Experimental research for new materials usable as strong thermal electron emitters

To find new materials usable as strong electron sources, a powdery sample (30–350 mesh) of metal hydride (NaH, LiH, MgH2, CaH2, SrH2, TiH2, ZrH2 or LiAlH4) was deposited on a molybdenum ribbon, and the thermal electron current (J−) emitted from the sample in vacuum was measured as a function of: (1) the sample temperature (T), (2) the pressure (P) of introduced hydrogen, or (3) the time elapsing after a change in T or P. The amount of H2 desorbed from the sample was measured by a mass spectrometer. Theoretical analysis of the data yields the following results. (1) By ageing of LiH at 705 K for 5 h, for example, J− is increased 103 times, corresponding to a work function change of −0.5 eV. (2) This is due to the formation of active sites (Li) by thermal decomposition of LiH. (3) J− from LiH becomes strongest when the decomposition reaches ∼75%. (4) The sites are destroyed by admission of H2 beyond ∼10–5 Torr, but readily constructed again by stopping the admission. (5) NaH is smallest in work function (ϕ≃2 eV), but thermally too unstable to keep J− constant in time. Finally, (6) CaH2 is high in ϕ (∼5 eV), but best as a thermal electron source material among the eight hydrides because it is stronger (>1 mA cm−2) and more stable (<±5%) in J− and longer in durability (>100 h) against heating up to ∼1000 K.