The Thermionic Properties of Tantalum

Tantalum was carefully outgassed and its photoelectric and thermionic properties simultaneously studied. As was shown in a previous paper, heating 1000 hours at temperatures up to 2200°K produced an apparent stable condition of the surface. Further heat treatment at temperatures up to 2500°K produced a final stable condition. The values obtained for the thermionic constants of the $T^2$ law are, when corrected for the Schottky effect, as follows: (1) For surface in first apparent stable condition, $b=52,532\mathrm{}$°K and $h=4.53\mathrm{}$ volts. (2) For surface in final stable condition, $b=47\mathrm{}$, 560°K, $h=4.10\mathrm{}$ volts and $A=37.2\mathrm{}$ amp./${\mathrm{cm}}^2$ ${\mathrm{degree}}^2$. A comparison of these results with the previously published photoelectric data shows that the value of the photoelectric work function determined according to Fowler's method agrees with the value of the thermionic work function of the same surface regardless of whether the surface is thoroughly outgassed.