GB721621A - Improvements in and relating to graded gas-tight seals - Google Patents

Abstract

721,621. Quartz-metal seals. BRITISH THOMSON-HOUSTON CO., Ltd. June 16, 1952 [June 19, 1951], No. 15087/52. Class 36. [Also in Group XL (a)] A method of making, particularly for a highpressure mercury discharge lamp, an electrically conducting gastight quartz stem tube including an electrical conductor made up of current lead wires 3, 4 extending from a molybdenum ribbon foil 2, consists in shaping an outer quartz tube 10 with a necked portion 11, covering the wires 3, 4 with metal foil 9, positioning about the foil 2 a vitreous spacer tube 15 of lower (e.g. about 200‹ C. lower) softening temperature and a higher (e.g. about twice) coefficient of expansion than quartz, inserting the conductor 2, 3, 4 and spacer tube 15 in the outer tube 10 with the wire 3 and foil 9 positioned in the necked portion 11 and with one end of the tube 15 butting against the necked portion, inserting in the tube 10 and into butting position against the tube 15 an inner tube 13 having a small bore end 12 closely surrounding the wire 4 and foil 9, heating and collapsing the portion of the tube 10 overlapping the tube 12 while maintaining a non-oxidizing atmosphere, e.g. nitrogen, within the tubes, evacuating and sealing the tube 13, heating and collapsing the portion of the tube 10 around the tube 15 and heating the portions of the composite tube surrounding the foil covered portions of the wires 3, 4 to form a joint between the wires and composite tube. The spacer tube 15 may consist of material known under the Registered Trade Mark " Vycor 7911 " and which contains approximately 96 per cent silica and has a coefficient of expansion lower than that of the molybdenum foil 2. As shown in Fig. 3, the foil 2 has its corner portions cut to form tabs 5, 6 which are folded over the wedge-shaped end 7 of the wire 3, and similarly for the wire 4. The tabs 5, 6 are welded to a flat side of the ends of wires 3, 4 and the portion of the foil 2 between each pair of tabs 5, 6 is welded to the other flat side of the wire ends. One or more transverse notches or slots 21 may be provided along the wires 3, 4 to key the wires to the surrounding quartz and prevent relative longitudinal or rotary displacements. With the above construction molybdenum foils with a thickness of up to about 0.004 inch have been used with high pressure lamps. A stem incorporating current leads 0.08 inch in diameter and a molybdenum foil 0.5 inch long, <SP>3</SP>/ 16 inch wide and etched to a thickness of 0.0023 inch at the longitudinal centre line with a tolerance of Œ0.0002 inch can be used.