WO2002081393A1

WO2002081393A1 - Method for producing a quartz glass component

Info

Publication number: WO2002081393A1
Application number: PCT/EP2002/003447
Authority: WO
Inventors: Clemens Schmitt; Karsten Bräuer; Peter Bauer; Frank Gänsicke
Original assignee: Heraeus Tenevo Ag; Shin-Etsu Quartz Products Co., Ltd.
Priority date: 2001-04-05
Filing date: 2002-03-27
Publication date: 2002-10-17
Also published as: DE10117153C1

Abstract

According to a known method for producing a quartz glass cylinder in a vertical drawing process comprising an attraction phase and a drawing phase, a quartz glass hollow cylinder comprising an inner bore hole having an upper end and a lower end is supplied to a heating zone by means of its lower end, is softened in areas and flattened, closing the inner bore hole. During the drawing phase, an inner pressure (Pi,Z) is applied in the inner bore hole of the hollow cylinder, said inner pressure being reduced in relation to an outer pressure (Pa) which is applied outside said bore hole. In order to provide a simple and cost-effective method based on the same, which ensures sufficient cleaning and drying of the inner bore hole of the hollow cylinder and which simultaneously prevents the inner wall from being contaminated by particles from the oven atmosphere, the lower end (19) is kept open at least temporarily during the attraction phase, and a gas window (20) is produced in the region of the open, lower end, by introducing a gas flow (10) into the inner bore hole (4) from the upper end, thus maintaining an inner pressure (Pi,A) which is increased in relation to the outer pressure (Pa) in the region of the open end (19).

Description

Process for producing a quartz glass component

The present invention relates to a method for producing a component made of quartz glass in a vertical drawing process comprising a tightening phase and a drawing phase, in that a hollow cylinder made of quartz glass, which has an inner bore with an upper end and a lower end, is supplied to a heating zone and begins to soften therein in regions and collapsing while the inner bore is being closed, an inner pressure (Pj, z) reduced in the inner bore of the hollow cylinder during the drawing phase being applied compared to an external pressure (P _a ) outside it.

Such components are quartz glass rods or preforms for optical fibers.

EP-B 598 349 describes a generic method for producing a preform for optical fibers by covering a core rod made of doped quartz glass with a cladding glass tube according to the so-called rod-in-tube

Technology known. In this case, a core rod is arranged in the bore of a thick-walled hollow cylinder made of quartz glass and this arrangement is fed in a vertical orientation, beginning with its lower end, to a drawing furnace and softening it zone by zone. The hollow cylinder is collapsed onto the core rod and the arrangement is elongated at the same time.

In the annular gap between the hollow cylinder and the core rod, a negative pressure of 200 to 1000 mm water column (corresponds to 20 mbar to 100 mbar) is maintained during the collapsing and elongation compared to the atmospheric pressure applied to the outside. In order to adjust the vacuum even during the tightening phase - during which the inner bore of the hollow cylinder is still is not completely collapsed - to facilitate, the lower end of the hollow cylinder is closed. Without this measure, particles from the furnace atmosphere would be sucked into the inner bore of the hollow cylinder.

The inner wall of the hollow cylinder is cleaned in hydrofluoric acid before the vertical drawing process. To clean and dry the inner walls, flushing with a reactive or an inert flushing gas is often advantageous even during the tightening phase. However, these cleaning and drying measures are made more difficult by closing the inner bore on one side.

The invention is based on the object of specifying a simple and inexpensive method which ensures adequate cleaning and drying of the inner bore of the hollow cylinder and which at the same time prevents contamination of the inner wall by particles from the furnace atmosphere.

Based on the above-mentioned method, this object is achieved in that the lower end is kept open at least temporarily during the tightening phase and that a gas window is created in the area of the open end by inserting into the inner bore from the upper end Gas flow is introduced and an internal pressure (Pj, _A ) which is higher than the external pressure (P _a ) is maintained in the region of the open end.

The vertical pulling process comprises a pulling phase and the actual pulling phase. During the tightening phase, an internal pressure (PJ, A) that is at least temporarily higher than the external pressure is present in the inner bore, and a reduced internal pressure (Pj _ιZ ) is present during the actual drawing phase.

The tightening phase continues until the lower end of the inner bore of the hollow cylinder is closed by collapsing or by melting the hollow cylinder onto a component arranged in the inner bore. The opening of the inner bore (hereinafter also referred to as “bore opening”) thus has a circular or an annular opening cross section. During the tightening phase, the inner bore is open at least temporarily, so that the inner bore can be flushed with an inert gas or with a treatment gas in the flow before the drawing process and during the tightening phase. This enables effective cleaning or treatment of the inner wall of the hollow cylinder.

Through diffusion, supported by abrasion (thermal) and flow (chimney effect), particles - even against a gas flow - can get into the inner bore. To prevent this, a gas window is created in the area of the hole opening. The gas window prevents particles from the furnace atmosphere from entering the inner bore while it is still open. The gas window is generated by introducing a gas flow into the inner bore from the upper end. However, the gas flow complicates the collapse of the inner bore. The flow rate of the gas stream is therefore ideally limited to the minimum necessary to produce an effective gas window. The minimum required flow rate depends on the opening cross-section and the thermal conditions in the individual case. With just a few experiments, the gas flow can be adjusted so that the gas flow velocity in the area of the bore opening is sufficient to effectively prevent particles from entering the inner bore. In this sense, inert gases - such as nitrogen, argon or inert gas mixtures - are particularly suitable as a gas stream,

According to the invention, the internal pressure (P _J , _A ) is used as a measure of the flow velocity in the region of the bore opening. During the tightening phase, this is kept in the area of the open end of the inner bore at a value which is higher than the external pressure (P _a ). During the transition from the tightening phase to the actual drawing phase, instead of the increased internal pressure (PJ, A), the reduced internal pressure (Pj, z) compared to the external pressure (P _a ) is applied.

It has proven useful to regulate the increased internal pressure (Pj, _A ) set during the tightening phase, the flow rate of the gas stream introduced into the inner bore being used as the control variable. In the course of Tightening phase reduces the free opening cross-section of the bore opening. With the same flow rate of the gas flow, this increases the flow velocity in the area of the bore opening and the internal pressure (PJ, A). In order to maintain an optimized flow rate (essentially equivalent to a minimal, still effective flow rate), the internal pressure in the inner bore is measured and regulated. As the internal pressure increases, the flow rate of the gas stream is reduced.

In a preferred procedure, the internal pressure (P |, _A ) is kept essentially constant during the tightening phase. As a result, the flow velocity in the region of the bore opening can also be kept essentially constant during the tightening phase.

Advantageously, in order to generate a negative pressure in the inner bore during the transition from the tightening phase to the drawing phase, the increased internal pressure (PJ, A) is gradually reduced to the reduced internal pressure (Pj, z). The gradual reduction in pressure prevents radial deformation of the inner bore in the softened area. Only when a phase that is critical with regard to this deformation is exceeded - for example as soon as the bore opening is completely closed, is the internal pressure in the internal bore set to the predetermined negative pressure. In this way, it is possible to set a comparatively low internal pressure (high negative pressure) in the actual drawing phase without radial deformation of the hollow cylinder. The lower internal pressure in turn enables the inner bore to collapse faster during the drawing phase and thus accelerate the drawing process.

The "absolute pressure" is the absolute value of the pressure difference between the external pressure applied outside the inner bore in the area of the softened zone and the pressure in the inner bore (internal pressure). In the simplest case, the external pressure corresponds - but not necessarily - to atmospheric pressure If the internal pressure is lower than the external pressure, the negative pressure values have a positive sign Reduction of the internal pressure means an increase in the negative pressure.

The "gradual" reduction in the internal pressure during the tightening phase preferably takes place continuously, but a reduction in small individual steps is not harmful to the technical success of the teaching according to the invention.

As a result of the bore opening being closed, the transition from the tightening phase to the drawing phase is initiated by a characteristic pressure increase in the inner bore, which can initially be counteracted by the pressure control. It has proven to be advantageous to measure the flow rate of the gas stream, whereby by definition the transition from the pull-in phase to the pull-in phase begins as soon as the flow rate is zero. Then the reduction of the internal pressure to the specified negative pressure is initiated.

It has also proven useful to reduce the internal pressure (Pj _ιA ) by 50 mbar per minute or more slowly. Slowly lowering the internal pressure reduces the risk of radial deformation of the hollow cylinder in the softened area. The specified negative pressure is only reached when the drawing process has stabilized, so that the risk of deformation during the remaining collapse of the hollow cylinder is low.

It has proven to be advantageous to first regulate the vacuum to a predetermined setpoint. The “target value” of the internal pressure is understood to mean an absolute value for the pressure in the inner bore, which is to be set approximately during the actual drawing phase

In a preferred embodiment of the method according to the invention, the negative pressure is set in the range between 0 and 500 mbar, preferably above 70 mbar. The comparatively high negative pressure accelerates the collapse of the inner bore in the softened area - and thus the entire drawing process. This procedure is particularly suitable if the inner bore of a hollow cylinder is to be completely collapsed. The method according to the invention has proven particularly useful when using a thick-walled hollow cylinder with an outside diameter of more than 120 mm and a ratio of outside diameter and inside diameter of at least 2. The process times required for the collapse of such thick-walled hollow cylinders are significantly shortened by the method according to the invention, so that the method enables a large quartz glass mass to be processed.

The method according to the invention is particularly suitable for the cost-effective production of a component in the form of a full cylinder for the production of an optical waveguide by using a hollow cylinder made of high-purity, synthetic quartz glass.

The method according to the invention is explained in more detail below on the basis of exemplary embodiments and a drawing. In the drawing, method steps for producing a quartz glass rod by collapsing and elon- gating a hollow cylinder by means of the vertical drawing method according to the invention are shown schematically, specifically in FIG

Figure 1 shows the tightening phase with gradual closure of the inner bore and in

Figure 2 shows the actual drawing phase.

FIG. 1 shows the method according to the invention during the tightening phase and a device suitable for carrying out the method. The device comprises a vertically arranged furnace 1 which can be heated to temperatures above 2300 ° C. A hollow cylinder 2 made of synthetic quartz glass (shown broken) with a vertically oriented longitudinal axis 3 is introduced into the furnace 1 from above. The inner bore 4 of the hollow cylinder 2 is open at the bottom and closed at the top with a stopper 5. A flushing line 6 and a vacuum line 7 are inserted into the inner bore 4 through the stopper 5. Basically, gas lines are shown as solid lines in FIG. 1 and FIG. 2, and electrical lines are shown as dotted lines.

The purge gas line 6 is connected via a first control and shut-off valve 8 and via a first flow measuring device 9 to a nitrogen source, from which a nitrogen stream 10 can be introduced into the furnace 1 via the purge gas line 6 and the inner bore 4.

A first pressure measuring cell 12 and - after a second shut-off valve 13 - a second pressure measuring cell 14 are provided on the vacuum line 7 between a vacuum pump 11. The exhaust air drawn off by the vacuum pump 11 is symbolized in FIG. 1 with the directional arrow 15.

Purge gas line 6 and vacuum line 7 are connected to one another via a connecting line 16, the connecting points of the connecting line 16 on the one hand (purge line 6) - seen in the flow direction of the nitrogen stream 10 - in front of the control and shut-off valve 8, and on the other hand (vacuum line 7) in front of the vacuum pump 11 lie. A third control and shut-off valve 17, which is connected to the second pressure measuring cell 14 via a second flow measuring device 18, is provided in the connecting line 16. Furthermore, the first pressure cell 12 is connected to the second shut-off valve 13 and - via the first flow meter 9 - to the control and shut-off valve 8.

The lower end 19 of the inner bore 4 is open during the tightening phase. Due to the nitrogen stream 10 introduced into the inner bore 4 from above, a gas window 20 is generated during the tightening phase in the region of the lower end 19, which reduces the entry of particles into the inner bore 4.

FIG. 2 shows the same device as in FIG. 1. If the same reference numbers are used in FIG. 2 as in FIG. 1, these designate the same or equivalent components and parts of the system as the corresponding reference numbers in FIG. 1. The above Explanations are referred to. FIG. 2 shows the process step following the tightening phase, the actual pulling phase. A quartz glass rod 21 is drawn from the hollow cylinder 2 softened in the furnace 1 using a trigger (not shown in FIG. 2), a reduced internal pressure (Pj, z) in the inner bore 4 compared to the atmospheric pressure present in the interior of the furnace 1. is set.

A procedure typical for the method according to the invention is described in more detail below with reference to FIGS. 1 and 2:

The hollow cylinder 2 has an outer diameter of 150 mm and a wall thickness of 50 mm. After the furnace 1 has been heated to its target temperature of approximately 2300 ° C., the hollow cylinder 2 is inserted with the lower end 19 into the furnace 1 from above and softened at a position approximately in the middle of the furnace 1. As soon as its lower end 19 melts, the hollow cylinder 1 is lowered at a lowering speed of 18 mm / min.

The inner bore 4, which is initially open at the lower end 19, is closed at the upper end with the stopper 5. Atmospheric pressure (P _a ) prevails in the interior of the furnace. In the tightening phase, a slight excess pressure compared to P _{a is} generated and maintained in the inner bore 4. The control and shut-off valve 8 is open and the shut-off valve 13 is closed. A nitrogen stream 10 is introduced into the inner bore 4 via the purge line 6. The flow of the nitrogen stream 10 is regulated by means of the control and shut-off valves 8 (or 17), the control and shut-off valve 8 forming a flow controller (MFC) together with the first flow measuring device 9. The nitrogen flow 10 is regulated in such a way that an essentially constant internal pressure (P _{i] A} ) of 1.5 mbar is established in the inner bore 4. For this purpose, the internal pressure (Pj _ιA ) is continuously measured by means of the first pressure _cell 12, which is connected to the first flow measuring device 9, and the flow of the nitrogen stream 10 is adjusted accordingly (manipulated variable of the pressure control). Due to the gas flow generated in this way and the internal pressure (P _{i | A} ), a gas window is formed in the region of the lower end of the inner bore 4 20, which effectively prevents particles from diffusing into the inner bore during the tightening phase.

In the area of a deformation zone, the hollow cylinder 2 softens and the inner bore 4 gradually collapses. The associated increase in internal pressure (Pj _{, A} ) is initially compensated for by a corresponding reduction in the nitrogen flow rate using the flow control described above (MFC; flow meter 9 and control and shut-off valve 8). When the inner bore 4 is completely closed, the nitrogen flow 10 goes to zero. The system then switches from overpressure control to vacuum control. For this purpose, the control and shut-off valve 8 is closed and the shut-off valve 13 is opened.

During the tightening phase, the vacuum pump 11 is supplied with a nitrogen stream via the open control and shut-off valve 17 and via the connecting line 16 in order to maintain a predetermined, constant negative pressure in the vacuum line 7 - up to the shut-off valve 13. It is regulated according to the negative pressure measured by the second pressure cell 14. This measure serves to ensure that a constant negative pressure is already present in the vacuum line 7 when switching from positive pressure control to negative pressure control.

In the actual drawing phase, the inner bore 4 is closed on both sides and the rod 21 - with a regulated diameter of approximately 90 mm - is drawn off with an approximately constant withdrawal speed. In this phase, a regulated vacuum is generated in the inner bore 4 by means of a vacuum pump 11, control and shut-off valve 17 and pressure measuring cell 12, the setpoint of which is initially set to 0 mbar and which is ramped to the setpoint vacuum of approximately 50 mbar / min -100 mbar (compared to atmospheric pressure (P _a )) is lowered. The control circuit for this includes the vacuum pump 11, the shut-off and control valve 17, the flow meter 18 and the pressure cell 12. The constant internal pressure (Pj, z) mentioned above of approximately 100 mbar is maintained during the drawing phase. This is a comparatively high negative pressure which, in particular in the case of the thick-walled hollow cylinder 2 used, contributes to the rapid collapse of the inner bore 4.

The rod 21 thus obtained is cut into suitable sections and used as a preform for the production of optical fibers.

Claims

claims

1. A method for producing a cylinder made of quartz glass in a vertical drawing process comprising a tightening phase and a pulling phase, in that a hollow cylinder made of quartz glass, which has an inner bore with an upper end and a lower end, is supplied to a heating zone starting at the lower end, softened therein and under closure of the internal bore is collapsed, wherein during the drawing phase in the inner bore of Hohizylinders an opposite an on outside thereof lying outside pressure (P _a) of reduced internal pressure (Pj z) is applied, characterized in that during the Anziehphase the lower end (19 ) is kept open at least at times, and that a gas window (20) is generated in the area of the open, lower end (19) by introducing a gas stream (10) into the inner bore (4) from the upper end and thereby in the area of the open end (19) an internal pressure (PJ, A) increased compared to the external pressure (P _a ) is maintained.

2. The method according to claim 1, characterized in that the internal pressure is regulated during the tightening phase (PJ, A), and that the flow rate of the gas stream (10) introduced into the inner bore (4) is used as the control variable for the regulation.

3. The method according to claim 1 or 2, characterized in that the internal pressure (PJ, A) is kept substantially constant during the tightening phase.

4. The method according to any one of the preceding claims, characterized in that to generate a negative pressure in the inner bore (4) during the transition from the tightening phase to the drawing phase, the increased internal pressure (PI, A) to the reduced internal pressure (Pj _, z) gradually is reduced.

5. The method according to claim 4, characterized in that the flow rate of the gas stream (10) is measured, and that the transition begins as soon as the flow rate is zero.

6. The method according to claim 4 or 5, characterized in that the internal pressure (P _{i | A} ) is reduced by 50 mbar per minute or slower. .

7. The method according to claim 4, characterized in that the negative pressure is adjusted to a predetermined setpoint.

8. The method according to any one of the preceding claims, characterized in that a negative pressure is set below 500 mbar.

9. The method according to claim 8, characterized in that a negative pressure of at least 70 mbar is set.

10. The method according to any one of the preceding claims, characterized in that a thick-walled hollow cylinder (2) with an outer diameter of more than 120 mm and a ratio of outer diameter and inner diameter of at least 2.0 is used.

11. The method according to any one of the preceding claims, characterized in that for the manufacture of a cylinder for the manufacture of an optical waveguide see a hollow cylinder (2) made of high-purity, synthetic

Quartz glass is used.