EP0204303A2

EP0204303A2 - High temperature tapered inlead for ceramic discharge lamps

Info

Publication number: EP0204303A2
Application number: EP86107492A
Authority: EP
Inventors: Edmund M. Passmore; George L. Duggan; Robert G. Jenkins
Original assignee: GTE Products Corp
Current assignee: Osram Sylvania Inc
Priority date: 1985-06-03
Filing date: 1986-06-03
Publication date: 1986-12-10
Also published as: EP0204303A3; JPS61284048A

Abstract

@ The present invention provides a ceramic arc tube seal comprising an electrical feed-through or conductive inlead for a high-intensity discharge lamp having a high pressure gas discharge tube made of polycrystalline aluminum oxide (PCA). The conductive inlead is formed in a frusto-conical shape such that a mating relationship is created with tapered apertures in the opposed ends of the PCA tube. A seal is provided between the conductive inlead and the PCA tube such that exposure of the sealing frit glass to the inner chamber of the discharge tube and its fill constituents is substantially reduced or ideally prevented. An additional benefit provided by this invention is improved alignment and centering of the electrode that is supported by the conductive inlead in the discharge tube.

Description

CROSS REFERENCE TO CO-PENDING APPLICATIONS

Co-pending Application entitled, "End Seal for Ceramic Arc Discharge Tubes". to Makar et al was filed on September 2, 1983, with Serial Number 528.716: Two co-pending applications entitled, "Electrode for High Intensity Discharge Lamps", to Passmore were filed on December 5, 1983 with Serial Numbers ₅₅7.812 and 557.813. The co-pending Applications have been assigned to the same assignee as the present Application.

TECHNICAL FIELD

This invention relates to ceramic arc tubes for high pressure discharge lamp applications. More particularly, the invention relates to end caps or plugs for sealing a ceramic arc tube which are capable of withstanding high temperatures while prolonging the life of the lamp.

BACKGROUND OF THE INVENTION

High pressure discharge lamps using translucent polycrystalline alumina ceramic arc tubes, which arc tubes withstand vapors of such corrosive metals as sodium (Na). Cadmium (_Cd). Zinc (Zn), and Thallium (Tl) as well as metal halides have a high luminous efficiency. A high-intensity discharge lamp of the metal vapor type, for example, comprises a ceramic arc tube holding metal vapors and a protective envelope surrounding the arc tube. Accordingly, the arc tube is required to have both good translucency of light and high corrosion resistivity against the light-emitting material sealed therein, such as sodium vapor or the metal halide vapor. Translucent alumina ceramics have been found to meet the need for high corrosion resistivity and good translucency. Sealing of the electrical feedthroughs into the ends of the alumina arc tube presents a problem, because the alumina cannot be press sealed like fused quartz. Thus, although the quartz arc tube for mercury vapor lamps can be melted and sealed simply by heating it to a high temperature, the sealing of the alumina ceramics arc tube with the light-emitting material disposed therein requires a comparatively complicated process.
In a typical conventional, prior art process of sealing a translucent alumina ceramic arc tube, the open ends of the fired alumina arc tube are sealed by means of glass frit material with the mounting caps made of either alumina ceramic or another heat-resistant material which has a coefficient of thermal expansion similar to that of the alumina arc tube. Furthermore. heat-resistant metallic electrodes provided with the through-holes for introducing the light-emitting materials are sealed in the center portion of the above caps by glass frit (e.g.. Figure lA).
The conventional sealing process is limited by leakage of the light-emitting materials enclosed in the glass frit sealed ceramic arc tube due to the chemical reactivity and the wide sealing area of glass frit. exposure to a high operating temperature and thermal shock caused by on-off operation of the lamp. The problem becomes more pronounced when the arc tube is used in a discharge lamp that provides high luminous efficacy and high color rendering, the alumina arc tube and end seals failing to attain the required reliability, including the corrosion resistivity at a high temperature and high pressure.
In summary, the problem with any of these seals utilizing glass frits is that the temperature of the polycrystalline aluminum oxide (PCA) arc tube end is limited to about 800° Celsius, i.e., about the same as quartz and well below the capability of the PCA tube, which operates typically at temperatures up to 1200°C or even higher in the center of the tube. This limitation is largely due to the aggressive corrosion of the sealing glass by sodium, metal halide, and other fill constituents at higher temperatures. Such corrosion results in the progressive loss of sodium or other plasma discharge constituents by leakage or by chemical reaction with the glass frit. as well as eventual failure of the seal itself and consequently, the lamp. Both the life of the lamp and the quality of light are thereby limited. If higher end temperatures could be used, then higher metal vapor pressures could be attained, so that additives such as sodium (Na). scandium iodide (ScI₃), cadmium (Cd), thallium (Tl) and zinc (Zn) could be successfully utilized at higher pressures to improve both luminous efficacy and color rendering of these lamps.
It is believed, therefore, that a sealing structure used to seal the ends of a ceramic arc tube that can be utilized in a high-intensity discharge lamp which allows for operation of the arc tube at high temperatures thereby extending lamp life and improving both luminous efficacy and color rendering would constitute a significant advancement in the art.

DISCLOSURE OF THE INVENTION

Therefore, it is a primary object of this invention to enhance the art of high-intensity discharge lamps and particularly the sealing of the ceramic arc tube contained therein.
It is another object of the invention to provide for hermetic sealing of the ceramic arc discharge tube while centering the electrode within the tube without additional steps.
It is still another object of the invention to provide a ceramic arc tube that is capable of operating at high temperatures and pressures while extending lamp life.
In accordance with one aspect of the instant invention, there is provided a hermetically sealed ceramic arc tube for high intensity discharge lamps comprising a substantially cylindrical light transmissive polycrystalline ceramic body having a chamber. opposed ends and a longitudinal axis. Each of the opposed ends has a centrally located aperture through which the longitudinal axis intersects and the aperture has a substantially frusto-conical shape that narrows along said axis toward the chamber. The arc tube also has a pair of electrically conductive inleads with electrodes having a substantially frusto-conical shaped portion disposed within and forming a mating relationship with each of the apertures. The inleads and the ceramic body form a seal and each of the inleads has an included angle X which provides for an improved seal and improved centering of the electrodes within the ceramic body. Finally, the arc tube has sealing glass disposed about the outside of the inleads and the opposed ends to ensure a hermetic seal, the inlead-ceramic body seal substantially preventing leakage of the sealing glass into the chamber.
In accordance with another aspect of the present invention there is provided an electric arc discharge lamp comprising an outer envelope and a hermetically-sealed ceramic arc discharge tube disposed within the outer envelope. The arc tube comprises a substantially cylindrical light transmissive polycrystalline ceramic body having a chamber, opposed ends and a longitudinal axis. Each of the opposed ends has a centrally located aperture through which said longitudinal axis intersects and the aperture has a substantially frusto-conical shape that narrows along said axis toward the chamber. The arc tube also has a pair of electrically conductive inleads with electrodes having a substantially frusto-conical shaped portion disposed within and forming a mating relationship with each of the apertures. The inleads and ceramic body form a seal about the opposed ends. The arc tube has sealing glass disposed about the outside of the inleads and the opposed ends to provide a hermetic seal, the inlead ceramic-body seal substantially preventing exposure of the sealing glass to the chamber. Finally. a fill comprised of several constituents is located within the chamber of the arc tube.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1A and 1B illustrate examples of prior art ceramic arc tube end seals:
Figures 2A and 2B are photographs of axial sections of conventional prior art seals in polycrystalline ceramic arc tubes:
Figures 3A, 3B and 3C illustrate one embodiment of the arc tube seal made in accordance with the teachings of the present invention:
Figures 4A and B are photographs of the axial section in the inlead seal of Figure 3A:
Figure 5 is a photograph taken along the interface between the end seal and the PCA monolith; and
Figure 6 is an x-ray map of Figure 5 illustrating by the calcium distribution the location of the glass frit about the seal made in accordance with the teachings of the present invention.

THE 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 claims in conjunction with the above described drawings.
-With reference now to the drawings, there is shown in Fjigures 1A and 1B examples of end portions of ceramic arc tube end seals found in the prior art for use in high-intensity discharge lamps. Figure 1A illustrates the end portion of a ceramic arc tube 10 typically made for high-pressure sodium lamps. Specifically, end portion 10 is comprised of a polycrystalline aluminum oxide (PCA) tube 12. a chamber 14 located within tube 12. a niobium tube 16 inserted into tube 12 at one end and an electrode 18 in contact with tube 16 having a light-emitting material 19 located about the electrode. Niobium tube 16 and electrode 18 are sealed by glass frit 20. the glass-sealing frit comprising alumina, calcium, magnesia and barium oxide being frequently used for this sealing purpose. (see e.g. U.S. Pat. No. 4.208,605. McVey et al). A portion of sealing frit 20 is shown protruding into chamber 14 at point 22.
Figure 1B illustrates the end portion 30 of a ceramic arc tube which may be used in either metal halide or metal vapor lamps. Arc tube portion 30 is comprised of a PCA arc tube 32, a chamber 34 located within arc tube 32, a cermet inlead 36, anelectrode 38 in contact with cermet 36, and a light-emitting material 39 located about electrode 38. The cermet-electrode combination are sealed into tube 32 by sealing frit 40, which is typically comprised of silica, alumina and yttria or magnesia. Cermets, such as cermet 36, are electrically conductive, refractory composites, typically comprising an interconnected network of molybdenum or tungsten within a matrix of dense, polycrystalline aluminum oxide. Arc tube portion 30 also illustrates a portion of sealing frit 40 protruding into chamber 34. illustrated at point 42.
In Figures 1A and 1B, expose portion 22 in arc tube 10 and expose portion 42 are areas where the glass sealing frit is exposed to the inner elements of the ceramic arc tube. Figure 2A shows a glass sealing frit inside the arc tube with a niobium inlead (see also Fig. lA) and Figure 2B illustrates the glass sealing frit inside the arc tube chamber when using a cermet inlead. (see also Fig. 1B) As also shown in Figures 2A and 2B, leakage of the glass frit around the prior art straight inleads occurs, whether niobium or cermet is used, so that a substantial area of the glass is exposed to the corrosive action of the sodium and/or other fill constituents. The corrosive action would particularly start at point 22 of end portion 10 and point 42 of end portion 30. Such corrosion is highly temperature-dependent, with the result that the operating temperature, constituent vapor pressures, light output, color rendering, and lamp life will fall far below the ultimate potential of the radiating species and the PCA tube. Figures 2A and 2B are photographs of axial sections of conventional prior art seals in PCA tubes.
In contrast, the present invention provides a seal, in which exposure of the glass frit to the discharge tube fill constituents is substantially reduced or even eliminated. One embodiment of the invention is illustrated in Figures 3A. 3B and 3C. Figure 3A illustrates ceramic arc tube 50 (having a longitudinal axis L) which is comprised of a ceramic body (e.g. yttrium oxide) or polycrystalline aluminum oxide tube 52, an inner chamber 54 within arc tube 50, an electrical feed-through or conductive inlead 56, an electrode 58 connected to inlead 56 and a light-emitting material 59 located about electrode 58. After electrical inlead 56 has been inserted into one of opposed ends 53 of ceramic tube 50, a sealing glass frit 60 is formed about the inlead 56 and opposed end 53 of arc tube 52. Figure 3A illustrates that a seal made in accordance with the teachings of the present invention will result in substantially reduced or no glass frit leaks past conductive inlead 56 into chamber 54. specifically, point 62 as indicated. A seal is formed by the ceramic body 52 and conductive inlead 56 interface, as indicated by the arrow as 64. The manner of obtaining the minimum spacing between the ceramic body and the cermet inlead at interface 64 will be discussed later.
Referring now to Figure 3B. conductive inlead 56 with electrode 58 and light-emitting material--39 is illustrated in an isometric view. Electrically conductive inlead 56 has a substantially frusto-conical shape and is formed to be located within an aperture at opposed end 53 and to form a mating relationship with the aperture at the opposed end. The apertures at each of the opposed ends are also formed in a substantially frusto-conical shape that narrow along a longitudinal axis L of tube 50 toward chamber 54. The aperture at opposed end 53 intersects longitudinal axis L. while conductive inlead 56 and electrode 58 are concentric to axis L and are self-centering when placed in the aperture. In this embodiment, inlead 56 is made from a cermet material.
Figure 3C illustrates conductive inlead 56 without electrode 58 for purposes of illustrating that inlead 56 has a particular included angle X which is defined as illustrated. The included angle X of inlead 56 has a value which falls within the range of about 3° to about 9⁰ (degrees), the value preferably being about 6°. Although a tapered cermet inlead is shown to illustrate one embodiment of the present invention, the latter is by no means limited to cermets or cermet material. (see e.g. 4,155,757 to Hing and 4,155,758 to Evans et al). Niobium tubes. wires, rods, or any other inlead material such as titanium or zirconium borides can also be tapered and utilized in the same manner with equivalent resultant benefits.
Referring back to Figure 3A. the ceramic body inlead interface 64 has a spacing or maximum clearance of about 0.001 inch. Theoretically, the object in a seal of this type is to achieve the lowest maximum clearance, that being zero spacing in the ceramic body-inlead interface 64 to prevent glass frit leakage into the chamber of the arc tube. Figures 4A and 4B illustrate photographs of the axial section of the tapered inlead seal of Figure 3A. illustrating that there is no leakage of frit glass by the inlead into the discharge tube chamber. Figures 5 and 6 are also provided to further illustrate how the glass sealing frit is not exposed to the inner chamber of the ceramic arc tube. Specifically. Figure 5 shows a film of glass sealing frit slightly less than 0.001" thick. along the interface between the cermet inlead and the PCA monolith. In Figure 6, the corresponding x-ray map shows by the calcium distribution that the frit does not extent beyond the interface into the discharge tube. In Figure 6 the width of the calcium distribution is indicated by the two arrows and is marked as "Y", Y being equal to about 0.001" or less.
In summary, this invention provides a tight seal by the use of a tapered electrical feed-through or cermet inlead. fitted snugly into a tapered aperture in the ends of the PCA tube. By this means, substantially zero spacing or low maximum clearance is provided between the conductive inlead and the PCA tube. The sealing glass frit then serves to provide a hermetic seal around the outside of the tube end and conductive inlead, but penetration of the glass frit around the inlead and into the discharge is minimized and ideally prevented. This in turn reduces the corrosive reactions between the fill constituents and the sealing glass to a negligible factor with consequent increases by 100° Celsius or more in the attainable operating temperature of the end seal. The corresponding increases in metallic and halide vapor pressures are typified by sodium. whose pressure is approximately tripled by such a 100⁰ Celsius cold spot temperature increase. Luminous output and color rendering are thereby enhanced without the decrease in lamp life, which otherwise results from such increased end seal temperatures in the high color rendering index (HCRI) version of the high pressure sodium (HPS) lamp. Furthermore, an additional benefit provided by this invention is improved alignment and centering of the electrode in the discharge tube.
While there have been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

Claims

1. A hermetically sealed ceramic arc tube for high intensity discharge lamps comprising:

a substantially cylindrical light transmissive polycrystalline ceramic body having a chamber. opposed ends and a longitudinal axis, each of said opposed ends having a centrally located aperture through which said longitudinal axis intersects and said aperture having a substantially frusto-conical shape that narrows along said axis toward said chamber:

a pair of electrically conductive inleads with electrodes having a substantially frusto-conical shaped portion disposed within and forming a mating relationship with each of said apertures, said inleads and said ceramic body forming a seal and each of said inleads having an included angle which provides for an improved seal and improved centering of said electrodes within said ceramic body; and

a sealing glass disposed about the outside of said inleads and said opposed ends to ensure a hermetic seal, said inlead-ceramic body seal substantially preventing leakage of said sealing glass into said Chamber,

2. The ceramic arc tube according to Claim 1 wherein said included angle of said electrically conductive inleads is within the range of about 3 degrees to about 9 degrees.

3. The ceramic arc tube according to Claim 1 wherein said conductive inleads are formed from a cermet material.

4. The ceramic arc tube according to Claim 1 wherein said conductive inleads are formed from niobium.

5. The ceramic arc tube according to Claim 1 wherein said conductive inleads are formed from titanium or zirconium borides.

6. The ceramic arc tube according to Claim 1 wherein said polycrystalline ceramic comprises aluminum oxide or yttrium oxide.

7. The ceramic arc tube according to Claim 6 further including a fill comprised of constituents that are selected from the group consisting of sodium, mercury, zinc, cadmium and thallium or combinations thereof.

8. An electric arc discharge lamp comprising:

an outer envelope:

a hermetically sealed ceramic arc discharge tube disposed within said outer envelope, said arc tube comprising a substantially cylindrical light transmissive polycrystalline ceramic body having a chamber, opposed ends and a longitudinal axis, each of said opposed ends having a centrally located aperture through which said longitudinal axis intersects and said aperture having a substantially frusto-conical shape that narrows along said axis toward said chamber, a pair of electrically conductive inleads with electrodes having a substantially frusto-conical-shaped portion disposed within and forming a mating relationship with each of said apertures, said inleads and said ceramic body forming a seal and a sealing glass disposed about the outside of said inleads and said opposed ends to provide a hermetic seal, said inlead-ceramic body seal substantially preventing exposure of said sealing glass to said chamber: and

a fill comprised of several constituents located within said chamber.

9. The arc discharge lamp according to Claim 8 wherein each of said inleads further includes an included angle X. which provides for an improved seal and an improved centering of said electrodes within said ceramic body.

10. The arc discharge lamp according to Claim 9 wherein said included angle is within the range of about 3 degrees to about 9 degrees.

11. The arc discharge lamp according to Claim 9 wherein said conductive inleads are formed from a cermet material.

12. The arc discharge lamp according to Claim 9 wherein said conductive inleads are formed from niobium.

13. The arc discharge lamp according to Claim 9 wherein said conductive inleads are formed from titanium or zirconium borides.

14. The are discharge lamp according to Claim 9 wherein said polycrystalline ceramic comprises aluminum oxide or yttrium oxide.

15. The arc discharge lamp according to Claim 14 wherein said fill constituents are selected from the group consisting of sodium. mercury, zinc. cadmium and thallium or combinations thereof.