The measurement of O diffusivities in polycrystalline oxides, by means of 18O/16O isotope exchange and secondary ion mass spectrometry depth profiling, was considered. It was noted that it was usual to plot the secondary ion mass spectrometry data on a logarithmic scale, as a function of z6/5 (where z was the depth scale), and to determine the slope of the linear tail. From the slope, it was then possible to calculate the grain boundary diffusion coefficient when the volume diffusion coefficient in the grains was known or could be determined from the first part of the secondary ion mass spectrometry depth profile. Surface roughness, and the need to deposit a conductive layer onto the surface of poorly conducting oxides (to prevent charging effects), could considerably reduce the accuracy of the diffusion coefficients which were deduced from shallow volume-diffusion profiles. Therefore, a different method was suggested for extracting volume diffusion coefficients from secondary ion mass spectrometry depth profiles. This method was successfully checked for O grain-boundary diffusion experiments on high-purity dense polycrystalline 3/2-mullite material. In order to obtain accurate diffusion coefficients from the secondary ion mass spectrometry depth profiles, it was important to choose suitable annealing conditions.

How to Measure Volume Diffusivities and Grain Boundary Diffusivities of Oxygen in Polycrystalline Oxides. P.Fielitz, G.Borchardt, M.Schmücker, H.Schneider: Solid State Ionics, 2003, 160[1-2], 75-83