WO2011101327A1

WO2011101327A1 - Method for producing a quartz glass crucible

Info

Publication number: WO2011101327A1
Application number: PCT/EP2011/052177
Authority: WO
Inventors: Walter Lehmann; Thomas Kayser; Michael Huenermann; Christian Nasarow
Original assignee: Heraeus Quarzglas Gmbh & Co. Kg
Priority date: 2010-02-16
Filing date: 2011-02-15
Publication date: 2011-08-25
Also published as: CN102753493A; DE102010008162B4; JP2013519624A; DE102010008162A1

Abstract

The invention is based on a known method for producing a quartz glass crucible for drawing a monocrystal, in which a granular layer is formed in a fusion mould and is sintered or melted forming the quartz glass crucible. In order to specify cost-effective production of quartz glass for a quartz glass crucible that has a high degree of purity and allows both the freedom of bubbles and a defined and homogeneous fine porosity to be set reproducibly, according to the invention at least a part of the granular layer is produced from silica glass granules, the production of which comprises the following method steps: mechanically compressing silica glass powder by means of a rolling and briquetting method forming pellets having a substantially uniform, spheroidal morphology, and thermally compressing the pellets or fragments thereof to form the silica glass granules.

Description

Process for producing a quartz glass crucible

The invention relates to a method for producing a quartz glass crucible for pulling a single crystal by forming a graining layer in a melt mold and sintering or melting the same to form the quartz glass crucible.

Quartz glass crucibles are used to hold the molten metal when pulling single crystals using the so-called Czochralski process. Their preparation is usually carried out by the fact that on the inner wall of a melt mold, a layer of SiO ₂ grain produced and this heated using an arc (plasma) and thereby sintered to the quartz glass crucible. The wall of such a quartz glass crucible is usually formed by a heat-insulating outer layer of opaque quartz glass, which is provided with an inner layer of transparent, bubble-free as possible quartz glass.

The transparent inner layer is in contact with the silicon melt during the drawing process and is subject to high mechanical, chemical and thermal loads. Bubbles remaining in the inner layer grow under the influence of temperature and pressure and eventually burst, causing debris and impurities to enter the silicon melt, thereby providing a lower yield of dislocation-free silicon single crystal.

In order to reduce the corrosive attack of the silicon melt and thus to minimize the release of impurities from the crucible wall, the inner layer is therefore as homogeneous as possible and low in bubbles. In contrast, for the outer layer for the purpose of thermal insulation usually sought a uniform, fine-grained as possible opacity. State of the art

From DE 10 2008 030 310 B3 a method of the type mentioned is known. Here, a vacuum melt mold is used to produce a quartz glass crucible. In this, a rotationally symmetrical, crucible-shaped graining layer of mechanically solidified quartz sand with a layer thickness of about 12 mm is formed using a forming template, onto which an inner grain layer of synthetically produced quartz glass powder is then likewise formed using a shaping template.

The synthetic quartz glass powder has particle sizes in the range of 50 to 120 μιτι, wherein the average particle size is about 85 μιτι. The middle

Layer thickness of the inner grain layer is about 12 mm. The sintering of the graining layers takes place from inside to outside by generating an arc in the interior of the melt mold, so that the finely divided quartz glass powder first sinters and forms a dense glass layer. For the production of such a synthetic quartz glass powder, sol-gel methods and granulation methods are known. For example, it is proposed in DE 102 43 953 A1 to produce synthetic quartz glass powder by granulation starting from a suspension of pyrogenically produced SiO ₂ powder, as obtained as filter dust in the manufacture of quartz glass. In the process, a suspension is first produced from the loose SiO ₂ soy dust by mixing in water and homogenization, this suspension is processed to SiO ₂ granules by means of a wet granulation method, and these become dense after drying and cleaning by heating in a chlorine-containing atmosphere Quarzglaskörnung with a mean diameter of 140 μιτι gesin- tert.

The known build-up granulation process requires a variety of process steps, some of which are tedious and associated with a high energy requirement, such as the drying of the porous SiO ₂ granules. This disadvantage is avoided by granulation processes in which finely divided starting powders are mechanically agglomerated and compacted by roll compaction into coarser particles, also with the addition of lubricants or binders. Such a method is described for example in WO 2007/08551 1 A1 and DE 10 2007 031 633 A1. In the process, finely divided silica powder is passed between counter-rotating rollers, which may be smooth or profiled, and compacted into SiO ₂ granules in the form of so-called "scabs." These form more or less band-shaped structures which are normally broken and sized Granules produced therefrom typically have rupture densities in the range of 185 to 700 g / l.

The slug fragments can be dried in a halogen-containing atmosphere at a temperature in the range from 400 to 1100 ° and densely sintered in the range from 1200 ° C. to 1700 ° C. to form a "silica glass granulate." EP 2 014 622 A1 describes one densely sintered silica glass granules produced in this way by breaking and sintering slugs, which are largely free of bubbles The individual granules have diameters in the range of 10 to 140 μιτι and the specific surface is less than 1 m ² / g.

DE 10 2007 049 158 A1 mentions possible uses of such "Kie selglasgra-nulate" with diameters between 1 μιτι and 5 mm and an impurity content of less than 50 ppm by weight for producing a variety of different components of high-purity quartz glass, inter alia for the production of cased pipes, crucibles, semiconductor devices and glass rods.

The silica glass "slugs" or their fragments result in a low-dust, readily flowable "silica glass granules" with increased bulk density, which is fundamentally suitable for cost-effective production of high-purity quartz glass products. It turns out, however, that the starting material can be improved for applications with particularly high demands on the homogeneity of the quartz glass. Technical task

The invention has for its object to provide a method that allows a cost-effective production of quartz glass for a quartz glass crucible, which is characterized by high purity and allows both the reproducible setting of freedom from bubbles and a defined and homogeneous fine porosity.

This object is achieved on the basis of the generic method in that at least part of the granulation layer is produced from a silica glass granule whose production comprises the following process steps: (a) mechanical compaction of silica powder by means of a roller briquetting process to form molded compacts having a substantially uniform, spheroidal morphology , and

(b) thermally compacting the molded compacts or fragments thereof into the silica glass granulate The silica powder provided according to process step (a) is present, for example, as pyrogenically produced, finely divided SiO ₂ in the form of a soot dust consisting of discrete SiO ₂ nanoparticles which may also be partially agglomerated to improve handling, for example by means of spray granulation.

In the roll briquetting process, the press rolls have mutually running, corresponding mold cavities which mutually terminate outwardly as the press rolls rotate, thereby producing tablet shaped moldings from the silica powder which is trapped between the adjacent mold cavities during rotation. These are usually mirror-symmetrical depending on the internal geometry of the mold cavities and are in spheroidal, uniform geometry, in particular in spherical shape or in the form of flattened (oblate) ellipsoids or up to the cylindrical shape elongated (prolater) ellipsoids. In this way moldings are obtained inexpensively and with high purity, in reproducible size, shape and density.

The density of the molded compacts is adjustable via the roller pressure. During roller compaction, the press rollers exert-in contrast to the flaring method-a unidirectional pressing pressure on the fumed silica powder, so that the silica powder enclosed in the mold cavities experiences a compressive pressure acting on all sides, which leads to a spatially uniform compaction. The adjustment of the density, which is variable in terms of the roller pressure interval, and its spatially uniform distribution also facilitate the production of a reproducible end product, even in the case of density-sensitive further processing of the molded parts, for example in sintering processes in which a quartz glass having a predetermined volatility or transparency is obtained by heating a mass of the molded parts shall be.

In the process according to the invention, the molded compacts - or fragments thereof - are used as raw material for the production of quartz glass for the production of crucibles. In this case, the most homogeneous possible distribution of the density in the molding briquettes is advantageous, regardless of whether bubble freedom or fine porosity is to be set in the respective wall region of the quartz glass crucible. Depending on the preset density of the molded compacts, preference is given to either thermal densification to transparent or thermal compression to bubble-containing silica glass granules. A high roll pressure in the roll briquetting process results in a high internal density of the molded compacts, so that a high density in the thermal densification and a faster formation of a quartz glass network is favored. On the other hand, in the roll briquetting process, a low roll pressure can be used to set a lower inner density of the molded compacts, which leads to the formation of closed bubbles during thermal compression and promotes the formation of fine porosity during sintering. In addition, the good reproducibility in the thermal densification of the porous SiO ₂ granulate can also be attributed to the uniform and well-defined morphology of the molded compacts.

The fused silica granulate particles of synthetic quartz glass obtained after thermally compacting the molded compacts have a comparatively large volume, which further improves the productivity and economy of quartz glass production. This comparatively large "pre-glazed volume" contributes to the fact that the silica glass granules sinter relatively easily and homogeneously to opaque quartz glass or melt into transparent quartz glass.

The silica glass granules are characterized by high purity, so that crystallization of the quartz glass and bubble growth can be prevented during sintering or melting.

Preferably, the molded compacts have a mean equivalent diameter in the range of 1 mm to 5 mm.

Diameters of less than 1 mm are on the order of the diameter of typical synthetic silica grain. With larger diameters of the molded articles and the silica glass granules produced therefrom, the productivity gain becomes more noticeable due to the larger, pre-glazed volume. Molded compacts with equivalent diameters greater than 5 mm result in large interstitial fillings, which can be unfavorable to sintering for the purpose of transparency or fine poredness.

Fragments of such large molded compacts are readily usable, but have no uniform morphology. The equivalent diameter refers here only to the size of the particles (mesh size).

It has proven useful if the individual molded compacts have an average volume in the range of 1 to 100 mm ³ . With an average volume of less than 1 mm ³ , there is no significant advantage in terms of the economy of the process and on a homogenization of the spatial density. Large-volume moldings can show a noticeable density gradient from outside to inside, which can affect the freedom from bubbles of the quartz glass to be produced. Therefore, molded compacts having an average volume of more than 100 mm ^{3 are} not preferred.

In particular with regard to freedom from bubbles of the quartz glass to be produced, it has also proved to be advantageous if the individual molded compacts have an average specific density in the range from 0.6 to 1.3 g / cm ³ .

The high specific gravity of the molded compacts facilitates the reproducible, bubble-free thermal densification of the porous SiO ₂ granules to the silica glass granules.

For the production of molded compacts by means of roll briquetting, for economic or technological considerations, an optimum can be found for the volume of the individual molded compacts which is above an optimum for the size or volume of the silica glass granulate particles.

In these cases, it is advantageous to comminute the molded compacts prior to sintering or melting, more preferably prior to the thermal densification according to method step (b).

By crushing the molded compacts, a particularly high bulk density of the granules can be achieved. Preferably, the crushed shaped compacts form a SiO ₂ granulate having a bulk density in the range of more than 0.45 g / cm ³ , preferably from 0.8 to 1.1 g / cm ³ . The high bulk density, which is determined according to DIN ISO 697 (1984), not only contributes to the dense packing, but also the high specific gravity of the individual granulate particles or of fragments thereof. A high bulk density facilitates melting and sintering into quartz glass. In the sense of a high purity and freedom from bubbles of the quartz glass, a procedure is preferred in which the molded compacts or fragments thereof are treated in a chlorine-containing atmosphere before the thermal densification according to process step (b). In the treatment at elevated temperature in the range of 800 ° C and 1200 ° C impurities are eliminated and hydroxyl groups are largely removed.

In view of a high freedom of bubbles or a homogeneous fine poredness of the quartz glass to be produced, it has proven useful if the thermal compression of the molded compacts or fragments thereof to silica glass granules according to process step (b) under an atmosphere containing at least 30 vol .-% helium and / or under a negative pressure of 0.5 bar or less.

It has also proved to be advantageous when silica glass granules particles are used which consist of hydrogen-doped quartz glass. Hydrogen is a gas that diffuses relatively easily in quartz glass and is released when heated. During sintering or melting of the silica glass granules, the escaping hydrogen reacts with existing gaseous oxygen to form H ₂ O, which is soluble in the form of hydroxyl groups in the quartz glass. This facilitates bubble-free sintering or melting. The hydrogen loading can be carried out during the thermal compression of the molded compacts by carrying out these under a hydrogen-containing atmosphere.

The silica glass granules are suitable for the production of transparent quartz glass and fine-pored quartz glass. In view of this, it has been found useful if the silica glass granules thermally compacted according to process step (b) are applied as an outer grain layer on an inner wall of the melt mold and sintered to a crucible wall outer layer of at least partially opaque quartz glass. To form the outer wall of the crucible wall, preferably a fine-pored, not completely compacted silica glass granulate is used.

Alternatively or additionally, the silica glass granules thermally compacted according to process step (b) are applied as an intermediate granulation layer on an outer granulation layer and sintered to a crucible wall intermediate layer or to a crucible wall outer layer of at least partially opaque quartz glass.

Depending on whether, during the melting of the crucible wall, the enamel front proceeding from inside to outside completely or only partially passes through the intergranular layer, the intergranular layer forms an intermediate layer within the crucible wall or forms the crucible wall outer layer. To produce these layers, it is likewise preferred to use a fine-pored, not completely compacted silica glass granulate.

Preferably, the silica glass granules thermally compacted according to process step (b) are applied as an inner granulation layer on an inner wall of a bony base body and melted to form a crucible wall inner layer of transparent quartz glass on the base body.

In this case, a bony base body made of quartz glass or quartz glass grain is provided with a transparent quartz glass layer, which serves as a diffusion barrier against any impurities in the intended use, which are contained in the quartz glass of the base body. In addition, the crucible wall inner layer improves the surface finish of the base body.

In this case, preference is given to using a bite-shaped, porous base body made from the silica glass granulate, the inner grain layer being sintered while applying a negative pressure to the inner wall of the inner wall of the crucible wall.

For the production of the inner layer, first of all a porous SiO ₂ granulation layer of a less dense silica glass granulate is produced on the inner wall of an evacuable melt mold and a further body of SiO _{2 is} applied thereon. applied layer of a silica glass granules of higher density. When sintering the granulation layers from inside to outside, a vacuum is applied from the outside of the melt mold. The inner crucible wall of higher density silica glass granules produced by virtue of the method according to the invention is characterized by high purity and low bubble content, and it can be reproduced and produced economically even in large layer thicknesses.

In contrast, the lower density silica glass granules provide an outer layer of which at least the outer region is opaque and is characterized by a uniform porosity, as explained above.

In an alternative and equally preferred variant of the method for producing such a quartz glass crucible, it is provided that the silica glass granules thermally compacted according to process step (b) are fed to an arc for sintering or melting, melted therein and deposited on an inner wall of a bell-shaped base body made of quartz glass to form a crucible wall Inner layer of transparent quartz glass is spun on.

In this case, an arc is ignited within the interior of a rotating about its longitudinal axis crucible base body. In these, the silica glass granules are interspersed, melted and thrown against the inner wall of the base body under the action of the arc pressure, where it adheres to form a crucible-wall inner layer of transparent quartz glass. Depending on the size interspersed silica glass granules particles and their impact point in the arc, it can lead to different degrees of fusion and to a considerable scattering of the thrown particles. A uniform particle size of the interspersed silica glass granules causes both a defined impact point in the arc and a uniform degree of fusion, which promotes the reproducibility of the production of the crucible wall inner layer. Preference is given to using silica glass granulate particles whose diameter deviates from a nominal diameter by a maximum of 10%.

Silica glass granulate particles of uniform diameter exhibit a similar sintering and melting behavior. In addition, beds of such particles have a comparatively low bulk density so that they are easier to treat or degasify with reactive gases.

embodiment

The invention will be explained in more detail with reference to embodiments and a drawing. 1 shows, in a schematic representation, a melting apparatus for producing a quartz glass crucible using silica glass granules with reference to a first method variant of the invention, and

FIG. 2 shows a schematic representation of a melting apparatus for producing a quartz glass crucible with a transparent inner layer produced using silica glass granules, using a second method variant.

Example 1 Production of Synthetic Silica Glass Granules

By flame hydrolysis of SiCl ₄ pyrogenic, finely divided silica is produced. For this purpose, oxygen and hydrogen are fed to a flame hydrolysis burner as burner gases and, as feed for the formation of the SiO ₂ particles, a gas stream containing SiCl ₄ . The size of the primary SiO ₂ particles is in the nanometer range, with several primary particles assembling in the reaction zone and occurring in the form of more or less spherical agglomerates or aggregates having a BET specific surface area in the range of 50 m ² / g. The synthetic, high-purity silica is compacted by means of a commercial Walzenbrikettieranlage into tablet-shaped, spheroidal compacts with the following properties:

Geometry: oblate ellipsoids Equivalent diameter: 3 mm

Volume: 20 mm ³

Specific gravity: 1.2 g / cm ³

The molded compacts form a SiO ₂ granulate with a specific surface area of about 50 m ² / g (BET) and a bulk density of about 0.7 g / cm ³ . They are then crushed by means of a crusher and classified by sieving. The particle size range of 120 to 600 μιτι is further processed, as described below; and the defective fraction is added to the silica input material of the roll briquetting plant.

The SiO ₂ granulate produced in this way consists of fragments and has a specific surface area of about 50 m ² / g (BET) and a bulk density of about 0.9 g / cm ^{3, which is} higher than that of the non-shredded granules.

It is subsequently cleaned by exposing it to a HCl-containing atmosphere in a shaft furnace at a temperature of around 850 ° C. for a treatment time of 6 hours, thereby being freed of impurities and hydroxyl groups. The treated SiO ₂ granules have total content of impurities of Li, Na, K, Mg, Ca, Fe, Cu and Mn of less than 200 wt. Ppb and a hydroxyl group content of 30 wt. Ppm.

The thus treated SiO ₂ granules are then placed in a graphite mold and vitrified by vacuum sintering. Here, the mold is heated to the sintering temperature of 1600 ° C and held for about 60 minutes after reaching the sintering temperature.

After cooling, a mass of more or less loosely connected, glassy SiO ₂ particles is obtained, which are separated by slight pressure. you can. The silica glass granules thus obtained consist of glassy, bubble-free quartz glass particles with equivalent diameters in the range from about 100 to 500 μm and with a specific surface area (according to BET) of less than 1 m ² / g. The silica glass granules are subsequently loaded with hydrogen by being exposed to a hydrogen-containing atmosphere at a temperature of 800 ° C. for a period of 5 hours.

Example 2 Production of Synthetic Silica Glass Granules

The synthetic, high-purity silica is compacted by means of a commercial roller briquetting plant into spherical compacts having the following properties:

Geometry: prolate ellipsoids Equivalent diameter: 1, 5 mm Volume: 2.5 mm ³ Specific gravity: 1, 2 g / cm ³

The SiO ₂ granules so produced consist of elongated molded compacts of identical geometry and size and have a specific surface area of about 50 m ² / g (BET) and a relatively low bulk density of about 0.7 g / cm ³ . It is purified and thermally densified as described above with reference to Example 1. The silica glass granules obtained after vacuum sintering consist of glassy, bubble-free quartz glass particles of uniform dimensions (without fines) and with a specific surface area (according to BET) of less than 1 m ² / g.

Example 3 Production of Synthetic Silica Glass Granules

Synthetic high purity silica is processed and pelletized by means of a roll briquetting machine into tablet-shaped, spheroidal compacts, as explained in Example 1. The SiO ₂ granules obtained thereafter are then placed in a graphite mold and vitrified under helium. For this purpose, the graphite mold is heated to the sintering temperature of 1600 ° C and held for about 60 minutes after reaching the sintering temperature. After cooling, a mass of more or less loosely connected, glassy SiO ₂ particles is obtained, which can be separated by slight pressure. The silica glass granules thus obtained consist of glassy, bubble-free quartz glass particles with uniform dimensions (equivalent diameter) of about 100 to 500 μιτι and with a specific surface area (according to BET) of less than 1 m ² / g.

The production of a quartz glass crucible using the synthetic silica glass granulate is explained below with reference to two exemplary embodiments:

Example 4: Production of a quartz glass crucible with inner layer

The melting apparatus according to FIG. 1 comprises a metal melt mold 1 with an inner diameter of 75 cm, which rests on a support 3 with an outer flange. The carrier 3 is rotatable about the central axis 4. A cathode 5 and an anode 6 (electrodes 5, 6) made of graphite protrude into the interior 10 of the melt mold 1 and can be moved within the melt mold 1 in all spatial directions, as indicated by the directional arrows 7. The open top side of the melt mold 1 is partially covered by a heat shield 11 in the form of a water-cooled metal plate with a central through-hole through which the electrodes 5, 6 protrude into the melt mold 1. The heat shield 1 1 is connected to a gas inlet 9 for hydrogen (alternatively also for the supply of helium). The heat shield 2 is horizontally movable in the plane above the mold 1 (in the x and y direction), as indicated by the directional arrows 22.

The space between the carrier 3 and the melt mold 1 can be evacuated by means of a vacuum device, which is represented by the directional arrow 17. The melt mold 1 has a plurality of passages 8 (these are shown in FIG 1 symbolically indicated in the bottom area), through which the voltage applied to the outside of the mold 1 vacuum 17 can pass through to the inside.

In a first method step, fused silica granules which have been produced on the basis of example 3 above are filled into the melt mold 1 rotating about its longitudinal axis 4. Under the action of the centrifugal force and by means of a shaping template, a rotationally symmetrical, bony-shaped granulation layer 12 of the mechanically solidified granules is formed on the inner wall of the melt mold 1. The average layer thickness of the granulation layer 12 is about 12 mm.

In a second method step, on the inner wall of the granulate layer 12, an inner granulation layer 14 of the synthetically produced silica glass granulate according to Example 1 above-likewise using a molding template and with continuous rotation of the melt mold 1 -is formed.

The average layer thickness of the inner graining layer 14 is also about 12 mm. For glazing the SiO ₂ grain layers 12, 14, the heat shield 11 is positioned over the opening of the melt mold 1 and helium is introduced via the inlet 9 into the crucible interior 10. The electrodes 5; 6 are inserted through the central opening of the heat shield 1 1 in the interior 10 and between the electrodes 5; 6 an arc ignited, which is characterized in Figure 1 by the plasma zone 13 as gray background area. At the same time, a vacuum is applied to the outside of the melt mold 1.

The electrodes 5; 6 are brought together with the heat shield 1 1 in the lateral position shown in Figure 1 and applied with a power of 600 kW (300 V, 2000 A) and, to the granulation layers 12; 14 glazed in the area of the side wall. The plasma zone 13 is moved slowly downwards, while the quartz glass powder of the inner granulation layer 14 is continuously and partially melted into a bubble-free inner layer 16. For vitrifying the granulation layers 12; 14 in the region of the bottom are heat shield 1 1 and electrodes 5; 6 brought into a central position and the electrodes 5; 6 lowered down. When the layer is sintered, a dense inner skin initially forms. Thereafter, the applied negative pressure (vacuum) can be increased, so that the vacuum can develop its full effect.

The melting process is terminated before the melt front reaches the inner wall of the melt mold 1. The transparent inner layer 16 is smooth, low-bubble and has an average thickness of about 8 mm. The outer layer 12 remains at least partially opaque.

Example 5: Production of a quartz glass crucible with inner layer

In the following, a modification of this procedure will be explained with reference to the melting device shown schematically in Figure 2. If the same reference numerals as in FIG. 1 are used in FIG. 2, then identical or equivalent components and components are referred to, as explained in more detail above with reference to FIG.

Here, the melting device has a spreading tube 18, which can be moved in all spatial directions (directional arrows 7) and projects into the interior of the melt mold 1 and which is connected to a storage container 19. The litter tube 18 is provided with a trouser piece 23 for the supply of compressed air - symbolized by the directional arrow 24.

The storage container 19 is filled with silica glass granulate particles 25 of pure, synthetically produced and hydrogen-doped quartz glass according to Example 2 above. The fused silica granules particles 25 have uniform dimension without fines and are accordingly defined in terms of their mechanical properties and easy to handle.

To produce the quartz glass crucible, an outer granular layer of crystalline granules of naturally occurring quartz sand having a grain size in the range from 90 μm to 315 μm, which has been previously purified by means of hot chlorination, is first of all shaped. Subsequently, a transparent and low-bubble inner layer 26 is produced on the inner wall of the outer granulation layer by means of the "arc bedding process." For this purpose, while the melt mold 1 is being continuously rotated via the spreader tube 18 and compressed air 24 is fed, the high-purity silica glass granulate 25 is poured into the crucible. Interior space 10. At the same time plasma 13 (arc) is ignited between cathode 5 and anode 6.

The interspersed silica glass granules particles 25 enter the plasma zone 13, are softened therein, and are thrown against the inner wall of the outer granulation layer by the pressure generated by the arc and are melted thereon to form the inner layer 26 of transparent quartz glass. In this case, a maximum temperature of over 2100 ° C is reached in the region of the inner wall, so that the outer grain layer is sintered to form an outer layer 27 of opaque quartz glass. Because of the uniform size and shape of the silica glass granulate particles 25, there is little local scattering and a defined structure of the inner layer 26.

The inner layer 26 of the quartz glass crucible thus produced has an average thickness of 2.5 mm. It is smooth, low-bubble and firmly connected to the outer layer 27 of opaque quartz glass.

Claims

claims

1 . A method for producing a quartz glass crucible for growing a single crystal by forming a graining layer (12; 14) in a melt mold (1) and sintering or melting the same to form the quartz glass crucible, characterized in that at least a part of the graining layer (12; 14) is produced from a silica glass granulate whose production comprises the following process steps:

(a) mechanically densifying silica powder by means of a roll briquetting process to form molded compacts having a substantially uniform, spheroidal morphology, and

(b) thermally compacting the molded compacts or fragments thereof into the silica glass granules.

2. The method according to claim 1, characterized in that the molded compacts have a mean equivalent diameter in the range of 1 to 5 mm.

3. The method according to claim 1 or 2, characterized in that the individual molded compacts have an average volume in the range of 1 to 100 mm.

4. The method according to any one of the preceding claims, characterized in that the individual molded compacts have a mean specific log te in the range of 0.6 to 1, 3 g / cm ³ .

5. The method according to any one of the preceding claims, characterized in that the molded compacts are crushed before sintering or melting.

6. The method according to claim 5, characterized in that the molded compacts before the thermal densification according to method step (b) are comminuted.

7. The method according to any one of claims 5 or 6, characterized in that the crushed molded compacts a SiO ₂ granules having a bulk density in the range of more than 0.45 g / cm ³ , preferably from 0.8 to 1, 1 g / cm ³ , form.

8. The method according to any one of the preceding claims, characterized in that the molded compacts or fragments thereof are treated before the thermal densification according to process step (b) in a chlorine-containing atmosphere.

9. The method according to any one of the preceding claims, characterized in that the thermal compression of the molded compacts or fragments thereof to silica glass granules according to process step (b) under an atmosphere containing at least 30 vol .-% helium and / or under a negative pressure of 0.5 bar or less.

10. The method according to any one of the preceding claims, characterized in that silica glass granulate particles are used, which consist of hydrogen-doped quartz glass.

1 1 .Verfahren according to any one of the preceding claims, characterized in that the method according to step (b) thermally compacted silica glass granules applied as an outer graining layer on an inner wall of the melt mold and sintered to a crucible wall outer layer of at least partially opaque quartz glass.

12. The method according to any one of the preceding claims, characterized in that the method according to step (b) thermally compacted silica glass granules applied as Zwischenkörnungsschicht on a Außenkörnungsschicht and sintered to a Tiegelwandungs intermediate layer or to a Tiegelwandungs outer layer of at least partially opaque quartz glass.

13. The method according to any one of the preceding claims, characterized in that the method according to step (b) thermally compacted silica glass granules for sintering or melting supplied to an arc, melted therein and on an inner wall of a tiegeiförmigen base body made of quartz glass to form a Tiegelwandungs inner layer of transparent quartz glass is spun on.

14. The method according to any one of claims 1 to 12, characterized in that the method according to step (b) thermally compacted silica glass granules applied as Innenkörnungsschicht on an inner wall of a bite-shaped base body and melted to a crucible wall inner layer of transparent quartz glass on the base body.

15. The method according to claim 14, characterized in that a porous, dangling base body of the silica glass granules is used, and that the Innenkörnungsschicht is sintered under application of a negative pressure to the crucible wall inner layer.

16. The method according to any one of the preceding claims, characterized in that silica glass granulate particles are used, the diameter of which deviates from a nominal diameter by a maximum of 10%.