JP2004131380A - Process for manufacturing opaque quartz glass composite, composite obtained thereby, and its use - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a process for manufacturing an opaque quartz glass, especially a large opaque quartz glass material, employing slip casting which reduces or removes defects associated with shrinkage at drying. <P>SOLUTION: In the process for manufacturing the opaque quartz glass material, amorphous SiO<SB>2</SB>particles having particle sizes and a particle size distribution wherein D<SB>50</SB>is ≤15 μm and D<SB>90</SB>is≤50 μm are obtained by finely pulverizing amorphous SiO<SB>2</SB>granules in a liquid; by lowering the pH value, a homogeneous base slip having a solid content of 75 wt.% or higher is prepared; and quartz glass granules are mixed into the homogeneous base slip to form a composite slip having a density of at least 1.85g/cm<SP>3</SP>. <P>COPYRIGHT: (C)2004,JPO

Description

The present invention relates to a method for producing an opaque quartz glass composite, and the production of a slip containing amorphous SiO ₂ particles having a size of 100 μm or less and quartz glass granules having a size of 100 μm or more, and homogeneity of the slip The present invention relates to a method for producing an opaque quartz glass composite material by forming a porous substrate by forming the slip into a mold, drying, and sintering the substrate.

The invention also relates to a composite comprising a SiO ₂ containing matrix in which quartz glass granules are embedded.

The present invention further relates to a method of using the composite material according to the present invention.

Quartz glass is characterized by a low expansion coefficient and high chemical resistance. Quartz glass materials can be used as semi-finished products such as tubes, rods, slabs or blocks, or as finished products in the field of thermal engineering where high thermal insulation with high temperature stability and thermal shock resistance is important. Is done. Examples include reactors, diffusion tubes, heat shields, bells, crucibles, nozzles, protective tubes, rails, flanges.

A method called slip casting is common in ceramic process engineering and quartz glass material production. U.S. Pat. No. 4,043,361 describes a method for producing a quartz glass crucible in connection with a slip casting process using synthetic quartz glass granules. The quartz glass granules are produced from SiO ₂ powder that is thermally generated as filter dust during flame hydrolysis of silicon compounds. First, the free SiO ₂ powder is mixed with water and stirred to form a gel. The solid content of the gel is adjusted to 30 to 45 weight percent according to the type and speed of the stirring step. After the gel is dried, the fragments obtained are sintered at 1150-1500 ° C. to obtain high-density coarse quartz glass granules, then finely divided into fine particles of 1 μm to 10 μm in diameter, and stirred in a water-soluble slip. To do. The slip is poured into a crucible mold, and the layer adhering to the periphery of the crucible is dried to form a porous substrate. When the substrate is melted and solidified at a temperature of 1800 to 1900 ° C., a desired quartz glass crucible is obtained.

In the known method, a large number of steps that consume a large amount of energy are required. For example, melting, solidification, and sintering of a base material at a high temperature are performed by using a coarse material as desired quartz glass particles. Required.

A method for producing heat-resistant quartz glass products by sintering of the types mentioned above is well known from German Patent 69306169 (T2). In the quartz glass product described therein, a fine SiO ₂ powder classified into two types forms a binder phase, and a SiO ₂ -containing material takes the form of coarse SiO ₂ with a particle size range of 40 to 1000 μm. Form. Of the two types, the finer SiO ₂ powder has a particle size range of 0.2 to 0.6 μm and exists as quartz dust consisting of substantially spherical particles. Desirable quartz dust for use comes from the dissolution process and reduction of zirconium oxide. The quartz glass starting material is premixed in a dry granulation process and then a slip is produced by the addition of a stabilizer. The weight of each raw material is 54% (coarse SiO ₂ particles), 33% (fine SiO ₂ particles), and 13% (quartz dust). The slip is degassed under vacuum and poured into a gypsum mold. The green body thus formed is dried and sintered in a furnace at 1050 ° C. to obtain a composite material.

Coarse quartz glass granules represent the microstructure of the composite that is the particles embedded in a relatively continuous matrix of finer particles and spherical particles of quartz dust. The composite has an open porosity of 13% and a density of 1.91 g / cm ³ . The cristobalite content by crystallographic analysis shows 2% or less.

When the density or high purity is important for the open porosity, that is, the continuous porosity, a known composite material cannot be used as a constituent material without limitation. The metal melt may permeate into the wall of the component through the pores and leak.

In a variant of said method according to German Patent 69306169 (T2), instead of quartz dust obtained during the dissolution process and reduction of zirconium oxide, a finely divided or finely divided quartz material is used. It is done. However, the manufactured quartz glass product has low density and low bending strength.

Next to the slip casting method, problems arise due to shrinkage of the substrate, especially during drying and sintering. Shrinkage cracks can occur and the dimensional stability of the components can often be very low.

Because of shrinkage at the time of drying, a component material manufacturing method called a core casting method in which slip is formed around the core and the core is taken out after drying becomes more difficult. Due to the shrinkage when drying, the substrate shrinks in the direction of the core, which prevents the core from being removed without breaking or damaging the substrate.

The slip casting method itself enables inexpensive production of components with complex geometric shapes, and it is highly desirable to avoid the above disadvantages in the production of quartz glass. Therefore, an object of the present invention is to provide a method for producing opaque quartz glass using a slip casting method while reducing or eliminating the drawbacks associated with shrinkage during drying. A further object of the present invention is to provide an opaque quartz glass composite that can be used in applications where temperature resistance and chemical resistance are important, and is particularly suitable for the production of large opaque quartz glass materials. A further object of the present invention is to present a suitable use of the composite material produced according to the present invention.

The achievement of the object according to the invention is, as far as the above-mentioned method is concerned, starting from the above-mentioned method, and by refining amorphous SiO ₂ granules in a liquid, the D ₅₀ value is 15 μm or less and the D ₉₀ value is 50 μm. A SiO ₂ particle having a particle size and particle size distribution characterized by the following is obtained, and a homogeneous base slip having a solid content of at least 75% by weight or more is produced by reducing the pH value. It is obtained by mixing the granules into the homogeneous base slip and forming a composite slip with a density of at least 1.85 g / cm ³ .

The method of the present invention makes it possible to produce a non-cracked opaque quartz glass composite material by a slip casting method or a core casting method. The precondition is that the shrinkage of the substrate during drying is minimized. This condition is achieved by:

1. Preparation of a base slip with a high solids content of at least 75% by weight. Achievement of high solids content, on the one hand, has a particle size and particle size distribution is characterized by containing solid qualities in the slip, i.e. D ₅₀ value is less and D ₉₀ values 15μm is 50μm or less This is realized by the well-separated amorphous SiO ₂ particles. However, this characteristic of the solid content is always due to the refinement of SiO ₂ granules in the liquid, and the high initial solid content of the base slip is also a prerequisite for effective refinement work. One of the conditions.

On the other hand, the decrease in the pH value of the base slip increases the solubility of SiO _{2 in} the liquid by combining it with the specific surface area that increases during the refinement of the SiO ₂ granule, resulting in a higher solid content. Contribute to.

The base slip obtained by this means is very viscous and has no tendency to settle at all. This is accompanied by a strong tendency to agglomerate, realizing high green density and homogeneity, and good porosity of the green matrix.

2. Furthermore, in order to adjust the high solid content, it is necessary that the solid content contained in the homogeneous base slip is further added to the solid content in the form of quartz glass granules. However, the silica glass granules can be added only after the refinement of the SiO ₂ granules and the homogenization of the base slip in order to prevent the slip molding ability from being reduced. In this process step, in order to achieve a low shrinkage of the substrate, it is necessary to add an amount of quartz glass granules that can yield a composite slip with a density of at least 1.85 g / cm ³ . The addition of the quartz glass granules not only increases the solid content of the slip, but also improves the shape stability of the substrate and reduces shrinkage during drying and sintering. Therefore, as a result of adding such a granule, the dimensional stability of the constituent material composed of the composite material is improved.

The particle size and particle size distribution of the amorphous SiO ₂ particles are characterized by values called D ₅₀ and D ₉₀ values in a particle size distribution curve (cumulative amount of SiO ₂ particles based on particle size). The D ₉₀ value indicates the particle size that does not reach 90% of the cumulative SiO ₂ particle amount, and the D ₅₀ value also indicates the corresponding average particle size. The particle size distribution is defined by diffuse spectroscopy and laser diffraction spectroscopy according to ISO 13320.

The quartz glass granules have a BET specific surface area of 1 m ² / g or less, whereas the amorphous SiO ₂ particles in the base slip have a BET specific surface area of 2 m ² / g or more. The SiO ₂ particles and quartz glass granules are obtained from the same or different raw materials. The same raw material or each raw material is amorphous and is synthetic or naturally derived.

The method according to the invention results in a composite with a low open porosity and is characterized by a total shrinkage of less than 1% (shrinkage by drying and sintering) and a high density of at least 1.90 g / cm ³ .

It is essential that the homogeneous base slip is first produced from the refinement of the SiO ₂ granules. The fine granulation process affects the homogenization of the base slip, and at the same time, the pH value decreases according to the stepwise dissolution up to the solubility limit of SiO ₂ in the fine granulation process, contributing to further dissolution of SiO _2. To do. It has been found that it is advantageous for the base slip to be refined and homogenized for at least 12 hours, and desirably the aforementioned stage of the invention takes at least 36 hours. The base slip prepared by this method can be used immediately for the production of a mold.

It is known that the above-mentioned refinement process is advantageous when SiO ₂ particles having a particle size and particle size distribution with a D ₅₀ value of 15 μm or less and a D ₉₀ value of 60 μm or less are produced.

In accordance with this index, the combination of added quartz glass granules results in high green density and high homogeneity, and good porosity of the green matrix, and drying and sintering as a whole of the green body results in less than 2.5% Low shrinkage. In a particularly desirable way, the D ₅₀ value of the finely divided SiO ₂ particles is a maximum of 9 μm and the D ₉₀ value is a maximum of 40 μm.

The amount of shrinkage due to drying and sintering is further reduced when the SiO ₂ particles in the base slip are 10% by weight or less and the particle size is 1 μm or less.

A small amount of well-separated SiO ₂ particles (up to about 10% by weight of the total weight of the SiO ₂ particles) is exerted as a binder in the substrate. Inducing “neck formation” during drying, assisting in sintering, and facilitating sintering contribute to an increase in density and mechanical strength of the substrate and thereby the composite material. When it becomes 10% by weight or more, the amount of liquid required increases and the amount of shrinkage increases. Moreover, such well-separated SiO ₂ particles tend to block the drainage holes of the mold into which the slip is injected. Therefore, well-separated SiO ₂ particles of 10% by weight or more of the base slip are avoided in principle.

The production of the base slip is desirably carried out with a solid content of 81 to 86% by weight.

At a solids content of less than 75% by weight, the base slip tends to settle, resulting in a significantly lower substrate density, resulting in a total shrinkage (shrinkage due to drying and sintering) of the composite material produced. The amount) is too high to allow the production of a high density quartz glass material without cracks. This effect has already been pointed out when the solid content is 81% by weight or less. When the solid content is equal to or higher than the upper limit described above, the slip molding ability may be impaired due to blockage by the SiO ₂ lump.

Further improvement of the method is achieved by diluting the base slip with liquid prior to mixing the quartz glass granules.

A liquid is added to improve the absorption capacity of the base slip with respect to the additional solid content. This is particularly effective when the absorption capacity of the base slip is limited or the desired mixing of quartz glass granules is no longer sufficient.

Desirably, the base slip is diluted with a solid content of 75 to 80% by weight prior to mixing of the quartz glass granules.

Generally, a base slip having a solid content of about 75% by weight does not require any dilution prior to mixing the quartz glass granules. In particular, the degree of dilution and the adjusted solid content are determined by the amount of quartz glass granules to be mixed.

It is known to be advantageous when the initial average particle size of the SiO ₂ granules is in the range of 0.2 to 3 mm.

If the particle size is 0.2 mm or less, the result is insufficient fine particle size in the range of particle size distribution limited to the fine SiO ₂ particle size. On the other hand, if the particle size is more than the above upper limit, fine particles are required. Energy and time increase disproportionately, and fine grain results are inhomogeneous.

Advantageously, the quartz glass granules added to the base slip have an average particle size (D ₅₀ value) in the range of 200-1000 μm, preferably 250-400 μm.

As already explained, the quartz glass granules reduce the amount of shrinkage by drying and sintering the substrate. That is, the addition of the granules improves the shape stability and dimensional stability of the quartz glass material made of the composite material. A total shrinkage of 2.5% or less is achieved. The quartz glass granules are in principle supplied as fillers, but can be selected according to the choice of physical or chemical properties of the composite. While the use of free quartz glass granules with a particle size of 1000 μm or more results in a heterogeneous structure and a relatively low strength composite, the aforementioned effects also diminish as the particle size of the quartz glass granules decreases.

In one particularly desirable variant of the method according to the invention, a particle size of 50 to 200 μm, provided that the amount of fine quartz glass particles present in the total weight of the quartz glass granules is in the range of 3 to 5% by weight. It is to use the fine quartz glass particle of the range.

Thus, it is known that a substrate having a high substrate density and strength can be obtained by adding fine silica glass particles covering the range of the particle size of the finely divided SiO ₂ granules and the silica glass granules. .

So far, the pH value of the base slip is reduced due to the refinement of the SiO ₂ granules. Furthermore, the pH value can be lowered by adding an acidifying material, for example, by adding nano-scale silicic acid particles or a sol containing such particles for the purpose of increasing the solubility of SiO ₂ . In a desirable procedure, the pH value of the base slip is adjusted by the addition of acid.

To reduce the total shrinkage as much as possible with the high density and high strength of the composite material, when the solid content of the base slip is 30 to 60% by weight in the total solid content in the composite slip It is known to be advantageous.

When the solid content in the base slip is 30% by weight or less, the cross-linking is reduced and the basic strength of the substrate is reduced. On the other hand, when the solid content is 60% by weight or more, the shrinkage amount of the composite material is large and the dimensional stability is low.

Generally, the density of the composite material is high, the total shrinkage is low, and the density of the composite slip is adjusted to be high. In particular, the density of the composite slip is desirably at least 1.95 g / cm ³ .

It is known that it is particularly advantageous when a seed crystal forming material is added to the composite slip.

In the intended use of the quartz glass material produced from the composite material according to the present invention, when the glass material is used at a high temperature of 1400 ° C. or higher, the seed crystal forming material or the aggregate material promotes the formation of cristobalite crystal nuclei. Since cristobalite has a higher melting point than quartz glass, the formation of cristobalite is effective for the dimensional stability of the quartz glass material under such a use environment. The addition of the agglomerated material suppresses the formation of coarse crystals of cristobalite that can destroy the structure, so that a well-separated structure of the quartz glass matrix formed from the SiO ₂ particles of the base slip is retained.

As the crystallization promoting substance of quartz glass, barium titanate, barium carbonate or barium silicate is desirable.

The above-mentioned substances are in powder form with a particle size in the range of several μm and are advantageously introduced into the base slip, and are further pulverized and homogeneously dispersed in this process.

A further improvement of the invention is made when the suspension is poured into a mold cooled to below the freezing point of the liquid.

The mold is cooled to below the freezing point of the suspension during or before molding. This promotes crystal formation in the liquid and, in the case of water, ice crystal formation, and the crystal formation particularly affects the aggregation of SiO ₂ particles having a size of 1 μm or less. As a result, it is known that a high-strength substrate can be obtained. SiO ₂ dissolved in the suspension has a strengthening effect as a binder after the removal of the liquid (particularly water). The surprising effect on the drying of the substrate by shock freezing and the heating in the next frozen state is clearly markedly increased by the low moisture content of the substrate.

However, the strength of the substrate is impaired by the growth of giant crystals (ice crystals). There is a fact that SiO ₂ already dissolved in the suspension works as a restraining factor of ice crystal growth. It is known to be advantageous when an organic substance that suppresses the formation of ice crystals is added to the suspension. The addition of the organic substance has the effect of making the formed large number of crystals as small as possible. This type of active substance is glycerol, polyethylene glycol, or polyacrylate.

It is known that it is particularly advantageous when the substrate is taken out in a frozen state, introduced into a drying chamber, and heated while removing moisture at a temperature of 40 to 80 ° C. inside.

It is known that high strength can be realized through rapid removal of moisture by heating the shock freezing substrate in a drying chamber. Removal of moisture from the drying chamber avoids water vapor agglomeration and refreezing on the surface that can damage the surface structure and reduce the substrate strength.

In the present invention, for the composite material based on the originally shown material, the above object is achieved by the material having a density of at least 1.94 g / cm ³ .

The composite material manufactured according to the present invention has a high density. Its high substrate strength and low shrinkage are particularly advantageous for the production of large opaque quartz glass materials.

The pre-composite material is obtained through the method of the present invention. Reference is made below to the above description.

When the composite material manufactured according to the present invention is used as a starting material for manufacturing a permanent mold for solar silicon melting, the above-mentioned object is achieved.

It is known that the composites according to the invention are particularly suitable as starting materials for the production of permanent molds for melting solar silicon. For this purpose, high chemical resistance against silicic acid melts is particularly important. Such permanent molds are usually fitted with a Si ₃ N ₄ separation layer. Such separation layers are known to adhere in a particularly rigid manner to a permanent mold made of the composite material according to the invention. In addition, the mechanical strength and thermal shock resistance of the permanent mold are required. In order to achieve this, a stabilizing layer is attached to the outer wall of the permanent mold, which is effective in improving the shape stability of the permanent mold at high temperatures as in this application.
The invention will now be described in more detail with reference to examples and drawings.

The graph of FIG. 1 shows the particle size distribution determined by the light scattering spectroscopy and laser diffraction spectroscopy specified in ISO13320 for the composite slip meaning the present invention. The particle size (X-axis) is plotted in logarithm relative to the relative frequency (in%) of the corresponding granule volume in μm units.

This distribution curve shows two notable maxima 1 and 2. The maximum value in front belongs to a portion corresponding to the particle size distribution of the base slip in principle. This part covers the range of particle size 0.1 or more. The maximum value 1 is a particle size slightly smaller than 2 μm.

The rear maximum 2 is a part corresponding to the particle size distribution of the quartz glass granule mixed with the homogeneous base slip in principle in the distribution curve. The site includes a particle size of up to 900 μm.

Since the two parts on the above-mentioned particle size distribution curve overlap at a part near 100 μm (part 3), the maximum particle size of the base slip and the minimum particle size of the mixed quartz glass granule can be estimated from the graph. Can not. According to individual measurements, the SiO ₂ particles in the base slip have a maximum particle size of about 60 μm, a D ₅₀ value of about 5 μm and a D ₉₀ value of about 23 μm. Have

In this example, the composite slip includes added SiO ₂ particles having a particle size of 160 μm or less (see FIG. 2) in addition to the finely divided SiO ₂ granules. These particles partially fill the gap between two significant particle size sites (site 3 between maxima 1 and maxima 2).

The preparation of the composite slip will be described in more detail below with reference to design examples.

Initially, a homogeneous base slip 14 is produced. For 10 kg of batch of base slip (SiO ₂ suspension), amorphous quartz glass granule 11, which is a natural raw material with a particle size of 1 to 3 mm, 8.2 kg, deionized water 12 with conductivity 3 μS or less, 1.8 kg is mixed in a drum mill with a volume of 14 liters coated with polyurethane.

The above-mentioned mixture 13 was obtained by using Al ₂ O ₃ milling balls (weights 2.3 kg and 4.5 kg) on a roll stand at 23 rpm for 5 days with a homogeneous base slip 14 having a solid content of 82% and a viscosity of 140 mPas. Refine until fine. In the process of refining, the pH value decreases to about 5 due to dissolution of SiO ₂ .

After the silica glass granules 11 are refined, the SiO ₂ particles obtained in the base slip 14 have a particle size distribution characterized by a D ₅₀ value of about 5 μm and a D ₉₀ value of about 23 μm.

A composite slip 19 is produced from the homogeneous base slip 14 obtained by this method. In order to finally obtain a composite slip 19 with a batch quantity of 15 kg, the base slip 14,6.65 kg with a solid content of 8% is first diluted and mixed with 15,0.250 kg of deionized water in a drum mill. . This gives a mixture 17 with a solids content of 78.2% and a viscosity of 75 mPas. The mixture 17 is mixed with 0.54 kg of dust 16, which is amorphous SiO ₂ having a particle size of 50 to 125 μm, and mixed in a drum mill at a rotation speed of 27 rpm.

Following mixing for 20 minutes, the mixture 17 is added about 7.6 kg of amorphous quartz glass granules 18 having a particle size of 335-710 μm and mixed in a drum mill at a rotational speed of 25 rpm. The composite slip 19 thus produced has a solid content of 90%, a viscosity of 1531 mPas, and a density of 2.0 g / cm ³ .

Next, a substrate 20 is formed from the homogeneous base slip 19. There are many ways to do this. Here, several examples are illustrated by an exemplary technique.

The base slip 19 is poured into a die casting mold of a commercially available die casting machine and dehydrated through the porous plastic film during the formation of the porous substrate 20.
In order to dehydrate the bound water, the substrate 20 is dried at about 200 ° C. in a ventilated furnace and then sintered at 1430 ° C. to obtain an opaque composite 21.

The composite slip 19 is injected into a plastic mold by adding a gel forming agent such as ammonium fluoride, and after solidifying, is taken out from the mold as a base 20. In this case, a slow drying process is necessary to avoid dry cracking.

-The homogeneous base slip 19 is added with a gel-forming agent such as ammonium fluoride. This results in a viscous mass that is pressed into a suitable mold and solidifies inside.

The composite slip 19 is injected into a film mold embedded in solid carbonic acid. This is effective for rapid solidification of the composite slip in the formation of the substrate 20 as well as rapid ice formation. By adding a commercially available preparation (Optapix PFA 35 from Tschimmer & Schwarz) to the composite slip 19, formation of coarse needle-like ice crystals is suppressed. The base material 20 subjected to impact freezing is taken out from the film mold, and immediately introduced into a circulating drying cabinet heated to 50 ° C. in a frozen state, and dried for several hours. This method avoids re-aggregation of water and refreezing on the surface with the destruction of needle-like ice crystals and substrate structure.
The substrate 20 after drying is sintered at 1250 ° C. for 2 hours to obtain the opaque quartz glass composite material 21.
The composite material has a density of 2.1 g / cm ³ . This includes a matrix having a homogeneous structure of uniform fine pores embedded with the quartz glass granules. The composite material is particularly characterized by high thermal shock resistance and excellent chemical resistance to silicon melts. This total shrinkage (shrinkage during drying and sintering) is 1.1% shrinkage compared to the mold dimensions.
From the composite material, a permanent mold having a rectangular cross section for melting solar silicon is produced. The permanent mold has dimensions of 850 mm × 850 mm × 500 mm. The separation layer in the form of a Si ₃ N ₄ coating applied to the inner wall of the mold exhibits high adhesion.

The particle size distribution graph of the SiO ₂ particles and quartz glass granules in the composite slip. The flowchart figure of the manufacturing process of the composite material based on the method of this invention.

Claims (20)

Production of slips containing amorphous SiO ₂ particles with a size of the order of 100 μm or less and quartz glass granules with a size of the order of 100 μm or more, homogenization of the slip, introduction of the slip into a mold, and porous substrate by drying And a method for producing an opaque quartz glass composite material, wherein an opaque quartz glass composite material is obtained by sintering the substrate,
A SiO ₂ particle having a particle size and particle size distribution characterized by having a D ₅₀ value of 15 μm or less and a D ₉₀ value of 50 μm or less is obtained by refining amorphous SiO ₂ granules in a liquid. and by lowering the pH value, to produce a homogeneous base slip having at least 75% by weight of the solids content, also by the quartz glass granules are mixed into the homogeneous base slip, of at least 1.85 g / cm ³ A method for producing an opaque quartz glass composite material comprising forming a composite slip having a density. The method of claim 1 wherein the pre-base slip refinement and homogenization is at least 12 hours, preferably at least 36 hours. _{3. The} SiO ₂ particles having a particle size and a particle size distribution, wherein the D ₅₀ value is 9 μm or less and the D ₉₀ value is 40 μm or less are produced by the fine graining. Method. The base 10 wt% of SiO ₂ particles in the slip below, The process of any one of claims above, characterized in that it has a particle size of less than 1 [mu] m. The method according to any one of the preceding claims, characterized in that the base slip is produced with a solids content in the range 81-85% by weight. A method according to any one of the preceding claims, wherein the base slip is diluted by addition of liquid prior to mixing of the quartz glass granules. The method of claim 6, wherein the base slip is diluted to a solids content of 75 to 80 wt% prior to mixing of the quartz glass granules. The method of any one of claims above, characterized in that the initial average particle diameter of the SiO ₂ granules is in the range of 0.2 to 3 mm. Any of the above, wherein the quartz glass granules added into the base slip have an average particle size (D ₅₀ value) in the range of 200-1000 μm, preferably in the range of 250-400 μm The method of one claim. Fine quartz glass particles having a particle size in the range of 50 to 200 μm are used on condition that the amount of fine quartz glass particles present in the total weight of the quartz glass granules is in the range of 3 to 5% by weight. The method of any one of the preceding claims. The method according to any one of the preceding claims, wherein the pH value of the base slip is adjusted by adding an acid. The method according to any one of the preceding claims, wherein the content of solids derived from the base slip present in the total solids weight of the composite slip is in the range of 30-60% by weight. The method of any one of the preceding claims, wherein the composite slip has a density of at least 1.95 g / cm ³ . A method according to any one of the preceding claims, characterized in that a seed crystal-forming substance is added to the composite slip. 15. The method according to claim 14, wherein barium titanate or barium carbonate is preferably used as a crystallization accelerator for quartz glass. A method according to any one of the preceding claims, wherein the composite slip is injected into a mold cooled to below the freezing point of the liquid. 16. The method of claim 15, wherein the composite slip is mixed with an organic material that inhibits the formation of ice crystals. The method according to claim 16 or 17, wherein the substrate is taken out of the mold in a frozen state, introduced into a drying chamber, and heated while removing moisture at a temperature in the range of 40 to 80 ° C. A composite comprising a SiO ₂ -containing matrix in which quartz glass granules are embedded, characterized in that it has a density of at least 1.94 g / cm ³ . Use of the composite material produced according to claims 1-18 as a starting material for the production of a permanent mold for melting solar silicon.