A New Pressed Dispenser Cathode

The development of a barium-activated refractory metal thermionic emitter consisting of alkaline earth aluminates dispersed in a porous matrix of tungsten-molybdenum alloy is described. The cathode is manufactured from a powdered mixture of the constituents by compressing in a die, and sintering the compact. The influences of composition of alkaline earth aluminate, composition of refractory metal, ratio of aluminate to refractory metal, and firing conditions on the properties of the cathode are considered. The importance of chemical factors in determining optimum formulation for cathodes of this type is emphasized. In particular, the necessity for avoiding types of barium compounds capable of oxidizing the refractory metal without generating barium, and the desirability of providing a chemical system (combination of barium compound and refractory metal) capable of generating and evolving barium plus barium oxide at a rate not greatly in excess of that required for maintaining thermionic activation are demonstrated. The composition chosen for manufacture of cathodes is shown to meet the foregoing requirements, and to provide the additional favorable property of stability to air exposure of finished cathodes. Cathodes made by the methods described here and consisting of calcium barium aluminate dispersed in a porous matrix of tungsten-molybdenum alloy have a Richardson work function of approximately 1.7 ev and an A constant of about 2. They are suitable for operation in the temperature range of 1,000-1,200°C (or higher, with shortened life). Emission density of 10 amperes/cm2 with life of more than 5,000 hours is obtainable at an operating temperature of 1,130°C.