Monolithic seal for sapphire CMH lamp

Electric lamp and discharge devices – With gas or vapor – Having electrode lead-in or electrode support sealed to...

Reexamination Certificate

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Details

C445S026000

Reexamination Certificate

active

06274982

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention generally relates to sealing arc tubes for high-pressure discharge lamps and, more particularly, to sealing arc tubes composed of sapphire for high-pressure discharge lamps.
High-pressure discharge lamps, such as ceramic-metal-halide (CMH) lamps, commonly utilize ceramic arc tubes which are transparent or translucent. The ceramic tube should have high-corrosion resistance, high-temperature capabilities, and high light transmissivity. The opposite ends of the ceramic arc tube are closed and sealed by ceramic end assemblies such as plugs or caps. The end assemblies also support discharge electrodes made of molybdenum or tungsten. The electrodes extend through the end assemblies and are hermetically sealed therein. An arc discharge is formed within the tube between the electrodes when current is applied to the electrodes.
The metal halide arc tubes can be composed of polycrystalline alumina which has superior chemical attack resistance and higher practical operating temperatures than customary quartz metal halide arc tube materials. Polycrystalline alumina is a preferred arc tube material in current commercial practice. The polycrystalline alumina arc tubes are typically sealed with polycrystalline end plugs.
It has been proposed to use sapphire (single crystal alumina) instead of polycrystalline alumina as the arc tube material in order to gain an additional increase in lamp performance. The increased performance is primarily due to sapphire's increased level of transmission, compared to polycrystalline alumina.
An issue with fabricating sapphire (single crystal alumina) arc tubes, however, is sealing the ends of the arc tube. Conventional methods of sealing quartz and polycrystalline arc tubes have not proven to be satisfactory. Different crystal orientations of sapphire have different thermal coefficients of expansion. The crystal orientation of the sapphire arc tube, therefore, must be precisely oriented so that its thermal expansion coefficient closely matches the thermal expansion coefficient of the plugs or caps in the direction of greatest expansion and/or contraction. When the crystal orientation of the sapphire tube is not precisely oriented in this manner, rapid changes in temperature can crack the sapphire arc tube. Accordingly, there is a need in the art for an improved method of joining end assemblies to sapphire arc tubes.
SUMMARY OF THE INVENTION
The present invention provides a method of making a tube assembly for a ceramic-metal-halide discharge lamp. The method includes the steps of providing a tube made of sapphire or single crystal alumina and providing an end cap made of unsintered polycrystalline alumina. The end cap is heated until it is presintered to remove binder material. The presintered end cap is then placed on an end portion of the tube to form an interface therebetween. The presintered end cap and the tube are heated until the end cap is sintered onto the tube and the sapphire crystal of the tube grows into the end cap to form a monolithic seal at the previous interface between the end cap and the tube.


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Abstract of JP54047380A (in English).

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