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 (f+ and fe) 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 (02000 K) high enough to keep the surface virtually clean. Consequently, the thermionic contrast (Df* BB f+ fe) was determined experimentally to be 0.58 0.03 eV. They are in good agreement with literature values (f+ = 5.11 0.04 eV, fe = 4.59 0.01 eV and Df* = 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, f+ and fe at 2000 2300 K are theoretically evaluated to be 5.15 0.03 and 4.63 0.01 eV, respectively, thereby yielding Df* = 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, Df* = 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 Df* = 0 for monocrystalline tungsten.