MOUNTED TO CATHODE SHIELD

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 luminous 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) and in one area in 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 causes localized blackenings that reduce 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 tubulation 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 Bl and in patent application US 2001/0038264 Al.

_ O .

This 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,S23,047 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-A-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 of known weight of a material with a known content of mercury mechanically fixed on a metal shielding member of at least one of the lamp cathodes.

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

Figure 1 shows the terminal part of a known lamp

Figures 2, 2a, 2b and 3 show various alternative embodiments of the mercury dispenser of the invention.

Figure 1 shows a broken view of the terminal part of a lamp of a 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 1 1 (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 shows a first type of mercury dispenser 20 of the invention, formed of a pellet 21 of a material capable of releasing mercury which is held on the shield 22 by means of loops which are obtained by folding the shield band, and form a seat for the pellet. Figure 2a shows, in a top plan view, the part of the same dispenser wherein the pellet is placed, in order to highlight the housing and mechanical fixing system of the pellet. In order to improve the mechanical fixing effect of the pellet, it is possibile to use the expedient illustrated in Figure 2b, which is a preferred embodiment of the dispenser of Figure 2: in this case, in addition to forming the loops shown in Figures 2 and 2a, the four corners of the loops are crimped, that is, they are closed by mechanical compression, so to avoid any possibility that the pellet slips outside its seat in the dispenser; in Figure 2b the two crimped upper comers 23 and 23' are shown, and the left part of the Figure is a broken view in order to show the deformation of the band. 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,

mechanically held on a band 32, which, being closed in a loop, forms the shield for one electrode of the lamp. The retention members 33, holding the pellet 31 in place are obtained in this case by forming, through cutting from the band, four "tongues" of a length slightly greater than the thickness of the pellet and of a height sensibly lower than that of the band (e. g. not more than one tenth of the band height) and folding them over onto the pellet.

Numerous other embodiments for the mechanical fixing of the pellet on the shield will obviously result possible for those skilled in the art, which, however, do not move away from the present invention. The shields shown in the Figures are generally obtained starting from a metal band rolled up and closed by means of welding spots not shown in the figures.

The material with which to form the pellets 21 or 31 can be an amalgam, for example with one or more metals like zinc, tin _. , silver, indium or bismuth. Alternatively, it is possible to use compounds of the general formula Ti _x Zr _y Hg _z , wherein x and y can vary between 0 and 13, the sum (x + y) can vary between 3 and 13 and z can be 1 or 2, described in patent US 3,657,589, and in particular the compound Ti ₃ Hg, or mixtures thereof with alloys promoters of mercury release by these compounds; these alloys, being copper based, are described for example in patents US 5,520,560, US 5,831 ,385 and US 5,876,205. The mercury is released from these pellets by heating from outside, e. g, by means of radio frequencies. As the amalgams generally have melting points lower than about 200 ⁰ C, these pellets can be used in the case wherein the shield is sufficiently far from the glass terminal part of the lamp, otherwise the heating treatment thereof for its sealing can cause mercury emission in an early step of the manufacturing process, the lamp being not yet closed; on the other hand, compounds of the type Ti _x Zr _y Hg _z show a mercury release which is practically zero until about 500 ⁰ C, thereby they are suitable for the production of devices according to the invention for the use in lamps of any type.

Being known the pellet weight and the percentage content of mercury, it is possible to control the quantity of mercury released by the dispenser in the lamp.