High-pressure, thermally highly loaded discharge lamp, and method to make electrodes therefor
FIELD OF THE INVENTION 
     The present invention relates to high-pressure discharge lamps, and more particularly to the electrode structure in high-power, high-pressure discharge lamps, and to a method to make the electrode. Discharge lamps of the kind to which the present invention relates are frequently used in projectors, for example motion-picture projectors, mask-work projections for exposure of semiconductor chips in micro lithography, and the like. 
     BACKGROUND 
     Motion-picture projectors use effectively exclusively xenon high-pressure discharge lamps having power ratings of between about 0.5 to 10 kW. In micro lithography, mercury high-pressure discharge lamps are used with power ratings of between about 0.2 kW and 2 kW. Lamps for both uses are direct current operated, and the light is generated by a d-c arc of high stability extending between a cathode and an anode. The thermal loading on the anode is extremely high. 
     It is believed that the thermal loading on the anode is caused primarily by entry of electrons during the discharge into the facing end surface of the anode. Usually, the anode has a blunt end surface, and typically a somewhat spherical or frusto-conical end region. Upon entry of electrons, anode heat is generated and has to be dissipated. This heat is caused, in part, by the entrance work or entrance or insertion energy (about 4.4 eV for tungsten), the anode drop (about 1 eV) and the average plasma electron energy (about 1 eV). This heat increases in proportion to the lamp current. 
     Anodes for such high-pressure discharge lamps usually are made of tungsten, the metal of all metals which has the highest melting point of about 3680 K., and the lowest vapor pressure. The anodes are as large as possible, while still assuring a stable arc. The size should be so selected that it provides the largest possible heat radiating surface in order to obtain lowering of the anode operating temperature. Of course, due to costs and design considerations, as well as size of the overall lamp, the size of the anode cannot be increased beyond relatively narrow limits. 
     To obtain a low anode end surface temperature which is as low as possible, it is desirable to also provide for effective conduction of anode heat away from the anode front end surface. This requires a high heat conductivity of the tungsten material in the region of the front end surface. 
     The tungsten material used in making the anode must have high resistance with respect to material creep due to heat. This is particularly important due to the high anode front end surface temperatures. High mechanical strength at high temperature over a long time is expected so that the anode surface does not deform in the region facing the arc. Such deformation may lead to premature failure of the lamp. 
     Anodes which have insufficient resistance to high temperature creep may generate craters or the like in the region of the front end surface. Additionally, heating and cooling processes occur when the lamp, when cold, is energized, and then, after deenergization, cools again. The resulting thermal stresses may cause fissures at which sooner or later local hot spots will occur during lamp operation which may reach temperatures above the melting point of the tungsten. This leads to increased vaporization of tungsten and thus premature blackening of the discharge vessel, again reducing the effective lifetime of the lamp. 
     Usually, the tungsten for use in anodes utilizes tungsten rod or stock material which is made in a powder metallurgy process. Pure or doped tungsten powder is compressed into rod shape, and sintered; the thus premanufactured rods are circumferentially swaged, in which the sintered tungsten rod material is reformed and compacted. 
     The two important parameters for long life of anodes are the heat conduction characteristics and the high temperature creep resistance characteristics. Both of these parameters increase with compaction of the tungsten material. It is believed that the high temperature creep resistance increases not only due to the higher density of the material, but also due to the finer grains of the material which arise upon compaction. The larger numbers of grain boundaries within a unit volume then counteract thermally caused material deformations or shifts and provide greater resistance against material shifting under heat loading. 
     The deformation and compaction process acting on rod material are hammer forces which act essentially radially on the rod material. Deformation and compaction is done from the lateral surface. As a result, both the high temperature creep resistance and the heat conductivity decrease from the outer or sleeve surface of the rod material towards the center of the rod material. 
     When a lamp is energized, the arc attaches at about the center region of the anode face surface. That is the region where the highest temperature results. That is also the region where the compaction in accordance with the prior art processes has a minimum effect. Consequently, that central region of the anode has the lowest density, hence the lowest heat conductivity, and the lowest resistance towards creep under high temperature loading. 
     THE INVENTION 
     It is an object to improve high-pressure discharge lamps by providing such lamps with improved electrodes and especially anodes, and in which the heat conductivity and creep resistance under thermal stress is improved over that of prior art anodes; and to provide a method for making such anodes. 
     Briefly, the material of at least part of the end face of at least the anode of the high-pressure lamp is mechanically forged and compacted. This structure can be obtained by providing an electrode blank, and then axially forging at least a portion of the end face of the blank to compact the material thereof, so that the region adjacent the end surface likewise will be forged and compacted. 
     Thus, forging and compacting the central region of the anode has the advantage that the heat conductivity and resistance against high temperature creep of the tungsten material in the central region of the anode end surface is improved, and thus deformation of the anode, and especially the front end surface, is effectively avoided, exactly at the region where the arc attaches. Resulting local increases in temperatures due to formation of protrusions and fissures, leading to increased vaporization of tungsten can be effectively eliminated or, at least, substantially decreased, so that the lifetime of the lamps is enhanced, and blackening of the lamp envelope surrounding the electrodes is eliminated or at least reduced, or substantially delayed with respect to operating time of the lamp. 
     The electrode in accordance with the present invention is particularly suitable as the anode of a d-c operated xenon or mercury high-pressure discharge lamp. The characteristics of the anode is that the surface facing the discharge is compacted by axial forging. 
     In accordance with a feature of the invention, anode blanks are placed in an electrically heated furnace, and heated to about 1400° C. They are then singly removed with suitable tongs from the furnace and placed in a closed die for closed die forging in the die recess, and hammered with a pneumatic forging hammer of about 400 kg weight. About 10-15 impacts within a few seconds forge the blank to the height of the closed die. 
     This forging operation results in an axial compression of the front or face portion of the anode blank which, usually, is frusto-conical. The compression is by a few millimeters and, in connection therewith, a marked densification of the tungsten material is obtained in this region. Additionally, the grain structure of the material is changed towards a fine grained structure. 
    
    
     DRAWINGS 
     FIG. 1 is a highly schematic side view of a xenon high-pressure discharge lamp of 2.5 kW rating and having the anode electrode of the present invention; 
     FIG. 2 is a top view of an anode of a 2.5 kW xenon high-pressure discharge lamp in accordance with the prior art after 1200 hours of operation; 
     FIG. 3 is a top view of the anode of a 2.5 kW xenon high-pressure discharge lamp having an axially forged end face, after 1200 hours of operation; 
     FIG. 4 is a photo micrograph showing an axial cross-sectional cut through an anode blank before axial forging; 
     FIG. 5 is a view similar to FIG. 4 of an anode after axial forging and illustrating the axially compacted densification of the end face of the anode; 
     FIG. 6 is a graphic representation of an anode similar to FIG. 5, not to scale, after axial forging,and illustrating the position of the measuring points with respect to hardness; and 
     FIG. 7 is a diagram illustrating the averaged hardness values measured across the axial cross-sectional cut of 7 anodes after axial forging, close to the end face surface (x marks) and about 10 mm below the end face surface (  marks). 
    
    
     DETAILED DESCRIPTION 
     Referring first to FIG. 1: 
     The lamp of FIG. 1 is a 2.5 kW (nominal rating) d-c xenon high-pressure short-arc discharge lamp. It has a quartz-glass bulb 1, filled with xenon at about 10 bar pressure. It has two necks, 2, 2&#39;, also made of quartz glass. Two electrodes are located within the lamp, diametrically opposite each other, spaced by a few millimeters from each other. The cathode electrode 3 is pointed; the anode electrode 3&#39; is substantially larger than the cathode electrode, since it is thermally highly loaded. To obtain high stability of the discharge arc, it is formed in frusto-conical shape at the front end facing the discharge. The electrodes 3, 3&#39; are secured, for current supply, to rods 4, 4&#39;, likewise made of tungsten. The electrode rods 4, 4&#39; are melt-sealed in the end portions of the necks 2, 2&#39; to be gas-tight therein, and connected to external current supply leads 6, 6&#39;. Lamp bases 5, 5&#39; are cemented to the necks 2, 2&#39;. The lamp construction, as a whole, is conventional, and standardized. The electrical connection to supply energy at the cathode is obtained by a clamp connection to the base pin 6. The anode is connected to a base cable 7, terminating in a terminal lug 6&#39;. Lamp voltage is only about 28 V, lamp current 90 A. The anode face surface temperature is about 2500 K. 
     Lamps of this type are used in motion picture projection primarily due to the radiation which matches daylight very closely and, further, due to the high brightness. The lifetime of such lamps with prior art anodes is between 1200 to 1500 hours, and is limited primarily due to blackening of the bulb or vessel 1. The materials used in the lamp--tungsten for the electrodes and connecting rods, quartz glass, xenon--and the relatively high proportion of manual work at the lamp causes the lamps to be expensive. Premature failure of the lamps, thus, is undesirable due to the high cost of the lamps. Increasing the lifetime provides competitive advantages. 
     FIG. 2 is a photo micrograph of the end face of an anode of a prior art lamp of the type shown in FIG. 1, namely a 2.5 kW rated power lamp, after 1200 hours of operation. As can be clearly seen, the end face surface is highly deformed and shows small melt balls. This indicates that the melting temperature of tungsten, namely 3680 K., has been locally exceeded. Exceeding the melting temperature leads to increased vaporization of the tungsten; the vaporized tungsten deposits on the inside surface of the vessel 1, and the resulting blackening leads to limitation of the useful life of the lamp. 
     FIG. 3 is a top view of an anode of a lamp identical to that shown in FIG. 2, in which, however, the anode is made of the forged, compacted material in accordance with the present invention. Again, the lamp has a rated power of 2.5 kW, and the anode is shown after 1200 hours of operation. As can be seen, the front end surface of the anode is practically free of deformation, and so local temperature not spots are absent. Thus, since there was no increased vaporization of tungsten, the blackening of the bulb was effectively eliminated or, at least, substantially reduced. Lamps of such anodes readily exceed the previously standard lifetime of between 1200 to 1500 operating hours. 
     FIG. 4 is a photo micrograph of the surface of an axial section of the frusto-conical anode blank before axial forging and compaction. Before forging, the frusto-conical portion of the anode blank had a height of about 8 mm, and the diameter of the end face was only about 3 mm. As can be seen, the grain structure is essentially uniform over the entire blank. 
     FIG. 4 further illustrates a line V--V. This line shows the position of the anode end surface after compaction (see FIG. 5). The numbers above FIG. 4 show the hardness of the sectioned surface measured as the Vickers hardness HV5 (kg/mm 2 ) of the axially uncompacted blank at the level of the line V--V. In essentially uniformly grained material, hardness is a measure for the density of the tungsten material. Thus, hardness can be used as a secondary measuring parameter for both the heat conductivity and the resistance to deformation under thermal stress. As can be seen, the values are a minimum at the central axis of the electrode, and increase towards the outer or lateral or jacket surface. 
     FIG. 5 is a photo micrograph of the surface of an axial section of the frusto-conical anode blank after axial forging and compaction. The height of the frusto-conical section has been reduced by axial compaction by about 2.5 mm, and after finish machining by material removal, will be only about 4 mm. The diameter of the end face surface is about 12 mm. The grain structure has become finer in a roughly hemispherical region centered about the central axis, and having a depth of a few mm, as can be clearly seen from FIG. 5. 
     The numbers above FIG. 5 again are the Vickers hardness HV5 (kg/mm 2 ), measured at approximately equal distances transversely across the sectioned anode surface. The values are clearly above the values of the yet uncompacted blank. The value of &#34;337 at the center&#34; is believed to have been a measuring inaccuracy. 
     Both compaction as well as change in the micro structure of the grains contribute to increase of the hardness measured at the anode facing surface, and thus improvement in heat conductivity and resistance against deformation under heat. 
     The numbers indicated on FIGS. 4 and 5 relate to measurements taken about 2 mm from each other, and are single values from a single measurement for the Vickers hardness HV5 (kg/mm 2 ). It is extremely difficult to accurately measure hardness over this surface in small increments. Referring again to FIG. 4, it can be clearly seen that the axial hardness value of 323 is above the immediately adjacent hardness values, but substantially below the lateral-hardness values where circumferential compaction had been done to the raw stock or raw rod material. Considering, however, unavoidable inaccuracies in measured data, it can still be seen that the hardness and, in a homogeneous grain structure, consequentially the density decreases from the outside surface to the central region, or the axis of the electrode. 
     In FIG. 5, the central value 337 is apparently an individual measuring error. Subsequent hardness measures with six further anode structures provided average values. Measuring points and hardness values measured are graphically shown in FIGS. 6 and 7, respectively; the values are reproduced on tables 1 and 2, in which Table 1 shows the Vickers hardness HV5 of six further anodes adjacent the face surface compacted and densified in accordance with the present invention, and Table 2 shows the hardness at a lower level of the anodes, that is, where the tungsten material is effectively unaffected by forging and hence undensified. The measuring points 1-7 of Table 1 correspond to the measuring points 1-7 on FIGS. 6 and 7; the measuring points 1&#39;-12&#39; correspond to the measuring points 1&#39;-12&#39; on FIGS. 6 and 7. As can be seen, there is substantial deviation in individual measurements at like points. Comparing Tables 1 and 2, and specifically points 4 and points 6&#39;, 7&#39;, it can be clearly seen that the hardness, as measured at point 4, is substantially higher than the hardness measurements at points 6&#39;, 7&#39;; likewise, the hardness at points 3 and 5 of the anode in accordance with the present invention is clearly above the hardness values of points 5&#39;, 8&#39; inwardly of the anode. 
     After axial forging, the outer surfaces of the anode are cut to size on a lathe, or similar machine tool, removing a layer of about 1.5 mm thickness of deformed material from the end facing surface as well as the conical jacket surface. This step results in a substantially homogeneous grain structure at the anode face surface and further to obtain appropriate geometry, size and shape of the anode. 
     Comparison of the Tables 1 and 2 also shows that the forging action effectively does not penetrate through the entire anode; thus, the measured hardness of Table 2, at the rear, effectively uncompacted region, varies from a higher value at the outer circumference (points 1&#39; and 12&#39;) towards a lower value in the central region (points 6&#39;, 7&#39;). 
     The photo micrographs of FIGS. 4 and 5 were made by axial cutting, polishing and etching of the respective surfaces. The Vickers hardness HV5 is then measured at measuring distances of 2 mm from each other on a line of about 12 mm length, close to the front end surface, about 1 mm below it, in the forward measuring range, as illustrated by the x marks in FIG. 6. Thus, the measuring line for the values of Table 1 is along the line VI--VI; the measuring line for the rear region of the anode surface was 
     about 10 mm below the anode surface, along line VIa--VIa to provide a comparison with the effectively uncompacted rear region. 
     As can be clearly seen from Table 1 and FIG. 7, the hardness in the axially compacted region close to the front surface of the anode is quite uniform between the circumference and the region or the center, or axis of the electrode. In the uncompacted, rear region of the electrode, the hardness as shown in Table 2 and in FIG. 7 is significantly less at the axial region, by more than 15 kg/mm 2 , on the average, than along the lateral surface. 
     
                                           TABLE 1                                 
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Vickers hardness (HV5 (kg/mm.sup.2) measured in the axially densified     
front part of                                                             
the anode specimen in accordance with the invention, along line VI-VI of  
FIG. 6                                                                    
Anode no.                                                                 
       Point 1                                                            
            Point 2                                                       
                 Point 3                                                  
                      Point 4                                             
                           Point 5                                        
                                Point 6                                   
                                     Point 7                              
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1      357  366  361  337  354  345  357                                  
2      332  332  341  341  341  329  336                                  
3      336  349  345  345  349  347  332                                  
4      336  341  345  341  341  345  341                                  
5      342  352  351  348  354  342  342                                  
6      332  329  321  329  336  341  341                                  
7      343  341  329  332  345  345  341                                  
Average                                                                   
       339.7                                                              
            344.3                                                         
                 341.9                                                    
                      339.0                                               
                           345.7                                          
                                342.0                                     
                                     341.4                                
Mean   ±8.8                                                            
            ±12.7                                                      
                 ±13.4                                                 
                      ±6.8                                             
                           ±6.9                                        
                                ±6.1                                   
                                     ±7.8                              
deviation                                                                 
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                                           TABLE 2                                 
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Vickers Hardness HV5 (kg/mm.sup.2) measured in the effectively axially    
undensified                                                               
rear part of the anode specimen along a line VIa-VIa of FIG. 6            
      Point                                                               
         Point                                                            
            Point                                                         
               Point                                                      
                  Point                                                   
                     Point                                                
                        Point                                             
                           Point                                          
                               Point                                      
                                  Point                                   
                                     Point                                
                                        Point                             
Anode no.                                                                 
      1&#39; 2&#39; 3&#39; 4&#39; 5&#39; 6&#39; 7&#39; 8&#39;  9&#39; 10&#39;                                     
                                     11&#39;                                  
                                        12&#39;                               
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1     348                                                                 
         345                                                              
            336                                                           
               334                                                        
                  334                                                     
                     327                                                  
                        321                                               
                           325 331                                        
                                  335                                     
                                     335                                  
                                        341                               
2     341                                                                 
         341                                                              
            338                                                           
               329                                                        
                  310                                                     
                     329                                                  
                        317                                               
                           325 332                                        
                                  329                                     
                                     345                                  
                                        347                               
3     336                                                                 
         336                                                              
            336                                                           
               329                                                        
                  323                                                     
                     321                                                  
                        323                                               
                           345 345                                        
                                  336                                     
                                     343                                  
                                        336                               
4     341                                                                 
         345                                                              
            345                                                           
               336                                                        
                  329                                                     
                     329                                                  
                        323                                               
                           349 345                                        
                                  345                                     
                                     349                                  
                                        345                               
5     341                                                                 
         341                                                              
            332                                                           
               321                                                        
                  332                                                     
                     321                                                  
                        321                                               
                           329 343                                        
                                  349                                     
                                     345                                  
                                        349                               
6     347                                                                 
         341                                                              
            345                                                           
               336                                                        
                  321                                                     
                     317                                                  
                        323                                               
                           323 341                                        
                                  349                                     
                                     349                                  
                                        345                               
7     349                                                                 
         343                                                              
            336                                                           
               332                                                        
                  332                                                     
                     325                                                  
                        321                                               
                           336 336                                        
                                  345                                     
                                     345                                  
                                        362                               
Average                                                                   
      343.3                                                               
         341.7                                                            
            338.3                                                         
               331.0                                                      
                  325.9                                                   
                     324.1                                                
                        321.3                                             
                           333.1                                          
                               339.0                                      
                                  341.1                                   
                                     344.4                                
                                        346.4                             
Mean  ±4.8                                                             
         ±3.1                                                          
            ±4.9                                                       
               ±5.3                                                    
                  ±8.5                                                 
                     ±4.6                                              
                        ±2.1                                           
                           ±10.4                                       
                               ±6.0                                    
                                  ±7.8                                 
                                     ±6.7                              
                                        ±8.1                           
deviation                                                                 
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     Various changes and modifications may be made within the scope of the inventive concept.