FABRICATION AND OPERATING CHARACTERISTICS OF SINTERED THORIA CATHODES

Sintered thoria cathodes, 3 inches long and 0.860 inch in diameter, have been prepared by subjecting a mixture of thoria and tungsten or molyb-denum powder to 115 tons/inch 2 pressure, and then sintering the resulting compact at 1750OC for one-half hour. The use of a split-segment die as developed by Raytheon seems to be best for the process. A volume concen-tration of conducting powder to thoria exceeding 0.212 is required to make the sintered cathode conducting and capable of being heated directly. The operating characteristics of two cathode mixtures, one containing 30 percent tungsten and 70 percent thoria and the other 18 percent molyb-denum and 82 percent thoria (by weight), were investigated. The cathode resistivity was found to decrease markedly as the cathode was first brought up to temperature, but once the highest operating temperature was reached no further appreciable decreases occurred. Considerable emission may be obtained at temperatures exoeedlng 1600-C (brightness); however, the cathode may be activated for short periods of time by flashing to a high tempera-ture. In this manner over 1 amp/om2 was obtained at 1600'C (brightness). The period of activation was found to be much longer for the molybdenum mixture; it approached 100 hours. The activation time for the tungsten mixture never exceeded 15 hours