Effective work functions for thermal positive-ionic and electronic emissions from tantalum heated in a high vacuum

Abstract

To elucidate the thermionic property of a polycrystal surface of Ta heated in an easily attainable high vacuum (~ 10 −5Pa), the effective work functions (f + and f θ ) for thermal positive-ionic and electronic emissions from the surface were measured as a function of surface temperature (T). (1) Above ~ 1800K, f + and f θ were constant at ~ 5.1 and 4.3 eV, respectively, corresponding to a clean surface of Ta. (2) As T decreased from ~ 1800 to 1500 K, f + and f θ were increased up to ~ 5.3 and 4.4 eV, respectively, mainly by adsorption of residual gases. (3) Below ~ 1500 K, f + showed different patterns according to the sample species (RbCl, RbBr, CsF or CsCl), ranging from ~ 4.9 up to 6.7 eV. This was caused by co-adsorption of each sample and the gases. (4) The thermionic contrast ( f + − f θ ) was constant at ~ 0.90 ± 0.03 eV, showing not to depend on T.