Abstract
Thermionic emission measurements have been used to determine the work function (φ) of pure and oxidized uranium samples between 1100 and 1300 K; Auger electron spectroscopy (AES) was used to verify the cleanliness and compositions of the samples. It was found that impurities present in ppm amounts in the bulk U segregated to the surface upon heating and had an appreciable effect on the zero-field emission currents as well as the slopes of the Schottky curves obtained at various temperatures. A combination of ion-sputtering and ultrahigh vacuum (UHV) annealing at high temperatures was successful in reducing the total impurity level on the hot surfaces to 5%. At this low concentration of impurities, well-behaved Richardson line plots were obtained with A=135 A cm−2 K−2 and φ=3.54 eV for pure U and A=128 A cm−2 K−2 and φ=3.19 eV for UO2. The Schottky coefficients for clean U approached their ideal values at fields >400 V/cm.
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