TECHNICAL FIELD
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.
Fourth, the material must have a melting point above the temperatures reached
(typically > 1000°C) within the filter during a regeneration cycle.
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 be 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 and fair mechanical properties, but relatively poor thermal shock
properties. Tialite is an aluminium titanate having the formula [AI2Ti20 ] 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. According to one example, a honeycomb consisting of 72 % 3:2 mullite,
13 % andalusite, 8 % amorphous phase and 7 % tialite was prepared. The honeycomb
had a total porosity of 47.5% and a standard three point modulus of rupture (MOR) test
along the axis of the sample showed a fracture force of 99N.
There is a need in the art for new ceramic filter materials showing properties
comparable to or improved over those of the prior art. It has now been found that a
ceramic material providing desirable mechanical strength in combination with excellent
thermal shock resistance can be manufactured which comprises a relatively low
amount of tialite phase (compared to conventional honeycombs having very high tialite
content) in combination with an amount of mullite. This is surprising given that mullite,
relative to tialite, is known to have relatively poor thermal shock properties.
SUMMARY OF THE INVENTION
In accordance with a first aspect, there is provided a ceramic composition comprising:
from about 25 wt. % to about 60 wt. % tialite;
from about 35 wt. % to about 75 vol. % mullite;
from about 0 wt. % to about 8 wt. % zirconia;
form about 0 wt. % to about 10 wt. % zirconium titanate;
from about 0 wt. % to about 10 wt. % of an amorphous phase;
from about 0 wt. % to about 5 wt. % of an alkaline earth metal oxide; and
from about 0 wt. % to about 10 wt. % alumina (calculated on the basis of the total
weight of mineral phases);
wherein said ceramic composition has a porosity of from about 30 % to about 70 %
(calculated on the basis of the total volume of the mineral phases and pore space).
In accordance with a second aspect, there is provided the ceramic composition of the
first aspect of the present invention in the form of a honeycomb structure.
In accordance with a third aspect, there is provided a ceramic precursor composition
suitable for sintering to form a ceramic composition according to the first aspect of the
present invention, said precursor composition comprising:
from about 20 wt. % to about 55 wt. % aluminosilicate;
from about 15 wt. % to about 35 wt. % titania and/or titania precursor;
from about 25 wt. % to about 45 wt. % alumina (Al20 3) ;
from 0 wt. % to about 5 wt. % alkaline earth metal oxide and/or alkaline earth
metal oxide precursor; and
from 0 wt. % to about 15 wt. % zirconia and/or zirconia precursor.
In accordance with a fourth aspect, there is provided a ceramic precursor composition
suitable for sintering to form a ceramic composition according to the first aspect of the
present invention, said precursor composition comprising:
from about 20 wt. % to about 55 wt. % aluminosilicate;
from about 45 wt. % to about 75 wt. % of a tialite- and mullite containing
chamotte;
from 0 wt. % to about 20 wt. % alumina
from 0 wt. % to about 5 wt. % alkaline earth metal oxide and/or alkaline earth
metal oxide precursor; and
from 0 wt. % to about 15 wt. % zirconia and/or zirconia precursor
In accordance with a fifth aspect, there is provided a ceramic precursor composition
suitable for sintering to form a ceramic composition according to the first aspect of the
present invention, said precursor composition comprising:
from about 30 wt. % to about 60 wt. % mullite;
from about 5 wt. % to about 35 wt. % titania and/or titania precursor;
from 15 wt. % to 35 wt. % alumina;
from 0 wt. % to about 5 wt. % alkaline earth metal oxide and/or alkaline earth
metal oxide precursor; and
from 0 wt. % to about 15 wt. % zirconia and/or zirconia precursor.
In accordance with a sixth aspect, there is provided a method for making a ceramic
honeycomb structure according to the second aspect of the present invention, said
method comprising the steps of:
(a) providing a dried green honeycomb structure formed from the ceramic
precursor composition according to any one of the third to fifth aspects of the
present invention; and
(b) sintering.
In accordance with a seventh aspect, there is provided a diesel particulate filter
comprising or made from the ceramic honeycomb structure according to the second
aspect of the present invention, or the ceramic honeycomb structure obtainable by the
method of the sixth aspect of the present invention.
In accordance with an eight aspect, there is provided a vehicle having a diesel engine
and a filtration system comprising the diesel particulate filter according to the seventh
aspect of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The "total volume of the mineral phases" of a ceramic composition, e.g., a ceramic
honeycomb structure, refers to the total volume of the honeycomb without the pore
volume, i.e., only solid phases are considered. The "total volume of the mineral phases
and pore space" refers to the apparent volume of the ceramic composition, e.g.,
ceramic honeycomb structure, i.e., including solid phases and pore volume.
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, 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 and passed
completely through a 45 mesh. After milling and sieving, 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 0.01 ° , a measurement time of 2 seconds per step and a measurement
range from 5 to 80° 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
D500/501 , 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 S 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 d10 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 d 0 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.
Ceramic composition
In embodiments, the ceramic composition, e.g., ceramic honeycomb structure,
comprises (on a weight % basis):
• 25-60 %, or 35-60 %, or 40-60 %, or 40-55 %, or 40-50%, or 45-55 %, or 45-50
%, or 50-55 % tialite;
• 35-75 %, or 35-60 %, or 40-60 %, or 40-55 %, or 40-50 %, or 45-55 %, or 45-50
%, or 50-55 % mullite;
• 0-8 %, or 0-5 %, or 0-3 %, or 0-2 %, or 0-1 % zirconia;
• 0-10 %, or 0-5 %, 0-3 %, or 0-2 %, or 0-1 % zirconium titanate;
• 0-10 %, or 0-5, or 0-3 %, or 0-2 %, or 0-1 % of an amorphous phase;
• 0-5 %, or 0-4 %, or 0-3 %, or 0-2 %, or 0-1 % of an alkaline earth metal oxide;
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 from about 35-60 % tialite
and from about 35-60 % mullite.
In certain embodiment, the ceramic composition comprises from about 25-30 % tialite
and from about 65-75 wt. % mullite.
In further embodiments, the ceramic composition comprises 40-55 % tialite and 40-55
% mullite, for example, 40-50 % tialite and 45-55 % mullite, or 45-55 % tialite and 40-
50 % mullite, or 40-45 % tialite and 40-55 % mullite, or 40-45 % tialite and 40-50 %
mullite, or 40-45 % tialite and 45-55 % mullite, or 40-45 % tialite and 45-50 % mullite.
In certain embodiments, the ratio mullite to tialite is lower than 2:1 , for example, lower
than 1 8:1 , or lower than 1.6:1 , or equal to or lower than about 1.5:1 , or equal to or
lower than about 1.4:1 , or equal to or lower than about 1.3:1 , or equal to or lower than
about 1.2:1, or equal to or lower than about 1. 1 : 1 , or equal to or lower than about 1:1 ,
or equal to or lower than about 1: 1 .2.
In certain embodiments, the mullite and tialite mineral phases constitute at least about
90 % of the total weight of the mineral phases, for example, 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 total weight of the
mineral phases.
In certain embodiment, the ceramic composition comprises less than about 50 wt. %
tialite, for example, less than about 49 wt. % tialite, or less than about 48 wt. % tialite,
or less than about 47 wt. % tialite, or less than about 46 % tialite, or less than about 45
wt. % tialite, or less than about 44 wt. % tialite, or less than about 43 wt. % tialite, or
less than about 42 wt. % tialite, or less than about 4 1 wt. % tialite, or less than about 40
wt. % tialite.
In certain embodiments, the ceramic composition comprises at least about 0.1 wt. %
zirconia, for example, at least about 0.25 wt. % zirconia, or at least about 0 . 5 wt. %
zirconia. Additionally or alternatively, the ceramic composition may comprise at least
about 0.1 wt. % of said amorphous phase, for example, at least about 0.25 wt. % of
said amorphous phase, or at least about 0.5 wt. % of said amorphous phase.
Additionally or alternatively, the ceramic composition may comprise at least about 0 .1
wt. % alumina, for example, at least about 0.25 wt. % alumina, or at least about 0.5 wt.
% alumina. Additionally or alternatively, the ceramic composition may comprise at
least about 0.1 wt. % alkaline earth metal oxide, for example, at least about 0.25 wt. %
alkaline earth metal oxide, or at least about 0.5 wt. % alkaline earth metal oxide.
Additionally or alternatively, the ceramic composition may comprise at least about 0.1
wt. % zirconium titanate, for example, at least about 0.25 wt. % zirconium titanate, or at
least about 0. 5 wt. % zirconium titanate.
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 and/or zirconium titanate mineral phases and/or alkaline
earth metal oxide mineral phases.
When present, the alkaline earth metal oxide may be strontium oxide, magnesium
oxide, calcium oxide, barium oxide or a combination thereof. In an embodiment, the
alkaline earth metal oxide is magnesium oxide.
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.
Depending on the starting materials, the zirconia phase may be Zr0 2 or may be a
zirconia phase comprising Ti, e.g., Tix Zr - 0 2, wherein x is from 0.1 to about 0.9, for
example, greater than about 0.5. In embodiments, the zirconia phase may comprise a
mixture of Zr0 2 and Tix - 0 2.
The ceramic composition, for example, ceramic honeycomb structure, is 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.
The ceramic compositions and ceramic honeycomb structures of any of the above
embodiments may have a coefficient of thermal expansion (CTE) of equal to or less
than about 3.5 x 10 6 °C as measured at 800°C by dilatometry according to DIN
51045. In certain embodiments, the CTE may be equal to or less than about 3.0 x 10 6
°C , for example, equal to or less than about 2.75 x 10~6 °C or equal to or less than
about 2.5 x 0~ °C 1 , or equal to or less than about 2.25 x 10 6 °C 1 , or equal to or less
than about 2.0 x 10 s °C equal to or less than about 1.9 x 10 6 °C 1 , or equal to or less
than about 1.8 x 10~5 C or equal to or less than about 1.7 x 10 6 °C 1 or equal to or
less than about 1.6 x 10 6 °C 1 or equal to or less than about 1.5 x 10 6 ° 1 . Typically,
the CTE will be greater than about 0.1 x 10 6 °C 1 , for example, greater than about 0.5 x
10 6 C or even greater than about 1.0 x 10 6 °C 1 .
The ceramic compositions and ceramic honeycomb structures of any of the above
embodiments may have a modulus of rupture (MOR) of at least about 1.5 MPa, 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 was measured following Test Method A
(see section 1.3.1 of ASTM C 1674-08) and it was a 3-point bending test with userdefined
specimen geometries. In the test method a test specimen with a user-defined
rectangular geometry rests on two supports and is loaded by means of a loading roller
midway between the two outer supports. A suitable test specimen geometry is defined
for the particular honeycomb structure being tested (composition, architecture, cell
size, mechanical properties) using the guidelines provided in section 9.2 of ASTM C
1674-08. The user defined specimen geometry is selected such that the test gives
valid test data (failure in the gage section without major crushing failure or shear
failure).
In certain embodiments, the ceramic composition has a MOR of at least about 2.0
MPa, for example, at least about 2.5 MPa, or at least about 3.0 MPa, or at least about
3 . 5 MPa, or at least about 4.0 MPa. Typically, the MOR will be less than about 20
MPa, for example, less than about 15 MPa, or less than about 10 MPa, or less than
about 8 MPa, or less than about 6 MPa.
The ceramic compositions and ceramic honeycomb structures of any of the above
embodiments may have a Young's Modulus of at least about 5 GPa, as measured in
accordance with DIN EN 843-2:2007. 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, or equal to or less
than about 12 GPa, or equal to or less than about 10 GPa.
The ceramic compositions and ceramic honeycomb structures of any of the above
embodiments may be characterized in terms of their ratio of MOR to CTE. This ratio
may be used to compare the thermomechanical behaviour of ceramic materials. The
ratio is calculated by dividing the numerical value of MOR (in Pa) by the numerical
value of CTE (in reciprocal °C). For example, a ceramic composition having a MOR of
3.0 MPa (i.e., 3 x106 Pa) and a CTE of 2.75 x 10 6 °C 1 has a MOR/CTE of 1.09 x 1012
Pa.°C. In certain embodiments, the ceramic composition has a ratio of MOR/CTE of
greater than about 6.0 x 10 Pa.°C, with the proviso that the CTE is equal to or less
than about 3.5 x 10 6 °C for example, greater than about 9.0 x 101 1 Pa.°C, for
example, greater than about 1. 1 x 1012 Pa.°C, or greater than about 1.3 x 1012 Pa.°C,
or greater than about 1.5 x 10 2 Pa.°C, or greater than about 1.7 x 1012 Pa.°C, or
greater than about 1.9 x 1012 Pa.°C.
The ceramic compositions and ceramic honeycomb structures of any of the above
embodiments may be characterized in terms of a factor designated R1*, which is
[MOR/(CTE x Young's Modulus)]. This factor is commonly used by persons of skill in
the art. In certain embodiments, the ceramic composition or honeycomb structure has
a R1* of at least about 30°C, for example, at least about 45°C, or at least about 60°C,
or at least about 75°C, or at least about 90°C, or at least about 105°C, or at least about
130°C, or at least about 150°C, or at least about 175°C. Typically, the R 1* is less than
about 500°C, or less than about 350°C, or less than about 250°C, or less than about
230°C, or less than about 210°C.
The presence of the tialite phase improves the thermal shock resistance of the ceramic
structure. Further, the tialite phase is stabilized at temperatures above 1350°C (tialite
per se is known to be normally unstable at such temperatures). Further despite the
relatively low amount of tialite in the ceramic structures, the ceramic structures exhibit
thermomechanical properties suitable for use in diesel particulate filter applications.
This finding is contrary to conventional wisdom in the art.
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 Y 0 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 Y20 3,
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 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.
In accordance with the third aspect of the present invention, the ceramic precursor
composition comprises (weight %):
• 20-55 %, 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;
• 0-5 %, or 0.5-4 %, or 1-4 %, or 1-3 %, or 1-2 %, 2-3 % alkaline earth metal oxide
and/or alkaline earth metal oxide precursor; and
• 0-15 %, or 0.5-10 %, or 0.5-4 %, or 1-4 %, or 1-3 %, or 1-2 %, 2-3 zirconia.
In a further embodiment, the ceramic precursor composition comprises (weight %):
• 28-50 %, or 30-50 %, or 28-40 % aluminosilicate; and/or
• 20-30 %, or 22-27 % titania; and/or
• 38-45, or 40-45 % alumina; and/or
• 1-3 % alkaline earth metal oxide and/or alkaline earth metal oxide precursor;
and/or
• 1-4 % zirconia.
In embodiments, the relative amounts of aluminosilicate, titania, alumina and zirconia, if
present, 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. In particularly advantageous
embodiments, the relative amounts of tialite-forming and mullite forming components of
the ceramic precursor composition 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, or 1550°C and above, the resulting ceramic composition or ceramic
honeycomb structure comprises 40-55 % tialite and 40-55 % mullite, for example, 40-
50 % tialite and 45-55 % mullite, or 45-55 % tialite and 40-50 % mullite, or 40-45 %
tialite and 40-55 % mullite, or 40-45 % tialite and 40-50 % mullite, or 40-45 % tialite and
45-55 % mullite, or 40-45 % tialite and 45-50 % mullite (calculated on the basis of the
total weight of mineral phases in the ceramic composition or honeycomb structure).
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 pm and 55 pm, or between 0.1 pm and 80 pm, or between 10 and 55
pm, or between 10 m and 75 m, or between 15 m and 55 m, or between 15 p
and 75 pm, or between 20 pm and 55 pm, or between 20 pm and 75 pm. In a further
embodiment the aluminosilicate, for example andalusite, is in the form of particles
having a size in the range between 0.1 pm and 125 pm, or between 0.1 pm and 100
pm, or between 0.1 pm and 75 pm, or between 25 pm and 100 pm, or between 25 pm
and 75 pm.
The titania may selected from one or more of rutile, anatase, brookite.
The zirconia may be selected from one or more of Zr0 2 and TixZr1 0 2 (as described
above). The present inventors have surprisingly found that the inclusion of zirconia in
the ceramic precursor composition can have an advantageous effect on process
characteristics and the thermomechanical properties of the sintered ceramic
composition. For example, the presence of a relatively small amount of Zr0 2, e.g., less
than about 2 wt. %, can accelerate the mullitization process (primary and secondary)
during sintering. Thus, with the inclusion of Zr0 2, the alumina content of the final
sintered ceramic can be reduced, which has a beneficial effect in reducing the CTE of
the sintered ceramic. In other words, it appears the presence of Zr0 2 enhances the
reactivity of alumina. 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 100 pm, or in the range between 0.1 to 75 pm, or in the range
between 0.1 to 50 pm, or in the range between 0.1 to 25 pm, or in the range between
0.1 to 10 pm, or in the range between 0.1 to 1 pm, or between 0.3 to 0.6 pm. 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 of 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 Al 0 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 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 of the third aspect of the present invention. 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 1550°C and
1600°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 of the third aspect of the present invention. In certain embodiments,, the
alumina 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 75 pm, or in the range between 0.1 to 50 pm, or
in the range between 0.1 to 25 , or in the range between 0.1 to 10 pm, or between
0.1 to 1 pm, or between 0.3 to 0.6 pm. 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 pm, or in
the range between 0.1 to 50 , or in the range between 0.1 to 10 m, or between 0.1
to 1 pm, or between 0.3 to 50 , or between 0.3 to 1 pm, or between 0.3 to 0.6 pm. In
a further embodiment, the titania is present in the form of particles having a size in the
range between 0.1 to 10 pm, or between 0.2 to 1 pm, or between 0.2 to 0.5 pm. 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 pm, for example a d 0 smaller
than 0.5 pm. 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. The present
inventors having surprisingly found that the inclusion of alkaline earth metal oxide or
oxide precursor, in particular magnesium carbonate, can have an advantageous effect
on process characteristics. Further the formation or presence of alkaline earth metal
oxide, for example, magnesium oxide, in the ceramic composition can have an
advantageous effect on the thermomechanical properties of the sintered ceramic
composition. For example, the presence of magnesium carbonate can promote
densification at lower temperatures compared to a ceramic precursor composition
which does not contain magnesium carbonate. Further, the presence or formation of
magnesium oxide during sintering can lead to a decrease in the CTE of the sintered
ceramic composition.
In accordance with the fourth aspect of the present invention, the ceramic precursor
composition comprises (weight %):
• 20-55 %, or 30-50, or 20-40 %, or 25-40 %, or 25-35 %, or 30-40 %, or 40-50 %,
or 35-45 % aluminosilicate;
• 45-75 %, or 50-75 %, or 50-70 %, or 50-60 %, or 5-55 %, or 55-75 %, or 60-75%,
60-70%, or 60-65 %, or 62-67 %, or 65-75 %, or 65-70 % of a tialite- and
mullite-containing chamotte;
• up to 20 %, or 5-20 % alumina, or 5-15 %, or 10-20 % or 5-10 %, or 6-12 %
alumina;
• 0-5 %, or 0.5-4 %, or 1-4 %, or 1-3 %, or 1-2 %, 2-3 % alkaline earth metal oxide
and/or alkaline earth metal oxide precursor; and
• 0-15 %, or 0.5-10 %, or 0.5-4 %, or 1-4 %, or 1-3 %, or 1-2 %, 2-3 zirconia.
In embodiments, the relative amounts of aluminosilicate and tialite/mullite-containing
chamotte 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. In particularly advantageous
embodiments, the relative amounts of aluminosilicate and tialite/mullite containing
chamotte are selected such that upon sintering the ceramic precursor composition at a
suitable temperature, e.g., above about 1500°C, or 1550°C and above, the resulting
ceramic composition or ceramic honeycomb structure comprises 40-55 % tialite and
40-55 % mullite, for example, 40-50 % tialite and 45-55 % mullite, or 45-55 % tialite
and 40-50 % mullite, or 40-45 % tialite and 40-55 % mullite, or 40-45 % tialite and 40-
50 % mullite, or 40-45 % tialite and 45-55 % mullite, or 40-45 % tialite and 45-50 %
mullite.
In certain embodiments, the ratio mullite to tialite is lower than 2:1 , for example, lower
than 1.8:1 , or lower than 1.6:1 , or equal to or lower than about 1.5:1 , or equal to or
lower than about 1.4:1 , or equal to or lower than about 1.3:1 , or equal to or lower than
about 1.2:1 , or equal to or lower than about 1. 1 : 1 , or equal to or lower than about 1:1 ,
or equal to or lower than about 1: 1 .2.
In certain embodiments, 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 total weight of the mineral phases.
In certain embodiments, at least about 80 % by weight of the tialite/mullite containing
chamotte is tialite and mullite mineral phases (calculated on the basis of the total
weight of the mineral phases in the tialite/mullite containing chamotte), for example, at
least about 85 % by weight, or at least about 90 % by weight, or at least about 92 % by
weight, or at least about 94 % by weight, or at least about 95 % by weight, or at least
about 96 % by weight, or at least about 97 % by weight, or at least about 98 % by
weight of the tialite/mullite containing chamotte is tialite and mullite mineral phases. In
these embodiments, the ratio of tialite to mullite may range from about 1: 1 to about 3:1 .
The mineral phase composition of the tialite/mullite containing chamotte may be
determined in accordance with the qualitative XRD method described above.
Typically, the alumina content of the tialite/mullite containing chamotte is less than
about 5 % by weight, for example, equal to or less than about 4 % by weight, equal to
or less than about 3 % by weight, equal to or less than about 2 % by weight, equal to or
less than about 1 % by weight. In some embodiments, the tialite/mullite containing
chamotte is substantially free of alumina. In other embodiments, the tialite/mullite
containing chamotte comprise at least about 0.1 % by weight alumina, for example, at
least about 0 5 % by weight alumina. If the tialite/mullite containing chamotte contains
alumina this may be consumed during sintering of the ceramic precursor composition,
for example, during a secondary mullitization step (by reaction with any excess silica
from the aluminosilicate).
The tialite/mullite containing chamotte may have a CTE of less than about 3.0 x 10 6 oC
as measured at 800°C by dilatometry according to DIN 51045. In certain
embodiments, the CTE may be equal to or less than about 2.50 x 0 6 °C 1 , for
example, equal to or less than about 2.0 x 10 6 °C 1, or equal to or less than about 1.75
x 10 6 °C 1 , or equal to or less than about 1.5 x 10 6 X 1 .
The tialite/mullite containing chamotte may be prepared by sintering a mixture of raw
materials in suitable amounts and at a suitable temperature. The person skilled in the
art will able to determine suitable raw materials, amounts and sintering temperature
depending on the desired composition of the tialite/mullite containing chamotte.
Suitable raw materials include aluminosilicate (including the types described above),
alumina (including types described above) and titania (including the types described
above). The amount of aluminosilicate may be in the range of about 10-30 wt. %,
based on the total weight of the raw inorganic mineral materials, for example, in the
range of about 15-25 wt. %, or 18-22 wt. %. The amount of alumina may be in the
range of about 35-55 wt. %, based on the total weight of the raw inorganic mineral
materials, for example, in the range of about 35-55 wt. %, or 40-50 wt. %, or 40-45 wt.
%. The amount of titania (or titania precursor) may be in the range of about 25-40 wt.
%, for example, from about 25-35 wt. %, or from about 30-35 wt. %. The raw material
mixture may comprise minor amounts of further materials suitable for sintering, e.g.,
zirconia. The raw materials are typically mixed using conventional mixing means with a
suitable amount of water and optional binder and/or auxiliant components (as
described below). Following mixing the mixture is typically dried, optionally heated in a
debindering step, optionally ground and then sintered at a suitable temperature,
typically greater than about 1500°C, or greater than about 1550X, or up to about
1600°C. The sintered chamotte material may subjected to a milling step, e.g., jetmilling,
to modify the particle size distribution, e.g., to obtain a monomodal particle size
distribution. For example, the sintered tialite/mullite containing chamotte may be milled
to obtain a chamotte having a d ranging from about 2 to about 30 , for
example, from about 5 m to about 25 .
For use in the ceramic precursor composition of the fourth aspect of the present
invention, tialite/mullite chamotte having the same mineral phase composition but
different particle size distribution may be used in combination. For example, a
tialite/mullite chamotte (of fixed composition) having a relatively fine particle size
distribution may be used in combination with a tialite/mullite chamotte (of the same
fixed composition) having a relatively coarse particle size distribution. Further, various
combinations of tialite/mullite containing chamotte (of fixed composition) and
aluminosilicate, each having a different particle size distribution may be used in the
ceramic precursor composition of the fourth aspect of the present invention. This may
result in a ceramic precursor composition characterized in having a bimodal or
multimodal particle size distribution. Alternatively, the particle size distribution of the
tialite/mullite containing chamotte and aluminosilicate may be selected such that the
particle size distribution of the ceramic precursor composition is characterized in having
a monomodal particle size distribution.
The type, form (i.e., particle size) and amounts of aluminosilicate, alumina, titiania,
zirconia and alkaline earth metal carbonate that may be used in the ceramic precursor
composition of the fourth aspect of the present invention are the same as those
described above in connection with the ceramic precursor composition of the third
aspect of the present invention.
In a further embodiment, either of the ceramic precursor compositions of the present
invention comprises a graphite component. The 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 %, 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 0 to about 15 wt. %. The
graphite material can be used in a particulate form, wherein the particles have a size of
less than 200 pm, or less than 150 pm, or less than 100 pm. In another embodiment,
the graphite particles have a median particle diameter (d ) between 0 and 100 pm; or
between 5 pm to 50 pm, or between 7 pm and 30 pm, or between 20 pm and 30 pm.
The graphite may be included as a pore former, as described below.
In accordance with a fifth aspect, a ceramic precursor composition comprises
preformed mullite, for example, a mullite-containing chamotte, and tialite-forming
precursor components, i.e., titania and alumina, and optionally one or more of
aluminosilicate, zirconia 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 accordance with the fifth aspect of the present invention, the ceramic
precursor composition comprises (weight %):
• from about 30-60 %, or 35-60 %, or 40-60 %, or 35-55 %, or 40-55 %, or 45-55
%, or 47-52 % mullite, e.g., mullite-containing chamotte;
• from about 15-30 %, or 20-30 %, or 18-25 %, or 20-25 titania or titania
precursor;
• from about 15-35 %, or 20-35 %, or 20-30 %, or 22-30 %, 22-28 % alumina;
• from 0-5 %, or 0.2-4 %, or 0.5-3 %, or 0.5-2 %, or 0.1-1 %, or 0.2-1 %, or 0.5-1
%, alkaline earth metal oxide and/or alkaline earth metal oxide precursor; and
• from 0-15 %, or 0.2-10, or 0.2-4, or 0.5-3, or 0.5-2 % zirconia and/or zirconia
precursor.
In certain embodiments of the fifth aspect of the present invention, the ceramic
precursor composition comprises (weight %);
• from about 40 wt. % to about 60 wt. % mullite, .e.g., mullite-containing chamotte;
and/or
• from about 15 wt. % to about 25 wt. % titania and/or titania precursor; and/or
• from about 20 wt. % to about 35 wt. % alumina;
• 0 wt. % or from about 0.5 wt. % to about 2 wt. % alkaline earth metal oxide
and/or alkaline earth metal oxide precursor; and
• from about 0.5 to about 3 wt. % zirconia and/or zirconia 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 1500X, the titania and
alumina (and any additional alumina present in a mullite-containing chamotte) form a
tialite mineral phase which constitutes from about 25 wt % to about 60 wt %, for
example, from about 35 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 embodiments, the relative amounts of mullite, e.g., mullite-containing chamotte,
titania and alumina, and optional aluminosilicate, zirconia and alkaline earth metal
carbonate, are selected such that upon sintering the ceramic precursor composition at
a suitable temperature, e.g., above about 1400°C, or above about 1500X, a ceramic
composition or ceramic honeycomb structure according to the first aspect and second
aspects of the present invention is obtained. In particularly advantageous
embodiments, the relative amounts of mullite, e.g., mullite-containing chamotte, titania
and alumina 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, or
1550°C and above, the resulting ceramic composition or ceramic honeycomb structure
comprises 40-55 % tialite and 40-55 % mullite, for example, 40-50 % tialite and 45-55
% mullite, or 45-55 % tialite and 40-50 % mullite, or 40-45 % tialite and 40-55 %
mullite, or 40-45 % tialite and 40-50 % mullite, or 40-45 % tialite and 45-55 % mullite,
or 40-45 % tialite and 45-50 % mullite.
In certain embodiments of the fifth aspect of the present invention, the titania is
selected from one or more of rutile, anatase, brookite and a titania precursor
compound. Additionally, the alumina may be selected from one or more of fused
alumina (e.g., corundum), sintered alumina, calcined alumina, reactive or semi-reactive
alumina and bauxite. Additionally, when present, the alkaline earth metal oxide
precursor may be magnesium carbonate.
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 of the third, fourth and fifth aspects of the
present invention 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 third, fourth and fifth aspects 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. Alternatively, the
drying step can be performed by exposing the green honeycomb structure to an
atmosphere with controlled humidity at predefined temperatures in the range between
20 °C and 90 °C over an extended period of time in a climate chamber, where the
humidity of the surrounding air is reduced in a step-by-step manner, while the
temperature is correspondingly increased. For example, one drying program for the
green honeycomb structures of the present invention is as follows:
• maintaining a relative air humidity of higher than about 70 % at room
temperature for two days;
• maintaining a relative air humidity of higher than about 60 % at 50 °C for three
hours;
• maintaining a relative air humidity of higher than about 50 % at 75 °C for three
hours; and
• maintaining a relative air humidity of lower than about 50 % at 85 °C for twelve
hours.
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.
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 1700X, or between about 1250°C and 1650°C, or between about 1350°C
and 1650°C, or between 1 00 °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. In certain embodiment, 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, e.g., a ceramic precursor composition according to
the fourth aspect of the present invention, 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 total weight 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 0 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 pm 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 cells/cm 2) , 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/cm 2) .
The thickness of the partition wail 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 00 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 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.
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).
For the avoidance of doubt, the present application is directed to the subject-matter
described in the following numbered paragraphs:
1. A ceramic composition comprising:
from about 25 wt. % to about 60 wt. % tialite;
from about 35 wt. % to about 75 vol. % mullite;
from about 0 wt. % to about 8 wt. % zirconia;
from about 0 wt. % to about 0 wt. % zirconium titanate;
from about 0 wt. % to 10 wt. % of an amorphous phase;
from about 0 wt. % to about 5 wt. % of an alkaline earth metal oxide; and
from about 0 wt. % to 0 wt. % alumina (calculated on the basis of the total
weight of mineral phases);
wherein said ceramic composition has a porosity of from about 30 % to about 70 %
(calculated on the basis of the total volume of the mineral phases and pore space).
2 . A ceramic composition according to paragraph , comprising at least about 40
wt.% tialite.
3 . A ceramic composition according to paragraph 2 , comprising from about 45 wt. %
to about 55 wt % tialite, for example, from about 45 wt. % to about 50 wt. % tialite or,
for example, from about 50 wt. % to about 55 wt. % tialite.
4 . A ceramic composition according to any preceding numbered paragraph,
comprising from about 40 wt.% to about 60 wt % mullite.
5 . A ceramic composition according to paragraph 4 , comprising from about 45 wt. %
to about 55 wt. % mullite, for example, from about 45 wt. % to about 50 wt. % mullite
or, for example, from about 50 wt. % to about 55 wt. % mullite.
6 . A ceramic composition according to any preceding numbered paragraph,
comprising equal to or less than about 5.0 wt % of said amorphous phase, for example,
equal to or less than about 3.0 wt. %, or equal to or less than about 2.0 wt. %, or equal
to or less than about 1.0 wt. % of said amorphous phase.
7 . A ceramic composition according to any preceding numbered paragraph,
comprising equal to or less than about 7 wt. % alumina, for example, equal to or less
than about 5 wt. % alumina, or equal to or less than about 4 wt. % alumina.
8 . A ceramic composition according to any preceding numbered paragraph,
comprising no greater than about 5 wt. % zirconia, for example, no greater than about
3 wt. % zirconia or, no greater than about 2 wt. % zirconia or, no greater than about 1
wt. % zirconia.
9 . A ceramic composition according to any preceding numbered paragraph, wherein
said composition has a porosity of from about 35 % to about 65 %.
10. A ceramic composition according to pararagph 10, wherein said composition has
a porosity of from about 35 % to about 45 %, for example, a porosity of from about 35
% to about 40 % or, from about 40 % to about 45 %.
11. A ceramic composition according to any preceding numbered paragraph, wherein
said composition has a modulus of rupture (MOR) of at least about 1.5 MPa and/or a
coefficient of thermal expansion (CTE) of equal to or less than about 3.5 x 10 6 C 1 .
12. A ceramic composition according to any preceding numbered paragraph, wherein
said composition has a MOR of at least about 3.0 MPa and/or a CTE of equal to or less
than about 3.0 x 10 °0 .
13. A ceramic composition according to any preceding numbered paragraph, wherein
said composition has a MOR of at least about 3.5 MPa and/or a CTE of equal to or less
than about 2.5 x 10 1.
14. A ceramic composition according to any preceding numbered paragraph, wherein
said composition has a MOR of at least about 4.0 MPa and/or a CTE of equal to or less
than about 2.0 x 10 oC 1 .
15. A ceramic composition according to any one of numbered paragraphs 11-14,
wherein the composition has a ratio of MOR/CTE of greater than about 9 x 101 1 Pa.°C,
for example, greater than about 1.3 x 1012 Pa.°C, or greater than about 1.5 x 10 2
Pa.°C.
16. A ceramic composition according to any preceding numbered paragraph in the
form of a honeycomb structure.
17. A ceramic precursor composition suitable for sintering to form a ceramic
composition according to numbered paragraph 1, said precursor composition
comprising:
from about 20 wt. % to about 55 wt. % aluminosilicate;
from about 15 wt. % to about 35 wt. % titania and/or titania precursor;
from about 25 wt. % to about 45 wt. % alumina (Al20 3) ;
from 0 wt. % to about 5 wt. % alkaline earth metal oxide and/or alkaline earth
metal oxide precursor; and
from 0 wt. % to about 15 wt. % zirconia and/or zirconia precursor.
18. A ceramic precursor composition according to numbered paragraph 17,
comprising:
from about 28 wt. % to about 50 wt. % aluminosilicate; and/or
from about 20 wt. % to about 30 wt. % titania and/or titania precursor; and/or
from about 38 wt. % to about 45 wt. % alumina; and/or
from about 1 wt. % to about 3 wt. % alkaline earth metal oxide and/or alkaline
earth metal oxide precursor; and/or
from about 1 wt. % to about 4 wt. % zirconia or zirconia precursor.
19. A ceramic precursor composition according to numbered paragraph 17 or 18,
wherein said aluminosilicate is 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.
20. A ceramic precursor composition according to any one of numbered paragraphs
17-19, wherein said aluminosilicate is andalusite.
2 . A ceramic precursor composition according to any one of numbered paragraphs
17-20, wherein said titania is selected from one or more of rutile, anatase, brookite and
a titania precursor compound.
22. A ceramic precursor composition according to any one of numbered paragraphs
17-21 , wherein said alumina is selected from one or more of fused alumina (e.g.,
corundum), sintered alumina, calcined alumina, reactive or semi-reactive alumina and
bauxite.
23. A ceramic precursor composition according to any one of numbered paragraphs
17-21 , comprising from about 1 wt. % to about 4 wt. % zirconia (Zr0 2) .
24. A ceramic precursor composition according to any one of numbered paragraphs
17-22, wherein said alkaline earth metal oxide precursor is an alkaline earth metal
carbonate, for example, magnesium carbonate.
25. A ceramic precursor composition suitable for firing to form a ceramic composition
according to paragraph 1, said precursor composition comprising:
from about 20 wt. % to about 55 wt. % aluminosilicate;
from about 45 wt. % to about 75 wt. % of a tialite- and mullite-containing
chamotte;
from 0 wt. % to about 20 wt. % alumina;
from 0 wt. % to about 5 wt. % alkaline earth metal oxide and/or alkaline earth
metal oxide precursor; and
from 0 wt. % to about 15 wt. % zirconia and/or zirconia precursor.
26. A ceramic precursor composition according to paragraph 24, comprising:
from about 30 wt. % to about 50 wt. % aluminosilicate; and/or
from about 50 wt. % to about 70 wt. % tialite- and mullite-containing chamotte;
and/or
from about 5 wt. % to about 20 wt. % alumina
0 wt. % or from about 1 wt. % to about 3 wt. % alkaline earth metal oxide and/or
alkaline earth metal oxide precursor; and/or
from about 1 wt. % to about 4 wt. % zirconia and/or zirconia precursor.
27. A ceramic precursor composition according to numbered paragraph 25 or 26,
wherein said aluminosilicate is 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.
28. A ceramic precursor composition according to any one of numbered paragraphs
25-27, wherein said aluminosilicate is andalusite.
29. A ceramic precursor composition according to any one of numbered paragraph
25-28, comprising from about 1 to about 4 wt. % zirconia (Zr0 2) .
30. A ceramic precursor composition suitable for firing to form a ceramic composition
according to numbered paragraph 1, said precursor composition comprising:
from about 30 wt. % to about 60 wt. % mullite;
from about 15 wt. % to about 35 wt. % titania or titania precursor;
from 15 wt. % to about 35 wt. % alumina;
from 0 wt. % to about 5 wt. % alkaline earth metal oxide and/or alkaline earth
metal precursor; and
from 0 wt. % to about 15 wt. % zirconia and/or zirconia precursor.
3 1. A ceramic precursor composition according to numbered paragraph 30,
comprising:
from about 40 wt. % to about 60 wt. % mullite; and/or
from about 15 wt. % to about 25 wt. % titania and/or titania precursor; and/or
from about 20 wt. % to about 35 wt. % alumina;
0 wt. % or from about 0.5 wt. % to about 2 wt. % alkaline earth metal oxide
and/or alkaline earth metal oxide precursor; and
from about 0.5 to about 3 wt. % zirconia and/or zirconia precursor.
32. A ceramic precursor composition according to any one of numbered paragraphs
30-31 , wherein said titania is selected from one or more of rutile, anatase, brookite and
a titania precursor compound.
33. A ceramic precursor composition according to any one of numbered paragraphs
30-32, wherein said alumina is selected from one or more of fused alumina (e.g.,
corundum), sintered alumina, calcined alumina, reactive or semi-reactive alumina and
bauxite.
34. A ceramic precursor composition according to any one of numbered paragraphs
30-33, wherein said alkaline earth metal oxide precursor is an alkaline earth metal
carbonate, for example, magnesium carbonate.
35. A ceramic precursor composition according to any one of numbered paragraphs
30-34, wherein said mullite is a mullite-containing chamotte, for example, a chamotte
consisting essentially of about 100 wt. % mullite.
36. A ceramic precursor composition according to any one of numbered paragraphs
17-34, further comprising (i) one or more binding agents, (ii) one or more mineral
binders, and/or (iii) one or more auxiliants.
37. A method for making a ceramic honeycomb structure according to numbered
paragraph 16, said method comprising the steps of:
(a) providing a dried green honeycomb structure formed from the ceramic
precursor composition according to any one of numbered paragraph 17-34; and
(b) sintering.
38. A method according to numbered paragraph 31, comprising the steps of:
(a) providing a green honeycomb structure formed from the ceramic precursor
composition according to any one of numbered paragraphs 17-34;
(b) drying the green honeycomb structure; and
(c) sintering.
39. A method according to numbered paragraph 37 or 38, comprising the steps of:
(a) providing an extrudable mixture formed from the ceramic precursor
composition according to any one of numbered paragraphs 17-34;
(b) extruding the mixture to form a green honeycomb structure;
(c) drying the green honeycomb structure; and
(d) sintering.
40. A method according to any one of numbered paragraphs 37-39, wherein sintering
step is performed at a temperature between about 1200°C and 1700°C, for example,
between about 1350°C and 1650°C, or between about 1400°C and 1600°C, or
between about 1500°C and 1600°C.
4 1. A method according to numbered paragraph 40, wherein the sintering step is
performed at temperature between about 1520°C and 1600°C, or between about
1530X and 1600X, or between about 1540X and 1600X, or between about 1550X
and 1600X.
42. A method according to any one of numbered paragraphs 37 to 4 1, wherein the
green honeycomb structure comprises one or more organic binding agents or organic
auxiliants, the method further comprising a step of debindering.
43. A method according to any one of numbered paragraphs 37 to 42, further
comprising plugging the green honeycomb structure or sintered honeycomb structure.
44. A diesel particulate filter comprising or made from the ceramic honeycomb
structure according to numbered paragraph 16 or the ceramic honeycomb structure
obtainable by the method of any one of numbered paragraphs 37-43.
45. A vehicle having a diesel engine and a filtration system comprising the diesel
particulate filter according to numbered paragraph 44.
46. A ceramic composition according to any one of numbered paragraphs 1 to 16,
comprising no greater than about 10 wt. % zirconium titanate, for example, no greater
than about 5 wt. % zirconium titanate, or no greater than about 2 wt. % zirconium
titanate, or no greater than about 1 wt. % zirconium titanate.
EXAMPLES
The invention will now be described, by way of example only and without limitation,
with reference to the following Figure and Examples, in which:
Figure 1 illustrates results obtained in connection with Example 6 .
Example 1
The components listed in Table 1 were mixed together.
Table 1.
The resulting product was dried in a drying-oven and then fired at a temperature of
1600°C for 4 hours. The resulting chamotte comprises 7 1 % tialite, 27 % mullite 3:2, 1
% corundum and 1 % of an amorphous phase. The chamotte had a CTE at 800°C of
0.8 x 10 6 °C 1 . At 1600°C the phases are completely formed.
Example 2
A series of ceramic compositions were obtained from a ceramic precursor composition
comprising varying amounts of the TM chamotte prepared in Example 1.
Compositional analysis and thermomechanical properties are summarised in Tables 2
and 3 . Samples R 1 and R2-1 were fired at a maximum temperature of 1520 °C (2 hour
soaking time). Samples R2-2 and R3 were fired a maximum temperature of 1530 °C (2
hour soaking time). R3 has a very high tialite content (79 wt. %) and is included for
comparative purposes.
Table 2 .
As can be seen, CTE decreases with increasing tialite content. Further, whilst MOR is
seen to decrease with increasing tialite content, it remains within acceptable ranges
suitable for diesel particulate filter applications.
Table 3 .
Example 3
A further series of samples were prepared. Ceramic precursor compositions are
described in Table 4 . The TM chamotte in each precursor composition was the TM
chamotte prepared in Example 1 which had been jet milled separately to obtain a d 0 of
about 15 m. Each sample was fired at 1550°C for 1 hour. Compositional analysis
and thermomechanical properties of the sintered material is summarized in Tables 5
and 6 .
Table 5 .
As can be seen, for a fixed sintering temperature (1550°C), as the Zr0 2 content
increases, the CTE and the alumina content decrease. Further, the presence of Zr0 2
in the ceramic precursor composition accelerates mullitization (primary and secondary).
On the other hand, shrinkage increases with increasing Zr0 2 content.
With regard to MgO content, it is found that as the amount of MgO is increased, CTE
increases. Differently to Zr0 2, the presence of the Mg precursor does not affect the
unreacted alumina content.
Table 4 .
Recipe R4 R5 R6 R7 R8 R9
Raw materials (weight %) (weight %) (weight %) (weight %) (weight %) (weight %)
Aluminosilicate precursor 24.1% 25.0% 24.9% 24.3% 23.8% 23.8%
Tialite - mullite chamotte 65.2% 67.1% 66.8% 66.0% 64.6% 64.6%
Alumina 7.8% 7.8% 7.8% 7.9% 7.8% 7.8%
Mg Precursor 0.0% 0.0% 0.6% 0.0% 0.0% 3.8%
Zr precursor 2.9% 0.0% 0.0% 1.7% 3.8% 0.0%
Total solid content 100.0% 100.0% 100.0% 100.0% 100.0% 100.0%
Pore former 8.7% 8.7% 8.7% 8.7% 8.7% 8.7%
Water and binders 36.0% 36.0% 36.0% 36.0% 36.0% 36.0%
Total 144.7% 144.7% 144.7% 144.7% 144.7% 144.7%
Example 4
A further series of samples were prepared. Ceramic precursor compositions are
described in Table 7 . Each sample was fired at 1550°C for 1 hour. Compositional
analysis and thermomechanical properties of the sintered material is summarized in
Table 8 .
Table 7 .
Example 5
A further series of samples were prepared. Ceramic precursor compositions are
described in Table 9. Each sample was fired at 1550°C for 1 hour. Compositional
analysis and thermomechanical properties of the sintered material is summarized in
Table 10.
Table 9 .
Example 6
A further series of samples were prepared. Ceramic precursor compositions are
described in Table 11. Each sample was fired at 1500°C for 1 hour and at 1600°C for
1 hour. Thermomechanical properties of the sintered materials are summarized in
Figure 1. Figure 1 also provides details of the particle size distribution of the mullite
used in the ceramic precursor composition in accordance with Table 1 .
Table 11.
In one aspect, the invention can provide physical properties (in concrete CTE and
Young Modulus) that would correspond to a recipe with a higher content of tialite, with
a recipe that presents a mullite-tialite ratio with low tialite content. Without wishing to
be bound by theory, this is thought to be related to the distribution of mullite and tialite
grains in the microstructure depending on the firing conditions.
In one aspect, the tialite can be distributed covering the mullite in the microstructure.
The macroscopic effect is the diminution of the CTE (and Young Modulus) of the
composition in a higher degree than in a homogenous mullite-tialite distribution. In this
sense, the grain size distribution of the raw materials and its repartition in the
microstructure can impact the physical properties in a fixed firing cycle.
In another aspect, the maximum temperature gradient supported by the composite
recipe may be correlated to the grain size distribution of the mullite and tialite coverage
distribution in the microstructure in the optimal firing conditions (optimal tialite
microcracking conditions).
IMS
A ceramic composition comprising:
from about 25 wt. % to about 60 wt. % tialite;
from about 35 wt. % to about 75 vol. % mullite;
from about 0 wt. % to about 8 wt. % zirconia;
from about 0 wt. % to about 0 wt. % zirconium titanate;
from about 0 wt. % to 10 wt. % of an amorphous phase;
from about 0 wt. % to about 5 wt. % of an alkaline earth metal oxide; and
from about 0 wt. % to 10 wt. % alumina (calculated on the basis of the total
weight of mineral phases);
wherein said ceramic composition has a porosity of from about 30 % to about 70
% (calculated on the basis of the total volume of the mineral phases and pore
space).
A ceramic composition according to claim 1, comprising at least about 40 wt.%
tialite.
A ceramic composition according to claim 2 , comprising from about 45 wt. % to
about 55 wt % tialite, for example, from about 45 wt. % to about 50 wt. % tialite
or, for example, from about 50 wt. % to about 55 wt. % tialite.
A ceramic composition according to any preceding claim, comprising from about
40 wt.% to about 60 wt % mullite.
A ceramic composition according to any preceding claim, comprising no greater
than about 5 wt. % zirconia, for example, no greater than about 3 wt. % zirconia
or, no greater than about 2 wt. % zirconia or, no greater than about 1 wt. %
zirconia.
A ceramic composition according to any preceding claim, wherein said
composition has a porosity of from about 35 % to about 65 %.
A ceramic composition according to any preceding claim, wherein said
composition has a modulus of rupture (MOR) of at least about 1.5 MPa and/or a
coefficient of thermal expansion (CTE) of equal to or less than about 3.5 x 10
A ceramic composition according to any preceding in the form of a honeycomb
structure.
A ceramic precursor composition suitable for sintering to form a ceramic
composition according to claim 1, said precursor composition comprising:
from about 20 wt. % to about 55 wt. % aluminosilicate;
from about 15 wt. % to about 35 wt. % titania and/or titania precursor;
from about 25 wt. % to about 45 wt. % alumina (Al 0 3) ;
from 0 wt. % to about 5 wt. % alkaline earth metal oxide and/or alkaline
earth metal oxide precursor; and
from 0 wt. % to about 15 wt. % zirconia and/or zirconia precursor.
10. A ceramic precursor composition suitable for firing to form a ceramic composition
according to claim 1, said precursor composition comprising:
from about 20 wt. % to about 55 wt. % aluminosilicate;
from about 45 wt. % to about 75 wt. % of a tialite- and mullite-containing
chamotte;
from 0 wt. % to about 20 wt. % alumina;
from 0 wt. % to about 5 wt. % alkaline earth metal oxide and/or alkaline
earth metal oxide precursor; and
from 0 wt. % to about 15 wt. % zirconia and/or zirconia precursor.
11. A ceramic precursor composition suitable for firing to form a ceramic composition
according to claim 1, said precursor composition comprising:
from about 30 wt. % to about 60 wt. % mullite;
from about 15 wt. % to about 35 wt. % titania or titania precursor;
from 15 wt. % to about 35 wt. % alumina;
from 0 wt. % to about 5 wt. % alkaline earth metal oxide and/or alkaline
earth metal precursor; and
from 0 wt. % to about 15 wt. % zirconia and/or zirconia precursor.
12. A method for making a ceramic honeycomb structure according to claim 16, said
method comprising the steps of:
(a) providing a dried green honeycomb structure formed from the ceramic
precursor composition according to any one of claims 9-1 1; and
(b) sintering.
13 . A method according to claim 12, wherein sintering is performed at a temperature
between about 1200°C and 1700°C, for example, between about 1350°C and
1650°C, or between about 1400X and 1600°C, or between about 1500°C and
1600°C.