DE102011109034A1 - Ceramic body with variable porosity and method of manufacture - Google Patents

Abstract

An open-pore, porous ceramic body with inlet and outlet channels for guiding a gas flow, as well as porous walls separating the inlet and outlet channels, is provided, the body having an asymmetrical pore size distribution over the thickness of the walls, which can be produced in a single process step is.

Description

Technical area

The invention generally relates to ceramic bodies. In particular, the invention relates to open-pored ceramic bodies with variable porosity. Furthermore, the invention relates to a method for producing such ceramic bodies.

State of the art

Porous ceramic bodies are used in many fields of technology. A focus of their use is, for example, in the field of pollutant reduction of exhaust gases, especially in the field of automotive technology. Thus, such ceramic bodies are used as ceramic functional elements in the field of catalyst and / or particle filter technology, for example in the context of diesel particle filters (DPF). Such filters generally have a porous ceramic support material, which is usually composed of porous metallic and / or ceramic support materials, such as, for example, silicon carbide, cordierite, aluminum titanate or a sintered metal.

The ceramic bodies can be used in different structures, which depend on the nature of the exhaust gas purification. In particular, the ceramic bodies can be used in the context of a honeycomb structure. So is out of the DE 602 17 084 T2 a honeycomb structure having a plurality of honeycomb segments connected to a unitary body. The major component of each of the honeycomb segments comprises at least one of silicon carbide, silicon nitride, cordierite, alumina, mullite, zirconia, zirconium phosphate, aluminum titanate, titanium dioxide, and combinations thereof.

For use in filters, especially in DPFs, the body may, for example, have mutually closed channels whose walls have a certain porosity. The gas stream entering through the inlet channels is then forced to pass through the pores of the channel walls.

The porosity of the ceramic body thus represents an essential criterion for the stated fields of application. This porosity is produced partly in a natural way in the sintering process of the ceramic. An important example of this natural pore generation are silicate ceramics, such as cordierite ceramics.

Recently, ceramic filter elements based on aluminum titanate have become known which have suitable properties for use at high temperatures, such as. As vehicle exhaust control and diesel exhaust aftertreatment systems such as DPFs have. Aluminum titanate is the stoichiometric mixed phase of alumina and titania. It is characterized by a low thermal conductivity, a low modulus of elasticity and a macroscopically low thermal expansion coefficient and an associated thermal shock resistance.

Apart from the said "natural" porosity of the ceramics, the porosity can additionally be increased by adding pore formers to the raw materials, which are burned out during sintering. For this purpose, cheap organic substances, such as cellulose, carbon fibers, starch or waxes, are added to the ceramic powder before shaping and burnt out during the sintering process. Depending on the shape and size of the pore formers, pores remain in the ceramic structure.

Also known are ceramic papers which are produced by admixing inorganic raw materials during the papermaking process. Using such, mostly folded, ceramic papers porosities of about 80% with very low back pressure are possible. Also known are ceramic films which undergo a similar production process (film casting) as in papermaking.

A disadvantage of these known ceramic elements that, due to the manufacturing process, a targeted influencing of the porosity, in particular perpendicular to the pressing or production direction is difficult or impossible.

With DPFs, there are recent developments to produce a variable porosity by a subsequent coating process of the already fired component. For example, z. B. the DE 10 2006 040 739 A1 a filter for removing particles from a gas stream, in particular soot particles from an exhaust gas stream of an internal combustion engine, with a filter body made of a ceramic filter substrate, wherein the filter substrate is coated with a porous protective layer of a coating material. The coating material is selected from the group consisting of aluminas, aluminum hydroxides, titania, silica, zirconia, ceria, aluminosilicates, magnesium aluminosilicates, cordierite, mullites, silicon carbide, aluminum titanate, zeolites, quartz, glasses, mixtures and mixed oxides thereof. However, this method has the disadvantage that it offers only limited possibilities (no stepless gradients, only 2-3 gradations) and requires an additional, separate process step.

In addition to the production by plastic molding processes, such. B. extrusions, such ceramic body or filter devices are also prepared in such a way that a non-combustible carrier web impregnated with a ceramic slip and then a rigid filter body is formed. In Patent Abstracts of Japan JP 63134020 A For example, a ceramic filter element for an exhaust gas filter in an internal combustion engine is described, which is constructed from a corrugated, spirally wound filter web. To produce the filter web, heat-resistant, inorganic fibers are mixed with ceramic powder in an aqueous suspension and processed to form a web. Several superimposed webs are rolled up to the desired shape of the filter body, wherein honeycomb flow channels are formed between adjacent filter webs. The filter body is then fired at high temperature.

The WO 2006/005668 discloses another method of manufacturing a ceramic filter element in an exhaust gas filter for internal combustion engines. Here, a combustible, non-ceramic carrier web is first soaked with a ceramic slip and then burned out in the desired geometric shape until the carrier web is burned and a rigid filter body is formed.

From the EP 1 554 472 B1 an exhaust aftertreatment arrangement is known with a body through which the exhaust gas of an internal combustion engine can flow, wherein the body has delimited flow areas delimited by in each case one boundary device and the limitation devices for the exhaust gas are permeable. The permeability of the limiting devices (and therefore also the porosity) varies as the permeability of at least two limiting devices differs from one another in regions which are close to the inlet opening and / or in the region of the inlet opening. This results in a variable porosity in the entire component. However, a different porosity across the wall thickness of the individual permeable for the exhaust gas limiting devices is not disclosed in this document.

Finally, the reveals DE 10 2009 008 299 A1 a method for producing a ceramic filter element in an exhaust filter for internal combustion engines, in particular a diesel particulate filter, in which a raw paper carrier web is impregnated with a ceramic slip and then fired. The raw paper carrier web used in this case has a thickness and / or structure varying over its width and / or length. In this way, a filter element is formed, which has different porosities over its diameter or its length. Also, this document does not disclose different porosities across the thickness of the walls formed between adjacent filter channels.

It is an object of the invention to provide open-pored, porous ceramic bodies having a different porosity over the wall thickness, wherein this variable porosity can be achieved in a simple manner. Another object of the invention is to provide a method for producing such a body.

Disclosure of the invention

The present invention solves these objects by providing an open-pored ceramic body having inlet and outlet channels for guiding a gas flow, as well as the porous walls separating the inlet and outlet channels, the body having an asymmetric pore size distribution across the thickness of the walls formed in a single Process step can be generated.

Brief description of the drawings

The invention will be described in more detail below with reference to the drawings. It shows / show

1 a schematic representation of an internal combustion engine with a filter device having a ceramic body according to the invention;

2 the asymmetric pore sizes of individual pores over the wall thickness between dirty and clean side of a filter body according to the invention;

3 the pore size distribution over the wall thickness of a filter body according to the invention;

4 to 7 SEM images of different types of pores on the dirty or clean side;

8th the pore size distribution over the wall thickness with a simple gradient; and

9 Examples of a possible pore size distribution over the wall thickness along the direction of flow in a filter body according to the invention.

Embodiment (s) of the invention

The ceramic body according to the invention can be used, for example, as a catalyst, wherein a gas stream sweeps over the body. Above all, however, it can be used as a hot gas filter and in particular as a diesel particulate filter (DPF). The following is an example of use as a DPF. The starting material of the ceramic body according to the invention may be a material selected from the group consisting of aluminum titanate, cordierite, mullite, SiC and the like.

1 shows a schematic representation of an internal combustion engine with a filter device having a ceramic body according to the invention. The internal combustion engine 10 is over an exhaust pipe 12 connected in which the filter device 14 is arranged with the ceramic body according to the invention. With the filter device 14 soot particles are out of the exhaust pipe 12 filtered exhaust gas filtered out. This is especially necessary for diesel engines to comply with legal requirements.

The filter device 14 includes in the embodiment shown a cylindrical housing 16 in which, for example, a rotationally symmetrical, overall also cylindrical filter element 18 is arranged from a ceramic body according to the invention. Other housing types are also possible.

The open-pored ceramic body according to the invention contains inlet and outlet channels for guiding a gas flow, as well as the inlet and outlet channels separating porous walls, wherein the body has an asymmetric pore size distribution over the thickness of the walls, which can be generated in a single process step.

In the case of a winding impregnated with a ceramic slurry, the asymmetric pore size distribution can be achieved, for example, from a non-ceramic material (for example from paper, cellulose or plastic fibers) by mixing different pore formers with the ceramic slurry. By type, amount and / or combination of different sized pore formers can be targeted storage of pore formers in the depth and / or on the surface of the medium, d. h., Which are achieved by the gas flow to flow through the wall. Grolle pore formers can z. B. are not infiltrated and store specifically on the surface of the wall. In this way an asymmetric pore size distribution results over the wall thickness. It is also possible to use known ceramic papers in which a ceramic powder is already added to the cellulose fibers during papermaking. The shaping of the coated paper takes place in a later process step.

As pore formers can, for example, carbon, polymers, resin or polymer particles such as polyamides (0.5-90 microns), polystyrenes, polyurethanes, polypropylene (PP), polyethylene (PE), PP-PE copolymers, latex particles, polyvinyl acetate (PVA ), Polyvinylpyrrolidone (PVP), acrylates, starches and wood flour. Combinations of two or more of these pore formers are conceivable. The pore formers can be added in amounts of 0-15%, carbon and polymers such. As polyamide, preferably in amounts of 0 to 10%, each based on the total amount of the slip.

2 shows the asymmetric pore size distribution over the wall thickness between upstream (AN) and downstream (AB) of a filter body according to the invention. Out 3 the pore size distribution can be seen. One recognizes the over the wall cross-section (over arrival and outflow side) different pore diameter (bimodale pore distribution, there are two different pore sizes prevailing). Depending on the application, a more open (eg SCR catalyst) or more closed (eg DPF) upstream side of Be an advantage. Thus, the type of filtration (depth or surface filtration) can be influenced. For DPFs, surface filtration is preferable for faster and complete regeneration of the filter. This also reduces the pressure increase during loading. For the surface filtration, the small pores on the upstream side are responsible, the larger pores to the downstream side (clean side) allow a low pressure loss. Likewise, so that the contact surface can be increased, for example, to be removed soot. The 4 to 7 show SEM images of different versions of the pores on the inflow (dirt) and outflow side. All pictures are shown on the same scale (100x magnification). 4 shows large pores (about 50-70 microns) on the clean side and 5 average pores on the clean side. In contrast, are off 6 small pores and out 7 very small (about 10-25 microns) to remove pores on the dirty side. Of course, even larger or smaller pores are possible. The original fiber structure can be clearly seen in all the pictures, which shows that this gradient in pore size from the dirty to the clean side was not produced by a separate coating step, but in the same process step as the ceramic shaping of the entire body.

This will also come from the 8th clearly, on which a changing porosity over the wall thickness (below smaller pores, above larger pores) with a simple gradient (from small to large) is recognizable, which is manufactured in a single process step, which results from the fact that no Phase or layer boundary is visible, which is always recognizable in a separate coating step.

9 shows examples of a possible pore size distribution over the wall thickness along the flow direction (arrow A) in a filter body according to the invention. Curve ➀ shows large pores on the dirty side and small pores on the clean side, curve ➁ shows the reverse arrangement as curve ➀. Curve ➂ shows a bilateral distribution of large pores on the surface and smaller pores in the wall, while curve ➃ shows the reverse arrangement to curve ➂. The pore distribution shown in curve ➀ is suitable for applications where depth filtration is desired, while the distribution shown in curve ➁ for soot filtration, e.g. B. in the DPF, is suitable. In this case, surface filtration with a rapid build-up of the filter cake is advantageous since it allows a reduced pressure loss. Curve ➂ shows a pore distribution, which is advantageous for catalysts, if both sides a large surface is desired. Finally, the pore distribution shown in curve ➃ is suitable for catalytically coated components, if, for example, a catalytic noble metal coating is applied on both sides (prevents too deep penetration of the noble metals.

Another way to achieve an asymmetric pore size distribution in a impregnated with a ceramic slurry winding of a non-ceramic material, is in a suitable embodiment of the infiltrating wall structure by a gradient in the fiber structure, eg. B. a denser fiber structure on one of the two surfaces of the paper, or the use of multilayer medium (multi-layered cellulose medium and cellulose media with polymer content). The fibrous structure of the paper can influence the infiltration of the slurry. As a result, the open porosity varies over the thickness, in particular in combination with rheology and pore formers.

The advantages of the inventive body when used as a filter are the ability to make small pores in the contact area to be filtered gas flow and large pores in the wall to produce high filtration efficiency with low pressure drop. The reverse arrangement may also be advantageous, namely if a larger surface area is desired (eg in connection with catalytic coatings).

process example

Preparation of a ceramic slip by stirring ceramic particles in liquid (eg water, alcohol, etc.) with the addition of a commercial dispersing aid. The addition of the pore-forming agent, for. As carbon, takes place in connection. To achieve the desired particle size, a grinding process can follow. Formulation Examples: Raw material 1 Raw material 2 Raw material 3 Raw material 4 pollution dispersing aid pore formers Al ₂ O ₃ : 50.6% TiO ₂ : 40.1% SiO ₂ : 2.4% MgO: 0.5% 1.4% Carboxylic acid 1% Graphite powder 10 μm: 5% Al ₂ O ₃ : 44.3% TiO ₂ : 35.2% SiO ₂ : 2.1% MgO: 0.4% 1.3% Carboxylic acid 2% Polymer powder 50 μm: 16.7% Al ₂ O ₃ : 52.6% TiO ₂ : 41.6% SiO ₂ : 2.4% MgO: 0.4% 1.5% Carboxylic acid 0.5% Polymer powder 5 μm: 1.5%

About the amount and size of the pore forming agent is influenced according to the porosity and the pore distribution.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 60217084 T2 [0003]
• DE 102006040739 A1 [0010]
• JP 63134020A [0011]
• WO 2006/005668 [0012]
• EP 1554472 B1 [0013]
• DE 102009008299 A1 [0014]

Claims (19)

Porous, porous ceramic body with inlet and outlet channels for guiding a gas flow, and the inlet and outlet channels separating porous walls, characterized in that the body has an asymmetric pore size distribution over the thickness of the walls, which can be generated in a single Prozeßsdiritt. Ceramic body according to claim 1, characterized in that it has smaller pores on the upstream side than on the downstream side. Ceramic body according to claim 1, characterized in that it has larger pores on the upstream side than on the downstream side. Ceramic body according to claim 1, characterized in that it has on both sides of the surface smaller pores than in the separating porous wall. Ceramic body according to claim 1, characterized in that it has on both sides of the surface larger pores than in the separating porous wall. A ceramic body according to any one of the preceding claims, characterized in that it consists of a material selected from the group consisting of aluminum titanate, cordierite, mullite, SiC and the like. Ceramic body according to one of the preceding claims, characterized in that the starting material is a ceramic slip. Ceramic body according to claim 7, characterized in that the body consists of a non-ceramic material on a winding impregnated with the ceramic slurry. Ceramic body according to claim 8, characterized in that the non-ceramic material is selected from the group consisting of paper, cellulose and plastic fibers. Ceramic body according to one of claims 1 to 6, characterized in that the starting material is a ceramic paper. A method for producing a ceramic body according to claim 9, characterized in that the paper to be used has a predetermined gradient in its fiber structure. Process for producing a ceramic body according to one of claims 7 to 9, characterized in that different pore formers are added to the slurry. A method according to claim 12, characterized in that the pore formers are selected from the group consisting of carbon, polymers, resin or polymer particles such as polyamides (0.5-90 microns), polystyrenes, polyurethanes, polypropylene (PP), polyethylene (PE ), PP-PE copolymers, latex particles, polyvinyl acetate (PVA), polyvinylpyrrolidone (PVP), acrylates, starches, wood flours and combinations thereof. A method according to claim 13, characterized in that the polyamide has a particle size in the range of 0.5 to 90 microns. A method according to claim 12 or 13, characterized in that the pore-forming agent in an amount of 0-15%, based on the total amount of the slip added. A method according to claim 15, characterized in that the pore formers carbon and / or PVA and / or PVP are added in an amount of 0-10%, based on the total amount of the slip. A method according to claim 10, characterized in that already in the papermaking process the cellulose fibers, a ceramic powder is added. Use of a porous, open-pore body according to one of claims 1 to 10 as a diesel particulate filter. Use of a porous, open-pored body according to any one of claims 1 to 10 as a catalyst.

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