Segregation of oxygen at metal/oxide-interfaces

By internal oxidation of AgMg- and PdMg-alloys small precipitates of MgO are formed within the Ag- or Pd-matrix. The total area of the metal/oxide interface produced this way becomes several square meters in one cubic centimeter of metal. Segregation of oxygen at the interface is determined by desorption of gaseous oxygen for the Ag/MgO-interface and by trapping of hydrogen for the Pd/MgO-interface. At 673 to 873 K the oxygen coverage of the Ag/MgO boundary does not obey a Langmuir-McLean relation. At the highest oxygen pressure of 10(5) Pa O-coverage reached a maximum value of about 2.2 . 10(14) O-atoms/cm(2). The average free enthalpy of segregation is about -70 kJ/mol O. The results are interpreted in terms of a structural model, where the interface contains vacancies in the terminating oxygen (111)-plane of MgO. Segregation, i.e., filling of the vacancies with oxygen from solid solution, leads to the formation of a two dimensional silver oxide. For Pd/MgO condition were less favorable and oxygen segregation can be determined indirectly by irreversible hydrogen trapping. Oxygen segregation does not occur if the sample is rapidly quenched from 1273 K by drooping it into liquid nitrogen. In order to understand the kinetics of segregation, the diffusion coefficient of oxygen was determined in Pd by measuring solubility and permeability. Measured values can be described by the following relation D = 2.3 . 10(-3) exp[-12,300/T] cm(2)/s.