MERCURY DISPENSER WELDED TO CATHODE SHIELD OF FLUORESCENT LAMPS

The present invention relates to a mercury dispenser for fluorescent lamps.

As it is known, fluorescent lamps are formed of glass tubes (linear, circular or having complex shapes) with a filling made of one or more rare gases, generally argon or neon, and some milligrams of mercury; there are also two electrodes, also called cathodes, for triggering and maintaining an electric discharge in the gaseous atmosphere of the lamp, which causes the light emission. The electrodes are metal filaments placed at the two ends of the tube in the case of linear lamps (or having more complex shapes, but with two distinct ends), or in one area in the case of circular lamps. The electrodes are generally shielded laterally by members made of metal band, coaxially arranged with respect to the lamp; these shielding members stop the cathode material that is vaporized during the discharge throughout the lamp life, preventing the material itself from condensing on the walls of the glass tube, which would cause localized blackenings resulting in a reduced light emission.

The dosage of mercury in the lamps must be as precise and reproducible as possible. In fact, this element must be present in a minimum quantity, below which the lamp does not work, whereas it is preferable not to introduce doses of the element unduly greater than the necessary minimum because, owing to the mercury toxicity, this would cause environmental problems in the case of breaks of the lamp or anyway at the end of the life of the same. The appearance on the market of lamps more and more differentiated by shape, size and constituting materials, as well as the trend of the international standards imposing lower and lower mercury quantities per lamp, make the traditional method of dosing the element in liquid form practically unusable, because of the difficulty of obtaining mercury droplets of precisely controlled size with volumes lower than the microliter (equivalent to a few milligrams of the element).

Several systems have been suggested in order to overcome the problem.

Some of these systems provide for the mercury release in a fubulation connected to the lamp, forming a side chamber thereof during the manufacturing step, but being detached afterwards when the lamp is finished; examples of the use of this system are disclosed in patent US 6,680,571 and in patent application US 2001/0038264. This

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system is effective, but results in a complication in the manufacturing process, requiring two different steps for the sealing of the lamp, a first sealing of the tubulation in an area far from the lamp before the mercury evaporation, and a second sealing as close as possible to the lamp body after said evaporation, in order to detach the residual of the tubulation.

Other documents disclose systems wherein the mercury dispenser is inserted in the lamp body.

Patents US 4,823,047 and US 4,754,193 suggest the use of capsules containing liquid mercury, which are opened in various ways when the lamp is finished and sealed, but also in this case the exact and reproducible dosage of the element is difficult, likewise the construction of small size capsules is difficult.

Patent US 4,808,136 and patent application EP 568,317 suggest the use of pellets or little spheres made of porous materials soaked with liquid mercury; also in this case the exact and reproducible dosage is difficult, as it depends on the volume of the pores of the pellet or little sphere, which is difficult to control; further, the fixing of these pellets or little spheres in the lamp can be problematic (obviously these pellets or little spheres should not move, not to risk that they come into contact with delicate parts such as the filaments, which could be damaged).

Finally, patent US 6,107,737 suggests to manufacture the shielding members of the electrodes with metal bands on which a material is rolled capable of releasing mercury (and in case also a getter material, capable of sorbing gaseous impurities existing in the lamp); the shield is then heated from the outside while the lamp is closed, e.g. by means of radio frequencies, causing the emission of mercury. The system of this patent is functional and allows to dose in an exact and reproducible way very small quantities of mercury; however it has the drawback of not being suitable for the dosage of relatively high quantities of mercury (e.g. for large size lamps), and anyway the manufacturing of metal strips with rolled deposits of powder materials with linear loading of material known and reproducible, is rather complex and expensive.

Object of the present invention is to provide a simple system for releasing exact and reproducible quantities of mercury in fluorescent lamps.

According to the present invention this object is achieved with a mercury

dispenser formed of a pellet, having known weight and known mercury content, of a compound of general formula Ti _x Zr _y Hg ₂ , wherein x and y can range between 0 and 13, the sum (x+y) can range between 3 and 13 and z can be 1 or 2, said pellet being fixed by means of at least one welding spot on a metal shielding member of at least one of the lamp cathodes.

The inventor has discovered that bodies made of mercury compounds having general formula Ti _x Zr _y Hg _z as above described may be welded on the shield by electric or laser welding without losing significant quantities of mercury; in particular, the quantity of mercury emitted by pellets of these materials during electric or laser welding is lower than 1% by weight, whereby essentially all mercury contained in the pellet when manufactured remains in the pellet itself during welding, so as not to invalidate the possibility of controlling, during pellet manufacturing, the quantity of mercury that will be possible to release afterwards in the lamp.

The invention will be illustrated hereinafter with reference to the drawings in which:

Figure 1 shows the terminal part of a known lamp

Figures 2, 2. a and 3 show two possible alternative embodiments of the mercury dispenser of the invention.

Figure 1 shows a broken view of the terminal part of a lamp of known type; an end of a linear lamp is represented, but the description is also valid for lamps of different and more complex geometries; lamp 10 has a cathode 11 (here represented as a coil filament, but other geometries are possible), carried by two supports 12 and 12', immersed in the glass part 13, closing the lamp, and passing therethrough, thus resulting in electrical contact with the two electrical supply contacts 14 and 14'; a metal shield 16, which is made of common or nickel-plated steel and has the function previously described, is carried by a further support, 15, one end of which is also immersed in part

13 without necessarily passing therethrough.

Figure 2 and 2. a show, from two opposite points of view, a first type of mercury dispenser 20 of the invention, formed of a pellet 21 of a material capable of releasing mercury fixed on the shield 22 by means of at least one welding spot 23 (only one shown in figure); Figure 2 shows the dispenser 20 from the side of the pellet, in order to

highlight it, while Figure 2. a shows the same dispenser seen from the opposite side, thus highlighting the welding spot. The drawings show also the welding spots 24 and 24' by means of which the two ends of the initial metal band are closed to form a ring.

Figure 3 shows another possible embodiment of dispenser of the invention. In this case the dispenser 30 is formed of a pellet 31 of mercury dispensing material, fixed by means of a welding spot 33 between the two ends 34 and 34' of a band 32 closed as a ring; in the drawing only one welding spot is shown, but more than one could be present. In this case, the same welding spot (or spots) that fix the pellet between the ends 34 and 34' also have the function of closing the metal band as a ring. The welds of the invention may be obtained by electric welding, interposing the parts to be welded between two electrodes, generally made of copper, and causing an intense current to pass between the electrodes so as to obtain a localized melting of the concerned parts; or by laser welding, as it is well known to those skilled in the art.

The compounds having general formula Ti _x Zr _y Hg _z , used for forming the pellets 21 or 31, are described in patent US 3,657,589. The preferred compound is Ti ₃ Hg, produced and sold by the applicant under the tradename St 505, or mixtures thereof with alloys promoting mercury release from these compounds; these alloys, being copper based, are described for example in patents US 5,520,560, US 5,831,3S5 and US 5,876,205. Mercury is released from these compounds by heating from the outside, e.g. by means of radio frequencies. The compounds of the Ti _x Zr _y Hg ₂ type start to release mercury at about 500 ⁰ C; since, as mentioned, the loss of mercury during welding the pellet onto the band is negligible, being known the initial pellet weight and its percentage content of mercury, it is possible to control the quantity of mercury released by the dispenser in the lamp.