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WEEK 03: GENERATION: ALTERNATIVE SOURCES: THERMIONIC CONVERSION Sections: Introduction | Principles | Facts | Future History1 The British physicist Sir Owen Willans Richardson, b. Apr. 26, 1879, d. Feb. 15, 1959, won the 1928 Nobel Prize for physics for his work in thermionics--the emission of electrons by heated electrical conductors--and for his discovery of Richardson's law, an empirical formula that describes the effect of heat on the interaction between electricity and matter. Richardson taught and conducted research at Princeton University (1906-13) and King's College, London (1914-44). Thermionic emission is the phenomenon by which electrons are emitted into a vacuum as a result of the thermal excitation, or heating, of a metal or oxide-coated conductor. The cathode of an electron tube, for instance, is a thermionic emitter, and an electron tube that relies for its current on a heated cathode--as do most vacuum tubes--is called a thermionic tube. Thermionic emission2 is also known as the Edison effect, because Thomas A. Edison discovered the phenomenon in 1883 while developing filaments for the electric lightbulb. Practical thermionic emitters may be heated directly or indirectly.3 A directly heated emitter may resemble the filament of an incandescent lamp. (In fact, such a filament is itself a thermionic emitter.) Indirectly heated thermionic emitters generally consist of an oxide-coated substrate placed in proximity to a heater element. Thermionic emission may also be achieved by directing a beam of electrons against a suitable target. Thermionic power generators are devices that make use of thermionic emission to produce power. A great deal of work in this field is still in the experimental stage. One field of research is the development of a thermionic power generator for spacecraft, making use of heat from a small nuclear reactor. Efficient thermionic emitters should be sufficiently robust to withstand mechanical vibration and the effects of residual gases in the evacuated envelope in which the emitter is contained. These constraints are more easily met by metal emitters. Thermionic emission can be observed in all metals. Tungsten and tantalum are particularly efficient emitters, but rare-earth oxides of barium and strontium are used as well. Oxide-coated emitters are more efficient at lower temperatures. 1. Forrest M. Mims III, The 1998 Grolier Multimedia Encyclopedia, Grolier Interactive Inc.: 1997 |
