Abstract
ABSTRACT Evaporation from a thoria coated tungsten filament for the temperature range 1900-2300°K was studied by measuring the thermionic emission of a nearby tungsten receiver. Below 2100°k the evaporated product was mainly thoria with a small amount of thorium. Above 2100°K there was an increased proportion of thorium. The thermionic emission of thoria coated iridium was studied after evaporating thoria from a source on to the clean metal. After only a few monolayers had been deposited on the iridium the thermionic emission decreased during operation below 2200°k due to diffusion of thorium into the metal. Later when more thoria had been deposited a relatively high and stable thermionic emission was obtained following operation of the iridium at 2400°K. This is explained by assuming that at 2400°K considerable crystal growth occurred and that the thorium was stored in the grain boundaries.
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