GB1586352A

GB1586352A - Process and apparatus for hot working glass

Info

Publication number: GB1586352A
Application number: GB2569476A
Authority: GB
Original assignee: Corning Ltd; UK Atomic Energy Authority
Current assignee: Corning Ltd; UK Atomic Energy Authority
Priority date: 1977-06-21
Filing date: 1977-06-21
Publication date: 1981-03-18

Description

(54) PROCESS AND APPARATUS FOR HOT WORKING GLASS (71) We, COXING LIMiTED, a British Company, of Wear Glass Works, Sunderland, Tyne and Wear, England, and UNITED KINGDOM AToMIC ENERGY AUThoRITY, a British Corporation of 11 Charles II Street, London SW1Y 4QP, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a process and an apparatus for heating glass or glass ceramic materials to a working temperature such as is required for sealing the glass or glass ceramic materials, for example, in joining elements formed of glass or glass ceramic materials.

It is known that glass when cold has a high impedance or electrical resistance which decreases as the temperature of the glass is raised and when the temperature of the glass is high enough (depending on its composition) a sufficiently high current may be made to flow through the glass to cause further intense heating without the application of impractically high voltages. This phenomenon is also encountered with certain glass ceramic materials and it is to such glass ceramic materials that the term glass ceramic used herein applies. Moreover, in general, reference to glass herein should be understood to include reference to such glass ceramic materials.

This characteristic phenomenon of glass has been employed in a known method of sealing glass elements, such as joining pipe ends together, by preheating the area of the glass to be softened and then passing a high frequency or mains frequency electric current through the glass so that due to the electrical resistance of the glass it is further heated by the electric current to the temperature at which the sealing of the glass elements can be effected.

The glass is usually preheated by gas burners and then by using the burners as electrodes, a current of 1-10 amps, is passed through the flames from the electrodes into the glass thus establishing an electrically conducting path through the glass to increase the glass temperature. The usual preheat temperature of the glass is between 700"C and 950"C but this varies according to the composition of the glass and the available voltage.

Difficulties may arise in the application of such known methods. The discharge may damage the electrodes and over heat them, and particulate metal or metal oxides from the electrodes may produce undesirable inclusions and discoloration in the glass; the discharge tends to wander sporadically e.g.

around the periphery of the gas flames increasing the width of the heated zone and the required current and producing random surface markings on the glass whilst a substantial voltage drop can occur in the gas flames which increases the required electric circuit voltage and reduces the proportion of heat developed within the thickness of the glass. Moreover, if high frequency spark generators are employed as a source of the electrical energy required a very high noise may be created so that the operatives ears have to be protected.

The main object of the present invention is to provide an improved process and apparatus for working glass including sealing glass elements e.g. joining glass elements in which these disadvantages are reduced compared with the above known processes or are minimised and in which improved performance of the process is obtained.

According to the present invention a process of heating a localised area of a glass element or element of glass ceramic material to a predetermined working temperature, comprises preheating the localised area of the element by conventional heating means until it is sufficiently electrically conducting and, using electrodes disposed within a protective stream of non-oxidising gas which is screened from the ambient atmosphere, feeding an electric current through a highly ionised path directed within the gas stream into the glass in the localised area whereby the resistance of the glass as the current flows through it causes the temperature of the glass in the localised area to rise to the predetermined working temperature; the non-oxidising gas being compatible with the heated glass and the material of the electrodes at the working temperature of the glass.

The preheating may be by any conventional means such as by gas flame jets directed onto the localised area of the glass to be worked.

For use in heating such glass elements such as pipe ends to be sealed together, where relative movement is required or desirable between the gas stream and the glass at least two electrodes are used, the electrodes being spaced apart with the localised area of the glass element in the current flow between them. The distance between the electrodes and the glass may be varied to provide optimum working conditions.

It is known that an electrically energised electrode with a pointed end or otherwise shaped so as to produce an electric field of high intensity will initiate ionisation in the surrounding gas with an applied potential of only a few kilovolts. The resulting discharge between two such electrodes using an alternating source of current is maintained by electrons emitted from the tip of each electrode in turn as it becomes cathodic and for a stable discharge with a low voltage drop and minimum electrode erosion it is desirable that a copious supply of thermionic electrons be emitted locally from the tip.

By completely surrounding the electrode with a shroud of suitable gas an electrode of refractory material and/or low work function may be chosen and protected from oxidation. This enables it to be pointed or shaped so as to produce the desired high field intensity and attain locally the temperature at which copious thermionic electrons are emitted, whilst maintaining its shape for a useful working life. By this means a well-defined, stable discharge is produced having high electric conductivity and low voltage drop between the tip of the electrode and the surface of the localised area of the glass to be heated.

In a preferred arrangement each electrode is a tungsten wire containing 1 to 2% of zirconia or thoria tapered to a needle point and its tip may be hollow ground. In the latter case the hollow ground cone-shaped tip preferably has a tip angle of substantially 5 and a tip diameter of about 0.1 mm, but other materials and forms of construction as hereinafter described may be used.

Such another construction comprises a thin walled tube which may have a sharpened or constricted end and which may carry the same or a different gas from that which shrouds the electrode. In yet another form the electrode is constructed so as to include a reservoir of low work function material which can migrate at the operating temperature to the tip of the electrode.

Suitable gases for forming the highly ionised gas stream include the inert gases of Group O of the Periodic System of the Elements such as argon, helium and neon, which gases are compatible with the electrode material and with the glass and may be used singly or in admixture. The gas is preferably chosen for easy ionisation and electrical breakdown, to be compatible with the glass and to allow the electrode material to be chosen for high electron emission, freedom from erosion and disintegration, and so that it maintains its chosen shape for a useful working life.

Desirable properties of electrode materials include optimum combinations of low workfunction, high melting point and low vapour pressure; composite materials or electrode arrangements may also be used to produce a low work function surface film on a suitable refractory substrate, from which the necessary localised emission of electrons may be drawn.

Hollow electrodes may be used consisting of a thin-walled tube made of any of the materials mentioned hereinafter for forming the electrode. The bore of the tube is typically in the region of 1-2mm and the wall thickness 0.5--lmm depending on the material and operating current. The tube may have a sharpened or constricted end and may carry the same gas as that which shrouds the electrode or be used to introduce a different gas into the discharge as described above.

In use the electrode emits a copious electrode emission into the gas stream in which it is disposed maintaining an internal ionisation of the gas and producing a well defined stable discharge stream of high conductivity and low voltage drop between the tip of the electrode and the surface of the glass which replaces the unstable conducting arc flame used in known processes.

From another aspect an apparatus for carrying out the process of the invention comprises a support for the glass element or element of glass ceramic material to be heated, means to pre-heat a localised area of each element to a temperature at which it is sufficiently electrically conducting, one or more nozzles each consisting of a shroud and an electrode disposed to lie in it by means of which an electric discharge can be initiated and maintained from the nozzle within the shroud so as to feed an electric current into the pre-heated localised area of the glass to cause the glass in said area to heat up to a predetermined temperature, and supply means to said shoud for non-oxidising gas to flow through the shroud thereby to isolate the electrode from the atmosphere.

At least two electrodes are preferably provided spaced apart with the glass to be worked disposed in the electric current flow between them. Each electrode is preferably disposed within a nozzle through which the gas stream flows and has its tip at substantially the outlet of the nozzle, the end of the electrode being shaped readily to initiate an electrical discharge within the gas stream and to promote a copious electron emission in the discharge in a restricted area close to the end of the tip.

The electrode is preferably an elongated wire, e.g. 0.2 to 3 mm diameter or strip, disposed within an elongated tubular nozzle, extending substantially to the nozzle end i.e.

with its tip at or not more than a few millimeters, e.g. 3 mm, inside or projecting outside the nozzle end, while its inner end is rooted in the base of the nozzle. The nozzle may be of ceramic or other electrically insulating refractory material connected to a source of gas supply and having an internal collet, with passages for the gas flow, to which the root of the electrode is secured.

Alternatively the nozzle may be of electrically conducting heat resistant material such as stainless steel or high nickel alloy which is electrically insulated from the electrode.

By the term refractory material is meant a material which is oxidation resistant at the nozzle temperature in the process.

If desired three or more electrodes are provided in which case a multiphase electric supply is connected to the electrodes so that the current path flows through the electrodes in succession. Alternatively groups of electrodes may be connected together and connected to a single phase supply.

The electrodes may be energised from a high frequency or main frequency supply but preferably by H.F. supply. The current may be in the range 2-20 amps and the apparatus will function at any frequency from 50 Hz to 400 KHz or more, the preferred frequency range being 200--400 KHz.

The distance between the electrode tip and the surface of the glass is about 10 mms although it will work satisfactorily at a distance in the region of 50 mms.

In order that the invention may be more fully understood some embodiments in accordance therewith will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a general diagrammatic layout in side elevation of an apparatus for working two glass elements; Figure 2 shows a diagrammatic end elevation of Fig. 1; Figure 3 is a diagrammatic cross section through one form of electrode' Figure 4 is a cross section along line IV IV of Fig. 3; and Figure 5 is similar to Fig. 3 showing an alternative form of electrode.

In the drawings the same references are used to designate the same or similar parts.

Referring to the drawings these show the process of the invention being employed in sealing or joining two tubular glass sections 1, la, the two sections being brought together in axial alignment on the mountings by a suitable mechanism well known in the art and needing no further description here. The sections may however be mounted out of alignment if the seal is between the sections which form an angle or bend at the seal.

If the work is to be carried out on a single glass element, then that element is clamped in one or both of the clamping members 2 of the mechanism in Figure 1.

The sections 1, la, are mounted on the clamps 2 which are rotatable in bearings 3 on a frame 4, the clamps being adjustable if desired to accommodate glass sections of different sizes. The clamps may be rotated by any suitable means and each clamp may be separately driven if desired provided they are maintained in angular synchronism.

Mountings are provided for gas burner jets 6 by which the sections are pre-heated.

A number of electrode assemblies 7 (two shown in Fig. 2) to be described are mounted on supports 8 round the sections, and these are spaced from the surface of the glass by a gap e.g. 1040 mm. to provide the optimum conditions for the sealing of the sections.

One or both of the clamps 2 and the mountings 5 and the electrode supports 8 are mounted on slides on the frame 4 to permit their desired disposition and adjustment during a sealing operation.

Figures 3 and 4 show one form of electrode assembly in which a pointed electrode 9 is disposed in a gas nozzle 10 and having its end substantially in the open end of the nozzle, although it will operate if within or projecting from the nozzle by up to several mms e.g. 3 mm from the nozzle end. The electrode comprises a wire or strip of electrically conducting material capable of remaining stable and inert to the gas under the operating conditions employed. Suitable materials for the electrode are refractory metals such as tungsten, tantalum and rhenium or carbon. Any of tungsten, tantalum or rhenium may be used containing thoria or zirconia (e.g. 12% of thoria or zirconia) to enhance the electron emission from the tip of the electrode and from around the tip.The electrode may also be carburised to enhance reduction of the thoria or zirconia to thoroum or zirconium and provide a carbide surface to increase the stability of the low work-function surface.

The end of the electrode provides the optimum ionic discharge when energised. A suitable electrode diameter is 0.2 to 3.0 mm but in the case of carbon larger diameters may be used.

Alternatively any other conventional form of thermionic emitting cathode may be used which is constructed in known manner to withstand the operating conditions.

The electrode is mounted or rooted in a collet 11 of conducting material such as brass, stainless steel or copper, and is secured by a grub screw 12. Initially the electrode projects through the collet so that in the event that during prolonged use its pointed end wears down, it can be advanced by releasing the screw 12 to bring its tip into alignment with the end of the nozzle.

The nozzle 10 is of alumina or other ceramic or oxidation resistant material capable of withstanding thermal shock and high temperature, which is inert to the gas and at its inner end is secured as shown by a screw thread or other means to a collar 13 into which the collet 11 is secured with good electrical contact. The square section of the collet 11 provides passages 15 for the flow of gas to the nozzle. Attached to the collar 13 is a pipe 16 of electrically conducting material such as brass. The pipe 16 is thus electrically connected by the collet to the electrode. The pipe is supported on an insulator 14 and is connected to a supply of gas inert to the electrode, the nozzle and the glass by means of an insulating hose or tube. It is also connected to an electric current supply by a suitable lead. The current supply is preferably HF but mains frequency may be employed.

In a preferred embodiment of Figs. 3 and 4 the gas is argon and the electrode is zirconiated tungsten wire 1.2 mm diameter containing 1% of zirconia. The electrode is hollow ground to a concave faced cone with a tip angle of about 5 having a diameter at the apex of about 0.1 mm. This electrode is shrouded by a hollow nozzle of alumina having an internal bore diameter of about 6 mm and being 25 to 80 mm in length.

This electrode is held, in the optimum position, concentrically within the nozzle by a collet and its position within the nozzle is such that the electrode tip is coincident with the outlet end of the nozzle.

Referring to Figure 5 this shows an alternative having the pointed electrode and nozzle similar to that in Figures 3 and 4 but the nozzle is of greater length and the electrode is supported entirely within the nozzle by an internal collet 18 and spider 17. The gas supply hose 19 is of insulating material and electrical connection to the electrode is made via the collet 18 by means of an internal wire carried within the insulating hose. Access to the collet for adjustment of the electrode is obtained by removing the hose. This arrangement provides a completely insulated assembly substantially reducing the possibility of electric shock or HF burn to the operator, enables the mounting insulator to be dispensed with, and results in a neater, smaller assembly more easily incorporated in an existing gas burner array.The assembly may be mounted by means of a metal strap 20 surrounding the nozzle at any convenient point, and a further advantage of this arrangement lies in the ionisation in the gas stream surrounding the electrode produced by the electric field between the electrode and the metal strap 20. The ions produced are carried forward in the gas stream and enable the discharge to be established more easily.

In the operation of joining two tubular glass sections each glass section is mounted in the apparatus of Figures 1 and 2 whereby relative movement of the sections of the gas burners and the electrodes is effected by moving one or both of the sections and/or the gas burners and/or electrodes. A small protective flow of argon is initiated through the electrode nozzles through the supply pipe 10 and a narrow band of glass is raised to the preheat temperature by the preheat burners. If desired the preheating burners may be turned off at the point where the glass becomes sufficiently electrically conducting to enable the electrical power dissipated in the glass to exceed the total heat losses and to cause further heating of the glass in the preheated area.

When the electrical heating of the glass is to be started the gas supply to the electrodes through the pipe 16 is increased and the gas is directed towards the glass surface in the narrow localised band e.g. 10-20 mm wide, thereof to be heated. The electric current supply is switched on to the electrodes which due to the discharge heat up rapidly, usually within a fraction of a second, to incandescence. The tip of the electrode then initiates a copious electron emission which maintains and enhances the intensity of the discharge towards the glass within the stream of gas so that an electric circuit of high conductivity to and through the preheated glass is established within the stream of argon gas. As the temperature of the glass rises its electrical resistance falls and the increasing current passing through the conducting path established in the glass heats it up to the required working temperature enabling the softened glass sections to be joined or the glass to be worked in any desired manner. For borosilicate glass of about 6 mm wall thickness the power supply is adjusted so that the initial current at the chosen preheat temperature in the region of 900"C is about 2 amps and increases to 5 amps as the resistance of the glass falls, but these values will be varied for other chosen preheat temperatures and other glass compositions and/or thicknesses.

In this example the preferred gas flow rate through the nozzle round the collet and the electrode is 3.5 to 4 litres/min although flow rates up to 10 litres/min or more may be used. The preferred distance between the electrode tip and the surface of the glass section is about 10 mms but this may be increased to about 50 mm.

By means of the invention the glass is heated with a lower applied voltage than when known flame electrodes are used, the control of the softened glass is limited to a narrower band of glass than heretofore, the joined, sealed or worked glass is subjected to substantially less injurious contamination by emission from the electrode and there is less distortion of the softened glass area than by the existing methods.

The enhancement of electron-emission of the electrodes and thermionic emission into the gas stream enables a lower applied voltage to be used than in the known processes and apparatus, and enables a stable discharge to be established at low currents.

This facilitates the establishment of the desired electrically heated zone in the glass with lower preheat temperatures at an early stage of seal-making than in known processes.

Sporadic arc wander is minimised compared with known processes and surface markings on the glass are avoided.

The glass sections may be of any glass for which the known processes are used, such as borosilicate glass or glass ceramic systems in which the resistive nature of the glass caues sufficient self-heating of the glass due to the passage of the current through it.

WHAT WE CLAIM IS: - 1. A process of heating a localised area of a glass element or element of glass ceramic material to a predetermined working temperature, comprising preheating the localised area of the element by conventional heating means until it is sufficiently electrically conducting and, using electrodes disposed within a protective stream of nonoxidising gas which is screened from the ambient atmosphere, feeding an electric current through a highly ionised path within the gas stream into the glass in the localised area whereby the resistance of the glass as the current flows through it causes the temperature of the glass in the localised area to rise to the predetermined working temperature, the non-oxidising gas being compatible with the heated glass and the material of the electrodes at the working temperature of the glass.

2. A process according to claim 1 wherein the preheating is by gas flame jets directed onto the localised area of the glass to be worked.

3. A process according to claim 1 or 2 wherein at least two spaced electrodes are used to straddle the localised area of the glass element to be heated in a current flow between the spaced electrodes.

4. A process according to any of claims 1-3 wherein the distance between the electrode or electrodes and the glass is variable.

5. A process according to any of claims 1 to 4 wherein each electrode comprises a tungsten wire containing 1 to 2% of zirconia or thoria tapered to a needle point.

6. A process according to claim 5 wherein the tip of the electrode is hollow ground.

7. A process according to claim 6 wherein the electrode tip is cone-shaped.

8. A process according to claim 7 wherein the tip angle is substantially 5 and has a diameter of about 0.lmm.

9. A process according to any of claims 1 to 4 wherein the electrode comprises a thin walled tube.

10. A process according to claim 9 wherein the tube has a sharpened or constricted end.

11. A process according to claim 9 or 10 wherein the tube carries the same or different gas from that shrouding the electrode.

12. A process according to any of claims 1 to 11 wherein the electrode includes a reservoir of low work function material which at the operating temperature is migratable to the electrode tip.

13. A process according to any of claims 1 to 12 wherein the non-oxidising gas is of one of the inert gases of Group O of the Periodic System of the Elements or a mixture of two or more such gases.

14. A process according to any of claims 1 to 13 wherein the electrode is of refractory material.

15. A process according to claim 14 wherein the electrode is tungsten, tantalum or rhenium, or carbon.

16. A process according to claim 9 and any of claims 10 to 15 when appendant to claim 9 wherein the electrode tube bore is 1-2 mm diameter with a wall thickness of 0.15-1.0 mm.

17. A process of heating a localised area of a glass element or glass ceramic material to a predetermined working temperature substantially as herein described with reference to Figs. 1 to 4 or Fig. 5 of the accompanying drawings.

18. An apparatus for carrying out the process of Claim 1, comprises a support for the glass element or element of glass ceramic material to be heated, means to pre-heat a localised area of each element to a temperature at which it is sufficiently electrically conducting, one or more nozzles each consisting of a shroud and an electrode disposed to lie in it by means of which an electric discharge can be initiated and maintained

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. rates up to 10 litres/min or more may be used. The preferred distance between the electrode tip and the surface of the glass section is about 10 mms but this may be increased to about 50 mm. By means of the invention the glass is heated with a lower applied voltage than when known flame electrodes are used, the control of the softened glass is limited to a narrower band of glass than heretofore, the joined, sealed or worked glass is subjected to substantially less injurious contamination by emission from the electrode and there is less distortion of the softened glass area than by the existing methods. The enhancement of electron-emission of the electrodes and thermionic emission into the gas stream enables a lower applied voltage to be used than in the known processes and apparatus, and enables a stable discharge to be established at low currents. This facilitates the establishment of the desired electrically heated zone in the glass with lower preheat temperatures at an early stage of seal-making than in known processes. Sporadic arc wander is minimised compared with known processes and surface markings on the glass are avoided. The glass sections may be of any glass for which the known processes are used, such as borosilicate glass or glass ceramic systems in which the resistive nature of the glass caues sufficient self-heating of the glass due to the passage of the current through it. WHAT WE CLAIM IS: -

1. A process of heating a localised area of a glass element or element of glass ceramic material to a predetermined working temperature, comprising preheating the localised area of the element by conventional heating means until it is sufficiently electrically conducting and, using electrodes disposed within a protective stream of nonoxidising gas which is screened from the ambient atmosphere, feeding an electric current through a highly ionised path within the gas stream into the glass in the localised area whereby the resistance of the glass as the current flows through it causes the temperature of the glass in the localised area to rise to the predetermined working temperature, the non-oxidising gas being compatible with the heated glass and the material of the electrodes at the working temperature of the glass.

7. A process according to claim 6 wherein the electrode tip is cone-shaped.

from the nozzle within the shroud so as to feed an electric current into the pre-heated localised area of the glass to cause the glass in said area to heat up to a predetermined temperature, and supply means to said shroud for non-oxidising gas to flow through the shroud thereby to isolate the electrode from the atmosphere.

19. An apparatus according to Claim 18 wherein at least two spaced electrodes are provided and the electric current is fed between them through an element to be heated.

20. An apparatus according to Claim 19 wherein the distance between the electrode or electrodes and the glass is variable.

21. An apparatus according to Claim 18, 19 or 20 wherein each electrode is disposed within a nozzle through which the gas stream flows and has its tip at substantially the outlet of the nozzle, the end of the electrode being shaped readily to initiate an electrical discharge within the gas stream and to promote a copious electron emission in the discharge in a restricted area close to the end of the tip.

22. An apparatus according to any of Claims 18 to 21 wherein the electrode is an elongated wire or strip, disposed within an elongated tubular nozzle extending substantially to the nozzle end while its inner end is rooted in the base of the nozzle.

23. An apparatus according to Claim 22 wherein the nozzle is of ceramic or other electrical insulating refractory material connected to a source of gas supply and having an internal collet, with passages for the gas flow, to which the root of the electrode is secured.

24. An apparatus according to Claim 22 wherein the nozzle is electrically conducting heat resistant material such as stainless steel or high nickel alloy which is electrically insulated from the electrode.

25. An apparatus according to any of Claims 18 to 24 wherein three or more electrodes are provided and a multiphase electric supply is connected to the electrodes so that the current path flows through the electrodes in succession.

26. An apparatus according to any of Claims 18 to 25 wherein the electrodes are energised by an alternating supply.

27. An apparatus according to Claim 26, wherein the alternating supply is in the range of 2-20 amps at a frequency of 50 Hz to 400 KHz.

28. An apparatus for carrying out the process of any of Claims 1 to 7 substantially as herein described with reference to Figs. 1 to 4 or Fig. 5 of the accompanying drawings.

29. An element which has been locally heated by the process of any of Claims 1-17 and with the apparatus according to any of Claims 18 to 28.