Microfabricated thermionic energy converters [electronic resource]
Keywords: Electrical Engineering.

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 dissertation presents research on five key areas of microfabricated thermionic energy converters ([mu]-TECs). First, the numerical calculation of the emitter-collector gap that maximizes the power conversion efficiency of thermionic energy converters (TECs) is discussed. Thermionic energy converters require emitter and collector work-functions that are relatively low, to reach useful efficiencies at typical operating temperatures of 1000 - 1500 ºC. The optimum arises because efficiency drops both at very large gaps, due to space-charge limitations on the TEC current, and at very small gaps, due to the increased heat loss via near-field radiative heat transfer. The numerical calculation results show that, for typical TECs made with cesiated tungsten electrodes, the optimal gaps range from 900 nm to 3 [micrometers]. I then discuss several prototypes of mechanically and thermally robust [mu]-TECs, including the stress-relieved emitter design, emitter-collector structural design, as well as a recent approach for the stand-alone (encapsulated) [mu]-TECs. Thermionic emission from the SiC emitter was demonstrated for the first time. The stress-relieved design emitters were analyzed, and the work-function of the SiC emitter was estimated at temperatures of up to 2900K. Also described are both the planar and the U-shaped suspension for microfabricated TECs ([mu]-TECs). Our initial planar [mu]-TECs achieved emitter temperatures of over 2000 K with incident optical intensity of approximately 1 W/mm2 (equivalent to 1000 Suns), remained structurally stable under thermal cycling, and maintained a temperature difference between the emitter and the collector of over 1000 K. Conformal sidewall deposition of poly-SiC on a sacrificial mold is used to fabricate stiff suspension legs with U-shaped cross sections, which increases the out-of-plane rigidity and prevents contact with the substrate during the heating of the suspended emitter. By extending the conventional technique of cesium coatings to SiC, we reduce the work-function from 4 eV to 1.65 eV at room temperature. Subsequently, we tested [mu]-TECs with both barium and barium oxide coatings. The coatings reduced the work-function of the SiC emitter to as low as ~2.14 eV and increased the thermionic current by 5-6 orders of magnitude, which is a key step toward realizing a efficient thermionic energy converter. Encapsulation of [mu]-TEC was achieved by an anodic bond between pyrex and the silicon substrate with via feedthroughs. Last, I introduce the photon-enhanced thermionic emission (PETE) concept, and show why a single crystal photo-emitter is needed. I cover my recent fabrication development of smart-cut layer transfer using Spin-on-Glass (SoG). In addition, a novel layer transfer technology that can transfer any device materials onto the glass substrate, which I call "Anything on Glass, " is briefly described. I, then, describe how the first demonstration of the photon-enhanced thermionic emission (PETE) from the microfabricated emitter was achieved. The p-type SiC emitter was used to demonstrate PETE in an uncesiated and microfabricated sample, bringing this energy conversion approach closer to practical applications.