Thermionic emission and surface composition of the lanthanum-boron and yttrium-boron systems
The importance of multi-constituent material behavior to thermionic energy conversion is explored and analyzed. The relationship between bulk composition, surface composition, and thermionic work function in the Yttrium-Boron and Lanthanum-Boron systems are investigated experimentally. On the basis of these data, new insights have been developed into the correlation of experimentally obtained thermionic properties with material properties. At thermionic temperatures, a difference between bulk and surface composition will exist unless the interior happens to be at the congruently vaporizing composition (CVC). Vaporization rates from the surface compete with diffusion rates in the bulk to cause this difference. The surface composition will tend toward the congruently vaporizing composition which is YB/sub 4/ in the Yttrium-Boron system and LaB/sub 6/ in the Lanthanum-Boron system. Knowledge of the CVC is thus essential to understanding the behavior of any thermionic materials, as it provides the limiting case of surface composition. Four Yttrium-Boron (Y-B) compounds, YB/sub 2/ /sub 5/, YB/sub 5/, YB/sub 6/ /sub 4/, YB/sub 14/ and three Lanthanum-Boron (La-B) compounds LaB/sub 6/ /sub 01/, LaB/sub 8/ /sub 5/ and LaB/sub 5/ /sub 9/ were tested in a variable spacing vacuum emission system with a guard assembly. The bare work function was determined for each compound. Emitted current measurements were made with interelectrode potentials between 250 and 1400 volts. Schottky plots were used to extrapolate the zero field currents.
- OSTI ID:
- 6109337
- Country of Publication:
- United States
- Language:
- English
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NESDPS Office of Nuclear Energy Space and Defense Power Systems
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YTTRIUM COMPOUNDS