CERAMIC DISCHARGE VESSEL WITH CHROMIUM-COATED NIOBIUM FEEDTHROUGH AND DISCHARGE LAMP CONTAINING SAME
There is described a lamp having a ceramic discharge vessel with a niobium feedthrough that has been coated with a layer of chromium to provide oxidation resistance. In a preferred method, a slurry containing chromium powder is applied to the niobium feedthrough and dried. A sub-assembly is formed by inserting the coated niobium feedthrough into an annular polycrystalline alumina (PCA) disc and heated in vacuum to 1700° C. to 1900° C. to bond the feedthrough to the PCA and form the sub-assembly and an oxidation-resistant coating on the feedthrough. The sub-assembly is then sealed to a ceramic body of the discharge vessel in a conventional manner.
1 . A discharge lamp comprising:
a ceramic body formed of polycrystalline alumina;
the ceramic body having at least one seal and enclosing a discharge space, the discharge space containing a discharge sustaining medium and at least one electrode; and
the at least one seal having a niobium feedthrough sealed therein for making an electrical connection between the at least one electrode and an external power source, the niobium feedthrough having a layer consisting of chromium or an alloy of chromium and niobium on an external surface to provide oxidation resistance.
2 . The discharge lamp of claim 1 wherein the discharge lamp is mounted in an open-air fixture having a reflecting body with an open end.
3 . The discharge lamp of claim 2 wherein the open end has a cover.
4 . A discharge lamp comprising:
a tubular body formed of polycrystalline alumina;
a pair of seals, each sealing an end of said body; and
said seals each comprising an annular polycrystalline alumina disc, the outer surface of said disc being bonded to said body, and a tubular niobium feedthrough sealed to the inner surface of said disc in a seal area, the niobium feedthrough having a layer consisting of chromium or an alloy of chromium and niobium on an external surface, the layer in the seal area being between the feedthrough and the disc.
5 . A method of making a ceramic discharge vessel, comprising:
forming a tubular polycrystalline body;
forming a pair of annular polycrystalline alumina sealing discs;
forming a pair of niobium feedthroughs, each of said feedthroughs having an area for sealing to said discs;
providing coating an external surface of the niobium feedthroughs with a layer consisting of chromium;
inserting said each of said feedthroughs into one of said discs to form sub-assemblies;
firing said sub-assemblies in a vacuum at 1700° C. to 1900° C. to seal the discs to the respective sealing area; and
subsequently inserting said fired sub-assemblies into opposite ends of said tubular body and sealing thereto.
6 . The method of claim 5 wherein said chromium layer is applied by coating the niobium feedthroughs with a slurry of chromium and methanol; and
drying said layer to provide a thickness of chromium between 100-500 μm.
7 . The method of claim 5 wherein the sub-assemblies are heated at about 1850° C.
8 . A feedthrough for a ceramic discharge vessel, the feedthrough comprising a niobium tube having a layer consisting of chromium or an alloy of chromium and niobium on an external surface to provide oxidation resistance.