DE102018116642A1 - Open cell ceramic network and process for its manufacture - Google Patents

Abstract

The invention relates to the field of ceramics and relates to an open cell ceramic network, e.g. as a depth filter, in particular as a molten metal filter, as a support body for filtration. The object of the present invention is to provide an open-cell ceramic network which has improved mechanical strength and hardness and improved resistance to high temperatures and oxidation. The object is achieved due to an open-cell ceramic network, in which the webs are made of pressureless sintered SiC (SSiC), and the SSiC structure of the webs has a structure porosity of 2 to 10% from completely or essentially completely closed pores, with up to 50% of all pores in the SSiC Structures have an average diameter of ≤ 1 µm, and up to 90% of all pores in the SSiC structure have an average diameter of ≤ 3 µm, and at least 50% of all pores in the SSiC structure have a circularity ≥ 0.8.

Description

The invention relates to the field of ceramics and relates to an open-cell ceramic network, e.g. as a depth filter, in particular as a molten metal filter, as a support body for the filtration, for heat exchangers, regenerators, electrically heatable thermostats, catalyst supports, as a burner element for surface radiation burners, pore burners and volume burners, for sound absorbers, diesel soot filters, heating elements or as additionally catalytically activated reactor internals a process for its manufacture.

Ceramic networks in the form of open-cell foam ceramics are known. Processes for producing such open-cell foam ceramics according to the so-called “Schwartzwalder process” are known, which is used industrially and is the most widespread. According to this method, the desired component is cut out of an open-cell polymer foam and then impregnated with a suspension of ceramic particles and water or solvent. The impregnated polymer foam is then mechanically pressed out one or more times and then dried. Now the polymer foam is burned out and then the remaining ceramic coating is sintered ( US 3,090,094 ).
An open-cell foam ceramic produced by this process is an impression of the cell-like polymer structure of the starting material. By burning out the polymer foam, the remaining ceramic webs are hollow. The cross section of the ceramic webs is usually triangular to round, the shape of the interior web cavity being triangular in accordance with the burned-out polymer foam. The ceramic coating on the edges of the cavities is usually particularly thin and is therefore characterized by cracks along the edge. The cavities and the cracks lead to a very low mechanical strength. Since the shrinkage of the ceramic coating increases the susceptibility to cracking during sintering, relatively low-shrinkage materials are used, but which have a high internal porosity after sintering. This also leads to low mechanical strength ( YES,. Ceram. Soc. 77 (6), 1467-72 (1994) ).

The ceramic foams produced from polymer foams according to the above-mentioned process therefore have cavities with a concave triangular cross section on the inside ( Cahn, RW, Haasen, P., Kramer, EJ (ed.): Material Science and Technology, Vol. 11, VCH 1994, p. 474 ). The shape of this cavity is very unfavorable for the mechanical strength of the webs of the ceramic foam, since the load-bearing component of the tips of the triangles is only very low. Due to the susceptibility of the brittle ceramic to the initiation of cracks, the very pointed shape of the triangular cavities is also problematic, since from there cracks almost always form, which further reduce the strength of the ceramic webs ( J. Amer. Ceram. Soc. 77 (6) 1467-72 (1994) ). Therefore, the foams produced by the Schwartzwalder process have a low mechanical strength, which is disadvantageous for the above-mentioned applications, as well as the handling and transportation of such ceramic foams.

The foams used for the impression are made by foaming a mixture of different chemical components. A gas is formed during the reaction of the liquid components with one another, as a result of which gas bubbles form and grow in the liquid. Furthermore, the starting components polymerize, which increases the viscosity of the liquid. At the end of the reaction, a solid polymer is formed that contains a large number of gas bubbles (polymer foam). The size of the bubbles in the polymer foam can be controlled within certain limits by the choice of the starting components and the reaction procedure.
An aftertreatment, the so-called reticulation, completely or chemically or thermally removes the cuticles between the gas bubbles, which creates the open-cell polymer foam required for ceramic production. This foam consists only of polymer webs that have formed between three neighboring gas bubbles (Klemper D. and Frisch KC (ed.): Handbook of Polymeric Foams and Foam Technology, Hanser 1991, p. 24).

The nature of gas bubble foaming means that the surfaces of the polymer foam are always concave. The cross-sections of the polymer webs forming the foam therefore have the shape of triangles with concave side surfaces with very pointed tips (Klemper D. and Frisch KC (ed.): Handbook of Polymeric Foams and Foam Technology, Hanser 1991, pp. 28/29) , This is the natural law for all foamed materials.

The gas bubbles that occur during the foaming of the polymers cannot be produced in unlimited size. If the gas bubbles are too large, the foam collapses before the polymerization has solidified the foam (Klemper D. and Frisch KC (ed.): Handbook of Polymeric Foams and Foam Technology, Hanser 1991, p. 9). The upper limit for the most widely used polymer foam made of flexible polyurethane foam is approximately 5 ppi (pores per inch) = a maximum of around 6 mm cell width. That is also over In this direction, the possibilities of using polymer foam for foam ceramic production were limited.

In order to reduce the problems of mechanical strength somewhat, after the DE 35 40 449 A1 or DE 35 39 522 A1 proposed to carry out a multiple coating of the polyurethane foam used. This increases the thickness of the ceramic webs and thus the mechanical strength of the sintered ceramic foam.
The increased process complexity for the multiple coating is problematic. In addition, the ceramic coating has only a low strength before sintering, and the mechanical stress on the coated polymer foam required to separate the excess suspension during the multiple coating therefore often leads to new defects in the coating. In principle, however, the multiple coating does not eliminate the disadvantage of the unfavorably shaped concave-triangular cavities of the webs.

It is also known to use ceramic fibers as mono- or multifilaments for the production of porous ceramics, which can be laid, knitted, sewn or glued (IChemE Symposium Series No99 (1986) 421-443; MTZ Motortechnische Zeitschrift 56 (1995) 2 , 88-94).
The disadvantage here is that ceramic fibers of this type are difficult and expensive to produce and are therefore very expensive and difficult to process because they are very brittle. For example, knitting techniques can only be used to a limited extent. Thus, only a limited selection of ceramic materials can be used for such fibers, whereby the modification of the properties of the porous ceramic produced therefrom is difficult or almost impossible. Such porous structures are also flexible because the fibers are not fixed to one another at the contact points. This is disadvantageous, for example in the case of filtrations or mechanical loads, since these ceramics are then generally not very stiff and, moreover, fiber abrasion is particularly produced in the case of multifilaments. Such fibers can also be fixed ( US 5,075,160 ), but is only interesting for typical applications if a ceramic fixation is created. This is also difficult and time-consuming to do, mostly using CVD or CVI techniques, but the choice of materials is again very limited.

Furthermore, from the DE 197 53 249 A1 a ceramic network of three-dimensionally interconnected ceramic bars, in which the cavities in the ceramic bars have a round cross-section.

Such a ceramic network is produced by producing a fiber network from fibers with a round fiber cross section, which is impregnated with a ceramic suspension one or more times and excess suspension is removed, and after the suspension has dried, the fiber network is removed and the remaining network is sintered.

Ceramic networks, whether foams or molded fibers or other structures have formed the starting network, are often characterized by their porosity. The principle is that three types of porosity can be found in structures that add up to the total porosity. The individual types are the cell porosity, the web core porosity and the actual structural porosity.
The cell porosity characterizes the volume fraction of the structure that is spanned by the network of webs. The structure is divided into cells that are connected to each other in the case of an open-cell structure. This porosity value is decisive for most applications of these structures, since it has a significant influence on properties such as flowability and the geometric surface.
As a result of the molding of a starting network, for example in the form of a foam or molded fibers, which decomposes completely or almost completely after coating with ceramic materials in the course of the heat treatment, a ceramic network of webs which have the second type of porosity subsequently arises. the web core porosity. Their share in the total porosity varies depending on the starting network used for the impression. A web core porosity of 2 to 4% of the total porosity is characteristic for simply molded, open-cell foams.
The third type of porosity of ceramic networks is the structural porosity of the ceramic coating material itself. Its share of the total porosity depends on the material used, its initial grain size and packing density, the degree of compaction during sintering and / or any subsequent infiltration through a suspension or melt.

Silicon carbide (SiC) is preferred because of its high hardness and thermal conductivity as well as its excellent oxidation resistance and high stability compared to chemical reagents in high temperature processes. The basic requirement for this is a structure that is as dense as possible. Oxidation processes or chemical interactions cause the surface of the material to react with the environment. Passivation takes place there in the presence of oxygen through the formation of a silicon oxide layer. The formation of the layer ideally follows one parabolic time law and slows down over time due to the longer transport routes from the surrounding medium to still reactive areas of the SiC. This protects the SiC from an intensive interaction with the environment.

Open-cell ceramic foams have proven to be particularly suitable for burner applications, as well as a substrate for catalytically active substances, since they combine low pressure loss with a large geometric surface. Open-cell ceramic structures, such as foams or networks, consist of a three-dimensionally networked web structure, the individual web sections of which are linked to one another via nodes and enclose the open cells in between. The essential properties of the structure can be varied by the type of ceramic and its degree of compaction.

While compact components made by extrusion or pressing from pressure-sintered SiC (SSiC) have a structure with a low proportion of micropores, there are no open-cell structures, such as ceramic foams made from SSiC materials, which have less than 10% residual porosity in the structure. This higher residual porosity is due to the lack of mechanical compression of the SiC powder particles.

Foam structures or three-dimensional structures made of silicon carbide are available in different material variants. Structures made of SSiC (pressureless sintered SiC) or SiSiC (silicon-infiltrated SiC), however, have the highest mechanical and thermal resistance.

SiSiC is also used as a firing aid or as a material for burner nozzles in the form of compact cast or extruded parts in the ceramic, glass and metal processing industries. Ceramic foams or open-cell structures produced using an impression process or 3D powder printing are also known from SiSiC ( EP 1 382 590 A1 ).

Furthermore, open-cell silicon carbide foam ceramics are known which consist of a three-dimensionally cross-linked ceramic structure made up of many interconnected webs and open cells lying between the webs, the webs essentially consisting of sintered silicon carbide with 5 to 30% pore volume in completely or essentially completely closed Pores with an average diameter of <20 µm ( DE 100 44 656 A1 ).

A disadvantage of the solutions of the known prior art is that the open-cell silicon carbide foam structures do not yet have sufficient mechanical strength, hardness, high temperature and oxidation resistance.

The object of the present invention is to provide an open-cell ceramic network which has improved mechanical strength and hardness and improved high temperature and oxidation resistance, and a simple and inexpensive method for its production.

The object is achieved by the invention specified in the claims. Advantageous refinements are the subject of the dependent claims.

The open-cell ceramic network according to the invention consists of a three-dimensionally networked ceramic structure composed of many interconnected webs and open cells lying between the webs, the webs essentially consisting of SiC (SSiC) sintered without pressure, and the SSiC structure of the webs having a structure porosity of 2 to 10 % of completely or essentially completely closed pores, with up to 50% of all pores in the SSiC structure having an average diameter of ≤ 1 µm, and up to 90% of all pores in the SSiC structure having an average diameter of ≤ 3 µm, and at least 50% of all pores in the SSiC structure have a circularity> 0.8.

The porosity of the SSiC structure of the webs is advantageously 2 to <5%.

Furthermore advantageously, 20% to 50% of all pores in the SSiC structure have an average diameter of 1 1 μm.

Also advantageously, 70% to 90% of all pores in the SSiC structure have an average diameter of 3 3 μm.

And also advantageously, 50% to 90%, advantageously 80% to 90%, of all pores in the SSiC structure have a circularity greater than 0.8.

It is also advantageous if at least 50% of all pores in the SSiC structure have a circularity between 0.8 and 0.95, advantageously between 0.9 and 0.95.

It is also advantageous if open structural pores, cavities and cracks are filled with SiC materials.

In the method according to the invention for producing open-cell ceramic networks from a three-dimensionally cross-linked ceramic structure from many interconnected webs and open cells lying between the webs, the webs consist essentially of pressure-free sintered SiC (SSiC), and the SSiC structure of the webs has a porosity of 2 to 10% from completely or essentially completely closed pores, with up to 50% of all pores in the SSiC structure having a medium one Have a diameter of ≤ 1 µm, and up to 90% of all pores in the SSiC structure have an average diameter of ≤ 3 µm, and at least 50% of all pores in the SSiC structure have a circularity greater than 0.8, is made from a coarse silicon carbide powder an average grain size of 1 to <5 microns and a mixture of a fine silicon carbide powder with an average grain size of 0.1 to 1 micron, the mixing ratio of coarse to fine silicon carbide powder between 10: 90 to 90: 10, from this mixture If a suspension is produced together with small amounts of sintering additives known per se, then an open-cell foam or a open-cell network coated one or more times with the suspension, the foam or network dried, the foam or network removed and the remaining green body sintered at temperatures ≥ 1800 ° C under a protective gas or vacuum.

It is advantageous if a coarse silicon carbide powder with an average grain size of 2 to 4 µm is used.

It is also advantageous if a fine silicon carbide powder with an average grain size of 0.2 to 0.9 µm is used.

It is also advantageous if a silicon carbide powder mixture with a mixing ratio of coarse to fine silicon carbide powder between 30:70 to 70:30 is used.

It is also advantageous if boron and carbon or boron, aluminum and carbon are used as known sintering aids and a basic binder based on polysaccharides, polyamides or polyacrylates is used as the sintering aid.

It is also advantageous if a polyurethane foam is used as the open-cell foam, or a network of organic synthetic or natural fibers is used as the open-cell network.

It is also advantageous if the removal of the foam or of the network is carried out by burning out, evaporating, etching or dissolving.

It is also advantageous if the green body is sintered at temperatures in the range from 1800 to 2300 ° C., advantageously at temperatures between 2000 and 2180 ° C.

Water is advantageously used as the suspending agent.

And also advantageously a linear shrinkage of 12 to 20% is set.

Furthermore, before and or after the sintering, pores, cavities and cracks are advantageously filled with SiC materials, with SiC material advantageously being introduced into the open pores, cavities and cracks by means of infiltration, immersion, soaking, spraying.

And there is also advantageously a material connection between the first and time coating.

The solution according to the invention makes it possible for the first time to provide an open-cell ceramic network which has improved mechanical strength and hardness and improved high-temperature and oxidation resistance, and a simple and inexpensive process for its production.

This is achieved through an open-cell ceramic network consisting of a three-dimensionally networked ceramic structure made up of many interconnected webs and open cells lying between the webs.
The open cells are formed by the ceramic webs and are the voids in the ceramic network. They are characterized by the cell porosity, which does not contribute to the structural porosity considered according to the invention.

The invention relates essentially to the material and structure of the webs of the open-cell ceramic network, which essentially consist of SiC (SSiC) sintered without pressure.
Minor production-related ceramic components or phases other than SiC sintered without pressure can be components of the material of the webs, but are present to such a small extent that they have no significant influence on the properties of the open-cell ceramic network.

According to the invention, the SSiC structure of the webs has a structure porosity of 2 to 10%, advantageously 2 to <5%, of completely or essentially completely closed pores.
Here, up to 50%, advantageously 20% to 50 % of all pores in the SSiC structure have an average diameter of ≤ 1 µm, and up to 90%, advantageously 70% to 90%, of all pores in the SSiC structure have an average diameter of ≤ 3 µm, and at least 50%, advantageously 50% to 90%, still advantageously 80% to 90%, of all pores in the SSiC structure have a circularity 0,8 0.8.

With regard to the circularity of the completely or essentially completely closed pores in the ceramic material of the webs, it should be said that this is advantageously between 0.8 and 0.95, still advantageously between 0.9 and 0.95.

In the context of this invention, circularity is understood to mean the roundness of pores, the value circularity = 1 being intended to describe the circular diameter of a pore.
It should be noted that the rounder the pores in a ceramic material, the greater the likelihood that the pores are closed pores.

Known open-cell ceramic structures made of SiC can have low residual porosities in the structure of the webs, but with such structures it is very disadvantageous that the reduction in the structure porosity is achieved by infiltration with silicon and thus free Si is present in the structure, which amounts to up to 30% can, but softens when the melting temperature is reached and is pushed out of the structure by the density anomaly when cooling. This gradually creates pores in the structure, which lead to an internal oxidation and thus to an accelerated degradation of the component properties, in particular the much shorter lifespan of these materials.

Known open-cell ceramic structures made of SSiC, on the other hand, have a significantly higher microstructural porosity within the webs than 10%, mostly up to 35%, since no mechanical compression of the material takes place during production and no mechanical post-compression of the green bodies can take place due to the low mechanical stability of the green bodies.

These disadvantages of the solutions of the prior art with regard to the internal oxidation and the too high microstructural porosity can essentially be comprised of SSiC with such low microstructural porosities of 1 to 10% and advantageously even of 2 to <5 by the inventive production and specification of the open-cell ceramic structure %, are resolved so that the present invention is new and based on an inventive step.

The open-cell ceramic network according to the invention is produced by using a coarse silicon carbide powder with an average grain size of 1 to <5 μm, advantageously 2 to 4 μm, and from a fine silicon carbide powder with an average grain size of 0.1 to 1 μm, advantageously 0.2 up to 0.9 µm, a mixture is produced.

The mixture ratio of coarse to fine silicon carbide powder is between 10:90 to 90:10, advantageously between 30:70 to 70:30.

A suspension is produced from this mixture of the two SiC powders together with known small amounts of sintering additives, advantageously boron and carbon or boron, aluminum and carbon or a basic binder, in the form of a polysaccharide solution with a very high carbon yield.
Water can advantageously be used as the suspending agent.

An existing open-cell foam or an open-cell network, such as, for example, a polyurethane foam, or a network of organic synthetic or natural fibers, or a printed polymer network, is then coated one or more times with the suspension and the suspension is dried. Subsequent or as part of the drying process, the open-cell foam or the open-cell network is removed, so that a green body is formed which, on the one hand, maps the open-cell structure of the previous open-cell foam or network as cell porosity, and on the other hand, in addition to the material-related structural porosity, the web core porosity and optionally has cracks in which the material of the open-cell foam or network was previously located. The reaction products of the foam or network materials that have been removed may have escaped through the cracks.

The removal of the foam or network can advantageously be achieved by burning out, evaporating, etching or dissolving.

The green body is then sintered at temperatures 1800 1800 ° C., advantageously in the range from 1800 to 2300 ° C., still advantageously at temperatures between 2000 and 2180 ° C. under a protective gas or vacuum.

The open-cell ceramic network according to the invention thus produced has a structural porosity of the web material of 2 to 10% from completely or essentially completely closed pores, with up to 50% of all pores in the SSiC structure having an average diameter of 1 1 μm, and up to 90% all pores in the SSiC structure have an average diameter of ≤ 3 µm, and at least 50% of all pores in the SSiC structure have a circularity greater than 0.8.

Of particular importance according to the invention is that by using the fine silicon carbide powder with average grain sizes of 0.1 to 1 μm As a starting material with the coarser silicon carbide powder with average grain sizes of 1 to <5 µm, a significantly higher packing density of the silicon carbide particles in the green body and also in the sintered body of the open-cell ceramic network according to the invention is realized, and at the same time the much higher sintering activity of the fine silicon carbide powder leads to a significantly increased compression of the SSiC materials during the sintering process.
As a result of these effects of the use of the fine and coarse SiC powder according to the invention, there is also a significantly higher linear shrinkage during the sintering of the open-cell ceramic network according to the invention, which can be set in the range from 12 to 20%.

In addition to the very low residual porosity in the ceramic material, the open-cell ceramic network sintered without pressure according to the invention also has a very fine distribution of the completely or essentially completely closed pores.
Due to the structure porosity of 2 to a maximum of 10% of completely or essentially completely closed pores in the ceramic material of the webs and their homogeneous distribution in the ceramic material of the webs, the number and distribution of the pores fall below the percolation limit, so that there is no continuous connection between them a plurality of pores occurs, it being possible for a few pores to be connected to one another by chance, but these form no or essentially no open pores to the surface of the webs.

Pores in the structure can also be called defects. If several defects are stored together, so-called defect clusters arise. The formation of these clusters depends largely on the geometry of the defects and their volume part. If the volume fraction exceeds a certain limit, the individual defect clusters form a coherent cluster in the structure. In the case of the pores, this means that there are continuous pore channels in the structure. This limit is called the percolation limit. For ideal spherical defects, for example, it is 0.289 by volume. The more elliptical the defects, the lower the percolation limit (fracture mechanics - with an introduction to micromechanics, 6th ed. Gross, D .; Seelig, T., Springer, Berlin, 2016, pp. 64 f.).

With the composition according to the invention and the structure of the open-cell ceramic network, the risk of internal oxidation is greatly reduced, or there is no such risk, so that components from this open-cell ceramic network according to the invention have a significantly longer service life and / or also significantly higher peak temperatures when used without damage can survive.

With regard to the known small amounts of the known sintering additives which are added to the suspension, it must be said that within the scope of the present invention these can also be polymeric binders, such as solutions or emulsions of polysaccharides, polyamides or polyacrylates, which ensure a sufficiently high dry fracture resistance in the green body of the open-cell ceramic network, so that the green body is also sufficiently mechanically stable. At the same time, such polymeric binders can also prevent the formation of cracks in the green body during drying due to their corresponding elasticity.

When such polymeric binders are used, highly branched ceramic networks can also be produced and sintered, despite the high linear shrinkage during sintering.

Furthermore, the mechanical strength of the green and sintered body of the open-cell ceramic network according to the invention can be significantly increased if the open pores present in the webs, the web core porosity and cracks are filled with SiC material before or after sintering.
This can advantageously be achieved by means of infiltration, immersion, soaking, spraying. In addition to the primary coating process (first coating), a second coating can also be carried out in the pyrolyzed state in order to fill the resulting cavities in the web core with the same type of material.

In the case of infiltration, for example after the green body has dried, the foam or the network can be removed by means of pyrolysis (burnout). During this process step, the organic constituents and sintering additives contained in the suspension form glass-like carbon which temporarily bonds the SiC particles to one another.
As a result of this bonding of the SiC particles, the green body has sufficient mechanical strength for the subsequent process steps.
The pores, voids and cracks in the green body of the open-cell ceramic network, which are still open, can then be filled, for example by means of soaking, with a suspension, which can advantageously be the SiC suspension for coating the foam or network, this advantageously being possible by means of a vacuum that the suspension can flow into the free cavities of the web core porosity or is drawn into it. This means that all open pores, cavities and cracks can be partially or completely filled.
The suspension for filling the open pores, voids and cracks can also be precursors from ceramic materials are used, which then react to ceramic materials during sintering and can advantageously form a material connection with the web materials.

This filling of the open pores, voids and cracks is particularly advantageous in the event that a polymer foam is used as the starting material for the coating with the SiC suspension. When the foam is removed, advantageously pyrolyzed, cavities with a triangularly concave cross section typically arise. In the unfilled state, the pointed cavity corners often lead to further cracks in the webs, so that by filling these cavities, the notch effect of these pointed cavity corners in the sintered open-cell ceramic network can be significantly reduced.

A particular advantage of filling the open pores, cavities and cracks in the webs of the open-cell ceramic network according to the invention in the course of the second coating is that the pyrolysis of the organic foam or network creates a coating on the surface of the open pores, cavities and cracks, which is only pyrolyzed and thus after filling, just like the filled material, advantageously SiC, also shows a shrinkage which is essentially the same size with the same or similar shrinkage behavior of the materials. This can create a cohesive bond between the already pyrolyzed coating and the filled material, which significantly improves the mechanical and chemical stability of the filled open-cell ceramic network made of SSiC. The mechanical compressive strength of such a filled open-cell ceramic network made of SSiC can thereby be increased by a factor of 2 to 3 and reaches a level similar to that of SiSiC foams with web cavities completely or partially filled with free silicon.

After the green or sintered body has been filled or soaked, the excess material can be removed by centrifugation.
By choosing the centrifugation parameters, you can choose between different coating thicknesses and filling parameters. At a low speed and thus low centrifugal force, only the excess material of the suspension is removed from the structure, which otherwise can, for example, close the cells and cell windows. The suspension adhering to the surfaces of the webs can be retained. At higher speeds, the excess material of the suspension can, for example, be reduced to a very thin layer on the surface of the webs or removed entirely, in which case only material would remain in the open pores, cavities and cracks. Centrifugation also ensures that the suspension is also introduced into the corners of the cavity, which otherwise might not have been filled due to the capillary action.

With the filling of the open pores, cavities and cracks and the subsequent targeted removal of the excess suspension in the areas that are to remain free and unfilled, a very high cell porosity of up to 93% can be achieved according to the invention, which means that the cells of the Ceramic network are open and essentially unfilled. This means that there is a very large usable, freely flowable cell space.
The web core porosity, which usually accounts for 2 to 4% of the volume in most polymeric impression bodies, can be significantly reduced or reduced to approximately 0% by filling the open pores, cavities and cracks. According to the invention, the web core porosity relates only to the open pores, cavities and cracks, not to the structure porosity of the completely or essentially completely closed pores, which according to the invention is present in the web of the SSiC.
This structure porosity cannot be reduced by filling the pores.

With the open-cell ceramic network according to the invention, there are open-cell SSiC structures which have improved high-temperature resistance with high thermal shock resistance and whose ceramic structure has no or only a very small proportion of secondary phases.

The reduction in the porosity of the SSiC structure of the webs to a porosity of 2 to 10% from completely or essentially completely closed pores prevents the internal oxidation and leads to a significant improvement in the service life of the open-cell ceramic networks according to the invention in an oxidative environment by one or two orders of magnitude from a few hundred to a few tens of thousands of hours.

The invention is explained in more detail below using several exemplary embodiments.

Comparative Example:

A ceramic water-based suspension is produced. This contains a bimodal SiC grain distribution, produced by mixing SiC powders with an average grain diameter of 1.2 and 18 µm in a ratio of 50:50; also 0.6% boron (carbide) and 9% of a water-soluble phenolic resin (corresponds to 3% carbon after pyrolysis) as a sintering additive. The suspension is adjusted to a solids content of 84%.

To produce the foam ceramic, a polyurethane foam with a cell size of 10 ppi (pores per inch) is soaked with the suspension and then the excess suspension is separated off with a centrifuge. For a sample measuring 40x40x25 mm, the coating weight is approx. 20 g. The body is then dried and the polyurethane is burned out to a temperature of 600 ° C. under an inert gas atmosphere. The remaining SiC powder structure is sintered under an argon atmosphere at a temperature of 2100 ° C with a pressure of approx. 105 kPa.

The sample shrinks linearly 4% during sintering, so that the final dimensions are 39x39x24 mm.
The average breaking load for a stamp impression with a diameter of 20 mm is 230 N.
The webs of the foam ceramic contain about 12% closed pores with an average diameter of about 5 µm.

After the ceramic has been heated to 1000 ° C in air, it is quenched in cold water. The strength measured after this is still 190 N.

example 1

A ceramic water-based suspension is produced. This contains a bimodal SiC grain distribution, produced by mixing SiC powders with an average grain diameter of 0.8 µm and 3.0 µm in a ratio of 70:30; also 0.6% boron (carbide) and 11% of a water-soluble polysaccharide (corresponds to 4% carbon after pyrolysis) as a sintering additive. The suspension is adjusted to a solids content of 78%.

To produce the foam ceramic, a polyurethane foam with a cell size of 10 ppi (pores per inch) is soaked with the suspension and then the excess suspension is separated off with a centrifuge. For a sample measuring 40x40x25 mm, the coating weight is approx. 17 g. The body is then dried and the polyurethane is burned out to a temperature of 800 ° C. under an inert gas atmosphere. The remaining SiC powder structure is sintered without pressure under an argon atmosphere at a temperature of 2100 ° C.

The sample shrinks linearly 14% during sintering, so that the final dimensions are approx. 34x34x22 mm.
The average breaking load for a stamp impression with a diameter of 20 mm is 110 N.

The webs of the foam ceramic contain about 5% closed pores, with 40% of all pores in the SSiC structure having an average diameter of 0.8 µm and up to 92% of all pores in the SSiC structure having an average diameter of 1 µm, and 72 % of all pores in the SSiC structure have a circularity ≥ 0.85.

After the ceramic has been heated to 1000 ° C in air, it is quenched in cold water. The strength measured after this is still approx. 80N.

Example 2

A ceramic water-based suspension is produced. This contains a bimodal SiC grain distribution, produced by mixing SiC powders with an average grain diameter of 0.5 µm and 3.0 µm in a ratio of 60:40; also 0.6% boron (carbide) and 13% of a water-soluble polysaccharide (corresponds to 4% carbon after pyrolysis) as a sintering additive. The suspension is adjusted to a solids content of 75%.

A printed polymeric template in the form of a tetrakaidecahedron structure with a web thickness of 0.9 mm and a web length of 3 mm is used to produce the ceramic network. The dimensions correspond roughly to a very coarse foam ceramic with 5 to 10 ppi (pores per inch). The cylindrical template has a diameter of 69 mm and a height of 23 mm. Since the printed template is not flexible, the suspension cannot be kneaded in or introduced by rolling. Instead, the template is immersed in the viscous suspension and excess material is expelled by centrifugation. This results in a coating weight of approx. 29 g.
Analogously to example 1, the coated template is dried and then subjected to a thermal treatment at up to 800 ° C. under an inert gas atmosphere. The polymeric template decomposes. The remaining SiC powder structure is fired under argon at a temperature of 2130 ° C.
The sample shrinks linearly about 18% during sintering, so that the diameter is about 56 mm and the height is about 19 mm after sintering.

The webs of the foam ceramic contain approximately 3% closed pores, with approximately 30% of all pores in the SSiC structure having an average diameter of 0.7 µm and up to 95% of all pores in the SSiC structure having an average diameter of 2.3 µm and 85% of all pores in the SSiC structure have a circularity of 0.92.

Example 3

In order to increase the mechanical strength, a foam ceramic or a network according to Examples 1 and 2 can additionally have a Infiltration process, i.e. a second coating to be reinforced. For this purpose, polyurethane foam, as in Example 1, can first be coated with the ceramic suspension described there and dried. This completes the first coating. This is also followed by pyrolysis, which results in the thermal decomposition of the polymer foam.

This is followed by an infiltration step, i.e. the second coating, with the same but slightly diluted suspension, the solids content of which is adjusted to approximately 65%. The infiltration takes place by soaking the pyrolyzed component in a suspension bath and simultaneously applying a vacuum to a remaining pressure of approximately 200 mbar. After the infiltration, the component is removed from the suspension and the excess suspension in the cells is separated off by centrifugation. The mass after soaking and centrifuging is then approx. 22 g. The dried structure is then pyrolyzed a second time at a temperature of up to 800 ° C. The remaining SiC powder structure is sintered under argon at a temperature of 2130 ° C.

The sample shrinks linearly 18% during sintering, so that the final dimensions are approximately 33x33x20 mm. Unaffected by the second coating step, the structure of the webs of the foam ceramic is characterized by a pore fraction of approximately 3%. These are predominantly closed pores, with approximately 30% of these pores in the SSiC structure having an average diameter of 0.7 µm, and up to 95% of all pores in the SSiC structure having an average diameter of 2.3 µm, and 85% of all pores in the SSiC structure have a circularity of 0.92.

However, the second coating step has a strong impact on the web core porosity, which is caused by the template or polyurethane foam. Due to the infiltration after the pyrolysis, almost all web core cavities are now filled with the suspension. In particular, tapered web edges are thereby filled in and their notching effect is thus prevented. This has a significant impact on the mechanical stability of the foam structure. The average breaking load for a stamp impression with a diameter of 20 mm is 610 N. After the ceramic has been heated to 1000 ° C in air, it is quenched in cold water. The strength measured after this is still 550 N.

QUOTES INCLUDE IN THE DESCRIPTION

This list of documents listed by the applicant has been generated automatically and is only included 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.

Patent literature cited

• US 3090094 [0002]
• DE 3540449 A1 [0007]
• DE 3539522 A1 [0007]
• US 5075160 [0008]
• DE 19753249 A1 [0009]
• EP 1382590 A1 [0016]
• DE 10044656 A1 [0017]

Non-patent literature cited

• YES,. Ceram. Soc. 77 (6), 1467-72 (1994) [0002]
• Cahn, R.W., Haasen, P., Kramer, E.J. (Ed.): Material Science and Technology, Vol. 11, VCH 1994, p. 474 [0003]
• J. Amer. Ceram. Soc. 77 (6) 1467-72 (1994) [0003]

Claims (20)

Open-cell ceramic network, consisting of a three-dimensionally networked ceramic structure consisting of many interconnected webs and open cells lying between the webs, the webs essentially consisting of pressure-free sintered SiC (SSiC), and the SSiC structure of the webs having a structure porosity of 2 to 10% has completely or essentially completely closed pores, with up to 50% of all pores in the SSiC structure having an average diameter of 1 1 μm, and up to 90% of all pores in the SSiC structure having an average diameter of 3 3 μm, and at least 50% of all pores in the SSiC structure have a circularity ≥ 0.8. Open-cell ceramic network after Claim 1 , in which the porosity of the SSiC structure of the webs is 2 to <5%. Open-cell ceramic network after Claim 1 , in which 20% to 50% of all pores in the SSiC structure have an average diameter of ≤ 1 µm. Open-cell ceramic network after Claim 1 , in which 70% to 90% of all pores in the SSiC structure have an average diameter of ≤ 3 µm. Open-cell ceramic network after Claim 1 , in which 50% to 90%, advantageously 80% to 90%, of all pores in the SSiC structure have a circularity greater than 0.8. Open-cell ceramic network after Claim 1 , in which at least 50% of all pores in the SSiC structure have a circularity between 0.8 and 0.95, advantageously between 0.9 and 0.95. Open-cell ceramic network after Claim 1 , in which open structure pores, cavities and cracks are filled with SiC materials. Method for producing open-cell ceramic networks from a three-dimensionally cross-linked ceramic structure made up of many interconnected webs and open cells lying between the webs, the webs essentially consisting of pressure-free sintered SiC (SSiC), and the SSiC structure of the webs having a porosity of 2 to 10% of completely or essentially completely closed pores, with up to 50% of all pores in the SSiC structure having an average diameter of ≤ 1 µm, and up to 90% of all pores in the SSiC structure having an average diameter of ≤ 3 µm, and at least 50% of all pores in the SSiC structure have a circularity greater than 0.8, in which a coarse silicon carbide powder with an average grain size of 1 to <5 μm and a fine silicon carbide powder with an average grain size of 0.1 to 1 μm a mixture is produced, the mixing ratio of coarse to fine silicon carbide powder between 10: 90 to 90: 10, a suspension is produced from this mixture together with small amounts of sintering additives known per se, then an open-cell foam or an open-cell network is coated one or more times with the suspension, the foam or the network is dried, the foam or the network is removed and the remaining green body is sintered at temperatures ≥ 1800 ° C under protective gas or vacuum. Procedure according to Claim 8 , in which a coarse silicon carbide powder with an average grain size of 2 to 4 µm is used. Procedure according to Claim 8 , in which a fine silicon carbide powder with an average grain size of 0.2 to 0.9 µm is used. Procedure according to Claim 8 , in which a silicon carbide powder mixture with a mixing ratio of coarse to fine silicon carbide powder is used between 30: 70 to 70: 30. Procedure according to Claim 8 , in which boron and carbon or boron, aluminum and carbon are used as known sintering aids and a basic binder based on polysaccharides, polyamides or polyacrylates is used as a sintering aid. Procedure according to Claim 8 , in which a polyurethane foam is used as the open-cell foam, or a network of organic synthetic or natural fibers is used as the open-cell network. Procedure according to Claim 8 , in which the removal of the foam or the network is realized by burning out, evaporating, etching or dissolving. Procedure according to Claim 8 , in which the green body is sintered at temperatures in the range from 1800 to 2300 ° C., advantageously at temperatures between 2000 and 2180 ° C. Procedure according to Claim 8 , in which water is used as a suspending agent. Procedure according to Claim 8 with a linear shrinkage of 12 to 20%. Procedure according to Claim 8 , in which open pores, cavities and cracks are filled with SiC materials before and or after sintering. Procedure according to Claim 18 , with the SiC material in the open pores, voids and cracks by infiltration, immersion, watering, spraying. Procedure according to Claim 18 and 19 , where there is a cohesive connection between the first and time coating.