Patent Publication Number: US-5022883-A

Title: Method of making a long life high current density cathode from aluminum oxide and tungsten oxide powders

Description:
GOVERNMENT INTEREST 
     The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to us of any royalty thereon. 
    
    
     FIELD OF INVENTION 
     This invention relates in general to a method of making a long life high current density cathode and in particular to a method of making such a cathode from a mixture of aluminum oxide, Al 2  O 3  and tungsten oxide, WO 3  powders. 
     BACKGROUND OF THE INVENTION 
     It is known in the cathode fabrication art that Al 2  O 3  and WO 3  react in a 1:3 molar ratio to form Al 2  (WO 4 ) 3  at temperatures greater than 900° C. It is also known in the cathode fabrication art that the presence of Al 2  (WO 4 ) 3  can be detected in a tungsten billet that has been impregnated with the well known impregnant, Ba 3  Al 2  O 6 . The Al 2  (WO 4 ) 3  and the impregnant Ba 3  Al 2  O 6  however react with each other to form BaAl 2  O 4  and BaWO 4  without the formation of free barium metal. This is significant since it is the free barium metal that is responsible for electron emission in the aforementioned impregnated cathode. It is believed that the formation of Al 2  (WO 4 ) 3  acts as a deterrent to barium metal formation, and that electron emission is reduced. This occurs in conventional cathodes such as a porous billet of tungsten impregnated with Ba 3  Al 2  O 6 . 
    
    
     SUMMARY OF THE INVENTION 
     The general object of this invention is to provide an improved method of making a long life high current density cathode. A more particular object of the invention is to provide such a method in which conventional impregnants such as Ba 3  Al 2  O 6  can be eliminated from cathode processing. 
     It has now been found that a long life high current density cathode can be made from a mixture of aluminum oxide and tungsten oxide powders in a molar ratio of 1:3 by fusing the mixture of powders into the fused powder Al 2  (WO 4 ) 3 , adding barium hydride to the fused powder Al 2  (WO 4 ) 3 , inverting the fused powders and heating to decompose the barium hydride and obtain barium metal. 
     More particularly, according to the invention, a mixture of Al 2  O 3  and WO 3  powders in a 1:3 molar ratio are added to a sleeve that is contained at one end in a cup. The assembly is then placed in a vacuum or inert gas furnace and the powders heated to about 1100° C. for about 5 to 15 minutes. The sleeve and powder that has now fused and formed into Al 2  (WO 4 ) 3  is removed from the furnace after the furnace has cooled. Residual Al 2  (WO 4 ) 3  is then cleaned from the outer parts of the sleeve using emery cloth and a jeweler&#39;s lathe. Barium hydride, BaH 2  is then added to the sleeve with the Al 2  (WO 4 ) 3  in it and the assembly placed in an inert gas or vacuum furnace. The sleeve is then inverted and the back side of the sleeve heated to about 700° C. for about 5 to 10 minutes to decompose the barium hydride and obtain barium metal. The sleeve with the Al 2  (WO 4 ) 3  and barium metal in it are allowed to cool and any barium metal that may have splattered on the side of the sleeve then cleaned in an inert gas atmosphere. The sleeve with the fused Al 2  (WO 4 ) 3  and barium metal are then prepared for a cathode environment by heating the sleeve under vacuum. 
     The sleeve used in the foregoing method can be any of those typically used in cathode technology such as molybdenum or tungsten. It is cylindrical, open ended, of any desired diameter, and can be contained at one end in a cup. 
     The method of the invention involves the generation of barium metal from barium hydride with the Al 2  (WO 4 ) 3  on top of the barium metal. In the method, the barium diffuses through the Al 2  (WO 4 ) 3  layer that is separated from the free barium metal, reacts with the Al 2  (WO 4 ) 3  and forms a dipole on top of the Al 2  (WO 4 ) 3  to give electron emission. 
     We wish it to be understood that we do not desire to be limited to the exact details of construction as described for obvious modifications will occur to a person skilled in the art.