WO1995029326A1

WO1995029326A1 - Exhaust gas after-treatment devices with increased friction between honeycomb monolith and encapsulation

Info

Publication number: WO1995029326A1
Application number: PCT/DK1995/000172
Authority: WO
Inventors: Per Stobbe; Henrik Guldberg Petersen
Original assignee: Per Stobbe; Henrik Guldberg Petersen
Priority date: 1994-04-25
Filing date: 1995-04-25
Publication date: 1995-11-02
Also published as: EP0795075A1; AU2304895A

Abstract

An after-treatment device for exhaust gas from a combustion engine, e.g. a wall flow filter for removing particulate matter from diesel exhaust, comprises a ceramic, usually extruded, honeycomb monolith body, e.g. of silicon carbide (SiC), normally in the form of a cylinder, covered by a flexible interface material such as a flexible ceramic mat containing an intumescent agent, a steel wire wrap or an alumina insulation felt, and a container typically a steel canning or canister, encapsulating the monolith body and the interface material, the circumference surface of the honeycomb monolith having a domain or domains which has/have been modified to obtain an increased roughness in order to increase the friction between the monolith and the interface material. Thereby, the monolith is kept in place with reduced pressure and thus with reduced mechanical stress. The modification to obtain increased roughness may be constituted by surface parts extending upwardly from the remaining surface, or particles bound to and embedded in the surface. The monolith body with the increased roughness domain may be produced by applying, to the green body, ceramic particles in such manner that they will become bound to and embedded in the surface and heat treating the particles-carrying monolith, thereby converting it to a refractory body with the particles ceramically bound to the surface. Alternatively or additionally, the inside surface of the metal container may be supplied with a domain of increased roughness, e.g. by applying a glaze or enamel to the inside surface and incorporating coarse refractory particles conferring an increased roughness in the glaze or enamel and then heat treating.

Description

EXHAUST GAS AFTER-TREATMENT DEVICES WITH INCREASED FRICTION BETWEEN HONEYCOMB MONOLITH AND ENCAPSULATION

The emission of harmful gases from the internal combustion engine is a serious world-wide problem requiring reduction of exhaust gas emissions by after-treatment with catalytic converters and/or particulate filters.

The most common method of treating the gases and reducing the noxious content of the gases is to force the gases through a substrate that: 1. oxidises the gases 2. separates the particulate matter from the gas. The substrate thus acts as a Flow Through Catalyst carrier (FTC) or as a Wall Flow Filter (WFF). In both cases the harmful emission is reduced before the exhaust gas is released into the atmosphere.

The most commonly used "type of diesel particulate filters is the extruded Wall Flow Filter since these filters combine good filtration efficiency and a large filtration area per unit volume. The WFF is manufactured by Corning Inc. in the USA under the registered trade name "CelCor".

Other filter materials such as corrugated mullite fibre mat from Panasonic (SAE paper 860010), coated Cordierite (Ceramem coating on Corning substrates) and ceramic foam (manufactured by Alusuisse-Lonza, SAE paper 910325) have also shown to have good properties as diesel particulate filters.

The most used type of flow through catalyst carrier support is the extruded monolith, since these substrates combine a small frontal face and a large area per unit volume. They are manufactured by Corning Inc. in the USA under the registered trade name "CelCor".

A commonly used ceramic material for diesel filters or catalyst carriers is the low thermal expansion material Cordierite (Mg₂Al₄Si₅0₁₈). This is, for example, manufactured by extrusion by Corning in the USA and by NGK in Japan. This substrate is known to have a very low friction coefficient on the mounting surface (i.e. a very smooth surface). This monolith exhibits important properties as low thermal conductivity < 0,5 W/mK, mechanical strength < 4 MPa and a melting point around 1260'C. Cast Cordierite catalyst carriers are manufactured by American Lava Inc. in the U.S.A.

Other important monoliths and WFF are monoliths manufactured of silicon carbide according to US Patent No. 5.195.319 and WO 94/22656 by NoTox Corporation in Denmark. This monolith exhibits important properties as high thermal conductivity > 7 W/mK, very high mechanical strength > 30 MPa and a high melting point > 1600°C. All ceramic substrates (catalyst support or diesel filter) must be encapsulated in a steel container or "canister" in order to be an integrated part of the exhaust system of the vehicle. The rigid steel container wall and the brittle ceramic substrate outside wall are not suited to work in direct contact with each other because of different coefficients of thermal expansion and mechanical strength. Because of the high temperatures in the exhaust gases, the different coefficients of thermal expansion will either cause a rapid destruction of the ceramic substrate or - after a longer period of time - eventually cause erosion or cracks of the ceramic substrate or the steel container. The erosion or destruction will degrade the function of the ceramic substrate and will result in a lower filtration efficiency in the case of a diesel particulate filter or in a lower catalyst area and, hence, lower catalytic conversion of the harmful exhaust gases. In the case of a catalyst carrier, destruction of the ceramic substrate will lead to an increased back pressure, hence a higher fuel consumption.

To meet the problems caused by the different thermal expansion coefficients (α's) (These are, for example, for stainless steels: 18*10^_6/K for AISI 316 and 12*10^'6/ for AISI 409, while for ceramic materials they are: 1*10^"6/K for Cordierite and app. 4*10^"6/K for SiC and Mullite), flexible material is used as the interface between the ceramic substrate and the steel canister.

Different materials can be used as the flexible interface in the annular space between the ceramic substrate and the steel container.

The materials normally used include "Interam"® manufactured by 3M Inc. (flexible ceramic mat containing an intumescent agent), steel wire wrap manufactured by Catalytic Support Systems Ltd. (flexible mat woven from thin steel wires), pure alumina insulation felt with the trade name "Saffil" from ICI Ltd. in the UK and "FiberFrax" felts from Carborundum Co., U.S.A., which are based on 1-10 μm thick melt-spun or melt-blown fibres of spun aluminum oxide or a combination of Si0₂ or A1₂0₃. The thickness of the blanket or mat is generally around 2-20 mm or 5-10 mm under compression.

The most commonly used interface material is the above-mentioned "Interam"® which is described in US Patent No. 3,916,057, US Patent No. 4,365,922 and US Patent No. 4,385,135). The refractory fibrous alumina silicate ceramic material incorporates the flake mineral Vermiculite as the special feature which ensures an expansion of the mat during heating. The Vermiculite acts as an intumescing agent.

When the expansion (up to 170 % of original thickness, depending on temperature and pressure) happens, the mat compresses the ceramic substrate inside the container and also compensates for manufacturing tolerances. The pressure, and thus the mechanical stress on the ceramic substrate from the mat, increases due to the intumescent effect of the Vermiculite. According to the technical description of the "Interam"® from 3M, the pressure increases up to a factor of 10 depending on the initial pressure and on the temperature. Gas sealing provided by the "Interam"® mat is very good. The thickness of Interam made for this kind of interface is usually between 2 and 8 mm.

A metallic interface, called Knitted Crimped Wire Wrap, is well known and often manufactured by knitting a relatively thin metal wire (0.05-0.2 mm thick) into a clothlike material with an un¬ compressed thickness of 2-15 mm, as it is done, for example, at the UK based company Catalytic Support Systems Ltd. This extremely porous and highly flexible material is compressed slightly during the canning process and does not show any extra additional expansion during heating. The bending strength and the Young's and shear moduli of the metal wire give the necessary canning pressure. This material is usually resilient.

A problem with the knitted wire mesh is the gas sealing. This can be solved by incorporating a band of "Interam"® or refractory fibre material at the ends or in the middle of the wrap assembly. This gas sealing band may be 10-80% wide, such as 20-40% wide, of the total length of the wrap assembly.

On all Cordierite/CelCor substrates the surface is slightly corrugated in the longitudinal direction due to the manufacturing process. Corning has done experiments in order to make this surface even smoother by applying a smooth layer on the surface of the substrates (SAE paper 932663). The reason for making the surface smoother is that it increases the bidirectional compressive strength which is of importance during the canning procedure. The results in the SAE paper do not indicate any significant improvement of the strength, but the lower friction coefficient makes it possible for the substrate to slide relative to the mat. This sliding will, however, cause problems in some cases as the differential pressure over the substrate (the back pressure) increases during the application in either the catalyst support where the back pressure increases with higher flow rates and higher temperatures of the exhaust gas or in the diesel filter, where the back pressure increases due to the collection of soot particles on the filter. The increased back pressure tends to move the substrate down stream inside the steel container. Acceleration of the vehicle and transfer of G forces to the substrate must also be taken into account. To prevent the sliding of the ceramic substrate inside the mat, a relatively high canning pressure is needed.

Even for smooth ceramic substrates, the friction coefficient between the flexible material and the steel container is lower compared to the one between the ceramic substrate and the flexible interface. Therefore, it is necessary to mount so called L-rings or ring-lands inside the steel container, at each canister end, towards the flexible material. This is an effective way of holding the flexible material in place. A corrugation of the steel container is also known to keep the interface in place.

The metal ring-lands may also be made so wide, that they, at the same time, also hold the ceramic substrate in the axial direction. This is especially true for WFF diesel filters. In this case a ring of flexible material is mounted between the metal L-ring and the ceramic substrate. In this case, the ceramic substrate is manufactured with a 10 mm wide band, a so-called "dead ring area", on the inlet face and the outlet face of the substrate.

This is a major disadvantage of the known WFF substrates because it reduces the active filtration area dramatically. For example, the "dead ring area" with a loss of the outermost 10 mm of a 200 mm diameter substrate will decrease the effective filtration area by as much as 19%. For a 300 mm diameter substrate, the lost filtration area is approximately 13 % for the same length of approximately 200 mm.

It is seen on catalytic converters for automobiles that the ring-land used to prevent axial movement of the monolith reduces the frontal face area, and by this also the active surface area of the system. Alternatively, the canister may be corrugated in order to increase the grip at the interface.

The required pressure applied during the encapsulation ("the canning") of the ceramic substrate is determined by a lower and a higher limit. The lower limit is determined from the axial force on the substrate. The friction between the ceramic substrate and the flexible material must be so large that the substrate is not able to move in the can during use in the exhaust system. This limit is given in the following formula (SAE paper 840074)

F_z == / J π x D x π x τ dz = π / 4 x μ x pmin x D x L

or

A-^ = 4 _z / π x μ x D x L

Here, F_z is the axial force exerted on the substrate due to differential pressure and acceleration of the vehicle. D is the substrate diameter. L is the length of the interface between the substrate and the flexible material, μ is the friction coefficient (in this case a constant).

The higher limit for the canning pressure is given by a combination of : - the compression strength - the tensile strength of the ceramic substrate - the shear modulus of the flexible material

- the thermal expansion coefficients of the ceramic substrate and the steel container

If the applied canning pressure is too low, the ceramic substrate will be moved or pressed against the end cone of the steel container due to the exerted axial forces. This will lead to erosion and destruction of the ceramic material. If the canning pressure is too high several things can happen. If it increases the compressive loading on the ceramic substrate, which will be crushed and destroyed. If the pressure is lower than the compressive strength but higher than a certain limit, the flexible interface is not flexible enough with respect to the shear stresses, and this will lead to too high tensile stresses due to the different thermal expansion coefficients of the steel container and the substrate. The expansion of the steel container is the same all over the total length of the substrate. The expansion of the substrate is considerably lower, often a factor of 10 in comparison to the steel. The high axial tensile stresses thus applied on the ceramic substrate will lead to the so-called "ring-off" cracks. In the case of a diesel particulate filter, the "ring-off cracks will give a deterioration in the filtration efficiency.

The risk of "ring-off cracks can be reduced by use of low expansion steel container material. This is based on high temperature resistant steels with high Cr and low Ni content.

18*10^"6/K expansion for AISI 304 with high Ni content 12*10"⁶/K expansion for AISI 409 with low Ni content

The total expansion of the improved steel material AISI 409 is reduced to 2/3 of that of AISI 304/316.

Thermal insulation is of major impr .tance in order to keep the steel container at a suitable temperature which will reduce the expansion and the differences in expansion between the substrate and the container. It is an advantage to increase the insulation interface thickness or the insulation value. This, however, has the drawback of increased cost and canister diameter.

This invention concerns the modification of the friction between the substrate and the flexible interface between the substrate and the steel container by modifying the monolith substrate outer surface.

Increased roughness of the outer surface of a ceramic substrate is a major advantage in the application of ceramic substrates - slip-cast or extruded - for diesel particulate filters and catalyst carriers. An increased roughness, hence increased friction coefficient, will allow the interface a better grip of the monolith and lead to a lower canning or encapsulation pressure necessary to hold the ceramic substrate during handling and use. The lower the necessary pressure is, the lower the mechanical load i.e. mechanical stress on the ceramic substrate will be. This will improve the integrity and the durability of the ceramic substrate. Specifically, the ceramic catalyst carrier development shows a higher number of thinner walls and hence lower mechanical strength.

If the flexible material, due to the higher friction coefficient of the ceramic substrate, is able to hold the ceramic substrate in the axial direction even with much lower canning pressures, then the so-called ring-lands on the diesel filter substrates can be completely avoided. An extra advantage of this new canning technique is a much better ratio of filtration area per unit volume of the filter canister, as the L-ring often used to ensure no axial movement is eliminated.

One important feature of this invention is the great improvement of the WFF effective filtration area as no "dead" ring-land is needed to secure the substrate in the axial direction.

Thus, an aspect of the invention relates to an after-treatment device for exhaust gas from a combustion engine, comprising

a ceramic honeycomb monolith body at least part of which defines a cylindrical shape having a circumference surface,

a flexible interface material covering at least part of the circumference surface of the monolith body,

and a container encapsulating the filter body and the interface material, the container having an inlet adapted to be connected to an exhaust duct from the combustion engine and an outlet for exhaust gas which has passed through monolith,

the circumference surface of the honeycomb monolith having a domain or domains which has/have been modified to obtain an increased roughness in order to increase the friction towards the interface material.

In the present context, the term "cylindrical" is to be understood in its generally accepted broad sense as defined, e.g., in Websters Encyclopedic Unabridged Dictionary of the English Language, Portland House, New York, 1989, that is, the cross-section of the cylinder is not necessarily circular, although it often will be. Other cross-sections used in the automotive industry are, e.g. ellipsoidal (race track) or unsymmetrical. While the term "cylindrical" indicates that the ends of the body are normally parallel and (as in a right cylinder) at a right angle to the circumferential surface, this is not necessarily the case in the bodies according to the invention, but certainly the most common shape. The term "monolith", as used about the cylindrical body, indicates that the body appears as one piece, but does not rule out that the body is constituted by a number of segments, such as is explained below. The term "honeycomb" is used in the same meaning as it is conventionally used in the art: it indicates that the monolith body has a number of symmetrical, parallel adjacent channels extending in the longitudinal direction of the monolith body, see also Figures 10 and 11.

The increased roughness is normally a roughness of at least 1.5 times the roughness of the corresponding non-modified surface (which is often defined as the circumferential surface of the extruded and heat-treated body), more often at least 2 times the roughness of the corresponding non-modified surface.

The increased roughness can also be defined with reference to a DIN norm, that is, as roughness of 100-6000 μm, as determined as R_z according to DIN 4766, preferably 200-4000 μm and more preferably a roughness of 500-2000 μm, as determined as R_z according to DIN 4766.

The surface domain or domains having increased roughness may have any desired shape and extension and may be coherent domains or discontinuous patterns such as dot patterns. In preferred embodiments, they will often be a band or bands extending on the circumference surface of the monolith, either in the axial direction, or, more often, perpendicular thereto, or, in an intermediate direction.

The roughness may suitably be constituted by surface parts extending upwardly from the remaining surface, such as refractory particles bound to and optionally embedded in the surface. The particles will normally be particles of the same ceramic material as constitutes the monolith, but it is, of course, also possible to use particles of a different ceramic material. As examples of types of refractory particles may be mentioned particles of silicon carbide, Cordierite, Corundum, Alumina, Silicon Nitride or a material blended of components selected from the groups I, II, III, rV, V, VI, VII, or Vπi of the elements. However, the use of the same ceramic material as constitutes the monolith will facilitate the effective production in that it permits simple conversion of the "green" monolith body with the particles on the surface to refractory bodies with the particles on the surface by heat treatment. The particles used for this purpose will normally have a size in the range of Mesh 8 to Mesh 220, such as Mesh 20-120, preferably Mesh 30-60.

The invention also relates to a ceramic honeycomb monolith body for use in a device as defined above, and to methods for producing such a body. One such method comprises producing a "green" ceramic honeycomb monolith body, applying, to a domain or domains of the surface of the green body, ceramic particles in such a manner that they become bound to and optionally embedded in the surface and extend upwardly from the surface, and heat treating the particle- carrying monolith body to convert it to a refractory body with the particles ceramically bound to the surface.

A suitable way of binding the particles to the surface is by first applying a layer of a slurry containing fine ceramic particles to the surface, and then applying coarser particles to the slurry layer, e.g. by sprinkling. In the later heat treatment, the slurry will be converted to contact "glue" points binding the particles to the surface. Often, an improved binding of the particles to the surface is obtained by applying an extra layer of slurry on top of the particles. It is also possible to press the particles into the surface of the green monolith body, whereby they will be permanently bonded during the heat treatment; however, it is often preferred to combine the pressing procedure with a simultaneous or subsequent application of a slurry as described above.

Another method for producing a ceramic honeycomb monolith body according to the invention is to produce a green ceramic honeycomb monolith body, create upwardly extending surface flaws in a domain or domains of the circumferential surface thereof, and heat treat the particle- carrying monolith body to convert it to a refractory body. The surface flaws may, e.g., be created by means of a needle or sprocket wheel.

As different strategy for producing a ceramic honeycomb monolith body according to the invention comprises cutting slices into the circumference surface of the body to thereby estabhsh a corrugated or roughened "landscape" or environment in all or part of the circumferential surface.

In another aspect of the invention, the improved fixation of the monolith is obtained by increasing the friction between the inside of the container and the interface material. This aspect, which can, of course, be combined with the aspect where the roughening modification is a modification of the monolith surface, can be defined as an after-treatment device for exhaust gas from a combustion engine, comprising

and a metal container encapsulating the filter body and the interface material, the container Laving an inlet adapted to be connected to an exhaust duct from the combustion engine and an outlet for exhaust gas which has passed through monolith,

the inside surface of the metal container having a domain or domains which has/have been modified to obtain an increased roughness in order to increase the friction towards the interface material.

As will be described in greater detail in the following, the increased roughness can be obtained by applying a glaze or enamel to the inside surface of the container, coarse refractory particles conferring an increased roughness being incorporated in the glaze or enamel or applied thereto, and then heat treating.

Another way of improving the friction between the inside of the container and the interface material is to apply a foil or tape carrying a glaze or enamel with coarse refractory particles (Mesh 8-220) embedded in the glaze or enamel to the outside of the interface material prior to encapsulation so that the coarse particles will increase the friction between the interface material and the metal container. In the use of the device, the high temperature conditions will burn away any organic material in the tape, and the particles will become entrapped/adhered between the interface material and the inside of the container.

As will be understood, the tape of foil application technique may also be used to apply roughening particles to the domain between the interface material and the monolith surface, either by applying the tape or foil to the monolith surface or by applying the tape or foil to interior surface of the interface material. Thus, an important tool for obtaining the roughening effect according to the invention is constituted by a foil or tape carrying a glaze or enamel with coarse refractory particles embedded in the glaze or enamel. The particles will normally be Mesh 8-220, usually Mesh 20-80, preferably Mesh 30-60, particles. It is, of course, practical for the application of the tape that it carries an adhesive layer on the side not carrying the glaze or enamel.

In the following, the invention will be described in greater detail with reference to the figures and working examples.

FIGURES

Figure 1 shows a longitudinal section of a "canned" monolith. In Figure 1, the canned monolith is άrcular-cynndrical, but it is evident that also either shapes such as ellipsoidal-cylindrical

("race track") or non-symmetric cross sections are possible. A ceramic monolith 1 is encapsulated in a cylindrical canister 2, with a flexible interface material such a Interam, 3 between the container metal and the monohth surface. Figure 1 can be used both to illustrate the known art and the present invention. According to the preferred method of mounting a monohth 1 in its canister, the diameter of the monohth is also the effective diameter D_eff. However, as the known monoliths have smooth and low friction outside surfaces, the mounting pressure (symbolized by P in figure 1) must necessarily be high. The steel container or canister 2 transfers the high mounting pressure forces through a flexible interface 3. This often results in a "ring crack". However, by supplying according to the invention, the monohth surface 5 with a friction area of higher roughness, the much better grip is obtained between the interface material and the monohth, whereby a lower canning pressure can be used. This makes it possible to utilize the optimal mounting method where the diameter of the monohth is also the effective diameter. According to a further feature of the present invention a friction increasing modification may also be applied on the inside 4 of the canister, thereby further enhancing the fixation of the monohth by enhancing the fixation between the flexible interface material and steel the canister.

Figure 2 illustrates a method commonly used in the prior art when canning a monohth. Here "L-rings" 1 and an extra ring 2 normally of a flexible material such as a wire mesh ring are mounted to take up any axial movement of the monohth. The drawback is the reduced effective diameter D_efa of the monohth. The reduction is often in a range of 0 10-25 mm.

Figure 3 is a cross section illustrating the general build up of the after-treatment devices as commonly used with the monohth 1 in the center and, in this case, two layer of interface material 2 all the way around the monohth. The outer encapsulation (canister) is constituted by a steel plate 3.

In Figure 4, a monohth 2 with a friction band 1 apphed to the monohth surface, i.e. described in Example 1 or 2, is mounted in a steel container or canister 6 with flexible interface material 5. The areas 3 of the monohth surface are smooth. While the interface material and the monohth are fixed relative to each other by means of the friction band 1, the interface material is fixed relative to the canister 6 by means of "L-rings" 4. As it will be noted, this construction makes it possible to utilize the full effective diameter of the monohth.

Figure 5 is a cross section of the device shown in Figure 4, showing the monohth 2 in the center surrounded by the interface material 3 and encapsulated by the container or canister 6. The essential friction band is shown at 1.

Figure 6 shows a section according to A-A in Figure 5, but in a slightly modified version where the container or canister 6 is provided with a groove 7, thereby fixing the interface material 5 relative to the container or canister 6. In this embodiment, no "L-ring" is needed, and the effective diameter is still the full diameter of the monohth. Figures 7-9 illustrate measurements of the improvements in friction obtained according to the invention and are discussed in greater detail further below.

Figure 10 illustrates an alternative design where the monolith is assembled from four identical segments 1, 2, 3, 4 each with two flat sides and one circular arc sides constituting a 90°C angle of the total circle. On the circular arc segments, a band 5 of high friction material has been apphed during the manufacturing process in accordance with the techniques discussed herein. As another alternative design, the unit may be assemble from a number of individual segments around a circular central segment. The surfaces pointing towards the interstice between adjacent segments may also be provided with friction-increasing areas in accordance with the techniques described herein.

Figure 11 illustrates a typical honeycomb monolith body 1 with a friction zone 2. The honeycomb structure, comprising a number of symmetrical adjacent cells extending in the longitudinal direction of the monolith, can be identified at 3.

EXAMPLE 1

Silicon carbide powder technology substrates were manufactured using a continuous extrusion process. The compound was composed of 66 wt% commercially available, large size Mesh 180 SIKA I grinding grain with particle size 55-75 μm and 13 wt% ultra fine SiC FCP 10-S, both from Arendal in Norway, mixed into a plastic paste composed of 5 wt% methyl cellulose (Tylose MH 300 P from Hoechst), 9 wt% water and 7 wt% ethanol. The compound was extruded in a water cooled single screw auger extruder with vacuum chamber through a honeycomb die head. The extrusion speed was 1.5 meter per minute.

After extruding, drying was performed in a controlled humidity starting at 95% and reduced to 65% relative humidity over approximately 4 days. In a similar manner and over the same period, the temperature was reduced from 80°C to 20^CC.

After drying, the green substrates were cut clean at each end in order to obtain the exact desired length of 250 mm and to prepare for channel closing.

After channel closing according to WO 94/22656 (PCT/DK94/00140), the substrates were painted with a slurry on the area where the friction band was intended to be created. The slurry was based on water with 10% sub-micron SiC powder (FCP 10-S, see above) and 3% methyl- hydroxy-ethyl cellulose (Tylose, see above). The slurry had high viscosity and was easily applied to the monolith surface using a brush (spraying would have been another useful method). The layer thickness was app. 0.5 mm. A layer of SiC grain Mesh 60 was sprinkled on the wet slurry surface. After air drying for one day, (or forced drying by being heated to 80°C for one hour for some of the samples), further painting with the same slurry as above was performed to soak the coarse grains with the slurry and thereby improve the adhesion of the individual grains to the surface. After air drying for one day (or forced drying by heating to 80^CC for one hour), the monoliths were ready for sintering.

(The Mesh values stated herein are according to EU standard FEPA or US standard ANSI B:74.12-1976.)

After a very high temperature sintering process, above 2000°C, in a protective atmosphere (argon; nitrogen and helium are other possibilities), the structure became a low density, rigid and highly porous filter element with an integrated friction band on selected areas.

The SiC (silicon carbide) based filter had an extremely high thermal conductivity (11 W/mK) and a very homogeneous and controlled pore size and distribution, measured to be around 15 μm.

By this new technique, on all the circumference of the monolith, a band, an area of high friction was sintered onto the otherwise smooth monolith surface. The high friction area was created by the large grains sintered to the monolith mounting surface. The width of the friction band was about 100 mm positioned in the middle of the length of the monolith.

EXAMPLE 2 A series of different oxide-based ceramic monohth substrates are manufactured from Cordierite, Spodumene and Mullite compositions by extrusion. The ceramic precursors are listed in Table l.Table 1. Ceramic precursors. wt%

Mix A Mix B Mix C

Cordierite Spodumene Mulhte

China Clay gr. E (APS 2-3) 40.4 65.8 51.5

Talc (APS 1-4) 43.6 - -

A1₂0₃ CT 3000 SG (APS 0.4-0.6) 16.0 - 48.5

Si0₂ Fyleverken (APS 3-6) - 15.3 -

Li₂C0₃ anal, quality - - 18.9 In the case of Cordierite and Spodumene ceramics the precursors are calcined/sintered to a grog and crushed into a coarse grained partly porous powder with a particle size similar to FEPA Mesh 180. As binder/plasticiser, a methyl-hydroxy-ethyl cellulose is used (Tylose MH 300 P from Hoechst). The green body compounds are composed according to Table 2 and mixed dry for 30 minutes Ethanol is added and after another 10 min of mixing, the water is introduced. Another 30 minutes of mixing remains.

Table 2. Green body compounds.

Cordierite Spodumene Mullit-

wt% wt% wt%

Mix A 46.5 - -

Mix B - 48.5 -

Mix C - - 73.7

Tylose 8.6 9.0 0.7

Water 15.7 11.1 19.0

Ethanol 29.2 31.4 6.7

Filler/Compound 56-64 58-62 58-64

(vol/vol)

Sintering temp. ^CC 1340 1270 1400

Linear shrinkage % 5.4 6.7 7.1

The compound is extruded in a single screw auger extruder with vacuum chamber through a honeycomb die head. The extruded bodies are dried at ambient temperature and controlled humidity.

After drying, the substrates were painted with a slurry on the area where the friction band is intended to be created. The slurry was prepared analogously to the slurry described in example 1, but sub-micron powder of the same ceramic material as the bodies instead of the sub-micron SiC powder. The layer thickness is of approximately 0.5 mm. A layer of Mesh 60 coarse grain of the corresponding ceramic material as the material of monolith is sprinkled on the wet slurry surface. The friction area is dried and supplied with another layer of slurry analogously to Example 1. After further drying, the monoliths are sintered in an electric furnace under the conditions stated in table 2.

The results and structures are a low density, rigid and highly porous filter elements useful as monoliths for exhaust gas after-treatment.

By this new technique, on all the circumference of the monolith, a band, an area of high friction was sintered onto the otherwise smooth monohth surface. The high friction area was created by the large grains sintered to the monolith mounting surface. The width of the friction band was about 100 mm positioned in the middle of the length of the monohth.

EXAMPLE 3

Flow Through Catalyst carriers manufactured from Cordierite according to US patent 3,790,654 and others will benefit from this invention as tolerance to vibration on the vehicle and in Hot Shaking Tests is greatly improved.

The Cordierite compound is extruded in a screw auger extruder with vacuum chamber through a honeycomb die head. The extruded bodies are dried at ambient temperature and controlled humidity or alternatively with micro-wave heating. After drying, the substrates were painted with a slurry on the area where the friction band is intended to be created. The slurry was prepared analogously to the slurry described in example 1, but sub-micron powder of the same ceramic material as the bodies instead of the sub-micron SiC powder. The layer thickness is of approximately 0.5 mm. A layer of Mesh 60 coarse grain of the corresponding ceramic material as the material of monolith is sprinkled on the wet slurry surface. The friction area was dried and supplied with another layer of slurry analogously to Example 1.

Sintering is performed in electrically heated batch furnaces at app. 1500'C in controlled atmosphere according to Table 2. Alternatively, sintering may take place in a gas fired furnace with a controlled composition of gasses.

The structure becomes a low density, rigid and low porosity monolith.

By this new technique, on all the circumference of the monolith, a band, an area of high friction was sintered onto the otherwise smooth monolith surface. The high friction area was created by the large grains sintered to the monohth mounting surface. The width of the friction band was about 100 mm positioned in the middle of the length of the monolith. EXAMPLE 4

Green bodies are made of SiC or Cordierite, Spodumene or Mullite as described in any of the Examples 1-3. On these green bodies, a friction area of high roughness is created by means of a tool (such as a needle wheel or sprocket wheel) that cuts of the otherwise smooth surface into approximately 2.0 mm high flaws, positioned at a distance of between e.g. 5-15 mm from each other. The flaws are preferably applied by moving the wheel in a longitudinal direction but may, in principal, be applied by moving the tool in any angle to the longitudinal direction of the monolith body.

The flaws may be applied in a friction band around the monolith or, preferably, over the whole circumference area of the monolith.

After the creation of the flaws, the green bodies are sintered in the same manner as described in the respective Examples 1-3.

EXAMPLE 5

An attractive option for applying a friction zone or band on monoliths which have been sintered at high temperature is to use a refractory cement as the "glue" with which the coarse grains creating the roughness are attached to the monohth surface.

Once such suitable cement material is Saureriesen No. 8, from Saureriesen, U.S.A. This is a phosphate-bonded Zircon-based cement available as a ready for use powder mix which is to be admixed with 14 g of water per 100 g of the dry powder. The resulting cement composition has been painted on 250 mm long SiC monoliths prepared otherwise as described in Example 1, thereby creating a layer of a thickness of about 0.3 mm of this very fine grained cement composition. Coarse SiC grains of the same type as in Example 1 was sprinkled into this wet cement layer, and the cement layer was allowed to dry (about 1 day at room temperature or about 2 hours at 50°C). In this manner, excellent friction zones were obtained on the already sintered monoliths.

Some of the samples were subjected to calcination at 1100°C for about 1 hour. Thereby, the resistance of the cement binder against water vapour was improved.

Other cements of suitable compositions may also be use, but the reason why A Zircon-based cement was selected as a preferred cement is that its thermal expansion coefficient of Zircon corresponds closely to thermal expansion coefficient of sintered SiC bodies. Alternatively, a glaze may be a suitable binder for use on presintered monolith. Examples of suitable glazes are described in Examples 8 and 9.

EXAMPLE 6

As an alternative to application of the coarse grains in a slurry applied on the green bodies, the coarse grains may be "pressed" into the surface of green bodies before drying.

Thus, into the surface of the green bodies prepared according to any of Examples 1-3, Mesh 60 coarse particles of the same material as the same material of the bodies are pressed into the circumference surface of the newly extruded and elastic monolith by means of a rubber roller.

After calcination or sintering, the coarse grains are bonded to the monolith surface.

The bond may be improved by applying a thin layer (less than 0.5 mm) of the slurry mentioned in the respective Examples 1-3 on the friction area, either simultaneous with or after the coarse grains have been pressed into the monohth surface. In stead of the slurry, a ceramic paste, fluid or glaze may be employed as to act as a "glue" for enhancing the fixation for the friction part of particles.

The rubber roller has a shore hardness which is sufficiently small to ensure that only minor or no destruction by penetration occurs on the inside of the honeycomb channels adjacent to the circumference.

EXAMPLE 7

Another interesting technique is to cut slices, e.g. circumferential groups or shorter groups, preferably in a direction having a circumferential component, in the surface or in the circumferential surface of the "green bodies" or the sintered bodies. The groups will interact with a flexible interface material to fix the interface material relative to monolith in the longitudinal direction, thereby preventing actual movement of the monolith in the canister. The slices or grooves may be cut to a depth of 0.1-20 mm, preferably 0.3-1 mm, and with a width of e.g. 0.1- 20 mm, preferably 2-10 mm such as. The slice or groove angle from actual flow angle may be between 10 and 170°, such as 45-135°, preferably 90°. The length of the slices may be endless all around the monolith or the length may be decided by the cutting tool. The slices may be cut symmetrically or non-symmetrically on the monohth surface.

The flexible interface material used in connection with this type of friction enhancement is preferably a thread- or fiber-based material such as a wire mesh ceramic fiber insulation mat. INTRODUCTION TO EXAMPLES 8 AND 9

A special technique for applying a roughness-increasing domain on the metal container inside (and, as is explained herein, for generally as an alternative to the above-described techniques) comprises the use of a glaze or enamel. The basic principle of this is as follows:

A mix of enamel/glaze, recrystallization agent and coarse grained refractories is applied on the inner surface of the canning container in order to increase the friction coefficient between the steel surface and the insulating material.

A band of the applied mix has a width that can be adjusted during the application of the mix. The width of the applied mix is normally 20-80 % of the total length of the steel container.

When the mix applied to the inner surface of the steel container is heated for the first time, the enamel melts and binds the coarse grained refractory particles to the steel surface.

After prolonged heating (either by the use of a furnace or by using the hot exhaust gas from the vehicle's engine) the recrystallization agent reacts with the enamel resulting in:

a) crystallization of refractory phases in the glaze b) reduced amount of melt in the enamel c) increase in viscosity of the melt d) enhanced binding between the coarse refractory grains, enamel/glaze and steel

For the application of the mix to the inner surface of the steel container, a temporary organic binder is necessary. This binder can either be a water based polymer or a glue/adhesive based on, for example, polyurethane. The temporary organic binder is decomposed on heating to a temperature of above 200-300°C. This temperature is lower than the temperature of the exhaust gas of the vehicle.

After removal of the temporary organic binder, the composition of the mix is usually in the range:

20-50 wt% of fritted enamel/glaze

15-35 wt% recrystallization agent (Halloysite, kaolinite, zirconia, titania)

10-50 wt% coarse grained refractory particles (SiC, quartz, corundum, zirconia)

The resulting mix increases the friction coefficient between the insulating material and the inner surface of the steel container of a magnitude of more than a factor 2. EXAMPLE 8

An enamel/glaze was made from the constituents stated in Table 3:

Table 3

Oxide Wt% Raw materials

Si0₂ 50 Quartz, Clay, Feldspar

A1₂0₃ 2 Clay

B₂0₃ 28 Borax

CaO 4 Carbonate

KsO 2 Feldspar

Na₂0 11 Borax, Feldspar

NiO 0.5 Pure oxides

CoO 0.5 Pure oxides

Mn₂0₃ 2 Pure oxides

Sum 100

The ingredients of the enamel/glaze were mixed dry using a bakery blender and fritted at 1200°C in an electrically heated surface in air. The resulting frit was ground to a mean grain size of approximately 10 μm.

1 kg of fritted and ground enamel/glaze was mixed with 650 g of kaolinite, 1 kg SiC grain Mesh 60 and 200 g Tylose MH 300 P using a bakery blender. After mixing for 10 minutes, 3500 g water and 1000 g water with 10 wt% Poly(vinylalcohol) (from Bie & Berntsen, Denmark) was added and the mixing was continued for further 15 minutes.

Approximately 75 g of the resulting friction mix was sprayed on the inner surface of a steel container in a 100 mm wide band. The steel container was a rolled from a 1.5 mm thick, 300 mm wide and 700 mm long AISI 304 stainless steel sheet. The glaze band thickness was approximately 0.75 mm.

After drying at 80°C for 30 minutes, a ceramic monolith manufactured as described in Example 2 with a diameter of 190 mm was packed with 6.4 mm thick Interam® type III mat from 3M around the substrate. The surrounding steel container was welded and the whole unit was placed in a electrically heated furnace at more than 700°C for 60 minutes in order to expand the Interam^® mat and to melt the enamel/glaze to bind the coarse grained SiC particles to the steel sheet.

The friction band on the interior surface of the steel container increased the friction coefficient of the steel surface towards the Interam® mat from 0.35 to 0.8.

EXAMPLE 9

1.5 kg of enamel/glaze produced as described in Example 8 was mixed with 1 kg kaohnite and 250 g Tylose MH 300 P in a bakery blender. After mixing for 10 minutes, 4000 g water was added and the mixing continued for further 15 minutes.

The mix was apphed on a 100 mm wide roll of 0.2 mm thick paper using a doctor blade method. In this continuous procedure, the mix was cast to a layer thickness of approximately 2 mm on the rolling sheet of paper. A sharp blade placed at a fixed position above the rolling paper ensured a wet layer thickness of approximately 0.5 mm.

On the paper side covered with wet glaze, sprinkling of SiC Mesh 60 coarse grain was performed followed by drying in a oven at 110 degree for 15 minutes. The reverse paper side was coated with a sticky organic tackifying agent. The resulting tape was mounted on the inside of a steel sheet container of the same type as described in Example 8.

A ceramic monohth of the same type as described in Example 8 was mounted in the steel container. The steel container was welded, and the whole unit was placed in a electrically heated furnace at 700°C for 60 minutes in order to expand the Interam mat and to melt the enamel/glaze to bind the coarse grained SiC particles to the steel sheet.

The increase in friction coefficient was the same as described in Example 8.

DETAILED DISCUSSION OF FIGURES 7, 8 AND 9

Figure 7 shows friction coefficients measured between a Interam® insulating mat and various sample surfaces. In order to examine the effect of the increased friction coefficient between the ceramic monohth and the insulating mat, measurements of the friction force between sample surfaces and a Interam® type HI mat was performed.

A 5 cm wide and 10 cm long piece of the Interam® mat was placed on the sample surface of with a 7 kg load applied evenly over the whole mat area.

The force necessary to make the Interam® mat with applied load slip relative to the sample surface was measured. This force is called the friction force. The friction coefficient was calculated as the friction force divided with the apphed loading force. An average from 10 measurements was used as result.

This measuring procedure was performed with the Interam® mat and six different sample surfaces. These were:

1) Surface of a 250 mm long, 190 mm diameter extruded, dried and sintered ceramic filter as described in example 1 without applying a friction band. This sample surface is designated "NoTox WFF Untreated".

2) Same surface as described as in 1) with a 100 mm wide friction band manufactured according to example 5 using SiC grains Mesh 60 as the coarse grains. This surface is designated "SiC grain cement bond".

3) Surface as described in 1) where the friction band manufactured according to example 1 is applied in full length. This surface is designated "SiC grain/SiC bond full length".

4) Surface of a untreated Cordierite FTC substrate. This surface is designated "CelCor FTC standard".

5) Surface of a Cordierite diesel filter with a smoothed surface. This surface is designated "CelCor WFF smooth surface".

6) Surface of stainless steel, AISI 304.

The resulting friction coefficients between the Interam® III mat and the sample surfaces are plotted in Figure 7.

Figure 8 shows friction coefficients measured between a coarse grade knitted stainless steel wire mesh and various sample surfaces.

The same experiment as described in the text for Figure 7 was performed using a coarse grade stainless steel wire mesh knitted with 6 mm openings instead of the Interam® mat and the same sample surfaces as above. The resulting friction coefficients are shown in Figure 8.

It will be seen that the friction coefficient was increased from 0.8 to 1.5 or more by the application of a high friction band. Figure 9 shows friction coefficients measured between a coarse grade knitted stainless steel wire mesh and different sample surfaces. The same experiment as described in the text for figure 7 was performed using a medium grade stainless steel wire mesh knitted with 2.5 mm openings instead of the Interam mat.

In this case the sample surfaces were:

1) Surface of a 250 mm long, 190 mm diameter extruded, dried and sintered ceramic filter as described in example 1 with a friction band applied in full length of the filter using SiC grains Mesh 60 as the coarse grains. This sample surface is designated "SiC Grain cement bond full length".

2) Surface as described in 1) where the friction band manufactured according to Example

1 is applied in full length. This surface is designated "SiC grain/SiC bond full length".

3) Surface as described in 2) where the friction band is applied in 1/3 of full length. This surface is designated "SiC grain/SiC bond full length".

6) Surface of stainless steel, AISI 304.

The resulting friction coefficients are shown in Figure 9.

It will be seen that the friction coefficient is increased from 1.2 to 1.6 or more by the application of a high friction band.

Claims

1. An after-treatment device for exhaust gas from a combustion engine, comprising

a ceramic honeycomb monohth body at least part of which defines a cylindrical shape having a circumference surface,

a flexible interface material covering at least part of the circumference surface of the monohth body,

and a container encapsulating the monohth body and the interface material, the container having an inlet adapted to be connected to an exhaust duct from the combustion engine and an outlet for exhaust gas which has passed through monohth,

the circumference surface of the honeycomb monohth having a domain or domains which has/have been modified to obtain an increased roughness in order to increase the friction towards the interface material.

2. An after-treatment device according to claim 1, wherein the increased roughness is a roughness of at least 1.5 the roughness of the corresponding non-modified surface.

3. An after-treatment device according to claim 2, wherein the increased roughness is a roughness of at least 2 times the roughness of the corresponding non-modified surface.

4. An after-treatment device according to claim 1, wherein the increased roughness is a roughness of 100-6000 μm, as determined as R_z according to DIN 4766.

5. An after-treatment device according to claim 4, wherein the increased roughness is a roughness of 500-2000 μm, as determined as R_z according to DIN 4766.

6. An after-treatment device according to any of the preceding claims, in which the surface domain or domains having increased roughness is/are a band or bands extending on the circumference surface of the monohth.

7. An after-treatment device according to any of the preceding claims, in which the modification of the surface to increase the roughness is constituted by surface parts extending upwardly from the remaining surface.

8. An after-treatment device according to claim 7, wherein the surface parts extending upwardly from the remaining surface are constituted by particles bound to and optionally embedded in the surface.

9. An after-treatment device according to claim 8, wherein the particles are particles of the same ceramic material as constitutes the monohth.

10. A ceramic honeycomb monohth body at least part of which defines a cylindrical shape having a circumference surface, the body being adapted to be incorporated in an after-treatment device for exhaust gas from a combustion engine, the circumference surface of the honeycomb monolith body having a domain or domains which has/have been modified to obtain an increased roughness.

11. A ceramic honeycomb monohth body according to claim 10, wherein the increased roughness is a roughness of at least 1.5 times the roughness of the corresponding non-modified surface.

12. A ceramic honeycomb monohth body according to claim 11, wherein the increased roughness is a roughness of at least 2 times the roughness of the corresponding non-modified surface.

13. A ceramic honeycomb monolith body according to claim 10, wherein the increased roughness is a roughness of 100-6000 μm, as determined as R_z according to DIN 4766.

14. A ceramic honeycomb monohth body according to claim 13, wherein the increased roughness is a roughness of 500-2000 μm, as determined as R_z according to DIN 4766.

15. A ceramic honeycomb monohth body according to any of the claims 10-14, in which the surface domain or domains having increased roughness is/are a band or bands extending on the circumference surface of the monohth.

16. A ceramic honeycomb monohth body according to any of claims 10-15, in which the modification of the surface to increase the roughness is constituted by surface parts extending upwardly from the remaining surface.

17. A ceramic honeycomb monohth body according to claim 16, wherein the surface parts extending upwardly from the remaining surface are constituted by particles bound to and optionally embedded in the surface.

18. A ceramic honeycomb monolith body according to claim 17, wherein the particles are particles of the same ceramic material as constitutes the monolith.

19. A method for producing a ceramic honeycomb monohth body according to claim 10, comprising producing a green ceramic honeycomb monolith body, applying, to a domain or domains of the surface of the green body, ceramic particles in such a manner that they become bound to and optionally embedded in the surface and extend upwardly from the surface, and heat treating the particle-carrying monohth body to convert it to a refractory body with the particles ceramically bound to the surface.

20. A method according to claim 19, wherein a layer of a slurry containing fine ceramic particles is first apphed to the surface, and the coarser particles to become bound to the surface are apphed to the slurry layer.

21. A method for producing a ceramic honeycomb monohth body according to claim 10, comprising producing a green ceramic honeycomb monohth body, creating upwardly extending surface flaws in a domain or domains of the circumferential surface thereof, and heat treating the particle-carrying monohth body to convert it to a refractory body.

22. A method for producing a ceramic honeycomb monohth body according to claim 10, comprising cutting slices into the circumference surface of the body.

23. An after-treatment device for exhaust gas from a combustion engine, comprising

and a metal container encapsulating the filter body and the interface material, the container having an inlet adapted to be connected to an exhaust duct from the combustion engine and an outlet for exhaust gas which has passed through monohth,

24. An after-treatment device according to claim 23, wherein the increased roughness has been obtained by applying a glaze or enamel to the inside surface, coarse refractory particles conferring an increased roughness being incorporated in the glaze or enamel or apphed thereto, and then heat treating.

25. An after-treatment device according to claim 23, wherein the increased roughness has been obtained by applying a foil or tape carrying a glaze or enamel with coarse refractory particles (Mesh 8-220) embedded in the glaze or enamel to the outside of the interface material prior to encapsulation so that the coarse particles will increase the friction between the interface material and the metal container.

26. A foil or tape carrying a glaze or enamel with coarse refractory particles (Mesh 8-220) embedded in the glaze or enamel.

27. A device according to claim 25 or a foil or tape according to claim 26, wherein the particles are Mesh 20-80, preferably Mesh 30-60, particles.

28. A foil or tape according to claim 26 or 26, which carries an adhesive layer on the side not carrying the glaze or enamel.

29. A method for preparing a ceramic honeycomb monolith body according to claim 10, comprising applying a tape according to any of claims 26-28 on the monolith surface.