Patent Application
20100219751
The invention relates to a direct current discharge lamp of the type specified in the preamble of patent claim 1.
Such a direct current discharge lamp may already be taken as known from the prior art and comprises an anode and a cathode that are arranged opposite one another at a predetermined distance inside a discharge vessel (14) filled with a filling gas. In order to produce light, an electric power can be applied to the anode and the cathode, the result being the formation of a gas discharge in the region of an arc.
A disadvantageous circumstance with the known direct current discharge lamps may be seen in the substantial limitation of their useful life by a blackening of the discharge vessel. This blackening results from geometric variations in the surface of the anode facing the cathode in the heated state during operation of the direct current discharge lamp. In this case, local growths occur that lead to a concentration of the attachment of the arc. Very high temperatures that lead to an increased evaporation of the material of the anode can occur at these attachment points. The evaporated anode material is then deposited on the inside of the discharge vessel and leads to said blackening.
It is therefore the object of the present invention to provide a direct current discharge lamp of the type mentioned at the beginning that has a reduced blackening of the discharge vessel and thus a lengthened service life.
This object is achieved according to the invention by a direct current discharge lamp having the features of patent claim 1. Particularly advantageous refinements are to be found in the dependent claims.
According to the invention, a direct current discharge lamp that has a reduced blackening of the discharge vessel and therefore a lengthened service life is characterized in that at least the distance between the anode and the cathode, the electric power and a geometry of the anode are adapted to one another in such a way that a region of a surface of the anode facing the cathode is free flowing in the heated state of the direct current discharge lamp. In other words, by adapting at least said parameters a free flowing state of the material of the anode is specifically produced during operation of the direct current discharge lamp in the region of its surface facing the cathode such that deformations of the surface occurring during operation are automatically compensated by subsequent flowing of the material, and a uniform anode plateau is ensured. This reliably prevents the occurrence of local growths with the associated high temperatures, and so there is a substantial reduction in the evaporation of the anode material. Owing to the self-healing ability of the anode, the direct current discharge lamp therefore exhibits a substantially weaker blackening of the discharge vessel and has a correspondingly lengthened service life.
In an advantageous refinement of the invention, it is provided that at least the distance between the anode and the cathode, the electric power and the geometry of the anode are adapted to one another in such a way that the region of the surface of the anode facing the cathode has a fluidity of at most 10−6 mPas, and preferably of at most 10−8 mPas in the heated state of the direct current discharge lamp. Such a limitation of the fluidity ensures that during operation of the direct current discharge lamp the material of the anode has a sufficiently high viscosity, and also that there is no macroscopic deformation owing to increased or frequent effects of force. The direct current discharge lamp can therefore, for example, also be used for illumination devices of motor vehicles or the like.
In a further advantageous refinement of the invention, it is provided that the anode consists of doped and/or undoped tungsten at least in the region of the surface facing the cathode. Owing to the high evaporation temperature and the chemical resistance of tungsten, the service life of the direct current discharge lamp can be additionally lengthened. Here, doped and/or undoped tungsten can be provided as a function of the desired illumination characteristic of the direct current discharge lamp. It is possible furthermore, in this case to provide that in addition to the parameters of electrode spacing, electric power and geometry of the anode, account is also taken of the characteristic properties of the respective material of the anode.
It has further proved to be advantageous in this case that the anode is of rotationally symmetrical design at least along a longitudinal region facing the cathode. During the heated state of the direct current discharge lamp, this permits on the surface of the anode the formation of a “melt pool” of large area and permanent stability. Because of the fact that the arc is attached over a large area and uniformly, the occurrence of operating temperatures above the respective evaporation temperature of the anode material is reliably avoided.
In a further advantageous refinement of the invention, it is provided that starting from the surface facing the cathode, the anode has a length of at least 5 mm. In this way, the anode acts in the heated state as a thermal heat store, thus ensuring that the temperature of the surface facing the cathode is as uniform as possible.
It has proved advantageously furthermore, that a quotient Q of the electric power in W and the distance between the anode and the cathode in mm is given in the heated state of the direct current discharge lamp by the relationship
a
1
*A
2
+a
2
*A+a
3
<Q<b
1
*A
2
+b
2
*A+b
3,
where:
a1=−0.0001 W*mm−7;
a2=0.42 W*mm−4;
a3=687 W*mm−1;
b1=−0.0003 W*mm−7;
b2=0.8967 W*mm−4; and
b3=88 W*mm−1,
A denoting the volume of the anode in mm3 on the first 5 mm length starting from the surface facing the cathode. This ensures an operation of the direct current discharge lamp in a region in which, given gas discharge lamps with anodes of sufficient length, on the one hand the required ability to free flow, and on the other hand a reliable reduction in the evaporation of the material of the anode in the region of the surface are attained.
The aim below is to explain the invention in more detail with the aid of an exemplary embodiment. Of the figures:
Q=a
1
*A
2
+a
2
*A+a
3
where:
Q=b
1
*A
2
+b
2
*A+b
3
where:
Owing to the high energy input, undesired fusings of the anode 10, instabilities of the arc and increased evaporation of the material of the anode 10 occur in the region above the compensation curve IIb. Conversely, in the region below the compensation curve IIa no sufficient ability to free flow, and therefore also no permanently stable “melt pool” are achieved on the surface 24 of the anode 10, which means that it is impossible to remedy irregularities in the surface 24 occurring during operation. Only lamps whose parameters Q and A fall into the middle range, which is essentially delimited by the two compensation curves IIa and IIb, exhibit a good operational performance.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2007/060042 | 9/21/2007 | WO | 00 | 3/22/2010 |
