Molybdenum foils with yttrium oxide and recrystallization grains no more than 50 microns within the pinch seals of a metallic halide lamp

#This invention relates generally to electric lamps and particularly to metallic halide lamps for use, for example, on motor vehicles or the like. More particularly, the invention deals with improvements concerning the molybdenum foils that are embedded in the pinch seals of a metallic halide lamp for electrically connecting the pair of discharge electrodes to leads.

#Generally, a metallic halide lamp has its envelope composed of a length of glass tube, with its opposite ends pinch sealed to provide a hermetically closed discharge chamber therebetween in which there are contained substances such as a metallic halide, xenon, and mercury. A pair of electrodes are partly disposed in the discharge chamber and partly embedded in the pinch seals. Also partly embedded in the pinch seals, a pair of leads are connected to the electrodes via molybdenum foils to which the instant invention pertains.

#There might be contemplated the use of either hard glass or silica glass for the lamp envelope. For its higher coefficient of linear expansion, 50.times.10.sup.-7 (1/.degree. C.), hard glass is undesirable because the envelope of a metallic halide lamp is heated to a temperature of as high as 900.degree. C. in use, at which hard glass would be deformed or lose transparency. As an additional disadvantage, hard glass contains substances in addition to silica. These additives would luminesce on arc discharge, imparting an undesired hue to the color in which the lamp is intended to give off light.

#FIG. 1 is a longitudinal section through the metallic halide lamp embodying the principles of the present invention;

#FIG. 2 is explanatory of accelerated lifetime tests that were conducted to determine the useful life of the improved lamp according to the invention in comparison with that of some prior art lamps;

#FIG. 3 is an enlarged sectional view of one of the improved molybdenum foils of the FIG. 1 lamp, the view being explanatory of the reduced thermal expansion of the molybdenum foil with its fine crystal grains;

#The invention will now be described more specifically as embodied in a metallic halide lamp for use as a vehicular headlamp. With reference to FIG. 1 the exemplified lamp has a silica glass bulb or envelope 2 defining a discharge chamber 3 of generally globular shape. The discharge chamber 3 conventionally contains mercury and one or more metallic iodides together with one or more of such inert gases as argon, xenon, and krypton under pressure. The bulb 2 is formed in one piece with a pair of pinch seals 4 extending in opposite directions therefrom in alignment with each other.

#At 8 are seen a pair of discharge electrodes of tungsten each including a stem 6 and a ball 7. Partly buried one in each pinch seal 4, the electrode stems 6 extend into the discharge chamber 3 to terminate in the balls 7.

#Within the pinch seals 4 the electrode stems 6 are joined respectively to a pair of rectangular shaped foils 5 of molybdenum and thence to a pair of lead wires 9 which also are of molybdenum. The molybdenum foils 5 are wholly embedded in the pinch seals 4 whereas the molybdenum lead wires 9 have only their minimal lengths buried in the pinch seals and other parts projecting therefrom.