Patent Application
20100026181
This application relates to discharge lamps and more particularly to ceramic discharge vessels therefor and methods of making such discharge vessels.
Recent developments in high intensity discharge lamps, in particular metal halide lamps, have led to the use of ceramic discharge vessels in place of the previous discharge vessels formed from quartz. The use of the ceramic discharge vessels has led to many advantages; however, sealing problems involved in hermetically sealing electrodes into the ceramic discharge vessels have limited their use somewhat. Sealing electrodes into the ceramic has involved using various glass frits or other sealing compounds to accommodate the differences in thermal expansion between the metallic electrodes and ceramic.
While the use of glass frits has proved workable, its use has many disadvantages. The main disadvantage relates to the fact that the glass frits are reactive with the standard metal halide fills. The higher the temperature at which the discharge vessel operates, the higher the reaction rate will be. To minimize the reaction rate, so as to minimize the effect such reactions have on lamp performance, the discharge vessel must be designed in such a way as to keep the glass frit sealing compound from reaching temperatures where they would react rapidly with the metal halide fill (typically mercury and a mixture of metal iodides). This temperature limitation typically necessitates the discharge vessel to be designed with long capillaries extending from the discharge vessel body. At the far end, that is, the end of the capillary remote from the discharge vessel body, the temperature is low enough so as not to cause a severe problem. It is at this remote end that the glass frit hermetically seals the electrode into the ceramic. This solution to the sealing problem presents its own constraints. First, the discharge vessel is more difficult and expensive to produce. Second, the long capillaries increase the size of the discharge vessel, limiting design flexibility, especially by hindering the miniaturization of the lamp employing the discharge vessel, a relatively constant demand of the marketplace. Third, the elongated capillaries provide an discharge vessel with “cold” spaces or reservoirs, where components of the fill can condense and remain permanently or temporarily out of the plasma discharge. These fill components entering and leaving the plasma discharge in an uncontrolled manner can, and do, cause unwanted color shifts in the lamp output.
Accordingly, it would be an advance in the art to provide a seal between a ceramic member and metal member without the use of intermediate sealing materials.
It is, therefore, an object of the invention to obviate the disadvantages of the prior art.
It is another object of the invention to enhance ceramic discharge vessels.
Yet another object of the invention is the improvement of ceramic discharge vessels and methods of making the same.
The objects are accomplished in one aspect of the invention by the provision of a ceramic discharge vessel comprising a hollow body having at least one tubular receptor extending from the hollow body. A molybdenum tube is joined to the receptor at a hermetic seal, the hermetic seal occurring in the absence of any intermediate sealing compound. An electrode is inserted into the molybdenum tube. The electrode has a rod portion that is welded to the molybdenum tube at a remote end of the molybdenum tube. The inner diameter of the molybdenum tube is no more than 0.02 mm greater than the outer diameter of the rod portion of the electrode so that a gap of 0.01 mm or less is formed between the rod portion and the molybdenum tube.
The objects are further accomplished by a method of making a ceramic discharge vessel comprising the steps of:
forming a hollow ceramic body having at least one tubular receptor projecting from the body and firing the body in air to remove binder material and pre-sinter the body;
inserting a molybdenum tube into the receptor to form a subassembly and firing the subassembly in a hydrogen-containing atmosphere to hermetically seal the receptors to the molybdenum tube without the use of any intermediate bonding agents;
inserting an electrode into the molybdenum tube, the electrode having a rod portion, the inner diameter of the molybdenum tube being no more than 0.02 mm greater than the outer diameter of the rod portion of the electrode so that a gap of 0.01 mm or less is formed between the rod portion and the molybdenum tube; and
welding the rod portion of the electrode to the molybdenum tube at a remote end of the molybdenum tube.
In a preferred embodiment, the method comprises the steps of:
forming a hollow, bulbous body of alumina, the body having two tubular receptors extending from opposite sides along a longitudinal axis of the discharge vessel;
firing the body at about 900° C. in air to remove binder material and pre-sinter the body;
inserting a molybdenum tube into each receptor to form a subassembly;
firing the subassembly at about 1820 to about 1850° C. in hydrogen to hermetically seal the receptors to the molybdenum tubes without the use of any intermediate bonding agents;
inserting a first of two electrodes into a first of the molybdenum tubes, the electrodes each having a rod portion, the inner diameter of the molybdenum tubes being no more than 0.02 mm greater than the outer diameter of the rod portions of the electrodes so that a gap of 0.01 mm or less is formed between the rod portions and the molybdenum tubes;
For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims taken in conjunction with the above-described drawings.
Referring now to the drawings with greater particularity, there is shown in
In a next step (
After sealing the molybdenum tubes to the receptors, electrodes 24 (
Referring now to
Before finally sealing the discharge vessel, the arc generating and sustaining medium (i.e., the fill, usually comprised of one or more metal salts, as is known) is inserted into the body 12 through an open tube, which then has an electrode 24 inserted and sealed, by welding, to the tube.
The following non-limiting examples illustrate the invention more particularly.
A molded discharge vessel body 12, such as one for a 70 W discharge vessel, and having a receptor inside diameter of about 1.11 mm (designed to have a finished inside diameter of 0.83 mm) is pre-sintered by firing in air at about 900° C. to remove any binder material. The pre-sintered body is then fitted with a molybdenum tube of 1.0 mm O.D. and 0.76 mm I.D. in each receptor. If desired, a stop wire 34 (
A molded discharge vessel body 12, for a 70 W discharge vessel, designed to have a capillary inside diameter of 0.95 mm upon completion, was pre-sintered as above. Molybdenum tubes 20, 22, having an O.D. of 1.2 mm and an I.D. of 0.8 mm, were inserted into each receptor and the assembly threaded on to a temporary tungsten rod as above. Final sintering was again carried out at between 1820 and 1850° C. in a hydrogen atmosphere for 240 minutes. During sintering, the capillaries shrank onto the molybdenum tubes with a shrink fit. After sintering, the bond was tested by helium leak testing and showed no leakage and no cracks. The shrink fit ratio of the molybdenum tubing with the ceramic capillaries was 1.2 mm divided by 0.95 mm or about 26.3%.
To determine the efficacy of this procedure if solid, as opposed to tubular, molybdenum structures were employed, the above tests were repeated with solid molybdenum rods used in place of the molybdenum tubes. In a first instance, a capillary designed to have an inside diameter of 0.95 after sintering was fitted with a solid molybdenum rod of 1.01 mm diameter. The final sinter procedure was as described above. After sintering, the ceramic appeared to be not as tightly conformed to the solid rods. Large cracks were apparent in the ceramic along the rod length and the bond was not considered to be leak tight. The shrink fit ratio of the molybdenum rod with the ceramic capillaries was 1.01 mm divided by 0.95 mm or about 6.3%.
In a second instance, a similar body to that described above was fitted with solid molybdenum rods of 1.11 mm diameter. Final sintering was as described above with reference to Examples I and II and the first instance of the molybdenum rods. Again, after sintering, the ceramic appeared to be not tightly bonded and large cracks were apparent. The bond was not leak-tight. The shrink fit ratio of the molybdenum rod with the ceramic capillaries was 1.11 mm divided by 0.95 mm or about 16.8%.
In the examples described above the receptors were trimmed to provide an overall capillary length of 38 mm for the discharge vessel after final sintering.
To determine if the ceramic capillary length could be effectively shortened, a molded discharge vessel designed to have a capillary inside diameter of 0.95 was used. The capillaries were trimmed to provide a receptor 15 with a shortened overall length of about 27.7 mm after final sintering. Hollow bodies with shorter receptors 15′, 15″ are shown in
The versatility of this construction is further illustrated by the embodiment shown in
The benefits derived from this discharge vessel and the method of making it are many. The bond that is formed requires no frit seals or extra material, as do the prior art procedures. This, alone, provides a cost saving. Also, the tenacity of the seal allows a given wattage discharge vessel to made smaller, by shortening the receptors or capillaries, also a highly desired result.
While there have been shown and described what are at present considered to be the preferred embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications can be made herein without departing from the scope of the invention as defined by the appended claims.
