Thermionic devices convert thermal energy to electricity using the thermionic effect [3, p. 182]. A thermionic device consists of a vacuum tube with electrodes in it. The metal cathode is heated until electrons start evaporating off the metal. The electrons collect at the anode which is at a colder temperature. In a typical device, the cathode may be at a temperature of 1500 $$^{\circ}$$C, and the distance between the anode and the cathode may be 10 $$\mu m$$ [60]. A device based on this effect was first patented by Thomas Edison in 1883. The Carnot efficiency limits this effect because a temperature differential is converted to electricity [5]. Efficiencies up to 12% have been measured. However, for a given temperature differential, other methods of converting temperature difference to electricity are often more efficient. Cathodes have been made from tungsten, molybdenum, tantalum, and barium oxide [3]. The cathode gets used up in the process and eventually needs to be replaced. Anodes have been made from copper, cesium, nickel, barium oxide, strontium oxide, and silver [3] [60]. Some gas chromatographs use nitrogen phosphorous thermionic detectors [151].