High Pressure Discharge Lamp Structure

Published European Patent Specification No. 0074188, Denbigh et al.;

Published European Patent Specification No. 0074720, Coaton et al.

The present invention relates to high-pressure lamps and more particularly to a construction of a high-pressure discharge lamp having a bulb operable at extremely high temperatures, and in which the current supply connection is melted into an end portion of the discharge vessel or bulb.

In metal halide high-pressure lamps having discharge vessels of quartz glass, not all optically active fill substances can be vaporized at operating temperature, so that some proportion thereof remains as a condensate within the discharge vessel, without participating in the discharge formation. The quartz glass, which can withstand the temperature only slightly above 1000.degree. C., provides an upper limit for the operating temperature. Quartz glass, further, is attacked by the highly aggressive fill substances used in metal halide high-pressure discharge lamps and becomes brittle due to recrystallization. This negatively affects the life of the lamp.

It has been proposed to utilize aluminum oxide ceramic as the bulb or arc vessel for a metal halide high-pressure lamp. Aluminum oxide ceramic is well known from the technology of sodium high-pressure lamps. This material is capable of withstanding a temperature of up to about 1300.degree. C., and is not attacked by the aggressive substances of the fill. Aluminum oxide ceramic, however, introduces a difficulty in the manufacture of a gas-tight, pressure-tight melt for the electrodes. Tungsten, which is customarily used for the current supplies, has a higher thermal coefficient of expansion than aluminum oxide ceramic. At the high temperatures which occur, the current supply lead made of tungsten would destroy the melt connection through the discharge vessel, if made of aluminum oxide ceramic.

It has been proposed to utilize current supply leads made of niobium rather than of tungsten. The thermal coefficient of expansion of nobium corresponds roughly to that of aluminum oxide ceramic, and the melt connection is not destroyed thereby. Niobium, however, has the disadvantage that it is attacked by the aggressive fill substances of the metal halides and thus the presence of niobium within the discharge vessel cannot be tolerated.

European published patent specification EP No. 0 074 188 describes a melt connection for a sodium high-pressure lamp in which the discharge vessel is made of aluminum oxide ceramic and a terminal connection for current supply of niobium is used. A current connecting lead of tungsten is welded to the niobium lead, the tungsten connection carrying the electrode. Melt connections with terminals made of Cermet are also proposed.

European published patent-specification EP NO. 0 074 720, assigned to the same assignee as the aforementioned EP 0 074 188, describes a further development in which the application of the current supply connection is extended to high-pressure discharge lamps with metal halide filling.

It has been found that the presence of even a portion of the niobium through-connection within the interior of the discharge vessel leads to deterioration thereof and premature failure of the lamp.

It is an object to provide a melt connection for a high-pressure discharge lamp vessel construction, and more particularly a melt connection, in which the bulb or discharge vessel is made of aluminum oxide ceramic, which is gas-tight, is not subject to premature failure due to deterioration of the melt connection, and which may be used for sodium and metal halide discharge lamps alike.

Briefly, a niobium melt connection is used. The niobium is isolated, however, from the interior of the lamp by constructing the niobium in form of a sealing disk embedded between an internal and an external disk of the aluminum ceramic. The side of the sealing disk facing the outside has a niobium electrode lead secured thereto, for example by welding, and the side of the niobium disk facing the inside has a tungsten electrode secured thereto, for example by welding. The two ceramic sealing disks are retained in the ceramic bulb or discharge vessel by a suitable glass melt, which has a well-known composition resistant to the aggressive fill substances within the lamp, and filling any capillary spaces between the ceramic disks and the aluminum oxide of the discharge vessel or bulb.

The arrangement has the advantage that the melt connection is gas- and pressure-tight and is suitable for sodium high-pressure discharge lamps, as well as for metal halide high-pressure discharge lamps. No niobium is present within the discharge space itself, which might be attacked by aggressive fill substances within the discharge vessel, and particularly such aggressive fill substances used in metal halide discharge lamps, thus substantially increasing the lifetime of the lamp.

The discharge vessel is highly temperature loaded, and thus complete vaporization of all fill components can be obtained, so that all optically active fill components will be fully effective and vaporized. This improves the color rendition characteristics of the lamps with respect to those of the prior art.

The melt connection between an inner disk and an outer disk, both of aluminum oxide ceramic, and embedding the sealing disk of niobium, further insures connection which is essentially free of stresses, and thus tightness throughout a wide temperature range and including the extreme operating temperature which the aluminum oxide ceramic can accept. This further increases the lifetime of the lamp.

The single figure is a schematic cross-section of a melt-through connection of an electrode suitable for a sodium or metal halide high-pressure discharge lamp.

A discharge vessel 1, of standard construction and, for example in the form of an elongated tube 2, is made of aluminum oxide ceramic. Only one end portion of the discharge vessel is shown; the other end portion and electrode can be identical. The tube 2 is closed off at the bottom by a closing structure. The closing structure is formed by an external disk 3 and a preferably identical inner disk 4. Disks 3,4 are made of aluminum oxide ceramic. The disks 3,4 are positioned tightly against each other. The tubular body 2 is formed with a shoulder 5 and a recess in the inner diameter thereof to accept the two disks 3,4; the position of the shoulder 5 corresponds approximately to the thickness of the two disks 3,4 taken together.

The tubular body 2, the external disk 3 and the internal disk 4 all are of aluminum oxide ceramic.

A sealing disk 6 of niobium is embedded between the disks 3,4. The diameter of the disk 6 is less than the diameter of the disks 3,4. The disks 3,4 are formed with recesses 3a,4a to receive the disk 6--see the figure. The recesses 3a,4a in the disks 3,4 have a depth of about half the thickness of the niobium disk 6. An external current supply lead 7 is secured to the disk 6. Lead 7 is made of niobium and, for example, welded to the disk 6. An internal current supply lead 8 is connected to the niobium disk 6. Internal current supply lead 8 is made of tungsten and, for example, welded to the niobium disk 6. The respective disks 3,4 are formed with concentric openings for the respective current supply leads 7,8. Electrically conductive welding connections are suitable for attaching the supply leads 7,8 to the disk 6. A coiled electrode 9 of tungsten is secured to the internal current supply lead 8, located within the discharge space of the discharge vessel 1, and, for example, of standard construction. The entire end connection of the discharge vessel 1 is closed off gas- and pressure-tight by means of a metal halide resistant glass melt of solder 10, filling the capillary spaces between the recessed tubular portion 2a of the discharge vessel 2, the external and internal disks 3,4, the space around the recesses 3a,4a and a space around the electrode supply leads 7,8. These spaces, actually, are capillary spaces and are shown in the drawing greatly enlarged and exaggerated for ease of illustration. A tube 8a of aluminum oxide ceramic surrounds the tungsten current supply lead 8. The aluminum oxide ceramic tube 8a extends between the electrode winding or wrapping 9, also of tungsten, and the niobium disk 6. The tube 8a has a slightly larger internal diameter than the lead 8. The play or gap is just large enough to accomodate the different expansions of the tungsten lead 8 and the tube 8a; this gap is too small, however, that condensate of the fill components within the vessel 1 can deposit or precipitate therein. The absolute width of the play or gap depends on the diameter of the current supply 8 which, again, varies with the power rating of the lamp. The end of the tube 8a which faces the niobium disk 6 is tightly surrounded by the glass melt, made of solder glass 10. The solder glass 10 is resistant to metal halides.

Thermal coefficients of expansions, for typical elements used in the lamp are as follows:

tungsten: 45.times.10.sup.-7

solder glass 10: 85-100.times.10.sup.-7 ; melting point between about 1250.degree. C. to 1370.degree. C.

aluminum oxide ceramic, used for sealing disks 3, 4, and tube 8a: about 86-100.times.10.sup.-7

niobium: about 85.times.10.sup.-7

The construction, thus effectively avoids direct contact of the tungsten lead 8 with a material having a substantially different thermal coefficient of expansion. The sealing disk of niobium is effectively shielded from contact with the fill materials within the discharge vessel, so that the niobium disk is effectively protected against attack by the fill components therein, and especially the metal halides within the fill.

The thermal coefficient of expansion of niobium, as well known and as recited above, is close to that of aluminum oxide ceramic. Consequently, thermal stresses which occur during operation of the lamp, and particularly when the melt connection reaches an operating temperature which, typically, is about 800.degree. C. and with an operating temperature of the discharge vessel which may go higher and be even in excess of 1000.degree. C., can be essentially ignored. Preferably, the external surface of the aluminium oxide ceramic disk 3 terminates essentially flush with the bottom of the recessed portion 2a of the tube 2.

The arrangement permits use of metal halide high-pressure lamps with fill substances which would attack niobium. Such lamps have superior color rendition with respect to sodium high-pressure lamps.