Electric Lamp

1. Field of the Invention

This invention generally relates to an electric lamp having a quartz glass envelope with a pinch seal and more particularly to a molybdenum foil coupling member used at the pinch seal.

2. Description of the Prior Art

Electric lamps having a quartz glass with a pinch seal in which a foil coupling member is incorporated as a current lead-through conductor are used widely in discharge lamps, in halogen lamps and so on.

The foil coupling member used in such lamps are usually made of pure molybdenum. In spite of the considerably different coefficients of thermal expansion of the quartz glass (approximately 5-6.times.10.sup.-7 per deg. C.) and of molybdenum (55.times.10.sup.-7 per deg. C.) lamps having vacuum-tight pinch seals are nevertheless obtained. This is due to the ductility of molybdenum, to the shape of the foil, the knife edge of the foil extending in the longitudinal direction of the pinch, and to the small thick thickness of the foil, which is about 20-40 microns.

In a halogen lamp, for example, the molybdenum foil coupling member is respectively connected to an internal current conductor, in most cases consisting of tungsten, which has a much higher melting point than molybdenum and of molybdenum.

Lamps of the type described above are known from U.S. Pat. No. 4,254,300. In such lamps, the molybdenum foil contains yttrium oxide (Y.sub.2 O.sub.3) in a quantity of from 1/4 to 1% of the molybdenum weight to avoid fracture at a welded joint between the molybdenum foil coupling member and the internal current conductor.

A more serious problem than the aforementioned fracture, is cracking or tearing off of the molybdenum foil itself which mainly occurs during the sealing step, i.e., after making the weld between the molybdenum foil coupling member and internal and external current conductors when the sealing step proceeds. At the sealing step between the molybdenum foil coupling member and the quartz glass envelope, the end of the quartz glass envelope is heated with sealing burners above its melting point (more than 1500.degree. C.). The pure molybdenum foil coupling member is simultaneously heated to a very high temperature by the sealing burners. After that, the end of the envelope is pinched with pinching jigs, and a pinch seal is formed.

Although molybdenum foil coupling members show high tensile strength, i.e., 70-90 kg/mm.sup.2 in the normal temperature range, the molybdenum foil coupling member occasionally becomes torn or cracked owing to high temperatures during the sealing step.

The cracking or tearing of the molybdenum foil coupling member is a cause of short lamp life, bad vacuum seal and results in unusable articles of inferior quality.

Accordingly, an object of this invention is to provide an electric lamp having a quartz glass envelope with a pinch seal using a foil coupling member which shows a good resistance to tearing and cracking upon exposure to high temperature.

Another object of this invention is to provide an electric lamp showing a good vacuum seal through lamp life.

Yet another object of this invention is to provide an electric lamp having a long life.

These and other objects have been achieved according to this invention by providing a novel electric lamp having a molybdenum foil coupling member in which potassium and silicon dioxide are disposed.

A more complete appreciation of this invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 is a schematic elevation of a halogen lamp for a copying machine; and

FIG. 2 is a schematic elevation of an arc tube for a mercury lamp.

Referring now to the drawings wherein like reference numerals designate identical or corresponding parts throughout the several views, and more particularly to FIGS. 1 and 2, thereof, a quartz glass envelope 11 of a 400 W halogen lamp has a tungsten filament 12 along its axis. The lamp envelope 11 has two pinch seals 13a and 13b, made by pinching at the both end thereof, each including potassium and silicon dioxide-containing molybdenum foil coupling members 14a and 14b, respectively, as current lead-through conductors. Each foil coupling member 14a, 14b is welded to an external current conductor 15 made of molybdenum and an internal current conductor 16 made of tungsten. The molybdenum foil coupling members 14a, 14b, each contain 500 ppm potassium (K) and 200 ppm silicon dioxide (SiO.sub.2).

FIG. 2 shows a quartz arc tube 21, normally disposed in an outer envelope (not shown) of a 1 kw mercury lamp. The arc tube 21 has two pinch seals 23a and 23b, made by pinching at both ends thereof, each including potassium and silicon dioxide containing molybdenum foil coupling members 24a and 24b, as current lead-through conductors, to which external current conductors 25a and 25b, respectively, and electrodes 22a and 22b made of tungsten, respectively, are welded. The molybdenum foil coupling member 24 contains 600 ppm potassium (K) and 500 ppm silicon dioxide (SiO.sub.2).

Crack testing of conventional molybdenum foil coupling members and the aforementioned molybdenum foil coupling member containing potassium and silicon dioxide was carried out in halogen lamps and mercury lamps. Average values of more than 1000 measurements are recorded in the following table.

The table shows that the foil coupling member of this invention has a good resistance to high temperatures and resulted in few defective articles.

The inventors also conducted lighting tests by selecting 100 conventional mercury lamps and 100 mercury lamps using the invention. With conventional lamps, three failed after lighting for 3000 hours. With lamps using the coupling member of the present invention, no failures occurred even after lighting for 6000 hours.

The molybdenum foil coupling member of this invention contains predetermined quantities of potassium (200-800 ppm) and silicon dioxide (200-2000 ppm), so that the recrystallization temperature of molybdenum rises. Consequently, the foil coupling member of this invention is better able to resist damage during the high temperature sealing step. It is therefore possible, using the invention, to heat even quartz glass envelopes to a sufficiently high temperature so that an excellent vacuum-tight pinch seal can be achieved. It is well known, of course, that quartz glass has a very high melting point.

The molybdenum foil coupling member of the invention, containing potassium and silicon dioxide, can be applied to other lamps using a quartz glass envelope, such as metal-halide lamps, xenon lamps, and so on.

Obviously other modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.