CONTACTLESS HEATING OF THIN FILAMENTS

Abstract: A contactless heating apparatus for thin filaments comprises means to linearly support a length of filament (10), an induction coil (11) provided adjacent to the filament with its axis substantially parallel to the length of filament, a source (15) of alternating current in the frequency range from HF to UHF connected to the coil, and a conducting rod (16) connected to one of the end turns of the induction coil (11) and disposed substantially parallel to the length of filament (11); the arrangement being such that heating of the filament occurs adjacent to the conducting rod. The filament (20) is run through a vertical cylindrical chamber (21) in which a reducing atmosphere is maintained; the filament (20) and the coil (11) being located in an induction furnace. The coil (11) is non-uniform in diameter, the diameter being larger at the end (14) to which the rod (16) is attached and the end (12) of the coil opposite to that to which the rod is attached is earthed (13). The rod (16) may be shaped such that the separation between rod and filament varies along the length of the rod to thereby vary the temperature profile along the heated portion of the filament. The AC power source (15) used is a Class C oscillator valve located in the induction furnace. This power source is rich in harmonics.

Description: Contactless Heating of Thin Filaments The invention relates to the heating of thin filaments and in particular to the heating of electrically conductive thin filaments for example for chemical vapour deposition or zone refining to produce improved characteristics. In the area of chemical vapour deposition (CVD) , high strength refractory filaments can be formed by coating fine refractory filaments with high strength materials such as boron and silicon carbide. Such composite filamentshaveahigh strength toweight ratioand areuseful as reinforcement materials for plastics/ metals and ceramics. The CVD method commonly used relies on heating the filament substrate to a temperature sufficiently high to cause reaction and deposition from raw materials in the vapour phase. Aproblem encountered in resistance heating of the filament substrate is inmaintaining good electrical contactwith the filament to prevent variations in resistance during the electrical heating process. While short lengths of filament could be fixed between carbon electrodes/ it is extremely difficult tomaintaingood electrical connection to a moving filament. Difficulties have been found when using brass/ carbon or other materials as electrical contacts for moving tungsten filaments when silicon carbide deposits/ for example/ begin tobuild up. This results partly from the lack of conductivity of the silicon carbide deposit. Mercury has been used by several previous investigators but difficulties due to build up of deposits remain and there is the added difficulty ofworkingwith a substancehaving a toxic vapour, use of mercury contacts also has the disadvantage that it leads to contamination of the filament by the mercury. The filament substrate is normally fed fromone reel to another and attempts tomaintain electrical connectionsat the feed reels rather thanbetween twopoints on the mςving filament has resulted in temperature fluctuations due to the changes which occur in filament resistance. Induction heating using for example copper coils in an induction furnace cannotbeusedtoheat fine filaments/ according toaccepted theory/ since the skin effect limits the heated volume to a thin layer around the periphery of the wire. The thickness of this layer is inversely dependent on the frequency of the induction furnace and is thicker than the filament diameter for60 microndiameter filaments forhighfrequency (HF) inductionfurnaces; thismeans that no net current can flow circumferentially around thewireto heat it up. As examples/ 450 kHz/ 13.5 MHz or 430 MHz induction furnaces wouldhaveskindepthsofabout0.05 cm/ 0.01cmor0.0016cmrespectively/ for tungsten wire at 1200OC. It is thus necessary for thewire diameter to be considerably greater than the skin depth for successful heating. This argument eliminates the possibility of using the first two frequencies to heat a60 micron tungsten filament. Afurthermajor problemis thatthegap between thefilament surface and thecopper induction coilmustbe small for efficientheatingbut ofcourse thisgapisverylargebetweenathinfilament and even the smallest diameter water-cooled copper coil. Moreover/ in the case of chemical vapour deposition a reactionchamber/ inwhich the reactant vapour phase is enclosed/ necessitates a large gap between the filament and the copper coil. Thus this problem also eliminates use of the 430 MHz UHF frequency for heating a tungsten filament of 60 micron diameter. Aplurality of coaxial tuned induction circuits spaced along thelengthof a wire or filament havebeen described byDeBolt inUS patentNo3754112 andby Douglas et al in US patent No 3811940. Such arrangements are complicated/ requiring individual induction circuits to be tuned - usually quarter wave tuned/ and rely on the interaction of electric fields produced thereby to cause a heating current to flow. Theobjectofthepresentinventionis toprovideacontactlessheatingsystem for thin filaments which alleviates the above-mentioned difficulties. Applications of such an inventionare extremelybroad and cover aspectrumof fields ranging from CVD to ceramic sintering and heating of filamentary conducting or semiconducting extrudates. The invention provides a contactless heating apparatus for thin filaments comprising: means to linearly support a length of filament; an inductioncoilprovidedadjacent tothefilamentwith itsaxisparallelto the length of filament; a source of HF/ VHF or UHF alternating current connected to the coil; and a conducting rod connected to the one non-earthed end of the induction coil and disposed substantially parallel to the length of filament; thearrangementbeingsuchthatheatingofthefilamentoccursadjacenttothe conducting rod. In the invention/ as distinct fromprior art arrangements/ the filament tobe heated is placed outside the coil and the attached rod/ preferably copper as withthecoil/ ispositionedat asuitabledistanceaway/ typicallyabout2cm/ and the length of the rod determines the length of the heating zone. Typically the rod is positioned parallel to the filament to provide uniform heating (at red hot temperatures and above) in theheating zone. Heating by this method was unexpected and the precise mechanism by which energy is transferred from the coil to the filament is not understood. The rod is preferably connected to the end turn but may be connected to an adjacent turn near the end of the coil. Preferably the coil is non-uniform in diameter/ the diameter being larger at the end to which the rod is attached. Advantageously by earthing the lower end of the filament at a point remote from the coil region thepower required to heat the filament is reduced and a longer heated region is possible. Advantageously the coil is the output coil of an induction furnaceand this is tuned and the AC voltage raised until the required power is reached whereby firing occurs ie thewire is heated. Once this value is known the furnace can be switched off and on for almost instant heating. Temperatures above red heat can be varied by changing the power. At the low temperature range/ heating is of a vibrating elongated beadlike nature. Reducing the temperature further leads to extinguishing. On the other hand/ raising the temperature producesmore uniformheating. Oxidation or coatingof thewire also leads tomore uniformheating. Byslightlychanging thepositionof the rod or altering its shape controlledvariations in temperaturealong the thin filament (red hot or hotter) can also be achieved. Materials other than copper can be used for the rod. In addition it can be in the form of a hollow tube. Typically the coil is made from 0.63 cm diameter copper tube/ with 9 turns of inside diameter 3.5 cm increasing to 5.2 cm at the non-earthed end (not critical)/ the coil lengthbeing8 cm. In one advantageous arrangement the AC power source is a Class C oscillator valve in the induction furnace. This oscillator is rich in harmonics which could contribute to the observed heating effect. More than one conducting rod connected to a coil may be disposed to heat several filaments. The inventionwill now be described byway of example onlywith reference to the accompanying Drawing of which: Figure 1 illustrates the principle of the invention; Figure 2 is a part side elevation of an arrangement of the invention; and Figure3 shows theoutput stageofaninductionfurnaceprovidingpowertothe coil of the invention. As shown in Figure 1 a filament 10/ of tungsten for exampler to be heated is su portedvertically. Acopper induction coil 11 isplaced adjacent to the filament 10 with its axis substantiallyvertical. The coil 11 is arranged such that its top three turns are of increasinglylarger diameter andwider spaced/ although this has been foundnot tobecritical. Thesmall diameter end12 isconnected toearth13whilethelargediameterend14 isconnectedto a resonating oscillator15 operated in the frequency range fromHFtoUHF. A copper rod16 is attachedby a clamp17 to the end turn of thelargediameter end 14 of the coil. The copper rod 16 has a straight portion AB disposed parallel to the filament 10 and a curved portion BCwhich is engaged by the clamp17. The straight lengthAB of the copper roddeterminesthelengthof f lament18 (heating zone) which is heatedwhen resonantACpower is applied tothecoil11. Typicallythecopper rod16 ispositionedabout2anfromthe filament 10. Figure2 shewsapartview of apractical arrangement inwhichathinfilament 20 can be fed through a cylindrical tube 21 containing a reducing gaseous mixture of argon and hydrogen at atmospheric pressure. Where chemical vapour deposition is to be adopted the appropriate gases are circulated through the cylindrical tube 21. Inthis arrangement copper tubing is used both for the coil 11 and the straight portion 16. The filament 20 is fed upwardlythroughthecylindricaltube21fromaspoolandthetreatedfilament is collectedby awooden take-upspoolpositionedabovethetube. Wherethe filamententers and leaves thetube21 gassealsarepreferablyused. These gas seals contain the reaction gases within the tube 21 while permitting frictionlessmovementofthefilamentandpreventingthe ingressofexternal gases andvapours into the tube. Advantageously/ the take-off spool maybe earthed/ but the take-up spool is made of wood (or other non-conductor) to avoid connection or capacitance to earth which could cause unwanted RF heatingofthewireas it emerges intotheairbetweentheglassapparatusand the take-up spool. Figure 3 shows a conventional output stage of a Class Coscillator induction furnace used to provide the RF output at terminals 31/32 to the heating coil 11. An HT source is connected totheanode of aYD1162 outputvalve33 and a 15 - 45 pF variable tuning capacitor 34 is provided. For40 and60microndiametertungstenwiresthepowerrequired isoftheorder of 500 Watts applied to the plate of a Class C oscillator valve in the induction furnace. The Class C oscillator is rich in harmonics and it is thought that theseharmonicsmaycontributetotheheating effect. Once the furnace is tuned and thepowerneededtofirethefilamenthasbeendetermined then ithasbeendiscoveredthatthefiringofthefilamentcanbeswitchedoff and on almost instantaneously by switching the power source. A minimum threshold exists below which firing will not occur. The induction furnace gives best results when slightly off-tune. Certain conditions produce a elongated beadlike heating effect which can be minimised by detuning. Larger diameter wires require higher powers to heat them. Heating is believed to start at about 800oc upwards/ with no upper limit. Below about 900oc the heating is slightly elongated beadlike and below about 800°C it extinguishes. Althoughnot shown/for radiationsafety/theapparatusshould be located in an earthed fine mesh screen cabinet. The power source may be operated over a frequency range fromHF to UHF. In addition to heating conducting filaments of tungsten/ for example to coat themwith silicon carbide/ the invention can alsobe used toheat composite filaments composed of a conducting inner core and an outer semiconducting coating. The rod or tube attached to the coil can be shaped to give a predetermined temperature profile along the filament/ different fran the uniform temperature achieved with rod and filament parallel. The filament has been found to heat significantly only opposite the rod and thus the heating zone is well defined. In order to tune the induction furnace it is necessary to have at least a few turns of the coil for resonance. Although only a singleheating rod has been shewn it maybe desirable to usemore than one spaced around the filament. In additiontherodmaybeattachedtoaturn of the coil adjacent to the end turn although this is not the preferred arrangement. Experimentstodatehavedemonstrated thattheinventionworks with tungsten wires diameters of 40 to 500 microns. When carbon fibreyarn was heated by this method/ the many very fine fibre ends/ of about 5 micron diameter which protrude profusely from such yarn/ were found to become readilywhitehot at alowpower settingatwhich thebulk oftheyarndidnot reach red heat. This shows that themethod is alsoveryefficientlyableto heat fine threads of very small diameter such as 5 microns. Mostoftheworkusingtheinventionhasbeendonewitha6kWinductionfurnace working at 13.56 MHz (an industrially approved frequency) with a YD 1162 output valve. When an induction furnace operating at a frequency of about 1/2MHzwasused/ belowthelowerlimitoftheHFbandheatingoffilamentsdid not occur. The rf power supplyto the coil and attached rod is providedvia condusting rods and these could be coupled to any point in the rf output circuit. Low gas pressures are not required since the invention works at atmosphericpressure. Theinventionhasalsobeenshowntoworkatpressures other than atmospheric and also in a vacuum. Although the invention has been described principally in relation to the heating of tunsten wires the invention has also been demonstrated with a 5 micron diameter silverplatedplatinumwire. Brassor aluminiummaybeused in place of copper for the coil and rod. In addition the circular rodmaybe replaced by a strip or sheet. The strip or sheet may be straight or/ for example/ helically or otherwise wrapped around the cylindrical containing vessel or the space throughwhich thewire or filamentpasses. The stripor sheet couldbe arranged so as towrap substantiallyorcompletelyaroundthe filament. This last arrangementhowever isnotsoconvenient sinceit isnot possible to observe the filament. Afurther possible application ofthepresent invention is thedepositionof insulating coatings on conductingwires. One examplewould be to coat iron with alumina so as to render thewire inert and suitable as a reinforcement material. Although the invention has been described using a continuous rf source it would be possible to use a pulsed rf sourse. Other modifications and applications of the invention will be apparent to those skilled in the art.

Claims: Claims 1. Acontactlessheating apparatusfor thinfilaments10 comprisingmeansto linearly support a length of filament characterised in that an induction coil 11 provided adjacent to the filamentwith itsaxisparallel to the length of filament; a source 15 of alternating current in the frequency range fran HF to UHF connected to the coil; and a conducting inductor element 16 connected to the one non-earthed end 14 of the induction coil and disposed substantially parallel to the length of filament; thearrangementbeingsuchthatheatingof thefilamentoccursadjacenttothe conducting inductor element. 2. A contactless heating apparatus for thin filaments as claimed in claim1 characterised in that the element 16 is connected to the end turn of the induction coil. 3. A contactless heating apparatus for thin filaments as claimed in claim1 or 2 characterised in that the coil 11 is non-uniform in diameter/ the diameter being larger at the end 14 to which the inductor element 16 is attached. 4. A contactless heating apparatus for thin filaments as claimed in one of claims 1 to 3 characterised in that the end 12 of the coil opposite to that to which the inductor element is attached is earthed. 5. A contactless heating apparatus for thin filaments as claimed in any one preceding claim characterised in that the filament 10 and the coil 11 are located in an induction furnace. 6. A contactless heating apparatus for thin filaments as claimed in any one preceding claim characterised in that the inductor element11 is formed such that the separation between inductor element and filament varies along the length of the inductor element to therebyvary the temperatureprofile along the heated portion of the filament. 7. A contactless heating apparatus for thin filaments as claimed in any one preceding claim characterised in that the inductor element 11 is ametal rod or wire. 8. A contactless heating apparatus for thin filaments as claimed in any one of claims 1-6 characterised in that the coil 11 and the inductor element 16 are made of copper tubing. 9. Acontactless heating apparatus forthin filaments as claimed in any one of claims 4 to 7 characterised in that the AC power source 15 is a Class C oscillator valve located in the induction furnace. 10. Acontactless heating apparatus for thin filaments as claimed inany one preceding claim characterised in that the frequency is substantially 13.6MHz. 11. Acontactless heating apparatus for thin filaments asclaimedinanyone preceding claim characterised in that the filament 20 is located in a cylindrical chamber 21 containing a reducing atmosphere and the coil 11 and inductor element 16 are adjacent to the chamber.

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: 1989
Main classification: H05B6
IPC codes: C23C16, H05B6