Electric lamp and discharge devices – With gas or vapor – Having particular electrode structure
Reexamination Certificate
1999-06-22
2003-03-18
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
With gas or vapor
Having particular electrode structure
C313S633000, C313S634000, C313S332000
Reexamination Certificate
active
06534918
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a high-pressure gas discharge lamp comprising:
a lamp vessel which is closed in a vacuumtight manner and has a quartz glass wall enclosing a discharge space;
metal foils embedded in the wall of the lamp vessel and each connected to a respective external current conductor;
tungsten electrode rods each connected to a respective one of said metal foils and projecting from the wall of the lamp vessel into the discharge space;
an ionizable filling in the discharge space;
the lamp being defined by the following relation
f
inw
>=40%
in which:
f
inw
=fraction of length of the electrode rod enclosed in the wall of the lamp vessel.
A high-pressure gas discharge lamp of this type is known from U.S. Pat. No. 5,461,277. The known lamp is suitable for use as a vehicle headlamp and has electrode rods of a thickness of 250 &mgr;m which may or may not have an envelope at their ends and may be made of, for example, thoriated tungsten.
Stringent requirements are imposed on the speed with which the lamp, after it has been ignited, provides a large fraction of the luminous flux during stable operation. It is also necessary that the lamp can be ignited while it is still hot due to a previous operating period. The lamp is ignited at a voltage of several kV and a frequency of several kHz in order to comply with these requirements.
In the manufacture of the known lamp, a seal is made in which one or several 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, the external current conductor and the electrode rod. Subsequently, the lamp, or the lamp-to-be, cools down. Due to its relatively high coefficient of linear thermal expansion (approximately 45*10
−7
K
−1
), the electrode rod then contracts more strongly than the quartz glass in which it is embedded. Quartz glass is a glass having an SiO
2
content of at least 98% by weight, the coefficient of expansion of the glass is approximately 6*10
−7
K
−1
. For a good adhesion between the rod and the quartz glass, obtained by an additive to the electrode rod tungsten, such as thorium oxide, a coating of quartz glass around the rod is obtained, which is mechanically unconnected with the quartz glass of the wall. If the electrode rod and the quartz glass adhere insufficiently to each other, a capillary space is created due to shrinkage around this rod. No such capillary space is created around the metal foil, often a molybdenum foil, because of the foil shape.
In the known lamp, there is often a good adhesion between the rod and the quartz glass and thus there is a coating of quartz glass around the rod. The quartz glass coating of the electrode rods in the known lamp enhances their thermal capacity (the energy which is necessary for the same rise of temperature) and also increases their thermal conductance (the quantity of heat which can be depleted per unit of time). On the other hand, their electrical conductivity is not affected. The higher thermal capacity retards the rise of temperature of the rods during ignition of the lamp, so that the permanent contact with the embedded metal foil enables the surrounding quartz glass of the wall to assume a higher temperature and to expand, also because of the heat developed in this foil due to the passage of current.
It has been found that the coatings of species of one type of lamp may have alternating lengths. This may be due to small variations of temperature of the quartz glass when the seal is being made. It is a drawback that the absence of a coating or an insufficient coating results in rejects during the lamp production and that the known lamp has only a short lifetime when there is no or not enough quartz glass coating and when this lamp is often switched on and switched off after a short operating period.
When such a lamp without coating is ignited, the temperature of the electrode rods rises steeply owing to the high current flowing through them and owing to heat transfer from the discharge. The quartz glass does not instantaneously follow this temperature rise. Owing to their higher temperature and their higher coefficient of expansion, the rods will come into contact with the quartz glass and exert pressure on it. It was found that damage, such as microcracks, then occurred in the quartz glass, which microcracks generally increase in number and size during subsequent ignition periods. This leads to a (premature) end of the lifetime of the lamp owing to leakage, causing constituents of the filling to escape so that the lamp no longer ignites, or the lamp vessel is broken.
Lamps complying with the relation f
inw
>=40% have a greater risk of occurrence of the above-mentioned detrimental phenomena, unless special circumstances are created, for example, a quartz glass coating around the electrode rod.
Another drawback is that the coating leads to unwanted and troublesome reflections of the light generated in the discharge.
U.S. Pat. No. 5,510,675-A discloses electrodes, a part of which is made of rhenium and has a thickness of 400 &mgr;m. However, the part made of rhenium does project very far into the discharge space and is only provided with a head at its very last end, with a thickness of, for example 1 mm or an enveloping winding of tungsten. However, this large head leads to the unfavorable effect of lamp flickering, i.e. the point of contact of the discharge arc displaces suddenly over the head.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a high-pressure gas discharge lamp, having a simple construction and counteracting said drawbacks.
According to the invention, the electrode rods have first parts projecting into the discharge space, which first parts are at least substantially made of tungsten, and second parts enclosed at least partly in the wall, which second parts have a thickness ranging between 250 &mgr;m and 350 &mgr;m and at least an envelope of rhenium, said first and second parts contacting and being connected to each other via facing ends.
Since the electrodes are composed of a first and a second part, it is possible to adapt the electrodes to circumstances. The first part is made in conformity with the end of the electrode of the known lamp projecting into the discharge space, so that it can withstand the heat developed by the high starting currents and the discharge during the lifetime of the lamp. The first part of the electrode is made of tungsten so that a strong evaporation of electrode material is prevented, as will occur if the first part consists of rhenium. The second part is designed in such a way that the problem of leakage or breakage of the lamp due to expansion and, consequently, exertion of pressure on the quartz glass by the electrode rod upon (re)ignition of the lamp at least substantially does not occur anymore. The first and the second part of the electrode may be secured to each other in accordance with conventional techniques such as laser welding.
In second parts having a relatively thick envelope of rhenium or being entirely made of rhenium, a greater thickness is necessary than when this second part is made of, for example, tungsten having a relatively thin envelope of rhenium due to the smaller coefficient of thermal conductance of rhenium compared with tungsten, S
Re
~0.3*S
W
. Experiments have proved that a thickness of minimally 250 &mgr;m is necessary for second parts substantially made of rhenium so as to ensure a sufficient depletion of heat.
It has been found that in lamps complying with the relation f
inw
>=40%, the occurring problems of leakage at least substantially do not occur at relatively small thicknesses of second parts of the electrode rods enclosed in the wall. The risk of leakage or breakage of the lamp is considerably reduced if the thickness of these second parts is chosen to be smaller than 350 &mgr;m. The successful use of relatively large thicknesses in second parts havin
Eijsermans Joseph F. R.
Jörres Angela
Kubon Marcus
Peterek Manfred
Seebode Dorothea
Halajian Dicran
Keegan Frank J.
Koninklijke Philips Electronics , N.V.
Patel Vip
Quarterman Kevin
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