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

Abstract

Abstract To elucidate the thermionic property of polycrystalline surfaces, a further study is made on the mean work functions ( φ − , φ e 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 − = φ e holds always with both mono- and polycrystalline surfaces. (2) The relation of φ − = φ e φ + applies to polycrystalline surfaces because they bear the thermionic contrast (Δ φ * ≡ φ + − φ e > 0). (3) The value of Δ φ * ranges from ∼0.4 to 0.9 eV depending upon the surface species of polycrystalline metals (e.g., W, Re and Pt), whilst Δ φ * = 0 for monocrystalline surfaces. (4) When the degree of monocrystallization ( δ m ) is less than ∼50%, the theoretical value of Δ φ * is virtually independent of δ m and agrees well with experimental data, nearly the same within ±0.1 eV among the so-called “polycrystalline” surfaces of W. (5) As δ m increases beyond ∼80 up to 100%, Δ φ * 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 δ m .