Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
5369127 | Applied Surface Science | 2006 | 10 Pages |
To elucidate the thermionic property of polycrystalline surfaces, a further study is made on the mean work functions (Ïâ, Ïe and Ï+) 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 Ïâ = Ïe holds always with both mono- and polycrystalline surfaces. (2) The relation of Ïâ = Ïe < Ï+ applies to polycrystalline surfaces because they bear the thermionic contrast (ÎÏ* â¡Â Ï+ â Ïe > 0). (3) The value of ÎÏ* ranges from â¼0.4 to 0.9 eV depending upon the surface species of polycrystalline metals (e.g., W, Re and Pt), whilst ÎÏ* = 0 for monocrystalline surfaces. (4) When the degree of monocrystallization (δm) is less than â¼50%, the theoretical value of ÎÏ* 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 â¼80 up to 100%, ÎÏ* 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.