Electric lamp having an envelope with an intermediate zirconium oxide coated layer
The invention relates to an electric lamp comprising a lamp vessel of glass having a silica content of at least 95% by weight. The lamp vessel is sealed in a vacuum-tight manner and has a seal through which at least one current supply conductor extends to a light source arranged inside the lamp vessel. The current supply conductor is connected to a contact on a lamp cap which is fixed with cement on the seal of the lamp vessel. Such a lamp is known from British patent specification No. 2,089,563. 
     Lamps of this kind may be subjected during operation to a high thermal load, especially when they are operated in a luminaire. The seal of the lamp vessel is liable to crack at the area at which it is secured in the lamp cap, which results in the lamp seal becoming defective. 
     The invention has for its object to provide an improved lamp in which the glass seal is prevented from cracking and the lamp seal is prevented from becoming defective. 
     According to the invention, this object is achieved in a lamp of the kind mentioned in the opening paragraph in that the seal of the lamp vessel has a coating comprising zirconium oxide at the area at which it is surrounded by the cement. 
     The coating of zirconium oxide causes the adherence of the cement to the seal to be reduced. Nevertheless, the lamp vessel is rigidly anchored in the lamp cap because the cement tightly surrounds the seal of the lamp vessel. In lamps according to the invention, cracks and leakage were no longer observed, probably due to the reduction of the adherence of cement to quartz glass. 
     The invention is applicable to lamps of different kinds and intended for different lamp applications. The light source may be a filament or a pair of electrodes in an ionizable gas filling. Furthermore, the light source may comprise an inner envelope arranged inside the lamp vessel. 
     The lamp vessel may have one seal through which current supply conductors of the lamp are passed, or it may have several seals, for example, two seals. At least one current supply conductor extends through each seal of the lamp vessel, and each seal is fixed by cement on a lamp cap. The lamp cap may mainly consist of ceramic material, of mainly metal or of a combination of ceramic material and metal. 
     The lamp may be used as a studio, theatre or projection lamp. 
     The coating of zirconium oxide is applied to the seal by immersing in, spraying or coating with, a suspension of zirconium oxide in, for example, butylacetate, or alternatively by applying zirconium oxide by flame spraying. The coating of zirconium oxide may contain, for example eight % by weight of a glass enamel, which itself consists, for example, of 37.7% by weight of SiO 2 , 22.0% by weight of B 2  O 3 , 0.05% by weight of Na 2  O, 3.8% by weight of CaO, 18.4% by weight of BaO, 17.8% by weight of Al 2  O 3 , 0.025% by weight of Fe 2  O 3 . 
    
    
     Embodiments of the lamp according to the invention will now be described, by way of example, with reference to the accompanying drawing. In the drawing: 
     FIG. 1 shows a first embodiment of a lamp in side elevation, 
     FIG. 2 shows a second embodiment of a lamp in side elevation with a lamp cap broken away. 
    
    
     The lamp shown in FIG. 1 has a quartz glass lamp vessel 1 sealed in a vacuum-tight manner and having a seal 7, through which current supply conductors 5 extend to a light source 10 arranged inside the lamp vessel 1. The light source 10 has a quartz glass inner envelope 11 in which a pair of electrodes 12 are arranged in an ionizable gas filling. 
     The seal 7 is fixed by cement in a lamp cap 2 which mainly consists of ceramic material and whose contacts 3,4 are connected to a respective current supply conductor 5. The cement is a mixture of powdered silica, sodium silicofluoride and sodium silicate. An alternative mixture of cement includes powdered talc, zinc oxide and potassium silicate. 
     The seal 7 is coated with zirconium oxide 8 at the area at which it is surrounded by the cement, so as to completely separate the seal from the cement. 
     FIG. 2 shows a lamp having a similar lamp vessel 1 and a similar lamp cap 2 as that shown in FIG. 1. A biplanar filament 20, arranged in a halogen-containing gas filling, constitutes a light source. Current supply conductors 6 extend through the seal 7 from the contacts 3 (contact 4 not shown compare FIG. 1) on the lamp cap 2 to the light source 20. The lamp cap 2 is filled for a considerable part with a granular material 13, such as sand or aluminium oxide, which is covered with cement 9. The cement 9 holds the seal 7 anchored in the lamp cap 2. 
     In experiments leading to the lamp according to the invention, besides the white ZrO 2  coating, various other coatings including black graphite, were tested. The black graphite was tested because it was believed to yield a good heat transfer from the seal to the lamp cap. 
     In comparison tests on 2000 W, 220 V film studio halogen incandescent lamps, a thermocouple was provided in the lamp cap in order to measure the temperature of the seal of the lamps. The results obtained are indicated in the Table below. 
     
                       TABLE                                                       
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Lamp     Temperature Adherence cement                                     
                                   Anchoring                              
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without  375° C.                                                   
                     yes           good                                   
coating                                                                   
ZrO.sub.2 coating                                                         
         357° C.                                                   
                     no            good                                   
graphite 388° C.                                                   
                     no            moderate                               
coating                                                                   
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     These results show the graphite coating does not lead to reduction of the temperature of the seal, but on the contrary leads to an increase thereof of 13° C. Furthermore, and unexpectedly, the zirconium oxide coating causes this temperature to be reduced by 18° C. so that the thermal load of the seal is lower than without a coating. Moreover, the lamp coated with zirconium oxide is anchored more satisfactorily than the lamp coated with graphite.