“Boiling” Electrons Out of Metals
Like the thermoelectric element, the thermionic converter is a heat
engine. In its simplest form it consists of two closely spaced metallic
plates and resembles the diode radio tube. Whereas thermoelectric
elements depend on heat to drive electrons and holes through
semiconductors to an external electricity-using device or _load_, the
salient feature of the thermionic diode is _thermionic emission_,[7] or,
simply, the boiling-off of electrons from a hot metal surface. The
thermionic converter shown in Figure 7 powers a small motor when heated
by a torch.
Metals, as we have already seen, have an abundance of loosely bound
conduction electrons roaming the atomic latticework. These electrons are
easily moved by electric fields while within the metal; but it takes
considerably more energy to boil them out of the metal into free space.
Work has to be done against the electric fields set up by the surface
layer of atoms, which have unattached valence bonds on the side facing
empty space.
The energy required to completely detach an electron from the surface is
called the metal’s _work function_. In the case of tungsten, for
example, the work function is about 4.5 electron volts[8] of energy.
[Illustration: Figure 7 _Vacuum type thermionic converter in
operation._
Courtesy General Electric Company.]
As we raise the temperature of a metal, the conduction electrons in the
metal also get hotter and move with greater velocity. We may think of
some of the electrons in a metal as forming a kind of _electron gas_.
Some electrons will gain such high speeds that they can escape the metal
surface. This happens when their kinetic energy exceeds the metal’s work
function.
Now that we have found a way to force electrons out of the metal, we
would like to make them do useful electrical work. To do this we have to
push the electrons across the gap between the plates as well as create a
voltage difference to go with the hoped-for current flow.