Honeycomb element with reinforced corners

Abstract

A monolithic honeycomb element includes a collection of adjacent ducts of mutually parallel axes separated by walls made of a porous material, the element having, in transverse cross section, a polygonal, notably quadratic cross section delimited by exterior wall elements, wherein at least one corner of the polygon, has, along the bisector of the angle at the corner, an additional thickness, such that the total thickness of the external wall, likewise measured along the bisector of the angle at the corner, is greater than the mean thickness of the exterior walls by a factor of at least 1.43, the additional thickness being obtained at least in part by an additional quantity of porous material on the external face of the corner.

Claims

1. A monolithic honeycomb element comprising a plurality of adjacent ducts of mutually parallel axes separated by internal walls made of a porous material, said element having, in transverse cross section, a shape of a polygon that is delimited by an exterior wall forming a periphery of the polygon, wherein a corner of said polygon has, along a bisector of an angle formed by said corner, an additional thickness, such that a total thickness of the exterior wall, measured along said bisector of the angle at said corner, is greater than a mean thickness of said exterior wall by a factor of at least 1.43, said additional thickness being obtained at least in part by an additional quantity of material provided on an external face of the exterior wall at said corner so that the external face of said at least one corner protrudes away from a central part of said element along said bisector.

2. The monolithic element as claimed in claim 1, wherein said exterior wall includes a plurality of exterior wall elements each forming a side of the polygon.

3. The monolithic element as claimed in claim 2, wherein the mean thickness of the exterior wall elements is between 100 and 1000 microns.

4. The monolithic element as claimed in claim 1, wherein said element has, in transverse cross section, a substantially quadratic or triangular shape, and of which the corners have vertex angles of between 60 and 120.

5. The monolithic element as claimed in claim 1, wherein said element has a transverse cross section of substantially square shape.

6. The monolithic element as claimed in claim 1, wherein said additional thickness has a substantially rounded external edge that can be inscribed inside a radius of curvature R of between 0.3 and 3 mm.

7. The monolithic element as claimed in claim 1, wherein, in transverse section, said additional thickness extends over a length l.sub.1 and has a maximum value e.sub.1 along a first exterior wall element that makes up the corner and over a length l.sub.2 and has a maximum value e.sub.2 along a second exterior wall element that makes up the corner, and in which:
0.5 l.sub.1<l.sub.2<2 l.sub.1
0.5 e.sub.1<e.sub.2<2 e.sub.1.

8. The monolithic element as claimed in claim 7, wherein the lengths l.sub.1 and l.sub.2 are substantially equal and/or wherein the additional thicknesses e.sub.1 and e.sub.2 are substantially equal.

9. The monolithic element as claimed in claim 1, wherein the factor is greater than or equal to 1.45.

10. The monolithic element as claimed in claim 9, wherein the factor is greater than 1.5.

11. The monolithic element as claimed in claim 10, wherein the factor is greater than 1.6.

12. The monolithic element as claimed in claim 1, wherein said ducts are plugged by plugs at one or other of their ends in order to delimit inlet ducts opening onto a gas intake face and outlet ducts opening onto a gas discharge face, so that the gas passes through the porous internal walls.

13. The monolithic element as claimed in claim 1, wherein said additional thickness is present over the entire length of the element.

14. The monolithic element as claimed in claim 1, wherein the porous material is silicon carbide or silicon nitride.

15. An assembled structure obtained by assembly of a plurality of monolithic elements as claimed in claim 1, said elements being bonded together using a cement jointing compound.

16. The assembled structure as claimed in claim 15, wherein a ratio of the additional thickness to a mean thickness of the cement jointing compound between two constituent elements, in said transverse plane, is less than or equal to 0.4 and/or wherein a ratio of the additional thickness to a mean thickness of the cement jointing compound, in said transverse plane, is less than or equal to 0.4.

17. The assembled structure as claimed in claim 15, wherein the assembled structure is a particulate filter.

18. The monolithic element as claimed in claim 1, wherein said cross-section is a quadratic cross-section.

19. The monolithic element as claimed in claim 1, wherein each corner of the polygon has, along the bisector of the angle at said corner, an additional thickness, such that a total thickness of the exterior wall, measured along said bisector of the angle at said corner, is greater than a mean thickness of said exterior wall by a factor of at least 1.43, said additional thickness being obtained at least in part by an additional quantity of material on the external face of said corner.

Description

(1) Advantageously, within the filter, said elements and the cement jointing compound essentially contain the same ceramic material, and are preferably based on silicon carbide (SiC).

(2) The invention will be better understood from reading the description of various embodiments of the invention which follow, these respectively being illustrated by FIGS. 1 and 2.

(3) FIG. 1 is a schematic and perspective cross section through a monolithic element according to the invention.

(4) FIG. 2 is a more detailed schematic view, deliberately exaggerated, in this same transverse cross section, of the corner part of the monolithic element according to the invention.

(5) According to well-known techniques, all the monolithic elements are advantageously obtained by extruding a loose paste, for example of silicon carbide, which after firing forms a porous honeycomb structure.

(6) The shape of the extruder head is configured according to conventional methods, for example such as described in patent U.S. Pat. No. 5,761,787, to obtain and form honeycomb elements that have additional thicknesses at the corners according to the invention, as schematically depicted in the following FIGS. 1 and 2.

(7) Without this implying any restriction, the extruded structure is, according to FIG. 1, in the form of a monolithic individual element or block 1, the exterior shape of which is that of a rectangular parallelepiped extending along a longitudinal axis between its upstream and downstream faces. The transverse cross section is substantially square. Opening onto the ends of the elements 1 are a plurality of adjacent channels 2, 3, the main axis of which is parallel to the longitudinal axis L of the block.

(8) In a way that is known but not depicted in the figures, the extruded porous structures may be alternately plugged on their upstream face or on their downstream face by respective upstream and downstream plugs, to form outlet channels 3 and inlet channels 2, respectively, for the formation of filtering structures. Each channel 2 or 3 therefore defines an interior volume delimited by internal walls 4, a plugging stopper (not depicted in the figures) arranged either on the upstream face in the case of an outlet channel or on the downstream face in the case of an inlet channel and an opening opening alternately toward the downstream face or the upstream face such that the inlet channels 2 and outlet channels 3 are in fluidic communication via the internal walls 4.

(9) In a way that is known and has not been depicted in the figures, several monolithic individual elements 1 are assembled with one another by bonding using a cement jointing compound of ceramic nature, for example likewise based on silicon carbide, into a filtration structure or assembled filter. The assembly thus formed has then to be machined in order, for example, to adopt a round or ovoid cross section, and then for example has to be covered with a cement sealant in order to make it gas tight and give it a smooth external surface.

(10) When the monolithic elements are plugged as previously described, this yields an assembled filter that can be inserted in an exhaust line, using well known techniques. In operation, the flow of exhaust gases enters the filter via the inlet channels 2 then passes through the filtering internal walls 4 of these channels to access the outlet channels 3. For further information regarding the structure of the monolithic elements and how they are assembled to form a filter, reference may, for example, be made to applications EP 1142619, WO 05/063462 or even WO 05/016491.

(11) In the embodiment depicted in FIG. 1, the transverse cross section of the inlet channels 2 differs from that of the outlet channels 3. Thus, the transverse cross sections of the inlet channels 2 are greater than those of the outlet channels 3, in order to increase the overall volume of the inlet channels at the expense of that of the outlet channels. In the embodiment illustrated in the figures, the walls 4 follow on from one another, in transverse section and in a horizontal (along the axis x) or vertical (along the axis y) row of channels to define a sinusoidal or wavy shape. The wall elements undulate for example substantially by a sinusoidal half period over the width of a channel.

(12) In one application of the structure as a filter, the particulate storage capability per individual element 1 is thus advantageously increased. However, it would not constitute a departure from the scope of the invention if the transverse cross sections of the inlet and outlet channels were identical and the walls 4 were planar.

(13) External wall elements 6, 7, 8, 9 of thickness E complete and surround the internal walls 4. These wall elements meet and intersect in pairs at a bisector, at corners that have vertex angles =90 in the case of the square-section structure depicted in FIG. 1.

(14) FIG. 2 schematically and in greater detail and in the same transverse section depicts the corner part of the monolithic element described in FIG. 1. More specifically, FIG. 2 illustrates in greater detail (and in exaggerated fashion in order to make it easier to study and to comprehend) the profile of the corners 11 that have the additional thicknesses 10. According to the invention, the filtering elements are therefore characterized, at the corner 11, by the presence of an additional thickness 10, in the form of an addition of material arranged at the external part 12 of the corner 11. This additional thickness is characterized by an additional layer of material by comparison with the conventional configuration described in the documents of the prior art and illustrated for example in application EP0816065.

(15) More specifically, according to the conventional configuration and as depicted by the dotted lines in FIG. 2, the two wall elements 6 and 7 meet to form the exterior corners of the individual element, at straight edges making an angle of 90, to form external edge corners along the entire length of the element.

(16) According to the invention and as depicted in FIG. 1, an additional quantity of material, of thickness e.sub.c measured along the bisector 13 of the angle formed by the wall elements 6 and 7, is arranged at said corner 11. According to the invention, this additional quantity of material is present over the exterior side (the edge corner) 12 of the corner of the element so that, along said bisector 13, the value e.sub.c contributes to the total thickness E.sub.c of the external wall, and in such a way that said total thickness E.sub.c of the wall, still along this same bisector, is greater than the mean thickness E of said walls 6 and 7 by a factor of at least 1.43, preferably of at least 1.45 or even of at least 1.5, or even highly preferably of at least 1.6.

(17) As depicted in FIG. 2, said additional thickness preferably has a rounded external edge, particularly one that can be inscribed inside a radius of curvature R of between 0.3 and 3 mm, the center of the circle inscribed along the rounded exterior edge being situated on said bisector 13.

(18) According to the invention, said additional thickness extends over a length l.sub.1 along the first wall element 6 of which the corner 11 is made (i.e. in the direction X) and over a length l.sub.2 along the second wall element 7 of which the corner 11 is made (i.e. in the direction Y). For preference, the ratio of the lengths l.sub.1 and l.sub.2 is between 0.5 and 2 and highly preferably is close to 1 or equal to 1.

(19) According to the invention, said additional thickness 10 has a maximum value e.sub.1 with respect to the first wall element 7 of which the corner is made and a maximum value e.sub.2 with respect to the second wall element 6 of which the corner is made. For preference, the ratio of the lengths e.sub.1 and e.sub.2 is between 0.5 and 2, and highly preferably is close to 1 or equal to 1.

(20) Typically, the elements channel density is between 1 and around 280 c/cm.sup.2, preferably between around 14 and around 62 c/cm.sup.2. According to the invention, the additional thickness at the corners preferably extends along the entire length L of the element, from the upstream face to the downstream face.

(21) The invention and the advantages thereof will be better understood from studying the following example which is given purely by way of illustration.

EXAMPLE

(22) The elements according to the invention were synthesized in the conventional way:

(23) More specifically, a population of monolithic honeycomb elements made of silicon carbide were synthesized in accordance with the techniques of the prior art, for example described in patents EP 816 065, EP 1 142 619, EP 1 455 923 or even WO 2004/090294.

(24) To do this, in a way comparable with the method described in application EP 1 142 619, 70 wt % of a SiC powder, the grains of which had a median diameter d.sub.50 of 10 microns, were first of all mixed with a second SiC powder, the grains of which had a median diameter d.sub.50 of 0.5 microns. Within the meaning of the present description, the median pore diameter d.sub.50 means the diameter of the particles such that respectively 50% of the total population of the grains has a size smaller than this diameter. Added to this mixture is a porogen of polyethylene type in a proportion equal to 5 wt % of the total weight of SiC grains and a forming additive of methyl cellulose type in a proportion equal to 10 wt % of the total weight of SiC grains.

(25) The necessary quantity of water is then added and the mixture is mixed until a homogeneous paste is obtained that has a plasticity that allows it to be extruded through a die configured to yield monolith blocks of square cross section and the internal channels of which have a transverse cross section illustrated schematically in FIG. 1. The half-period P of the undulations is 1.83 mm.

(26) The unfired monoliths obtained are dried by microwave for long enough to bring the content of chemically unbonded water down to under 1 wt %.

(27) The channels of each face of the monolith are alternately plugged using well known techniques, for example described in application WO 2004/065088.

(28) The monoliths (elements) are rid of their binder then fired in an atmosphere of argon with a temperature-rise gradient of 20 C./hour until a maximum temperature of 2200 C. is reached, which temperature is then held for 6 hours.

(29) The porous material obtained has an open porosity of 47% and a median pore diameter of the order of 15 micrometers.

(30) The structural features of the elements thus obtained are given in table 1 below, in conjunction with the data already described in the foregoing description of FIGS. 1 and 2.

(31) To form the filter, 16 (44) elements are then assembled with one another by bonding using a cement compound with the following chemical composition: 72 wt % SiC, 15 wt % Al.sub.2O.sub.3, 11 wt % SiO.sub.2, the rest consisting of impurities predominantly of Fe.sub.2O.sub.3 and alkali metal and alkaline-earth metal oxides. The mean thickness of the joint between two adjacent blocks is of the order of 2 mm. The thermal conductivity of the cement jointing compound after heat treatment is around 2.1 W/m.Math.K at ambient temperature and its measured open porosity is around 38%.

(32) The assembly is then machined by abrasion, the most peripheral parts being removed to form assembled filters of cylindrical shape. A cement with the same composition as the cement jointing compound is applied at the periphery of the machined filter at a mean thickness of 1 mm in order to smooth the external surface of the cylindrically shaped filters.

(33) A plurality of assembled filters were thus produced from individual elements.

(34) TABLE-US-00001 TABLE 1 Size of elements Cross (cross Length of Geometry of Channels section of section) elements internal density elements (mm mm) (cm) channels (c/cm.sup.2) square 35.8 35.8 25.4 wavy Around 30 Final Mean Number of Thickness diameter of Thickness thickness E assembled of joint assembled of internal of external elements (mm) filter (mm) walls (m) walls (m) 16 1.2 144 370 650 Additional Total thickness e.sub.c thickness E.sub.c along the along the Max Max bisector bisector thickness e.sub.1 thickness e.sub.2 1.sub.1 (m) (m) (m) (m) (mm) 420 1136 300 300 10 Radius of 1.sub.2 curvature R (mm) (mm) 10 2

(35) During the step of resizing the filters, unlike the filters known in the art, no tearing out of peripheral individual elements from the preassembled structures under the forces used to abrade the peripheral parts of the filter was observed. Such a result incontestably demonstrates that the polygonal filtering elements according to the present invention, having an additional quantity of porous material on the external faces of the corners, allows significant improvement in the cohesion of the filtering elements within the structure, notably when this structure has to be subjected to high radial mechanical stresses.

(36) In the foregoing description, the advantages of the present invention described chiefly in relation to the honeycomb structures used as particulate filters in an internal combustion engine exhaust line to eliminate the soot produced by the combustion of a diesel or gasoline fuel.

(37) Quite obviously, the invention is not restricted to such an application and can also be applied in any field in which the aforementioned problems arise, particularly in the field of heat exchangers.