Direct Power Generation Using Tunneling and Thermionic Emission

Thermionic emission in vacuum could be a highly efficient cooler or power generator if the work function, the minimum work for electrons to go into vacuum, is around 0.3–0.4 eV for heat source at a temperature below 500C [Mahan, 1994]. Unfortunately, the work function of existing materials is currently above 1 eV. Theoretical and experimental studies have shown that the work function can be reduced to 0.3–0.4 eV if the distance between the two electrodes (cathode and anode) of the thermionic emission cooler/power generator is below 10 nm [Hishinuma, et al, 2001, 2003]. At this nanometer scale, electron transport between the two electrodes takes two paths: electron tunneling and thermionic emission. The combined physical processes result in a desired work function. However, maintaining a nanometer gap for two parallel plates within an area larger than 1 cm2 is a daunting task, if not impossible, especially if the power generator is mounted on a moving or vibrating device. Even a slight vibration or thermal expansion of the two plates (electrodes) could cause direct contact between the two plates (electrodes), and thus shorten the circuits. Thus vacuum thermionic power generator based on difficult to make and to operate [Tavkhelidze, et al., 2002]. In this study, we propose to use a solid insulating spacer for preventing the shortening and for feasibility of manufacturing. The spacer is less than 5nm, and electron transport as thermionic emission and tunneling concurrently. In this study, we first investigate electron and phonon transport in single-layer (spacer) double heterostructures by including the tunneling effects. It is found that single-layer generator can have a high efficiency, but small power intensity due to the small temperature difference between the two electrodes. We then investigate the efficiency of multilayer-layer power generator. Calculations show that the solid power generator operating at a temperature below 500°C, can have an efficiency of larger than 40% of the Carnot efficiency.