Electrode for a discharge lamp, discharge lamp and method for producing an electrode

Abstract

The invention relates to an electrode for a discharge lamp, wherein the electrode has a base body having an electrode plateau providing an end face of the electrode, wherein the base body is delimited by the electrode plateau in a longitudinal extension direction of the electrode. Furthermore, the electrode has a coating, arranged in at least a first region of the base body that is different from the electrode plateau, to increase an emission of heat. In addition, the electrode has an at least partially contiguous free region of the base body extending at least partly in the longitudinal extension direction as far as the electrode plateau, in which the coating for increasing the emission of heat is not arranged, and wherein the first region adjoins at least one section of the free region in the circumferential direction of the electrode.

Claims

1. An electrode for a discharge lamp, the electrode comprising: a base body having an electrode plateau providing an end face of the electrode, the base body being bounded by the electrode plateau in a longitudinal extent direction of the electrode; and a coating for increasing a thermal emission, arranged in at least one first region of the base body other than the electrode plateau, characterized in that the electrode comprises at least one at least regionally continuous free region of the base body, which extends at least partly in the longitudinal extent direction as far as the electrode plateau and in which the coating for increasing the thermal emission is not arranged, the first region adjoining at least one section of the free region in a circumferential direction of the electrode.

2. The electrode as claimed in claim 1, characterized in that the at least one free region is configured in a form of a strip with a width of at least 1 mm.

3. The electrode as claimed in claim 1, characterized in that the at least one free region is configured in a form of a strip with a length that is at least as large as half of a maximum diameter of the electrode perpendicular to its longitudinal extent direction.

4. The electrode as claimed in claim 1, characterized in that the at least one free region extends in a straight line along the longitudinal extent direction.

5. The electrode as claimed in claim 1, characterized in that the at least one free region extends at least partly in a form of a spiral around the longitudinal extent direction.

6. The electrode as claimed in claim 1, characterized in that the coating for increasing the thermal emission is arranged on at least half of an overall surface of the base body.

7. The electrode as claimed in claim 1, characterized in that the coating for increasing the thermal emission comprises a ceramic substance, and in particular is formed to at least 50 percent by volume from the ceramic substance.

8. The electrode as claimed in claim 1, characterized in that the coating for increasing the thermal emission comprises a matrix layer of ZrO.sub.2 with incorporated tungsten particles, the tungsten particles providing in particular between 2 percent by volume and 40 percent by volume of the coating.

9. The electrode as claimed in claim 1, characterized in that the electrode comprises a tungsten coating, which is arranged in at least one second region of the base body adjacent to an edge of the coating for increasing the thermal emission facing toward the electrode plateau.

10. The electrode as claimed in claim 9, characterized in that the at least one second region constitutes a region which is annular in the circumferential direction of the electrode, and is in particular continuous or discontinuous in the circumferential direction.

11. The electrode as claimed in claim 9, characterized in that the base body comprises a plurality of second regions, which are respectively arranged adjacent to the edge of the coating for increasing the thermal emission and at a distance from one another in the circumferential direction of the electrode, the tungsten coating being arranged in a respective one of the plurality of second regions.

12. The electrode as claimed in claim 9, characterized in that the at least one second region has a width which extends in a direction.

Description

(1) Further advantages, features and details of the invention may be found from the following description of preferred exemplary embodiments and with the aid of the drawing.

(2) FIG. 1 shows a schematic representation of an electrode for a discharge lamp having an uncoated free region extending in a straight line to the electrode plateau, according to one exemplary embodiment of the invention;

(3) FIG. 2 shows a schematic representation of an electrode having a free region extending in the form of a spiral to the electrode plateau, according to a further exemplary embodiment of the invention;

(4) FIG. 3 shows a schematic representation of an electrode having a partially applied tungsten coating at the edge of the coating for increasing the thermal emission; and

(5) FIG. 4 shows a schematic representation of a discharge lamp according to one exemplary embodiment of the invention.

(6) FIG. 1 shows a schematic representation of an electrode 10 for a discharge lamp 12 (cf. FIG. 4) according to one exemplary embodiment of the invention. The electrode 10 is configured as the cathode 10a in this exemplary embodiment. It may, however, also be configured in a similar way as the anode 10b (cf. FIG. 4). In this example, the electrode 10 thus comprises a base body 14. The latter in turn comprises an electrode plateau 16 providing an end face of the electrode 10. In this case, the base body 14 is bounded in a longitudinal extent direction L, which extends parallel to the electrode axis A, by the electrode plateau 16.

(7) In this example, the base body 14 is composed of a cylindrical section 14a and a cone-shaped, or conical, section 14b. The electrode 10 may, however, also be shaped in any other desired way, for example if it is configured as the anode 10b, as represented in FIG. 4. In this example, essentially the conical section 14b is omitted and the electrode plateau 16 follows on substantially directly from the cylindrical section 14a.

(8) In order to improve the thermal radiation power, the surface is coated in a first region 18 of the base body 14 other than the electrode plateau 16 with a coating 20 for increasing the thermal emission. This coating 20 preferably constitutes a particle composite coating consisting of a ZrO.sub.2 matrix layer with incorporated tungsten particles. To this end, for example, a powder mixture consisting of 10 percent by volume tungsten and 90 percent by volume ZrO.sub.2 may be sintered on.

(9) During operation of a discharge lamp, for example the discharge lamp 12 as represented in FIG. 4, a corresponding discharge arc is finally formed between the respective electrode plateaus 16 of the cathode 10a and of the anode 10b.

(10) When igniting short-arc discharge lamps, it often occurs that the charge carrier density in the plasma is not sufficient to form a stable discharge arc between the tips of the electrodes 10 from the start. In this case, the arc initially starts at positions on the electrode 10 which are further away from the electrode plateaus 16. In conventional electrodes, it then very often occurs that this arc starts on the emission-increasing coating, which generally destroys such coatings partially, and furthermore leads to increased clouding or blackening of the bulb and overall to a reduction of the lifetime of the discharge lamp.

(11) In order to prevent this, a free region 22, in which the coating 20 for increasing the thermal emission is not arranged, is now advantageously provided on the base body 14. This free region 22 is configured in the form of a strip and extends at least partly in the direction of the longitudinal extent direction L as a continuous area as far as the electrode plateau 16. In general, this free region 22 has a length LF and a width B locally perpendicular thereto, as represented in FIG. 1. The length LF is in this case greater than the width B. The length LF of the free region 22 is preferably at least as large as half of the diameter D of the electrode 10, particularly in relation to the widest region of the electrode 10, or in relation to the cylindrical section 14a.

(12) In this example represented here in FIG. 1, the free region 22 extends in the form of a strip in a straight line over the entire length of the electrode 10 as far as the electrode plateau 16. The free region 22 may, however, also be configured to be shorter. It is furthermore preferred for the width B of the free region 22 to be at least 1 millimeter, preferably at least 2 millimeters, in particular between 2 millimeters and 5 millimeters.

(13) The discharge arcs are therefore advantageously provided with the possibility of starting in the free region 22, and therefore directly on the thermally stable surface of the base body 14 of the electrode 10, which is essentially formed from tungsten. This strip 22 extending toward the electrode plateau advantageously makes it possible that the discharge arc is not hindered by any coating 20 during its contraction between the electrode tips, and is therefore not held back on the way to the electrode plateau 16. In order to offer the arc alternative initial positions, a plurality of such strips, in particular such free regions 22, may also be provided.

(14) The discharge arc may advantageously therefore sweep along on the way from the initial arc position to the electrode plateau 16 without destroying the heat-radiating coating 20. By virtue of the free region 22 being small in terms of area, the efficiency of the thermal emission, which is promoted by the coating 20, is not impaired, or is impaired only slightly.

(15) So that the arc also does not destroy the edge 20a of the cathode coating, that is to say the coating 20 for increasing the thermal emission, close to the plateau 16 in the second start-up phase, in the first minutes after the ignition, the coating edge 20a is also provided with a coating of tungsten powder sintered on, i.e. a tungsten coating 24, particularly in the region close to the cathode plateau, or generally of the electrode plateau 16. This coating ensures that the arc preferably starts there and the thermally less stable coating 20 for improved thermal radiation remains undamaged.

(16) This tungsten coating 24 preferably has a minimum width BW of at least 0.5 millimeters, which extends from the edge 20a of the coating 20 in the direction of the electrode plateau 16. Furthermore, in the example represented in FIG. 1, the free region 22 extends in a straight line as described, and the tungsten coating 24 is arranged extending annularly around the axis A in a second region 26 of the base body 14, the free region 22 also extending through the tungsten coating 24 and therefore interrupting its annular structure. Both the free region 22 and the tungsten coating 24 may respectively also assume different shapes, without thereby losing their function. This will now be explained in more detail with the aid of FIG. 2 and FIG. 3.

(17) FIG. 2 shows a schematic representation of an electrode 10 according to a further exemplary embodiment of the invention. In particular, the electrode 10 may be configured as described with reference to FIG. 1 except for the differences described below. In this example, the free region 22 is configured at least partly in the form of a spiral in the direction of the longitudinal extent direction L of the electrode 10, particularly in the region of the conical section 14b of the base body 14. In the case of such a spiral configuration of the free region 22, however, it is preferred for the free region 22 to fully encircle the electrode axis A at most once, in order to offer the discharge arc a path that is as simple as possible to the electrode plateau 16. The helical structure of the free region 22 in the vicinity of the electrode plateau 16 in turn leads directly into the tungsten coating 24 for protecting the edge 20a of the coating 20, the tungsten coating 24 here as well in turn being arranged in a second continuous region 26 which extends in an open ring shape around the electrode axis A.

(18) The function of this tungsten coating 24 is, however, also provided if this tungsten coating 24 is partially applied, as is represented in FIG. 3. FIG. 3 in turn shows an electrode 10 according to a further exemplary embodiment of the invention, which in particular may in turn be configured as described with reference to FIG. 1 except for the differences described below. These differences are in this case restricted in particular only to the configuration, or arrangement, of the tungsten coating 24. The latter is now in this example arranged in a plurality of mutually separated second regions 26, which are arranged extending around the axis A of the electrode 10 and adjacent to the edge 20a of the coating 20 for increasing the thermal emission.

(19) FIG. 4 shows a schematic representation of a discharge lamp 12, which in this example is configured as a high-pressure discharge lamp in short-arc technology. The discharge lamp 12 furthermore comprises two electrodes 10 according to exemplary embodiments of the invention, one of which is configured as the anode 10b and one as the cathode 10a. In this case, the cathode 10a and/or the anode 10b may be configured as described with reference to FIG. 1 to FIG. 3. The anode 10b additionally differs from the cathode 10a by its shaping, as may be seen from FIG. 4. In particular, the anode 10b has a much larger diameter D, preferably in the range of between 2 and 4 centimeters, while the cathode 10a has a much smaller diameter D, preferably in the range of less than or up to at most 3.0 centimeters, particularly preferably up to at most 2.5 centimeters.

(20) The discharge lamp 12 furthermore comprises the components which are conventional for such a lamp, such as a discharge tube 28, a discharge space 30, which is filled with a gas mixture and in which the electrodes 10 are located, and further components. The respective electrode plateaus 16 of the anode 10b and of the cathode 10a in this case face toward one another and the electrode axes A lie approximately along a line, that is to say the cathode 10a and the anode 10b are arranged coaxially with one another. Both electrodes 10 may comprise a free region 22 as described above. These respective free regions 22 are preferably arranged on the same side in relation to the axis A, i.e. for example they both extend at the top on the anode 10b and at the top on the cathode 10a, or underneath in relation to the two electrodes 10, or on any desired side, but always on the same one in relation to the respective axis A and/or longitudinal extent direction L of the anode 10b and of the cathode 10a. Even though the discharge arc often starts during ignition on the electrode plateau 16 at one of the two electrodes 10 and on the cone or lateral surface of the electrode 10 only at the other of the two electrodes, often the cathode, in infrequent cases it may nevertheless occur that the arc starts away from the plateau 16 at both electrodes 10. In this case, the unilateral arrangement of the free region 22 facilitates the contraction of the arc, and the probability that it will touch the coating 20 or cross it is additionally reduced.

(21) Overall, an electrode, a discharge lamp and a production method are thus provided, which advantageously make it possible to increase the lifetime of the discharge lamp further by particularly economical measures, namely by a free region extending toward the electrode plateau.

LIST OF REFERENCES

(22) 10 electrode

(23) 10a cathode

(24) 10b anode

(25) 12 discharge lamp

(26) 14 base body

(27) 14a cylindrical section

(28) 14b conical section

(29) 16 electrode plateau

(30) 18 first region

(31) 20 coating for increasing the thermal emission

(32) 20a edge of the coating for increasing the thermal emission free region

(33) 24 tungsten coating

(34) 26 second region

(35) 28 discharge tube

(36) 30 discharge space

(37) A electrode axis

(38) B width of the free region

(39) D diameter of the electrode

(40) BW width of the tungsten coating

(41) L longitudinal extent direction of the electrode

(42) LF length of the free region