GB2125209A - Mercury dispenser for electric discharge lamps - Google Patents

Abstract

A mercury dispenser 55 for an electric discharge lamp, e.g. a tubular fluorescent lamp, comprises two metallic walls shaped and sealed together. One of the walls is formed with dimples or depressions filled with mercury or a mercury-containing intermetallic compound before the other wall is sealed over it. One of the walls is the cathode disintegration shield 10 itself while the other is either a discrete metallic "patch" member welded to the shield or is formed from the shield itself by cutting, bending over and welding. Either one of the walls is perforated and the perforation is sealed by fusible non-metallic material e.g. glass 64, whereby in use on heating the dispenser 55 the glass softens and permits mercury 65 to escape from the dispenser through the perforation 62. <IMAGE>

Description

SPECIFICATION Mercury dispenser for electric discharge lamps This invention concerns a mercury dispenser for electric discharge lamps, especially lamps having a sealed transparent or translucent envelope containing at least one cathode, at least one gas at substantially reduced pressure and a certain amount of mercury. Such discharge lamps include fluorescent lamps and low pressure mercury discharge lamps, but may also include cold cathode glow discharge tubes.

In the manufacture of fluorescent tubes the introduction of an accurately metered amount of mercury into an already sealed and evacuated lamp envelope presents a great problem, not only technologically but also from the point of view of preventing the escape of mercury which is, of course, biologically toxic.

The conventional technique involves the use of an electromagnetic valve dispenser to dispense liquid mercury into a portion of an exhausting machine adjacent the exhaust tube (sometimes referred to as "tubulation") and then blowing or dropping the droplet of mercury into the interior of the envelope by means of a stream of argon, which is also the fill gas. This technique suffers from several drawbacks. Firstly, the dispenser cannot dispense an exact amount of mercury.

Secondly, tiny amounts of mercury may never reach the tube envelope but may instead get stuck along the dispensing path, e.g. in the dispenser itself or in the exhaust tube. Thirdly, as the dispensing takes place in a hot environment, evaporation losses may occur. Because of these disadvantageous factors the amount of mercury usually dispensed considerably exceeds the actually desired amount and this is wasteful of a not-expensive raw material of finite abundance.

Furthermore, on breakage of a tube, excessive amounts of harmful mercury may escape into the environment.

One prior proposal to overcome this drawback is disclosed and claimed in our European patent application No. 81304874.1 (Published No.

0 050 509). In this application, there are described numerous embodiments based on the concept of forming a mercury dispenser in the manner of a metallic 'patch' on the anti-sputtering or disintegration shield, the outer surface of the shield constituting at least one wall of the 'patch'.

The 'patch' contains or traps the required amount of mercury by virtue of having one of its walls dimpled; in other words, there is provided a mercury dispenser for an electric discharge lamp comprising at least two metallic walls shaped and sealed together so as to form a heat-rupturable container for mercury or a mercury-containing inter-metallic compound, characterised in that at least one of said walls forms part of a cathode disintegration shield or of a blank for a cathode disintegration shield.

The present invention represents in some aspects thereof a further development of the 'patch' concept of the aforesaid earlier application and is directed at facilitating the opening of the 'patch' dispenser and thus the release of the mercury vapour: in other aspects it aims generally at reducing or eliminating the risks of contamination of the internal atmosphere of discharge lamps due to the phenomenon known as 'outgassing' when metallic bodies are heated to high temperatures.

According therefore to one aspect of this invention, there is provided a mercury dispenser for an electric discharge lamp comprising two metallic walls shaped and sealed together so as to form a heat-rupturable container containing mercury or a mercury-containing intermetallic compound, wherein one of said walls is perforated and the perforation is sealed by a fusible nonmetallic material, preferably glass, whereby in use on heating the dispenser the glass softens and permits mercury to escape from the dispenser through the perforation.

In one alternative, the dispenser set forth above is a discrete unit or capsule mountable at any desired location within the discharge lamp envelope. However, in a preferred embodiment, it is in the form of a 'patch' on the cathode disintegration shield, as in our aforesaid patent application; in other words, at least one of said walls forms an integral part of the cathode disintegration shield, or a blank or strip from which such a cathode disintegration shield is to be made.

The invention is another aspect also extends to an article of manufacture comprising a continuous flat ribbon of metal provided with a plurality of discrete heat-rupturabie containers for mercury or a mercury-containing intermetallic compound, said ribbon is preferably but not necessarily disintegration shield blank material severable into individual blanks each of which is foldable into discrete shields, wherein each of said discrete containers or dispensers is as set forth in the last but one preceding paragraph.

A further aspect of the invention embraces an electric discharge lamp having a sealed and evacuated envelope a mount sealed to the envelope the mount supporting a cathode and a disintegration shield around said cathode, said shield forming at least one wall of a mercury dispenser or container which is as set forth in the last but second preceding paragraph.

In a further aspect of the present invention there is provided a method of manufacturing a mercury dispenser comprising forming a continuous ribbon with depressions formed at a predetermined spacing, making a perforation in each said depression, sealing said perforation by an amount of fusible non-metallic material, e.g.

glass, placing in each depression liquid mercury or a mercury-containing intermetallic compound and securing a metallic wall over each depression so as to form a sealed container or dispenser of mercury. In an alternative, the depression is left unperforated but instead the metallic wall or lid is perforated and the perforation plugged with a fusible, e.g. glass, material.

Preferably, said ribbon is a blank of disintegration shield material and said wall is a discrete metallic member or a cut-and-folded part of said blank.

In one preferred embodiment the reduced shield cross-section is obtained by notching the shield with generally L-shaped notches to produce lugs or tags on either side of the "patch" and the lugs or tags are then bent inwardly (i.e. towards the position of the cathode) to prevent, in use, material sputtered off the cathode from reaching the envelope wall.

Alternatively, the "patch" may be off-centre with respect to the width of the shield and in the larger portion of the shield adjacent the "patch" an aperture is formed.

In yet another alternative a tongue or tag is formed on one circumferential end of an openlooped shield, and is then welded to the other circumferential end of the shield, the "patch" being located on this tongue or tag, to form a closed loop.

Preferably, the shield is formed with overlapping ends having a transverse (radial) gap therebetween, the container being disposed in said gap.

Yet another embodiment for reducing the cross-sectional area of the disintegration shield in the vicinity of the 'patch' or dispenser consists in a pair of circular or other apertures cut in the shield disposed symmetrically about the patch along the central longitudinal axis of the shield.

Preferred embodiments of the invention, purely by way of example, are illustrated in and will be described with reference to the accompanying schematic drawings, wherein Figure 1 is a perspective view, partially broken away, of a mercury dispenser for an electric discharge lamp, mounted on and forming part of the anti-sputtering cathode shield and the stay wire that carries the latter; Figure 2 shows successive steps A-A' to E-E' in the manufacture of a mercury dispenser according to Figure 1; the views A to E being perspective views while the views A' to E' are the corresponding views in cross-section; Figure 3 is a perspective view illustrating an embodiment similar to that shown in Figure 1 but on a smaller scale and taken from the opposite side of the mercury dispenser, wherein the capsule includes circular apertures not present in the embodiment shown in Figure 1; Figures 4 to 9 are respective schematic elevations of further preferred embodiments of the invention; Figures 10 and 11 illustrate another preferred embodiment of the invention, respectively in schematic elevation and fragmentary perspective view;; Figures 12 to 19 illustrate still further embodiments of ways of reducing the cross section of the disintegration shield in the vicinity of the mercury dispenser in schematic form, Figures 12 to 1 5 and 17 being in fragmentary elevation; Figure 1 6 being in fragmentary perspective view on an enlarged scale showing a stage of manufacture subsequent to that shown in Figure 1 5; Figure 1 8 is a side view taken on the arrow A in Figure 1 9 which in turn is a perspective view corresponding to Figure 17; and Figure 20 is a view corresponding to Figure 2E' but illustrating a further variant.

Referring first to Figures 1 to 3, there is shown a disintegration shield 10 surrounding the cathode of a fluorescent lamp tube. The shield is of metallic material and is welded to one end of a stay wire 11 the other end of which is sealed to a glass pinched steam. The assembly or sub-unit consisting of stem, lead-in wires and cathode filament is collectively referred to as a 'mount' 40.

The mount consists of a stem 41 with a flare 42, an exhaust tube 43 terminating in a hole 44 in the stem 41, a pair of spaced lead-in wires 45 pinchsealed in the stem, a cathode filament 46 secured between the upper ends (as viewed) of the lead-in wires 45, and the stay wire 11 sealed at one end in the stem 41 and welded at the other end to the shield 10. A tubular envelope 50 is fused to the flare 42.

The shield 10 has overlapping ends 12 welded together at 13. The inner end is bent inwardly to provide shielding for notches 14 cut into the shield 10 to form the reduced-section portion 1 5. In this embodiment this portion 1 5 is formed in the region of overlap between the ends. The 'patch' or mercury dispenser is disposed between the notches 14 and is generally designated 55.

Figure 2 illustrates successive stages of manufacture of a disintegration shield blank with an integral mercury dispenser 55.

Figure 2 A-A' illustrate the first step in which an elongated flat strip 60, e.g. of iron, is formed with a dimple 61, by any suitable means. Each dimple 61 is then perforated, e.g. by punching, to produce a small aperture 62 at the peak region of the dimple 61, Figure 2 B-B'. Typically, the diameter of the aperture 62 is 0.7 mm. Next, as seen in Figure 2 C-C', the strip 60 is turned over by 1800 (arrow 63) and a predetermined amount of glass 64, molten or melted in situ, is placed into the dimple 61. As the glass 64 cools and solidifies, it fills and partially protrudes through the aperture 62 to form a plug therein. The glass is chosen to have a softening point of about 4500C or more, and to have a coefficient of linear expansion compatible with that of the material of the strip 60 to prevent cracking of the glass during the manufacturing process as it is sealed to the metallic strip 60.

Now a predetermined amount of mercury (or a mercury-releasing amalgam or intermetallic compound) 65 is placed in the dimple 61 (Figure 2 D-D') and then the dimple 61 is sealed by welding a small, e.g. iron, plate 66 across the dimple 61 and over the mercury 65, Figure 2 E-E'. The plate 66 and the strip 60 may be of equal thickness or the plate 66 may be thinner.

The notches 14 are made subsequently.

The resulting semi-finished product may then be severed between adjacent dimples 61, bent into a shield in a conventional manner and assembled with the mount in a conventional machine, known as a 'mount mill', not shown.

In use, to release the mercury 65 from the dispenser 55 the latter is heated from an external source.

The heating may be effected e.g. external irradiation by an electron beam or laser beam or by induced electric currents. To this end, radio frequency (R.F.) coils (not shown) are used. As the metallic parts of the dispenser 55 is heated by induced currents, the glass 64 is also heated by conduction. By suitable choice of the softening ,* point of the glass, it is softened without, however, melting and dripping.

Thus the mercury 65, vaporised by the heating, can push softened glass through the hole 62 and escape from the dispenser 55. On either side of the dispenser 55 and of the notches 14 the shield 10 is provided with a respective circular hole 66, for a reason explained below.

Figures 4 to 9 will not be described in detail: they show variants of the disposition of the mercury dispenser 1 5 and of the manner of overlapping the ends of the disintegration shield 10. It is noteworthy that in Figures 4, 5, 7 and 8 the glass plug 64 faces towards the interior of the shield 10 but in each case the internal end of the shield 10 acts as a physical barrier to a purely radial mercury vapour flow path. This may be a useful feature where there is a risk that heat from the cathode in the cathode activation or aging step of fluorescent lamp manufacture could prematurely release the mercury from the 'patch'.

Conversely, in Figures 6 and 9 the flow path is radially outward but the outer end or flap of the shield 10 acts as a barrier against direct impact on the delicate 'phosphor' on the internal wall of the envelope 50.

Figures 10 and 11 illustrate an embodiment wherein, in order further to reduce any risk of damage to the phosphor from the bursting of the mercury dispenser the notched portion 1 5 is formed with integral bent tabs 70 which may be seen more clearly in Figure 11 where the inner one of the overlapping ends 1 2 has been removed, for clarity.

Figures 12 to 1 9 show various ways of notching the shield 10 and thus of forming the reduced-section portion 1 5. In each case, the reduced section portion 1 5 alone, or in combination with holes such as the holes 66 shown in Figure 3, is effective to create a current flow path of higher current density in order to concentrate the heating effect in the area of the 'patch' without wasting energy by heating the rest of the shield 10 to a high temperature.

Figure 1 2 shows straight-sided lateral notches 14. Figure 13 shows an alternative embodiment wherein the dispenser is offset from the longitudinal centreline of the strip 60 and the latter is not notched. However, the same enhanced current density can be achieved by forming a hole 67 alongside the patch. Such holes 67 along the length of the strip 60 may then be utilised as sprocket holes for engagement by a gear tooth or other projection of a feeding mechanism for feeding the strip 60 and/or as locating holes for correctly positioning the strip 60 in the severing operation or any other subsequent manipulation of the strip 60.

Alternatively, as in the Figure 6 embodiment, notches 14 running in a direction making an acute angle with the central longitudinal axis of the strip 60 may be made.

In Figures 15 and 16, the notches 14 are generally L-shaped and of mirror symmetry in relation to the central longitudinal axis of the strip 60, resulting in tabs 68 which are bent away from the reduced portion 1 5 and inwardly towards the cathode. In this way, the tabs 68 block a purely radial path of movement for sputtered-off particles.

In Figures 17 to 19, there is shown an embodiment in which the mercury dispenser is formed wholly and exclusively of the shield 10. A pair of parallel, transverse cuts are made from one edge of the strip 60 up to a fold line 80 to form a reduced portion 1 5. The uncut portion receives a depression 61 for the mercury. The cut portion 81 is then folded about line 80 to overlie the uncut, dimpled and mercury-filled portion and is welded to it at 13. The cuts may extend beyond the fold line 80, to form slits 82 further to reduce the width of the current flow path and so to increase current density in the region of the "patch".

Finally, in Figure 20 there is shown an embodiment wherein the dimple or depression 61 is not apertured: instead, it is the cover plate 66 which is apertured and the aperture is plugged by a glass plug 64. In adapting this variant to the embodiments shown in Figures 4 to 11, the orientation of the dimples 61 should be reversed.

Thus, e.g. in Figure 4 (as viewed) the dimple 61 is to be 'rotated' by 1 800 about the axis 90 indicated in broken lines whereupon the unperforated dimple will be on the outside and the plugged cover plate 66 will be between the overlapping shield ends 12. Similar transposition would be required in adapting the Figure 20 embodiment to that of Figure 6.

In any of the foregoing embodiments the shield may have two separate patches, the first one containing mercury and the second a metal or alloy, such as indium or indium-bismuth, which can form an amalgam with the mercury. This second patch may be slightly open all the time. In this way the atmosphere inside the envelope may be better controlled.

Claims (13)

1. A mercury dispenser for an electric discharge lamp comprising two metallic walls shaped and sealed together so as to form a heat-rupturable container containing mercury or a mercurycontaining intermetallic compound, wherein one of said walls is provided with at least one perforation which is sealed by a fusible nonmetallic material, whereby in use on heating the dispenser the said non-metallic material fuses and permits mercury to escape from the dispenser through the or each perforation.

2. A mercury dispenser according to claim 1 wherein the said non-metallic material is a glass.

3. A mercury dispenser according to claim 1 or claim 2 wherein the dispenser is a discrete unit or capsule mountable at any desired location within the discharge lamp envelope.

4. A mercury dispenser according to claim 1 or claim 2 wherein at least one of said metallic walls forms an integral part of a cathode disintegration shield of the discharge lamp or of a blank or strip from which such a cathode disintegration shield is to be made.

5. A mercury dispenser according to claim 4 wherein the cross-sectional area of the disintegration shield is reduced in the vicinity of the dispenser by the provision of at least two circular or other apertures in the shield and disposed symmetrically about the dispenser along the central longitudinal axis of the shield.

6. A mercury dispenser substantially as herein described with reference to any of the embodiments illustrated in the accompanying drawings.

7. An article of manufacture comprising a continuous flat ribbon of metal provided with a plurality of discrete heat-rupturable dispensers as claimed in any preceding claim.

8. An article according to claim 7 wherein said ribbon is of disintegration shield blank material severable into individual blanks each of which is foldable into discrete shields.

9. An electric discharge lamp having a sealed and evacuated envelope, a mount sealed to the envelope, the mount supporting a cathode and a disintegration shield around said cathode, said shield forming at least one wall of a mercury dispenser which is as claimed in any of claims 1 to 6.

10. A method of manufacturing a mercury dispenser comprising providing a continuous ribbon with depressions at a predetermined spacing, making a perforation in each said depression, sealing each said perforation with fusible non-metailic material, placing in each sealed depression liquid mercury or a mercurycontaining intermetallic compound and securing a metallic wall or lid over each depression so as to form a sealed dispenser of mercury.

11. A method of manufacturing a mercury dispenser wherein the method according to claim 10 is modified by leaving the ribbon unperforated but instead the metallic wall or lid is perforated and the perforation plugged with a fusible material.

12. A method according to claim 9 or claim 10 wherein said ribbon is a blank of disintegration shield material and said wall is a discrete metallic member or a cut-and-folded part of said blank.

13. An article according to claim 7, or a lamp according to claim 9, or a method according to claim 11, substantially as herein described with reference to any of the embodiments illustrated in the accompanying drawings.