Electric lamp and discharge devices – Electrode and shield structures – With lead wire or connector
Reexamination Certificate
1999-08-06
2001-07-24
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
Electrode and shield structures
With lead wire or connector
C313S626000, C313S623000
Reexamination Certificate
active
06265817
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an electric lamp comprising:
a light-transmissive lamp vessel which is closed in a vacuumtight manner and has a quartz glass wall enclosing a space, accommodating an electric element;
a metal foil completely embedded in the wall and having knife edges formed by knife planes;
at least an internal current conductor which is connected to the embedded metal foil and projects into the space;
at least an external current conductor which is connected to the embedded metal foil, projects from the wall of the lamp vessel and is provided with a coating.
A lamp of this type is known from U.S. Pat. No. 3,420,944. During operation of the known lamp, a part of the external current conductor and the metal foil, generally of molybdenum with an additive of, for example, 0.5-1.0% by weight of Y
2
O
3
, has a temperature of more than 450° C. In a lamp in which no measures were taken to inhibit corrosion of the external current conductor and the metal foil, these metal parts would corrode due to the high temperature in so far as the metal parts have an open connection with the atmosphere outside the lamp via a capillary around the external current conductor. Corrosion of the metal foil and/or the external current conductor leads to failure of the lamp due to the interruption of the current supply. The known lamp is protected against corrosion by providing, prior to its manufacture, a chromium coating on the external current conductor and at least parts of the metal foil, the knife edges and the knife planes. At locations where the coating is provided, the protection after manufacture of the lamp has remained intact, but the coating is partly converted into a chromium-containing protective coating. Both the coating and the protective coating retard the corrosion during operation of the lamp.
It is known that, in addition to corrosion of the current feed-through as a cause of premature failure of the lamp, there are various other causes of premature failure. Other causes may be, for example, leakage of the lamp vessel or, for example, an explosion of the lamp. The risk of failure of the lamp due to these other causes has appeared to be small in practice if the lamp has operated for less than a thousand hours.
The corrosion protection of the lamp as is known from U.S. Pat. No. 3,420,944 has the drawback that this leads to such a long lifetime of the lamp, for example, more than a thousand operating hours, that the risk of the lamp failure due to an explosion of the lamp and the risk of follow-up damage are unacceptably greater. The coating has a coating thickness and a quality level determining the corrosion protection and influencing the lifetime of the lamp. However, the quality level and the coating thickness in the known lamp are not controlled to such an extent that a lifetime limitation of a thousand operating hours is adjustable, which leads to an unacceptably large spread of the lamplife.
Another drawback of the known lamp is that the coating must be provided on the metal foil. Due to the extra treatments with the vulnerable metal foil, there is a great risk that the knife edges of the metal foil are damaged. The damaged knife edges of the metal foil embedded in the finished lamp lead to high tensions in the wall of the lamp vessel so that the risk of failure during manufacture of the lamp or due to premature leakage of the lamp vessel will be unacceptably greater.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an electric lamp, having a simple construction which can easily be made and obviates the above-mentioned drawbacks.
According to the invention, this object is achieved in that a protective coating is present on the metal foil and on the external current conductor, the protective coating comprising a low melting point reaction product of the coating with SiO
2
, and at least the knife planes being free from the protective coating. In the manufacture of the lamp, a seal is made in which one or more of said metal foils are enclosed in the wall. During this operation, the quartz glass is softened at the area where this seal is to be created in the presence of the metal foil and the external current conductor. The quartz glass then reaches a temperature of more than 1900° C. As soon as the quartz glass comes into contact with the external current conductor, this conductor and the coating provided thereon become so hot that the coating melts and flows out on the quartz glass and parts of the metal foil. The molten coating reacts substantially immediately and forms relatively low melting point reaction products with the molybdenum of the external current conductor and the metal foil, and with the quartz glass. Subsequently, the seal thus formed is cooled down. Owing to its comparatively high coefficient of linear thermal expansion (approximately 50*10
−7
K
−1
), the external current conductor contracts more strongly than the quartz glass, glass having an SiO
2
content of at least 95% by weight (linear thermal expansion coefficient of approximately 6*10
−7
K
−1
) in which it is embedded. This creates a capillary space around this current conductor. No such capillary space is created around the metal foil because of the foil shape.
After some cooling, the capillary space has formed around the external current conductor but the low melting point reaction products are still fluid for some time. Due to capillary action, the low melting point reaction products mainly contract in corners and narrow portions of the capillary space, with a large, substantially cylindrical hollow space remaining behind in the capillary. The hollow space has an open connection with the atmosphere outside the lamp. The capillary-adjacent parts of the quartz glass, the external current conductor and the metal foil are, however, shielded from the atmosphere outside the lamp in that the low melting point reaction products have remained behind as a thin protective coating on the parts adjacent the capillary, which protective coating is relatively thick in the corners and the narrow portions of the capillary. The knife planes, preferably at least up to a distance of the knife edges having a largest thickness D of the metal foil, and the knife edges have remained free from the protective coating.
Corrosion of the external current conductor and/or the metal foil results in an expansion and is most critical in the corners of the capillary. In the corners of the capillary, this expansion soon leads to high tensile stresses in the quartz glass because the capillary in the corners has little room for this expansion. Thus there is a great risk of breakage in the quartz glass, starting in one of the corners of the capillary. If corrosion of the metal foil and the external current conductor occurs near one of the corners of the capillary, the accompanying expansion has a wedge effect. Due to the acute angles at which the quartz glass engages the metal foil, the tension building up in the quartz glass as a result of the expansion will concentrate near the acute angles of the capillary in the quartz glass. The risk of breakage in the quartz glass, starting at one of the angles of the capillary, is thereby further increased. Since a relatively thick protective coating has come in the lamp according to the invention, notably in the corners, these corners are well protected against corrosion and there is a small risk that the above-mentioned phenomena occur too quickly. However, corrosion of the metal foil and the external current conductor still occurs. It has been found that the moment of failure, for example at a lifetime of 800-1000 operating hours, has become satisfactorily adjustable in the lamp according to the invention by varying the coating thickness of the coating. This is in contrast to the known lamp in which it has been found that the quality level and the coating thickness cannot be controlled to such an extent that a lifetime limitation of a thousand operating hours is adjustable, resulting in an unacceptably large spread
Baken Petronella C. M.
Steinmann Maarten W.
Halajian Dicran
Patel Ashok
U.S. Philips Corporation
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