Question

Which among the following cannot be used as a thermionic emitter?

• A. Tungsten
• B. Thoriated Tungsten
• C. Tungsten coated with alkali metal oxide
• D. Tungsten coated with iron oxide

Answer 1

The correct option is D Tungsten coated with iron oxide

Thermionic emission is the thermally induced flow of charge carriers from a surface or over a potential-energy barrier. This occurs because the thermal energy given to the carrier overcomes the work function of the material.

So, we can use Tungsten and Thoriated Tungsten (Thoriated tungsten electrodes are widely used in commercial industries, in a process known as tungsten inert gas, or “TIG” welding).

Now if we use alkali metal oxide in coating the emitted electron will be emitted from the surface but in case of iron oxide it has other option also.

For alkali metals changes in oxidation state is not easy and stable also.

But for iron we have two different oxidation state (Ferrous and Ferric), so the electron will be partially (if a mix iron oxide is used) used in oxidation so overall wastage of electron made it insufficient.