Mention the reason why tungsten is a suitable metal for Thermionic emission

Tungsten is a suitable metal for Thermionic emission, has a high melting point of 3655 K but its work function is high at about 4.52 eV(electron volt-energy unit). It starts emission at 2500 K. If thoriated tungsten (coated with carbon and thorium) is used work function reduces to 2.6 eV and emission begins at 2000 K. If tungsten is coated with barium and strontium oxide work function reduces to 1 eV and emission begins at 1000 K and a good supply of electrons is obtained.

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