Electric lamp and discharge devices – With gas or vapor – Envelope with particular structure
Reexamination Certificate
2000-09-06
2003-05-27
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
With gas or vapor
Envelope with particular structure
C313S636000, C313S623000, C313S025000
Reexamination Certificate
active
06570328
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an electric lamp comprising:
a lamp vessel closed in a gastight manner and having a quartz glass wall enclosing a space in which an electric element is arranged;
a feedthrough comprising:
a foil-type metal sealing part completely embedded in the wall of the lamp vessel so as to form a gastight seal with the quartz glass wall,
an inner current conductor connected to the metal sealing part, extending into the space, and connected to the electric element;
an outer current conductor connected to the metal sealing part at a connection area and extending through the wall to the exterior.
Such a lamp is known from GB 496 679. In the known lamp the metal sealing part is a metal strip, e.g. made of molybdenum. Tensile stresses in the quartz glass wall are present owing to different coefficients of linear thermal expansion, approximately 50*10
−7
K
−1
for molybdenum and approximately 6*10
−7
K
−1
for quartz glass, i.e. glass having an SiO
2
content of at least 95% by weight, in the gastight seal between the metal strip and the quartz glass wall. Seals are stronger when these stresses are relatively low, and as a result the risk of early failure of the lamp is reduced. To lower these tensile stresses in the quartz glass, the metal strip has a special shape, i.e. is crinkled or provided with holes. High tensile stresses between the quartz glass wall and the metal strip are avoided during the manufacture of the lamp because of this shape. However, the known lamp has the disadvantage that the metal strip should extend for a substantial distance outside the quartz glass wall, both into the space and to the exterior. Since the metal strip extends into the space of the lamp vessel, the metal strip is excessively exposed to the corrosive atmosphere inside the lamp vessel. As a result there is a significant risk of corrosion of the metal strip, leading to a relatively fast blackening of the quartz glass wall involving relatively bad lumen maintenance. Since the metal strip extends to the exterior, the risk of a person unintentionally touching live electric parts is significantly increased.
Another disadvantage of the metal strip is that its manufacture involves a serious risk of fracture of the metal strip; besides, the manufacture of the metal strip is cumbersome. To obtain a good functioning of the crinkled metal strip, the crinkles are obtained by making bends in the foil through a predetermined angle. However, on the one side these bends should be as sharp as possible to lower the risk of too high stresses in the quartz glass, on the other hand these bends should be rounded to lower the risk of fracture and weakening of the metal strip owing to too sharp bends. In the case of the crinkled metal strip, furthermore, high demands are imposed on the manufacture of the gastight seal since care should be taken that the shape of the strip and in particular the shape of the sharp bends should withstand the sealing process. To obtain a metal strip with holes, openings have to be made in a strip having a completely closed surface. This may be done, for example, by punching or chemical etching. In the known lamp, the holes are made by punching. However, punching involves a mechanical load and hence the risk of fracture or at least serious weakening of the (brittle) metal strip. Hence, the manufacture of the lamp is relatively cumbersome, because special care has to be taken to avoid breakage of the weakened metal strip during the sealing process.
Yet another disadvantage of the known lamp is that corrosion and subsequent expansion of the external current conductor and/or the metal strip in the quartz glass wall relatively soon leads to high tensile stresses in the quartz glass. Since there is little room for this expansion in the quartz glass wall and since there is an abundance of oxidation-sensitive material, there is a great risk for these tensile stresses to reach a critical value and a subsequent breakage of the quartz glass. Such breakage heightens the risk of failure of the known lamp by explosion, hence the known lamp is relatively unsafe.
SUMMARY OF THE INVENTION
It is an object of the invention to supply a lamp of the type described in the opening paragraph, which has a relatively good lumen maintenance, which can be easily manufactured, and which is of a relatively safe construction.
This object is obtained according to the invention by an electric lamp of the kind described in the opening paragraph which is characterized in that the metal sealing part comprises a gauze at the connection area. In the lamp of the invention the metal sealing part does not extend from the quartz glass wall into the space of the lamp vessel. Hence, the risk of corrosion of the metal sealing part is significantly reduced and the prospect of relatively little blackening taking place, involving a good lumen maintenance, is enhanced.
Furthermore, the metal sealing part comprising the gauze is relatively robust, hence the sealing process can readily be done. Therefore the manufacture of the lamp can be relatively easy. In the manufacture of the- lamps a seal is made in which one or more of said metal sealing parts comprising gauzes are enclosed in the wall. During the manufacture, the glass is softened at the area where this seal is to be created in the presence of the metal sealing part 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 becomes so hot that the quartz glass flows out over the metal sealing part and into the openings of the gauze. The molten quartz glass fuses itself substantially immediately to the metal sealing part and to the quartz glass on the other side of the openings, forming a tight bond. 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 during this cooling down than does the quartz glass (linear coefficient of thermal expansion of approximately 6*10
−7
K
−1
) in which it is embedded. Under these circumstances a capillary space is formed around this current conductor. It appears that no such capillary space is forming around the metal sealing part because of its foil-like shape. The areas adjacent to the areas where either the internal or the external current conductor and the metal sealing part overlap are the connection areas.
The capillary space around the external current conductor is in an open connection with the atmosphere outside the lamp, which renders the external current conductor and the gauze of the metal sealing part easily accessible to oxygen. Corrosion of the external current conductor and/or the gauze will result in an expansion, which expansion is especially critical at the connection area. The time needed for tensile stresses to reach a critical value is increased in the lamp according to the invention, because less oxidation of metal will occur because in the case of the gauze there is less material which has an open connection with the atmosphere outside the lamp in comparison with a seal construction having an ordinary or crinkled metal foil. To avoid excessive oxidation outside the connection area, it is not necessary for the metal sealing parts to have a gauze structure outside this area.
It has been found that, due to the increase in time needed for tensile stresses to reach a critical value, the risk of explosion of the lamp of the invention has become negligibly small, since the lamp is likely to fail through oxidation of the metal gauze,. Most probably this oxidation will have caused the end of the electrical contact between the external current conductor and the metal gauze before the tensile stresses could reach a critical value. Hence the lamp is relatively safe.
The capillary space around the internal current conductor is in an open connection with
Keegan Frank
Koninklijke Philips Electronics , N.V.
Patel Vip
Quarterman Kevin
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