To clarify the thermionic property of solids, the mean work function effective for positive ion from polycrystalline surfaces (ϕ+) is theoretically studied together with that for electron emission (ϕe). The theoretical values of ϕ+ and ϕe thus evaluated typically with W are in good agreement respectively with those experimental ones accepted today. The thermionic contrast (Δϕ* ≡ ϕ+ – ϕe) is determined to be 0.56–0.57eV for W. For other polycrystalline metals (Nb, Mo, Ta, Re, Ir and Pt), it ranges from about 0.4 to 0.8eV in contrast with monocrystalline ones having Δϕ* = 0. Consequently, the fact of Δϕ* > 0 should be taken into consideration with any polycrystalline surface, and ϕ+ should be adopted whenever we try to analyze those data on positive ion emission due to any of the processes such as thermal stimulation and ion bombardment at heterogeneous surfaces. This article concludes that such typical data on thermal- and secondary-positive ion emissions are analyzed reasonably by adoption of ϕ+ instead of ϕe.
