed into a green body
substrate of the desired configuration and subsequently dried and
fired for a time and at temperature sufficient to form a structure
having the aforementioned CTE and I-ratio properties.
Inventors: |
Beall, Douglas M.; (Painted
Post, NY) ; Merkel, Gregory A.; (Big Flats,
NY) |
Correspondence
Address: |
CORNING INCORPORATED
SP-TI-3-1
CORNING
NY
14831
|
Family ID: |
22337072 |
Appl. No.: |
09/872414 |
Filed: |
May 31, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09872414 |
May 31, 2001 |
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09451309 |
Nov 30, 1999 |
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60111192 |
Dec 7, 1998 |
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Current U.S.
Class: |
501/128 ;
264/631; 264/669; 428/116 |
Current CPC
Class: |
C04B 2235/3418 20130101;
C04B 2235/3218 20130101; C04B 35/6365 20130101; C04B 2111/0081
20130101; C04B 2235/3445 20130101; C04B 2235/96 20130101; C04B
2235/449 20130101; F01N 3/2828 20130101; C04B 35/63 20130101; C04B
35/632 20130101; C04B 38/0006 20130101; C04B 2111/32 20130101; C04B
35/634 20130101; C04B 2111/0037 20130101; C04B 2111/0025 20130101;
F01N 2330/06 20130101; C04B 35/195 20130101; C04B 2235/5445
20130101; C04B 2235/349 20130101; C04B 2235/5409 20130101; C04B
35/6263 20130101; C04B 2235/5292 20130101; Y10T 428/24149 20150115;
C04B 2111/00129 20130101; C04B 2235/3206 20130101; C04B 2235/9607
20130101; C04B 2235/3217 20130101; C04B 2235/5296 20130101; C04B
2235/77 20130101; C04B 38/0006 20130101; C04B 35/195 20130101; C04B
35/6365 20130101 |
Class at
Publication: |
501/128 ;
264/631; 264/669; 428/116 |
International
Class: |
C04B 035/195; C04B
033/32 |
Claims
We claim:
1. A ceramic article having sintered phase composition, by weight,
comprising at least about 93% cordierite, consisting essentially of
about, by weight, 49-53% SiO.sub.2, 33-38% Al.sub.2O.sub.3, 12-16%
MgO and which exhibits a coefficient of thermal expansion in at
least one direction no greater than about
4.0.times.10.sup.-7/.degree. C. over the temperature range of about
25.degree. C. to about 800.degree. C. and a transverse I-ratio of
not less than about 0.92.
2. A ceramic article in accordance with claim 1 which exhibits a
coefficient of thermal expansion no greater than about
3.0.times.10.sup.-7/.degree. C. over the temperature range of about
25.degree. C. to about 800.degree. C.
3. A ceramic article in accordance with claim 1 that exhibits a
transverse I-ratio of at least about 0.93.
4. A ceramic article in accordance with claim 1 which further
exhibits an axial I-ratio of less than about 0.41.
5. A ceramic article in accordance with claim 1 wherein the ceramic
article comprises a honeycomb configuration.
6. A ceramic article in accordance with claim 1 wherein the article
exhibits a porosity greater than about 15%.
7. A method of producing a ceramic body having cordierite as its
primary phase, comprising the following steps: selecting raw
materials to form a plasticizable inorganic raw material mixture
having a chemical composition consisting essentially of about, by
weight, 49-53% SiO.sub.2, 33-38% Al.sub.2O.sub.3, 12-16% MgO, the
mixture comprising a magnesium-source comprising a platy talc
having a morphology index of greater than 0.75 and an SiO.sub.2
forming source comprising either crystalline or amorphous silica
and one of the two following additional components, (a) a
substantially clay-free Al.sub.2O.sub.3 forming source having a
surface area of greater than about 5 m.sup.2/g; (b) a combination
of clay and an Al.sub.2O.sub.3-forming source wherein the clay
comprises no greater than about 30% clay, by weight, of total
inorganic mixture, and the Al.sub.2O.sub.3 forming source exhibits
a surface area of greater than about 40 m.sup.2/g adding an organic
binder system, including water to the inorganic mixture and
kneading the mixture and thereafter forming a green body. drying
the green body and thereafter firing the green body at a time and
at a temperature between 1380.degree. C. and 1450.degree. C. to
result in sintered ceramic body having at least 93% cordierite
which exhibits a transverse I-ratio of not less than about 0.92 and
a coefficient of thermal expansion no greater than about
4.0.times.10.sup.-7/.degree. C. over the temperature range of about
25.degree. C. to about 800.degree. C.
8. The method of claim 7 wherein the Al.sub.2O.sub.3-forming source
in component (a) comprises a highly reactive cc-alumina or aluminum
trihydrate having a median particle diameter of no greater than
about 1 .mu.m.
9. The method of claim 7 wherein the Al.sub.2O.sub.3-forming source
in component (b) is selected from the group consisting of a
transition alumina and aluminum oxyhydroxide having a median
particle diameter no greater than about 1 .mu.m.
10. The method of claim 7 wherein the coefficient of thermal
expansion of the ceramic body is no greater than about
3.0.times.10.sup.-7/.degree. C.
11. The method of claim 7 in which the green body is a honeycomb
monolith.
12. The method of claim 7 wherein the organic binder system
comprises a cellulose ether binder component selected from the
group consisting of methylcellulose, methylcellulose derivatives,
and combinations thereof, a surfactant component selected from the
group consisting of, stearic acid, sodium stearate, ammonium
stearate, ammonium lauryl sulfate, lauric acid, oleic acid,
palmitic acid and combinations thereof and a solvent comprising
water.
13. The method of claim 7 wherein the inorganic raw material
mixture comprises 100 parts by weight and about 0.2 to 2 parts by
weight of the surfactant component, about 2.5 to 5 parts by weight
of the cellulose ether binder component, and about 20 to 50 parts
by weight of the water.
Description
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/111,192, filed Dec. 7, 1998, entitled
"Fabrication of Ultra Low Thermal Expansion Cordierite Structures",
by Beall et al.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to cordierite ceramic bodies for use
as catalyst carriers, particularly to cordierite bodies, having an
ultra low thermal expansion, for use as catalyst carriers for
purifying automobile exhaust gas, and a method for producing the
cordierite structures.
[0004] 2. Discussion of the Related Art
[0005] The exhaust gases emitted by internal combustion systems
utilizing hydrocarbon fuels, such as hydrocarbon gases, gasoline or
diesel fuel, can cause serious pollution of the atmosphere. Among
the many pollutants in these exhaust gases are hydrocarbons and
oxygen-containing compounds, the latter including nitrogen oxides
(NOx) and carbon monoxide (CO). The automotive industry has for
many years attempted to reduce the quantities of pollutants from
automobile engine systems, the first automobiles equipped with
catalytic converters having been introduced in the mid 1970's.
[0006] Cordierite substrates, typically in the form of a honeycomb
body, have long been preferred for use as substrates to support
catalytically active components for catalytic converters on
automobiles, in part due to cordierite ceramics' high thermal shock
resistance. The thermal shock resistance is inversely proportional
to the coefficient of thermal expansion. That is, honeycombs with a
low thermal expansion have a good thermal shock resistance and can
survive the wide temperature fluctuations that are encountered in
the application. It is generally known that the coefficient of
thermal expansion of cordierite bodies is about
18.times.10.sup.-7/.degree. C. in the range of 25.degree.
C.-800.degree. C. for those polycrystalline cordierite bodies in
which the cordierite crystals are randomly oriented.
[0007] The production of cordierite
(2MgO.2Al.sub.2O.sub.3.5SiO.sub.2) ceramics from mineral batches
containing sources of magnesium, aluminum and silicon such as clay
and talc is well known. Such a process is described in U.S. Pat.
No. 2,684,919. U.S. Pat. No. 3,885,977 discloses the manufacture of
thermal-shock-resistant cordierite honeycomb ceramics from
clay/talc batches by extruding the batches and firing the extrudate
to provide ceramics with very low expansion coefficients along at
least one direction. Furthermore, this reference describes the
principle of orienting the cordierite crystals with their
crystallographic c-axis in the plane of the honeycomb webs,
resulting in thermal expansion values as low as
5.5.times.10.sup.-7/.degree. C.
[0008] Manufacturers work continuously to optimize the
characteristics of cordierite substrates to enhance their utility
as catalyst carriers. Specifically, manufacturers continually
strive to optimize the thermal shock resistance and strength of the
cordierite substrates. The following patents each relate to the
manufacture of ceramic honeycombs exhibiting improved thermal shock
resistance or coefficient of thermal expansion (CTE).
[0009] U.S. Pat. No. 4,434,117 (Inoguchi et al.) discloses the use
of a raw material mixture comprising plate-shaped talc particles
and non-plate shaped particles of other ceramic materials and
thereafter anisostatically forming the mixed batch so as to impart
a planar orientation to the plate-shaped talc particles and then
drying and firing to obtain a formed ceramic body. The ceramic
bodies formed in the Inoguchi reference exhibited thermal expansion
coefficients as low as 7.0.times.10.sup.-7/.degree. C.
[0010] U.S. Pat. No. 5,144,643 (Beall et al.) and U.S. Pat. No.
5,144, 643 (Beall et al.) disclose a method of fabricating a
cordierite body involving selecting specific raw materials that
will form the desired cordierite body. Specifically, these raw
material selections should not include any clay or talc, should
include a MgO-yielding component and an Al.sub.2O.sub.3-yielding
component having a particle size of no greater than 15 and 8 .mu.m,
respectively. The raw materials are mixed together, subsequently
dried and fired for a time and a temperature sufficient to form the
aforementioned cordierite body. The ceramic bodies formed by these
Beall references exhibited thermal expansion coefficients of less
than about 9.times.10.sup.-7/.degree. C. from about 25 to about
1000.degree. C.
[0011] Lastly, U.S. Pat. No. 5,258,150 (Merkel et al.) discloses a
method of forming a cordierite body involving a raw material batch
mixture comprising certain selected raw materials including platy
talc, 0 to 48% of a platelet-type or delaminated clay and an
aluminum oxide-yielding component having an average particle size
of between 3 to 8 .mu.m, or less than 3 .mu.m. The method involves
mixing the raw materials with a binder system and extruding the
mixture to form a green body and thereafter firing the green body
at a temperature of at least 1390.degree. C. to result in a
sintered cordierite body. The ceramic bodies formed by this Merkel
reference exhibited thermal expansion coefficients of less than
about 4.times.10.sup.-7/.degree. C. from about 25 to about
1000.degree. C., a porosity greater than about 42% and a median
pore diameter of between about 5 to 40 .mu.m; however, these bodies
are disclosed therein as exhibiting I-ratios of no greater than
about 0.91.
[0012] While such ceramics represent an improvement in the thermal
expansion coefficient properties over extruded cordierite ceramics
produced using pre-existing processes, still further improvements
in this thermal expansion characteristic, particularly without a
measurable reduction in the ceramics' strength would be desirable.
Strength has become an increasingly important consideration in the
production of cordierite honeycomb substrates as a result of the
move to producing thinner-walled, higher cell density, increased
catalytic conversion efficiency and lower back pressure cordierite
honeycomb catalyst carriers.
[0013] It is therefore a principal object of the present invention
to provide improved cordierite ceramics, and method for making
them, that exhibit adequate strength in combination with an
ultra-low thermal expansion.
SUMMARY OF THE INVENTION
[0014] The present invention provides for a sintered ceramic
substrate and method for making the ceramic substrate, having a
primary crystalline phase comprising cordierite and exhibiting an
ultra-low coefficient of thermal expansion and a higher than
expected strength.
[0015] Specifically, the sintered ceramic article of the invention
exhibits a primary crystalline phase of cordierite and analytical
oxide composition, in weight percent, of 49-53% SiO.sub.2, 33-38%
Al.sub.2O.sub.3, 12-16% MgO and exhibits a coefficient of thermal
expansion in at least one direction no greater than about
4.0.times.10.sup.-7/.degree. C. over the temperature range of about
25.degree. C. to about 800.degree. C. and a transverse-I ratio of
not less than about 0.92.
[0016] This invention also relates to a method for producing a
sintered cordierite ceramic article involving preparing a
plasticizable raw material mixture, comprising a magnesium source,
a SiO.sub.2-forming source and an additional component of either:
(a) a clay-free, Al.sub.2O.sub.3-forming source having a surface
area of greater than about 5 m.sup.2/g; or, (b) a clay and
Al.sub.2O.sub.3-forming source combination wherein the clay
comprises no greater than about 30%, by weight, of the total
inorganic mixture, and the Al.sub.2O.sub.3-forming source exhibits
a surface area of greater than about 40 m.sup.2/g. The magnesium
source comprises a platy talc having a morphology index of greater
than about 0.75. The Al.sub.2O.sub.3-forming source having a
surface area of greater than about 5 m.sup.2/g preferably comprises
a reactive alumina or aluminum hydroxide having an median particle
diameter of no greater than about 1 .mu.m, while the
Al.sub.2O.sub.3-forming source having a surface area of greater
than about 40 m.sup.2/g preferably comprises "transition" aluminas
or aluminum oxyhydroxide having a median particle diameter of no
greater than about 1 .mu.m, where median particle diameters are
measured by a particle size analyzer employing the sedimentation
technique. The mixture is thereafter formed into a green body
substrate of the desired configuration and subsequently dried and
fired for a time and at temperature sufficient to form a structure
having the aforementioned CTE and I-ratio properties.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a graphical illustration comparing the CTE v.
I-ratio relationship of the inventive and comparative examples.
DETAILED DESCRIPTION OF THE INVENTION
[0018] The present invention relates to ceramic articles exhibiting
a primary crystalline phase of cordierite and to a method of
producing these articles using a selected combination of either low
clay or clay-free raw materials including specified sources of
aluminum, magnesium and silicon. Specifically, the inventive
ceramic articles are formed from a plasticizable batch mixture
comprised of raw materials and the relative amounts of each
selected to form the sintered ceramic article consisting
essentially, on an analytical oxide basis, of about, by weight,
49-53% SiO.sub.2, 33-38% Al.sub.2O.sub.3, 12-16% MgO.
[0019] As previously mentioned, it has been found that by utilizing
the specific magnesium yielding and alumina-forming raw materials
in the batch mixture, the mixture described herein results in
sintered ceramic articles, characterized by a primary crystalline
phase of cordierite, having a property combination of an ultra-low
coefficient of thermal expansion (CTE) and a high transverse
I-ratio. Specifically, a CTE of less than about
4.0.times.10.sup.-7/.degree. C. at 25.degree. C. to 800.degree. C.
and a transverse I-ratio of not less than about 0.92 characterize
the ceramic bodies of the present invention. Preferably, the CTE
exhibited by the inventive cordierite bodies is no greater than
about 3.0.times.10.sup.-7/.degree. C. at 25.degree. C. to
800.degree. C. The cordierite ceramic bodies are further
characterized by a relatively high strength, given the low CTE, of
at least about 2400 psi.
[0020] In accordance with the present invention, provided is a
plasticizable mixture for use in preparing the ceramic article
above with the mixture comprising a SiO.sub.2-forming source, a
magnesium source comprising a platy talc having a morphology index
greater than about 0.75 and one of the following additional
components: (a) a clay-free, Al.sub.2O.sub.3-forming source having
a surface area of greater than about 5 m.sup.2/g; or, (b) a
combination of clay and an Al.sub.2O.sub.3-forming source, wherein
the clay comprises no greater than about 30%, by weight of the
total inorganic mixture, and the Al.sub.2O.sub.3-forming source
exhibits a surface area of greater than about 40 m.sup.2/g.
[0021] The silica forming source comprises silica raw materials
including fused SiO.sub.2; colloidal silica; crystalline silica,
such as quartz or cristobalite, or a low-alumina substantially
alkali-free zeolite. Additionally, the silica-forming source can
comprise a compound that forms free silica, when heated, for
example, silicic acid or a silicon organometallic compound.
[0022] The Al.sub.2O.sub.3-forming source, for the purposes of the
instant invention is a compound which, when heated, forms
Al.sub.2O.sub.3. Regarding the 5 m.sup.2/g or greater
Al.sub.2O.sub.3 source, that material is selected from the group
consisting of alumina, aluminum hydroxide, aluminum oxyhydroxide,
and combinations thereof. A particularly preferred source comprises
a highly reactive cc-alumina or aluminum hydroxide having a median
particle diameter of about one .mu.m micron or less. Regarding the
Al.sub.2O.sub.3 forming materials having a surface area greater
than 40 m.sup.2/g, that material includes a compound selected from
the group consisting of "transition" or activated aluminas, such as
gamma alumina, and aluminum oxyhydroxide, wherein the median
particle size of the alumina source is no greater than about 1
.mu.m; preferably this source comprises boehmite or
pseudoboehmite.
[0023] Clay as used herein means either calcined or raw clay, the
clay preferably comprising a kaolin.
[0024] The magnesium source comprises a platy talc, that is a talc
that exhibits a platelet particle morphology, that is, particles
having two long dimensions and one short dimension, or, a length
and width of the platelet that is much larger than its thickness.
It is preferred that the talc possess a morphology index greater
than about 0.75. The morphology index (refer to U.S. Pat. No.
5,141,686) is a measure of the degree of platiness of the talc. One
typical procedure for measuring the morphology index is to place
the sample in a holder so that the orientation of the platy talc is
maximized within the plane of the sample holder. The x-ray
diffraction pattern is then determined for this oriented talc. The
morphology index semi-quantitatively relates the platy character of
the talc to its XRD peak intensities using the following equation:
1 M = I x I x + 2 I y
[0025] where I.sub.x is the intensity of the (004) peak and I.sub.y
is that of the (020) reflection.
[0026] The aforementioned raw materials of which the plasticized
mixture is comprised are combined in a mixing step sufficient to
produce an intimate mixing of the raw material phases to allow
complete reaction in thermal processing. A binder system is added
at this point to help create an extrudable mixture that is formable
and moldable. A preferred binder system for use in the present
invention comprises a cellulose ether binder component selected
from the group consisting of methylcellulose, methylcellulose
derivatives, and combinations thereof, a surfactant component,
preferably stearic acid or sodium stearate, and a solvent
comprising water. Excellent results have been obtained utilizing a
binder system which comprises the following amounts, assuming 100
parts by weight of the inorganic, alumina and silica forming
sources and talc, raw material mixture: about 0.2 to 2 parts by
weight of the sodium stearate, about 2.5 to 6.0 parts by weight of
a methylcellulose or a hydroxypropyl methylcellulose binder, and
about 20-50 parts by weight of the water.
[0027] The individual components of the binder system are mixed
with a mass of the inorganic powder material, e.g., the talc,
alumina and silica forming sources mixture, in a suitable known
manner, to prepare an intimate mixture of the ceramic material and
the binder system capable of being formed into a ceramic body by,
for example, extrusion. For example, all components of the binder
system may be previously mixed with each other, and the mixture is
added to the ceramic powder material. In this case, the entire
portion of the binder system may be added at one time, or divided
portions of the binder system may be added one after another at
suitable intervals. Alternatively, the components of binder system
may be added to the ceramic material one after another, or each
previously prepared mixture of two or more components of the binder
system may be added to the ceramic powder material. Further, the
binder system may be first mixed with a portion of the ceramic
powder material. In this case, the remaining portion of the ceramic
powder is subsequently added to the prepared mixture. In any case,
the binder system must be uniformly mixed with the ceramic powder
material in a predetermined portion. Uniform mixing of the binder
system and the ceramic powder material may be accomplished in a
known kneading process.
[0028] The resulting stiff, uniform and extrudable batch mixture is
then shaped into a green body by any known conventional ceramic
forming process, such as, e.g. extrusion, injection molding, slip
casting, centrifugal casting, pressure casting, dry pressing, etc.
For the preparation of a thin-walled honeycomb substrate suitable
for use as a catalyst support, extrusion through a die is
preferable.
[0029] The prepared ceramic green body is then dried for a period
of about 5-20 minutes prior to firing by any conventional method
such as either hot-air or dielectric drying. The dried green body
is thereafter fired at a sufficient temperature for a sufficient
time to result in a fired ceramic body containing cordierite as its
primary phase. The firing conditions can vary depending on the
process conditions such as specific composition, size of the green
body, and nature of the equipment. However, some preferred firing
conditions are as follows:
[0030] heating the green body to a temperature of between about
1380.degree. C. to about 1450.degree. C. holding at this
temperature for about 6 hours to about 16 hours, and thereafter
cooling the green body to room temperature.
[0031] As described above the cordierite ceramic bodies, when
formed as a honeycomb structure for example, by extrusion, are
further characterized by their preferred orientation of the
cordierite crystallites as evidenced by their characteristic high
transverse and low axial I-ratios. Specifically, a transverse
I-ratio of not less than about 0.92, and an axial I-ratio of less
than about 0.41 characterize the cordierite bodies of the present
invention.
[0032] This I-ratio characteristic is measured through the use of
an x-ray diffraction analysis of a section of the fired web of a
cordierite honeycomb body. If the negative expansion c-axes of the
crystallites comprising cordierite body are preferentially oriented
in a particular direction, then the (001) reflections measured from
a slice cut normal to that direction should be more intense than if
the crystallites were randomly oriented. At the same time, (hk0)
reflections that are diffracted from crystallographic planes
parallel to the negative expansion c-axes (and perpendicular to the
001 planes) should be less intense than when there is no preferred
orientation. The following ratio, the I-ratio (I.sub.R), as first
described in U.S. Pat. No. 3,885,977, is used to describe the
degree of preferred orientation: 2 I R = I ( 110 ) I ( 110 ) + I (
002 )
[0033] where I.sub.(110) and I.sub.(002) are the peak heights of
the X-ray reflections from the (110) and (002) crystallographic
planes respectively, based on hexagonal cordierite crystal
structure; these reflections correspond to d-spacings of about 4.9
and 4.68 .ANG., respectively.
[0034] The axial and transverse I-ratio refer to different
orientations of a cordierite honeycomb sample in the x-ray beam.
The x-ray beam impinges a planar surface at an angle. Referring
specifically to the measurement of the transverse I-ratio, this
measurement is taken on the planar surface of the sample when that
planar surface on which the x-rays impinge is the flat surface made
up of the as-formed wall surfaces of the honeycomb. Put
differently, this measurement of the transverse I-ratio is
performed by slicing the cordierite honeycomb substrate to expose a
flat section of a web of the honeycomb and subjecting this web to
X-ray diffraction and calculating the intensity of the observed
diffraction peaks. If the obtained value is greater than 0.65,
which is the I-ratio for a body of completely randomly oriented
crystals (i.e., a powder), then it can be inferred that the
cordierite crystallites have a preferred orientation; i.e., a
majority of the cordierite crystallites are oriented with their
c-axes in the plane of the web. An I-ratio of 1.00 would imply that
all of the cordierite crystallites were oriented with their
negative expansion axis within the plane of the web, and thus the
closer the transverse I-ratio (I.sub.T) is to a value of 1.00, the
higher the degree of this planar orientation. Referring
specifically now to the measurement of the axial I ratio, this
measurement is taken on a plane which is perpendicular to the
length of the cell channels (and, therefore, also perpendicular to
that for a transverse I-ratio) where the planar surface on which
the x-rays impinge consists of the cross-sectional ends of the
honeycomb webs. Put differently, this X-ray measurement is
performed on the surface of the cordierite honeycomb that is normal
to the direction of extrusion. If the axial I-ratio (I.sub.A) is
less than 0.65 it can again be inferred that the cordierite
crystallites exhibit a preferred orientation. Specifically, since
the cordierite crystallites are preferentially oriented with their
c-axes in the plane of the webs, the intensity of the reflections
from the (002) planes is expected to be greater than that for a
body with completely randomly oriented cordierite crystallites.
[0035] Simply stated, if the I-ratio measured in the transverse
direction with respect to the extrusion direction of the body
exceeds about 0.65 or the axial I-ratio with respect to the
extrusion direction is less than about 0.65, then the cordierite
crystallites are becoming substantially oriented with their c-axes
within plane of the webs.
[0036] It is well established that the coefficient of thermal
expansion of cordierite cellular bodies in the axial direction
(parallel to the cell channels) is affected by non-random
crystallographic orientation of the cordierite crystals in the
microstructure, by the degree of microcracking present in the body
after firing, and by the presence of high-expansion extraneous
phases. Typically, higher values of transverse I-ratio, and
correspondingly lower values of axial I-ratio correlate with low
values of thermal expansion measured axially. Essentially, this is
due to the combined effect of the negative expansion direction of
the cordierite crystallites being oriented in the plane of the webs
coupled with the generation of microcracks forming from strains
associated with thermal expansion anisotropy of large regions of
oriented crystallites.
[0037] As indicated previously, a primary utility of the mixtures
described herein is for preparing high strength cordierite
honeycomb substrates useful as catalyst carriers. Although the
invention is particularly advantageous for preparing thin-walled
honeycombs, the claimed mixtures can also be used for thicker
walled structures. Methods of applying catalysts to the honeycomb
structures, and utilizing those structures, for example, in
automobile exhaust systems, are well known in the art. The mixtures
may also be useful for preparing other high strength cordierite
structures, such as filters.
EXAMPLES
[0038] To further illustrate the principles of the present
invention, there will be described several examples of the ceramic
bodies formed according to the invention, as well as several
comparison examples. However, it is to be understood that the
examples are given for illustrative purpose only, and the invention
is not limited thereto, but various modifications and changes may
be made in the invention, without departing from the spirit of the
invention.
Examples 1-14
[0039] Inorganic powder batch mixtures, as listed in percent by
weight, suitable for the formation of a ceramic body having
cordierite as its primary crystalline phase are listed in Table I;
Examples 1-7 inventive, 8-14 comparison. Each of compositions 1-14
was prepared by combining and dry mixing together the components of
the designated inorganic mixture as listed in Table I. To these
mixtures was added the amount of the organic binder system listed
in Table I and this intermediate mixture was thereafter further
mixed with deionized water to form a plasticized ceramic batch
mixture. The binder system components, as detailed in Table I are
listed in parts by weight, based on 100 parts total inorganics.
[0040] Table II reports various properties of the commercially
available raw materials utilized in Examples, specifically those
properties discussed as being important in the formation of the low
CTE, high I-ratio cordierite examples. Included in the table are
the following important raw material characteristic properties:
morphology index, the surface area (m.sup.2/g) as measured by the
B.E.T. method and average particle diameter (.mu.m) as measured by
the sedimentation technique.
[0041] Each of the various plasticized mixtures was extruded
through an extruder under conditions suitable to form 1/4 in. (6.35
mm) rods, as well as conditions suitable to form 4 in. (101.6 mm)
long, 400 cell/in.sup.2 (62 cells/cm.sup.2) honeycomb substrates
having a 1.0 in. (25.4 mm) diameter and 8 mils (0.20 mm) thick cell
walls. The green ceramic rods and honeycombs formed from each of
the 14 batch compositions were sufficiently dried to remove any
water or liquid phases that might be present and thereafter
subjected to a heating and firing cycle sufficient to remove the
organic binder system from, and to sinter, the extruded rods and
honeycombs. Specifically, the green bodies of each type of
substrate were fired to between 1405 and 1430.degree. C. and held
for a period of about 10 hours; i.e., firing conditions suitable
for forming ceramic bodies having cordierite as their primary
phase.
[0042] Table I additionally reports selected properties for the
ceramics produced from the batches reported in the Table.
Properties included for each of the ceramic bodies are the modulus
of rupture strength (MOR) of the rods measured by four-point
loading, in psi, except for Example 5; the average coefficient of
thermal expansion (CTE) of the ceramic rod over the temperature
range from about 25.degree. C. to 800.degree. C.
(.times.10.sup.-7/.degree. C.) as measured by dilatometry; and the
volume percent open porosity and median pore diameter in .mu.m, as
measured by Hg porosimetry. Furthermore, Table I includes the
transverse I-ratio, I.sub.T, and, for some examples, the axial
I-ratios, I.sub.A, each as measured in the manner as described
above.
1 TABLE I 1 2 3 4 5 6 7 8* 9* 10* 11* 12* 13* 14* INORGANICS Talc 1
-- -- -- -- -- -- -- 40.78 40.86 42.29 -- -- -- -- Talc 2 -- 42.29
41.92 39.88 39.88 -- 40.13 -- -- -- -- -- -- -- Talc 3 42.29 -- --
-- -- 42.29 -- -- -- -- -- 42.29 41.14 42.29 Calcined Clay -- -- --
-- -- -- 15.71 26.48 32.60 -- -- -- -- 20.0 Raw Clay -- -- -- -- --
-- 9.12 15.37 12.82 -- -- -- 19.46 -- Calcinated -- 13.7 MgO Coarse
.alpha.- -- -- -- -- -- -- 15.35 -- -- -- 34.20 -- -- alumina Fine
.alpha.-alumina 34.20 34.20 28.94 -- -- 34.20 -- -- 13.72 34.20
34.90 -- 25.57 25.01 Boebmite -- --5.83 37.94 37.94 -- 25.46 -- --
Silica 1 23.52 23.52 23.31 22.18 16.60 -- 2.03 -- 22.52 51.40 23.52
13.83 12.71 Silica 2 - -- -- -- -- 5.55 9.59 -- -- colloidal sil-
ica solution** Silica 3 - fused -- -- -- -- -- 23.5 -- -- --
ORGANIC BINDER SYSTEM Methylcellu- 4.0 4.0 4.0 2.7 4.0 3.0 4.0 4.0
4.0 4.0 2.73 4.0 4.0 4.03 lose Stearic Acid 1.0 1.0 1.0 0.6 1.0 0.6
1.0 1.0 1.0 1.0 0.6 1.0 1.0 1.0 Water 28.0 31.0 32.0 38.0 38.0 28.0
40.0 30.0 30.0 27.0 23.0 29.0 29.0 32.0 OXIDE PERCENT SiO.sub.2
51.35 51.35 51.50 52.42 52.41 51.34 52.07 51.45 51.42 51.35 51.40
51.35 51.36 51.36 Al.sub.2O.sub.3 34.90 34.90 34.70 33.53 33.54
34.90 33.99 35.00 35.05 34.90 34.90 34.90 34.89 MgO 13.76 13.76
13.80 14.05 14.05 13.76 13.95 13.55 13.53 13.76 13.70 13.76 13.76
13.76 PROPERTIES CTE 1.5 1.1 0.0 -0.2 -0.5 1.8 1.9 6.3 4.2 2.7 5.5
12.4 3.5 5.2 (10.sup.-7/.degree. C.) I.sub.T 0.96 0.93 0.93 0.93
0.94 0.93 0.93 0.80 0.86 0.88 0.80 0.88 0.91 0.90 I.sub.A 0.34 --
-- -- -- 0.37 -- 0.50 0.44 0.39 -- 0.42 0.39 0.42 % Open 33.0 33.7
33.6 21.0 19.8 32.7 14.3 36.0 28.5 28.2 29.6 43.6 34.1 35.6
Porosity Median Pore 2.2 2.7 2.4 2.7 2.7 2.5 2.5 3.3 3.1 4.1 2.6
7.2 2.2 2.0 size (.mu.m) MOR (psi) 2900 2600 2600 2400 -- 2800 4000
2500 3300 2800 2900 2700 3300 3200 *Comparative Examples
**Colloidal silica - In Example 5, 13.9 parts by weight of a
colloidal silica solution were used, which contributed 5.55 parts
by weight silica and 8.3 parts by weight water; this water is
included in the total 38.0 parts by weight water used in this
batch. In Example 7, 24.0 parts by weight of a colloidal silica
solution were used, which contributed 9.59 parts by weight silica
and 14.4 parts by weight water; this water is included in the 40
parts by weight water used in the batch.
[0043]
2 TABLE II MEDIAN PARTICLE MORPHOLOGY DIAMETER SURFACE AREA INDEX
(.mu.m) (m.sup.2/g) Talc 1 0.75 8.0 Talc 2 0.95 1.6 Talc 3 0.95 3.4
Calcined Clay 1.6 Raw Clay 0.8 Calcined MgO 0.8 Coarse
.alpha.-alumina 4.5 .about.1.0 Fine .alpha.-alumina 0.3 9.4
Boehmite 0.1 180
[0044] An examination of the results of Table I reveals that
composition Examples 1-7 represent cordierite ceramic bodies
according to the present invention each of which exhibits an
ultra-low thermal expansion of less than about
2.0.times.10.sup.-7/.degree. C. (-0.5 to
1.9.times.10.sup.-7/.degree. C.). Furthermore, it should be noted
that all of the inventive examples exhibit a transverse I-ratio
(I.sub.T) of not less than 0.92, specifically ranging from 0.93 to
0.96 indicating a very high degree of preferred cordierite
orientation with the crystal c-axes preferentially oriented within
the plane of the cell walls of the ceramic honeycomb. This high
degree of orientation, not previously reported in the prior art, is
attributed to the novel combination of the highly platy nature of
the talc raw materials, the high surface area of the
alumina-yielding raw material and the low clay contents used in
these examples. It should be noted, Examples 3, 4 and 5 demonstrate
that the use of an aluminum oxyhydroxide raw material, specifically
boehmite, produces cordierite bodies exhibiting CTE's of
0.times.10.sup.-7/.degree. C. or less and I-ratios of at least
0.93.
[0045] While not intending to be limited by theory, it is thought
that the very low CTE's of the present inventive bodies are due not
only to an increase in the degree of preferred planar orientation
of the cordierite crystallites, but also to an increased degree of
microcracking. Before the increase in microcracking can be proved,
it is first necessary to establish the relationship between the
CTE, as measured parallel to the channels of the honeycomb body,
and the transverse I-ratio. This requires definition of the
relationship between CTE and crystal orientation, and between
crystal orientation and I-ratio.
[0046] It is well known that cordierite exhibits different
coefficients of thermal expansion along its three crystallographic
axes. Specifically, the mean CTEs from 25.degree. to 800.degree. C.
for orthorhombic cordierite have been reported to be
31.9.times.10.sup.-7/.degree. C. along the a-axis,
25.9.times.10.sup.-7/.degree. C. along the b-axis, and
-14.5.times.10.sup.-7/.degree. C. along the c-axis (Derek Taylor,
1988, Thermal Expansion Data XIII. Complex oxides with chain, ring
and layer structures and the apatites, Br. Cer. Trans. & Jour.,
vol. 87, no. 3, pp. 88-95, Table 3).
[0047] From the foregoing discussion of the cordierite I-ratio as
measured by x-ray diffractometry, if all of the cordierite crystals
are oriented with their c-axes perpendicular to the plane of the
wall of honeycomb channels, the transverse I-ratio will be found to
be equal to zero. This condition is referred to as complete
reversed orientation. Also, because the c-axes of all of the
crystals are orthogonal to the axial direction of the honeycomb
along which the CTE is measured, then only the a-axis and b-axis
CTEs will contribute to the axial CTE. Thus, assuming a cordierite
ceramic body with no microcracking, the axial CTE of a cordierite
honeycomb body with completely reverse-oriented cordierite crystals
will be equal to the average of the a-axis and b-axis expansions,
CTE(a,b), which is computed to be (31.9.times.10.sup.-7/.degr- ee.
C.+25.9.times.10.sup.-7/.degree.
C.)/2=28.9.times.10.sup.-7/.degree. C.
[0048] Next, one considers the case in which all of the cordierite
crystals are oriented with their c-axes within the plane of the
honeycomb walls, but in which the c-axes have a net random
orientation within that plane. This condition is referred to as
complete planar orientation. The transverse I-ratio for such a body
would be equal to 1.0. The CTE along the axial direction of such a
body, exclusive of microcracking, will have a value that is the
average of the c-axis CTE and the mean of the a-axis and b-axis
expansions (CTE(a,b)), which is calculated to be
(-14.5.times.10.sup.-7/.degree. C.+28.9.times.10.sup.-7/.degree.
C.)/2=7.2.times.10.sup.-7/.degree. C.
[0049] From this analysis, it can be concluded that the rate at
which CTE decreases due to an increase in the preferred, planar
orientation of the cordierite, and with no change in microcracking
or amount of extraneous phases, is equal to the difference in axial
CTE for complete planar and complete reverse orientation, divided
by the difference in transverse I-ratio for these two end-member
conditions. This rate of change is thus computed to be
(7.2.times.10.sup.-7/.degree. C.-28.9.times.10.sup.-7/.deg- ree.
C.)/(1.0-0.0)=-21.7.times.10/.degree. C..sup.-1 per unit change in
I-ratio.
[0050] Referring now to FIG. 1, the CTE values of the inventive and
comparative examples, except for Example 12, from Table I are
plotted against their respective transverse I-ratios. The solid
black line is a least-squares fit to the data for the comparative
examples. The dashed line represents the decrease in CTE, relative
to comparative Example 8, that would occur for an increase in
degree of crystal orientation with no change in microcracking. A
comparison of the data for the inventive examples reveals that
their thermal expansions are lower than would be predicted based
upon the trend of the data for the comparative examples, as
represented by the solid line. The data also illustrate that the
CTEs of inventive examples are 1.3.times.10.sup.-7/.degree. C. to
3.8.times.10.sup.-7/.degree. C. lower than would be predicted by
only an increase in the planar orientation of the cordierite
crystallites relative to the comparative examples. This difference
demonstrates that the unexpectedly low CTEs of the inventive bodies
are due not only to improved crystal orientation, but also to an
increase in the degree of microcracking, which further lowers
thermal expansion.
[0051] Although, the resultant ultra-low CTE is due to an increase
in the particle orientation (i.e., high I-ratio) and a
corresponding increase in the microcracking, it is worth noting
that the strength of the inventive cordierite ceramic bodies
remains acceptably high. Specifically, the inventive examples
exhibit strength values of no less than about 2400 psi (2400 to
4000 psi), which is a strength expected for cordierite bodies
having a 2 to 3 times greater CTE.
[0052] Referring now to the comparative examples, all of the
comparison examples, Examples 8-14 exhibit a thermal expansion of
greater than 2.0.times.10.sup.-7/.degree. C. (2.6 to
12.4.times.10.sup.-7/.degree. C.) and a transverse I-ratio
(I.sub.T) of 0.91 or less, 0.80 to 0.91. This lowered degree of
orientation demonstrates that the use of clay without a
sufficiently high surface area alumina source and/or talc having an
insufficient degree of platiness results in ceramic bodies having a
coefficient of thermal expansion and an I-ratio outside the scope
of the invention.
[0053] Comparative Examples 8 and 9, representative of standard
commercially available cordierite ceramics, specifically
demonstrate that the use of a clay raw material component and a
talc having an insufficient degree of platiness results in the
formation of cordierite bodies having a decreased degree of
orientation, as evidenced by I-ratios of 0.80 and 0.86,
respectively, and thus a CTE of greater than
2.0.times.10-7/.degree. C.
[0054] Comparative Example 10 demonstrates that the use of a talc
exhibiting an insufficient degree of platiness, a morphology index
of 0.75, even in combination with an Al.sub.2O.sub.3-forming source
having the requisite surface area of greater than 5 m.sup.2/g for a
clay-free batch mixture, produces a cordierite body that exhibits a
less than desirable degree of orientation, I-ratio of 0.88, and
thus a cordierite body exhibiting an increased CTE,
2.7.times.10.sup.-7/.degree. C.
[0055] Comparative Example 12 demonstrates that the use of an
alumina raw material comprising coarse alumina, even though used in
combination with a talc having a platy morphology, results in the
formation of a cordierite ceramic body which exhibits both an
I-ratio and a CTE, 0.88 and 12.4.times.10.sup.-7/.degree. C.,
respectively, outside the scope of the invention.
[0056] Comparative Examples 13 and 14 demonstrate that the use of a
clay in the batch mixture, kaolin or calcined kaolin, in
combination with the use of an Al.sub.2O.sub.3 forming source
having less than the requisite surface area of greater than 40
m.sup.2/g for clay containing batches, even in combination with a
sufficiently platy talc, yields cordierite ceramic bodies having
properties that lie outside the scope of the present invention.
[0057] It should be understood that while the present invention has
been described in detail with respect to certain illustrative and
specific embodiments thereof, it should not be considered limited
to such, as numerous modifications are possible without departing
from the broad spirit and scope of the present invention as defined
in the appended claims.
* * * * *