Patent Application
20100045185
This application claims priority from Japanese Patent Application Serial No. 2008-211264 filed Aug. 20, 2008, the contents of which are incorporated herein by reference in its entirety.
The present invention generally relates to a cathode for a discharge lamp, and a discharge lamp having the cathode containing an emitter material, wherein a carbide layer is formed on an outer surface thereof.
A xenon lamp or discharge lamp, such as a short arc type high-pressure mercury lamp, is used as a light source of, for example, a projector, a semiconductor exposure apparatus, a liquid crystal exposure apparatus or a printed circuit board exposure apparatus, which uses the DLP® (Digital Light Processing) technology. Such a discharge lamp is turned on by direct-current power source, and electrons are emitted toward an anode from a cathode during lighting. The electrode of such a discharge lamp is made of high melting point metal, such as tungsten, and specifically the cathode contains an emitter material which consists of a metal oxide, such as thorium oxide, in order to stably emit electrons from the cathode. In the cathode heated by lighting of a discharge lamp, metal atoms are taken out from the metal oxide which forms the emitter material, by reduction action caused by the tungsten which forms the cathode, so that the metal atoms act as an emitter. Moreover, in order to take out the metal atoms at high efficiency from the metal oxide which forms the emitter material and to stably supply them to the tip of the cathode from which electrons are emitted, it is known to form a carbide layer which consists of, for example, tungsten carbide, on an outer surface of an area extending from the tip portion of the cathode, by carrying out carbonization processing of the outer surface thereof (refer to Patent Publication No. H05-86026 and Patent Application Publication No. 2000-21349).
In addition, there is a demand for much more brightness in a discharge lamp in recent years. As a possible means for obtaining high brightness in such a discharge lamp, shortening of a distance between the electrodes may be considered. However, in such a discharge lamp, when high electrical current flows between the electrodes during lighting, the cathode is heated to very high temperature so that the emitter material contained in the cathode evaporates thereby decreasing or being depleted at an early stage. As a result, electrons are not stably emitted from the cathode, so that there is a problem that flickering of the radiation light, that is, a flicker occurs at an early stage.
In view of the above background, described herein is a cathode for a discharge lamp and a discharge lamp having such a cathode which is capable of emitting light stably for a long time without flickering of radiation light at an early stage
The present cathode for a discharge lamp contains emitter material, wherein a carbide layer is formed on an outer surface of the cathode, the carbide layer is formed on an area extending from a tip portion of the cathode, and two or more grooves extending toward the tip portion of the cathode are formed the area where the carbide layer is formed.
In the present cathode for a discharge lamp, the two or more grooves may be formed so as to be apart from each other. Moreover, it is desirable that the thickness of the carbide layer may be smaller than the depth of the groove.
Another aspect of the present invention is that the present discharge lamp has the above-mentioned cathode for discharge lamp.
In the present cathode for a discharge lamp, since evaporation of the emitter material is suppressed by forming the two or more grooves extending toward the tip of the cathode for a discharge lamp on the area in which the carbide layer is formed, the emitter material does not decrease or is not depleted at an early stage, so that flickering of a radiation light does not occur at such an early stage, whereby it is possible to realize a discharge lamp capable of stably emitting light for a long time. Moreover, since loss of the carbide layer is prevented or suppressed by the two or more grooves which are arranged apart from one another, the shape of these grooves is maintained so that the evaporation of the emitter material is suppressed certainly, whereby it is possible to stably emit electrons over a long time. Moreover, since loss of the carbide layer formed in portions other than a groove (top part) is suppressed by forming the carbide layer of thickness smaller than the depth of these grooves, the shape of grooves is maintained and the evaporation of the emitter material is controlled certainly, whereby it is possible to stably emit electrons over a long time.
Other features and advantages of the present cathode for a discharge lamp and the present discharge lamp using the cathode will be apparent from the ensuing description, taken in conjunction with the accompanying drawings, in which:
A description will now be given, referring to embodiments of the present cathode for a discharge lamp and the present discharge lamp using the cathode. While the claims are not limited to such embodiments, an appreciation of various aspects of the present cathode for a discharge lamp and the present discharge lamp using the cathode is best gained through a discussion of various examples thereof.
The emitter material is contained in the basis material of the cathode 10 in form of an oxide of one kind or two or more kinds of metals, such as thorium oxide, barium oxide, lanthanum oxide or a compound oxide of lanthanum oxide and zirconium oxide. The number of the grooves D formed in the cathode 10 is 20-70. Moreover, the depth of each groove D is 60-500 μm (micrometers). Moreover, the length of the tip portion on which the carbide layer of the cathode 10 is not formed along the axis direction thereof, is 1-4 mm.
The carbide layer can be formed by carrying out a carbonization process on the outer surface of the basis material which forms the cathode 10. Metal atoms are more easily taken out from the metal oxide which forms the emitter material, as the thickness of this carbide layer is large, so that they are easily supplied to the tip portion of the cathode 10. However, it is desirable that the thickness of the carbide layer is smaller than that of the grooves D. That is, since the strength of the carbide such as tungsten carbide is lower than that of the high melting point metal, for example, tungsten, which is the basis material of the cathode 10, when the thickness of the carbide layer is larger than the depth of the grooves D, the carbide layer formed in portions other than the grooves D (the top part) tends to be lost. Moreover, the desirable thickness of the carbide layer is 30-100 μm (micrometers). Here, the thickness of the carbide layer can be measured by using an electron scanning microscope etc., after grinding a cutting plane of the cathode 10 and carrying out an etching process with etching liquid.
The following is the specification of an example, such as the size of each part of this cathode 10. The full length of the cathode 10 is 18 mm, the length of the first taper portion 11 in the axial direction in is 4 mm, the length of the second taper portion 12 in the axial direction is 9 mm, and the length of the body portion in the axial direction 13 is 5 mm. The diameter of the tip P is 0.6 mm, and that of the body portion 13 is 10 mm. The inclination angle of the outer surface of the first taper portion 11 with respect to the central axis of the cathode 10 is 60 degrees, and that of the outer surface of the second taper portion 12 with respect to the central axis of the cathode 10 is 30 degrees. Twenty four (24) grooves D whose width is 0.5 mm and whose depth is 0.4 mm are formed apart from one another at equal intervals in the circumferential direction (whose angles with respect to the central axis of the cathode 10 are 15 degrees). Moreover, the carbide layer is formed over areas between the position of 3 mm from the tip P and the base end of the second taper portion 12, and the thickness thereof is 30 μm (micrometers).
In such a cathode 10, evaporation of the emitter material is suppressed by forming two or more grooves D which extend from a top portion of the cylindrical body portion 13 toward the tip of the cathode 10 on the areas on which a carbide layer is formed. It is believed that since the grooves D are surrounded by the outer surface of the cathode 10 so that this surface area of the cathode 10 is large, the emitter material which is once evaporated is again trapped to the cathode 10, thereby being supplied to the tip portion of the cathode 10 along the grooves D. As a result, the emitter material in the cathode 10 does not decrease or is not depleted at an early stage. Therefore, it is possible to realize a discharge lamp in which light is emitted stably for a long time without flickering of a radiation light at an early stage. Moreover, since the two or more grooves D are formed so as to be apart from each other, formation of a pointed portion(s) on the outer surface of the cathode 10 is avoided. Therefore, since loss of the carbide layer is suppressed or prevented, the shape of the grooves D is maintained and evaporation of the emitter material is suppressed certainly, whereby it is possible to stably emit electrons for a long time. Moreover, since loss of the carbide layer formed in portions other than the grooves D (top part) is suppressed by forming the carbide layer of the thickness smaller than the depth of the grooves D, the shape of the grooves is maintained and evaporation of the emitter material is suppressed certainly, whereby it is possible to stably emit electrons over a long time.
The following is the specification of an example, such as the size of each part of this cathode 10. The full length of the cathode 10 is 18 mm, the length of the taper portion 14 in the axial direction is 13 mm, and the length of the body portion 13 in the axial direction is 5 mm. The diameter of the tip P is 0.6 mm, and the diameter of the body portion 13 is 10 mm. The inclination angle of the outer surface of the taper portion 14 with respect to the central axis of the cathode 10 is 40 degrees. Twenty (20) grooves D whose width is 0.5 mm and whose depth is 0.4 mm are formed apart from each other (one another) at equal intervals in the circumferential direction (whose angles with respect to the central axis of the cathode 10 are 18 degrees). Moreover, the carbide layer is formed over areas between the position of 3 mm from the tip P and the base end of the taper portion 14, and the thickness thereof is 30 μm (micrometers).
It is believed that in such a cathode 10, the emitter material in the cathode 10 does not decrease or is not depleted at an early stage, since evaporation of an emitter material is suppressed by forming two or more grooves D which extend toward the tip of the cathode 10 on the areas in which a carbide layer is formed. Therefore, it is possible to realize a discharge lamp in which light is emitted stably for a long time without flickering of a radiation light at an early stage. Therefore, since the two or more grooves D are formed so as to be apart from each other, so that loss of the carbide layer is suppressed or prevented, the shape of the grooves D is maintained and evaporation of the emitter material is suppressed certainly, whereby it is possible to stably emit electrons for a long time. Moreover, since loss of the carbide layer formed in portions other than the grooves D (top part) is suppressed by forming the carbide layer of the thickness smaller than the depth of the grooves D, the shape of the grooves is maintained and evaporation of the emitter material is suppressed certainly, whereby it is possible to stably emit electrons over a long time.
In such a discharge lamp 20, since the present cathode 10 is provided, flickering of a radiation light does not occur at an early stage, so that light can be stably emitted for a long time. Such a discharge lamp 20 can be suitably used as a light source of, for example, a projector, a semiconductor exposure apparatus, a liquid crystal exposure apparatus or a printed circuit board exposure apparatus, which use the DLP technology (DLP is a TRADEMARK).
As mentioned above, although the embodiments according to the present invention are explained above, it is possible to make various changes. For example, the grooves D may not be limited to V-shaped grooves, and may be U-shaped grooves, or grooves in other shape. Moreover, the grooves D are not necessarily formed in the shape of a straight line, and, for example, as shown in
Although a concrete example will be described below. However, the present invention is not limited to this concrete example.
As in the structure shown in
After turning on the above-mentioned discharge lamp 20 with 4 kW electric power for 2 hours, the light of the discharge lamp was turned off for 30 minutes. When a cycle of lighting on and off was repeated, a flicker occurred after total light time of 630 hours. In addition, it was regarded that a flicker occurred when the voltage impressed to the discharge lamp 20 changed by 1.2 V or more.
Except the cathode having no grooves was used, a discharge lamp having the same specification as the above-mentioned concrete embodiment was produced. In this discharge lamp, after total light time of 480 hours, a flicker occurred like the above-mentioned concrete embodiment.
Thus, in the present cathode 10 according to the embodiments, it was confirmed that the discharge lamp 20 in which the flickering of a radiation light did not occur at an early stage, and light was emitted stably for a long time.
The preceding description has been presented only to illustrate and describe exemplary embodiments of the present cathode for a discharge lamp and the present discharge lamp using the cathode. It is not intended to be exhaustive or to limit the invention to any precise form disclosed. It will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the claims. The invention may be practiced otherwise than is specifically explained and illustrated without departing from its spirit or scope.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-211264 | Aug 2008 | JP | national |
