Why are iron nickel and carbon suitable materials for anodes?

In an electron tube, the anode is typically constructed from materials such as iron, nickel, or carbon because they are good electrical conductors that do not readily emit electrons at typical operating temperatures.1

While cathodes are designed to emit electrons—often through thermionic emission—the anode serves to attract those electrons. Using materials that resist electron emission at operating temperatures ensures that the anode does not become an unintended source of electron emission itself, which would interfere with the tube's intended function.12