US3869772A - Method of incorporating amalgam-forming material in a fluorescent lamp - Google Patents

Abstract

In the manufacture of a fluorescent lamp, a method of incorporating an amalgam-forming material, such as indium, as an integral part of the lamp for providing mercury vapor pressure regulation. During fabrication of the lamp mount structure, indium is applied to the hot flare of the glass stem immediately after the flare making step and prior to scoring and cutting. The non-flared end of the glass stem is then heated to the melting point and pressed to fuse a pair of lead wires and an exhaust tube into the stem. After completion of the mount, it is attached to the lamp by sealing the periphery of the stem flare to the end of the lamp envelope, and the lamp is further processed to completion.

Description

United States Patent Latassa et al.

[451 Mar/l1, 1975 METHOD OF INCORPORATING AMALGAM-FORMING MATERIAL IN A FLUORESCENT LAMP [75] Inventors: Frank M. Latassa, Magnolia;

Howard W. Milke, Danvers; .Tadius T. Sadoski, Salem, all of Mass.

[73] Assignee: GTE Sylvania Incorporated,

Danvers, Mass.

[22] Filed: Apr. 25, 1974 [21] Appl. No.: 463,952

[52] U.S. Cl. 29/25.l3, 316/25 [51] Int. Cl. HOlj 9/18, HOlj 17/26 [58] Field of Search 29/251, 25.11, 25.13,

[56] References Cited UNITED STATES PATENTS 3,048,737 8/1962 Rimbach 313/174 3,287,587 11/1966 Menelly 313/178 X 3,548,241 12/1970' Rash et a1. 316/25 X Primary ExaminerRoy Lake Assistant Examiner.1. W. Davie Attorney, Agent, or FirmEdward .1. Coleman [57] ABSTRACT In the manufacture of a fluorescent lamp, a method of incorporating an amalgam-forming material, such as indium, as an integral part of the lamp for providing mercury vapor pressure regulation. During fabrication of the lamp mount structure, indium is applied to the hot flare of the glass stem immediately after the flare making step and prior to scoring and cutting. The nonflared end of the glass stem is then heated to the melting point and pressed to fuse a pair of lead wires and an exhaust tube into the stem. After completion of the mount, it is attached to the lamp by sealing the periphery of the stem flare to the end of the lamp envelope, and the lamp is further processed to completion.

9 Claims, 7 Drawing Figures PATENTEU 1 75 HEAT SOURCE I 7 FiG.2

HEAT SOURCE METHOD OF INCORPORATING AMALGAM-FORMING MATERIAL IN A FLUORESCENT LAMP BACKGROUND OF THE INVENTION This invention relates to the manufacture of lowpressure mercury vapor discharge lamps and, more particularly, to a method of incorporating an amalgamforming material within the envelope of a fluorescent lamp at a location such that it regulates the mercury vapor pressure and permits the lamp to be operated at high power loadings and under high ambient temperature conditions with improved efficiency and lumen output.

It is well-known that the light output of a fluorescent lamp is a function of the mercury vapor pressure, which in turn often depends upon the temperature of the coldest region of the glass envelope of the lamp. It is further known that the envelope cold spot temperature for most efficient lamp operation is approximately 40C, which causes a mercury vapor pressure of approximately 4 to 6 10 Torr to occur inside the lamp. Often, due to high lamp loading or high ambient temperature, the envelope temperature and mercury vapor pressure rise above the optimum value.

Various methods of cooling portions of the lamp envelope to regulate vapor pressure have beem em ployed. Shields have been placed between the electrodes and the ends of the envelope; heat sinks have been attached to the envelope; and the lamp envelope has been increased in size, and made with grooves, depressions and the like. It has also been well-known that mercury vapor pressure may be reduced by the use of an amalgam-forming metal, such as cadmium or indium. U.S. Pat. No. 2,966,602 mentions such an application of an amalgam of mercury at column 4, lines 60-64. It has been observed that the location of the amalgam or amalgam-forming metal in the lamp is an important factor in providing the desired improvement in lamp operation. For example, U.S. Pat. No. 3,007,071 discloses the use of an amalgam-forming metal as a strip or powder located along the length of the tubular lamp envelope where it is not exposed to temperatures much higher than those in the discharge. In a lamp described by U.S. Pat. 3,392,298, the mercury vapor pressure is controlled by a coating ofindium in the form of a ring at the center of the tubular lamp envelope. Such use of an amalgam-forming metal is suitable for fixing the mercury vapor pressure after the lamp reaches thermal equilibrium, but the vapor pressure in the lamp will be extremely low when the lamp is first started since a considerable time is required for the middle part of the lamp, where the indium is located, to warm up.

Accordingly, the use of two sources of amalgam within a fluorescent lamp has been employed-- one which heats up rather slowly when the lamp is energized, and then controls the mercury vapor pressure during operation, and a secondary source of amalgam which is located closer to the electrodes and thus heats up at a faster rate and provides a sufficient amount of mercury vapor to enable the lamp to reach its output more rapidly. A fluorescent lamp of this type is disclosed in U.S. Pat. No. 3,227,907. According to this patent, the cool spot deposit of amalgam-forming material is provided by a band of indium at the center of the glass tube as described in U.S. Pat. 3,392,298, and

the hot spot deposit of indium is located on auxiliary electrodes, such as flag anodes, connected to the cathode coil lead-in wires. Such auxiliary electrodes generally rise quickly in temperature, and may reach temperatures as high as 300C to 400C. Whatever mercury is picked up by the indium in this location will be quickly vaporized into the atmosphere of the lamp and quickly diffused through it.

Although the above-discussed cool spot locations of amalgamsyviz a lengthwise strip or deposit of amalgam-forming metal or a center band of indium, can provide effective pressure regulation, it is difficult to apply the amalgam-forming metal with sufficient adherence at such locations on the interior surface of the lamp envelope, and since the lamp is heated to high temperatures during the preheating and baking operations, the amalgam has a tendency to melt and flow away from the desired location. In addition, some amalgam-forming metals or amalgams have such a low melting point that they become liquid at the operating temperatures within the lamp and thus create a situation where the amalgam may not remain at the desired location within the lamp. Further, the use of an amalgam center band or lengthwise strip will block radiation and thus cause an appearance defect and some loss of light output, which may be objectionable in specific applications. Also, amalgam locations toward the center of the lamp envelope are more sensitive to ambient temperature and thereby cause shifts in the mercury control point of the lamp.

According to one approach for overcoming the abovementioned disadvantages of prior cool spot locations of amalgam for providing the main vapor pressure control means within the lamp, a strip of the amalgamforming metal is placed in a wire mesh holder which is wrapped and tied or clamped about the cylindrical portion of the glass'mount stem at one or both ends of the lamp envelope. The temperature of the amalgam during lamp operation is dictated by the length of the stem and the selected axial portion of the mesh wrap along the stem (and thus its distance from the electrode). Such a structure is described in the following US. Pat. Nos: 3,373,303; 3,442,299; 3,526,802; 3,526,804; 3,534,212 and 3,619,697. A disadvantage of this approach is that it complicates the lamp making process by the additional manual or mechanical steps required to properly place the indium into the wire meshand attach the mesh about the mount.

In another approach to overcome the disadvantages of prior cool spot locations, the main vapor pressure control means is provided by depositing the amalgamforming material in a band about the glass mount stem at one or both ends of the fluorescent lamp. For example, U.S. Pat. No. 3,287,587 describes a method ofproviding such a band by rubbing a pellet of indium against the barrel portion of a glass stem which has been heated to a temperature of about 160C. A later reference, viz. U.S. Pat. No. 3,548,241, describes a method whereby a suitable amalgam-forming metal, such as indium, is heated to the liquid state and then sprayed onto the flared portion of one of the glass mount stems before it is sealed into the envelope. Preferably the stem is rotated while the spray is controlled to-deposit a band of the amalgam-forming metal having a thickness of less than microns which extends around the circumference of the flare portion of the stem. Starting is facilitated by an auxiliary (hot spot) source of amalgarn secured to an electrode cap (disintegration shield).

SUMMARY OF THE INVENTION In view of the foregoing, it is an object of the present invention to provide an improved method of manufacturing low-pressure mercury vapor discharge lamps.

A principal object of the invention is to provide an improved method of incorporating an amalgamforming material within a fluorescent lamp.

These and other objects, advantages and features are attained, in accordance with the principles of this invention, by a method comprising: heating a vitreous tube from which the stem is to be made to the softening point; flaring the heated stem tube at one end; and applying the amalgam-forming material onto the exterior surface of the flared portion of the heated stem tube. Thereafter, an exhaust tube and lead wires are inserted into the stem tube, and the non-flared end of the stem tube is heated to the melting point and then is pressed to fuse the exhaust tube and lead wires therein. After the mount structure is completed, it is attached to the lamp by sealing the periphery of the stem flare to the end of the lamp envelope, and the remaining fabrication of the lamp is completed. Several advantages accrue from this method of incorporating the amalgamforming material. Little or no modification of the automatic lamp and lamp mount processing equipment are required. A minimum of material loss results if the flare is improperly coated; i.e. only the flare, not a finished mount structure, must be discarded. The high temperature heating of the stem press subsequent to application of an amalgam-forming material, such as indium, produces a more intimate indium to glass bond and serves to decontaminate the free indium metals from impurities and trapped gases immediately prior to lamp fabrication. A resulting oxide layer on the indium serves as a protective crust during lamp operation to reduce or eliminate the absorption of contaminants and prevent the softened indium from running. Further, higher light outputs have been observed with lamps using stem flares precoated with indium in accordance with the invention, as compared with lamps employing stem flares coated with indium after fabrication of the mount structure.

BRIEF DESCRIPTION OF THE DRAWINGS This invention will be more fully described hereinafter in conjunction with the accompanying drawings in which:

FIG. 1 is a perspective view, partly in section, ofa fluorescent lamp made in accordance with the present invention, a portion of the bulb being removed for convenience;

and, in fabricating a mount structure according to the invention;

FIG. 2 illustrates a step of heating the end of the glass stem tube;

FIG.3 shows the stem tube after flaring one end;

FIG. 4 illustrates the application of indium to the hot stem flare by wire feed;

FIG. 5 illustrates the results of the score and cut operation;

FIG. 6 shows insertion of the exhaust tube and lead wires and illustrates heating of the non-flared end of the stem; and

FIG. 7 shows the partially fabricated mount structure after pressing and piercing.

DESCRIPTION OF-THE PREFERRED EMBODIMENT Referring to FIG. 1, a fluorescent lamp is shown comprising an elongated tubular glass envelope 10 having the customary coating 11 of phosphor on its inner surface and electrode mounts l2 sealed into each of its ends. The light-transmitting envelope is filled with a small amount of rare gas, such as argon, at low pressure, e.g., one to three Torr, and a small quantity of mercury, say 45 mgs., after which it is hermetically sealed in the usual manner by tipping off the exhaust tube 13 (see FIGS. 6 and 7) at one or both ends of the lamp.

Each mount structure 12 includes the typical reentrant vitreous stem 14 having at one end a flare portion 15 which is sealed about its periphery to the end of the tubular glass envelope, and a press 16 at the inward end supporting an electrode arrangement 17. More specifically the electrode comprises a cathode coil 18, preferably consisting of a coiled tungsten filament carrying the usual alkaline-earth oxide (electron-emissive) coating, supported on a pair of lead wires 19 and 20 sealed through the stem press 16 and extending to terminal pins 21 and 22 insulatively mounted in the lamp bases 23 attached to each end of the hermetically sealed, light-transmitting envelope 10. A pair of flag anodes 24 and 25 are also mounted on either side of and in parallel relationship with the cathode coil 18, preferably by attaching them to the ends of the lead wires.

The lamp of FIG. 1 further includes a band 26 of amalgam-forming material, such as indium, deposited on the flared portion 15 (cool spot location) of at least one of the mount stems 14 for regulating the mercury vapor pressure when the lamp is in operation. Another coating 27 of a much smaller quantity of amalgamforming material is located on at least one of the electrically connected flag anodes (hot spot location) for improving the starting characteristics.

The quantities of amalgam-forming material required at each location are variable from one lamp type to another. In addition, changes in the design of a lamp type (eg a change in the heat output ofa coil, or a change in the mercury content) may necessitate changes in indium quantities or placement. Satisfactory results have been obtained in F40Tl2 lamps employing approximately 2.5 mgs. of indium on each of four flags in a lamp (it may range from 3 to 10 mgs. of indium per inch of flag length) and from 40 mgs. to I40 mgs. of indium on each of two flares per lamp. The mercury content of the lamps was approximately 45 mgs. As previously mentioned, the main location of indium need not be applied to both flares. Also, the hot spot location of indium for facilitating starting may be applied to one flag rather than all four flags.

In accordance with the present invention, the main deposit of amalgam forming material, viz., band 26, is incorporated in the lamp of FIG. 1 by the following method of manufacture to provide a number of signifi cant advantages with respect to reduced manufacturing cost, a more reliable lamp construction, and improved operating results. With reference to the fabrication of a single electrode mount structure 12, a tube of glass or some other suitable vitreous material (see FIG. 2), from which the stem 14 is to be made, is initially heated at one end to the softening point. Next, the heated end of the glass stem tube is flared to provide portion (see FIG. 3). The next step on a conventional flaremaking machine would be a cutting and scoring operation to remove excess tubing, after which the mount 12 would be completed and the indium band would be applied prior to joining the mount to the lamp envelope. According to the present invention, however, the indium, or other amalgam-forming material, is applied to a hot flare on the flare-making machine after the flaring operation and before the scoring operation. Typically, the heated stem is rotated about its longitudinal axis on the flare-making machine, and the amalgam-forming material is applied inwardly from the rim of the flare 15 so that it is deposited near the bend region of the rotating flare in a layer of annular configuration, viz. band 26, that extends around the circumference of the flare. For example, as illustrated in FIG. 4, a preferred method of applying the indium, or other amalgamforming material, is to feed it as a wire 28 onto the hot flare portion 15 of the rotating stem tube. The rate of feed can control the specific amount of indium deposited. With smaller quantities of indium, it might be more desirable to cover only a segment of the flare circumference.

An alternative method of application is to use indium powder in 5 suspension and spray it onto the hot rotating flare. The weight of the deposited indium can then be controlled by the size of the nozzle and spraying time. Spraying of liquid indium metal could also be successfully used since the indium is being applied to an unfinished mount flare which allows for improved indium to glass bonding through subsequent high temperature heating of the flare in mount fabrication. Again, other amalgam-forming materials may be used in lieu of indium in the above described spray methods of application.

After applying the amalgam-forming material to the flare, the rotating stem tube is scored and cut to re move a segment 29 of the nonflared end thereof (see FIG. 5). Of course, the cutting step may be unnecessary if the desired stem length is used in the flare forming process. Next, an exhaust tube 13 and the pair of lead wires 19 and are inserted into the flared and cut stem tube, and the non-flared end of the stem tube is heated to the melting point (see FIG. 6) in open air. Then, the very hot non-flared end of the stem tube is pressed to fuse the exhaust tube 13 and lead wires therein. After piercing a hole through the hot press 16 into the exhaust tube 13, the partially fabricated mount appears as shown in FIG. 7. When the mount is heated to these high temperatures in the mount fabrication process in open air, a quite noticeable, dark oxide layer forms on the otherwise silvery appearing indium band. This has proved to be considerably advantageous in that it provides a protective crust over the indium, as will be discussed in more detail hereinafter.

Thereafter, fabrication of the mount structure is completed by shaping the leads extending from the press, attaching the cathode coil 18 across the ends of the lead wires 19 and 20, such as by clamping, and then attaching the flag anodes 24 and 25 to the lead wires, such as by welding, with the coating 27 of indium being applied to the outside surface of each flag anode either before or after attachment. For various lamp types and applications, of course, the flag coating 27 may not be required, or flag anodes may not be employed.

The described method of applying the primary deposit of amalgam-forming material (band 26) after flaring and before scoring provides several significant advantages. The only additional step in the normal lamp making process is the insertion of a wire-feed or nozzlespray operation while the stem tube is still on the flare making machine and, thus, early in the fabrication of the mount structure. Otherwise, there is no modification of automatic lamp and lamp mount processing equipment required, and the indium application (band 26) requires no other change in the normal sequence of manufacturing the lamp or lamp parts. Further, there is a minimum of material loss if the flare is im properly coated, as only the flare and not the finished mount need be discarded.

Application of the indium band to the hot flare before scoring and cutting allows for subsequent high temperature heating of the flare in the normal mount process (the non flared end of the stem is heated to the melting point prior to pressing) which produces a more intimate indium to glass bond. Further the normal heating of the flare during mount fabrication, particularly the heating of the non-flared end of the stem to the melting point prior to pressing, serves to outgas and de' contaminate the indium band 26 from impurities and trapped gases immediately prior to lamp fabrication. Further, the distinctive oxide layer over the indium band 26, resulting from the high temperature heating and pressing operations, provides a protective crust on the band 26 as the indium oxide has a higher melting point than the pure metal. Accordingly, when the indium is softened during sealing of the mount flare to the end of the glass envelope, the harder oxide overcrust will significantly reduce or eliminate the contaminants picked up by the soft indium. In addition the oxide layer helps to prevent the indium from running during lamp processing and operation. Of course. pre vious conventional methods of applying the oxide band 26, such as sprayingon the completed mount structure prior to sealing the lamp envelope, do leave a trace of oxide layer which is barely visible, however, the above noted advantages, obtainable with the obviously much heavier oxide layer produced by the present method invention, are not evident. The minimal nature of the oxide layer produced in conventional methods during the lamp sealing process is due to the fact that the indium is shielded by the glass envelope, tangential fires in the sealing operation heat a very small portion of the mount flare at its end producing less heat toward the indium layer, and the glass envelope acts as a heat sink.

Finally, testing oflamps made in accordance with the invention, as compared to the same type lamps with the indium applied to the stern flare after mount fabrication but before sealing the mount to the envelope, has exhibited a definite 2 to 3 percent increase in light output over an ambient temperature range of from 45F to F.

Although the invention has been described with respect to specific embodiments, it will be appreciated that modificationsand changes may be made by those skilled in the art without departing from the true spirit and scope of the invention.

What we claim is:

1. in the manufacture of a low-pressure mercury vapor discharge lamp having an electrode mount structure including a vitreous stem sealed to and extending inwardly from one end of the lamp envelope, the

method of incorporating an amalgam-forming material as an integral part of the lamp and at a location therein such that said material will regulate the mercury vapor pressure when the lamp is operated, said method comprising:

heating a vitreous tube from which said stem is to be made to the softening point thereof,

flaring said heated stem tube at one end,

applying said amalgam-forming material "onto the exterior surface of the flared portion of said heated stern tube,

inserting an exhaust tube and lead wires into said stem tube,

heating the non-flared end of said stem tube to the melting point thereof, pressing the non-flared end of said heated stem tube to fuse said exhaust tube and lead wires therein,

completing fabrication of the electrode mount structure which includes said stem tube, exhaust tube and lead wires, and

attaching said mount structure to the lamp by sealing the periphery of the flared portion of said stem tube to an end of said lamp envelope, and then completing the fabrication of the lamp.

2. The method of claim 1 wherein said amalgamforming material is applied inwardly from the rim of said flared portion while said heated stem tube is being rotated about its longitudinal axis so that siad material said deposited in a layer of annular configuration that extends around the circumference of said flared portion.

3. The method of claim 2 including the further steps of scoring and cutting said rotating stem tube to remove a segment of the non-flared end thereof subsequent to applying the amalgam-forming material and prior to'inserting the lead wires and exhaust tube.

4. The method of claim 2 wherein said amalgamforming material is applied by feeding a wire thereof onto the heated flared portion of said rotating stem tube.

5. The method of claim 4 wherein the amalgamforming material is indium.

6. The method of claim 1 including the further steps of scoring and cutting said stem tube to remove 21 segment of the non-flared end thereof subsequent to applying the amalgam-forming material and prior to inserting the lead wires and exhaust tube.

7. The method of claim 1 wherein said vitreous tube is glass, one of said glass tube is heated to the softening point thereof prior to said flaring step, and said amalgam-forming material is indium and is applied inwardly from the rim of said flared portion about the circumference thereof in a layer of annular configuration.

8. The method of claim 1 wherein said amalgamforming material is applied by using a powder thereof in a suspension and spraying it onto the heated flare portion of said stem tube.

9. The method of claim 1 wherein said amalgamforming material is applied by spraying a liquid thereof onto the heated flare portion of said stem tube.

Claims (9)

1. IN THE MANUFACTURE OF A LOW-PRESSURE MERCURY VAPOR DISCHARGE LAMP HAVING AN ELECTRODE MOUNT STRUCTURE INCLUDING A VITREOUS STEM SEALED TO AND EXTENDING INWARDLY FROM ONE END OF THE LAMP ENVELOPE, THE MTHOD OF INCORPORATING AN AMALGAM-FORMING MATERIAL AS AN INTEGRAL PART OF THE LAMP AND AT A LOCATION THEREIN SUCH THAT SAID MATERIAL WILL REGULATE THE MERCURY VAPOR PRESSURE WHEN THE LAMP IS OPERATED, SAID METHOD COMPRISING: HEATING A VITREOUS TUBE FROM WHICH SAID STEM IS TO BE MADE TO THE SOFTENING POINT THEREOF, FLARING SAID HEATED STEM TUBE AT ONE END, APPLYING SAID AMALGAM-FORMING MATERIAL ONTO THE EXTERIOR SURFACE OF THE FLARED PORTION OF SAID HEATED STEM TUBE, INSERTING AN EXHAUST TUBE AND LEAD WIRES INTO SAID STEM TUBE, HEATING THE NON-FLARED END OF SAID STEM TUBE TO THE MELTING POINT THEREOF, PRESSING THE NON-FLARED END OF SAID HEATED STEM TUBE TO FUSE SAID EXHAUST TUBE AND LEAD WIRES THEREIN, COMPLETING FABRICATION OF THE ELECTRODE MOUNT STRUCTURE WHICH INCLUDES SAID STEM TUBE, EXHAUST TUBE AND LEAD WIRES, AND ATTACHING SAID MOUNT STRUCTURE TO THE LAMP BY SEALING THE PERIPHERY OF THE FLARED PORTION OF SAID STEM TUBE TO AN END OF SAID LAMP ENVELOPE, AND THEN COMPLETING THE FABRICATION OF THE LAMP.

1. In the manufacture of a low-pressure mercury vapor discharge lamp having an electrode mount structure including a vitreous stem sealed to and extending inwardly from one end of the lamp envelope, the method of incorporating an amalgam-forming material as an integral part of the lamp and at a location therein such that said material will regulate the mercury vapor pressure when the lamp is operated, said method comprising: heating a vitreous tube from which said stem is to be made to the softening point thereof, flaring said heated stem tube at one end, applying said amalgam-forming material onto the exterior surface of the flared portion of said heated stem tube, inserting an exhaust tube and lead wires into said stem tube, heating the non-flared end of said stem tube to the melting point thereof, pressing the non-flared end of said heated stem tube to fuse said exhaust tube and lead wires therein, completing fabrication of the electrode mount structure which includes said stem tube, exhaust tube and lead wires, and attaching said mount structure to the lamp by sealing the periphery of the flared portion of said stem tube to an end of said lamp envelope, and then completing the fabrication of the lamp.

2. The method of claim 1 wherein said amalgam-forming material is applied inwardly from the rim of said flared portion while said heated stem tube is being rotated about its longitudinal axis so that said material is deposited in a layer of annular configuration that extends around the circumference of said flared portion.

3. The method of claim 2 including the further steps of scoring and cutting said rotating stem tube to remove a segment of the non-flared end thereof subsequent to applying the amalgam-forming material and prior to inserting the lead wires and exhaust tube.

4. The method of claim 2 wherein said amalgam-forming material is applied by feeding a wire thereof onto the heated flared portion of said rotating stem tube.

5. The method of claim 4 wherein the amalgam-forming material is indium.

6. The method of claim 1 including the further steps of scoring and cutting said stem tube to remove a segment of the non-flared end thereof subsequent to applying the amalgam-forming material and prior to inserting the lead wires and exhaust tube.

7. The method of claim 1 wherein said vitreous tube is glass, one of said glass tube is heated to the softening point thereof prior to said flaring stEp, and said amalgam-forming material is indium and is applied inwardly from the rim of said flared portion about the circumference thereof in a layer of annular configuration.

8. The method of claim 1 wherein said amalgam-forming material is applied by using a powder thereof in a suspension and spraying it onto the heated flare portion of said stem tube.

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