WO2006046166A2

WO2006046166A2 - Electric lamp

Info

Publication number: WO2006046166A2
Application number: PCT/IB2005/053411
Authority: WO
Inventors: Jacobus H. A. Selen
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-10-25
Filing date: 2005-10-18
Publication date: 2006-05-04
Also published as: WO2006046166A3

Abstract

The invention relates to an electric lamp with a longer service life, comprising a quartz-glass envelope having at least one sealed end, containing at least one current feedthrough comprising a thin molybdenum foil entirely embedded within said sealed end, a first conductor connected to said molybdenum foil by an overlapping connection and extending interiorly of said envelope, and a second conductor connected to said molybdenum foil and extending exteriorly of said envelope, wherein the overlapping connection is a weld, which has a length which is between 0.01 and 1.5 times the diameter of the first conductor, and the foil is in a plane, of which the distance to a centre line through the first conductor is less than 0.9 times, preferably less than 0.5 times, the radius of the first conductor.

Description

Electric lamp

The invention relates to an electric lamp comprising a quartz-glass envelope having at least one sealed end, containing at least one current feedthrough comprising a thin molybdenum foil entirely embedded within said sealed end, a first conductor connected to said molybdenum foil by an overlapping weld and extending interiorly of said envelope, and a second conductor connected to said molybdenum foil and extending exteriorly of said envelope.

In lamps having a lamp envelope of quartzglass, i.e. glass having a SiO₂ content of at least 95% by weight, a molybdenum foil is frequently used as a current lead through. In spite of the considerably different coefficients of thermal expansion of quartzglass (approximately 7x10^"7 per deg. C.) and molybdenum (approximately 54x10^"7 per deg. C), lamps having vacuum-tight seals are nevertheless obtained. This is due to the ductility of molybdenum, to the shape of the foil, knife edges of the foil extending in the longitudinal direction of the seal, and to the small thickness of the foil, which is a few tens of μm. Current conductors which are much thicker than the foil, namely a few hundred μm thick, are generally welded to the foil. In a typical weld the foil is connected to the surface of the current conductor as depicted in Fig. 2A. Around the weld of the foil and the current conductor cracks may occur in the quartzglass due to cooling stresses. These cracks, propagating due to temperature cycles of the lamp, may cause premature failure. The material and shape of the current feedthrough of electrical lamps having a glass bulb quite substantially determine the manufacture, function and service life of such lamps. The term "lamps" especially comprises halogen filament lamps and discharge lamps such as high pressure mercury lamps, metal-halide lamps, and high-pressure xenon lamps. Much attention has been paid in the past to make a gas-tight current feedthrough. A current feedthrough in a lamp with or without gas filling is, as a rule, fused in quartz glass or squeezed into the latter. This results in, respectively, a shrink seal or a pressed seal. Molybdenum, owing to its high melting point and in spite if its different coefficient of thermal expansion as compared to glass, has been found to be a suitable conductor material for a current lead through. Other material requirements a molybdenum conductor is expected to satisfy are ductility, good mouldability, high mechanical strength, resistance to oxidation or corrosion, especially versus halides, and fusibility with other components of the conductor.

US 3,668,456 describes a quartz-glass envelope having a pressed sealed end, containing at least one current feedthrough comprising a thin molybdenum foil entirely embedded within said sealed end, a first conductor connected to said molybdenum foil by an overlapping connection and extending interiorly of said envelope, and a second conductor connected to said molybdenum foil and extending exteriorly of said envelope. The connection between the first conductor and the molybdenum foil is a pinched connection formed by a slot in the embedded end of the first conductor, the sides of said slot being physically deformed in order to be securely closed on one end of the foil.

An advantage of such a pinched connection is its symmetry with respect to a plane through rod and foil. Connections wherein rod and foil are just resistance welded are asymmetric with respect to said plane. This asymmetry causes cooling stresses in the quartz after sealing of the end of the envelope. These cooling stresses increase the sensitivity of the lamp to explosions or fatal cracking and therefore decrease the service life of the lamp.

A disadvantage of the pinched connection is that the resistance of this connection increases during the service life of the lamp due to cyclic heating and cooling. This shortens the lifetime of the lamp.

Another disadvantage of the pinched connection is that at the temperature required for sealing of the envelope, the material of the first conductor, often tungsten, has a much lower yield point than at room temperature, as a result of which the pinch force relaxes and may disappear completely already during sealing of the lamp. This also causes a poor electical connection between the current connector and the foil.

An object of the present invention is to provide a lamp with a longer service life.

According to the invention, a lamp of the kind described in the opening paragraph for this purpose comprises the characterizing features of claim 1.

In the lamp of the invention, the overlapping connection is a weld, which has a length, which is between 0.01 and 1.5 times the diameter of the first conductor. An overlap of between 0.01 and 1.5, preferably between 0.1 and 1.0, turned out to cause a level of internal stresses in the quartz after sealing of the envelope, which is sufficiently low to obtain a longer service life.

In the lamp of the invention, the overlapping connection is in a plane, of which the distance to a centre line through the first conductor is less than 0.9 times, preferably less than 0.5 times, the radius of the first conductor. Although the best results with respect to lifetime were obtained with a connection which is as symmetric as the connection described in US 3,668,456, a certain asymmetry, such that the distance from the foil to a centre line through the first conductor is less than 0.9 times, preferably less than 0.5 times, the radius of the conductor, still results in an improved service life of the lamp.

An additional advantage of the lamp of the invention is that its production is cheaper and faster than the cumbersome process required for making a pinched connection as described in US 3,668,456. A frequently used alternative to the pinched connection is resistance welding as described in, for example, US 4,254,300. However, this requires a long overlap between the conductor and the foil. A disadvantage of the resistance weld as shown in Fig. 2A is its asymmetry. The connection between the first conductor and the foil in the lamp of the invention can be made by laser butt welding. In doing so, the assembly of first conductor and foil is heated, while the foil is either placed over the end surface of the conductor, or pressed against its end surface. A seal in the lamp of the invention can be a pressed seal as well as a shrink seal, depending on the pressure in the lamp. In order to achieve vacuum-tight pressing or fusing of the molybdenum foil in the glass despite the different coefficients of thermal expansion in particular of silica glass or glass materials with a high SiO₂ content and molybdenum, the foil is configured to be very thin (typically 15 to 50 μm), with a high width to thickness ratio (typically >50), and has side edges which taper in the form of a cutting blade.

An additional advantage of the weld in the lamp of the invention is that, compared to a lamp with a similar service life, the internal pressure in the lamp can be higher, without additional risk of explosions. Therefore, the lamp of the invention is preferably a high pressure discharge lamp; the envelope encloses a discharge zone with pressed sealed ends on both sides thereof, and the first conductors are electrode rods.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment described hereinafter.

Fig. 1 is a cross-sectional view of a part of an embodiment of the electric lamp in accordance with the invention. The Figure is purely schematic and not drawn to scale. Particularly for clarity, some dimensions are exaggerated strongly.

Fig. 2 A shows a side view of a typical weld according to the state of the art. Figs. 2B and 2C shows the weld as used in the lamp of the invention.

Fig. 1 shows a cross-sectional view of a seal in a high-pressure discharge lamp according to the invention. A first conductor 3 with a diameter of 250 μm extends in the discharge space 1, enclosed by a quartz-glass envelope 2. A molybdenum foil 5 with a thickness of 30 μm is entirely embedded within the pressed seal 6 and welded to the first conductor 3 with an overlapping connection 8 having a length of 200 μm, such that the weld is in a plane, of which the distance to a centre line 7 through the first conductor is less than 20 μm. In this Figure the weld between the foil and the second current conductor 4 is made similar to the weld to the first conductor. However, as the temperature changes at a larger distance from the discharge space are less critical, said weld can also be a weld according to the state of the art, as depicted in Fig. 2A.

Figs. 2A, 2B and 2C show a weld between a detail of the first current conductor 3 and the foil 5, the weld shown in Fig. 2A being a weld according the state of the art and the weld shown in Fig. 2B, side view, and Fig 2C, top view, being used in the lamp of the invention.

Claims

CLAIMS:

1. Electric lamp comprising a quartz-glass envelope 2 having at least one sealed end 6, containing at least one current feedthrough comprising a thin molybdenum foil 5 entirely embedded within said sealed end 6, a first conductor 3 connected to said molybdenum foil 5 by an overlapping connection and extending interiorly of said envelope 2, and a second conductor 4 connected to said molybdenum foil and extending exteriorly of said envelope, characterized in that the overlapping connection 8 is a weld, which has a length which is between 0.01 and 1.5 times the diameter of the first conductor 3, and the foil is in a plane, of which the distance to a centre line through the first conductor is less than 0.9 times the radius of the first conductor 3.

2. Lamp according to claim 1, wherein the distance to a centre line 7 through the first conductor is less than 0.5 times the radius of the first conductor 3.

3. Lamp according to claim 1 or claim 2, wherein the lamp is a high pressure discharge lamp, the envelope encloses a discharge zone with press or shrink sealed ends on both sides thereof, and the first conductors being electrode rods.