Method of making an arc tube for electrodeless lamp

Abstract: An arc tube (18) for an electrodeless metal halide discharge lamp has an arc chamber fabricated from a material selected from the group consisting of magnesia-doped polycrystalline alumina, silicon dioxide doped polycrystalline alumina and mono-crystalline alumina. The arc chamber is tubular and has at least one end and has a given outside diameter. At least one end cap (22,22a) closes the at least one end of the arc chamber, the end cap (22,22a) being formed from magnesia-doped polycrystalline alumina and comprising a substantially cup-shaped member having an inside diameter which is sealed to the outside diameter of the arc chamber by a shrink-fit.

Description: TECHNICAL FIELD This invention relates to arc discharge lamps and more particularly to a method of making an arc tube for an electrodeless lamp. BACKGROUND ART Electrodeless lamps are known; see, for example, U.S. Patent Nos. 3,942,058; 4,427,924; 4,427,922 and 4,783,615. Such lamps have been fabricated preferably from quartz arc tubes. Greater efficiencies could be realized if rare earth fills could be employed; however, to take advantage of some of these fill it is necessary, because of the low vapour pressure of some of the ingredients when in the iodide form, to increase the operating temperature of the arc tube to the point that the lifetime of the lamps using these arc tubes becomes too limited. US-A 4 810 938 discloses an electrodeless arc discharge lamp. Its arc tube is made of fused quartz or alumina shaped as a flattened spheroid or as a short cylinder with rounded edges. From US-A 3 564 328 a method for producing a ceramic body from at least two component members is known. The members are formed of sinterable ceramic material. The ceramic body is described by way of example as being suitable for a gas discharge lamp with electrodes. DISCLOSURE OF THE INVENTION 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 electrodeless lamps. These objects are accomplished, in one aspect of the invention, by the provision of an arc tube for an electrodeless metal halide discharge lamp which comprises an arc chamber fabricated from a material selected from the group consisting of magnesia-doped polycrystalline alumina and mono-crystalline alumina. The arc chamber is tubular and has at least one end and has a given outside diameter. At least one end cap closes the at least one end of the arc chamber. The end cap comprises a substantially cup-shaped member having an inside diameter which is sealed to the outside diameter of the arc chamber by a shrink-fit. The arc tube is fabricated by a method which comprises the steps of first forming from polycrystalline alumina doped with 0.08 weight percent magnesium oxide a green arc chamber having a substantially tubular configuration, and prefiring the green arc chamber at about 1350 °C for about 120 minutes, preferably in air. A sealing disc is formed to fit inside one end of the arc chamber, the sealing disc being formed from polycrystalline alumina doped with 0.08 weight percent magnesium oxide. The disc is fired, preferably in air, at 1200 °C for about 120 minutes and sintered, preferably in 92 % N2-8% H2, at 1850 °C for about one minute. The sintered disc is inserted into an end of the arc chamber to form a first assembly and this first assembly is sintered at 1950 °C for about 30 minutes, preferably in an inert atmosphere or using a mixture of 92 %N2-8% H2, to form an hermetic seal between the arc chamber and the disc. An end cap is formed from polycrystalline alumina doped with 0.08 weight percent magnesium oxide for sealing an open end of the arc chamber, the end cap being cup-shaped and having an inside diameter which is formed to fit over the outside diameter of the arc chamber. The end cap is prefired at 1200 °C for about 120 minutes, preferably in air. An arc generating and sustaining fill is introduced into the arc chamber, the end cap is fitted over the open end of the arc chamber to form a second assembly, and the second assembly is rapidly heated to about 1800 °C and held there for about one minute to form an interference-fit, hermetic bond between the end cap and the arc chamber to complete the arc tube. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an elevational, sectional view of a component used in the invention; Fig. 2 is a perspective view of a disc used with the invention; Fig. 3 is an elevational, sectional view of a step in the sealing operation; Fig. 4 is a perspective view of an arc tube made by means of the method of the invention; Fig. 5 is an elevational, sectional view taken along the line 5-5 of Fig.4; Fig. 6 is a partial, elevational, sectional view of an alternate portion of an arc tube; Fig. 7 is a perspective view of yet another arc tube, not made by means of the method of the invention; and Fig. 8 is a graph of the spectrum of an excited lamp. BEST MODE FOR CARRYING OUT THE INVENTION 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 claim taken in conjunction with the above-described drawings 1 to 6. Polycrystalline alumina (PCA) powder doped with 0.08 weight percent was compacted and fabricated to a small grain size (about 15 µm) with an equiaxed microstructure by known techniques into an open ended, green tube 10. These green tubes were prefired in air at about 1350°C for about 120 minutes. Green PCA discs 12, were machined from previously constructed logs to predetermined dimensions that would shrink to be slightly smaller than the inner diameter (ID) of the prefired green tubes 10 after firing of the discs at 1200°C in air for about 120 minutes and sintering in 92% N 2 -8% H 2 at 1850°C for 1 minute. The fired discs 12 were then inserted into an end 14 of tube 10 to form a first assembly 16 (Fig. 3). The first assembly 16 was then sintered at 1950°C for 30 minutes in dry N 2 -8%H 2 . This latter firing forms the polycrystalline alumina which may include a secondary spinel phase (as is known) and causes a 10-14% shrinkage in the diameter of tube 10 and forms a fritless, hermetic seal between the ID and the disc 12. Total transmittance of the tube was typically 95-96% and in-line transmittance was about 5-6%. To form an arc tube 18 (Fig. 4), a predetermined amount of desired fill material is placed in the tube, preferably in the form of a pellet 20, and a prefired PCA hat 22, also containing 0.08 weight percent MgO, is placed over the open end of tube 10 to form a second assembly. The second assembly is placed in a furnace containing a suitable atmosphere and heated rapidly to about 1800 °C to form a fritless seal due to the hat 22 shrinking about 12-18 % against the previously sintered and pre-shrunk tube 10. The hat 22 was machined from a prefired PCA log to fit the dimensions of the fully sintered tube 10. In a preferred form of the invention, the fill comprises NdI 3 , CsI, Hg and Xe which is sealed into the arc tube in the absense of water since the rare earth halides are extremely hygroscopic. Electrodeless lamps so made were excited in a dual-ended power applicator, such as that shown in U.S. Patent No. 5,070,277, at 915 MHz, and the spectrum was dominated by rare earth emission lines as shown in Fig. 8. Alternatively, PCA doped with SiO 2 or pure monocrystalline alumina (sapphire) can by employed as the arc tube material.

Claims: A method of making an arc tube (18) for an electrodeless lamp by producing a ceramic body from at least two component members formed of sinterable ceramic material, at least one of said members being a tubular body, said method comprising the following steps: I. forming from alumina a green arc chamber (10) having a substantially tubular configuration; II. forming from polycrystalline alumina doped with magnesium oxide an end cap (22) for an open end of said arc chamber, said end cap (22) being cup-shaped and having an inside diameter which is formed to fit over an outside diameter of said arc chamber (10); III. introducing an arc generating and sustaining fill into said arc chamber (10); IV. fitting said end cap (22) over said open end of said arc chamber (10) to form a second assembly; V. and heating said second assembly to about 1800 °C to form an interference-fit, hermetic bond between said end cap (22) and said arc chamber (10), and being characterized by comprising the following steps: a. forming the green arc chamber (10) in step I. from the group consisting essentially of polycrystalline alumina doped with 0.08 weight percent magnesium oxide, polycrystalline alumina doped with silicon dioxide, or monocrystalline alumina; b. prefiring said green arc chamber (10) at about 1350 °C for about 120 minutes; c. forming a sealing disc (12) to fit inside one end (14) of said arc chamber (10), said sealing disc (12) being formed from polycrystalline alumina doped with 0.08 weight percent magnesium oxide; d. firing said disc (12) at 1200 °C for about 120 minutes and sintering said disc (12) at 1850 °C for about one minute; e. inserting said sintered disc (12) into said end (14) of said arc chamber to form a first assembly and sintering said first assembly at 1950 °C for about 30 minutes to form an hermetic seal between said arc chamber and said disc (12); f. forming said end cap (22) in step II. from polycrystalline alumina doped with 0.08 weight percent magnesium oxide, g. prefiring said end cap (22) at 1200 °C for about 120 minutes; h. performing steps III. and IV.; i. heating said second assembly in step V. to about 1800 °C in about 1,5 to 3 minutes and holding the assembly at about this temperature for about one minute to form the interference-fit, hermetic bond between said end cap and said arc chamber.

Inventors: KRAMER JERRY, LAPATOVICH WALTER P, WALSH JOHN, WEI GEORGE C, KRAMER, JERRY, LAPATOVICH, WALTER P., WALSH, JOHN, WEI, GEORGE C.
Applicants: OSRAM SYLVANIA INC, OSRAM SYLVANIA INC.
Application year: 1996
Main classification: H01J61
IPC codes: H01J61, H01J65, H01J9