Phenomena in Oxide Coated Filaments

Abstract

A theory of the changes in activity in oxide coated filaments is proposed. From a comparison of the behavior of these filaments and filaments with composite surfaces such as thorium on tungsten, caesium on tungsten, and caesium on oxygen on tungsten it appears probable that oxide coated filaments owe their high activity to adsorbed metallic barium. The changes in emission from a coated filament produced by changes in plate potential and by currents sent into or drawn from it, are ascribed to electrolysis of the oxide. When electrons are sent into a coated filament barium is deposited on the surface and the activity increases until an optimum is reached beyond which the activity decreases. When current is drawn from the oxide, oxygen is deposited on the surface. If the oxygen is beneath the adsorbed barium, it increases the activity; if it is above the barium, it decreases the activity. Both barium and oxygen diffuse readily from the surface into the oxide and vice versa. This theory is tested, confirmed, and extended by numerous experiments.An experimental technique is employed by which relative rates of evaporation of small amounts of electropositive and electronegative materials can be determined with considerable precision. The same technique might be useful in a number of similar investigations. Metallic barium or oxygen which evaporate from a coated filament are allowed to deposit on one side of a flat tungsten ribbon whose thermionic activity is followed. When the plausible assumption is made that an optimum activity is obtained when the tungsten is covered with a single layer of electropositive material, the relative rates of evaporation can be converted to absolute rates. This technique is also employed to determine the factors which control the evaporation of oxygen from a coated filament.Experimental results.---(1) Metallic barium evaporates from a well broken down coated filament provided that the temperature is not too high and that no appreciable currents are drawn. (2) Oxygen evaporates if an emission limited current is drawn and the temperature is high enough. (3) Metallic barium which is deposited on the surface of the oxide either by electrolysis or by evaporation from an outside source, produces changes in activity of the same sort as are produced by barium on tungsten. (4) Barium or oxygen diffuse readily to and from the surface of the oxide (5) Oxygen brought to the surface of the oxide by drawing current from it, increases the activity under some circumstances and decreases it under others. (6) Most of the current through the oxide is carried by electrons but a small part is carried by ions; the ratio depends upon the temperature, the composition of the oxide, and the plate potential. It appears that the conductivity of the oxide does not obey Ohm's law and that some of the conductivity is due to a diffusion current which may flow against an opposing field. (7) The complex time changes that occur in coated filaments are described in a systematic manner.