PRODUCTION OF CERAMIC FILAMENTS

Abstract: Apparatus for producing refractory material filaments comprises an enclosure (11), means for heating the constituents of the refractory material, non-contacting means to support a drop (17) of molten material such as silicon, being one of the constituents of the refractory material within the enclosure and means to transport a filament (10) such as carbon, being a second constituent of the refractory material, through the drop of molten material in use. A levitation coil and/or an inert gas jet is used to support the drop of molten material and a silica glass or other refractory tube (13) is used to direct the gas jet. The enclosure has opposed side arms (15, 16) respectively for entry and exit of the filament (10) or the filament may be passed vertically through the molten silicon drop. An inclined side tube may be provided through which grains of silicon may be introduced into the enclosure for addition to the molten drop. In one form, initial heating of the silicon prior to levitation is achieved by providing a resistively heated silicon nitride platform on which the silicon is melted, the platform then being moved from the field of the levitation coil to leave a levitated silicon drop.

Description: Production of Ceramic Filaments The invention relates to refractory fibres and in particular but not exclusively to the production of silicon carbide and silicon carbide coated carbon fibres. GB Patent Application No 8819760 describes a method of manufacture of refractory fibres of silicon carbide or silicon carbide coated carbon by passing fine carbon fibres through molten silicon. Molten silicon is contained within a crucible provided with one or more holes in its base through which fibre(s) can be drawn. The holes are dimensioned such that surface tension forces in the molten silicon prevent silicon from flowing out of the crucible. The object of the present invention is to provide apparatus and method for manufacture of refractory filaments, particularly silicon carbide, utilising molten material such as silicon without problems associated with crucible containment. The invention provides in one form apparatus for producing refractory material filaments comprising: an enclosure; means for heating the constituents of the refractory material; non-contacting means to support a drop of molten material, being one of the constituents of the refractory material, within the enclosure; and means to transport a filament made of material, being a second constituent of the refractory material, through the drop of molten material in use. In one arrangement the apparatus may be used with a carbon filament and molten silicon. The means to support or levitate the silicon drop may be a levitation coil and /or a gas jet. A silica glass or other refractory tube may be provided to direct the gas jet and the gas may be selected to provide an inert, protective atmosphere to prevent oxidation. Advantageously the enclosure includes opposed side arms which are respectively for entry and exit of the filament. Alternatively the filament may be passed vertically through the molten silicon drop. An inclined side tube may be provided through which grains of silicon may be introduced into the enclosure for addition to the molten drop. In one form, initial heating of the silicon prior to levitation may be achieved by providing a resistively heated silicon nitride platform on which the silicon can be melted, the platform being removable from the field of the levitation coil so as to leave a levitated silicon drop. Alternatively a laser may be provided to melt the silicon. The invention also provides in a second form a method of making silicon carbide filaments comprising the steps of: a) forming a molten drop of silicon; b) levitating the molten drop; and c) passing a carbon filament through the molten drop. The molten drop may be formed by: a) placing silicon on a platform; b) heating the platform; then c) removing the platform. In one form the platform is made of silicon nitride such that it is not wetted by molten silicon. The silicon can then be melted by resistively heating the platform or heating by means of a laser. Once the silicon is melted the means to levitate the molten silicon may be applied and the platform removed to leave the levitated drop of silicon. The levitation may be achieved by application of an electromagnetic field from a levitation coil or by forming an upward jet of supporting gas. Preferably the gas is selected such that oxidation of the molten silicon does not occur. The carbon filament ' may conveniently be passed horizontally through the molten drop. Preferably the molten drop of silicon is replenished by addition of solid silicon to the molten drop to thereby enable continuous manufacture of silicon carbide filaments. The invention will now be described with reference to the accompanying Figure which shows a carbon filament 10 being fed into a reaction enclosure 11. The reaction enclosure 11 is formed by a vertical silica tube 12, open at its lower end 13 and closed at its upper end 1- Two opposed lateral side tubes 15, 16 are provided through which the carbon filament 10 is fed from a take-off spool (not shown) to a take-up spool (not shown). As shown, a spherical drop 17 of molten silicon is levitated by means of an external levitation coil (not shown for clarity) such that the carbon fibre 10 passes through the molten silicon. An inert gas is introduced into the silica tube 13 to prevent oxidation of the heated silica such that the gas enters the open end 13 of the tube and flows out through the side tubes 15 and 16. Silicon will not normally levitate when cold because of its low room temperature electrical conductivity. To overcome this problem a silicon nitride platform can be provided to support solid silicon within the levitation coil and within the heated region in the silica tube 12. The silicon nitride platform is then heated resistively and once the silicon is red hot, the platform is slowly moved downwards through the tube 12 while the levitation coil is energised. Silicon nitride is a particularly suitable material for the platform as it is not wetted by the silicon. In use one or more carbon filaments are drawn through the molten silicon, the filament diameter and drawing speed determining whether: the carbon is coated with silicon; the carbon surface is converted to silicon carbide; or the carbon is completely converted to silicon carbide. The upward flow of gas through the reaction enclosure 11 assists in supporting the molten silicon drop 17 and it may be possible to dispense with the levitation coil and make use of the gas lift alone. In an alternative arrangement the side tubes 15 and 16 may be dispensed with and the filament may be drawn upwardly through the molten silicon drop and through a suitable orifice at the top of the silica tube 12. Although described in relation to silicon carbide filament manufacture the invention may be applied to any filamentary material which is to be coated or reacted with a further material which is capable of being levitated within a reaction enclosure. A further sloping side tube may be provided in the wall of the silica tube such that grains of silicon can be introduced therethrough so as to fall into the molten silicon drop to replenish lost silicon.

Claims: Claims 1. Apparatus for producing refractory material filaments 10 comprising: an enclosure 11 and means for heating the constituents of the refractory material; characterised in that there is provided: non-contacting means to support a drop 17 of molten material, being one of the constituents of the refractory material, within the enclosure; and means to transport a filament made of material, being a second constituent of the refractory material, through the drop of molten material in use. 2. Apparatus for producing refractory material filaments as claimed in claim 1 characterised in that the filament 10 is carbon and the molten material 17 is silicon. 3. Apparatus for producing refractory material filaments as claimed in aclaim 1 or 2 characterised in that the means to support or levitate the silicon drop 17 is a levitation coil and /or a gas jet (13). >*, . Apparatus for producing refractory material filaments as claimed in any one preceding claim characterised in that a silica glass tube 13 is provided to direct the gas jet and the gas is inert thereby providing a protective atmosphere to prevent oxidation, 5. Apparatus for producing refractory material filaments as claimed in any one preceding claim characterised in that the enclosure includes opposed side arms 15.16 which are respectively for entry and exit of the filament 10. 6. Apparatus for producing refractory material filaments as claimed in any one of claims 1 to -1 characterised in that the filament 10 is passed vertically through the molten silicon drop 17. 7. Apparatus for producing refractory material filaments as claimed in any one preceding claim characterised in that an inclined side tube is provided through which grains of silicon can be introduced into the enclosure for addition to the molten drop 17. 8. Apparatus for producing refractory material filaments as claimed in any one of claims 2 to 7 characterised in that there is provided a resistively heated silicon nitride platform on which the silicon can be melted prior to levitation, the platform being located in a position in the field of the levitation coil and removable therefrom so as to leave a levitated silicon drop 17. 9. Apparatus for producing refractory material filaments as claimed in any one of claims 1 to 7 characterised in that a laser is provided to melt the silicon prior to leviation. 10. A method of making silicon carbide filaments comprising the steps of: a) forming a molten drop 17 of silicon; b) levitating the molten drop; and c) passing a carbon filament 10 through the molten drop 17. 11. A method of making silicon carbide filaments as claimed in claim 10 characterised in that the molten drop 17 is formed by: a) placing silicon on a platform; b) heating the platform; then c) removing the platform. 12. A method of making silicon carbide filaments as claimed in claim 10 or 11 characterised in that the molten drop 17 is levitated by application of an electromagnetic field from a levitation coil. 13- A method of making silicon carbide filaments as claimed in any one of claims 10 to 12 characterised in that an upward jet of supporting gas is provided. 14. A method of making silicon carbide filaments as claimed in claim 13 characterised in that the gas is selected such that oxidation of the molten silicon 17 does not occur. 15- A method of making silicon carbide filaments as claimed in any one of claims 10 to 14 characterised in that the carbon filament 10 is passed horizontally through the molten drop 17• 16. A method of making silicon carbide filaments as claimed in any one of claims 10 to 15 characterised in that the molten drop 17 of silicon is replenished by addition of solid silicon to the molten drop to thereby enable continuous manufacture of silicon carbide filaments.

Inventors: HOCKING MICHAEL GWYN, SIDKY PAULETTE SHAFIK, HOCKING, MICHAEL, GWYN, SIDKY, PAULETTE, SHAFIK
Applicants: SECR DEFENCE BRIT, THE SECRETARY OF STATE FOR DEFENCE IN HER BRITANNI
Application year: 1991
Main classification: D01F11
IPC codes: C04B35, C04B41, D01F9, D01F11, D06M11, D06M101