Let's look at the CATHODE today. Of the two types of CATHODE construction we commonly see, I am today going to describe just one - the directly heated cathode.
Directly heated cathodes are often seen in battery valves and power rectifiers, these tend to be filament wires or filament tapes coated with an emissive cathode material. Just take a look at the shadowgram below where we have, from L to R:- a human hair, a coated directly heated filament for a DF66 valve, an uncoated filament for a DF66 valve.
The filament wire used for diretly heated cathodes tend to be either pure metallic nickel, pure metallic tungsten or a nickel alloy. The emissive coating is a proprietary admixture of barium and strontium carbonates which are oxidized to barium and strontium oxide as in the firing process of a valve's manufacture.