Thermionic Work Function of Thin-Oxide-Coated Aluminum Electrodes in Vacuum and in Cesium Vapor

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Abstract
The thermionic work function of aluminum electrodes with anodized aluminum oxide coatings of 30 and 2000 Å was determined for each electrode from the contact potential difference of each electrode with a well outgassed and aged polycrystalline tungsten wire emitter. In vacuum, the thermionic work function of the 30-Å coated electrode was 4.3±0.1 eV, relative to a polycrystalline tungsten work function of 4.6 eV and a tungsten emission constant of 120 A/cm2· °K2. In cesium vapor at reservoir temperatures up to 70°C, the 30-Å coated electrode had a minimum observed work function of 1.4±0.1 eV relative to the same tungsten reference electrode. The resistance of the 2000-Å film coated collector electrode was too large to allow accurate determination of the work function of this electrode. In both vacuum and cesium vapor, however, there was an indication that the work function of the 2000-Å coated electrode was a few tenths of an electron volt lower than that for the 30-Å coated electrode. It is suggested that similar but perhaps less stable oxide-coated electrodes could occur naturally on the collector of a thermionic energy converter due to oxidation of the collector surface or buildup on the collector surface of outgassing products from the emitter. Auxillary experiments were performed on the secondary-electron emission from these oxide-coated electrodes. No lingering secondary electron emission current corresponding to a Malter effect was observed. The true secondary yield from the 2000-Å coated electrode was 3 in vacuum and 7 in cesium vapor for a primary electron energy of 150 eV. The secondary yield for the 30-Å coated electrode was similar in cesium vapor but was not studied in vacuum.

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