Patent Publication Number: US-7218046-B2

Title: Unitized chemically etched tungsten electrode

Description:
FIELD OF THE INVENTION 
     This invention relates to an electrode design and method. 
     BACKGROUND OF THE INVENTION 
     The present invention relates in general to electrodes suitable for use in arc lamps and more specifically to a one piece or unitized electrode where the final dimensions are formed by chemical etching. 
     In the prior art, electrodes, i.e., anodes and cathodes which are used in lamps, such as arc lamps, are generally mechanically formed. In some cases the electrodes are mechanically ground to the required tolerances and dimensions, which is labor intensive and costly. Furthermore, these mechanical forming methods result in undesirable stresses created by the electrode geometry resulting from the manufacturing process and contribute to a short lamp life. Electrodes formed in this manner are also subject when sealed in a glass envelope to cracking and/or stresses to the glass at the distal end of the anode or cathode due to a relative large geometry and sharp edges which are inherent in the forming process. 
     In another prior art technique, the electrode nose or head portion can be welded onto the shank portion to avoid some of the grinding requirements described above. This manufacturing technique is characterized and is the cause of embrittlement of the electrode shank when welded to form a two piece electrode, thus introducing another failure mode. The electrodes of the type described above are characterized in that they often produce streamers in the arc coming off multiple points on the edge of the electrode tip due to the manufacturing technique and geometry irregularities resulting from these methods. 
     It therefore can be seen that there is a need in the field for an electrode which eliminates the sharp geometry at the distal end of the electrode shank and uneven performance of the tip portion to provide and improve overall lamp life. 
     SUMMARY OF THE INVENTION 
     It is therefore an objective of the present invention to provide for a unitized electrode which overcomes the problems of the prior art described above. 
     It is a further object of the present invention to provide an electrode which results in improved lamp life because of improved electrode geometry. 
     It is a further object of the present invention to provide an electrode which eliminates sharp geometry at the distal end of the electrode. 
     It is a further object of the present invention to provide an electrode which exhibits reduced stress when sealed in a glass envelope. 
     It is a further object of the present invention to provide an improved electrode having a rounded head which improves the focus of the arc on the center of the electrode. 
     It is yet another object of the present invention to provide unitized tungsten electrode which when sealed in a glass envelope eliminates embrittlement and misalignment. 
     The present invention is directed to a method of making an unitized tungsten electrode which exhibits superior mechanical and electrical properties and the electrode produced by such method. The method includes providing a length of cylindrical cut stock having a predetermined diameter and length. The stock is ground to form a rough unfinished electrode having an enlarged tip or nose portion at one end integrally connected to an elongated shank section. The unfinished electrode is formed into a finished electrode to the desired dimensions by exposing the electrode to a chemical etchant for a time sufficient to form a finished electrode characterized by a smooth nose and shank surface, and rounded shoulders. In one embodiment, the chemical treatment is carried out by immersion of the unfinished electrode in a bath containing sodium hypochlorite at an elevated temperature. 
     The finished product consists of an unitized tungsten electrode having a smooth rounded tip or nose portion and a shank portion having a rounded distal end. The electrode further is characterized by rounded shoulders and a chemically etched smooth surface. The unique geometry of the electrode functions to reduce stress when sealed in a glass envelope, and contributes to eliminating embrittlement and misalignment, with the rounded tip functioning to focus the arc to the center of the electrode resulting in greater efficiency and longer lamp life. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a fuller understanding of the nature and objects of the invention, reference should be made to the following detailed description of a preferred mode of practicing the invention, read in connection with the accompanying drawings, in which: 
         FIGS. 1   a, b  and  c  illustrate the basic steps in forming the electrode in the present invention. 
         FIGS. 2   a  and  b  illustrate the finished electrode and its relative proportions and a typically winding when the electrode is used as an anode. 
         FIG. 3  illustrates a side sectional view of a conventional arc lamp utilizing the electrode of the present invention. 
         FIG. 4  is a perspective view of the anode and cathode illustrating the arc formed by the anode when used in a lamp. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The manufacture of the electrode of the present invention is illustrated in  FIGS. 1   a–c  of the present invention in which a cylindrical piece of stock  10  ( FIG. 1   a ) is mechanically ground by conventional means into a rough unfinished electrode  20  ( FIG. 1   b ) having an enlarged tip or nose portion  22  and an elongated shank  24  having a distal end  26  with sharp edges  28  at both the distal end and nose portion. 
     The unfinished electrode  20  is then immersed in a bath containing sodium hypochlorite maintained at a temperature in the range of about 180–212° F. for about three to fifteen minutes. Typically the electrode is chemically etched in a bath in batch 10 to 50 electrodes at a time. The unfinished electrodes typically are made from tungsten stock 0.016 to 0.040 inches in diameter, 0.250 to 0.500 inches in length. The unfinished electrodes are then immersed in a suitable 500 ml heated bath (180–212° F.) of 6% sodium hypochlorite in water. The sodium hypochlorite functions to preferably attack the sharp edges of the electrode and results in the formation of electrode  30  ( FIG. 1   c ) having a rounded tip or nose portion  32 , smooth undercut  34 , smooth elongated shank  36  having a rounded distal end  38 . The electrode is further characterized by a chemically etched smooth surface. Furthermore, the sodium hypochlorite functions to eliminate rough grind marks on the surface of the electrodes. 
       FIG. 2   a  illustrates the relative proportions and dimensions of the finished electrode  30 , with a metallic wire tungsten winding  40  shown in  FIG. 2   b.    
       FIG. 3  of the drawings illustrates a glass envelope  50  of the present invention which is made of a quartz glass having a chamber  52 , neck portions  54  and a pair of electrodes  56  (anode) and  58  (cathode) having tip portions  60  and  62  and shank portions  64  and  66 , respectively. Typically the end of each shank is connected to a metal foil  68 , usually made of molybdenum. 
     In operation the anode of the present invention is used in combination with the cathode as illustrated broadly in  FIG. 3 .  FIG. 4  illustrates the hot arc center  70  of arc  72  in a stable condition between the electrodes, anode  74  and cathode  76 , of the present invention. In this embodiment, the anode and cathode have tungsten wire windings  78  and  80 , respectively. The stability is determined by observing a projected image of the arc. Because of the geometry of the anode formed by the present invention, the arc center does not translate or dance across the electrode surface but is confined to the arc center as illustrated in  FIG. 4 . 
     In another embodiment of the present invention the electrode may be in the form of a straight shank in which the ends of the stock are rough ground and the surface and ends of the electrode are treated and finished in the chemical etchant as described above. 
     While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawings, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims.