CERAMIC HONEYCOMB STRUCTURES
FIELD OF THE SNVENTSON
The present invention relates to ceramic honeycomb structures
comprising an alternating pattern of inlet cells and outlet cells of defined shape,
to a process for the preparation of these structures, and to their use in exhaust
gas particulate filters, such as diesel filters.

BACKGROUND OF THE INVENTION
Ceramic honeycomb structures are commonly used in the art in the
manufacture of filters for liquid and gaseous media, and in particular in the
manufacture of filters for the removal of fine particles from exhaust gases; the
filters are positioned in the exhaust lines of vehicle diesel engines, in order to
remove the soot component of the exhausts. These filters can be monoliths or
segmented ceramics honeycombs, which comprise cells or channels of
dimension commonly ranging from 500 to 2000 microns, with controlled wall
porosity. The cells are alternatively plugged on the inlet and outlet side, so that
the exhaust gas is forced through the porous ceramic wall between the
channels and filtration occurs when the gas crosses the wall.
Suitable honeycomb structures provide a balance of several desirable
properties, such as sufficient filtering efficiency, i.e., the exhaust gas passing
the filter should be substantially free of diesel particulates; limited pressure
drop, i.e. the filter must show a sufficient ability to let the exhaust gas stream
pass through its walls; and sufficient chemical resistance against the
compounds present in exhaust gas of diesel engines over a broad temperature
range.
A low thermal expansion coefficient and a high thermal shock resistance
are also desirable, as they can help a filter to survive the several regeneration
cycles that it normally undergoes during its lifetime, which involve rapid heating
to temperatures substantially higher than the normal operating temperature. In
fact, during filtering activities, the inlet channels of the honeycomb structures
are progressively filled with soot, thus reducing filtering activities of the
structures. Therefore the filter must be regenerated periodically; the cleaning of
the filter is performed by heating the filter to a temperature sufficient to ignite the
collected diesel particulates at high temperatures (normally higher than
1000°C), thus causing the combustion of the soot. If filters do not possess
sufficient thermal shock resistance, mechanical and/or thermal tensions may
cause cracks in the ceramic material, resulting in a decrease or loss of filtering
efficiency and consequently of the filter lifetime.
In order to increase the lifetime and their filtering efficiency of the
honeycomb filters, various attempts have been made in the art to develop
ceramic materials with improved properties, such as of silicon carbide (SiC),
mullite, tialite or sillimanite minerals.
Further efforts have been directed to develop asymmetric designs of the
cells, where the inlet cells are larger than the outlet cells; two main ways of
creating asymmetry have been investigated in the art. The first solution
comprises the use of curved walls of the channels, as described for instance in
Figure 6 of EP-A-1 676 622; in these designs the cells, which have normally
square or rectangular cross-sections, may be partially deformed to create the
asymmetry. As also shown in Figure 1, all sides of the inlet cells are bulged
outwardly (inlet channels are "inflated") while the corresponding sides of the
outlet cells are bulged inwardly to give a reduced cross-section area; the result
is an undulation of the walls and a bulged pattern having the inlet cells of
slightly greater area than the outlet cells. Nevertheless, this design requires the
use of complex and costly dies in the manufacture of the filter; moreover, the
many constraints accumulated in the structure may lead to problems with the
ceramics performance. A further drawback of this solution is that adjacent inlet
channels are very close to each other, thus decreasing filtration efficiency.
Therefore, these designs have proved shortcomings when used for honeycomb
filters, in particular for monolith filters.
A second way of creating asymmetry, known in the art, involves the use
inlet channels having a cross-section higher than the cross-section of outlet
channels, as shown in Figure 2 . For instance, WO 03/020407 describes a
honeycomb structure wherein the cell channels have non-equal, square crosssection.
This design has the disadvantage that the distance separating two
adjacent inlet squares becomes smaller, thus creating areas of brittleness for
the structure which may originate fractures. This drawback may be partly
compensated by creating chamfers on the square, therefore creating octagonal
cells; nevertheless, the surface of the chamfer leads to a decrease of filtering
efficiency, as a significant portion of the inlet cell walls is closer to an adjacent
inlet cell than to the nearest outlet cell, which necessitates a longer flow path
through the wall.
Therefore, there is a need in the art for a new ceramic honeycomb
structure having asymmetric design, able to provide honeycomb filters of
increased lifetime and filtering efficiency, at the same time avoiding the
problems of the asymmetric designs known in the art.

SUMMARY OF THE INVENTION
The Applicant has unexpectedly found that the above problems are
solved by ceramic honeycomb structures having a defined pattern of alternating
inlet and outlet cells, of defined cross-sectional shapes. The ceramic
honeycomb structures of the invention have an inlet face and an outlet face,
comprising a plurality of inlet cells and a plurality of outlet cells extending
through the structure from the inlet face to the outlet face, the inlet cells being
open at the inlet face and closed where adjoining the outlet face, and the outlet
cells being open at the outlet face and closed where adjoining the inlet face.
According to a first aspect, the present invention is directed to a ceramic
honeycomb structure wherein:
the inlet and outlet cells are rhombic in cross-section and are arranged in
an alternating pattern, and
the outlet cells have a cross-sectional area generally smaller than that of
inlet cells and have an acute interior angle.
The outlet cells may have a diamond cross-section, while the inlet cells may
have a diamond or square cross-section. In the ceramic honeycomb structure of
this embodiment, preferably no point of a given inlet cell is closer to an adjacent
inlet cell than to an adjacent outlet cell.
According to a second aspect, the present invention is directed to a
ceramic honeycomb structure wherein:
the inlet and outlet cells are arranged in a checkerboard arrangement,
the inlet cells are quadrangular in cross-section; and
adjacent inlet cells along a given diagonal of the checkerboard
arrangement are rotated relative to one another.
Adjacent inlet cells along a given diagonal of the checkerboard arrangement are
angularly offset relative to one another by an angle higher than 1 degree.
The outlet cells may have a cross-sectional area generally smaller than the inlet
cells. The inlet cells may have a diamond or square cross-section, and
preferably have an acute interior angle. In the ceramic honeycomb structure of
this embodiment, preferably no point of a given inlet cell is closer to an adjacent
inlet cell than to an adjacent outlet cell.
According to a third aspect, the present invention is directed to a ceramic
honeycomb structure wherein:
the inlet and outlet cells are quadrangular in cross-section and are
arranged in an alternating pattern, and
no point of a given inlet cell is closer to an adjacent inlet celi than to an
adjacent outlet cell.
In the ceramic honeycomb structures of the invention, the inlet and/or
outlet cells may have cross-sectional shapes, such as rhombic or quadrangular,
wherein one or more corners are chamfered or rounded.
The specific geometrical configuration of the cells in the honeycomb
structures of the invention leads to an improved outlet over inlet ratio, increased
cell density, higher filtration surface and improved filtration efficiency; moreover,
when no point of a given inlet cell is closer to an adjacent inlet cell than to an
adjacent outlet cell, the honeycomb structures show reduced structural failures
due to thermal shock.
The present invention also provides a process for preparing a ceramic
honeycomb structure comprising the steps of:
(a) providing a green honeycomb structure having an pattern of inlet cells
and outlet cells as described in any of the above aspects of the invention;
(b) optionally drying the green honeycomb structure, and
(c) sintering the green honeycomb structure.
In an embodiment of the method of the invention, step (a) comprises providing
an extrudable ceramic mixture and extruding the mixture to form the green
honeycomb structure.
The present invention also provides a diesel particulate filter comprising
one or more ceramic honeycomb structures as described in any of the above
aspects of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS
Figures 1 and 2 show enlarged schematic plan views of asymmetric
designs of iniet and outlet channels not in accordance with the present
invention.
Figures 3-6 are enlarged schematic plan views of the asymmetric
designs of inlet and outlet channels in the ceramic honeycomb structures of the
invention.

DETAILED DESCRIPTION OF THE INVENTION
According to a first aspect, the present invention is directed to a ceramic
honeycomb structure wherein both inlet and outlet cells are rhombic in crosssection
and are arranged in an alternating pattern, wherein the outlet cells have
a cross-sectional area generally smaller than that of inlet cells and have an
acute interior angle.
The outlet cells have preferably a diamond cross-section with an acute interior
angle (a); the inlet cells may have a diamond or square cross-section, and
preferably have an acute interior angle () . In the ceramic honeycomb structure
of this embodiment, preferably no point of a given inlet cell is closer to an
adjacent inlet cell than to an adjacent outlet cell.
According to a second aspect of the present invention, the outlet and
inlet cells are arranged in a checkerboard arrangement; the inlet cells are
quadrangular in cross-section and adjacent inlet cells along a given diagonal of
the checkerboard arrangement are rotated relative to one another by an angle
higher than 1 degree. Adjacent inlet cells along a given diagonal of the
checkerboard arrangement are angularly offset by an angle higher than 1
degree; the "angular offset" means the deviation from perpendicular of the
diagonals of the adjacent inlet cells. The inlet cells may have a square or
diamond cross-section, and preferably have an acute interior angle () . When
the inlet cells have a diamond cross-section, the "angular offset" means the
deviation from perpendicular of the two major diagonals of the adjacent inlet
cells. By major diagonal of a cell there is meant the longer of the two diagonals
of the cell. Preferably, the major diagonals of adjacent inlet cells along a given
diagonal of the checkerboard arrangement are angularly offset by 1 to 30
degrees, or by 3 to 20 degrees.
The outlet cells may have a cross-sectional area generally smaller than
the inlet cells. The outlet cells can be square, rectangular, octagonal, polygonal
or any other shape or combination of shapes that are suitable for arrangement
in a repeating pattern; the outlet cells are preferably quadrangular in crosssection.
The outlet cells may have an acute interior angle (a), preferably smaller
than () ; adjacent inlet cells along a given diagonal of the checkerboard
arrangement may be angularly offset by an angle equal to 90 - (a). In the
ceramic honeycomb structure of this embodiment, preferably no point of a given
inlet cell is closer to an adjacent inlet cell than to an adjacent outlet cell.
According to a third aspect of the present invention, the inlet and outlet
cells are quadrangular in cross-section and are arranged in an alternating
pattern, and no point of a given inlet cell is closer to an adjacent inlet cell than to
an adjacent outlet cell.
In the ceramic honeycomb structures of any of the aspects of the
invention as described above, the cross-sectional shapes of the inlet and/or
outlet cells may have one or more chamfered or rounded corners.
The cross-sectional shape of the cells for obtaining the configurations of
any of the aspects of the invention described above is not especially limited;
examples of cells cross-sectional shapes include rhombus and rectangle. By
"rhombus" is meant a quadrilateral with all the sides equal, like a square or a
diamond.
The inlet and outlet cells have preferably a diamond cross-section,
wherein outlet cells have an acute angle (a) and inlet cells have an acute angle
() , separated by straight walls. The diamonds are preferably organized so that
four (a) diamonds delimitate between themselves a () diamond, and vice
versa () is preferably higher than (a). The outlet cells may have an acute
interior angle (a) ranging from 50 to 85 degrees, preferably from 60 to 85
degrees.
In the ceramic honeycomb structures of any of the above aspects of the
invention, the surface of filtration per volume of filter, expressed in mm /mm3
may be comprised between 0.8 and 1.
The aperture ratio, defined as the surface of cross section of inlet
channels with respect to the total surface of cross section of the filter, may be
higher than 35%; the aperture ratio is preferably smaller than 45%. This ratio is
typically measured by dividing the surface of the inlet channels in one
elementary cell of the filter (that is reproduced as many times as necessary to
represent the global surface of the filter) by the surface of such elementary cell,
multiplied by 100.
The ceramic honeycomb structures of any of the above aspects of the
invention have a coefficient of thermal expansion that may be comprised
between 0 and 9 - 0 6 K , or between 4.5-1 0 6 and 7- 0 6 K~ , measured by
dilatometry according to DIN 51045.
Figures 3 and 4 show variations in the arrangement of inlet and outlet
cells according to the invention, although numerous other configurations may be
utilised. The description of the cells pattern is given as they would be viewed in
a plane extending normal to the longitudinal axis of the honeycomb structure.
The inlet cells are shaded to indicate that they are blocked at their outlet ends,
while the outlet passages are clear to indicate that they are open at their outlet
ends. The honeycomb structure may be a cylindrical body having a circular
outer bounding wall; the outer bounding wall may take any desired curvilinear or
geometric configuration, such as elliptical, oval, rectangular, triangular or the
like.
Figure 3 shows a schematic cross-sectional view of a portion of a
honeycomb structure in which inlet and outlet cells are rhombic in shape. Inlet
cells have an acute interior angle () of 84 degrees, while outlet cells have an
acute interior angle (a) of 69 degrees; therefore, () is higher than (a).
In the embodiment of Figure 4 , both inlet and outlet cells have rhombic cross
sections with rounded corners, and are arranged in a checkerboard pattern, as
viewed in cross section. Inlet cells have an acute interior angle () of 83
degrees, while outlet cells have an acute interior angle (a) of 70 degrees;
therefore, () is higher than (a). The inlet and outlet cells are arranged in
vertical and horizontal rows, with the inlet cells alternating with outlet cells in a
checkerboard pattern. Each interior wall portion of the honeycomb structure lies
between an inlet cell and an outlet cell at every point of its surface except where
it engages another wall, as it does at the corners of the cell; therefore, except
for the corner engagement, the inlet cells are spaced from one another by
intervening outlet cells and vice versa. The major diagonals of adjacent inlet
cells disposed along a given diagonal of the checkerboard arrangement are
angularly offset by an angle of 20 degrees, i.e. 90 - (a). As indicated above,
"angular offset" means the deviation from perpendicular of the diagonals of the
adjacent inlet cells.
The new cell configuration of the ceramic honeycomb structures of the
invention provide more degrees of freedom and better possibility to adapt the
filter to the requirements imposed by the filtering purpose, and in particular the
thickness of the cell walls, the acute interior angle (a) of the outlet cells, the
acute interior angle () of the inlet cells and the distance between adjacent inlet
cells. The new cell configuration also offers the advantages of an increased cell
density (measured as number of cells per square centimetre, or as number of
cells per square inch cpsi) for a given cell inlet cross-sectional area, and
increased aperture ratio. In particular, the smaller the (a) and () angles, the
larger the cell density for a given length of the side of the diamonds. The larger
the () to (a) ratio, the larger the outlet over inlet ratio of the honeycomb
structure.
Moreover, the solution of the invention avoids the use of chamfers, thus
avoiding any the loss of filtration area. The entire wall surface is available for
filtration, since no point of a given inlet channel is closer to another inlet channel
than the closest point of the adjacent outlet channel. The flow is channelled in a
trapezoidal shape from the inlet channels to the outlet channels.
Moreover, the ceramic honeycomb structures of the invention can have a very
homogeneous wall thickness. The parameters of the cell configuration can be
easily adjusted so that wall thickness is constant in the whole design. This
allows to obtain a structure without points of specific wall accumulation
(increased thickness), that could generate discontinuities in the exhaust flow
and consequent soot accumulation, as well as specific hot points during the
regeneration phase.
In the honeycomb structures of the invention, the inlet and outlet cells
side by side in a longitudinal direction may be separated by porous walls and
plugged in an alternating fashion as indicated above. The interior walls of the
honeycomb structure may be porous, so as to permit the passage of exhaust
gases through the walls from the inlet to the outlet cells. The porosity of the
walls is sized appropriately to filter out a substantial portion of the particulates
present in exhaust gases.
The ceramic honeycomb structure of the invention may have a total
porosity in the range between 20 and 80%, or between 35 and 70%, measured
by mercury porosimetry (the volume percentages are calculated on the basis of
the total volume of mineral phases and pore space). Porosity is determined by
mercury diffusion as measured using a Thermo Scientific Mercury Porosimeter -
Pascal 40, with a contact angle of 30 degrees. The pore diameter d50 ,
measured by mercury porosimetry, may be in the range of 1 to 60 microns, or 5
to 50 microns, or 8 to 30 microns. 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 10 and 20
microns, while for impregnated structures, the range is usually between 20 and
25 microns prior to impregnating. The catalyst material deposited in the pore
space will result in a reduction of the original pore diameter.
An average cell density of the honeycomb filter 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 1 cells/cm 2) , or between 50 and 1000
cells/square inch (7.8 to 155 cells/cm 2) , or between 100 and 400 cells/square
inch ( 15.5 to 62.0 cells/cm 2) . Cell density is defined as the ratio between the
surface of the inlet or outlet of the filter, once sintered, divided by the surface of
two inlet and two outlet channels and associated walls, this ratio being in turn
multiplied by 4. The associated walls are the walls adjacent to the cells, chosen
so that the elementary drawing made of the inlet and outlet cells and associated
walls can be reproduced as much as needed by translation to form the
checkerboard arrangement.
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.
The cells may have a surface roughness Ra comprised between 1 and
100 microns, or 10-50 microns, as measured in accordance with JIS B 0601
(1994).
In the present invention, the material constituting the honeycomb
structure is not limited; the honeycomb structure of the invention may be formed
of any suitable ceramic material. Suitable ceramic materials comprise silicon
carbide (SiC), silicon nitride, mullite, cordierite, zirconia, titania, silica,
magnesia, alumina, spinel, tialite, kyanite, sillimanite, andalusite, lithium
aluminum silicate, aluminum titanate and mixtures thereof. The ceramic material
may contain metals, such as Fe-Cr-AI-based metal, metal silicon and the like.
According to a preferred embodiment, the ceramic material comprises a
high amount of a mullite phase in combination with a minor amount of tialite
(i.e., the mullite phase is the dominant phase), as described in WO
2009/076985, the content of which is incorporated herein by reference; this
ceramic material provides increased mechanical strength and high thermal
shock resistance.
The ceramic honeycomb structures may comprise a mineral phase of
mullite and a mineral phase of tialite, wherein the volume ratio of mullite to tialite
is > 2:1 , or >4:1 , or > 10:1 . The tialite phase may be enclosed by the mullite
phase, and may be in the form of crystals substantially parallel. The amount of
mullite in the ceramic honeycomb structure may be greater than 50%, or greater
than 75%, even or greater than 80%, by volume (calculated on the basis of the
total volume of the mineral phases of the honeycomb).
The ceramic honeycomb structures may comprise a mineral phase
consisting of andalusite; the andalusite phase may be present in an amount
from 0.5% to less than 50%, or 5 % to 30 %, or 0.5% to 15% by volume (based
on the volume of the solid phases of the ceramic honeycomb structure). A
suitable andalusite-containing ceramic honeycomb structure comprises:
- 0.5 to 15.0 %, or 5.0 to 8.0 % of andalusite;
- 60.0 to 90.0 %, or 75.0 to 90.0 % of mullite;
- 2.5 to 20.0%, or 4.0 to 7.0 %, of tialite;
- 0 to 2.0 % of rutile and/or anatase; and
- 3.0 to 20.0 % of an amorphous silica phase;
wherein the total amount of the above components is 100 % by volume (based
on the volume of the solid compounds).
The material of the sealing portion formed by sealing the cells is not
limited, but the material preferably contains one or more ceramics and/or metals
selected from the ceramics and metals described above as preferable for the
partition wall of the honeycomb structure.
The method for producing the above ceramic honeycomb structures,
according to the invention, comprises the steps of:
(a) providing a green honeycomb structure having an alternating pattern
of inlet cells and outlet cells as described above;
(b) optionally drying the green honeycomb structure, and
(c) sintering the green honeycomb structure.
Step (a) may comprise providing an extrudable ceramic mixture and
extruding the mixture to form the green honeycomb structure.
The extrudable mixture or the green honeycomb structure may comprise
one or more binding agents; 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 step. Suitable binding agents may be
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, alginates and mixtures thereof. The binding
agents can be present in a total amount between 1.5 % and 15 % by weight, or
between 2 % and 9 % by weight (based on the dry weight of the extrudable
mixture or the green honeycomb structure).
The extrudable mixture or the green honeycomb structure may comprise
one or more mineral binders; suitable mineral binder may be selected from the
group including, but not limited to, bentonite, aluminum phosphate, boehmite,
sodium silicates, boron silicates and mixtures thereof.
The extrudable mixture or the green honeycomb structure may comprise
one or more auxiliants, which provide the raw material with advantageous
properties for the extrusion step (plasticizers, glidants, lubricants, and the like).
Suitable auxiliants may be 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, lauric acid and mixtures thereof. The
auxiliants can be present in a total amount between 1.5 % and 15 % by weight,
or between 2 % and 9 % by weight (based on the dry weight of the extrudable
mixture or the green honeycomb structure; if liquid auxiliants are used, the
weight is included into the dry weight of the extrudable mixture or the green
honeycomb structure). The "dry weight" of the extrudable mixture or of the
green honeycomb structure refers to the total weight of any compounds
discussed herein to be suitable to be used in the extrudable mixture, i.e., the
total weight of the mineral phases and of the binders/auxiliants. The "dry weight"
is thus understood to include such auxiliants that are liquid under ambient
conditions, but it does not include water in aqueous solutions of minerals,
binders or auxiliants if such are used to prepare the mixture.
The preparation of an extrudable mixture from the mineral compounds
(optionally in combination with binders and/or auxiliants) is performed according
to methods and techniques known in the art. The raw materials can be mixed in
a conventional kneading machine with the addition of a sufficient amount of a
suitable liquid phase as needed (normally water), to obtain a 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 on 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 and arrangement of the green honeycomb structures can
be determined by selecting extruder dies of desired size and shape. The
honeycomb structure can be made using extrusion dies having pins arranged in
a quadrangular symmetry. The corners of the pins may or may not be rounded.
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.
In optional step (b) of the method of the invention, 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 70 % at room temperature for 48 hours;
- maintaining a relative air humidity of 60 % at 50°C for 3 hours;
- maintaining a relative air humidity of 50 % at 75°C for 3 hours; and
- maintaining a relative air humidity of 50 % at 85°C for 12 hours.
The dried green honeycomb structure may be then heated in a
conventional oven or kiln for the 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 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).
The sintering step (c) may be carried out at a temperature between
1250°C and 1700°C, or between 350°C and 1600°C, or between 1400°C and
1580°C, or between 1400°C and 500°C. According to an embodiment, the
method comprises the step of heating the green honeycomb structure to a
temperature in the range of between 650°C and 950°C, or between 700°C and
900°C, or between 800°C and 850°C prior to the sintering step.
For the use as diesel particulate filters, the ceramic honeycomb
structures of the present invention, or the green ceramic honeycomb structures
can be further processed by plugging, i.e., by closing 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.
Another object of the present invention is a particulate filter comprising
one or more ceramic honeycomb structures as indicated above; the filter may
be for instance a diesel particulate filter or a filter for selective catalytic reduction
for the removal of NOx from exhaust gases.
The particulate filter may be formed by one ceramic honeycomb structure
of the invention, in the form of a monolith, or may be constituted of a plurality of
integrated structures. In the latter case, where the honeycomb filter is
segmented and then integrated, a size or shape of each structure is not limited;
the cross-sectional area of each structure may range between 900 and 10000
mm2, or between 900 and 5000 mm2, or between 900 and 3600 mm2. As a
preferable shape of the structure, for example, the cross-sectional shape is
quadrangular. The whole cross-sectional shape of the particulate filter is not
especially limited, and may be circular, elliptic, quadrangular and polygonal in
shape. To form the particulate filter into a constitution in which a plurality of
structures are integrated, after obtaining the structures as indicated above, the
structures can be bonded using, for example, ceramic cement, and dried/
hardened to obtain the filter.

EXAMPLES
The following examples, which are not intended to limit the scope of the
present invention, illustrate the advantages obtained with the cell geometry of
the honeycomb structures of the invention over that of the prior art.

Example 1
Honeycomb structures according to the invention, having a configuration
according to the parameters reported in Table 1, were evaluated; the meaning
of the parameters , , a, e , f is evident from Figure 5. The side of the inlet
diamond may be calculated as (a - 2f), while the side of the outlet diamond may
be calculated as (a - 2e).

Table 1
In the following Table 2, honeycomb configurations according to the present
invention were compared to the square design of the prior art, wherein (a) and
() are 90 degrees.

Table 2
In Table 2 , the honeycomb configurations of the three first lines were
made at constant cross section surface of the wall, while the honeycomb
configurations of the last three lines were made at constant wall thickness. As
evident from the above table, the honeycomb configurations according to the
present invention wherein (a) is 65 degrees and () is 90 or 80 degrees offer a
much higher filtration surface with respect to the square design of the prior art.
Moreover, as evident from Table 2, the honeycomb configurations of the
invention offer improved asymmetry ratio and higher cell density, thus providing
improved filtration surface.
The above examples demonstrate that the honeycomb configurations
according to the present invention are able to deliver suitable asymmetry ratios
with outlet hydraulic diameter around 1000 microns and filtration surface even
higher than 0.98. The above examples also show the degrees of freedom
offered by the honeycomb structure configuration of the present invention; in
particular, once the space devoted to the wall has been determined, inlet and
outlet areas may be easily adjusted by varying (a) and () angles. In
comparison with the chamfered designs of the prior art, the honeycomb
structure configuration of the present invention allow to obtain increased
filtration area and filtration efficiency, since there is no loss in the filtration
efficiency due to the chamfers.
Finally, honeycomb configurations according to the present invention,
wherein no point of a given inlet cell is closer to an adjacent inlet cell than to an
adjacent outlet cell are obtained when the following inequalities ( 1 ) and (2) are
fulfilled:
( 1) f > e(1 - cosa) / ( 1 + cos ) and
(2) f > e(1 + cosa) / ( 1 - )

Example 2
Honeycomb structures according to the invention, wherein inlet and
outlet cells had rounded angles, were evaluated; the honeycomb configurations
and the parameters , , a , e and f are reported in Table 3 and Figure 6. These
honeycomb structures offer manufacturing advantages, since electrodes having
variable diameters A and B may be used for the preparation of the asymmetric
cells; in the embodiments of the invention, the radius of both electrodes A and B
was 00 microns.
Table 3
The above configurations, wherein the overall space devoted to the walls
was maintained constant, gave larger inlet channel with respect to the
corresponding structures with sharp angles. The configurations wherein (a) was
65 degrees and () was 70, 75 or 85 degrees gave the best balance of high
filtration surface, high cell density and improved outlet over inlet ratio.
The foregoing description is directed to particular embodiments of the
present invention for the purpose of illustrating it. It will be apparent, however, to
one skilled in the art, that many modifications and variations to the
embodiments described herein are possible. Ail such modifications and
variations are intended to be within the scope of the present invention, as
defined in the appended claims.

CLAIMS

1. A ceramic honeycomb structure having an inlet face and an outlet
face, comprising a plurality of inlet cells and a plurality of outlet cells extending
through the structure from the inlet face to the outlet face, the inlet cells being
open at the inlet face and closed where adjoining the outlet face, and the outlet
cells being open at the outlet face and closed where adjoining the inlet face,
wherein:
the inlet and outlet cells are rhombic in cross-section and are arranged in
an alternating pattern,
the outlet cells have a cross-sectional area generally smaller than that of
inlet cells and have an acute interior angle.
2 . A ceramic honeycomb structure having an inlet face and an outlet
face, comprising a plurality of inlet cells and a plurality of outlet cells extending
through the structure from the inlet face to the outlet face, the inlet cells being
open at the inlet face and closed where adjoining the outlet face, and the outlet
cells being open at the outlet face and closed where adjoining the inlet face,
wherein:
the inlet and outlet cells are arranged in a checkerboard arrangement,
the inlet cells are quadrangular in cross-section; and
adjacent inlet cells along a given diagonal of the checkerboard
arrangement are rotated relative to one another.
3 . The ceramic honeycomb structure of claim 2, wherein adjacent inlet
cells along a given diagonal of the checkerboard arrangement are rotated
relative to one another by an angle higher than 1 degree.
4. The ceramic honeycomb structure according to any of claims 1 to 3,
wherein no point of a given inlet cell is closer to an adjacent inlet cell than to an
adjacent outlet cell.
5. A ceramic honeycomb structure having an inlet face and an outlet
face, comprising a plurality of inlet cells and a plurality of outlet cells extending
through the structure from the inlet face to the outlet face, the inlet cells being
open at the inlet face and closed where adjoining the outlet face, and the outlet
cells being open at the outlet face and closed where adjoining the inlet face,
wherein the inlet and outlet cells are quadrangular in cross-section and are
arranged in an alternating pattern and no point of a given inlet cell is closer to
an adjacent inlet cell than to an adjacent outlet cell.
6 . The ceramic honeycomb structure according to any of the preceding
claims, wherein the surface of filtration per volume of filter is comprised
between 0.8 and 1 mm2/mm3.
7. The ceramic honeycomb structure according to any of the preceding
claims, wherein the aperture ratio is higher than 35%.
8 . The ceramic honeycomb structure according to any of the preceding
claims, wherein the outlet cells have an acute interior angle (a) ranging from 50
to 85 degrees.
9 . The ceramic honeycomb structure according to any of the preceding
claims, wherein the inlet cells have an acute interior angle () and the outlet
cells has an acute interior angle (a), and () is higher than (a).
10. The ceramic honeycomb structure according to any of the preceding
claims, wherein the rhombic or quadrangular cells have one or more chamfered
or rounded corners.
. The ceramic honeycomb structure according to any of the preceding
claims, wherein adjacent cells are separated by partition walls having a
thickness ranging from 100 to 500 microns.
12. The ceramic honeycomb structure according to any of the preceding
claims, comprising one or more minerals selected from the group consisting of
silicon carbide (SiC), silicon nitride, mullite, cordierite, zirconia, titania, silica,
magnesia, alumina, spinel, tialite, kyanite, sillimanite, andalusite, lithium
aluminum silicate and aluminum titanate.
13. The ceramic honeycomb structure according to any of the preceding
claims, having a coefficient of thermal expansion comprised between 0 and
9-1 0 6 K .
14. The ceramic honeycomb structure according to any of the preceding
claims, wherein the cells have a surface roughness Ra comprised between 1
and 100 microns.
15. The ceramic honeycomb structure according to any of the preceding
claims, having a total porosity comprised between 20 and 80%.
16. The ceramic honeycomb structure according to any of the preceding
claims, having pore diameter d50 comprised between 1 to 60 microns.
17. A process for preparing a ceramic honeycomb structure according to
any of the preceding claims, comprising the steps of:
(a) providing a green honeycomb structure having a pattern of inlet cells
and outlet cells as defined in any of claims 1 to 11;
(b) optionally drying the green honeycomb structure, and
(c) sintering the green honeycomb structure.
8. The method according to claim 17, wherein step (a) comprises
providing an extrudable ceramic mixture and extruding the mixture to form the
green honeycomb structure.
19. A particulate filter comprising one or more ceramic honeycomb
structure according to any one of claims 1 to 16.