How to measure volume diffusivities and grain boundary diffusivities of oxygen in polycrystalline oxides

In this paper we discuss the special aspects of measuring oxygen diffusivities in polycrystalline oxides by 18O/16O isotope exchange experiments via SIMS depth profiling. It is common to plot the SIMS data on a logarithmic scale as a function of z6/5 (where z is the depth scale) and to determine the slope of the linear tail. From the slope one can then calculate the grain boundary diffusion coefficient when the volume diffusion coefficient in the grains is known or determined from the first part of the SIMS depth profile. However, surface roughness and the necessity to deposit a conductive layer on the surface of poorly conducting oxides to prevent charging effects during the SIMS analysis can considerably reduce the accuracy of diffusion coefficients determined from shallow volume diffusion profiles. Therefore, we suggest a different method to extract volume diffusion coefficients from SIMS depth profiles. This method was successfully checked for oxygen grain boundary diffusion experiments on high purity dense polycrystalline 3/2-mullite material. In order to obtain accurate diffusion coefficients from the SIMS depth profiles it is important to choose appropriate annealing conditions. Therefore, this paper also gives a guideline for the choice of suitable annealing time-temperature intervals for oxygen grain boundary diffusion experiments.