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.
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.
Reducing the Space Charge
The emitted or boiled-off electrons between the converter plates ([Figure 8]) form a cloud of negative charges that will repel subsequently emitted electrons back to the emitter plate unless counteraction is taken. To circumvent these space charge effects, we fill the space between the plates with a gas containing positively charged particles. These mix with the electrons and neutralize their charge. The mixture of positively and negatively charged particles is called a plasma.
The presence of the plasma makes the gas a good conductor. The emitted electrons can now move easily across it to the collector where, to continue the gas analogy, they condense on the cooler surface.