Variation of Concentration with Depth of Absorbed Oxygen in Niobium during Oxidation

Abstract

The dissolution of oxygen in niobium is governed by Fick's second law applied with the appropriate boundary conditions. Two different boundary conditions have been proposed to describe the physical situation: (i) That the surface concentration of oxygen atoms is constant; and (ii) that the flux of oxygen into the metal is directly proportional to the difference between the actual concentration on the surface at any time and the concentration which would be in equilibrium with the partial pressure of the oxygen. The absorption of oxygen in (001) niobium at temperatures of 450–650 °C and pressures of 3×10⁻⁶−3×10⁻⁴ Torr O₂ has been measured using ellipsometry. The results indicate that for the pressure and temperature ranges investigated the latter boundary condition is the appropriate one.