DE10134524A1 - Foam ceramic with a directed open pore structure - Google Patents

Abstract

The invention relates to ceramic materials based on aluminium oxide or silicate and/or carbide and/or zirconium oxide and/or hydroxylapatite, and having a directed open or closed pore structure. The invention also relates to a method for producing the same. The ceramic materials are suitable for producing construction materials, fire-proof materials, combustion aids, catalyst carriers and filter materials.

Description

The invention relates to ceramic material Alumina / silicate base and / or carbide base, a process for its manufacture and its use for the manufacture of Building materials, refractory materials, catalyst supports and Filter materials.

Porous stones based on aluminum oxide / silicate or carbide are produced in a conventional manner by burning organic materials or by degassing carbonates or sulfates at high temperatures (see P. Sepulveda, JGP Binner, Processing of Cellular Ceramics by foaming and in situ polymerization of Organic Monomers, Journal of the European Ceramic Soc. 19 (1999) 2059-2066 and TJ Fitzgerald, A. Mortensen, Processing of microcellular SiC foams, J. Mater. Sci. 30 (1995) 1025-1032). A disadvantage of this technology is that when organic substances are burned out, strongly reducing gases are produced which can lead to so-called "black cores". Furthermore, the pore distribution of the fired ceramic is limited by the grain size distribution of the material to be burned out. At the moment there are few lightweight fire bricks with a classification temperature above 1200 ° C and a bulk density below 1000 kg / m ³ . In addition, there are no light kaolin-based fire bricks with an application temperature above 1200 ° C to 1250 ° C. Furthermore, there are few lightweight fire bricks with a bulk density of less than 1000 kg / m ³ and a cold compressive strength of more than 2 MPa (refractory materials for the ceramic industry, R. Sladek, publishing group German Economic Service, 1994, and product information: Morgan "Thermal Ceramics"). It is also known that the manufacturing process of light mullite or light corundum ceramics is very complicated and expensive. In the conventional porosity process, one also works with the burning out of organic additives (e.g. plastics) and therefore also causes environmental problems (emission of HCl, HF, etc.). Hollow spherical corundum powders, some of which are very energy-intensive, are sometimes processed and sintered. Another known porosity method is the use of peroxides in an acidic environment. The stones produced in this way also only reach application temperatures of a maximum of 1250 ° C. Furthermore, the porous materials produced by the methods of the prior art run the risk of cracking during the shaping, drying or burnout process, which has disadvantageous physical properties, and furthermore the cold compressive strengths of the materials produced are also not always satisfactory. When the masses are burned with organic burn-out substances, smoldering gases (HCl, HF, CO, CO ₂ , etc.) are created and as a result an increased environmental impact. There is also the possibility of forming black cores. When using soaps, there are disadvantages that the possibility of adjusting the porosity is limited, that the stability of the raw material is low, and that the soaps mostly introduce alkalis, which lower the operating temperature of the ceramic. In many cases, the thermal conductivity is also not sufficiently low.

The object of the invention is porous ceramic Alumina / silicate and / or carbide based materials and to provide processes for their production, in which the disadvantages of the prior art described above Technology does not occur.

The solution to the problem is a ceramic material based on alumina / silicate and / or carbide, which is characterized in that it is in the macro and micro range has a directed, open pore structure. The ceramic Material based on alumina / silicate stands out further characterized by the fact that after sintering there is an almost quartz and cristoballite-free material with oriented mullites and Forms spinels.

The process of making the ceramic materials consists essentially in that slip-like masses Alumina / silicate base and / or carbide base under Use of conventional modifiers, binders and Thixotropic agents at temperatures below 100 ° C and pH values foamed from 7 to 12 with metal pastes or powders become. The foamed ceramic material will then dried and at temperatures from 900 ° to 1800 ° C burned. Drying is optionally carried out using Microwaves.

The ceramic materials are suitable for the production of Building materials and refractories as well as due to their directed, open pore structure also for the production of Catalyst supports and filter materials.

With bulk densities of about 0.35 to 1.0 g / cm ³ and classification temperatures of 1200 ° C to 1650 ° C, the thermal conductivity of the ceramic materials according to the invention is very low.

By selecting certain raw materials - aluminum oxides / silicates (Clays, kaolins, etc.) and carbides (SiC, etc.) and mixtures both - and mixing with those necessary for gas development Metal powders or pastes, and additives, such as. B. Modifiers, binders, thixotropic agents, the a pore formation process at room temperature or slightly elevated temperatures below 100 ° C in a ceramic mass enable to be environmentally friendly and without organic Burnout materials Light stones with high application temperatures and open porosity. The foaming is said to take shape with the final dimensions of the stones or in large blocks that later cut into the desired formats, respectively. Drying is conventional, or can be done with the help accelerated by microwaves, or even made possible. The firing takes place, depending on the application class of the products, between 900 ° C and 1800 ° C.

The use of metal powders or pastes for foaming is known in principle from the production of aerated concrete but so far not for the production of foamed ceramic have been considered since the foaming at Aerated concrete production based on the reaction of the strongly alkaline Suspension (pH> 13) with the metal, using hydrogen as Propellant is set (see "The AAC manual", Prof. Dr.-Ing. Dr. H. c. Helmut Weber, Bauverlag GmbH, Wiesbaden, 1992). In the case of ceramic materials, this would high alkalinity due to the considerable proportion of alkali or alkaline earth elements to a significant reduction in Fire resistance. Surprisingly, the process in the manufacture of ceramic materials at a great deal lower pH values are carried out.

As starting materials for the production of the invention ceramic materials are customary slip-like masses Alumina / silicate base and / or carbide base, for example masses based on kaolin, alumina and / or Silicon carbide used. Depending on your needs, they include slurry-like masses customary in the present field Additives such as B. modifiers, binders or Thixotropic agents that are free of alkalis and phosphate should.

The metal pastes are then in the slurry-like masses or powder mixed in and at temperatures below 100 ° C and pH values from 7 to 12 foamed in molds.

All possible metals come into consideration as metals, for example Al, Mg, Zn, Ti, etc. Preferably Metals Al, Mg and Zn used.

The metals can be used in the form of metal powders or metal pastes. In the case of metal pastes, they are preferably water or glycol pastes. The D ₅₀ value of the powders or pastes is preferably in a range from 10 to 200 μm.

The advantage of the method according to the invention is that that in the formation of the pore structure, the total porosity and the pore size distribution can be adjusted within wide limits are (e.g. 0.2-5 mm). Because of the applied The pore formation process gives a directed, continuous Macro-pore structure, which also makes the body manufactured as Catalyst supports or used as filter media can. Also in the micro range (up to 5 µm) special processes an orientation of the particles (e.g. House of cards structure). This helps improve of shrinkage behavior.

So far, porosity of inorganic building materials has been achieved Metal powders or pastes only in alkaline to strong alkaline environment (pH> 12 - see above). Metal pastes also enable one - as carried out here intensive gas development process in alkali-free or very low-alkali ceramic masses (pH 7-pH 9). However, it can the entire range between pH 7 and pH 14 is also covered become.

Porosity with metal powders or metal pastes reduces the risk of cracking due to the shaping, Drying or burnout process, which improved physical Properties (also in the individual process stages) result Has.

The drying can, depending on the bulk density of the masses, additionally accelerated significantly with the help of microwave drying or be made possible in the first place.

With a porosity of lightweight fire bricks with Aluminum pastes or powders are used because of the cheaper Pore distribution a higher cold compressive strength with comparable Bulk densities achieved compared to conventionally porous stones.

The metal particles of the powders or pastes influence the Phase formation process during sintering (spinel, Mullite formation) of the stones favorably. Oriented mullites are formed. As a result, the fire resistance increases as well Compressive strength of the stones. You get an (almost) quartz and critoballite-free refractory material without "regrowth" of the Stones, which increases their lifespan.

The technology of porosity of ceramic masses Aluminum powder or pastes is simple and also allows the production of small formats by foaming in small ones Forms the production of larger stones by casting into large shapes and then cutting the desired ones Formats.

The previously known light ceramic stones are - as already mentioned above - with ordinary organic foaming agents - Polymer materials, soaps, etc., or by burning out organic placeholder porous.

The method according to the invention, however, works with a inorganic material, a metal powder or one Metal paste, with a direct in an aqueous ceramic mass Reaction with gas formation between the minerals (e.g. Kaolinite) and the metal powder (e.g. aluminum) expires.

For the reaction of clay minerals (without and with additives) the pH of the mass is very low with the metal; approximately neutral to pH 9. The amount used is also very low, it lies e.g. B. at 0.1-5.0 mass%, based on the Dry matter. For the fixation of the porous structure during and after the rheological properties of the gas development Mass composition by means of modifiers, binders, Thixotropic agent set.

All mass components are made with about 10-50 mass% water mixed to a uniform consistency and in shapes filled. The pore formation process takes place at room temperature or at higher temperatures in about 2 to 60 minutes. The Porous green body is formed in or under Use of microwaves dried. The dried material is at selected temperatures between 900 ° C and 1800 ° C, depending on the composition, burned.

The green or burned stones may still be sawn into the desired formats.

Examples example 1 mullite

A slip composition of 27.0 mass% Zettlizer kaolin 1a, 27.0 mass% alumina (Al ₂ O ₃ ) and 36.0 mass% water is mixed thoroughly. Then 0.5% by mass of aluminum powder is mixed in and the mixture is poured into a mold for foaming. The pH of the mixture is 7.5. The pore formation process takes about 2 to 20 minutes. The ceramic material obtained is then dried conventionally or in a microwave oven to a moisture content of <0.5% by mass. Then the ceramic material is fired in an oven at up to 1800 ° C depending on the desired application temperature.

The ceramic obtained has the following properties, the determination method being given in brackets:
Bulk density [g / cm ³ ]: 1.12 (DIN 51065)
Thermal conductivity (30 ° C) [W / Km]: 0.26 (DIN 52612)
Cold compressive strength [N / mm ² ]: 6.5 (DIN 51067)
Reversible linear expansion [%]: 0.6 (dillatometry)
Aftershortening [%]: 1.0 (dillatometry)
Softening temperature [° C]: 1485 (dillatometry)
Classification temperature [° C]: 1400 (dillatometry)
Average macro pore size [µm]: 500 (Hg porosimetry)
Main components:
Al ₂ O ₃ - 71% (X-ray diffractometry)
SiO ₂ - 27.2%
Fe ₂ O ₃ - 0.4%
Phase composition of the ceramic: mullite (X-ray diffractometry)

Example 2 Alumina

From a slip composition of 13.6 mass% Zettlizer kaolin 1a, 54.4 mass% alumina (Al ₂ O ₃ ), 32.0 mass% water and 0.7 mass% aluminum paste (pH value of the mixture: 8.5), a corundum ceramic is produced by the method of Example 1, the properties of which are given in the table below:
Bulk density [g / cm ³ ]: 0.73
Thermal conductivity (30 ° C) [W / Km]: 0.14
Cold compressive strength [N / mm ² ]: 2.2
Reversible linear expansion [%]: 1.1
Aftershortening [%]: 0.6
Softening temperature [° C]: 1550
Classification temperature [° C]: 1500
Average macro pore size [µm]: 350
Main components:
Al ₂ O ₃ - 89.1%
SiO ₂ - 9.1%
Fe ₂ O ₃ - 0.2%
Phase composition of the ceramic: corundum

Example 3 cermet

A slip composition of 23.5 mass% Zettlizer kaolin 1a, 45.5 mass% alumina (Al ₂ O ₃ ), 31.0 mass% water and 0.4 mass% aluminum powder (the pH of the mixture is 8.0) is processed according to the method of Example 1 to a mixed ceramic which has the properties listed in the table below.
Bulk density [g / cm ³ ]: 0.94
Thermal conductivity (30 ° C) [W / Km]: 0.24
Cold compressive strength [N / mm ² ]: 4.2
Reversible linear expansion [%]: 0.85
Aftershortening [%]: 0.25
Softening temperature [° C]:> 1550
Classification temperature [° C]: 1550
Average macro pore size [µm]: 300
Main components:
Al ₂ O ₃ - 81.5%
SiO ₂ - 18.3%
Fe ₂ O ₃ - 0.3%
Phase composition of the ceramic: mullite corundum

Example 4 silicon carbide

A slip composition consisting of 23.5% by mass of Zettlizer kaolin 1a, 45.5% by mass of SiC, 31.0% by mass of water and 1.5% by mass of aluminum paste (the pH of the mixture is 7.0) is then processed the process of Example 1 to a silicon carbide ceramic with the following properties:
Bulk density [g / cm ³ ]: 0.84
Thermal conductivity (30 ° C) [W / Km]: -
Cold compressive strength [N / mm ² ]: 11.0
Reversible linear expansion [%]: -
Aftershortening [%]: -
Softening temperature [° C]: 1530
Classification temperature [° C]: 1350
Average macro pore size [µm]: -
Main components:
Al ₂ O ₃ - 81.5%
SiO ₂ - 18.3%
Fe ₂ O ₃ - 0.3%
Phase composition of the ceramic: SiC ceramic. bound

Claims (6)

1. Ceramic material based on aluminum oxide / silicate and / or carbide, characterized in that it has a directed, open pore structure in the macro and micro range. 2. Ceramic material based on alumina / silicate Claim 1, characterized in that according to the Sintering is almost quartz and cristoballite free Forms material with oriented mullites and spinels. 3. Process for the production of ceramic material Aluminum oxide / silicate base and / or carbide base with directed open pore structure, characterized in that that slurry-like masses on Alumina / silicate base and / or carbide base using conventional ones Modifiers, binders and Thixotropic agents at temperatures below 100 ° C and pH values of 7 up to 12 can be foamed with metal pastes or powders. 4. The method according to claim 3, characterized in that the foamed ceramic material dried and at Temperatures from 900 ° to 1800 ° C is burned. 5. The method according to claim 4, characterized in that the Drying is carried out using microwaves. 6. Use of the ceramic materials according to the claims 1 and 2 or according to the method of claims 3 to 5 obtained ceramic materials for the production of Building materials, refractories, catalyst carriers and Filter materials.