Thermionic contrast between the mean work functions effective for thermal positive-ionic and electronic emissions from polycrystalline tungsten surfaces heated in vacuum: comparison between theory and experiment

摘要

To study the thermionic contrast of polycrystalline surfaces consisting of many patchy faces with different local work functions, those mean work functions (φ~+ and φ~e) effective for thermal positive-ionic and electronic emissions from a polycrystalline tungsten surface heated in a high vacuum were measured as a function of surface temperature and found to be 5.18 ± 0.02 and 4.60 ± 0.02 eV, respectively, in a temperature range (approx > 2000 K) high enough to keep the surface virtually clean. Consequently, the thermionic contrast (Δφ~* ≡ φ~+-φ~e) was determined experimentally to be 0.58 ± 0.03 eV. They are in good agreement with literature values (φ~+ = 5.11 ± 0.04 eV, φ~e = 4.59 ± 0.01 eV and Δφ~* = 0.52 ± 0.04 eV) reported with essentially clean surfaces of polycrystalline tungsten. By using those data on both local work function and fractional area reported with patchy faces of polycrystalline tungsten, φ~+ and φ~e at 2000—2300 K are theoretically evaluated to be 5.15 ± 0.03 and 4.63 ± 0.01 eV, respectively, thereby yielding Δφ~* = 0.52 ± 0.03 eV. Each of these theoretical values agrees well with each of the experimental ones within the errors of ±0.04 eV. In addition, Δφ~* = 0 deduced theoretically with monocrystalline tungsten consists with literature values (from —0.05 to 0.06 eV, affording 0.01 ± 0.04 eV as average) determined experimentally by several groups of workers. These results lead to the conclusion that the thermionic contrast for polycrystalline tungsten is 0.54 ± 0.04 eV in contrast to Δφ~* = 0 for monocrystalline tungsten.