WO2007014823A2

WO2007014823A2 - Molded sio2 element from two layers, method for producing the same and use thereof

Info

Publication number: WO2007014823A2
Application number: PCT/EP2006/063934
Authority: WO
Inventors: Fritz Schwertfeger
Original assignee: Wacker Chemie Ag
Priority date: 2005-08-04
Filing date: 2006-07-06
Publication date: 2007-02-08
Also published as: WO2007014823A3; DE102005036746A1

Abstract

The invention relates to an amorphous porous open-pore molded SiO2 element which is characterized by being constituted of two or more SiO2 layers that have a different composition or that have a different structure.

Description

SiO ₂ molding of two layers, process for their preparation and use

The invention relates to amorphous porous open-pore SiC> 2 molded bodies made of two layers, to processes for their production and to their use.

Amorphous porous open-pored SiC> ₂ shaped bodies are used in many technical fields. Examples include filter materials, thermal insulation materials or heat shields.

Furthermore, from sintered and / or melted amorphous porous open-pored SiO ₂ -formed bodies, it is possible to produce quartz goods of all kinds. Highly pure SiO 2 ~ shaped bodies can serve as "preform" for glass fibers or optical fibers, for example. In addition, crucibles for pulling silicon single crystals can also be produced in this way.

Amorphous porous open-pored SiO 2 ~ shaped bodies can in principle be produced by pressing corresponding SiO ₂ ~ powders or by a wet chemical process.

In the known from the ceramic method for pressing powders, e.g. Cold or hot isostatic pressing process must usually be added organic binders to obtain a stable shaped body. These binders must be removed again or burned in a later step. This is technically complex, expensive and leads to impurities, which must be avoided in particular in the production of dies for pulling silicon single crystals.

The preferred way to prepare porous SiC> ₂ shaped bodies is therefore the wet-chemical route. A distinction is made between a so-called sol-gel route, in which the amorphous porous open-pored SiC> 2 molded body is produced by hydrolysis and condensation of organosilicon compounds in a solvent and a colloidal sol-gel route, in which the system additionally still SiC> 2 particles are added.

The main disadvantage of the sol-gel route is the low resulting solids content in the molding. This leads to large fractures and fracture problems, especially with larger geometries, and to enormous shrinkage in the case of later sintering. In the colloidal sol-gel route, a higher degree of filling of the dispersion is achieved by the addition of SiC> 2 particles, so that the resulting solids content in the shaped body is higher. Such a method is described in EP 705797 and in EP 318100.

EP 1506947 A2 describes a process in which a solids content of more than 80% by weight can be achieved by using different particle sizes. This leads to a much higher strength of the SiO 2 ~ molded body, however, the preparation of such a dispersion is very expensive.

EP 653381 and DE-OS 2218766 disclose a slip casting method in which a dispersion of quartz glass particles in water is prepared and the SiC> ₂ shaped body is formed by slow removal of water from a porous mold. Here, too, solid contents are achieved that are over 80Gew .-%. However, due to the diffusion-dependent removal of water, the slip casting process is very time-consuming and can only be used for thin-walled molded parts.

This disadvantage can be avoided by using a die casting process. Such as As described in EP 1245703 or EP 0196717 Bl, while a SiC> 2 particles containing dispersion in a die-casting mold of a die-casting machine poured and dewatered over a porous plastic membrane to form the SiC> 2 shaped body.

All known amorphous porous open-pored SiC> ₂ molding bodies have the great disadvantage of being very fragile, crack and fracture-prone in the wet or dried state, ie in the state not yet solidified by heat treatment. This is attempted in the art by the addition of binders to improve. Since it is very often necessary to dispense with the addition of binders in the production of high-purity SiO ₂ ~ moldings for applications in the optical waveguide or semiconductor sector, the crack problem here is even greater. DE 10339676 describes a process in which a shaped body is formed, which consists of two layers, which are constructed in the same structure and composition. Micro-cracks in this shaped body do not automatically lead to crack formation or breakage of the shaped body.

If such a shaped body is to be used as a crucible for drawing silicon single crystals, it must consist of very pure and therefore very expensive SiC> 2 material. But especially in the semiconductor sector there is a high and constant cost pressure.

The object of the present invention was therefore to provide a binder-free amorphous porous open-pore SiC> ₂ shaped body which is inexpensive to manufacture.

Another object of the present invention was to provide a process for producing such an amorphous porous open-pore SiC> ₂ shaped body.

The former object is achieved by an amorphous porous open-pored SiC> ₂ shaped body, which is characterized in that it consists of two or more SiO 2 layers, which ne have different composition, or are structured differently in their structure.

The shaped body according to the invention preferably consists of two or more SiC> ₂ layers, which have a different composition, and which have different structure in structure.

It preferably consists of 2 to 5 SiO 2 layers, more preferably 2 SiO ₂ layers.

A different structure is present, for example, when the particle sizes or the particle size distributions are different. A different composition is, for example, when the contamination of the layers is different.

The further object is achieved by a method in which, in a first step, a first dispersion containing SiC> 2 particles is pumped into a die-casting mold of a die-casting machine, in which the dispersion is dewatered to form a SiC> 2 shaped body via an inner and an outer porous plastic membrane Is, the SiC> 2 molded body is unilaterally demolded and in a second step by means of a second dispersion containing SiC> 2 particles, which differs from the first dispersion in their composition, on the molded side of the SiC> 2 shaped body another Layer is formed by means of a further die-casting mold and the resulting molded body is removed from the mold.

The die used in the process consists of two porous membrane parts which form a closed space having the shape of the desired shaped body. At one or more points there is a corresponding supply line in the membrane, which is the filling of the closed Die casting mold allows. The two die-cast parts are held together with a closing pressure, which allows filling and a body formation. After filling with a first dispersion and dewatering in a customary manner for die-casting processes, the shaped body obtained is demolded either on the inner or the outer side.

In the second step of the process, a further layer is formed on the demoulded side of the SiC> 2 shaped body by means of a further die casting mold.

If a further layer is to be formed on the inner side of the SiC> 2 shaped body, unilateral demoulding of the formed SiO ₂ ~ shaped body takes place with the two die cast parts moving apart and simultaneous pressurization of the inner porous membranes with compressed air and / or water. The compressed air or water dissolves the shards from the inner porous membrane by pushing some of the water that has penetrated into the porous membrane in the opposite direction to the shard and forms a thin film of water between shards and membrane.

A second die-casting mold, again consisting of a porous membrane, is now moved together with the external die casting mold which contains the already formed shaped body. The inner die has such a shape and size that it forms a closed gap to the already formed moldings, which corresponds in shape and thickness of the desired second layer. Now a further dispersion containing SiC> 2 particles is pumped into the die casting in which the dispersion is dewatered over the inner and partly over the already present SiC> 2 shaped body to form a new layer on the already present SiC> 2 shaped body , At one or more points there is again a corresponding supply line in the inner membrane, which allows the filling of the closed die casting mold. The two die-casting molds Parts are held together with a closing pressure that allows filling and potting under the pressures used in each case.

After the formation of the new layer, a two-sided demolding of the two-layer molded body formed takes place while the two die-cast moldings are moved apart and simultaneous exposure of the porous membranes to compressed air and / or water. The compressed air or the water dissolves the two-layer shaped body of the two porous membranes by some of the water penetrated into the porous membranes is pressed in the reverse direction to the two-layer molding and forms a thin film of water between two-layer molding and membrane.

If, instead of a further layer on the inner side of the SiC> 2 molded body, a further layer on the outer side are formed, carried out a one-sided demoulding of the formed SiC> ₂ shaped body with moving apart of the two die-cast moldings and simultaneous loading of the outer porous membranes with compressed air and /or water. The compressed air or the water dissolves the shards in this case, as already described by the outer porous membrane.

A second outer die, again consisting of a porous membrane, is now collapsed with the inner die casting containing the formed body already formed. The outer die has such a shape and size that it forms a closed gap to the already formed moldings, which corresponds in shape and thickness of the desired second layer. Now another dispersion containing SiC> 2 particles is pumped into the die casting in which the dispersion is dewatered over the outer and partly over the already present SiC> 2 shaped body to form a new layer on the already present SiC> 2 shaped body , At one or more points is again a corresponding supply line in the outer membrane, which allows the filling of the closed die casting mold. The two die-cast parts are held together with a closing pressure, which allows filling and a body formation under the pressures used in each case.

After the formation of the new layer, a two-sided demoulding of the formed two-layer molded article takes place again, with the two die-cast moldings moving out of contact with each other and simultaneous exposure of the porous membranes to compressed air and / or water. The compressed air or water dissolves the two-layer molded article from the two porous membranes by pushing some of the water penetrated into the porous membranes in the reverse direction to the two-layer molded article and forming a thin film of water between the two-layer molded article and the membrane.

By means of both process variants, SiC> 2 moldings are obtained, whose two layers have different composition and / or structure, since the respective ones used

SiC> ₂ dispersions differ in their composition.

Preference is given to producing SiC> 2 shaped bodies in crucible form, the inner layer of which contains fewer metal atoms than their outer layer.

The inner layer preferably has a foreign atom content, in particular of metals of <300 ppmw (parts per million by weight), preferably <100 ppmw, more preferably <10 ppmw and most preferably <1 ppmw.

In principle it is possible to produce SiC> 2 shaped bodies with more than two layers by the process according to the invention. In order to achieve this, in analogy to the layer formation for the production of a two-layer SiC> 2 shaped body, several layers are used successively formed in the manner described on a SiC> 2 molded body produced in a first step.

The dispersions used have a degree of filling of amorphous SiC> ₂ particles between 10 and 80% by weight, preferably between 50 and 80% by weight and particularly preferably between 65 and 75% by weight.

As dispersing agents, polar or nonpolar organic solvents, such as e.g. Alcohols, ethers, esters, organic acids, saturated or unsaturated hydrocarbons, or water or mixtures thereof.

Preferably, there are alcohols such as methanol, ethanol, propanol, or acetone or water or mixtures thereof. Particular preference is given to acetone and water or mixtures thereof, most preferably water.

Most preferably, the above-described dispersants are used in a highly pure form, as described, for example, in US Pat. can be obtained by literature methods or are commercially available.

When using water, it is preferable to use specially purified water having a resistance of> 18 megohm * cm.

Preferably, the water is treated with a mineral acid, e.g.

HCl, HF, H ₃ POoH ₂ SCU or silica or ionogenic additives such as fluorine salts. Particularly preferred is the addition of HCl or HF, most preferably HF.

It is also possible to use mixtures of the compounds mentioned. In this case, a pH of 2-7, preferably 3-6 should be set in the dispersion. Alternatively and also preferably, a mineral base may be added to the water, such as NH ₃ , NaOH or KOH. Particularly preferred is NH ₃ and NaOH, most preferably NH ₃ . However, it is also possible to use mixtures of the compounds mentioned. In this case, a pH of 7-11, preferably 9-10 should be set.

The use of a dispersion with reduced or increased pH usually leads to a firmer shard during the body formation, so that a more stable SiO 2 shaped body is formed.

The specific gravity of the amorphous SiO 2 particles should preferably be between 1.0 and 2.2 g / cm ³ . Particularly preferably, the particles have a specific gravity of between 1.8 and 2.2 g / cm ³ . Most preferably, the particles have a specific gravity between 2.0 and 2.2 g / cm ³ .

Also preferred are amorphous SiO 2 particles having ≦ 3 OH groups per nm ² on their outer surface, more preferably ≦ 2

OH groups per nm ² , and very particularly preferably <1 OH groups per nm ² .

The amorphous SiO 2 particles should preferably have a particle size distribution with a D 50 value between 1-200 μm, preferably between 1-100 μm, particularly preferably between 10-50 μm and very particularly preferably between 10-30 μm.

Preference is given to amorphous SiO 2 particles having a BET surface area of 0.001 m ² / g-50 m ² / g, more preferably of 0.001 m ² / g-5m ² / g, very particularly preferably of 0.01 m ² / g. 0.5 m ² / g.

The amorphous SiO 2 particles should preferably have a crystalline content of at most 1%. Preferably should they also show as possible no interaction with the dispersant.

These properties have amorphous Si02 particles of different origin, such as silica sintered (fused silica) and any type of amorphous sintered or compacted SiC> 2. They are therefore preferably suitable for the preparation of the dispersion according to the invention.

Corresponding material can be produced in a conventional manner in the oxyhydrogen flame. It is also commercially available, e.g. under the name Exelica® at Tokoyama, Japan. Furthermore, such a material can be prepared in a known manner via a sol-gel process. It is also commercially available, for. B. under the name MKC® Mitsubishi Chemical, Japan.

If the above criteria are met, particles of other origin may also be used, e.g. Natural quartz, quartz glass sand, glassy silica, ground quartz glass or ground quartz glass waste, and chemically produced silica glass, such as silica glass. precipitated silica, fumed silica (fumed silica prepared by flame pyrolysis), xerogels, or aerogels.

The amorphous SiU ₂ particles are preferably precipitated silicas, finely divided silicas, fused silica or compacted Si02 particles, more preferably highly dispersed silicic acid or fused silica, most preferably fused silica. Mixtures of said different Si02 particles are also possible and preferred.

Furthermore, preference is given to using amorphous SiO 2 particles having a different, preferably bimodal, particle size distribution. Such Si02 particles are obtained by admixture on SiC> ₂ particles, such as fused or fumed silica with a particle size of 1-100 nm, preferably 10 to 50 nm, in an amount of 0.1 to 50 wt.%, Particularly preferably in an amount of 1 to 30 % By weight, very particularly preferably in an amount of from 1 to 10% by weight to the abovementioned amorphous SiC> 2 particles.

The nanoscale SiC> 2 particles act as a kind of inorganic binder between the much larger SiC> ₂ particles, but not as filler material to achieve a higher degree of filling. Such SiO ₂ particles preferably have a bimodal particle size distribution in the dispersion.

In a preferred embodiment, the particles described above are present in the dispersion for the inner layer in a highly pure form, i. preferably with a sum of the impurity at metal atoms smaller than 5 ppm, preferably smaller than 1 ppm, particularly preferred smaller than 500 ppm, especially preferred smaller than 200 ppm. The sum of metal impurities in the outer layer is less critical and therefore may be higher.

In a further preferred embodiment, the dispersion for the outer layer additionally contains metal particles, metal compounds or metal salts. Compounds that are soluble in the dispersing agent are preferred, particularly preferred are water-soluble metal salts. Depending on the nature and amount of these additives, the molded article is exposed at high temperatures, e.g. a sintering process additional positive properties, as are familiar to those skilled in the glass production.

The metal particles, metal compounds or metal salts may be added during and / or after the preparation of the dispersion. In the preparation of the dispersion, the dispersant is introduced and the SiC> 2 particles slowly and preferably continuously added. The Siθ ₂ ~ particles can also be added in several steps (in portions).

By selecting the SiO 2 particle size and particle sizes, the pore size and distribution in the SiC> ₂ shaped bodies produced from the dispersion can be adjusted in a targeted manner.

As dispersing devices, it is possible to use all devices and devices known to the person skilled in the art. Preference is given to devices which contain no metal parts which could come into contact with the dispersion in order to avoid metal contamination due to abrasion.

The dispersion should be at temperatures between O ⁰ C and 5O ⁰ C, preferably between 5 ⁰ C and 3O ⁰ C take place.

Before, and / or during and / or after dispersion, methods known to those skilled in the art, e.g. Vacuum, the gases possibly contained in the dispersion, e.g. Air are removed. This is preferably carried out during and / or after complete dispersion.

In a homogeneous dispersion thus produced, no sedimentation of the particles occurs for at least 5 minutes, preferably for at least 30 minutes.

Subsequently, the dispersion is transferred to the die casting mold of a die casting machine, in which the dispersion is dewatered under pressure and to form the SiC> 2 shaped body.

The filling of the die casting mold with the dispersion takes place in a manner known to the person skilled in the art, such as, for example, by pumping. The filling can be carried out at any pressure, but is preferably carried out at pressures between 0 and 100 bar, more preferably at pressures between 5 and 30 bar and most preferably between 5 and 10 bar.

Shard formation preferably takes place under pressures between 0 and 100 bar, more preferably at pressures between 5 and 30 bar and most preferably between 5 and 10 bar.

Depending on the desired shaped body, the shards thicknesses of the individual layers formed are between 1 and 50 mm, preferably between 3 and 10 mm.

Depending on the body thickness, the porous membrane and the existing pressure, formation of dimensionally stable shards between 5 and 90 minutes is required.

The conversion of the dispersion and the body formation can be carried out at temperatures of from ⁰ ° C. to the boiling point of the dispersant. Preference is given to temperatures between 2O ⁰ C and 3O ⁰ C.

In the die casting process, membranes are preferably used as porous membranes which have an open porosity between 5 and 60% by volume, preferably between 10 and 30% by volume. The pore size of the membrane may be larger, smaller or equal to the size of the SiO 2 particles used. Preferably, a membrane with a pore size between 10 nanometers and 100 micrometers, more preferably between 100 nanometers and 50 micrometers, most preferably between 100 nanometers and 30 micrometers is used. The porous membrane is preferably completely wettable by the solvent of the dispersion, preferably water, so that a uniform body formation can take place.

As a material for the membrane, any known in the art plastic is chemically resistant and contains no free, in particular no metallic residues. Preference is given to plastics which are already used in commercial pressure slip casting. Particularly preferred are polymethyl methacrylates and polymethyl methacrylates.

The thickness of the porous membrane depends on the shape of the shaped body to be produced.

The formed fragments have a solids content between 80 and 95 wt .-%.

The desiccated SiC> 2 molded body is dried by means of methods known to the person skilled in the art, such as e.g. Vacuum drying, drying by means of hot gases, e.g. Nitrogen or air,

Contact drying or microwave drying. A combination of the individual drying methods is possible. Preference is given to drying by means of a microwave.

The drying takes place at temperatures in the molding between 25 ⁰ C and the boiling point of the dispersant in the pores of the molding.

The drying times depend on the volume of the shaped body to be dried, the maximum layer thickness, the dispersant and the pore structure of the shaped body.

During drying of the molded body occurs a low shrinkage. The shrinkage depends on the degree of filling of the layers of the moist molded body. At a degree of filling of 80% by weight, the volume shrinkage is <2.5% and the linear shrinkage is <1, 0% by volume. At a higher degree of filling, the shrinkage is correspondingly lower.

In order to ensure a crack-free drying of the molding, the shrinkage during drying must be approximately equal. This can be done with layers of different structure (particle morphology or particle size distribution) e.g. be achieved by a variation of the degree of filling and or the variation of the particle size distribution.

The density of the molding according to the invention is between 1.4 g / cm ³ and 1.8 g / cm ³ .

The moldings obtainable in this way are an amorphous, open-pore, SiC> ₂ shaped body of any dimensions and shape, which consists of at least two layers of different composition and / or structure.

The shaped articles described can be used in a variety of ways due to their special properties, e.g. are used as filter materials, thermal insulation materials, heat shields, catalyst support materials as well as "preform" for glass fibers, optical fibers, optical glasses or quartz goods of all kinds.

In a further specific embodiment, the porous shaped bodies can be completely or partially mixed with a wide variety of molecules, substances and substances. Preference is given to molecules, substances and substances which are catalytically active. All methods known to those skilled in the art can be used, as described, for example, in US Pat. No. 5,655,046. The shaped bodies according to the invention can still be subjected to sintering. In this case, all methods known to those skilled in the art, such as vacuum sintering, zone sintering, arc sintering, sintering by means of plasma or laser, inductive sintering or sintering in a gas atmosphere or gas stream can be used. Thus, the shaped bodies according to the invention as described in DE C 10158521, DE A 10260320 and DE A 10324440 can still be glazed by means of CO2 lasers.

During sintering, the layer structure of the amorphous porous open-pore shaped bodies is lost. If a complete sintering, so no layer structure in the molding is no longer available. If the layers of the shaped body differ in their proportion of metal atoms, this difference is still present after complete sintering.

The invention thus also relates to a silica glass molded body which is characterized in that it has a gradient with respect to the metal atom concentration.

The silica glass moldings produced in this way are suitable in principle for all applications in which silica glass is used. Preferred Anwendungsfeider are quartz goods of all kinds, glass fibers, optical fibers and optical glasses.

A particularly preferred field of application are high-purity silica glass crucibles for drawing silicon single crystals.

If the outer layer has been mixed with metal particles, metal compounds or metal salts as described above, the sintered silica glass body has additional properties.

In such an embodiment, the dispersion for the outer layer or the outer layer is wholly or partly with Added compounds that promote or cause cristobalite formation. All compounds known to those skilled in the art which promote and / or effect cristobalite formation can be used, as described, for example, in EP 0753605, US Pat. No. 5,053,359 or GB 1,428,788. BaOH and / or aluminum compounds are preferred here.

After sintering of such a shaped body, in particular crucibles are obtained for crystal pulling of Si single crystals which have a cristobalite layer on the outside. These crucibles are particularly suitable for crystal pulling, since they are more stable in temperature and z. B. contaminate a silicon melt less. As a result, a higher yield in crystal pulling can be achieved.

Fig. 1 shows schematically the sequence of the method according to the invention.

2 shows a piece of shards consisting of two different layers (5 mm inside) consisting of fused silica

Fig. 3 shows a crack-free dried two-layer 14 "green body crucible

Fig. 4 shows a fully sintered body in which the boundary layer between the layers is no longer detectable

The following example serves to further explain the invention. Example 1 :

A) Preparation of the SiC> 2 dispersion for the inner layer.

In a 10 liter plastic cup 3800 g were redistilled. H ₂ O submitted. 712 g of fumed silica (Wacker HDK, BET surface area 200 m ² / g) were first stirred in over 30 minutes using a plastic-coated propeller stirrer. 8188 g of ground fused silica (MKC 400® from Mitsubishi Chemical, average particle size 25 μm) were then added in portions over a period of 30 minutes and dispersed.

Following complete dispersion, the dispersion was subjected to a slight vacuum (0.8 bar) for 10 minutes to remove any trapped air bubbles.

The dispersion thus prepared consisted of 8900 g of solid, which corresponds to a solids content of 70 wt .-% (of which in turn 92% fused silica and 8% fumed silica).

B) Production of a molded article in 14 "crucible geometry with a layer thickness of 5 mm.

The SiC> ₂ dispersion is pressed from a feed tank at a pressure of 10 bar through a pipe system between two open-pored plastic membranes made of methyl methacrylate. The membranes have a porosity of 30% by volume and an average pore radius of 20 μm. The distance between the two membranes to each other allow the formation of a 5 mm thick shards. The two diaphragms are subjected to a closing pressure of 200 bar.

Due to the pressure on the dispersion, most of the water in the dispersion is forced into the membrane. It forms the SiC> 2 shards. After the formation of a clay body of 45 min, the pressure in the storage tank is raised to 0 bar. reduced pressure. Special air and water pipes laid in the outer membrane make it possible to apply air or water to the shaped bodies formed by the porous membrane for final shaping. In this case, the shaped body of the outer membrane dissolves. The inner membrane is moved upwards. The molding now hangs on the inner membrane.

The inner membrane is now lowered to another outer open-pored plastic membranes of methyl methacrylate. This membrane also has a porosity of 30% by volume and an average pore radius of 20 μm. The distance of the new outer membranes to the already formed cullet allows the formation of another layer of 5 mm on the cullet. The two diaphragms are subjected to a closing pressure of 200 bar.

C) Preparation of the SiC> 2 dispersion for the outer layer.

In a 10 liter plastic cup 3800 g were redistilled. H ₂ O submitted. 712 g of fumed silica (Wacker HDK, BET surface area 200 m ² / g) were first stirred in over 30 minutes using a plastic-coated propeller stirrer. 8188 g of fused silica (Exelica® SE 15 from Tokuyama, average particle size 30 μm) were then added in portions over a period of 30 minutes and dispersed.

The dispersion thus prepared consisted of 8900 g of solid, which corresponds to a solids content of 70% by weight (of which in turn 92% fused silica and 8% fumed silica). D) Preparation of a two-layer molding in 14 "crucible geometry

The SiC> ₂ dispersion is pressed from a feed tank at a pressure of 10 bar through a conduit system into the space between the already formed shard and the second outer membrane.

Due to the pressure on the dispersion, most of the water in the dispersion is forced into the membrane. The second SiC> 2 shard forms. After expiry of the body formation of 45 min, the pressure in the storage tank is reduced to 0 bar overpressure. Special air and water pipes laid in the membrane make it possible to pressurize the shaped bodies formed by the porous membrane with air or water for final shaping. First, the molding is released from the outer membrane. The inner membrane is moved upwards. The molding now hangs on the inner membrane. A positive base is positioned under the molded body. Thereafter, the molding is deposited on the pad and released from the inner membrane. The inner membrane is in turn moved upwards. The amorphous open-pored porous shaped body produced in this way has a solids content of 89% by weight and a residual water content of 11% by weight. It consists of two different layers with regard to the fused silica particles contained. Fig. 2 shows these layers in section through the shards.

After drying at 9O ⁰ C for 3 hours, the shaped body is free of cracks and completely dried Fig. 3 shows this molding. After a vacuum sintering (10 ^-3 mbar) no longer visually distinguish the two layers for 2 hours at 1600 ⁰ C Can (see Fig. 4).

Claims

Claims:

1. Amorphous porous open-pore SiC> ₂ shaped body, characterized in that it consists of two or more SiO 2 layers, which have a different composition, or which are structured differently in structure.

2. Shaped body according to claim 1, characterized in that it consists of two or more Siθ ₂ ~ layers, which have a different composition, and which are structured differently in structure.

3. Shaped body according to claim 1 or 2, characterized marked, that it consists of 2 to 5 SiO 2 ~ layers, more preferably 2 SiC> ₂ layers.

4. Shaped body according to one of claims 1 to 3, characterized in that it has a crucible shape.

5. Shaped body according to one of claims 1 to 4, characterized in that the individual layers have a thickness between 1 and 50 mm, preferably between 3 and 10mm,

6. Shaped body according to one of claims 1 to 5, characterized in that it has a solids content of 80 to 95 wt .-%.

7. Shaped body according to one of claims 1 to 6, character- ized in that it has a density between 1.4 g / cm ³ and

1.8 g / cm ³ .

8. A process for producing a shaped article according to any one of claims 1 to 7, characterized in that in a first step, a SiC> 2 particles containing first Dispersion is pumped into a die casting mold of a die casting machine, in which the dispersion is dewatered to form a SiC> 2 shaped body via an inner and an outer porous plastic membrane, the SiC> 2 molded body is unilaterally demolded and in a second step by means of a SiC> 2 particles containing second dispersion, which differs from the first dispersion in its composition, on the demoulded side of the SiC> 2 shaped body, a further layer is formed by means of a further die-casting mold and the resulting molded body is removed from the mold.

9. The method according to claim 8, characterized in that the dispersions are filled with a pressure between 0 and 100 bar, more preferably between 5 and 30 bar and most preferably between 5 and 10 bar in the die.

10. The method according to claim 8 or 9, marked thereby, that it is carried out over a period of 5 to 90 min.

11. The method according to claim 8, 9 or 10, characterized in that it is carried out at a temperature of ^{0 0} C to the boiling point of the dispersant, preferably at a temperature between 2O ⁰ C and 3O ⁰ C.

12. Use of a shaped body according to one of claims 1 to 7 as filter materials, thermal insulation materials, heat shields, catalyst support materials or as preform for glass fibers, optical fibers, optical glasses or quartz goods.

13. fused silica molded body which is characterized in that it has a gradient with respect to the metal atom concentration