Work functions of rhenium and tungsten determined by thermionic and mass spectrometric measurements

Abstract

Measurements have been made of the changes in the effective work function of polycrystalline tungsten, rhenium, and tungsten—rhenium alloys at high temperatures, as a function of the ambient oxygen pressures in the range of 10 −8 to 10 −5 Torr. The observed work function changes were determined by two techniques: (1) the standard thermionic emission method, utilizing Richardson's equation and (2) a mass spectrometric method wherein the positive ion emission rates of the specimen were monitored as a function of specimen temperature. In this latter method, calculations were made on the basis of the Saha—Langmuir equation that relates ion emission rates to the ionization potential, work function, and absolute temperature of the sample. Experimental data from both methods in the temperature range 1700 K to 2150 K yielded consistent work function values for the pure metals. A maximum work function increase about 0.7 eV was noted for both tungsten and rhenium at 1710 K at 10 −5 Torr, compared to a 10 −8 Torr reference. In the case of the alloys, there is a large difference in the effective work function change as determined by thermionic and mass spectrometric measurements. Ion emission is demnated by the high work function element, while electron emission is dominated by the low work function element.