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首页> 美国政府科技报告 >Thermionic Emission and Surface Composition of the Lanthanum-Boron and Yttrium-Boron Systems
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Thermionic Emission and Surface Composition of the Lanthanum-Boron and Yttrium-Boron Systems

机译：镧 - 硼和钇 - 硼体系的热离子发射和表面组成

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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. The CVC is also a function of temperature and may vary slightly for the same bulk composition at different temperature. 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. Emitted current measurements were made with interelctrode potentials between 250 and 1400 volts. Schottky plots were used to extrapolate the zero field currents. When a sample is taken from equilibrium to a new temperature, a definite time lag appears while vaporization rates change to bring about a new equilibrium surface composition. This manifests itself in the recorded emission currents. After thermal equilibrium is reached a distinct change is seen in emission currents. A higher density is measured, reflecting the emission of a surface that has been raised to a higher temperature. But with time, at this temperature, the surface reacts through vaporization and a new composition appears that is closer to the congruently vaporizing composition, and hence, has a work function nearer that of the CVC. (ERA citation 08:018280)

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