Abstract:
A mercury capsule for use in a fluorescent lamp comprises a shell defining a chamber and a bore extending through the shell. A body of mercury is disposed in the chamber. A plug is disposed in the bore to seal the bore. The plug exhibits a melting point reached in manufacture of the fluorescent lamp, to melt from the bore to open an exit passageway for the mercury. A method for making the capsule is provided.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to fluorescent lamps which contain mercury, and is directed more particularly to means by which mercury is released into a lamp during manufacture of the lamp.  
           [0003]    2. Description of the Prior Art  
           [0004]    In FIGS. 1 and 2 there is shown an illustrative capsule  10  of the type to which the present invention pertains. The capsule  10  comprises a metal ribbon  12  comprising a first portion  14  having a depression  16  formed in a surface  18  thereof for receiving and retaining a body of liquid mercury  30  (FIG. 3). The ribbon  12  further comprises a second portion  20  having a protrusion  22  formed on a surface  24  thereof. The protrusion  22  and depression  16  are of complementary configuration. The ribbon  12  still further includes a bendable portion  26  which interconnects the first and second portions  14 ,  20 . The first and second portions  14 ,  20  are bendably movable to the position shown in FIGS. 2 and 3, and thence to the positions shown in FIGS. 5 and 6, wherein the protrusion  23  is clamped into sealing engagement with the depression  16 , to form an enclosed chamber  32  (FIG. 6) in which the mercury  30  is sealingly captured.  
           [0005]    The capsule  10  may then be handled in a fluorescent lamp fabrication environment without special provisions for handling mercury, and insuring safety to the environment and to personnel, inasmuch as the mercury is securely sealed in the capsule. Once in the lamp, however, the capsule  10  must be ruptured to permit the mercury to enter the lamp envelope. Rupturing of the capsule is accomplished by means of application of heat to the capsule by way of radio frequency energy directed to the metal of the capsule, preferably nickel plated stainless steel, and the mercury, to raise the temperature of the metal and the pressure of the mercury. The heating of the metal and the pressurization of the mercury serve to rupture the capsule, permitting the mercury to escape into the lamp envelope. Unfortunately, a substantial portion of the lamp is heated during the capsule rupturing step, including portions which can be deleteriously affected by exposure to high heat.  
           [0006]    It is deemed beneficial to provide a capsule of similar structure, but with facility for releasing mercury at lower temperatures which do not risk damage to other portions of the lamp.  
         SUMMARY OF THE INVENTION  
         [0007]    An object of the invention is, then, to provide a mercury capsule for use in fluorescent lamps, which capsule retains the advantages of the above described capsule, and which, in addition, is capable of releasing the mercury into the lamp when acted upon by a relatively low temperature.  
           [0008]    A still further object of the invention is to provide such a capsule capable of releasing its full content of mercury in a relatively short time.  
           [0009]    With the above and other objects in view, as will hereinafter appear, a feature of the present invention is the provision of a mercury capsule for use in a fluorescent lamp, the capsule comprising a shell defining a chamber and a bore extending through the shell, a body of mercury disposed in the chamber, and a plug sealing the bore, the plug having a melting point less than a melting point of the capsule otherwise, to melt from the bore to open an exit passageway for the mercury.  
           [0010]    In accordance with a further feature of the invention, there is provided a fluorescent lamp having an envelope of light-transmitting vitrous material, having opposed end portions and containing an inert gas. First and second electrodes are respectively disposed within the opposed end portions, and a pair of lead-in wires are connected to each of the electrodes. A mercury capsule is secured to one of the lead-in wires. The mercury capsule comprises a shell defining a chamber and a bore extending through the shell, a body of mercury disposed in the chamber, and a plug sealing the bore, the plug having a melting point less than a melting point of the capsule otherwise, to melt from the bore to open an exit passageway for the mercury.  
           [0011]    In accordance with a still further feature of the invention, there is provided a method for making a mercury capsule for use in a fluorescent lamp. The method comprises the steps of forming a metal shell for receiving a body of mercury, forming a bore in the shell, closing the bore with molten metal, and permitting the molten metal to solidify to form a plug in the bore. The method includes the further steps of depositing a body of mercury in the shell, and sealing the shell closed with the body of mercury therein. The plug exhibits a melting point reached in manufacture of the lamp, to melt from the bore to open an exit passageway for the mercury.  
           [0012]    The above and other features of the invention, including various novel details of construction and combinations of parts and method steps, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular devices and methods embodying the invention are shown by way of illustration only and not as limitations of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    Reference is made to the accompanying drawings in which are shown illustrative embodiments of the invention, from which its novel features and advantages will be apparent.  
         [0014]    In the drawings:  
         [0015]    [0015]FIG. 1 is a top plan view of a blank from which is made a capsule of the type to which the invention pertains;  
         [0016]    [0016]FIG. 2 is a side elevational view of the blank of FIG. 1;  
         [0017]    [0017]FIGS. 3 and 4 are similar to FIGS. 1 and 2, respectively, but showing steps in making of the capsule;  
         [0018]    [0018]FIG. 5 is a top plan view of the capsule;  
         [0019]    [0019]FIG. 6 is a sectional view of a portion of the capsule;  
         [0020]    [0020]FIG. 7 is a centerline sectional view of the blank of FIG. 1 but showing a step in the making of a capsule in accordance with the present invention;  
         [0021]    [0021]FIGS. 8 and 9 are similar to FIG. 7 but showing additional steps in the making of the capsule;  
         [0022]    [0022]FIG. 10 is a centerline sectional view of a portion of the completed capsule;  
         [0023]    [0023]FIG. 11 is a partially sectional, partially elevational, view of a lamp electrode assembly with the capsule of FIG. 10 fixed thereto;  
         [0024]    [0024]FIG. 12 is a side elevational view of a fluorescent lamp having the electrode assembly and capsule of FIG. 11 therein; and  
         [0025]    [0025]FIG. 13 is similar to FIG. 10 but diagrammatically illustrating release of mercury in manufacture of the lamp of FIG. 12.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0026]    Referring to FIG. 7, it will be seen that the ribbon  12  is provided with a small bore  60  extending through the wall of the depression  16 . The bore preferably is formed with a diameter of about 0.018 inch.  
         [0027]    The bore  60  is covered by a plug  62  of an alloy of zinc and aluminum, preferably 95-98% zinc and 2-5% aluminum, by weight. The ribbon  12  preferably is about 0.006 inch in thickness. The plug  62  exhibits a melting temperature of about 382° C.-422° C. It has been found that an alloy of 95% zinc and 5% aluminum exhibits a melting point of about 382° C.; an alloy of 98% zinc and 2% aluminum exhibits a melting point of about 402-422° C.  
         [0028]    To effect plugging of the bore  60 , the ribbon first portion  14  is dipped into a pool of molten alloy. Prior to dipping, the outside surface of the capsule may be coated with a flux which provides an interface between the metal ribbon and the alloy, which aids in the adhesion of the alloy to the ribbon. The capsule is removed from the molten alloy with a small film of alloy adhering to the capsule. Upon solidification of the alloy, the bore  60  is thereby covered and sealed (FIG. 8).  
         [0029]    After the plug  62  is in place, the mercury  30 , in liquid form, is placed in the depression  16 . The second portion  20  of the ribbon  12  is moved by bending the portion  26 , and the protrusion  22  is clamped into the depression  16  to form a shell and to seal the mercury  30  in the chamber  32  of the shell.  
         [0030]    The capsule  10  is then attached to a lead-in wire  40  of a first electrode assembly  42  (FIG. 11) fixed in a first end portion  44  of a fluorescent lamp  46  (FIG. 12) defined in part by an envelope  48  of vitrous material and provided with a second electrode assembly  50  fixed in a second end portion  52 , and filled with an inert gas, as is known in the art.  
         [0031]    The capsule ribbon  12  preferably is provided with a clamp portion  34  including integral tabs  36 ,  36 ′, as shown in FIGS. 1 and 3, which may be crimped upon a lead wire  40 , as shown in FIG. 11. The tabs  36  are spaced from each other to define a notch  38  which is configured to receive the tab  36 ′. Thus, the tabs  36 ,  36 ′ may be bent around the lead wire  40  to clamp the capsule  10  to the lead wire  40 .  
         [0032]    The lamp  46  is then subjected to RF heat, producing a temperature sufficient to melt the plug  62 , which opens the bore  60  and allows the mercury  30  to escape (FIG. 13) into the envelope  48  of the lamp. The plug material and the length and diameter of the bore  60  are important considerations. Too small a bore may require too long a heating time for the plug to melt and the mercury to escape from the capsule into the lamp. For example, a bore plugged with the above-described alloy, and having a diameter of 0.0008 inch, and 0.005 inch long, when subjected to 400° C., has been found by calculation to require over ten seconds to release 5 mg of mercury, an unacceptable length of time in a typical lamp production line. However, a bore plugged with the same alloy, having a diameter of 0.018 inch, and a length of 0.006 inch, when subjected to 400° C., has been found by experiment to exhibit a release time of about five seconds.  
         [0033]    There is thus provided a mercury capsule for fluorescent lamps, which capsule is adapted to release mercury at a low release temperature, and a temperature unlikely to deleteriously affect portions of the lamp, including the capsule other than the plug  62 . The capsule is further adapted to release all its mercury in about five seconds, which is acceptable for production purposes. Prior art capsules having a heat-activated release facility commonly require a release temperature of more than 600° C. The reduced release temperature requirement of the inventive capsule reduces the heating time required to reach release temperature.  
         [0034]    The temperature required to open a capsule which is hermetically sealed, such that no mercury leaks out of the capsule during processing, depends on the sealing process. The sealing process must be suitable for subsequent lamp operation, which rules out commonly used epoxies and other adhesives. Known and useable hermetic sealing methods, such as arc welding, result in a seal which cannot be opened without excessive heating. There is no known sealing method for a capsule configuration of the type shown in FIGS.  1 - 6 , which provides both a hermetic seal and an opening temperature compatible with manufacturing. The use of a melting plug for releasing mercury thus divorces the capsule opening means from the capsule sealing means.  
         [0035]    It is to be understood that the present invention is by no means limited to the particular construction and method steps herein disclosed and/or shown in the drawings, but also comprises any modification or equivalent within the scope of the claims.