Mean work functions effective for negative-ionic, electronic and positive-ionic emissions from polycrystalline surfaces

To elucidate the thermionic property of polycrystalline surfaces, a further study is made on the mean work functions (φ−, φe and φ+) effective for negative-ionic, electronic and positive-ionic emissions. Comparison between theoretical analyses and experimental data yields the conclusions as follows. (1) The equation of φ− = φe holds always with both mono- and polycrystalline surfaces. (2) The relation of φ− = φe < φ+ applies to polycrystalline surfaces because they bear the thermionic contrast (Δφ* ≡ φ+ − φe > 0). (3) The value of Δφ* ranges from ∼0.4 to 0.9 eV depending upon the surface species of polycrystalline metals (e.g., W, Re and Pt), whilst Δφ* = 0 for monocrystalline surfaces. (4) When the degree of monocrystallization (δm) is less than ∼50%, the theoretical value of Δφ* is virtually independent of δm and agrees well with experimental data, nearly the same within ±0.1 eV among the so-called “polycrystalline” surfaces of W. (5) As δm increases beyond ∼80 up to 100%, Δφ* decreases rapidly down to 0 eV, showing again a good agreement between theory and experiment. (6) Our theoretical model is valid in evaluating the effective mean work functions, irrespective of the range of δm.