Field Emission Studies on Kinetics of Barium Oxide on Tungsten

Abstract

The field emission microscope has been used to investigate the state of adsorption of barium oxide molecules deposited on tungsten by evaporation. Precautions are taken so as to keep the tube pressure at ultra high vacuum. Pulse method is adopted to protect the state of adsorption from changing with steady application of intense fields.
Surface migration of adsorbed molecules over the entire surface is pronounced only above 700°C and its activation energy is about 4.6 eV.
Work function decreases very rapidly for the first 0.15 layer with the rate which can be explained quantitatively by considering the adsorbed molecules as oriented dipoles. The mean dipole moment of an adsorbed molecule is estimated to be (13.5±8.1)×10⁻¹⁸ esu. Crystallites of barium oxide are formed when covering factor amounts to about unity.
Chemical reaction takes place between barium oxide and tungsten for temperatures above 800°C and the reaction product persists stably unless temperature is raised above 1000°C. The activation energy of the reaction is about 3.3eV