Patent Publication Number: US-7898179-B2

Title: Discharge lamp exhibiting reduced thermal stress and method of making such a lamp

Description:
TECHNICAL FIELD 
     This invention relates to discharge lamp. More particularly it relates to discharge lamps having ceramic arc tubes and still more particularly it relates to high pressure sodium discharge lamps. 
     BACKGROUND ART 
     Discharge lamps, such as the afore-mentioned sodium lamps, are staple articles of commerce. The lamps comprise a ceramic arc tube mounted in a vitreous envelope and various electrical lead-ins to conduct current to the arc tube electrodes. Because of the different materials used, such as the ceramic arc tube and various metallic components and their differing thermal expansion coefficients (TEC), many compromises have to be made to ensure economic manufacturing costs, reasonable light output and reasonable life expectancy. Anytime any of these contingencies can be enhanced or improved, a decided advance in the art is achieved. For example, in high pressure sodium lamps it has been necessary to provide an auxiliary, U-shaped, current-carrying niobium wire welded between a niobium component and a nickel-plated steel (NPS) support attached to a lead-in at one end and the niobium feed-through at the other end. The U-shaped niobium wire was necessary because, while niobium provides a close TEC with the ceramic (usually, alumina), its TEC is far removed from most other metals. This creates a large thermal strain at the weld joint of the niobium component and NPS support rod. The U-shaped niobium wire, or U-bend, places the weld joint farther away from the heat generated by the operating arc tube and reduces the thermal strain on the niobium-NPS juncture. However, while this technique worked successfully, it added parts and cost to the lamp. Further, it would be an advance in the art to eliminate the NPS support because the nickel plating process is not environmentally friendly and, as fewer and fewer companies undertake its manufacture, its cost has greatly increased. 
     DISCLOSURE OF INVENTION 
     It is, therefore, an object of the invention to obviate disadvantages in the prior art. 
     It is another object of the to enhance discharge lamps. 
     Yet another object of the invention is to improve discharge lamps. 
     These objects are accomplished, in one aspect of the invention, by the provision of a discharge lamp comprising: an outer envelope; first and second electrical lead-ins sealed into the base of the envelope; a ceramic arc tube operatively mounted within the envelope, the arc tube having at least one electrode therein; a tubular, niobium feed-through connected to the at least one electrode and sealed to the ceramic body; and a stainless steel rod electrically connected between a first of the electrical lead-ins and the tubular niobium feed-through, the stainless steel rod being the only electrical connection between the lead-in and the niobium feed-through. The difficulty of achieving a good, long-lasting weld between the niobium and the stainless steel (necessary because a suitable stainless steel, such as 430SS, has a lower thermal conductivity and higher electrical resistance then the previously employed NPS) is surmounted by a new welding method that involves adding an argon shield gas to the welding station. The elimination of what was previously considered to be an indispensable component has reduced the cost of the lamp and the consummation of a better connection with a more suitable support structure has greatly enhanced and improved the lamp. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic, sectional view of a prior art lamp; and 
         FIG. 2  is a similar view of a preferred embodiment of the invention. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims taken in conjunction with the above-described drawings. 
     Referring now to the drawings with greater particularity, there is shown in  FIG. 1  a prior art high pressure sodium lamp  10  having a vitreous outer envelope  12 . Lead-ins  14 ,  16  are sealed into the base  18  of the envelope. A ceramic arc tube  20 , for example, one constructed of polycrystalline alumina, is mounted in the outer envelope  12 . Electrodes  22  (only one of which is shown) are sealed into opposite ends of the arc tube  20  by known techniques. For example, the electrode  22  can be a tungsten rod  22   a  sealed into a niobium tube  24 . The niobium tube  24  is itself hermetically sealed into a ceramic endcap  23  by a joint  26  that comprises a glass frit  26   a . A support rod  28 , such as one of NPS, is fitted into the niobium tube  24  at one end and has its other end fixed to one of the lead-ins, for example,  14 . A U-bend  31 , which comprises a niobium wire, has one end welded to the niobium tube  24  and a second end welded to the support rod  28 . The U-bend  31  is a current-carrying member that places the weld joint farther away from the heat generated by the operating arc tube  20  and reduces the thermal strain on the niobium-NPS juncture. 
     Referring now to  FIG. 2  there is shown a discharge lamp  100  in accordance with an embodiment of the invention. The lamp  100  comprises an outer envelope  120  having first and second electrical lead-ins  140 ,  160  sealed into a base  180  of the envelope  120 . A ceramic arc tube  200  is operatively mounted within the envelope  120 , the arc tube  200  having at least one electrode  220  therein. A tubular, niobium feed-through  240  is connected to the at least one electrode  220  and sealed to the ceramic body  120  at a joint  260  that can comprise a glass frit  260   a . A stainless steel rod  280  is electrically connected between the electrical lead-in  140  and the tubular niobium feed-through  240 , the stainless steel rod  280  being the only electrical connection between the lead-in  140  and the niobium feed-through  240 . In a preferred embodiment of the invention, the stainless steel rod is comprised of 430SS, a composition of 84% iron and 16% chromium and has an end  300  enclosed within the niobium tube  240  and a second end  320  attached to lead-in  140 . A second current-carrying element  340  is attached to the second lead-in  160  and connects to the second electrode, which is not shown. 
     To accomplish the weld between the niobium tube  240  and the stainless steel rod  280  it is necessary to reduce excessive heating and oxidation of the niobium tube during the welding process and this is accomplished adding an argon gas shield to the welding station. A suitable flow rate for the argon is 2 scfh. Further, if pulse welding is used, it has been found desirable to reduce the welding current from 1.7 KA to 1.2 KA in the second pulse. This helps to prevent the niobium from overheating. 
     A second technique for removing excess heat comprises using a large chill-block as the grounding electrode at the welding station. Any other technique which adequately draws heat away from the welding station is also within the purview of the invention. 
     While there have been shown and described what are at present considered to be the preferred embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications can be made herein without departing from the scope of the invention as defined by the appended claims.