Abstract:
A lead-in seal of the type having a metal foil seal member sealed into an envelope, comprising a relatively narrow intermediate foil tab member connected between the foil seal member and a conductor. The result is reduced stress in the seal and less likelihood of cracking.

Description:
BACKGROUND OF THE INVENTION 
     The invention is in the field of lamps and other devices having current lead-in conductors sealed in and through a bulb or envelope by means of current-conductive foil members. 
     U.S. Pat. No. 3,685,880 to John C. Sobieski, for example, discloses a lamp containing a pair of electrodes each connected to a separate lead-in conductor by a thin metal foil member, the foil members being hermetically sealed in the lamp envelope at seal regions, the end regions of the electrodes and lead-in conductors being embedded in the seal regions. The electrodes and lead-in conductors are attached to the respective foil members by positioning their end regions to overlap regions of the foil member, and spot welding these overlapped regions together. Typically, the lead-in conductors and the foil members are made of molybdenum, and, in an arc lamp, the electrodes are tungsten rods. To improve the spot-welded connections, both electrically and mechanically, small platinum-coated molybdenum foil pads have been placed at the spot-weld locations, between the molybdenum seal foil member and the tungsten electrodes and/or the molybdenum lead-in conductors, prior to spot welding. The improvement thus achieved is relatively greater for the welds of the foil member to the tungsten electrode rods. However, it has been found that this improvement in weld bonding is accompanied by an increased tendency for cracking of the envelope material at the seal region. 
     SUMMARY OF THE INVENTION 
     Objects of the invention are to provide an improved foil-type seal for lamps and the like, and to provide such a seal which has good electrical and mechanical properties, and good reliability of remaining intact without cracking. 
     The invention comprises, briefly and in a preferred embodiment, a lead-in seal having a thin conductive foil member sealed into an envelope, such as into a stem integral with a lamp bulb or housing, a thin conductive tab member attached to the foil member and extending therefrom, and an electrode attached to the extending portion of the tab member and extending into the interior of the lamp bulb, the attached end of the electrode being spaced from the adjacent edge of the foil. Preferably, the width of the tab member is about the same as the diameter of the electrode to which it is attached, and may be thicker than the foil, if required for conducting the lamp operating current without overheating. The tab preferably is shaped in the form of a double bend such that its end regions lie in parallel planes. In the preferred embodiment, the electrode is a tungsten rod, the tab member is a platinum-coated molybdenum foil, and the seal member is molybdenum foil, and a molybdenum lead-in conductor is attached to the seal foil at the end thereof opposite from the tab, the electrode and lead-in conductor being substantially axially aligned. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a top view of a compact high intensity arc lamp provided with a foil lead-in seal in accordance with a preferred embodiment of the invention. 
     FIG. 2 is an enlarged side view of a lead-in seal region of the lamp of FIG. 1. 
     FIGS. 3 and 4 are side and top views, respectively, of a prior art type of foil seal arrangement. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The compact high intensity arc lamp 11 in FIG. 1 comprises an envelope of quartz or other suitable vitreous material having a bulb portion 12 and stem portions 13 and 14 extending therefrom on opposite sides thereof, on a common axis. Elongated tungsten rod anode and cathode electrodes 16 and 17 are positioned on the axis of the lamp with the inner ends thereof spaced apart within the bulb portion 12, and respectively extend into the anode stem 13 and cathode stem 14. The tungsten anode rod 16 is part of the anode lead-in assembly, which further comprises a molybdenum lead-in wire 18 having an end 19 spaced from the outer end of the anode 16, the lead-in 18 and anode rod 16 being substantially axially aligned. The inner end region 19 of the lead-in wire 18 overlaps and is spot-welded to an end portion of a molybdenum thin foil member 21. The end region 19 of the lead-in wire may be swaged or otherwise flattened to facilitate a good weld. A thin foil tab member 22 has an end region 23 positioned to overlap a portion of the other end of the foil 21, and is spot-welded thereto. The remaining end region 24 of the tab 22 is positioned to overlap an end region of the anode rod 16, and is spot-welded thereto. Preferably, the tab 22 is provided with a double bend, as shown in FIG. 2, so that the end regions 23 and 24 thereof lie in spaced apart parallel planes, so that the anode 16 and lead-in wire 18 will be substantially in axial alignment. The intermediate portion 26 of the tab 22 between its end regions 23 and 24 may be inclined at an angle of approximately 45° as shown in FIG. 2. 
     The outer end of the cathode rod 17 is shown spot-welded directly to a molybdenum seal foil 31, and a cathode lead-in wire 32, preferably of molybdenum, is spot-welded to the remaining end region of the foil 31, the welded end region 33 of the lead-in wire 32 having been flattened or swaged to facilitate the welding. 
     After the anode and cathode lead-in electrode assemblies have been prepared, they are properly positioned with their inner ends within the bulb portion 12, and the bulb is evacuated and filled with suitable gas, and a metal halide such as indium triiodide, if desired, and the end regions of the stems 13 and 14 are heated and flattened by pairs of jaws, for example as disclosed in the above-referenced Sobieski patent, to form anode and cathode seal regions 36 and 37, respectively, in which seal regions the quartz material of the bulb wets the foils 21 and 31, thus providing hermetic sealing. 
     The foil tab 22 of the invention is preferably molybdenum coated with platinum, similar to the spot-welding pads that have been used previously. The tab 22 preferably has a width approximately equal to the diameter of the tungsten rod 16 to which it is welded, and the tab 22 may be thicker than the seal foil 21, in order to adequately carry the operating current of the lamp without overheating. In a practical embodiment, the platinum-plated molybdenum tab 22 is 3/1000 of an inch thick, and the molybdenum sealing foil 21 is 9/10,000 of an inch thick. 
     The invention, comprising the elongated connector tab 22 between the sealing foil 21 and the electrode 16, achieves its objective of considerably reducing the number of seals 36 which crack during operation of the lamp, the anode seal 36 having been particularly prone to cracking. This achievement is believed due not only to the elongated feature of the tab 22, but also due to its double bend which provides the oblique intermediate section 26. Both of these features appear to reduce stress in the seal 36 after the lamp is completed. The reason for this improvement is believed due to a tendency for a stress to develop and remain in the seal region 36 when the former technique was employed of positioning a foil platinum-coated pad between the sealing foil 21 and electrode 16. Tabs similar to the tab 22 can be interposed between the foil 21 and lead-in wire 18, and between the foil 31 and electrode 17 and lead-in wire 32, respectively. 
     FIGS. 3 and 4 illustrate this prior construction, in which a platinum-coated molybdenum foil pad 41 was interposed between the weld regions of the anode 16 and the sealing foil 21 prior to the step of spot welding these members together. It is believed that during the spot welding, the foil pad 41 curled slightly upwardly at its edges, toward the round rod 16, causing narrow spaces between these curled edges and the sealing foil 21, and also narrow spaces between these curled edges and the electrode rod 16, and during the sealing operation the quartz material of the envelope was unable to adequately penetrate into and fill these narrow spaces, which resulted in setting up a stress in the seal. It should be noted that, due to its extreme hardness, it is relatively difficult to swage or flatten the spot-welded end of the tungsten rod 16, as can more readily be done at the spot-welded end 19 of the molybdenum lead-in wire 18, to provide a broader area of spot welding. 
     Another achievement of the invention is the prevention of corrosion of the molybdenum seal foil 21 by metal halide or other corrosive vapor contained in the bulb 12 and which can seep along the electrode 16 within the stem 13, to the outer welded end of the electrode. The platinum coating on the tab 22 resists corrosion, and the corrosion-prone molybdenum foil 21 is completely sealed and encased by the quartz housing material so the corrosive vapors cannot reach it. The use of a single platinum-coated foil interconnecting the members 16 and 18 is undesirable because platinum is not wetted by quartz as well as is bare molybdenum foil. 
     While preferred embodiments of the invention have been shown and described, various other embodiments and modifications thereof will become apparent to persons skilled in the art and will fall within the scope of the invention as defined in the following claims.