Patent Publication Number: US-6657388-B2

Title: High-pressure discharge lamp

Description:
The invention relates to a high-pressure discharge lamp which is provided with a discharge vessel that encloses a discharge space, has a ceramic wall and is closed by a ceramic plug, said discharge space accommodating an electrode which is connected to an electric current conductor by means of a leadthrough element which projects into the ceramic plug with a tight fit, is connected thereto in a gastight manner by means of a sealing ceramic and has a first part which forms a cermet at the area of the gastight connection. 
     A lamp of the kind set forth is known from U.S. Pat. No. 5,424,609 (=EP 0587238). The filling of the known lamp contains metal halide in addition to mercury. 
     In the context of the present description and the claims the term “ceramic wall” is to be understood to mean a wall of metal oxide, for example sapphire, sintered polycrystalline Al 2 O 3  or YAG, as well as a wall of metal nitride, for example AIN. 
     The known lamp has a comparatively low power of 150 W at the most at an arc voltage of approximately 90 V. Because the electrode in such a lamp conducts comparatively small currents during operation of the lamp, the dimensions of the electrode may remain comparatively small so that a comparatively small internal diameter of the projecting plug suffices. In the case of a lamp having a rated power in excess of 150 W, or a substantially lower arc voltage, so in the case of large electrode currents, electrodes of larger dimensions are required. Consequently, the internal plug diameter will be larger accordingly. It has been found that in such lamps there is an increased risk of premature failure, for example due to breaking off of the electrode or cracking of the plug. 
     It is an object of the present invention to provide a way to mitigate said drawbacks. 
     To this end, a high-pressure discharge lamp of the kind set forth is characterized accordance with the invention in that the leadthrough element also includes a second part which is a metal part and extends from the cermet in the direction of the electrode. 
     It is an advantage of the lamp in accordance with the invention that surprisingly it has been found that an internal plug diameter of more than one millimeter can be used in the case of lamps that are suitable for larger electrode currents, and that premature failure of the lamp due to breaking off of the electrode or cracking of the plug is effectively counteracted. As a result of this construction of the leadthrough element the first part of the leadthrough element can be optimized in respect of the coefficient of expansion relative to the ceramic plug. To this end, the cermet preferably has a metal content of at the most 45% by volume, but preferably no more than 35% by volume. This is beneficial to the realization of a gastight connection that is capable of withstanding thermal shocks to a high degree. Because of the presence of the second, metal part of the leadthrough element the cermet is exposed to less high temperatures during operation of the lamp. Processes of attack, if any, will thus be delayed; this has a favorable effect on the service life of the lamp. The cermet is a sintered composition of a ceramic material and a metal. The metal of the cermet preferably corresponds to that of the metal part of the leadthrough element. This is beneficial to the realization of a solid connection between the cermet and the metal part of the leadthrough element. Metals that are suitable in this respect are preferably Mo and W, because each of these metals has a very high melting point and is capable of withstanding halogenide to a significant degree. 
    
    
     The above aspects and further aspects of the lamp in accordance with the invention will be described in detail hereinafter with reference to a drawing (not to scale). In the drawing: 
     FIG. 1 is a diagrammatic view of a lamp in accordance with the invention, and 
     FIG. 2 is a detailed view of the discharge vessel of the lamp shown in FIG.  1 . 
    
    
     FIG. 1 shows a high-pressure discharge lamp which includes a discharge vessel  1  which encloses a discharge space  11  and has a ceramic wall, said discharge space containing a filling that can be ionized. Two electrodes  50 ,  60  are arranged in the discharge space. The discharge vessel is enclosed by an outer bulb  101 , one end of which is provided with a lamp base  2 . A discharge occurs between the electrodes  50 ,  60  in the operating condition of the lamp. The electrode  50  is connected, via a current conductor  90 , to a first electric contact which forms part of the lamp base  2 . The electrode  60  is connected, via a current conductor  100 , to a second electric contact which forms part of the lamp base  2 . The discharge vessel, being shown in greater detail in FIG. 2 (not to scale), has a ceramic wall  10  and is sealed by a ceramic plug  30 ,  40 . The discharge space  11  accommodates the electrode  50 ,  60  which is connected, by way of a leadthrough element  70 ,  80 , to the electric current conductor  90 ,  100 , said leadthrough element projecting into the ceramic plug  30 ,  40  with a tight fit  37 ,  48  and is connected thereto in a gastight manner by means of a sealing ceramic  15 . At the area of the gastight connection the leadthrough element  70 ,  80  includes a first part  71 ,  81  which forms a cermet. The leadthrough element also includes a second part  72 ,  82  which is a metal part which extends from the cermet in the direction of the electrode  50 ,  60 . 
     A practical embodiment of a lamp in accordance with the invention as described above has a rated power of 400 W. Each of the electrodes consists of a tungsten bar of a diameter of 0.7 mm, one free end of which is provided with an electrode winding. The electrode is connected to a molybdenum rod which constitutes the second part of the leadthrough element. The Mo rod has a diameter of 1.45 mm. A first part of the leadthrough element, being formed by an Al 2 O 3  Mo cermet with 35% Mo by weight, is connected to the Mo rod. The cermet also has a diameter of 1.45 mm. The cermet is connected to an Nb rod having a diameter of 1 mm. The Nb rod constitutes the electric current conductor. The electrode, the first part and the second part of the leadthrough element all have a length of 7 mm. The ceramic plug has an internal diameter of 1.50 mm. 
     The filling of the discharge vessel includes 50 mg Hg, 20 mg metal halide in a ratio of 83% mol Nal, 9.8 mol % TII and 7.2 mol % DyI 3 . The discharge vessel also contains Ar under a pressure of 30 kPa in the cold condition of the lamp. 
     The lamp was subjected to an endurance test which consisted partly of continuous operation of the lamp and partly of a test during which the lamp was periodically switched on and off. After a continuous period of operation of 11,000 hours, the lamp was still in good condition; no cracking of one of the projecting plugs had occurred and attack had occurred to a very minor extent only at the area of the cermet in each of the leadthrough elements. It was found that the lamp and the leadthrough elements were still in good condition after a switching endurance test during which the lamp was switched on and off 300 times in a period of 3000 hours.