GB2064211A

GB2064211A - Short-arc discharge lamp anode

Info

Publication number: GB2064211A
Application number: GB8031667A
Authority: GB
Original assignee: Tokyo Shibaura Electric Co Ltd
Current assignee: Toshiba Corp
Priority date: 1979-10-01
Filing date: 1980-10-01
Publication date: 1981-06-10
Also published as: JPS592145B2; DE3036746C2; JPS5650047A; GB2064211B; DE3036746A1

Abstract

A short-arc discharge lamp 1 comprising an envelope 2, an anode 3 and a cathode 4 each disposed inside said envelope 2, and a rare gas sealed in said envelope, said anode 3 being made of tungsten containing silicon, aluminum or potassium; or a mixture of at least two of these. Preferably the additive content is in the range of 20-300 ppm. Numerous examples giving proportions of materials are specified. The inclusion of the additive(s) is said to prevent darkening and whitening of the lamp envelope and to prevent deformation of the anode. <IMAGE>

Description

SPECIFICATION Short-arc discharge lamp The present invention relates to a short-arc discharge lamp and, more particularly, to a short-arc discharge lamp wherein the material of the anode is improved.

Short-arc discharge lamps, for example, xenon discharge lamps are widely used as the light source of movie projectors. In a conventional short-arc discharge lamp, an anode of highly pure tungsten with no additives and a cathode of tungsten containing thorium oxide with good electron emissivity are sealed in a translucent airtight container or envelope of a material such as quartz, and xenon gas is sealed therein to a pressure of several atmospheres. When the arc discharge is generated during use, the temperature of the electrodes is raised and tungsten is evaporated. Some of the evaported tungsten is deposited on the airtight container wall and darkens it, and the rest combines with the small amount of oxygen remaining inside the airtight container of the discharge lamp to form tungsten oxide and whitens the airtight container wall.The whitening and darkening of the airtight container wall have the defect of absorbing the light flux irradiated from the short-arc discharge lamp, thus degrading the luminous intensity.

The evaporation of tungsten from these electrodes not only causes the whitening and darkening of the airtight container wall, but also deforms the anode. This is attributed to the fact that some of the tungsten oxide near the short arc is reduced by the heat of the short arc, and tungsten is deposited on the anode.

The whitening and darkening of the airtight container wall leads to a defect in that the luminous intensity of the light source is degraded and the brightness of the projecting screen is decreased in a short-arc discharge lamp, in particular, in a xenon lamp used as the light source of a movie projector.

The deposition of tungsten on the anode results in the deformation of the electrode. This leads to a shift of the high luminance point which has been set at the focal point of the light source reflector of the projector, degrading the illuminance distribution on the projecting film surface and the resolution of the image at the film surface.

It is, therefore, the primary object of the present invention to provide a short-arc discharge lamp which is capable of preventing darkening and whitening of the lamp envelope and deformation of the anode.

The short-arc discharge lamp according to the present invention comprising an envelope, an anode and a cathode each disposed inside said envelope, and a rare gas sealed in said envelope, said anode being made of tungsten containing at least one element selected from the group consisting of silicon, aluminum, and potassium.

According to the present invention, the amount of the above-mentioned element included in tungsten is preferably 20-300 ppm and more preferably 20-200 ppm, and the anode is heat-treated preferably at a temperature above 2,0000C in a vacuum. The preferred anode is made of tungsten containing the above-mentioned all three elements.

This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a front view of a short-arc discharge lamp in accordance with one embodiment of the present invention, with part of it shown in section; Fig. 2 is an enlarged partial view of Fig. 1; Fig. 3 is a graph showing the relation of the lighting time to the horizontal luminous intensity maintaining rate for the present invention and comparative examples; and Figs. 4(a) and 4(b) are enlarged perspective views of the anodes of the short-arc discharge lamp of the present invention and a conventional lamp after 1 ,000 hours lighting time, respectively.

An embodiment of the present invention will be described with reference to Figs. 1 and 2.

Referring to Fig. 1, at both ends of an airtight container 2 of quartz of a xenon lamp 1 are fixed an anode 3 and a cathode 4 separated by a predetermined distance 5. The airtight container 2 is evacuated, and xenon (Xe) at a high pressure of about 6 atmospheres is sealed therein. The container 2 has a sealing end 6 for sealing it. The anode 3 and the cathode 4 are each conductively connected to one of lead wires 7, 7, and the other ends of the lead wires 7, 7 are connected to external terminals 9, 9 of bases 8, 8.

In Fig. 2, the same parts are designated by the same reference numerals as in Fig. 1.

The present invention is characterized in that the anode 3 of the xenon discharge lamp 1 shown in Figs. 1 and 2 is made of tungsten containing one or a plurality of elements selected from the group consisting of silicon, aluminum, and potassium. The effects of the present invention will be described in detail with reference to the examples.

The present inventors have conducted tests by changing the material of the anode as a means for preventing the darkening and whitening of the airtight container wall and the deformation of the anode of the xenon discharge lamp.

For performing the test, one or more elements selected from the group consisting of silicon, aluminum, and potassium are added in the form of an oxide or chloride to tungsten. The obtained mixture is reduced, molded by the general power metallurgical method, sintered, forged, machined, and heat-treated at a temperature above 2,0000C in a vacuum to obtain a desired anode. The xenon discharge lamps having anodes of tungsten with no additives as comparative examples were tested.

The results are shown in the table below. In the table, the additive remaining amount is the value obtained by analysis of the remaining amounts of every additive, this analysis being performed by taking samples after sintering of 1 9 kinds of anode materials tested; the additive remaining amount corresponds to the content of Si, Al and K contained in the anode assembled in the xenon discharge lamp. For evaluation after lighting, movie projector xenon discharge lamps of 26 V, 80 A were manufactured in the amount of 10 for each sample number using the respective anode materials. These lamps were horizontally lit with a lamp current such that the load on the airtight container walls was 30 W/cm2, corresponding to a 120% overload condition of the xenon discharge lamp relative to normal use.

The degree of discoloration of the airtight container wall for every hour was evaluated according to the evaluation standards shown below.

TABLE Additive remaining Evaluation after lighting Sample Total remaining after after after Overall Number Kind Si Al K amount (ppm) 200 hrs 600 hrs 1000 hrs evaluation 1 Tungsten 8 10 24 42 A A A' 0 containing 2 Si, Al, K 13 24 66 103 A A A' 0 3 12 16 100 128 A A' B 0' 4 28 15 117 160 A A' B 0' 5 27 38 132 197 A' B B' # 6 65 21 178 264 B B' B' # 7 Tungsten 15 7 - 22 A A A' 0 containing 8 Si, Al 47 35 - 82 A A A' 0 9 Tungsten 11 - 106 117 A A A' 0 containing 10 Si, K 27 - 129 156 A A' B 0' 11 Tungsten - 5 42 47 A A A' 0 containing 12 Al, K - 7 108 115 A A A' 0 13 Tungsten 30 - - 30 A A A' 0 containing 14 Si 60 - - 60 A A' B 0' TABLE (Continued) Additive remaining Evaluation after lighting amout (ppm) Sample Total remaining after after after Overall Number Kind Si Al K amount (ppm) 200 hrs 600 hrs 1000 hrs evaluation 15 Tungsten - 40 - 40 A A A' 0 containing 16 Al - 85 - 85 A A A' 0 17 Tungsten - - 73 73 A A A' 0 containing 18 K - - 58 58 A A A' 0 19 Tungsten - - - - B C C X containing no additives Evaluation Evaluation Standard A No discoloration noted A' Slight discoloration noted B Partial discoloration noted at the upper part of the airtight container wall B' Discoloration noted all along the upper part of the airtight container wall C Discoloration is considerable and the area of discoloration is great.

The overall evaluation was based on whether the lamp is serviceable for movie projection with the discoloration of the airtight container wall, and the evaluations were totalled after lighting the lamp for 200 hours, 600 hours, and 1 ,000 hours. A circle mark indicates that the lamp is usable without any problems. A circle mark with a dash indicates that the lamp is usable although partial discoloration is noted. A triangle mark indicates that the lamp is usable although discoloration is noted. An x mark indicates that the lamp is unusable due to considerable discoloration.

As may be apparent from the results shown in the table, as compared with tungsten with no additives, tungsten mixed with one or more of silicon, aluminum and potassium show good results, although some differences are noted in the area of darkening or whitening of the airtight container wall after 1,000 hours. The results were obtained with a 120% load exerted on the airtight container wall.

The results obtained after 1,000 hours under this condition are considered to correspond to the results to be expected after 2,000 hours under 100% load. Thus, it is seen that the results are satisfactory for normal use.

When the above results are considered in more detail, with tungsten containing the three elements of silicon, aluminum and potassium, the discoloration of the airtight container wall is small and the obtained results are good even after lighting for 600 hours and even when these elements are added in any combination, as long as the total additive remaining amount is less than 300 ppm. It is further apparent from the table that the discoloration of the airtight container wall is almost unnoticeable even after 1,000 hours lighting, when the total remaining amount is less than 120 ppm.

The tungstens with the circle mark for the overall evaluation, that is, the tungstens of sample number 1; 2; 7; 8; 9; 11, 12; 13; 15; 16; 17, and 18 were grouped into group A. Those with the circle and dash mark for the overall evaluation, that is, the tungstens of sample numbers 3, 4; 10; 14 were grouped into group B. Those with the triangle mark for the overall evaluation, that is, the tungstens of sample numbers 5 and 6 were grouped into group C. The tungsten with no additives, the sample number 19, and three of the lamps randomly sampled from each group were tested. They were lit at the rated voltage, that is, at 26 V. The horizontal luminous intensity was measured after 200, 600 and 1,000 hours, respectively. The variation in the mean horizontal luminous intensity of the three lamps, taking the initial horizontal luminous intensity as 100%, is shown in Fig.3. In Fig. 3, (I) denotes the horizontal luminous intensity maintaining rate for the group A; (II) denotes that for group B; (III) denotes that for group C; (IV) denotes that for the lamps of the sample number 19. As may be apparent from Fig. 3, the xenon discharge lamp using conventional tungsten with no additives for the anode showed about 76% horizontal luminous intensity maintaining rate after 1,000 hours relative to the initial luminous intensity.

The xenon discharge lamp using the tungsten of group C for the anode showed about 78%. The horizontal luminous intensity maintaining rates of the xenon lamps using the tungstens of groups A and B showed 94% and 91%, respectively, after 1,000 hours lighting. Thus, it is seen that the discoloration of the airtight container apparently correlates with the performance characteristics of the horizontal luminous intensity.

Next, observations were made with respect to the anode shape which is related to the degradation of the illuminance distribution of the xenon discharge lamp on the projecting screen and the image resolution at the film surface. Figs. 4(a) and 4(b) are perspective views of the anode surface after 1,000 hours lighting at the above-mentioned rated voltage. Fig. 4(a) shows the anode of tungsten with no additives, that is, of the sample number 1 9. Tungsten is deposited on and protrudes from the surface of the anode, and the deformation is significant. Fig. 4(b) shows the anode of sample number 1. Since the temperature at the electrode surface is raised after 1 ,000 hours lighting, the slight evaporation of the electrode material only cleans up the anode, providing a glossy surface. No deformation was noted.The phenomenon of Fig. 4(b) was noted for all of the lamps using the anodes of tungsten containing one or more of the elements selected from the group consisting of silicon, aluminum, and potassium, i.e., the tungstens of the groups A, B and C. Further, by using for the anode a tungsten containing one our more of the elements selected from the group consisting of silicon, aluminum, and potassium the deformation of the cathode of tungsten containing thorium oxide was also improved. This is attributed to the fact that the oxygen concentration in the vicinity of the cathode is decreased by using the above-mentioned materials for the anode.

With a tungsten containing one or more elements selected from the group consisting of silicon, aluminum and potassium, it is seen that the additives prevent the formation of tungsten oxide, the darkening or whitening of the airtight container wall by tungsten and tungsten oxide is eliminated, the decrease in the horizontal luminous intensity is reduced, and deposition of tungsten on the anode is prevented.

In the present invention, the contents of silicon, aluminum, and potassium to be added to tungsten are preferably 5 ppm or more, 5 ppm or more, and 15 ppm are more, respectively.

Although the above description has been made with reference to a xenon discharge lamp in accordance with one embodiment of the present invention, the present invention is also applicable to short-arc discharge lamps filled with other gases with which discharge is easy, such as argon and neon.

Thus, it is to be understood that the present invention is applicable to short-arc discharge lamps in general.

In summary, according to the present invention, a short-arc discharge lamp with excellent characteristics is advantageously provided wherein tungsten containing one or more elements selected from the group consisting of silicon, aluminum, and potassium is used for the anode so as to prevent darkening and whitening of the airtight container wall and deformation of the anode as occur in conventional short-arc discharge lamps using anode of tungsten containing no additives; the decrease in the horizontal luminous intensity of the short-arc discharge lamp while lit is vastly improved; and the shift in the high luminance point which causes deformation of the electrode is eliminated.

Claims

1. A short-arc discharge lamp comprising an envelope, an anode and a cathode each disposed inside said envelope, and a rare gas sealed in said envelope, said anode being made of tungsten containing at least one element selected from the group consisting of silicon, aluminum and potassium.

2. A short-arc discharge lamp as claimed in claim 1 wherein the content of said element is 20-300 ppm.

3. A short-arc discharge lamp as claimed in claim 1 wherein said anode is heat-treated at a temperature above 2,0000C in a vacuum.

4. A short-arc discharge lamp as claimed in claim 1 wherein said rare gas is xenon gas.

5. A short-arc discharge lamp, substantially as hereinbefore described with reference to the accompanying drawings.