GB2134099A

GB2134099A - Tube manufacture

Info

Publication number: GB2134099A
Application number: GB08330089A
Authority: GB
Inventors: Oliver Sanders Johnson
Original assignee: Associated Electrical Industries Ltd
Current assignee: Associated Electrical Industries Ltd
Priority date: 1983-01-24
Filing date: 1983-11-11
Publication date: 1984-08-08
Also published as: GB8330089D0; GB2134099B

Abstract

Vitreous silica tubing is formed by supporting a relatively thin-walled silica tube (1) coaxially around an inner tube (2) having in its wall a multiplicity of perforations (4), causing a gaseous mixture consisting of oxygen and at least one vapour capable of reacting therewith to form silica to flow into the annular space (5) between the tubes, with at least the vapour or vapours being introduced into the space through the perforations, and causing a reaction to take place within the space and silica (13) to be deposited simultaneously on the inner surface of the whole of outer tube, the coated tube being finally removed, heated to its softening temperature and drawn to required dimensions whilst maintaining a sufficiently high pressure within it to prevent its collapse. <IMAGE>

Description

SPECIFICATION Tube manufacture This invention relates to the manufacture of vitreous silica tubing, and especially, though not exclusively, for use as the support tubes in the manufacture of preforms for the production of optical fibres.

In accordance with the invention a method of manufactuing vitreous silica tubing, comprises supporting a relatively thin-walled tube of vitreous silica coaxially around an inner tube with an annular space between them, the wall of which inner tube is formed with a multiplicity of perforations, causing a gaseous mixture, consisting of oxygen and at least one vapor capable of reacting therewith to produce silica, to flow into the annular space, at least the vapour or vapours being introduced into said space through the perforations in the inner tube, and maintaining the thin-walled tube at an elevated temperature, such as to cause a chemical reaction to take place in the annular space by generating energy in the space and a coating of silica, resulting from the reaction, to be formed simultaneously on the whole of the inner surface of the heated length of the thin-walled tube, while residual gases and gaseous reaction products are withdrawn from the annular space, and subsequently heating the coated tube to its softening temperature and drawing the heated tube whilst maintaining a sufficiently high pressure within it to prevent the collapse thereof.

By this means tubes having at least the radially inner region of substantially pure silica can readily be obtained.

Coatings of several times the wall thickness of the silica tube can be deposited in accordance with the invention, thereby enabling tubes of substantially greater thickness and/or length than the original thin-walled tube to be produced.

The relatively thick-walled tubes formed by the initial thin-walled tube with the deposited coating, in accordance with the invention, are readily produced without strain and with a high degree of uniformity.

A tube formed in accordance with the invention may, for example, be drawn to appropriate dimensions to constitute, when cut into appropriate lengths, a plurality of thin-walled tubes similar to the initial tube, and suitable for coating in carrying out the method of the invention.

Alternatively a tube may be drawn to dimensions suitable for forming a plurality of support tubes for receiving internal coatings of doped silica in the formation of preforms for the production of optical fibres.

However the tubes can be drawn to a variety of dimensions and utilised for many different purposes. Moreover tubes which are formed of a material other than pure silica may sometimes be required, and in such a case they may readily be formed by introduing one or more additional vapours into the annular space through the performations in the inner tube, which vapour or vapours are capable of reacting with the oxygen within the annular space to form required additive material or materials which are deposited on the thin-walled tube with the silica. The proportions of the additive material or materials, deposited in such a case, can readily be controlled, by controlling the rate of flow of the respective vapour or vapours.

Heating of the silica tube during the deposition process may be effected by enclosing it in a tubular electric furnace.

The energy generated in the annular space is conveniently produced by a plasma-exciting device, operated to produce a plasma column through substantially the whole length of the space. Preferably the device is maintained in a stationary position with respect to the assembly of the silica and perforated tubes. The device may, for example, be a RF coil surrounding the silica tube, although preferably it is a microwave cavity located around one end of the silica tube and supplied with sufficient power to provide a plasma column of the required length in the annular space.

The pressure within the annular space must be kept sufficiently low to ensure the production of the plasma, whilst the pressure within the perforated tube should be high enough to prevent a plasma being produced within this tube and also to ensure a continuous flow of gas and vapour into the annular space through the perforations.

A suitable form of plasma-exciting device for use in carrying out the method of the invention is a microwave cavity of the asymmetrical type formed of two concentric cylinders disposed around one end of the silica tube, the outer cylinder being of greater length than the inner, the power input to the microwave cavity being maintained at an approximately constant level during a coating process and being conveniently provided by a microwave generator suitably coupled thereto.

Heating of the coated tube during the drawing process is conveniently effected by feeding the tube slowly through a Furnace whilst an appropriate degree of tension is applied to its emerging end, and with the interior of the tube maintained at an appropriate pressure to obtain the required diameter and wall thickness.

The invention will now be further explained by reference to the accompanying drawing which shows schematically, one form of apparatus suitable for forming tubes in accordance with the invention.

Thus referring to the drawing a vitreous silica tube 1 approximately 1 metre in length having an external diameter of approximately 10 centimetres and a wall thickness of 2 millimetres is mounted vertically and has supported coaxially within it, as by spacers 3, a further tube 2 having an external diameter of approximately 6 mm, the wall of the latter being pierced by a multiplicity of small perforations 4, show greatly enlarged. The annular space 5 between the inner and outer tubes is closed at its upper end apart from an inlet tube 7.

In carrying out the process in accordance with the invention argon is supplied to the annular space through the inlet tube 7, and the lower end of the silica tube 1 is connected to a vacuum pump (not shown).

The lower end of the inner tube 2 is closed and its open upper end 6 is connected to means (not shown) for supplying silicon tetrachloride vapour in a carrier gas, consisting of oxygen or a mixture of oxygen and argon, into this tube, the carrier gas and vapour emerging into the annular space 5 through the perforations 4.

A microwave cavity 8, formed of an outer cylinder 9 and an inner cylinder 10 of predetermined height, surrounds the lower end of the outer tube 1 ,the cylinder 9 being surmounted by a circular-sectioned waveguide tube 11 which extends over almost the full height of the silica tube 1. The waveguide tube 11 is surrounded, in turn, by a tubular electric furnace 12.

Initially a stream of the carrier gas alone is passed into the inner tube 2 while argon alone, or a mixture of argon and oxygen is fed into the annular space 5 through the inlet tube 7, and substantially constant power is applied to the microwave cavity from a microwave generator 1 4 sufficient to establish a plasma column 1 5 in the annular space 5, which is evacuated to low pressure, the presence of the argon facilitating the establishment of the plasma.The furnace is also energised to heat the wall of the tube to approximately 1 0000 C, and when this is at the required tempcrature silicon tetrachloride vapour is introduced into the annular space 5 through the perforations 4 in the tube 2 by bubbling a stream of oxygen through liquid silicon tetrachloride and conveying the oxygen and vapour into the inner tube 2 through its open upper ènd 6.

A chemical reaction then takes place within the annular space 5 and solid silica is deposited as a coating 13 over the whole of the circumference of the inner surface of the tube 1 along a region extending from about 1 cm below the upper perforations in the inner tube 2 to about 4 cm being the lowest perforations, the process being continued until the coating has a thickness of up to 4 centimetres.

After completion of the deposition process, which will take several hours, depending upon the thickness of coating required, the coated tube 1 is removed from the apparatus, and is drawn slowly through a furnace heated to a temperature sufficient to soften the silica, whilst a pressure in excess of atmospheric is maintained within the interior of the tube sufficient to prevent collapse of the tube. The pressure and rate of drawing are adjusted to obtain silica tubes of desired diameter and wall thickness.

For example the pressure within the tube may in some cases be such that the internal diameter increases to approximately 9.6 cm, with the rate of drawing adjusted to give a wall thickness of 2 mm. The tube can then be cut into a plurality of 1 metre lengths and used as further starting tubes for the production of tubes in accordance with the invention.

Alternatively the tube can be drawn to a crosssection suitable, when cut into appropriate lengths, for forming supports on which one or more layers of doped silica may be applied to form a preform from which an opticai cable can be produced by collapsing and drawing in known manner.

However silica tubes of different dimensions for a variety of other applications can also be produed by the process of the invention.

The process enables vitreous silica tubes of very high purity and uniformity to be readily produced, although, as previously mentioned, it also enables silica tubes incorporating accurately controlled amounts of additive oxides to be produced with equal facility.

Claims

1. A method of manufacturing vitreous silica tubing, comprising the steps of: supporting a relatively thin-walled tube of vitreous silica coaxially around an inner tube with an annular space between them, the wall of which inner tube is formed with a multiplicity of perforations, causing a gaseous mixture, consisting of oxygen and at least one vapour capable of reacting therewith to produce silica, to flow into the annular space, at least the vapour or vapours being introduced into said space through perforations in the inner tube, maintaining the thin-walled tube at an elevated temperature, such as to cause a chemical reaction to take place in the annular space by generating energy in the space and a coating of silica, resulting from the reaction, to be formed simultaneously on the whole of the inner surface of the heated length of the thin-walled tube, while residual gases and gaseous reaction products are withdrawn from the annular space, and subsequently heating the coated tube to its softening temperature and draining the heated tube whilst maintaining a sufficiently high pressure within it to prevent collapse thereof.

2. The method according to Claim 1 wherein heating of the silica tube during the deposition process is effected by enclosing it in a tubular electric furnace.

3. The method according to Claims 1 or 2 wherein the energy generated in the annular space is produced by a plasma-exciting device operated to produce a plasma column through substantially the whole length of the space.

4. The method according to Claim 3 wherein the device is maintained in a stationary position with respect to the assembly of the silica and perforated tubes.

5. The method according to Claims 3 or 4 wherein the device is an RF coil surrounding the silica tube.

6. The method according to Claims 3 or 4 wherein the device is a microwave cavity located around one end of the silica tube and supplied with sufficient power to provide a plasma column of the required length in the annular space.

7. The method according to any of Claims 3 to 6 wherein the pressure within the annular space is kept sufficiently low to ensure production of the plasma, whilst the-pressure within the perforated tube is high enough to prevent a plasma being produced within this tube and also to ensure a continuous flow of gas and vapour into the annular space through the perforations.

8. The method according to Claims 3, 4 or 6 wherein the device is a microwave cavity of the asymmetrical type formed of two concentric cylinders disposed around one end of the silica tube, the outer cylinder being of greater length than the inner, the power input to the microwave cavity being maintained at an approximately constant level during a coating process and being provided by a microwave generator.

9. The method according to any preceding Claim wherein the heating of the coated tube during the drawing process is effected by feeding the tube slowly through a furnace whilst an appropriate degree of tension is applied to its emerging end, and with the interior of the tube maintained at an appropriate pressure to obtain the required diameter and wall thickness.

10. A method of manufacturing vitreous silica tubing substantially as herein described with reference to the drawing.

11. Vitreous silica tubing manufactured by the method of any preceding Claim.