The Effect of Oxygen Potential on the Surface Self-Diffusion Coefficient of Silver

The surface self-diffusion coefficient of silver was measured by the grain boundary grooving method in atmospheres with various oxygen potentials. In an argon atmosphere with PO2=10−11 Pa, the surface self-diffusion coefficient can be expressed by the following equation:Ds=0.249exp(−166kJ/mol·RT)m2⁄s(973–1073K).This equation coincides well with Gjostein’s experimental formula for fcc metals.As the oxygen potential increased from PO2=10−11 Pa, the surface self-diffusion coefficient decreased and became minimum at PO2=10−2 Pa. At PO2=10−2 Pa, the temperature dependence of the surface self-diffusion coefficient cannot be expressed by Arrhenius’ equation. As the oxygen potential further increased above PO2=10−2 Pa, the surface self-diffusion coefficient increased and the activation energy for the diffusion at PO2=105 Pa was smaller than that at PO2=10−11 Pa. The surface self-diffusion coefficient at PO2=105 Pa can be expressed by the following equation:Ds=1.71×10−3exp(−106kJ/mol·RT)m2⁄s(973–1073K).The following model for the surface structure of silver at various oxygen potentials has been proposed. At PO2=10−11 Pa, oxygen ions are hardly adsorbed and the surface of silver remains clean. At PO2=10−2 Pa, oxygen ions are adsorbed locally, and neighboring silver atoms are ionized and stabilized. Then the number of sites, from which mobile ad-atoms that contribute to the diffusivity are produced, decreases. At PO2=105 Pa, the adsorbed oxygen ions are arranged in order and a 2-dimensional ionic crystal is formed on the surface. Then the jump distance becomes longer and the activation energy for the diffusion becomes smaller by an overlapping of long range Coulomb’s force. As a result, the surface self-diffusion coefficient increases.