RADIOFREQUENCY HEATING OF THIN FILAMENTS

Abstract: A radiofrequency heating apparatus for heating a linearly supported thin electrically conductive filament (10) comprises a coil (15), earthed at one end, located adjacent to the filament; and a source (12) of radiofrequency (RF) signals in the frequency range from H.F. to U.H.F. connected to the coil. A conducting element (16) is connected to the non-earthed end of the coil (15) and arranged to form or lie on the surface of a cylinder (11) through which the filament (10) passes coaxially whereby the heating of the filament occurs within the cylindrical conducting element (16) and the RF signals are amplitude modulated. Preferably the modulation applied to the RF source is within the audio frequency band, conveniently double or single sideband modulation. The source of modulation may be pulsed and preferably the radiofrequency is approximately 13,56 MHz.

Description: 1 Radiofrequency Heating of Thin Filaments The invention relates to the manufacture and or processing of filamentary material and in particular to heating of filaments by remote contactless means. In one application of processing filamentary materials a fine tungsten wire is drawn through a chemical vapour deposition (CVD) reaction chamber where coatings of refractory material are deposited. In these processes difficulties have been encountered when making electrical contact to the filament. PCT Patent application No. PCT/GB89/00505 describes a non-contacting method of heating filaments employing a high frequency alternating current applied to a coil with an attached energy radiating element. Such an arrangement enables the filament to be heated uniformly by apparatus which is located outside of the reaction chamber. The heating effect on the filament was found to be a function of the location and disposition of the radiating element which is a straight rod in its simplest form. The object of the invention is to provide a novel non-contacting filament heating arrangement which can be used over long lengths of filament and assembled to a heating chamber in a convenient manner. The invention provides a radiofrequency heating apparatus for heating thin electrically conductive filaments comprising a means to linearly support a length of filament, characterised in that there are provided: SUBSTITUTE SHEET a coil, earthed at one end, located adjacent to the filament; a source of radio frequency signals in the frequency range from H.F. to U.H.F. connected to the coil; and a conducting element connected to the non-earthed end of the coil and arranged to form or lie adjacent to the filament whereby the heating of the filament occurs within the cylindrical conducting element; characterised in that: the source of radio frequency signals is amplitude modulated. Preferably the conducting element is so formed as to lie on the surface of a cylinder through which the filament passes coaxially. Advantageously the apparatus includes a cylindrical enclosure through which the wire is drawn and the conducting element is helically wound around the enclosure. The amplitude modulation may be at audio frequencies, using double or single sideband modulation. It has been found that uniform heating of filaments or wires by this contactless heatinf apparatus can be ensured by modulation of the RF source. In the above arrangements it is also possible to provide uniform heating of the filament by pulsing the modulation or pulsing the RF source at a high repetition rate. In one further arrangement two close RF signals may be applied so as to produce a beat frequency. The invention will now be described with reference to the accompanying schematic Drawing which shows a thin filament 10 being drawn upwardly SUBSTITUTE SHEET through a cylindrical heating enclosure 11 from a take-off spool (not shown) to a take-up spool (not shown) . The filament 10 is drawn axially through the cylindrical enclosure 11 which may be made of glass. A 13-56 MHz radio frequency source 12 is connected via a variable capacitor 13 to one end 14 of an earthed coil 15- The coil 15 is made of copper tubing through which cooling water is passed and is formed with eight turns of diameter 5cm and an overall length of about 5cm. The variable capacitor 13 is adjusted to tune the arrangement to the RF supply frequency of 13- 6 MHz. An electrical connection is also provided connecting the end 14 of the coil 15 to a conducting wire element 16, helically wound around the glass cylinder 11. A shortwave transmitter or other suitable RF generator 12 may be used. With a pure radiofrequency waveform of frequency 13> MHz, for example, such as that from a good quality short wave radio transmitter, it has been found that there is a tendfency to obtain a type of heating which has the appearance of a few white-hot beads along the filament rather than a uniformly heated filament. The inventors have discovered that this type of beady heating can readily be converted to a more uniform heating by amplitude modulating the radiofrequency with (for example) audio frequencies or a white noise source. The AM type modulation need not be restriceted to audio frequencies, higher frequencies such as ulreasonic frequencies can also be used. Typically, the bright beads then become immediately much lower in temperature and spread out, leading to a much more uniformly heated filament. In this way the heating can be very uniform over a long length of filament and is affected by the audio frequencies used for the modulation. The ends of the filament may be optionally grounded remotely as a means to provide SUBSTITUTE SHEET improved heating in some circumstances. Heating of the filament according to the present invention occurs only within the helix. Conveniently a standard type of commercially available shortwave 2 kW output can be used. Instead of the helical aerial shown in the Figure, other shapes of aerial can be used. An advantage of the helix is that it can be stretched out (i.e. fewer turns per inch) locally to give a locally cooler region, or locally compressed to give a locally hotter region. Although described in relation to a helical wire element wrapped around the cylindrical heating vessel it is possible for the wire element to be formed as a printed circuit arrangement on the outer surface of the vessel. The path of the conducting element could then be made to differ from a simple helical form while retaining the overall cylindrical form. A cylindrical copper aerial, cut axially to allow viewing, has been found to give excellent results. To produce a better heating pattern over longer lengths of filament, grounded resistors may be connected to one or both ends of the heated filament at points remote from the aerial (heated) region. The heating apparatus described above can be used for chemical vapour deposition onto a core filament, wire annealing, etc. Other simple alternative configurations of components are possible and will be apparent to those skilled in the art. SUBSTITUTESHEET

Claims: Claims 1. A radiofrequency heating apparatus for heating thin electrically conductive filaments comprising a means to linearly support a length of filament; a coil, earthed at oner end, located adjacent to the filament; and a source of radiofrequency (RF) signals in the frequency range from H.F. to U.H.F. connected to the coil; and a conducting element connected to the non-earthed end of the coil and arranged to form or lie adjacent to the filament whereby the heating of the filament occurs adjacent to the conducting element; characterised in that there is provided: means to amplitude modulate the RF signals. 2. A radiofrequency heating apparatus as claimed in claim 1 characterised in that the conducting element lies on the surface of a cylinder through which the filament passes coaxially. 3. A radiofrequency heating apparatus as claimed in claim 2 characterised in that the apparatus includes a cylindrical enclosure through which the wire is drawn and the conducting element is helically wound around the enclosure. h. A radiofrequency heating apparatus as claimed in any preceding claim characterised in that the modulation is within the audio frequency band. 5- A radiofrequency heating apparatus as claimed in any one preceding claim characterised in that the modulation is double or single sideband modulation. 6. A radiofrequency heating apparatus as claimed in any one preceding claim characterised in that the source of modulation is pulsed. 7. A radiofrequency heating apparatus as claimed any one preceding SUBSTITUTE SHEET claim characterised in that the RF source is pulsed. 8. A radiofrequency heating apparatus as claimed in claim 1 characterised in that the radiofrequency source produces two close radiofrequency signals such that a beat frequency is produced. 9- A radiofrequency heating apparatus as claimed in any one preceding claim characterised in that the radiofrequency is approximately 13.56MHz. SUBSTITUTE SHEET

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: H05B6
IPC codes: C21D9, H05B6