Schottky Effect and Contact Potential Measurements on Thoriated Tungsten Filaments

The Schottky effect for thoriated tungsten filaments is investigated and Schottky's relation that log i∝(V)1/2 is verified at high fields, but fails at gradients below 10000 volts/cm, even for fully activated surfaces. This lack of saturation at low fields is accentuated by the effect of bombardment with high velocity positive ions, such bombardment apparently producing a surface roughening and consequently increased fields in local areas. Investigation with low accelerating and retarding voltages, while varying the temperature and state of activation of the filament, allows a comparison of the contact potential and the work function at the absolute zero of temperature (φ₀) for the thoriated surface. The two values (contact potential and φ₀) are found to vary with a one-to-one correspondence if φ, the work function at the temperature of measurement, be allowed a temperature coefficient. This coefficient is much larger than can be attributed to the specific heat of the electrons in the metal. The electrons with high velocities show a Maxwellian velocity distribution. Near the zero of potential there are certain anomalous effects and a "patch surface" is postulated which, in a qualitative manner, explains this behavior and the lack of current saturation with voltage gradients below a limiting value.