Impregnated cathode
This is a continuation of application Ser. No. 058,362, filed June 4, 1987, which was abandoned upon the filing hereof. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an impregnated cathode used in an electron tube or the like and, more particularly, to a surface coating layer thereof, used for thermionic emission 
     An impregnated cathode is obtained by impregnating pores of a porous pellet with an electron-emission material such as barium oxide, calcium oxide, aluminum oxide, etc. Such a cathode can provide a current density higher than a conventional oxide thermal cathode, and has a longer service life, since it is resistant to a harmful gas, contained in a tube, and which interferes with electron emission. Consequently, cathodes of this type are employed in a travelling-wave tube used in, for example, artificial satellites, in a high-power klystron used for plasma heating in a nuclear fusion reactor, etc. 
     In the above fields, high reliability (long service life, stable operation, and so on) and high current density are required of a cathode. As a means of increasing the reliability, a layer of an element of the platinum group, such as iridium, osmium, ruthenium, etc. or an alloy thereof, is coated on the cathode surface, in order to decrease the work function of the cathode surface, thereby to decrease the operating temperature In contrast to a case wherein such a coating layer is not provided, the operating temperature of a cathode having a coating layer can be decreased by several tens to one hundred and several tens °C., to obtain the same current density. Since evaporation of the electron emission material can then be limited, this is advantageous for a cathode, with regard to prolongation of its service life, and provides an improvement in the intratube withstand voltage characteristics. 
     However, the operating temperature in this case is still as high as 900° to 1,000° C. Therefore, W for forming a pellet is diffused in the surface coating layer during operation, and forms an alloy together with a metal constituting the surface coating layer. Alloying of the surface coating layer changes the electron-emission characteristics, and interferes with the achieving of stable characteristics from an early stage of operation, and with the prolongation of the service life. 
     SUMMARY OF THE INVENTION 
     The present invention has as its object to provide an impregnated cathode which maintains stable electron emission characteristics from the early stage of operation, and a method of manufacturing the same. 
     The present invention provides an impregnated cathode wherein an alloy layer of iridium and tungsten is formed on a surface of a porous pellet impregnated with an oxide of an alkali earth metal, wherein the crystal structure of the alloy has an εII phase comprising an hcp (hexagonal close-packed) structure whose lattice constants a and c satisfy 2.76≦a≦2.78 and 4.44≦c≦4.46. When this impregnated cathode is manufactured, a layer of iridium is coated on the surface of the porous pellet. Then, the porous pellet is heated in a vacuum or inert atmosphere at 1,100° to 1,260° C., for a predetermined period of time. 
     The heating process of the present invention is considerably practical, since it has a good reproducibility. The appropriate thickness of the Ir coating layer is 50 to 10,000 Å, because of the ease in controlling the heating time, and in order to preserve the electron emission characteristics of the pellet. The thickness of the alloy layer is about twice that of the Ir coating layer, as will be described later. However, when the alloy layer is thinner than 100 Å, the service life of the cathode is decreased; when it is thicker than 20,000 Å, it is necessary for the operating temperature to remain high. 
     The heating time in this case is arbitrarily determined within the range of 1 to 360 minutes. If the heating temperature is higher than 1,260° C., the amount of electron emission material evaporating from the pellet is excessive, thereby degrading electron emission characteristics. When the heating temperature is 1,100° C. or lower, an extended period of time is required for alloying of the ε II  phase; therefore, this is impractical. 
     Alternatively, an alloy layer of εII phase of iridium and tungsten, can be used as the coating layer, in place of the iridium layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of part of an impregnated cathode according to the present invention; 
     FIG. 2 is a graph showing the time and temperature in each heating process of Example 1 of the present invention; 
     FIG. 3 shows X-ray diffraction pattern of the cathode surface in the respective processes shown in FIG. 2; 
     FIG. 4 shows a graph comparing εI phase and εII phase; 
     FIGS. 5A and 5B show graphs of relative concentrations of W and Ir after lighting and aging processes are completed, respectively; 
     FIG. 6 shows a graph indicating a relationship between the aging time and the intensity ratio of the X-ray diffraction peak; 
     FIG. 7 shows a graph indicating a relationship between the aging time and MISC; and 
     FIG. 8 shows a graph indicating the relationship between the thickness of the Ir coating layer and the alloy layer. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An Ir layer having a thickness of 3500 Å was coated on a porous pellet, and the change in the crystal structure in the surface layer of the Ir-coated porous pellet was measured in situ using a vacuum high-temperature X-ray diffractometer. When the change in the X-ray diffraction pattern was observed along the heating schedule of the cathode shown in FIG. 2, it was confirmed that the change was as shown in FIG. 3. 
     It is seen in FIG. 3 that the ε phase of the intermetallic compound of Ir and W appears after the lighting process (IV). The ε phase has an hcp structure. In the aging process, a series of diffraction peaks exhibiting the same crystal type appeared on the low-angle sides of the respective diffraction peaks of ε phase. As the aging process proceeds, the peaks that appeared in the lighting process disappeared and were replaced by the pattern that appeared in the aging process. The ε phase which appeared in the lighting process will be referred to as εI phase and the phase that appeared in the aging process will be referred to as εII phase. The discrete changes in the diffraction pattern from εI to εII phase correspond to the discrete changes in the lattice constants a and c. Namely, 2.735≦a≦2.745 Å and 4.385≦c≦4.395 Å were obtained in εI phase, whereas 2.760≦a≦2.780 Å and 4.440≦c≦4.460 Å were obtained in εII phase. 
     The relationship between these values of lattice constants a and c and the W concentration in the Ir-W alloy has already been reported. This relationship is indicated by solid lines in FIG. 4. Dotted lines indicate the values of the lattice constants of the εI and εII phases obtained by the experiments conducted by the present inventors. The corresponding W concentrations are about 20 to 25 atm % in εI phase and about 40 to 50 atm % in εII phase. It is seen in FIG. 4 that the change in the composition of the surface layer occurs quite discretely by the transition from εI to εII phase. εII phase exhibited a considerably stable crystal structure. Its lattice constants did not substantially change in the subsequent heating process. 
     The compositions of the alloy layers after the lighting and aging processes were analyzed by sputtering from the surface in the direction of depth (indicated by a corresponding sputtering time) with an Auger electron spectroscope, and the results shown in FIGS. 5A and 5B were obtained. FIGS. 5A and 5B show relative concentration profiles after lighting and aging processes, respectively. Curves 51 and 53 indicate relative iridium concentrations, and curves 52 and 54 indicate relative tungsten concentrations. It is seen that, in the alloy layer after completion of the lighting process, tungsten was quickly diffused in iridium since the tungsten concentration gradient near the surface was small. The tungsten concentration near the surface was about 25 atm %. In the alloy layer after completion of the aging process, the tungsten concentration in the surface and in the layer is 40 to 50 atm %. These facts coincide with the results of changes in the composition in the surface coating layer shown in FIG. 4. 
     The relationship between the thickness of the iridium layer and the aging conditions was studied. FIG. 6 shows the results obtained by X-ray diffraction. The X-ray diffraction intensity ratios plotted along the axis of ordinate are ratios of the εII phase diffraction peak intensities to the sum of the diffraction peak intensities of Ir layer, εI and εII phases. Curves 61, 62, 63, and 64 indicate ratios when the thicknesses of the iridium coating layers are 1,000, 2,000, 3,500, and 5,000 Å, respectively. The heating temperature was 1,180° C. 
     It is seen in FIG. 6 that the aging time required for the transition from εI to εII phase depends on the thickness of the Ir coating layer and that the thicker the Ir layer, the longer the εII phase formation time. Therefore, when the aging time is set constant, in order to form a perfect εII phase, the thicker the Ir coating layer, the higher the heating temperature. 
     FIG. 7 shows a change in the maximum emission value in a space charge limiting region, i.e., MISC (Maximum I k  Saturated Current) with respect to the aging time for each Ir layer thickness. Curves 71, 72, 73, and 74 indicate MISC&#39;s when the thicknesses of the Ir coating layers are 1,000, 2,000, 3,500, and 5,000 Å, respectively. An MISC is a value measured 1 second after the start of an anode voltage application. It is seen from these results that the thicker the Ir coating layer, the less the increase in MISC, and that a longer heating time is required to activate emission. 
     The electron emission characteristics of MISC were measured in a plane-parallel diode glass dummy tube. During measurement of the electron emission characteristics, the cathode temperature was decreased to 1,000° C. so that aging did not proceed. 
     It is also apparent from FIGS. 5 to 7 that the electron emission characteristics are closely related to the formation ratio of εII phase, and that a stable, maximum electron emission current can be obtained when the εII phase is completely formed in the surface of the alloy layer. 
     Finally, the section of the cathode after alloying was observed by a scanning electron microscope to examine the relationship between the thickness of the alloy layer and thickness of the Ir coating layer. FIG. 8 shows its result. It is seen in FIG. 8 that the thickness of the alloy layer formed is about twice that of the thickness of the Ir layer before the heating process. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     EXAMPLE 1 
     A mixture of barium oxide, calcium oxide, and aluminum oxide (in a molar ratio of about 4:1:1) was melted and impregnated in a porous tungsten pellet having a diameter of 1.5 mm, a thickness of 0.4 mm, and a porosity of about 20%. The surface of the pellet was cleaned to remove excessive Ba, thereby forming impregnated pellet 11 shown in FIG. 1. Subsequently, pellet 11 was welded to tantalum cup 13 having a thickness of 25 μm through rhenium wire 15. Cup 13 was welded to an opening at one end of tantalum support sleeve 17. Sleeve 17 was fixed to a support cylinder (not shown) through three support straps of a rheniummolybdenum alloy, thereby forming a cathode. An Ir layer having a thickness of 3,500 Å was formed by sputtering on the surface of pellet 11. 
     The cathode was placed in a vacuum bell jar evacuated to 10 -7  Torr or less. A heater (not shown) was powered to heat the cathode at a predetermined temperature for a predetermined period of time. FIG. 2 shows the time and temperature in this heating process. The heating process consists of a lighting process (I, II, III, IV, V, and VI) for gradually heating the cathode for the purpose of degassing, and an aging process (VII, VIII, and IX) for heating the cathode at a constant temperature of a brightness temperature of about 1,180° C. for a predetermined period of time. The brightness temperature was that of the cathode surface measured with a optical eyrometer with 650 nm filter. 
     In this manner, Ir-W alloy coating layer 19 of ε phase having an hcp structure wherein the lattice constants a and c (unit: Å) satisfy 2.76≦a≦2.78 and 4.44≦c≦4.46 was formed. This impregnated cathode was incorporated in a travelling-wave tube for an artificial satellite and was started. Electron emission characteristics having a considerably excellent stability were obtained even after a lapse of a long time from the initial stage of operation. 
     EXAMPLES 2-20 
     Samples obtained by coating Ir layers to thicknesses of 50 to 10,000 Å on the surfaces of porous pellets by sputtering were prepared and were subjected to predetermined heating. This surface alloying treatment was practiced by two methods; an inside-the-tube heating method to assemble a cathode in an electron tube, that uses this cathode, and energize the heater in the cathode; and a single body heating method to heat the cathode in a vacuum bell jar before assembly in an election tube. The inside-the-tube heating method is suitable for a comparatively low-voltage electron tube or the like, and the single body heating method is suitable for a large or high-voltage electron tube or the like. 
     A cathode shown in FIG. 1 was formed by using each of these samples, and the following tests were conducted. A change in electron-emitting current value was measured at an operating temperature of 1,000° C. and under an anode voltage wherein the initial emitting current density was 0.8 A/cm 2  in the space charge limiting region. The ratios of the electron-emitting current values immediately after the start of operation and 3,000 hours after the start to the electron-emitting current value 100 hours after the start of the operation test were respectively evaluated as the initial and service life characteristics. Table 1 shows the result. Reference symbols x, Δ,  ○ , and  ⊚  indicate the cases wherein the above ratios were 59% or less, 60 to 79%, 80 to 89%, and 90 to 100%, respectively. The closer to 100%, the more superior the electron-emitting characteristics. 
     In Table 1, cathodes in which the εII phase was observed substantially in the entire portions of their alloy layers are grouped as Examples, and cathodes in which the εI phase only or both the εI and εII phases were observed in their alloy layers are grouped as Controls. 
     
                                           TABLE 1                                 
__________________________________________________________________________
                                       Service                            
Thickness of Alloying Method Crystal                                      
                                  Initial                                 
                                       life                               
      Ir Coating                                                          
             Heating         structure                                    
                                  Charac-                                 
                                       Charac-                            
Sample No.                                                                
      Layer (Å)                                                       
             Method                                                       
                  Condition (°C., hr(s))                           
                             of alloy                                     
                                  teristics                               
                                       teristics                          
__________________________________________________________________________
Control 1                                                                 
      49     Inside                                                       
                  1000, 1    εI                                   
                                  ○                                
                                       x                                  
             Tube                                                         
Example 2                                                                 
      48     Inside                                                       
                  1100, 1    εII                                  
                                  ○                                
                                       ○                           
             Tube                                                         
Example 3                                                                 
      53     Inside                                                       
                  1180, 1    εII                                  
                                  ⊚                        
                                       ○                           
             Tube                                                         
Control 2                                                                 
      495    Inside                                                       
                  1100, 5    εI, εII                      
                                  Δ                                 
                                       Δ                            
             Tube                                                         
Control 3                                                                 
      498    Inside                                                       
                  1100, 5    εI, εII                      
                                  Δ                                 
                                       Δ                            
             Tube                                                         
Example 4                                                                 
      503    Inside                                                       
                  1180, 5    εII                                  
                                  ○                                
                                       ○                           
             Tube                                                         
Control 4                                                                 
      988    Inside                                                       
                  1100, 10   εI, εII                      
                                  Δ                                 
                                       Δ                            
             Tube                                                         
Control 5                                                                 
      995    Inside                                                       
                  1180, 5    εI, εII                      
                                  Δ                                 
                                       Δ                            
             Tube                                                         
Example 5                                                                 
      997    Inside                                                       
                  1180, 10   εII                                  
                                  ○                                
                                       ○                           
             Tube                                                         
Example 6                                                                 
      1003   Single                                                       
                  1180, 10   εII                                  
                                  ⊚                        
                                       ○                           
             Member                                                       
Example 7                                                                 
      1005   Single                                                       
                  1180, 30   εII                                  
                                  ⊚                        
                                       ⊚                   
             Member                                                       
Control 6                                                                 
      1988   Inside                                                       
                  1100, 60   εI, εII                      
                                  Δ                                 
                                       x                                  
             Tube                                                         
Example 8                                                                 
      1993   Inside                                                       
                  1100, 150  εII                                  
                                  ○                                
                                       ○                           
             Tube                                                         
Example 9                                                                 
      1995   Inside                                                       
                  1100, 300  εII                                  
                                  ⊚                        
                                       ○                           
             Tube                                                         
Example 10                                                                
      2002   Single                                                       
                  1180, 30   εII                                  
                                  ⊚                        
                                       ○                           
             Member                                                       
Example 11                                                                
      2005   Single                                                       
                  1180, 60   εII                                  
                                  ⊚                        
                                       ⊚                   
             Member                                                       
Control 7                                                                 
      3490   Inside                                                       
                  1100, 60   εI, εII                      
                                  Δ                                 
                                       Δ                            
             Tube                                                         
Control 8                                                                 
      3496   Inside                                                       
                  1100, 120  εI, εII                      
                                  Δ                                 
                                       Δ                            
             Tube                                                         
Example 12                                                                
      3501   Single                                                       
                  1180, 60   εII                                  
                                  ○                                
                                       ○                           
             Member                                                       
Example 13                                                                
      3505   Single                                                       
                  1180, 120  εII                                  
                                  ⊚                        
                                       ⊚                   
             Member                                                       
Example 14                                                                
      3507   Single                                                       
                  1250, 120  εII                                  
                                  ⊚                        
                                       ⊚                   
             Member                                                       
Control 9                                                                 
      4987   Inside                                                       
                  1100, 60   εI, εII                      
                                  x    x                                  
             Tube                                                         
Control 10                                                                
      4995   Inside                                                       
                  1100, 120  εI, εII                      
                                  x    Δ                            
             Tube                                                         
Control 11                                                                
      5012   Single                                                       
                  1180, 120  εI, εII                      
                                  Δ                                 
                                       Δ                            
             Member                                                       
Example 15                                                                
      5020   Single                                                       
                  1180, 180  εII                                  
                                  ⊚                        
                                       ○                           
             Member                                                       
Example 16                                                                
      5023   Single                                                       
                  1180, 240  εII                                  
                                  ⊚                        
                                       ⊚                   
             Member                                                       
Control 12                                                                
      7444   Single                                                       
                  1100, 120  εI, εII                      
                                  x    x                                  
             Member                                                       
Control 13                                                                
      7456   Single                                                       
                  1100, 180  εI, εII                      
                                  x    x                                  
             Member                                                       
Control 14                                                                
      7459   Single                                                       
                  1180, 360  εII                                  
                                  ○                                
                                       ○                           
             Member                                                       
Example 17                                                                
      7480   Single                                                       
                  1220, 180  εII                                  
                                  ○                                
                                       ○                           
             Member                                                       
Example 18                                                                
      7490   Single                                                       
                  1260, 180  εII                                  
                                  ⊚                        
                                       ⊚                   
             Member                                                       
Control 15                                                                
      9958   Single                                                       
                  1180, 180  εI, εII                      
                                  Δ                                 
                                       x                                  
             Member                                                       
Example 19                                                                
      9970   Single                                                       
                  1180, 240  εII                                  
                                  ○                                
                                       ○                           
             Member                                                       
Example 20                                                                
      10046  Single                                                       
                  1260, 240  εII                                  
                                  ○                                
                                       ○                           
             Member                                                       
__________________________________________________________________________
 
    
     It is apparent from Table 1 that, when the heating conditions are changed in accordance with the thickness of the Ir layer to be coated first and the εII phase is formed on the entire surface of the alloy phase, stable electron emission characteristics that last for a long period of time from the early stage of operation can be obtained.