Patent Publication Number: US-5841222-A

Title: Low-pressure discharge lamp

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a low-pressure discharge lamp provided with a discharge vessel which is closed in a gastight manner and which contains an ionisable filling comprising an inert gas, electrodes being arranged inside the discharge vessel, between which electrodes a discharge path extends. At least one of the electrodes has a coil of a refractory metal which is electrically connected to current supply conductors which extend to outside the discharge vessel, the coil having a central zone covered with an electron emitting material, and, at either side, a boundary zone between the central zone and a respective current supply conductor, the boundary zones having a covering of a protective material. 
     Such a discharge lamp is known from U.S. Pat. No. 5,233,268. The known lamp is a low-pressure mercury discharge lamp having a tubular discharge vessel in which an electrode of the kind described above is arranged on either side. The coil of each electrode has a central zone covered with an electron emitting material composed of a mixture of oxides of the alkaline earth metals barium, calcium and strontium. Usually the emitter is applied in such lamps by covering the central zone with a suspension of carbonates of the alkaline earth metals mentioned above. After the electrodes have been mounted in the discharge vessel, but before the discharge vessel has been closed, the electrodes are resistively heated by a current passed through them. This results in a conversion of the carbonates into oxides, carbon dioxide being released during this process. The boundary zones adjoining the current supply conductors are usually left free of emitter, as these zones remain too cool during resistive heating to result in a conversion of the carbonates. Taking into account tolerances during the manufacturing process, the boundary zone of the coil is of the order of one to a few mm. An incomplete conversion of the carbonates would result in a conversion thereof in the finished lamp. The release of carbon dioxide in the discharge vessel would seriously hamper further lamp operation. 
     Circuits for supplying low-pressure discharge lamps can be subdivided into so-called hot starting and cold starting circuits. With the former type of circuit the electrode of the lamp is preheated before ignition of the lamp. In cold starting circuits the lamp is ignited without preheating. The latter type of circuit can be relatively simple and cheap as no additional means for heating the electrodes are required. However, the life of lamps operating on such a circuit is mainly determined by the switching life, i.e. the number of times they can be switched on. 
     A filling comprising an inert gas with a low atomic weight, such as neon, and at a relatively low pressure, of the order of a few mbar, is favourable for obtaining a high light output. However, these circumstances negatively influence switching life. Short operation periods also tend to reduce switching life. 
     In the known lamp the current supply conductors to the coil are covered by an insulating glass sleeve which also covers the boundary zones of the coil. This measure forces the arc to strike at the central zone. The measure significantly increases the switching life of the lamp as it prevents the discharge arc from applying itself to the metal of the boundary zone of the coil and thus damaging the latter during a cold start of the lamp. A drawback of the known lamp is, however, that the sleeves are relatively difficult to mount. In particular, this is a drawback in large-scale manufacture of the lamp. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a low pressure discharge lamp of the kind mentioned above which can be manufactured relatively easily and which still has a relatively long switching life. 
     According to the invention, the protective material comprises a ceramic material having a specific resistance less than 1000 μΩ.cm. Surprisingly, while these materials, in contrast to the materials used in the known lamp, have a relatively good electrical conduction, they do not negatively affect switching life, but instead increase the switching life. It is presumed that the discharge arc strikes at the covering on the boundary zones during cold ignition until the electron emitting material is sufficiently hot to function as such. Unlike insulating ceramics, ceramic materials of the kind mentioned above show a good adhesion to metals. This renders it possible to apply these materials to the coil relatively easily. The ceramic materials may, for example, be applied to the coil as a powder in a suspension with a suspension agent such as butyl acetate and a binder such as nitrocellulose. Alternatively, an aqueous suspension may be used for example. The suspension agent and the binder may subsequently be expelled through heating of the electrode. 
     A favorable embodiment of the low pressure discharge lamp of the invention is characterised in that specific resistance of the ceramic material is less than 100 μΩ.cm. In this embodiment, the covering of protective material comprises preferably at least one compound selected from the group of borides, carbides, silicides, and nitrides of at least one metal selected from the group of titanium, zirconium, hafnium, niobium, tantalum, molybdenum and tungsten. These materials have a specific resistance in the order of a few to a few tens μΩ.cm. Furthermore, these materials have melting points of over 2000° C., which is sufficiently high compared with temperatures normally prevailing in electrodes of low-pressure discharge lamps. 
     Borides of the metals from the above-mentioned group are particularly suitable for this purpose. Ceramic materials of this kind have a very high melting point, above 3000° C., and also a high dissociation energy of the order of 2000 kJ/mole. These properties render them also very suitable for use in lamps having electrodes operating under extreme circumstances. 
     The coil may have end zones extending beyond the current supply conductors to which it is connected. These zones may additionally be covered with the protective material. The covering of protective material may further overlap, for example, a few turns of the covering of electron emitting material. 
     Since the current conductors are generally relatively thick with respect to the coil, lamp life is not seriously affected when the discharge arc strikes them. However, it may result in sputtering of the current conductors, which can in its turn cause a discoloring of the discharge vessel and can have a negative effect on the functioning of the emitter. In a favorable embodiment, the low-pressure discharge lamp of the invention is characterised in that the protective material also covers a portion of the current conductors adjoining the coil. In this way discoloring of the discharge vessel and deterioration of the emitter is prevented. 
     The discharge lamp of the invention may have one electrode which is provided with a covering of protective material as described above. Such a lamp is suitable for operation on a power supply providing at least DC ignition, the electrode provided with the protective coating being the cathode. A favorable embodiment of the low-pressure discharge lamp according to the invention is characterised in that either electrode has a coil provided with a covering as described above. Such a lamp is also suitable for ignition on an AC power supply. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a low-pressure discharge lamp according to the invention in longitudinal section, and FIG. 2 shows a detail of the lamp of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The low-pressure discharge lamp shown in FIG. 1 is provided with a discharge vessel 10 which is closed in a gastight manner and contains an ionisable filling, here comprising mercury and an inert gas. For this purpose, an amalgam 14 of 5.5 mg of mercury with 180 mg of an alloy of PbBiSn is arranged in an exhaust tube 13 which is in communication with the discharge vessel. The inert gas is formed by 5 mbar of a mixture of neon and argon in a volume ratio of 75-25%. The discharge vessel 10 is provided with a luminescent layer 11 at an inner surface. Electrodes are arranged inside the discharge vessel 10, between which electrodes a discharge path extends. At least one of the electrodes, here both electrodes, has a coil 20 A , 20 B  of a refractory metal which is electrically connected and mounted to current supply conductors 30 A , 30 A  &#39;; 30 B , 30 B  &#39; at end portions 12 A , 12 B  of the discharge vessel 10. In this case the refractory metal is tungsten, but molybdenum or another refractory metal may alternatively be used. The current supply conductors 30 A , 30 A  &#39;; 30 B , 30 B  &#39; extend through the end portions 12 A , 12 B  to outside the discharge vessel 10. The end portion 12 A  with its current supply conductors 30 A , 30 A  &#39; and the electrode with coil 20 A  mounted thereto are shown in more detail in FIG. 2. The construction at the other end portion 12 B  is identical to that shown in FIG. 2. 
     The coil 20 A  has a central zone 21 A  covered with an electron-emitting material 22 A  comprising a mixture of barium oxide, calcium oxide and strontium oxide. On either side, the coil 20 A  has a boundary zone 23 A , 23 A  &#39; between the central zone 21 A  and a respective current supply conductor 30 A , 30 A  &#39;. The boundary zones 23 A , 23 A  &#39; have a protective covering 24 A , 24 A  &#39; comprising a ceramic material having a specific resistance of less than 1000 μΩ.cm. Suitable materials include a boride, a carbide, a silicide, or a nitride of at least one metal from the group of titanium, zirconium, hafnium, niobium, tantalum, molybdenum and tungsten. Preferably the ceramic material is a boride of a metal selected from the group. 
     In the embodiment of FIGS. 1 and 2, the boundary zones 23 A , 23 A  &#39; of the coil 20 A  have a covering 24 A , 24 A  &#39; comprising a layer of particles of zirconium boride having a particle size mainly below 5 μm. Zirconium boride has a melting point of about 3200° C. and a dissociation energy of 1952 kJ/mole. Its specific resistance is 9.7 μΩ.cm. The protective covering 24 A , 24 A  &#39; overlaps about 1 mm of the central zone 21 A  covered with the electron-emitting material 22 A . 
     Lamps according to this embodiment were manufactured as follows. A suspension of a mixture of the alkaline earth metal carbonates: barium carbonate, calcium carbonate and strontium carbonate in butyl acetate as a suspension agent and nitrocellulose as a binder was applied to the central zone 21 A  of the coil 20 A . After this suspension was sufficiently dried, a suspension of zirconium boride particles, also in butylacetate and nitrocellulose in the present case was applied to the boundary zones 23 A , 23 A  &#39;. After the coil 20 A  had been mounted in the discharge vessel 10, it was heated resistively so as to convert the carbonates into oxides, while at the same time the binder and the suspension agent were expelled both from the electron-emitting material and from the protective material. 
     In an endurance test five lamps of the invention as described with reference to FIG. 1 and 2 and five lamps not according to the invention were periodically switched on for one minute and off for three minutes, to measure the switching life of each lamp. The lamps were supplied by a cold starting circuit. In the five lamps not according to the invention the end turns of the coils were not covered, but in other respects these lamps were identical to the lamp of the invention. The lamps not according to the invention had an average switching life of 3000±1000 switching operations. For the lamps according to the invention, a switching life of 7500±1000 switching operations was found. It is apparent that the lamps of the invention, while being relatively easily to manufacture, have a relatively long switching life compared with lamps without a covering of protective material. Accordingly, the measure of the invention allows for a relatively long switching life despite the inert gas consisting mainly of neon, which has a low atomic weight, and the pressure of the inert gas being relatively low.