An Investigation of Short-Time Thermionic Emission from Oxide-Coated Cathodes

Vacuum-tube circuits were developed to measure thermionic current as a function of time for times ranging from 0.2 to 300 μsec. The decay with time of the temperature-saturated electron emission from oxide-coated cathodes was observed with this apparatus. Simultaneously with the application of anode voltage to an experimental diode the thermionic current density rose to an initial value and subsequently decayed to a steady value 1/5 to 1/15 of the initial value. The rate of decay was proportional to the current density, the decay requiring about 20 μsec. at 1200°K and about 500 μsec. at 900°K. The initial current exhibited a somewhat greater anode-voltage effect than did the steady-state current. The range of decay was independent of the thickness of cathode coating over the region 1 to 30 mg of oxide per ${\mathrm{cm}}^2$. An electrolytic conduction hypothesis of the decay process is proposed. The form of the observed current as a function of time agreed with the form predicted by this theory. The observed rate of decay, as interpreted on the basis of the electrolytic conduction hypothesis, indicated that within a single crystal of barium or strontium oxide between 0.05 and 0.5 of the total conduction current is ionic. The possible connection of this decay and the "flicker effect" is noted.