GB2538590A

GB2538590A - Light-absorbing quartz glass and method of producing it

Info

Publication number: GB2538590A
Application number: GB1602796.3A
Authority: GB
Inventors: Bar Michael; Ellrich Rene
Original assignee: Iqs GmbH
Current assignee: Iqs GmbH
Priority date: 2015-02-20
Filing date: 2016-02-17
Publication date: 2016-11-23
Anticipated expiration: 2036-02-17
Also published as: GB2538590B; GB201602796D0

Abstract

The invention relates to a method of producing a light-absorbing, in particular black or grey, quartz glass. The method involves producing a sol; gelling the sol to obtain a gel part; drying and setting the gel part to obtain a xerogel;purifying the xerogel by means of a first temperature treatment; and compacting the purified xerogel to obtain a glass body by means of a second temperature treatment, the temperature of the second temperature treatment being higher than that of the first temperature treatment. Compaction of the xerogel may take place under an inert gas atmosphere. Carbon particles are dispersed in the sol prior to gelling. A quartz glass incorporating a plurality of individual carbon particles is also provided, where the carbon particles are distributed so that the quartz glass has a black or grey colour tone, where the carbon particles are within a size range of 1 nm to 2000 nm.

Description

Light-absorbing quartz glass and method of producing it This invention relates firstly to a method of producing a light-absorbing, in particular black or grey, quartz glass. The invention further relates to a black or grey synthetic quartz glass which is made from a sol and which is used to absorb light beams from specific directions, for example.

By black quartz glass is meant a quartz glass which for all practical purposes totally absorbs light at least in the visible spectral range and which also has the properties typical of quartz glass, such as low thermal expansion, high temperature resistance and high purity. By a grey quartz glass is meant a quartz glass which uniformly reduces a light beam at least in the visible spectral range irrespective of its wavelength and the reduction is based on absorption. As used below, the term black quartz glass is also intended to include grey quartz glasses, the light absorbing effects of which are somewhat less than is the case with black glass. In general terms, therefore, these glasses can also be described as light-absorbing quartz glasses.

In black quartz glass, the light absorption is caused by sub-microscopic particles. These darkening substances are usually formed by transition metals or combinations of them. However, the disadvantage of using such transition metals is that they are often not tolerated in quartz glass equipment of the semiconductor industry. Furthermore, for the absorption required in the infrared spectral range, relatively high concentrations of these elements are necessary, the disadvantage of which is that other properties of the quartz glass are altered. Methods of producing black quartz glasses are known from the prior art for which carbon is used as a darkening substance. With these methods, the carbon is generated by chemical reactions during the production process. A defined adjustment of the size and distribution of the carbon particles in the quartz glass cannot be obtained or can be so but not with sufficient accuracy. The disadvantage of this is that properties such as the degree of absorption cannot be adjusted uniformly through the volume of the quartz glass.

DE 33 90 375 C2 describes a method of producing silica glass based on the sol-gel process using a silicon alkoxide as the raw material. The aim is to provide a cheap method of producing high quality silica glass of large dimensions that offers high yields. To this end, a sol solution of silicon alkoxide is produced by hydrolysis and mixed with colloidal silicon dioxide. This solution gels and is then dried to obtain a dry gel. The dry gel is sintered by increasing the temperature with a view to obtaining silica glass.

DE 10 201 022 534 Al relates to a method of producing a quartz glass body from a gel body, whereby the gel body produced from a sol is at least moulded and compacted to obtain the quartz glass body. To this end, displacement bodies are added to the sol prior to the gelling process to obtain the gel body, which are totally removed from the gel body after the gelling process, and cavities are created at the positions from which the displacement bodies have been removed so that a translucent or opaque quartz glass body is produced.

JP 2000 281430 A discloses black silica glass and a method of producing it. In this instance, fine silicon dioxide granules are moulded with a binding agent to produce a body. The body is subjected to a thermal treatment and carbon is produced in the gaps between the grains of the granules due to pyrolysis of the binding agent and the body turns black. The disadvantage of the resultant silica glass in this instance is that it has too coarse a structure when viewed under a microscope and foams when fused with normal quartz glass or when subjected to similar thermal stress and therefore cannot be used for the desired optical applications.

US 2009/0098370 Al describes a black quartz glass for the production of which a porous silica body is used, which is filled with an organosilicon gas. The gas phase reaction with the hydroxyl groups of the inner surface are said to lead to the formation of carbon and gaseous reaction products, some of which are released from the porous silica body prior to compaction to obtain compact black quartz glass.

US 2014/0072811 Al discloses a method of producing a black quartz glass with a transparent layer. In order to produce the black quartz glass, a porous silica body containing a hydroxyl group is subjected to a gas phase reaction in an atmosphere conducive to volatile organosilicon compounds at temperatures of at least 100°C and at most 1200°C. After this reaction, the body is fired at 1200°C to 2000°C. The porous silica body may be a body of the type produced by means of the sol-gel method, for example.

Against the background of the prior art, it is an objective of this invention to provide a light-absorbing, in particular black, quartz glass and a method of producing it, in which carbon particles are incorporated in a homogeneous distribution so that the absorption capacity in the material is homogeneous. It should be possible to define the material-specific absorption capacity on the basis of the quantity of incorporated carbon particles within broad ranges without detriment to the homogeneity of the absorption capacity as a result. It should also be possible to combine the black quartz glass with other quartz glass without foaming and its production should be inexpensive. In particular, it should be possible to produce directly black quartz glass bodies of an exact size having structured surfaces in the finished state.

This objective is achieved by a method of producing a black or grey quartz glass as defined in appended claim 1. The objective is also achieved by a quartz glass as defined in appended independent claims 6, 9 and 10.

The method proposed by the invention is used to produce light-absorbing, in particular black/grey, quartz glass by means of a sol-gel process in which carbon particles which absorb light are added to the sol. The sol-gel process is known per se and involves converting a sol into quartz glass. For details of the sol-gel process, reference may be made in particular to DE 33 90 375 C2 and DE 10 201 022 534 Al.

The sol is produced from hydrolysed silicon alkoxide and colloidal silicon dioxide. Silicon alkoxide is used for this purpose and hydrolyses forming a sol solution. Colloidal silicon dioxide is added to this sol solution whilst stirring, so that the sol is formed.

In another method step, carbon particles are then added to the sol. These carbon particles are added to the sol either in the form of dry powder or within a dispersion and are dispersed until they are homogeneously distributed in the sol.

The dispersion used by preference is aqueous and contains wetting and dispersing agent and carbon particles so that this dispersion may also be termed a colloidal carbon dispersion. The dispersion is also water soluble and functions on the basis of an electrostatic, steric or electrosteric type of stabilisation.

The carbon particles added to the sol have a monomodal size distribution and are incorporated in the sol with a defined nanoscale size.

In a next method step, the carbon-containing sol is gelled to obtain a gel part. To this end, a base is added to the sol in a first step so that the pH value rises. The sol is then poured into a container of a predefined size and made from a predefined material. This container is then closed and left to stand for a time. As a result, the sol gels to form a gel part and a silicon network is formed. The homogeneously distributed and freely moving carbon particles previously added to the sol are immobilised by the formation of the silicon network during gelling and are surrounded by the silicon network.

Gelling of the carbon-containing sol is followed by drying and setting of the gel part to obtain a xerogel. To this end, the cover of the container in which the gel part is disposed is replaced by a cover which is not completely closed. The container is heated and left to stand for several days at the same temperature. As a result, the aqueous part of the gel part is evaporated and the gel part shrinks. The resultant xerogel is a hard, dry and porous solid which preferably exhibits no crack formation.

In a subsequent method step, the xerogel is purified by a first temperature treatment which takes place alternating between a vacuum and inert gas. Accordingly, the xerogel is heated in several steps at predefined heating rates in each case to predefined temperature levels and left to stand for a predefined time. In order to remove water and gaseous oxygen, processing preferably takes place under an inert gas atmosphere. An oxygen deficit is created as a result, which prevents the formation of free oxygen so that oxidation of the carbon particles in the xerogel is prevented. Due to another increase in temperature, volatile impurities such as salts, for example, are removed.

By applying a flowing inert gas atmosphere, any other thermal decomposition products which might possibly occur can be removed from the xerogel during the temperature treatment. The use of helium or argon by preference as the inert gas promotes the collapse of micropores in the xerogel, the result of which is that a quartz glass part will subsequently be obtained without bubbles and inclusions. The purpose of using an inert gas atmosphere is initially to prevent oxidation of the carbon particles to CO and CO2.

Alternatively, instead of the inert gas atmosphere, the temperature treatment may be operated under appropriately good vacuum conditions. In this case, the oxygen left in the micropores is gradually removed from the xerogel over the duration of the temperature treatment and can therefore no longer lead to oxidation of the carbon particles.

The temperature treatment may also be run under reducing conditions. This is achieved, for example, by adding hydrogen gas. Taking account of a few chemical reactions (Bcudouard equilibrium) this also enables black quartz glass to be produced in this manner.

In a final step of the method, the purified xerogel is compacted by another temperature treatment immediately following the first temperature treatment. To this end, the temperature is increased at a predefined heating rate one more time and the purified xerogel is sintered under an inert gas atmosphere to obtain a dense, pore-free silica body which constitutes the quartz glass. This quartz glass is of a grey colour or black due to the carbon particles included in the structure of the quartz glass. The absorption capacity is homogeneous and isotropic due to the preferably homogeneous distribution of the carbon particles.

One particular advantage of the method proposed by the invention is that defined shapes of carbon particles can be used which undergo no further change during the production process. Accordingly, darkening and hence the absorption value of the quartz glass can be set simply on the basis of the number, size and shape of the carbon particles in the structure of the quartz glass. In addition to black quartz glasses, therefore, quartz glasses with different tones of grey can also be produced.

Furthermore, the method proposed by the invention enables quartz glasses to be produced which geometrically assume their final shape when finished or are close to their final shape, which reduces the work needed for mechanical finishing processes. The surface structures of the quartz glass proposed by the invention are also close to the final structures. Accordingly, the method is more cost-effective than comparable methods of producing black quartz glass because the cost of additional mechanical shaping processes is kept low.

Due to the preferred use of nanoscale carbon particles, a relatively high reboil temperature of the quartz glass is achieved, which makes thermal processing of the quartz glass proposed by the invention possible without the material foaming. In addition, such small carbon particles allow for a very homogeneous particle distribution in the sol solution without the occurrence of any settlement or sedimentation of the carbon particles to speak of. This homogeneous distribution prevents agglomeration of the carbon particles. The quartz glass produced exhibits a homogeneous and isotropic absorption as a result.

Another advantage of the method proposed by the invention is that the production of black quartz glass by this method based on a sol-gel process is more cost-effective than methods known to date. Another advantage of the sol-gel process applied in the method is that black quartz glasses of bigger volumes can be produced. The quartz glasses produced by this method also have the advantage of exhibiting a significantly higher degree of absorption than comparable black quartz glasses known from the prior art.

In preferred embodiments of the method proposed by the invention, the silicon alkoxide used is ethyl silicate or methyl silicate which can be mixed with hydrochloric acid by vigorous stirring and then hydrolysed. The sol is obtained by adding colloidal silicon dioxide whilst stirring.

The homogeneous distribution of the carbon particles needed to obtain uniform darkening and hence the uniform degree of absorption throughout the entire volume of the quartz glass to be produced requires the use of carbon particles in a size range that is as small as possible. Preferably, carbon particles within a size range of 1 nm to 500 nm, particularly preferably in a size range of 1 nm to 100 nm, are incorporated in the sol. The size distribution of the carbon particles is preferably monomodal. Also preferred is a size distribution characterised by an unsymmetrical intensity distribution curve in which the flank in the region of the large particles is significantly steeper than that of the small ones. The carbon particles can be more easily dispersed in the sol if the carbon particles are added to the sol in an aqueous dispersion which contains wetting and dispersing agent and is water soluble. The content of wetting and dispersing agent is preferably less than 1 percent by weight. As a result, the ultimate purity of the black quartz glass produced by the method proposed by the invention is affected to only a negligible degree so that the physical properties of the quartz glass produced are not or barely altered.

In preferred embodiments of the method proposed by the invention, the container into which the sol is poured for gelling is made from polystyrene, polypropylene or glass and its shape is preferably such that the gel part largely corresponds to the subsequent product at least in terms of scale. The container is preferably heated to room temperature and left to stand for several hours.

In particularly preferred embodiments of the method proposed by the invention, the container with the gel part which is dried and set to obtain a xerogel is preferably heated to a temperature within a temperature range of 50°C to 90°C, particularly preferably to a temperature of 60°C, at a constant heating rate.

In preferred embodiments of the method proposed by the invention, in order to purify the gel part, the xerogel is heated in several steps, preferably to temperatures within a temperature range of 100°C to 1000°C, particularly preferably a temperature range of 150°C to 800°C.

In particularly preferred embodiments of the method proposed by the invention, the purified xerogel is preferably heated to a temperature in a temperature range of 800°C to 1500°C at a constant heating rate in order to compact it to quartz glass, and the upper limit of the temperature is determined by the shaping method used.

The quartz glass proposed by the invention is used for absorbing beams in the spectral range from infrared to ultraviolet. It may be used wherever light beams from specific directions are undesirable and have to be eliminated. Examples of typical applications are quartz glass vessels for fluorescence tests, laser beam profilers and anti-glare devices. The quartz glass proposed by the invention may also be used to block heat radiation if the black quartz glass also has a high absorption capacity in the infrared spectral range. Another application is the use of black quartz glass as a measuring spot for taking pyrometric temperature measurements inside quartz glass reactors.

The quartz glass proposed by the invention is characterised by the fact that nanoscale carbon particles are homogeneously distributed in the structure of the quartz glass and respectively included. Depending on the number and size of the carbon particles incorporated in the structure of the quartz glass, the quartz glass is grey or black in colour.

One particularly preferred embodiment of the quartz glass proposed by the invention is characterised by the fact that for a layer thickness of 0.5 mm and a carbon content of 1000 ppm/weight, it has a transmission in the range of 190 nm to 3200 nm that is less than 0.25 % but nevertheless has the properties that are typical of quartz glass such as low thermal expansion, high temperature resistance and high purity. In addition, the quartz glass proposed by the invention is free of inclusions and bubbles in particular.

One advantage of the quartz glass proposed by the invention is that for a material thickness of 0.5 mm, it already has a very high degree of absorption of more than 99.75 % across a wavelength range of 190 nm to 3200 nm. The quartz glass preferably also has a high reboil temperature due to the homogeneous distribution of the nanoscale carbon particles and enables thermal processing without foaming. Such thermal processing steps might be sagging or welding, for example.

The carbon particles contained in the quartz glass are preferably of a size in a range of 1 nm to 500 nm, particularly preferably in a range of 1 nm to 100 nm.

The carbon particles contained in the quartz glass are also preferably homogeneously distributed, as a result of which the quartz glass has a homogeneous and isotropic absorption capacity.

Another object of the invention is a quartz glass which can be produced by means of the method proposed by the invention.

Another object of the invention is a quartz glass which has the specific properties described above and can be produced by means of the method proposed by the invention.

Other advantages, details and additional features of the invention will become apparent from the following description of several examples of embodiments.

Example 1:

292 ml of 0.01 mol/litre hydrochloric acid are added to 208 g (1 mol) of commercially available ethyl silicate whilst being vigorously stirred. The ethyl silicate is hydrolysed as a result. 89 g (1.5 mol) of colloidal silicon dioxide (surface area 50 m2/g) are added to the solution, again being vigorously stirred. 448 mg of carbon powder with a mean particle size of 25 nm are also added to the solution. The solution is then subjected to an ultrasound treatment. The pH value of the solution is adjusted to 4.5 by adding 0.1 mol/litre of ammonia solution. The sol produced in this manner is poured to a thickness of 3 cm into an appropriate container (e.g. made from polystyrene, polypropylene or borosilicate glass) with a width of 13 cm, a length of 13 cm and a height of 5 cm. The container is then closed by means of an appropriate cover. The container filled with sol is left to stand at 20°C. The sol contained in it gels within 30 minutes. The gel part is then left to stand for several hours.

The cover of the container is then replaced by a cover with an open proportion of 0.8 % and the gel part is heated at a heating rate of 3°C/h from 20°C to 60°C. The gel is dried for 7 days at 60°C so as to form a hard, dry xerogel with dimensions of 8.5cm x 8.5cm x 2cm.

During a practical test, the sol produced in the manner described above was poured into 20 identical containers and dried under the same conditions. No crack formation occurred in any of the 20 gels, corresponding to a yield of 100 %.

In the manner described above, another 20 xerogels were produced. In a next step, they were heated at a heating rate of 60°C/h to 150°C and left for 3 hours at 150°C. The samples were then heated under an inert gas atmosphere at a heating rate of 60°C/h to 300°C and left for 5 hours at 300°C in order to remove the adsorbed water and gaseous oxygen. These xerogels were then heated at a heating rate of 100°C/h to 800°C and left for 18 hours at 800°C in order to remove volatile impurities such as ammonium chloride and other salts, for example. After further heating to 1300°C at a heating rate of 100°C/h and being left for one hour at 1300°C, the samples were compacted so as to be free of pores. The result of the processing sequence described above were synthetic, silica glasses with a visually black appearance having dimensions of 6.5cm x 6.5cm x 1.5cm. When using the sintering process described above, no crack formation occurred in the glass bodies of any of the 20 samples. This corresponds to a yield of 100 %. Furthermore, all of the silica glasses obtained in this manner were free of devitrification or bubble formation and proved to be of high quality.

A chemical analysis of the silica glasses produced in this manner to test the content of inorganic foreign elements resulted in values comparable with customary transparent synthetic quartz glasses.

Fig. 1 illustrates the recorded transmission spectra of a silica glass produced by the method described above (sample B) compared with a conventional black quartz glass (sample A). The glass produced as proposed by the invention exhibits exceptional optical properties. The degree of absorption of this glass considerably exceeds the degree of absorption of the previously known black silica glass across the entire measured wavelength range of 190 nm to 3,200 nm. Even with a further reduction of the layer thickness of the measured object by the factor >10, a significantly higher absorption of the above-mentioned wavelength spectrum was still found compared with previously known glasses.

In order to determine the homogeneous absorption behaviour of the silica glasses produced as described above, a sample of identical thickness was tested in a total of 6 mutually remote positions to ascertain their transmission properties. No significant differences were found on evaluating the different spectra. This leads to the conclusion that there is a homogeneous distribution of the carbon particles incorporated in the silica glass. The synthetic black quartz glass produced in this manner was also found to thermally fuse (bond) with customary synthetic transparent quartz glasses without any problem.

Example 2:

349 ml 0.01 mol/litre of hydrochloric acid were added to 131 g (0.86 Mol) of commercially available methyl silicate whilst being vigorously stirred. The methyl silicate is hydrolysed as a result. 89 g (1.5 mol) of colloidal silicon dioxide (surface area 50 m2/g) were added to the solution whilst also being vigorously stirred. 1111 mg of a 38 percent colloidal carbon dispersion with 1 percent by weight of wetting and dispersing agent were also added to the solution. The mean particle size of the carbon particles in the carbon dispersion is 25 nm. The solution was then subjected to an ultrasound treatment. The pH value of the solution was adjusted to 4.5 by adding 0.1 mol/litre of ammonia solution. The sol produced in this manner is poured to a thickness of 3 cm into an appropriate container (e.g. made from polystyrene, polypropylene or borosilicate glass) with a width of 13 cm, a length of 13 cm and a height of 5 cm. The container is then closed with an appropriate cover. The container filled with sol is left to stand at 20°C. The sol contained in it gels within 30 minutes. The gel part is then left to stand for several hours.

The cover of the container is then replaced by a cover with an open proportion of 0.8 % and the gel part is heated at a heating rate of 3°C/h from 20°C to 50°C. The gel is dried for 7 days at 50°C to form a hard, dry xerogel having dimensions of 8.5cm x 8.5cm x 2cm.

During a practical test, the sol produced in the manner described above is poured into 20 identical containers and dried under the same conditions. No crack formation was found in any of the 20 gels, which corresponds to a yield of 100 %.

In the manner described above, another 20 xerogels were produced. In a next step, they were heated at a heating rate of 60°C/h to 150°C and left for 3 hours at 150°C. The samples were then heated under an inert gas atmosphere at a heating rate of 60°C/h to 300°C and left for 5 hours at 300°C in order to remove the adsorbed water and gaseous oxygen. These xerogels were then heated at a heating rate of 100°C/h to 800°C and left for 18 hours at 800°C in order to remove volatile impurities such as ammonium chloride and other salts, for example. After further heating to 1300°C at a heating rate of 100°C/h and being left for one hour at 1300°C, the samples were compacted so as to be free of pores. The result of the above-mentioned processing sequence were synthetic, silica glasses with a visually black appearance having dimensions of 6.5cm x 6.5cm x 1,5cm. When using the sintering process described above, there was no crack formation in the glass bodies of any of the 20 samples. This corresponds to a yield of 100 %. In addition, all of the silica glasses obtained in this manner were free of devitrification or bubble formation and proved to be of high quality.

A chemical analysis of the silica glasses produced in this manner to test the content of inorganic foreign elements resulted in values comparable with customary transparent synthetic quartz glasses. The measured carbon content in the silica glass produced in this manner is approx.

3,000 ppm / weight.

The recorded transmission spectra of silica glass produced by the method described above show exceptional optical properties. The degree of absorption of this glass considerably exceeds the degree of absorption of previously known black silica glasses across the entire measured wavelength range of 190 nm to 3,200 nm. Even with a further reduction of the layer thickness of the measured object by the factor >10, a significantly higher absorption of the above-mentioned wavelength spectrum was still found compared with previously known glasses.

The synthetic black quartz glass produced in the manner described above was also found to thermally fuse (bond) with customary synthetic transparent quartz glasses without any problem.

Example 3:

405 ml of 0.01 mol/litre hydrochloric acid were added to 208 g (1 mol) of commercially available ethyl silicate whilst being vigorously stirred. The ethyl silicate is hydrolysed as a result. 124 g (2.1 mol) of colloidal silicon dioxide (surface area 50 m2/g) are added to the solution whilst also being vigorously stirred. 552 mg of carbon powder with a mean particle size of 25 nm were also added to the solution. The solution was then subjected to an ultrasound treatment. The pH value of the solution was then adjusted to 4.5 by adding 0.1 mol/litre of ammonia solution. The sol produced in this manner was then poured into a casting mould with a special geometric shape made from polystyrene and the casting mould was closed. The casting mould has the geometric shape of a hollow, upturned cone and is provided with defined sub-micrometre structures on the internal face. The internal surface of the casting mould with the defined structures forms the boundary surface between the casting mould and the sol with which it is filled so that these structures will be imprinted in the sol as a negative during gelling. The container filled with sol was left to stand at 20°C. The sol contained in it gels within 30 minutes. The cone-shaped gel part was then released from the casting mould and transferred to a larger container. This container was closed and sealed and left to stand for several hours.

The cover of the container was then replaced by a cover with an open proportion of 0.8 % and the gel part was heated at a heating rate of 3°C/h from 20 to 60°C. The gel part was dried for 7 days at 60°C to form a hard dry xerogel.

During a practical test, the sol produced in the manner described above was poured into 20 identical containers and dried under the same conditions. No crack formation occurred in any of the 20 gels, which corresponds to a yield of 100 %.

The 20 dry gels produced in this manner were heated at a heating rate of 60°C/h to 150°C and left to stand for 3 hours at 150°C. The samples were then heated under an inert gas atmosphere at a heating rate of 60°C/h to 300°C and left for 5 hours at 300°C in order to remove the adsorbed water and gaseous oxygen. These xerogels were then heated at a heating rate of 100°C/h to 800°C and left for 18 hours at 800°C in order to remove volatile impurities such as ammonium chloride and other salts, for example. After further heating to 1300°C at a heating rate of 100°C/h and being left for one hour at 1300°C, the samples were compacted so as to be free of pores. The result of the above-mentioned processing sequence was conically shaped mouldings of synthetic black silica glass, which exhibited shrinkage of exactly 50 % compared with the casting mould dimensions. The sintering process described above did not cause any crack formation in any of the 20 samples, which corresponds to a yield of 100 %. In addition, all of the conically shaped silica mouldings thus obtained were free of devitrification or bubble formation and proved to be of high quality.

The superficial microstructures tested on the mouldings exhibited the negative impressions of the microstructures provided in the casting mould and also exhibited a homogeneous geometric shrinkage of 50 % compared with the mould structures.

In terms of other physical properties, the synthetic black quartz glass produced in this manner is comparable with customary synthetic transparent quartz glasses: e.g. a density of 2.2 g/cm3, a Vickers hardness of 770 N/mm2 and a coefficient of heat expansion of 5.8 x 10 K1.

Claims

Claims 1. Method of producing a light-absorbing, in particular black or grey, quartz glass, comprising the following steps: a) producing a sol; b) gelling the sol to obtain a gel part; c) drying and setting the gel part to obtain a xerogel; d) purifying the xerogel by means of a first temperature treatment; and e) compacting the purified xerogel to obtain a glass body by means of a second temperature treatment, the temperature of the second temperature treatment being higher than that of the first temperature treatment; characterised in that carbon particles are added to the sol and dispersed in the sol prior to gelling.
2. Method as claimed in claim 1, characterised in that the carbon particles added to the sol are within a size range of 1 nm to 2000 nm, preferably 5 nm to 500 nm, particularly preferably 10 nm to 100 nm, and preferably have a monomodal size distribution.
3. Method as claimed in claim 1 or 2, characterised in that compaction of the xerogel to obtain the glass body takes place under an inert gas atmosphere.
4. Method as claimed in one of claims 1 to 3, characterised in that the carbon particles are added to the sol either in the form of dry powder or within an aqueous dispersion.
5. Method as claimed in claim 4, characterised in that the dispersion contains a wetting and dispersing agent with a base of the electrostatic, steric or electrosteric stabilisation type.
6. Quartz glass incorporating a plurality of individual carbon particles, the carbon particles being distributed in the structure of the quartz glass and respectively included so that the quartz glass has a black or grey colour tone, characterised in that the carbon particles are within a size range of 1 nm to 2000 nm.
7. Quartz glass as claimed in claim 6, characterised in that the carbon particles are homogeneously distributed in the structure of the quartz glass.
8. Quartz glass as claimed in claim 6 or 7, characterised in that the degree of absorption of the quartz glass at a material thickness of 0.5 mm is already greater than 99.75 96.
9. Quartz glass which can be produced by a method as claimed in one of claims 1 to 5.
10. Quartz glass incorporating a plurality of individual carbon particles, the carbon particles being distributed in the structure of the quartz glass and respectively included so that the quartz glass has a black or grey colour tone, which can be produced by a sol-gel process whereby carbon particles within a size range of 1 nm to 2000 nm are added to the sol.