Oxidation of Barium: A Confirmation of Mott's Theory of Oxidation

Abstract

MEASUREMENTS of the rate of taking up of oxygen by barium films¹ have been continued in an attempt to discover the mechanism of the growth of the oxide layer. The results obtained support the ionic theory of oxidation of metals developed by Mott². In this theory it is supposed that positive metal ions and electrons diffuse from the metal through the oxide, the metal ions moving via interstitial positions in the oxide lattice. At the free surface of the oxide, these ions combine with adsorbed oxygen. In this way the thickness of the oxide layer is increased. In oxide films less than about 50 A. thick, the movement of ions is influenced principally by the strong electric field which is set up across the oxide because of the contact potential difference between the metal and the adsorbed oxygen. In thicker films the movement of ions, and hence the rate of growth of the oxide, is influenced chiefly by temperature. From the theory it is further predicted that, whatever the thickness of oxide, the movement of ions becomes controlled by temperature when this is raised above a critical value. Therefore, above the critical temperature the oxide layer should continue to grow until all the metal has been oxidized. Below the critical temperature, it is predicted that the growth of oxide stops at a limiting thickness. Further, the limiting thickness itself should vary with temperature when this is less than the critical value.