CERAMIC STRUCTURES
The present application is directed to a ceramic composition, optionally in the form of a
honeycomb structure, to ceramic precursor compositions suitable for sintering to form
said ceramic composition, to a method for preparing said ceramic composition and
ceramic honeycomb structure, to a diesel particulate filter comprising said ceramic
honeycomb structure, and to a vehicle comprising said diesel particulate filter.
BACKGROUND OF THE INVENTION
Ceramic structures, particularly ceramic honeycomb structures, are known in the art for
the manufacture of filters for liquid and gaseous media. The most relevant application
today is in the use of such ceramic structures as particle filters for the removal of fine
particles from the exhaust gas of diesel engines of vehicles (diesel particulates), since
those fine particulates have been shown to have negative influence on human health.
The ceramic material has to fulfil several requirements. First, the material should have
sufficient filtering efficiency, i.e., the exhaust gas passing the filter should be
substantially free of diesel particulates, but the filter should not produce a substantial
pressure drop, i.e., it must show a sufficient ability to let the exhaust gas stream pass
through its walls. These parameters generally depend upon the wall parameters
(thickness, porosity, pore size, etc.) of the filter.
Second, the material must show sufficient chemical resistance against the compounds
in exhaust gas of diesel engines over a broad temperature range.
Third, the material must be resistant against thermal shock due to the high temperature
differences that apply during its life cycle. Thus, the material should have a low
coefficient of thermal expansion to avoid mechanical tensions during heating and
cooling periods particularly for monolithic honeycombs.
Fourth, the material must have a melting point above the temperatures reached
(typically > 1000°C) within the filter during a regeneration cycle.
Fifth, the ceramic material should have favourable high temperature properties as
during use (as a diesel filer) and regeneration the ceramic material will be exposed to
high temperatures.
If the above requirements are not fulfilled, mechanical and/or thermal tension may
cause cracks in the ceramic material, resulting in decrease of filter efficiency or even
filter failure.
Further, since the filters for vehicles are produced in high numbers, the ceramic
material should be relatively inexpensive, and the process for its manufacture should
be cost-effective.
A summary on the ceramic materials known for this application is given in the paper of
J. Adler, Int. J. Appl. Ceram. Technol. 2005, 2(6), p429-439, the content of which is
incorporated herein in its entirety for all purposes.
Several ceramic materials have been described for the manufacture of ceramic
honeycomb filters suitable for that specific application.
For example, honeycombs made from ceramic materials based on mullite and tialite
have been used for the construction of diesel particulate filters. Mullite is an aluminium
and silicon containing silicate mineral of variable composition between the two defined
phases [3AI20 3
«2Si0 2] (the so-called "stoichiometric" mullite or "3:2 mullite") and
[2AI20 3
«1Si0 2] (the so-called "2:1 mullite"). The material is known to have a high
melting point, refractoriness and fair mechanical properties. Tialite is an aluminium
titanate having the formula [AI2Ti20 5] . The material is known to show a high thermal
shock resistance, low thermal expansion and a high melting point.
Owing to these properties, tialite has traditionally been a favoured material of choice for
the manufacture of honeycomb structures. For example, US-A-20070063398
describes porous bodies for use as particulate filters comprising over 90 % tialite.
Similarly, US-A-201 00230870 describes ceramic bodies suitable for use as particulate
filters having an aluminium titanate content of over 90 mass %.
Attempts have also been made to combine the positive properties of mullite and tialite,
e.g., by developing ceramic materials comprising both phases.
WO-A-2009/076985 describes a ceramic honeycomb structure comprising a mineral
phase of mullite and a mineral phase of tialite. The examples describe a variety of
ceramic structures typically comprising at least about 65 vol. % mullite and less than 15
vol. % tialite.
There is a need in the art for new ceramic filter materials showing properties
comparable to or improved over those of the prior art.
SUMMARY OF THE INVENTION
In accordance with a first aspect of the present invention, there is provided a ceramic
composition comprising: from about 15 wt. % to less than about 50 wt. % mullite; from
about 40 wt. % to about 75 wt. % tialite; and at least about 1.0 wt. % of a Zr-containing
mineral phase, for example, at least about 1.5 wt. % of a Zr-containing mineral phase.
The weight ratio of tialite to mullite is greater than 1: 1 , and the ceramic composition has
a coefficient of thermal expansion (CTE) of equal to or less than about 1.5 x 10 6 °C ,
and a thermal strength parameter (TSP) of at least about 150 °C.
In accordance with a second aspect of the present invention, there is provided a
ceramic composition according to the first aspect of the present invention in the form of
a honeycomb structure.
In accordance with a third aspect of the present invention, there is provided a ceramic
precursor composition suitable for sintering to form a ceramic composition according to
to the first aspect of the present invention, said precursor composition comprising:
mullite and/or one or more mullite-forming compounds or compositions; tialite and/or
one or more tialite-forming compounds or compositions; and Zr-containing mineral
phase and/or one or more Zr-containing mineral phase-forming compounds or
compositions.
In accordance with a fourth aspect of the present invention, there is provided a method
for making a honeycomb structure according to the second aspect of the present
invention, said method comprising: (a) providing a dried green honeycomb structure
formed from the ceramic precursor composition according to third aspect of the present
invention; and (b) sintering.
In accordance with a fifth aspect of the present invention, there is provided a diesel
particulate filter comprising or made from the ceramic honeycomb structure according
to second aspect of the present invention or obtainable by the method of according to
the third fourth aspect of the present invention.
In accordance with a sixth aspect of the present invention, there is provided a vehicle
having a diesel engine and a filtration system comprising the diesel particulate filer
according to the fifth aspect of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a graph summarizing a thermomechanical property of a ceramic
honeycomb structure prepared in accordance with the present invention and a
comparative ceramic honeycomb structure.
DETAILED DESCRIPTION OF THE INVENTION
The amounts of tialite, mullite and other mineral phases in the ceramic composition or
ceramic honeycomb structure may be measured using qualitative X-ray diffraction (Cu
Ka radiation, 40 KV, 30 mA, Rietveld analysis with a 30 wt. % ZnO standard), or any
other measurement method which gives an equivalent result. As will be understood by
the skilled person, in the X-ray diffraction method, the sample is milled. After milling,
the powder is homogenized, and then filled into the sample holder of the X-ray
diffractometer. The powder is pressed into the holder and any overlapping powder is
removed to ensure an even surface. After placing the sample holder containing the
sample into the X-ray diffractometer, the measurement is started. Typical
measurement conditions are a step width of 0.015°, a measurement time of 2 seconds
per step and a measurement range from 10 to 60° 2. The resulting diffraction pattern
is used for the quantification of the different phases, which the sample material consists
of, by using appropriate software capable of Rietveld refinement. A suitable
diffractometer is a SIEMENS D5000, and suitable Rietveld software is BRUKER AXS
DIFFRAC , TOPAS. The amount of each mineral phase in the ceramic composition,
e.g., ceramic honeycomb structure, is expressed as a weight % based on the total
weight of the mineral phases.
Unless otherwise stated, the particle size properties referred to herein for the mineral
starting material are as measured by the well known conventional method employed in
the art of laser light scattering, using a Malvern Mastersizer 2000 machine as supplied
by Malvern Instruments Ltd (or by other methods which give essentially the same
result). In the laser light scattering technique, the size of particles in powders,
suspensions and emulsions may be measured using the diffraction of a laser beam,
based on an application of Mie theory. Such a machine provides measurements and a
plot of the cumulative percentage by volume of particles having a size, referred to in
the art as the 'equivalent spherical diameter' (e.s.d), less than given e.s.d values. The
mean particle size d50 is the value determined in this way of the particle e.s.d at which
there are 50% by volume of the particles which have an equivalent spherical diameter
less than that d50 value. The d and d90 are to be understood in similar fashion.
Unless otherwise stated, in each case, the lower limit of a range is the d value and
the upper limit of the range is the d90 value.
In the case of colloidal titania, the particle size is measured using transmission electron
microscopy.
Unless otherwise stated, the measurement of the particle sizes of components which
are present in the sintered ceramic composition or honeycomb structure in a particulate
form may be accomplished by image analysis.
In embodiments, the ceramic composition, e.g., ceramic honeycomb structure,
comprises (on a weight % basis):
• 15-59 %, or 19-49 %, or 22-49 %, or 25-49 %, or 19-48 %, or 25-48 %, or 30-48
%, or 22-47 %, or 25-47 %, or 30-47 %, or 35-47 %, or 35-46 %, or 35-45%, or
36-45 %, or 37-45 %, or 37-44 %, or 37-43 %; or 35-43 %, or 35-42 %, or 35-41
%, or 35-40 %, or 40-48 %, or 40-45 % mullite;
• 40-75 %, or 40-72 %, or 40-70 %, or 40-68 %, or 40-66 %, or 40-64 %, or 40-62
%, or 40-60 %, or 42-60 %, or 44-60 %, or 44-58 %, or 44-56 %, or 44-54 %, or
44-52 %, or 44-50 %, or 45-50 %, or 50-65 %, or 50-60 %, or 55-65%, or 50-55
%, or 45-55 % tialite;
• 1.0-8.0 %, or 1.5-8.0 %, or 2.0-8.0 %, or 2.5-8.0 %, or 3.0-8.0 %, or 3.0-7.0 %, or
3.5-7.0 %, or 3.5-6.5 %, or 3.5-6.0 %, or 3.5-5.5%, or 4.0-6.0 %, or 4.0-5.0 % of
Zr-containing mineral phase;
• 0-10 %, or 0-5, or 0-3 %, or 0-2 %, or 0-1 % of an amorphous phase;
« 0-10 %, or 0.5-8 %, or 0.5-7 %, or 1.0-6.0 %, or 1.5-5.5 %, 2.0-5.0 %, or 2.5-5.0
%, or 3.0-5.0 %, or 3.5-5.0 % of alkaline earth metal-containing mineral phase;
and
• 0-10 %, or 0-7 %, or 0-5 %, or 0-4 %, or 0-3 %, or 0-2 %, or 0-1 % alumina.
In embodiments, the ceramic composition, e.g., ceramic honeycomb structure,
comprises (on a weight % basis):
• 15-44 %, or 19-42 %, or 22-40 %, or 25-38 %, or 19-35 %, or 25-35 %, or 30-40
%, or 15-35 %, or 15-32 %, or 15-30 % mullite;
• 56-75 %, or 58-72 %, or 60-72 %, or 60-70 %, or 62-72 %, or 64-72 %, or 64-70
% tialite;
• 1.0-8.0 %, or 1.5-8.0 %, or 1.5-7.0 %, or 2.0-6.0 %, or 2.0-5.0 %, or 2.0-4.0 %,
2.0-3.5 %, or 2.0-3.0 % of Zr-containing mineral phase;
• 0-10 %, or 0-5, or 0-3 %, or 0-2 %, or 0-1 % of an amorphous phase;
• 0-10 %, or 0.5-8 %, or 0.5-7 %, or 1.0-6.0 %, 1.0-4.0 %, or 1.0-3.0, or 1.0-2.5 %,
or 1.0-2.0 %, of alkaline earth metal-containing mineral phase; and
• 0-10 %, or 0-7 %, or 0-5 %, or 0-4 %, or 0-3 %, or 0-2 %, or 0-1 % alumina.
In certain embodiments, the ceramic composition comprises up to about 45 wt. %
mullite and greater than about 45 wt. % tialite, for example, up to about 44 wt. %
mullite, or up to about 43 wt. % mullite, or up to about 42 wt. % mullite, or up to about
4 1 wt. % mullite, or up to about 40 wt. % mullite. In certain embodiments, the ceramic
composition comprises at least about 46 wt. % tialite, or at least about 47 wt. % tialite,
or at least about 48 wt. % tialite, or at least about 49 wt. % tialite, or at least about 50
wt. % tialite.
In certain embodiments, the ceramic composition comprises greater than about 45 wt.
% tialite.
In certain embodiments, the ceramic composition comprises equal to or greater than
about 50 wt. % tialite.
In certain embodiments, the ceramic composition comprises equal to or greater than
about 60 wt. % tialite.
In certain embodiments, the ceramic composition comprises equal to or greater than
about 65 wt. % tialite.
In certain embodiments, the weight ratio of tialite to mullite is equal to or greater than
about 1. 1 : 1 , for example, equal to or greater than about 1.2:1 , or equal to or greater
than about 1.3:1 , or equal to or greater than about 1.4:1 , or equal to or greater than
about 1.5:1 , or equal to or greater than about 1.6:1 , or equal to or greater than about
1.7:1 , or equal to or greater than about 1.9:1 , or equal to or greater than about 2.0:1 , or
equal to or greater than about 2.1 : 1 , or equal to or greater than about 2.2:1 , or equal to
or greater than about 2.3:1 , or equal to or greater than about 2.4:1 , or equal to or
greater than about 2.5:1 . In certain embodiments, the weight ratio of tialite to mullite is
less than about 3.8:1 , for example, equal to or less than about 3.6:1 , or equal to or less
than about 3.4:1 , or equal to or less than about 3.2:1 , or equal to or less than about
3.0:1 , or equal to or less than about 2.9:1 , or equal to or less than about 2.8:1 , or equal
to or less than about 2.7:1 , or equal to or less than about 2.6:1 . In certain
embodiments, the weight ratio of tialite to mullite is from about 1. 1 : 1 to less than about
3:1 , for example, from about 1. 1 : 1 to equal to or less than about 2.8:1 , or from about
1. 1 : 1 to equal to or less than about 2.6:1 .
In certain embodiments, the mullite and tialite mineral phases constitute at least about
80 wt. % of the total weight of the mineral phases, for example, at least about 85 wt. %
of the total weight of the mineral phases, or at least about 88 wt. %, or at least about 90
wt. %, or at least about 92 wt. %, or at least about 94 wt. % of total weight of the
mineral phases. In certain embodiments, the mullite and tialite mineral phases
constitute up to about 98.5 wt. % of the mineral phases, for example, up to about 98.0
wt. % of the mineral phases, or up to about 97.5 % of the mineral phases, or up to
about 97.0 % of the mineral phases, or up to about 96.5 % of the mineral phases, or up
to about 96.0 % of the mineral phases, or up to about 95.5 % of the mineral phases, or
up to about 95.0 % of the mineral phases.
In certain embodiments, the ceramic composition comprises from about 1.0-8.0 % of
Zr-containing mineral phase, for example, from about 1.5-8.0 %, or about 2.0-8.0 %, or
about 2.5-8.0 %, or about 3.0-8.0 %, or about 3.0-7.0 %, or about 3.5-7.0 %, or about
3.5-6.5 %, or about 3.5-6.0 %, or about 3.5-5.5%, or about 4.0-6.0 %, or about 4.0-5.0
% of Zr-containing mineral phase. In such embodiments, the ceramic composition may
further comprise from about 0.5-3.0 wt. % of alkaline earth metal-containing mineral
phase, for example, from about 0.5-2.5 wt. %, or about 1.0-2.5 wt. %, or about 1.0-2.0
wt. %, or about 1.0-1 .5 wt. % of alkaline earth metal-containing mineral phase.
In certain embodiments, for example, embodiments in which the ceramic composition
comprises at least about 56 wt. % tialite, or at least about 60.0 wt. % tialite, the ceramic
composition comprises from about 1.0-8.0 wt. % of Zr-containing mineral phase, for
example, from about 1.5-8.0 wt. %, or about 1.5-5.0 wt. %, or about 1.5-4.0 wt. %, or
about 2.0-4.0 wt. %, or about 2.0-3.5 wt. %, or about 2.0-3.0 wt. % of Zr-containing
mineral phase; In such embodiments, the ceramic composition may further comprise
from about 0.5-3.0 wt. % of alkaline earth metal-containing mineral phase, for example,
from about 0.5-2.5 wt. %, or about 1.0-2.5 wt. %, or about 1.0-2.0 wt. %, or about 1.0-
1.5 wt. % of alkaline earth metal-containing mineral phase.
In certain embodiments, the ceramic composition comprises at least about 1.0 wt. % of
Zr-containing mineral phase.
In certain embodiments, the ceramic composition comprises at least about 3.0 wt. % of
Zr-containing mineral phase.
In certain embodiments, the ceramic composition comprises at least about 3.5 wt. % of
Zr-containing mineral phase.
In certain embodiments, the ceramic composition comprises at least about 4.0 wt. % of
Zr-containing mineral phase.
In certain embodiments, the ceramic composition comprises at least about 4.5 wt. % of
Zr-containing mineral phase.
In certain embodiments, the ceramic composition comprises at least about 4.5 wt. % of
Zr-containing mineral phase.
In the embodiments described above, the ceramic composition typically comprises no
more than about 8.0 wt. % Zr-containing mineral phase, for example, no more than
about 7.0 wt. % Zr-containing mineral phase, or no more than about 6.5 wt. % Zrcontaining
mineral phase, or no more than about 6.0 wt. % Zr-containing mineral phase
The ceramic composition, e.g., ceramic honeycomb structure, of any of the above
embodiments has a coefficient of thermal expansion (CTE) of equal to or less than
about 1.5 x 10 6 °C , as measured at 800°C by dilatometry according to DIN 51045
using a Dilatometer Adamel Lhomargy - model DI-24, and a sample length of 40 mm
+/- 5 mm. In certain embodiments, the CTE may be equal to or less than about 1.4 x
10 6 °C , for example, equal to or less than about 1.3 x 10 6 °C , or equal to or less
than about 2.5 x 10 6 °C 1 , or equal to or less than about 1.2 x 0 6 °C 1 .
In certain embodiments, the ceramic composition, e.g., ceramic honeycomb structure,
has a CTE of equal to or less than about 1. 1 x 10 6 °C , or equal to or less than about
1.0 x 10 6 °C , equal to or less than about 0.9 x 10 6 °C , or equal to or less than about
0.8 x 10 6 °C , or equal to or less than about 0.7 x 10 6 °C or equal to or less than
about 0.6 x 10 6 °C 1 . Typically, the CTE will be greater than about 0.1 x 10 6 °C 1 , for
example, greater than about 0.2 x 10 6 °C , or greater than about 0.3 x 10 6 °C .
The thermal strength parameter (TSP) of the ceramic composition is determined in
accordance with the following equation:
TSP = [MOR/(CTE x Young's Modulus)] ( 1 )
MOR is the modulus of rupture (MOR) of the ceramic composition, e.g., ceramic
honeycomb structure, as measured in accordance with ASTM C 1674-08 (Standard
Test Method for Flexural Strength of Advanced Ceramics with Engineered Porosity
(Honeycomb Cellular Channels at Ambient Temperature). MOR is measured following
Test Method B (see section 1.3.2 of ASTM C 1674-08) and it was a 4-point bending
test. In the test method a test specimen rests on two supports and is loaded by means
of a loading roller midway between the two outer supports. The press equipment was
Model MEM-102/M3 available from Suzpecar.
The Young's modulus is determined in accordance with DIN EN 843-2:2007 using
Pundit Plus ultra sound equipment (Reference E0646) available from Controlab. The
test sample is a honeycomb sample cut with dimensions of 55 mm x 55 mm +/- 10 mm.
The measurement is made in the longitudinal channels direction (with 82 KHz
transducers with diameter 33 mm) with a resolution of greater than 0.1 s .
In certain embodiments, the ceramic composition, e.g., ceramic honeycomb structure
has a TSP of at least about 175 °C, for example, at least about 200 °C, or at least
about 210 °C, or at least about 220 °C, or at least about 230 °C, or at least about
240°C, or at least about 250 °C. In certain embodiments, the ceramic composition,
e.g., ceramic honeycomb structure has a TSP of from about 150 °C to about 350 °C,
for example, from about 150 °C to about 275 °C, or from about 175°C to about 250 °C,
or from about 200 °C to about 250 °C.
In certain embodiments, the ceramic composition, e.g., ceramic honeycomb structure,
has a MOR of from about 0.5 MPa to about 10 MPa and a Young's Modulus of from
about 5 GPa to about 25 GPa, with the proviso that the MOR and Young's Modulus are
such that a TSP calculated in accordance with equation (1) is at least 150 °C.
The ceramic composition and ceramic honeycomb structure of any of the above
embodiments may have a MOR of from about 0.5 MPa to about 8 MPa, or from about
1.0 to about 6 MPa, or from about 1.25 to about 5 MPa, or from about 1.5 MPa to about
5 MPa, or from about 0.5 MPa to about 4 MPa, or from about 0.5 MPa to about 3. MPa,
or from about 0.5 MPa to about 2 MPa.
The ceramic composition and ceramic honeycomb structure of any of the above
embodiments may have a Young's Modulus of at least about 5 GPa. In certain
embodiments, the Young's Modulus may be between about 5 and 25 GPa, for
example, equal to or less than about 22 GPa, or equal to or less than about 20 GPa, or
equal to or less than about 18 GPa, or equal to or less than about 16 GPa, or equal to
or less than about 14 GPa. In certain embodiments, the Young's Modulus may be from
about 5 GPa to about 15 GPa, for example, from about 6 GPa to about 12 GPa, or
from about 6 GPa to about 10 GPa.
In certain embodiments, the ceramic composition comprises greater than about 45 wt.
% tialite, no more than about 44 wt. % mullite, from about 1.0-8.0 wt. % Zr-containing
mineral phase, for example, from about 1.5-8.0 wt. % Zr-containing mineral phase, has
a CTE of equal to or less than about 1.5 x 10 6 °C , and a TSP of greater than about
150 °C. In such embodiments, the ceramic composition may further comprise from
about 0.5-3.0 wt. % of alkaline earth metal-containing mineral phase, for example, from
about 0.5-2.5 wt. %, or about 1.0-2.5 wt. %, or about 1.0-2.0 wt. %, or about 1.0-1 .5 wt.
% of alkaline earth metal-containing mineral phase.
In certain embodiments, the ceramic composition comprises greater than about 46 wt.
% tialite, no more than about 44 wt. % mullite, from about 1.0-8.0 wt. % Zr-containing
mineral phase, for example, from about 3.0-8.0 wt. % Zr-containing mineral phase, has
a CTE of equal to or less than about 1. 1 x 10 6 °C , and a TSP of greater than about
150 °C, for example, a TSP of equal to or greater than about 200°C. In such
embodiments, the ceramic composition may further comprise from about 0.5-3.0 wt. %
of alkaline earth metal-containing mineral phase, for example, from about 0.5-2.5 wt.
%, or about 1.0-2.5 wt. %, or about 1.0-2.0 wt. %, or about 1.0-1 .5 wt. % of alkaline
earth metal-containing mineral phase.
In certain embodiments, the ceramic composition comprises greater than about 47 wt.
% tialite, no more than about 44 wt. % mullite, from about 1.0-8.0 wt. % Zr-containing
mineral phase, for example, from about 4.0-8.0 wt. % Zr-containing mineral phase, has
a CTE of equal to or less than about 1.0 x 10 6 °C , and a TSP of equal to or greater
than about 200°C. In such embodiments, the ceramic composition may further
comprise from about 0.5-3.0 wt. % of alkaline earth metal-containing mineral phase, for
example, from about 0.5-2.5 wt. %, or about 1.0-2.5 wt. %, or about 1.0-2.0 wt. %, or
about 1.0-1 .5 wt. % of alkaline earth metal-containing mineral phase.
In certain embodiments, the ceramic composition comprises greater than about 47 wt.
% tialite, no more than about 44 wt. % mullite, from about 5.0-8.0 wt. % Zr-containing
mineral phase, has a CTE of less than about 1.0 x 10 6 C , and a TSP of equal to or
greater than about 220°C. In such embodiments, the ceramic composition may further
comprise from about 0.5-3.0 wt. % of alkaline earth metal-containing mineral phase, for
example, from about 0.5-2.5 wt. %, or about 1.0-2.5 wt. %, or about 1.0-2.0 wt. %, or
about 1.0-1 .5 wt. % of alkaline earth metal-containing mineral phase. In such
embodiments, the ceramic composition may further comprise from about 0.5-3.0 wt. %
of alkaline earth metal-containing mineral phase, for example, from about 0.5-2.5 wt.
%, or about 1.0-2.5 wt. %, or about 1.0-2.0 wt. %, or about 1.0-1 .5 wt. % of alkaline
earth metal-containing mineral phase.
In certain embodiments, the ceramic composition comprises greater than about 50 wt.
% tialite, no more than about 44 wt. % mullite, from about 3.0-8.0 wt. % Zr-containing
mineral phase, has a CTE of less than about 1.5 x 10 6 C , and a TSP of equal to or
greater than about 150°C, for example, a TSP of equal to or greater than about 175°C.
In such embodiments, the ceramic composition may further comprise from about 0.5-
3.0 wt. % of alkaline earth metal-containing mineral phase, for example, from about
0.5-2.5 wt. %, or about 1.0-2.5 wt. %, or about 1.0-2.0 wt. %, or about 1.0-1 .5 wt. % of
alkaline earth metal-containing mineral phase. Advantageously, the alkaline earth
metal-containing mineral phase is a Mg-containing mineral phase.
In certain embodiments, the ceramic composition comprises from about 60 wt. % to 75
wt. % tialite, e.g., from about 60 wt. % to about 70 Wt. % tialite, from about 1.0-8.0 wt.
% Zr-containing mineral phase, for example, from about 1.5-8.0 wt. % Zr-containing
mineral phase, has a CTE of equal to or less than about 1. 1 x 10 6 °C , and a TSP of
greater than about 150 °C, for example, a TSP of equal to or greater than about 175°C.
In certain embodiments, the ceramic composition comprises from about 65 wt. % to 70
wt. % tialite, from about 1.5.0-5.0 wt. % Zr-containing mineral phase, for example, from
about 1.5-4.0 wt. % Zr-containing mineral phase, has a CTE of equal to or less than
about 1.0 x 10 6 °C , and a TSP of greater than about 150 °C, for example, a TSP of
equal to or greater than about 175°C.
In certain embodiments, the ceramic composition comprises from about 60 wt. % to 75
wt. % tialite, for example, from about 60 wt. % to about 70 wt. % tialite, from about 1.5-
8.0 wt. % Zr-containing mineral phase, for example, from about 1.5-5.0 wt. % Zrcontaining
mineral phase, or from about 1.5-3.5 wt. % Zr-containing mineral phase, has
a CTE of equal to or less than about 1.0 x 10 6 °C , and a TSP of greater than about
150 °C, for example, a TSP of equal to or greater than about 200°C. In certain
embodiments, the ceramic composition has a CTE of equal to or less than about 0.9 x
10 6 C 1 , or equal to or less than about 0.8 x 10 6 C 1 , equal to or less than about 0.7 x
10 6 °C , equal to or less than about 0.6 x 10 6 °C .
In certain embodiments, Zr-containing mineral phase comprises ZrO (i.e., zirconia). In
certain embodiments, the Zr-containing mineral phase comprises zirconium titanate. In
certain embodiments, the Zr-containing mineral phase comprises ZrO and zirconium
titanate. In certain embodiments, zirconium titanate has the chemical formula Tix Zr .
x0 2, wherein x is from 0.1 to about 0.9, for example, greater than about 0.5. In
embodiments, the Zr-containing mineral phase comprise a mixture of Zr0 2 and Tix Zr-i.
In certain embodiments, at least about 10 wt. % of the Zr-containing mineral phase is
zirconium titanate, as may be determined in accordance with the XRD method
described above. In certain embodiments, at least about 20 wt. % of the Zr-containing
mineral phase is zirconium titanate, for example, at least about 30 wt. % of the Zrcontaining
mineral phase is zirconium titanate, or at least about 40 wt. % of the Zrcontaining
mineral phase is zirconium titanate, or at least about 50 wt. % of the Zrcontaining
mineral phase is zirconium titanate.
In certain embodiments, the ceramic composition comprises from about 1.0-6.0 wt.% of
alkaline earth metal-containing mineral phase, for example, from about 1.0-5.0 %, or
from about 1.0-4.0 wt. %, or from about 1.0-3.5 wt. %, or from about 1.0-3.0 wt. %, or
1.0-2.5 wt. %, or from about 1.0-2.0 wt. % of alkaline earth metal-containing mineral
phase. The alkaline earth metal may be selected from Mg, Ca and Ba, or mixtures
thereof. In certain embodiments, the alkaline earth metal is Mg.
In embodiments in which the alkaline earth metal is Mg, the Mg-containing mineral
phase may comprise MgO and/or magnesium titanate.
In certain embodiments, the total amount of Zr-containing mineral phase and Mgcontaining
mineral phase constitutes from about 1.0-8.0 wt. % of the ceramic
composition, e.g., ceramic honeycomb structure, for example, from about 1.5-8.0 wt.
%, of from about 2.5-7.5 wt. %, or from about 3.0-6.5 wt. %, or from about 3.5-6.0 wt.
%, or from about 4.0-6.0 wt. %, or from about 4.5-6.0 wt. %, or from about 5.0 -6.0 wt.
% of the ceramic composition. In such embodiments, the weight ratio of Zr-containing
mineral phase and Mg-containing mineral phase may be at least about 1.25:1 , for
example, at least about 1.5:1 , or at least about 1.75:1 , or at least about 2:1 . Typically,
the weight ratio of Zr-containing mineral phase and Mg-containing mineral phase is no
more than about 5:1 , for example, no more than about 4:1 , or no more than about 3:1 .
Conventional wisdom in the art is that as the tialite content of a tialite-mullite ceramic
increases, the MOR of the ceramic would be expected to decrease, and thus, following
equation ( 1 ) , so would the TSP. However, the present inventors have surprisingly
found that the presence of a relatively small (relative to the tialite and mullite content)
amount of a Zr-containing mineral phase and optionally an alkaline earth metalcontaining
mineral phase, can offset, at least partially, the decrease in MOR, and thus,
TSP. Without wishing to be bound by theory, it is believed that the crystalline structure
of the Zr-containing mineral phase, e.g., Zr titanate adopting a perovskite-type
structure, has a beneficial effect on the strength properties of the ceramic composition.
Further, the present inventors have surprisingly found that the presence of a relatively
small amount of a Zr-containing mineral phase and optionally an alkaline earth metalcontaining
mineral phase has a beneficial effect in lowering the CTE of a tialite-mullite
ceramic comprising more tialite than mullite. Again, without wishing to be bound by
theory, it is believed that the crystalline structure of the Zr-containing mineral phase,
e.g., Zr titanate adopting a perovskite-type structure, is able to "absorb" the impact of
structural expansion at increased temperatures, meaning that the CTE of the tialitemullite
ceramic comprising the Zr-containing mineral phase is lower that it would have
been without the Zr-containing mineral phase. From equation ( 1 ) it is seen that a lower
CTE will lead to a higher TSP.
In certain embodiments, the ceramic composition is substantially free of alumina
mineral phases and/or aluminosilicate mineral phases and/or titania mineral phases
and/or an amorphous phase.
As used herein, the term "substantially free" refers to the total absence of or near total
absence of a specific compound or composition or mineral phase. For example, when
the ceramic composition is said to be substantially free of alumina, there is either no
alumina in the ceramic composition or only trace amounts of alumina in the
composition. A person skilled in the art will understand that a trace amount is an
amount which may be detectable by the XRD method described above, but not
quantifiable and moreover, if present, would not adversely affect the properties of the
ceramic composition or ceramic honeycomb structure.
The amorphous phase may comprise, consist essentially of, or consist of a glassy silica
phase. The glassy silica phase may form from decomposition of aluminosilicate, for
example, andalusite, during mullitization, typically at sintering temperatures between
about 1300°C and 1600°C.
In an embodiment, the amount of iron in the ceramic composition or ceramic
honeycomb structure, measured as Fe20 3, is less than 5 % by weight, and for example
may be less than about 2 wt. %, or for example less than about 1 wt. %, or for example
less than about 0.75 wt. %, or for example less than about 0.50 wt. %, or for example
less than about 0.25 wt. %. The structure may be essentially free from iron, as may be
achieved for example by using starting materials which are essentially free of iron. Iron
content, measured as Fe20 3, may be measured by XRF.
In an embodiment, the amount of strontium, measured as SrO, is less than about 2 wt.
%, and for example less than about 1 wt. %, or for example less than about 0.75 wt. %,
or for example less than about 0.50 wt. %, or for example less than about 0.25 wt. %.
The structure may be essentially free from strontium, as may be achieved for example
by using starting materials which are essentially free of strontium. Strontium content,
measured as Sr0 2, may be measured by XRF.
In an embodiment, the amount of chromium, measured as Cr20 3, is less than about 2
wt. %, and for example less than about 1 wt. %, or for example less than about 0.75 wt.
%, or for example less than about 0.50 wt. %, or for example less than about 0.25 wt.
%. The structure may be essentially free from chromium, as may be achieved for
example by using starting materials which are essentially free of chromium. Chromium
content, measured as Cr20 3, may be measured by XRF.
In an embodiment, the amount of tungsten, measured as W20 3, is less than about 2 wt.
%, and for example less than about 1 wt. %, or for example less than about 0.75 wt. %,
or for example less than about 0.50 wt. %, or for example less than about 0.25 wt. %.
The structure may be essentially free from tungsten, as may be achieved for example
by using starting materials which are essentially free of tungsten. Tungsten content,
measured as W20 3, may be measured by XRF.
In an embodiment, the amount of yttria, measured as Y20 3, is less than about 2.5 wt.
%, for example, less than about 2.0 wt. %, for example, less than about 1.5 wt. %, for
example, less than about 1 wt. %, for example, less than about 0.5 wt. %, for example,
in the range of about 0.3-0.4 wt. %. Any yttria present may be derived from yttriastabilized
zirconia which in embodiments may be used as a source of zirconia. The
structure may be essentially free from yttria, as may be achieved for example by using
starting materials which are essentially free of yttria. Yttria content, measured as Y2O3,
may be measured by XRF.
In an embodiment, the amount of rare earth metals, measured as Ln20 3 (wherein Ln
represents any one or more of the lanthanide elements La, Ce, Pr, Nd, Pm, Sm, Eu,
Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu), is less than about 2 wt. %, and for example less
than about 1 wt. %, or for example less than about 0.75 wt. %, or for example less than
about 0.50 wt. %, or for example less than about 0.25 wt. %. The structure may be
essentially free from rare earth metals, as may be achieved for example by using
starting materials which are essentially free of rare earth metals. Rare earth content,
measured as Ln20 3, may be measured by XRF.
The ceramic composition, for example, ceramic honeycomb structure, is typically
porous having a porosity in the range of from about 30 % to about 70 %. In one
embodiment, the porosity is from about 35 % to about 65 %, for example, from about
40 % to about 65 %, or from about 35 % to about 60 %, or from about 40 % to about 60
%. In a further embodiment, the porosity is from about 35 % to about 50 %, for
example, from about 35 % to about 45 %, or from about 35 % to about 40 %, or from
about 40 % to about 45 % (calculated on the basis of the total volume of the mineral
phases and pore space). Pore space (e.g., total pore volume) may be determined by
mercury diffusion as measured using a Thermo Scientific Mercury Porosimiter - Pascal
140, with a contact angle of 130 degrees, or any other measurement method which
gives an equivalent result.
In certain embodiments, the ceramic composition has a pore size (d50) of from about
8.0 to 25.0 , for example, from about 10.0 to 20.0 microns, or from about 12.0 to
20.0 microns, or from about 13.0 to 20.0 , or from about 14.0 to 20.0 . Pore size
may be determined by mercury porosimetry using a Pascal 140 series mercury
porosimeter from Thermo Scientific (Thermo Fisher). The software employed is
S.O.L.I.D. S/W, version 1.3.3 from Thermo Scientific. A sample weight of 1.0 g +/- 0.5
g is typically used for this measurement.
In certain embodiments, the ceramic composition, e.g., ceramic honeycomb structure,
exhibits favourable high temperature properties. For example, a mechanical strength
property of the ceramic composition, e.g., ceramic honeycomb structure may increase
at elevated temperatures, e.g., at a temperature of at least about 800 °C. Put another
way, a mechanical strength property of the ceramic composition, e.g., ceramic
honeycomb structure, at elevated temperatures (e.g., at least about 800 °C) may be
greater than the mechanical strength property of the ceramic composition at lower
temperature, e.g., room temperature (e.g., about 25 °C).
In certain embodiments, the mechanical strength property is the nominal beam
strength, SNB (in MPa), which may be determined in accordance with the three-point
flexure test described in ASTM C 1674-08, section 11.2. The nominal beam strength in
a three point flexure test may be calculated using the standard 3-point flexure elastic
beam formula as follows:
3PL
(3 pt) = 3 Point Bend
where:
P = breaking force (N),
L = outer (support) span (mm,
b = specimen width (mm), and
d = specimen thickness.
In certain embodiments, the SNB of the ceramic composition, e.g., ceramic honeycomb
structure, increases at elevated temperature, e.g., at a temperature of at least about
800 °C, compared to the SNB at room temperature (e.g., about 25°C). In certain
embodiments, the SNB increases from between about 0.5 and 1.5 MPa at about room
temperature to between about 2.5 and 3.5 MPa at about 800 °C. In certain
embodiments, the SNB at about 800°C is about 50 % greater than the SNB at room
temperature, for example, at least about 100 % greater, or at least about 125 %
greater, or at least about 150 % greater, or at least about 175 % greater, or at least
about 200 % greater, or at least about 225 % greater, or at least about 250 % greater.
In certain embodiments, the SNB at about 800 °C is from about 50 % to about 250 %
greater than the SNB at room temperature, for example, from about 100 % to about 225
% greater. In such embodiments, the ceramic composition may comprise greater than
about 46 wt. % tialite, no more than about 44 wt. % mullite, from about 3.0-8.0 wt. %
Zr-containing mineral phase, have a CTE of equal to or less than about 1. 1 x 10 6 C ,
and a TSP of greater than about 150 °C, for example, a TSP of equal to or greater than
about 175 °C, or equal to or greater than about 200°C. In such embodiments, the
ceramic composition may comprise greater than about 56 wt. % tialite, no more than
about 40 wt. % mullite, from about 1.5-8.0 wt. % Zr-containing mineral phase, for
example, from about 1.5-5.0 wt. % Zr-containing mineral phase, or from about 1.5-3.5
wt. % Zr-containing mineral phase, have a CTE of equal to or less than about 1. x 0 6
°C , and a TSP of greater than about 150 °C, for example, a TSP of equal to or greater
than about 175 °C, or equal to or greater than about 200°C.
In certain embodiments, the SNB increases from between about 0.5 and 1.5 MPa at
about room temperature to between about 2.5 and 3.5 MPa at about 1300 °C. In
certain embodiments, the SNB at about 1300°C is about 50 % greater than the SNB at
room temperature, for example, at least about 100 % greater, or at least about 125 %
greater, or at least about 150 % greater, or at least about 175 % greater, or at least
about 200 % greater, or at least about 225 % greater, or at least about 250 % greater.
In certain embodiments, the SNB at about 1300 °C is from about 50 % to about 250 %
greater than the SNB at room temperature, for example, from about 100 % to about 225
% greater.
The ceramic composition, for example, ceramic honeycomb structure, is formed by
sintering a ceramic precursor composition, as described below.
Ceramic precursor compositions
Unless otherwise stated, the amounts expressed in wt. % (or 'weight %' or '% by
weight') below are based on the total weight of inorganic mineral components in each
of the ceramic precursor compositions, i.e., excluding solvent (e.g., water), binder,
auxiliant, pore forming agents and any other non-inorganic mineral components.
The solid mineral compounds suitable for use as raw materials in the present invention
(aluminosilicate, alumina, titania, tialite, mullite, chamotte, etc.) can be used in the form
of powders, suspensions, dispersions, and the like. Corresponding formulations are
commercially available and known to the skilled person in the art. For example,
powdered andalusite having a particle size range suitable for the present invention is
commercially available under the trade name Kerphalite (Damrec), powdered alumina
and alumina dispersions are available from Evonik Gmbh or Nabaltec, and powdered
titania and titania dispersions are available from Cristal Global.
The ceramic precursor composition suitable for sintering to form a ceramic composition
according to first aspect of the invention comprises: mullite and/or one or more mulliteforming
compounds or compositions; tialite and/or one or more tialite-forming
compounds or compositions; and Zr-containing mineral phase and/or one or more Zrcontaining
mineral phase-forming compounds or compositions. The ceramic precursor
composition may further comprise an alkaline earth metal-containing mineral phase
and/or alkaline earth metal-containing mineral phase-forming compounds or
compositions.
The relative amounts of: mullite and/or one or more mullite-forming compounds or
compositions; tialite and/or one or more tialite-forming compounds or compositions;
and Zr-containing mineral phase and/or one or more Zr-containing mineral phaseforming
compounds or compositions (e.g., aluminosilicate, titania, alumina and
zirconia) are selected such that upon sintering the ceramic precursor composition at a
suitable temperature, e.g., above about 1400°C, or above about 1500°C, a ceramic
composition or ceramic honeycomb structure according to the first aspect and second
aspects of the present invention is obtained.
The mullite and/or one or more mullite-forming compounds or compositions, and tialite
and/or one or more tialite-forming compounds or compositions may be selected from
mullite, tialite, aluminosilicate, titania and alumina.
The aluminosilicate may be selected from one or more of andalusite, kyanite,
sillimanite, mullite, molochite, a hydrous kandite clay such as kaolin, halloysite or ball
clay, or an anhydrous (calcined) kandite clay such as metakaolin or fully calcined
kaolin. In further embodiments, the aluminosilicate is selected from one or more of
andalusite and kaolin. In a further embodiment, the aluminosilicate is andalusite.
In a further embodiment, the aluminosilicate, for example andalusite, is present in the
ceramic precursor composition in the form of particles present in a size in the range
between 0.1 and 55 , or between 0.1 and 80 , or between 10 and 55
, or between 10 and 75 , or between 15 and 55 , or between 15 
and 75 , or between 20 and 55 , or between 20 and 75 . In a further
embodiment, the aluminosilicate, for example andalusite, is in the form of particles
having a size in the range between 0.1 and 125 , or between 0.1 and 100
, or between 0.1 and 75 , or between 25 and 100 , or between 25 
and 75 .
The titania may be selected from one or more of rutile, anatase, brookite.
The zirconia may be selected from one or more of Zr0 2 and TixZr -x0 2 (as described
above). Besides the advantageous effects the presence of Zr-containing mineral
phase in the ceramic composition can have on thermomechanical properties of the
sintered ceramic composition, the inclusion of Zr0 2 in the ceramic precursor
composition appears to enhance the reactivity of alumina, such that the alumina
content of the final sintered ceramic can be reduced or eliminated. Further, the
inclusion of Zr0 2 may facilitate primary and secondary mullitization at lower reaction
temperatures.
In embodiments, the alumina is selected from one or more of fused alumina (e.g.,
corundum), sintered alumina, calcined alumina, reactive or semi-reactive alumina, and
bauxite.
In a further embodiment, the alumina is present in the form of particles having a size in
the range between 0.1 to 150 , or in the range between 0.1 to 100 or in the
range between 0.1 to 75 , or in the range between 0.1 to 50 , or in the range
between 0.1 to 25 , or in the range between 0.1 to 10 , or in the range between
0.1 to 1 , or between 0.3 to 0.6 . In a further embodiment, the alumina is used in
the form of colloidal/nanometric solutions.
In all of the above embodiments comprising the use of alumina (Al20 3) , titania (Ti0 2)
and zirconia (Zr0 2) , the alumina, titania and/or zirconia may be partially or fully
replaced by alumina, titania and/or zirconia precursor compounds. By the term
"alumina precursor compounds", such compounds are understood which may comprise
one or more additional components to aluminum (Al) and oxygen (O), which additional
components are removed during subjecting the alumina precursor compound to
sintering conditions, and wherein the additional components are volatile under sintering
conditions. Thus, although the alumina precursor compound may have a total formula
different from Al20 3, only a component with a formula Al20 3 (or its reaction product with
further solid phases) is left behind after sintering. Thus, the amount of alumina
precursor compound present in an extrudable mixture or green honeycomb structure
according to the invention can be easily recalculated to represent a specific equivalent
of alumina (Al20 3) . The terms "titania precursor compound" and "zirconia precursor
compound" are to be understood in similar fashion.
Examples for alumina precursor compounds include, but are not limited to aluminum
salts such as aluminum phosphates, and aluminum sulphates, or aluminum hydroxides
such as boehmite (AIO(OH) and gibbsite (AI(OH)3) . The additional hydrogen and
oxygen components present in those compounds are set free during sintering in the
form of water. Usually, alumina precursor compounds are more reactive in solid phase
reactions occurring under sintering conditions, than alumina (Al20 3) itself. Moreover,
several of the alumina precursor compounds are available in preparations showing
very small particle sizes, which also leads to an increased reactivity of the particles
under sintering conditions.
The aluminosilicate and in (part) alumina are the main mullite-forming components of
the ceramic precursor composition. During primary mullitization, aluminosilicate
decomposes and mullite forms. In secondary mullitization, excess silica from the
aluminosilicate reacts with any remaining alumina, forming further mullite. As
described below, the ceramic precursor composition may be sintered to a suitably high
temperature, for example, between about 1500°C and 1600°C, e.g., between about
1525 and 1575 °C, such that substantially all aluminosilicate and alumina has been
consumed in the primary and secondary mullitization stages.
Alumina and titania are the main tialite-forming components of the ceramic precursor
composition. In certain embodiments, the alumina is present in the form of particles
having a size in the range between 0.1 to 150 , or in the range between 0.1 to 100
or in the range between 0.1 to 75 , or in the range between 0.1 to 50 , or in
the range between 0.1 to 25 , or in the range between 0.1 to 10 , or between 0.1
to 1 , or between 0.3 to 0.6 . In a further embodiment, the alumina is used in the
form of colloidal/nanometric solutions. In a further embodiment, the titania is present in
the form of particles having a size in the range between 0.1 to 100 , or in the range
between 0.1 to 50 , or in the range between 0.1 to 10 , or between 0.1 to 1 ,
or between 0.3 to 50 , or between 0.3 to 1 , or between 0.3 to 0.6 . In a further
embodiment, the titania is present in the form of particles having a size in the range
between 0.1 to 10 , or between 0.2 to 1 , or between 0.2 to 0.5 . In a further
embodiment, the titania is used in the form of colloidal/nanometric solutions. Where
colloidal titania is used, this may be employed together with a non-colloidal form of
titania, for example one having a d50 smaller than 1 , for example a d50 smaller than
0.5 . In a further embodiment, the size of the titania particles is larger than the size
of the alumina particles. In a further embodiment, the amount of the alumina in the
ceramic precursor composition is higher than the amount of titania.
Because the components of the ceramic precursor composition may have different
particle size ranges, the ceramic precursor composition may have a bimodal or
multimodal particle size distribution. In other embodiments, particle size ranges of
components may be selected such that the ceramic precursor composition has a
monomodal particle size distribution. In further embodiments, the ceramic precursor
composition may be subjected to size classification step, for example, by milling or
sieving, prior to a forming step (e.g., extrusion) to homogenize the mixture particle size
distribution, e.g., milling to obtain a ceramic precursor composition having a
monomodal particle size distribution.
In certain embodiments, the ceramic precursor composition comprises an amount of
alkaline earth metal oxide or alkaline earth metal oxide precursor, or combinations
thereof. The alkaline earth metal oxide may be magnesium oxide, calcium oxide,
barium oxide, or combinations thereof. The alkaline earth metal oxide precursor may
be an alkaline earth metal salt, for example, an alkaline earth metal sulphide, sulphate,
chloride, nitrate or carbonate, in which the alkaline earth metal may be magnesium,
strontium, calcium, barium or combinations thereof. In certain embodiments, the
alkaline earth metal oxide precursor is an alkaline earth metal carbonate, which may be
magnesium carbonate, strontium carbonate, calcium carbonate, barium carbonate or
mixtures thereof. In embodiments, the carbonate is magnesium or calcium carbonate,
or combinations thereof. In an advantageous embodiment, the carbonate is
magnesium carbonate. The amount of alkaline earth metal oxide and/or alkaline earth
metal oxide precursor, for example, magnesium carbonate, may be from about 1-4 wt.
%, based on the total weight of the ceramic precursor composition.
Thus, in certain embodiments, the ceramic precursor composition comprises (weight
%):
• 15-55 %, or 20-50 %, or 20-45 %, or 20-40 %, or 25-40 %, or 30-50 %, or 25-35
%, or 30-40 %, or 40-50 %, or 35-45 % aluminosilicate;
• 15-35 %, or 20-30 %, or 22-27 % titania;
• 25-45 %, or 30-45 %, or 35-40 %, or 30-40 %, or 35-45 %, 37-45 %, or 40-45 %
alumina;
• 1.0-8.0 %, or 1.5-8.0 %, or 2.0-8.0 %, or 2.5-8.0 %, or 3.0-8.0 %, or 3.0-7.0 %, or
3.5-7.0 %, or 3.5-6.5 %, or 3.5-6.0 %, or 3.5-5.5%, or 4.0-6.0 %, or 4.0-5.0 %
zirconia and/or zirconium titanate; and
• 0-10 %, or 0.5-8 %, or 0.5-7 %, or 1.0-6.0 %, or 1.5-5.5 %, 2.0-5.0 %, or 2.5-5.0
%, or 3.0-5.0 %, or 3.5-5.0 % alkaline earth metal oxide and/or alkaline earth
metal oxide precursor.
In a further embodiment, the ceramic precursor compositions of the present invention
comprises a pore former, for example, a graphite component. The pore former, for
example, graphite, can be present in an amount of up to about 55 % (based on the
total weight of the inorganic mineral components), for example, from about 1 to about
40 wt. %, or from about 1 to about 25 wt. %, or from about 5 to about 20 wt. %, or from
about 5 to about 15 wt. %, or form about 5 to about 10 wt. %, or from about 10 to about
20 wt. % , or from about 10 to about 15 wt. %. The pore former, for example, graphite
material, can be used in a particulate form, wherein the particles have a size of less
than 200 , or less than 150 , or less than 100 . In another embodiment, the
graphite particles have a median particle diameter (d50) between 0 and 100 ; or
between 5 to 50 , or between 7 and 30 , or between 20 and 30 .
The graphite may be included as a pore former, as described below.
In certain embodiments, the ceramic precursor composition comprises preformed
mullite, for example, a mullite-containing chamotte, and tialite-forming precursor
components, i.e., titania and alumina, and the zirconia precursor, and optionally one or
more of aluminosilicate and alkaline earth metal carbonate. In an embodiment, the
preformed-mullite is a mullite-containing chamotte, for example, a chamotte comprising
at least about 90 wt. % mullite, or at least about 95 wt. % mullite, or at least about 99
wt. % mullite, or consisting essentially of 100 wt. % mullite.
Thus, in certain embodiments, the ceramic precursor composition comprises (weight
%):
• from about 15 % to less than about 50 %, or 25-49 %, or 30-48 %, or 35-47 %, or
35-46 %, or 35-45%, or 36-45 %, or 37-45 %, or 37-44 %, or 37-43 %; or 35-43
%, or 35-42 %, or 35-41 %, or 35-40 %, or 40-48 %, or 40-45 % mullitecontaining
chamotte;
• from about 15-35 %, or 20-35 %, or 18-30 %, or 20-28, or 20-25 titania or titania
precursor;
• from about 15-35 %, or 20-35 %, or 20-30 %, or 22-30 %, 22-28 % alumina;
• 1.0-8.0 %, or 1.5-8.0 %, or 2.0-8.0 %, or 2.5-8.0 %, or 3.0-8.0 %, or 3.0-7.0 %, or
3.5-7.0 %, or 3.5-6.5 %, or 3.5-6.0 %, or 3.5-5.5%, or 4.0-6.0 %, or 4.0-5.0 %
zirconia and/or zirconium titanate; and
• 0-10 %, or 0.5-8 %, or 0.5-7 %, or 1.0-6.0 %, or 1.5-5.5 %, 2.0-5.0 %, or 2.5-5.0
%, or 3.0-5.0 %, or 3.5-5.0 % alkaline earth metal oxide and/or alkaline earth
metal oxide precursor.
Generally, the amount of titania will be such that, upon sintering at a suitable
temperature, e.g., above about 1400°C, or above about 1500°C, the titania and
alumina (and any additional alumina present in a mullite-containing chamotte) form a
tialite mineral phase which constitutes from about 40 wt % to less than about 75 wt %,
for example, from about 45 wt. % to about 60 wt. %, of the ceramic composition or
ceramic honeycomb structure obtained following sintering. The person skilled in the art
will be able to determine suitable raw materials, amounts and sintering temperature
depending on the desired composition of the mullite-containing chamotte. Suitable raw
materials include aluminosilicate (including the types described above), alumina
(including types described above), titania (including the types described above),
zirconia (including the types described above) and alkaline earth metal oxide and/or
alkaline earth metal oxide precursor (including the types described above).
In certain embodiments, the relative amounts of mullite, e.g., mullite-containing
chamotte, titania and alumina, zirconia and optional aluminosilicate and alkaline earth
metal oxide/carbonate, are selected such that upon sintering the ceramic precursor
composition at a suitable temperature, e.g., above about 1400°C, or above about
1500°C, a ceramic composition or ceramic honeycomb structure according to the first
aspect and second aspects of the present invention is obtained.
The binding agents and auxiliants that may be used in the present invention are all
commercially available from various sources known to the skilled person in the art.
The function of the binding agent is to provide a sufficient mechanical stability of the
green honeycomb structure in the process steps before the heating or sintering. The
additional auxiliants provide the raw material, i.e., ceramic precursor composition, with
advantageous properties of the extrusion step (e.g., plasticizers, glidants, lubricants,
and the like).
In embodiments, the ceramic precursor composition (or the extrudable mixture or green
honeycomb structure formed therefrom) comprises one or more binding agents
selected from the group consisting of, methyl cellulose, hydroxymethylpropyl cellulose,
polyvinyl butyrals, emulsified acrylates, polyvinyl alcohols, polyvinyl pyrrolidones,
polyacrylics, starch, silicon binders, polyacrylates, silicates, polyethylene imine,
lignosulfonates, and alginates.
In a further embodiment, the ceramic precursor composition (or the extrudable mixture
or green honeycomb structure formed therefrom) comprises one or more mineral
binders. Suitable mineral binder may be selected from the group including, but not
limited to, one or more of bentonite, aluminum phosphate, boehmite, sodium silicates,
boron silicates, or mixtures thereof.
The binding agents can be present in a total amount between about 0.5 and 20 %, for
example, from about 0.5 % and 15 %, or between about 2 % and 9 % (based on the
total weight of inorganic mineral components in the ceramic precursor composition or
extrudable mixture or the green honeycomb structure).
In a further embodiment, the ceramic precursor composition (or the extrudable mixture
or green honeycomb formed therefrom) comprises one or more auxiliants (e.g.
plasticizers and lubricants) selected from the groups consisting of polyethylene glycols
(PEGs), glycerol, ethylene glycol, octyl phthalates, ammonium stearates, wax
emulsions, oleic acid, Manhattan fish oil, stearic acid, wax, palmitic acid, linoleic acid,
myristic acid, and lauric acid.
The auxiliants can be present in a total amount between about 0.5 % and about 40 %,
for example, from about 0.5 % to about 30 %, or from about 0.5 % and about 25 %, or
from about 0.5 % and about 15 %, or between 2 % and 9 % (based on the total weight
of inorganic mineral components in the ceramic precursor composition or extrudable
mixture or the green honeycomb structure).
Each of the ceramic precursor compositions may be combined with solvent. The
solvent may be an organic or aqueous liquid medium. In certain embodiments, the
solvent is water. The solvent, e.g., water, may be present in an amount ranging from
about 1-55 wt. %, based on the total weight of inorganic mineral components in the
ceramic precursor composition, for example, from about 5 to about 40 wt. %, or from
about 10 to about 35 wt. %, or from about 15 to about 30 wt. %, or from about 20 to
about 30 wt. %, or from about 22 to about 28 wt. %.
Each of the ceramic precursor compositions of the present invention may further
comprise an amount of pore former. The pore former is any chemical entity which,
when included in the ceramic precursor composition, induces or otherwise facilitates
the creation of porosity in the ceramic composition formed by sintering the ceramic
precursor composition. Suitable pore formers include graphite (as described above) or
other forms of carbon, cellulose and cellulose derivatives, starch, organic polymers and
mixtures thereof. Pore former may be present in an amount ranging from about 1 and
70 wt. %, based on the total weight of inorganic mineral components in the ceramic
precursor composition, for example, from about 1 to about 60 wt. %, or from about 1 to
about 50 wt. %, or from about 1 to about 40 wt. %, or from about 1 to about 30 wt. %,
or from about 2 to about 25 wt. %, or from about 2 to about 20 wt. %, or from about 2 to
about 15 wt. %, or from about 4 to about 12 wt. %, or from about 4 to about 10 wt. %,
or from about 5 to about 8 wt. %.
Preparative methods
The preparation of an extrudable mixture from the mineral compounds, i.e., the ceramic
precursor composition (optionally in combination with binding agent(s), mineral
binder(s) and/or auxiliant(s)) is performed according to methods and techniques known
in the art. For, example, the components of the ceramic precursor composition can be
mixed in a conventional kneading machine with the addition of a suitable amount of a
suitable liquid phase as needed (normally water) to a slurry or paste suitable for
extrusion. Additionally, conventional extruding equipment (such as, e.g., a screw
extruder) and dies for the extrusion of honeycomb structures known in the art can be
used. A summary of the technology is given in the textbook of W. Kollenberg (ed.),
Technische Keramik, Vulkan-Verlag, Essen, Germany, 2004, which is incorporated
herein by reference.
The diameter of the green honeycomb structures can be determined by selecting
extruder dies of desired size and shape. After extrusion, the extruded mass is cut into
pieces of suitable length to obtain green honeycomb structures of desired format.
Suitable cutting means for this step (such as wire cutters) are known to the person
skilled in the art.
The extruded green honeycomb structure can be dried according to methods known in
the art (e.g., microwave drying, hot-air drying) prior to sintering.
The dried green honeycomb structure is then heated in a conventional oven or kiln for
preparation of ceramic materials. Generally, any oven or kiln that is suitable to subject
the heated objects to a predefined temperature is suitable for the process of the
invention.
In certain embodiments, the green honeycomb structure maybe plugged prior to
sintering. In other embodiments, the plugging may be carried out after sintering.
Further details of the plugging process are described below.
When the green honeycomb structure comprises organic binder compound and/or
organic auxiliants, usually the structure is heated to a temperature in the range
between 200°C and 400°C, for example, between about 200°C and 300°C, prior to
heating the structure to the final sintering temperature, and that temperature is
maintained for a period of time that is sufficient to remove the organic binder and
auxiliant compounds by means of combustion (for example, between one and three
hours). For example, one heating program for the manufacture of ceramic honeycomb
structures of the present invention is as follows:
• heating from ambient temperature to 250 °C with a heating rate of 0.5 °C/min;
• maintaining the temperature of 250 °C for up to about two hours;
• heating to the final sintering temperature with a heating rate of 2.0 °C/min; and
• maintaining the final sintering temperature for about 1 hour to about four hours.
The honeycomb structure may be sintered at a temperature in the range from between
1200°C and 1700°C, or between about 1250°C and 1650°C, or between about 1350°C
and 1650°C, or between 1400 °C and 1600°C, or between about 1450°C and 1600°C,
or between about 1500°C and 1600°C. In certain embodiments, the sintering step is
performed at temperature between about 1520°C and 1600°C, or between about
1530°C and 1600°C, or between about 1540°C and 1600°C, or between about 1550°C
and 1600°C, or between about 1525°C and about 1575°C. In certain embodiments,
the sintering temperature is less than about 1575°C.
For embodiments of the invention in which the ceramic precursor composition
comprises a major amount of mullite-forming components/compositions and tialiteforming
components compositions, e.g., a ceramic precursor composition according to
the third aspect of the invention, the above components/compositions undergo
chemical reactions resulting in the formation of mullite and tialite. These reactions, as
well as the required reactions conditions, are known to persons skilled in the art. A
summary is given in the textbook of W. Kollenberg (ed.), Technische Keramik, Vulkan-
Verlag, Essen, Germany, 2004, which is incorporated herein by reference.
For embodiments in which at least part of the mullite and tialite are already formed in
the ceramic precursor composition, the number of competing reactions during sintering
is reduced and comprise substantially only primary and secondary mullitization. A
further advantage in using a precursor composition comprising already formed mullite
and tialite is better control of the amounts of these mineral phases in the sintered
ceramic composition or honeycomb structure.
Sintering may be performed for a suitable period of time and a suitable temperature
such that the mullite and tialite mineral phases constitute at least about 80 % of the
total weight of the mineral phases, for example, at least about 85 % of the total weight
of the mineral phases, or at least about 90 % of the total weight of the mineral phases,
or at least about 92 % of the total weight of the mineral phases, or at least about 94 %,
or at least about 96 %, or at least about 97 %, or at least about 98 %, or at least about
99 % of the total weight of the mineral phases, or up to about 98.5 wt. % of the mineral
phases, or up to about 98.0 wt. % of the mineral phases, or up to about 97.5 % of the
mineral phases, or up to about 97.0 % of the mineral phases, or up to about 96.5 % of
the mineral phases, or up to about 96.0 % of the mineral phases, or up to about 95.5 %
of the mineral phases, or up to about 95.0 % of the mineral phases.
Ceramic Honeycomb Structures:
In the ceramic honeycomb structures described in the above embodiments, the optimal
pore diameter is in the range between 5 to 30 , or 10 to 25 . Depending on the
intended use of the ceramic honeycombs, in particular with regard to the question
whether the ceramic honeycomb structure is further impregnated, e.g., with a catalyst,
the above values may be varied. For non-impregnated ceramic honeycomb structures,
the pore diameter is usually in the range between 7 and 15 , while for impregnated
structures, the range is usually between 10 and 25 prior to impregnating, for
example, between 15 and 25 , or between about 20 and 25 prior to
impregnating. The catalyst material deposited in the pore space will result in a
reduction of the original pore diameter.
The honeycomb structure of the invention can typically include a plurality of cells side
by side in a longitudinal direction that are separated by porous partitions and plugged
in an alternating (e.g., checkerboard) fashion. In one embodiment, the cells of the
honeycomb structure are arranged in a repeating pattern. The cells can be square,
round, rectangular, octagonal, polygonal or any other shape or combination of shapes
that are suitable for arrangement in a repeating pattern. Optionally, the opening area
at one end face of the honeycomb structural body can be different from an opening
area at the other end face thereof. For example, the honeycomb structural body can
have a group of large volume through-holes plugged so as to make a relatively large
sum of opening areas on its gas inlet side and a group of small volume through-holes
plugged so as to make a relatively small sum of opening areas on its gas outlet side.
In certain embodiments, the cells of the honeycomb structure are arranged in
accordance with the structures described in WO-A-201 1/1 17385, the entire contents of
which are hereby incorporated by reference.
An average cell density of the honeycomb structure of the present invention is not
limited. The ceramic honeycomb structure may have a cell density between 6 and
2000 cells/square inch (0.9 to 3 11 cells/cm2) , or between 50 and 1000 cells/square
inch (7.8 to 155 cells/cm2) , or between 100 and 400 cells/square inch (15.5 to 62.0
cells/cm2) .
The thickness of the partition wall separating adjacent cells in the present invention is
not limited. The thickness of the partition wall may range from 100 to 500 microns, or
from 200 to 450 microns.
Moreover, the outer peripheral wall of the structure is preferably thicker than the
partition walls, and its thickness may be in a range of 100 to 700 microns, or 200 to 400
microns. The outer peripheral wall may be not only a wall formed integrally with the
partition wall at the time of the forming but also a cement coated wall formed by
grinding an outer periphery into a predetermined shape.
In certain embodiments, the ceramic honeycomb structure is of a modular form in
which a series of ceramic honeycomb structures are prepared in accordance with the
present invention and then combined to form a composite ceramic honeycomb
structure. The series of honeycomb structures may be combined whilst in the green
state, prior to sintering or, alternatively, may be individually sintered, and then
combined. In certain embodiments, the composite ceramic honeycomb structure may
comprise a series of ceramic honeycomb structures prepared in accordance with
present invention and ceramic honeycomb structures not in accordance with the
present invention.
For the use as diesel particulate filters, the ceramic honeycomb structures of the
present invention, or the green ceramic honeycomb structures of the present invention
can be further processed by plugging, i.e., close certain open structures of the
honeycomb at predefined positions with additional ceramic mass. Plugging processes
thus include the preparation of a suitable plugging mass, applying the plugging mass to
the desired positions of the ceramic or green honeycomb structure, and subjecting the
plugged honeycomb structure to an additional sintering step, or sintering the plugged
green honeycomb structure in one step, wherein the plugging mass is transformed into
a ceramic plugging mass having suitable properties for the use in diesel particulate
filters. It is not required that the ceramic plugging mass is of the same composition as
the ceramic mass of the honeycomb body. Generally, methods and materials for
plugging known to the person skilled in the art may be applied for the plugging of the
honeycombs of the present invention. In an exemplary process about 50 % of the inlet
channels are plugged on one side of the honeycomb piece and on the opposite side a
further 50 % of the channels are plugged in order such that, in use, exhaust gas is
forced to pass through walls of the honeycomb structure.
The plugged ceramic honeycomb structure may then be fixed in a box suitable for
mounting the structure into the exhaust gas line of a diesel engine, for example, the
diesel engine of a vehicle (e.g., automobile, truck, van, motorbike, digger, excavator,
tractor, bulldozer, dump-truck, and the like).
EXAMPLES
Example 1
A series of ceramic honeycomb structures were obtained from the ceramic precursor
compositions described in Tables 1 and 2 below. Compositional analysis and
thermomechanical properties were determined in accordance with the methods
described above. Results are summarised in Tables 3-6.
Samples RMT1-RMT4 were extruded as square honeycomb barrels and fired in a
laboratory electric kiln at a maximum temperature of 1550°C ( 1 hour soaking time).
Samples RMT5-RMT9 were extruded as complete honeycomb prototypes and fired in
an industrial gas kiln at a maximum temperature of 1550°C (2 hour soaking time)
Table 1.
Table 2.
RMT 6 RMT 7 RMT 8 RMT 9
Raw materials (wt. %) (wt. %) (wt. %) (wt. %)
Aluminosilicate precursor 25.44% 22.55% 19 .10% 26.61
Tialite precursor 28.52% 30.37% 32.72% 27.69
Alumina 42.2% 43.2% 44.3% 4 1.9
Zirconia precursor 2.60% 2.60% 2.60% 2.60
Mg Precursor 1.24% 1.24% 1.24% 1.25
Total solid content 100.0% 100.0% 100.0% 100.0
Water and binders 39.84% 39.84% 39.84% 40.06
Total 139.8% 139.8% 139.8% 140. 1
Table 3.
Table 4.
*4-point test
Table 6.
Recipe RMT6 RMT7 RMT8 RMT 9
Tialite precursor (% weight) 28.5% 30.4% 32.7% 27.7%
Porosity (%) 4 1.5% 42.9% 43.0% 47%
Pore size (d50) (microns) 14.7 12.4 13.4 17.7
MOR (SNB) (MPa) 1.5 1.2 0.9 0.6
CTE (800°C) (10E-6°C 1) 1. 1 0.8 0.7 1.0
Young Modulus (GPa) 7.8 7.9 6.3 3.0
TSP °C 175 190 204 200
Example 2
The nominal beam strength, SNB (in MPa), of sample RMT 6 was determined in
accordance with the three-point flexure test described in ASTM C 1674-08, section
11.2 (as described above) between 25 °C and 1300 °C. In the same way, a
comparative sample formed of tialite was analysed. Results are summarized in Figure
1. It is seen that SNB for RMT-6 increases significantly at elevated temperatures,
whereas there is little variation in SNBof the tialite honeycomb.
CLAIMS
A ceramic composition comprising:
from about 15 wt. % to less than about 50 wt. % mullite;
from about 40 wt. % to about 75 wt. % tialite; and
at least about 1.0 wt. % of a Zr-containing mineral phase;
wherein the weight ratio of tialite to mullite is greater than 1: 1 , and
wherein the ceramic composition has a coefficient of thermal expansion (CTE) of
equal to or less than about 1.5 x 10 6 °C , and a thermal strength parameter
(TSP) of at least about 150 °C.
A ceramic composition according to claim 1, comprising: ( 1 ) from about 40 wt. %
to about 55 wt. % tialite; or (2), from about 56 wt. % to about 75 wt. % tialite, for
example, from about 60 wt. % to about 70 wt. % tialite.
A ceramic composition according to claim 1 or 2 having a CTE of less than about
1.5 x 10 6 °C 1 , and a TSP of at least about 170 °C, for example, at least about
200 °C.
A ceramic composition according to claim 1, 2 or 3, comprising from about 1.0 wt.
% to about 8 wt. % of the Zr-containing mineral phase, for example, from about
1.5 wt. % to about 8 wt. % of the Zr-containing mineral phase, or from about 2 wt.
% to about 6 wt. % of the Zr-containing mineral phase.
A ceramic composition according to claim 4, wherein: ( 1 ) the composition
comprises from about 45 wt. % to about 55 wt. % tialite and from about 3.0 wt. %
to about 8.0 wt. % of the Zr-containing mineral phase; or (2) the composition
comprises from about 56 wt. % to about 75 wt. % tialite and from about 1.5 wt. %
to about 5.0 wt. % of the Zr-containing mineral phase.
6. A ceramic composition according to any preceding claim, wherein the Zrcontaining
phase comprises at least about 20 wt. % zirconium titanate, for
example, at least about 50 wt. % zirconium titanate.
7. A ceramic composition according to any preceding claim, further comprising from
about 0.5 wt. % to about 8 wt. % of an alkaline earth metal-containing mineral
phase, for example, a Mg-containing mineral phase.
8 . A ceramic composition according to any preceding claim, wherein: (i) the nominal
beam strength, S B, of the ceramic composition increases at elevated
temperatures, for example, at a temperature of at least about 800°C; and/or (ii)
the SNB of the ceramic composition at elevated temperatures (e.g., at least about
800°C) is greater than the SNB of the ceramic composition at room temperature
(e.g., about 25 °C).
9. A ceramic composition according to any preceding claim, comprising from about
55 wt. % to about 70 wt. % tialite.
10. A ceramic composition according to any preceding claim in the form of a
honeycomb structure.
11. A ceramic precursor composition suitable for sintering to form a ceramic
composition according to claim 1, said precursor composition comprising:
mullite and/or one or more mullite-forming compounds or compositions;
tialite and/or one or more tialite-forming compounds or compositions; and
Zr-containing mineral phase and/or one or more Zr-containing mineral phaseforming
compounds or compositions.
12. A ceramic precursor composition according to claim 11, further comprising an
alkaline earth metal-containing mineral phase and/or alkaline earth metalcontaining
mineral phase-forming compounds or compositions.
13. A ceramic precursor composition according to claim 11 or 12, further comprising:
(i) one or more binding agents;
(ii) one or more mineral binders;
(iii) one or more pore forming agents;
(iv) one or more auxiliants; and/or
(v) water.
14. A method for making a honeycomb structure according to claim 11, said method
comprising:
(a) providing a dried green honeycomb structure formed from the ceramic
precursor composition according to any one of claims 11-13; and
(b) sintering.
15. A method according to claim 14, comprising the steps of:
(a)(i) providing an extrudable mixture formed from the precursor composition
according to any one of claims 11-13;
(a)(ii) extruding the mixture to form a green honeycomb structure;
(a)(iii) drying the green honeycomb structure; and
(b) sintering, for example, at a temperature of from about 1200 °C to about 1700
°C.
16. A method according to any to claim 14 or 15, further comprising plugging the
green honeycomb structure or sintered honeycomb structure.
17. A diesel particulate filter comprising or made from the ceramic honeycomb
structure according to claim 10 or obtainable by the method of according to claim
14, 15 or 16.
18. A vehicle having a diesel engine and a filtration system comprising the diesel
particulate filer according to claim 17.