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

To study the thermionic contrast of polycrystalline surfaces consisting of many patchy faces with different local work functions, those mean work functions (ϕ+ and ϕe) 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 (≳2000 K) high enough to keep the surface virtually clean. Consequently, the thermionic contrast (Δϕ*  ϕ+−ϕe) was determined experimentally to be 0.58 ± 0.03 eV. They are in good agreement with literature values (ϕ+ = 5.11 ± 0.04 eV, ϕe = 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 ϕe 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.