WO2009090257A1

WO2009090257A1 - Heat treatment furnaces

Info

Publication number: WO2009090257A1
Application number: PCT/EP2009/050520
Authority: WO
Inventors: Ian George Sayce; Alan Mundy
Original assignee: Heraeus Quartz Uk Limited
Priority date: 2008-01-18
Filing date: 2009-01-16
Publication date: 2009-07-23
Also published as: GB0800895D0

Abstract

A heat treatment furnace for zone-sintering of silica bodies is described. The furnace comprises: a vacuum envelope; an internal space within the vacuum envelope to receive a silica body; and a plurality of heating elements exposed to the interior of the furnace. The heating elements are situated in proximity to the internal space and are axially separated from each other. The temperature of each of the heating elements is individually controllable, so that different selected portions of a silica body disposed within the internal space may be heated to different selected temperatures. Silica bodies of large diameter may be zone- sintered in the furnace, and optionally re-shaped by slumping, without the need for a muffle tube between the heating elements and the silica body.

Description

HEAT TREATMENT FURNACES

Technical Field

This invention relates to the heat treatment of silica bodies and to furnaces for use in such heat treatment.

Background Art

A commonly used method of manufacturing high purity synthetic vitreous silica involves vapour-phase oxidation of a silicon-containing precursor in a flame, and the deposition of silica to form a porous silica soot body. Porous bodies of vitreous silica may alternatively be made by sol-gel, by slip-casting or by other techniques. Such a porous silica body may then be heated in an appropriate gas atmosphere (or vacuum) to effect dehydration and then heated at higher temperature when the body may be sintered to transparent silica glass.

These heat treatment processes may be undertaken within a single furnace, and there are many examples of furnaces adapted for this purpose, generally intended for the manufacture of optical fibre preforms. These typically employ heat treatment in an atmosphere containing chlorine, optionally diluted in helium, in order to effect dehydration, before zone sintering in helium to give a transparent pore-free body. These methods are not optimal for large soot bodies as required for example to make glass ingots suitable for the manufacture of jigs for semiconductor manufacture. The use of chlorine as dehydrating agent is costly and hazardous. The use of helium contributes greatly to the cost, and the use of a conventional zone-sintering furnace, in which the silica body is lowered through the hot zone, demands a furnace facility of overall height at least three times the length of the preform being treated.

The inventors have found that vacuum dehydration and vacuum sintering provide a satisfactory solution to the problems associated with the use of chlorine or helium, and have used a form of vacuum zone-sintering furnace as described in US 5713979, which is well suited to the treatment of bodies of intermediate dimensions. For heating during vacuum dehydration, it employs an inductively heated graphite susceptor which extends the full length of the body being treated, and vacuum dehydration is followed by zone-sintering, where the body is lowered through a second inductively heated graphite susceptor, which is of reduced length, and operates at higher temperature.

While such a construction has proved satisfactory in the past for heat-treating porous silica bodies of limited diameter, for larger soot bodies the construction is less practical. It necessitates a vacuum envelope made from an electrically insulating material, and requires expensive cylindrical susceptors of large diameter. Additionally, conventional zone-sintering would require facilities for lowering the body through the hot zone, and a longer vacuum chamber than might appear desirable. The requirement for vertical movement of the body through a hot zone is not however essential. It would be possible to effect a form of zone sintering by using a series of susceptors arranged vertically along the furnace, and heated sequentially but, using inductive heating, this would be a relatively expensive option.

There are a number of examples of furnaces for treating silica soot bodies, which avoid vertical movement of the body relative to the furnace. In some of these the body is dehydrated and sintered in a substantially uniform thermal environment, and in some there is the option progressively to raise or lower a localised zone of higher temperature by sequential heating of sections of the furnace. In some of these examples the process can be achieved under reduced pressure. These examples generally employ heating elements which are external to a muffle tube (or work tube) which surrounds the silica body being treated. The more relevant of these will now be described.

JP 63-201025 describes vacuum sintering of a soot preform for an optical fibre where the entire body is sintered simultaneously within an electrically heated furnace with a muffle tube of fused quartz or high purity graphite. Zone- sintering may be preferable, and JP 01-224236 describes a furnace with four hot zones which may be heated independently so that zone-sintering can be effected by raising the temperature of each hot zone successively to cause a progressively advancing melt front to move up the preform, contained within a muffle tube, starting at the bottom.

The furnaces used in this field generally have heating elements made from graphite, but JP 03-223133 describes a furnace for whole-body heating while dehydrating, where the elements are made from silicon carbide or molybdenum disilicide. Silicon carbide may be used to coat the inner surface of a graphite muffle tube of a vacuum furnace used for whole-body heat treatments, as in

US 5513983, and gas may be provided at low pressure to facilitate dehydration, while such flow may be stopped during sintering to permit operation at further reduced pressure (EP 0547560).

Whole body heat treatment is again proposed in US 5693115, which describes a vacuum furnace with three independently controlled hot zones provided by heaters which are external to a graphite muffle tube which envelopes the silica body. The heat treatment occurs in three stages. The body is heated for a prolonged period at a first temperature for dehydration, and then for a shorter time at a higher temperature permitting some shrinkage of the body, before finally being heated briefly at still higher temperature to sinter the porous soot to glass. During stage 2 the hot zone temperatures are adjusted so that the lowest zone is at higher temperature than the middle zone, which is again at higher temperature than the upper zone. This means that when the body is eventually heated to sintering temperature, the lower regions sinter first, and again a form of progressive zone sintering is brought about.

Tighter control of the progressive sintering process appears possible using the furnace described in JP 2004-115330. This comprises five (or more) graphite ring heaters which heat a muffle tube, on the axis of which a soot body is suspended, and optionally rotated. The soot body may be dehydrated by heating the entire body in an atmosphere of helium mixed with chlorine, and then zone sintered in an atmosphere of helium by locally heating to consolidation temperature with one or more heaters, and progressively advancing the hot zone by sequential adjustment of the heaters. - A -

JP 2004-292195 describes a similar furnace, in which external graphite heaters heat a muffle tube, and it appears that a synthetic silica soot body may be dehydrated by heating along the full length in an atmosphere of chlorine mixed with helium, and that zone sintering is possible by appropriate sequential adjustment of the element temperatures while holding the soot body in an atmosphere of helium.

The above furnaces have been developed for the manufacture of optical fibre preforms, where the required body is of relatively small diameter, compared to its length, i.e. of high aspect ratio. But there is a requirement for an inexpensive facility of compact design, capable of permitting dehydration and sintering of soot bodies of large diameter, e.g. greater than 500 mm diameter, and smaller aspect ratio.

The product glass ingots are in some cases solid bodies, but there is also a need to manufacture hollow bodies, for example hollow cylinders, as may be generated by sintering a hollow cylindrical soot body. For this purpose it is normal to sinter such hollow soot body while supported on a removable mandrel, optionally a substantially non-reactive material such as graphite which may be removed after sintering, or alternatively of vitreous silica, which may be subsequently removed by machining.

For some applications there is a further requirement, whereby the silica body may be caused to slump (reflow) to modify the diameter or shape. Typically such slumping can employ a mould, made from a non-reactive material e.g. graphite, which may be round, square, or of some alternative shape, into which the vitreous silica may be allowed to flow. Such flow may be facilitated by the application of pressure to the upper surface of the body, when it is possible to reshape the body at somewhat lower temperature than when flow is induced by gravity alone. This slumping can be effected during the sintering of the porous soot body, on completion of the sintering, or may be undertaken on a vitreous silica ingot made in a separate operation, and loaded into the furnace for the purpose of reshaping to the required dimensions. Such a process may be undertaken under reduced gas pressure, or alternatively under higher pressure, e.g. near-ambient pressure, in a substantially inert atmosphere e.g. argon or nitrogen.

Summary of Invention

The inventors have now found that dehydration and sintering operations of the type described above may be effected using a vacuum furnace and, by employing multiple heating elements exposed to the interior of the furnace, it is possible to effect zone-sintering without the use of a muffle tube, and without the need for vertical movement of the soot body during the sintering operation.

It has also be found that such a furnace may be used additionally or alternatively to effect the slumping of a vitreous silica body into a mould to permit reshaping to alternative cross-sectional shape, either under vacuum or under an atmosphere of inert gas e.g. argon or nitrogen.

Furthermore, considerable economy is possible if the heating elements take the form of arrays of resistively heated rods, plates or hairpins disposed around the central axis of the furnace. Graphite elements have proved satisfactory, but alternative materials are possible, e.g. carbon fibre reinforced carbon (CFRC), molybdenum disilicide, silicon carbide, etc. The elements in each array may be arranged in the form of a square, a hexagon or other polygonal form.

The invention accordingly provides, in one aspect, apparatus for heat treatment of a silica body, the apparatus comprising: a vacuum envelope; an internal space within the vacuum envelope to receive a silica body; and a plurality of heating elements exposed to the interior of the furnace, said heating elements being situated in proximity to said internal space and axially separated from each other; the temperature of each of the heating elements being individually controllable so that different selected portions of a silica body disposed within the internal space may be heated to different selected temperatures. Furnaces according to the invention may be used in a number of different manners. In one application, a furnace according to the invention may be employed to effect zone sintering of an elongate silica body, without the need for axial movement of the body within the furnace. The body to be heated may, for example, be either a solid body (e.g. of deposited silica soot, ready for consolidation) or a hollow body (such as a tubular body of silica soot deposited on a mandrel, which may be removable). In another application, a furnace according to the invention may be used to establish fine control of the heating of different portions of a silica body (e.g. an ingot derived from a soot body which has already been subjected to consolidation) to effect slumping of the body into a suitable mould. The body undergoing slumping may, as desirable, be placed free-standing in the mould, or it may be suspended on a shaft (which may optionally be rotatable), or it may be mounted on an axially moveable shaft and actively pressed into the mould.

The invention accordingly provides, in a further general aspect, a method of heat treatment of a silica body in a furnace as described above.

In a more specific alternative aspect, the invention provides a method of zone heat treatment of an elongate silica body, comprising the steps of: placing the silica body in an elongate internal space of a vacuum furnace, defined by a plurality of individually controllable axially spaced heating elements exposed to the interior of the furnace; and raising the temperature of successive heating elements along the length of the silica body so as to heat successive portions of said silica body without the need for axial movement thereof.

In a further alternative aspect, the invention provides a method of slumping a silica body, comprising the steps of: placing the silica body in an internal space of a vacuum furnace, defined by a plurality of individually controllable axially spaced heating elements exposed to the interior of the furnace, the silica body being held in or suspended above a mould; and raising the temperature of successive heating elements along the length of the silica body so as to bring successive portions of said silica body to a sufficiently high temperature to permit flow of silica, whereby said body may be slumped into said mould in a controlled fashion.

Brief Description of Drawings The invention is hereinafter described in more detail by way of example only, with reference to the accompanying drawings, in which:

Fig. 1 is a schematic vertical section of an embodiment of furnace according to the invention, with a silica body inserted for heat treatment.

Figs. 2, 3a and 3b are graphical representations of the relative amounts of power supplied to each of the heating elements of the Fig. 1 apparatus in various successive stages of an illustrative heat treatment process.

Fig. 4 is a schematic vertical section of a further embodiment of furnace according to the invention, with a silica body in process of sintering.

Fig. 5 is a schematic vertical section of a further embodiment of furnace according to the invention, with a hollow silica body supported on a graphite mandrel in process of sintering.

Fig. 6 is a schematic vertical section of a further embodiment of furnace according to the invention, with a silica body in process of slumping.

Fig. 7 is a schematic vertical section of a further embodiment of furnace according to the invention, with a silica body suspended and rotated in process of slumping with homogenisation (mixing).

Description of Preferred Embodiments The furnace depicted schematically in Fig. 1 comprises an evacuable chamber 21, having removable end closures 22 and 23. The chamber is lined with appropriate insulation 24, e.g. graphite felt, and within the chamber is an array of heating elements 25. For the purpose of illustration, the diagram shows 10 such elements, with indication 26 of individual element number, although the precise number of heating elements may be varied as desired. Each element represents an independently heated zone, optionally a ring element, but preferably a set of four or more individual units in a polygonal array, made for example from resistively-heated rods, plates or hairpins made from graphite or CFRC.

Within the furnace is the soot body 27, supported on shaft 28, so that it may be rotated during the heat treatment process. A gas-tight seal is provided by vacuum-seal 29, permitting rotation and optional vertical displacement of shaft

28.

A typical treatment cycle proceeds as follows. A soot body, supported on shaft 28, is mounted within the furnace chamber, the furnace is evacuated, back-filled with nitrogen, and evacuated once more. The furnace is then heated to a temperature typically in the range 1100 - 1350 ⁰C to permit dehydration of the body. The temperature may be monitored using an optical pyrometer to view the soot body, or a target placed on the wall of the furnace, but conveniently operating conditions may alternatively be established by monitoring the power input into each of the elements in the array. Fig. 2 shows the power distribution to each element from top (element 1) to bottom (element 10) during the dehydration process, which may last for example for a period of 10 to 24 hours or more, depending on the desired residual OH content of the product glass.

On completion of the dehydration cycle, the power to the lowest elements is increased in the manner shown in Fig. 3a, Stage 1. By appropriate adjustment of the input power to each element, the hot zone is then progressively raised so that it traverses the entire soot body, at a rate which permits effective de-gassing or the porous soot body, and sintering to transparent pore-free glass. A total of six successive stages are shown in Figs. 3a and 3b. Local temperature within the moving hot zone is typically in the region 1500 to 1600 ⁰C, but it has been found convenient to control the process by monitoring the power distribution to the elements, and adjusting the duration of each stage, i.e. the rate of advance of the moving hot zone. The advance of the hot zone may be in steps, as suggested by the successive stages shown in Figs. 3a and 3b, but advantageously the changes in power distribution between the stages may be effected smoothly and in minor increments under computer control, to ensure a steady and progressive advance of the fusion front within the silica soot body.

Fig. 4 shows schematically the situation near the mid-point of the sintering operation, as it might appear near to Stage 4 in Fig. 3 b, when the energy input to the elements is a maximum at element numbers 4 and 5. The lower region of ingot 27 is transparent, and substantially pore-free, while the upper region has been subjected to dehydration, but is still comprised of porous material. The advancing melt front is at the interface between these two regions.

The top end of the soot body is sintered during Stage 5 (Fig. 3b) and, on completion, the power to the elements is reduced, and the fully sintered synthetic silica body is allowed to cool under control, with a cooling rate which is adjusted according to the body temperature, to ensure adequate stress relief, e.g. a reduced cooling rate may employed over the temperature range 1100 to 650 ⁰C. Cooling rate may be enhanced over parts of the cooling cycle to increase productivity. Any conventional method may be used to enhance the cooling rate, for example recirculation of inert gas (e.g. nitrogen) which may be cooled externally using a heat exchanger.

In a further embodiment of the furnace of the invention, it may be used for the dehydration and sintering of a hollow cylindrical porous silica soot body, supported on a removable mandrel made for example from graphite. Fig. 5 illustrates schematically the appearance of the ingot when partially sintered. On completion of the sintering operation, and after cooling, the graphite mandrel, 30, may be removed, and may be re-used. The hollow ingot of synthetic silica may then be machined internally and externally to the required dimensions and converted to the desired products, for example annular components such as semiconductor jigs. The moving hot zone in the present furnace enables heat treatment of large porous silica bodies (e.g. soot diameter in the range 500-1000mm, and length up to 3000mm) in a furnace of minimal vertical dimensions. The use of vacuum during dehydration and sintering avoids the need for expensive or toxic gases (helium and chlorine). The element design is economical to build at large effective diameter (unlike elements machined from solid graphite).

As noted above, a furnace of the invention may alternatively be used for slumping or reflow of a silica ingot to provide a product ingot of larger cross- section, or alternative shape. Such a process is illustrated schematically in Fig. 6, where an ingot of vitreous silica 31 is shown descending under the influence of gravity, and partially slumped into crucible or mould, 32, which is made from graphite, or other suitably refractory material. In this application the upper elements are operated at reduced power (if at all), but the lower elements e.g. elements numbers 7 to 10 are operated at increased power, in order to raise the ingot and crucible 32 to sufficiently high temperature to permit flow of the glass, typically in the region of 1700 - 1750⁰C. For this purpose it is advantageous to incorporate an additional heating element or element array, 11, under the base of the crucible. This feature, coupled with independent control of the heating elements in the furnace wall (e.g. elements 7 to 10) ensures controlled melting and flow of the ingot into the crucible, and vertical descent into the molten mass. This in turn ensures that the upper regions of the ingot being treated do not soften prematurely, which would risk toppling of the ingot and the introduction of wrinkles leading to defects in the surface of the product ingot.

In an alternative mode of operation, the ingot being slumped may be suspended from shaft 28. Under these circumstances it is possible to avoid this wrinkling of the ingot surface, and to allow natural flow of silica glass into the crucible below. When operating in this mode it is possible to operate at higher temperatures in the critical region of the furnace, when it is additionally possible to rotate the ingot during slumping. This leads to advantageous mixing and homogenisation of the glass in the product ingot. Such a process is illustrated schematically in Fig. 7, where extensive mixing occurs in the constricted zone 33, due to rotation of the ingot above, and further mixing and diffusion of any impurities occurs in the course of radial outward flow of glass beneath this constriction, as the glass fills the mould. The mixing process may be further enhanced by counter- rotation of the crucible, but this significantly adds to the cost and complexity of the furnace.

In a further embodiment of the furnace when used for re-shaping an ingot, it is possible forcibly to lower the ingot while attached to shaft 28. In this way it is possible to apply pressure to the upper surface of the ingot being treated. Under these circumstances it is possible to slump or re-shape the ingot at reduced temperature (e.g. 1600 - 1700⁰C). At such reduced temperature, reaction of silica with the container material is reduced, and the resultant superficial contamination of the glass (for example from mobile impurities arising from the material of the mould) may be minimised. Also it is possible to achieve a better shape of product, in that the glass may thus be forced into the corners of the mould, and the product ingot better fits the required dimensions.

Industrial Applicability

Applications of furnaces according to the invention thus include:

• Dehydration and sintering of a solid body made from porous silica.

• Dehydration and sintering of a hollow porous silica body supported on a removable mandrel, which may be made from graphite or CFRC.

• Dehydration and sintering of a hollow porous silica body supported on a vitreous silica mandrel which is subsequently removed by drilling.

• Slumping of a solid cylindrical ingot of silica glass into a mould - under gravity.

• Slumping of a solid cylindrical ingot of silica glass into a mould - while under pressure from shaft 8. • Slumping of a solid cylindrical ingot of silica glass into a mould - where the ingot is suspended from the movable shaft, and rotated so as to achieve homogenisation of the glass.

Surprisingly, these processes may be achieved in a furnace according to the invention which employs no muffle tube, yet substantially maintains the very high purity levels of the deposited synthetic silica soot (typically less than 10 ppB of any contaminant metal ions). Such performance is maintained in a versatile furnace of economical construction, in which replacement of the elements is relatively inexpensive when compared to prior art designs which require heating elements, susceptors or muffle tubes, machined from large diameter solid graphite cylinders. The performance also compares favourably with that of comparable smaller scale furnaces which conventionally employ a muffle tube to separate heating elements from the work zone. Other advantages include the ability to effect zone-sintering without vertical movement of the body, and control is aided by the tightly defined hot zone possible with the present design, whereby, in addition to pyrometric measurement of temperature, local control of hot-zone temperature profile may be effected by monitoring and controlling the power fed to each element in the array.

The benefits arising from the present design also extend to its versatility. In addition to being readily adaptable to a multiplicity of applications, by adjustment of the dimensions of the polygonal heater arrays it may readily be adapted and optimised for the treatment of silica bodies of alternative diameters.

Claims

1. Apparatus comprising a vacuum zone-sintering furnace having multiple individually controllable heating elements exposed to the interior of the furnace, whereby a porous silica body may be zone-sintered without the need for vertical movement of the body, characterised in that the furnace lacks a muffle tube between the heating elements and the space to be occupied by the silica body.

2. Apparatus for heat treatment of a silica body, the apparatus comprising: a vacuum envelope; an internal space within the vacuum envelope to receive a silica body; and a plurality of heating elements exposed to the interior of the furnace, said heating elements being situated in proximity to said internal space and axially separated from each other; the temperature of each of the heating elements being individually controllable so that different selected portions of a silica body disposed within the internal space may be heated to different selected temperatures.

3. Apparatus according to claim 2, lacking a muffle tube between said heating elements and said internal space.

4. Apparatus according to any preceding claim, wherein the heaters comprise an array of resistively heated rods, plates or hairpins disposed around the central axis of the furnace.

5. Apparatus according to any preceding claim, wherein the heating elements are composed of a material selected from the group consisting of graphite, carbon fibre reinforced carbon, molybdenum disilicide and silicon carbide.

6. Apparatus according to any preceding claim, wherein the heating elements are arranged in the form of a square, a hexagon or other polygonal form.

7. Apparatus according to any preceding claim, further comprising a mould for re-shaping the silica body.

8. A method of heat treating a silica body, comprising the steps of: placing the silica body in apparatus according to any preceding claim; and heating one or more of the heating elements.

9. A method of zone heat treatment of an elongate silica body, comprising the steps of: placing the silica body in an elongate internal space of a vacuum furnace, defined by a plurality of individually controllable axially spaced heating elements exposed to the interior of the furnace; and raising the temperature of successive heating elements along the length of the silica body so as to heat successive portions of said silica body without the need for axial movement thereof.

10. A method according to claim 8 or claim 9, wherein the apparatus comprises a mould for re-shaping the silica body and the method further comprising the step of remoulding the silica body in said mould.

11. A method of slumping a silica body, comprising the steps of: placing the silica body in an internal space of a vacuum furnace, defined by a plurality of individually controllable axially spaced heating elements exposed to the interior of the furnace, the silica body being held in or suspended above a mould; and raising the temperature of successive heating elements along the length of the silica body so as to bring successive portions of said silica body to a sufficiently high temperature to permit flow of silica, whereby said body may be slumped into said mould in a controlled fashion.

12. A method according to any of claims 8 to 11, wherein the apparatus comprises a shaft upon which the silica body may be mounted and the method further comprises the step of mounting the silica body on the shaft prior to heating the heating elements.

13. A method according to claim 12, wherein the shaft is rotatable and the method further comprises the step of rotating the shaft during heating of the silica body.

14. A method according to claim 13, as appendant to claim 10 or claim 11, whereby rotation of the ingot relative to the mould results in mixing or homogenisation of the product glass.

15. A method according to any of claims 12 to 14, as appendant to claim 10 or claim 11, wherein the shaft is axially moveable and the method further comprises the step of actively pressing the silica body into the mould.

16. A method according to any of claims 8 to 15, wherein the heat treatment is carried out under an atmosphere of nitrogen, argon or another inert gas.