Abstract:
This invention is about a CMH lamp arc tube seal construction where the feedthrough electrode contains a cermet in such a manner that the said cermet is either not exposed outside the ceramic capillary (which in most cases is polycrystalline alumina, PCA) or if it is exposed to the outside of the arc tube, the part that is exposed has no current carrying function. The invention provides safe ways of assembling the cermet so as to avoid breakage of the said cermet due to mechanical stresses in the electrical connections.

Description:
BACKGROUND OF THE INVENTION 
   This invention is about Ceramic Metal Halide lamps (CMH) and the sealing technology of such lamps. 
   Often times one of the components in the electrode feedthrough of such lamps is made of a cermet material. Cermets have been known for a long time to provide acceptable solutions for the sealing of electrical feedthroughs to surrounding nonconductive materials. For example cermet materials have been made as early as 1979 by mixing course refractory oxide granules with fine metallic powders, such as tungsten, nickel and molybdenum to obtain electrical conductivity and yet a thermal expansion coefficients compatible with ceramic materials. 
   In later years, up to the early 1990&#39;s, the details of making the cermets with various particle size materials, their construction forms and their initial use in ceramic metal halide lamps were described by various lamp developers, but did not yet result in a practical ceramic metal halide lamp. Later on in the mid 1990&#39;s the first commercially viable CMH lamp was introduced and the whole field of metal halide lamps got a big boost as a result since the color characteristics, the kind of chemistries that one could use and the efficacies obtained were far superior to the quartz metal halide technology. While the initial lamps introduced had an electrode construction made out of Nb, Mo and W metals later CMH lamp introductions used cermets quite frequently. Much of the work attempted to either shorten the overall size of the extended plug construction, lower the cost of the materials used, increase reliability of the seal under high temperature conditions or provide an alternative seal that may be more manufacturable or some combination of these. In many of the cermet constructions the brittleness of the cermet is still an issue and needs to be solved. 
   BRIEF SUMMARY OF THE INVENTION 
   This invention is about a CMH lamp arc tube seal construction where the feedthrough electrode contains a cermet in such a manner that the said cermet is either not exposed outside the ceramic capillary (which in most cases is polycrystalline alumina, PCA) or if it is exposed to the outside of the arc tube, the part that is exposed has no current carrying function. The invention provides safe ways of assembling the cermet so as to avoid breakage of the said cermet due to mechanical stresses in the electrical connections. The need for protecting the cermet arises due to its brittle nature and its susceptibility to mechanical stress. In the majority of the embodiments described the cermet is protected by the PCA capillary completely surrounding it. In a few embodiments the cermet extends to the outside of the capillary and beyond the frit fillet; however in those cases the part of the cermet that is likely to break off has no current carrying function such that electrical continuity is maintained in spite of the break. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a cross section of a known extended plug construction for a CMH arc tube, 
       FIG. 1   a  shows a one side cross section of a known extended plug construction for a CMH arc tube, 
       FIG. 2  shows a one side cross section of a plug construction embodying the present invention, 
       FIG. 3  shows a one side cross section of another plug construction embodying the present invention, 
       FIG. 4  shows a one side cross section of yet another plug construction embodying the present invention, 
       FIG. 5  shows a one side cross section of still another plug construction embodying the present invention, 
       FIG. 6  shows a one side cross section of a different plug construction embodying the present invention, 
       FIG. 7  shows a one side cross section of still a different plug construction embodying the present invention, 
       FIG. 7   a  shows a detail of the  FIG. 7  embodiment, 
       FIG. 8  shows a one side cross section of yet a different plug construction embodying the present invention, 
       FIG. 9  shows a one side cross section of yet another plug construction embodying the present invention, 
       FIG. 10  shows a one side cross section of a different plug construction embodying the present invention, and 
       FIGS. 11   a  and  11   b  show one side cross-section of two additional plug constructions embodying the present invention. 
   

   DETAILED DESCRIPTION 
   As is well known the extended plug construction of such lamps as shown in  FIG. 1  allows the seal temperature to be considerably lower than a non-extended plug construction. The reason for this is the fact that the extended plug construction removes the seal further from the electrode which is the heat source, compared to the non-extended plug that happens to have the seal very close to the electrode and the main chamber of the discharge (essentially without a capillary PCA extension) close to the electrode. This feature enables these types of lamps to have a reasonable lifetime and be commercially viable. One of the construction techniques provides the use of cermets (ceramic-metal composites) that have an expansion coefficient intermediary to the two joining materials (which provide for the cermet)—most often polycrystalline alumina (PCA) and molybdenum (Mo). In as much as the cermet successfully provides a hermetic seal between the electrode and the PCA of the capillary tube via the frit material, it tends to be fairly brittle and hard to spot weld to. Therefore it is quite a task to handle the leads with an exposed cermet piece sticking out of the PCA capillary. 
   The current invention disclosure addresses this particular issue and provides acceptable technical and economical solutions which are superior to the existing approaches as can be seen in the following paragraphs. 
   As mentioned above cermets do provide a good solution for an electrically conductive and yet thermal expansion wise compatible structure for CMH lamps. However, as mentioned above the brittleness of the cermets and the difficulty of spot welding to them, make them a difficult choice in manufacturing unless a suitable solution is found that attaches the cermets to an electrical conductor. 
   The present invention is an improvement over the arc tube whose cross section is shown in  FIG. 1 . This is an arc tube of a 150 W ceramic metal halide lamp that uses an extended plug construction. Here a discharge tube  10  includes a cylindrical main tube  15  smoothly joined with tapered capillaries  11   a  and  11   b . The main tube  15  as well as the capillary part of the main tube  11   a  and  11   b  may be made of translucent ceramic material in which alumina is a main component. Sealing member  16   a , a first lead-through wire  19   a  and a first main electrode shaft  21   a  are integrated and inserted in tube  11   a . Specifically one end of lead-through-wire  19   a  is connected with one end of sealing member  16   a  by welding, and the other end of lead-through-wire  19   a  is connected with one end of main electrode shaft  21   a  by welding. Then sealing member  16   a  is fixed to the inner surface of tube  11   a  by a frit  17   a  such that tube  11   a  is sealed hermetically. When sealing member  16   a  first lead-through-wire  19   a  and first main electrode shaft  21   a  are disposed in the tube  11   a , an end of sealing member  16   a  may be positioned outside tube  11   a  as described in the various embodiments of the invention. An electrode coil  22   a  is integrated and mounted to the tip portion of main electrode shaft  21   a  by welding, so that main electrode  23   a  includes main electrode shaft  21   a  and electrode coil  22   a . The lead-through-wire  19   a  serves as a lead-through for positioning the main electrode  23   a  at a predetermined position in main tube  15 . The sealing member  16   a  is typically formed by a metal wire compatible with the frit expansion coefficient. For example the diameter of the sealing member  16   a  may be 0.9 mm and the diameter of the first main electrode shaft may be 0.5 mm. 
   An alternative to the  FIG. 1  construction is shown in  FIG. 1   a  where the cross section of one side of the PCA capillary is shown. Here instead of attaching the cermet  25  directly to the W pin  21   a  one inserts a Mo or W mandrel  12  and a fine Mo coil  13  surrounding the mandrel between the cermet and the W pin. The advantage of this construction, described in prior art, is the fact that the salts do not penetrate as far into the end of the capillary and therefore they are not as cold, thereby, yielding reasonable performance of the light source. In addition the combination of the Mo or W mandrel and Mo coil is compatible with the expansion/contraction of the capillary PCA so as not to lead to cracking. 
   In the following  FIGS. 2-11  we show the different designs of the extended plug invention which basically change the structures of items  16 - 22   a  and  b . In all cases the cermet is composed of a metal such as Mo (unless specified as Nb or Mo—Nb) and aluminum oxide powder while the frit is composed of oxides of Al—Dy—Si in a variety of proportions. A cermet composed of 2 or 3 different metals and PCA powder is within the scope of the invention and applies to all the different designs. In addition all the designs we describe below can have portions other than the electrodes with the cermet material and the seal thereabout with the construction of similar portions of either  FIG. 1  or  FIG. 1   a.    
     FIG. 2  shows a cross section of a design where the cermet&#39;s  25  integrity is protected by keeping it inside the PCA capillary  11   a  and welding (most conveniently by using a laser) a Mo pin  29  to the end of the cermet protruding to the outside of the capillary so that the current can be transferred from a power source to the tip of the electrode and then to the gaseous discharge. This electrode construction requires two laser welds  26  and  27  one at each end of the cermet and the whole structure is prepared ahead of time before inserting it into the arc tube. Typically some cross wire  29   a  is inserted perpendicular to the Mo pin and just above the capillary PCA  11   a  edge so as to make sure that the electrode does not fall through the capillary opening to the inside of the arc tube. In other words the cross wire basically stops the electrode at the right length. The cross wire most conveniently may be spot welded to the Mo pin as  29   b . The material of this cross wire could be either Mo, Nb, Ta, Ti or other not limited by these choices. Since the frit material  17   a  typically bonds to the cermet and the PCA well; hermeticity can be accomplished during the thermal cycling when the lamp is in use. This approach while protecting the integrity of the cermet will provide a conduit for the current via the Mo pin-cermet-W coil electrode construction. 
     FIG. 3  shows a cross section view in which the cermet  31  is still kept inside the PCA capillary  11   a  of the arc tube while the external lead is made out of Nb  34  and the interface of the cermet  31  and the Nb lead  34  (which lead is outside the capillary) is provided by a bead of Mo  33 . The advantages of this approach are as follows: The frit material  17   a  wets the external lead Nb well and as such the seal outside the capillary is likely to be very sturdy. The small Mo stub  33 , which can be a small cylinder cut out of Mo wire, inside the capillary is a good transition from the Mo—PCA cermet to Nb and as such does not experience as much stress as the direct seal without the Mo stub during expansion and contraction cycles due to thermal cycling in ordinary use of the lamp. A typical size of the Mo part in the electrode construction would be no more than a tenth of the total length of the electrode. This would easily be welded to the Nb and the cermet with one operation of the laser welder which is done as a separate operation before the assembly of the arc tube. 
     FIG. 4  shows another embodiment of the same concept whereby the size of the Nb pin  45  is larger than the Mo diameter  46  or the cermet  42  by at least a factor of two. This difference in diameters provides for a stopping point of the electrode so that one need not worry about how to stop the electrode from falling into the arc tube through the capillary or the use of a wire to mark the stopping point of the electrode. The Mo stub  33 , the Nb  45  and the cermet  42  can most conveniently be laser welded in one operation. Similarly, the cermet  42  can be conveniently welded with a laser to the W pin  21   a . Otherwise the advantages of this approach are the same as mentioned above in connection with  FIG. 3 . 
     FIGS. 5 and 6  show two different designs of the idea of using a cermet  54  made out of PCA and Nb, or PCA—Nb—Mo combination such that the volume percentage of metals versus the total is no more than about 50%. This particular combination of materials in the cermet tends to have a stronger and better binding to the frit and thereby PCA than just Mo. While some of this can be explained by the expansion coefficient of Nb the rest has to do with the physicochemical properties of the metal and the frit material  17   a  (composed typically of the oxides of Al, Si and Dy). The difference between  FIGS. 5 and 6  is essentially the length of the cermet rod where in  FIG. 5  the rod ends at the edge of the PCA capillary  11   a  while in  FIG. 6  it extends beyond the edge of the PCA capillary up to the top of the frit fillet  17   a . The  FIG. 5  design we believe will give a somewhat sturdier protection to the cermet while the  FIG. 6  design will have a more practical advantage of providing a larger piece of the cermet to weld onto. The choice between these two designs might depend on the power level of the lamp which would have different diameter cermet rods. The  FIG. 5  design would be preferred for the lower power lamps with a smaller diameter cermet rod while  FIG. 6  design would be preferable for the higher power lamps. Again the different pieces are welded as shown in  FIG. 5  by laser welds  57  and  58  and in  FIG. 6  by laser welds  67  and  68 . The cross wires  55  and  65  in  FIGS. 5 and 6  respectively may be made out of a material compatible with the frit expansion coefficient such as Nb, Mo etc. 
   Another embodiment of attaching the cermet to an electrical conductor in a safe manner such that the brittleness of the cermet is not a problem is shown in  FIG. 7 . Here the cermet  73  has a horizontal groove  76  on the part external to the capillary  11   a  such that a tight Nb or Mo (or some other metal compatible with the frit composition) hairpin (“U” shape)  75  holds the cermet in place and from falling into the capillary  11   a  of the arc tube. This horizontal hairpin is preferably made out of Nb metal and sits outside the capillary, in fact on top of it holding the cermet mechanically. The cermet could have a small groove to accommodate the hairpin which is acting both as stop wire as well as conducting current to the cermet and holding it in place at the right distance while the frit  17   a  is being melted during processing. When the frit melts it binds the cermet and the PCA and the hairpin altogether hermetically. The basic idea here is the use of a metal hairpin or some other shape and hold the cermet mechanically until the frit melts and binds the pieces together permanently allowing for the current source to be connected to the hairpin without experiencing the brittleness of the cermet or mechanically stressing it. The reason the cermet&#39;s brittleness is not an issue anymore is because the cermet is buried inside the frit and is not susceptible to much direct mechanical stress.  FIG. 7   a  shows more detail of the hairpin-cermet arrangement. The electrode of  FIG. 7  requires only one weld resulting in a lower cost electrode. 
   Yet another embodiment of the present invention is shown in  FIG. 8  where the cermet  84  is made into a hollow cylinder rather than a solid one. This geometry of the cermet allows the use of a metallic pin of say Mo  85  to go through the opening of the cermet and carry the current to the tungsten pin  21   a  and tungsten coil  22   a  for the electrical discharge to take place. We should reiterate here that the cermet provides a better seal to the PCA capillary  11   a  via the frit material  17   a  rather than Mo alone and as such it is still a necessary and preferred material to use. A further advantage is that a single piece of Mo metal is all is needed to attach to the tungsten (W) pin  21   a  and W coil  22   a  and as such it offers a more economical solution to the entire electrode construction. Clearly either a cross wire or a flattened part on the Mo wire  88  would be needed to prevent the pin from falling through during the sealing process. Flattening of the Mo wire can be accomplished with a simple squeeze of the wire at the right place using a pair of pliers. This will distort the circular cross section of the wire and make it impossible to go through the tight opening of the PCA capillary. A slight flattening at  82  prevents the cermet from falling through while a larger flattening at  88  prevents the whole assembly from falling through the capillary  11   a.    
   It is also conceivable to have the part of the pin sticking out of the capillary and the cross wire  97  both made out of Nb instead of Mo as shown in  FIG. 9  since Nb tends to stick better to the frit material and provides somewhat better bonding. The cross wire, of course is simply spot welded to the vertical pin made out of Mo. The cermet again is held in place by the slight flattening of the pin as shown in  92 . 
   In  FIG. 10  we show a one side cross section of the designs of  FIGS. 5 and 6  with the exception that the end of the cermet closer to the W electrode has the Mo mandrel/Mo coil combination of  FIG. 1   a  as a construct instead of directly attaching to the W pin/W coil combination as shown in  FIGS. 5 and 6 . 
   In  FIG. 11   a  we show yet another embodiment of the invention where the cermet  110  is protected from breakage by a hollow tube  113  made out of a metal compatible with the frit and the cermet expansion coefficients. This metal could be made out of Mo, Nb or some other metal. The hollow tube  113  is either spot welded or laser welded, as shown in  114 , to the cermet ahead of time before the entire feedthrough is inserted into the capillary. If the cermet is laser welded to the hollow tube hermetically the frit during processing will not penetrate into the hollow tube, it will simply seal the cermet/hollow tube combination to the capillary. If, however, the cermet is spot welded to the hollow tube non-hermetically, then the frit will penetrate into the inside of the hollow tube and provide an additional sealing of the cermet/hollow tube combination. The advantage of the  FIG. 11   a  construction is the fact that the hollow tube metal&#39;s expansion coefficient does not have to be compatible with the PCA capillary material since it is not in contact with the PCA. This results in more choices for the hollow tube material. A similar construction is shown in  FIG. 11   b  where the end of the cermet closest to the gas discharge inside the arc tube is terminated by a Mo or W mandrel  111  and Mo coil  112  as an alternate construction explained above. 
   Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.