EP1152988A1

EP1152988A1 - Method for producing a quartz glass body and holding element made of quartz glass

Info

Publication number: EP1152988A1
Application number: EP00974527A
Authority: EP
Inventors: Klaus Dittmer; Heinz Fabian; Susanne Elsesser
Original assignee: Heraeus Tenevo GmbH
Current assignee: Heraeus Quarzglas GmbH and Co KG
Priority date: 1999-11-24
Filing date: 2000-11-14
Publication date: 2001-11-14
Also published as: WO2001038239A1; TW477777B; JP4634684B2; CN1336903A; DE19956570A1; CN1213955C; KR20010092796A; DE19956570B4; JP2003514755A

Abstract

In a prior art method for producing a quartz glass body, SiO2 particles are deposited on the cylindrical outer surface of a cylindrical mandrel (4), which rotates around the longitudinal axis thereof, while forming an essentially cylindrical porous blank (1), whereby a holding element (12) made of quartz glass is provided in the area of one of the ends of said blank (1). The aim of the invention is to improve, based on existing methods, the reproducibility of the properties of the products made of the quartz glass body. To this end, the invention provides that a holding element (2) is used which has a metastable OH content of less than 30 ppm by weight, and that the holding element (2) is subjected to a thermal pretreatment. A suitable holding element (2) for holding a blank made of quartz glass is characterized in that it is comprised of quartz glass that has a metastable OH content of less than 30 ppm by weight.

Description

Patent application

Heraeus Quarzglas GmbH & Co.KG Shin-Etsu Quartz Products Co., Ltd.

Process for the production of a quartz glass body and

Holding element made of quartz glass

The present invention relates to a method for producing a quartz glass body, comprising depositing SiO ₂ particles on the cylindrical surface of a dome rotating about its longitudinal axis to form a substantially cylindrical, porous blank, which is in the region of one of the ends of the blank attacking holding element made of quartz glass is connected, and sintering of the blank.

Furthermore, the invention relates to a holding element made of quartz glass which is connected to one of the ends of the blank to hold a cylindrical blank made of porous quartz glass.

A method and a holding element of this type are known from DE-A1 196 49 935. In the process described therein, a hollow cylindrical blank made of porous quartz glass is produced according to the so-called "OVD process" (Outside Vapor Depostion), in that fine SiO ₂ particles are deposited in layers on a carrier tube rotating about its longitudinal axis tubular holder made of quartz glass is stably embedded in one of the ends of the blank that is formed, and after removal of the carrier tube from the blank, it can be used on the embedded tube for further processing steps, for example a dehydration treatment in a chlorine-containing atmosphere

Hold the holder in a vertical orientation. A transparent quartz glass preform is formed from the porous blank by sintering. When glazing, the holder initially remains in the preform and is then removed with the relevant part of the preform. DE 196 49 935 (A1) discloses a method for producing a quartz glass body, in which SiO ₂ particles are produced by flame hydrolysis of silicon-containing starting materials and are deposited on a rotating mandrel to form a porous blank (soot body). A quartz glass tube is stably embedded in the area of one of the ends of the blank by setting the density of the blank in this area to at least 60% of the theoretical density of quartz glass.

Because the preform and holder (or tube) are made of quartz glass, differences in expansion and the associated problems with glazing are avoided. However, it has been shown that using the known

Holder preforms can be obtained optical fibers with partially fluctuating optical properties.

The object of the invention is to improve the known method with regard to the reproducibility of the properties of the products produced from the quartz glass body. Furthermore, the invention is based on the object of specifying a suitable holding element for holding a blank made of porous quartz glass.

With regard to the method, this object is achieved, based on the method mentioned at the outset, by using a holding element with a metastable OH content of less than 30 ppm by weight and subjecting the holding element to a thermal pretreatment.

It has been shown that the fluctuations in the optical properties mentioned can be attributed to the fact that the blank has an axial gradient in the hydroxyl group content (hereinafter also referred to as OH content). In the known method, the OH content is greatest in the area of the blank - and the quartz glass body produced therefrom - to which the holding element was connected.

This can be attributed to the fact that OH groups are formed during the production process, and in particular during process steps at high temperature diffuse out of the holding element and are delivered to the blank and to the quartz glass body. Surprisingly, such a release of OH groups cannot be clearly correlated with the hydroxyl group content of the quartz glass. The hydroxyl group content of the quartz glass is rather composed of a chemically firmly bound OH content and a metastable OH content, only the latter being problematic with regard to the release of OH groups. The method according to the invention therefore makes it possible to use a holding element made of quartz glass with a high OH content, with the proviso that its metastable OH content is less than 30 ppm by weight.

Because a holding element with the lowest possible metastable OH content (less than 30 ppm by weight) is used, the release of OH groups is largely avoided, so that an essentially axially homogeneous distribution of hydroxyl groups in the blank and in the blank produced therefrom transparent quartz glass body is guaranteed.

A "metastable OH content" generally means that

Understand hydroxyl group content that can be removed by a temperature treatment of the quartz glass. The solution of metastable OH groups from quartz glass requires a certain activation energy. Regardless of this, the diffusion of metastable OH groups from a quartz glass component is determined by the level of the temperature, the duration of the temperature treatment and the length of the

Diffusion paths determined. With a larger component, the diffusion path (= maximum distance of a point within the component to the free component surface) is longer. For the purposes of this invention, the “metastable OH content” is defined as a hydroxyl group content which is removed by heating a holding element to a temperature of 1040 ° C. over a period of 48 hours with a diffusion path of 5 mm.

A low metastable OH content can be achieved by using a holding element made of a quartz glass, which is specified by a correspondingly low OH content. However, such quartz glass is usually expensive and special measures are required to keep the low OH content during the To maintain the shape of the holding element. According to the invention, it is therefore proposed to subject the holding element to a thermal pretreatment before it is used as intended. The metastable OH content of the quartz glass is reduced. As a result, inexpensive quartz glass with an initially higher metastable OH content can be used for the production of the holding element. First, the holding element is formed from a quartz glass with a high metastable OH content. The higher metastable OH content of the quartz glass is reduced before the use of the holding element by a thermal aftertreatment and adjusted according to the invention. Because of the aftertreatment, special precautionary measures regarding the OH content of the quartz glass are not necessary when shaping the holding element.

A holding element with a metastable OH content of less than 1 ppm by weight is preferably used. Taking into account the above definition, this is a component made of quartz glass, from which the original OH content is reduced by at most 1% by weight by heating to a temperature of 1040 ° C over a period of 48 hours (in one Diffusion path of 5 mm). Accordingly, the release of metastable OH groups is largely avoided and an essentially axially homogeneous distribution of hydroxyl groups in the blank and in the transparent quartz glass body produced therefrom is achieved.

The higher the temperature and the longer the treatment time in the thermal aftertreatment, the lower the metastable OH content of the aftertreated holding element. A thermal aftertreatment has proven itself, which comprises heating the holding element at a temperature of at least 900 ° C. The holding element is preferably heated at a temperature of at least 1000 ° C. The heating time in the context of the thermal aftertreatment is advantageously 5 hours, preferably 20 hours and more.

The method according to the invention has proven itself when using a cylindrical one Haltele ent, which is partially embedded in the area of one of the ends of the blank being formed, has proven to be particularly favorable.

With regard to the holding element, the above-mentioned object is achieved according to the invention starting from the holding element of the type mentioned at the outset in that the holding element consists of quartz glass with a metastable OH content of less than 30 ppm by weight.

A holding element made of quartz glass with the lowest possible metastable OH content releases little or no OH groups to the blank to be held or to the quartz glass body, so that there is an essentially axially homogeneous distribution of hydroxyl groups in the blank and in the transparent material produced therefrom

Quartz glass body guaranteed. A low metastable OH content can be achieved on the one hand by a low-hydroxyl quartz glass, on the other hand by a suitable thermal aftertreatment of a quartz glass with an initially higher metastable OH content.

With regard to further advantages and effects of a low “metastable OH content” with regard to the technical task mentioned above and the definition thereof, reference is made to the above explanations regarding the method according to the invention.

In view of the lowest possible release of OH groups and the lowest possible axial gradient of the OH content in the blank and in the quartz glass body, a holding element is preferred which consists of quartz glass with a metastable OH content of less than 1 ppm by weight.

The holding element according to the invention preferably has a cylindrical region which is at least partially embedded in the tube for the intended mounting. The cylindrical area embedded in the blank ensures a firm connection.

The invention is explained in more detail below on the basis of exemplary embodiments and a drawing. In the drawing, a schematic representation shows in detail: 1 shows the step of the deposition of Si0 ₂ particles to form a blank made of porous quartz glass with a holder embedded on the end face according to the invention, and

Figure 2 shows the same process step as in Figure 1, but using a holder according to the prior art, wherein areas of the blank are marked that contribute to an increased OH content in the preform.

Example 1: FIG. 1 shows a section of the production of a blank 1 made of porous quartz glass by embedding a holder in the form of a tubular holder 2 in the region of one of the end faces 3 of the blank 1. The blank 1, each of which has an outer diameter of approximately 80 mm and a length of approximately 700 mm, is made according to the known OVD method by flame hydrolysis of SiCI and GeCU with layer-by-layer deposition of SiO ₂ particles on its

Longitudinal axis rotating mandrel 4 made. The inner region of the blank 1 is doped with germanium dioxide. A deposition burner 5 is provided, which is moved back and forth along the surface of the blank 1 with layer-by-layer deposition of Si0 ₂ particles (and Ge0 ₂ particles). In the course of the deposition process, the holder 2 is stably embedded over part of its length in the region of the end face 3 of the blank 1. The holder 2 has an outer diameter of 25 mm and a wall thickness of 9 mm. The diffusion path for the diffusion of metastable OH groups in holder 2 is thus 4.5 mm. A particularly stable embedding of the holder 2 is achieved in that a higher temperature and thus a higher density is set in the region of the end face 3 by means of an additional burner 7, so that the holder 2 melts on the surface with the blank 1.

The blank 1 thus produced can weigh more than 2 kg. After the dome 2 has been removed, it can be handled for its further processing by means of the part of the holder 2 protruding from the blank 1. Electrically melted quartz glass is used to produce the holder 2. The OH content of this quartz glass is about 80 ppm by weight. By annealing at a temperature of 1040 ° C for 48 hours, the OH content is reduced to 4.8 ppm by weight. This means that the metastable OH content of the stop 2 before tempering. approx. 75 ppm by weight. When the holder is used as intended, the holder 2 no longer contains any measurable metastable OH content as defined above (temperature = 1040 ° C., treatment time = 48 h, diffusion path <5 mm).

After the deposition process, the blank is cleaned and dried at 900 ° C in a chlorine-containing atmosphere and then sintered at a temperature of approx. 1350 ° C. A preform for so-called monomode fibers is produced from the core rod thus obtained by covering the core rod with a "jacket tube". The fiber drawn from this preform shows homogeneous optical properties over the entire length of the fiber. In particular, the optical attenuation at a wavelength of 1385 nm (in the range of an OH absorption band) is less than 0.7 dB / km.

Comparative Example 1:

In a comparative embodiment, the blank 1 was produced as described in Example 1, with the only exception that a holder 12 (see FIG. 2) was used which was not tempered before it was used. The OH content of the holder 12 was approximately 55 ppm by weight. A monomode optical fiber was also drawn from the preform made using the holder 12. Their optical attenuation was inhomogeneous. At the wavelength of 1385 nm, it was in the range between 0.6 dB / km and 7 dB / km, the maximum attenuation values occurring in the preform regions which were originally in the vicinity of the holder 12. A measurement of the OH content of the holder 12 after use showed an OH content of only 5.6 ppm by weight. It follows from this that approx. 50 ppm by weight of OH were released from the holder 12 as “metastable OH groups” during the production process and were found in the blank 1 and solved in the preform and thus contributed to the above-mentioned high attenuation values at 1385 nm.

In FIG. 2, the hatched area 8 schematically shows the area of increased OH concentration, as is typically found after the blank has been sintered in a method according to the prior art. The increased OH

Concentration reflects the distribution of OH groups in the blank 1 which are released from the holder 12 in the course of the deposition process. These released OH groups diffuse into the porous quartz glass surrounding the holder 12 and thus generate an axial OH gradient in the later preform and the optical fiber drawn therefrom. Those treatment layers that take place at a higher temperature, such as chlorination at approx. 900 ° C and sintering at approx. 1350 ° C, are particularly critical. In particular during sintering, during which generally no chlorine or only very little chlorine is available for the reaction, metastable OH groups are released due to the high sintering temperature.

Claims

claims

1. A method for the production of a quartz glass body, comprising a deposition of Si0 ₂ particles on the cylindrical surface of one around it

Longitudinal axis rotating, cylindrical mandrel to form a substantially cylindrical, porous blank, which is connected to a quartz glass holding element engaging in the region of one of the ends of the blank, and sintering the blank, characterized in that a holding element with a metastable OH content of less than 30 ppm by weight is used, and that the holding element is subjected to a thermal pretreatment.

2. The method according to claim 1, characterized in that a holding element with a metastable OH content of less than 1 ppm by weight is used.

3. The method according to claim 1 or 2, characterized in that the thermal pretreatment involves heating the holding element at a

Temperature of at least 900 ° C.

4. The method according to claim 3, characterized in that the heating of the holding element takes place at a temperature of at least 1000 ° C.

5. The method according to any one of claims 1 to 4, characterized in that the thermal pretreatment involves heating the holding element during a

Duration of at least 5 hours.

6. The method according to claim 5, characterized in that the heating of the holding element lasts at least 20 hours.

7. The method according to any one of the preceding claims, characterized in that a cylindrical holding element is used, which is partially embedded in the region of one of the ends of the blank being formed.

8. holding element made of quartz glass, which is connected to hold a cylindrical blank made of porous quartz glass with one of the ends of the blank, characterized in that the holding element consists of quartz glass with a metastable OH content of less than 30 ppm by weight.

9. Holding element according to claim 8, characterized by a metastable OH content of less than 1 ppm by weight.

10. Holding element according to claim 8 or 9, characterized in that it has a cylindrical region which is at least partially embedded in the blank for holding the blank.