Method of making a vacuum-tight seal between a ceramic and a metal part,

Abstract

A vacuum-tight seal, particularly for an arc tube or discharge vessel for a metal halide or sodium high-pressure discharge lamp, is made by forming the arc tube of transparent ceramic, typically Al.sub.2 O.sub.3, having two open ends, into which connecting plugs (3), with central openings, are sealed. Open metal tubes (4) fitting into the openings of the plugs (3) are then vacuum-tightly sintered therein. An electrode system (5, 10, 10'), forming a subassembly with a current supply lead or lead-through (6, 11, 14, 15, 20, 24) is provided, having an outer diameter fitting into the metal tube. After mounting one of the electrode systems and lead-through into the metal tube (4), and welding the lead-through to the metal tube, the vessel can be evacuated through the still open tube (4) at the other end, supplied with suitable fill substances and an ignition gas, such as argon or xenon, and the second electrode system then introduced, and welded, for example by a laser, to the outer end of the other metal tube (4). The arrangement permits use of molybdenum tubes, molybdenum lead-throughs, in Al.sub.2 O.sub.3 ceramic, in spite of different thermal coefficients of expansion. Preferably, the metal tube extends beyond the end face of the vessel and/or sealing plug (3), to permit free access during the welding step and reduce heat transfer into the interior of the arc tube or discharge vessel (1).