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 (w , we and w+) 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 w = we holds always with both mono- and polycrystalline surfaces. (2) The relation of w = we < w+ applies to polycrystalline surfaces because they bear the thermionic contrast (Dw* w+ we > 0). (3) The value of Dw* ranges from 0.4 to 0.9 eV depending upon the surface species of polycrystalline metals (e.g., W, Re and Pt), whilst Dw* = 0 for monocrystalline surfaces. (4) When the degree of monocrystallization (dm) is less than 50%, the theoretical value of Dw* is virtually independent of dm and agrees well with experimental data, nearly the same within 0.1 eV among the so-called ‘‘polycrystalline’’ surfaces of W. (5) As dm increases beyond 80 up to 100%, Dw* 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 dm.