WO2006111454A1

WO2006111454A1 - Method and apparatus for producing a quartz glass body

Info

Publication number: WO2006111454A1
Application number: PCT/EP2006/061090
Authority: WO
Inventors: Juergen Roeper
Original assignee: Heraeus Tenevo Gmbh
Priority date: 2005-04-19
Filing date: 2006-03-28
Publication date: 2006-10-26
Also published as: DE102005018209B4; DE102005018209A1

Abstract

In a known method for producing a quartz glass body, glass starting material and combustible gas are supplied to a deposition burner, SiO2 particles are formed therefrom, and said particles are deposited in layers on a carrier with formation of an SiO2 blank. Starting therefrom, to indicate a reproducible method for producing a high-purity homogeneous quartz glass body, it is suggested according to the invention that a deposition burner should be used which consists at least in part of graphite. An apparatus according to the invention for producing a quartz glass body comprises a deposition burner including a burner head having a burner mouth, in which burner head a plurality of media supply lines extend for supplying glass starting material and combustible gas, said lines terminating at the burner mouth in the form of nozzle openings. To manufacture the deposition burner with narrow manufacturing tolerances at low costs and to prevent the risk of contamination at the same time, it is suggested according to the invention that the burner head should consist at least in part of graphite.

Description

Method and apparatus for producing a quartz glass body

The present invention relates to a method for producing a quartz glass body by supplying glass starting material and combustible gas to a deposition burner, forming SiO₂ particles therefrom by flame hydrolysis, and depositing said particles in layers on a carrier with formation of an SiO₂ blank.

Furthermore the present invention relates to an apparatus for producing a quartz glass body, comprising a deposition burner including a burner head comprising a burner mouth, in which burner head a plurality of media supply lines extend for supplying glass starting material and combustible gas, said lines terminating at the burner mouth in the form of nozzle openings.

When quartz glass bodies are produced according to the so-called OVD (outside vapor deposition) method, SiO₂ particles are formed by flame hydrolysis by using one or several deposition burners and are deposited on the outer surface of a carrier which is rotating about its longitudinal axis, resulting in the formation of a cylindrical blank of porous quartz glass (hereinafter also called ,,soot body"). The deposition burners used therefor consist of quartz glass or of metal.

A plurality of deposition burners are often used for accelerating the deposition process. These burners are reversingly moved in a joint burner row along the soot body, each deposition burner sweeping only over a partial length of the soot body.

Particular attention is here paid that all deposition burners exhibit deposition characteristics that are as uniform as possible because otherwise the soot body might be built up in a non-uniform manner and disorders might particularly arise in the contact area of neighboring deposition zones. Therefore, a great number of measures have been described for ensuring a uniform or reproducibly adjustable deposition characteristic of the deposition burners.

DE 10O 18 857 A1 , for instance, from which a method and an apparatus of the above-mentioned type are known, suggests a deposition burner of quartz glass which consists of four concentrically arranged quartz glass tubes forming a central nozzle and three annular gap nozzles on the whole. The central nozzle is fed with SiCI₄ and the outer annular gap nozzles with the combustible gases in the form of hydrogen and oxygen. A separation gas nozzle is provided between the central nozzle and the outer portion for passing an oxygen stream therethrough, said oxygen stream first shielding the SiCI₄ stream from the combustible gas streams. To achieve an exchange of burners of one burner row without any great matching or adjusting efforts, attention is paid to an exact dimensional stability of the annular gaps, and each burner is equipped with a separate positioning unit.

The known deposition burner of quartz glass is distinguished by a simple construction. Contamination of the SiO₂ soot body due to wear from the burner material need not be feared. On the other hand, it is complicated to precisely manufacture the known quartz glass burner according to traditional gas-blowing methods and to compensate for still existing differences in the burner characteristics by individually positioning the deposition burners.

A metal burner consisting of special steel or aluminum is for example described in US 5,599,371 A. The deposition burner is composed of a plurality of metallic nozzle parts which are interconnected by screws and form fluidically separated gas chambers. Sealing elements are provided between the nozzle parts for separating the individual gas chambers from one another.

Since in deposition burners of metal the individual nozzle parts can be manufactured in a precise and reproducible manner with the help of known machining methods, such as drilling, punching, milling, or the like, even complex constructions with narrow manufacturing tolerances can be realized in a comparatively easy way. Specifically adapted tools are normally used for series production, manufacture of these tools constituting a considerable cost factor. A complex burner construction requires the provision of a plurality of such tools. As has been shown, these are however subject to rapid wear because of the hardness of the metallic material, resulting in a decreasing manufacturing precision. Moreover, contamination of the quartz glass caused by constituents of the high temperature-resistant metallic material must be expected in the case of metal burners. It is therefore the object of the present invention to provide a deposition burner which can be manufactured with narrow manufacturing tolerances at low costs and in which the risk of contamination of the deposited material is small at the same time. Furthermore, it is the object of the present invention to provide a reproducible method for producing a homogeneous quartz glass body of high purity by using a flame hydrolysis burner according to the invention.

As for the method, this object starting from the method of the above-mentioned type is achieved according to the invention in that a deposition burner is used which comprises a burner head consisting at least in part of graphite.

Thanks to the use of a deposition burner comprising a burner head consisting fully or in part of graphite (hereinafter also called "graphite burner"), the essential advantages of a quartz glass burner and of a metal burner are achieved and the corresponding drawbacks are avoided.

Burner head means in this context that part of the deposition burner, including the burner month, in which a plurality of media supply lines for the supply of glass starting material and combustible gas extend towards the deposition burner and terminate at the burner mouth in the form of nozzle openings.

• Graphite is thermally stable and commercially available in a very high purity. Bodies of ultrapure graphite are used for applications where high demands are made on purity, e.g. for cuvettes in analytical technology or crucibles for semiconductor production. Possible impurities of the SiO₂ blank in the form of carbon can easily be removed by oxidation and do not pose any problems in this respect. Hence, quartz glass bodies of high purity can be made from the SiO₂ blank.

• Graphite can be mechanically treated in an easy way. With the help of the known machining methods the deposition burners or parts thereof can be produced precisely and reproducibly with narrow manufacturing tolerances. Even complex constructions can be realized in a comparatively easy way. As a result, the deposition and burner characteristic of the graphite burner is reproducible, which has an advantageous effect on the uniformity of the deposition process.

• In contrast to high temperature resistant metal, graphite is characterized by low hardness. As a consequence, tools for the series manufacture of the graphite parts are subject to comparatively little wear and achieve long service lives together with a permanently high manufacturing precision.

Thus, with the help of the method of the invention using a graphite burner it is possible to reproducibly produce a uniform SiO₂ blank of high purity by flame hydrolysis. A quartz glass body is obtained from the blank in possibly subsequent treatment steps comprising cleaning, dehydration, doping, vitrification or machining.

It has turned out to be advantageous when a deposition burner is used that is coated at least in part with a protective layer.

At high temperatures graphite can react with oxygen and hydrogen with formation of carbon dioxide and methane, respectively. The protective layer serves the passivation of the surface. To avoid or reduce loss of graphite, it is enough when the protective layer is provided in the particularly exposed portions around the burner mouth.

Preferably, the protective layer consists of SiC, Si₃N₄, SiO₂ or of pyrolytically produced vitreous carbon.

Said materials are distinguished by a corrosion resistance to hydrogen or oxygen that is higher in comparison with pure graphite and by an appropriate wettability with graphite and thus by high adhesion to the graphite surface.

In an advantageous variant of the method, the production of the quartz glass body includes deposition of a soot body using the deposition burner and subsequent treatment under oxidizing conditions.

The soot body obtained by virtue of the deposition process consists of porous SiO₂ and can subsequently be cleaned or doped by gas phase treatments. Especially through the treatment in oxygen-containing atmosphere the elimination of possible carbon impurities or the compensation of an oxygen deficit in the SiO₂ network is possible. The treatment under oxidizing conditions is carried out together with or before the vitrification of the soot body.

A particularly preferred development of the method of the invention is characterized in that a plurality of deposition burners of graphite are used which are arranged in a row and reversingly moved along the outer surface of a carrier rotating about its longitudinal axis and by means of which SiO₂ particles are deposited on the carrier with formation of a substantially cylindrical SiO₂ blank.

The dimensionally stable and nevertheless inexpensive series production of the graphite burner facilities its use in a burner row as is often used for accelerating the deposition process, for a substantially uniform deposition characteristic of all deposition burners of the burner row can be guaranteed in a comparatively easy way. This prevents an irregular build-up of the blank and permits an exchange of a defective deposition burner without any great adjusting efforts. To be more specific, disorders in the contact area of neighboring deposition zones caused by a non-uniform SiO₂ deposition at both sides of the zone are avoided. The SiO₂ blank is normally a soot body, as has been described above.

As for the apparatus, the above-mentioned object starting from an apparatus of the above-mentioned type is achieved according to the invention in that the burner head consists at least in part of graphite.

A deposition burner of graphite combines the essential advantages of a quartz glass burner and a metal burner and avoids the respective drawbacks thereof.

• Graphite is thermally stable and commercially available in a very high purity. Bodies of ultrapure graphite are already used for applications where high demands are made on purity, e.g. for cuvettes in analytical technology or for crucibles in semiconductor production. Possible impurities of the quartz glass body in the form of carbon from the graphite burner can be easily removed by oxidation and do not pose any problems in this respect.

• Graphite can be mechanically treated in an easy way. With the help of the known machining methods the burner head or parts thereof can be produced precisely, so that a reproducible deposition and burner characteristic of the graphite burner can be ensured. Even complex constructions of the deposition burner can be realized in an easy way.

• Moreover, graphite is characterized by low hardness so that tools for the series manufacture of the graphite parts are subject to comparatively little wear, thereby achieving long service lives together with a permanently high manufacturing precision.

Thus, the deposition burner of the invention can be produced with small manufacturing tolerances at low costs. During its use for performing the above- explained method there is only a small risk of contamination of the deposited SiO₂ material.

It has turned out to be advantageous when the burner head is coated at least in part with a protective layer.

Said materials are distinguished by a corrosion resistance to hydrogen or oxygen that is higher in comparison with pure graphite and by an appropriate wettability with graphite and thus by high adhesion at the same time.

Alternatively or in addition, it has also turned out to be useful when the area of the burner head which is oriented towards the burner mouth consists of quartz glass, aluminum oxide or of a ceramic material.

The area of the burner head which is oriented towards the burner mouth is thermally and corrosively subjected to the greatest stresses. The said materials are distinguished by a high chemical and thermal resistance. As an alternative, the deposition burner of the invention or at least the area of the burner head which is oriented towards the burner mouth consists of SiC-infiltrated graphite.

In comparison with pure graphite, SiC-infiltrated graphite exhibits a higher density and better chemical resistance, particularly to oxygen and hydrogen.

The use of graphite which is isostatically pressed and has a bulk density of at least 1.77 g/cm³ has turned out to be particularly useful.

Graphite bodies produced by isostatic pressing are distinguished by a uniform density. A high density is needed for ensuring a sufficiently low gas permeability and an adequate separation of the various gas chambers inside the burner head. The bulk density is determined according to the buoyancy method according to DIN 51918.

In this connection it has also turned out to be advantageous when the graphite has a porosity of not more than 15%.

The open porosity is determined by impregnation with water, also according to DIN 51918.

Depending on the use and function of the corresponding graphite part of the burner head, the respectively used graphite qualities may differ. For instance, it has turned out to be advantageous particularly for corrosively highly loaded graphite parts provided with a protective layer when the porosity thereof is at least 6%.

A higher porosity improves the adhesion of the protective layer.

Especially with respect to a high dimensional stability it has turned out to be useful when graphite is employed that consists of graphite granules having a grain size of not more than 15 μm, preferably not more than 10 μm.

The coarser the graphite granules are, the more difficult it gets to work edges in an exact manner. Preferably, the media supply lines are configured in the form of bores in the burner head.

In comparison with annular gaps between hollow cylinders that are staggered within each other, bores can be produced particularly easily and with high precision.

Furthermore, it has turned out to be useful when the burner head comprises a nozzle body terminating at the burner mouth, in which the bores are extending and which is connected to a basic body with connections for media supply lines.

The burner head comprises a nozzle body and a basic body. Bores extend in the nozzle body for the supply of media to the burner mouth. The basic body is provided with connections for the media supply lines which terminate in the bores in the nozzle body. The nozzle body is exchangeably connected to the basic body.

It faces the burner flame and is subject to greater wear than the basic body.

Moreover, the deposition characteristic of the deposition burner can be easily changed through a changed geometry of the nozzle body.

Other components of the deposition burner may also be arranged between the nozzle body and the basic body. In the constructionally simplest case the nozzle body and the basic body are adjacent to each other via a seal.

The burner head of the deposition burner of the invention just comprises two essential components, namely the nozzle body and the basic body which are adjacent to each other via a seal ring or another sealing element. When the two components are made from graphite either fully or for their greatest part, no problems are created because of different thermal expansion coefficients.

In a particularly preferred embodiment of the apparatus according to the invention, a plurality of deposition burners which are arranged in a row, each having a burner head consisting of graphite, form a burner arrangement which is reversingly movable along the outer surface of a carrier rotating about its longitudinal axis and by means of which SiO₂ particles are deposited on the carrier with formation of a substantially cylindrical SiO₂ blank. The dimensionally stable and nevertheless inexpensive series production of the graphite burner permits its use in a burner row for accelerating the deposition process.

The invention shall now be explained in more detail with reference to an embodiment and a drawing which shows in detail in

Figure 1 a side view of an embodiment of the deposition burner of graphite according to the invention, in a longitudinal section; and in

Figure 2 a top view on the burner mouth of the deposition burner according to Figure 1.

Figure 1 shows a longitudinal section through the burner head 1 of an oxyhydrogen burner for producing an SiO₂ soot body according to the OVD method. The burner head 1 consists of a basic body 2 and of a nozzle body 3, each made from high-purity graphite.

The nozzle body 3 consists of high-purity graphite produced by the company Schunk Kohlenstofftechnik GmbH, which is commercially available under the name FE 779. This graphite is distinguished by a bulk density of 1.8 g/cm³ and by a mean porosity of 1 1 %. The nozzle body is configured to be substantially rotationally symmetric with respect to the longitudinal axis 6. It comprises a rear cylindrical section having an outer diameter of 50 mm, which rests through a sealing ring (not shown in the figure) on the basic body 2, and a front conical section slightly tapering in two steps towards the burner mouth 5 to reach an outer diameter of 30 mm. The front side of the nozzle body 3 which forms the burner mouth 5 is provided with an SiC layer.

A plurality of bores for the supply of SiCI₄, a separation gas stream (oxygen) and combustible gases to the burner mouth 5 extend from one front end to the other front end of the nozzle body 3. In detail, there is a central nozzle 7 for the supply of SiCI₄ (and optionally dopants), a plurality of tubular separation gas nozzles 8 for the supply of oxygen that surround the central nozzle 7 in an inner coaxial enveloping circle, a plurality of tubular hydrogen nozzles 9 which are radially staggered in a central enveloping circle relative to the separation gas nozzles 8, and a plurality of oxygen nozzles 10 which surround the central nozzle 7 in an outer coaxial enveloping circle.

The basic body 2 is present as a substantially cylindrical graphite block with an outer diameter of 50 mm and a length of 40 mm. The basic body 2 consists of high-purity graphite obtained from the above-mentioned company under the name FE 779. This graphite is distinguished by a bulk density of 1.95 g/cm³ and a mean porosity of 6%. The basic body 2 is provided with connections 1 1 for the supply of SiCI₄, the separation gas stream and the combustible gases, the connections being extended in the form of bores 12 to the nozzles 7, 8, 9 and 10 in the nozzle body 3.

Basic body 2 and nozzle body 3 are detachably interconnected by means of screws 13 (see Fig. 2), the individual nozzles 7, 8, 9 and 10 being fluidically separated by the sealing ring 4. The total length of the burner head 1 is 65 mm.

The top view of Fig. 2 shows the openings of the above-mentioned nozzles 7, 8, 9 and 10 on the burner mouth 5, like reference numerals as in Fig. 1 being here used for like components.

The deposition burner of the invention can be manufactured with narrow manufacturing tolerances at low costs by means of the known mechanical treatment methods. The material used is a high purity material and it is substantially inert with respect to the quartz glass to be produced.

The method of the invention shall now be explained in more detail with reference to the manufacturing example of an SiO₂ soot body according to the OVD method as a prestage for a preform for optical fibers using the deposition burner according to Figs. 1 and 2.

Ten deposition burners 1 are mounted at a distance of 15 cm on a joint burner bank and are exactly aligned with the bottom side of a cylindrical carrier rotating about its longitudinal axis. The burner bank is reversingly moved along the carrier with a motion amplitude of 15 cm. For igniting an oxyhydrogen gas flame and for forming and depositing SiO₂ particles on the outer cylindrical surface of the carrier, glass starting material and fuels are supplied to the deposition burners as follows: 32.5 g/minSiCU and 2.0 l/min carrier gas oxygen are passed through the central nozzle 7, a total of 4.5 l/min separation gas oxygen through the separation gas nozzles 8, a total of 46.0 l/min hydrogen through the hydrogen nozzles 9 and a total of 6.2 l/min oxygen through the oxygen nozzles 10.

The distance of the burner mouth 5 from the surface of the developing SiO₂ soot body is kept constant at 210 mm. After a build-up time of 45 minutes the deposition process is completed.

On the whole, a deposition efficiency of 44% and a mean build-up rate of 304 g per hour and deposition burner were achieved. The visual assessment of the resulting soot body revealed no flaws.

After removal of the carrier the soot body is cleaned according to the generally known method and then subjected to an after-treatment under oxidizing conditions in an oxygen-containing atmosphere and then sintered to obtain a hollow cylinder of quartz glass.

Claims

Patent Claims

1. A method for producing a quartz glass body by supplying glass starting material and combustible gas to a deposition burner, forming SiO₂ particles therefrom by flame hydrolysis, and depositing said particles in layers on a carrier with formation of an SiO₂ blank, characterized in that a deposition burner is used which comprises a burner head consisting at least in part of graphite.

2. The method according to claim 1 , characterized in that a deposition burner is used whose burner head is coated at least in part with a protective layer.

3. The method according to claim 2, characterized in that the protective layer consists of SiC, Si₃N₄, SiO₂ or of pyrolytically produced vitreous carbon.

4. The method according to any one of the preceding claims, characterized in that the production of the quartz glass body includes the deposition of a soot body using the deposition burner and subsequent treatment under oxidizing conditions.

5. The method according to any one of the preceding claims, characterized in that a plurality of deposition burners are used which are arranged in a row and comprise a burner head consisting at least in part of graphite and which are reversingly moved along the outer surface of a carrier rotating about its longitudinal axis, and by means of which SiO₂ particles are deposited on the carrier with formation of a substantially cylindrical SiO₂ blank.

6. An apparatus for producing a quartz glass body, comprising a deposition burner including a burner head (1 ) comprising a burner mouth (5), in which burner head a plurality of media supply lines (7, 8, 9, 10) extend for supplying glass starting material and combustible gas, said lines terminating at the burner mouth (5) in the form of nozzle openings, characterized in that the burner head (1 ) consists at least in part of graphite.

7. The apparatus according to claim 6, characterized in that the burner head (1 ) is coated at least in part with a protective layer.

8. The apparatus according to claim 7, characterized in that the protective layer consists of SiC, Si₃N₄ or SiO₂.

9. The apparatus according to any one of the preceding claims 6 to 8, characterized in that the area of the burner head (1 ) which is oriented towards the burner mouth (5) consists of quartz glass, aluminum oxide or of a ceramic material.

10. The apparatus according to any one of the preceding claims 6 to 9, characterized in that the area of the burner head (1 ) which is oriented towards the burner mouth (5) consists of SiC-infiltrated graphite.

1 1. The apparatus according to any one of the preceding claims 6 to 10, characterized in that the graphite is isostatically pressed and has a bulk density of 1.77 g/cm³.

12. The apparatus according to any one of the preceding claims 6 to 1 1 , characterized in that the graphite has a porosity of not more than 15%.

13. The apparatus according to claim 12, characterized in that the porosity is at least 6%.

14. The apparatus according to any one of the preceding claims 6 to 13, characterized in that the graphite consists of graphite granules having a grain size of not more than 15 μm, preferably not more than 10 μm.

15. The apparatus according to any one of the preceding claims 6 to 14, characterized in that the media supply lines are configured in the form of bores (7, 8, 9, 10) in the burner head.

16. The apparatus according to claim 15, characterized in that the burner head (1 ) comprises a nozzle body (3) terminating at the burner mouth (5), in which the bores (7, 8, 9, 10) are extending and which is connected to a basic body (2) with connections for media supply lines.

17. The apparatus according to claim 16, characterized in that nozzle body (3) and basic body (2) are adjacent to each other through a seal.

18. The apparatus according to any one of the preceding claims 6 to 17, characterized in that a plurality of deposition burners which are arranged in a row, each having a burner head consisting at least in part of graphite, form a burner arrangement which is reversingly movable along the outer surface of a carrier rotating about its longitudinal axis and by means of which SiO₂ particles are deposited on the carrier with formation of a substantially cylindrical SiO₂ blank.