Why are thermionic converters considered direct energy-conversion devices?

Thermionic converters are classified as direct energy-conversion devices because they transform thermal energy directly into electricity using thermionic emission, bypassing the need to first convert that heat into intermediate forms of energy, such as mechanical work.1

In these devices, one electrode (the emitter) is heated to a high temperature, causing electrons to be liberated from its surface. These electrons flow across a gap to a cooler electrode (the collector), creating an electric current when the two electrodes are connected by an external load. Because this process relies on the physical phenomenon of electron emission rather than a multi-step cycle involving moving parts or intermediate mechanical energy, it is considered a direct conversion process.1

Functionally, a thermionic converter acts as an electronic diode that utilizes an electron-rich gas or vacuum to facilitate this flow, effectively serving as a heat engine at the electronic level.1