Patent Publication Number: US-7586263-B2

Title: Discharge lamp having a cathode and emitter assembly composed of tungsten and thorium oxide and joined through a carbide interface region

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a high pressure discharge lamp filled with xenon gas as the emitter, which is used as a light source, for example, in a projection apparatus or the like using DLP® (digital light processing) technology. 
   2. Description of Related Art 
   Conventionally, a high pressure discharge lamp is one with the arrangement shown, for example, in  FIG. 8 . This high pressure discharge lamp  10  consists of a bulb of silica glass which has an arc tube  11 , and sealing parts  12 , with a cathode  19  and an anode  14  arranged opposite each other in the arc tube  11 . 
   Furthermore, tungsten electrode rods  191 ,  141 , which support the cathode  19  and anode  14 , respectively, are inserted into a silica glass cylindrical retaining body  16 , and thus, retained. Each retaining body  16  is permanently located in a respective one of the sealing parts  12  which has an axial through opening through which the electrode rod extends in the axial direction. Moreover, the electrode rods are sealed in the sealing part  12  by graded glass  15 . The electrode rods  191 ,  141  extend outward from the outer end of the bulb and act as outer lead pins which supply power to the cathode  19  and the anode  14 . The arc tube  11  is filled with xenon gas. 
   In a high pressure discharge lamp  10  with the above described arrangement, as is shown in  FIG. 9 , the cathode  19  is formed of a cylindrical body  192  and a truncated cone-shaped conical part  194  which is located on an end of the body  192  in one piece with it. Furthermore, the diameter is gradually reduced along the cathode axis L in the tip-shaped area  196  (to the left in  FIG. 9 ) and a round, flat tip surface  193  is formed on its tip. 
   In such a discharge lamp, in order to obtain stable radiant light over a long time, an arc discharge which forms between the electrodes must be stabilized over a long time. For the cathode  19 , thoriated tungsten is used in which an emissive material of thorium dioxide (ThO 2 ) is provided. A carbide layer A of tungsten carbide (W 2 C) is formed on the surface of the base side region  195 , which is a region outside of the tip-shaped area  196 . This technology is described in JP-A-HEI 10-283921.  FIG. 9  is a top view of the cathode in which the carbide layer A is advantageously shown using a broken line. 
   In this carbide layer A, during the arc discharge, the oxygen of the thorium dioxide (ThO 2 ) is trapped by the tungsten carbide (W 2 C) and thorium (Th) is supplied with high efficiency to the tip surface  193  of the cathode  19 . This thorium feed phenomenon occurs optimally at a temperature of 1400° C. to 1800° C. of the carbide layer A of tungsten carbide (W 2 C) and is described by the following chemical formulas (formulas 1 and 2).
 
ThO 2 +W 2 C→Th+2W+CO 2   (formula 1)
 
ThO 2 +2W 2 C→Th+4W+2CO  (formula 2)
 
   The thorium feed phenomenon is further described below. The carbide layer formed on the cathode surface does not penetrate only through the cathode surface, but reaches into the interior of the cathode to a depth of roughly 100 μm from the cathode surface. The reactions described above using formulas 1 and 2, therefore, occur not only on the cathode surface, but also within the cathode. The thorium (Th) which forms within the cathode passes through between the grain boundary of the tungsten and is deposited on the cathode surface, while it passes partially through the grain boundaries of the tungsten, moves simultaneously within the cathode and is deposited on the tip surface  193  of the cathode  19 . 
   As a result, after lamp operation over a long time, thorium (Th) can be reliably supplied with high efficiency to the tip surface  193  of the cathode  19  and stable radiant light is obtained over a long time. 
   If a carbide layer is applied as far as the tip-side region  196  of the cathode  19 , the tip-side region  196  reaches roughly 2900° C., melting the tungsten carbide (W 2 C). This results in the disadvantages of premature wear of the cathode tip, and thus, shortening of the service life and the danger of blackening of the arc tube resulting in premature reduction of the intensity of the radiant light. Therefore, the tip-shaped region  196  of the cathode  19  is no longer provided with a carbide layer. 
   The thorium feed phenomenon also optimally occurs at a temperature of the carbide layer of 1400° C. to 1800° C. of the carbide layer. A cathode has been suggested in which, as shown in  FIG. 10 , on the cathode surface, a surface  197  perpendicular to the axis L is formed and is irradiated with light from the arc so that the carbide layer applied to the cathode reliably reaches 1400° C. to 1800° C., and in which, thus, the carbide layer A which has formed on the vertical surface  197  is reliably fixed at the range from 1400° C. to 1800° C. so that the thorium feed phenomenon can be carried out even more flexibly. This technology is described in Japanese Application Publication JP-A 2005-142071 (U.S. Patent Application Publication 2005/0099121 A1).  FIG. 10  is a top view of the cathode in which the carbide layer A is advantageously shown using broken lines. 
   Recently, however, in the field of projector apparatus using DLP® technology, a discharge lamp with an increased amount of xenon gas added, an increased operating pressure of the lamp and a simultaneously reduced distance between the electrodes has been developed, since there is more and more a demand for a point light source lamp with high radiance. 
   However, in this discharge lamp, there is a tendency for the cathode temperature to increase due to the effect of the radiant heat of the anode, since the anode temperature is higher than in the past. As a result, the thorium (Th) contained in the cathode is prematurely reduced and dried out in a short time. Furthermore, since the tip area of the cathode is shifted into a high-temperature state, the crystal grain size of the tip area of the cathode increases. The movement of the thorium (Th) in the cathode is prevented by the crystal grains. The thorium (Th) is no longer supplied to the cathode tip. 
   This means that thorium (Th) can no longer be supplied to the cathode tip after lamp operation, resulting in the disadvantages that the flicker phenomenon occurs and images on the screen flicker. 
   SUMMARY OF THE INVENTION 
   The invention was devised to eliminate the above described disadvantages in the prior art. Thus, a primary object of the invention is to devise a high pressure discharge lamp with a long service life in which, after lamp operation over a long time, thorium (Th) can be stably supplied to the cathode tip, and in which formation of the flicker phenomenon is suppressed over a long time. 
   According to a first aspect of the invention, in a high pressure discharge lamp in which there are an anode and a cathode opposite each other in the bulb, this object is achieved in that the above described cathode is made of tungsten which contains thorium oxide, that on its surface, apart from the tip end, a carbide layer of tungsten carbide is formed, and that the above described cathode is bordered by an emitter containing body of tungsten which contains thorium dioxide, for which, at least in one region bordering the cathode, a carbide layer of tungsten carbide is formed. 
   In a development of the invention, in a high pressure discharge lamp according to a first aspect, the object is achieved especially in that the above described emitter containing body is a solid body, that it is provided with a through opening, and which forms a fit with the cathode, and that it is retained by the cathode. 
   The object is also achieved by the invention in a high pressure discharge lamp especially in that the above described emitter containing body is a linear body and that it is retained by the cathode being wound around it. 
   ACTION OF THE INVENTION 
   The discharge lamp in accordance with the invention supplies thorium (Th) as the emissive material from the emitter containing body which is retained by the cathode to the cathode surface so that over a long time thorium (Th) can be stably supplied to the cathode tip. Thus, a discharge lamp with a long service life is obtained in which formation of the flicker phenomenon over a long time is suppressed. 
   The invention is further described below with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic cross-sectional view of a high pressure discharge lamp in accordance with the invention; 
       FIG. 2  is a schematic depiction of the cathode of a high pressure discharge lamp in accordance with the invention; 
       FIG. 3  is a schematic perspective view of the support body for emissive material which is held fast by the cathode in a high pressure discharge lamp in accordance with the invention; 
       FIG. 4  is a schematic cross-sectional view of the cathode of a high pressure discharge lamp in accordance with the invention which retains the emitter containing body; 
       FIG. 5  illustrates the motion of the thorium (Th) in the cathode in a high pressure discharge lamp in accordance with the invention; 
       FIG. 6  is a schematic cross-sectional view of the cathode of a high pressure discharge lamp in accordance with the invention which retains another emitter containing body; 
       FIG. 7  is a plot of the data of test results in which the situation of flicker formation was studied; 
       FIG. 8  is a schematic cross-sectional view of a conventional high pressure discharge lamp; 
       FIG. 9  is an enlarged view of the cathode of a conventional high pressure discharge lamp; and 
       FIG. 10  is an enlarged view of cathode of another conventional high pressure discharge lamp. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The high pressure discharge lamp  10  in accordance with the invention is described below using  FIG. 1 . For comparison purposes, parts of lamp  10  which correspond to like parts of the conventional lamp of  FIG. 8  are identified by the same reference characters. 
   The high pressure discharge lamp  10  comprises a silica glass bulb which has an arc tube  11  and sealing parts  12 . Furthermore, a cathode  13  and an anode  14  are arranged opposite each other in the arc tube  11 . 
   The cathode  13  and the anode  14  are supported by tungsten electrode rods  131 ,  141 , respectively, which are inserted into a silica glass cylindrical retaining body  16  which has a through opening which extends in a sealing part  12  in the axial direction. Furthermore, the electrode rods  131 ,  141 , are sealed in the sealing part  12  by graded glass  15 . The electrode rods  131 ,  141  extend outward from the outer end of the bulb and also act as outer lead pins which supply power to the cathode  13  and the anode  14 . The arc tube  11  is filled with xenon gas. 
   For the cathode  13 , thoriated tungsten is used in which an emissive material of thorium dioxide (ThO 2 ) is doped. It is a sintered electrode of 98% tungsten (W) and 2% thorium dioxide (ThO 2 ). The anode  14  is made of tungsten which does not contain thorium dioxide (ThO 2 ). 
     FIG. 2  is an enlarged top view of the cathode  13 . The cathode  13  has a cylindrical body part  132 , a reduced-diameter cylindrical body part  133 , which is formed in one piece on the electrode tip-side end of the body part  132 , and of a truncated cone-shaped conical part  135  with a diameter which gradually decreases along the electrode axis L toward the electrode tip (to the left in  FIG. 2 ), and a round flat tip surface  134  is formed on its tip which constitutes the tip end facing the anode. 
   On the surface of the reduced diameter body part  133  of the cathode  13 , a carbide layer A of tungsten carbide (W 2 C) is formed. This carbide layer A is obtained by a dispersion liquid of a carbon powder being applied to the surface of the reduced diameter body part  133  and being heat treated in a vacuum. As a result, this carbide layer A is present not only on the surface of the reduced diameter body part  133 , but is also cemented from the surface up to roughly 100 μm to the inside and is located there. 
     FIG. 3  is a perspective view in which only the emitter containing body  17  is shown. The emitter containing body  17  is a disk-like, solid body with a through opening  171  in its middle. Like the cathode  13  it is made of thoriated tungsten in which an emissive material of thorium dioxide (ThO 2 ) is doped. It is a sinter body of 98% tungsten (W) and 2% thorium dioxide (ThO 2 ). On the entire surface, including the inner surface of the through opening  171  of the emitter containing body  17 , a carbide layer A of tungsten carbide (W 2 C) is formed, and like the carbide layer of the cathode, is obtained by a dispersion liquid of carbon powder being applied to the surface of the emitter containing body and being heat treated in a vacuum. As a result, this carbide layer A is present not only on the inside surface of the through opening  171 , but is also produced so as to extend inward from the surface to roughly 100 μm below the surface. 
     FIG. 4  shows a cross section in which an emitter containing body is applied to the cathode. The reduced diameter body part  133  of the cathode  13  forms a fit with the through opening  171  of the emitter containing body  17 , by which the emitter containing body  17  is fixed on the cathode  13 . The outer peripheral surface of the reduced diameter body part  133  of the cathode  13  in which the carbide layer A is formed, and the inner peripheral surface of the through opening  171  of the body  17  for the emissive material in which the carbide layer A is formed, border one another. 
   The region in which the emitter containing body  17  borders the cathode  13  is therefore the inner peripheral surface of the through opening  171  of the emitter containing body  17 . 
   In  FIG. 4 , the gap between the inner peripheral surface of the through opening  171  of the emitter containing body  17  and the reduced diameter body part  133  is shown exaggerated. The inner peripheral surface of the through opening  171  of the emitter containing body  17 , however, in fact, borders the outer peripheral surface of the reduced diameter body part  133 . 
   The reduced diameter body part  133  is, if necessary, wound proceeding from the emitter containing body  17  toward the anode side with a wire formed of a metal with a high melting point, for example, a molybdenum wire W, and the wire is thus attached to prevent the emitter containing body  17  from falling off of the reduced diameter body part  133  of the cathode  13 . 
     FIG. 5  schematically illustrates the movement of the thorium (Th) in the cathode of the high pressure discharge lamp in accordance with the invention by the arrows. In the reduced diameter body part  133  of the cathode  13 , a carbide layer A is formed. Carbon is located on the surface of the reduced diameter body part  133  and proceeding therefrom, up to roughly 100 μm below the surface. If, during lamp operation, the temperature of the reduced diameter body part  133  reaches 1400° C. to 1800° C., as is shown using the above described formulas 1 and 2, by the trapping of the oxygen of the thorium dioxide (ThO 2 ) by tungsten carbide (W 2 C), thorium (Th) is produced. 
   The thorium (Th) produced on the surface of the cathode  13  and within the cathode  13  passes through, by grain boundary diffusion, between the grain boundaries of the tungsten and is deposited on the surface of the cathode  13 . Due to the high temperature of the tip region of the cathode, surface diffusion takes place. This means that thorium moves along the surface of the reduced diameter body part  133 , and also, along the surface of the cone part  135  and is supplied to the tip surface  134  of the cathode  13 . 
   Furthermore, part of the thorium (Th) which has been produced within the cathode  13  moves within the cathode  13  by it passing through between the grain boundaries of the tungsten and is supplied to the tip surface of the cathode  13 . 
   The surface of the emitter containing body  17  and the interior thereof proceeding from the surface up to 100 μm which is an adjacent region in which the cathode  13  borders the emitter containing body  17 , due to the heat of the cathode, has essentially the same temperature as the cathode, i.e. 1400° C. to 1800° C. 
   As a result, in the emitter containing body  17  a carbide layer A is formed. By trapping the oxygen of the thorium dioxide (ThO 2 ) by carbon which is present on the inner surface of the through opening  171  and in the interior proceeding from the inner surface up to a depth of roughly 100 μm in which the emitter containing body  17  borders the cathode  13 , thorium (Th) is also produced in the emitter containing body  17 . 
   The thorium (Th) which has been deposited on the inner surface of the through opening  171  of the emitter containing body  17  is supplied to the surface of the reduced diameter body part  133  of the cathode  13 , moves on the surface of the reduced diameter body part  133  and along the surface of the conical part  135  so that it is supplied to the tip surface of the cathode  13 . 
   As a result, thorium (Th) is supplied to the cathode  13  from the emitter containing body  17 . After lamp operation has commenced, thus over a long time, thorium (Th) can be stably supplied to the cathode tip. Thus, the formation of the flicker phenomenon can be suppressed as a result of drying out of thorium (Th) atoms over a long time. 
   Furthermore, after operation, the tip of the cathode  13  reaches a high temperature so that the grain size of the tip of the cathode  13  increases. Therefore, the thorium (Th) is also supplied from the emitter containing body  17  to the surface of the cathode  13  when it becomes impossible for the thorium (Th) produced within the cathode  13  to pass between the grain boundaries and move within the cathode. Thus, thorium (Th) can be reliably, and moreover, stably supplied to the tip surface  134  of the cathode  13 , and after lamp operation has commenced, thorium (Th) can be stably supplied to the cathode tip over a long time. Thus, the formation of the flicker phenomenon can be suppressed over a long time. 
     FIG. 6  shows a schematic of another example in which an emitter containing body  18  is attached to the cathode, which body is a linear body with a diameter from 6 mm to 12 mm, which is a wire of thoriated tungsten, in which the emissive material of thorium dioxide (ThO 2 ) is doped and which is made of 98% tungsten (W) and 2% thorium dioxide (ThO 2 ). 
   On the surface of the linear body  18  for the emitter containing body, a carbide layer A of tungsten carbide (W 2 C) is formed which is obtained by a dispersion liquid of a carbon powder being applied to the surface of the emitter containing body  18  and being heat treated in a vacuum. In  FIG. 6 , the dimensions of the carbide layer A of the emitter containing body  18  are shown exaggerated. The carbide layer A of the reduced diameter body part  133  is advantageously shown using broken lines. 
   This emitter containing body  18  in the state in which the carbide layer A is formed beforehand on the surface is wound tightly around the reduced diameter body part  133  of the cathode  13  in which the carbide layer A is formed and is thus retained by the cathode  13 . 
   On this cathode  13 , a carbide layer A is formed on the reduced diameter body part  133  of the cathode  13 . On the surface of the reduced diameter body part  133  and within it there is carbon proceeding from the surface down to a depth of 100 μm. 
   If, during lamp operation, the temperature of the reduced diameter body part  133  reaches 1400° C. to 1800° C., as is shown using the above described formulas 1 and 2, by the trapping of the oxygen of the thorium dioxide (ThO 2 ) by tungsten carbide (W 2 C), thorium (Th) is produced. 
   The thorium (Th) produced on the surface of and within the cathode  13  passes through by grain boundary diffusion between the grain boundaries of the tungsten and is deposited on the surface of the cathode  13 . Due to the high temperature of the tip region of the cathode, surface diffusion takes place. This means that thorium moves along the surface of the reduced diameter body part  133  and along the surface of the conical part  135  and is supplied to the tip surface  134  of the cathode  13 . 
   Furthermore, part of the thorium (Th) which has been produced within the cathode  13  moves within the cathode  13  by it passing through between the grain boundaries of the tungsten and is supplied to the tip surface of the cathode  13 . 
   The emitter containing body  18 , due to the heat of the cathode, has essentially the same temperature as the cathode, i.e. 1400° C. to 1800° C. 
   As a result, in the emitter containing body  18 , by trapping the oxygen of the thorium dioxide (ThO 2 ) by carbon which is present on the surface and in the interior proceeding from the inner surface down to a depth of roughly 100 μm, in which the emitter containing body  18  borders the carbon  13 , thorium (Th) is also produced. 
   The thorium (Th) which has been deposited on surface of the emitter containing body  18  is supplied to the surface of the reduced diameter body part  133  of the cathode  13 , moves on the surface of the reduced diameter body part  133  and also along the surface of the conical part  135  and is supplied to the tip surface  134  of the cathode  13 . 
   As a result, thorium (Th) is also supplied to the cathode  13  from the emitter containing body  18 . After lamp operation over a long time thorium (Th) can thus be stably supplied to the cathode tip. Thus, formation of the flicker phenomenon as a result of drying out of thorium (Th) atoms over a long time can be suppressed. 
   A high pressure discharge lamp filled with xenon as the emitter was described above. However, the invention can also be applied to a high pressure discharge lamp in which the emitter is mercury. 
   Next, a test was run in which using the high pressure discharge lamp in which the cathode shown in  FIG. 1  and  FIG. 4  with an emitter containing body was used, the situation for formation of flicker was studied. 
   If the amplitude of the voltage during operation exceeds 1 V, flickering of the images on the screen becomes apparent. At the instant at which the voltage amplitude exceeds 1 V, therefore, flickering of the images on the screen is regarded as disadvantageous. This instant was therefore defined as lamp service life by flickering. 
   The high pressure discharge lamp which was used for this test is operated with rated values of 24 V, 78 A and 2 kW. The comparison lamp was a lamp with the same specifications except for using the cathode shown in  FIG. 9  without the emitter containing body for the test.  FIG. 7  shows the results. 
   In  FIG. 7 , the x-axis plots the duration of operation in hours and the y-axis plots the amplitude of the voltage (V). The curve A shows the data of the high pressure discharge lamp in accordance with the invention and the curve B shows the data of the comparison lamp. As  FIG. 7  shows, in the comparison lamp, the voltage amplitude 900 hours after operation commenced is greater than or equal to 1 V, where flickering of the images on the screen has become apparent and thus the service life of the lamp is reached. 
   On the other hand, in the high pressure discharge lamp in accordance with the invention, the voltage amplitude does not become greater than or equal to 1 V until 1100 hours after operation, where flickering of images on the screen has become apparent and thus the service life of the lamp is reached. Therefore, the end of the lamp service life as a result of flickering was increased by 200 hours. 
   From this result, it becomes apparent that, for the high pressure discharge lamp in accordance with the invention, after lamp operation over a long time, thorium (Th) is stably supplied to the cathode tip and that thus a lamp with a long service life is obtained in which formation of the flicker phenomenon is suppressed over a long time.