Abstract
The invention describes an electrode (1) for use in a lamp (3) comprising a quartz glass envelope (30) enclosing a chamber (31), which electrode (1) comprises a tip for extending into the chamber (31) and base for embedding in a sealed portion (33) of the quartz glass envelope (30), characterized in that the base comprises a plurality of essentially smooth concave channels (2) arranged around the body of the electrode (2) and wherein the depth (d.sub.ch) of a channel (2) is preferably at most 8 percent, more preferably at most 5 percent, most preferably at most 3 percent of a diameter (D.sub.e) of the electrode (2). The invention further describes a method of manufacturing an electrode (1) for use in a lamp (3) comprising a chamber (11) in a quartz glass envelope (30), which method comprises the step of removing material from the body of the electrode (1) to form a plurality of channels (2) around the body of the electrode such that a channel (2) comprises channel side walls (62) and an essentially concave channel floor (60), and such that depth (d.sub.ch) of a channel (2) is preferably at most 8 percent, more preferably at most 5 percent, most preferably at most 3 percent of a diameter (D.sub.e) of the electrode (2). The invention also describes a lamp (3) comprising such electrodes (1), and a method of manufacturing such a lamp (3).
Claims
1. A lamp comprising a quartz glass envelope enclosing a chamber, the lamp including an electrode, wherein the electrode comprises: a tip for extending into the chamber; and a base for embedding in a sealed portion of the quartz glass envelope, characterized in that the base comprises a plurality of essentially smooth U-shaped concave channels, each channel being arranged circumferentially around a body of the electrode, wherein each channel comprises side walls and a concave channel floor, and wherein the depth (d.sub.ch) of a channel is not greater than 8 percent of a diameter (d.sub.e) of the electrode, wherein the glass envelope contacts at least some of the channels in the completed lamp, and wherein the glass envelope includes relief cracks neighboring the base in the completed lamp during manufacturing of the lamp.
2. The lamp according to claim 1, wherein the ratio of channel width to channel depth is not greater than 8:1.
3. The lamp according to claim 1, wherein the ratio of channel width to a diameter of the channel floor is not greater than 10:1.
4. The lamp according to claim 1, wherein, at a transition between the surface of the electrode and a channel, material of the electrode is deposited to form a plurality of brush-like protrusions.
5. The lamp according to claim 1, wherein, at a transition between the surface of the electrode and a channel, material of the electrode is deposited to form a low ridge with a height of not greater than 20 m.
6. The lamp according to claim 1, wherein the plurality of channels comprises a helical channel around the body of the electrode.
7. The lamp according to claim 1, wherein the base comprises a region treated to comprise a plurality of channels, which channel region is flanked on at least one side by an untreated region in which the surface of the electrode is essentially smooth.
8. The lamp according to claim 1, wherein the depth of a channel is not greater than 3 percent of a diameter of the electrode.
9. The lamp according to claim 1, wherein the ratio of channel width to channel depth is not greater than 2:1.
10. The lamp according to claim 1, wherein the ratio of channel width to a diameter of the channel floor is not greater than 1:1.
11. A lamp comprising a quartz glass envelope enclosing a chamber and a pair of electrodes disposed to extend into the chamber from opposite sides, wherein each electrode is partially embedded in a sealed portion of the quartz glass envelope, wherein each electrode comprises: a tip for extending into the chamber; and a base for embedding in a sealed portion of the quartz glass envelope, characterized in that the base comprises a plurality of essentially smooth U-shaped concave channels, each channel being arranged circumferentially around a body of the electrode, wherein each channel comprises side walls and a concave channel floor, wherein the glass envelope contacts at least some of the channels in the completed lamp, and wherein the glass envelope includes relief cracks neighboring the base in the completed lamp during manufacturing of the lamp.
12. A lamp according to claim 11, the lamp being a gas-discharge lamp, and wherein the electrodes comprise tungsten rods with a diameter in the range of 200 m to 500 m and are disposed in the lamp such that tips of the electrodes extend into the discharge chamber from opposite sides, and the other end of each electrode is embedded in the sealed portion of the lamp such that channels arranged around the body of the electrode are enclosed in the sealed portion.
13. The lamp according to claim 11 wherein, at a transition between the surface of the electrode and a channel, material of the electrode is deposited to form a substantially smooth low ridge with a height of not greater than 20 m, wherein a space between adjacent low ridges is substantially smooth.
14. The lamp according to claim 13, wherein, at the transition between the surface of the electrode and a channel, material of the electrode is deposited to form the low ridge with a height of not greater than 6 m.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) Other objects and features of the present invention will become apparent from the following detailed descriptions considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention.
(2) FIG. 1 shows a prior art electrode in a sealed portion of a quartz glass lamp and a number of different cracks in the sealing portion of the type that arise during cooling;
(3) FIG. 2 shows an enlarged schematic view of a groove in a prior art electrode;
(4) FIG. 3 is a schematic representation of a notched tension specimen;
(5) FIG. 4 shows an enlarged view of a groove in the electrode of FIG. 2 after failure during a cooling step in manufacture;
(6) FIG. 5 shows an enlarged schematic view of a first embodiment of an electrode according to the invention;
(7) FIG. 6a shows a cross-section of the electrode of FIG. 5;
(8) FIG. 6b shows a cross-section of a second embodiment of an electrode according to the invention;
(9) FIG. 7 shows a scanning electron microscope image of a prior-art grooved electrode at a magnification of 330 times;
(10) FIG. 8 shows a scanning electron microscope image of a groove in a prior-art electrode, at a magnification of 1500 times;
(11) FIG. 9 shows a scanning electron microscope image of an electrode with channels according to the invention, at a magnification of 330 times;
(12) FIG. 10 shows a scanning electron microscope image of a channel of an electrode according to the invention, at a magnification of 1500 times;
(13) FIG. 11 shows a gas-discharge lamp according to the invention.
(14) In the diagrams, like numbers refer to like objects throughout. Elements of the diagrams are not necessarily drawn to scale.
DETAILED DESCRIPTION OF THE EMBODIMENTS
(15) FIG. 1 shows quartz glass gas-discharge lamp 40 with a discharge chamber 41. Two electrodes 42 protrude into the discharge chamber 41 and embedded in sealed portions 43 of the lamp 40. The end of the electrode 42 embedded in the sealed portion 43 is attached to a molybdenum foil 47, which in turn is connected to a lead-in wire 48. During operation, a voltage is applied across the lead-in wires 48 so that a discharge arc can be established between the tips of the electrodes 42 and so that a current can flow through the electrodes 42. The electrodes 42 become very hot during operation, causing the quartz glass in the sealed portions 43 to heat up as well. During a cooling step, but also during the lifetime of the lamp 40, cracks 44, 45, 46 can develop in the sealed portions 43 as a result of the different thermal expansion behavior of the electrode metal and the quartz glass. Initially, small cracks 44 may develop. As the lamp age progresses with use, some of the smaller bead-like cracks 45 may develop into radially extending cracks 46 Particularly the larger types of crack 45, 46 can lead to failure of the lamp 40.
(16) FIG. 2 shows an enlarged schematic view of a prior art electrode 50. This electrode 50 has been treated so that it has a number of relatively deep grooves 51 about its circumference in a region 54 of the sealed portion between a molybdenum foil and the discharge chamber. In known prior art electrodes 50 of this type, the depth of such a groove 51 is about 10% of the electrode diameter. The purpose of the grooves 51 is to lessen the mechanical stress exerted by the expanding electrode 50 when the temperature rises during operation. In lateral regions 55, 56 on either side of the grooved region 54, the body of the electrode 50 is left smooth and adheres to the quartz glass. However, as explained in the introduction, such grooves 51 lead to an unfavorable side-effect during manufacture. In effect, the deep grooves 51 cause the electrode 50 to behave as a notched tensile specimen during cooling, so that the electrode 50 is not able to withstand the resulting triaxial stresses in the notch 51. The forces acting on the electrode 50 during a cooling stage in manufacture are shown schematically in FIG. 3. Here, a tension specimen 30 with a notch 31 is being subject to axial loads F.sub.A exerted along an axis A.sub.e. The notch 31 effectively weakens the specimen 30. If the axial forces F.sub.A are strong enough, the notch 31 will develop into a crack 32 in the body of the specimen 30, originating at the base 33 of the notch 31, and the specimen 30 will break. This behavior will be known to the skilled person and is described by Griffith's criterion, which, for brittle materials, relates the notch depth in a notched tension specimen to the critical tensile force (i.e. the force at which the specimen fails). Most metals, when cooled, behave as brittle materials, so that this criterion can also be applied to explain the tendency of such electrodes 50 to fail during cooling. FIG. 4 shows a close-up of the electrode 50 of FIG. 2 after failure during such a cooling step in manufacture. Axial loads F.sub.A were exerted on the electrode 50 along its axis A.sub.e on account of the forces of adhesion F.sub.Q between the quartz glass and the surface of the electrodes 50 in regions 55, 56 on either side of the grooved region 54, which effectively held the electrode regions 55, 56 in a vice-like grip while the electrode 50 was contracting. Because of the inability of the groove 51 to withstand the axial forces F.sub.A, the groove 51 developed into a crack 52 travelling through the body of the electrode 50, so that the electrode 50 (and therefore its lamp) is rendered useless.
(17) FIG. 5 shows an enlarged schematic view of an electrode 1 according to the invention, with a diameter D.sub.e of 400 m. The diagram clearly shows the U-shaped channel 2 running along the surface of the electrode in a helical fashion. FIG. 6a shows a cross-section of the electrode 1 of FIG. 5, in which the geometry of the channel 2 is more clearly shown. Here, the channel 2 has a channel width w.sub.ch of 30 m and a channel depth d.sub.ch of 20 m. The walls of the channel taper towards the channel floor 60, and the channel floor 60 is curved with a radius r.sub.ch of 7.5 m. Here, the ratio of channel width w.sub.ch to channel depth d.sub.ch is 30:20 or 1.5:1, and the ratio of channel width w.sub.ch to channel diameter 2r.sub.ch is 30:15 or 2:1. The ratio of channel depth d.sub.ch to electrode diameter D.sub.e is 20:400, i.e. the channel depth is only 5% of the electrode diameter. The dimensions given here are only exemplary, so that other dimensions are possible, of course. For example, for an electrode with a diameter of 400 m, a channel depth of 6 m would yield a ratio of channel depth d.sub.ch to electrode diameter D.sub.e of 6:400, or about 1.5%. For the increased resilience of the electrode to breakages during cooling, it is only important that the dimensions satisfy the relationships already described in the above. FIG. 6b shows an alternative embodiment of the electrode 1 according to the invention, in which the material of the electrode 1 has been converted or transformed during a laser treatment step to give a series of bristle-like or brush-like protrusions 63 along the transition between channel 2 and electrode outer surface.
(18) FIG. 7 shows a scanning electron microscope image of a prior-art grooved electrode 70 at a magnification of 330 times. The grooves 71 have been gouged out of the surface of the electrode 70 by a laser beam. The material of the electrode 70 displaced to form the grooves 71 is deposited to give clearly raised walls 72 on either side of the grooves 71, as shown in FIG. 8, a magnification of one groove 71 at 1500 times. Furthermore, the grooves 71 have clearly visible pits 73 or deep holes 73. Any of these pits 73 can result in the electrode 71 acting as a notched tensile specimen when the electrode 70 is cooled during manufacture, and may contribute to the failure of the electrode 70 in the manner described above, so that the electrode 70 does not survive the manufacturing process.
(19) In contrast, FIG. 9 shows a scanning electron microscope image of an electrode 1 with channels 2 according to the invention, also at a magnification of 330 times. This image clearly shows that the channels 2 are smoothly formed, and that there are no pits or marked unevenness on the lower surface or floor of the channels 2, unlike in the prior art grooved electrode of FIGS. 7 and 8. This is shown in more detail in FIG. 10, which is an image, magnified 1500 times, of a single channel 2 in an electrode 1 according to the invention. This image also shows the favorable low ridges 61 on either side of the channel 2. The smoothness or regularity of the channel floor 62 can be clearly seen. Since there is effectively no pit, hole or other similar irregularity on the channel floor that would act as a notch, the channels 2 ensure that this inventive electrode 1 is much less likely to suffer breakage during cooling in manufacture, so that the production yield can be increased.
(20) FIG. 11 shows a gas-discharge lamp 3 according to the invention, made of a quartz glass envelope 30, with a pair of electrodes 1 extending at their tips into a discharge chamber 31. The base of each electrode 1 is held in a sealed portion 33. The channels 2, made using any of the techniques described above, are shallow, essentially concave channels 2, indicated here only by the parallel slanted lines. During the cooling step in manufacture, the quartz glass and electrode metal cooled at different rates, and therefore contracted at different rates. The favorable geometry of the channels 2 allowed the controlled formation of micro-cracks 34 or relief cracks 34, shown here enlarged for the sake of clarity. These relief cracks 34 prohibit or restrict the development of spontaneous large bead cracks resulting in RECs.
(21) While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art from a study of the drawings, the disclosure, and the appended claims. For the sake of clarity, it is to be understood that the use of a or an throughout this application does not exclude a plurality, and comprising does not exclude other steps or elements. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.
