Abstract
Negative surface ionization of various elements on low-work-function emitters (pure polycrystalline tantalum, thorium oxide, lanthanum hexaboride, tantalum and zirconium carbides) has been investigated. We find a good agreement between the experimental values of the negative surface-ionization coefficients and the theoretical values calculated from the Saha—Langmuir relationship using the work function of the emitters measured in this work. Chemical reactivity and adsorption processes limit the utilization of thorium oxide and lanthanum hexaboride. On the other hand, tantalum and zirconium carbides which are refractory, stable and chemically inert, seem to be of interest for negative surface-ionization applications. In particular, the negative surface-ionization coefficients on zirconium carbide emitters (Φ = 3.2 ± 0.2 eV) are two or three orders of magnitude greater than on pure tantalum emitters.
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