Work function of refractory metals and its dependence upon working conditions

To investigate the stability of work function f. of polycrystalline refractory metals Nb, Mo, Ta, W and Re., the thermal electron current J. from a thin wire heated at a residual gas pressure of Prf1=10y7 Torr was measured as a function of wire temperature Tf1200–2200 K., elapsed time tf0–20 min. after flashing at T)2000 K or the pressure Pa or POf10y7–10y5 Torr.of introduced gas air or oxygen.. Theoretical analysis of experimental data yields the following results. 1. At Pr, the lowest boundary temperature Tb. keeping each metal surface virtually clean ranges from ;1500–1900 K, above which the clean surface work functions fc.of Nb, Mo, Ta, W and Re are kept constant at 4.02, 4.39, 4.28, 4.54 and 4.96 eV, respectively. 2. Below Tb, f is increased by ;0.1–0.9 eV, depending upon metal species. 3. This increase is caused by adsorption of residual gases especially of oxygen.. 4. When air or oxygen is introduced up to ;2.5=10y5 or 5=10y6 Torr, f of Re at 1700 K, for example, is increased from ;5.0 to 5.5 eV, which is governed not by Pabut by Par5 equivalent to PO. 5. The lowest temperature Tc. keeping fc in O2 depends upon PO according to Tcsaqb log PO, where as2795 K and bs115 K in the case of W. 6. Among the five metals, Re is highest in fc , but chemically most stable against oxidation and also mechanically strongest, thereby most stable in J and longest in durability as a thermal electron source metal. q1999 Elsevier Science B.V. All rights reserved.