Patent GB-772826-A (GB)
Title: Method and apparatus for the manufacture of tubes and other profiled parts made of fused silica
IPC: C03B5/02
Filed: 1955-06-29
Abstract: 772,826. Working Silica. QUARTZ & SILICE. June 29, 1955 [June 30, 1954, Sept. 4, 1954. June 6, 1955, June 10, 1955]. No. 18801/55. Class 56 [Also in Group XI) Tubes, or rods of profiled section, are made from silica by first melting, the grains; or powder, of ground quartz, in a high-frequency induction furnace, to form an ingot which is subsequently re-heated and drawn to the required section. As shown, the quartz is held in a crucible, formed by superposed graphite sections 98, enclosed in a heating-tube 92 likewise of superposed graphite sections. A silica wall 90 supports the heating-tube 92, and an outer silica tube 28 is spaced from the tube 92 by a packing of granular silicon carbide which material also fills the bottom of the tube 92 beneath the crucible. The space above. the crucible is insulated by boxes 112 containing silicon carbide. Heating is effected by an inductor winding consisting of a copper coil 60 of rectangular section through which flows cooling water. The turns of the winding are separated by mica blocks and turns of asbestos cord, and they are less widely spaced at the ends than at the central parts to compensate for greater heat losses at the ends. The coil 60 is carried by a silica tube 36 held between asbestos rings 32; 50 the whole heater assembly resting upon a silica tube 30. The base plate 14 and the top plate 70 are cooled by water coils. The quartz is subjected to vacuum, applied through a pump connection 84, for about 15 minutes, and then the full power of the furnace is applied for about 30 minutes, a rapid rise to about 370‹C. being thereby effected. The furnace power is then considerably reduced, e.g. from 8 Kw. to 5 Kw., for about one hour, the temperature of the quartz rising more slowly during this time. The slow temperature rise is continued, and observed by an optical pyrometer, and the furnace power increased as necessary until the desired fusing temperature, e.g. 1750‹C., is reached. This slow heating to the fusing temperature is stated to be important, in that the vaporization of silica grains is avoided, and the temperature gradient through the quartz is reduced, thereby allowing a progressive melting from the exterior towards the centre and encouraging a migration of impurities through the mass and the liberation of the same. To enhance this effect, a central perforated chimney 100 of superposed graphite sections, may be placed in the crucible, the vacuum being thus applied to the inner surface of the mass in the crucible which latter may be formed with external screw threads or rings to assist the breaking thereof away from the cooled ingot. The ingot 225, Fig. 8, is placed, for drawing, in a graphite crucible 222, where, it rests upon a frusto-conical member 226 of graphite, a die 224, also of graphite, defining with the member 226, an annular drawing passage. A hollow graphite mandrel 230 is secured to the member 226 by a hollow graphite bolt 228. The ingot is re-heated by an inductor winding 214 on a silica tube 212 spaced from the crucible 222 by a packing of pure lamp black, and, when the proper temperature about 2000‹C., is reached, -the plug 242 is removed by unscrewing graphite bolts 248 with tongs. The silica is too viscous to flow appreciably, so the operator pulls it with tongs into a number of tongues which join into a ball below the end of the mandrel 230. He then exerts a continuous pull downwards so that a tube forms around the mandrel, the pull being maintained until the tube can be engaged between drawing rollers. As the drawing progresses, the power of the furnace is reduced so that overheating of the diminishing amount of silica to be drawn is avoided. The member 226 is carried by a hollow graphite rod 238 which is carried by a tube 300. A stirrup-like support for the tube 300 is supported on pillars on the furnace base and is swingable out of position for re-charging the furnace. The stirrup is provided with means for centering the members 226, 230 and is formed of tubes which supply cooling water to, and withdraw it from, the tube 300 which is closed at its bottom. The mandrel does not actually form the silica tube, but stabilizes its final dimensions, as shown in Fig. 12. As the silica leaves the mandrel, it is of high viscosity, and the subsequent decrease in the diameter of the tube is small. During drawing, air rises in the silica tube, passes through the mandrel 230 and the member 226, and leaves through holes 241 in the rod 238. Passages 236 in the large head of the mandrel 230 vent the space enclosed by the flowing silica under the member 226. Nitrogen or other inert gas may be circulated through the mandrel, member 226 and rod 238. In a modification, Fig. 13, the heating .tube 222 is lined with graphite sleeves 320, 322; the rod 238 is surrounded by a thin graphite sleeve 324 which leaves a space 326 communicating with passages 334 leading from the troughs of a wave-like surface of the member 328. The die 340 also is grooved at 342, the grooves being vented by passages 344. The grooves in the members 328. 340 reduce the surface area in contact with the silica and render the drawing easier. The passages 344, 334, 326 allow silica vapours or impurities to leave the furnace without being enclosed in the silica.