METHOD FOR PRODUCE A MOLDED BODY AND MOLDED BODY

Description

[0001] The present invention relates to a method for producing a molded body.

[0002] From DE 10 2005 036 394 A1, a method for producing a shaped body is known in which a workpiece is formed from a base material and then sintered at temperatures above 1,550 °C. The sintering process transforms the starting material.

[0003] The following documents disclose various devices, products, processes and uses relating to ceramic materials, in particular cordierite and mullite materials: G. Routschka / H. Wuthnow: "Practical Handbook of Refractory Materials; 5th Edition", October 14, 2011 (2011-10-14), Vulkan Verlag, Essen (DE), ISBN: 978-3-8027-3161-7 ACIMOVIC-PAVLOVIC Z ET AL: "Application Effects of Refractory Linings Based on Cordierite and Talc during the Lost Foam Casting Process", INTERCERAM, SCHMID, FREIBURG, DE, Vol. 49, No. 6, January 1, 2000 (2000-01-01), pages 438-441, , ISSN: 0020-5214 HOMBACH ET AL: "Observations on Cordierite-Mullite Materials", SILIKAT-JOURNAL, GEKT : GLASS, ENAMEL, KERAMO-TECHNIK, Vol. 15, January 1, 1976 (1976-01-01), pages 285-289, 291, ISSN: 0560-0421 EP 1 428 807 A2 EP 1 447 130 A1 JP S59 159296 A GB 579 515 A GROSJEAN P: "Cordierite-Ceramics", INTERCERAM, SCHMID, FREIBURG, DE, Vol. 42, No. 1, January 1, 1993 (1993-01-01), pages 11-15, ISSN: 0020-5214 SCHULLE W: "Trends in the Development of Firing Aid Materials" SILIKATTECHNIK, VERLAG FUER BAUWESEN, BERLIN, DE, Vol. 36, No. 10, 1.January 1985 (1985-01-01), pages 313-315, ISSN: 0037-5233 GB 1 527 566 A US 5 305 726 A .

[0004] The present invention is based on the objective of providing a method for producing a molded body by means of which a molded body with good high-temperature resistance can be easily produced.

[0005] This problem is solved according to the invention by a method according to claim 1. The method for producing a molded body comprises: Providing a mixture comprising: a powdered base material which includes a pre-burned and / or ground mullite material; producing a shaped body by shaping the mixture;Firing the shaped body at a temperature below the melting point of the base material, such that particles of the base material are interconnected while retaining their particle properties, wherein clear particle boundaries are visible in a scanning electron microscope image of the finished shaped body, wherein the particles of the base material are cross-linked, and wherein the particles of the base material are not completely melted and are fused with other particles to form a larger body, and wherein the shaped body is fired in a firing cycle with a temperature gradient of > 2200 K / h, and wherein the mixture comprises an alkali metal-containing and / or an alkaline earth metal-containing material as a sintering aid, and / or wherein an alkali metal-containing and / or an alkaline earth metal-containing material is additionally added to the mixture as a sintering aid for shaping it.

[0006] Because the particles of the base material are bonded together in the process according to the invention while preserving their particle properties, the shaped body can be produced particularly energy-efficiently and quickly. In particular, time-consuming sintering at high temperatures after shaping the body is unnecessary.

[0007] The powdered base material preferably comprises pre-calcined mullite.

[0008] A mullite material, especially mullite, can be, for example, mullite chamotte, sintered mullite and / or melted mullite.

[0009] The term "fireclay" refers to a material that has been fired or pre-fired and then ground.

[0010] The mullite material is preferably present in the powdered base material in a significant quantity, i.e., not merely as an impurity. In particular, it may be provided that the powdered base material comprises at least approximately 20 wt% (mass%), more specifically at least approximately 40 wt%, for example at least approximately 60 wt% mullite material.

[0011] In this description and the attached claims, "particle property" means that clear particle boundaries are visible in a scanning electron microscope image (SEM image) of the finished molded body. The particles are interconnected, for example, bonded together. The particles are not completely melted and fused with other particles to form a larger body.

[0012] A powder or powdery substance is understood to be a substance consisting of particles which are of a small size, in particular smaller than approximately 0.1 mm, for example smaller than approximately 0.05 mm.

[0013] Preferably, the shaped body obtains its dimensional stability through firing.

[0014] It can be advantageous if the mixture includes corundum, zircon mullite and / or zircon silicate.

[0015] Corundum can be, for example, sintered corundum and / or fused corundum. Preferably, the mixture comprises corundum, zirconium mullite and / or zirconium silicate in powder form.

[0016] The mixture additionally comprises an alkali metal-containing and / or alkaline earth metal-containing material and / or an alkali metal-containing and / or an alkaline earth metal-containing material is additionally added to the mixture for forming it.

[0017] In particular, it may be provided that the powdered base material and / or the mixture is sieved, for example to obtain a preferred particle size distribution. The alkali metal-containing and / or alkaline earth metal-containing material is preferably added to the powdered base material and / or the mixture after sieving.

[0018] The alkali metal-containing and / or alkaline earth metal-containing material is preferably a plasticizing agent, particularly to simplify the shaping process when forming the mixture.

[0019] The alkali metal-containing and / or alkaline earth metal-containing material preferably comprises lithium (Li), sodium (Na), potassium (K), rubidium (Rb), beryllium (Be), magnesium (Mg), calcium (Ca) and / or strontium (Sr).

[0020] In particular, the alkali metal-containing and / or alkaline earth metal-containing material may comprise potassium ions. Preferably, the alkali metal-containing and / or alkaline earth metal-containing material comprises at least approximately 0.1 wt%, for example at least approximately 0.5 wt%, and / or at most approximately 2 wt%, in particular at most approximately 1.5 wt%, potassium ions.

[0021] For example, the alkali metal and / or alkaline earth metal-containing material may be an aqueous solution with approximately 1 wt% potassium ions. For example, an aqueous solution with approximately 1.6 wt% potassium carbonate (K₂CO₃) may be specified.

[0022] The alkali metal-containing and / or alkaline earth metal-containing material is a sintering aid material.

[0023] The alkali metal-containing and / or alkaline earth metal-containing material enables the particles of the base material to be joined together, for example, a metallurgical bonding of the particles of the base material together.

[0024] In one embodiment of the invention, it is provided that at least one substance of the mixture, in particular the base substance, is sieved before being combined with other substances of the mixture and / or that the entire mixture is sieved in order to obtain a desired particle size distribution.

[0025] In particular, it may be provided that mullite, zirconium mullite and / or corundum, each taken separately, are sieved and then combined with other substances of the mixture, in particular mullite, zirconium mullite and / or corundum.

[0026] Alternatively or additionally, it may be provided that at least one substance of the mixture, in particular the base substance, and / or the entire mixture is supplied with a desired particle size distribution, for example with one of the particle size distributions described below.

[0027] In one embodiment of the invention, it is provided that at least one substance of the mixture is sieved before being combined with other substances of the mixture and / or that the entire mixture is sieved in such a way as to that the d10 value of the at least one substance and / or the mixture is at least approximately 1 µm, that the d50 value of the at least one substance and / or the mixture is at least approximately 38 µm and / or that the d90 value of the at least one substance and / or the mixture is at least approximately 60 µm.

[0028] A d10 value is understood to be the particle size which is below 10% of the particles of the at least one substance or of the entire mixture, while 90% of the particles of the at least one substance or of the mixture are larger than the d10 value.

[0029] The d50 value is understood to be the particle size which is below 50% of the particles of the at least one substance or of the entire mixture, while 50% of the particles of the at least one substance or of the mixture are larger than the d50 value.

[0030] The d90 value is understood to be the particle size which is below 90% of the particles of the at least one substance or of the entire mixture, while 10% of the particles of the at least one substance or of the mixture are larger than the d90 value.

[0031] In one embodiment of the invention, it is provided that at least one substance of the mixture is sieved before being combined with other substances of the mixture and / or that the entire mixture is sieved in such a way as to that the d10 value of the at least one substance and / or the mixture is at most approximately 2 µm, that the d50 value of the at least one substance and / or the mixture is at most approximately 49 µm and / or that the d90 value of the at least one substance and / or the mixture is at most approximately 80 µm.

[0032] A median of the at least one substance and / or the mixture is preferably at least approximately 10 µm and / or at most approximately 20 µm, for example approximately 15 µm.

[0033] Preferably, the molded body has a pore volume (volume fraction of the pores to the total volume) of at least approximately 35%, in particular at least approximately 40%, and / or at most approximately 65%, for example at most approximately 58%.

[0034] It may be provided that the mixture includes a pore-enhancing agent. For example, it may be provided that the mixture contains at least approximately 5 wt% and / or at most approximately 20 wt%, for example approximately 10 wt%, a pore-enhancing agent, and / or that at least approximately 5 wt% and / or at most approximately 20 wt%, for example approximately 10 wt%, a pore-enhancing agent is added to the mixture.

[0035] In one embodiment of the invention, it may be provided that at least one substance of the mixture is sieved before being combined with other substances of the mixture and / or that the entire mixture is sieved in such a way, that the d10 value of the at least one substance and / or the mixture is at least approximately 5 µm, that the d50 value of the at least one substance and / or the mixture is at least approximately 35 µm and / or that the d90 value of the at least one substance and / or the mixture is at least approximately 70 µm.

[0036] Furthermore, it may be provided that at least one substance of the mixture is sieved before being combined with other substances of the mixture and / or that the entire mixture is sieved in such a way, that the d10 value of the at least one substance and / or the mixture is at most approximately 10 µm, that the d50 value of the at least one substance and / or the mixture is at most approximately 40 µm and / or that the d90 value of the at least one substance and / or the mixture is at most approximately 80 µm.

[0037] An amorphous fraction of the molded body preferably amounts to at most approximately 2%, for example between approximately 0.5% and approximately 1%.

[0038] Furthermore, it may be provided that the mixture before firing and / or the shaped body after firing comprises between approximately 5 wt% and approximately 50 wt%, in particular between approximately 5 wt% and approximately 30 wt%, for example approximately 15 wt%, corundum.

[0039] Furthermore, it can be advantageous if the mixture before firing and / or the molded body after firing contains between approximately 5 wt% and approximately 30 wt%, in particular between approximately 15 wt% and approximately 30 wt%, for example approximately 25 wt%, mullite.

[0040] In a further embodiment of the invention, it can be provided that the mixture before firing and / or the shaped body after firing comprises at least approximately 50 wt% mullite, at least approximately 10 wt% corundum and / or at least approximately 10 wt% zirconium mullite.

[0041] It can be advantageous if the mixture before firing and / or the shaped body after firing contains at most approximately 80 wt% mullite, at most approximately 50 wt% corundum and / or at most approximately 30 wt% zirconium mullite.

[0042] "Mullit" refers in particular to "sintered mullite".

[0043] It can be advantageous if the mixture before firing and / or the molded body after firing comprises between approximately 50 wt% and approximately 80 wt%, in particular between approximately 60 wt% and approximately 80 wt%, for example approximately 70 wt%, mullite, especially sintered mullite.

[0044] Furthermore, it may be advantageous if the mixture before firing and / or the shaped body after firing comprises between approximately 10 wt% and approximately 50 wt%, in particular between approximately 10 wt% and approximately 30 wt%, for example approximately 20 wt%, corundum, in particular sintered corundum.

[0045] Furthermore, it may be provided that the mixture before firing and / or the shaped body after firing comprises between approximately 5 wt% and approximately 30 wt%, in particular between approximately 10 wt% and approximately 20 wt%, for example approximately 10 wt%, zirconium mullite.

[0046] The shaped body is fired with a temperature gradient of > 2200 K / h, in particular > 2200 K / m and > 2200 K / h.

[0047] Preferably, the molded part is fired at a temperature of no more than approximately 1,400 °C. This allows the particles of the base material to bond together while preserving their particle properties. In particular, it prevents undesirable transformations of the base material, such as melting of the particles, at temperatures above approximately 1,400 °C.

[0048] In one embodiment of the invention, the molded body is fired at a temperature of approximately 1,300 °C. At this temperature, a strong bond between the particles of the base material is ensured while preserving their particle properties.

[0049] It can be advantageous if the molded part is fired in a firing cycle lasting at least approximately 300 minutes and / or at most approximately 480 minutes, for example approximately 360 minutes.

[0050] The duration of the firing cycle is understood to be the time between the start of the firing process of the molded part and its end, in particular the removal of the molded part from a furnace. Specifically, this includes the time between the start of the firing process and the cooling of the fired molded part.

[0051] The shaped body is, in particular, a honeycomb body.

[0052] Preferably, the molded body is a ceramic molded body, in particular a porous ceramic molded body.

[0053] The inventive method makes it possible to produce a shaped body that has good high-temperature resistance and is easy to manufacture.

[0054] The molded body preferably has one or more of the features and / or advantages described in connection with the method according to the invention. Furthermore, the molded body preferably has one or more features and / or advantages that result from carrying out one or more of the process steps described above.

[0055] The molded body is preferably made from a powdered material. Preferably, the surfaces and / or interfaces of the mullite material particles in the molded body are essentially like the surfaces of particles of a ground mullite material. Clear boundaries between the mullite material particles and their surroundings are visible using a scanning electron microscope image. The mullite material particles are not melted, but merely cross-linked, for example, by point-like, metallurgical bonds.

[0056] The shaped body is preferably a honeycomb body, in particular a filter body.

[0057] In particular, the molded body is suitable for use as a wall flow filter and / or as a flow filter, possibly after further processing.

[0058] The shaped body can be formed, for example, using an extrusion process.

[0059] A shaped body designed as a honeycomb preferably has honeycombs arranged in a matrix. In particular, it can be provided that the honeycomb body has a square cross-section and comprises honeycombs (cells) which are arranged in a matrix and are cylindrical, wherein the honeycombs (cells) preferably also have a square cross-section.

[0060] It can be advantageous if the mold body contains 100, 200 or 300 cells per square inch.

[0061] Furthermore, the inventive method for producing a molded body and / or the molded body produced by the method may have one or more of the following features and / or advantages: The particles of the base material preferably remain chemically and / or physically essentially unchanged during the firing of the molded body.

[0062] Preferably, the mullite material of the molded body is not produced, or only to an insignificant extent, during the firing of the molded body by converting a base material.

[0063] Preferably, during the firing of the molded body, no or only an insignificant phase transformation of the base material into a desired mineral phase, in particular into mullite, takes place.

[0064] The molded body preferably exhibits no anisotropy of expansion.

[0065] A thermal shock parameter of the molded body is preferably greater than 300 K.

[0066] The molded body can be heated and / or flammable with a temperature gradient of at least approximately 2,000 K per hour.

[0067] Furthermore, it may be provided that a combination of sintered mullite, sintered corundum, zirconium mullite, zirconium silicate and / or a sintering additive from the group of alkali metals and / or alkaline earth metals is used to produce a shaped body.

[0068] The molded body can be used in particular as a soot filter, especially as a diesel soot particle filter.

[0069] The molded body is preferably dimensionally stable up to temperatures of more than approximately 1,600 °C, especially when using a mullite material with a thermal shock parameter of more than 200 K.

[0070] The base material is preferably a pre-sintered material which can be fired to form a dimensionally stable body, in particular through reactive sintering behavior with a sinter additive.

[0071] By selective sieving of at least one substance, in particular the base substance, and / or the mixture, a pore volume, in particular with a pore radius distribution between approximately 0.004 mm and approximately 0.045 mm, can preferably be specifically adjusted.

[0072] Particularly when the molded part is a fine-walled honeycomb structure, the use of a pore-enhancing agent to adjust the pore volume and the median of the particle size distribution can be advantageous. This is especially beneficial when the distance between the centers of two adjacent, parallel honeycomb walls of the molded part (so-called pitch P) is relatively small, for example, less than 0.27 mm.

[0073] The molded body according to the invention is preferably more alkali-resistant and corrosion-resistant than cordierite molded bodies and / or mullite molded bodies which are sintered at very high temperatures according to known manufacturing processes.

[0074] It can be provided that the coefficient of expansion of the shaped body, which for example mainly comprises mullite, is preferably essentially identical in all three spatial directions between approximately 20 °C and approximately 800 °C and is, for example, approximately 5.2 x 10 -6< / K.

[0075] It can be advantageous if the molded body is or is provided with a coating, in particular with a catalytically effective coating.

[0076] The molded part can be produced, for example, using a pressing process and / or an extrusion process.

[0077] It may be provided that the powdered base material, in particular the mullite material, is pre-fired, for example at a temperature of up to approximately 1,200 °C. During the firing of the molded part, the powdered base material is then preferably fully fired to obtain its final chemical properties. The particle structure of the base material particles is retained in this process.

[0078] Preferably, the mixture and / or the formed body after firing comprises aluminium oxide (Al 2 O 3 ), glass, in particular magnesium aluminium silicate, mullite and / or aluminosilicate.

[0079] Preferably, aluminum oxide (Al₂O₃) is added to the mixture in a superstoichiometric amount. This allows the molded part to re-sinter in the event of unwanted overheating during its use, for example as a particle filter, before damage occurs.

[0080] It can be advantageous if the mixture includes an additive to increase the strength of the molded part.

[0081] The thermal shock parameter is, in particular, a relative measure of an acceptable temperature difference, based on the ratio between the flexural strength of the material and the stress imposed by a temperature gradient (in Kelvin per minute or Kelvin per hour). A shaped body made of SiC has a thermal shock parameter of, for example, at most approximately 160 K. A shaped body according to the invention preferably has a thermal shock parameter of at least approximately 180 K.

[0082] The temperature gradient refers to a spatially or temporally dependent temperature gradient and is measured in Kelvin per meter, Kelvin per minute, or Kelvin per hour. Specifically, to describe the heating rate, the time-dependent temperature gradient in Kelvin per minute is given in the range from approximately 120 °C to, for example, approximately 2,000 °C. A smaller temperature gradient can lead to an increase in the coefficient of thermal expansion, changes in the pore structure, and / or a decrease in strength. A large temperature gradient, for example, greater than approximately 2,300 °C / h, can result in a dimensionally stable molded body, especially honeycomb structures, with sufficient strength.

[0083] Examples of pore-enhancing agents to adjust the degree of porosity and / or pore size include graphite, acrylate, acrylic glass, coconut flour and / or corn.

[0084] Further preferred features and / or advantages of the invention are the subject of the following description of individual embodiments.

[0085] In a first embodiment of a method for producing a shaped body, in particular a honeycomb body, for example a diesel particulate filter (DPF), which is not according to the invention, a mixture is used as the starting material. The mixture comprises a powdered base material which includes pre-calcined, calcined substances, for example cordierite, mullite, sintered mullite, corundum, sintered corundum and zirconium mullite.

[0086] The materials are ground and / or sieved to obtain a desired particle size distribution.

[0087] In particular, the mixture, especially the powdered base material, is milled and / or sieved such that the d10 value is between approximately 1 µm and approximately 2 µm, the d50 value is between approximately 38 µm and approximately 49 µm, and the d90 value is between approximately 60 µm and approximately 80 µm. This yields a pore volume of approximately 40% to approximately 58%. A median pore size distribution is preferably between approximately 10 µm and approximately 20 µm.

[0088] With a median pore size distribution of approximately 10 µm, the use of a pore-enhancing agent is preferably unnecessary. However, a pore-enhancing agent can also be added in amounts of, for example, approximately 5 wt% to approximately 20 wt% to obtain, for example, a modified pore size distribution, particularly with a median of approximately 20 µm.

[0089] Alternatively, it can be provided that a pore size distribution with a median of approximately 20 µm is obtained by modifying the sieving of the mixture, in particular the base material. For this purpose, the mixture, in particular the powdered base material, is sieved such that the d10 value is between approximately 5 µm and approximately 10 µm, that the d50 value is between approximately 35 µm and approximately 40 µm, and that the d90 value is between approximately 70 µm and approximately 80 µm.

[0090] In the first embodiment not according to the invention, the mineral composition of the base material is approximately 60 wt% cordierite, approximately 15 wt% corundum, and approximately 25 wt% mullite. A sintering aid is preferably added to the mixture. In particular, an aqueous solution comprising approximately 0.1 wt% to approximately 2 wt% alkali and / or alkaline earth ions is added.

[0091] The mixture is then shaped into the desired form using a pressing or extrusion process.

[0092] The resulting shaped body can then be freeze-dried and further processed, for example.

[0093] In particular, the molded body is then fired to achieve its final strength.

[0094] The molded part is fired, for example, in a continuous kiln with a feed rate of 10 to 30 cm / min, for example approximately 20 cm / min, and a temperature gradient of more than 2,200 K / h. The firing time from the start of the firing process until the molded part has cooled is between approximately 300 minutes and approximately 480 minutes, for example approximately 360 minutes.

[0095] The temperature of the molded body during firing is preferably no more than approximately 1,400 °C, for example approximately 1,330 °C. This ensures that the particles of the powdered base material, in particular the cordierite material and / or the mullite material, retain their particle properties unchanged even after the firing step. In particular, clear boundaries or edges of the base material particles in the molded body are visible in a suitably magnified image even after the firing step.

[0096] After completion of the molded body, in particular after firing and cooling of the molded body, the molded body has, for example, a coefficient of expansion between approximately 1.7 and 4 x 10 -6< / K, especially in the temperature range between approximately 20 °C and approximately 800 °C.

[0097] The molded body can be used, in particular as a honeycomb structure, for example as a diesel soot particle filter. For this purpose, the molded body can be provided with a catalytic coating, for example.

[0098] A second embodiment of a method for producing a shaped body, and the shaped body obtained thereby, differ from the method and the shaped body according to the first embodiment essentially in that the powdered base material comprises approximately 70 wt% sintered mullite, approximately 20 wt% sintered corundum, and approximately 10 wt% zirconium mullite. The shaped body obtained thereby has a coefficient of thermal expansion of approximately 5.2 x 10⁻⁶ K / K in the temperature range between approximately 20 °C and approximately 800 °C.

[0099] Furthermore, the method for producing a molded body and the molded body according to the second embodiment correspond with regard to the manufacturing steps, structure, properties and function to the method for producing a molded body or to the molded body according to the first non-inventive embodiment, so that reference is made to its preceding description in this respect.

[0100] Because in all processes for manufacturing a shaped body, particles of the base material are joined together while retaining their particle properties during the firing of the shaped body, the shaped body can be manufactured easily and exhibit high temperature resistance.

Claims

1. A method of producing a molded body, comprising: - providing a mixture, comprising: - a powdered base material which comprises a pre-fired and / or ground mullite material; - producing a molded body by molding the mixture; - firing the molded body at a temperature below the melting temperature of the base material, so that particles of the base material are bonded to each other while preserving the particle property, wherein clear boundaries of the particles are visible in a scanning electron micrograph of the finished molded body, wherein the particles of the base material are cross-linked with each other, and wherein the particles of the base material are not fully melted and are fused with further particles to form a larger body, and wherein the molded body is fired in a firing cycle at a temperature gradient of > 2200 K / h, and wherein the mixture comprises an alkali metal-containing and / or an alkaline earth metal-containing material as a sintering aid and / or that an alkali metal-containing and / or an alkaline earth metal-containing material is additionally admixed to the mixture as a sintering aid for molding the mixture.

2. The method according to claim 1, wherein the mixture additionally comprises corundum, zirconia mullite and / or zirconium silicate.

3. The method according to either one of claims 1 or 2, wherein at least one substance of the mixture is screened prior to a combination with further substances of the mixture and / or that the entire mixture is screened to obtain a desired grain size distribution.

4. The method according to any one of claims 1 to 3, wherein at least one substance of the mixture is screened prior to a combination with further substances of the mixture and / or that the entire mixture is screened such that - the D10 value of the at least one substance and / or of the mixture is at least 1 µm, - the D50 value of the at least one substance and / or of the mixture is at least 38 µm, and / or - the D90 value of the at least one substance and / or of the mixture is at least 60 µm.

5. The method according to any one of claims 1 to 4, wherein at least one substance of the mixture is screened prior to a combination with further substances of the mixture and / or that the entire mixture is screened such that - the D10 value of the at least one substance and / or of the mixture is at most 2 µm, - the D50 value of the at least one substance and / or of the mixture is at most 49 µm, and / or - the D90 value of the at least one substance and / or of the mixture is at most 80 µm.

6. The method according to any one of claims 1 to 5, wherein at least one substance of the mixture is screened prior to a combination with further substances of the mixture and / or that the entire mixture is screened such that - the D10 value of the at least one substance and / or of the mixture is at least 5 µm, - the D50 value of the at least one substance and / or of the mixture is at least 35 µm, and / or - the D90 value of the at least one substance and / or of the mixture is at least 70 µm.

7. The method according to any one of claims 1 to 6, wherein at least one substance of the mixture is screened prior to a combination with further substances of the mixture and / or that the entire mixture is screened such that - the D10 value of the at least one substance and / or of the mixture is at most 10 µm, - the D50 value of the at least one substance and / or of the mixture is at most 40 µm, and / or - the D90 value of the at least one substance and / or of the mixture is at most 80 µm.

8. The method according to any one of claims 1 to 7, wherein the mixture prior to firing and / or the molded body after firing comprises at least 50% by mass of mullite, at least 10% by mass of corundum, and / or at least 10% by mass of zirconia mullite.

9. The method according to any one of claims 1 to 8, wherein the mixture prior to firing and / or the molded body after firing comprises at most 80% by mass of mullite, at most 50% by mass of corundum, and / or at most 30% by mass of zirconia mullite.

10. The method according to any one of claims 1 to 9, wherein the molded body is fired at a temperature of at most 1400 °C.

11. The method according to any one of claims 1 to 10, wherein the molded body is fired in a firing cycle having a duration of at least 300 min and / or at most 480 min.