Patent GB-1424694-A (GB)
Title: Forming of articles having radially varying compositions
IPC: C03B37/027
Filed: 1973-03-30
Abstract: 1424694 Light focusing waveguides CORNING GLASS WORKS 30 March 1973 [30 March 1972] 15432/73 Heading C1M A method of forming a transparent light focusing waveguide having a varying chemical composition comprises the steps of taking a cylindrical member, rotating the member along its longitudinal axis, entraining in a gaseous carrier medium predetermined amounts of each of a plurality of constituents in vapour form, hydrolysing the vapours in a flame to form a soot having a predetermined initial composition, applying the soot to a curved surface of the cylindrical member, and continually varying in a predetermined desired manner the amount of vapour of at least one of the constituents entrained in the gaseous carrier medium, thereby building up on the cylindrical member a single layer of soot having a continually radially varying composition. The soot layer may be sintered as deposited if the temperature is high enough, or it may thereafter be sintered. The soot is preferably fused silica doped with germanium alone or in combination with other oxides, but other dopants such as Ti, Ta, Sn, Nb, Zr, Al or La may be used. Preferably the soot at any point is doped with not more than 25% by weight of dopant. The cylindrical member having a coefficient of expansion compatible with the layers to be applied may be of glass fused quartz or graphite, and may be drilled out by means of a diamond reamer either before of after the sintering step in an induction furnace having an oxygen atmosphere at about 1500‹ C. The tubular member so formed is then washed with hydrofluoric acid, after which the structure is placed in an induction furnace and drawn in an oxygen atmosphere at about 2000‹ C. As the structure is drawn the central hole collapses. The cylindrical member may be hollow and the layer of soot applied to the internal or external surface of the member. In one embodiment (Fig. 1) a carrier gas is supplied by tubes 28 and 30 to containers 20 and 22 respectively which contain liquid constituents 24 and 26. As the carrier gas is bubbled through vapours of the liquids become entrained and are exhausted by pipe 44 to a conventional flame hydrolysis burner 14 wherein the gasvapour mixture is hydrolysed to form a glass soot which is deposited as a layer of soot 10 on the cylinder 12. The amount of vapour entrained in the gaseous carrier medium may be controlled by varying the flow rate of the carrier gas to at least one of the containers of liquid constituents in a predetermined manner. One means for controlling the amount of carrier gas to 20 is to have a valve 32 connected to a valve motor 52, signal connector 54 and a controller 56. Controller 56 is programmed to provide a signal to 54 which correspondingly drives valve motor 52 which controls the amount of carrier gas flowing through 32. Another means for controlling the amount of carrier gas supplied to 20 is accomplished by having the gas flow through valve 32 controlled by a cam. In a specific example a layer of doped silica initially comprising 97% by weight SiO 2  and 3% by weight TiO 2  is applied to a rotating and translating member, and during build up the flow of carrier gas through the vaporizable liquid providing the Ti component, TiCl 4 , is slowly decreased to yield a soot layer of continually radially decreasing TiO 2  content. The finished thickness of glass after sintering is 1¥ inches.