FR2965489A1 - FRAME STRUCTURE OF MICROFINED BEES. - Google Patents

Abstract

The present invention relates to a honeycomb structure comprising a ceramic material consisting of sintered grains, said honeycomb structure having an open crack having a length L of between 3 mm and 30 mm and a maximum width of less than three. times the maximum size D of said grains.

Description

TECHNICAL FIELD The invention relates to a honeycomb structure, especially to a honeycomb structure intended for the filtration of exhaust gases from an internal combustion engine or intended to to a heat exchanger. It relates in particular to a monolithic body, an assembled body comprising an assembly of a plurality of unit blocks secured by means of seals interposed between said unit blocks, and a unitary block for the manufacture of such an assembled body. The invention finally relates to a method of manufacturing a honeycomb structure according to the invention.

STATE OF THE ART Before being vented to the open air, the exhaust gases of a motor vehicle 15 may be purified by means of a lubricating body such as those shown in FIGS. 1, 3 and 4, which are known. of the prior art. As indicated by the arrows shown in FIG. 4, the flow F of the exhaust gas enters the filter body 3 through the inlet openings of the inlet channels, passes through the filtering walls of these channels to join the inlet channels. 20 out, then escapes outward through the outlet openings. After a certain period of use, the particles, or "soot" accumulated in the channels increase the pressure drop and thus impair the performance of the engine. For this reason, the filter body must be regenerated regularly, for example every 500 kilometers. Regeneration, or "declogging", consists of oxidizing the soot. To do this, it is necessary to heat them to a temperature that allows them to ignite. During the regeneration phases, the temperature differs according to the zones of the filtering body 3 and does not vary uniformly. Indeed, the exhaust gases transport downstream the heat energy released by the combustion of soot. In addition, the soot does not deposit uniformly in the different channels, accumulating for example preferably in the area of the filter body near its longitudinal axis. The combustion zones are therefore not uniformly distributed in the filter body 3. The soot combustion thus causes a temperature increase in the core of the filter body greater than that in the peripheral zones. Finally, the peripheral zones of the filter body 3 are cooled, through the metal casing 5, by the surrounding air. The inhomogeneity of the temperatures within the filter body 3 generates local stresses of high amplitudes. These stresses can lead to fractures or local cracks directly affecting the filtration performance and leading to abnormally high levels of emission downstream of the filter body (post-filter). The filter body 3 must then be changed. SUMMARY OF THE INVENTION There is a continuing need for a honeycomb structure having excellent thermomechanical strength, particularly for use in filtering exhaust gases from internal combustion engines, especially diesel. An object of the present invention is to satisfy this need. The invention provides a honeycomb structure comprising, preferably consisting of, a ceramic material made of sintered grains, said honeycomb structure having an open crack having a length Lf of between 3 mm and 30 mm and a maximum width 1R, a, less than three times the maximum size D9g, 5 of said grains. Such a crack is hereinafter referred to as "microcracking". The presence of cracks is classically considered to be detrimental. Indeed, the cracks that appear during regenerations usually lead to destruction of the honeycomb structure. Contrary to this prejudice, the inventors have discovered that, surprisingly, the presence of one or more microcracks improves the thermomechanical resistance of the honeycomb structure. Without being bound by this theory, they consider that the microcracks allow the thermomechanical stresses associated with the deformations resulting from the expansion phenomena to be absorbed more efficiently during the regenerations, and can therefore contribute to increasing the duration of the filter. Microfissure Said microcrack may have one or more of the following optional features: The ratio between the maximum width of the microcrack and the maximum grain size, Ga> / D99.5, is less than 2.5, less than 2.0, less than 1.5, less than 1.2, and / or greater than 0.1, greater than 0.5, or even greater than 0.8; At least 90% of its length, the microcrack has a width to maximum I, a, less than 90 microns, less than 8o microns, less than 6o microns, less than 50 microns, less than 40 microns, or even less than 30 microns and / or has a minimum width I, greater than 3 microns, or even greater than 5 microns; The microcrack has a maximum depth pr, preferably less than 100 microns, less than 80 microns, less than 60 microns, less than 40 microns, less than 30 microns, and / or greater than 10 microns, greater than 20 microns; The microcrack has a length Lf preferably greater than 5 mm, preferably greater than 7 mm, and / or preferably less than 40 mm, preferably less than 30 mm, preferably less than 25 mm; The microcrack has a length less than 25%, less than 20%, less than 15%, less than 12%, less than 10%, less than 8%, less than 6%, less than 4%, and / or greater than 1% of the length of the honeycomb structure. The microcrack extends substantially in the longitudinal direction of the honeycomb structure. In particular, the angle between a plane perpendicular to the main direction of the microcrack and a transverse plane (plane perpendicular to the longitudinal direction) is preferably less than 45 °, less than 30 °, or even less than 20 °, Preferably, the microcrack opens out on one side of the honeycomb structure exposed to the outside of this honeycomb structure. In particular, the microcrack is preferably arranged on a lateral face of a unitary block.

Microfissured region Preferably, the honeycomb structure has a plurality of microcracks, more than 500%, more than 70%, even more than 90%, or even more than 95% or substantially 100% of the microcracks being in accordance with the invention. invention and preferably having one or more of the optional features mentioned above. The microcracks can be evenly distributed in the honeycomb structure or grouped together. The region of the block with microcracks is called the "microfissured region". The microfissured region is delimited by the closed envelope having the smallest possible volume, containing the set fo microfissures of the honeycomb structure and delimited by the side surface of the honeycomb structure and by two planes transverse. Preferably, the concentration of microcracks leading to the side surface of the honeycomb structure is greater than 0.001, preferably greater than 0.01 and / or less than 0.1 microcracks per cm 2 of said lateral surface delimiting the microcracked region. . The inventors have also discovered that it is preferable that the microfissured region does not extend over the entire length of the honeycomb structure. Preferably, elfe is less than 50%, less than 40%, or even less than 30% of the length of the honeycomb structure. The position of the microfissured region also modifies the obtained performances. Preferably, this region extends entirely into the part of the honeycomb structure which extends, from one of the admission and discharge faces, over 40%, 30% or even 20% of the length L of the honeycomb structure. A honeycomb structure according to the invention may comprise more than 2, more than 3, more than 5, more than 10, more than 15, more than 20 microcracks. Preferably, more than 50%, more than 70% or more than 90% of the microcracks open out of the honeycomb structure. Preferably more than 50%, more than 700% or even more than 90% of the microcracks, in particular microcracks, or even all microcracks, in particular through microcracks, open onto the lateral (or "external") surface. of the honeycomb structure, the side surface being the outer surface of the honeycomb structure which extends between the intake face and the discharge face. 4 Honeycomb structure Generally, the length L of a honeycomb structure according to the invention is less than 50 cm, less than 40 cm, less than 30 cm, less than 25 cm, or even less than 20 cm. cm, or even less than 15 cm.

A honeycomb structure according to the invention may be made of a material having a coefficient of thermal expansion greater than 1.0 10 6C, between 20 and 1000 ° C. The invention is more particularly suitable for honeycomb structures of a material having a coefficient of thermal expansion greater than 2.5 × 10 -6 ° C at 20 ° to 1000 ° C. It may in particular comprise or be made of silicon carbide SiC. This feature is particularly useful in an assembled filter body. During regeneration, the peripheral and central unit blocks present a very different deformation, which generates transverse stresses (in transverse planes), and eventually leads to longitudinal cracks in the joints. The presence of microcracks in the unit blocks has proved particularly advantageous in this situation. A honeycomb structure according to the invention may be a monolithic body, an assembled body or a unitary block for the manufacture of an assembled body. It may be a preform or a sintered structure. It can be filtering (the channels being closed at one of their ends) or not. Preferably, a honeycomb structure according to the invention is an assembled body, formed by assembling unit blocks with the interposition of joints, continuous or not. The invention relates to an assembled body, remarkable in that it comprises at least one unit block according to the invention. Preferably at least 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, preferably all the unit blocks of the assembled body are in accordance with the invention. When the assembled body is intended for the filtration of particles, in particular of the exhaust particles of an internal combustion engine, each unit block comprises a set of adjacent channels extending between intake and discharge faces. and separated by filtering walls, said channels being closed by upstream and downstream plugs arranged alternately near the intake face and the discharge face. Preferably, each unit block according to the invention is arranged so that its microfissured region is closer to the discharge face than to the intake face, preferably so that it extends from said evacuation face, more preferably over a length less than 30% of the length of the unit block. Thermomechanical stresses, particularly on joints, are particularly severe with assembled bodies comprising unit blocks with asymmetrical structure, that is to say in which the cross sections of the input channels are different from those of the output channels. . The provision of the microfissured region near the discharge face is particularly advantageous with this type of unit blocks. The inventors believe that this particular arrangement makes it possible to relax the thermomechanical stresses close to this face. The invention also relates to a device chosen from a heat exchanger and a particulate filter, this device being remarkable in that it comprises at least one sintered honeycomb structure according to the invention. The invention also relates to an exhaust line of a motor vehicle equipped with a sintered honeycomb structure according to the invention, in particular an exhaust line of a diesel engine of a motor vehicle.

The invention also relates to a method of manufacturing a honeycomb structure comprising the following successive steps: a) extruding a ceramic material through a die so as to form a honeycomb preform, b) drying, c) debinding and sintering said preform to obtain a sintered honeycomb structure, d) optionally, before or after sintering, capping channels of said sintered honeycomb structure.

This process is remarkable in that, during the debinding operation, at least one region is exposed to an environment capable of generating a microcracking, called the "microfissuring environment", in the sintered honeycomb structure.

The microcracking character of an environment is characterized by a triplet: environmental temperature - concentration of oxidizing gas of the environment; duration of maintaining this environment in contact with said region.

Preferably, the temperature is below 700 ° C. Preferably, the environment is gaseous and the oxygen concentration of the environment is greater than 15%, by volume, and preferably less than 25% by volume, a concentration of 21% being adapted. Air can be used. Preferably, to maintain such an environment in contact with said region, consumed oxygen is renewed, for example by permanent insufflation or not, of oxygen-laden gas. For example, an average airflow of between 5 and 20 × 10 -2 m.sup.3 / m2 of surface of the honeycomb structure exposed to this air may be suitable. This flow rate, however, depends on the mass content of organic compounds to be decomposed added to the extrusion mixture of honeycomb structures, in particular binders, plasticizers and porogens of organic nature. The average air flow rate mentioned above is particularly suitable for a mass content of organic compounds greater than 10% and less than 30%. Preferably, the duration is greater than 1 hour.

Simple tests will enable those skilled in the art to easily determine acceptable triplets. Preferably, at least two regions of the preform are exposed to environments with different oxidizing potentials. Advantageously, it is thus possible to microfissure only a part of the sintered honeycomb structure, and in particular to only microcrack the regions of the honeycomb structure which, in the application to which it is intended, suffer the highest thermomechanical stresses. Preferably, the extruded ceramic material comprises more than 5%, preferably more than 7%, or even more than 10% of organic material, for example methylcellulose, as a percentage by weight. A high content of organic matter, removed during debinding, advantageously promotes microcracking.

Definitions A crack is said to be "open" when it opens outwardly from the material that defines it (unlike a closed crack which is formed exclusively within the material). A microcrack is called "through" when it passes through a wall. The length of a microcrack is measured on the surface on which the microcrack (usually in a photo of this surface) opens, following the microcrack line. The width is measured on the surface on which the microfissure opens, perpendicular to the guideline. The main direction of a microcrack is the direction defined by the line joining its two ends. The "longitudinal" or "lengthwise" direction of a honeycomb structure (CC or DD in FIGS. 1 to 3) is defined by the general direction of fluid flow through this nest structure. bee. Conventionally, all the channels of a honeycomb structure extend parallel to the longitudinal direction. In an assembled honeycomb structure, the unit blocks are generally assembled so that the joint faces between which a seal extends is, at least locally, substantially parallel to the longitudinal direction and preferably parallel to the longitudinal direction. one to another. A "longitudinal" plane is a plane parallel to the longitudinal direction of the honeycomb structure. A "transverse" plane is a plane perpendicular to the longitudinal direction. In a honeycomb structure whose channels are all parallel, a transverse plane is a plane perpendicular to the direction of these channels. In a particle powder, the "size" of a particle is the average between its largest dimension and its smallest dimension. The percentiles or "percentiles" (D, Q), 50 (D50), 90 (D90) and 99.5 (D99.5) of a set of particles in a powder are the sizes of these particles corresponding to percentages. , by volume, of 10%, 50%, 90% and 99.5%, respectively, on the cumulative size distribution curve of the particles, the sizes being ranked in ascending order. For example, 10%, by volume, of the particles have a size less than Dao and 90% of the particles by volume have a size greater than or equal to D, o. In a sintered material, the "size" of a grain in a section of the material is the average between its largest dimension and its smallest dimension. The size is conventionally measured by observation with a scanning microscope. For a set of grains in a sintered material, the percentiles (Dlo), 50 (D50), 90 (Dgo) and 99.5 (D99.5) are the sizes of these grains. in a cutting plane, corresponding to the percentages, in number of grains, of 10 ° / a, 50%, 90% and 99.5 respectively, on the curved grain size distribution, the to sizes being classified by order increasing. For example, 10% by number of grains have a size smaller than D10 and 90% of the grains in number have a size greater than or equal to D, p. Percentiles can be determined, using suitable software, by image analysis of a metallographic section of the material taken with a scanning microscope. Since a sintered material is substantially homogeneous, the cumulative size distribution curve of the grains does not depend substantially on the section plane considered. Conventionally, the "D50" percentile and the "maximum size" percentile "D99.5" are called "median size". "Debinding" is a heat treatment for removing binders and temporary pore formers. Conventionally, these binders are organic binders. Pore forming agents include cellulose-type agents, such as polyethylene, polystyrene, starch and graphite. They are described, for example, in applications JP 08-281036 or EP 1 541 538. Conventionally, debinding is carried out at a maximum temperature greater than 300 ° C., preferably greater than 400 ° C. and / or less than 700 ° C. ° C or less than 500 ° C, preferably for a period greater than 1 hour and less than 15 hours. Unless otherwise indicated, by "comprising a" or "presenting a", it is necessary to include "having at least one" or "having at least one". Bremen description of the figures The following description, made with reference to the accompanying drawings, will better understand and appreciate the advantages of the invention. In these drawings: - Figure 1 shows schematically in perspective an assembled filter body; FIG. 2 diagrammatically shows in perspective a unitary block of the assembled filter body represented in FIG. 1; - Figure 3 shows schematically in perspective a monolithic filter body; - Figure 4 shows schematically in median longitudinal section along the sectional plane P, the assembled filter body shown in Figure 1, after insertion into its envelope; Figures 5 to 8 show photos of microcracks; FIG. 9 represents a debinding device making it possible to obtain microcracks in unit blocks; and - Figure 10 schematically shows a microcrack on a unit block. In these nonlimiting figures, the various elements are not necessarily represented on the same scale. In particular, for the sake of clarity, the microcrack 120 of Figure 10 has not been shown to scale. Identical references have been used in the various figures to designate identical or similar elements. In order to improve the clarity of the figures, the number of channels represented is very much smaller than that of unitary units or filtering bodies conventionally marketed. Detailed Description Honeycomb structure A particle filter 1 conventionally comprises at least one filter body 3, generally cylindrical with a longitudinal axis CC, of length L, typically between 10 and 50 cm, inserted in a metal casing 5 , or "canning" (Figure 4).

The filter body 3 can be a monolithic body, that is to say in one piece, without a joint, as shown in FIG. 3. In order to improve its thermomechanical resistance, particularly during the regeneration phases, it is nevertheless advantageous It results from the assembly of a plurality of unit blocks 11 by means of seals 12. The filter body is then an "assembled" body, as shown in FIGS. 1 and 4. A unit block 11 (FIG. ) conventionally has the shape of a rectangular parallelepiped, axis DD, length "L", width "1" and height "h", as shown in Figure 2. The width "1" and the height "h" of a side face 14a-d of such a unit block are typically between 30 mm and 100 mm. The set of side faces 14a-d of the unit block forms its "lateral surface". A monolithic body and a unitary block 11 are examples of honeycomb structures, a "honeycomb" configuration corresponding to the presence of a set of channels 18, or "ducts", adjacent to each other. form, in section, a regular pattern, for example checkerboard. The "outer wall" is the peripheral wall of a honeycomb structure that defines its lateral surface. The honeycomb structure may be of a porous material having greater than 30% or even more than 40% and / or less than 65% or even less than 50% open porosity. In one embodiment, the honeycomb structure is made of a material having, at a temperature of between 20 ° C. and 600 ° C., a thermal conductivity of less than 60 W / m ° C., or even less than 40 Wlm. C., and / or greater than 25 .mu.l ° C or even greater than 10 W / m. The honeycomb structure may be made of a material having a coefficient of expansion greater than 2.5 × 10 -6 ° C. between 20 ° and 1000 ° C., for which the invention is particularly useful. Preferably, the honeycomb structure is of a sintered material and preferably has more than 50%, more than 80%, even more than 95%, or even more than 98%, or even 100%. mass of silicon carbide SiC, preferably recrystallized. The silicon carbide can be sintered and / or bonded with silicon. The honeycomb structure may also be of a material comprising at least 50% by weight aluminum titanate and / or mullite and / or cordierite (Mg2Al4Si2O1 $) and / or silicon nitride and / or sintered metals.

In an assembled body, all the unit blocks can be of identical material. Channels In an application to filtration in particular, the channels 18, each delimited by a side wall 22, are generally rectilinear, and extend parallel to each other along the length L. The thickness of the side walls (or "filter walls ") May especially be between 180 and 500 microns. The cross section of the channels, preferably constant along their length, may especially be between 0. 4 and 9 mm 2. Each channel opens outwards through an upstream opening 24e on an inlet face 26e, and a downstream opening 24s on a discharge face 26s. In an application to filtration, the number of channels of the honeycomb structure is typically between 7.75 and 62 per cm 2 of the admission face, preferably between 150 to 400 cpsi. Conventionally, to manufacture a honeycomb structure, a ceramic material is extruded through a suitable die. The extrusion die is conventionally shaped so that the preform has the shape of a cylinder of circular or ellipsoidal section to manufacture a monolithic body or a cylinder of polygonal section, for example square, hexagonal or rectangular to manufacture a unitary block . Caps The channels of a honeycomb structure for the manufacture of a filter body are then plugged near the inlet face 26e or near the discharge face 26s by 30s upstream and 30th downstream plugs , respectively, as is well known, to form so-called "output channels" 18s and 2965489 [3 "input channels" 18e, respectively (see FIG. 4). A "filtering" honeycomb structure is then obtained. More preferably, the upstream and downstream plugs extend along the intake face and the discharge face, respectively. Thus, the inlet and outlet channels define inlet and outlet chambers 34e and 34s, respectively delimited each by a side wall 22, a blanking plug, and an opening opening outwardly. Two adjacent input and output channels are in fluid communication through their common sidewall. In one embodiment, the input channels 18e and the output channels 18s are adjacent, and arranged relative to one another so that all of the gas filtered by any input channel passes through channels. output adjacent to said input channel. Thus, there are no zones of one or more input channels that open into another input channel, areas that can not be useful for filtration. The filtration area (i.e., the effective area of the walls of the inlet channels) available for a given honeycomb structure volume is optimized. Preferably, the sets of input channels and output channels are nested within each other so as to form, in cross-section, a checkered pattern where said input channels alternate with said output channels. . The inlet channels preferably have a larger cross-section than the outlet channels to increase the volume available for storing the residues. Advantageously, the cleaning frequency of the filter and the pressure drop are reduced. For this purpose, the walls of the input channels are "deformed" to increase the overall volume of the input channels at the expense of that of the output channels. For example, these walls can be concave on the side of an input channel and convex on the side of the output channels that are adjacent to it. The intermediate walls separating two rows or two columns of channels may in particular have, in cross-section, a wavy or "wavy" shape, the intermediate wall substantially waving half a wavy-wave length. the width of a channel. The term "length" of a ripple is the distance separating two points of this ripple located at the same height, with the same direction of variation of slope. In the case of a periodic ripple, the "length" of the ripple is called "period". Preferably the corrugation is periodic, but the amplitude of the corrugations can be constant or variable. Preferably this amplitude is constant. More preferably, the corrugation has a sinusoidal shape whose half-period is equal to the pitch "p" of the channel network, or a succession of adjacent arcs of circles, each arc having a length equal to the pitch "p", Preferably, finally, all the intermediate walls extending between two lines and / or extending between two columns of channels have, in cross section, an undulation of identical shape. The "degree of asymmetry" of a "wavy" structure designates the ratio between the amplitude "h" and the half-length of said waviness, that is to say, in the case of a periodic waviness , the ratio between the amplitude "h, N" and the half-period. Preferably, the degree of asymmetry is less than 40%, preferably less than 30%, preferably less than 20%, even more preferably less than or equal to 1% and / or greater than 2%. preferably greater than 5%. Advantageously, the pressure drop induced by the unit block after accumulation of soot is thus substantially reduced, and the frequency of the regeneration of the filter body is therefore limited.

With this asymmetrical configuration of the channels, the total accumulated volume of the input channels is greater than that of the output channels and the total accumulated surface of the openings of the input channels on the admission face, that is to say the sum of the areas of these openings is greater than that of the openings of the outlet channels on the discharge face.

For optimum efficiency, the ratio r of the total accumulated volume Ve of the input channels to the total accumulated volume Vs of the output channels or the ratio r 'of the total cumulative interior surface of the input channels to the total cumulative interior surface output channels is preferably greater than 1.03, greater than 1.10, greater than 1.15 and / or less than 3, less than 2.5, preferably less than 2.

After capping, the extruded honeycomb structures are sintered. To manufacture an assembled body, unit blocks are assembled, as for example described below. As a rule, the lateral surface 38 of the monolithic or assembled filter body is covered with a peripheral coating 36, also called "external coating" or "coating", in a thermally insulating coating cement and gas-tight, as described for example in EP 1 142 619 or EP 1 632 657. This peripheral coating may be manufactured using a cement identical to that used for joints. The tightness of the coating cement makes it possible to protect, in the thickness of the outer wall of the filter body, through microcracks, without any detrimental effect on the efficiency of the filtration (by arranging this cement so as to cover the openings of these microcracks). The filter body 3, assembled or monolithic, can then be inserted into the casing 5, an exhaust gas-tight peripheral seal 40 being disposed between the lateral surface 38 of the filter body and the casing 5. Examples of Filtering bodies are described in particular in patent applications EP 816 065, EP 1 142 619, EP 1 455 923 or WQ 20041065088 to which reference may be made for further details on their structure or manufacture. Assembled Body A method of manufacturing an assembled body typically comprises the following successive steps: A) manufacturing a plurality of sintered unit blocks 11; B) preparation of fresh assembly cement; C) bonding the unit blocks using fresh assembly cement; D) curing the fresh joining cement so as to form seals 12 between said joint faces; E) optionally, heat treatment. In step A), the unit blocks can be manufactured as previously described. In step B), the fresh assembly cement is prepared according to the desired characteristics for the joints. The two seal faces between which a seal extends may be of generally planar shape. The joining cement forming the joints is generally made of silica and / or silicon carbide and / or aluminum nitride. Preferably, the bonding cement is substantially leakproof to the exhaust gases to be filtered. The joining cement may have, at a temperature of between 20 ° C. and 600 ° C., a thermal conductivity greater than 0.1 WIm.K, and preferably less than 5 WIm.K, or even less than 1 WIm.K to limit thermomechanical stresses. Typically, the average thickness of a seal 12 is between 0.3 and 4 mm. The total porosity of the joint-joining cement may be greater than 5% and less than 90%, preferably greater than 30% and / or less than 85%. The joining cement may advantageously comprise more than 0.05% and less than 5% of a thermosetting resin, in percentage relative to the mass of the dry mineral matter. The joining cement may be obtained from a mineral powder comprising, in percentage by weight relative to the mineral matter, at least 5% of particles whose median size is between 0.1 and 10 microns, preferably between 0.3 and 5 microns. Preferably, silicon carbide (SiC), alumina (Al2O3), zirconia (ZrO2), titanium oxide (TiO2), magnesium oxide, silica (SiO2) and mixed compounds derived therefrom Oxides, such as aluminum titanate, mullite or zircon, together account for more than 85% of the mass of the dry mineral material of the joining cement. The joining cement may have a CaO lime content of less than 0.5%, by weight percent relative to the dry mineral matter. The joining cement may be a composite material comprising grains of an inorganic material bonded by a geopolymeric matrix. The joining cement may comprise more than 30%, more than 50%, more than 60% or even more than 75% of silicon carbide, in percentage by weight relative to the dry mineral material, in particular to obtain thermal conductivity. High, preferably, the silicon carbide is present in the form of particles whose median size is less than 200 microns.

Silicon carbide, however, has a relatively high coefficient of expansion. The silicon carbide content must therefore be limited to ensure a thermomechanical resistance adapted to the application to particulate filters. A silicon carbide content of between 30 and 90%, preferably between 55% and 75%, is particularly suitable. The assembly cement preferably comprises less than 10%, preferably less than 5%, preferably less than 1%, of mineral fibers, in particular ceramics, as a percentage by weight on the basis of the dry mineral matter. Preferably, the joining cement does not comprise such fibers.

Preferably, the assembly cement comprises an inorganic and / or organic binder. Conventionally, the assembly cement is impervious to filtered gases. The tightness of the assembly cement advantageously makes it possible to protect, in the thickness of the outer wall of a unitary block, through microcracks, without any detrimental effect on the efficiency of the filtration (by arranging this cement so as to cover the openings of these microcracks). In step C), two adjacent unit blocks are glued to each other by facing, and preferably parallel to each other, the respective lateral faces 14a-14d, then applying cement fresh assembly between said facing side faces (called "joint faces"). In step D), after having been placed between the joint faces of the unit blocks, the fresh assembly cement is dried, preferably at a temperature of between 100 ° C. and 200 ° C., preferably under air or atmosphere. controlled humidity, preferably so that the residual moisture is between 0 and 20%. Preferably, the drying time is between a few seconds and 10 hours, in particular depending on the size of the joint and the ceramic body assembled. The unit blocks are preferably held in position to prevent expansion of the fresh joining cement being cured, for example by wedging the unit blocks with spacers, as described for example in EP 1 435 348, and strapping of the unit blocks thus calibrated. In step E), heat treatment at a temperature sufficient to remove any particles and / or organic fibers can then be performed to remove them and thus create macroporosity {closed porosity}, advantageous for reducing the thermal conductivity. The heat treatment is carried out, preferably under an oxidizing atmosphere, preferably at atmospheric pressure, at a temperature of between 300 ° C and 1200 ° C. The heat treatment leads to the polymerization of the optional thermosetting resin, to the hardening of the inorganic binder, to the elimination of possible organic components, or even to a more or less complete sintering of the assembly cement when the temperature exceeds 700 ° C. This heat treatment is generally accompanied by an improvement in the mechanical strength. Its duration preferably between 1 and 20 hours of cold cold, is variable depending on the materials but also the size and shape of the joints. The heat treatment can also be carried out in situ. In particular, in the case of assembled filter bodies for motor vehicle filters, the filter bodies can be installed in the motor vehicle before removing the organic components of the assembly cement, the regeneration temperature being sufficient to eliminate them. For example, the combustion temperature of the cellulose fibers is about 200 ° C while the regeneration temperature is typically about 500 ° C or higher.

The assembled body thus formed can then be machined to obtain a shape adapted to the casing 5, for example to manufacture a cylindrical body of circular section. Alternatively to the method described above, a method according to the invention may comprise the following steps: A »manufacturing a plurality of unsintered unit blocks 11; B ') preparing a fresh assembly cement; C ') bonding the unit blocks using the fresh assembly cement; D "curing the fresh joining cement so as to form seals 12 between said joint faces; E ') sintering heat treatment so as to sinter said unit blocks and said assembly cement and to microfissure said unit blocks.

Method of manufacturing microcracks A honeycomb structure according to the invention can be manufactured from all currently used processes, provided that this method is adapted to lead to the appearance of microcracks, in particular by adapting the step of debinding.

The invention relates in particular to a method of manufacturing a honeycomb structure comprising the steps a) to d) described above. Preferably, in step b), the drying can be obtained by a heat treatment and / or by the use of microwaves, for a time and a temperature sufficient to bring the water content not bound chemically to less than 1% to mass. Of course, other known equivalent means can be envisaged without departing from the scope of the present invention. In step c), the elimination of the binder (or debinding) is preferably carried out under an oxidizing atmosphere, in particular in air, at a temperature preferably below 700 ° C., so as to ensure sufficient mechanical strength before Sintering and avoid uncontrolled oxidation, including any SiC. Debinding can be carried out under a less oxidizing atmosphere than air so as to prevent excessive oxidation of the silicon carbide possibly present, but the debinding step is then longer. Debinding methods and devices are described, for example, in EP 1 541 638, EP 1 902 766, US2007 054229 and WO20081063538. The sintering firing is carried out at a temperature above 1600 ° C, or even higher than 1800 ° C, preferably greater than 2000 ° C, more preferably still greater than 2100 ° C. This temperature is preferably lower than 2400 ° C. Preferably, said cooking is conducted under a non-oxidizing atmosphere, for example Argon. According to this method, the conditions imposed in step c) are adapted to microfissure the sintered honeycomb structure. In particular, microcracks can appear during a debinding step performed under air, under the conditions mentioned above. To control the microfissuring environment, it is possible to implement an oxidant gas injection device, in particular air, having calibrated orifices 19 oriented so as to renew the oxidizing gas in contact with the region to be treated. microfissurer. It is thus possible to locally expose one of a honeycomb structure, in particular the intake face or the discharge face, to a variable oxidizing gas flow rate and / or an oxidizing gas having a concentration in oxygen higher or lower. Debinding causes a reaction that is all the more exothermic since the oxygen concentration of the oxidizing gas and the flow rate of the oxidizing gas are high. The greater the heat release, the more likely the formation of microcracks. Preferably, the injection device is adapted to generate microcracks locally in the honeycomb structure, in particular to provide microcracks only on the outer wall of the honeycomb structure. In particular, it may be advantageous to blow oxidizing gas to channels of the honeycomb structure. To better delineate the microfissured region, the preform is preferably placed in a "gazette". As shown in FIG. 9, a journal 100 is a tray, usually made of metal. This plate comprises a bottom 102 on which are placed the honeycomb structures, for example sintered unit blocks 11 forming filter blocks of dimensions 25.4 X 3.6 X 3.6 cm 3, lateral cheeks 104 and 106 and, connected by the lateral cheeks 104 and 106, an upstream frontage 108 and a downstream frontage 110. The gazettes are superimposed on each other so that, during debinding, the unitary blocks 11 are confined between the bottom 102 of the gasket 100 on which they are laid, and the bottom 103 of the upper gasket 100 ', shown in phantom, placed on the gasket 100. The length Lloo and the width 1, 00 may be for example between 20 and 40 cm, for example 30 cm, and the height hl. for example about 5 cm. In the preferred embodiment, the unit blocks 11 are arranged substantially parallel to each other, substantially in the direction of the length L 00, as shown, spaced apart by a distance 8 for example of about 1 cm. More preferably, the upstream face 108 is pierced with injection orifices 112 having for example a diameter of between 1 and 5 mm, for example about 2 mm. Preferably, each unit block 11 is disposed facing an injection orifice 112, preferably so that its axis D-D corresponds to that of the corresponding injection orifice 112. Preferably, the upstream frontage 108 does not have an injection orifice that is not facing the inlet face or the discharge face of a unitary block 11.

More preferably, a unit block 11 faces an injection port 112 through its inlet face 26e. In the case where the structure is asymmetrical, the total mass of the plugs on the intake face 26e is greater than the total mass of plugs on the discharge face 26s. The downstream face comprises a discharge mouth 114 extending substantially over the entire height and the width of the downstream face 110, for example rectangular, for example with a surface area of approximately 100 to 150 cm 2. Stacking of gasettes is introduced into an oven raised to debinding temperature, for example at 450 ° C. An oxidizing gas G, for example air, preferably at the debinding temperature, is then injected through the injection orifices 112, for example by means of a suction or ventilation device. The oxidizing gas G is thus oriented towards the evacuation face 26e of a unitary block 11, traverses the outside of the unitary block substantially parallel to the lateral faces of the unitary block 11, along its axis DD, then emerges through the mouth of 114. The unit blocks are maintained in this microfissuring environment for more than one hour, for example 2 hours. The temperature, the flow of oxidizing gas and the orientation of the unit blocks lead to a microcracking of the unit blocks, in particular when the unit blocks are asymmetrical. Typically, with a flow of air through the injection port 112 facing the inlet face 26e of a unit block of between 5 and 20 × 10 -2 m 3 / s per m 2 of the surface of said face 26e admission (surface directly exposed to the air entering the injection port 112), microcracks appear on the side surface of the unit block 11, in particular near its discharge face 26s. Generally, the microcracks obtained with the device described above are not branched. As shown in FIG. 10, a microcrack 120 may extend in particular on the upper lateral face 14c of the unitary block 11, opposite to the lateral face 14b through which the unitary block 11 rests on the bottom 102 of the gasket 100. microfissure 120 may extend in particular from the evacuation face 26s. The main direction D120 joining the two ends 122 and 124 of the microcrack 120 is substantially longitudinal, the angle a between a transverse plane P1 and a plane P120 perpendicular to the main direction D120 is preferably less than 45 °, less than 30 ° or less than 20 °. The length of the microcrack 120 measured along its guideline 126 is typically between 3 and 30 mm. The maximum width I, ax of the microcrack 120 is less than 3 times the maximum size of the grains constituting the unit block 11. The microcracks generated during the debinding are maintained during the cooking, without however affecting the modulus of rupture or the modulus of elasticity of the sintered honeycomb structure. In step c) debinding and sintering operations can be separated or simultaneous. Preferably, these steps are separated, debinding is preferably carried out under air and sintering under a controlled atmosphere of Argon. The debinding and sintering operations can be carried out in the same oven, the oven temperature not being brought back to room temperature after debinding. According to the invention, FIGS. 5 to 8 represent the type of microcracks obtained (intergranular). The thickness of the microcrack represented is of the order of the median size D50 of the grains constituting the unit block. The microcracks obtained are thinner than the cracks created during severe regenerations, which typically have a width of 1 mm. In addition, these regeneration cracks may, in use, encircle a unitary block, thus reducing its filtration capacity but also threatening its integrity. The microcrack can be through, especially when it is formed in the thickness of the outer wall of the honeycomb structure. It can also not be through, that is to say, only lead to a single opening. In other words, it does not cross the wall of the honeycomb structure on which it opens. Advantageously, such a microcracking does not degrade the efficiency of filtration, even when it is provided on a wall extending inside the honeycomb structure, called "internal wall", or on the outer wall of a monolithic filter body having no peripheral coating. The maximum depth pmax of the microcrack may in particular be greater than 5 0.8, greater than 0.9 times the thickness of the outer wall of the honeycomb structure, or even equal to this thickness (through microcrack). The maximum depth p, ax of the microcrack may in particular be less than 0.7, less than 0.6, or even less than 0.5 times the thickness of the wall of the honeycomb structure on which it is arranged, especially when this wall is Io an inner wall. Advantageously, the filtration efficiency then remains very satisfactory. Microcracks can be generated specifically near the inlet face or the discharge face, preferably near the discharge face, by local modification of the air flow. The greater the oxidant gas flow provided locally during debinding is important, the more the exothermic debinding reaction generates local stresses generating microcracks. To manufacture an assembled body according to the invention, at least one of the unit blocks is preferably manufactured in step A) according to steps a) to d) above. EXAMPLES All the unit blocks of the examples were manufactured according to the following method. In a kneader, silicon carbide powders, a polyethylene-type blowing agent and an organic methylcellulose binder were blended. More specifically, 70% by weight of a SiC powder whose grains have a median diameter D50 of 10 microns is mixed with a second SiC powder - 30% by weight - whose grains have a median diameter D 50 of 0.5 micron. To this mixture is added a pore-forming agent of the polyethylene type in a proportion equal to 4.5% by weight of the total weight of the SiC grains and a methylcellulose-type shaping additive in a proportion equal to 10% by weight of the total weight. grains of SiC.

Water is added and kneaded to obtain a homogeneous paste whose plasticity allows extrusion through a die. This paste is then extruded through a die adapted to the manufacture of a honeycomb preform having the dimensional characteristics of Table 1.

The preforms are then dried by microwave and then under air for a time sufficient to bring the water content not chemically bound to less than 1% by weight. The channels are then alternately closed on the intake face and on the evacuation face of the preforms according to known techniques, for example as described in Application WO 2004/1065088. The preforms are then debonded at 500 ° C. for one hour under air and then fired in a cycle having a rise in temperature of 20 ° C / hour to a temperature of the order of 2200 ° C, and then maintained at this temperature for 2 hours. Thus, a series of substantially identical silicon carbide unit blocks are obtained, the characteristics of which are given in the following Table 1: Cross Section of the Channels Square Channel Density 1 $ 0 cpsi (channels per square inch) Wall Thickness 350 microns Length 25.4 cm Width 3.6 cm Open porosity About 47% (measured by mercury porometry) Median pares diameter About 15 microns (measured by mercury porometry) Table 1 The unit blocks according to the invention each had a through microcrack on one of their side faces. The length of this microcrack was about 1 cm and its maximum width Imax was about one time the maximum size D99.5 grains. In accordance with the teaching of patent application EP 816 065, 16 unit blocks are then assembled together by gluing.

For the assembled body 1 according to the invention, 14 of the 16 unit blocks were in accordance with the invention. The unit blocks had undergone debinding in gazettes having injection ports, as described above, the air flow towards the inlet faces of the unit blocks being of the order of 8.10 -2, but per m2 surface of said intake faces (each intake face being placed facing an injection orifice) .For the assembled assembled body C1, none of the 16 unitary units was in accordance with the invention. debinding in gaskets having injection orifices, as described above, the air flow towards the admission face of the unit blocks being of the order of 2.10-2 mals per m 2 of surface of said intake side.

For the bonding of the unit blocks, an assembly cement is prepared by mixing the following constituents: 1.5 - 66% by weight of a SiC powder with a particle size of between 10 and 200 microns, - 3% by weight of a powder reactive alumina marketed by Almatis, with a median size of about 3 microns, -24 ° / Q of hollow spheres marketed by Enviro-spheres under the name "e-spheres", which have a typical chemical composition comprising 60 % SiO2 and 40% Al2O3 and a median size of the order of 100 microns, - 6% silica fume of the Elkem 971 type, - 0.8% by weight of a temporary binder and plasticizer of the cellulose type, - 0, 2% by weight of a deflocculant of the TPPNa type (sodium tripolyphosphate). An amount of water corresponding to about 150% of the weight of this mixture is added to obtain a fresh blend cement of adequate viscosity. In all the examples, the fresh joining cement was applied without pressure and uniformly between the unit blocks so as to obtain joints with an average thickness of about 1 mm filling the entire gap between the joint faces. The assemblies of the unit blocks are then dried in air at 100 ° C. for 1 hour and then fired at 1000 ° C. under air for 1 hour in order to obtain assembled filtering bodies. The assembled filter bodies are then machined to obtain assembled bodies of circular section, volume of the order of 4.1 liters (5.66 "x 10"). The assembled bodies were then subjected to the following tests: Thermomechanical resistance control

The resulting assembled bodies were mounted on an exhaust line of a direct injection 2.0 L diesel engine operated at full power (4000 rpm) for 30 minutes, then disassembled and weighed to determine their initial mass IO. The assembled bodies are then reassembled on a motor bench with a speed of 3000 rpm and a torque of 50 N.m for durations adapted to obtain a soot load of 8 g / liter (in volume of the assembled body). The assembled bodies thus loaded are then reassembled on the exhaust line 1.5 to undergo a severe regeneration: after stabilization at an engine speed of 1700 revolutions / minute for a torque of 95 Nm for 2 minutes, a post-injection is carried out with 70 ° phasing for a post-injection rate of 18 mm3 / stroke. Once the soot combustion initiated, that is to say when the pressure drop decreases for at least 4 seconds, the engine speed is lowered to 1050 rpm for a torque of 40 Nm for 5 minutes to accelerate the combustion of soot. The assembled bodies are then subjected to an engine speed of 4000 rpm for 30 minutes to remove the remaining soot. The regenerated assembled bodies are inspected after cutting to reveal the possible presence of cracks visible to the naked eye. The thermomechanical resistance is appreciated in view of the number of cracks, a small number of cracks reflecting an acceptable thermomechanical resistance for use as a filter body. The following notes can be attributed: 30 +++: presence of numerous fissures, ++: presence of numerous cracks, +: presence of some cracks,: no cracks or rare cracks. The regeneration being carried out under extreme conditions, the presence of some cracks ("+" rating) is acceptable. The notes "++" and "+++" are however representative of a bad thermomechanical resistance.

Filtration Efficiency Index The Filtration Efficiency Index is defined as the difference between the amount of soot particles upstream of an assembled body and the amount of soot particles downstream of said body in relation to the amount of particulates of soot upstream of said assembled body. The filtration efficiency index is measured by means of fumimeters placed upstream and downstream of the assembled body, the latter being placed on the exhaust line of an engine operated at full power at 4000 revolutions / minute during 30 minutes. Measurements of the Younq modules of the assembly cement and the filter unit blocks The dynamic Young's modulus (E) is measured, in accordance with the ASTM C1259-09 standard, with a device marketed under the reference Grindosonic MK5 by the company JW. Lemmens. At the ambient temperature, the natural vibration frequency of a specimen in "dynamic" mode is measured. For this purpose, the test specimen is placed on two rubbery supports so as not to interact with the vibration mode of the specimen. The supports are placed symmetrically with respect to the middle of the specimen length, the distance between supports being 900 mm. The test piece is then excited by a mechanical pulse in the center of its upper face (opposite the bearing face on the supports), for example by means of a stick, a pencil or a hammer. supplied with the device, the excitation energy being low. This excitation induces a vibration within the material of the specimen. A piezoelectric detector placed in contact with the specimen then records this vibration and converts it into an electrical signal from which the natural vibration frequency f (or "bending resonance frequency") is determined.

The dynamic Young's modulus E (in GPa) is calculated as a function of the mass m (in g) of the specimen and of f (in Hz) according to the following formula: E = 9.15 $ 4.10- 'x in xf 2 The dynamic Young's modulus was measured on filter unit blocks of dimensions 36x36x264mm3. The dynamic Young's modulus was mis-measured on a 22x22x143mm3 test specimen made of molded assembly cement which had undergone the same heat treatment as that undergone by the bonding cement when used in an assembled body. The sample was then dried at 110 ° C before being cooled to room temperature. Results: Cl 1 E (GPa) unit blocks 60 60 E (GPa) of the assembly cement 6 6 Number of microcracks per unit block according to the invention 0 1 E Number of microcracks for the assembled body 0 14 Thermomechanical resistance of the assembled body + Number of cracks, after regeneration, on the assembled body 22 2 Filtration efficiency ~ 85% X85 '/ p C1: Comparative example Example 1: Example according to the invention Table 2

An increase in the number of microcracks significantly improves the thermomechanical behavior. This improvement, however, does not lead to a significant reduction in the efficiency of the filtration.

Additional measurements have confirmed that the presence of microcracks does not substantially modify the porosity and the median pore diameter of the material constituting the unit blocks.

The preceding description makes it possible to illustrate a few possible embodiments of the invention. It is understood that this description is however not limiting and that the skilled person is able to achieve other variants of the invention without departing from its scope.

In particular, a honeycomb structure according to the invention could be used in other applications than filtration, and in particular for catalysis. In one embodiment, a portion of the honeycomb structure is coated with a catalytic coating, or "wash coat", for example suitable for the treatment of pollutants of CO, HC or NOx type. For example, for optimum performance, the catalytic coating may be applied only on the surfaces delimiting a portion of the channels, for example on the only surfaces defining the inlet channels of a filter body. Especially for catalysis application, the channels may not be clogged. Furthermore, the examples use cylindrical unit blocks of square base, called "parallelepipedic". The invention is however not limited to such blocks. Finally, in other embodiments, the input channels and the adjacent output channels are not arranged with respect to each other so that all of the gas filtered through any input channel goes into output channels adjacent to said input channel. 20

Claims (10)

REVENDICATIONS1. A honeycomb structure comprising a plurality of channels opening on inlet and outlet faces, said structure comprising a ceramic material consisting of sintered grains, said honeycomb structure having an open crack having a length Lf included entered 3 mm and 30 mm and a maximum width lma, less than three times the maximum size D99.5 of said grains, called "microcrack". 2. honeycomb structure according to the preceding claim, wherein the ratio between the maximum width of the microcrack and the maximum grain size, lm), 1 D99.5, is less than 2.0. 3. honeycomb structure according to any one of the preceding claims, wherein the microcrack has a length Lt greater than 5 mm. Honeycomb structure according to any one of the preceding claims, wherein the microcrack opens onto the outer surface of the honeycomb structure which extends between said inlet and outlet faces. 5. honeycomb structure according to any one of the preceding claims, comprising a microfissured region defined by the closed envelope having the smallest possible volume; containing all the microcracks and delimited by the outer surface of the honeycomb structure and by two planes perpendicular to the longitudinal direction of the honeycomb structure, said microcracked region extending over less than 50% of the length of the honeycomb structure. 6. honeycomb structure according to any one of the preceding claims, consisting of a material having a coefficient of thermal expansion greater than 2.5.10-6C 'between 20 and 1000 ° C. 7. Assembled body according to any one of the preceding claims, constituted by an assembly of honeycomb structures, at least one of said honeycomb structures being in accordance with any one of the preceding claims. 8. Assembled body according to the preceding claim, wherein said honeycomb structure is according to claim 5, said microcracked region being closer to the discharge face than to the intake face of said honeycomb structure. 'bee. 9. Device selected from a heat exchanger and a particulate filter, comprising at least one sintered honeycomb structure according to any one of claims 1 to 6. 10. A method of manufacturing a honeycomb structure, comprising the following successive steps: a) extruding a ceramic material through a die so as to form a honeycomb preform, b) drying, c ) debinding and sintering of said. preform for obtaining a sintered honeycomb structure according to any one of claims 1 to 6, d) optionally, before or after sintering, capping channels. of said sintered honeycomb structure.