WO2012126820A1

WO2012126820A1 - Refractory shaped ceramic bodies, in particular firing auxiliaries, and process for the production thereof

Info

Publication number: WO2012126820A1
Application number: PCT/EP2012/054640
Authority: WO
Inventors: Edda BÜCKMANN
Original assignee: BEGO Bremer Goldschlägerei Wilh. Herbst GmbH & Co. KG
Priority date: 2011-03-22
Filing date: 2012-03-16
Publication date: 2012-09-27
Also published as: DE102011005914A1

Abstract

A process for producing refractory shaped ceramic bodies comprising at least 60% by volume of mullite, based on the total mass of the refractory shaped ceramic body, which has the following steps: - provision or production of a freezing-sensitive and flowable slip consisting of or comprising water and (i) mullite particles, (ii) colloidally dispersed silicon dioxide particles and preferably (iii) aluminium oxide particles and optionally comprising (iv) further ceramic constituents and/or (v) further nonceramic constituents, - provision of a casting mould, - introduction of the slip into the casting mould, - freezing of the slip in the casting mould so as to form an intermediate body, - drying of the intermediate body and - sintering of the dried intermediate body in such a way that silicon dioxide particles are entirely or partly reacted to form mullite and the refractory shaped ceramic body, is described. Corresponding shaped bodies and their use as firing auxiliaries are also described.

Description

Refractory ceramic shaped body, in particular kiln furniture, and method for its production

The present invention relates to refractory ceramic shaped bodies, in particular kiln furniture, and to processes for their production. The refractory ceramic shaped bodies according to the invention, in particular kiln furniture, comprise mullite as the essential ceramic constituent. The invention also relates to the use of shaped bodies according to the invention as kiln furniture.

The present invention relates in particular to refractory ceramic shaped bodies and to processes for their production, wherein the particular shaped body is a kiln furniture, in particular a kiln furniture, as used in sintering dental ceramic components at a temperature> 1500 ° C. Such kiln furniture are usually and preferably designed as pads, trays, capsules, discs, stacking supports, rings, hollow columns or stack bare rings, disc systems.

The use of kiln furniture (sintering aids) in firing or sintering of ceramic products, in particular dental ceramic components, but also porcelain, sanitary ware, electroceramics parts, roof tiles or chimney pipes, is known. Kiln furniture must generally be stable at temperatures above 1000 ° C. They should have a high individual and / or predicted thermal shock resistance and be suitable for multiple heating and cooling cycles. Kiln furniture are combined with a kiln (For example, a ceramic component mentioned above) heated in a kiln. Kiln furniture should therefore generally have a low mass with high mechanical strength.

Kiln furniture with a content of mullite are already known, see, for example, the document WO 94/03410, which relates to refractory moldings made of silicon carbide with mullite bond, which can be used in particular as kiln furniture. In the context of the present text, the term "mullite" is understood as meaning a silicate whose composition is in the range from 3ΑΙ ₂ 0 ₃ · 2Si0 ₂ to 2AI ₂ 0 ₃ · Si0 ₂ Irrespective of whether or not a kiln furniture comprises mullite In the past, it has conventionally been produced by pouring a slurry into a porous, absorbent plaster mold, whereby the slip comprises suspension liquid which is absorbed by the absorbent plaster mold When the desired layer thickness of the ceramic layer is reached, the resulting green body is removed from the mold and then sintered to form the finished firing aid, In some cases, a plaster mold is used as the plaster mold In this case, after pouring the slurry and after reaching the desired Wall thickness of the resulting green body of about poured off some slate. Subsequently, the green body obtained is also demolded and sintered in accordance with this process design, so that the kiln furniture results.

A disadvantage of methods using a porous, expandable plaster mold is that the plaster mold used is expensive to produce and can only be used to produce a limited number of kiln furniture. As a result, these methods are relatively expensive. Aging of the plaster mold also leads to changes in the geometry and the plaster surface, so that the produced firing aids have only a small dimensional stability. A low dimensional stability means that kiln furniture, which are produced by a per se constant process, differ greatly in their geometric dimensions and can also deviate greatly from the average geometric dimensions of the manufactured firing aids. A low dimensional stability is therefore synonymous with a low reproducibility of kiln furniture produced. If a process has low reproducibility, no small dimensional tolerances are achievable. The Dimensional tolerance is the permissible deviation in a geometric dimension. Furthermore, no small form tolerances can be achieved with the methods of the prior art mentioned. But if small form tolerances are given, only those kiln furniture are accepted, which differ from a given geometric shape only by a predetermined maximum geometric deviation (shape tolerance); the rest is scrap. This means that kiln furniture, which are produced with the help of the same plaster mold under otherwise identical process conditions according to one of the known methods, can differ in their geometric dimensions so much from the given geometric shape that a high proportion of rejects. Kiln furniture, which are prepared according to the specified method, also have an uneven surface, which is disadvantageous for their subsequent use. Another disadvantage is that the wall thickness of kiln furniture, which are produced by the above-mentioned methods, can vary widely, which contributes to the low dimensional stability. The already mentioned above WO 94/03410 discloses a method for producing a refractory plate and a combustion capsule, wherein from clay (as Si0 ₂ -containing component) and alumina (as Al ₂ 0 ₃ -containing component) in the presence of large amounts of silicon carbide and In the presence of a binder solution during firing mullite is to be formed, which serve as a binder phase and bind the silicon carbide. WO 94/03410 teaches that it is also possible to use pure Si0 ₂ , for example in the form of amorphous silica, as Si0 ₂ component for the formation of mullite, but that there are no further advantages associated with this, one would have to pay much more attention to this in that the amorphous silicic acid does not convert to cristobalite during the firing process, which hinders mullite formation and degrades the properties of the refractory castings. WO 94/03410 does not disclose a process for the production of sophisticated shaped refractory molded articles (kiln furniture), but specifically discloses molding only by pressing.

It was an object of the present invention to overcome or at least alleviate the disadvantages discussed above in the processes known from the prior art for producing refractory molded bodies, in particular kiln furniture.

In particular, a method should be given, which preferably leads very well reproducible to refractory moldings having a high dimensional stability. Preferably, small dimensional tolerances should be achieved. In addition, that should to specify given method preferably to refractory ceramic moldings with a uniform surface and exact geometry. In addition, the method to be specified should preferably offer the possibility of more precisely setting wall thicknesses of molded bodies to be produced.

It was a further object of the present invention to provide a corresponding refractory ceramic molded body, which can be produced by a method according to the invention. Such a refractory ceramic molded body should preferably be used as a kiln furniture for sintering dental ceramic components at a temperature> 1350 ° C.

With regard to the method to be specified, the object is achieved by a method according to claim 1. Preferred embodiments of a method according to the invention will become apparent from the dependent claims and the description below. Further solved objects as well as advantages of a method according to the invention as well as further aspects of the invention are indicated below.

The present invention thus relates in a first aspect to a method for producing a refractory (ie suitable for use at a temperature of over 600 ° C) ceramic molding comprising at least 60 vol .-% of mullite, based on the total mass of the refractory ceramic molding, with the following steps:

Providing or producing a freeze-sensitive and flowable slurry consisting of or comprising water and

(i) mullite particles,

(ii) colloidally dispersed silica particles, and preferably

(iii) alumina particles

as well as optional

(iv) other ceramic constituents (colloidally and / or non-colloidally dispersed) and / or

(v) other non-ceramic ingredients

Providing a mold, Filling the slip into the mold,

Freeze the slurry in the mold so that a preform is formed

Drying of the preform (so that a dried preform is formed) and

Sintering of the dried preform, so that silica particles are fully or partially reacted to form mullite and the refractory ceramic molded body is formed.

Alternatively, to produce a refractory ceramic shaped body comprising at least 60% by volume of mullite, based on the total mass of the refractory ceramic shaped body, such a dried preform is provided, i. a preform, which can be produced by a method with the steps

(i) mullite particles,

(ii) colloidally dispersed silica particles, and preferably

(iii) alumina particles

as well as optional

(iv) other ceramic constituents and / or

(v) other non-ceramic ingredients

Providing a mold,

Filling the slip into the mold,

Freeze the slurry in the mold to form a preform, drying the preform to form the dried preform.

And then the final step is then carried out using the provided preform, namely:

Sintering the dried precursor body so that silica particles are wholly or partially reacted to form mullite and the refractory ceramic body is formed. The invention also relates to a method for producing such a dried preform, the method comprising the steps indicated above.

The invention also relates to a dried preform for producing a refractory ceramic shaped body comprising at least 60% by volume of mullite, based on the total mass of the refractory ceramic shaped body, wherein the dried preform can be produced by a method comprising the following steps:

(i) mullite particles,

(ii) colloidally dispersed silica particles, and preferably

(iii) alumina particles

as well as optional

(iv) other ceramic constituents and / or

(v) other non-ceramic ingredients

Providing a mold,

Filling the slip into the mold,

Freeze the slurry in the mold so that a preform is formed

Drying of the pre-body, so that the dried pre-body is formed.

Finally, the invention also relates to the use of a dried preform according to the invention for producing a preferably refractory ceramic shaped body according to the invention comprising at least 60% by volume of mullite, based on the total mass of the refractory ceramic shaped body. In the context of this use, the dried preform according to the invention is regularly sintered in such a way that the silicon dioxide particles contained in it are completely or partially reacted to form mullite and the refractory ceramic shaped body is produced.

Insofar as the invention is described above and below with a view to the refractory ceramic shaped bodies according to the invention, the statements made (in particular those relating to preferred embodiments) apply mutatis mutandis to the dried preforms according to the invention, and vice versa. Here is Of course, it should be noted regularly that the shaped body is first produced from the dried preform by sintering and thus the properties of the preform and shaped body differ.

Fireproof ceramic moldings produced according to the invention preferably have a cone drop point greater than SK17, the softening point of the skimmer cone being determined according to DIN EN 993-12. The cone drop point is preferably above SK27, particularly preferably above SK32. The Seiler cone to be used for the determination is to be prepared from a slip, as it is to be prepared or prepared in the corresponding process according to the invention; The casting mold used is a casting mold which is suitable for producing a sailer cone. The further steps for producing the sail cone then again correspond to those of the method according to the invention.

The process steps filling, freezing, drying and sintering correspond to process steps as they are already known from freeze-casting process. It is an essential finding of the present invention that it is possible by means of a freeze-casting process using a slip composed in accordance with the invention to produce refractory ceramic shaped bodies with a content of at least 60% by volume of mullite and at least some of the abovementioned ones (Partial) to solve tasks in an advantageous manner.

In the process according to the invention, a freeze-sensitive and free-flowing slip is used, as required for carrying out a freeze-casting process. A slip is considered "freeze-sensitive" in the context of the present text if it is suitable for use in a freeze-casting process A slip is considered to be freeze-sensitive in particular if it is fluid at a first temperature (outlet temperature) A freeze-sensitive slurry is provided in the flowable state according to the invention) when it solidifies on cooling below a certain temperature (freeze-hardening temperature) so that it carries its own weight with sufficient wall thickness, and when the subsequent reheating to the first temperature (starting temperature) The freezing hardening temperature is below the freezing point of the dispersing agent of the slip, for example in the range between -200 ° C and -5 ° C, preferably in the range between -. 175 ° C and - 15 ° C, when using Water as the predominant dispersing agent, more preferably between-150 ° C and -25 ° C.

In the process according to the invention, water is used as the dispersant, but the slurry may comprise one or more further dispersants as further non-ceramic constituent. Preferably, however, the liquid fraction of the slip used according to the invention consists of at least 80% by volume of water

Based on the total weight of the slip, the proportion of water is in the range of 20-60% by volume, preferably 25-50% by volume, particularly preferably 27.5-40% by volume.

In the context of the present text, a slip is said to be "free-flowing" when it runs at 21 ° C due to gravity.

In a process according to the invention (for the production of dried preforms and for the production of molded articles therefrom), a slurry is used which comprises water (as dispersant) (for preferred amounts of water in the liquid phase of the slurry, see above). When freezing the slurry in the mold thus creates a preform that includes ice crystals. When the precursor is dried, pores thus regularly result which, in their shape and position, correspond to the previously formed ice crystals. The total porosity as well as the size of the pores of the refractory ceramic shaped body (in particular kiln furniture) can be adjusted by a person skilled in the art, in particular by varying the water content in the slurry and by the special conditions during freezing of the slurry and during drying of the preform. Since the porosity of the refractory ceramic molded body produced is relevant to its individual and / or predicted thermal shock resistance, the skilled person is able to adjust this important material property according to his wishes. Of course, the properties of the refractory ceramic molded body produced depend not only on the amount of water contained in the slurry and the freezing or drying conditions, but z. B. also of the other constituents of the slip used.

In the step of freezing the slurry in the mold, the skilled person will particularly appreciate the cooling rate, the glass transition temperature (freeze-curing temperature), and the freezing direction and the thermal conductivity of the Select mold according to his wishes and the requirements of the case. The freezing direction describes in which direction the heat conduction took place.

As further non-ceramic constituents (component (v)) of a slip to be used according to the invention, it is possible to provide additives which lower the freezing point of the dispersant below 0.degree. In our own experiments, for example, it has been found to be advantageous to provide a certain amount of glycerine or another substance which lowers the freezing point of water, in addition to water as the primary dispersing agent. The presence of such a substance in the slurry results in the resulting refractory ceramic shaped body having comparatively small pores. In particular, glycerol promotes a uniform freezing of the slip and leads overall to more homogeneous shaped bodies.

The freezing point of the aqueous phase of the slurry is preferably below -5 ° C.

The slip used in the process according to the invention (for the production of dried preforms and for the production of shaped articles therefrom) comprises mullite particles as component (i). In addition, it comprises as component (ii) colloidally dispersed silicon dioxide particles, which are preferably amorphous silica particles. The use of colloidally dispersed crystalline silica is less preferred. All colloidally dispersed silica particles of the slurry are considered as part of component (ii) and thus can not be considered as part of component (iv). Other non-colloidally dispersed silica particles may be present as component (iv) or as component of component (iv). During sintering of the dried preform, silicon dioxide particles (colloidally dispersed silicon dioxide particles and / or further silicon dioxide particles, preferably colloidally dispersed silicon dioxide particles) are completely or partially converted to form mullite. The reaction takes place either with existing aluminum oxide particles which are present as (optional) component (iii) in the slip to be used according to the invention or by reaction with alumina-rich mullite particles of component (i). It is preferred if the slip used as component (iii) comprises aluminum oxide particles, wherein during sintering of the preform silica particles (component (ii) and / or (iv)) wholly or partially with the formation of mullite with the Alumina particles are reacted, wherein preferably colloidally dispersed silica particles are fully or partially reacted to form mullite with the alumina particles.

Preferably, all ceramic constituents of the slip are oxide ceramic constituents. Preferably, the slurry does not comprise silicon carbide, preferably no non-oxide ceramic at all.

The present invention combines, for the first time and in a surprising manner, a finding of the freeze-casting technology, according to which colloidally dispersed silicon dioxide particles can be used to produce moldings, with the recognition that silica (colloidally dispersed or as a further ceramic constituent) in a sintering process can be reacted with alumina or alumina-rich mullite to mullite. In the method according to the invention, the colloidally dispersed silicon dioxide particles are used in the freeze-casting process in order to guarantee the stability of the preform after freezing and after appropriate formation of a stabilizing network, which it requires in particular during drying for the dried preform according to the invention and during the subsequent sintering process. In addition, silicon dioxide is converted to mullite during the sintering process. The resulting shaped body thus comprises further mullite beyond the amount of mullite used as component (i), while the amount of silica (in free form, amorphous or crystalline) has decreased in favor of the level of mullite.

The presence of at least 60% by volume mullite in the ceramic shaped body produced leads to a number of advantages of the shaped body, which can be summarized as follows:

- high melting point;

high creep stability even at high temperatures;

low thermal expansion

good individual and / or predicted thermal shock resistance (see below);

- high shear modulus;

favorable corrosion properties even under extreme chemical conditions. Because of their excellent properties, refractory ceramic shaped bodies produced by a process according to the invention can be used in a wide range of applications. A preferred field of application of refractory ceramic moldings produced by the process according to the invention is the use as kiln furniture. Kiln furniture serve as stable documents for a kiln (see also above). Preferably, the inventive method is carried out so that a refractory ceramic molded body results, which is glass phase poor. Here, a body is referred to as glass phase, in which the volume fraction of glass phase is less than 10 vol .-%, based on the volume of the total solids. Preferably, the volume fraction of glass phase is less than 5 vol .-%, more preferably less than 2.5 vol .-%.

The high-temperature creep resistance of a refractory ceramic shaped body which was produced according to a method according to the invention and thus comprises a high proportion of mullite is regularly clearly superior to that of a conventionally produced aluminum oxide molded body.

The invention also relates to refractory ceramic shaped bodies which can be produced by a method according to the invention. Preferred embodiments of a method according to the invention (as explained above and below in detail) lead to preferred refractory ceramic (preferably oxide-ceramic) shaped bodies. Preferred ceramic shaped bodies are kiln furniture; they are preferably selected from the group consisting of documents, trays, capsules, discs, stacking supports, rings, hollow columns, and stackable rings-disc systems.

Refractory ceramic shaped bodies according to the invention are preferably used as kiln furniture, preferably as kiln furniture for sintering dental ceramic components at a temperature of 1350 ° C., preferably> 1450 ° C., more preferably 1500 ° C.

The use as a kiln furniture can be done especially in the selection of only oxide ceramic ceramic components in an oxidizing atmosphere.

Refractory ceramic shaped bodies according to the invention can also be used as carrying rollers in high-temperature furnaces. In own investigations it proved to be advantageous, if as component

(Iii) of a slip to be used according to the invention aluminum oxide particles are used and they have a particle size d ₉₀ <5μηι, preferably <3 m. The particle size is determined in accordance with IS013320-1 and, for example, with a device LS-13320 from Beckman Coulter GmbH. It is understood that in the presence of aluminum oxide particles as component (iii) all alumina particles present in the slip used are assigned to this component; In other words, alumina particles can not be considered as another ceramic component (component (iv)). Of course, an analogous consideration also applies to the mullite particles used as component (i). Mullite particles are assigned exclusively to component (i) and can not be used as another ceramic constituent according to the component

(iv).

Aluminum oxide particles with a particle size d ₉₀ <5 μm (preferably <3 μm) react easily with amorphous silicon dioxide, as it is present in a dried preform.

The sintering of the preform to the molding is carried out in a process according to the invention preferably at a temperature> 950 ° C, preferably> 1300 ° C, more preferably> 1450 ° C, preferably at a temperature <1600 ° C.

On the one hand, the preferred sintering temperatures lead to particularly stable refractory ceramic shaped bodies which have the aforementioned advantageous properties; Moreover, at the preferred sintering temperatures, the conversion of the silica (preferably the reaction with alumina particles) to the mullite proceeds sufficiently rapidly.

It has already been explained above that the person skilled in the art can design the process according to the invention in such a way that a particularly advantageous porosity of a refractory ceramic shaped body to be produced results. Fireproof ceramic shaped bodies which have a porosity in the range of 20 to 50%, preferably 30 to 40%, have proved to be particularly advantageous, in particular for use as kiln furniture. The porosity is determined according to DIN EN 993-1: 1995. A preferred process according to the invention is thus one in which the proportion of water in the slurry and the further process conditions are adjusted be that the produced refractory ceramic molded body has a total porosity in the range of 15 to 55%, preferably 20 to 45%, particularly preferably 25 to 35%.

The refractory ceramic shaped body produced by a process according to the invention comprises at least 60% by volume of mullite (based on the total solids content). The volume fraction of mullite in the molded article produced is determined in accordance with DIN 623-5: 2009 (Advanced Ceramics - Monolithic Ceramics - General and Structural Properties; Determination of the Volume Content of Phases by Evaluation of Microstructure Images). It has been found that refractory ceramic shaped bodies, which comprise a proportion of mullite of at least 60 vol.% And are produced by a method according to the invention, realize the desired properties in a particularly advantageous manner. Preferably, the produced refractory ceramic molded body comprises at least 75 vol.% Mullite, preferably at least 90 vol.% Mullite (based on the total solids content). Once again, it should be pointed out that the total mullite present in the final refractory ceramic shaped body comprises a proportion of mullite which is formed by reacting silicon dioxide particles during sintering.

Each refractory ceramic shaped body produced by a process according to the invention has an individual thermal shock resistance, which is determined by a method given below.

In addition, specimens of a defined geometry and a defined manufacturing process have a predicted thermal shock resistance, which can be determined by examining specimens of the same geometry and the same manufacturing process. It is specified below how such a "predicted thermal shock resistance" is determined for a body of defined geometry and defined production method.

Refractory ceramic moldings produced by a process according to the invention have excellent individual and / or predicted thermal shock resistance (for the determination, see below) and are therefore also usable in an outstanding manner as kiln furniture. Preferably, a inventive method designed so that the refractory ceramic molded body produced an individual and / or predicted thermal shock resistance of 30 at a test temperature of 300 ° C, preferably an individual and / or predicted thermal shock resistance of 30 at a test temperature of 400 ° C, particularly preferably an individual and / or has predicted thermal shock resistance of 30 at a test temperature of 500 ° C.

In a method according to the invention (for producing dried preforms and for producing molded articles therefrom), one or more of the colloidally dispersed silicon dioxide particles of component (ii) preferably have a particle size of <100 nm, preferably <50 nm, particularly preferably <20 nm. The particle size of the colloidally dispersed silicon dioxide particles is determined according to ISO 13322-1: 2004-12 using a transmission electron microscope (TEM).

In preferred processes according to the invention (for the production of dried preforms and for the production of molded articles therefrom), the provided or prepared freeze-sensitive and flowable slip comprises

(i) from 20 to 75% by weight of mullite particles, preferably from 30 to 65% by weight, particularly preferably from 40 to 60% by weight,

(ii) 1 to 20% by weight of colloidally dispersed silicon dioxide particles, preferably 2 to 15% by weight, particularly preferably 3 to 10% by weight,

(iii) from 0 to 60% by weight of aluminum oxide particles, preferably from 2.5 to 60% by weight, particularly preferably from 5 to 45% by weight, very particularly preferably from 7.5 to 35% by weight,

in each case based on the total mass of the slip.

In preferred processes according to the invention (for producing dried preforms and for producing molded articles therefrom), the freeze-sensitive and flowable slip provided or prepared comprises:

(iv) other (colloidally or non-colloidally dispersed) ceramic constituents selected from the group consisting of: zirconia, non-colloidal dispersed silica particles, magnesium dioxide, cordierite, spinel (MgAl ₂ 0 ₄ ), other colloidally dispersed oxide ceramic particles

and or

(v) other non-ceramic constituents selected from the group consisting of:

Means for lowering the freezing point of the dispersing agent (eg glycerin), organic or inorganic binders, stabilizers ((a) electrostatic stabilizers: acids or bases for adjusting the pH - eg ammonia dissolved in water, (b) steric stabilizers: on the particle surface adsorbed polymers and / or (c) electrosteric stabilizers: polyelectrolytes).

The slip may also comprise short fibers (preferred length: between 5mm and 20mm) as another ceramic or other non-ceramic component. In individual cases, a 2- or 3-dimensional fiber fabric is inserted into the mold to achieve special product properties, before it is filled up with slurry. The fibers are preferably alumina or mullite fibers.

In a method according to the invention, a freeze-sensitive and flowable slip is provided or prepared:

When producing a freeze-sensitive and flowable slurry for use in a process according to the invention, preference is given to initially introducing a dispersion of colloidally dispersed silicon dioxide particles. Dispersions of colloidally dispersed silica particles are commercially available under the name silica sol or silica sol. Preferably, such a dispersion of colloidally dispersed silicon dioxide particles, which is to be used in the preparation of a slurry to be used according to the invention, has a specific surface area of at least 300 m ² / g. A dispersion for use in the production of a slip to be used according to the invention should moreover have a Si0 ₂ content of at least 6% by weight. The proportion of colloidally dispersed silicon dioxide particles in the dispersion is preferably from 10 to 60% by weight, preferably from 25 to 50% by weight, based on the mass of the dispersion presented.

The preferably initially introduced dispersion of colloidally dissolved silicon dioxide particles (the silica sol) typically contains mullite particles for producing a freeze-sensitive and flowable slurry to be used according to the invention, and optionally the further constituents of the slip to be used, such as (in each case optional) alumina particles, other ceramic constituents and / or other non-ceramic constituents.

In processes according to the invention (for the production of dried preforms and for the production of shaped articles from them), the slip previously filled into a casting mold is frozen in this casting mold, so that a preform is formed. For this purpose, the slurry in the mold is cooled to a temperature which is below the (composition-dependent) freezing temperature of the dispersant of the slurry.

In a large number of cases, it has proved to be advantageous to introduce the casting mold with the slip filled therein into a cooling space which is cooled to a temperature in the range from -200 ° C. to -5 ° C., preferably to a temperature in the range from - 175 ° C to - 15 ° C, more preferably to a temperature in the range of - 150 ° C to - 25 ° C.

Alternatively, it is possible, for example, to cool the mold itself to one of the abovementioned temperatures. According to a preferred embodiment, the casting mold already has a temperature below the freezing point of the dispersing agent of the slip when the slip is introduced; Preferably, in this case, the temperature of the mold is then maintained at a level below the freezing temperature of the dispersant. The casting mold is then preferably moved so that a uniformly thick ceramic layer forms on the casting wall and deposits there.

The mold used in a process according to the invention (for the production of dried preforms and for the production of moldings therefrom) preferably consists of or preferably comprises material with a thermal conductivity of more than 10 W / Km at 23 ° C. and 1013 hPa, e.g. As metal (see below) and / or plastic and / or rubber products and / or silicones. An advantage of the use of molds comprising or consisting of materials of the specified thermal conductivity is that in this way a rapid temperature equalization takes place between the slurry in the mold and the environment. This will shorten the time until the slurry is completely frozen. In that regard, it should be noted that the time required until the complete freezing of the slurry has an influence on the size of the ice crystals present in the frozen slurry. The specialist will Set the freezing rate as needed for the application. An advantage of the use of plastic as the material of the mold is its low price, so that corresponding molds are particularly inexpensive to produce. An advantage of the use of rubber products and silicones is their elasticity, which facilitates the removal of the preform body.

In processes according to the invention, a casting mold is preferably used which consists of metal, at least in some places which are in contact with the slurry on freezing. Preference is given in this respect to metals having a high thermal conductivity, in particular those having a conductivity of more than 150 W / Km at 23 ° C. and 1013 hPa. Preferred metals are aluminum, copper and stainless steel. Particularly preferably, the casting mold as a whole consists of metal, preferably one of the abovementioned metals.

The drying of the preform takes place in a process according to the invention either after the removal of the preform (that is to say after removal from the casting mold) or still in the casting mold. In the latter case, the preform is removed from the mold after drying and before sintering (i.e., demolded).

The drying of the preform is preferably carried out at a temperature in the range of 40 ° C to 100 ° C and / or at a relative humidity of less than 30%.

The drying of the preform is usually carried out at ambient pressure. In some cases, however, it may be advantageous to set a pressure that is less than 1013 hPa. By setting such a low pressure, the drying is accelerated.

Preferably, the refractory ceramic shaped body produced according to a method of the invention is a kiln furniture. Preferably, the kiln furniture is selected from the group consisting of documents, trays, capsules, discs, stacking supports, rings, hollow columns, and stackable rings-disc systems. The kiln auxiliary material preferably has an individual and / or predicted thermal shock resistance, as indicated above.

The filling of the slurry into the mold takes place in a method according to the invention, preferably by casting, by means of a peristaltic pump or by means of a centrifuge. It is understood that the abovementioned preferred embodiments of a method according to the invention are preferably combined with one another. For example, therefore, a method according to the invention for producing a refractory ceramic shaped body comprising at least 60% by volume of mullite, based on the total mass of the refractory ceramic shaped body, which comprises the following steps is preferred:

(i) mullite particles,

(ii) colloidally dispersed silica particles, wherein any or all of the colloidally dispersed silica particles having a particle size

<100 nm, preferably <50 nm, particularly preferably <20 nm,

(Iii) alumina particles having a particle size d ₉₀ <5μηι, preferably

<3μηι,

as well as optional

(iv) other ceramic constituents and / or

(v) other non-ceramic ingredients

Providing a mold,

Filling the slip into the mold,

- Freeze the slip in the mold so that a preform is formed

Drying of the preform for the (inventive) dried preform and (for the production of a shaped article according to the invention)

Sintering of the dried preform at a temperature> 950 ° C, preferably> 1300 ° C, more preferably> 1450 ° C, preferably at a temperature <1600 ° C, so that silica particles are fully or partially reacted to form mullite and the refractory ceramic Shaped body arises.

The individual and the predicted thermal shock resistance is determined as follows:

Determination of the individual and the predicted thermal shock resistance of a refractory ceramic shaped body produced in accordance with the invention: The individual thermal shock resistance of a refractory ceramic shaped body is determined on the basis of DIN 51068: 2008-1 1. It is the resistance of the refractory ceramic molded body against destruction by a sudden change in temperature between a test temperature and a temperature of 10 ° C to 20 ° C (immersion in running water) determined. The test temperature is set to a value between 300 ° C and 750 ° C. (It should be noted that in the present case for the determination of thermal shock resistance in deviation from DIN 51068: 2008-1 1 is not heated to a temperature of 950 ° C.)

The test specimen used is the refractory ceramic molding itself. Cylindrical shaped bodies, as used in accordance with section 5.2 of DIN 51068: 2008-1 1, are not used, as far as the refractory ceramic molded body does not itself have such a shape.

The measure of thermal shock resistance (TWB) is the quench rate, i. the number of sustained to the destruction of the refractory ceramic shaped body rugged temperature changes (quenching). As a measure of TWB, the quenching rate is given as a single value for the specified test temperature. In this case, that deterrent at which a respective specimen is destroyed is counted.

The specimens (refractory ceramic shaped bodies) are dried in an oven according to DIN 51068: 2008-1 1 point 4.3, to (1 10 ± 5) ° Celsius before the test. The dried specimens are then placed in an oven preheated to the test temperature (300 ° C to 750 ° C) (see the above note on deviation of the test method from DIN 51068: 2008-1 1). The oven temperature acting directly on the specimens must never fall below the test temperature (300 ° C to 750 ° C) by more than 50 ° Celsius. This is checked with a temperature measurement independent of the furnace control.

The heat capacity of the oven must be sufficient to bring the temperature back to the test temperature within 15 minutes to 30 minutes after insertion of the test specimen. Once the test temperature has been reached, the specimens must be left in the oven for an additional 15 minutes. Thereafter, they are immersed in running water of 10 ° Celsius to 20 ° Celsius and left there for 3 minutes. The specimens are then dried for 30 minutes in the oven at (1 10 ± 5) ° Celsius and then put back into the oven.

If the test has to be interrupted, for example overnight, care must be taken that the samples do not remain in the oven but in the oven.

The test cycles are repeated until the specimens have broken into two or more large pieces. The test is stopped when the specimens have endured 30 deterrents.

If a test specimen endures 30 quenching, then this individual test specimen is considered to be resistant to the thermal cycling at the respective test temperature (individual thermal shock resistance).

If this is determined for three test specimens of the same geometry and the same manufacturing process at the test temperature, then bodies of this geometry and this production method are considered to be resistant to the thermal cycling at the respective test temperature (predicted thermal shock resistance). In this case, with a next higher test temperature and three new specimens, the entire test can be repeated on three specimens until the three specimens can no longer bear all 30 deterrents. Then the last / highest test temperature applies, at which all three specimens endured the 30 quenchings, as the test temperature at which bodies of this geometry and this manufacturing process are resistant to thermal cycling.

If, on the other hand, the 30 quenchings are not tolerated by all three specimens at a specified test temperature, then the test temperature should be lowered until all three (new) test specimens endure the 30 quenchings. The invention is explained in more detail below with reference to an embodiment: Embodiment 1

In the following, a method for producing a refractory disk is described, which, due to its shape according to utility model DE 20 2007 008 520 U1 (see FIGS. 3 and 4), makes it possible to carry firing material, but at the same time can be stacked using wall structures with additional disks , The disc has a diameter of 1 15mm and a thickness of 15mm. Notwithstanding drawing 3 and 4 of the utility model, however, both surfaces are without honeycomb structure. To produce the freeze-sensitive and flowable slurry, aqueous silica sol is mixed with alumina and mullite powder by means of a stirrer; Glycerol is added - the respective proportions are listed in Table 1 together with the product names.

The slurry is poured into a two-piece metallic mold (material: aluminum) at room temperature and ambient pressure. The two parts are connected by screws. After filling the mold, it is placed in a refrigerated freezer compartment. The air temperature in the freezer is -40 ° C, the pressure 1013 hPa. In the freezer cool the metallic mold and the slip contained therein. After four hours the slip is completely frozen.

After this time, the metallic mold is removed from the freezer and placed in a drying oven for 6 hours; Here, the preform dries at 60 ° C and 1013 hPa at 30% relative humidity. During drying, the water escapes, but the precursor body does not collapse, since the freezing has formed a stable framework based on ceramic particles, which is retained, so that the preform carries its own weight. After drying, the liberated from the water preform remains. Subsequently, the (inventive) dried preform of the metallic mold is removed and sintered. The sintering takes place for 15 hours at a holding temperature of 1450 ° C.

The total porosity of the disc thus produced (i.e., a molded article according to the invention) is 36% according to DIN EN 993-1: 1995. It has an individual temperature change resistance of 30 at a test temperature of 350 ° C; For the method of determination see above. The content of mullite is more than 90% by volume according to DIN EN 623-5: 2009.

Claims

claims:

1 . Process for producing a refractory ceramic shaped body comprising at least 60% by volume of mullite, based on the total mass of the refractory ceramic shaped body,

with the following steps:

(i) mullite particles,

(ii) colloidally dispersed silica particles, and preferably

(iii) alumina particles

as well as optional

(iv) other ceramic constituents and / or

(v) other non-ceramic ingredients

Providing a mold,

Filling the slip into the mold,

Freezing of the slip in the mold, so that a preform arises drying of the preform, so that a dried preform arises or

Providing such a dried pre-body

and subsequently

2. The method of claim 1, wherein the slurry comprises alumina particles, wherein the sintering of the preform silica particles are fully or partially reacted to form mullite with the alumina particles, preferably colloidally dispersed silica particles are wholly or partially reacted to form mullite with the alumina particles. The method of claim 1 or 2, wherein the aluminum oxide particles have a particle size d ₉₀ <5μηι, preferably <3 μηι possess.

Method according to one of the preceding claims, wherein the sintering of the preform at a temperature> 950 ° C, preferably> 1300 ° C, more preferably> 1450 ° C, preferably at a temperature <1600 ° C.

Method according to one of the preceding claims, wherein the water content in the slurry and the process conditions are adjusted so that the produced refractory ceramic molded body has a total porosity in the range of 15 to 55%, preferably 20 to 45%.

A method according to any one of the preceding claims, wherein the refractory ceramic shaped article produced comprises at least 75% by volume mullite, preferably at least 90% by volume mullite.

Method according to one of the preceding claims, wherein the refractory ceramic molded body produced an individual and / or predicted thermal shock resistance of 30 at a test temperature of 300 ° C, preferably an individual and / or predicted

Thermal shock resistance of 30 at a test temperature of 400 ° C, particularly preferably has an individual and / or predicted thermal shock resistance of 30 at a test temperature of 500 ° C.

Method according to one of the preceding claims, wherein some or all of the colloidally dispersed silica particles have a particle size <100 nm, preferably <50 nm, more preferably <20 nm.

The method of any one of the preceding claims, wherein the provided or prepared freeze-sensitive and flowable slurry comprises:

(ii) 1 to 20% by weight of colloidally dispersed silicon dioxide particles, preferably 2 to 15% by weight, particularly preferably 3 to 10% by weight, (iii) from 0 to 60% by weight of aluminum oxide particles, preferably from 2.5 to 60% by weight, particularly preferably from 5 to 45% by weight, very particularly preferably from 7.5 to 35% by weight,

in each case based on the total mass of the slip.

(iv) other ceramic constituents selected from the group consisting of: zirconia, non-colloidally dispersed silica particles, magnesia, cordierite, spinel (MgAl ₂ 0 ₄ ), other colloidally dispersed metal oxide ceramic particles

and or

(v) other non-ceramic ingredients selected from the group consisting of: means for lowering the freezing point of the dispersant, organic or inorganic binders, stabilizers.

Method according to one of the preceding claims, wherein for freezing the slurry in the casting mold, the casting mold is introduced into a cooling space which is cooled to a temperature in the range of -200 ° C to -5 ° C, preferably to a temperature in the range of - 175 ° C to -15 ° C, more preferably to a temperature in the range of -150 ° C to -25 ° C.

A method according to any one of the preceding claims, wherein the refractory ceramic shaped body is a kiln furniture, preferably selected from the group consisting of pads, shells, capsules, discs, stack supports, rings, hollow columns, and stackable ring-disc systems. 13. A method for producing a refractory ceramic shaped body according to one of the preceding claims, comprising at least 60 vol .-% mullite, based on the total mass of the refractory ceramic shaped body,

with the following steps:

Providing or producing a freeze-sensitive and flowable slurry consisting of or comprising water and (i) mullite particles,

(ii) colloidally dispersed silicon dioxide particles, individual or all of the colloidally dispersed silicon dioxide particles having a particle size <100 nm, preferably <50 nm, particularly preferably <20 nm,

(iii) aluminum oxide particles with a particle size d ₉₀ <5 μm, preferably <3 μm,

as well as optional

(iv) other ceramic constituents and / or

(v) other non-ceramic ingredients

Providing a mold,

Filling the slip into the mold,

Freezing of the slip in the mold, so that a preform arises drying of the preform

and

Refractory ceramic shaped body, producible by a process according to one of claims 1 to 13.

The refractory ceramic body of claim 14, wherein the refractory ceramic body is a kiln furniture, preferably selected from the group consisting of pads, shells, capsules, discs, stack supports, rings, hollow columns, and stackable disc-ring systems.

Use of a refractory ceramic shaped body according to one of claims 14 to 15 as kiln furniture, preferably as kiln furniture Sintering dental ceramic components at a temperature> 1350 ° C, preferably> 1450, more preferably> 1500 ° C.

17. Dried preform for producing a refractory ceramic shaped body comprising at least 60% by volume of mullite, based on the total mass of the refractory ceramic shaped body, wherein the dried preform can be produced by a process comprising the following steps:

(i) mullite particles,

(ii) colloidally dispersed silica particles, and preferably

(iii) alumina particles

as well as optional

(iv) other ceramic constituents and / or

(v) other non-ceramic ingredients

Providing a mold,

Filling the slip into the mold,

Freezing of the slip in the mold, so that a preform arises drying of the preform, so that the dried preform is formed.

18. Use of a dried preform according to claim 17 for the production of a refractory ceramic shaped body comprising at least 60 vol .-% mullite, based on the total mass of the refractory ceramic shaped body.