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Thermionic Energy Conversion

Thermionic energy converters (TECs) are heat engines that convert heat directly to electricity at very high temperatures. This energy conversion process is based on thermionic emission—the evaporation of electrons from conductors at high temperatures. In its simplest form, the converter consists of two electrodes in the parallel plate capacitor geometry, and it uses the thermionically emitted current to drive an electrical load. This technology has existed for over half a century, with applications mostly limited to space use. However, with recent advancement in nano/micro fabrication, together with engineered high-temperature materials, two dimensional materials, and powerful computing methods, e.g. density functional theory (DFT), we are committed to revolutionizing this technology to be more robust with higher efficiency. To be specific, our work is focused on fabricating thermally robust emitters, ultra-low work function collectors, and micron-meter scale inter-electrode gap.

Modern microfabrication techniques promise to overcome many of the challenges for TEC devices: large electrode spacings, thermal shorting, and thermal expansion mismatches. In the last few years, we have demonstrated thermionic energy converters based on suspended cathodes. Silicon carbide (SiC) is an attractive material for electronic devices operating at high temperatures and high power. The three most common SiC polymorphs (3C, 4H, and 6H) have relatively large bandgaps and are chemically and thermally stable, with thermal decomposition temperatures of over 2500°C. Recent developments in SiC surface micromachining have made possible the fabrication and testing of poly-SiC MEMS devices that operate at temperatures of 1000°C or higher. These superior properties allowed us to design a thermionic emitter.

The work function is the surface property that determines how easily electrons can escape into a vacuum or gas environment, with lower work functions generally facilitating electron emission. Discovery of thermally stable materials with low work functions has promising applications in thermionic energy converters (TECs) and photon-enhanced thermionics energy converters (PETECs). Traditional methods of work function lowering rely on alkali coatings, which were first developed in the first half of the 20th century. However, these coatings typically enable work functions only as low as 1.5 to 1.0 eV. For applications such as the (PE)TEC, whose output efficiency is highly dependent on its anode’s work function, a work function of over 1eV is not low enough for efficient operation. Previous calculations have shown that with a work function of 0.5eV a TEC device can theoretically reach an efficiency of over 50% under 1000x concentrated solar radiation, which almost doubles the efficiency of the theoretical limit of single junction solar cell. By using DFT, combining with various of experimental approaches, we are looking for ultra-low work function materials.

Density functional theory (DFT) is a first-principles-based quantum mechanically motivated method that can offer new insights into the atomistic processes that control the work functions of various surfaces. DFT enables a systematic approach in the discovery of new nanostructured multilayer materials with low work functions. Therefore, many promising film coating combinations can be efficiently investigated for the first time.