Thermionic energy converters were conceived in 1915, demonstrated in
1939, and were the focus of astronomical investments during the Cold
War by the U.S.  and the Soviet Union.  A 6 kW thermionic converter,
fabricated using precision machining technology, was flown in 1987 by
the Soviet Union .

Thermionic converters can be fabricated using extensions of MEMS
technology, in which advances in materials, micromachining, and vacuum
encapsulation processes can be used to enhance performance and reduce
manufacturing costs.  Potential commercial applications include
small-scale co-generation.  Recently, a new conversion concept has been
demonstrated at Stanford, in which a semiconductor photocathode replaces
the conventional metal cathode. This photon-enhanced thermionic energy
(PETE) converter harvests photon energies above the bandgap, as well
as broad-spectrum radiation through heating of the photocathode ,
making it attractive as a high-temperature topping cycle for solar-thermal
power stations.  Micro- and nano-structured, high-temperature
materials and micromachining processes are also essential to fabricating
wafer-scale, cost-effective PETE converters.  I will conclude by
summarizing the research directions that are needed to bring thermionic
and PETE converters into the mix of energy conversion technologies.