Patent Publication Number: US-6669523-B1

Title: Method of dimensionally stabilizing a tungsten filament

Description:
BACKGROUND OF THE INVENTION 
     Tungsten filaments used primarily in halogen and incandescent lamps are coiled coils, i.e. the tungsten filament or wire is coiled once around a primary mandrel to form the primary coil or the primary set of coils, then that set of coils itself is coiled (the “secondary set of coils”). In the development of a new lamp, it was found that the secondary set of coils of the tungsten filament would become very distorted after the lamp was operated for only a minute. The distortion was so severe that the vertical legs of this filament actually shorted out against the secondary set of coils. Shorting out only one secondary coil reduces the life of the product to approximately 20% of the design life for a filament with only six secondary coils. In order to achieve design life, the coil dimensions needed to be stabilized. 
     A known method for achieving coil dimensional stability in a tungsten filament with vertical legs is to screw the completed filament (with no primary mandrel in the coil) on a mandrel form, clamp the vertical legs in place, and heat the filament in a vacuum furnace for 2 to 10 hours. After this process, a tungsten plug is inserted into the primary set of coils&#39; leg to aid in welding the filament to molybdenum foil. For high wattage coils with large wire diameters this is possible. In fine wire coils, high shrinkage (losses of coils) would be incurred due to handling. Furthermore, with both large and fine coils, this process is both labor and time intensive. 
     Thus, there is a need for a cost effective method of stabilizing the coil dimensions of a tungsten filament with vertical legs by thermally stress relieving the coil prior to mounting in a lamp. This process is useful to prevent the filament from distorting and shorting out the secondary set of coils when the lamp is operated. 
     SUMMARY OF THE INVENTION 
     A method of dimensionally stabilizing a tungsten filament prior to installation in a lamp comprising the steps of providing a tungsten filament which has been manipulated into a desired shape around a primary mandrel to provide a primary coil, inducing a current in the primary coil, and maintaining the current until the tungsten filament at least partially recrystallizes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a perspective view of a tungsten filament used in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a typical tungsten filament  10  that is used primarily in halogen and incandescent lamps, but also in fluorescent lamps. The radii and lengths of these filaments are known in the art. A length of tungsten filament or wire  12  is first wrapped around a primary mandrel  22 , for example, a length of molybdenum wire, to form a primary coil. This gives the tungsten its primary set of coils. While the tungsten is still wrapped around the primary mandrel  22 , the tungsten and the primary mandrel  22 , which form the primary coil, are wrapped around a retractable secondary mandrel. This process forms a secondary set of coils  16 . The secondary mandrel is usually a steel chuck, although other retractable mandrels as are known in the art may be used. The chuck has an outside diameter that is equal to the desired inside diameter of the secondary coils  16 . Sections at both ends of the length of tungsten are not wrapped around the secondary mandrel. These sections become the vertical legs  18  of the primary coil and of the tungsten filament  10 . In some situations the invention can be practiced without the secondary set of coils  16 , for example when the primary mandrel is more than 5 times the diameter of the tungsten wire, for example when the tungsten wire is a fine wire such as less than 1.2 mils diameter. 
     After the secondary coiling on the retractable mandrel, the filament is held at the ends of the vertical legs  18  by a pair of electrical contacts  20 . The retractable mandrel is removed from the tungsten filament  10 . However, the primary mandrel  22  (the molybdenum wire) is still intact and serves to maintain the dimensions of the tungsten filament  10  as wound. If the primary mandrel  22  is not present during this process, the filament  10  “squirms” or becomes dimensionally unstable, causing the secondary coils  16  to distort to the point of making contact with each other and short circuiting the coil. 
     The electrical contacts  20  holding the vertical legs  18 , the tungsten filament  10  and the primary mandrel  22  are all placed into a reducing atmosphere that is known in the art. The reducing atmosphere comprises substantially N 2  with sufficient H 2  to effectively prevent oxidizing of the tungsten filament  10 . Preferably, the reducing atmosphere comprises about 90% N 2  and about 10% H 2 . A current is then passed through the primary coil via the electrical contacts  20 . The current causes the tungsten filament to heat sufficiently to recrystallize or partially recrystallize while held in the desired coil shape. This recrystallization provides the necessary dimensional stability. 
     The amount of current necessary varies, depending on the degree of recrystallization desired and the size of the tungsten filament  10  and the primary coil. As the coil size increases, the current necessary to achieve a constant degree of recrystallization will also increase. Likewise, for a given coil size, as the desired degree of recrystallization increases, the current necessary will also increase. The amount or degree of recrystallization is preferably at least 30, 40, 50, 60, 70, 80, 90 or 95% and is optionally less than 95%. As is known in the art, for a certain size filament, the current and time necessary to achieve a preselected degree of recrystallization can be ascertained by running a series of trials. For example, to achieve about 80% recrystallization for tungsten filaments having a diameter of between 1.0 to 1.5 to 1.9 to 2.5 to 3.0 to 3.5 to 4.0 to 4.3 to 4.5 mils, the voltage used to induce the current can range from 1 to 2 to 3 to 4 to 5 to 6 to 7 to 8 to 9 to 10 to 15 to 20V, if the voltage is maintained from 1 to 2 to 3 to 4 to 5 to 10 to 20 to 30 to 40 seconds. However, the method of the present invention can be used with any size filament. 
     When the filament  10  achieves the desired degree of recrystallization, it is cooled. The first mandrel  22  is then removed by dipping the recrystallized filament and mandrel  22  in an acid bath that will dissolve the molybdenum, but not the tungsten. However, it is desired to keep the primary mandrel  22  in the vertical legs  18 , as the presence of the molybdenum is helpful when the filament is resistance welded to the molybdenum foils inside the lamp. Thus, the vertical legs  18  are covered when the filament  10  is dipped in the acid bath, to prevent dissolving the molybdenum in the vertical legs  18 . Any material which will not dissolve in the acid bath which is known in the art, for example, wax, may be used to cover the vertical legs  18 . 
     It is also advantageous, while welding the vertical legs  18  to the molybdenum foils, if the vertical legs  18  are still fibrous, i.e., they have not recrystallized. This result is achieved in the present invention by the presence of the electrical contacts  20 . The contacts  20  hold the vertical legs  18  while the electrical current passes through the body of the filament  10 . The contacts  20  can be made from, for example, tungsten, although any appropriate conducting metal may be used. The contacts  20  preferably have a diameter of about ⅛″ and are about 1½″ long, although any size may be used, as long as the contacts are large enough to hold the filament. The current heats the tungsten filament to the point of recrystallization. As the filament  10  heats up, the contacts  20  act as heat sinks, absorbing the heat from the vertical legs  18 . The vertical legs  18  never heat to a point where they will recrystallize because the heat is transferred from the vertical legs  18  to the electrical contacts  20 . Thus, the vertical legs  18  remain fibrous and less brittle, which aids in welding the vertical legs  18  to the molybdenum foils. 
     The following Example further illustrates various aspects of the invention. 
     EXAMPLE 
     A tungsten filament having a diameter of 4.3 mils, to be used in a 5 W 24V lamp, is coiled around a primary mandrel of molybdenum. The tungsten and molybdenum are coiled around a retractable secondary mandrel, which is then removed. The legs of the filament are held in electrical contacts while the filament and contacts are in a reducing atmosphere (90% N 2 , 10% H 2 ). A voltage is applied across the filament and primary coil to induce a current in order to thermally stress relieve the filament. The current is varied to achieve various degrees of recrystallization of the tungsten wire in the coil body, while the legs of the filament remain fibrous. The voltage was maintained for a constant time of 2.8 seconds with current varying to achieve the degrees of recrystallization as noted below: 
     
       
         
           
               
            
               
                   
               
               
                 Coil Body Length (mm) 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                   
                 Coil 
               
               
                   
                 % 
                 After 2nd 
                 In Finished 
                 Contraction 
               
               
                   
                 Recrystallized 
                 Coiling (A) 
                 Lamp (L) 
                 (L − A) 
               
               
                   
                   
               
               
                   
                 45 
                 3.63 
                 3.24 
                 −.39 
               
               
                   
                 60 
                 3.53 
                 3.24 
                 −.29 
               
               
                   
                 65 
                 3.67 
                 3.45 
                 −.22 
               
               
                   
                 80 
                 3.53 
                 3.44 
                 −.09 
               
               
                   
                   
               
            
           
         
       
     
     In order to achieve 80% recrystallization, the voltage was set at 5V. After the recrystallization process, the coil body length was measured. The coils were finished and mounted into lamps. The lamps were operated 1 minute and the coil body was measured again. From the data above, as the degree of recrystallization increases, the amount of coil contraction decreases. Therefore, the dimensional stability of the coil increases. 
     Although the preferred embodiments of the invention have been shown and described, it should be understood that various modifications may be resorted to without departing from the scope of the invention as disclosed and claimed herein.