Theory for oxygen vacancy mediated electron transport in barrier oxides from atomistic modeling
Oxidation of high-temperature alloys is commonly subdivided into anode, cathode and transport processes. The anode process takes place at the metal/oxide interface while the cathode process is at the gas/oxide interface. In case of inward oxide growth, the anode related processes are mainly associated with electrons and charged oxygen vacancies diffusing to the gas/oxide interface, i.e. to the cathode, where oxygen reduction takes place. The lattice is continuously being "healed" by the recombination of oxygen ions and charged oxygen vacancies. In as series of studies we have previously addressed the formations and diffusivities of charged and uncharged oxygen vacancies in the large band gap oxides, alumina and zirconia, on good grounds assuming transport of electrons not to be rate limiting. Indeed, we have previously underscored the roles of oxygen vacancies in offering impurity states in the band gap. The present application concerns quantifying oxygen vacancy mediated electron transport to test the validity of said assumption. Thus, impact of differential polaronic lattice relaxations in the vicinity of charged and uncharged oxygen vacancies in the bulk as well as at grain boundaries is sought. We perform atomistic first principles calculations by means of density functional theory in order to determine parameters for a free-energy "toy model" analogous to Marcus theory. This project comprises part of a collaborative effort jointly with experiment. It is undertaken within the competence center for high-temperature corrosion.