GB2133786A

GB2133786A - Silica tube manufacture

Info

Publication number: GB2133786A
Application number: GB08330090A
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: 1984-08-01
Also published as: GB8330090D0

Abstract

Vitreous silica tubing is made by introducing into a relatively thin walled tube of silica (1) a gaseous mixture consisting of oxygen and at least one vapour capable of reacting therewith to produce silica, causing a chemical reaction to take place within the tube to cause a coating of silica (6) to be deposited on the inner surface of the tube, building up the coating to a thickness greater than that of the tube wall, heating the coated tube to its softening temperature and drawing it 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.

According to the invention a method of manufacturing vitreous silica tubing comprises introducing into a relatively thin-walled tube of vitreous silica a gaseous mixture consisting of oxygen and at least one vapour capable of reacting therewith to produce silica, causing a chemical reaction to take place within the tube such that a coating of silica is deposited on the inner surface of the tube, building up the coating to a thickness greater than that of the tube wall, and subsequently heating the coated tube to its softening temperature and drawing the heated composite 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 introducing one or more additional vapours into the tube, which vapour or varpours are capable of reacting with the oxygen to form required additive material or materials which are deposited on the inner surface of the 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.

The gaseous mixture is preferably fed continuously through the tube, and is caused to react and so form a silica coating on the internal surface of the tube, by the generation of energy within the tube.

Conveniently the energy generated within the tube is produced by a plasma generator.

the tube wall being simultaneously heated to a suitably elevated temperature to cause a vitreous silica coating to be formed directly on the tube wall.

The plasma generator may be arranged to form a plasma extending along a fraction only of the length of the tube, means being provided for producing a relative movement between the generator and the tube, so as to cause the plasma to reciprocate along the tube at a suitably controlled speed; by this means a thin layer of silica is deposited along the length of the tube on each passage of the plasma, and the coating is built up by the deposition of a multiplicity of successive layers.

Although the whole length of the tube may be maintained at an elevated temperature, for example between 800"C and 1 200 C, in such a process, it will generally be more convenient, and more economic, for the means for heating the tube wall to be reciprocated along the length of the tube with the plasma generator.

The latter conveniently consists of an RF coil or a microwave resonator of appropriate length surrounding the tube, which may be supported with its axis vertical during the coating process.

Heating of the composite 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.

One method of manufacturing vitreous silica tubing in accordance with the invention will now be described by way of example with reference to the accompanying schematic 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 supported vertically, and its lower end is connected by means of an inlet tube 2 to means (not shown) for suppiying silicon tetrachloride vapour in a carrier gas consisting of oxygen or a mixture of oxygen and argon.

The tube 1 is surrounded by a heater 3, for example in the form of an electric resistance heating element, which heater extends over part of the length of the tube only. The central part of the heater 3 is surrounded in turn by a microwave resonator 4 by which a plasma 5 can be produced in the gas/vapour mixture as it is passed through the heated zone of the tube, the heater 3 and resonator being mounted on a support (not shown) which can be reciprocated in a vertical direction.

In carrying out the method of the invention the heater 3 and resonator 4 are energised to cause a zone of the tube wall surrounded by the heater to be heated to a temperature of approximately 1 000'C and a plasma 5 to be formed within the heated zone; the gas/vapour mixture is simultaneously fed upwards through the tube 1 at a controlled rate whilst the heater 3 and resonator 4 are reciprocated slowly along substantially the whole length of the tube.

The combined effect of the plasma and the heating of the tube wall is to cause a chemical reaction to take place within the gas/vapour mixture and a thin layer of vitreous silica to be deposited directly on the inner surface of the tube wall in the heated zone, so that as the heater 3 and resonator 4 are reciprocated along the tube a thin layer of silica is formed along the region traversed; residual gases and gaseous reaction products are expelled from the upper end of the tube. Repeated traversal of the tube thus results in the deposition of further layers, and the process is continued for a time sufficient to cause a relatively thick deposit of silica to be built up as indicated at 6.

After the completion of the deposition process, which will take several hours depending upon the thickness of the coating required, the coated tube 1 is removed 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 tube of the required 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 cross-section suitable, when cut into appropriate lengths for forming support tubes on which one or more layers of doped silica may be applied to form a preform from which an optical fibre can be produced by collapsing and drawing in known manner.

However silica tubes of different dimensions for a variety of applications can also be produced 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 accu rately controlled amounts of additive oxides to be produced with equal facility.

Claims

1. A method of manufacturing vitreous silica tubing comprises introducing into a relatively thin-walled tube of vitreous silica a gaseous mixture consisting of oxygen and at least one vapour capable of reacting therewith to produce silica, causing a chemical reaction to take place within the tube such that a coating of silica is deposited on the inner surface of the tube, building up the coating to a thickness greater than that of the tube wall, and subsequently heating the coated tube to its softening temperature and drawing the heated composite tube whilst maintaining a sufficiently high pressure within it to prevent collapse thereof.

2. A method of manufacturing vitreous silica tubing according to Claim 1 wherein the heated composite tube is drawn to appropriate dimensions to constitute, when cut into its appropriate lengths, a plurality of thin-walled tubes similar to the initial tube and suitable for coating in carrying out the method of Claim 1.

3. A method of manufacturing vitreous silica tubing according to Claim 1 wherein the heated composite tube is 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.

4. A method of manufacturing vitreous silica tubing according to any preceding claim wherein one or more additional vapours are introduced into the tube, which vapour or vapours are capable of reacting with the oxygen to form required additive material or materials which are deposited on the inner surface of the tube with the silica, so as to modify the properties of the tubing which is formed.

5. A method of manufacturing vitreous silica tubing according to any preceding claim wherein the gaseous mixture is fed continuously through the tube and is caused to react, and so form a silica coating on the internal surface of the tube, by the generation of energy within the tube.

6. A method of manufacturing vitreous silica tubing according to Claim 5 wherein the energy generated within the tube is produced by a plasma generator, the tube wall being simultaneously heated to a suitably elevated temperature to cause vitreous silica coating to be formed directly on the tube wall.

7. A method of manufacturing vitreous silica tubing according to Claim 6 wherein the plasma generaor is arranged to form a plasma extending along a fraction only of the length af the tube, means being provided for produc ing a relative movement between the generator and the tube, so as to cause the plasma to reciprocate along the tube at a suitably controlled speed so that a thin layer of silica is deposited along the length of the tube on each passage of the plasma, and the coating is built up by the deposition of a multiplicity of successive layers.

8. A method of manufacturing vitreous silica tubing according to Claim 7 wherein the means for heating the tube wall is reciprocated along the length of the tube with the plasma generator.

9. A method of manufacturing vitreous silica tubing according to any of Claims 6, 7 or 8 wherein the plasma generator consists of an RF coil or a microwave resonator of appropriate length surrounding the tube.

10. A method of manufacturing vitreous silica tubing according to any preceding claim wherein heating of the composite 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 obtained the required diameter and wall thickness.

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

1 2. Vitreous silica tubing manufactured by a method according to any preceding claim.