Thermionic converters are devices used to convert heat energy into electricity through a process involving thermionic emission. They have been used in certain space applications, particularly in the past, though their usage has declined with the development of more efficient technologies.
The basic principle behind thermionic converters is the thermionic emission, which is the phenomenon where electrons are emitted from a heated surface. Here's a general overview of how electricity is generated in thermionic converters for space applications:
Heat Source: A thermionic converter requires a heat source to function. In space applications, this heat is typically provided by a radioactive material, such as a radioisotope thermoelectric generator (RTG). RTGs use the natural decay of certain radioactive isotopes to generate heat.
Emitter Surface: The thermionic converter consists of two main electrodes: the emitter and the collector. The emitter is usually made of a refractory material (high melting point material) capable of withstanding high temperatures. Common materials used are tungsten or thoriated tungsten.
Thermionic Emission: The emitter is heated to a high temperature (typically hundreds to thousands of degrees Celsius) using the heat from the radioactive source. When the temperature is sufficiently high, electrons in the emitter gain enough energy to overcome the material's work function, resulting in their emission from the surface.
Electron Flow: The emitted electrons form an electron cloud around the emitter surface due to their thermal energy. This cloud is often referred to as the "space charge."
Electrical Collection: The collector electrode is positioned near the emitter and is maintained at a lower temperature than the emitter. This temperature difference creates an electric potential between the emitter and the collector.
Electrical Output: The emitted electrons, being negatively charged, are attracted to the positively charged collector. As they move towards the collector, they create an electrical current, which can be harvested as usable electricity for the spacecraft's systems.
Efficiency and Limitations: Thermionic converters can be relatively efficient in certain temperature ranges, but they have some limitations. The efficiency depends on factors like the temperature difference between the emitter and the collector and the properties of the materials used. However, they can be heavy, complex, and less efficient compared to other power generation technologies, which is one reason their use in space applications has diminished over time.
It's worth noting that while thermionic converters have been used in space missions, newer and more efficient power generation technologies, like solar panels and advanced radioisotope power systems, have largely replaced them in modern space exploration.